4.0.1.RELEASE
Copyright © 2004-2013
Table of Contents
The Spring Framework is a lightweight solution and a potential one-stop-shop for building your enterprise-ready applications. However, Spring is modular, allowing you to use only those parts that you need, without having to bring in the rest. You can use the IoC container, with Struts on top, but you can also use only the Hibernate integration code or the JDBC abstraction layer. The Spring Framework supports declarative transaction management, remote access to your logic through RMI or web services, and various options for persisting your data. It offers a full-featured MVC framework, and enables you to integrate AOP transparently into your software.
Spring is designed to be non-intrusive, meaning that your domain logic code generally has no dependencies on the framework itself. In your integration layer (such as the data access layer), some dependencies on the data access technology and the Spring libraries will exist. However, it should be easy to isolate these dependencies from the rest of your code base.
This document is a reference guide to Spring Framework features. If you have any requests, comments, or questions on this document, please post them on the user mailing list or on the support forums at http://forum.spring.io/.
This reference guide provides detailed information about the Spring Framework. It provides comprehensive documentation for all features, as well as some background about the underlying concepts (such as "Dependency Injection") that Spring has embraced.
If you are just getting started with Spring, you may want to begin with the lighter "Getting Started" guides that are available from http://spring.io. As well as being easier to digest, these guide are very task focused. They also cover other projects from the Spring portfolio that you might want to consider when solving a particular problem.
Getting Started Building a RESTful Web Service would be an excellent first choice to get your feet wet.
Spring Framework is a Java platform that provides comprehensive infrastructure support for developing Java applications. Spring handles the infrastructure so you can focus on your application.
Spring enables you to build applications from "plain old Java objects" (POJOs) and to apply enterprise services non-invasively to POJOs. This capability applies to the Java SE programming model and to full and partial Java EE.
Examples of how you, as an application developer, can use the Spring platform advantage:
Java applications — a loose term that runs the gamut from constrained applets to n-tier server-side enterprise applications — typically consist of objects that collaborate to form the application proper. Thus the objects in an application have dependencies on each other.
Although the Java platform provides a wealth of application development functionality, it lacks the means to organize the basic building blocks into a coherent whole, leaving that task to architects and developers. True, you can use design patterns such as Factory, Abstract Factory, Builder, Decorator, and Service Locator to compose the various classes and object instances that make up an application. However, these patterns are simply that: best practices given a name, with a description of what the pattern does, where to apply it, the problems it addresses, and so forth. Patterns are formalized best practices that you must implement yourself in your application.
The Spring Framework Inversion of Control (IoC) component addresses this concern by providing a formalized means of composing disparate components into a fully working application ready for use. The Spring Framework codifies formalized design patterns as first-class objects that you can integrate into your own application(s). Numerous organizations and institutions use the Spring Framework in this manner to engineer robust, maintainable applications.
The Spring Framework consists of features organized into about 20 modules. These modules are grouped into Core Container, Data Access/Integration, Web, AOP (Aspect Oriented Programming), Instrumentation, and Test, as shown in the following diagram.
The Core Container consists of the Core, Beans, Context, and Expression Language modules.
The Core and Beans modules provide the fundamental parts of
the framework, including the IoC and Dependency Injection features. The BeanFactory
is
a sophisticated implementation of the factory pattern. It removes the need for
programmatic singletons and allows you to decouple the configuration and specification
of dependencies from your actual program logic.
The Context module builds on the solid base provided by the
Core and Beans modules: it is a means to access objects in a
framework-style manner that is similar to a JNDI registry. The Context module inherits
its features from the Beans module and adds support for internationalization (using, for
example, resource bundles), event-propagation, resource-loading, and the transparent
creation of contexts by, for example, a servlet container. The Context module also
supports Java EE features such as EJB, JMX ,and basic remoting. The ApplicationContext
interface is the focal point of the Context module.
The Expression Language module provides a powerful expression language for querying and manipulating an object graph at runtime. It is an extension of the unified expression language (unified EL) as specified in the JSP 2.1 specification. The language supports setting and getting property values, property assignment, method invocation, accessing the context of arrays, collections and indexers, logical and arithmetic operators, named variables, and retrieval of objects by name from Spring’s IoC container. It also supports list projection and selection as well as common list aggregations.
The Data Access/Integration layer consists of the JDBC, ORM, OXM, JMS and Transaction modules.
The JDBC module provides a JDBC-abstraction layer that removes the need to do tedious JDBC coding and parsing of database-vendor specific error codes.
The ORM module provides integration layers for popular object-relational mapping APIs, including JPA, JDO, and Hibernate. Using the ORM package you can use all of these O/R-mapping frameworks in combination with all of the other features Spring offers, such as the simple declarative transaction management feature mentioned previously.
The OXM module provides an abstraction layer that supports Object/XML mapping implementations for JAXB, Castor, XMLBeans, JiBX and XStream.
The Java Messaging Service (JMS) module contains features for producing and consuming messages.
The Transaction module supports programmatic and declarative transaction management for classes that implement special interfaces and for all your POJOs (plain old Java objects).
The Web layer consists of the Web, Web-Servlet, WebSocket and Web-Portlet modules.
Spring’s Web module provides basic web-oriented integration features such as multipart file-upload functionality and the initialization of the IoC container using servlet listeners and a web-oriented application context. It also contains the web-related parts of Spring’s remoting support.
The Web-Servlet module contains Spring’s model-view-controller (MVC) implementation for web applications. Spring’s MVC framework provides a clean separation between domain model code and web forms, and integrates with all the other features of the Spring Framework.
The Web-Portlet module provides the MVC implementation to be used in a portlet environment and mirrors the functionality of Web-Servlet module.
Spring’s AOP module provides an AOP Alliance-compliant aspect-oriented programming implementation allowing you to define, for example, method-interceptors and pointcuts to cleanly decouple code that implements functionality that should be separated. Using source-level metadata functionality, you can also incorporate behavioral information into your code, in a manner similar to that of .NET attributes.
The separate Aspects module provides integration with AspectJ.
The Instrumentation module provides class instrumentation support and classloader implementations to be used in certain application servers.
The building blocks described previously make Spring a logical choice in many scenarios, from applets to full-fledged enterprise applications that use Spring’s transaction management functionality and web framework integration.
Spring’s declarative transaction management features make
the web application fully transactional, just as it would be if you used EJB
container-managed transactions. All your custom business logic can be implemented with
simple POJOs and managed by Spring’s IoC container. Additional services include support
for sending email and validation that is independent of the web layer, which lets you
choose where to execute validation rules. Spring’s ORM support is integrated with JPA,
Hibernate and and JDO; for example, when using Hibernate, you can continue to use
your existing mapping files and standard Hibernate SessionFactory
configuration. Form
controllers seamlessly integrate the web-layer with the domain model, removing the need
for ActionForms
or other classes that transform HTTP parameters to values for your
domain model.
Sometimes circumstances do not allow you to completely switch to a different framework.
The Spring Framework does not force you to use everything within it; it is not an
all-or-nothing solution. Existing front-ends built with Struts, Tapestry, JSF
or other UI frameworks can be integrated with a Spring-based middle-tier, which allows
you to use Spring transaction features. You simply need to wire up your business logic
using an ApplicationContext
and use a WebApplicationContext
to integrate your web
layer.
When you need to access existing code through web services, you can use Spring’s
Hessian-
, Burlap-
, Rmi-
or JaxRpcProxyFactory
classes. Enabling remote access to
existing applications is not difficult.
The Spring Framework also provides an access and abstraction layer for Enterprise JavaBeans, enabling you to reuse your existing POJOs and wrap them in stateless session beans for use in scalable, fail-safe web applications that might need declarative security.
Dependency management and dependency injection are different things. To get those nice
features of Spring into your application (like dependency injection) you need to
assemble all the libraries needed (jar files) and get them onto your classpath at
runtime, and possibly at compile time. These dependencies are not virtual components
that are injected, but physical resources in a file system (typically). The process of
dependency management involves locating those resources, storing them and adding them to
classpaths. Dependencies can be direct (e.g. my application depends on Spring at
runtime), or indirect (e.g. my application depends on commons-dbcp
which depends on
commons-pool
). The indirect dependencies are also known as "transitive" and it is
those dependencies that are hardest to identify and manage.
If you are going to use Spring you need to get a copy of the jar libraries that comprise
the pieces of Spring that you need. To make this easier Spring is packaged as a set of
modules that separate the dependencies as much as possible, so for example if you don’t
want to write a web application you don’t need the spring-web modules. To refer to
Spring library modules in this guide we use a shorthand naming convention spring-*
or
spring-*.jar,
where *
represents the short name for the module (e.g. spring-core
,
spring-webmvc
, spring-jms
, etc.). The actual jar file name that you use is normally
the module name concatenated with the version number
(e.g. spring-core-4.0.1.RELEASE.jar).
Each release of the Spring Framework will publish artifacts to the following places:
spring-*-<version>.jar
and the Maven groupId
is org.springframework
.
So the first thing you need to decide is how to manage your dependencies: we generally recommend the use of an automated system like Maven, Gradle or Ivy, but you can also do it manually by downloading all the jars yourself. We provide detailed instructions later in this chapter.
Although Spring provides integration and support for a huge range of enterprise and other external tools, it intentionally keeps its mandatory dependencies to an absolute minimum: you shouldn’t have to locate and download (even automatically) a large number of jar libraries in order to use Spring for simple use cases. For basic dependency injection there is only one mandatory external dependency, and that is for logging (see below for a more detailed description of logging options).
Next we outline the basic steps needed to configure an application that depends on Spring, first with Maven and then with Gradle and finally using Ivy. In all cases, if anything is unclear, refer to the documentation of your dependency management system, or look at some sample code - Spring itself uses Gradle to manage dependencies when it is building, and our samples mostly use Gradle or Maven.
If you are using Maven for dependency management you don’t even need to supply the logging dependency explicitly. For example, to create an application context and use dependency injection to configure an application, your Maven dependencies will look like this:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>4.0.1.RELEASE</version> <scope>runtime</scope> </dependency> </dependencies>
That’s it. Note the scope can be declared as runtime if you don’t need to compile against Spring APIs, which is typically the case for basic dependency injection use cases.
The example above works with the Maven Central repository. To use the Spring Maven repository (e.g. for milestones or developer snapshots), you need to specify the repository location in your Maven configuration. For full releases:
<repositories> <repository> <id>io.spring.repo.maven.release</id> <url>http://repo.spring.io/release/</url> <snapshots><enabled>false</enabled></snapshots> </repository> </repositories>
For milestones:
<repositories> <repository> <id>io.spring.repo.maven.milestone</id> <url>http://repo.spring.io/milestone/</url> <snapshots><enabled>false</enabled></snapshots> </repository> </repositories>
And for snapshots:
<repositories> <repository> <id>io.spring.repo.maven.snapshot</id> <url>http://repo.spring.io/snapshot/</url> <snapshots><enabled>true</enabled></snapshots> </repository> </repositories>
It is possible to accidentally mix different versions of Spring JARs when using Maven. For example, you may find that a third-party library, or another Spring project, pulls in a transitive dependency to an older release. If you forget to explicitly declare a direct dependency yourself, all sorts of unexpected issues can arise.
To overcome such problems Maven supports the concept of a "bill of materials" (BOM)
dependency. You can import the spring-framework-bom
in your dependencyManagement
section to ensure that all spring dependencies (both direct and transitive) are at
the same version.
<dependencyManagement> <dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-framework-bom</artifactId> <version>4.0.1.RELEASE</version> <type>pom</type> <scope>import</scope> </dependency> </dependencies> </dependencyManagement>
An added benefit of using the BOM is that you no longer need to specify the <version>
attribute when depending on Spring Framework artifacts:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> </dependency> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-web</artifactId> </dependency> <dependencies>
To use the Spring repository with the Gradle build system,
include the appropriate URL in the repositories
section:
repositories { mavenCentral() // and optionally... maven { url "http://repo.spring.io/release" } }
You can change the repositories
URL from /release
to /milestone
or /snapshot
as
appropriate. Once a repository has been configured, you can declare dependencies in the
usual Gradle way:
dependencies { compile("org.springframework:spring-context:4.0.1.RELEASE") testCompile("org.springframework:spring-test:4.0.1.RELEASE") }
If you prefer to use Ivy to manage dependencies then there are similar configuration options.
To configure Ivy to point to the Spring repository add the following resolver to your
ivysettings.xml
:
<resolvers> <ibiblio name="io.spring.repo.maven.release" m2compatible="true" root="http://repo.spring.io/release/"/> </resolvers>
You can change the root
URL from /release/
to /milestone/
or /snapshot/
as
appropriate.
Once configured, you can add dependencies in the usual way. For example (in ivy.xml
):
<dependency org="org.springframework" name="spring-core" rev="4.0.1.RELEASE" conf="compile->runtime"/>
Although using a build system that supports dependency management is the recommended way to obtain the Spring Framework, it is still possible to download a distribution zip file.
Distribution zips are published to the Spring Maven Repository (this is just for our convenience, you don’t need Maven or any other build system in order to download them).
To download a distribution zip open a web browser to
http://repo.spring.io/release/org/springframework/spring and select the appropriate
subfolder for the version that you want. Distribution files end -dist.zip
, for example
spring-framework-4.0.1.RELEASE-RELEASE-dist.zip
. Distributions are also published
for milestones and
snapshots.
Logging is a very important dependency for Spring because a) it is the only mandatory external dependency, b) everyone likes to see some output from the tools they are using, and c) Spring integrates with lots of other tools all of which have also made a choice of logging dependency. One of the goals of an application developer is often to have unified logging configured in a central place for the whole application, including all external components. This is more difficult than it might have been since there are so many choices of logging framework.
The mandatory logging dependency in Spring is the Jakarta Commons Logging API (JCL). We
compile against JCL and we also make JCL Log
objects visible for classes that extend
the Spring Framework. It’s important to users that all versions of Spring use the same
logging library: migration is easy because backwards compatibility is preserved even
with applications that extend Spring. The way we do this is to make one of the modules
in Spring depend explicitly on commons-logging
(the canonical implementation of JCL),
and then make all the other modules depend on that at compile time. If you are using
Maven for example, and wondering where you picked up the dependency on
commons-logging
, then it is from Spring and specifically from the central module
called spring-core
.
The nice thing about commons-logging
is that you don’t need anything else to make your
application work. It has a runtime discovery algorithm that looks for other logging
frameworks in well known places on the classpath and uses one that it thinks is
appropriate (or you can tell it which one if you need to). If nothing else is available
you get pretty nice looking logs just from the JDK (java.util.logging or JUL for short).
You should find that your Spring application works and logs happily to the console out
of the box in most situations, and that’s important.
Unfortunately, the runtime discovery algorithm in commons-logging
, while convenient
for the end-user, is problematic. If we could turn back the clock and start Spring now
as a new project it would use a different logging dependency. The first choice would
probably be the Simple Logging Facade for Java ( SLF4J), which is
also used by a lot of other tools that people use with Spring inside their applications.
There are basically two ways to switch off commons-logging
:
spring-core
module (as it is the only module that
explicitly depends on commons-logging
)
commons-logging
dependency that replaces the library with
an empty jar (more details can be found in the
SLF4J FAQ)
To exclude commons-logging, add the following to your dependencyManagement
section:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.0.1.RELEASE</version> <exclusions> <exclusion> <groupId>commons-logging</groupId> <artifactId>commons-logging</artifactId> </exclusion> </exclusions> </dependency> </dependencies>
Now this application is probably broken because there is no implementation of the JCL API on the classpath, so to fix it a new one has to be provided. In the next section we show you how to provide an alternative implementation of JCL using SLF4J as an example.
SLF4J is a cleaner dependency and more efficient at runtime than commons-logging
because it uses compile-time bindings instead of runtime discovery of the other logging
frameworks it integrates. This also means that you have to be more explicit about what
you want to happen at runtime, and declare it or configure it accordingly. SLF4J
provides bindings to many common logging frameworks, so you can usually choose one that
you already use, and bind to that for configuration and management.
SLF4J provides bindings to many common logging frameworks, including JCL, and it also
does the reverse: bridges between other logging frameworks and itself. So to use SLF4J
with Spring you need to replace the commons-logging
dependency with the SLF4J-JCL
bridge. Once you have done that then logging calls from within Spring will be translated
into logging calls to the SLF4J API, so if other libraries in your application use that
API, then you have a single place to configure and manage logging.
A common choice might be to bridge Spring to SLF4J, and then provide explicit binding
from SLF4J to Log4J. You need to supply 4 dependencies (and exclude the existing
commons-logging
): the bridge, the SLF4J API, the binding to Log4J, and the Log4J
implementation itself. In Maven you would do that like this
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.0.1.RELEASE</version> <exclusions> <exclusion> <groupId>commons-logging</groupId> <artifactId>commons-logging</artifactId> </exclusion> </exclusions> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>jcl-over-slf4j</artifactId> <version>1.5.8</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.5.8</version> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-log4j12</artifactId> <version>1.5.8</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.14</version> </dependency> </dependencies>
That might seem like a lot of dependencies just to get some logging. Well it is, but it
is optional, and it should behave better than the vanilla commons-logging
with
respect to classloader issues, notably if you are in a strict container like an OSGi
platform. Allegedly there is also a performance benefit because the bindings are at
compile-time not runtime.
A more common choice amongst SLF4J users, which uses fewer steps and generates fewer
dependencies, is to bind directly to Logback. This removes the
extra binding step because Logback implements SLF4J directly, so you only need to depend
on two libraries not four ( jcl-over-slf4j
and logback
). If you do that you might
also need to exclude the slf4j-api dependency from other external dependencies (not
Spring), because you only want one version of that API on the classpath.
Many people use Log4j as a logging framework for configuration and management purposes. It’s efficient and well-established, and in fact it’s what we use at runtime when we build and test Spring. Spring also provides some utilities for configuring and initializing Log4j, so it has an optional compile-time dependency on Log4j in some modules.
To make Log4j work with the default JCL dependency ( commons-logging
) all you need to
do is put Log4j on the classpath, and provide it with a configuration file (
log4j.properties
or log4j.xml
in the root of the classpath). So for Maven users this
is your dependency declaration:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-core</artifactId> <version>4.0.1.RELEASE</version> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.14</version> </dependency> </dependencies>
And here’s a sample log4j.properties for logging to the console:
log4j.rootCategory=INFO, stdout log4j.appender.stdout=org.apache.log4j.ConsoleAppender log4j.appender.stdout.layout=org.apache.log4j.PatternLayout log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %t %c{2}:%L - %m%n log4j.category.org.springframework.beans.factory=DEBUG
Many people run their Spring applications in a container that itself provides an
implementation of JCL. IBM Websphere Application Server (WAS) is the archetype. This
often causes problems, and unfortunately there is no silver bullet solution; simply
excluding commons-logging
from your application is not enough in most situations.
To be clear about this: the problems reported are usually not with JCL per se, or even
with commons-logging
: rather they are to do with binding commons-logging
to another
framework (often Log4J). This can fail because commons-logging
changed the way they do
the runtime discovery in between the older versions (1.0) found in some containers and
the modern versions that most people use now (1.1). Spring does not use any unusual
parts of the JCL API, so nothing breaks there, but as soon as Spring or your application
tries to do any logging you can find that the bindings to Log4J are not working.
In such cases with WAS the easiest thing to do is to invert the class loader hierarchy (IBM calls it "parent last") so that the application controls the JCL dependency, not the container. That option isn’t always open, but there are plenty of other suggestions in the public domain for alternative approaches, and your mileage may vary depending on the exact version and feature set of the container.
The Spring Framework was first released in 2004; since then there have been significant
major revisions: Spring 2.0 provided XML namespaces and AspectJ support; Spring 2.5
embraced annotation-driven configuration; Spring 3.0 introduced a strong Java 5+ foundation
across the framework codebase, and features such as the Java-based @Configuration
model.
Version 4.0 is the latest major release of the Spring Framework and the first to fully support Java 8 features. You can still use Spring with older versions of Java, however, the minimum requirement has now been raised to Java SE 6. We have also taken the opportunity of a major release to remove many deprecated classes and methods.
A migration guide for upgrading to Spring 4.0 is available on the Spring Framework GitHub Wiki.
The new spring.io website provides a whole series of "Getting Started" guides to help you learn Spring. You can read more about the guides in the Chapter 1, Getting Started With Spring section in this document. The new website also provides a comprehensive overview of the many additional projects that are released under the Spring umbrella.
If you are a Maven user you may also be interested in the helpful bill of materials POM file that is now published with each Spring Framework release.
All deprecated packages, and many deprecated classes and methods have been removed with version 4.0. If you are upgrading from a previous release of Spring, you should ensure that you have fixed any deprecated calls that you were making to outdated APIs.
For a complete set of changes, check out the API Differences Report.
Note that optional third-party dependencies have been raised to a 2010/2011 minimum (i.e. Spring 4 generally only supports versions released in late 2010 or later now): notably, Hibernate 3.6+, EhCache 2.1+, Quartz 1.8+, Groovy 1.8+, and Joda-Time 2.0+. As an exception to the rule, Spring 4 requires the recent Hibernate Validator 4.3+, and support for Jackson has been focused on 2.0+ now (with Jackson 1.8/1.9 support retained for the time being where Spring 3.2 had it; now just in deprecated form).
Spring Framework 4.0 provides support for several Java 8 features. You can make use of
lambda expressions and method references with Spring’s callback interfaces. There
is first-class support for java.time
(JSR-310),
and several existing annotations have been retrofitted as @Repeatable
. You can also
use Java 8’s parameter name discovery (based on the -parameters
compiler flag) as an
alternative to compiling your code with debug information enabled.
Spring remains compatible with older versions of Java and the JDK: concretely, Java SE 6 (specifically, a minimum level equivalent to JDK 6 update 18, as released in January 2010) and above are still fully supported. However, for newly started development projects based on Spring 4, we recommend the use of Java 7 or 8.
Note that the Java 8 bytecode level (-target 1.8
, as required by -source 1.8
) is only
fully supported as of Spring Framework 4.0. In particular, Spring 3.2 based applications
need to be compiled with a maximum of Java 7 as the target, even if they happen to be
deployed onto a Java 8 runtime. Please upgrade to Spring 4 for Java 8 based applications.
Java EE version 6 or above is now considered the baseline for Spring Framework 4, with the JPA 2.0 and Servlet 3.0 specifications being of particular relevance. In order to remain compatible with Google App Engine and older application servers, it is possible to deploy a Spring 4 application into a Servlet 2.5 environment. However, Servlet 3.0+ is strongly recommended and a prerequisite in Spring’s test and mock packages for test setups in development environments.
Note | |
---|---|
If you are a WebSphere 7 user, be sure to install the JPA 2.0 feature pack. On WebLogic 10.3.4 or higher, install the JPA 2.0 patch that comes with it. This turns both of those server generations into Spring 4 compatible deployment environments. |
On a more forward-looking note, Spring Framework 4.0 supports the Java EE 7 level of applicable specifications now: in particular, JMS 2.0, JTA 1.2, JPA 2.1, Bean Validation 1.1, and JSR-236 Concurrency Utilities. As usual, this support focuses on individual use of those specifications, e.g. on Tomcat or in standalone environments. However, it works equally well when a Spring application is deployed to a Java EE 7 server.
Note that Hibernate 4.3 is a JPA 2.1 provider and therefore only supported as of Spring Framework 4.0. The same applies to Hibernate Validator 5.0 as a Bean Validation 1.1 provider. Neither of the two are officially supported with Spring Framework 3.2.
With Spring Framework 4.0 it is now possible to define external bean configuration using a Groovy DSL. This is similar in concept to using XML bean definitions, but allows for a more concise syntax. Using Groovy also allows you to easily embed bean definitions directly in your bootstrap code. For example:
def reader = new GroovyBeanDefinitionReader(myApplicationContext) reader.beans { dataSource(BasicDataSource) { driverClassName = "org.hsqldb.jdbcDriver" url = "jdbc:hsqldb:mem:grailsDB" username = "sa" password = "" settings = [mynew:"setting"] } sessionFactory(SessionFactory) { dataSource = dataSource } myService(MyService) { nestedBean = { AnotherBean bean -> dataSource = dataSource } } }
For more information consult the GroovyBeanDefinitionReader
Javadoc.
There have been several general improvements to the core container:
Repository
you can now easily inject a specific implementation:
@Autowired Repository<Customer> customerRepository
.
@Ordered
annotation and Ordered
interface are
supported.
@Lazy
annotation can now be used on injection points, as well as on @Bean
definitions.
@Description
annotation has been introduced for
developers using Java-based configuration.
@Conditional
annotation. This is similar to @Profile
support but
allows for user-defined strategies to be developed programmatically.
LocaleContext
.
Deployment to Servlet 2.5 servers remains an option, but Spring Framework 4.0 is now focused primarily on Servlet 3.0+ environments. If you are using the Spring MVC Test Framework you will need to ensure that a Servlet 3.0 compatible JAR is in your test classpath.
In addition to the WebSocket support mentioned later, the following general improvements have been made to Spring’s Web modules:
@RestController
annotation with Spring
MVC applications, removing the need to add @ResponseBody
to each of your
@RequestMapping
methods.
AsyncRestTemplate
class has been added, allowing
non-blocking asynchronous support when developing REST clients.
A new spring-websocket
module provides comprehensive support for WebSocket-based,
two-way communication between client and server in web applications. It is compatible with
JSR-356, the Java WebSocket API, and in addition
provides SockJS-based fallback options (i.e. WebSocket emulation) for use in browsers
that don’t yet support the WebSocket protocol (e.g. Internet Explorer < 10).
A new spring-messaging
module adds support for STOMP as the WebSocket sub-protocol
to use in applications along with an annotation programming model for routing and
processing STOMP messages from WebSocket clients. As a result an @Controller
can now contain both @RequestMapping
and @MessageMapping
methods for handling
HTTP requests and messages from WebSocket-connected clients. The new spring-messaging
module also contains key abstractions from the
Spring Integration project such as
Message
, MessageChannel
, MessageHandler
, and others to serve as a foundation
for messaging-based applications.
For further details, including a more thorough introduction, see the Chapter 20, WebSocket Support section.
In addition to pruning of deprecated code within the spring-test
module, Spring
Framework 4.0 introduces several new features for use in unit and integration testing.
spring-test
module (e.g., @ContextConfiguration
,
@WebAppConfiguration
, @ContextHierarchy
, @ActiveProfiles
, etc.) can now be used
as meta-annotations to create custom
composed annotations and reduce configuration duplication across a test suite.
ActiveProfilesResolver
and registering it via the resolver
attribute of @ActiveProfiles
.
SocketUtils
class has been introduced in the spring-core
module
which enables you to scan for free TCP and UDP server ports on localhost. This
functionality is not specific to testing but can prove very useful when writing
integration tests that require the use of sockets, for example tests that start
an in-memory SMTP server, FTP server, Servlet container, etc.
org.springframework.mock.web
package is
now based on the Servlet 3.0 API. Furthermore, several of the Servlet API mocks
(e.g., MockHttpServletRequest
, MockServletContext
, etc.) have been updated with
minor enhancements and improved configurability.
This part of the reference documentation covers all of those technologies that are absolutely integral to the Spring Framework.
Foremost amongst these is the Spring Framework’s Inversion of Control (IoC) container. A thorough treatment of the Spring Framework’s IoC container is closely followed by comprehensive coverage of Spring’s Aspect-Oriented Programming (AOP) technologies. The Spring Framework has its own AOP framework, which is conceptually easy to understand, and which successfully addresses the 80% sweet spot of AOP requirements in Java enterprise programming.
Coverage of Spring’s integration with AspectJ (currently the richest - in terms of features - and certainly most mature AOP implementation in the Java enterprise space) is also provided.
Finally, the adoption of the test-driven-development (TDD) approach to software development is certainly advocated by the Spring team, and so coverage of Spring’s support for integration testing is covered (alongside best practices for unit testing). The Spring team has found that the correct use of IoC certainly does make both unit and integration testing easier (in that the presence of setter methods and appropriate constructors on classes makes them easier to wire together in a test without having to set up service locator registries and suchlike)… the chapter dedicated solely to testing will hopefully convince you of this as well.
This chapter covers the Spring Framework implementation of the Inversion of Control (IoC) [1] principle. IoC is also known as dependency injection (DI). It is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies by using direct construction of classes, or a mechanism such as the Service Locator pattern.
The org.springframework.beans
and org.springframework.context
packages are the basis
for Spring Framework’s IoC container. The
BeanFactory
interface provides an advanced configuration mechanism capable of managing any type of
object.
ApplicationContext
is a sub-interface of BeanFactory
. It adds easier integration with Spring’s AOP
features; message resource handling (for use in internationalization), event
publication; and application-layer specific contexts such as the WebApplicationContext
for use in web applications.
In short, the BeanFactory
provides the configuration framework and basic
functionality, and the ApplicationContext
adds more enterprise-specific functionality.
The ApplicationContext
is a complete superset of the BeanFactory
, and is used
exclusively in this chapter in descriptions of Spring’s IoC container. For more
information on using the BeanFactory
instead of the ApplicationContext,
refer to
Section 4.17, “The BeanFactory”.
In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a bean is simply one of many objects in your application. Beans, and the dependencies among them, are reflected in the configuration metadata used by a container.
The interface org.springframework.context.ApplicationContext
represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the
aforementioned beans. The container gets its instructions on what objects to
instantiate, configure, and assemble by reading configuration metadata. The
configuration metadata is represented in XML, Java annotations, or Java code. It allows
you to express the objects that compose your application and the rich interdependencies
between such objects.
Several implementations of the ApplicationContext
interface are supplied
out-of-the-box with Spring. In standalone applications it is common to create an
instance of
ClassPathXmlApplicationContext
or FileSystemXmlApplicationContext
.
While XML has been the traditional format for defining configuration metadata you can
instruct the container to use Java annotations or code as the metadata format by
providing a small amount of XML configuration to declaratively enable support for these
additional metadata formats.
In most application scenarios, explicit user code is not required to instantiate one or
more instances of a Spring IoC container. For example, in a web application scenario, a
simple eight (or so) lines of boilerplate web descriptor XML in the web.xml
file
of the application will typically suffice (see Section 4.16.4, “Convenient ApplicationContext instantiation for web applications”). If you are using the
SpringSource Tool Suite Eclipse-powered development
environment this boilerplate configuration can be easily created with few mouse clicks or
keystrokes.
The following diagram is a high-level view of how Spring works. Your application classes
are combined with configuration metadata so that after the ApplicationContext
is
created and initialized, you have a fully configured and executable system or
application.
As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata; this configuration metadata represents how you as an application developer tell the Spring container to instantiate, configure, and assemble the objects in your application.
Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most of this chapter uses to convey key concepts and features of the Spring IoC container.
Note | |
---|---|
XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. These days many developers choose Java-based configuration for their Spring applications. |
For information about using other forms of metadata with the Spring container, see:
@Configuration
, @Bean
, @Import
and @DependsOn
annotations.
Spring configuration consists of at least one and typically more than one bean
definition that the container must manage. XML-based configuration metadata shows these
beans configured as <bean/>
elements inside a top-level <beans/>
element. Java
configuration typically uses @Bean
annotated methods within a @Configuration
class.
These bean definitions correspond to the actual objects that make up your application.
Typically you define service layer objects, data access objects (DAOs), presentation
objects such as Struts Action
instances, infrastructure objects such as Hibernate
SessionFactories
, JMS Queues
, and so forth. Typically one does not configure
fine-grained domain objects in the container, because it is usually the responsibility
of DAOs and business logic to create and load domain objects. However, you can use
Spring’s integration with AspectJ to configure objects that have been created outside
the control of an IoC container. See Using AspectJ to
dependency-inject domain objects with Spring.
The following example shows the basic structure of XML-based configuration metadata:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions go here --> </beans>
The id
attribute is a string that you use to identify the individual bean definition.
The class
attribute defines the type of the bean and uses the fully qualified
classname. The value of the id attribute refers to collaborating objects. The XML for
referring to collaborating objects is not shown in this example; see
Dependencies for more information.
Instantiating a Spring IoC container is straightforward. The location path or paths
supplied to an ApplicationContext
constructor are actually resource strings that allow
the container to load configuration metadata from a variety of external resources such
as the local file system, from the Java CLASSPATH
, and so on.
ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});
Note | |
---|---|
After you learn about Spring’s IoC container, you may want to know more about Spring’s
|
The following example shows the service layer objects (services.xml)
configuration file:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- services --> <bean id="petStore" class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl"> <property name="accountDao" ref="accountDao"/> <property name="itemDao" ref="itemDao"/> <!-- additional collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions for services go here --> </beans>
The following example shows the data access objects daos.xml
file:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="accountDao" class="org.springframework.samples.jpetstore.dao.jpa.JpaAccountDao"> <!-- additional collaborators and configuration for this bean go here --> </bean> <bean id="itemDao" class="org.springframework.samples.jpetstore.dao.jpa.JapItemDao"> <!-- additional collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions for data access objects go here --> </beans>
In the preceding example, the service layer consists of the class PetStoreServiceImpl
,
and two data access objects of the type JpaAccountDao
and JpaItemDao
(based
on the JPA Object/Relational mapping standard). The property name
element refers to the
name of the JavaBean property, and the ref
element refers to the name of another bean
definition. This linkage between id
and ref
elements expresses the dependency between
collaborating objects. For details of configuring an object’s dependencies, see
Dependencies.
It can be useful to have bean definitions span multiple XML files. Often each individual XML configuration file represents a logical layer or module in your architecture.
You can use the application context constructor to load bean definitions from all these
XML fragments. This constructor takes multiple Resource
locations, as was shown in the
previous section. Alternatively, use one or more occurrences of the <import/>
element
to load bean definitions from another file or files. For example:
<beans> <import resource="services.xml"/> <import resource="resources/messageSource.xml"/> <import resource="/resources/themeSource.xml"/> <bean id="bean1" class="..."/> <bean id="bean2" class="..."/> </beans>
In the preceding example, external bean definitions are loaded from three files,
services.xml
, messageSource.xml
, and themeSource.xml
. All location paths are
relative to the definition file doing the importing, so services.xml
must be in the
same directory or classpath location as the file doing the importing, while
messageSource.xml
and themeSource.xml
must be in a resources
location below the
location of the importing file. As you can see, a leading slash is ignored, but given
that these paths are relative, it is better form not to use the slash at all. The
contents of the files being imported, including the top level <beans/>
element, must
be valid XML bean definitions according to the Spring Schema.
Note | |
---|---|
It is possible, but not recommended, to reference files in parent directories using a relative "../" path. Doing so creates a dependency on a file that is outside the current application. In particular, this reference is not recommended for "classpath:" URLs (for example, "classpath:../services.xml"), where the runtime resolution process chooses the "nearest" classpath root and then looks into its parent directory. Classpath configuration changes may lead to the choice of a different, incorrect directory. You can always use fully qualified resource locations instead of relative paths: for example, "file:C:/config/services.xml" or "classpath:/config/services.xml". However, be aware that you are coupling your application’s configuration to specific absolute locations. It is generally preferable to keep an indirection for such absolute locations, for example, through "${…}" placeholders that are resolved against JVM system properties at runtime. |
The ApplicationContext
is the interface for an advanced factory capable of maintaining
a registry of different beans and their dependencies. Using the method T getBean(String
name, Class<T> requiredType)
you can retrieve instances of your beans.
The ApplicationContext
enables you to read bean definitions and access them as follows:
// create and configure beans ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"}); // retrieve configured instance PetStoreService service = context.getBean("petStore", PetStoreService.class); // use configured instance List<String> userList = service.getUsernameList();
You use getBean()
to retrieve instances of your beans. The ApplicationContext
interface has a few other methods for retrieving beans, but ideally your application
code should never use them. Indeed, your application code should have no calls to the
getBean()
method at all, and thus no dependency on Spring APIs at all. For example,
Spring’s integration with web frameworks provides for dependency injection for various
web framework classes such as controllers and JSF-managed beans.
A Spring IoC container manages one or more beans. These beans are created with the
configuration metadata that you supply to the container, for example, in the form of XML
<bean/>
definitions.
Within the container itself, these bean definitions are represented as BeanDefinition
objects, which contain (among other information) the following metadata:
This metadata translates to a set of properties that make up each bean definition.
Table 4.1. The bean definition
Property | Explained in… |
---|---|
class | |
name | |
scope | |
constructor arguments | |
properties | |
autowiring mode | |
lazy-initialization mode | |
initialization method | |
destruction method |
In addition to bean definitions that contain information on how to create a specific
bean, the ApplicationContext
implementations also permit the registration of existing
objects that are created outside the container, by users. This is done by accessing the
ApplicationContext’s BeanFactory via the method getBeanFactory()
which returns the
BeanFactory implementation DefaultListableBeanFactory
. DefaultListableBeanFactory
supports this registration through the methods registerSingleton(..)
and
registerBeanDefinition(..)
. However, typical applications work solely with beans
defined through metadata bean definitions.
Every bean has one or more identifiers. These identifiers must be unique within the container that hosts the bean. A bean usually has only one identifier, but if it requires more than one, the extra ones can be considered aliases.
In XML-based configuration metadata, you use the id
and/or name
attributes
to specify the bean identifier(s). The id
attribute allows you to specify
exactly one id. Conventionally these names are alphanumeric (myBean,
fooService, etc.), but may contain special characters as well. If you want to
introduce other aliases to the bean, you can also specify them in the name
attribute, separated by a comma (,
), semicolon (;
), or white space. As a
historical note, in versions prior to Spring 3.1, the id
attribute was
defined as an xsd:ID
type, which constrained possible characters. As of 3.1,
it is defined as an xsd:string
type. Note that bean id
uniqueness is still
enforced by the container, though no longer by XML parsers.
You are not required to supply a name or id for a bean. If no name or id is supplied
explicitly, the container generates a unique name for that bean. However, if you want to
refer to that bean by name, through the use of the ref
element or
Service Locator style lookup, you must provide a name.
Motivations for not supplying a name are related to using inner
beans and autowiring collaborators.
In a bean definition itself, you can supply more than one name for the bean, by using a
combination of up to one name specified by the id
attribute, and any number of other
names in the name
attribute. These names can be equivalent aliases to the same bean,
and are useful for some situations, such as allowing each component in an application to
refer to a common dependency by using a bean name that is specific to that component
itself.
Specifying all aliases where the bean is actually defined is not always adequate,
however. It is sometimes desirable to introduce an alias for a bean that is defined
elsewhere. This is commonly the case in large systems where configuration is split
amongst each subsystem, each subsystem having its own set of object definitions. In
XML-based configuration metadata, you can use the <alias/>
element to accomplish this.
<alias name="fromName" alias="toName"/>
In this case, a bean in the same container which is named fromName
, may also,
after the use of this alias definition, be referred to as toName
.
For example, the configuration metadata for subsystem A may refer to a DataSource via
the name subsystemA-dataSource
. The configuration metadata for subsystem B may refer to
a DataSource via the name subsystemB-dataSource
. When composing the main application
that uses both these subsystems the main application refers to the DataSource via the
name myApp-dataSource
. To have all three names refer to the same object you add to the
MyApp configuration metadata the following aliases definitions:
<alias name="subsystemA-dataSource" alias="subsystemB-dataSource"/> <alias name="subsystemA-dataSource" alias="myApp-dataSource" />
Now each component and the main application can refer to the dataSource through a name that is unique and guaranteed not to clash with any other definition (effectively creating a namespace), yet they refer to the same bean.
A bean definition essentially is a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.
If you use XML-based configuration metadata, you specify the type (or class) of object
that is to be instantiated in the class
attribute of the <bean/>
element. This
class
attribute, which internally is a Class
property on a BeanDefinition
instance, is usually mandatory. (For exceptions, see
the section called “Instantiation using an instance factory method” and Section 4.7, “Bean definition inheritance”.)
You use the Class
property in one of two ways:
new
operator.
static
factory method that will be
invoked to create the object, in the less common case where the container invokes a
static
, factory method on a class to create the bean. The object type returned
from the invocation of the static
factory method may be the same class or another
class entirely.
When you create a bean by the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being developed does not need to implement any specific interfaces or to be coded in a specific fashion. Simply specifying the bean class should suffice. However, depending on what type of IoC you use for that specific bean, you may need a default (empty) constructor.
The Spring IoC container can manage virtually any class you want it to manage; it is not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument) constructor and appropriate setters and getters modeled after the properties in the container. You can also have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.
With XML-based configuration metadata you can specify your bean class as follows:
<bean id="exampleBean" class="examples.ExampleBean"/> <bean name="anotherExample" class="examples.ExampleBeanTwo"/>
For details about the mechanism for supplying arguments to the constructor (if required) and setting object instance properties after the object is constructed, see Injecting Dependencies.
When defining a bean that you create with a static factory method, you use the class
attribute to specify the class containing the static
factory method and an attribute
named factory-method
to specify the name of the factory method itself. You should be
able to call this method (with optional arguments as described later) and return a live
object, which subsequently is treated as if it had been created through a constructor.
One use for such a bean definition is to call static
factories in legacy code.
The following bean definition specifies that the bean will be created by calling a
factory-method. The definition does not specify the type (class) of the returned object,
only the class containing the factory method. In this example, the createInstance()
method must be a static method.
<bean id="clientService" class="examples.ClientService" factory-method="createInstance"/>
public class ClientService { private static ClientService clientService = new ClientService(); private ClientService() {} public static ClientService createInstance() { return clientService; } }
For details about the mechanism for supplying (optional) arguments to the factory method and setting object instance properties after the object is returned from the factory, see Dependencies and configuration in detail.
Similar to instantiation through a static
factory method, instantiation with an instance factory method invokes a non-static
method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class
attribute empty, and in the factory-bean
attribute,
specify the name of a bean in the current (or parent/ancestor) container that contains
the instance method that is to be invoked to create the object. Set the name of the
factory method itself with the factory-method
attribute.
<!-- the factory bean, which contains a method called createInstance() --> <bean id="serviceLocator" class="examples.DefaultServiceLocator"> <!-- inject any dependencies required by this locator bean --> </bean> <!-- the bean to be created via the factory bean --> <bean id="clientService" factory-bean="serviceLocator" factory-method="createClientServiceInstance"/>
public class DefaultServiceLocator { private static ClientService clientService = new ClientServiceImpl(); private DefaultServiceLocator() {} public ClientService createClientServiceInstance() { return clientService; } }
One factory class can also hold more than one factory method as shown here:
<bean id="serviceLocator" class="examples.DefaultServiceLocator"> <!-- inject any dependencies required by this locator bean --> </bean> <bean id="clientService" factory-bean="serviceLocator" factory-method="createClientServiceInstance"/> <bean id="accountService" factory-bean="serviceLocator" factory-method="createAccountServiceInstance"/>
public class DefaultServiceLocator { private static ClientService clientService = new ClientServiceImpl(); private static AccountService accountService = new AccountServiceImpl(); private DefaultServiceLocator() {} public ClientService createClientServiceInstance() { return clientService; } public AccountService createAccountServiceInstance() { return accountService; } }
This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and configuration in detail.
Note | |
---|---|
In Spring documentation, factory bean refers to a bean that is configured in the
Spring container that will create objects through an
instance or
static factory method. By contrast,
|
A typical enterprise application does not consist of a single object (or bean in the Spring parlance). Even the simplest application has a few objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand alone to a fully realized application where objects collaborate to achieve a goal.
Dependency injection (DI) is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies on its own by using direct construction of classes, or the Service Locator pattern.
Code is cleaner with the DI principle and decoupling is more effective when objects are provided with their dependencies. The object does not look up its dependencies, and does not know the location or class of the dependencies. As such, your classes become easier to test, in particular when the dependencies are on interfaces or abstract base classes, which allow for stub or mock implementations to be used in unit tests.
DI exists in two major variants, Constructor-based dependency injection and Setter-based dependency injection.
Constructor-based DI is accomplished by the container invoking a constructor with a
number of arguments, each representing a dependency. Calling a static
factory method
with specific arguments to construct the bean is nearly equivalent, and this discussion
treats arguments to a constructor and to a static
factory method similarly. The
following example shows a class that can only be dependency-injected with constructor
injection. Notice that there is nothing special about this class, it is a POJO that
has no dependencies on container specific interfaces, base classes or annotations.
public class SimpleMovieLister { // the SimpleMovieLister has a dependency on a MovieFinder private MovieFinder movieFinder; // a constructor so that the Spring container can inject a MovieFinder public SimpleMovieLister(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually uses the injected MovieFinder is omitted... }
Constructor argument resolution matching occurs using the argument’s type. If no potential ambiguity exists in the constructor arguments of a bean definition, then the order in which the constructor arguments are defined in a bean definition is the order in which those arguments are supplied to the appropriate constructor when the bean is being instantiated. Consider the following class:
package x.y; public class Foo { public Foo(Bar bar, Baz baz) { // ... } }
No potential ambiguity exists, assuming that Bar
and Baz
classes are not related by
inheritance. Thus the following configuration works fine, and you do not need to specify
the constructor argument indexes and/or types explicitly in the <constructor-arg/>
element.
<beans> <bean id="foo" class="x.y.Foo"> <constructor-arg ref="bar"/> <constructor-arg ref="baz"/> </bean> <bean id="bar" class="x.y.Bar"/> <bean id="baz" class="x.y.Baz"/> </beans>
When another bean is referenced, the type is known, and matching can occur (as was the
case with the preceding example). When a simple type is used, such as
<value>true</value>
, Spring cannot determine the type of the value, and so cannot match
by type without help. Consider the following class:
package examples; public class ExampleBean { // Number of years to calculate the Ultimate Answer private int years; // The Answer to Life, the Universe, and Everything private String ultimateAnswer; public ExampleBean(int years, String ultimateAnswer) { this.years = years; this.ultimateAnswer = ultimateAnswer; } }
In the preceding scenario, the container can use type matching with simple types if
you explicitly specify the type of the constructor argument using the type
attribute.
For example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg type="int" value="7500000"/> <constructor-arg type="java.lang.String" value="42"/> </bean>
Use the index
attribute to specify explicitly the index of constructor arguments. For
example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg index="0" value="7500000"/> <constructor-arg index="1" value="42"/> </bean>
In addition to resolving the ambiguity of multiple simple values, specifying an index resolves ambiguity where a constructor has two arguments of the same type. Note that the index is 0 based.
You can also use the constructor parameter name for value disambiguation:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg name="years" value="7500000"/> <constructor-arg name="ultimateanswer" value="42"/> </bean>
Keep in mind that to make this work out of the box your code must be compiled with the debug flag enabled so that Spring can look up the parameter name from the constructor. If you can’t compile your code with debug flag (or don’t want to) you can use @ConstructorProperties JDK annotation to explicitly name your constructor arguments. The sample class would then have to look as follows:
package examples; public class ExampleBean { // Fields omitted @ConstructorProperties({"years", "ultimateAnswer"}) public ExampleBean(int years, String ultimateAnswer) { this.years = years; this.ultimateAnswer = ultimateAnswer; } }
Setter-based DI is accomplished by the container calling setter methods on your
beans after invoking a no-argument constructor or no-argument static
factory method to
instantiate your bean.
The following example shows a class that can only be dependency-injected using pure setter injection. This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces, base classes or annotations.
public class SimpleMovieLister { // the SimpleMovieLister has a dependency on the MovieFinder private MovieFinder movieFinder; // a setter method so that the Spring container can inject a MovieFinder public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually uses the injected MovieFinder is omitted... }
The ApplicationContext
supports constructor- and setter-based DI for the beans it
manages. It also supports setter-based DI after some dependencies are already injected
through the constructor approach. You configure the dependencies in the form of a
BeanDefinition
, which you use with PropertyEditor
instances to convert properties
from one format to another. However, most Spring users do not work with these classes
directly (programmatically), but rather with an XML definition file that is then
converted internally into instances of these classes, and used to load an entire Spring
IoC container instance.
The container performs bean dependency resolution as follows:
ApplicationContext
is created and initialized with configuration metadata that
describes all the beans. Configuration metadata can be specified via XML, Java code or
annotations.
int
,
long
, String
, boolean
, etc.
The Spring container validates the configuration of each bean as the container is created, including the validation of whether bean reference properties refer to valid beans. However, the bean properties themselves are not set until the bean is actually created. Beans that are singleton-scoped and set to be pre-instantiated (the default) are created when the container is created. Scopes are defined in Section 4.5, “Bean scopes”. Otherwise, the bean is created only when it is requested. Creation of a bean potentially causes a graph of beans to be created, as the bean’s dependencies and its dependencies' dependencies (and so on) are created and assigned.
You can generally trust Spring to do the right thing. It detects configuration problems,
such as references to non-existent beans and circular dependencies, at container
load-time. Spring sets properties and resolves dependencies as late as possible, when
the bean is actually created. This means that a Spring container which has loaded
correctly can later generate an exception when you request an object if there is a
problem creating that object or one of its dependencies. For example, the bean throws an
exception as a result of a missing or invalid property. This potentially delayed
visibility of some configuration issues is why ApplicationContext
implementations by
default pre-instantiate singleton beans. At the cost of some upfront time and memory to
create these beans before they are actually needed, you discover configuration issues
when the ApplicationContext
is created, not later. You can still override this default
behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.
If no circular dependencies exist, when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being injected into the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container completely configures bean B prior to invoking the setter method on bean A. In other words, the bean is instantiated (if not a pre-instantiated singleton), its dependencies are set, and the relevant lifecycle methods (such as a configured init method or the InitializingBean callback method) are invoked.
The following example uses XML-based configuration metadata for setter-based DI. A small part of a Spring XML configuration file specifies some bean definitions:
<bean id="exampleBean" class="examples.ExampleBean"> <!-- setter injection using the nested <ref/> element --> <property name="beanOne"> <ref bean="anotherExampleBean"/> </property> <!-- setter injection using the neater ref attribute --> <property name="beanTwo" ref="yetAnotherBean"/> <property name="integerProperty" value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public void setBeanOne(AnotherBean beanOne) { this.beanOne = beanOne; } public void setBeanTwo(YetAnotherBean beanTwo) { this.beanTwo = beanTwo; } public void setIntegerProperty(int i) { this.i = i; } }
In the preceding example, setters are declared to match against the properties specified in the XML file. The following example uses constructor-based DI:
<bean id="exampleBean" class="examples.ExampleBean"> <!-- constructor injection using the nested <ref/> element --> <constructor-arg> <ref bean="anotherExampleBean"/> </constructor-arg> <!-- constructor injection using the neater ref attribute --> <constructor-arg ref="yetAnotherBean"/> <constructor-arg type="int" value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public ExampleBean( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { this.beanOne = anotherBean; this.beanTwo = yetAnotherBean; this.i = i; } }
The constructor arguments specified in the bean definition will be used as arguments to
the constructor of the ExampleBean
.
Now consider a variant of this example, where instead of using a constructor, Spring is
told to call a static
factory method to return an instance of the object:
<bean id="exampleBean" class="examples.ExampleBean" factory-method="createInstance"> <constructor-arg ref="anotherExampleBean"/> <constructor-arg ref="yetAnotherBean"/> <constructor-arg value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { // a private constructor private ExampleBean(...) { ... } // a static factory method; the arguments to this method can be // considered the dependencies of the bean that is returned, // regardless of how those arguments are actually used. public static ExampleBean createInstance ( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { ExampleBean eb = new ExampleBean (...); // some other operations... return eb; } }
Arguments to the static
factory method are supplied via <constructor-arg/>
elements,
exactly the same as if a constructor had actually been used. The type of the class being
returned by the factory method does not have to be of the same type as the class that
contains the static
factory method, although in this example it is. An instance
(non-static) factory method would be used in an essentially identical fashion (aside
from the use of the factory-bean
attribute instead of the class
attribute), so
details will not be discussed here.
As mentioned in the previous section, you can define bean properties and constructor
arguments as references to other managed beans (collaborators), or as values defined
inline. Spring’s XML-based configuration metadata supports sub-element types within its
<property/>
and <constructor-arg/>
elements for this purpose.
The value
attribute of the <property/>
element specifies a property or constructor
argument as a human-readable string representation. Spring’s
conversion service is used to convert these
values from a String
to the actual type of the property or argument.
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <!-- results in a setDriverClassName(String) call --> <property name="driverClassName" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/mydb"/> <property name="username" value="root"/> <property name="password" value="masterkaoli"/> </bean>
The following example uses the p-namespace for even more succinct XML configuration.
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close" p:driverClassName="com.mysql.jdbc.Driver" p:url="jdbc:mysql://localhost:3306/mydb" p:username="root" p:password="masterkaoli"/> </beans>
The preceding XML is more succinct; however, typos are discovered at runtime rather than design time, unless you use an IDE such as IntelliJ IDEA or the SpringSource Tool Suite (STS) that support automatic property completion when you create bean definitions. Such IDE assistance is highly recommended.
You can also configure a java.util.Properties
instance as:
<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <!-- typed as a java.util.Properties --> <property name="properties"> <value> jdbc.driver.className=com.mysql.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/mydb </value> </property> </bean>
The Spring container converts the text inside the <value/>
element into a
java.util.Properties
instance by using the JavaBeans PropertyEditor
mechanism. This
is a nice shortcut, and is one of a few places where the Spring team do favor the use of
the nested <value/>
element over the value
attribute style.
The idref
element is simply an error-proof way to pass the id (string value - not
a reference) of another bean in the container to a <constructor-arg/>
or <property/>
element.
<bean id="theTargetBean" class="..."/> <bean id="theClientBean" class="..."> <property name="targetName"> <idref bean="theTargetBean" /> </property> </bean>
The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:
<bean id="theTargetBean" class="..." /> <bean id="client" class="..."> <property name="targetName" value="theTargetBean" /> </bean>
The first form is preferable to the second, because using the idref
tag allows the
container to validate at deployment time that the referenced, named bean actually
exists. In the second variation, no validation is performed on the value that is passed
to the targetName
property of the client
bean. Typos are only discovered (with most
likely fatal results) when the client
bean is actually instantiated. If the client
bean is a prototype bean, this typo and the resulting exception
may only be discovered long after the container is deployed.
Additionally, if the referenced bean is in the same XML unit, and the bean name is the
bean id, you can use the local
attribute, which allows the XML parser itself to
validate the bean id earlier, at XML document parse time.
<property name="targetName"> <!-- a bean with id theTargetBean must exist; otherwise an exception will be thrown --> <idref bean="theTargetBean"/> </property>
A common place (at least in versions earlier than Spring 2.0) where the <idref/>
element
brings value is in the configuration of AOP interceptors in a
ProxyFactoryBean
bean definition. Using <idref/>
elements when you specify the
interceptor names prevents you from misspelling an interceptor id.
The ref
element is the final element inside a <constructor-arg/>
or <property/>
definition element. Here you set the value of the specified property of a bean to be a
reference to another bean (a collaborator) managed by the container. The referenced bean
is a dependency of the bean whose property will be set, and it is initialized on demand
as needed before the property is set. (If the collaborator is a singleton bean, it may
be initialized already by the container.) All references are ultimately a reference to
another object. Scoping and validation depend on whether you specify the id/name of the
other object through the bean
, local,
or parent
attributes.
Specifying the target bean through the bean
attribute of the <ref/>
tag is the most
general form, and allows creation of a reference to any bean in the same container or
parent container, regardless of whether it is in the same XML file. The value of the
bean
attribute may be the same as the id
attribute of the target bean, or as one of
the values in the name
attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean through the parent
attribute creates a reference to a bean
that is in a parent container of the current container. The value of the parent
attribute may be the same as either the id
attribute of the target bean, or one of the
values in the name
attribute of the target bean, and the target bean must be in a
parent container of the current one. You use this bean reference variant mainly when you
have a hierarchy of containers and you want to wrap an existing bean in a parent
container with a proxy that will have the same name as the parent bean.
<!-- in the parent context --> <bean id="accountService" class="com.foo.SimpleAccountService"> <!-- insert dependencies as required as here --> </bean>
<!-- in the child (descendant) context --> <bean id="accountService" <-- bean name is the same as the parent bean --> class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="target"> <ref parent="accountService"/> <!-- notice how we refer to the parent bean --> </property> <!-- insert other configuration and dependencies as required here --> </bean>
Note | |
---|---|
The |
A <bean/>
element inside the <property/>
or <constructor-arg/>
elements defines a
so-called inner bean.
<bean id="outer" class="..."> <!-- instead of using a reference to a target bean, simply define the target bean inline --> <property name="target"> <bean class="com.example.Person"> <!-- this is the inner bean --> <property name="name" value="Fiona Apple"/> <property name="age" value="25"/> </bean> </property> </bean>
An inner bean definition does not require a defined id or name; the container ignores
these values. It also ignores the scope
flag. Inner beans are always anonymous and
they are always created with the outer bean. It is not possible to inject inner
beans into collaborating beans other than into the enclosing bean.
In the <list/>
, <set/>
, <map/>
, and <props/>
elements, you set the properties
and arguments of the Java Collection
types List
, Set
, Map
, and Properties
,
respectively.
<bean id="moreComplexObject" class="example.ComplexObject"> <!-- results in a setAdminEmails(java.util.Properties) call --> <property name="adminEmails"> <props> <prop key="administrator">[email protected]</prop> <prop key="support">[email protected]</prop> <prop key="development">[email protected]</prop> </props> </property> <!-- results in a setSomeList(java.util.List) call --> <property name="someList"> <list> <value>a list element followed by a reference</value> <ref bean="myDataSource" /> </list> </property> <!-- results in a setSomeMap(java.util.Map) call --> <property name="someMap"> <map> <entry key="an entry" value="just some string"/> <entry key ="a ref" value-ref="myDataSource"/> </map> </property> <!-- results in a setSomeSet(java.util.Set) call --> <property name="someSet"> <set> <value>just some string</value> <ref bean="myDataSource" /> </set> </property> </bean>
The value of a map key or value, or a set value, can also again be any of the following elements:
bean | ref | idref | list | set | map | props | value | null
The Spring container also supports the merging of collections. An application
developer can define a parent-style <list/>
, <map/>
, <set/>
or <props/>
element,
and have child-style <list/>
, <map/>
, <set/>
or <props/>
elements inherit and
override values from the parent collection. That is, the child collection’s values are
the result of merging the elements of the parent and child collections, with the child’s
collection elements overriding values specified in the parent collection.
This section on merging discusses the parent-child bean mechanism. Readers unfamiliar with parent and child bean definitions may wish to read the relevant section before continuing.
The following example demonstrates collection merging:
<beans> <bean id="parent" abstract="true" class="example.ComplexObject"> <property name="adminEmails"> <props> <prop key="administrator">[email protected]</prop> <prop key="support">[email protected]</prop> </props> </property> </bean> <bean id="child" parent="parent"> <property name="adminEmails"> <!-- the merge is specified on the child collection definition --> <props merge="true"> <prop key="sales">[email protected]</prop> <prop key="support">[email protected]</prop> </props> </property> </bean> <beans>
Notice the use of the merge=true
attribute on the <props/>
element of the
adminEmails
property of the child
bean definition. When the child
bean is resolved
and instantiated by the container, the resulting instance has an adminEmails
Properties
collection that contains the result of the merging of the child’s
adminEmails
collection with the parent’s adminEmails
collection.
[email protected] [email protected] [email protected]
The child Properties
collection’s value set inherits all property elements from the
parent <props/>
, and the child’s value for the support
value overrides the value in
the parent collection.
This merging behavior applies similarly to the <list/>
, <map/>
, and <set/>
collection types. In the specific case of the <list/>
element, the semantics
associated with the List
collection type, that is, the notion of an ordered
collection of values, is maintained; the parent’s values precede all of the child list’s
values. In the case of the Map
, Set
, and Properties
collection types, no ordering
exists. Hence no ordering semantics are in effect for the collection types that underlie
the associated Map
, Set
, and Properties
implementation types that the container
uses internally.
You cannot merge different collection types (such as a Map
and a List
), and if you
do attempt to do so an appropriate Exception
is thrown. The merge
attribute must be
specified on the lower, inherited, child definition; specifying the merge
attribute on
a parent collection definition is redundant and will not result in the desired merging.
With the introduction of generic types in Java 5, you can use strongly typed collections.
That is, it is possible to declare a Collection
type such that it can only contain
String
elements (for example). If you are using Spring to dependency-inject a
strongly-typed Collection
into a bean, you can take advantage of Spring’s
type-conversion support such that the elements of your strongly-typed Collection
instances are converted to the appropriate type prior to being added to the Collection
.
public class Foo { private Map<String, Float> accounts; public void setAccounts(Map<String, Float> accounts) { this.accounts = accounts; } }
<beans> <bean id="foo" class="x.y.Foo"> <property name="accounts"> <map> <entry key="one" value="9.99"/> <entry key="two" value="2.75"/> <entry key="six" value="3.99"/> </map> </property> </bean> </beans>
When the accounts
property of the foo
bean is prepared for injection, the generics
information about the element type of the strongly-typed Map<String, Float>
is
available by reflection. Thus Spring’s type conversion infrastructure recognizes the
various value elements as being of type Float
, and the string values 9.99, 2.75
, and
3.99
are converted into an actual Float
type.
Spring treats empty arguments for properties and the like as empty Strings
. The
following XML-based configuration metadata snippet sets the email property to the empty
String
value ("").
<bean class="ExampleBean"> <property name="email" value=""/> </bean>
The preceding example is equivalent to the following Java code:
exampleBean.setEmail("")
. The <null/>
element handles null
values. For example:
<bean class="ExampleBean"> <property name="email"> <null/> </property> </bean>
The above configuration is equivalent to the following Java code:
exampleBean.setEmail(null)
.
The p-namespace enables you to use the bean
element’s attributes, instead of nested
<property/>
elements, to describe your property values and/or collaborating beans.
Spring supports extensible configuration formats with namespaces, which are
based on an XML Schema definition. The beans
configuration format discussed in this
chapter is defined in an XML Schema document. However, the p-namespace is not defined in
an XSD file and exists only in the core of Spring.
The following example shows two XML snippets that resolve to the same result: The first uses standard XML format and the second uses the p-namespace.
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean name="classic" class="com.example.ExampleBean"> <property name="email" value="[email protected]"/> </bean> <bean name="p-namespace" class="com.example.ExampleBean" p:email="[email protected]"/> </beans>
The example shows an attribute in the p-namespace called email in the bean definition. This tells Spring to include a property declaration. As previously mentioned, the p-namespace does not have a schema definition, so you can set the name of the attribute to the property name.
This next example includes two more bean definitions that both have a reference to another bean:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean name="john-classic" class="com.example.Person"> <property name="name" value="John Doe"/> <property name="spouse" ref="jane"/> </bean> <bean name="john-modern" class="com.example.Person" p:name="John Doe" p:spouse-ref="jane"/> <bean name="jane" class="com.example.Person"> <property name="name" value="Jane Doe"/> </bean> </beans>
As you can see, this example includes not only a property value using the p-namespace,
but also uses a special format to declare property references. Whereas the first bean
definition uses <property name="spouse" ref="jane"/>
to create a reference from bean
john
to bean jane
, the second bean definition uses p:spouse-ref="jane"
as an
attribute to do the exact same thing. In this case spouse
is the property name,
whereas the -ref
part indicates that this is not a straight value but rather a
reference to another bean.
Note | |
---|---|
The p-namespace is not as flexible as the standard XML format. For example, the format
for declaring property references clashes with properties that end in |
Similar to the the section called “XML shortcut with the p-namespace”, the c-namespace, newly introduced in Spring
3.1, allows usage of inlined attributes for configuring the constructor arguments rather
then nested constructor-arg
elements.
Let’s review the examples from the section called “Constructor-based dependency injection” with the c:
namespace:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:c="http://www.springframework.org/schema/c" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="bar" class="x.y.Bar"/> <bean id="baz" class="x.y.Baz"/> <!-- traditional declaration --> <bean id="foo" class="x.y.Foo"> <constructor-arg ref="bar"/> <constructor-arg ref="baz"/> <constructor-arg value="[email protected]"/> </bean> <!-- c-namespace declaration --> <bean id="foo" class="x.y.Foo" c:bar-ref="bar" c:baz-ref="baz" c:email="[email protected]"/> </beans>
The c:
namespace uses the same conventions as the p:
one (trailing -ref
for bean
references) for setting the constructor arguments by their names. And just as well, it
needs to be declared even though it is not defined in an XSD schema (but it exists
inside the Spring core).
For the rare cases where the constructor argument names are not available (usually if the bytecode was compiled without debugging information), one can use fallback to the argument indexes:
<!-- c-namespace index declaration --> <bean id="foo" class="x.y.Foo" c:_0-ref="bar" c:_1-ref="baz"/>
Note | |
---|---|
Due to the XML grammar, the index notation requires the presence of the leading |
In practice, the constructor resolution mechanism is quite efficient in matching arguments so unless one really needs to, we recommend using the name notation through-out your configuration.
You can use compound or nested property names when you set bean properties, as long as
all components of the path except the final property name are not null
. Consider the
following bean definition.
<bean id="foo" class="foo.Bar"> <property name="fred.bob.sammy" value="123" /> </bean>
The foo
bean has a fred
property, which has a bob
property, which has a sammy
property, and that final sammy
property is being set to the value 123
. In order for
this to work, the fred
property of foo
, and the bob
property of fred
must not be
null
after the bean is constructed, or a NullPointerException
is thrown.
If a bean is a dependency of another that usually means that one bean is set as a
property of another. Typically you accomplish this with the <ref/>
element in XML-based configuration metadata. However, sometimes dependencies between
beans are less direct; for example, a static initializer in a class needs to be
triggered, such as database driver registration. The depends-on
attribute can
explicitly force one or more beans to be initialized before the bean using this element
is initialized. The following example uses the depends-on
attribute to express a
dependency on a single bean:
<bean id="beanOne" class="ExampleBean" depends-on="manager"/> <bean id="manager" class="ManagerBean" />
To express a dependency on multiple beans, supply a list of bean names as the value of
the depends-on
attribute, with commas, whitespace and semicolons, used as valid
delimiters:
<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao"> <property name="manager" ref="manager" /> </bean> <bean id="manager" class="ManagerBean" /> <bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
Note | |
---|---|
The |
By default, ApplicationContext
implementations eagerly create and configure all
singleton beans as part of the initialization
process. Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately, as opposed to hours
or even days later. When this behavior is not desirable, you can prevent
pre-instantiation of a singleton bean by marking the bean definition as
lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean
instance when it is first requested, rather than at startup.
In XML, this behavior is controlled by the lazy-init
attribute on the <bean/>
element; for example:
<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/> <bean name="not.lazy" class="com.foo.AnotherBean"/>
When the preceding configuration is consumed by an ApplicationContext
, the bean named
lazy
is not eagerly pre-instantiated when the ApplicationContext
is starting up,
whereas the not.lazy
bean is eagerly pre-instantiated.
However, when a lazy-initialized bean is a dependency of a singleton bean that is
not lazy-initialized, the ApplicationContext
creates the lazy-initialized bean at
startup, because it must satisfy the singleton’s dependencies. The lazy-initialized bean
is injected into a singleton bean elsewhere that is not lazy-initialized.
You can also control lazy-initialization at the container level by using the
default-lazy-init
attribute on the <beans/>
element; for example:
<beans default-lazy-init="true"> <!-- no beans will be pre-instantiated... --> </beans>
The Spring container can autowire relationships between collaborating beans. You can
allow Spring to resolve collaborators (other beans) automatically for your bean by
inspecting the contents of the ApplicationContext
. Autowiring has the following
advantages:
When using XML-based configuration metadata [2], you specify autowire
mode for a bean definition with the autowire
attribute of the <bean/>
element. The
autowiring functionality has five modes. You specify autowiring per bean and thus
can choose which ones to autowire.
Table 4.2. Autowiring modes
Mode | Explanation |
---|---|
no | (Default) No autowiring. Bean references must be defined via a |
byName | Autowiring by property name. Spring looks for a bean with the same name as the
property that needs to be autowired. For example, if a bean definition is set to
autowire by name, and it contains a master property (that is, it has a
setMaster(..) method), Spring looks for a bean definition named |
byType | Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. |
constructor | Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised. |
With byType or constructor autowiring mode, you can wire arrays and
typed-collections. In such cases all autowire candidates within the container that
match the expected type are provided to satisfy the dependency. You can autowire
strongly-typed Maps if the expected key type is String
. An autowired Maps values will
consist of all bean instances that match the expected type, and the Maps keys will
contain the corresponding bean names.
You can combine autowire behavior with dependency checking, which is performed after autowiring completes.
Autowiring works best when it is used consistently across a project. If autowiring is not used in general, it might be confusing to developers to use it to wire only one or two bean definitions.
Consider the limitations and disadvantages of autowiring:
property
and constructor-arg
settings always override
autowiring. You cannot autowire so-called simple properties such as primitives,
Strings
, and Classes
(and arrays of such simple properties). This limitation is
by-design.
In the latter scenario, you have several options:
autowire-candidate
attributes
to false
as described in the next section.
primary
attribute of its <bean/>
element to true
.
On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set
the autowire-candidate
attribute of the <bean/>
element to false
; the container
makes that specific bean definition unavailable to the autowiring infrastructure
(including annotation style configurations such as @Autowired
).
You can also limit autowire candidates based on pattern-matching against bean names. The
top-level <beans/>
element accepts one or more patterns within its
default-autowire-candidates
attribute. For example, to limit autowire candidate status
to any bean whose name ends with Repository, provide a value of *Repository. To
provide multiple patterns, define them in a comma-separated list. An explicit value of
true
or false
for a bean definitions autowire-candidate
attribute always takes
precedence, and for such beans, the pattern matching rules do not apply.
These techniques are useful for beans that you never want to be injected into other beans by autowiring. It does not mean that an excluded bean cannot itself be configured using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.
A solution is to forego some inversion of control. You can make
bean A aware of the container by implementing the ApplicationContextAware
interface,
and by making a getBean("B") call to the container ask for (a
typically new) bean B instance every time bean A needs it. The following is an example
of this approach:
// a class that uses a stateful Command-style class to perform some processing package fiona.apple; // Spring-API imports import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; public class CommandManager implements ApplicationContextAware { private ApplicationContext applicationContext; public Object process(Map commandState) { // grab a new instance of the appropriate Command Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } protected Command createCommand() { // notice the Spring API dependency! return this.applicationContext.getBean("command", Command.class); } public void setApplicationContext( ApplicationContext applicationContext) throws BeansException { this.applicationContext = applicationContext; } }
The preceding is not desirable, because the business code is aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this use case to be handled in a clean fashion.
Lookup method injection is the ability of the container to override methods on container managed beans, to return the lookup result for another named bean in the container. The lookup typically involves a prototype bean as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to generate dynamically a subclass that overrides the method.
Note | |
---|---|
For this dynamic subclassing to work, the class that the Spring container will subclass
cannot be |
Looking at the CommandManager
class in the previous code snippet, you see that the
Spring container will dynamically override the implementation of the createCommand()
method. Your CommandManager
class will not have any Spring dependencies, as can be
seen in the reworked example:
package fiona.apple; // no more Spring imports! public abstract class CommandManager { public Object process(Object commandState) { // grab a new instance of the appropriate Command interface Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } // okay... but where is the implementation of this method? protected abstract Command createCommand(); }
In the client class containing the method to be injected (the CommandManager
in this
case), the method to be injected requires a signature of the following form:
<public|protected> [abstract] <return-type> theMethodName(no-arguments);
If the method is abstract
, the dynamically-generated subclass implements the method.
Otherwise, the dynamically-generated subclass overrides the concrete method defined in
the original class. For example:
<!-- a stateful bean deployed as a prototype (non-singleton) --> <bean id="command" class="fiona.apple.AsyncCommand" scope="prototype"> <!-- inject dependencies here as required --> </bean> <!-- commandProcessor uses statefulCommandHelper --> <bean id="commandManager" class="fiona.apple.CommandManager"> <lookup-method name="createCommand" bean="command"/> </bean>
The bean identified as commandManager calls its own method createCommand()
whenever it needs a new instance of the command bean. You must be careful to deploy
the command
bean as a prototype, if that is actually what is needed. If it is deployed
as a singleton, the same instance of the command
bean is returned each time.
Tip | |
---|---|
The interested reader may also find the |
A less useful form of method injection than lookup method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section until the functionality is actually needed.
With XML-based configuration metadata, you can use the replaced-method
element to
replace an existing method implementation with another, for a deployed bean. Consider
the following class, with a method computeValue, which we want to override:
public class MyValueCalculator { public String computeValue(String input) { // some real code... } // some other methods... }
A class implementing the org.springframework.beans.factory.support.MethodReplacer
interface provides the new method definition.
/** * meant to be used to override the existing computeValue(String) * implementation in MyValueCalculator */ public class ReplacementComputeValue implements MethodReplacer { public Object reimplement(Object o, Method m, Object[] args) throws Throwable { // get the input value, work with it, and return a computed result String input = (String) args[0]; ... return ...; } }
The bean definition to deploy the original class and specify the method override would look like this:
<bean id="myValueCalculator" class="x.y.z.MyValueCalculator"> <!-- arbitrary method replacement --> <replaced-method name="computeValue" replacer="replacementComputeValue"> <arg-type>String</arg-type> </replaced-method> </bean> <bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>
You can use one or more contained <arg-type/>
elements within the <replaced-method/>
element to indicate the method signature of the method being overridden. The signature
for the arguments is necessary only if the method is overloaded and multiple variants
exist within the class. For convenience, the type string for an argument may be a
substring of the fully qualified type name. For example, the following all match
java.lang.String
:
java.lang.String String Str
Because the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by allowing you to type only the shortest string that will match an argument type.
When you create a bean definition, you create a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, as with a class, you can create many object instances from a single recipe.
You can control not only the various dependencies and configuration values that are to
be plugged into an object that is created from a particular bean definition, but also
the scope of the objects created from a particular bean definition. This approach is
powerful and flexible in that you can choose the scope of the objects you create
through configuration instead of having to bake in the scope of an object at the Java
class level. Beans can be defined to be deployed in one of a number of scopes: out of
the box, the Spring Framework supports five scopes, three of which are available only if
you use a web-aware ApplicationContext
.
The following scopes are supported out of the box. You can also create a custom scope.
Table 4.3. Bean scopesThread-scoped beans
Scope | Description |
---|---|
(Default) Scopes a single bean definition to a single object instance per Spring IoC container. | |
Scopes a single bean definition to any number of object instances. | |
Scopes a single bean definition to the lifecycle of a single HTTP request; that is,
each HTTP request has its own instance of a bean created off the back of a single bean
definition. Only valid in the context of a web-aware Spring | |
Scopes a single bean definition to the lifecycle of an HTTP | |
Scopes a single bean definition to the lifecycle of a global HTTP |
Note | |
---|---|
As of Spring 3.0, a thread scope is available, but is not registered by default. For
more information, see the documentation for
|
Only one shared instance of a singleton bean is managed, and all requests for beans with an id or ids matching that bean definition result in that one specific bean instance being returned by the Spring container.
To put it another way, when you define a bean definition and it is scoped as a singleton, the Spring IoC container creates exactly one instance of the object defined by that bean definition. This single instance is stored in a cache of such singleton beans, and all subsequent requests and references for that named bean return the cached object.
Spring’s concept of a singleton bean differs from the Singleton pattern as defined in the Gang of Four (GoF) patterns book. The GoF Singleton hard-codes the scope of an object such that one and only one instance of a particular class is created per ClassLoader. The scope of the Spring singleton is best described as per container and per bean. This means that if you define one bean for a particular class in a single Spring container, then the Spring container creates one and only one instance of the class defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as a singleton in XML, you would write, for example:
<bean id="accountService" class="com.foo.DefaultAccountService"/> <!-- the following is equivalent, though redundant (singleton scope is the default) --> <bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>
The non-singleton, prototype scope of bean deployment results in the creation of a new
bean instance every time a request for that specific bean is made. That is, the bean
is injected into another bean or you request it through a getBean()
method call on the
container. As a rule, use the prototype scope for all stateful beans and the singleton
scope for stateless beans.
The following diagram illustrates the Spring prototype scope. A data access object (DAO) is not typically configured as a prototype, because a typical DAO does not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.
The following example defines a bean as a prototype in XML:
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>
In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean: the container instantiates, configures, and otherwise assembles a prototype object, and hands it to the client, with no further record of that prototype instance. Thus, although initialization lifecycle callback methods are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle callbacks are not called. The client code must clean up prototype-scoped objects and release expensive resources that the prototype bean(s) are holding. To get the Spring container to release resources held by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans that need to be cleaned up.
In some respects, the Spring container’s role in regard to a prototype-scoped bean is a
replacement for the Java new
operator. All lifecycle management past that point must
be handled by the client. (For details on the lifecycle of a bean in the Spring
container, see Section 4.6.1, “Lifecycle callbacks”.)
When you use singleton-scoped beans with dependencies on prototype beans, be aware that dependencies are resolved at instantiation time. Thus if you dependency-inject a prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated and then dependency-injected into the singleton bean. The prototype instance is the sole instance that is ever supplied to the singleton-scoped bean.
However, suppose you want the singleton-scoped bean to acquire a new instance of the prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a prototype-scoped bean into your singleton bean, because that injection occurs only once, when the Spring container is instantiating the singleton bean and resolving and injecting its dependencies. If you need a new instance of a prototype bean at runtime more than once, see Section 4.4.6, “Method injection”
The request
, session
, and global session
scopes are only available if you use
a web-aware Spring ApplicationContext
implementation (such as
XmlWebApplicationContext
). If you use these scopes with regular Spring IoC containers
such as the ClassPathXmlApplicationContext
, you get an IllegalStateException
complaining about an unknown bean scope.
To support the scoping of beans at the request
, session
, and global session
levels
(web-scoped beans), some minor initial configuration is required before you define your
beans. (This initial setup is not required for the standard scopes, singleton and
prototype.)
How you accomplish this initial setup depends on your particular Servlet environment..
If you access scoped beans within Spring Web MVC, in effect, within a request that is
processed by the Spring DispatcherServlet
, or DispatcherPortlet
, then no special
setup is necessary: DispatcherServlet
and DispatcherPortlet
already expose all
relevant state.
If you use a Servlet 2.5 web container, with requests processed outside of Spring’s
DispatcherServlet (for example, when using JSF or Struts), you need to register the
org.springframework.web.context.request.RequestContextListener
ServletRequestListener
.
For Servlet 3.0+, this can done programmatically via the WebApplicationInitializer
interface. Alternatively, or for older containers, add the following declaration to
your web application’s web.xml
file:
<web-app> ... <listener> <listener-class> org.springframework.web.context.request.RequestContextListener </listener-class> </listener> ... </web-app>
Alternatively, if there are issues with your listener setup, consider the provided
RequestContextFilter
. The filter mapping depends on the surrounding web
application configuration, so you have to change it as appropriate.
<web-app> ... <filter> <filter-name>requestContextFilter</filter-name> <filter-class>org.springframework.web.filter.RequestContextFilter</filter-class> </filter> <filter-mapping> <filter-name>requestContextFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> ... </web-app>
DispatcherServlet
, RequestContextListener
and RequestContextFilter
all do exactly
the same thing, namely bind the HTTP request object to the Thread
that is servicing
that request. This makes beans that are request- and session-scoped available further
down the call chain.
Consider the following bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>
The Spring container creates a new instance of the LoginAction
bean by using the
loginAction
bean definition for each and every HTTP request. That is, the
loginAction
bean is scoped at the HTTP request level. You can change the internal
state of the instance that is created as much as you want, because other instances
created from the same loginAction
bean definition will not see these changes in state;
they are particular to an individual request. When the request completes processing, the
bean that is scoped to the request is discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
The Spring container creates a new instance of the UserPreferences
bean by using the
userPreferences
bean definition for the lifetime of a single HTTP Session
. In other
words, the userPreferences
bean is effectively scoped at the HTTP Session
level. As
with request-scoped
beans, you can change the internal state of the instance that is
created as much as you want, knowing that other HTTP Session
instances that are also
using instances created from the same userPreferences
bean definition do not see these
changes in state, because they are particular to an individual HTTP Session
. When the
HTTP Session
is eventually discarded, the bean that is scoped to that particular HTTP
Session
is also discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>
The global session
scope is similar to the standard HTTP Session
scope
(described above), and applies only in the context of
portlet-based web applications. The portlet specification defines the notion of a global
Session
that is shared among all portlets that make up a single portlet web
application. Beans defined at the global session
scope are scoped (or bound) to the
lifetime of the global portlet Session
.
If you write a standard Servlet-based web application and you define one or more beans
as having global session
scope, the standard HTTP Session
scope is used, and no
error is raised.
The Spring IoC container manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject (for example) an HTTP request scoped bean into another bean, you must inject an AOP proxy in place of the scoped bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped object but that can also retrieve the real, target object from the relevant scope (for example, an HTTP request) and delegate method calls onto the real object.
Note | |
---|---|
You do not need to use the |
The configuration in the following example is only one line, but it is important to understand the "why" as well as the "how" behind it.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- an HTTP Session-scoped bean exposed as a proxy --> <bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <!-- instructs the container to proxy the surrounding bean --> <aop:scoped-proxy/> </bean> <!-- a singleton-scoped bean injected with a proxy to the above bean --> <bean id="userService" class="com.foo.SimpleUserService"> <!-- a reference to the proxied userPreferences bean --> <property name="userPreferences" ref="userPreferences"/> </bean> </beans>
To create such a proxy, you insert a child <aop:scoped-proxy/>
element into a scoped
bean definition. See the section called “Choosing the type of proxy to create” and
Chapter 33, XML Schema-based configuration.) Why do definitions of beans scoped at the request
, session
,
globalSession
and custom-scope levels require the <aop:scoped-proxy/>
element ?
Let’s examine the following singleton bean definition and contrast it with what you need
to define for the aforementioned scopes. (The following userPreferences
bean
definition as it stands is incomplete.)
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
In the preceding example, the singleton bean userManager
is injected with a reference
to the HTTP Session
-scoped bean userPreferences
. The salient point here is that the
userManager
bean is a singleton: it will be instantiated exactly once per
container, and its dependencies (in this case only one, the userPreferences
bean) are
also injected only once. This means that the userManager
bean will only operate on the
exact same userPreferences
object, that is, the one that it was originally injected
with.
This is not the behavior you want when injecting a shorter-lived scoped bean into a
longer-lived scoped bean, for example injecting an HTTP Session
-scoped collaborating
bean as a dependency into singleton bean. Rather, you need a single userManager
object, and for the lifetime of an HTTP Session
, you need a userPreferences
object
that is specific to said HTTP Session
. Thus the container creates an object that
exposes the exact same public interface as the UserPreferences
class (ideally an
object that is a UserPreferences
instance) which can fetch the real
UserPreferences
object from the scoping mechanism (HTTP request, Session
, etc.). The
container injects this proxy object into the userManager
bean, which is unaware that
this UserPreferences
reference is a proxy. In this example, when a UserManager
instance invokes a method on the dependency-injected UserPreferences
object, it
actually is invoking a method on the proxy. The proxy then fetches the real
UserPreferences
object from (in this case) the HTTP Session
, and delegates the
method invocation onto the retrieved real UserPreferences
object.
Thus you need the following, correct and complete, configuration when injecting
request-
, session-
, and globalSession-scoped
beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <aop:scoped-proxy/> </bean> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
By default, when the Spring container creates a proxy for a bean that is marked up with
the <aop:scoped-proxy/>
element, a CGLIB-based class proxy is created.
Note | |
---|---|
CGLIB proxies only intercept public method calls! Do not call non-public methods on such a proxy; they will not be delegated to the scoped target object. |
Alternatively, you can configure the Spring container to create standard JDK
interface-based proxies for such scoped beans, by specifying false
for the value of
the proxy-target-class
attribute of the <aop:scoped-proxy/>
element. Using JDK
interface-based proxies means that you do not need additional libraries in your
application classpath to effect such proxying. However, it also means that the class of
the scoped bean must implement at least one interface, and that all collaborators
into which the scoped bean is injected must reference the bean through one of its
interfaces.
<!-- DefaultUserPreferences implements the UserPreferences interface --> <bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session"> <aop:scoped-proxy proxy-target-class="false"/> </bean> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
For more detailed information about choosing class-based or interface-based proxying, see Section 8.6, “Proxying mechanisms”.
The bean scoping mechanism is extensible; You can define your own
scopes, or even redefine existing scopes, although the latter is considered bad practice
and you cannot override the built-in singleton
and prototype
scopes.
To integrate your custom scope(s) into the Spring container, you need to implement the
org.springframework.beans.factory.config.Scope
interface, which is described in this
section. For an idea of how to implement your own scopes, see the Scope
implementations that are supplied with the Spring Framework itself and the
Scope
Javadoc, which explains the methods you need to implement in more detail.
The Scope
interface has four methods to get objects from the scope, remove them from
the scope, and allow them to be destroyed.
The following method returns the object from the underlying scope. The session scope implementation, for example, returns the session-scoped bean (and if it does not exist, the method returns a new instance of the bean, after having bound it to the session for future reference).
Object get(String name, ObjectFactory objectFactory)
The following method removes the object from the underlying scope. The session scope implementation for example, removes the session-scoped bean from the underlying session. The object should be returned, but you can return null if the object with the specified name is not found.
Object remove(String name)
The following method registers the callbacks the scope should execute when it is destroyed or when the specified object in the scope is destroyed. Refer to the Javadoc or a Spring scope implementation for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The following method obtains the conversation identifier for the underlying scope. This identifier is different for each scope. For a session scoped implementation, this identifier can be the session identifier.
String getConversationId()
After you write and test one or more custom Scope
implementations, you need to make
the Spring container aware of your new scope(s). The following method is the central
method to register a new Scope
with the Spring container:
void registerScope(String scopeName, Scope scope);
This method is declared on the ConfigurableBeanFactory
interface, which is available
on most of the concrete ApplicationContext
implementations that ship with Spring via
the BeanFactory property.
The first argument to the registerScope(..)
method is the unique name associated with
a scope; examples of such names in the Spring container itself are singleton
and
prototype
. The second argument to the registerScope(..)
method is an actual instance
of the custom Scope
implementation that you wish to register and use.
Suppose that you write your custom Scope
implementation, and then register it as below.
Note | |
---|---|
The example below uses |
Scope threadScope = new SimpleThreadScope(); beanFactory.registerScope("thread", threadScope);
You then create bean definitions that adhere to the scoping rules of your custom Scope
:
<bean id="..." class="..." scope="thread">
With a custom Scope
implementation, you are not limited to programmatic registration
of the scope. You can also do the Scope
registration declaratively, using the
CustomScopeConfigurer
class:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer"> <property name="scopes"> <map> <entry key="thread"> <bean class="org.springframework.context.support.SimpleThreadScope"/> </entry> </map> </property> </bean> <bean id="bar" class="x.y.Bar" scope="thread"> <property name="name" value="Rick"/> <aop:scoped-proxy/> </bean> <bean id="foo" class="x.y.Foo"> <property name="bar" ref="bar"/> </bean> </beans>
Note | |
---|---|
When you place <aop:scoped-proxy/> in a |
To interact with the container’s management of the bean lifecycle, you can implement the
Spring InitializingBean
and DisposableBean
interfaces. The container calls
afterPropertiesSet()
for the former and destroy()
for the latter to allow the bean
to perform certain actions upon initialization and destruction of your beans.
Tip | |
---|---|
The JSR-250 If you don’t want to use the JSR-250 annotations but you are still looking to remove coupling consider the use of init-method and destroy-method object definition metadata. |
Internally, the Spring Framework uses BeanPostProcessor
implementations to process any
callback interfaces it can find and call the appropriate methods. If you need custom
features or other lifecycle behavior Spring does not offer out-of-the-box, you can
implement a BeanPostProcessor
yourself. For more information, see
Section 4.8, “Container Extension Points”.
In addition to the initialization and destruction callbacks, Spring-managed objects may
also implement the Lifecycle
interface so that those objects can participate in the
startup and shutdown process as driven by the container’s own lifecycle.
The lifecycle callback interfaces are described in this section.
The org.springframework.beans.factory.InitializingBean
interface allows a bean to
perform initialization work after all necessary properties on the bean have been set by
the container. The InitializingBean
interface specifies a single method:
void afterPropertiesSet() throws Exception;
It is recommended that you do not use the InitializingBean
interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PostConstruct
annotation or
specify a POJO initialization method. In the case of XML-based configuration metadata,
you use the init-method
attribute to specify the name of the method that has a void
no-argument signature. For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean { public void init() { // do some initialization work } }
…is exactly the same as…
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean { public void afterPropertiesSet() { // do some initialization work } }
but does not couple the code to Spring.
Implementing the org.springframework.beans.factory.DisposableBean
interface allows a
bean to get a callback when the container containing it is destroyed. The
DisposableBean
interface specifies a single method:
void destroy() throws Exception;
It is recommended that you do not use the DisposableBean
callback interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PreDestroy
annotation or
specify a generic method that is supported by bean definitions. With XML-based
configuration metadata, you use the destroy-method
attribute on the <bean/>
. For
example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean { public void cleanup() { // do some destruction work (like releasing pooled connections) } }
is exactly the same as:
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean { public void destroy() { // do some destruction work (like releasing pooled connections) } }
but does not couple the code to Spring.
When you write initialization and destroy method callbacks that do not use the
Spring-specific InitializingBean
and DisposableBean
callback interfaces, you
typically write methods with names such as init()
, initialize()
, dispose()
, and so
on. Ideally, the names of such lifecycle callback methods are standardized across a
project so that all developers use the same method names and ensure consistency.
You can configure the Spring container to look
for named initialization and destroy
callback method names on every bean. This means that you, as an application
developer, can write your application classes and use an initialization callback called
init()
, without having to configure an init-method="init"
attribute with each bean
definition. The Spring IoC container calls that method when the bean is created (and in
accordance with the standard lifecycle callback contract described previously). This
feature also enforces a consistent naming convention for initialization and destroy
method callbacks.
Suppose that your initialization callback methods are named init()
and destroy
callback methods are named destroy()
. Your class will resemble the class in the
following example.
public class DefaultBlogService implements BlogService { private BlogDao blogDao; public void setBlogDao(BlogDao blogDao) { this.blogDao = blogDao; } // this is (unsurprisingly) the initialization callback method public void init() { if (this.blogDao == null) { throw new IllegalStateException("The [blogDao] property must be set."); } } }
<beans default-init-method="init"> <bean id="blogService" class="com.foo.DefaultBlogService"> <property name="blogDao" ref="blogDao" /> </bean> </beans>
The presence of the default-init-method
attribute on the top-level <beans/>
element
attribute causes the Spring IoC container to recognize a method called init
on beans
as the initialization method callback. When a bean is created and assembled, if the bean
class has such a method, it is invoked at the appropriate time.
You configure destroy method callbacks similarly (in XML, that is) by using the
default-destroy-method
attribute on the top-level <beans/>
element.
Where existing bean classes already have callback methods that are named at variance
with the convention, you can override the default by specifying (in XML, that is) the
method name using the init-method
and destroy-method
attributes of the <bean/>
itself.
The Spring container guarantees that a configured initialization callback is called immediately after a bean is supplied with all dependencies. Thus the initialization callback is called on the raw bean reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. If the target bean and the proxy are defined separately, your code can even interact with the raw target bean, bypassing the proxy. Hence, it would be inconsistent to apply the interceptors to the init method, because doing so would couple the lifecycle of the target bean with its proxy/interceptors and leave strange semantics when your code interacts directly to the raw target bean.
As of Spring 2.5, you have three options for controlling bean lifecycle behavior: the
InitializingBean
and
DisposableBean
callback interfaces; custom
init()
and destroy()
methods; and the
@PostConstruct
and @PreDestroy
annotations. You can combine these mechanisms to control a given bean.
Note | |
---|---|
If multiple lifecycle mechanisms are configured for a bean, and each mechanism is
configured with a different method name, then each configured method is executed in the
order listed below. However, if the same method name is configured - for example,
|
Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are called as follows:
@PostConstruct
afterPropertiesSet()
as defined by the InitializingBean
callback interface
init()
method
Destroy methods are called in the same order:
@PreDestroy
destroy()
as defined by the DisposableBean
callback interface
destroy()
method
The Lifecycle
interface defines the essential methods for any object that has its own
lifecycle requirements (e.g. starts and stops some background process):
public interface Lifecycle { void start(); void stop(); boolean isRunning(); }
Any Spring-managed object may implement that interface. Then, when the
ApplicationContext itself starts and stops, it will cascade those calls to all Lifecycle
implementations defined within that context. It does this by delegating to a
LifecycleProcessor
:
public interface LifecycleProcessor extends Lifecycle { void onRefresh(); void onClose(); }
Notice that the LifecycleProcessor
is itself an extension of the Lifecycle
interface. It also adds two other methods for reacting to the context being refreshed
and closed.
The order of startup and shutdown invocations can be important. If a "depends-on"
relationship exists between any two objects, the dependent side will start after its
dependency, and it will stop before its dependency. However, at times the direct
dependencies are unknown. You may only know that objects of a certain type should start
prior to objects of another type. In those cases, the SmartLifecycle
interface defines
another option, namely the getPhase()
method as defined on its super-interface,
Phased
.
public interface Phased { int getPhase(); }
public interface SmartLifecycle extends Lifecycle, Phased { boolean isAutoStartup(); void stop(Runnable callback); }
When starting, the objects with the lowest phase start first, and when stopping, the
reverse order is followed. Therefore, an object that implements SmartLifecycle
and
whose getPhase() method returns Integer.MIN_VALUE
would be among the first to start
and the last to stop. At the other end of the spectrum, a phase value of
Integer.MAX_VALUE
would indicate that the object should be started last and stopped
first (likely because it depends on other processes to be running). When considering the
phase value, it’s also important to know that the default phase for any "normal"
Lifecycle
object that does not implement SmartLifecycle
would be 0. Therefore, any
negative phase value would indicate that an object should start before those standard
components (and stop after them), and vice versa for any positive phase value.
As you can see the stop method defined by SmartLifecycle
accepts a callback. Any
implementation must invoke that callback’s run() method after that implementation’s
shutdown process is complete. That enables asynchronous shutdown where necessary since
the default implementation of the LifecycleProcessor
interface,
DefaultLifecycleProcessor
, will wait up to its timeout value for the group of objects
within each phase to invoke that callback. The default per-phase timeout is 30 seconds.
You can override the default lifecycle processor instance by defining a bean named
"lifecycleProcessor" within the context. If you only want to modify the timeout, then
defining the following would be sufficient:
<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor"> <!-- timeout value in milliseconds --> <property name="timeoutPerShutdownPhase" value="10000"/> </bean>
As mentioned, the LifecycleProcessor
interface defines callback methods for the
refreshing and closing of the context as well. The latter will simply drive the shutdown
process as if stop() had been called explicitly, but it will happen when the context is
closing. The refresh callback on the other hand enables another feature of
SmartLifecycle
beans. When the context is refreshed (after all objects have been
instantiated and initialized), that callback will be invoked, and at that point the
default lifecycle processor will check the boolean value returned by each
SmartLifecycle
object’s isAutoStartup()
method. If "true", then that object will be
started at that point rather than waiting for an explicit invocation of the context’s or
its own start() method (unlike the context refresh, the context start does not happen
automatically for a standard context implementation). The "phase" value as well as any
"depends-on" relationships will determine the startup order in the same way as described
above.
Note | |
---|---|
This section applies only to non-web applications. Spring’s web-based
|
If you are using Spring’s IoC container in a non-web application environment; for example, in a rich client desktop environment; you register a shutdown hook with the JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your singleton beans so that all resources are released. Of course, you must still configure and implement these destroy callbacks correctly.
To register a shutdown hook, you call the registerShutdownHook()
method that is
declared on the AbstractApplicationContext
class:
import org.springframework.context.support.AbstractApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public final class Boot { public static void main(final String[] args) throws Exception { AbstractApplicationContext ctx = new ClassPathXmlApplicationContext( new String []{"beans.xml"}); // add a shutdown hook for the above context... ctx.registerShutdownHook(); // app runs here... // main method exits, hook is called prior to the app shutting down... } }
When an ApplicationContext
creates a class that implements the
org.springframework.context.ApplicationContextAware
interface, the class is provided
with a reference to that ApplicationContext
.
public interface ApplicationContextAware { void setApplicationContext(ApplicationContext applicationContext) throws BeansException; }
Thus beans can manipulate programmatically the ApplicationContext
that created them,
through the ApplicationContext
interface, or by casting the reference to a known
subclass of this interface, such as ConfigurableApplicationContext
, which exposes
additional functionality. One use would be the programmatic retrieval of other beans.
Sometimes this capability is useful; however, in general you should avoid it, because it
couples the code to Spring and does not follow the Inversion of Control style, where
collaborators are provided to beans as properties. Other methods of the
ApplicationContext provide access to file resources, publishing application events, and
accessing a MessageSource. These additional features are described in
Section 4.16, “Additional Capabilities of the ApplicationContext”
As of Spring 2.5, autowiring is another alternative to obtain reference to the
ApplicationContext
. The "traditional" constructor
and byType
autowiring modes (as
described in Section 4.4.5, “Autowiring collaborators”) can provide a dependency of type
ApplicationContext
for a constructor argument or setter method parameter,
respectively. For more flexibility, including the ability to autowire fields and
multiple parameter methods, use the new annotation-based autowiring features. If you do,
the ApplicationContext
is autowired into a field, constructor argument, or method
parameter that is expecting the ApplicationContext
type if the field, constructor, or
method in question carries the @Autowired
annotation. For more information, see
Section 4.9.2, “@Autowired”.
When an ApplicationContext creates a class that implements the
org.springframework.beans.factory.BeanNameAware
interface, the class is provided with
a reference to the name defined in its associated object definition.
public interface BeanNameAware { void setBeanName(string name) throws BeansException; }
The callback is invoked after population of normal bean properties but before an
initialization callback such as InitializingBean
afterPropertiesSet or a custom
init-method.
Besides ApplicationContextAware
and BeanNameAware
discussed above, Spring offers a
range of Aware
interfaces that allow beans to indicate to the container that they
require a certain infrastructure dependency. The most important Aware
interfaces
are summarized below - as a general rule, the name is a good indication of the
dependency type:
Table 4.4. Aware interfaces
Name | Injected Dependency | Explained in… |
---|---|---|
| Declaring | |
| Event publisher of the enclosing | Section 4.16, “Additional Capabilities of the ApplicationContext” |
| Class loader used to load the bean classes. | |
| Declaring | |
| Name of the declaring bean | |
| Resource adapter | |
| Defined weaver for processing class definition at load time | Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework” |
| Configured strategy for resolving messages (with support for parametrization and internationalization) | Section 4.16, “Additional Capabilities of the ApplicationContext” |
| Spring JMX notification publisher | |
| Current | |
| Current | |
| Configured loader for low-level access to resources | |
| Current | |
| Current |
Note again that usage of these interfaces ties your code to the Spring API and does not follow the Inversion of Control style. As such, they are recommended for infrastructure beans that require programmatic access to the container.
A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information such as initialization method, static factory method name, and so on. A child bean definition inherits configuration data from a parent definition. The child definition can override some values, or add others, as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.
If you work with an ApplicationContext
interface programmatically, child bean
definitions are represented by the ChildBeanDefinition
class. Most users do not work
with them on this level, instead configuring bean definitions declaratively in something
like the ClassPathXmlApplicationContext
. When you use XML-based configuration
metadata, you indicate a child bean definition by using the parent
attribute,
specifying the parent bean as the value of this attribute.
<bean id="inheritedTestBean" abstract="true" class="org.springframework.beans.TestBean"> <property name="name" value="parent"/> <property name="age" value="1"/> </bean> <bean id="inheritsWithDifferentClass" class="org.springframework.beans.DerivedTestBean" parent="inheritedTestBean" init-method="initialize"> <property name="name" value="override"/> <!-- the age property value of 1 will be inherited from parent --> </bean>
A child bean definition uses the bean class from the parent definition if none is specified, but can also override it. In the latter case, the child bean class must be compatible with the parent, that is, it must accept the parent’s property values.
A child bean definition inherits constructor argument values, property values, and
method overrides from the parent, with the option to add new values. Any initialization
method, destroy method, and/or static
factory method settings that you specify will
override the corresponding parent settings.
The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.
The preceding example explicitly marks the parent bean definition as abstract by using
the abstract
attribute. If the parent definition does not specify a class, explicitly
marking the parent bean definition as abstract
is required, as follows:
<bean id="inheritedTestBeanWithoutClass" abstract="true"> <property name="name" value="parent"/> <property name="age" value="1"/> </bean> <bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean" parent="inheritedTestBeanWithoutClass" init-method="initialize"> <property name="name" value="override"/> <!-- age will inherit the value of 1 from the parent bean definition--> </bean>
The parent bean cannot be instantiated on its own because it is incomplete, and it is
also explicitly marked as abstract
. When a definition is abstract
like this, it is
usable only as a pure template bean definition that serves as a parent definition for
child definitions. Trying to use such an abstract
parent bean on its own, by referring
to it as a ref property of another bean or doing an explicit getBean()
call with the
parent bean id, returns an error. Similarly, the container’s internal
preInstantiateSingletons()
method ignores bean definitions that are defined as
abstract.
Note | |
---|---|
|
Typically, an application developer does not need to subclass ApplicationContext
implementation classes. Instead, the Spring IoC container can be extended by plugging in
implementations of special integration interfaces. The next few sections describe these
integration interfaces.
The BeanPostProcessor
interface defines callback methods that you can implement to
provide your own (or override the container’s default) instantiation logic,
dependency-resolution logic, and so forth. If you want to implement some custom logic
after the Spring container finishes instantiating, configuring, and initializing a bean,
you can plug in one or more BeanPostProcessor
implementations.
You can configure multiple BeanPostProcessor
instances, and you can control the order
in which these BeanPostProcessor
s execute by setting the order
property. You can
set this property only if the BeanPostProcessor
implements the Ordered
interface; if
you write your own BeanPostProcessor
you should consider implementing the Ordered
interface too. For further details, consult the Javadoc for the BeanPostProcessor
and
Ordered
interfaces. See also the note below on
programmatic
registration of BeanPostProcessors
Note | |
---|---|
To change the actual bean definition (i.e., the blueprint that defines the bean),
you instead need to use a |
The org.springframework.beans.factory.config.BeanPostProcessor
interface consists of
exactly two callback methods. When such a class is registered as a post-processor with
the container, for each bean instance that is created by the container, the
post-processor gets a callback from the container both before container
initialization methods (such as InitializingBean’s afterPropertiesSet() and any
declared init method) are called as well as after any bean initialization callbacks.
The post-processor can take any action with the bean instance, including ignoring the
callback completely. A bean post-processor typically checks for callback interfaces or
may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as
bean post-processors in order to provide proxy-wrapping logic.
An ApplicationContext
automatically detects any beans that are defined in the
configuration metadata which implement the BeanPostProcessor
interface. The
ApplicationContext
registers these beans as post-processors so that they can be called
later upon bean creation. Bean post-processors can be deployed in the container just
like any other beans.
Note | |
---|---|
Programmatically registering BeanPostProcessors While the recommended approach for |
Note | |
---|---|
BeanPostProcessors and AOP auto-proxying Classes that implement the For any such bean, you should see an informational log message: "Bean foo is not eligible for getting processed by all BeanPostProcessor interfaces (for example: not eligible for auto-proxying)". Note that if you have beans wired into your |
The following examples show how to write, register, and use BeanPostProcessors
in an
ApplicationContext
.
This first example illustrates basic usage. The example shows a custom
BeanPostProcessor
implementation that invokes the toString()
method of each bean as
it is created by the container and prints the resulting string to the system console.
Find below the custom BeanPostProcessor
implementation class definition:
package scripting; import org.springframework.beans.factory.config.BeanPostProcessor; import org.springframework.beans.BeansException; public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor { // simply return the instantiated bean as-is public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { return bean; // we could potentially return any object reference here... } public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("Bean " + beanName + " created : " + bean.toString()); return bean; } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:lang="http://www.springframework.org/schema/lang" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd"> <lang:groovy id="messenger" script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy"> <lang:property name="message" value="Fiona Apple Is Just So Dreamy."/> </lang:groovy> <!-- when the above bean (messenger) is instantiated, this custom BeanPostProcessor implementation will output the fact to the system console --> <bean class="scripting.InstantiationTracingBeanPostProcessor"/> </beans>
Notice how the InstantiationTracingBeanPostProcessor
is simply defined. It does not
even have a name, and because it is a bean it can be dependency-injected just like any
other bean. (The preceding configuration also defines a bean that is backed by a Groovy
script. The Spring dynamic language support is detailed in the chapter entitled
Chapter 28, Dynamic language support.)
The following simple Java application executes the preceding code and configuration:
import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.scripting.Messenger; public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml"); Messenger messenger = (Messenger) ctx.getBean("messenger"); System.out.println(messenger); } }
The output of the preceding application resembles the following:
Bean messenger created : org.springframework.scripting.groovy.GroovyMessenger@272961
org.springframework.scripting.groovy.GroovyMessenger@272961
Using callback interfaces or annotations in conjunction with a custom
BeanPostProcessor
implementation is a common means of extending the Spring IoC
container. An example is Spring’s RequiredAnnotationBeanPostProcessor
- a
BeanPostProcessor
implementation that ships with the Spring distribution which ensures
that JavaBean properties on beans that are marked with an (arbitrary) annotation are
actually (configured to be) dependency-injected with a value.
The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor
. The semantics of
this interface are similar to those of the BeanPostProcessor
, with one major
difference: BeanFactoryPostProcessor
operates on the bean configuration metadata;
that is, the Spring IoC container allows a BeanFactoryPostProcessor
to read the
configuration metadata and potentially change it before the container instantiates
any beans other than BeanFactoryPostProcessors
.
You can configure multiple BeanFactoryPostProcessors
, and you can control the order in
which these BeanFactoryPostProcessors
execute by setting the order
property.
However, you can only set this property if the BeanFactoryPostProcessor
implements the
Ordered
interface. If you write your own BeanFactoryPostProcessor
, you should
consider implementing the Ordered
interface too. Consult the Javadoc for the
BeanFactoryPostProcessor
and Ordered
interfaces for more details.
Note | |
---|---|
If you want to change the actual bean instances (i.e., the objects that are created
from the configuration metadata), then you instead need to use a Also, |
A bean factory post-processor is executed automatically when it is declared inside an
ApplicationContext
, in order to apply changes to the configuration metadata that
define the container. Spring includes a number of predefined bean factory
post-processors, such as PropertyOverrideConfigurer
and
PropertyPlaceholderConfigurer
. A custom BeanFactoryPostProcessor
can also be used,
for example, to register custom property editors.
An ApplicationContext
automatically detects any beans that are deployed into it that
implement the BeanFactoryPostProcessor
interface. It uses these beans as bean factory
post-processors, at the appropriate time. You can deploy these post-processor beans as
you would any other bean.
Note | |
---|---|
As with |
You use the PropertyPlaceholderConfigurer
to externalize property values from a bean
definition in a separate file using the standard Java Properties
format. Doing so
enables the person deploying an application to customize environment-specific properties
such as database URLs and passwords, without the complexity or risk of modifying the
main XML definition file or files for the container.
Consider the following XML-based configuration metadata fragment, where a DataSource
with placeholder values is defined. The example shows properties configured from an
external Properties
file. At runtime, a PropertyPlaceholderConfigurer
is applied to
the metadata that will replace some properties of the DataSource. The values to replace
are specified as placeholders of the form ${property-name}
which follows the Ant /
log4j / JSP EL style.
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations" value="classpath:com/foo/jdbc.properties"/> </bean> <bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean>
The actual values come from another file in the standard Java Properties
format:
jdbc.driverClassName=org.hsqldb.jdbcDriver jdbc.url=jdbc:hsqldb:hsql://production:9002 jdbc.username=sa jdbc.password=root
Therefore, the string ${jdbc.username}
is replaced at runtime with the value sa, and
the same applies for other placeholder values that match keys in the properties file.
The PropertyPlaceholderConfigurer
checks for placeholders in most properties and
attributes of a bean definition. Furthermore, the placeholder prefix and suffix can be
customized.
With the context
namespace introduced in Spring 2.5, it is possible to configure
property placeholders with a dedicated configuration element. One or more locations can
be provided as a comma-separated list in the location
attribute.
<context:property-placeholder location="classpath:com/foo/jdbc.properties"/>
The PropertyPlaceholderConfigurer
not only looks for properties in the Properties
file you specify. By default it also checks against the Java System
properties if it
cannot find a property in the specified properties files. You can customize this
behavior by setting the systemPropertiesMode
property of the configurer with one of
the following three supported integer values:
Consult the Javadoc for the PropertyPlaceholderConfigurer
for more information.
Tip | |
---|---|
You can use the <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations"> <value>classpath:com/foo/strategy.properties</value> </property> <property name="properties"> <value>custom.strategy.class=com.foo.DefaultStrategy</value> </property> </bean> <bean id="serviceStrategy" class="${custom.strategy.class}"/> If the class cannot be resolved at runtime to a valid class, resolution of the bean
fails when it is about to be created, which is during the |
The PropertyOverrideConfigurer
, another bean factory post-processor, resembles the
PropertyPlaceholderConfigurer
, but unlike the latter, the original definitions can
have default values or no values at all for bean properties. If an overriding
Properties
file does not have an entry for a certain bean property, the default
context definition is used.
Note that the bean definition is not aware of being overridden, so it is not
immediately obvious from the XML definition file that the override configurer is being
used. In case of multiple PropertyOverrideConfigurer
instances that define different
values for the same bean property, the last one wins, due to the overriding mechanism.
Properties file configuration lines take this format:
beanName.property=value
For example:
dataSource.driverClassName=com.mysql.jdbc.Driver dataSource.url=jdbc:mysql:mydb
This example file can be used with a container definition that contains a bean called dataSource, which has driver and url properties.
Compound property names are also supported, as long as every component of the path except the final property being overridden is already non-null (presumably initialized by the constructors). In this example…
foo.fred.bob.sammy=123
sammy
property of the bob
property of the fred
property of the foo
bean
is set to the scalar value 123
.
Note | |
---|---|
Specified override values are always literal values; they are not translated into bean references. This convention also applies when the original value in the XML bean definition specifies a bean reference. |
With the context
namespace introduced in Spring 2.5, it is possible to configure
property overriding with a dedicated configuration element:
<context:property-override location="classpath:override.properties"/>
Implement the org.springframework.beans.factory.FactoryBean
interface for objects that
are themselves factories.
The FactoryBean
interface is a point of pluggability into the Spring IoC container’s
instantiation logic. If you have complex initialization code that is better expressed in
Java as opposed to a (potentially) verbose amount of XML, you can create your own
FactoryBean
, write the complex initialization inside that class, and then plug your
custom FactoryBean
into the container.
The FactoryBean
interface provides three methods:
Object getObject()
: returns an instance of the object this factory creates. The
instance can possibly be shared, depending on whether this factory returns singletons
or prototypes.
boolean isSingleton()
: returns true
if this FactoryBean
returns singletons,
false
otherwise.
Class getObjectType()
: returns the object type returned by the getObject()
method
or null
if the type is not known in advance.
The FactoryBean
concept and interface is used in a number of places within the Spring
Framework; more than 50 implementations of the FactoryBean
interface ship with Spring
itself.
When you need to ask a container for an actual FactoryBean
instance itself instead of
the bean it produces, preface the bean’s id with the ampersand symbol ( &
) when
calling the getBean()
method of the ApplicationContext
. So for a given FactoryBean
with an id of myBean
, invoking getBean("myBean")
on the container returns the
product of the FactoryBean
; whereas, invoking getBean("&myBean")
returns the
FactoryBean
instance itself.
An alternative to XML setups is provided by annotation-based configuration which rely on
the bytecode metadata for wiring up components instead of angle-bracket declarations.
Instead of using XML to describe a bean wiring, the developer moves the configuration
into the component class itself by using annotations on the relevant class, method, or
field declaration. As mentioned in the section called “Example: The RequiredAnnotationBeanPostProcessor”, using
a BeanPostProcessor
in conjunction with annotations is a common means of extending the
Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing
required properties with the @Required annotation. Spring
2.5 made it possible to follow that same general approach to drive Spring’s dependency
injection. Essentially, the @Autowired
annotation provides the same capabilities as
described in Section 4.4.5, “Autowiring collaborators” but with more fine-grained control and wider
applicability. Spring 2.5 also added support for JSR-250 annotations such as
@PostConstruct
, and @PreDestroy
. Spring 3.0 added support for JSR-330 (Dependency
Injection for Java) annotations contained in the javax.inject package such as @Inject
and @Named
. Details about those annotations can be found in the
relevant section.
Note | |
---|---|
Annotation injection is performed before XML injection, thus the latter configuration will override the former for properties wired through both approaches. |
As always, you can register them as individual bean definitions, but they can also be
implicitly registered by including the following tag in an XML-based Spring
configuration (notice the inclusion of the context
namespace):
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> </beans>
(The implicitly registered post-processors include
AutowiredAnnotationBeanPostProcessor
,
CommonAnnotationBeanPostProcessor
,
PersistenceAnnotationBeanPostProcessor
,
as well as the aforementioned
RequiredAnnotationBeanPostProcessor
.)
Note | |
---|---|
|
The @Required
annotation applies to bean property setter methods, as in the following
example:
public class SimpleMovieLister { private MovieFinder movieFinder; @Required public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
This annotation simply indicates that the affected bean property must be populated at
configuration time, through an explicit property value in a bean definition or through
autowiring. The container throws an exception if the affected bean property has not been
populated; this allows for eager and explicit failure, avoiding NullPointerException
s
or the like later on. It is still recommended that you put assertions into the bean
class itself, for example, into an init method. Doing so enforces those required
references and values even when you use the class outside of a container.
As expected, you can apply the @Autowired
annotation to "traditional" setter methods:
public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Note | |
---|---|
JSR 330’s @Inject annotation can be used in place of Spring’s |
You can also apply the annotation to methods with arbitrary names and/or multiple arguments:
public class MovieRecommender { private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public void prepare(MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) { this.movieCatalog = movieCatalog; this.customerPreferenceDao = customerPreferenceDao; } // ... }
You can apply @Autowired
to constructors and fields:
public class MovieRecommender { @Autowired private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) { this.customerPreferenceDao = customerPreferenceDao; } // ... }
It is also possible to provide all beans of a particular type from the
ApplicationContext
by adding the annotation to a field or method that expects an array
of that type:
public class MovieRecommender { @Autowired private MovieCatalog[] movieCatalogs; // ... }
The same applies for typed collections:
public class MovieRecommender { private Set<MovieCatalog> movieCatalogs; @Autowired public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) { this.movieCatalogs = movieCatalogs; } // ... }
Tip | |
---|---|
Your beans can implement the |
Even typed Maps can be autowired as long as the expected key type is String
. The Map
values will contain all beans of the expected type, and the keys will contain the
corresponding bean names:
public class MovieRecommender { private Map<String, MovieCatalog> movieCatalogs; @Autowired public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) { this.movieCatalogs = movieCatalogs; } // ... }
By default, the autowiring fails whenever zero candidate beans are available; the default behavior is to treat annotated methods, constructors, and fields as indicating required dependencies. This behavior can be changed as demonstrated below.
public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired(required=false) public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Note | |
---|---|
Only one annotated constructor per-class can be marked as required, but multiple non-required constructors can be annotated. In that case, each is considered among the candidates and Spring uses the greediest constructor whose dependencies can be satisfied, that is the constructor that has the largest number of arguments.
|
You can also use @Autowired
for interfaces that are well-known resolvable
dependencies: BeanFactory
, ApplicationContext
, Environment
, ResourceLoader
,
ApplicationEventPublisher
, and MessageSource
. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext
or ResourcePatternResolver
, are
automatically resolved, with no special setup necessary.
public class MovieRecommender { @Autowired private ApplicationContext context; public MovieRecommender() { } // ... }
Note | |
---|---|
|
Because autowiring by type may lead to multiple candidates, it is often necessary to
have more control over the selection process. One way to accomplish this is with
Spring’s @Qualifier
annotation. You can associate qualifier values with specific
arguments, narrowing the set of type matches so that a specific bean is chosen for each
argument. In the simplest case, this can be a plain descriptive value:
public class MovieRecommender { @Autowired @Qualifier("main") private MovieCatalog movieCatalog; // ... }
The @Qualifier
annotation can also be specified on individual constructor arguments or
method parameters:
public class MovieRecommender { private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public void prepare(@Qualifier("main")MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) { this.movieCatalog = movieCatalog; this.customerPreferenceDao = customerPreferenceDao; } // ... }
The corresponding bean definitions appear as follows. The bean with qualifier value "main" is wired with the constructor argument that is qualified with the same value.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier value="main"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <qualifier value="action"/> <!-- inject any dependencies required by this bean --> </bean> <bean id="movieRecommender" class="example.MovieRecommender"/> </beans>
For a fallback match, the bean name is considered a default qualifier value. Thus you
can define the bean with an id "main" instead of the nested qualifier element, leading
to the same matching result. However, although you can use this convention to refer to
specific beans by name, @Autowired
is fundamentally about type-driven injection with
optional semantic qualifiers. This means that qualifier values, even with the bean name
fallback, always have narrowing semantics within the set of type matches; they do not
semantically express a reference to a unique bean id. Good qualifier values are "main"
or "EMEA" or "persistent", expressing characteristics of a specific component that are
independent from the bean id, which may be auto-generated in case of an anonymous bean
definition like the one in the preceding example.
Qualifiers also apply to typed collections, as discussed above, for example, to
Set<MovieCatalog>
. In this case, all matching beans according to the declared
qualifiers are injected as a collection. This implies that qualifiers do not have to be
unique; they rather simply constitute filtering criteria. For example, you can define
multiple MovieCatalog
beans with the same qualifier value "action"; all of which would
be injected into a Set<MovieCatalog>
annotated with @Qualifier("action")
.
Tip | |
---|---|
If you intend to express annotation-driven injection by name, do not primarily use
As a specific consequence of this semantic difference, beans that are themselves defined
as a collection or map type cannot be injected through
|
You can create your own custom qualifier annotations. Simply define an annotation and
provide the @Qualifier
annotation within your definition:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Genre { String value(); }
Then you can provide the custom qualifier on autowired fields and parameters:
public class MovieRecommender { @Autowired @Genre("Action") private MovieCatalog actionCatalog; private MovieCatalog comedyCatalog; @Autowired public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) { this.comedyCatalog = comedyCatalog; } // ... }
Next, provide the information for the candidate bean definitions. You can add
<qualifier/>
tags as sub-elements of the <bean/>
tag and then specify the type
and
value
to match your custom qualifier annotations. The type is matched against the
fully-qualified class name of the annotation. Or, as a convenience if no risk of
conflicting names exists, you can use the short class name. Both approaches are
demonstrated in the following example.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier type="Genre" value="Action"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> _<qualifier type="example.Genre" value="Comedy"/> <!-- inject any dependencies required by this bean --> </bean> <bean id="movieRecommender" class="example.MovieRecommender"/> </beans>
In Section 4.10, “Classpath scanning and managed components”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Section 4.10.8, “Providing qualifier metadata with annotations”.
In some cases, it may be sufficient to use an annotation without a value. This may be useful when the annotation serves a more generic purpose and can be applied across several different types of dependencies. For example, you may provide an offline catalog that would be searched when no Internet connection is available. First define the simple annotation:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Offline { }
Then add the annotation to the field or property to be autowired:
public class MovieRecommender { @Autowired @Offline private MovieCatalog offlineCatalog; // ... }
Now the bean definition only needs a qualifier type
:
<bean class="example.SimpleMovieCatalog"> <qualifier type="Offline"/> <!-- inject any dependencies required by this bean --> </bean>
You can also define custom qualifier annotations that accept named attributes in
addition to or instead of the simple value
attribute. If multiple attribute values are
then specified on a field or parameter to be autowired, a bean definition must match
all such attribute values to be considered an autowire candidate. As an example,
consider the following annotation definition:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface MovieQualifier { String genre(); Format format(); }
In this case Format
is an enum:
public enum Format {
VHS, DVD, BLURAY
}
The fields to be autowired are annotated with the custom qualifier and include values
for both attributes: genre
and format
.
public class MovieRecommender { @Autowired @MovieQualifier(format=Format.VHS, genre="Action") private MovieCatalog actionVhsCatalog; @Autowired @MovieQualifier(format=Format.VHS, genre="Comedy") private MovieCatalog comedyVhsCatalog; @Autowired @MovieQualifier(format=Format.DVD, genre="Action") private MovieCatalog actionDvdCatalog; @Autowired @MovieQualifier(format=Format.BLURAY, genre="Comedy") private MovieCatalog comedyBluRayCatalog; // ... }
Finally, the bean definitions should contain matching qualifier values. This example
also demonstrates that bean meta attributes may be used instead of the
<qualifier/>
sub-elements. If available, the <qualifier/>
and its attributes take
precedence, but the autowiring mechanism falls back on the values provided within the
<meta/>
tags if no such qualifier is present, as in the last two bean definitions in
the following example.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier type="MovieQualifier"> <attribute key="format" value="VHS"/> <attribute key="genre" value="Action"/> </qualifier> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <qualifier type="MovieQualifier"> <attribute key="format" value="VHS"/> <attribute key="genre" value="Comedy"/> </qualifier> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <meta key="format" value="DVD"/> <meta key="genre" value="Action"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <meta key="format" value="BLURAY"/> <meta key="genre" value="Comedy"/> <!-- inject any dependencies required by this bean --> </bean> </beans>
In addition to the @Qualifier
annotation, it is also possible to use Java generic types
as an implicit form of qualification. For example, suppose you have the following
configuration:
@Configuration public class MyConfiguration { @Bean public StringStore stringStore() { return new StringStore(); } @Bean public IntegerStore integerStore() { return new IntegerStore(); } }
Assuming that beans above implement a generic interface, i.e. Store<String>
and
Store<Integer>
, you can @Autowire
the Store
interface and the generic will
be used as a qualifier:
@Autowired private Store<String> s1; // <String> qualifier, injects the stringStore bean @Autowired private Store<Integer> s2; // <Integer> qualifier, injects the integerStore bean
Generic qualifiers also apply when autowiring Lists, Maps and Arrays:
// Inject all Store beans as long as they have an <Integer> generic // Store<String> beans will not appear in this list @Autowired private List<Store<Integer>> s;
The
CustomAutowireConfigurer
is a BeanFactoryPostProcessor
that enables you to register your own custom qualifier
annotation types even if they are not annotated with Spring’s @Qualifier
annotation.
<bean id="customAutowireConfigurer" class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer"> <property name="customQualifierTypes"> <set> <value>example.CustomQualifier</value> </set> </property> </bean>
The AutowireCandidateResolver
determines autowire candidates by:
autowire-candidate
value of each bean definition
default-autowire-candidates
pattern(s) available on the <beans/>
element
@Qualifier
annotations and any custom annotations registered
with the CustomAutowireConfigurer
When multiple beans qualify as autowire candidates, the determination of a "primary" is
the following: if exactly one bean definition among the candidates has a primary
attribute set to true
, it will be selected.
Spring also supports injection using the JSR-250 @Resource
annotation on fields or
bean property setter methods. This is a common pattern in Java EE 5 and 6, for example
in JSF 1.2 managed beans or JAX-WS 2.0 endpoints. Spring supports this pattern for
Spring-managed objects as well.
@Resource
takes a name attribute, and by default Spring interprets that value as the
bean name to be injected. In other words, it follows by-name semantics, as
demonstrated in this example:
public class SimpleMovieLister { private MovieFinder movieFinder; @Resource(name="myMovieFinder") public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
If no name is specified explicitly, the default name is derived from the field name or setter method. In case of a field, it takes the field name; in case of a setter method, it takes the bean property name. So the following example is going to have the bean with name "movieFinder" injected into its setter method:
public class SimpleMovieLister { private MovieFinder movieFinder; @Resource public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
Note | |
---|---|
The name provided with the annotation is resolved as a bean name by the
|
In the exclusive case of @Resource
usage with no explicit name specified, and similar
to @Autowired
, @Resource
finds a primary type match instead of a specific named bean
and resolves well-known resolvable dependencies: the BeanFactory
,
ApplicationContext
, ResourceLoader
, ApplicationEventPublisher
, and MessageSource
interfaces.
Thus in the following example, the customerPreferenceDao
field first looks for a bean
named customerPreferenceDao, then falls back to a primary type match for the type
CustomerPreferenceDao
. The "context" field is injected based on the known resolvable
dependency type ApplicationContext
.
public class MovieRecommender { @Resource private CustomerPreferenceDao customerPreferenceDao; @Resource private ApplicationContext context; public MovieRecommender() { } // ... }
The CommonAnnotationBeanPostProcessor
not only recognizes the @Resource
annotation
but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support
for these annotations offers yet another alternative to those described in
initialization callbacks and
destruction callbacks. Provided that the
CommonAnnotationBeanPostProcessor
is registered within the Spring
ApplicationContext
, a method carrying one of these annotations is invoked at the same
point in the lifecycle as the corresponding Spring lifecycle interface method or
explicitly declared callback method. In the example below, the cache will be
pre-populated upon initialization and cleared upon destruction.
public class CachingMovieLister { @PostConstruct public void populateMovieCache() { // populates the movie cache upon initialization... } @PreDestroy public void clearMovieCache() { // clears the movie cache upon destruction... } }
Note | |
---|---|
For details about the effects of combining various lifecycle mechanisms, see the section called “Combining lifecycle mechanisms”. |
Most examples in this chapter use XML to specify the configuration metadata that
produces each BeanDefinition
within the Spring container. The previous section
(Section 4.9, “Annotation-based container configuration”) demonstrates how to provide a lot of the configuration
metadata through source-level annotations. Even in those examples, however, the "base"
bean definitions are explicitly defined in the XML file, while the annotations only
drive the dependency injection. This section describes an option for implicitly
detecting the candidate components by scanning the classpath. Candidate components
are classes that match against a filter criteria and have a corresponding bean
definition registered with the container. This removes the need to use XML to perform
bean registration, instead you can use annotations (for example @Component), AspectJ
type expressions, or your own custom filter criteria to select which classes will have
bean definitions registered with the container.
Note | |
---|---|
Starting with Spring 3.0, many features provided by the Spring JavaConfig project are
part of the core Spring Framework. This allows you to define beans using Java rather
than using the traditional XML files. Take a look at the |
The @Repository
annotation is a marker for any class that fulfills the role or
stereotype (also known as Data Access Object or DAO) of a repository. Among the uses
of this marker is the automatic translation of exceptions as described in
Section 14.2.2, “Exception translation”.
Spring provides further stereotype annotations: @Component
, @Service
, and
@Controller
. @Component
is a generic stereotype for any Spring-managed component.
@Repository
, @Service
, and @Controller
are specializations of @Component
for
more specific use cases, for example, in the persistence, service, and presentation
layers, respectively. Therefore, you can annotate your component classes with
@Component
, but by annotating them with @Repository
, @Service
, or @Controller
instead, your classes are more properly suited for processing by tools or associating
with aspects. For example, these stereotype annotations make ideal targets for
pointcuts. It is also possible that @Repository
, @Service
, and @Controller
may
carry additional semantics in future releases of the Spring Framework. Thus, if you are
choosing between using @Component
or @Service
for your service layer, @Service
is
clearly the better choice. Similarly, as stated above, @Repository
is already
supported as a marker for automatic exception translation in your persistence layer.
Many of the annotations provided by Spring can be used as "meta-annotations" in
your own code. A meta-annotation is simply an annotation, that can be applied to another
annotation. For example, The @Service
annotation mentioned above is meta-annotated with
with @Component
:
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Component // Spring will see this and treat @Service in the same way as @Component public @interface Service { // .... }
Meta-annotations can also be combined together to create composed annotations. For
example, the @RestController
annotation from Spring MVC is composed of
@Controller
and @ResponseBody
.
With the exception of value()
, meta-annotated types may redeclare attributes from the
source annotation to allow user customization. This can be particularly useful when you
want to only expose a subset of the source annotation attributes. For example, here is a
custom @Scope
annotation that defines session
scope, but still allows customization
of the proxyMode
.
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Scope("session") public @interface SessionScope { ScopedProxyMode proxyMode() default ScopedProxyMode.DEFAULT }
Spring can automatically detect stereotyped classes and register corresponding
BeanDefinition
s with the ApplicationContext
. For example, the following two classes
are eligible for such autodetection:
@Service public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public SimpleMovieLister(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
@Repository public class JpaMovieFinder implements MovieFinder { // implementation elided for clarity }
To autodetect these classes and register the corresponding beans, you need to include the following element in XML, where the base-package element is a common parent package for the two classes. (Alternatively, you can specify a comma-separated list that includes the parent package of each class.)
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="org.example"/> </beans>
Tip | |
---|---|
The use of |
Note | |
---|---|
The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, make sure that you do not activate the files-only switch of the JAR task. Also, classpath directories may not get exposed based on security policies in some environments, e.g. standalone apps on JDK 1.7.0_45 and higher (which requires Trusted-Library setup in your manifests; see http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources). |
Furthermore, the AutowiredAnnotationBeanPostProcessor
and
CommonAnnotationBeanPostProcessor
are both included implicitly when you use the
component-scan element. That means that the two components are autodetected and
wired together - all without any bean configuration metadata provided in XML.
Note | |
---|---|
You can disable the registration of |
By default, classes annotated with @Component
, @Repository
, @Service
,
@Controller
, or a custom annotation that itself is annotated with @Component
are the
only detected candidate components. However, you can modify and extend this behavior
simply by applying custom filters. Add them as include-filter or exclude-filter
sub-elements of the component-scan
element. Each filter element requires the type
and expression
attributes. The following table describes the filtering options.
Table 4.5. Filter Types
Filter Type | Example Expression | Description |
---|---|---|
annotation |
| An annotation to be present at the type level in target components. |
assignable |
| A class (or interface) that the target components are assignable to (extend/implement). |
aspectj |
| An AspectJ type expression to be matched by the target components. |
regex |
| A regex expression to be matched by the target components class names. |
custom |
| A custom implementation of the |
The following example shows the XML configuration ignoring all @Repository
annotations
and using "stub" repositories instead.
<beans> <context:component-scan base-package="org.example"> <context:include-filter type="regex" expression=".*Stub.*Repository"/> <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Repository"/> </context:component-scan> </beans>
Note | |
---|---|
You can also disable the default filters by providing use-default-filters="false" as
an attribute of the <component-scan/> element. This will in effect disable automatic
detection of classes annotated with |
Spring components can also contribute bean definition metadata to the container. You do
this with the same @Bean
annotation used to define bean metadata within
@Configuration
annotated classes. Here is a simple example:
@Component public class FactoryMethodComponent { @Bean @Qualifier("public") public TestBean publicInstance() { return new TestBean("publicInstance"); } public void doWork() { // Component method implementation omitted } }
This class is a Spring component that has application-specific code contained in its
doWork()
method. However, it also contributes a bean definition that has a factory
method referring to the method publicInstance()
. The @Bean
annotation identifies the
factory method and other bean definition properties, such as a qualifier value through
the @Qualifier
annotation. Other method level annotations that can be specified are
@Scope
, @Lazy
, and custom qualifier annotations.
Tip | |
---|---|
In addition to its role for component initialization, the |
Autowired fields and methods are supported as previously discussed, with additional
support for autowiring of @Bean
methods:
@Component public class FactoryMethodComponent { private static int i; @Bean @Qualifier("public") public TestBean publicInstance() { return new TestBean("publicInstance"); } // use of a custom qualifier and autowiring of method parameters @Bean protected TestBean protectedInstance( @Qualifier("public") TestBean spouse, @Value("#{privateInstance.age}") String country) { TestBean tb = new TestBean("protectedInstance", 1); tb.setSpouse(tb); tb.setCountry(country); return tb; } @Bean @Scope(BeanDefinition.SCOPE_SINGLETON) private TestBean privateInstance() { return new TestBean("privateInstance", i++); } @Bean @Scope(value = WebApplicationContext.SCOPE_SESSION, proxyMode = ScopedProxyMode.TARGET_CLASS) public TestBean requestScopedInstance() { return new TestBean("requestScopedInstance", 3); } }
The example autowires the String
method parameter country
to the value of the Age
property on another bean named privateInstance
. A Spring Expression Language element
defines the value of the property through the notation #{ <expression> }
. For @Value
annotations, an expression resolver is preconfigured to look for bean names when
resolving expression text.
The @Bean
methods in a Spring component are processed differently than their
counterparts inside a Spring @Configuration
class. The difference is that @Component
classes are not enhanced with CGLIB to intercept the invocation of methods and fields.
CGLIB proxying is the means by which invoking methods or fields within @Configuration
classes @Bean
methods create bean metadata references to collaborating objects.
Methods are not invoked with normal Java semantics. In contrast, calling a method or
field within a @Component
classes @Bean
method has standard Java semantics.
When a component is autodetected as part of the scanning process, its bean name is
generated by the BeanNameGenerator
strategy known to that scanner. By default, any
Spring stereotype annotation ( @Component
, @Repository
, @Service
, and
@Controller
) that contains a name
value will thereby provide that name to the
corresponding bean definition.
If such an annotation contains no name
value or for any other detected component (such
as those discovered by custom filters), the default bean name generator returns the
uncapitalized non-qualified class name. For example, if the following two components
were detected, the names would be myMovieLister and movieFinderImpl:
@Service("myMovieLister") public class SimpleMovieLister { // ... }
@Repository public class MovieFinderImpl implements MovieFinder { // ... }
Note | |
---|---|
If you do not want to rely on the default bean-naming strategy, you can provide a custom
bean-naming strategy. First, implement the
|
<beans> <context:component-scan base-package="org.example" name-generator="org.example.MyNameGenerator" /> </beans>
As a general rule, consider specifying the name with the annotation whenever other components may be making explicit references to it. On the other hand, the auto-generated names are adequate whenever the container is responsible for wiring.
As with Spring-managed components in general, the default and most common scope for
autodetected components is singleton. However, sometimes you need other scopes, which
Spring 2.5 provides with a new @Scope
annotation. Simply provide the name of the scope
within the annotation:
@Scope("prototype") @Repository public class MovieFinderImpl implements MovieFinder { // ... }
Note | |
---|---|
To provide a custom strategy for scope resolution rather than relying on the
annotation-based approach, implement the
|
<beans> <context:component-scan base-package="org.example" scope-resolver="org.example.MyScopeResolver" /> </beans>
When using certain non-singleton scopes, it may be necessary to generate proxies for the scoped objects. The reasoning is described in the section called “Scoped beans as dependencies”. For this purpose, a scoped-proxy attribute is available on the component-scan element. The three possible values are: no, interfaces, and targetClass. For example, the following configuration will result in standard JDK dynamic proxies:
<beans> <context:component-scan base-package="org.example" scoped-proxy="interfaces" /> </beans>
The @Qualifier
annotation is discussed in Section 4.9.3, “Fine-tuning annotation-based autowiring with qualifiers”.
The examples in that section demonstrate the use of the @Qualifier
annotation and
custom qualifier annotations to provide fine-grained control when you resolve autowire
candidates. Because those examples were based on XML bean definitions, the qualifier
metadata was provided on the candidate bean definitions using the qualifier
or meta
sub-elements of the bean
element in the XML. When relying upon classpath scanning for
autodetection of components, you provide the qualifier metadata with type-level
annotations on the candidate class. The following three examples demonstrate this
technique:
@Component @Qualifier("Action") public class ActionMovieCatalog implements MovieCatalog { // ... }
@Component @Genre("Action") public class ActionMovieCatalog implements MovieCatalog { // ... }
@Component @Offline public class CachingMovieCatalog implements MovieCatalog { // ... }
Note | |
---|---|
As with most annotation-based alternatives, keep in mind that the annotation metadata is bound to the class definition itself, while the use of XML allows for multiple beans of the same type to provide variations in their qualifier metadata, because that metadata is provided per-instance rather than per-class. |
Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations (Dependency Injection). Those annotations are scanned in the same way as the Spring annotations. You just need to have the relevant jars in your classpath.
Note | |
---|---|
If you are using Maven, the <dependency> <groupId>javax.inject</groupId> <artifactId>javax.inject</artifactId> <version>1</version> </dependency> |
Instead of @Autowired
, @javax.inject.Inject
may be used as follows:
import javax.inject.Inject; public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
As with @Autowired
, it is possible to use @Inject
at the class-level, field-level,
method-level and constructor-argument level. If you would like to use a qualified name
for the dependency that should be injected, you should use the @Named
annotation as
follows:
import javax.inject.Inject; import javax.inject.Named; public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(@Named("main") MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Instead of @Component
, @javax.inject.Named
may be used as follows:
import javax.inject.Inject; import javax.inject.Named; @Named("movieListener") public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
It is very common to use @Component
without
specifying a name for the component. @Named
can be used in a similar fashion:
import javax.inject.Inject; import javax.inject.Named; @Named public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
When using @Named
, it is possible to use
component-scanning in the exact same way as when using Spring annotations:
<beans> <context:component-scan base-package="org.example"/> </beans>
When working with standard annotations, it is important to know that some significant features are not available as shown in the table below:
Table 4.6. Spring annotations vs. standard annotations
Spring | javax.inject.* | javax.inject restrictions / comments |
---|---|---|
@Autowired | @Inject | @Inject has no required attribute |
@Component | @Named | - |
@Scope("singleton") | @Singleton | The JSR-330 default scope is like Spring’s
|
@Qualifier | @Named | - |
@Value | - | no equivalent |
@Required | - | no equivalent |
@Lazy | - | no equivalent |
The central artifacts in Spring’s new Java-configuration support are
@Configuration
-annotated classes and @Bean
-annotated methods.
The @Bean
annotation is used to indicate that a method instantiates, configures and
initializes a new object to be managed by the Spring IoC container. For those familiar
with Spring’s <beans/>
XML configuration the @Bean
annotation plays the same role as
the <bean/>
element. You can use @Bean
annotated methods with any Spring
@Component
, however, they are most often used with @Configuration
beans.
Annotating a class with @Configuration
indicates that its primary purpose is as a
source of bean definitions. Furthermore, @Configuration
classes allow inter-bean
dependencies to be defined by simply calling other @Bean
methods in the same class.
The simplest possible @Configuration
class would read as follows:
@Configuration public class AppConfig { @Bean public MyService myService() { return new MyServiceImpl(); } }
The AppConfig
class above would be equivalent to the following Spring <beans/>
XML:
<beans> <bean id="myService" class="com.acme.services.MyServiceImpl"/> </beans>
The @Bean
and @Configuration
annotations will be discussed in depth in the sections
below. First, however, we’ll cover the various ways of creating a spring container using
Java-based configuration.
The sections below document Spring’s AnnotationConfigApplicationContext
, new in Spring
3.0. This versatile ApplicationContext
implementation is capable of accepting not only
@Configuration
classes as input, but also plain @Component
classes and classes
annotated with JSR-330 metadata.
When @Configuration
classes are provided as input, the @Configuration
class itself
is registered as a bean definition, and all declared @Bean
methods within the class
are also registered as bean definitions.
When @Component
and JSR-330 classes are provided, they are registered as bean
definitions, and it is assumed that DI metadata such as @Autowired
or @Inject
are
used within those classes where necessary.
In much the same way that Spring XML files are used as input when instantiating a
ClassPathXmlApplicationContext
, @Configuration
classes may be used as input when
instantiating an AnnotationConfigApplicationContext
. This allows for completely
XML-free usage of the Spring container:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
As mentioned above, AnnotationConfigApplicationContext
is not limited to working only
with @Configuration
classes. Any @Component
or JSR-330 annotated class may be supplied
as input to the constructor. For example:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class, Dependency1.class, Dependency2.class); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
The above assumes that MyServiceImpl
, Dependency1
and Dependency2
use Spring
dependency injection annotations such as @Autowired
.
An AnnotationConfigApplicationContext
may be instantiated using a no-arg constructor
and then configured using the register()
method. This approach is particularly useful
when programmatically building an AnnotationConfigApplicationContext
.
public static void main(String[] args) { AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(); ctx.register(AppConfig.class, OtherConfig.class); ctx.register(AdditionalConfig.class); ctx.refresh(); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
Experienced Spring users will be familiar with the following commonly-used XML
declaration from Spring’s context:
namespace
<beans> <context:component-scan base-package="com.acme"/> </beans>
In the example above, the com.acme
package will be scanned, looking for any
@Component
-annotated classes, and those classes will be registered as Spring bean
definitions within the container. AnnotationConfigApplicationContext
exposes the
scan(String...)
method to allow for the same component-scanning functionality:
public static void main(String[] args) { AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(); ctx.scan("com.acme"); ctx.refresh(); MyService myService = ctx.getBean(MyService.class); }
Note | |
---|---|
Remember that |
A WebApplicationContext
variant of AnnotationConfigApplicationContext
is available
with AnnotationConfigWebApplicationContext
. This implementation may be used when
configuring the Spring ContextLoaderListener
servlet listener, Spring MVC
DispatcherServlet
, etc. What follows is a web.xml
snippet that configures a typical
Spring MVC web application. Note the use of the contextClass
context-param and
init-param:
<web-app> <!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext instead of the default XmlWebApplicationContext --> <context-param> <param-name>contextClass</param-name> <param-value> org.springframework.web.context.support.AnnotationConfigWebApplicationContext </param-value> </context-param> <!-- Configuration locations must consist of one or more comma- or space-delimited fully-qualified @Configuration classes. Fully-qualified packages may also be specified for component-scanning --> <context-param> <param-name>contextConfigLocation</param-name> <param-value>com.acme.AppConfig</param-value> </context-param> <!-- Bootstrap the root application context as usual using ContextLoaderListener --> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <!-- Declare a Spring MVC DispatcherServlet as usual --> <servlet> <servlet-name>dispatcher</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext instead of the default XmlWebApplicationContext --> <init-param> <param-name>contextClass</param-name> <param-value> org.springframework.web.context.support.AnnotationConfigWebApplicationContext </param-value> </init-param> <!-- Again, config locations must consist of one or more comma- or space-delimited and fully-qualified @Configuration classes --> <init-param> <param-name>contextConfigLocation</param-name> <param-value>com.acme.web.MvcConfig</param-value> </init-param> </servlet> <!-- map all requests for /app/* to the dispatcher servlet --> <servlet-mapping> <servlet-name>dispatcher</servlet-name> <url-pattern>/app/*</url-pattern> </servlet-mapping> </web-app>
@Bean
is a method-level annotation and a direct analog of the XML <bean/>
element.
The annotation supports some of the attributes offered by <bean/>
, such as:
init-method,
destroy-method,
autowiring and name
.
You can use the @Bean
annotation in a @Configuration
-annotated or in a
@Component
-annotated class.
To declare a bean, simply annotate a method with the @Bean
annotation. You use this
method to register a bean definition within an ApplicationContext
of the type
specified as the method’s return value. By default, the bean name will be the same as
the method name. The following is a simple example of a @Bean
method declaration:
@Configuration public class AppConfig { @Bean public TransferService transferService() { return new TransferServiceImpl(); } }
The preceding configuration is exactly equivalent to the following Spring XML:
<beans> <bean id="transferService" class="com.acme.TransferServiceImpl"/> </beans>
Both declarations make a bean named transferService
available in the
ApplicationContext
, bound to an object instance of type TransferServiceImpl
:
transferService -> com.acme.TransferServiceImpl
Any classes defined with the @Bean
annotation support the regular lifecycle callbacks
and can use the @PostConstruct
and @PreDestroy
annotations from JSR-250, see
JSR-250 annotations for further
details.
The regular Spring lifecycle callbacks are fully supported as
well. If a bean implements InitializingBean
, DisposableBean
, or Lifecycle
, their
respective methods are called by the container.
The standard set of *Aware
interfaces such as BeanFactoryAware,
BeanNameAware,
MessageSourceAware,
ApplicationContextAware, and so on are also fully supported.
The @Bean
annotation supports specifying arbitrary initialization and destruction
callback methods, much like Spring XML’s init-method
and destroy-method
attributes
on the bean
element:
public class Foo { public void init() { // initialization logic } } public class Bar { public void cleanup() { // destruction logic } } @Configuration public class AppConfig { @Bean(initMethod = "init") public Foo foo() { return new Foo(); } @Bean(destroyMethod = "cleanup") public Bar bar() { return new Bar(); } }
Of course, in the case of Foo
above, it would be equally as valid to call the init()
method directly during construction:
@Configuration public class AppConfig { @Bean public Foo foo() { Foo foo = new Foo(); foo.init(); return foo; } // ... }
Tip | |
---|---|
When you work directly in Java, you can do anything you like with your objects and do not always need to rely on the container lifecycle! |
You can specify that your beans defined with the @Bean
annotation should have a
specific scope. You can use any of the standard scopes specified in the
Bean Scopes section.
The default scope is singleton
, but you can override this with the @Scope
annotation:
@Configuration public class MyConfiguration { @Bean @Scope("prototype") public Encryptor encryptor() { // ... } }
Spring offers a convenient way of working with scoped dependencies through
scoped proxies. The easiest way to create such
a proxy when using the XML configuration is the <aop:scoped-proxy/>
element.
Configuring your beans in Java with a @Scope annotation offers equivalent support with
the proxyMode attribute. The default is no proxy ( ScopedProxyMode.NO
), but you can
specify ScopedProxyMode.TARGET_CLASS
or ScopedProxyMode.INTERFACES
.
If you port the scoped proxy example from the XML reference documentation (see preceding
link) to our @Bean
using Java, it would look like the following:
// an HTTP Session-scoped bean exposed as a proxy @Bean @Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS) public UserPreferences userPreferences() { return new UserPreferences(); } @Bean public Service userService() { UserService service = new SimpleUserService(); // a reference to the proxied userPreferences bean service.setUserPreferences(userPreferences()); return service; }
By default, configuration classes use a @Bean
method’s name as the name of the
resulting bean. This functionality can be overridden, however, with the name
attribute.
@Configuration public class AppConfig { @Bean(name = "myFoo") public Foo foo() { return new Foo(); } }
As discussed in Section 4.3.1, “Naming beans”, it is sometimes desirable to give a single bean
multiple names, otherwise known asbean aliasing. The name
attribute of the @Bean
annotation accepts a String array for this purpose.
@Configuration public class AppConfig { @Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" }) public DataSource dataSource() { // instantiate, configure and return DataSource bean... } }
Sometimes it is helpful to provide a more detailed textual description of a bean. This can be particularly useful when beans are exposed (perhaps via JMX) for monitoring purposes.
To add a description to a @Bean
the
@Description
annotation can be used:
@Configuration public class AppConfig { @Bean @Desciption("Provides a basic example of a bean") public Foo foo() { return new Foo(); } }
@Configuration
is a class-level annotation indicating that an object is a source of
bean definitions. @Configuration
classes declare beans via public @Bean
annotated
methods. Calls to @Bean
methods on @Configuration
classes can also be used to define
inter-bean dependencies. See Section 4.12.1, “Basic concepts: @Bean and @Configuration” for a general introduction.
When @Bean
s have dependencies on one another, expressing that dependency is as simple
as having one bean method call another:
@Configuration public class AppConfig { @Bean public Foo foo() { return new Foo(bar()); } @Bean public Bar bar() { return new Bar(); } }
In the example above, the foo
bean receives a reference to bar
via constructor
injection.
Note | |
---|---|
This method of declaring inter-bean dependencies only works when the |
As noted earlier, lookup method injection is an advanced feature that you should use rarely. It is useful in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this type of configuration provides a natural means for implementing this pattern.
public abstract class CommandManager { public Object process(Object commandState) { // grab a new instance of the appropriate Command interface Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } // okay... but where is the implementation of this method? protected abstract Command createCommand(); }
Using Java-configuration support , you can create a subclass of CommandManager
where
the abstract createCommand()
method is overridden in such a way that it looks up a new
(prototype) command object:
@Bean @Scope("prototype") public AsyncCommand asyncCommand() { AsyncCommand command = new AsyncCommand(); // inject dependencies here as required return command; } @Bean public CommandManager commandManager() { // return new anonymous implementation of CommandManager with command() overridden // to return a new prototype Command object return new CommandManager() { protected Command createCommand() { return asyncCommand(); } } }
The following example shows a @Bean
annotated method being called twice:
@Configuration public class AppConfig { @Bean public ClientService clientService1() { ClientServiceImpl clientService = new ClientServiceImpl(); clientService.setClientDao(clientDao()); return clientService; } @Bean public ClientService clientService2() { ClientServiceImpl clientService = new ClientServiceImpl(); clientService.setClientDao(clientDao()); return clientService; } @Bean public ClientDao clientDao() { return new ClientDaoImpl(); } }
clientDao()
has been called once in clientService1()
and once in clientService2()
.
Since this method creates a new instance of ClientDaoImpl
and returns it, you would
normally expect having 2 instances (one for each service). That definitely would be
problematic: in Spring, instantiated beans have a singleton
scope by default. This is
where the magic comes in: All @Configuration
classes are subclassed at startup-time
with CGLIB
. In the subclass, the child method checks the container first for any
cached (scoped) beans before it calls the parent method and creates a new instance. Note
that as of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because
CGLIB classes have been repackaged under org.springframework and included directly
within the spring-core JAR.
Note | |
---|---|
The behavior could be different according to the scope of your bean. We are talking about singletons here. |
Note | |
---|---|
There are a few restrictions due to the fact that CGLIB dynamically adds features at startup-time:
|
Much as the <import/>
element is used within Spring XML files to aid in modularizing
configurations, the @Import
annotation allows for loading @Bean
definitions from
another configuration class:
@Configuration public class ConfigA { @Bean public A a() { return new A(); } } @Configuration @Import(ConfigA.class) public class ConfigB { @Bean public B b() { return new B(); } }
Now, rather than needing to specify both ConfigA.class
and ConfigB.class
when
instantiating the context, only ConfigB
needs to be supplied explicitly:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class); // now both beans A and B will be available... A a = ctx.getBean(A.class); B b = ctx.getBean(B.class); }
This approach simplifies container instantiation, as only one class needs to be dealt
with, rather than requiring the developer to remember a potentially large number of
@Configuration
classes during construction.
The example above works, but is simplistic. In most practical scenarios, beans will have
dependencies on one another across configuration classes. When using XML, this is not an
issue, per se, because there is no compiler involved, and one can simply declare
ref="someBean"
and trust that Spring will work it out during container initialization.
Of course, when using @Configuration
classes, the Java compiler places constraints on
the configuration model, in that references to other beans must be valid Java syntax.
Fortunately, solving this problem is simple. Remember that @Configuration
classes are
ultimately just another bean in the container - this means that they can take advantage
of @Autowired
injection metadata just like any other bean!
Let’s consider a more real-world scenario with several @Configuration
classes, each
depending on beans declared in the others:
@Configuration public class ServiceConfig { @Autowired private AccountRepository accountRepository; @Bean public TransferService transferService() { return new TransferServiceImpl(accountRepository); } } @Configuration public class RepositoryConfig { @Autowired private DataSource dataSource; @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } } @Configuration @Import({ServiceConfig.class, RepositoryConfig.class}) public class SystemTestConfig { @Bean public DataSource dataSource() { // return new DataSource } } public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class); // everything wires up across configuration classes... TransferService transferService = ctx.getBean(TransferService.class); transferService.transfer(100.00, "A123", "C456"); }
In the scenario above, using @Autowired
works well and provides the desired
modularity, but determining exactly where the autowired bean definitions are declared is
still somewhat ambiguous. For example, as a developer looking at ServiceConfig
, how do
you know exactly where the @Autowired AccountRepository
bean is declared? It’s not
explicit in the code, and this may be just fine. Remember that the
SpringSource Tool Suite provides tooling that
can render graphs showing how everything is wired up - that may be all you need. Also,
your Java IDE can easily find all declarations and uses of the AccountRepository
type,
and will quickly show you the location of @Bean
methods that return that type.
In cases where this ambiguity is not acceptable and you wish to have direct navigation
from within your IDE from one @Configuration
class to another, consider autowiring the
configuration classes themselves:
@Configuration public class ServiceConfig { @Autowired private RepositoryConfig repositoryConfig; @Bean public TransferService transferService() { // navigate through the config class to the @Bean method! return new TransferServiceImpl(repositoryConfig.accountRepository()); } }
In the situation above, it is completely explicit where AccountRepository
is defined.
However, ServiceConfig
is now tightly coupled to RepositoryConfig
; that’s the
tradeoff. This tight coupling can be somewhat mitigated by using interface-based or
abstract class-based @Configuration
classes. Consider the following:
@Configuration public class ServiceConfig { @Autowired private RepositoryConfig repositoryConfig; @Bean public TransferService transferService() { return new TransferServiceImpl(repositoryConfig.accountRepository()); } } @Configuration public interface RepositoryConfig { @Bean AccountRepository accountRepository(); } @Configuration public class DefaultRepositoryConfig implements RepositoryConfig { @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(...); } } @Configuration @Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config! public class SystemTestConfig { @Bean public DataSource dataSource() { // return DataSource } } public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class); TransferService transferService = ctx.getBean(TransferService.class); transferService.transfer(100.00, "A123", "C456"); }
Now ServiceConfig
is loosely coupled with respect to the concrete
DefaultRepositoryConfig
, and built-in IDE tooling is still useful: it will be easy for
the developer to get a type hierarchy of RepositoryConfig
implementations. In this
way, navigating @Configuration
classes and their dependencies becomes no different
than the usual process of navigating interface-based code.
It is often useful to conditionally enable to disable a complete @Configuration
class,
or even individual @Bean
methods, based on some arbitrary system state. One common
example of this it to use the @Profile
annotation to active beans only when a specific
profile has been enabled in the Spring Environment
(see Section 4.13, “Bean definition profiles and environment abstraction”
for details).
The @Profile
annotation is actually implemented using a much more flexible annotation
called @Conditional
.
The @Conditional
annotation indicates specific
org.springframework.context.annotation.Condition
implementations that should be
consulted before a @Bean
is registered.
Implementations of the Condition
interface simply provide a matches(...)
method that returns true
or false
. For example, here is the actual
Condition
implementation used for @Profile
:
@Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { if (context.getEnvironment() != null) { // Read the @Profile annotation attributes MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName()); if (attrs != null) { for (Object value : attrs.get("value")) { if (context.getEnvironment().acceptsProfiles(((String[]) value))) { return true; } } return false; } } return true; }
See the
@Conditional
Javadoc for more detail.
Spring’s @Configuration
class support does not aim to be a 100% complete replacement
for Spring XML. Some facilities such as Spring XML namespaces remain an ideal way to
configure the container. In cases where XML is convenient or necessary, you have a
choice: either instantiate the container in an "XML-centric" way using, for example,
ClassPathXmlApplicationContext
, or in a "Java-centric" fashion using
AnnotationConfigApplicationContext
and the @ImportResource
annotation to import XML
as needed.
It may be preferable to bootstrap the Spring container from XML and include
@Configuration
classes in an ad-hoc fashion. For example, in a large existing codebase
that uses Spring XML, it will be easier to create @Configuration
classes on an
as-needed basis and include them from the existing XML files. Below you’ll find the
options for using @Configuration
classes in this kind of "XML-centric" situation.
Remember that @Configuration
classes are ultimately just bean definitions in the
container. In this example, we create a @Configuration
class named AppConfig
and
include it within system-test-config.xml
as a <bean/>
definition. Because
<context:annotation-config/>
is switched on, the container will recognize the
@Configuration
annotation, and process the @Bean
methods declared in AppConfig
properly.
@Configuration public class AppConfig { @Autowired private DataSource dataSource; @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } @Bean public TransferService transferService() { return new TransferService(accountRepository()); } }
system-test-config.xml <beans> <!-- enable processing of annotations such as @Autowired and @Configuration --> <context:annotation-config/> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> <bean class="com.acme.AppConfig"/> <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> </beans>
jdbc.properties jdbc.url=jdbc:hsqldb:hsql://localhost/xdb jdbc.username=sa jdbc.password=
public static void main(String[] args) { ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml"); TransferService transferService = ctx.getBean(TransferService.class); // ... }
Note | |
---|---|
In |
Because @Configuration
is meta-annotated with @Component
, @Configuration
-annotated
classes are automatically candidates for component scanning. Using the same scenario as
above, we can redefine system-test-config.xml
to take advantage of component-scanning.
Note that in this case, we don’t need to explicitly declare
<context:annotation-config/>
, because <context:component-scan/>
enables all the same
functionality.
system-test-config.xml <beans> <!-- picks up and registers AppConfig as a bean definition --> <context:component-scan base-package="com.acme"/> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> </beans>
In applications where @Configuration
classes are the primary mechanism for configuring
the container, it will still likely be necessary to use at least some XML. In these
scenarios, simply use @ImportResource
and define only as much XML as is needed. Doing
so achieves a "Java-centric" approach to configuring the container and keeps XML to a
bare minimum.
@Configuration @ImportResource("classpath:/com/acme/properties-config.xml") public class AppConfig { @Value("${jdbc.url}") private String url; @Value("${jdbc.username}") private String username; @Value("${jdbc.password}") private String password; @Bean public DataSource dataSource() { return new DriverManagerDataSource(url, username, password); } }
properties-config.xml <beans> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> </beans>
jdbc.properties jdbc.url=jdbc:hsqldb:hsql://localhost/xdb jdbc.username=sa jdbc.password=
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class); TransferService transferService = ctx.getBean(TransferService.class); // ... }
Bean definition profiles is a mechanism in the core container that allows for registration of different beans in different environments. This feature can help with many use cases, including:
Find out more about Environment, XML Profiles and the @Profile annotation.
Spring’s Environment abstraction provides search operations over a configurable hierarchy of property sources.
You can find out more about
Unified
Property Management, the
PropertySource
class
and the @PropertySource
annotation.
The LoadTimeWeaver
is used by Spring to dynamically transform classes as they are
loaded into the Java virtual machine (JVM).
To enable load-time weaving add the @EnableLoadTimeWeaving
to one of your
@Configuration
classes:
@Configuration @EnableLoadTimeWeaving public class AppConfig { }
Alternatively for XML configuration use the context:load-time-weaver
element:
<beans> <context:load-time-weaver/> </beans>
Once configured for the ApplicationContext
. Any bean within that ApplicationContext
may implement LoadTimeWeaverAware
, thereby receiving a reference to the load-time
weaver instance. This is particularly useful in combination with Spring’s JPA
support where load-time weaving may be necessary for JPA class transformation. Consult
the LocalContainerEntityManagerFactoryBean
Javadoc for more detail. For more on
AspectJ load-time weaving, see Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”.
As was discussed in the chapter introduction, the org.springframework.beans.factory
package provides basic functionality for managing and manipulating beans, including in a
programmatic way. The org.springframework.context
package adds the
ApplicationContext
interface, which extends the BeanFactory
interface, in addition to extending other
interfaces to provide additional functionality in a more application
framework-oriented style. Many people use the ApplicationContext
in a completely
declarative fashion, not even creating it programmatically, but instead relying on
support classes such as ContextLoader
to automatically instantiate an
ApplicationContext
as part of the normal startup process of a J2EE web application.
To enhance BeanFactory
functionality in a more framework-oriented style the context
package also provides the following functionality:
MessageSource
interface.
ResourceLoader
interface.
ApplicationListener
interface,
through the use of the ApplicationEventPublisher
interface.
HierarchicalBeanFactory
interface.
The ApplicationContext
interface extends an interface called MessageSource
, and
therefore provides internationalization (i18n) functionality. Spring also provides the
interface HierarchicalMessageSource
, which can resolve messages hierarchically.
Together these interfaces provide the foundation upon which Spring effects message
resolution. The methods defined on these interfaces include:
String getMessage(String code, Object[] args, String default, Locale loc)
: The basic
method used to retrieve a message from the MessageSource
. When no message is found
for the specified locale, the default message is used. Any arguments passed in become
replacement values, using the MessageFormat
functionality provided by the standard
library.
String getMessage(String code, Object[] args, Locale loc)
: Essentially the same as
the previous method, but with one difference: no default message can be specified; if
the message cannot be found, a NoSuchMessageException
is thrown.
String getMessage(MessageSourceResolvable resolvable, Locale locale)
: All properties
used in the preceding methods are also wrapped in a class named
MessageSourceResolvable
, which you can use with this method.
When an ApplicationContext
is loaded, it automatically searches for a MessageSource
bean defined in the context. The bean must have the name messageSource
. If such a bean
is found, all calls to the preceding methods are delegated to the message source. If no
message source is found, the ApplicationContext
attempts to find a parent containing a
bean with the same name. If it does, it uses that bean as the MessageSource
. If the
ApplicationContext
cannot find any source for messages, an empty
DelegatingMessageSource
is instantiated in order to be able to accept calls to the
methods defined above.
Spring provides two MessageSource
implementations, ResourceBundleMessageSource
and
StaticMessageSource
. Both implement HierarchicalMessageSource
in order to do nested
messaging. The StaticMessageSource
is rarely used but provides programmatic ways to
add messages to the source. The ResourceBundleMessageSource
is shown in the following
example:
<beans> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="basenames"> <list> <value>format</value> <value>exceptions</value> <value>windows</value> </list> </property> </bean> </beans>
In the example it is assumed you have three resource bundles defined in your classpath
called format
, exceptions
and windows
. Any request to resolve a message will be
handled in the JDK standard way of resolving messages through ResourceBundles. For the
purposes of the example, assume the contents of two of the above resource bundle files
are…
# in format.properties message=Alligators rock!
# in exceptions.properties
argument.required=The {0} argument is required.
A program to execute the MessageSource
functionality is shown in the next example.
Remember that all ApplicationContext
implementations are also MessageSource
implementations and so can be cast to the MessageSource
interface.
public static void main(String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("message", null, "Default", null); System.out.println(message); }
The resulting output from the above program will be…
Alligators rock!
So to summarize, the MessageSource
is defined in a file called beans.xml
, which
exists at the root of your classpath. The messageSource
bean definition refers to a
number of resource bundles through its basenames
property. The three files that are
passed in the list to the basenames
property exist as files at the root of your
classpath and are called format.properties
, exceptions.properties
, and
windows.properties
respectively.
The next example shows arguments passed to the message lookup; these arguments will be converted into Strings and inserted into placeholders in the lookup message.
<beans> <!-- this MessageSource is being used in a web application --> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="basename" value="exceptions"/> </bean> <!-- lets inject the above MessageSource into this POJO --> <bean id="example" class="com.foo.Example"> <property name="messages" ref="messageSource"/> </bean> </beans>
public class Example { private MessageSource messages; public void setMessages(MessageSource messages) { this.messages = messages; } public void execute() { String message = this.messages.getMessage("argument.required", new Object [] {"userDao"}, "Required", null); System.out.println(message); } }
The resulting output from the invocation of the execute()
method will be…
The userDao argument is required.
With regard to internationalization (i18n), Spring’s various MessageResource
implementations follow the same locale resolution and fallback rules as the standard JDK
ResourceBundle
. In short, and continuing with the example messageSource
defined
previously, if you want to resolve messages against the British (en-GB
) locale, you
would create files called format_en_GB.properties
, exceptions_en_GB.properties
, and
windows_en_GB.properties
respectively.
Typically, locale resolution is managed by the surrounding environment of the application. In this example, the locale against which (British) messages will be resolved is specified manually.
# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.
public static void main(final String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("argument.required", new Object [] {"userDao"}, "Required", Locale.UK); System.out.println(message); }
The resulting output from the running of the above program will be…
Ebagum lad, the userDao argument is required, I say, required.
You can also use the MessageSourceAware
interface to acquire a reference to any
MessageSource
that has been defined. Any bean that is defined in an
ApplicationContext
that implements the MessageSourceAware
interface is injected with
the application context’s MessageSource
when the bean is created and configured.
Note | |
---|---|
As an alternative to |
Event handling in the ApplicationContext
is provided through the ApplicationEvent
class and ApplicationListener
interface. If a bean that implements the
ApplicationListener
interface is deployed into the context, every time an
ApplicationEvent
gets published to the ApplicationContext
, that bean is notified.
Essentially, this is the standard Observer design pattern. Spring provides the
following standard events:
Table 4.7. Built-in Events
Event | Explanation |
---|---|
| Published when the |
| Published when the |
| Published when the |
| Published when the |
| A web-specific event telling all beans that an HTTP request has been serviced. This
event is published after the request is complete. This event is only applicable to
web applications using Spring’s |
You can also create and publish your own custom events. This example demonstrates a
simple class that extends Spring’s ApplicationEvent
base class:
public class BlackListEvent extends ApplicationEvent { private final String address; private final String test; public BlackListEvent(Object source, String address, String test) { super(source); this.address = address; this.test = test; } // accessor and other methods... }
To publish a custom ApplicationEvent
, call the publishEvent()
method on an
ApplicationEventPublisher
. Typically this is done by creating a class that implements
ApplicationEventPublisherAware
and registering it as a Spring bean. The following
example demonstrates such a class:
public class EmailService implements ApplicationEventPublisherAware { private List<String> blackList; private ApplicationEventPublisher publisher; public void setBlackList(List<String> blackList) { this.blackList = blackList; } public void setApplicationEventPublisher(ApplicationEventPublisher publisher) { this.publisher = publisher; } public void sendEmail(String address, String text) { if (blackList.contains(address)) { BlackListEvent event = new BlackListEvent(this, address, text); publisher.publishEvent(event); return; } // send email... } }
At configuration time, the Spring container will detect that EmailService
implements
ApplicationEventPublisherAware
and will automatically call
setApplicationEventPublisher()
. In reality, the parameter passed in will be the Spring
container itself; you’re simply interacting with the application context via its
ApplicationEventPublisher
interface.
To receive the custom ApplicationEvent
, create a class that implements
ApplicationListener
and register it as a Spring bean. The following example
demonstrates such a class:
public class BlackListNotifier implements ApplicationListener<BlackListEvent> { private String notificationAddress; public void setNotificationAddress(String notificationAddress) { this.notificationAddress = notificationAddress; } public void onApplicationEvent(BlackListEvent event) { // notify appropriate parties via notificationAddress... } }
Notice that ApplicationListener
is generically parameterized with the type of your
custom event, BlackListEvent
. This means that the onApplicationEvent()
method can
remain type-safe, avoiding any need for downcasting. You may register as many event
listeners as you wish, but note that by default event listeners receive events
synchronously. This means the publishEvent()
method blocks until all listeners have
finished processing the event. One advantage of this synchronous and single-threaded
approach is that when a listener receives an event, it operates inside the transaction
context of the publisher if a transaction context is available. If another strategy for
event publication becomes necessary, refer to the JavaDoc for Spring’s
ApplicationEventMulticaster
interface.
The following example shows the bean definitions used to register and configure each of the classes above:
<bean id="emailService" class="example.EmailService"> <property name="blackList"> <list> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </list> </property> </bean> <bean id="blackListNotifier" class="example.BlackListNotifier"> <property name="notificationAddress" value="[email protected]"/> </bean>
Putting it all together, when the sendEmail()
method of the emailService
bean is
called, if there are any emails that should be blacklisted, a custom event of type
BlackListEvent
is published. The blackListNotifier
bean is registered as an
ApplicationListener
and thus receives the BlackListEvent
, at which point it can
notify appropriate parties.
Note | |
---|---|
Spring’s eventing mechanism is designed for simple communication between Spring beans within the same application context. However, for more sophisticated enterprise integration needs, the separately-maintained Spring Integration project provides complete support for building lightweight, pattern-oriented, event-driven architectures that build upon the well-known Spring programming model. |
For optimal usage and understanding of application contexts, users should generally
familiarize themselves with Spring’s Resource
abstraction, as described in the chapter
Chapter 5, Resources.
An application context is a ResourceLoader
, which can be used to load Resource
s. A
Resource
is essentially a more feature rich version of the JDK class java.net.URL
,
in fact, the implementations of the Resource
wrap an instance of java.net.URL
where
appropriate. A Resource
can obtain low-level resources from almost any location in a
transparent fashion, including from the classpath, a filesystem location, anywhere
describable with a standard URL, and some other variations. If the resource location
string is a simple path without any special prefixes, where those resources come from is
specific and appropriate to the actual application context type.
You can configure a bean deployed into the application context to implement the special
callback interface, ResourceLoaderAware
, to be automatically called back at
initialization time with the application context itself passed in as the
ResourceLoader
. You can also expose properties of type Resource
, to be used to
access static resources; they will be injected into it like any other properties. You
can specify those Resource
properties as simple String paths, and rely on a special
JavaBean PropertyEditor
that is automatically registered by the context, to convert
those text strings to actual Resource
objects when the bean is deployed.
The location path or paths supplied to an ApplicationContext
constructor are actually
resource strings, and in simple form are treated appropriately to the specific context
implementation. ClassPathXmlApplicationContext
treats a simple location path as a
classpath location. You can also use location paths (resource strings) with special
prefixes to force loading of definitions from the classpath or a URL, regardless of the
actual context type.
You can create ApplicationContext
instances declaratively by using, for example, a
ContextLoader
. Of course you can also create ApplicationContext
instances
programmatically by using one of the ApplicationContext
implementations.
You can register an ApplicationContext
using the ContextLoaderListener
as follows:
<context-param> <param-name>contextConfigLocation</param-name> <param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <!-- or use the ContextLoaderServlet instead of the above listener <servlet> <servlet-name>context</servlet-name> <servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> -->
The listener inspects the contextConfigLocation
parameter. If the parameter does not
exist, the listener uses /WEB-INF/applicationContext.xml
as a default. When the
parameter does exist, the listener separates the String by using predefined
delimiters (comma, semicolon and whitespace) and uses the values as locations where
application contexts will be searched. Ant-style path patterns are supported as well.
Examples are /WEB-INF/*Context.xml
for all files with names ending with "Context.xml",
residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml
, for all such files
in any subdirectory of "WEB-INF".
You can use ContextLoaderServlet
instead of ContextLoaderListener
. The Servlet uses
the contextConfigLocation
parameter just as the listener does.
It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the context and all of its required bean classes and library JARs in a J2EE RAR deployment unit. This is the equivalent of bootstrapping a standalone ApplicationContext, just hosted in J2EE environment, being able to access the J2EE servers facilities. RAR deployment is a more natural alternative to scenario of deploying a headless WAR file, in effect, a WAR file without any HTTP entry points that is used only for bootstrapping a Spring ApplicationContext in a J2EE environment.
RAR deployment is ideal for application contexts that do not need HTTP entry points but
rather consist only of message endpoints and scheduled jobs. Beans in such a context can
use application server resources such as the JTA transaction manager and JNDI-bound JDBC
DataSources and JMS ConnectionFactory instances, and may also register with the
platform’s JMX server - all through Spring’s standard transaction management and JNDI
and JMX support facilities. Application components can also interact with the
application server’s JCA WorkManager through Spring’s TaskExecutor
abstraction.
Check out the JavaDoc of the
SpringContextResourceAdapter
class for the configuration details involved in RAR deployment.
For a simple deployment of a Spring ApplicationContext as a J2EE RAR file: package
all application classes into a RAR file, which is a standard JAR file with a different
file extension. Add all required library JARs into the root of the RAR archive. Add a
"META-INF/ra.xml" deployment descriptor (as shown in SpringContextResourceAdapter
s
JavaDoc) and the corresponding Spring XML bean definition file(s) (typically
"META-INF/applicationContext.xml"), and drop the resulting RAR file into your
application server’s deployment directory.
Note | |
---|---|
Such RAR deployment units are usually self-contained; they do not expose components to the outside world, not even to other modules of the same application. Interaction with a RAR-based ApplicationContext usually occurs through JMS destinations that it shares with other modules. A RAR-based ApplicationContext may also, for example, schedule some jobs, reacting to new files in the file system (or the like). If it needs to allow synchronous access from the outside, it could for example export RMI endpoints, which of course may be used by other application modules on the same machine. |
The BeanFactory
provides the underlying basis for Spring’s IoC functionality but it is
only used directly in integration with other third-party frameworks and is now largely
historical in nature for most users of Spring. The BeanFactory
and related interfaces,
such as BeanFactoryAware
, InitializingBean
, DisposableBean
, are still present in
Spring for the purposes of backward compatibility with the large number of third-party
frameworks that integrate with Spring. Often third-party components that can not use
more modern equivalents such as @PostConstruct
or @PreDestroy
in order to remain
compatible with JDK 1.4 or to avoid a dependency on JSR-250.
This section provides additional background into the differences between the
BeanFactory
and ApplicationContext
and how one might access the IoC container
directly through a classic singleton lookup.
Use an ApplicationContext
unless you have a good reason for not doing so.
Because the ApplicationContext
includes all functionality of the BeanFactory
, it is
generally recommended over the BeanFactory
, except for a few situations such as in an
Applet
where memory consumption might be critical and a few extra kilobytes might make
a difference. However, for most typical enterprise applications and systems, the
ApplicationContext
is what you will want to use. Spring makes heavy
use of the BeanPostProcessor
extension point (to
effect proxying and so on). If you use only a plain BeanFactory
, a fair amount of
support such as transactions and AOP will not take effect, at least not without some
extra steps on your part. This situation could be confusing because nothing is actually
wrong with the configuration.
The following table lists features provided by the BeanFactory
and
ApplicationContext
interfaces and implementations.
Table 4.8. Feature Matrix
Feature | BeanFactory | ApplicationContext |
---|---|---|
Bean instantiation/wiring | Yes | Yes |
Automatic | No | Yes |
Automatic | No | Yes |
Convenient | No | Yes |
| No | Yes |
To explicitly register a bean post-processor with a BeanFactory
implementation, you
must write code like this:
ConfigurableBeanFactory factory = new XmlBeanFactory(...); // now register any needed BeanPostProcessor instances MyBeanPostProcessor postProcessor = new MyBeanPostProcessor(); factory.addBeanPostProcessor(postProcessor); // now start using the factory
To explicitly register a BeanFactoryPostProcessor
when using a BeanFactory
implementation, you must write code like this:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml")); // bring in some property values from a Properties file PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer(); cfg.setLocation(new FileSystemResource("jdbc.properties")); // now actually do the replacement cfg.postProcessBeanFactory(factory);
In both cases, the explicit registration step is inconvenient, which is one reason why
the various ApplicationContext
implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially when
using BeanFactoryPostProcessors
and BeanPostProcessors
. These mechanisms implement
important functionality such as property placeholder replacement and AOP.
It is best to write most application code in a dependency-injection (DI) style, where
that code is served out of a Spring IoC container, has its own dependencies supplied by
the container when it is created, and is completely unaware of the container. However,
for the small glue layers of code that are sometimes needed to tie other code together,
you sometimes need a singleton (or quasi-singleton) style access to a Spring IoC
container. For example, third-party code may try to construct new objects directly (
Class.forName()
style), without the ability to get these objects out of a Spring IoC
container.If the object constructed by the third-party code is a small stub or proxy,
which then uses a singleton style access to a Spring IoC container to get a real object
to delegate to, then inversion of control has still been achieved for the majority of
the code (the object coming out of the container). Thus most code is still unaware of
the container or how it is accessed, and remains decoupled from other code, with all
ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java
implementation object, retrieved from a Spring IoC container. While the Spring IoC
container itself ideally does not have to be a singleton, it may be unrealistic in terms
of memory usage or initialization times (when using beans in the Spring IoC container
such as a Hibernate SessionFactory
) for each bean to use its own, non-singleton Spring
IoC container.
Looking up the application context in a service locator style is sometimes the only
option for accessing shared Spring-managed components, such as in an EJB 2.1
environment, or when you want to share a single ApplicationContext as a parent to
WebApplicationContexts across WAR files. In this case you should look into using the
utility class
ContextSingletonBeanFactoryLocator
locator that is described in this
SpringSource
team blog entry.
Java’s standard java.net.URL
class and standard handlers for various URL prefixes
unfortunately are not quite adequate enough for all access to low-level resources. For
example, there is no standardized URL
implementation that may be used to access a
resource that needs to be obtained from the classpath, or relative to a
ServletContext
. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:
), this is generally
quite complicated, and the URL
interface still lacks some desirable functionality,
such as a method to check for the existence of the resource being pointed to.
Spring’s Resource
interface is meant to be a more capable interface for abstracting
access to low-level resources.
public interface Resource extends InputStreamSource { boolean exists(); boolean isOpen(); URL getURL() throws IOException; File getFile() throws IOException; Resource createRelative(String relativePath) throws IOException; String getFilename(); String getDescription(); }
public interface InputStreamSource { InputStream getInputStream() throws IOException; }
Some of the most important methods from the Resource
interface are:
getInputStream()
: locates and opens the resource, returning an InputStream
for
reading from the resource. It is expected that each invocation returns a fresh
InputStream
. It is the responsibility of the caller to close the stream.
exists()
: returns a boolean
indicating whether this resource actually exists in
physical form.
isOpen()
: returns a boolean
indicating whether this resource represents a handle
with an open stream. If true
, the InputStream
cannot be read multiple times, and
must be read once only and then closed to avoid resource leaks. Will be false
for
all usual resource implementations, with the exception of InputStreamResource
.
getDescription()
: returns a description for this resource, to be used for error
output when working with the resource. This is often the fully qualified file name or
the actual URL of the resource.
Other methods allow you to obtain an actual URL
or File
object representing the
resource (if the underlying implementation is compatible, and supports that
functionality).
The Resource
abstraction is used extensively in Spring itself, as an argument type in
many method signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various ApplicationContext
implementations), take a
String
which in unadorned or simple form is used to create a Resource
appropriate to
that context implementation, or via special prefixes on the String
path, allow the
caller to specify that a specific Resource
implementation must be created and used.
While the Resource
interface is used a lot with Spring and by Spring, it’s actually
very useful to use as a general utility class by itself in your own code, for access to
resources, even when your code doesn’t know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this small set of
utility classes, which are serving as a more capable replacement for URL
, and can be
considered equivalent to any other library you would use for this purpose.
It is important to note that the Resource
abstraction does not replace functionality:
it wraps it where possible. For example, a UrlResource
wraps a URL, and uses the
wrapped URL
to do its work.
There are a number of Resource
implementations that come supplied straight out of the
box in Spring:
The UrlResource
wraps a java.net.URL
, and may be used to access any object that is
normally accessible via a URL, such as files, an HTTP target, an FTP target, etc. All
URLs have a standardized String
representation, such that appropriate standardized
prefixes are used to indicate one URL type from another. This includes file:
for
accessing filesystem paths, http:
for accessing resources via the HTTP protocol,
ftp:
for accessing resources via FTP, etc.
A UrlResource
is created by Java code explicitly using the UrlResource
constructor,
but will often be created implicitly when you call an API method which takes a String
argument which is meant to represent a path. For the latter case, a JavaBeans
PropertyEditor
will ultimately decide which type of Resource
to create. If the path
string contains a few well-known (to it, that is) prefixes such as classpath:
, it will
create an appropriate specialized Resource
for that prefix. However, if it doesn’t
recognize the prefix, it will assume the this is just a standard URL string, and will
create a UrlResource
.
This class represents a resource which should be obtained from the classpath. This uses either the thread context class loader, a given class loader, or a given class for loading resources.
This Resource
implementation supports resolution as java.io.File
if the class path
resource resides in the file system, but not for classpath resources which reside in a
jar and have not been expanded (by the servlet engine, or whatever the environment is)
to the filesystem. To address this the various Resource
implementations always support
resolution as a java.net.URL
.
A ClassPathResource
is created by Java code explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API method which
takes a String
argument which is meant to represent a path. For the latter case, a
JavaBeans PropertyEditor
will recognize the special prefix classpath:
on the string
path, and create a ClassPathResource
in that case.
This is a Resource
implementation for java.io.File
handles. It obviously supports
resolution as a File
, and as a URL
.
This is a Resource
implementation for ServletContext
resources, interpreting
relative paths within the relevant web application’s root directory.
This always supports stream access and URL access, but only allows java.io.File
access
when the web application archive is expanded and the resource is physically on the
filesystem. Whether or not it’s expanded and on the filesystem like this, or accessed
directly from the JAR or somewhere else like a DB (it’s conceivable) is actually
dependent on the Servlet container.
A Resource
implementation for a given InputStream
. This should only be used if no
specific Resource
implementation is applicable. In particular, prefer
ByteArrayResource
or any of the file-based Resource
implementations where possible.
In contrast to other Resource
implementations, this is a descriptor for an already
opened resource - therefore returning true
from isOpen()
. Do not use it if you need
to keep the resource descriptor somewhere, or if you need to read a stream multiple
times.
The ResourceLoader
interface is meant to be implemented by objects that can return
(i.e. load) Resource
instances.
public interface ResourceLoader { Resource getResource(String location); }
All application contexts implement the ResourceLoader
interface, and therefore all
application contexts may be used to obtain Resource
instances.
When you call getResource()
on a specific application context, and the location path
specified doesn’t have a specific prefix, you will get back a Resource
type that is
appropriate to that particular application context. For example, assume the following
snippet of code was executed against a ClassPathXmlApplicationContext
instance:
Resource template = ctx.getResource("some/resource/path/myTemplate.txt");
What would be returned would be a ClassPathResource
; if the same method was executed
against a FileSystemXmlApplicationContext
instance, you’d get back a
FileSystemResource
. For a WebApplicationContext
, you’d get back a
ServletContextResource
, and so on.
As such, you can load resources in a fashion appropriate to the particular application context.
On the other hand, you may also force ClassPathResource
to be used, regardless of the
application context type, by specifying the special classpath:
prefix:
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");
Similarly, one can force a UrlResource
to be used by specifying any of the standard
java.net.URL
prefixes:
Resource template = ctx.getResource("file:/some/resource/path/myTemplate.txt");
Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt");
The following table summarizes the strategy for converting String
s to Resource
s:
Table 5.1. Resource strings
Prefix | Example | Explanation |
---|---|---|
classpath: |
| Loaded from the classpath. |
file: |
| Loaded as a |
http: |
| Loaded as a |
(none) |
| Depends on the underlying |
[1] But see also Section 5.7.3, “FileSystemResource caveats”. |
The ResourceLoaderAware
interface is a special marker interface, identifying objects
that expect to be provided with a ResourceLoader
reference.
public interface ResourceLoaderAware { void setResourceLoader(ResourceLoader resourceLoader); }
When a class implements ResourceLoaderAware
and is deployed into an application
context (as a Spring-managed bean), it is recognized as ResourceLoaderAware
by the
application context. The application context will then invoke the
setResourceLoader(ResourceLoader)
, supplying itself as the argument (remember, all
application contexts in Spring implement the ResourceLoader
interface).
Of course, since an ApplicationContext
is a ResourceLoader
, the bean could also
implement the ApplicationContextAware
interface and use the supplied application
context directly to load resources, but in general, it’s better to use the specialized
ResourceLoader
interface if that’s all that’s needed. The code would just be coupled
to the resource loading interface, which can be considered a utility interface, and not
the whole Spring ApplicationContext
interface.
As of Spring 2.5, you can rely upon autowiring of the ResourceLoader
as an alternative
to implementing the ResourceLoaderAware
interface. The "traditional" constructor
and
byType
autowiring modes (as described in Section 4.4.5, “Autowiring collaborators”) are now capable
of providing a dependency of type ResourceLoader
for either a constructor argument or
setter method parameter respectively. For more flexibility (including the ability to
autowire fields and multiple parameter methods), consider using the new annotation-based
autowiring features. In that case, the ResourceLoader
will be autowired into a field,
constructor argument, or method parameter that is expecting the ResourceLoader
type as
long as the field, constructor, or method in question carries the @Autowired
annotation. For more information, see Section 4.9.2, “@Autowired”.
If the bean itself is going to determine and supply the resource path through some sort
of dynamic process, it probably makes sense for the bean to use the ResourceLoader
interface to load resources. Consider as an example the loading of a template of some
sort, where the specific resource that is needed depends on the role of the user. If the
resources are static, it makes sense to eliminate the use of the ResourceLoader
interface completely, and just have the bean expose the Resource
properties it needs,
and expect that they will be injected into it.
What makes it trivial to then inject these properties, is that all application contexts
register and use a special JavaBeans PropertyEditor
which can convert String
paths
to Resource
objects. So if myBean
has a template property of type Resource
, it can
be configured with a simple string for that resource, as follows:
<bean id="myBean" class="..."> <property name="template" value="some/resource/path/myTemplate.txt"/> </bean>
Note that the resource path has no prefix, so because the application context itself is
going to be used as the ResourceLoader
, the resource itself will be loaded via a
ClassPathResource
, FileSystemResource
, or ServletContextResource
(as appropriate)
depending on the exact type of the context.
If there is a need to force a specific Resource
type to be used, then a prefix may be
used. The following two examples show how to force a ClassPathResource
and a
UrlResource
(the latter being used to access a filesystem file).
<property name="template" value="classpath:some/resource/path/myTemplate.txt">
<property name="template" value="file:/some/resource/path/myTemplate.txt"/>
An application context constructor (for a specific application context type) generally takes a string or array of strings as the location path(s) of the resource(s) such as XML files that make up the definition of the context.
When such a location path doesn’t have a prefix, the specific Resource
type built from
that path and used to load the bean definitions, depends on and is appropriate to the
specific application context. For example, if you create a
ClassPathXmlApplicationContext
as follows:
ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");
The bean definitions will be loaded from the classpath, as a ClassPathResource
will be
used. But if you create a FileSystemXmlApplicationContext
as follows:
ApplicationContext ctx = new FileSystemXmlApplicationContext("conf/appContext.xml");
The bean definition will be loaded from a filesystem location, in this case relative to the current working directory.
Note that the use of the special classpath prefix or a standard URL prefix on the
location path will override the default type of Resource
created to load the
definition. So this FileSystemXmlApplicationContext
…
ApplicationContext ctx = new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");
FileSystemXmlApplicationContext
. If it is subsequently used as a ResourceLoader
, any
unprefixed paths will still be treated as filesystem paths.
The ClassPathXmlApplicationContext
exposes a number of constructors to enable
convenient instantiation. The basic idea is that one supplies merely a string array
containing just the filenames of the XML files themselves (without the leading path
information), and one also supplies a Class
; the ClassPathXmlApplicationContext
will derive the path information from the supplied class.
An example will hopefully make this clear. Consider a directory layout that looks like this:
com/ foo/ services.xml daos.xml MessengerService.class
A ClassPathXmlApplicationContext
instance composed of the beans defined in the
'services.xml'
and 'daos.xml'
could be instantiated like so…
ApplicationContext ctx = new ClassPathXmlApplicationContext( new String[] {"services.xml", "daos.xml"}, MessengerService.class);
Please do consult the Javadocs for the ClassPathXmlApplicationContext
class for
details of the various constructors.
The resource paths in application context constructor values may be a simple path (as
shown above) which has a one-to-one mapping to a target Resource, or alternately may
contain the special "classpath*:" prefix and/or internal Ant-style regular expressions
(matched using Spring’s PathMatcher
utility). Both of the latter are effectively
wildcards
One use for this mechanism is when doing component-style application assembly. All
components can publish context definition fragments to a well-known location path, and
when the final application context is created using the same path prefixed via
classpath*:
, all component fragments will be picked up automatically.
Note that this wildcarding is specific to use of resource paths in application context
constructors (or when using the PathMatcher
utility class hierarchy directly), and is
resolved at construction time. It has nothing to do with the Resource
type itself.
It’s not possible to use the classpath*:
prefix to construct an actual Resource
, as
a resource points to just one resource at a time.
When the path location contains an Ant-style pattern, for example:
/WEB-INF/-context.xml
com/mycompany//applicationContext.xml
file:C:/some/path/-context.xml
classpath:com/mycompany//applicationContext.xml
zip:
" in WebLogic, " wsjar
" in WebSphere, etc.), then a java.io.File
is
obtained from it and used to resolve the wildcard by traversing the filesystem. In the
case of a jar URL, the resolver either gets a java.net.JarURLConnection
from it or
manually parses the jar URL and then traverses the contents of the jar file to resolve
the wildcards.
If the specified path is already a file URL (either explicitly, or implicitly because
the base ResourceLoader
is a filesystem one, then wildcarding is guaranteed to work in
a completely portable fashion.
If the specified path is a classpath location, then the resolver must obtain the last
non-wildcard path segment URL via a Classloader.getResource()
call. Since this is just
a node of the path (not the file at the end) it is actually undefined (in the
ClassLoader
Javadocs) exactly what sort of a URL is returned in this case. In
practice, it is always a java.io.File
representing the directory, where the classpath
resource resolves to a filesystem location, or a jar URL of some sort, where the
classpath resource resolves to a jar location. Still, there is a portability concern on
this operation.
If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to
get a java.net.JarURLConnection
from it, or manually parse the jar URL, to be able to
walk the contents of the jar, and resolve the wildcard. This will work in most
environments, but will fail in others, and it is strongly recommended that the wildcard
resolution of resources coming from jars be thoroughly tested in your specific
environment before you rely on it.
When constructing an XML-based application context, a location string may use the
special classpath*:
prefix:
ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");
This special prefix specifies that all classpath resources that match the given name
must be obtained (internally, this essentially happens via a
ClassLoader.getResources(...)
call), and then merged to form the final application
context definition.
Note | |
---|---|
The wildcard classpath relies on the |
The " classpath*:
" prefix can also be combined with a PathMatcher
pattern in the
rest of the location path, for example " classpath*:META-INF/*-beans.xml
". In this
case, the resolution strategy is fairly simple: a ClassLoader.getResources() call is
used on the last non-wildcard path segment to get all the matching resources in the
class loader hierarchy, and then off each resource the same PathMatcher resolution
strategy described above is used for the wildcard subpath.
Please note that " classpath*:
" when combined with Ant-style patterns will only work
reliably with at least one root directory before the pattern starts, unless the actual
target files reside in the file system. This means that a pattern like "
classpath*:*.xml
" will not retrieve files from the root of jar files but rather only
from the root of expanded directories. This originates from a limitation in the JDK’s
ClassLoader.getResources()
method which only returns file system locations for a
passed-in empty string (indicating potential roots to search).
Ant-style patterns with " classpath:
" resources are not guaranteed to find matching
resources if the root package to search is available in multiple class path locations.
This is because a resource such as
com/mycompany/package1/service-context.xml
may be in only one location, but when a path such as
classpath:com/mycompany/**/service-context.xml
is used to try to resolve it, the resolver will work off the (first) URL returned by
getResource("com/mycompany")
;. If this base package node exists in multiple
classloader locations, the actual end resource may not be underneath. Therefore,
preferably, use " classpath*:
" with the same Ant-style pattern in such a case, which
will search all class path locations that contain the root package.
A FileSystemResource
that is not attached to a FileSystemApplicationContext
(that
is, a FileSystemApplicationContext
is not the actual ResourceLoader
) will treat
absolute vs. relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root of the
filesystem.
For backwards compatibility (historical) reasons however, this changes when the
FileSystemApplicationContext
is the ResourceLoader
. The
FileSystemApplicationContext
simply forces all attached FileSystemResource
instances
to treat all location paths as relative, whether they start with a leading slash or not.
In practice, this means the following are equivalent:
ApplicationContext ctx = new FileSystemXmlApplicationContext("conf/context.xml");
ApplicationContext ctx = new FileSystemXmlApplicationContext("/conf/context.xml");
As are the following: (Even though it would make sense for them to be different, as one case is relative and the other absolute.)
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");
In practice, if true absolute filesystem paths are needed, it is better to forgo the use
of absolute paths with FileSystemResource
/ FileSystemXmlApplicationContext
, and
just force the use of a UrlResource
, by using the file:
URL prefix.
// actual context type doesn't matter, the Resource will always be UrlResource ctx.getResource("file:/some/resource/path/myTemplate.txt");
// force this FileSystemXmlApplicationContext to load its definition via a UrlResource ApplicationContext ctx = new FileSystemXmlApplicationContext("file:/conf/context.xml");
There are pros and cons for considering validation as business logic, and Spring offers
a design for validation (and data binding) that does not exclude either one of them.
Specifically validation should not be tied to the web tier, should be easy to localize
and it should be possible to plug in any validator available. Considering the above,
Spring has come up with a Validator
interface that is both basic ands eminently usable
in every layer of an application.
Data binding is useful for allowing user input to be dynamically bound to the domain
model of an application (or whatever objects you use to process user input). Spring
provides the so-called DataBinder
to do exactly that. The Validator
and the
DataBinder
make up the validation
package, which is primarily used in but not
limited to the MVC framework.
The BeanWrapper
is a fundamental concept in the Spring Framework and is used in a lot
of places. However, you probably will not have the need to use the BeanWrapper
directly. Because this is reference documentation however, we felt that some explanation
might be in order. We will explain the BeanWrapper
in this chapter since, if you were
going to use it at all, you would most likely do so when trying to bind data to objects.
Spring’s DataBinder and the lower-level BeanWrapper both use PropertyEditors to parse
and format property values. The PropertyEditor
concept is part of the JavaBeans
specification, and is also explained in this chapter. Spring 3 introduces a
"core.convert" package that provides a general type conversion facility, as well as a
higher-level "format" package for formatting UI field values. These new packages may be
used as simpler alternatives to PropertyEditors, and will also be discussed in this
chapter.
Spring features a Validator
interface that you can use to validate objects. The
Validator
interface works using an Errors
object so that while validating,
validators can report validation failures to the Errors
object.
Let’s consider a small data object:
public class Person { private String name; private int age; // the usual getters and setters... }
We’re going to provide validation behavior for the Person
class by implementing the
following two methods of the org.springframework.validation.Validator
interface:
supports(Class)
- Can this Validator
validate instances of the supplied Class
?
validate(Object, org.springframework.validation.Errors)
- validates the given object
and in case of validation errors, registers those with the given Errors
object
Implementing a Validator
is fairly straightforward, especially when you know of the
ValidationUtils
helper class that the Spring Framework also provides.
public class PersonValidator implements Validator { /* * This Validator validates *just Person instances */ public boolean supports(Class clazz) { return Person.class.equals(clazz); } public void validate(Object obj, Errors e) { ValidationUtils.rejectIfEmpty(e, "name", "name.empty"); Person p = (Person) obj; if (p.getAge() < 0) { e.rejectValue("age", "negativevalue"); } else if (p.getAge() > 110) { e.rejectValue("age", "too.darn.old"); } } }
As you can see, the static
rejectIfEmpty(..)
method on the ValidationUtils
class
is used to reject the 'name'
property if it is null
or the empty string. Have a look
at the Javadoc for the ValidationUtils
class to see what functionality it provides
besides the example shown previously.
While it is certainly possible to implement a single Validator
class to validate each
of the nested objects in a rich object, it may be better to encapsulate the validation
logic for each nested class of object in its own Validator
implementation. A simple
example of a 'rich' object would be a Customer
that is composed of two String
properties (a first and second name) and a complex Address
object. Address
objects
may be used independently of Customer
objects, and so a distinct AddressValidator
has been implemented. If you want your CustomerValidator
to reuse the logic contained
within the AddressValidator
class without resorting to copy-and-paste, you can
dependency-inject or instantiate an AddressValidator
within your CustomerValidator
,
and use it like so:
public class CustomerValidator implements Validator { private final Validator addressValidator; public CustomerValidator(Validator addressValidator) { if (addressValidator == null) { throw new IllegalArgumentException("The supplied [Validator] is " + "required and must not be null."); } if (!addressValidator.supports(Address.class)) { throw new IllegalArgumentException("The supplied [Validator] must " + support the validation of [Address] instances."); } this.addressValidator = addressValidator; } /** * This Validator validates Customer instances, and any subclasses of Customer too */ public boolean supports(Class clazz) { return Customer.class.isAssignableFrom(clazz); } public void validate(Object target, Errors errors) { ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required"); ValidationUtils.rejectIfEmptyOrWhitespace(errors, "surname", "field.required"); Customer customer = (Customer) target; try { errors.pushNestedPath("address"); ValidationUtils.invokeValidator(this.addressValidator, customer.getAddress(), errors); } finally { errors.popNestedPath(); } } }
Validation errors are reported to the Errors
object passed to the validator. In case
of Spring Web MVC you can use <spring:bind/>
tag to inspect the error messages, but of
course you can also inspect the errors object yourself. More information about the
methods it offers can be found from the Javadoc.
We’ve talked about databinding and validation. Outputting messages corresponding to
validation errors is the last thing we need to discuss. In the example we’ve shown
above, we rejected the name
and the age
field. If we’re going to output the error
messages by using a MessageSource
, we will do so using the error code we’ve given when
rejecting the field (name and age in this case). When you call (either directly, or
indirectly, using for example the ValidationUtils
class) rejectValue
or one of the
other reject
methods from the Errors
interface, the underlying implementation will
not only register the code you’ve passed in, but also a number of additional error
codes. What error codes it registers is determined by the MessageCodesResolver
that is
used. By default, the DefaultMessageCodesResolver
is used, which for example not only
registers a message with the code you gave, but also messages that include the field
name you passed to the reject method. So in case you reject a field using
rejectValue("age", "too.darn.old")
, apart from the too.darn.old
code, Spring will
also register too.darn.old.age
and too.darn.old.age.int
(so the first will include
the field name and the second will include the type of the field); this is done as a
convenience to aid developers in targeting error messages and suchlike.
More information on the MessageCodesResolver
and the default strategy can be found
online with the Javadocs for
MessageCodesResolver
and
DefaultMessageCodesResolver
respectively.
The org.springframework.beans
package adheres to the JavaBeans standard provided by
Oracle. A JavaBean is simply a class with a default no-argument constructor, which follows
a naming convention where (by way of an example) a property named bingoMadness
would
have a setter method setBingoMadness(..)
and a getter method getBingoMadness()
. For
more information about JavaBeans and the specification, please refer to Oracle’s website (
javabeans).
One quite important class in the beans package is the BeanWrapper
interface and its
corresponding implementation ( BeanWrapperImpl
). As quoted from the Javadoc, the
BeanWrapper
offers functionality to set and get property values (individually or in
bulk), get property descriptors, and to query properties to determine if they are
readable or writable. Also, the BeanWrapper
offers support for nested properties,
enabling the setting of properties on sub-properties to an unlimited depth. Then, the
BeanWrapper
supports the ability to add standard JavaBeans PropertyChangeListeners
and VetoableChangeListeners
, without the need for supporting code in the target class.
Last but not least, the BeanWrapper
provides support for the setting of indexed
properties. The BeanWrapper
usually isn’t used by application code directly, but by
the DataBinder
and the BeanFactory
.
The way the BeanWrapper
works is partly indicated by its name: it wraps a bean to
perform actions on that bean, like setting and retrieving properties.
Setting and getting properties is done using the setPropertyValue(s)
and
getPropertyValue(s)
methods that both come with a couple of overloaded variants.
They’re all described in more detail in the Javadoc Spring comes with. What’s important
to know is that there are a couple of conventions for indicating properties of an
object. A couple of examples:
Table 6.1. Examples of properties
Expression | Explanation |
---|---|
| Indicates the property |
| Indicates the nested property |
| Indicates the third element of the indexed property |
| Indicates the value of the map entry indexed by the key COMPANYNAME of the Map
property |
Below you’ll find some examples of working with the BeanWrapper
to get and set
properties.
(This next section is not vitally important to you if you’re not planning to work with
the BeanWrapper
directly. If you’re just using the DataBinder
and the BeanFactory
and their out-of-the-box implementation, you should skip ahead to the section about
PropertyEditors
.)
Consider the following two classes:
public class Company { private String name; private Employee managingDirector; public String getName() { return this.name; } public void setName(String name) { this.name = name; } public Employee getManagingDirector() { return this.managingDirector; } public void setManagingDirector(Employee managingDirector) { this.managingDirector = managingDirector; } }
public class Employee { private String name; private float salary; public String getName() { return this.name; } public void setName(String name) { this.name = name; } public float getSalary() { return salary; } public void setSalary(float salary) { this.salary = salary; } }
The following code snippets show some examples of how to retrieve and manipulate some of
the properties of instantiated Companies
and Employees
:
BeanWrapper company = BeanWrapperImpl(new Company()); // setting the company name.. company.setPropertyValue("name", "Some Company Inc."); // ... can also be done like this: PropertyValue value = new PropertyValue("name", "Some Company Inc."); company.setPropertyValue(value); // ok, let's create the director and tie it to the company: BeanWrapper jim = BeanWrapperImpl(new Employee()); jim.setPropertyValue("name", "Jim Stravinsky"); company.setPropertyValue("managingDirector", jim.getWrappedInstance()); // retrieving the salary of the managingDirector through the company Float salary = (Float) company.getPropertyValue("managingDirector.salary");
Spring uses the concept of PropertyEditors
to effect the conversion between an
Object
and a String
. If you think about it, it sometimes might be handy to be able
to represent properties in a different way than the object itself. For example, a Date
can be represented in a human readable way (as the String
' 2007-14-09
'), while
we’re still able to convert the human readable form back to the original date (or even
better: convert any date entered in a human readable form, back to Date
objects). This
behavior can be achieved by registering custom editors, of type
java.beans.PropertyEditor
. Registering custom editors on a BeanWrapper
or
alternately in a specific IoC container as mentioned in the previous chapter, gives it
the knowledge of how to convert properties to the desired type. Read more about
PropertyEditors
in the Javadoc of the java.beans
package provided by Oracle.
A couple of examples where property editing is used in Spring:
PropertyEditors
. When mentioning
java.lang.String
as the value of a property of some bean you’re declaring in XML
file, Spring will (if the setter of the corresponding property has a
Class
-parameter) use the ClassEditor
to try to resolve the parameter to a Class
object.
PropertyEditors
that you can manually bind in all subclasses of the
CommandController
.
Spring has a number of built-in PropertyEditors
to make life easy. Each of those is
listed below and they are all located in the org.springframework.beans.propertyeditors
package. Most, but not all (as indicated below), are registered by default by
BeanWrapperImpl
. Where the property editor is configurable in some fashion, you can of
course still register your own variant to override the default one:
Table 6.2. Built-in PropertyEditors
Class | Explanation |
---|---|
| Editor for byte arrays. Strings will simply be converted to their corresponding byte
representations. Registered by default by |
| Parses Strings representing classes to actual classes and the other way around. When a
class is not found, an |
| Customizable property editor for |
| Property editor for Collections, converting any source |
| Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user registered as needed with appropriate format. |
| Customizable property editor for any Number subclass like |
| Capable of resolving Strings to |
| One-way property editor, capable of taking a text string and producing (via an
intermediate |
| Capable of resolving Strings to |
| Capable of resolving Strings to JDK 1.5 |
| Capable of converting Strings (formatted using the format as defined in the Javadoc
for the java.lang.Properties class) to |
| Property editor that trims Strings. Optionally allows transforming an empty string
into a |
| Capable of resolving a String representation of a URL to an actual |
Spring uses the java.beans.PropertyEditorManager
to set the search path for property
editors that might be needed. The search path also includes sun.bean.editors
, which
includes PropertyEditor
implementations for types such as Font
, Color
, and most of
the primitive types. Note also that the standard JavaBeans infrastructure will
automatically discover PropertyEditor
classes (without you having to register them
explicitly) if they are in the same package as the class they handle, and have the same
name as that class, with 'Editor'
appended; for example, one could have the following
class and package structure, which would be sufficient for the FooEditor
class to be
recognized and used as the PropertyEditor
for Foo
-typed properties.
com chank pop Foo FooEditor // the PropertyEditor for the Foo class
Note that you can also use the standard BeanInfo
JavaBeans mechanism here as well
(described
in
not-amazing-detail here). Find below an example of using the BeanInfo
mechanism for
explicitly registering one or more PropertyEditor
instances with the properties of an
associated class.
com chank pop Foo FooBeanInfo // the BeanInfo for the Foo class
Here is the Java source code for the referenced FooBeanInfo
class. This would
associate a CustomNumberEditor
with the age
property of the Foo
class.
public class FooBeanInfo extends SimpleBeanInfo { public PropertyDescriptor[] getPropertyDescriptors() { try { final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true); PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.class) { public PropertyEditor createPropertyEditor(Object bean) { return numberPE; }; }; return new PropertyDescriptor[] { ageDescriptor }; } catch (IntrospectionException ex) { throw new Error(ex.toString()); } } }
When setting bean properties as a string value, a Spring IoC container ultimately uses
standard JavaBeans PropertyEditors
to convert these Strings to the complex type of the
property. Spring pre-registers a number of custom PropertyEditors
(for example, to
convert a classname expressed as a string into a real Class
object). Additionally,
Java’s standard JavaBeans PropertyEditor
lookup mechanism allows a PropertyEditor
for a class simply to be named appropriately and placed in the same package as the class
it provides support for, to be found automatically.
If there is a need to register other custom PropertyEditors
, there are several
mechanisms available. The most manual approach, which is not normally convenient or
recommended, is to simply use the registerCustomEditor()
method of the
ConfigurableBeanFactory
interface, assuming you have a BeanFactory
reference.
Another, slightly more convenient, mechanism is to use a special bean factory
post-processor called CustomEditorConfigurer
. Although bean factory post-processors
can be used with BeanFactory
implementations, the CustomEditorConfigurer
has a
nested property setup, so it is strongly recommended that it is used with the
ApplicationContext
, where it may be deployed in similar fashion to any other bean, and
automatically detected and applied.
Note that all bean factories and application contexts automatically use a number of
built-in property editors, through their use of something called a BeanWrapper
to
handle property conversions. The standard property editors that the BeanWrapper
registers are listed in the previous section. Additionally,
ApplicationContexts
also override or add an additional number of editors to handle
resource lookups in a manner appropriate to the specific application context type.
Standard JavaBeans PropertyEditor
instances are used to convert property values
expressed as strings to the actual complex type of the property.
CustomEditorConfigurer
, a bean factory post-processor, may be used to conveniently add
support for additional PropertyEditor
instances to an ApplicationContext
.
Consider a user class ExoticType
, and another class DependsOnExoticType
which needs
ExoticType
set as a property:
package example; public class ExoticType { private String name; public ExoticType(String name) { this.name = name; } } public class DependsOnExoticType { private ExoticType type; public void setType(ExoticType type) { this.type = type; } }
When things are properly set up, we want to be able to assign the type property as a
string, which a PropertyEditor
will behind the scenes convert into an actual
ExoticType
instance:
<bean id="sample" class="example.DependsOnExoticType"> <property name="type" value="aNameForExoticType"/> </bean>
The PropertyEditor
implementation could look similar to this:
// converts string representation to ExoticType object package example; public class ExoticTypeEditor extends PropertyEditorSupport { public void setAsText(String text) { setValue(new ExoticType(text.toUpperCase())); } }
Finally, we use CustomEditorConfigurer
to register the new PropertyEditor
with the
ApplicationContext
, which will then be able to use it as needed:
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer"> <property name="customEditors"> <map> <entry key="example.ExoticType" value="example.ExoticTypeEditor"/> </map> </property> </bean>
Another mechanism for registering property editors with the Spring container is to
create and use a PropertyEditorRegistrar
. This interface is particularly useful when
you need to use the same set of property editors in several different situations: write
a corresponding registrar and reuse that in each case. PropertyEditorRegistrars
work
in conjunction with an interface called PropertyEditorRegistry
, an interface that is
implemented by the Spring BeanWrapper
(and DataBinder
). PropertyEditorRegistrars
are particularly convenient when used in conjunction with the CustomEditorConfigurer
(introduced here), which exposes a
property called setPropertyEditorRegistrars(..)
: PropertyEditorRegistrars
added to a
CustomEditorConfigurer
in this fashion can easily be shared with DataBinder
and
Spring MVC Controllers
. Furthermore, it avoids the need for synchronization on custom
editors: a PropertyEditorRegistrar
is expected to create fresh PropertyEditor
instances for each bean creation attempt.
Using a PropertyEditorRegistrar
is perhaps best illustrated with an example. First
off, you need to create your own PropertyEditorRegistrar
implementation:
package com.foo.editors.spring; public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar { public void registerCustomEditors(PropertyEditorRegistry registry) { // it is expected that new PropertyEditor instances are created registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor()); // you could register as many custom property editors as are required here... } }
See also the org.springframework.beans.support.ResourceEditorRegistrar
for an example
PropertyEditorRegistrar
implementation. Notice how in its implementation of the
registerCustomEditors(..)
method it creates new instances of each property editor.
Next we configure a CustomEditorConfigurer
and inject an instance of our
CustomPropertyEditorRegistrar
into it:
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer"> <property name="propertyEditorRegistrars"> <list> <ref bean="customPropertyEditorRegistrar"/> </list> </property> </bean> <bean id="customPropertyEditorRegistrar" class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>
Finally, and in a bit of a departure from the focus of this chapter, for those of you
using Spring’s MVC web framework, using PropertyEditorRegistrars
in
conjunction with data-binding Controllers
(such as SimpleFormController
) can be very
convenient. Find below an example of using a PropertyEditorRegistrar
in the
implementation of an initBinder(..)
method:
public final class RegisterUserController extends SimpleFormController { private final PropertyEditorRegistrar customPropertyEditorRegistrar; public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) { this.customPropertyEditorRegistrar = propertyEditorRegistrar; } protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder) throws Exception { this.customPropertyEditorRegistrar.registerCustomEditors(binder); } // other methods to do with registering a User }
This style of PropertyEditor
registration can lead to concise code (the implementation
of initBinder(..)
is just one line long!), and allows common PropertyEditor
registration code to be encapsulated in a class and then shared amongst as many
Controllers
as needed.
Spring 3 introduces a core.convert
package that provides a general type conversion
system. The system defines an SPI to implement type conversion logic, as well as an API
to execute type conversions at runtime. Within a Spring container, this system can be
used as an alternative to PropertyEditors to convert externalized bean property value
strings to required property types. The public API may also be used anywhere in your
application where type conversion is needed.
The SPI to implement type conversion logic is simple and strongly typed:
package org.springframework.core.convert.converter; public interface Converter<S, T> { T convert(S source); }
To create your own Converter, simply implement the interface above. Parameterize S as the type you are converting from, and T as the type you are converting to. For each call to convert(S), the source argument is guaranteed to be NOT null. Your Converter may throw any Exception if conversion fails. An IllegalArgumentException should be thrown to report an invalid source value. Take care to ensure your Converter implementation is thread-safe.
Several converter implementations are provided in the core.convert.support
package as
a convenience. These include converters from Strings to Numbers and other common types.
Consider StringToInteger
as an example Converter implementation:
package org.springframework.core.convert.support; final class StringToInteger implements Converter<String, Integer> { public Integer convert(String source) { return Integer.valueOf(source); } }
When you need to centralize the conversion logic for an entire class hierarchy, for
example, when converting from String to java.lang.Enum objects, implement
ConverterFactory
:
package org.springframework.core.convert.converter; public interface ConverterFactory<S, R> { <T extends R> Converter<S, T> getConverter(Class<T> targetType); }
Parameterize S to be the type you are converting from and R to be the base type defining the range of classes you can convert to. Then implement getConverter(Class<T>), where T is a subclass of R.
Consider the StringToEnum
ConverterFactory as an example:
package org.springframework.core.convert.support; final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> { public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) { return new StringToEnumConverter(targetType); } private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> { private Class<T> enumType; public StringToEnumConverter(Class<T> enumType) { this.enumType = enumType; } public T convert(String source) { return (T) Enum.valueOf(this.enumType, source.trim()); } } }
When you require a sophisticated Converter implementation, consider the GenericConverter interface. With a more flexible but less strongly typed signature, a GenericConverter supports converting between multiple source and target types. In addition, a GenericConverter makes available source and target field context you can use when implementing your conversion logic. Such context allows a type conversion to be driven by a field annotation, or generic information declared on a field signature.
package org.springframework.core.convert.converter; public interface GenericConverter { public Set<ConvertiblePair> getConvertibleTypes(); Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType); }
To implement a GenericConverter, have getConvertibleTypes() return the supported source→target type pairs. Then implement convert(Object, TypeDescriptor, TypeDescriptor) to implement your conversion logic. The source TypeDescriptor provides access to the source field holding the value being converted. The target TypeDescriptor provides access to the target field where the converted value will be set.
A good example of a GenericConverter is a converter that converts between a Java Array and a Collection. Such an ArrayToCollectionConverter introspects the field that declares the target Collection type to resolve the Collection’s element type. This allows each element in the source array to be converted to the Collection element type before the Collection is set on the target field.
Note | |
---|---|
Because GenericConverter is a more complex SPI interface, only use it when you need it. Favor Converter or ConverterFactory for basic type conversion needs. |
Sometimes you only want a Converter to execute if a specific condition holds true. For example, you might only want to execute a Converter if a specific annotation is present on the target field. Or you might only want to execute a Converter if a specific method, such as static valueOf method, is defined on the target class. ConditionalGenericConverter is an subinterface of GenericConverter that allows you to define such custom matching criteria:
public interface ConditionalGenericConverter extends GenericConverter { boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType); }
A good example of a ConditionalGenericConverter is an EntityConverter that converts between an persistent entity identifier and an entity reference. Such a EntityConverter might only match if the target entity type declares a static finder method e.g. findAccount(Long). You would perform such a finder method check in the implementation of matches(TypeDescriptor, TypeDescriptor).
The ConversionService defines a unified API for executing type conversion logic at runtime. Converters are often executed behind this facade interface:
package org.springframework.core.convert; public interface ConversionService { boolean canConvert(Class<?> sourceType, Class<?> targetType); <T> T convert(Object source, Class<T> targetType); boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType); Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType); }
Most ConversionService implementations also implement ConverterRegistry
, which
provides an SPI for registering converters. Internally, a ConversionService
implementation delegates to its registered converters to carry out type conversion logic.
A robust ConversionService implementation is provided in the core.convert.support
package. GenericConversionService
is the general-purpose implementation suitable for
use in most environments. ConversionServiceFactory
provides a convenient factory for
creating common ConversionService configurations.
A ConversionService is a stateless object designed to be instantiated at application startup, then shared between multiple threads. In a Spring application, you typically configure a ConversionService instance per Spring container (or ApplicationContext). That ConversionService will be picked up by Spring and then used whenever a type conversion needs to be performed by the framework. You may also inject this ConversionService into any of your beans and invoke it directly.
Note | |
---|---|
If no ConversionService is registered with Spring, the original PropertyEditor-based system is used. |
To register a default ConversionService with Spring, add the following bean definition
with id conversionService
:
<bean id="conversionService" class="org.springframework.context.support.ConversionServiceFactoryBean"/>
A default ConversionService can convert between strings, numbers, enums, collections,
maps, and other common types. To supplement or override the default converters with your
own custom converter(s), set the converters
property. Property values may implement
either of the Converter, ConverterFactory, or GenericConverter interfaces.
<bean id="conversionService" class="org.springframework.context.support.ConversionServiceFactoryBean"> <property name="converters"> <set> <bean class="example.MyCustomConverter"/> </set> </property> </bean>
It is also common to use a ConversionService within a Spring MVC application. See
Section 6.6.5, “Configuring Formatting in Spring MVC” for details on use with <mvc:annotation-driven/>
.
In certain situations you may wish to apply formatting during conversion. See
Section 6.6.3, “FormatterRegistry SPI” for details on using
FormattingConversionServiceFactoryBean
.
To work with a ConversionService instance programmatically, simply inject a reference to it like you would for any other bean:
@Service public class MyService { @Autowired public MyService(ConversionService conversionService) { this.conversionService = conversionService; } public void doIt() { this.conversionService.convert(...) } }
As discussed in the previous section, core.convert
is a
general-purpose type conversion system. It provides a unified ConversionService API as
well as a strongly-typed Converter SPI for implementing conversion logic from one type
to another. A Spring Container uses this system to bind bean property values. In
addition, both the Spring Expression Language (SpEL) and DataBinder use this system to
bind field values. For example, when SpEL needs to coerce a Short
to a Long
to
complete an expression.setValue(Object bean, Object value)
attempt, the core.convert
system performs the coercion.
Now consider the type conversion requirements of a typical client environment such as a web or desktop application. In such environments, you typically convert from String to support the client postback process, as well as back to String to support the view rendering process. In addition, you often need to localize String values. The more general core.convert Converter SPI does not address such formatting requirements directly. To directly address them, Spring 3 introduces a convenient Formatter SPI that provides a simple and robust alternative to PropertyEditors for client environments.
In general, use the Converter SPI when you need to implement general-purpose type conversion logic; for example, for converting between a java.util.Date and and java.lang.Long. Use the Formatter SPI when you’re working in a client environment, such as a web application, and need to parse and print localized field values. The ConversionService provides a unified type conversion API for both SPIs.
The Formatter SPI to implement field formatting logic is simple and strongly typed:
package org.springframework.format; public interface Formatter<T> extends Printer<T>, Parser<T> { }
Where Formatter extends from the Printer and Parser building-block interfaces:
public interface Printer<T> { String print(T fieldValue, Locale locale); }
import java.text.ParseException; public interface Parser<T> { T parse(String clientValue, Locale locale) throws ParseException; }
To create your own Formatter, simply implement the Formatter interface above.
Parameterize T to be the type of object you wish to format, for example,
java.util.Date
. Implement the print()
operation to print an instance of T for
display in the client locale. Implement the parse()
operation to parse an instance of
T from the formatted representation returned from the client locale. Your Formatter
should throw a ParseException or IllegalArgumentException if a parse attempt fails. Take
care to ensure your Formatter implementation is thread-safe.
Several Formatter implementations are provided in format
subpackages as a convenience.
The number
package provides a NumberFormatter
, CurrencyFormatter
, and
PercentFormatter
to format java.lang.Number
objects using a java.text.NumberFormat
.
The datetime
package provides a DateFormatter
to format java.util.Date
objects with
a java.text.DateFormat
. The datetime.joda
package provides comprehensive datetime
formatting support based on the Joda Time library.
Consider DateFormatter
as an example Formatter
implementation:
package org.springframework.format.datetime; public final class DateFormatter implements Formatter<Date> { private String pattern; public DateFormatter(String pattern) { this.pattern = pattern; } public String print(Date date, Locale locale) { if (date == null) { return ""; } return getDateFormat(locale).format(date); } public Date parse(String formatted, Locale locale) throws ParseException { if (formatted.length() == 0) { return null; } return getDateFormat(locale).parse(formatted); } protected DateFormat getDateFormat(Locale locale) { DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale); dateFormat.setLenient(false); return dateFormat; } }
The Spring team welcomes community-driven Formatter contributions; see http://jira.springframework.org to contribute.
As you will see, field formatting can be configured by field type or annotation. To bind an Annotation to a formatter, implement AnnotationFormatterFactory:
package org.springframework.format; public interface AnnotationFormatterFactory<A extends Annotation> { Set<Class<?>> getFieldTypes(); Printer<?> getPrinter(A annotation, Class<?> fieldType); Parser<?> getParser(A annotation, Class<?> fieldType); }
Parameterize A to be the field annotationType you wish to associate formatting logic
with, for example org.springframework.format.annotation.DateTimeFormat
. Have
getFieldTypes()
return the types of fields the annotation may be used on. Have
getPrinter()
return a Printer to print the value of an annotated field. Have
getParser()
return a Parser to parse a clientValue for an annotated field.
The example AnnotationFormatterFactory implementation below binds the @NumberFormat Annotation to a formatter. This annotation allows either a number style or pattern to be specified:
public final class NumberFormatAnnotationFormatterFactory implements AnnotationFormatterFactory<NumberFormat> { public Set<Class<?>> getFieldTypes() { return new HashSet<Class<?>>(asList(new Class<?>[] { Short.class, Integer.class, Long.class, Float.class, Double.class, BigDecimal.class, BigInteger.class })); } public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) { return configureFormatterFrom(annotation, fieldType); } public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) { return configureFormatterFrom(annotation, fieldType); } private Formatter<Number> configureFormatterFrom(NumberFormat annotation, Class<?> fieldType) { if (!annotation.pattern().isEmpty()) { return new NumberFormatter(annotation.pattern()); } else { Style style = annotation.style(); if (style == Style.PERCENT) { return new PercentFormatter(); } else if (style == Style.CURRENCY) { return new CurrencyFormatter(); } else { return new NumberFormatter(); } } } }
To trigger formatting, simply annotate fields with @NumberFormat:
public class MyModel { @NumberFormat(style=Style.CURRENCY) private BigDecimal decimal; }
A portable format annotation API exists in the org.springframework.format.annotation
package. Use @NumberFormat to format java.lang.Number fields. Use @DateTimeFormat to
format java.util.Date, java.util.Calendar, java.util.Long, or Joda Time fields.
The example below uses @DateTimeFormat to format a java.util.Date as a ISO Date (yyyy-MM-dd):
public class MyModel { @DateTimeFormat(iso=ISO.DATE) private Date date; }
The FormatterRegistry is an SPI for registering formatters and converters.
FormattingConversionService
is an implementation of FormatterRegistry suitable for
most environments. This implementation may be configured programmatically or
declaratively as a Spring bean using FormattingConversionServiceFactoryBean
. Because
this implementation also implements ConversionService
, it can be directly configured
for use with Spring’s DataBinder and the Spring Expression Language (SpEL).
Review the FormatterRegistry SPI below:
package org.springframework.format; public interface FormatterRegistry extends ConverterRegistry { void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser); void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter); void addFormatterForFieldType(Formatter<?> formatter); void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory); }
As shown above, Formatters can be registered by fieldType or annotation.
The FormatterRegistry SPI allows you to configure Formatting rules centrally, instead of duplicating such configuration across your Controllers. For example, you might want to enforce that all Date fields are formatted a certain way, or fields with a specific annotation are formatted in a certain way. With a shared FormatterRegistry, you define these rules once and they are applied whenever formatting is needed.
The FormatterRegistrar is an SPI for registering formatters and converters through the FormatterRegistry:
package org.springframework.format; public interface FormatterRegistrar { void registerFormatters(FormatterRegistry registry); }
A FormatterRegistrar is useful when registering multiple related converters and formatters for a given formatting category, such as Date formatting. It can also be useful where declarative registration is insufficient. For example when a formatter needs to be indexed under a specific field type different from its own <T> or when registering a Printer/Parser pair. The next section provides more information on converter and formatter registration.
In a Spring MVC application, you may configure a custom ConversionService instance
explicitly as an attribute of the annotation-driven
element of the MVC namespace. This
ConversionService will then be used anytime a type conversion is required during
Controller model binding. If not configured explicitly, Spring MVC will automatically
register default formatters and converters for common types such as numbers and dates.
To rely on default formatting rules, no custom configuration is required in your Spring MVC config XML:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <mvc:annotation-driven/> </beans>
With this one-line of configuration, default formatters for Numbers and Date types will be installed, including support for the @NumberFormat and @DateTimeFormat annotations. Full support for the Joda Time formatting library is also installed if Joda Time is present on the classpath.
To inject a ConversionService instance with custom formatters and converters registered, set the conversion-service attribute and then specify custom converters, formatters, or FormatterRegistrars as properties of the FormattingConversionServiceFactoryBean:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <mvc:annotation-driven conversion-service="conversionService"/> <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean"> <property name="converters"> <set> <bean class="org.example.MyConverter"/> </set> </property> <property name="formatters"> <set> <bean class="org.example.MyFormatter"/> <bean class="org.example.MyAnnotationFormatterFactory"/> </set> </property> <property name="formatterRegistrars"> <set> <bean class="org.example.MyFormatterRegistrar"/> </set> </property> </bean> </beans>
Note | |
---|---|
See Section 6.6.4, “FormatterRegistrar SPI” and the |
By default, date and time fields that are not annotated with @DateTimeFormat
are
converted from strings using the the DateFormat.SHORT
style. If you prefer, you can
change this by defining your own global format.
You will need to ensure that Spring does not register default formatters, and instead
you should register all formatters manually. Use the
org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar
or
org.springframework.format.datetime.DateFormatterRegistrar
class depending on whether
you use the Joda Time library.
For example, the following Java configuration will register a global ' yyyyMMdd
'
format. This example does not depend on the Joda Time library:
@Configuration public class AppConfig { @Bean public FormattingConversionService conversionService() { // Use the DefaultFormattingConversionService but do not register defaults DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService(false); // Ensure @NumberFormat is still supported conversionService.addFormatterForFieldAnnotation(new NumberFormatAnnotationFormatterFactory()); // Register date conversion with a specific global format DateFormatterRegistrar registrar = new DateFormatterRegistrar(); registrar.setFormatter(new DateFormatter("yyyyMMdd")); registrar.registerFormatters(conversionService); return conversionService; } }
If you prefer XML based configuration you can use a
FormattingConversionServiceFactoryBean
. Here is the same example, this time using Joda
Time:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd> <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean"> <property name="registerDefaultFormatters" value="false" /> <property name="formatters"> <set> <bean class="org.springframework.format.number.NumberFormatAnnotationFormatterFactory" /> </set> </property> <property name="formatterRegistrars"> <set> <bean class="org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar"> <property name="dateFormatter"> <bean class="org.springframework.format.datetime.joda.DateTimeFormatterFactoryBean"> <property name="pattern" value="yyyyMMdd"/> </bean> </property> </bean> </set> </property> </bean> </beans>
Note | |
---|---|
Joda Time provides separate distinct types to represent |
If you are using Spring MVC remember to explicitly configure the conversion service that
is used. For Java based @Configuration
this means extending the
WebMvcConfigurationSupport
class and overriding the mvcConversionService()
method.
For XML you should use the 'conversion-service'
attribute of the
mvc:annotation-driven
element. See Section 6.6.5, “Configuring Formatting in Spring MVC” for details.
Spring 3 introduces several enhancements to its validation support. First, the JSR-303 Bean Validation API is now fully supported. Second, when used programmatically, Spring’s DataBinder can now validate objects as well as bind to them. Third, Spring MVC now has support for declaratively validating @Controller inputs.
JSR-303 standardizes validation constraint declaration and metadata for the Java platform. Using this API, you annotate domain model properties with declarative validation constraints and the runtime enforces them. There are a number of built-in constraints you can take advantage of. You may also define your own custom constraints.
To illustrate, consider a simple PersonForm model with two properties:
public class PersonForm { private String name; private int age; }
JSR-303 allows you to define declarative validation constraints against such properties:
public class PersonForm { @NotNull @Size(max=64) private String name; @Min(0) private int age; }
When an instance of this class is validated by a JSR-303 Validator, these constraints will be enforced.
For general information on JSR-303/JSR-349, see the Bean Validation website. For information on the specific capabilities of the default reference implementation, see the Hibernate Validator documentation. To learn how to setup a Bean Validation provider as a Spring bean, keep reading.
Spring provides full support for the Bean Validation API. This includes convenient
support for bootstrapping a JSR-303/JSR-349 Bean Validation provider as a Spring bean.
This allows for a javax.validation.ValidatorFactory
or javax.validation.Validator
to
be injected wherever validation is needed in your application.
Use the LocalValidatorFactoryBean
to configure a default Validator as a Spring bean:
<bean id="validator" class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>
The basic configuration above will trigger Bean Validation to initialize using its default bootstrap mechanism. A JSR-303/JSR-349 provider, such as Hibernate Validator, is expected to be present in the classpath and will be detected automatically.
LocalValidatorFactoryBean
implements both javax.validation.ValidatorFactory
and
javax.validation.Validator
, as well as Spring’s
org.springframework.validation.Validator
. You may inject a reference to either of
these interfaces into beans that need to invoke validation logic.
Inject a reference to javax.validation.Validator
if you prefer to work with the Bean
Validation API directly:
import javax.validation.Validator; @Service public class MyService { @Autowired private Validator validator;
Inject a reference to org.springframework.validation.Validator
if your bean requires
the Spring Validation API:
import org.springframework.validation.Validator; @Service public class MyService { @Autowired private Validator validator; }
Each Bean Validation constraint consists of two parts. First, a @Constraint annotation
that declares the constraint and its configurable properties. Second, an implementation
of the javax.validation.ConstraintValidator
interface that implements the constraint’s
behavior. To associate a declaration with an implementation, each @Constraint annotation
references a corresponding ValidationConstraint implementation class. At runtime, a
ConstraintValidatorFactory
instantiates the referenced implementation when the
constraint annotation is encountered in your domain model.
By default, the LocalValidatorFactoryBean
configures a
SpringConstraintValidatorFactory
that uses Spring to create ConstraintValidator
instances. This allows your custom ConstraintValidators to benefit from dependency
injection like any other Spring bean.
Shown below is an example of a custom @Constraint declaration, followed by an associated
ConstraintValidator
implementation that uses Spring for dependency injection:
@Target({ElementType.METHOD, ElementType.FIELD}) @Retention(RetentionPolicy.RUNTIME) @Constraint(validatedBy=MyConstraintValidator.class) public @interface MyConstraint { }
import javax.validation.ConstraintValidator; public class MyConstraintValidator implements ConstraintValidator { @Autowired; private Foo aDependency; ... }
As you can see, a ConstraintValidator implementation may have its dependencies @Autowired like any other Spring bean.
The default LocalValidatorFactoryBean
configuration should prove sufficient for most
cases. There are a number of configuration options for various Bean Validation
constructs, from message interpolation to traversal resolution. See the JavaDocs of
LocalValidatorFactoryBean
for more information on these options.
Since Spring 3, a DataBinder instance can be configured with a Validator. Once
configured, the Validator may be invoked by calling binder.validate()
. Any validation
Errors are automatically added to the binder’s BindingResult.
When working with the DataBinder programmatically, this can be used to invoke validation logic after binding to a target object:
Foo target = new Foo(); DataBinder binder = new DataBinder(target); binder.setValidator(new FooValidator()); // bind to the target object binder.bind(propertyValues); // validate the target object binder.validate(); // get BindingResult that includes any validation errors BindingResult results = binder.getBindingResult();
A DataBinder can also be configured with multiple Validator
instances via
dataBinder.addValidators
and dataBinder.replaceValidators
. This is useful when
combining globally configured Bean Validation with a Spring Validator
configured
locally on a DataBinder instance. See the section called “Configuring a Validator for use by Spring MVC”.
Beginning with Spring 3, Spring MVC has the ability to automatically validate @Controller inputs. In previous versions it was up to the developer to manually invoke validation logic.
To trigger validation of a @Controller input, simply annotate the input argument as @Valid:
@Controller public class MyController { @RequestMapping("/foo", method=RequestMethod.POST) public void processFoo(@Valid Foo foo) { /* ... */ }
Spring MVC will validate a @Valid object after binding so-long as an appropriate Validator has been configured.
Note | |
---|---|
The @Valid annotation is part of the standard JSR-303 Bean Validation API, and is not a Spring-specific construct. |
The Validator instance invoked when a @Valid method argument is encountered may be configured in two ways. First, you may call binder.setValidator(Validator) within a @Controller’s @InitBinder callback. This allows you to configure a Validator instance per @Controller class:
@Controller public class MyController { @InitBinder protected void initBinder(WebDataBinder binder) { binder.setValidator(new FooValidator()); } @RequestMapping("/foo", method=RequestMethod.POST) public void processFoo(@Valid Foo foo) { ... } }
Second, you may call setValidator(Validator) on the global WebBindingInitializer. This allows you to configure a Validator instance across all @Controllers. This can be achieved easily by using the Spring MVC namespace:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <mvc:annotation-driven validator="globalValidator"/> </beans>
To combine a global and a local validator, configure the global validator as shown above and then add a local validator:
@Controller public class MyController { @InitBinder protected void initBinder(WebDataBinder binder) { binder.addValidators(new FooValidator()); } }
With Bean Validation, a single javax.validation.Validator
instance typically validates
all model objects that declare validation constraints. To configure such a JSR-303
backed Validator with Spring MVC, simply add a Bean Validation provider, such as
Hibernate Validator, to your classpath. Spring MVC will detect it and automatically
enable Bean Validation support across all Controllers.
The Spring MVC configuration required to enable Bean Validation support is shown below:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <!-- JSR-303/JSR-349 support will be detected on classpath and enabled automatically --> <mvc:annotation-driven/> </beans>
With this minimal configuration, anytime a @Valid @Controller input is encountered, it will be validated by the Bean Validation provider. That provider, in turn, will enforce any constraints declared against the input. Any ConstraintViolations will automatically be exposed as errors in the BindingResult renderable by standard Spring MVC form tags.
The Spring Expression Language (SpEL for short) is a powerful expression language that supports querying and manipulating an object graph at runtime. The language syntax is similar to Unified EL but offers additional features, most notably method invocation and basic string templating functionality.
While there are several other Java expression languages available, OGNL, MVEL, and JBoss EL, to name a few, the Spring Expression Language was created to provide the Spring community with a single well supported expression language that can be used across all the products in the Spring portfolio. Its language features are driven by the requirements of the projects in the Spring portfolio, including tooling requirements for code completion support within the eclipse based SpringSource Tool Suite. That said, SpEL is based on a technology agnostic API allowing other expression language implementations to be integrated should the need arise.
While SpEL serves as the foundation for expression evaluation within the Spring portfolio, it is not directly tied to Spring and can be used independently. In order to be self contained, many of the examples in this chapter use SpEL as if it were an independent expression language. This requires creating a few bootstrapping infrastructure classes such as the parser. Most Spring users will not need to deal with this infrastructure and will instead only author expression strings for evaluation. An example of this typical use is the integration of SpEL into creating XML or annotated based bean definitions as shown in the section Expression support for defining bean definitions.
This chapter covers the features of the expression language, its API, and its language syntax. In several places an Inventor and Inventor’s Society class are used as the target objects for expression evaluation. These class declarations and the data used to populate them are listed at the end of the chapter.
The expression language supports the following functionality
This section introduces the simple use of SpEL interfaces and its expression language. The complete language reference can be found in the section Language Reference.
The following code introduces the SpEL API to evaluate the literal string expression Hello World.
ExpressionParser parser = new SpelExpressionParser(); Expression exp = parser.parseExpression("'Hello World'"); String message = (String) exp.getValue();
The value of the message variable is simply Hello World.
The SpEL classes and interfaces you are most likely to use are located in the packages
org.springframework.expression
and its sub packages and spel.support
.
The interface ExpressionParser
is responsible for parsing an expression string. In
this example the expression string is a string literal denoted by the surrounding single
quotes. The interface Expression
is responsible for evaluating the previously defined
expression string. There are two exceptions that can be thrown, ParseException
and
EvaluationException
when calling ' parser.parseExpression
' and ' exp.getValue
'
respectively.
SpEL supports a wide range of features, such as calling methods, accessing properties, and calling constructors.
As an example of method invocation, we call the concat method on the string literal.
ExpressionParser parser = new SpelExpressionParser(); Expression exp = parser.parseExpression("'Hello World'.concat('!')"); String message = (String) exp.getValue();
The value of message is now Hello World!.
As an example of calling a JavaBean property, the String property Bytes can be called as shown below.
ExpressionParser parser = new SpelExpressionParser(); // invokes getBytes() Expression exp = parser.parseExpression("'Hello World'.bytes"); byte[] bytes = (byte[]) exp.getValue();
SpEL also supports nested properties using standard dot notation, i.e. prop1.prop2.prop3 and the setting of property values
Public fields may also be accessed.
ExpressionParser parser = new SpelExpressionParser(); // invokes getBytes().length Expression exp = parser.parseExpression("'Hello World'.bytes.length"); int length = (Integer) exp.getValue();
The String’s constructor can be called instead of using a string literal.
ExpressionParser parser = new SpelExpressionParser(); Expression exp = parser.parseExpression("new String('hello world').toUpperCase()"); String message = exp.getValue(String.class);
Note the use of the generic method public <T> T getValue(Class<T> desiredResultType)
.
Using this method removes the need to cast the value of the expression to the desired
result type. An EvaluationException
will be thrown if the value cannot be cast to the
type T
or converted using the registered type converter.
The more common usage of SpEL is to provide an expression string that is evaluated
against a specific object instance (called the root object). There are two options here
and which to choose depends on whether the object against which the expression is being
evaluated will be changing with each call to evaluate the expression. In the following
example we retrieve the name
property from an instance of the Inventor class.
// Create and set a calendar GregorianCalendar c = new GregorianCalendar(); c.set(1856, 7, 9); // The constructor arguments are name, birthday, and nationality. Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian"); ExpressionParser parser = new SpelExpressionParser(); Expression exp = parser.parseExpression("name"); EvaluationContext context = new StandardEvaluationContext(tesla); String name = (String) exp.getValue(context);
In the last line, the value of the string variable name will be set to "Nikola Tesla".
The class StandardEvaluationContext is where you can specify which object the "name"
property will be evaluated against. This is the mechanism to use if the root object is
unlikely to change, it can simply be set once in the evaluation context. If the root
object is likely to change repeatedly, it can be supplied on each call to getValue
, as
this next example shows:
/ Create and set a calendar GregorianCalendar c = new GregorianCalendar(); c.set(1856, 7, 9); // The constructor arguments are name, birthday, and nationality. Inventor tesla = new Inventor("Nikola Tesla", c.getTime(), "Serbian"); ExpressionParser parser = new SpelExpressionParser(); Expression exp = parser.parseExpression("name"); String name = (String) exp.getValue(tesla);
In this case the inventor tesla
has been supplied directly to getValue
and the
expression evaluation infrastructure creates and manages a default evaluation context
internally - it did not require one to be supplied.
The StandardEvaluationContext is relatively expensive to construct and during repeated usage it builds up cached state that enables subsequent expression evaluations to be performed more quickly. For this reason it is better to cache and reuse them where possible, rather than construct a new one for each expression evaluation.
In some cases it can be desirable to use a configured evaluation context and yet still
supply a different root object on each call to getValue
. getValue
allows both to be
specified on the same call. In these situations the root object passed on the call is
considered to override any (which maybe null) specified on the evaluation context.
Note | |
---|---|
In standalone usage of SpEL there is a need to create the parser, parse expressions and perhaps provide evaluation contexts and a root context object. However, more common usage is to provide only the SpEL expression string as part of a configuration file, for example for Spring bean or Spring Web Flow definitions. In this case, the parser, evaluation context, root object and any predefined variables are all set up implicitly, requiring the user to specify nothing other than the expressions. |
As a final introductory example, the use of a boolean operator is shown using the Inventor object in the previous example.
Expression exp = parser.parseExpression("name == Nikola Tesla"); boolean result = exp.getValue(context, Boolean.class); // evaluates to true
The interface EvaluationContext
is used when evaluating an expression to resolve
properties, methods, fields, and to help perform type conversion. The out-of-the-box
implementation, StandardEvaluationContext
, uses reflection to manipulate the object,
caching java.lang.reflect
's Method
, Field
, and Constructor
instances for
increased performance.
The StandardEvaluationContext
is where you may specify the root object to evaluate
against via the method setRootObject()
or passing the root object into the
constructor. You can also specify variables and functions that will be used in the
expression using the methods setVariable()
and registerFunction()
. The use of
variables and functions are described in the language reference sections
Variables and Functions. The
StandardEvaluationContext
is also where you can register custom
ConstructorResolver
s, MethodResolver
s, and PropertyAccessor
s to extend how SpEL
evaluates expressions. Please refer to the JavaDoc of these classes for more details.
By default SpEL uses the conversion service available in Spring core (
org.springframework.core.convert.ConversionService
). This conversion service comes
with many converters built in for common conversions but is also fully extensible so
custom conversions between types can be added. Additionally it has the key capability
that it is generics aware. This means that when working with generic types in
expressions, SpEL will attempt conversions to maintain type correctness for any objects
it encounters.
What does this mean in practice? Suppose assignment, using setValue()
, is being used
to set a List
property. The type of the property is actually List<Boolean>
. SpEL
will recognize that the elements of the list need to be converted to Boolean
before
being placed in it. A simple example:
class Simple { public List<Boolean> booleanList = new ArrayList<Boolean>(); } Simple simple = new Simple(); simple.booleanList.add(true); StandardEvaluationContext simpleContext = new StandardEvaluationContext(simple); // false is passed in here as a string. SpEL and the conversion service will // correctly recognize that it needs to be a Boolean and convert it parser.parseExpression("booleanList[0]").setValue(simpleContext, "false"); // b will be false Boolean b = simple.booleanList.get(0);
SpEL expressions can be used with XML or annotation based configuration metadata for
defining BeanDefinitions. In both cases the syntax to define the expression is of the
form #{ <expression string> }
.
A property or constructor-arg value can be set using expressions as shown below
<bean id="numberGuess" class="org.spring.samples.NumberGuess"> <property name="randomNumber" value="#{ T(java.lang.Math).random() * 100.0 }"/> <!-- other properties --> </bean>
The variable systemProperties is predefined, so you can use it in your expressions as shown below. Note that you do not have to prefix the predefined variable with the # symbol in this context.
<bean id="taxCalculator" class="org.spring.samples.TaxCalculator"> <property name="defaultLocale" value="#{ systemProperties[user.region] }"/> <!-- other properties --> </bean>
You can also refer to other bean properties by name, for example.
<bean id="numberGuess" class="org.spring.samples.NumberGuess"> <property name="randomNumber" value="{ T(java.lang.Math).random() * 100.0 }"/> <!-- other properties --> </bean> <bean id="shapeGuess" class="org.spring.samples.ShapeGuess"> <property name="initialShapeSeed" value="{ numberGuess.randomNumber }"/> <!-- other properties --> </bean>
The @Value
annotation can be placed on fields, methods and method/constructor
parameters to specify a default value.
Here is an example to set the default value of a field variable.
public static class FieldValueTestBean @Value("#{ systemProperties[user.region] }") private String defaultLocale; public void setDefaultLocale(String defaultLocale) { this.defaultLocale = defaultLocale; } public String getDefaultLocale() { return this.defaultLocale; } }
The equivalent but on a property setter method is shown below.
public static class PropertyValueTestBean private String defaultLocale; @Value("#{ systemProperties[user.region] }") public void setDefaultLocale(String defaultLocale) { this.defaultLocale = defaultLocale; } public String getDefaultLocale() { return this.defaultLocale; } }
Autowired methods and constructors can also use the @Value
annotation.
public class SimpleMovieLister { private MovieFinder movieFinder; private String defaultLocale; @Autowired public void configure(MovieFinder movieFinder, @Value("#{ systemProperties[user.region] }") String defaultLocale) { this.movieFinder = movieFinder; this.defaultLocale = defaultLocale; } // ... }
public class MovieRecommender { private String defaultLocale; private CustomerPreferenceDao customerPreferenceDao; @Autowired public MovieRecommender(CustomerPreferenceDao customerPreferenceDao, @Value("#{systemProperties[user.country]}") String defaultLocale) { this.customerPreferenceDao = customerPreferenceDao; this.defaultLocale = defaultLocale; } // ... }
The types of literal expressions supported are strings, dates, numeric values (int, real, and hex), boolean and null. Strings are delimited by single quotes. To put a single quote itself in a string use two single quote characters. The following listing shows simple usage of literals. Typically they would not be used in isolation like this, but as part of a more complex expression, for example using a literal on one side of a logical comparison operator.
ExpressionParser parser = new SpelExpressionParser(); // evals to "Hello World" String helloWorld = (String) parser.parseExpression("Hello World").getValue(); double avogadrosNumber = (Double) parser.parseExpression("6.0221415E+23").getValue(); // evals to 2147483647 int maxValue = (Integer) parser.parseExpression("0x7FFFFFFF").getValue(); boolean trueValue = (Boolean) parser.parseExpression("true").getValue(); Object nullValue = parser.parseExpression("null").getValue();
Numbers support the use of the negative sign, exponential notation, and decimal points. By default real numbers are parsed using Double.parseDouble().
Navigating with property references is easy, just use a period to indicate a nested property value. The instances of Inventor class, pupin and tesla, were populated with data listed in the section Classes used in the examples. To navigate "down" and get Tesla’s year of birth and Pupin’s city of birth the following expressions are used.
// evals to 1856 int year = (Integer) parser.parseExpression("Birthdate.Year + 1900").getValue(context); String city = (String) parser.parseExpression("placeOfBirth.City").getValue(context);
Case insensitivity is allowed for the first letter of property names. The contents of arrays and lists are obtained using square bracket notation.
ExpressionParser parser = new SpelExpressionParser(); // Inventions Array StandardEvaluationContext teslaContext = new StandardEvaluationContext(tesla); // evaluates to "Induction motor" String invention = parser.parseExpression("inventions[3]").getValue( teslaContext, String.class); // Members List StandardEvaluationContext societyContext = new StandardEvaluationContext(ieee); // evaluates to "Nikola Tesla" String name = parser.parseExpression("Members[0].Name").getValue( societyContext, String.class); // List and Array navigation // evaluates to "Wireless communication" String invention = parser.parseExpression("Members[0].Inventions[6]").getValue( societyContext, String.class);
The contents of maps are obtained by specifying the literal key value within the brackets. In this case, because keys for the Officers map are strings, we can specify string literals.
// Officer's Dictionary Inventor pupin = parser.parseExpression("Officers[president]").getValue( societyContext, Inventor.class); // evaluates to "Idvor" String city = parser.parseExpression("Officers[president].PlaceOfBirth.City").getValue( societyContext, String.class); // setting values parser.parseExpression("Officers[advisors][0].PlaceOfBirth.Country").setValue( societyContext, "Croatia");
Lists can be expressed directly in an expression using {} notation.
// evaluates to a Java list containing the four numbers List numbers = (List) parser.parseExpression("{1,2,3,4}").getValue(context); List listOfLists = (List) parser.parseExpression("{{a,b},{x,y}}").getValue(context);
{} by itself means an empty list. For performance reasons, if the list is itself entirely composed of fixed literals then a constant list is created to represent the expression, rather than building a new list on each evaluation.
Arrays can be built using the familiar Java syntax, optionally supplying an initializer to have the array populated at construction time.
int[] numbers1 = (int[]) parser.parseExpression("new int[4]").getValue(context); // Array with initializer int[] numbers2 = (int[]) parser.parseExpression("new int[]{1,2,3}").getValue(context); // Multi dimensional array int[][] numbers3 = (int[][]) parser.parseExpression("new int[4][5]").getValue(context);
It is not currently allowed to supply an initializer when constructing a multi-dimensional array.
Methods are invoked using typical Java programming syntax. You may also invoke methods on literals. Varargs are also supported.
// string literal, evaluates to "bc" String c = parser.parseExpression("abc.substring(2, 3)").getValue(String.class); // evaluates to true boolean isMember = parser.parseExpression("isMember(Mihajlo Pupin)").getValue( societyContext, Boolean.class);
The relational operators; equal, not equal, less than, less than or equal, greater than, and greater than or equal are supported using standard operator notation.
// evaluates to true boolean trueValue = parser.parseExpression("2 == 2").getValue(Boolean.class); // evaluates to false boolean falseValue = parser.parseExpression("2 < -5.0").getValue(Boolean.class); // evaluates to true boolean trueValue = parser.parseExpression("black < block").getValue(Boolean.class);
In addition to standard relational operators SpEL supports the instanceof and regular expression based matches operator.
// evaluates to false boolean falseValue = parser.parseExpression( "'xyz' instanceof T(int)").getValue(Boolean.class); // evaluates to true boolean trueValue = parser.parseExpression( "'5.00' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class); //evaluates to false boolean falseValue = parser.parseExpression( "'5.0067' matches '^-?\\d+(\\.\\d{2})?$'").getValue(Boolean.class);
Each symbolic operator can also be specified as a purely alphabetic equivalent. This avoids problems where the symbols used have special meaning for the document type in which the expression is embedded (eg. an XML document). The textual equivalents are shown here: lt (<), gt (>), le (⇐), ge (>=), eq (==), ne (!=), div (/), mod (%), not (!). These are case insensitive.
The logical operators that are supported are and, or, and not. Their use is demonstrated below.
// -- AND -- // evaluates to false boolean falseValue = parser.parseExpression("true and false").getValue(Boolean.class); // evaluates to true String expression = "isMember(Nikola Tesla) and isMember(Mihajlo Pupin)"; boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class); // -- OR -- // evaluates to true boolean trueValue = parser.parseExpression("true or false").getValue(Boolean.class); // evaluates to true String expression = "isMember(Nikola Tesla) or isMember(Albert Einstein)"; boolean trueValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class); // -- NOT -- // evaluates to false boolean falseValue = parser.parseExpression("!true").getValue(Boolean.class); // -- AND and NOT -- String expression = "isMember(Nikola Tesla) and !isMember(Mihajlo Pupin)"; boolean falseValue = parser.parseExpression(expression).getValue(societyContext, Boolean.class);
The addition operator can be used on both numbers and strings. Subtraction, multiplication and division can be used only on numbers. Other mathematical operators supported are modulus (%) and exponential power (^). Standard operator precedence is enforced. These operators are demonstrated below.
// Addition int two = parser.parseExpression("1 + 1").getValue(Integer.class); // 2 String testString = parser.parseExpression( "test + ' ' + string").getValue(String.class); // test string // Subtraction int four = parser.parseExpression("1 - -3").getValue(Integer.class); // 4 double d = parser.parseExpression("1000.00 - 1e4").getValue(Double.class); // -9000 // Multiplication int six = parser.parseExpression("-2 * -3").getValue(Integer.class); // 6 double twentyFour = parser.parseExpression("2.0 * 3e0 * 4").getValue(Double.class); // 24.0 // Division int minusTwo = parser.parseExpression("6 / -3").getValue(Integer.class); // -2 double one = parser.parseExpression("8.0 / 4e0 / 2").getValue(Double.class); // 1.0 // Modulus int three = parser.parseExpression("7 % 4").getValue(Integer.class); // 3 int one = parser.parseExpression("8 / 5 % 2").getValue(Integer.class); // 1 // Operator precedence int minusTwentyOne = parser.parseExpression("1+2-3*8").getValue(Integer.class); // -21
Setting of a property is done by using the assignment operator. This would typically be
done within a call to setValue
but can also be done inside a call to getValue
.
Inventor inventor = new Inventor(); StandardEvaluationContext inventorContext = new StandardEvaluationContext(inventor); parser.parseExpression("Name").setValue(inventorContext, "Alexander Seovic2"); // alternatively String aleks = parser.parseExpression( "Name = Alexandar Seovic").getValue(inventorContext, String.class);
The special T operator can be used to specify an instance of java.lang.Class (the
type). Static methods are invoked using this operator as well. The
StandardEvaluationContext
uses a TypeLocator
to find types and the
StandardTypeLocator
(which can be replaced) is built with an understanding of the
java.lang package. This means T() references to types within java.lang do not need to be
fully qualified, but all other type references must be.
Class dateClass = parser.parseExpression("T(java.util.Date)").getValue(Class.class); Class stringClass = parser.parseExpression("T(String)").getValue(Class.class); boolean trueValue = parser.parseExpression( "T(java.math.RoundingMode).CEILING < T(java.math.RoundingMode).FLOOR") .getValue(Boolean.class);
Constructors can be invoked using the new operator. The fully qualified class name should be used for all but the primitive type and String (where int, float, etc, can be used).
Inventor einstein = p.parseExpression( "new org.spring.samples.spel.inventor.Inventor(Albert Einstein, German)") .getValue(Inventor.class); //create new inventor instance within add method of List p.parseExpression( "Members.add(new org.spring.samples.spel.inventor.Inventor( Albert Einstein, German))").getValue(societyContext);
Variables can be referenced in the expression using the syntax #variableName. Variables are set using the method setVariable on the StandardEvaluationContext.
Inventor tesla = new Inventor("Nikola Tesla", "Serbian"); StandardEvaluationContext context = new StandardEvaluationContext(tesla); context.setVariable("newName", "Mike Tesla"); parser.parseExpression("Name = #newName").getValue(context); System.out.println(tesla.getName()) // "Mike Tesla"
The variable #this is always defined and refers to the current evaluation object (against which unqualified references are resolved). The variable #root is always defined and refers to the root context object. Although #this may vary as components of an expression are evaluated, #root always refers to the root.
// create an array of integers List<Integer> primes = new ArrayList<Integer>(); primes.addAll(Arrays.asList(2,3,5,7,11,13,17)); // create parser and set variable primes as the array of integers ExpressionParser parser = new SpelExpressionParser(); StandardEvaluationContext context = new StandardEvaluationContext(); context.setVariable("primes",primes); // all prime numbers > 10 from the list (using selection ?{...}) // evaluates to [11, 13, 17] List<Integer> primesGreaterThanTen = (List<Integer>) parser.parseExpression( "#primes.?[#this>10]").getValue(context);
You can extend SpEL by registering user defined functions that can be called within the
expression string. The function is registered with the StandardEvaluationContext
using
the method.
public void registerFunction(String name, Method m)
A reference to a Java Method provides the implementation of the function. For example, a utility method to reverse a string is shown below.
public abstract class StringUtils { public static String reverseString(String input) { StringBuilder backwards = new StringBuilder(); for (int i = 0; i < input.length(); i++) backwards.append(input.charAt(input.length() - 1 - i)); } return backwards.toString(); } }
This method is then registered with the evaluation context and can be used within an expression string.
ExpressionParser parser = new SpelExpressionParser(); StandardEvaluationContext context = new StandardEvaluationContext(); context.registerFunction("reverseString", StringUtils.class.getDeclaredMethod("reverseString", new Class[] { String.class })); String helloWorldReversed = parser.parseExpression( "#reverseString(hello)").getValue(context, String.class);
If the evaluation context has been configured with a bean resolver it is possible to lookup beans from an expression using the (@) symbol.
ExpressionParser parser = new SpelExpressionParser(); StandardEvaluationContext context = new StandardEvaluationContext(); context.setBeanResolver(new MyBeanResolver()); // This will end up calling resolve(context,"foo") on MyBeanResolver during evaluation Object bean = parser.parseExpression("@foo").getValue(context);
You can use the ternary operator for performing if-then-else conditional logic inside the expression. A minimal example is:
String falseString = parser.parseExpression( "false ? trueExp : falseExp").getValue(String.class);
In this case, the boolean false results in returning the string value falseExp. A more realistic example is shown below.
parser.parseExpression("Name").setValue(societyContext, "IEEE"); societyContext.setVariable("queryName", "Nikola Tesla"); expression = "isMember(#queryName)? #queryName + ' is a member of the ' " + "+ Name + ' Society' : #queryName + ' is not a member of the ' + Name + ' Society'"; String queryResultString = parser.parseExpression(expression) .getValue(societyContext, String.class); // queryResultString = "Nikola Tesla is a member of the IEEE Society"
Also see the next section on the Elvis operator for an even shorter syntax for the ternary operator.
The Elvis operator is a shortening of the ternary operator syntax and is used in the Groovy language. With the ternary operator syntax you usually have to repeat a variable twice, for example:
String name = "Elvis Presley"; String displayName = name != null ? name : "Unknown";
Instead you can use the Elvis operator, named for the resemblance to Elvis' hair style.
ExpressionParser parser = new SpelExpressionParser(); String name = parser.parseExpression("null?:'Unknown'").getValue(String.class); System.out.println(name); // Unknown
Here is a more complex example.
ExpressionParser parser = new SpelExpressionParser(); Inventor tesla = new Inventor("Nikola Tesla", "Serbian"); StandardEvaluationContext context = new StandardEvaluationContext(tesla); String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class); System.out.println(name); // Nikola Tesla tesla.setName(null); name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class); System.out.println(name); // Elvis Presley
The Safe Navigation operator is used to avoid a NullPointerException
and comes from
the Groovy
language. Typically when you have a reference to an object you might need to verify that
it is not null before accessing methods or properties of the object. To avoid this, the
safe navigation operator will simply return null instead of throwing an exception.
ExpressionParser parser = new SpelExpressionParser(); Inventor tesla = new Inventor("Nikola Tesla", "Serbian"); tesla.setPlaceOfBirth(new PlaceOfBirth("Smiljan")); StandardEvaluationContext context = new StandardEvaluationContext(tesla); String city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class); System.out.println(city); // Smiljan tesla.setPlaceOfBirth(null); city = parser.parseExpression("PlaceOfBirth?.City").getValue(context, String.class); System.out.println(city); // null - does not throw NullPointerException!!!
Note | |
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The Elvis operator can be used to apply default values in expressions, e.g. in an
@Value("#{systemProperties[pop3.port] ?: 25}") This will inject a system property |
Selection is a powerful expression language feature that allows you to transform some source collection into another by selecting from its entries.
Selection uses the syntax ?[selectionExpression]
. This will filter the collection and
return a new collection containing a subset of the original elements. For example,
selection would allow us to easily get a list of Serbian inventors:
List<Inventor> list = (List<Inventor>) parser.parseExpression( "Members.?[Nationality == Serbian]").getValue(societyContext);
Selection is possible upon both lists and maps. In the former case the selection
criteria is evaluated against each individual list element whilst against a map the
selection criteria is evaluated against each map entry (objects of the Java type
Map.Entry
). Map entries have their key and value accessible as properties for use in
the selection.
This expression will return a new map consisting of those elements of the original map where the entry value is less than 27.
Map newMap = parser.parseExpression("map.?[value<27]").getValue();
In addition to returning all the selected elements, it is possible to retrieve just the
first or the last value. To obtain the first entry matching the selection the syntax is
^[...]
whilst to obtain the last matching selection the syntax is $[...]
.
Projection allows a collection to drive the evaluation of a sub-expression and the
result is a new collection. The syntax for projection is ![projectionExpression]
. Most
easily understood by example, suppose we have a list of inventors but want the list of
cities where they were born. Effectively we want to evaluate placeOfBirth.city for
every entry in the inventor list. Using projection:
// returns [Smiljan, Idvor ] List placesOfBirth = (List)parser.parseExpression("Members.![placeOfBirth.city]");
A map can also be used to drive projection and in this case the projection expression is
evaluated against each entry in the map (represented as a Java Map.Entry
). The result
of a projection across a map is a list consisting of the evaluation of the projection
expression against each map entry.
Expression templates allow a mixing of literal text with one or more evaluation blocks.
Each evaluation block is delimited with prefix and suffix characters that you can
define, a common choice is to use #{ }
as the delimiters. For example,
String randomPhrase = parser.parseExpression( "random number is #{T(java.lang.Math).random()}", new TemplateParserContext()).getValue(String.class); // evaluates to "random number is 0.7038186818312008"
The string is evaluated by concatenating the literal text 'random number is ' with the
result of evaluating the expression inside the #{ } delimiter, in this case the result
of calling that random() method. The second argument to the method parseExpression()
is of the type ParserContext
. The ParserContext
interface is used to influence how
the expression is parsed in order to support the expression templating functionality.
The definition of TemplateParserContext
is shown below.
public class TemplateParserContext implements ParserContext { public String getExpressionPrefix() { return "#{"; } public String getExpressionSuffix() { return "}"; } public boolean isTemplate() { return true; } }
Inventor.java
package org.spring.samples.spel.inventor; import java.util.Date; import java.util.GregorianCalendar; public class Inventor { private String name; private String nationality; private String[] inventions; private Date birthdate; private PlaceOfBirth placeOfBirth; public Inventor(String name, String nationality) { GregorianCalendar c= new GregorianCalendar(); this.name = name; this.nationality = nationality; this.birthdate = c.getTime(); } public Inventor(String name, Date birthdate, String nationality) { this.name = name; this.nationality = nationality; this.birthdate = birthdate; } public Inventor() { } public String getName() { return name; } public void setName(String name) { this.name = name; } public String getNationality() { return nationality; } public void setNationality(String nationality) { this.nationality = nationality; } public Date getBirthdate() { return birthdate; } public void setBirthdate(Date birthdate) { this.birthdate = birthdate; } public PlaceOfBirth getPlaceOfBirth() { return placeOfBirth; } public void setPlaceOfBirth(PlaceOfBirth placeOfBirth) { this.placeOfBirth = placeOfBirth; } public void setInventions(String[] inventions) { this.inventions = inventions; } public String[] getInventions() { return inventions; } }
PlaceOfBirth.java
package org.spring.samples.spel.inventor; public class PlaceOfBirth { private String city; private String country; public PlaceOfBirth(String city) { this.city=city; } public PlaceOfBirth(String city, String country) { this(city); this.country = country; } public String getCity() { return city; } public void setCity(String s) { this.city = s; } public String getCountry() { return country; } public void setCountry(String country) { this.country = country; } }
Society.java
package org.spring.samples.spel.inventor; import java.util.*; public class Society { private String name; public static String Advisors = "advisors"; public static String President = "president"; private List<Inventor> members = new ArrayList<Inventor>(); private Map officers = new HashMap(); public List getMembers() { return members; } public Map getOfficers() { return officers; } public String getName() { return name; } public void setName(String name) { this.name = name; } public boolean isMember(String name) { for (Inventor inventor : members) { if (inventor.getName().equals(name)) { return true; } } return false; } }
Aspect-Oriented Programming (AOP) complements Object-Oriented Programming (OOP) by providing another way of thinking about program structure. The key unit of modularity in OOP is the class, whereas in AOP the unit of modularity is the aspect. Aspects enable the modularization of concerns such as transaction management that cut across multiple types and objects. (Such concerns are often termed crosscutting concerns in AOP literature.)
One of the key components of Spring is the AOP framework. While the Spring IoC container does not depend on AOP, meaning you do not need to use AOP if you don’t want to, AOP complements Spring IoC to provide a very capable middleware solution.
AOP is used in the Spring Framework to…
Note | |
---|---|
If you are interested only in generic declarative services or other pre-packaged declarative middleware services such as pooling, you do not need to work directly with Spring AOP, and can skip most of this chapter. |
Let us begin by defining some central AOP concepts and terminology. These terms are not Spring-specific… unfortunately, AOP terminology is not particularly intuitive; however, it would be even more confusing if Spring used its own terminology.
@Aspect
annotation (the @AspectJ
style).
IsModified
interface, to simplify caching. (An introduction is known as an
inter-type declaration in the AspectJ community.)
Types of advice:
Around advice is the most general kind of advice. Since Spring AOP, like AspectJ,
provides a full range of advice types, we recommend that you use the least powerful
advice type that can implement the required behavior. For example, if you need only to
update a cache with the return value of a method, you are better off implementing an
after returning advice than an around advice, although an around advice can accomplish
the same thing. Using the most specific advice type provides a simpler programming model
with less potential for errors. For example, you do not need to invoke the proceed()
method on the JoinPoint
used for around advice, and hence cannot fail to invoke it.
In Spring 2.0, all advice parameters are statically typed, so that you work with advice
parameters of the appropriate type (the type of the return value from a method execution
for example) rather than Object
arrays.
The concept of join points, matched by pointcuts, is the key to AOP which distinguishes it from older technologies offering only interception. Pointcuts enable advice to be targeted independently of the Object-Oriented hierarchy. For example, an around advice providing declarative transaction management can be applied to a set of methods spanning multiple objects (such as all business operations in the service layer).
Spring AOP is implemented in pure Java. There is no need for a special compilation process. Spring AOP does not need to control the class loader hierarchy, and is thus suitable for use in a Servlet container or application server.
Spring AOP currently supports only method execution join points (advising the execution of methods on Spring beans). Field interception is not implemented, although support for field interception could be added without breaking the core Spring AOP APIs. If you need to advise field access and update join points, consider a language such as AspectJ.
Spring AOP’s approach to AOP differs from that of most other AOP frameworks. The aim is not to provide the most complete AOP implementation (although Spring AOP is quite capable); it is rather to provide a close integration between AOP implementation and Spring IoC to help solve common problems in enterprise applications.
Thus, for example, the Spring Framework’s AOP functionality is normally used in conjunction with the Spring IoC container. Aspects are configured using normal bean definition syntax (although this allows powerful "autoproxying" capabilities): this is a crucial difference from other AOP implementations. There are some things you cannot do easily or efficiently with Spring AOP, such as advise very fine-grained objects (such as domain objects typically): AspectJ is the best choice in such cases. However, our experience is that Spring AOP provides an excellent solution to most problems in enterprise Java applications that are amenable to AOP.
Spring AOP will never strive to compete with AspectJ to provide a comprehensive AOP solution. We believe that both proxy-based frameworks like Spring AOP and full-blown frameworks such as AspectJ are valuable, and that they are complementary, rather than in competition. Spring seamlessly integrates Spring AOP and IoC with AspectJ, to enable all uses of AOP to be catered for within a consistent Spring-based application architecture. This integration does not affect the Spring AOP API or the AOP Alliance API: Spring AOP remains backward-compatible. See the following chapter for a discussion of the Spring AOP APIs.
Note | |
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One of the central tenets of the Spring Framework is that of non-invasiveness; this is the idea that you should not be forced to introduce framework-specific classes and interfaces into your business/domain model. However, in some places the Spring Framework does give you the option to introduce Spring Framework-specific dependencies into your codebase: the rationale in giving you such options is because in certain scenarios it might be just plain easier to read or code some specific piece of functionality in such a way. The Spring Framework (almost) always offers you the choice though: you have the freedom to make an informed decision as to which option best suits your particular use case or scenario. One such choice that is relevant to this chapter is that of which AOP framework (and which AOP style) to choose. You have the choice of AspectJ and/or Spring AOP, and you also have the choice of either the @AspectJ annotation-style approach or the Spring XML configuration-style approach. The fact that this chapter chooses to introduce the @AspectJ-style approach first should not be taken as an indication that the Spring team favors the @AspectJ annotation-style approach over the Spring XML configuration-style. See Section 8.4, “Choosing which AOP declaration style to use” for a more complete discussion of the whys and wherefores of each style. |
Spring AOP defaults to using standard J2SE dynamic proxies for AOP proxies. This enables any interface (or set of interfaces) to be proxied.
Spring AOP can also use CGLIB proxies. This is necessary to proxy classes, rather than interfaces. CGLIB is used by default if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally will implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface, or where you need to pass a proxied object to a method as a concrete type.
It is important to grasp the fact that Spring AOP is proxy-based. See Section 8.6.1, “Understanding AOP proxies” for a thorough examination of exactly what this implementation detail actually means.
@AspectJ refers to a style of declaring aspects as regular Java classes annotated with annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring interprets the same annotations as AspectJ 5, using a library supplied by AspectJ for pointcut parsing and matching. The AOP runtime is still pure Spring AOP though, and there is no dependency on the AspectJ compiler or weaver.
Note | |
---|---|
Using the AspectJ compiler and weaver enables use of the full AspectJ language, and is discussed in Section 8.8, “Using AspectJ with Spring applications”. |
To use @AspectJ aspects in a Spring configuration you need to enable Spring support for configuring Spring AOP based on @AspectJ aspects, and autoproxying beans based on whether or not they are advised by those aspects. By autoproxying we mean that if Spring determines that a bean is advised by one or more aspects, it will automatically generate a proxy for that bean to intercept method invocations and ensure that advice is executed as needed.
The @AspectJ support can be enabled with XML or Java style configuration. In either
case you will also need to ensure that AspectJ’s aspectjweaver.jar
library is on the
classpath of your application (version 1.6.8 or later). This library is available in the
'lib'
directory of an AspectJ distribution or via the Maven Central repository.
To enable @AspectJ support with Java @Configuration
add the @EnableAspectJAutoProxy
annotation:
@Configuration @EnableAspectJAutoProxy public class AppConfig { }
To enable @AspectJ support with XML based configuration use the aop:aspectj-autoproxy
element:
<aop:aspectj-autoproxy/>
This assumes that you are using schema support as described in Chapter 33, XML Schema-based configuration. See Section 33.2.7, “the aop schema” for how to import the tags in the aop namespace.
With the @AspectJ support enabled, any bean defined in your application context with a
class that is an @AspectJ aspect (has the @Aspect
annotation) will be automatically
detected by Spring and used to configure Spring AOP. The following example shows the
minimal definition required for a not-very-useful aspect:
A regular bean definition in the application context, pointing to a bean class that has
the @Aspect
annotation:
<bean id="myAspect" class="org.xyz.NotVeryUsefulAspect"> <!-- configure properties of aspect here as normal --> </bean>
And the NotVeryUsefulAspect
class definition, annotated with
org.aspectj.lang.annotation.Aspect
annotation;
package org.xyz; import org.aspectj.lang.annotation.Aspect; @Aspect public class NotVeryUsefulAspect { }
Aspects (classes annotated with @Aspect
) may have methods and fields just like any
other class. They may also contain pointcut, advice, and introduction (inter-type)
declarations.
Autodetecting aspects through component scanning | |
---|---|
You may register aspect classes as regular beans in your Spring XML configuration, or autodetect them through classpath scanning - just like any other Spring-managed bean. However, note that the @Aspect annotation is not sufficient for autodetection in the classpath: For that purpose, you need to add a separate @Component annotation (or alternatively a custom stereotype annotation that qualifies, as per the rules of Spring’s component scanner). |
Advising aspects with other aspects? | |
---|---|
In Spring AOP, it is not possible to have aspects themselves be the target of advice from other aspects. The @Aspect annotation on a class marks it as an aspect, and hence excludes it from auto-proxying. |
Recall that pointcuts determine join points of interest, and thus enable us to control
when advice executes. Spring AOP only supports method execution join points for Spring
beans, so you can think of a pointcut as matching the execution of methods on Spring
beans. A pointcut declaration has two parts: a signature comprising a name and any
parameters, and a pointcut expression that determines exactly which method
executions we are interested in. In the @AspectJ annotation-style of AOP, a pointcut
signature is provided by a regular method definition, and the pointcut expression is
indicated using the @Pointcut
annotation (the method serving as the pointcut signature
must have a void
return type).
An example will help make this distinction between a pointcut signature and a pointcut
expression clear. The following example defines a pointcut named 'anyOldTransfer'
that
will match the execution of any method named 'transfer'
:
@Pointcut("execution(* transfer(..))")// the pointcut expression private void anyOldTransfer() {}// the pointcut signature
The pointcut expression that forms the value of the @Pointcut
annotation is a regular
AspectJ 5 pointcut expression. For a full discussion of AspectJ’s pointcut language, see
the AspectJ
Programming Guide (and for extensions, the
AspectJ 5
Developers Notebook) or one of the books on AspectJ such as "Eclipse AspectJ" by Colyer
et. al. or "AspectJ in Action" by Ramnivas Laddad.
Spring AOP supports the following AspectJ pointcut designators (PCD) for use in pointcut expressions:
Because Spring AOP limits matching to only method execution join points, the discussion
of the pointcut designators above gives a narrower definition than you will find in the
AspectJ programming guide. In addition, AspectJ itself has type-based semantics and at
an execution join point both ' this
' and ' target
' refer to the same object - the
object executing the method. Spring AOP is a proxy-based system and differentiates
between the proxy object itself (bound to ' this
) and the target object behind the
proxy (bound to ' target
).
Note | |
---|---|
Due to the proxy-based nature of Spring’s AOP framework, protected methods are by definition not intercepted, neither for JDK proxies (where this isn’t applicable) nor for CGLIB proxies (where this is technically possible but not recommendable for AOP purposes). As a consequence, any given pointcut will be matched against public methods only! If your interception needs include protected/private methods or even constructors, consider the use of Spring-driven native AspectJ weaving instead of Spring’s proxy-based AOP framework. This constitutes a different mode of AOP usage with different characteristics, so be sure to make yourself familiar with weaving first before making a decision. |
Spring AOP also supports an additional PCD named ' bean
. This PCD allows you to limit
the matching of join points to a particular named Spring bean, or to a set of named
Spring beans (when using wildcards). The ' bean
PCD has the following form:
bean(idOrNameOfBean)
The ' idOrNameOfBean
' token can be the name of any Spring bean: limited wildcard
support using the ' *
' character is provided, so if you establish some naming
conventions for your Spring beans you can quite easily write a ' bean
' PCD expression
to pick them out. As is the case with other pointcut designators, the ' bean
' PCD can
be &&'ed, ||'ed, and ! (negated) too.
Note | |
---|---|
Please note that the ' The ' |
Pointcut expressions can be combined using &&, || and !. It is also possible to
refer to pointcut expressions by name. The following example shows three pointcut
expressions: anyPublicOperation
(which matches if a method execution join point
represents the execution of any public method); inTrading
(which matches if a method
execution is in the trading module), and tradingOperation
(which matches if a method
execution represents any public method in the trading module).
@Pointcut("execution(public * (..))") private void anyPublicOperation() {} @Pointcut("within(com.xyz.someapp.trading..)") private void inTrading() {} @Pointcut("anyPublicOperation() && inTrading()") private void tradingOperation() {}
It is a best practice to build more complex pointcut expressions out of smaller named components as shown above. When referring to pointcuts by name, normal Java visibility rules apply (you can see private pointcuts in the same type, protected pointcuts in the hierarchy, public pointcuts anywhere and so on). Visibility does not affect pointcut matching.
When working with enterprise applications, you often want to refer to modules of the application and particular sets of operations from within several aspects. We recommend defining a "SystemArchitecture" aspect that captures common pointcut expressions for this purpose. A typical such aspect would look as follows:
package com.xyz.someapp; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Pointcut; @Aspect public class SystemArchitecture { / * A join point is in the web layer if the method is defined * in a type in the com.xyz.someapp.web package or any sub-package * under that. / @Pointcut("within(com.xyz.someapp.web..)") public void inWebLayer() {} / * A join point is in the service layer if the method is defined * in a type in the com.xyz.someapp.service package or any sub-package * under that. / @Pointcut("within(com.xyz.someapp.service..)") public void inServiceLayer() {} / * A join point is in the data access layer if the method is defined * in a type in the com.xyz.someapp.dao package or any sub-package * under that. / @Pointcut("within(com.xyz.someapp.dao..)") public void inDataAccessLayer() {} / * A business service is the execution of any method defined on a service * interface. This definition assumes that interfaces are placed in the * "service" package, and that implementation types are in sub-packages. * * If you group service interfaces by functional area (for example, * in packages com.xyz.someapp.abc.service and com.xyz.def.service) then * the pointcut expression "execution(* com.xyz.someapp..service..(..))" * could be used instead. * * Alternatively, you can write the expression using the bean * PCD, like so "bean(Service)". (This assumes that you have * named your Spring service beans in a consistent fashion.) */ @Pointcut("execution( com.xyz.someapp.service..(..))") public void businessService() {} /* * A data access operation is the execution of any method defined on a * dao interface. This definition assumes that interfaces are placed in the * "dao" package, and that implementation types are in sub-packages. */ @Pointcut("execution( com.xyz.someapp.dao..(..))") public void dataAccessOperation() {} }
The pointcuts defined in such an aspect can be referred to anywhere that you need a pointcut expression. For example, to make the service layer transactional, you could write:
<aop:config> <aop:advisor pointcut="com.xyz.someapp.SystemArchitecture.businessService()" advice-ref="tx-advice"/> </aop:config> <tx:advice id="tx-advice"> <tx:attributes> <tx:method name="*" propagation="REQUIRED"/> </tx:attributes> </tx:advice>
The <aop:config>
and <aop:advisor>
elements are discussed in Section 8.3, “Schema-based AOP support”. The
transaction elements are discussed in Chapter 11, Transaction Management.
Spring AOP users are likely to use the execution
pointcut designator the most often.
The format of an execution expression is:
execution(modifiers-pattern? ret-type-pattern declaring-type-pattern? name-pattern(param-pattern)
throws-pattern?)
All parts except the returning type pattern (ret-type-pattern in the snippet above),
name pattern, and parameters pattern are optional. The returning type pattern determines
what the return type of the method must be in order for a join point to be matched. Most
frequently you will use *
as the returning type pattern, which matches any return
type. A fully-qualified type name will match only when the method returns the given
type. The name pattern matches the method name. You can use the *
wildcard as all or
part of a name pattern. The parameters pattern is slightly more complex: ()
matches a
method that takes no parameters, whereas (..)
matches any number of parameters (zero
or more). The pattern (*)
matches a method taking one parameter of any type,
(*,String)
matches a method taking two parameters, the first can be of any type, the
second must be a String. Consult the
Language
Semantics section of the AspectJ Programming Guide for more information.
Some examples of common pointcut expressions are given below.
execution(public * *(..))
execution(* set*(..))
AccountService
interface:
execution(* com.xyz.service.AccountService.*(..))
execution(* com.xyz.service..(..))
execution(* com.xyz.service...(..))
within(com.xyz.service.*)
within(com.xyz.service..*)
AccountService
interface:
this(com.xyz.service.AccountService)
Note | |
---|---|
this is more commonly used in a binding form :- see the following section on advice for how to make the proxy object available in the advice body. |
AccountService
interface:
target(com.xyz.service.AccountService)
Note | |
---|---|
target is more commonly used in a binding form :- see the following section on advice for how to make the target object available in the advice body. |
Serializable
:
args(java.io.Serializable)
Note | |
---|---|
args is more commonly used in a binding form :- see the following section on advice for how to make the method arguments available in the advice body. |
Note that the pointcut given in this example is different to execution(*
*(java.io.Serializable))
: the args version matches if the argument passed at runtime is
Serializable, the execution version matches if the method signature declares a single
parameter of type Serializable
.
@Transactional
annotation:
@target(org.springframework.transaction.annotation.Transactional)
Note | |
---|---|
@target can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body. |
@Transactional
annotation:
@within(org.springframework.transaction.annotation.Transactional)
Note | |
---|---|
@within can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body. |
@Transactional
annotation:
@annotation(org.springframework.transaction.annotation.Transactional)
Note | |
---|---|
@annotation can also be used in a binding form :- see the following section on advice for how to make the annotation object available in the advice body. |
@Classified
annotation:
@args(com.xyz.security.Classified)
Note | |
---|---|
@args can also be used in a binding form :- see the following section on advice for how to make the annotation object(s) available in the advice body. |
tradeService
':
bean(tradeService)
*Service
':
bean(*Service)
During compilation, AspectJ processes pointcuts in order to try and optimize matching performance. Examining code and determining if each join point matches (statically or dynamically) a given pointcut is a costly process. (A dynamic match means the match cannot be fully determined from static analysis and a test will be placed in the code to determine if there is an actual match when the code is running). On first encountering a pointcut declaration, AspectJ will rewrite it into an optimal form for the matching process. What does this mean? Basically pointcuts are rewritten in DNF (Disjunctive Normal Form) and the components of the pointcut are sorted such that those components that are cheaper to evaluate are checked first. This means you do not have to worry about understanding the performance of various pointcut designators and may supply them in any order in a pointcut declaration.
However, AspectJ can only work with what it is told, and for optimal performance of matching you should think about what they are trying to achieve and narrow the search space for matches as much as possible in the definition. The existing designators naturally fall into one of three groups: kinded, scoping and context:
A well written pointcut should try and include at least the first two types (kinded and scoping), whilst the contextual designators may be included if wishing to match based on join point context, or bind that context for use in the advice. Supplying either just a kinded designator or just a contextual designator will work but could affect weaving performance (time and memory used) due to all the extra processing and analysis. Scoping designators are very fast to match and their usage means AspectJ can very quickly dismiss groups of join points that should not be further processed - that is why a good pointcut should always include one if possible.
Advice is associated with a pointcut expression, and runs before, after, or around method executions matched by the pointcut. The pointcut expression may be either a simple reference to a named pointcut, or a pointcut expression declared in place.
Before advice is declared in an aspect using the @Before
annotation:
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; @Aspect public class BeforeExample { @Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation()") public void doAccessCheck() { // ... } }
If using an in-place pointcut expression we could rewrite the above example as:
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Before; @Aspect public class BeforeExample { @Before("execution(* com.xyz.myapp.dao..(..))") public void doAccessCheck() { // ... } }
After returning advice runs when a matched method execution returns normally. It is
declared using the @AfterReturning
annotation:
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.AfterReturning; @Aspect public class AfterReturningExample { @AfterReturning("com.xyz.myapp.SystemArchitecture.dataAccessOperation()") public void doAccessCheck() { // ... } }
Note | |
---|---|
Note: it is of course possible to have multiple advice declarations, and other members as well, all inside the same aspect. We’re just showing a single advice declaration in these examples to focus on the issue under discussion at the time. |
Sometimes you need access in the advice body to the actual value that was returned. You
can use the form of @AfterReturning
that binds the return value for this:
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.AfterReturning; @Aspect public class AfterReturningExample { @AfterReturning( pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()", returning="retVal") public void doAccessCheck(Object retVal) { // ... } }
The name used in the returning
attribute must correspond to the name of a parameter in
the advice method. When a method execution returns, the return value will be passed to
the advice method as the corresponding argument value. A returning
clause also
restricts matching to only those method executions that return a value of the specified
type ( Object
in this case, which will match any return value).
Please note that it is not possible to return a totally different reference when using after-returning advice.
After throwing advice runs when a matched method execution exits by throwing an
exception. It is declared using the @AfterThrowing
annotation:
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.AfterThrowing; @Aspect public class AfterThrowingExample { @AfterThrowing("com.xyz.myapp.SystemArchitecture.dataAccessOperation()") public void doRecoveryActions() { // ... } }
Often you want the advice to run only when exceptions of a given type are thrown, and
you also often need access to the thrown exception in the advice body. Use the
throwing
attribute to both restrict matching (if desired, use Throwable
as the
exception type otherwise) and bind the thrown exception to an advice parameter.
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.AfterThrowing; @Aspect public class AfterThrowingExample { @AfterThrowing( pointcut="com.xyz.myapp.SystemArchitecture.dataAccessOperation()", throwing="ex") public void doRecoveryActions(DataAccessException ex) { // ... } }
The name used in the throwing
attribute must correspond to the name of a parameter in
the advice method. When a method execution exits by throwing an exception, the exception
will be passed to the advice method as the corresponding argument value. A throwing
clause also restricts matching to only those method executions that throw an exception
of the specified type ( DataAccessException
in this case).
After (finally) advice runs however a matched method execution exits. It is declared
using the @After
annotation. After advice must be prepared to handle both normal and
exception return conditions. It is typically used for releasing resources, etc.
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.After; @Aspect public class AfterFinallyExample { @After("com.xyz.myapp.SystemArchitecture.dataAccessOperation()") public void doReleaseLock() { // ... } }
The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements (i.e. don’t use around advice if simple before advice would do).
Around advice is declared using the @Around
annotation. The first parameter of the
advice method must be of type ProceedingJoinPoint
. Within the body of the advice,
calling proceed()
on the ProceedingJoinPoint
causes the underlying method to
execute. The proceed
method may also be called passing in an Object[]
- the values
in the array will be used as the arguments to the method execution when it proceeds.
Note | |
---|---|
The behavior of proceed when called with an Object[] is a little different than the behavior of proceed for around advice compiled by the AspectJ compiler. For around advice written using the traditional AspectJ language, the number of arguments passed to proceed must match the number of arguments passed to the around advice (not the number of arguments taken by the underlying join point), and the value passed to proceed in a given argument position supplants the original value at the join point for the entity the value was bound to (Don’t worry if this doesn’t make sense right now!). The approach taken by Spring is simpler and a better match to its proxy-based, execution only semantics. You only need to be aware of this difference if you are compiling @AspectJ aspects written for Spring and using proceed with arguments with the AspectJ compiler and weaver. There is a way to write such aspects that is 100% compatible across both Spring AOP and AspectJ, and this is discussed in the following section on advice parameters. |
import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.ProceedingJoinPoint; @Aspect public class AroundExample { @Around("com.xyz.myapp.SystemArchitecture.businessService()") public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable { // start stopwatch Object retVal = pjp.proceed(); // stop stopwatch return retVal; } }
The value returned by the around advice will be the return value seen by the caller of the method. A simple caching aspect for example could return a value from a cache if it has one, and invoke proceed() if it does not. Note that proceed may be invoked once, many times, or not at all within the body of the around advice, all of these are quite legal.
Spring offers fully typed advice - meaning that you declare the parameters you need
in the advice signature (as we saw for the returning and throwing examples above) rather
than work with Object[]
arrays all the time. We’ll see how to make argument and other
contextual values available to the advice body in a moment. First let’s take a look at
how to write generic advice that can find out about the method the advice is currently
advising.
Any advice method may declare as its first parameter, a parameter of type
org.aspectj.lang.JoinPoint
(please note that around advice is required to declare
a first parameter of type ProceedingJoinPoint
, which is a subclass of JoinPoint
. The
JoinPoint
interface provides a number of useful methods such as getArgs()
(returns
the method arguments), getThis()
(returns the proxy object), getTarget()
(returns
the target object), getSignature()
(returns a description of the method that is being
advised) and toString()
(prints a useful description of the method being advised).
Please do consult the Javadocs for full details.
We’ve already seen how to bind the returned value or exception value (using after
returning and after throwing advice). To make argument values available to the advice
body, you can use the binding form of args
. If a parameter name is used in place of a
type name in an args expression, then the value of the corresponding argument will be
passed as the parameter value when the advice is invoked. An example should make this
clearer. Suppose you want to advise the execution of dao operations that take an Account
object as the first parameter, and you need access to the account in the advice body.
You could write the following:
@Before("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)") public void validateAccount(Account account) { // ... }
The args(account,..)
part of the pointcut expression serves two purposes: firstly, it
restricts matching to only those method executions where the method takes at least one
parameter, and the argument passed to that parameter is an instance of Account
;
secondly, it makes the actual Account
object available to the advice via the account
parameter.
Another way of writing this is to declare a pointcut that "provides" the Account
object value when it matches a join point, and then just refer to the named pointcut
from the advice. This would look as follows:
@Pointcut("com.xyz.myapp.SystemArchitecture.dataAccessOperation() && args(account,..)") private void accountDataAccessOperation(Account account) {} @Before("accountDataAccessOperation(account)") public void validateAccount(Account account) { // ... }
The interested reader is once more referred to the AspectJ programming guide for more details.
The proxy object ( this
), target object ( target
), and annotations ( @within,
@target, @annotation, @args
) can all be bound in a similar fashion. The following
example shows how you could match the execution of methods annotated with an
@Auditable
annotation, and extract the audit code.
First the definition of the @Auditable
annotation:
@Retention(RetentionPolicy.RUNTIME) @Target(ElementType.METHOD) public @interface Auditable { AuditCode value(); }
And then the advice that matches the execution of @Auditable
methods:
@Before("com.xyz.lib.Pointcuts.anyPublicMethod() && @annotation(auditable)") public void audit(Auditable auditable) { AuditCode code = auditable.value(); // ... }
Spring AOP can handle generics used in class declarations and method parameters. Suppose you have a generic type like this:
public interface Sample<T> { void sampleGenericMethod(T param); void sampleGenericCollectionMethod(Collection>T> param); }
You can restrict interception of method types to certain parameter types by simply typing the advice parameter to the parameter type you want to intercept the method for:
@Before("execution(* ..Sample+.sampleGenericMethod(*)) && args(param)") public void beforeSampleMethod(MyType param) { // Advice implementation }
That this works is pretty obvious as we already discussed above. However, it’s worth pointing out that this won’t work for generic collections. So you cannot define a pointcut like this:
@Before("execution(* ..Sample+.sampleGenericCollectionMethod(*)) && args(param)") public void beforeSampleMethod(Collection<MyType> param) { // Advice implementation }
To make this work we would have to inspect every element of the collection, which is not
reasonable as we also cannot decide how to treat null
values in general. To achieve
something similar to this you have to type the parameter to Collection<?>
and manually
check the type of the elements.
The parameter binding in advice invocations relies on matching names used in pointcut expressions to declared parameter names in (advice and pointcut) method signatures. Parameter names are not available through Java reflection, so Spring AOP uses the following strategies to determine parameter names:
@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)", argNames="bean,auditable") public void audit(Object bean, Auditable auditable) { AuditCode code = auditable.value(); // ... use code and bean }
If the first parameter is of the JoinPoint
, ProceedingJoinPoint
, or
JoinPoint.StaticPart
type, you may leave out the name of the parameter from the value
of the "argNames" attribute. For example, if you modify the preceding advice to receive
the join point object, the "argNames" attribute need not include it:
@Before(value="com.xyz.lib.Pointcuts.anyPublicMethod() && target(bean) && @annotation(auditable)", argNames="bean,auditable") public void audit(JoinPoint jp, Object bean, Auditable auditable) { AuditCode code = auditable.value(); // ... use code, bean, and jp }
The special treatment given to the first parameter of the JoinPoint
,
ProceedingJoinPoint
, and JoinPoint.StaticPart
types is particularly convenient for
advice that do not collect any other join point context. In such situations, you may
simply omit the "argNames" attribute. For example, the following advice need not declare
the "argNames" attribute:
@Before("com.xyz.lib.Pointcuts.anyPublicMethod()") public void audit(JoinPoint jp) { // ... use jp }
'argNames'
attribute is a little clumsy, so if the 'argNames'
attribute
has not been specified, then Spring AOP will look at the debug information for the
class and try to determine the parameter names from the local variable table. This
information will be present as long as the classes have been compiled with debug
information ( '-g:vars'
at a minimum). The consequences of compiling with this flag
on are: (1) your code will be slightly easier to understand (reverse engineer), (2)
the class file sizes will be very slightly bigger (typically inconsequential), (3) the
optimization to remove unused local variables will not be applied by your compiler. In
other words, you should encounter no difficulties building with this flag on.
Note | |
---|---|
If an @AspectJ aspect has been compiled by the AspectJ compiler (ajc) even without the debug information then there is no need to add the argNames attribute as the compiler will retain the needed information. |
AmbiguousBindingException
will be
thrown.
IllegalArgumentException
will be thrown.
We remarked earlier that we would describe how to write a proceed call with arguments that works consistently across Spring AOP and AspectJ. The solution is simply to ensure that the advice signature binds each of the method parameters in order. For example:
@Around("execution(List<Account> find*(..)) && " + "com.xyz.myapp.SystemArchitecture.inDataAccessLayer() && " + "args(accountHolderNamePattern)") public Object preProcessQueryPattern(ProceedingJoinPoint pjp, String accountHolderNamePattern) throws Throwable { String newPattern = preProcess(accountHolderNamePattern); return pjp.proceed(new Object[] {newPattern}); }
In many cases you will be doing this binding anyway (as in the example above).
What happens when multiple pieces of advice all want to run at the same join point? Spring AOP follows the same precedence rules as AspectJ to determine the order of advice execution. The highest precedence advice runs first "on the way in" (so given two pieces of before advice, the one with highest precedence runs first). "On the way out" from a join point, the highest precedence advice runs last (so given two pieces of after advice, the one with the highest precedence will run second).
When two pieces of advice defined in different aspects both need to run at the same
join point, unless you specify otherwise the order of execution is undefined. You can
control the order of execution by specifying precedence. This is done in the normal
Spring way by either implementing the org.springframework.core.Ordered
interface in
the aspect class or annotating it with the Order
annotation. Given two aspects, the
aspect returning the lower value from Ordered.getValue()
(or the annotation value) has
the higher precedence.
When two pieces of advice defined in the same aspect both need to run at the same join point, the ordering is undefined (since there is no way to retrieve the declaration order via reflection for javac-compiled classes). Consider collapsing such advice methods into one advice method per join point in each aspect class, or refactor the pieces of advice into separate aspect classes - which can be ordered at the aspect level.
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.
An introduction is made using the @DeclareParents
annotation. This annotation is used
to declare that matching types have a new parent (hence the name). For example, given an
interface UsageTracked
, and an implementation of that interface DefaultUsageTracked
,
the following aspect declares that all implementors of service interfaces also implement
the UsageTracked
interface. (In order to expose statistics via JMX for example.)
@Aspect public class UsageTracking { @DeclareParents(value="com.xzy.myapp.service.*+", defaultImpl=DefaultUsageTracked.class) public static UsageTracked mixin; @Before("com.xyz.myapp.SystemArchitecture.businessService() && this(usageTracked)") public void recordUsage(UsageTracked usageTracked) { usageTracked.incrementUseCount(); } }
The interface to be implemented is determined by the type of the annotated field. The
value
attribute of the @DeclareParents
annotation is an AspectJ type pattern :- any
bean of a matching type will implement the UsageTracked interface. Note that in the
before advice of the above example, service beans can be directly used as
implementations of the UsageTracked
interface. If accessing a bean programmatically
you would write the following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
Note | |
---|---|
(This is an advanced topic, so if you are just starting out with AOP you can safely skip it until later.) |
By default there will be a single instance of each aspect within the application
context. AspectJ calls this the singleton instantiation model. It is possible to define
aspects with alternate lifecycles :- Spring supports AspectJ’s perthis
and pertarget
instantiation models ( percflow, percflowbelow,
and pertypewithin
are not currently
supported).
A "perthis" aspect is declared by specifying a perthis
clause in the @Aspect
annotation. Let’s look at an example, and then we’ll explain how it works.
@Aspect("perthis(com.xyz.myapp.SystemArchitecture.businessService())") public class MyAspect { private int someState; @Before(com.xyz.myapp.SystemArchitecture.businessService()) public void recordServiceUsage() { // ... } }
The effect of the 'perthis'
clause is that one aspect instance will be created for
each unique service object executing a business service (each unique object bound to
this at join points matched by the pointcut expression). The aspect instance is
created the first time that a method is invoked on the service object. The aspect goes
out of scope when the service object goes out of scope. Before the aspect instance is
created, none of the advice within it executes. As soon as the aspect instance has been
created, the advice declared within it will execute at matched join points, but only
when the service object is the one this aspect is associated with. See the AspectJ
programming guide for more information on per-clauses.
The 'pertarget'
instantiation model works in exactly the same way as perthis, but
creates one aspect instance for each unique target object at matched join points.
Now that you have seen how all the constituent parts work, let’s put them together to do something useful!
The execution of business services can sometimes fail due to concurrency issues (for
example, deadlock loser). If the operation is retried, it is quite likely to succeed
next time round. For business services where it is appropriate to retry in such
conditions (idempotent operations that don’t need to go back to the user for conflict
resolution), we’d like to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException
. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an
aspect.
Because we want to retry the operation, we will need to use around advice so that we can call proceed multiple times. Here’s how the basic aspect implementation looks:
@Aspect public class ConcurrentOperationExecutor implements Ordered { private static final int DEFAULT_MAX_RETRIES = 2; private int maxRetries = DEFAULT_MAX_RETRIES; private int order = 1; public void setMaxRetries(int maxRetries) { this.maxRetries = maxRetries; } public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } @Around("com.xyz.myapp.SystemArchitecture.businessService()") public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { int numAttempts = 0; PessimisticLockingFailureException lockFailureException; do { numAttempts++; try { return pjp.proceed(); } catch(PessimisticLockingFailureException ex) { lockFailureException = ex; } } while(numAttempts <= this.maxRetries); throw lockFailureException; } }
Note that the aspect implements the Ordered
interface so we can set the precedence of
the aspect higher than the transaction advice (we want a fresh transaction each time we
retry). The maxRetries
and order
properties will both be configured by Spring. The
main action happens in the doConcurrentOperation
around advice. Notice that for the
moment we’re applying the retry logic to all businessService()s
. We try to proceed,
and if we fail with an PessimisticLockingFailureException
we simply try again unless
we have exhausted all of our retry attempts.
The corresponding Spring configuration is:
<aop:aspectj-autoproxy/> <bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor"> <property name="maxRetries" value="3"/> <property name="order" value="100"/> </bean>
To refine the aspect so that it only retries idempotent operations, we might define an
Idempotent
annotation:
@Retention(RetentionPolicy.RUNTIME) public @interface Idempotent { // marker annotation }
and use the annotation to annotate the implementation of service operations. The change
to the aspect to only retry idempotent operations simply involves refining the pointcut
expression so that only @Idempotent
operations match:
@Around("com.xyz.myapp.SystemArchitecture.businessService() && " + "@annotation(com.xyz.myapp.service.Idempotent)") public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { ... }
If you prefer an XML-based format, then Spring also offers support for defining aspects using the new "aop" namespace tags. The exact same pointcut expressions and advice kinds are supported as when using the @AspectJ style, hence in this section we will focus on the new syntax and refer the reader to the discussion in the previous section (Section 8.2, “@AspectJ support”) for an understanding of writing pointcut expressions and the binding of advice parameters.
To use the aop namespace tags described in this section, you need to import the spring-aop schema as described in Chapter 33, XML Schema-based configuration. See Section 33.2.7, “the aop schema” for how to import the tags in the aop namespace.
Within your Spring configurations, all aspect and advisor elements must be placed within
an <aop:config>
element (you can have more than one <aop:config>
element in an
application context configuration). An <aop:config>
element can contain pointcut,
advisor, and aspect elements (note these must be declared in that order).
Warning | |
---|---|
The |
Using the schema support, an aspect is simply a regular Java object defined as a bean in your Spring application context. The state and behavior is captured in the fields and methods of the object, and the pointcut and advice information is captured in the XML.
An aspect is declared using the <aop:aspect> element, and the backing bean is referenced
using the ref
attribute:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> ... </aop:aspect> </aop:config> <bean id="aBean" class="..."> ... </bean>
The bean backing the aspect (" aBean
" in this case) can of course be configured and
dependency injected just like any other Spring bean.
A named pointcut can be declared inside an <aop:config> element, enabling the pointcut definition to be shared across several aspects and advisors.
A pointcut representing the execution of any business service in the service layer could be defined as follows:
<aop:config> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service..(..))"/> </aop:config>
Note that the pointcut expression itself is using the same AspectJ pointcut expression language as described in Section 8.2, “@AspectJ support”. If you are using the schema based declaration style, you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression. Another way of defining the above pointcut would be:
<aop:config> <aop:pointcut id="businessService" expression="com.xyz.myapp.SystemArchitecture.businessService()"/> </aop:config>
Assuming you have a SystemArchitecture
aspect as described in the section called “Sharing common pointcut definitions”.
Declaring a pointcut inside an aspect is very similar to declaring a top-level pointcut:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service..(..))"/> ... </aop:aspect> </aop:config>
Much the same way in an @AspectJ aspect, pointcuts declared using the schema based definition style may collect join point context. For example, the following pointcut collects the this object as the join point context and passes it to advice:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service..(..)) && this(service)"/> <aop:before pointcut-ref="businessService" method="monitor"/> ... </aop:aspect> </aop:config>
The advice must be declared to receive the collected join point context by including parameters of the matching names:
public void monitor(Object service) { ... }
When combining pointcut sub-expressions, && is awkward within an XML document, and so the keywords and, or and not can be used in place of &&, || and ! respectively. For example, the previous pointcut may be better written as:
<aop:config> <aop:aspect id="myAspect" ref="aBean"> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service..(..)) and this(service)"/> <aop:before pointcut-ref="businessService" method="monitor"/> ... </aop:aspect> </aop:config>
Note that pointcuts defined in this way are referred to by their XML id and cannot be used as named pointcuts to form composite pointcuts. The named pointcut support in the schema based definition style is thus more limited than that offered by the @AspectJ style.
The same five advice kinds are supported as for the @AspectJ style, and they have exactly the same semantics.
Before advice runs before a matched method execution. It is declared inside an
<aop:aspect>
using the <aop:before> element.
<aop:aspect id="beforeExample" ref="aBean"> <aop:before pointcut-ref="dataAccessOperation" method="doAccessCheck"/> ... </aop:aspect>
Here dataAccessOperation
is the id of a pointcut defined at the top ( <aop:config>
)
level. To define the pointcut inline instead, replace the pointcut-ref
attribute with
a pointcut
attribute:
<aop:aspect id="beforeExample" ref="aBean"> <aop:before pointcut="execution(* com.xyz.myapp.dao..(..))" method="doAccessCheck"/> ... </aop:aspect>
As we noted in the discussion of the @AspectJ style, using named pointcuts can significantly improve the readability of your code.
The method attribute identifies a method ( doAccessCheck
) that provides the body of
the advice. This method must be defined for the bean referenced by the aspect element
containing the advice. Before a data access operation is executed (a method execution
join point matched by the pointcut expression), the "doAccessCheck" method on the aspect
bean will be invoked.
After returning advice runs when a matched method execution completes normally. It is
declared inside an <aop:aspect>
in the same way as before advice. For example:
<aop:aspect id="afterReturningExample" ref="aBean"> <aop:after-returning pointcut-ref="dataAccessOperation" method="doAccessCheck"/> ... </aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the return value within the advice body. Use the returning attribute to specify the name of the parameter to which the return value should be passed:
<aop:aspect id="afterReturningExample" ref="aBean"> <aop:after-returning pointcut-ref="dataAccessOperation" returning="retVal" method="doAccessCheck"/> ... </aop:aspect>
The doAccessCheck method must declare a parameter named retVal
. The type of this
parameter constrains matching in the same way as described for @AfterReturning. For
example, the method signature may be declared as:
public void doAccessCheck(Object retVal) {...
After throwing advice executes when a matched method execution exits by throwing an
exception. It is declared inside an <aop:aspect>
using the after-throwing element:
<aop:aspect id="afterThrowingExample" ref="aBean"> <aop:after-throwing pointcut-ref="dataAccessOperation" method="doRecoveryActions"/> ... </aop:aspect>
Just as in the @AspectJ style, it is possible to get hold of the thrown exception within the advice body. Use the throwing attribute to specify the name of the parameter to which the exception should be passed:
<aop:aspect id="afterThrowingExample" ref="aBean"> <aop:after-throwing pointcut-ref="dataAccessOperation" throwing="dataAccessEx" method="doRecoveryActions"/> ... </aop:aspect>
The doRecoveryActions method must declare a parameter named dataAccessEx
. The type of
this parameter constrains matching in the same way as described for @AfterThrowing. For
example, the method signature may be declared as:
public void doRecoveryActions(DataAccessException dataAccessEx) {...
After (finally) advice runs however a matched method execution exits. It is declared
using the after
element:
<aop:aspect id="afterFinallyExample" ref="aBean"> <aop:after pointcut-ref="dataAccessOperation" method="doReleaseLock"/> ... </aop:aspect>
The final kind of advice is around advice. Around advice runs "around" a matched method execution. It has the opportunity to do work both before and after the method executes, and to determine when, how, and even if, the method actually gets to execute at all. Around advice is often used if you need to share state before and after a method execution in a thread-safe manner (starting and stopping a timer for example). Always use the least powerful form of advice that meets your requirements; don’t use around advice if simple before advice would do.
Around advice is declared using the aop:around
element. The first parameter of the
advice method must be of type ProceedingJoinPoint
. Within the body of the advice,
calling proceed()
on the ProceedingJoinPoint
causes the underlying method to
execute. The proceed
method may also be calling passing in an Object[]
- the values
in the array will be used as the arguments to the method execution when it proceeds. See
the section called “Around advice” for notes on calling proceed with an Object[]
.
<aop:aspect id="aroundExample" ref="aBean"> <aop:around pointcut-ref="businessService" method="doBasicProfiling"/> ... </aop:aspect>
The implementation of the doBasicProfiling
advice would be exactly the same as in the
@AspectJ example (minus the annotation of course):
public Object doBasicProfiling(ProceedingJoinPoint pjp) throws Throwable { // start stopwatch Object retVal = pjp.proceed(); // stop stopwatch return retVal; }
The schema based declaration style supports fully typed advice in the same way as
described for the @AspectJ support - by matching pointcut parameters by name against
advice method parameters. See the section called “Advice parameters” for details. If you wish
to explicitly specify argument names for the advice methods (not relying on the
detection strategies previously described) then this is done using the arg-names
attribute of the advice element, which is treated in the same manner to the "argNames"
attribute in an advice annotation as described in the section called “Determining argument names”.
For example:
<aop:before pointcut="com.xyz.lib.Pointcuts.anyPublicMethod() and @annotation(auditable)" method="audit" arg-names="auditable"/>
The arg-names
attribute accepts a comma-delimited list of parameter names.
Find below a slightly more involved example of the XSD-based approach that illustrates some around advice used in conjunction with a number of strongly typed parameters.
package x.y.service; public interface FooService { Foo getFoo(String fooName, int age); } public class DefaultFooService implements FooService { public Foo getFoo(String name, int age) { return new Foo(name, age); } }
Next up is the aspect. Notice the fact that the profile(..)
method accepts a number of
strongly-typed parameters, the first of which happens to be the join point used to
proceed with the method call: the presence of this parameter is an indication that the
profile(..)
is to be used as around
advice:
package x.y; import org.aspectj.lang.ProceedingJoinPoint; import org.springframework.util.StopWatch; public class SimpleProfiler { public Object profile(ProceedingJoinPoint call, String name, int age) throws Throwable { StopWatch clock = new StopWatch("Profiling for " + name + " and " + age + ""); try { clock.start(call.toShortString()); return call.proceed(); } finally { clock.stop(); System.out.println(clock.prettyPrint()); } } }
Finally, here is the XML configuration that is required to effect the execution of the above advice for a particular join point:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- this is the object that will be proxied by Spring's AOP infrastructure --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this is the actual advice itself --> <bean id="profiler" class="x.y.SimpleProfiler"/> <aop:config> <aop:aspect ref="profiler"> <aop:pointcut id="theExecutionOfSomeFooServiceMethod" expression="execution(* x.y.service.FooService.getFoo(String,int)) and args(name, age)"/> <aop:around pointcut-ref="theExecutionOfSomeFooServiceMethod" method="profile"/> </aop:aspect> </aop:config> </beans>
If we had the following driver script, we would get output something like this on standard output:
import org.springframework.beans.factory.BeanFactory; import org.springframework.context.support.ClassPathXmlApplicationContext; import x.y.service.FooService; public final class Boot { public static void main(final String[] args) throws Exception { BeanFactory ctx = new ClassPathXmlApplicationContext("x/y/plain.xml"); FooService foo = (FooService) ctx.getBean("fooService"); foo.getFoo("Pengo", 12); } }
StopWatch Profiling for 'Pengo and 12': running time (millis) = 0 ----------------------------------------- ms % Task name ----------------------------------------- 00000 ? execution(getFoo)
When multiple advice needs to execute at the same join point (executing method) the
ordering rules are as described in the section called “Advice ordering”. The precedence
between aspects is determined by either adding the Order
annotation to the bean
backing the aspect or by having the bean implement the Ordered
interface.
Introductions (known as inter-type declarations in AspectJ) enable an aspect to declare that advised objects implement a given interface, and to provide an implementation of that interface on behalf of those objects.
An introduction is made using the aop:declare-parents
element inside an aop:aspect
This element is used to declare that matching types have a new parent (hence the name).
For example, given an interface UsageTracked
, and an implementation of that interface
DefaultUsageTracked
, the following aspect declares that all implementors of service
interfaces also implement the UsageTracked
interface. (In order to expose statistics
via JMX for example.)
<aop:aspect id="usageTrackerAspect" ref="usageTracking"> <aop:declare-parents types-matching="com.xzy.myapp.service.*+" implement-interface="com.xyz.myapp.service.tracking.UsageTracked" default-impl="com.xyz.myapp.service.tracking.DefaultUsageTracked"/> <aop:before pointcut="com.xyz.myapp.SystemArchitecture.businessService() and this(usageTracked)" method="recordUsage"/> </aop:aspect>
The class backing the usageTracking
bean would contain the method:
public void recordUsage(UsageTracked usageTracked) { usageTracked.incrementUseCount(); }
The interface to be implemented is determined by implement-interface
attribute. The
value of the types-matching
attribute is an AspectJ type pattern :- any bean of a
matching type will implement the UsageTracked
interface. Note that in the before
advice of the above example, service beans can be directly used as implementations of
the UsageTracked
interface. If accessing a bean programmatically you would write the
following:
UsageTracked usageTracked = (UsageTracked) context.getBean("myService");
The only supported instantiation model for schema-defined aspects is the singleton model. Other instantiation models may be supported in future releases.
The concept of "advisors" is brought forward from the AOP support defined in Spring 1.2 and does not have a direct equivalent in AspectJ. An advisor is like a small self-contained aspect that has a single piece of advice. The advice itself is represented by a bean, and must implement one of the advice interfaces described in Section 9.3.2, “Advice types in Spring”. Advisors can take advantage of AspectJ pointcut expressions though.
Spring supports the advisor concept with the <aop:advisor>
element. You will most
commonly see it used in conjunction with transactional advice, which also has its own
namespace support in Spring. Here’s how it looks:
<aop:config> <aop:pointcut id="businessService" expression="execution(* com.xyz.myapp.service..(..))"/> <aop:advisor pointcut-ref="businessService" advice-ref="tx-advice"/> </aop:config> <tx:advice id="tx-advice"> <tx:attributes> <tx:method name="*" propagation="REQUIRED"/> </tx:attributes> </tx:advice>
As well as the pointcut-ref
attribute used in the above example, you can also use the
pointcut
attribute to define a pointcut expression inline.
To define the precedence of an advisor so that the advice can participate in ordering,
use the order
attribute to define the Ordered
value of the advisor.
Let’s see how the concurrent locking failure retry example from Section 8.2.7, “Example” looks when rewritten using the schema support.
The execution of business services can sometimes fail due to concurrency issues (for
example, deadlock loser). If the operation is retried, it is quite likely it will
succeed next time round. For business services where it is appropriate to retry in such
conditions (idempotent operations that don’t need to go back to the user for conflict
resolution), we’d like to transparently retry the operation to avoid the client seeing a
PessimisticLockingFailureException
. This is a requirement that clearly cuts across
multiple services in the service layer, and hence is ideal for implementing via an
aspect.
Because we want to retry the operation, we’ll need to use around advice so that we can call proceed multiple times. Here’s how the basic aspect implementation looks (it’s just a regular Java class using the schema support):
public class ConcurrentOperationExecutor implements Ordered { private static final int DEFAULT_MAX_RETRIES = 2; private int maxRetries = DEFAULT_MAX_RETRIES; private int order = 1; public void setMaxRetries(int maxRetries) { this.maxRetries = maxRetries; } public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } public Object doConcurrentOperation(ProceedingJoinPoint pjp) throws Throwable { int numAttempts = 0; PessimisticLockingFailureException lockFailureException; do { numAttempts++; try { return pjp.proceed(); } catch(PessimisticLockingFailureException ex) { lockFailureException = ex; } } while(numAttempts <= this.maxRetries); throw lockFailureException; } }
Note that the aspect implements the Ordered
interface so we can set the precedence of
the aspect higher than the transaction advice (we want a fresh transaction each time we
retry). The maxRetries
and order
properties will both be configured by Spring. The
main action happens in the doConcurrentOperation
around advice method. We try to
proceed, and if we fail with a PessimisticLockingFailureException
we simply try again
unless we have exhausted all of our retry attempts.
Note | |
---|---|
This class is identical to the one used in the @AspectJ example, but with the annotations removed. |
The corresponding Spring configuration is:
<aop:config> <aop:aspect id="concurrentOperationRetry" ref="concurrentOperationExecutor"> <aop:pointcut id="idempotentOperation" expression="execution(* com.xyz.myapp.service..(..))"/> <aop:around pointcut-ref="idempotentOperation" method="doConcurrentOperation"/> </aop:aspect> </aop:config> <bean id="concurrentOperationExecutor" class="com.xyz.myapp.service.impl.ConcurrentOperationExecutor"> <property name="maxRetries" value="3"/> <property name="order" value="100"/> </bean>
Notice that for the time being we assume that all business services are idempotent. If
this is not the case we can refine the aspect so that it only retries genuinely
idempotent operations, by introducing an Idempotent
annotation:
@Retention(RetentionPolicy.RUNTIME) public @interface Idempotent { // marker annotation }
and using the annotation to annotate the implementation of service operations. The
change to the aspect to retry only idempotent operations simply involves refining the
pointcut expression so that only @Idempotent
operations match:
<aop:pointcut id="idempotentOperation" expression="execution(* com.xyz.myapp.service..(..)) and @annotation(com.xyz.myapp.service.Idempotent)"/>
Once you have decided that an aspect is the best approach for implementing a given requirement, how do you decide between using Spring AOP or AspectJ, and between the Aspect language (code) style, @AspectJ annotation style, or the Spring XML style? These decisions are influenced by a number of factors including application requirements, development tools, and team familiarity with AOP.
Use the simplest thing that can work. Spring AOP is simpler than using full AspectJ as there is no requirement to introduce the AspectJ compiler / weaver into your development and build processes. If you only need to advise the execution of operations on Spring beans, then Spring AOP is the right choice. If you need to advise objects not managed by the Spring container (such as domain objects typically), then you will need to use AspectJ. You will also need to use AspectJ if you wish to advise join points other than simple method executions (for example, field get or set join points, and so on).
When using AspectJ, you have the choice of the AspectJ language syntax (also known as the "code style") or the @AspectJ annotation style. Clearly, if you are not using Java 5+ then the choice has been made for you… use the code style. If aspects play a large role in your design, and you are able to use the AspectJ Development Tools (AJDT) plugin for Eclipse, then the AspectJ language syntax is the preferred option: it is cleaner and simpler because the language was purposefully designed for writing aspects. If you are not using Eclipse, or have only a few aspects that do not play a major role in your application, then you may want to consider using the @AspectJ style and sticking with a regular Java compilation in your IDE, and adding an aspect weaving phase to your build script.
If you have chosen to use Spring AOP, then you have a choice of @AspectJ or XML style. There are various tradeoffs to consider.
The XML style will be most familiar to existing Spring users and it is backed by genuine POJOs. When using AOP as a tool to configure enterprise services then XML can be a good choice (a good test is whether you consider the pointcut expression to be a part of your configuration you might want to change independently). With the XML style arguably it is clearer from your configuration what aspects are present in the system.
The XML style has two disadvantages. Firstly it does not fully encapsulate the implementation of the requirement it addresses in a single place. The DRY principle says that there should be a single, unambiguous, authoritative representation of any piece of knowledge within a system. When using the XML style, the knowledge of how a requirement is implemented is split across the declaration of the backing bean class, and the XML in the configuration file. When using the @AspectJ style there is a single module - the aspect - in which this information is encapsulated. Secondly, the XML style is slightly more limited in what it can express than the @AspectJ style: only the "singleton" aspect instantiation model is supported, and it is not possible to combine named pointcuts declared in XML. For example, in the @AspectJ style you can write something like:
@Pointcut(execution(* get*())) public void propertyAccess() {} @Pointcut(execution(org.xyz.Account+ *(..)) public void operationReturningAnAccount() {} @Pointcut(propertyAccess() && operationReturningAnAccount()) public void accountPropertyAccess() {}
In the XML style I can declare the first two pointcuts:
<aop:pointcut id="propertyAccess" expression="execution(* get*())"/> <aop:pointcut id="operationReturningAnAccount" expression="execution(org.xyz.Account+ *(..))"/>
The downside of the XML approach is that you cannot define the '
accountPropertyAccess
' pointcut by combining these definitions.
The @AspectJ style supports additional instantiation models, and richer pointcut composition. It has the advantage of keeping the aspect as a modular unit. It also has the advantage the @AspectJ aspects can be understood (and thus consumed) both by Spring AOP and by AspectJ - so if you later decide you need the capabilities of AspectJ to implement additional requirements then it is very easy to migrate to an AspectJ-based approach. On balance the Spring team prefer the @AspectJ style whenever you have aspects that do more than simple "configuration" of enterprise services.
It is perfectly possible to mix @AspectJ style aspects using the autoproxying support,
schema-defined <aop:aspect>
aspects, <aop:advisor>
declared advisors and even
proxies and interceptors defined using the Spring 1.2 style in the same configuration.
All of these are implemented using the same underlying support mechanism and will
co-exist without any difficulty.
Spring AOP uses either JDK dynamic proxies or CGLIB to create the proxy for a given target object. (JDK dynamic proxies are preferred whenever you have a choice).
If the target object to be proxied implements at least one interface then a JDK dynamic proxy will be used. All of the interfaces implemented by the target type will be proxied. If the target object does not implement any interfaces then a CGLIB proxy will be created.
If you want to force the use of CGLIB proxying (for example, to proxy every method defined for the target object, not just those implemented by its interfaces) you can do so. However, there are some issues to consider:
final
methods cannot be advised, as they cannot be overridden.
To force the use of CGLIB proxies set the value of the proxy-target-class
attribute of
the <aop:config>
element to true:
<aop:config proxy-target-class="true"> <!-- other beans defined here... --> </aop:config>
To force CGLIB proxying when using the @AspectJ autoproxy support, set the
'proxy-target-class'
attribute of the <aop:aspectj-autoproxy>
element to true
:
<aop:aspectj-autoproxy proxy-target-class="true"/>
Note | |
---|---|
Multiple To be clear: using ' |
Spring AOP is proxy-based. It is vitally important that you grasp the semantics of what that last statement actually means before you write your own aspects or use any of the Spring AOP-based aspects supplied with the Spring Framework.
Consider first the scenario where you have a plain-vanilla, un-proxied, nothing-special-about-it, straight object reference, as illustrated by the following code snippet.
public class SimplePojo implements Pojo { public void foo() { // this next method invocation is a direct call on the this reference this.bar(); } public void bar() { // some logic... } }
If you invoke a method on an object reference, the method is invoked directly on that object reference, as can be seen below.
public class Main { public static void main(String[] args) { Pojo pojo = new SimplePojo(); // this is a direct method call on the pojo reference pojo.foo(); } }
Things change slightly when the reference that client code has is a proxy. Consider the following diagram and code snippet.
public class Main { public static void main(String[] args) { ProxyFactory factory = new ProxyFactory(new SimplePojo()); factory.addInterface(Pojo.class); factory.addAdvice(new RetryAdvice()); Pojo pojo = (Pojo) factory.getProxy(); // this is a method call on the proxy! pojo.foo(); } }
The key thing to understand here is that the client code inside the main(..)
of the
Main
class has a reference to the proxy. This means that method calls on that
object reference will be calls on the proxy, and as such the proxy will be able to
delegate to all of the interceptors (advice) that are relevant to that particular method
call. However, once the call has finally reached the target object, the SimplePojo
reference in this case, any method calls that it may make on itself, such as
this.bar()
or this.foo()
, are going to be invoked against the this reference,
and not the proxy. This has important implications. It means that self-invocation is
not going to result in the advice associated with a method invocation getting a
chance to execute.
Okay, so what is to be done about this? The best approach (the term best is used loosely here) is to refactor your code such that the self-invocation does not happen. For sure, this does entail some work on your part, but it is the best, least-invasive approach. The next approach is absolutely horrendous, and I am almost reticent to point it out precisely because it is so horrendous. You can (choke!) totally tie the logic within your class to Spring AOP by doing this:
public class SimplePojo implements Pojo { public void foo() { // this works, but... gah! ((Pojo) AopContext.currentProxy()).bar(); } public void bar() { // some logic... } }
This totally couples your code to Spring AOP, and it makes the class itself aware of the fact that it is being used in an AOP context, which flies in the face of AOP. It also requires some additional configuration when the proxy is being created:
public class Main { public static void main(String[] args) { ProxyFactory factory = new ProxyFactory(new SimplePojo()); factory.adddInterface(Pojo.class); factory.addAdvice(new RetryAdvice()); factory.setExposeProxy(true); Pojo pojo = (Pojo) factory.getProxy(); // this is a method call on the proxy! pojo.foo(); } }
Finally, it must be noted that AspectJ does not have this self-invocation issue because it is not a proxy-based AOP framework.
In addition to declaring aspects in your configuration using either <aop:config>
or
<aop:aspectj-autoproxy>
, it is also possible programmatically to create proxies that
advise target objects. For the full details of Spring’s AOP API, see the next chapter.
Here we want to focus on the ability to automatically create proxies using @AspectJ
aspects.
The class org.springframework.aop.aspectj.annotation.AspectJProxyFactory
can be used
to create a proxy for a target object that is advised by one or more @AspectJ aspects.
Basic usage for this class is very simple, as illustrated below. See the Javadocs for
full information.
// create a factory that can generate a proxy for the given target object AspectJProxyFactory factory = new AspectJProxyFactory(targetObject); // add an aspect, the class must be an @AspectJ aspect // you can call this as many times as you need with different aspects factory.addAspect(SecurityManager.class); // you can also add existing aspect instances, the type of the object supplied must be an @AspectJ aspect factory.addAspect(usageTracker); // now get the proxy object... MyInterfaceType proxy = factory.getProxy();
Everything we’ve covered so far in this chapter is pure Spring AOP. In this section, we’re going to look at how you can use the AspectJ compiler/weaver instead of, or in addition to, Spring AOP if your needs go beyond the facilities offered by Spring AOP alone.
Spring ships with a small AspectJ aspect library, which is available standalone in your
distribution as spring-aspects.jar
; you’ll need to add this to your classpath in order
to use the aspects in it. Section 8.8.1, “Using AspectJ to dependency inject domain objects with Spring” and Section 8.8.2, “Other Spring aspects for AspectJ” discuss the
content of this library and how you can use it. Section 8.8.3, “Configuring AspectJ aspects using Spring IoC” discusses how to
dependency inject AspectJ aspects that are woven using the AspectJ compiler. Finally,
Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework” provides an introduction to load-time weaving for Spring applications
using AspectJ.
The Spring container instantiates and configures beans defined in your application
context. It is also possible to ask a bean factory to configure a pre-existing
object given the name of a bean definition containing the configuration to be applied.
The spring-aspects.jar
contains an annotation-driven aspect that exploits this
capability to allow dependency injection of any object. The support is intended to
be used for objects created outside of the control of any container. Domain objects
often fall into this category because they are often created programmatically using the
new
operator, or by an ORM tool as a result of a database query.
The @Configurable
annotation marks a class as eligible for Spring-driven
configuration. In the simplest case it can be used just as a marker annotation:
package com.xyz.myapp.domain; import org.springframework.beans.factory.annotation.Configurable; @Configurable public class Account { // ... }
When used as a marker interface in this way, Spring will configure new instances of the
annotated type ( Account
in this case) using a bean definition (typically
prototype-scoped) with the same name as the fully-qualified type name (
com.xyz.myapp.domain.Account
). Since the default name for a bean is the
fully-qualified name of its type, a convenient way to declare the prototype definition
is simply to omit the id
attribute:
<bean class="com.xyz.myapp.domain.Account" scope="prototype"> <property name="fundsTransferService" ref="fundsTransferService"/> </bean>
If you want to explicitly specify the name of the prototype bean definition to use, you can do so directly in the annotation:
package com.xyz.myapp.domain; import org.springframework.beans.factory.annotation.Configurable; @Configurable("account") public class Account { // ... }
Spring will now look for a bean definition named " account
" and use that as the
definition to configure new Account
instances.
You can also use autowiring to avoid having to specify a dedicated bean definition at
all. To have Spring apply autowiring use the ' autowire
' property of the
@Configurable
annotation: specify either @Configurable(autowire=Autowire.BY_TYPE)
or
@Configurable(autowire=Autowire.BY_NAME
for autowiring by type or by name
respectively. As an alternative, as of Spring 2.5 it is preferable to specify explicit,
annotation-driven dependency injection for your @Configurable
beans by using
@Autowired
or @Inject
at the field or method level (see Section 4.9, “Annotation-based container configuration”
for further details).
Finally you can enable Spring dependency checking for the object references in the newly
created and configured object by using the dependencyCheck
attribute (for example:
@Configurable(autowire=Autowire.BY_NAME,dependencyCheck=true)
). If this attribute is
set to true, then Spring will validate after configuration that all properties (which
are not primitives or collections) have been set.
Using the annotation on its own does nothing of course. It is the
AnnotationBeanConfigurerAspect
in spring-aspects.jar
that acts on the presence of
the annotation. In essence the aspect says "after returning from the initialization of a
new object of a type annotated with @Configurable
, configure the newly created object
using Spring in accordance with the properties of the annotation". In this context,
initialization refers to newly instantiated objects (e.g., objects instantiated with
the ' new
' operator) as well as to Serializable
objects that are undergoing
deserialization (e.g., via
readResolve()).
Note | |
---|---|
One of the key phrases in the above paragraph is in essence. For most cases, the
exact semantics of after returning from the initialization of a new object will be
fine… in this context, after initialization means that the dependencies will be
injected after the object has been constructed - this means that the dependencies
will not be available for use in the constructor bodies of the class. If you want the
dependencies to be injected before the constructor bodies execute, and thus be
available for use in the body of the constructors, then you need to define this on the
@Configurable(preConstruction=true) You can find out more information about the language semantics of the various pointcut types in AspectJ in this appendix of the AspectJ Programming Guide. |
For this to work the annotated types must be woven with the AspectJ weaver - you can
either use a build-time Ant or Maven task to do this (see for example the
AspectJ Development
Environment Guide) or load-time weaving (see Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”). The
AnnotationBeanConfigurerAspect
itself needs configuring by Spring (in order to obtain
a reference to the bean factory that is to be used to configure new objects). If you are
using Java based configuration simply add @EnableSpringConfigured
to any
@Configuration
class.
@Configuration @EnableSpringConfigured public class AppConfig { }
If you prefer XML based configuration, the Spring context
namespace defines a convenient context:spring-configured
element:
<context:spring-configured/>
Instances of @Configurable
objects created before the aspect has been configured
will result in a message being issued to the debug log and no configuration of the
object taking place. An example might be a bean in the Spring configuration that creates
domain objects when it is initialized by Spring. In this case you can use the
"depends-on" bean attribute to manually specify that the bean depends on the
configuration aspect.
<bean id="myService" class="com.xzy.myapp.service.MyService" depends-on="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect"> <!-- ... --> </bean>
Note | |
---|---|
Do not activate |
One of the goals of the @Configurable
support is to enable independent unit testing of
domain objects without the difficulties associated with hard-coded lookups. If
@Configurable
types have not been woven by AspectJ then the annotation has no affect
during unit testing, and you can simply set mock or stub property references in the
object under test and proceed as normal. If @Configurable
types have been woven by
AspectJ then you can still unit test outside of the container as normal, but you will
see a warning message each time that you construct an @Configurable
object indicating
that it has not been configured by Spring.
The AnnotationBeanConfigurerAspect
used to implement the @Configurable
support is an
AspectJ singleton aspect. The scope of a singleton aspect is the same as the scope of
static
members, that is to say there is one aspect instance per classloader that
defines the type. This means that if you define multiple application contexts within the
same classloader hierarchy you need to consider where to define the
@EnableSpringConfigured
bean and where to place spring-aspects.jar
on the classpath.
Consider a typical Spring web-app configuration with a shared parent application context
defining common business services and everything needed to support them, and one child
application context per servlet containing definitions particular to that servlet. All
of these contexts will co-exist within the same classloader hierarchy, and so the
AnnotationBeanConfigurerAspect
can only hold a reference to one of them. In this case
we recommend defining the @EnableSpringConfigured
bean in the shared (parent)
application context: this defines the services that you are likely to want to inject
into domain objects. A consequence is that you cannot configure domain objects with
references to beans defined in the child (servlet-specific) contexts using the
@Configurable mechanism (probably not something you want to do anyway!).
When deploying multiple web-apps within the same container, ensure that each
web-application loads the types in spring-aspects.jar
using its own classloader (for
example, by placing spring-aspects.jar
in 'WEB-INF/lib'
). If spring-aspects.jar
is
only added to the container wide classpath (and hence loaded by the shared parent
classloader), all web applications will share the same aspect instance which is probably
not what you want.
In addition to the @Configurable
aspect, spring-aspects.jar
contains an AspectJ
aspect that can be used to drive Spring’s transaction management for types and methods
annotated with the @Transactional
annotation. This is primarily intended for users who
want to use the Spring Framework’s transaction support outside of the Spring container.
The aspect that interprets @Transactional
annotations is the
AnnotationTransactionAspect
. When using this aspect, you must annotate the
implementation class (and/or methods within that class), not the interface (if
any) that the class implements. AspectJ follows Java’s rule that annotations on
interfaces are not inherited.
A @Transactional
annotation on a class specifies the default transaction semantics for
the execution of any public operation in the class.
A @Transactional
annotation on a method within the class overrides the default
transaction semantics given by the class annotation (if present). Methods with public
,
protected
, and default visibility may all be annotated. Annotating protected
and
default visibility methods directly is the only way to get transaction demarcation for
the execution of such methods.
For AspectJ programmers that want to use the Spring configuration and transaction
management support but don’t want to (or cannot) use annotations, spring-aspects.jar
also contains abstract
aspects you can extend to provide your own pointcut
definitions. See the sources for the AbstractBeanConfigurerAspect
and
AbstractTransactionAspect
aspects for more information. As an example, the following
excerpt shows how you could write an aspect to configure all instances of objects
defined in the domain model using prototype bean definitions that match the
fully-qualified class names:
public aspect DomainObjectConfiguration extends AbstractBeanConfigurerAspect { public DomainObjectConfiguration() { setBeanWiringInfoResolver(new ClassNameBeanWiringInfoResolver()); } // the creation of a new bean (any object in the domain model) protected pointcut beanCreation(Object beanInstance) : initialization(new(..)) && SystemArchitecture.inDomainModel() && this(beanInstance); }
When using AspectJ aspects with Spring applications, it is natural to both want and
expect to be able to configure such aspects using Spring. The AspectJ runtime itself is
responsible for aspect creation, and the means of configuring the AspectJ created
aspects via Spring depends on the AspectJ instantiation model (the ' per-xxx
' clause)
used by the aspect.
The majority of AspectJ aspects are singleton aspects. Configuration of these
aspects is very easy: simply create a bean definition referencing the aspect type as
normal, and include the bean attribute 'factory-method="aspectOf"'
. This ensures that
Spring obtains the aspect instance by asking AspectJ for it rather than trying to create
an instance itself. For example:
<bean id="profiler" class="com.xyz.profiler.Profiler" factory-method="aspectOf"> <property name="profilingStrategy" ref="jamonProfilingStrategy"/> </bean>
Non-singleton aspects are harder to configure: however it is possible to do so by
creating prototype bean definitions and using the @Configurable
support from
spring-aspects.jar
to configure the aspect instances once they have bean created by
the AspectJ runtime.
If you have some @AspectJ aspects that you want to weave with AspectJ (for example,
using load-time weaving for domain model types) and other @AspectJ aspects that you want
to use with Spring AOP, and these aspects are all configured using Spring, then you will
need to tell the Spring AOP @AspectJ autoproxying support which exact subset of the
@AspectJ aspects defined in the configuration should be used for autoproxying. You can
do this by using one or more <include/>
elements inside the <aop:aspectj-autoproxy/>
declaration. Each <include/>
element specifies a name pattern, and only beans with
names matched by at least one of the patterns will be used for Spring AOP autoproxy
configuration:
<aop:aspectj-autoproxy> <aop:include name="thisBean"/> <aop:include name="thatBean"/> </aop:aspectj-autoproxy>
Note | |
---|---|
Do not be misled by the name of the |
Load-time weaving (LTW) refers to the process of weaving AspectJ aspects into an application’s class files as they are being loaded into the Java virtual machine (JVM). The focus of this section is on configuring and using LTW in the specific context of the Spring Framework: this section is not an introduction to LTW though. For full details on the specifics of LTW and configuring LTW with just AspectJ (with Spring not being involved at all), see the LTW section of the AspectJ Development Environment Guide.
The value-add that the Spring Framework brings to AspectJ LTW is in enabling much finer-grained control over the weaving process. Vanilla AspectJ LTW is effected using a Java (5+) agent, which is switched on by specifying a VM argument when starting up a JVM. It is thus a JVM-wide setting, which may be fine in some situations, but often is a little too coarse. Spring-enabled LTW enables you to switch on LTW on a per-ClassLoader basis, which obviously is more fine-grained and which can make more sense in a single-JVM-multiple-application environment (such as is found in a typical application server environment).
Further, in certain environments, this support enables
load-time weaving without making any modifications to the application server’s launch
script that will be needed to add -javaagent:path/to/aspectjweaver.jar
or (as we
describe later in this section)
-javaagent:path/to/org.springframework.instrument-{version}.jar
(previously named
spring-agent.jar
). Developers simply modify one or more files that form the
application context to enable load-time weaving instead of relying on administrators who
typically are in charge of the deployment configuration such as the launch script.
Now that the sales pitch is over, let us first walk through a quick example of AspectJ LTW using Spring, followed by detailed specifics about elements introduced in the following example. For a complete example, please see the Petclinic sample application.
Let us assume that you are an application developer who has been tasked with diagnosing the cause of some performance problems in a system. Rather than break out a profiling tool, what we are going to do is switch on a simple profiling aspect that will enable us to very quickly get some performance metrics, so that we can then apply a finer-grained profiling tool to that specific area immediately afterwards.
Note | |
---|---|
The example presented here uses XML style configuration, it is also possible to
configure and use @AspectJ with Java Configuration. Specifically the
|
Here is the profiling aspect. Nothing too fancy, just a quick-and-dirty time-based profiler, using the @AspectJ-style of aspect declaration.
package foo; import org.aspectj.lang.ProceedingJoinPoint; import org.aspectj.lang.annotation.Aspect; import org.aspectj.lang.annotation.Around; import org.aspectj.lang.annotation.Pointcut; import org.springframework.util.StopWatch; import org.springframework.core.annotation.Order; @Aspect public class ProfilingAspect { @Around("methodsToBeProfiled()") public Object profile(ProceedingJoinPoint pjp) throws Throwable { StopWatch sw = new StopWatch(getClass().getSimpleName()); try { sw.start(pjp.getSignature().getName()); return pjp.proceed(); } finally { sw.stop(); System.out.println(sw.prettyPrint()); } } @Pointcut("execution(public * foo...(..))") public void methodsToBeProfiled(){} }
We will also need to create an ' META-INF/aop.xml
' file, to inform the AspectJ weaver
that we want to weave our ProfilingAspect
into our classes. This file convention,
namely the presence of a file (or files) on the Java classpath called '
META-INF/aop.xml
' is standard AspectJ.
<!DOCTYPE aspectj PUBLIC "-//AspectJ//DTD//EN" "http://www.eclipse.org/aspectj/dtd/aspectj.dtd"> <aspectj> <weaver> <!-- only weave classes in our application-specific packages --> <include within="foo.*"/> </weaver> <aspects> <!-- weave in just this aspect --> <aspect name="foo.ProfilingAspect"/> </aspects> </aspectj>
Now to the Spring-specific portion of the configuration. We need to configure a
LoadTimeWeaver
(all explained later, just take it on trust for now). This load-time
weaver is the essential component responsible for weaving the aspect configuration in
one or more ' META-INF/aop.xml
' files into the classes in your application. The good
thing is that it does not require a lot of configuration, as can be seen below (there
are some more options that you can specify, but these are detailed later).
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <!-- a service object; we will be profiling its methods --> <bean id="entitlementCalculationService" class="foo.StubEntitlementCalculationService"/> <!-- this switches on the load-time weaving --> <context:load-time-weaver/> </beans>
Now that all the required artifacts are in place - the aspect, the ' META-INF/aop.xml
'
file, and the Spring configuration -, let us create a simple driver class with a
main(..)
method to demonstrate the LTW in action.
package foo; import org.springframework.context.support.ClassPathXmlApplicationContext; public final class Main { public static void main(String[] args) { ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml", Main.class); EntitlementCalculationService entitlementCalculationService = (EntitlementCalculationService) ctx.getBean("entitlementCalculationService"); // the profiling aspect is woven around this method execution entitlementCalculationService.calculateEntitlement(); } }
There is one last thing to do. The introduction to this section did say that one could
switch on LTW selectively on a per- ClassLoader
basis with Spring, and this is true.
However, just for this example, we are going to use a Java agent (supplied with Spring)
to switch on the LTW. This is the command line we will use to run the above Main
class:
java -javaagent:C:/projects/foo/lib/global/spring-instrument.jar foo.Main
The ' -javaagent
' is a flag for specifying and enabling
agents
to instrument programs running on the JVM. The Spring Framework ships with such an
agent, the InstrumentationSavingAgent
, which is packaged in the
spring-instrument.jar
that was supplied as the value of the -javaagent
argument in
the above example.
The output from the execution of the Main
program will look something like that below.
(I have introduced a Thread.sleep(..)
statement into the calculateEntitlement()
implementation so that the profiler actually captures something other than 0
milliseconds - the 01234
milliseconds is not an overhead introduced by the AOP :) )
Calculating entitlement
StopWatch ProfilingAspect: running time (millis) = 1234
------ ----- ----------------------------
ms % Task name
------ ----- ----------------------------
01234 100% calculateEntitlement
Since this LTW is effected using full-blown AspectJ, we are not just limited to advising
Spring beans; the following slight variation on the Main
program will yield the same
result.
package foo; import org.springframework.context.support.ClassPathXmlApplicationContext; public final class Main { public static void main(String[] args) { new ClassPathXmlApplicationContext("beans.xml", Main.class); EntitlementCalculationService entitlementCalculationService = new StubEntitlementCalculationService(); // the profiling aspect will be woven around this method execution entitlementCalculationService.calculateEntitlement(); } }
Notice how in the above program we are simply bootstrapping the Spring container, and
then creating a new instance of the StubEntitlementCalculationService
totally outside
the context of Spring… the profiling advice still gets woven in.
The example admittedly is simplistic… however the basics of the LTW support in Spring have all been introduced in the above example, and the rest of this section will explain the why behind each bit of configuration and usage in detail.
Note | |
---|---|
The |
The aspects that you use in LTW have to be AspectJ aspects. They can be written in either the AspectJ language itself or you can write your aspects in the @AspectJ-style. It means that your aspects are then both valid AspectJ and Spring AOP aspects. Furthermore, the compiled aspect classes need to be available on the classpath.
The AspectJ LTW infrastructure is configured using one or more ' META-INF/aop.xml
'
files, that are on the Java classpath (either directly, or more typically in jar files).
The structure and contents of this file is detailed in the main AspectJ reference
documentation, and the interested reader is
referred to
that resource. (I appreciate that this section is brief, but the ' aop.xml
' file is
100% AspectJ - there is no Spring-specific information or semantics that apply to it,
and so there is no extra value that I can contribute either as a result), so rather than
rehash the quite satisfactory section that the AspectJ developers wrote, I am just
directing you there.)
At a minimum you will need the following libraries to use the Spring Framework’s support for AspectJ LTW:
spring-aop.jar
(version 2.5 or later, plus all mandatory dependencies)
aspectjweaver.jar
(version 1.6.8 or later)
If you are using the Spring-provided agent to enable instrumentation, you will also need:
spring-instrument.jar
The key component in Spring’s LTW support is the LoadTimeWeaver
interface (in the
org.springframework.instrument.classloading
package), and the numerous implementations
of it that ship with the Spring distribution. A LoadTimeWeaver
is responsible for
adding one or more java.lang.instrument.ClassFileTransformers
to a ClassLoader
at
runtime, which opens the door to all manner of interesting applications, one of which
happens to be the LTW of aspects.
Tip | |
---|---|
If you are unfamiliar with the idea of runtime class file transformation, you are
encouraged to read the Javadoc API documentation for the |
Configuring a LoadTimeWeaver
for a particular ApplicationContext
can be as easy as
adding one line. (Please note that you almost certainly will need to be using an
ApplicationContext
as your Spring container - typically a BeanFactory
will not be
enough because the LTW support makes use of BeanFactoryPostProcessors
.)
To enable the Spring Framework’s LTW support, you need to configure a LoadTimeWeaver
,
which typically is done using the @EnableLoadTimeWeaving
annotation.
@Configuration @EnableLoadTimeWeaving public class AppConfig { }
Alternatively, if you prefer XML based configuration, use the
<context:load-time-weaver/>
element. Note that the element is defined in the '
context
' namespace.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:load-time-weaver/> </beans>
The above configuration will define and register a number of LTW-specific infrastructure
beans for you automatically, such as a LoadTimeWeaver
and an AspectJWeavingEnabler
.
The default LoadTimeWeaver
is the DefaultContextLoadTimeWeaver
class, which attempts
to decorate an automatically detected LoadTimeWeaver
: the exact type of
LoadTimeWeaver
that will be automatically detected is dependent upon your runtime
environment (summarized in the following table).
Table 8.1. DefaultContextLoadTimeWeaver LoadTimeWeavers
Runtime Environment | LoadTimeWeaver implementation |
---|---|
Running in BEA’s Weblogic 10 |
|
Running in IBM WebSphere Application Server 7 |
|
Running in GlassFish |
|
Running in JBoss AS |
|
JVM started with Spring |
|
Fallback, expecting the underlying ClassLoader to follow common conventions (e.g.
applicable to |
|
Note that these are just the LoadTimeWeavers
that are autodetected when using the
DefaultContextLoadTimeWeaver
: it is of course possible to specify exactly which
LoadTimeWeaver
implementation that you wish to use.
To specify a specific LoadTimeWeaver
with Java configuration implement the
LoadTimeWeavingConfigurer
interface and override the getLoadTimeWeaver()
method:
@Configuration @EnableLoadTimeWeaving public class AppConfig implements LoadTimeWeavingConfigurer { @Override public LoadTimeWeaver getLoadTimeWeaver() { return new ReflectiveLoadTimeWeaver(); } }
If you are using XML based configuration you can specify the fully-qualified classname
as the value of the ' weaver-class
' attribute on the <context:load-time-weaver/>
element:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:load-time-weaver weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/> </beans>
The LoadTimeWeaver
that is defined and registered by the configuration can be later
retrieved from the Spring container using the well-known name ' loadTimeWeaver
'.
Remember that the LoadTimeWeaver
exists just as a mechanism for Spring’s LTW
infrastructure to add one or more ClassFileTransformers
. The actual
ClassFileTransformer
that does the LTW is the ClassPreProcessorAgentAdapter
(from
the org.aspectj.weaver.loadtime
package) class. See the class-level Javadoc for the
ClassPreProcessorAgentAdapter
class for further details, because the specifics of how
the weaving is actually effected is beyond the scope of this section.
There is one final attribute of the configuration left to discuss: the '
aspectjWeaving
' attribute (or ' aspectj-weaving
' if you are using XML). This is a
simple attribute that controls whether LTW is enabled or not, it is as simple as that.
It accepts one of three possible values, summarized below, with the default value if the
attribute is not present being ' autodetect
'
Table 8.2. AspectJ weaving attribute values
Annotation Value | XML Value | Explanation |
---|---|---|
|
| AspectJ weaving is on, and aspects will be woven at load-time as appropriate. |
|
| LTW is off… no aspect will be woven at load-time. |
|
| If the Spring LTW infrastructure can find at least one ' |
This last section contains any additional settings and configuration that you will need when using Spring’s LTW support in environments such as application servers and web containers.
Apache Tomcat's default class loader does not support class
transformation which is why Spring provides an enhanced implementation that addresses
this need. Named TomcatInstrumentableClassLoader
, the loader works on Tomcat 5.0 and
above and can be registered individually for each web application as follows:
org.springframework.instrument.tomcat.jar
into $CATALINA_HOME/lib, where
$CATALINA_HOME represents the root of the Tomcat installation)
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/> </Context>
Apache Tomcat 6.0.x (similar to 5.0.x/5.5.x) series supports several context locations:
For efficiency, the embedded per-web-app configuration style is recommended because it will impact only applications that use the custom class loader and does not require any changes to the server configuration. See the Tomcat 6.0.x documentation for more details about available context locations.
org.springframework.instrument.tomcat.jar
into $CATALINA_HOME/server/lib,
where $CATALINA_HOME represents the root of the Tomcat installation.
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader"/> </Context>
Tomcat 5.0.x and 5.5.x series supports several context locations:
For efficiency, the embedded web-app configuration style is recommended recommended because it will impact only applications that use the class loader. See the Tomcat 5.x documentation for more details about available context locations.
Tomcat versions prior to 5.5.20 contained a bug in the XML configuration parsing that
prevented usage of the Loader
tag inside server.xml configuration, regardless of
whether a class loader is specified or whether it is the official or a custom one. See
Tomcat’s bugzilla for more
details.
In Tomcat 5.5.x, versions 5.5.20 or later, you should set
useSystemClassLoaderAsParent to false
to fix this problem:
<Context path="/myWebApp" docBase="/my/webApp/location"> <Loader loaderClass="org.springframework.instrument.classloading.tomcat.TomcatInstrumentableClassLoader" useSystemClassLoaderAsParent="false"/> </Context>
This setting is not needed on Tomcat 6 or higher.
Alternatively, consider the use of the Spring-provided generic VM agent, to be specified in Tomcat’s launch script (see above). This will make instrumentation available to all deployed web applications, no matter what ClassLoader they happen to run on.
Recent versions of WebLogic Server (version 10 and above), IBM WebSphere Application
Server (version 7 and above), Resin (3.1 and above) and JBoss (5.x or above) provide a
ClassLoader that is capable of local instrumentation. Spring’s native LTW leverages such
ClassLoaders to enable AspectJ weaving. You can enable LTW by simply activating
load-time weaving as described earlier. Specifically, you do not need to modify the
launch script to add -javaagent:path/to/spring-instrument.jar
.
Note that GlassFish instrumentation-capable ClassLoader is available only in its EAR environment. For GlassFish web applications, follow the Tomcat setup instructions as outlined above.
Note that on JBoss 6.x, the app server scanning needs to be disabled to prevent it from
loading the classes before the application actually starts. A quick workaround is to add
to your artifact a file named WEB-INF/jboss-scanning.xml
with the following content:
<scanning xmlns="urn:jboss:scanning:1.0"/>
When class instrumentation is required in environments that do not support or are not
supported by the existing LoadTimeWeaver
implementations, a JDK agent can be the only
solution. For such cases, Spring provides InstrumentationLoadTimeWeaver
, which
requires a Spring-specific (but very general) VM agent,
org.springframework.instrument-{version}.jar
(previously named spring-agent.jar
).
To use it, you must start the virtual machine with the Spring agent, by supplying the following JVM options:
-javaagent:/path/to/org.springframework.instrument-{version}.jar
Note that this requires modification of the VM launch script which may prevent you from using this in application server environments (depending on your operation policies). Additionally, the JDK agent will instrument the entire VM which can prove expensive.
For performance reasons, it is recommended to use this configuration only if your target environment (such as Jetty) does not have (or does not support) a dedicated LTW.
More information on AspectJ can be found on the AspectJ website.
The book Eclipse AspectJ by Adrian Colyer et. al. (Addison-Wesley, 2005) provides a comprehensive introduction and reference for the AspectJ language.
The book AspectJ in Action by Ramnivas Laddad (Manning, 2003) comes highly recommended; the focus of the book is on AspectJ, but a lot of general AOP themes are explored (in some depth).
The previous chapter described the Spring’s support for AOP using @AspectJ and schema-based aspect definitions. In this chapter we discuss the lower-level Spring AOP APIs and the AOP support used in Spring 1.2 applications. For new applications, we recommend the use of the Spring 2.0 and later AOP support described in the previous chapter, but when working with existing applications, or when reading books and articles, you may come across Spring 1.2 style examples. Spring 4.0 is backwards compatible with Spring 1.2 and everything described in this chapter is fully supported in Spring 4.0.
Let’s look at how Spring handles the crucial pointcut concept.
Spring’s pointcut model enables pointcut reuse independent of advice types. It’s possible to target different advice using the same pointcut.
The org.springframework.aop.Pointcut
interface is the central interface, used to
target advices to particular classes and methods. The complete interface is shown below:
public interface Pointcut { ClassFilter getClassFilter(); MethodMatcher getMethodMatcher(); }
Splitting the Pointcut
interface into two parts allows reuse of class and method
matching parts, and fine-grained composition operations (such as performing a "union"
with another method matcher).
The ClassFilter
interface is used to restrict the pointcut to a given set of target
classes. If the matches()
method always returns true, all target classes will be
matched:
public interface ClassFilter { boolean matches(Class clazz); }
The MethodMatcher
interface is normally more important. The complete interface is
shown below:
public interface MethodMatcher { boolean matches(Method m, Class targetClass); boolean isRuntime(); boolean matches(Method m, Class targetClass, Object[] args); }
The matches(Method, Class)
method is used to test whether this pointcut will ever
match a given method on a target class. This evaluation can be performed when an AOP
proxy is created, to avoid the need for a test on every method invocation. If the
2-argument matches method returns true for a given method, and the isRuntime()
method
for the MethodMatcher returns true, the 3-argument matches method will be invoked on
every method invocation. This enables a pointcut to look at the arguments passed to the
method invocation immediately before the target advice is to execute.
Most MethodMatchers are static, meaning that their isRuntime()
method returns false.
In this case, the 3-argument matches method will never be invoked.
Tip | |
---|---|
If possible, try to make pointcuts static, allowing the AOP framework to cache the results of pointcut evaluation when an AOP proxy is created. |
Spring supports operations on pointcuts: notably, union and intersection.
Since 2.0, the most important type of pointcut used by Spring is
org.springframework.aop.aspectj.AspectJExpressionPointcut
. This is a pointcut that
uses an AspectJ supplied library to parse an AspectJ pointcut expression string.
See the previous chapter for a discussion of supported AspectJ pointcut primitives.
Spring provides several convenient pointcut implementations. Some can be used out of the box; others are intended to be subclassed in application-specific pointcuts.
Static pointcuts are based on method and target class, and cannot take into account the method’s arguments. Static pointcuts are sufficient - and best - for most usages. It’s possible for Spring to evaluate a static pointcut only once, when a method is first invoked: after that, there is no need to evaluate the pointcut again with each method invocation.
Let’s consider some static pointcut implementations included with Spring.
One obvious way to specify static pointcuts is regular expressions. Several AOP
frameworks besides Spring make this possible.
org.springframework.aop.support.JdkRegexpMethodPointcut
is a generic regular
expression pointcut, using the regular expression support in JDK 1.4+.
Using the JdkRegexpMethodPointcut
class, you can provide a list of pattern Strings. If
any of these is a match, the pointcut will evaluate to true. (So the result is
effectively the union of these pointcuts.)
The usage is shown below:
<bean id="settersAndAbsquatulatePointcut" class="org.springframework.aop.support.JdkRegexpMethodPointcut"> <property name="patterns"> <list> <value>.set.</value> <value>.*absquatulate</value> </list> </property> </bean>
Spring provides a convenience class, RegexpMethodPointcutAdvisor
, that allows us to
also reference an Advice (remember that an Advice can be an interceptor, before advice,
throws advice etc.). Behind the scenes, Spring will use a JdkRegexpMethodPointcut
.
Using RegexpMethodPointcutAdvisor
simplifies wiring, as the one bean encapsulates both
pointcut and advice, as shown below:
<bean id="settersAndAbsquatulateAdvisor" class="org.springframework.aop.support.RegexpMethodPointcutAdvisor"> <property name="advice"> <ref bean="beanNameOfAopAllianceInterceptor"/> </property> <property name="patterns"> <list> <value>.set.</value> <value>.*absquatulate</value> </list> </property> </bean>
RegexpMethodPointcutAdvisor can be used with any Advice type.
Dynamic pointcuts are costlier to evaluate than static pointcuts. They take into account method arguments, as well as static information. This means that they must be evaluated with every method invocation; the result cannot be cached, as arguments will vary.
The main example is the control flow
pointcut.
Spring control flow pointcuts are conceptually similar to AspectJ cflow pointcuts,
although less powerful. (There is currently no way to specify that a pointcut executes
below a join point matched by another pointcut.) A control flow pointcut matches the
current call stack. For example, it might fire if the join point was invoked by a method
in the com.mycompany.web
package, or by the SomeCaller
class. Control flow pointcuts
are specified using the org.springframework.aop.support.ControlFlowPointcut
class.
Note | |
---|---|
Control flow pointcuts are significantly more expensive to evaluate at runtime than even other dynamic pointcuts. In Java 1.4, the cost is about 5 times that of other dynamic pointcuts. |
Spring provides useful pointcut superclasses to help you to implement your own pointcuts.
Because static pointcuts are most useful, you’ll probably subclass StaticMethodMatcherPointcut, as shown below. This requires implementing just one abstract method (although it’s possible to override other methods to customize behavior):
class TestStaticPointcut extends StaticMethodMatcherPointcut { public boolean matches(Method m, Class targetClass) { // return true if custom criteria match } }
There are also superclasses for dynamic pointcuts.
You can use custom pointcuts with any advice type in Spring 1.0 RC2 and above.
Because pointcuts in Spring AOP are Java classes, rather than language features (as in AspectJ) it’s possible to declare custom pointcuts, whether static or dynamic. Custom pointcuts in Spring can be arbitrarily complex. However, using the AspectJ pointcut expression language is recommended if possible.
Note | |
---|---|
Later versions of Spring may offer support for "semantic pointcuts" as offered by JAC: for example, "all methods that change instance variables in the target object." |
Let’s now look at how Spring AOP handles advice.
Each advice is a Spring bean. An advice instance can be shared across all advised objects, or unique to each advised object. This corresponds to per-class or per-instance advice.
Per-class advice is used most often. It is appropriate for generic advice such as transaction advisors. These do not depend on the state of the proxied object or add new state; they merely act on the method and arguments.
Per-instance advice is appropriate for introductions, to support mixins. In this case, the advice adds state to the proxied object.
It’s possible to use a mix of shared and per-instance advice in the same AOP proxy.
Spring provides several advice types out of the box, and is extensible to support arbitrary advice types. Let us look at the basic concepts and standard advice types.
The most fundamental advice type in Spring is interception around advice.
Spring is compliant with the AOP Alliance interface for around advice using method interception. MethodInterceptors implementing around advice should implement the following interface:
public interface MethodInterceptor extends Interceptor { Object invoke(MethodInvocation invocation) throws Throwable; }
The MethodInvocation
argument to the invoke()
method exposes the method being
invoked; the target join point; the AOP proxy; and the arguments to the method. The
invoke()
method should return the invocation’s result: the return value of the join
point.
A simple MethodInterceptor
implementation looks as follows:
public class DebugInterceptor implements MethodInterceptor { public Object invoke(MethodInvocation invocation) throws Throwable { System.out.println("Before: invocation=[" + invocation + "]"); Object rval = invocation.proceed(); System.out.println("Invocation returned"); return rval; } }
Note the call to the MethodInvocation’s proceed()
method. This proceeds down the
interceptor chain towards the join point. Most interceptors will invoke this method, and
return its return value. However, a MethodInterceptor, like any around advice, can
return a different value or throw an exception rather than invoke the proceed method.
However, you don’t want to do this without good reason!
Note | |
---|---|
MethodInterceptors offer interoperability with other AOP Alliance-compliant AOP implementations. The other advice types discussed in the remainder of this section implement common AOP concepts, but in a Spring-specific way. While there is an advantage in using the most specific advice type, stick with MethodInterceptor around advice if you are likely to want to run the aspect in another AOP framework. Note that pointcuts are not currently interoperable between frameworks, and the AOP Alliance does not currently define pointcut interfaces. |
A simpler advice type is a before advice. This does not need a MethodInvocation
object, since it will only be called before entering the method.
The main advantage of a before advice is that there is no need to invoke the proceed()
method, and therefore no possibility of inadvertently failing to proceed down the
interceptor chain.
The MethodBeforeAdvice
interface is shown below. (Spring’s API design would allow for
field before advice, although the usual objects apply to field interception and it’s
unlikely that Spring will ever implement it).
public interface MethodBeforeAdvice extends BeforeAdvice { void before(Method m, Object[] args, Object target) throws Throwable; }
Note the return type is void
. Before advice can insert custom behavior before the join
point executes, but cannot change the return value. If a before advice throws an
exception, this will abort further execution of the interceptor chain. The exception
will propagate back up the interceptor chain. If it is unchecked, or on the signature of
the invoked method, it will be passed directly to the client; otherwise it will be
wrapped in an unchecked exception by the AOP proxy.
An example of a before advice in Spring, which counts all method invocations:
public class CountingBeforeAdvice implements MethodBeforeAdvice { private int count; public void before(Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
Tip | |
---|---|
Before advice can be used with any pointcut. |
Throws advice is invoked after the return of the join point if the join point threw
an exception. Spring offers typed throws advice. Note that this means that the
org.springframework.aop.ThrowsAdvice
interface does not contain any methods: It is a
tag interface identifying that the given object implements one or more typed throws
advice methods. These should be in the form of:
afterThrowing([Method, args, target], subclassOfThrowable)
Only the last argument is required. The method signatures may have either one or four arguments, depending on whether the advice method is interested in the method and arguments. The following classes are examples of throws advice.
The advice below is invoked if a RemoteException
is thrown (including subclasses):
public class RemoteThrowsAdvice implements ThrowsAdvice { public void afterThrowing(RemoteException ex) throws Throwable { // Do something with remote exception } }
The following advice is invoked if a ServletException
is thrown. Unlike the above
advice, it declares 4 arguments, so that it has access to the invoked method, method
arguments and target object:
public class ServletThrowsAdviceWithArguments implements ThrowsAdvice { public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) { // Do something with all arguments } }
The final example illustrates how these two methods could be used in a single class,
which handles both RemoteException
and ServletException
. Any number of throws advice
methods can be combined in a single class.
public static class CombinedThrowsAdvice implements ThrowsAdvice { public void afterThrowing(RemoteException ex) throws Throwable { // Do something with remote exception } public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) { // Do something with all arguments } }
Note | |
---|---|
If a throws-advice method throws an exception itself, it will override the original exception (i.e. change the exception thrown to the user). The overriding exception will typically be a RuntimeException; this is compatible with any method signature. However, if a throws-advice method throws a checked exception, it will have to match the declared exceptions of the target method and is hence to some degree coupled to specific target method signatures. Do not throw an undeclared checked exception that is incompatible with the target method’s signature! |
Tip | |
---|---|
Throws advice can be used with any pointcut. |
An after returning advice in Spring must implement the org.springframework.aop.AfterReturningAdvice interface, shown below:
public interface AfterReturningAdvice extends Advice { void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable; }
An after returning advice has access to the return value (which it cannot modify), invoked method, methods arguments and target.
The following after returning advice counts all successful method invocations that have not thrown exceptions:
public class CountingAfterReturningAdvice implements AfterReturningAdvice { private int count; public void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
This advice doesn’t change the execution path. If it throws an exception, this will be thrown up the interceptor chain instead of the return value.
Tip | |
---|---|
After returning advice can be used with any pointcut. |
Spring treats introduction advice as a special kind of interception advice.
Introduction requires an IntroductionAdvisor
, and an IntroductionInterceptor
,
implementing the following interface:
public interface IntroductionInterceptor extends MethodInterceptor { boolean implementsInterface(Class intf); }
The invoke()
method inherited from the AOP Alliance MethodInterceptor
interface must
implement the introduction: that is, if the invoked method is on an introduced
interface, the introduction interceptor is responsible for handling the method call - it
cannot invoke proceed()
.
Introduction advice cannot be used with any pointcut, as it applies only at class,
rather than method, level. You can only use introduction advice with the
IntroductionAdvisor
, which has the following methods:
public interface IntroductionAdvisor extends Advisor, IntroductionInfo { ClassFilter getClassFilter(); void validateInterfaces() throws IllegalArgumentException; } public interface IntroductionInfo { Class[] getInterfaces(); }
There is no MethodMatcher
, and hence no Pointcut
, associated with introduction
advice. Only class filtering is logical.
The getInterfaces()
method returns the interfaces introduced by this advisor.
The validateInterfaces()
method is used internally to see whether or not the
introduced interfaces can be implemented by the configured IntroductionInterceptor
.
Let’s look at a simple example from the Spring test suite. Let’s suppose we want to introduce the following interface to one or more objects:
public interface Lockable { void lock(); void unlock(); boolean locked(); }
This illustrates a mixin. We want to be able to cast advised objects to Lockable,
whatever their type, and call lock and unlock methods. If we call the lock() method, we
want all setter methods to throw a LockedException
. Thus we can add an aspect that
provides the ability to make objects immutable, without them having any knowledge of it:
a good example of AOP.
Firstly, we’ll need an IntroductionInterceptor
that does the heavy lifting. In this
case, we extend the org.springframework.aop.support.DelegatingIntroductionInterceptor
convenience class. We could implement IntroductionInterceptor directly, but using
DelegatingIntroductionInterceptor
is best for most cases.
The DelegatingIntroductionInterceptor
is designed to delegate an introduction to an
actual implementation of the introduced interface(s), concealing the use of interception
to do so. The delegate can be set to any object using a constructor argument; the
default delegate (when the no-arg constructor is used) is this. Thus in the example
below, the delegate is the LockMixin
subclass of DelegatingIntroductionInterceptor
.
Given a delegate (by default itself), a DelegatingIntroductionInterceptor
instance
looks for all interfaces implemented by the delegate (other than
IntroductionInterceptor), and will support introductions against any of them. It’s
possible for subclasses such as LockMixin
to call the suppressInterface(Class intf)
method to suppress interfaces that should not be exposed. However, no matter how many
interfaces an IntroductionInterceptor
is prepared to support, the
IntroductionAdvisor
used will control which interfaces are actually exposed. An
introduced interface will conceal any implementation of the same interface by the target.
Thus LockMixin
extends DelegatingIntroductionInterceptor
and implements Lockable
itself. The superclass automatically picks up that Lockable can be supported for
introduction, so we don’t need to specify that. We could introduce any number of
interfaces in this way.
Note the use of the locked
instance variable. This effectively adds additional state
to that held in the target object.
public class LockMixin extends DelegatingIntroductionInterceptor implements Lockable { private boolean locked; public void lock() { this.locked = true; } public void unlock() { this.locked = false; } public boolean locked() { return this.locked; } public Object invoke(MethodInvocation invocation) throws Throwable { if (locked() && invocation.getMethod().getName().indexOf("set") == 0) { throw new LockedException(); } return super.invoke(invocation); } }
Often it isn’t necessary to override the invoke()
method: the
DelegatingIntroductionInterceptor
implementation - which calls the delegate method if
the method is introduced, otherwise proceeds towards the join point - is usually
sufficient. In the present case, we need to add a check: no setter method can be invoked
if in locked mode.
The introduction advisor required is simple. All it needs to do is hold a distinct
LockMixin
instance, and specify the introduced interfaces - in this case, just
Lockable
. A more complex example might take a reference to the introduction
interceptor (which would be defined as a prototype): in this case, there’s no
configuration relevant for a LockMixin
, so we simply create it using new
.
public class LockMixinAdvisor extends DefaultIntroductionAdvisor { public LockMixinAdvisor() { super(new LockMixin(), Lockable.class); } }
We can apply this advisor very simply: it requires no configuration. (However, it is
necessary: It’s impossible to use an IntroductionInterceptor
without an
IntroductionAdvisor.) As usual with introductions, the advisor must be per-instance,
as it is stateful. We need a different instance of LockMixinAdvisor
, and hence
LockMixin
, for each advised object. The advisor comprises part of the advised object’s
state.
We can apply this advisor programmatically, using the Advised.addAdvisor()
method, or
(the recommended way) in XML configuration, like any other advisor. All proxy creation
choices discussed below, including "auto proxy creators," correctly handle introductions
and stateful mixins.
In Spring, an Advisor is an aspect that contains just a single advice object associated with a pointcut expression.
Apart from the special case of introductions, any advisor can be used with any advice.
org.springframework.aop.support.DefaultPointcutAdvisor
is the most commonly used
advisor class. For example, it can be used with a MethodInterceptor
, BeforeAdvice
or
ThrowsAdvice
.
It is possible to mix advisor and advice types in Spring in the same AOP proxy. For example, you could use a interception around advice, throws advice and before advice in one proxy configuration: Spring will automatically create the necessary interceptor chain.
If you’re using the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects - and you should be! - you will want to use one of Spring’s AOP FactoryBeans. (Remember that a factory bean introduces a layer of indirection, enabling it to create objects of a different type.)
Note | |
---|---|
The Spring AOP support also uses factory beans under the covers. |
The basic way to create an AOP proxy in Spring is to use the org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts and advice that will apply, and their ordering. However, there are simpler options that are preferable if you don’t need such control.
The ProxyFactoryBean
, like other Spring FactoryBean
implementations, introduces a
level of indirection. If you define a ProxyFactoryBean
with name foo
, what objects
referencing foo
see is not the ProxyFactoryBean
instance itself, but an object
created by the ProxyFactoryBean
's implementation of the getObject()
method. This
method will create an AOP proxy wrapping a target object.
One of the most important benefits of using a ProxyFactoryBean
or another IoC-aware
class to create AOP proxies, is that it means that advices and pointcuts can also be
managed by IoC. This is a powerful feature, enabling certain approaches that are hard to
achieve with other AOP frameworks. For example, an advice may itself reference
application objects (besides the target, which should be available in any AOP
framework), benefiting from all the pluggability provided by Dependency Injection.
In common with most FactoryBean
implementations provided with Spring, the
ProxyFactoryBean
class is itself a JavaBean. Its properties are used to:
Some key properties are inherited from org.springframework.aop.framework.ProxyConfig
(the superclass for all AOP proxy factories in Spring). These key properties include:
proxyTargetClass
: true
if the target class is to be proxied, rather than the
target class' interfaces. If this property value is set to true
, then CGLIB proxies
will be created (but see also Section 9.5.3, “JDK- and CGLIB-based proxies”).
optimize
: controls whether or not aggressive optimizations are applied to proxies
created via CGLIB. One should not blithely use this setting unless one fully
understands how the relevant AOP proxy handles optimization. This is currently used
only for CGLIB proxies; it has no effect with JDK dynamic proxies.
frozen
: if a proxy configuration is frozen
, then changes to the configuration are
no longer allowed. This is useful both as a slight optimization and for those cases
when you don’t want callers to be able to manipulate the proxy (via the Advised
interface) after the proxy has been created. The default value of this property is
false
, so changes such as adding additional advice are allowed.
exposeProxy
: determines whether or not the current proxy should be exposed in a
ThreadLocal
so that it can be accessed by the target. If a target needs to obtain
the proxy and the exposeProxy
property is set to true
, the target can use the
AopContext.currentProxy()
method.
Other properties specific to ProxyFactoryBean
include:
proxyInterfaces
: array of String interface names. If this isn’t supplied, a CGLIB
proxy for the target class will be used (but see also Section 9.5.3, “JDK- and CGLIB-based proxies”).
interceptorNames
: String array of Advisor
, interceptor or other advice names to
apply. Ordering is significant, on a first come-first served basis. That is to say
that the first interceptor in the list will be the first to be able to intercept the
invocation.
The names are bean names in the current factory, including bean names from ancestor
factories. You can’t mention bean references here since doing so would result in the
ProxyFactoryBean
ignoring the singleton setting of the advice.
You can append an interceptor name with an asterisk ( *
). This will result in the
application of all advisor beans with names starting with the part before the asterisk
to be applied. An example of using this feature can be found in Section 9.5.6, “Using global advisors”.
getObject()
method is called. Several FactoryBean
implementations offer
such a method. The default value is true
. If you want to use stateful advice - for
example, for stateful mixins - use prototype advices along with a singleton value of
false
.
This section serves as the definitive documentation on how the ProxyFactoryBean
chooses to create one of either a JDK- and CGLIB-based proxy for a particular target
object (that is to be proxied).
Note | |
---|---|
The behavior of the |
If the class of a target object that is to be proxied (hereafter simply referred to as
the target class) doesn’t implement any interfaces, then a CGLIB-based proxy will be
created. This is the easiest scenario, because JDK proxies are interface based, and no
interfaces means JDK proxying isn’t even possible. One simply plugs in the target bean,
and specifies the list of interceptors via the interceptorNames
property. Note that a
CGLIB-based proxy will be created even if the proxyTargetClass
property of the
ProxyFactoryBean
has been set to false
. (Obviously this makes no sense, and is best
removed from the bean definition because it is at best redundant, and at worst
confusing.)
If the target class implements one (or more) interfaces, then the type of proxy that is
created depends on the configuration of the ProxyFactoryBean
.
If the proxyTargetClass
property of the ProxyFactoryBean
has been set to true
,
then a CGLIB-based proxy will be created. This makes sense, and is in keeping with the
principle of least surprise. Even if the proxyInterfaces
property of the
ProxyFactoryBean
has been set to one or more fully qualified interface names, the fact
that the proxyTargetClass
property is set to true
will cause CGLIB-based
proxying to be in effect.
If the proxyInterfaces
property of the ProxyFactoryBean
has been set to one or more
fully qualified interface names, then a JDK-based proxy will be created. The created
proxy will implement all of the interfaces that were specified in the proxyInterfaces
property; if the target class happens to implement a whole lot more interfaces than
those specified in the proxyInterfaces
property, that is all well and good but those
additional interfaces will not be implemented by the returned proxy.
If the proxyInterfaces
property of the ProxyFactoryBean
has not been set, but
the target class does implement one (or more) interfaces, then the
ProxyFactoryBean
will auto-detect the fact that the target class does actually
implement at least one interface, and a JDK-based proxy will be created. The interfaces
that are actually proxied will be all of the interfaces that the target class
implements; in effect, this is the same as simply supplying a list of each and every
interface that the target class implements to the proxyInterfaces
property. However,
it is significantly less work, and less prone to typos.
Let’s look at a simple example of ProxyFactoryBean
in action. This example involves:
<bean id="personTarget" class="com.mycompany.PersonImpl"> <property name="name" value="Tony"/> <property name="age" value="51"/> </bean> <bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty" value="Custom string property value"/> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"> </bean> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces" value="com.mycompany.Person"/> <property name="target" ref="personTarget"/> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
Note that the interceptorNames
property takes a list of String: the bean names of the
interceptor or advisors in the current factory. Advisors, interceptors, before, after
returning and throws advice objects can be used. The ordering of advisors is significant.
Note | |
---|---|
You might be wondering why the list doesn’t hold bean references. The reason for this is that if the ProxyFactoryBean’s singleton property is set to false, it must be able to return independent proxy instances. If any of the advisors is itself a prototype, an independent instance would need to be returned, so it’s necessary to be able to obtain an instance of the prototype from the factory; holding a reference isn’t sufficient. |
The "person" bean definition above can be used in place of a Person implementation, as follows:
Person person = (Person) factory.getBean("person");
Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary Java object:
<bean id="personUser" class="com.mycompany.PersonUser"> <property name="person"><ref bean="person"/></property> </bean>
The PersonUser
class in this example would expose a property of type Person. As far as
it’s concerned, the AOP proxy can be used transparently in place of a "real" person
implementation. However, its class would be a dynamic proxy class. It would be possible
to cast it to the Advised
interface (discussed below).
It’s possible to conceal the distinction between target and proxy using an anonymous
inner bean, as follows. Only the ProxyFactoryBean
definition is different; the
advice is included only for completeness:
<bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty" value="Custom string property value"/> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces" value="com.mycompany.Person"/> <!-- Use inner bean, not local reference to target --> <property name="target"> <bean class="com.mycompany.PersonImpl"> <property name="name" value="Tony"/> <property name="age" value="51"/> </bean> </property> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
This has the advantage that there’s only one object of type Person
: useful if we want
to prevent users of the application context from obtaining a reference to the un-advised
object, or need to avoid any ambiguity with Spring IoC autowiring. There’s also
arguably an advantage in that the ProxyFactoryBean definition is self-contained.
However, there are times when being able to obtain the un-advised target from the
factory might actually be an advantage: for example, in certain test scenarios.
What if you need to proxy a class, rather than one or more interfaces?
Imagine that in our example above, there was no Person
interface: we needed to advise
a class called Person
that didn’t implement any business interface. In this case, you
can configure Spring to use CGLIB proxying, rather than dynamic proxies. Simply set the
proxyTargetClass
property on the ProxyFactoryBean above to true. While it’s best to
program to interfaces, rather than classes, the ability to advise classes that don’t
implement interfaces can be useful when working with legacy code. (In general, Spring
isn’t prescriptive. While it makes it easy to apply good practices, it avoids forcing a
particular approach.)
If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.
CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this generated subclass to delegate method calls to the original target: the subclass is used to implement the Decorator pattern, weaving in the advice.
CGLIB proxying should generally be transparent to users. However, there are some issues to consider:
Final
methods can’t be advised, as they can’t be overridden.
There’s little performance difference between CGLIB proxying and dynamic proxies. As of Spring 1.0, dynamic proxies are slightly faster. However, this may change in the future. Performance should not be a decisive consideration in this case.
By appending an asterisk to an interceptor name, all advisors with bean names matching the part before the asterisk, will be added to the advisor chain. This can come in handy if you need to add a standard set of global advisors:
<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="target" ref="service"/> <property name="interceptorNames"> <list> <value>global*</value> </list> </property> </bean> <bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/>
Especially when defining transactional proxies, you may end up with many similar proxy definitions. The use of parent and child bean definitions, along with inner bean definitions, can result in much cleaner and more concise proxy definitions.
First a parent, template, bean definition is created for the proxy:
<bean id="txProxyTemplate" abstract="true" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributes"> <props> <prop key="*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
This will never be instantiated itself, so may actually be incomplete. Then each proxy which needs to be created is just a child bean definition, which wraps the target of the proxy as an inner bean definition, since the target will never be used on its own anyway.
<bean id="myService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MyServiceImpl"> </bean> </property> </bean>
It is of course possible to override properties from the parent template, such as in this case, the transaction propagation settings:
<bean id="mySpecialService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MySpecialServiceImpl"> </bean> </property> <property name="transactionAttributes"> <props> <prop key="get*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="find*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="load*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="store*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute, as described previously, so that it may not actually ever be instantiated. Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. It is therefore important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set the abstract attribute to true, otherwise the application context will actually try to pre-instantiate it.
It’s easy to create AOP proxies programmatically using Spring. This enables you to use Spring AOP without dependency on Spring IoC.
The following listing shows creation of a proxy for a target object, with one interceptor and one advisor. The interfaces implemented by the target object will automatically be proxied:
ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl);
factory.addAdvice(myMethodInterceptor);
factory.addAdvisor(myAdvisor);
MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();
The first step is to construct an object of type
org.springframework.aop.framework.ProxyFactory
. You can create this with a target
object, as in the above example, or specify the interfaces to be proxied in an alternate
constructor.
You can add advices (with interceptors as a specialized kind of advice) and/or advisors, and manipulate them for the life of the ProxyFactory. If you add an IntroductionInterceptionAroundAdvisor, you can cause the proxy to implement additional interfaces.
There are also convenience methods on ProxyFactory (inherited from AdvisedSupport
)
which allow you to add other advice types such as before and throws advice.
AdvisedSupport is the superclass of both ProxyFactory and ProxyFactoryBean.
Tip | |
---|---|
Integrating AOP proxy creation with the IoC framework is best practice in most applications. We recommend that you externalize configuration from Java code with AOP, as in general. |
However you create AOP proxies, you can manipulate them using the
org.springframework.aop.framework.Advised
interface. Any AOP proxy can be cast to this
interface, whichever other interfaces it implements. This interface includes the
following methods:
Advisor[] getAdvisors(); void addAdvice(Advice advice) throws AopConfigException; void addAdvice(int pos, Advice advice) throws AopConfigException; void addAdvisor(Advisor advisor) throws AopConfigException; void addAdvisor(int pos, Advisor advisor) throws AopConfigException; int indexOf(Advisor advisor); boolean removeAdvisor(Advisor advisor) throws AopConfigException; void removeAdvisor(int index) throws AopConfigException; boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException; boolean isFrozen();
The getAdvisors()
method will return an Advisor for every advisor, interceptor or
other advice type that has been added to the factory. If you added an Advisor, the
returned advisor at this index will be the object that you added. If you added an
interceptor or other advice type, Spring will have wrapped this in an advisor with a
pointcut that always returns true. Thus if you added a MethodInterceptor
, the advisor
returned for this index will be an DefaultPointcutAdvisor
returning your
MethodInterceptor
and a pointcut that matches all classes and methods.
The addAdvisor()
methods can be used to add any Advisor. Usually the advisor holding
pointcut and advice will be the generic DefaultPointcutAdvisor
, which can be used with
any advice or pointcut (but not for introductions).
By default, it’s possible to add or remove advisors or interceptors even once a proxy has been created. The only restriction is that it’s impossible to add or remove an introduction advisor, as existing proxies from the factory will not show the interface change. (You can obtain a new proxy from the factory to avoid this problem.)
A simple example of casting an AOP proxy to the Advised
interface and examining and
manipulating its advice:
Advised advised = (Advised) myObject; Advisor[] advisors = advised.getAdvisors(); int oldAdvisorCount = advisors.length; System.out.println(oldAdvisorCount + " advisors"); // Add an advice like an interceptor without a pointcut // Will match all proxied methods // Can use for interceptors, before, after returning or throws advice advised.addAdvice(new DebugInterceptor()); // Add selective advice using a pointcut advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice)); assertEquals("Added two advisors", oldAdvisorCount + 2, advised.getAdvisors().length);
Note | |
---|---|
It’s questionable whether it’s advisable (no pun intended) to modify advice on a business object in production, although there are no doubt legitimate usage cases. However, it can be very useful in development: for example, in tests. I have sometimes found it very useful to be able to add test code in the form of an interceptor or other advice, getting inside a method invocation I want to test. (For example, the advice can get inside a transaction created for that method: for example, to run SQL to check that a database was correctly updated, before marking the transaction for roll back.) |
Depending on how you created the proxy, you can usually set a frozen
flag, in which
case the Advised
isFrozen()
method will return true, and any attempts to modify
advice through addition or removal will result in an AopConfigException
. The ability
to freeze the state of an advised object is useful in some cases, for example, to
prevent calling code removing a security interceptor. It may also be used in Spring 1.1
to allow aggressive optimization if runtime advice modification is known not to be
required.
So far we’ve considered explicit creation of AOP proxies using a ProxyFactoryBean
or
similar factory bean.
Spring also allows us to use "auto-proxy" bean definitions, which can automatically proxy selected bean definitions. This is built on Spring "bean post processor" infrastructure, which enables modification of any bean definition as the container loads.
In this model, you set up some special bean definitions in your XML bean definition file
to configure the auto proxy infrastructure. This allows you just to declare the targets
eligible for auto-proxying: you don’t need to use ProxyFactoryBean
.
There are two ways to do this:
The org.springframework.aop.framework.autoproxy
package provides the following
standard auto-proxy creators.
The BeanNameAutoProxyCreator
class is a BeanPostProcessor
that automatically creates
AOP proxies for beans with names matching literal values or wildcards.
<bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator"> <property name="beanNames" value="jdk*,onlyJdk"/> <property name="interceptorNames"> <list> <value>myInterceptor</value> </list> </property> </bean>
As with ProxyFactoryBean
, there is an interceptorNames
property rather than a list
of interceptors, to allow correct behavior for prototype advisors. Named "interceptors"
can be advisors or any advice type.
As with auto proxying in general, the main point of using BeanNameAutoProxyCreator
is
to apply the same configuration consistently to multiple objects, with minimal volume of
configuration. It is a popular choice for applying declarative transactions to multiple
objects.
Bean definitions whose names match, such as "jdkMyBean" and "onlyJdk" in the above
example, are plain old bean definitions with the target class. An AOP proxy will be
created automatically by the BeanNameAutoProxyCreator
. The same advice will be applied
to all matching beans. Note that if advisors are used (rather than the interceptor in
the above example), the pointcuts may apply differently to different beans.
A more general and extremely powerful auto proxy creator is
DefaultAdvisorAutoProxyCreator
. This will automagically apply eligible advisors in the
current context, without the need to include specific bean names in the auto-proxy
advisor’s bean definition. It offers the same merit of consistent configuration and
avoidance of duplication as BeanNameAutoProxyCreator
.
Using this mechanism involves:
DefaultAdvisorAutoProxyCreator
bean definition.
The DefaultAdvisorAutoProxyCreator
will automatically evaluate the pointcut contained
in each advisor, to see what (if any) advice it should apply to each business object
(such as "businessObject1" and "businessObject2" in the example).
This means that any number of advisors can be applied automatically to each business object. If no pointcut in any of the advisors matches any method in a business object, the object will not be proxied. As bean definitions are added for new business objects, they will automatically be proxied if necessary.
Autoproxying in general has the advantage of making it impossible for callers or dependencies to obtain an un-advised object. Calling getBean("businessObject1") on this ApplicationContext will return an AOP proxy, not the target business object. (The "inner bean" idiom shown earlier also offers this benefit.)
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="customAdvisor" class="com.mycompany.MyAdvisor"/> <bean id="businessObject1" class="com.mycompany.BusinessObject1"> <!-- Properties omitted --> </bean> <bean id="businessObject2" class="com.mycompany.BusinessObject2"/>
The DefaultAdvisorAutoProxyCreator
is very useful if you want to apply the same advice
consistently to many business objects. Once the infrastructure definitions are in place,
you can simply add new business objects without including specific proxy configuration.
You can also drop in additional aspects very easily - for example, tracing or
performance monitoring aspects - with minimal change to configuration.
The DefaultAdvisorAutoProxyCreator offers support for filtering (using a naming
convention so that only certain advisors are evaluated, allowing use of multiple,
differently configured, AdvisorAutoProxyCreators in the same factory) and ordering.
Advisors can implement the org.springframework.core.Ordered
interface to ensure
correct ordering if this is an issue. The TransactionAttributeSourceAdvisor used in the
above example has a configurable order value; the default setting is unordered.
This is the superclass of DefaultAdvisorAutoProxyCreator. You can create your own
auto-proxy creators by subclassing this class, in the unlikely event that advisor
definitions offer insufficient customization to the behavior of the framework
DefaultAdvisorAutoProxyCreator
.
A particularly important type of auto-proxying is driven by metadata. This produces a
similar programming model to .NET ServicedComponents
. Instead of defining metadata in
XML descriptors, configuration for transaction management and other enterprise services
is held in source-level attributes.
In this case, you use the DefaultAdvisorAutoProxyCreator
, in combination with Advisors
that understand metadata attributes. The metadata specifics are held in the pointcut
part of the candidate advisors, rather than in the auto-proxy creation class itself.
This is really a special case of the DefaultAdvisorAutoProxyCreator
, but deserves
consideration on its own. (The metadata-aware code is in the pointcuts contained in the
advisors, not the AOP framework itself.)
The /attributes
directory of the JPetStore sample application shows the use of
attribute-driven auto-proxying. In this case, there’s no need to use the
TransactionProxyFactoryBean
. Simply defining transactional attributes on business
objects is sufficient, because of the use of metadata-aware pointcuts. The bean
definitions include the following code, in /WEB-INF/declarativeServices.xml
. Note that
this is generic, and can be used outside the JPetStore:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource"> <property name="attributes" ref="attributes"/> </bean> </property> </bean> <bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/>
The DefaultAdvisorAutoProxyCreator
bean definition (the name is not significant, hence
it can even be omitted) will pick up all eligible pointcuts in the current application
context. In this case, the "transactionAdvisor" bean definition, of type
TransactionAttributeSourceAdvisor
, will apply to classes or methods carrying a
transaction attribute. The TransactionAttributeSourceAdvisor depends on a
TransactionInterceptor, via constructor dependency. The example resolves this via
autowiring. The AttributesTransactionAttributeSource
depends on an implementation of
the org.springframework.metadata.Attributes
interface. In this fragment, the
"attributes" bean satisfies this, using the Jakarta Commons Attributes API to obtain
attribute information. (The application code must have been compiled using the Commons
Attributes compilation task.)
The /annotation
directory of the JPetStore sample application contains an analogous
example for auto-proxying driven by JDK 1.5+ annotations. The following configuration
enables automatic detection of Spring’s Transactional
annotation, leading to implicit
proxies for beans containing that annotation:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/> </property> </bean>
The TransactionInterceptor
defined here depends on a PlatformTransactionManager
definition, which is not included in this generic file (although it could be) because it
will be specific to the application’s transaction requirements (typically JTA, as in
this example, or Hibernate, JDO or JDBC):
<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
Tip | |
---|---|
If you require only declarative transaction management, using these generic XML definitions will result in Spring automatically proxying all classes or methods with transaction attributes. You won’t need to work directly with AOP, and the programming model is similar to that of .NET ServicedComponents. |
This mechanism is extensible. It’s possible to do auto-proxying based on custom attributes. You need to:
It’s possible for such advisors to be unique to each advised class (for example, mixins):
they simply need to be defined as prototype, rather than singleton, bean definitions.
For example, the LockMixin
introduction interceptor from the Spring test suite,
shown above, could be used in conjunction with a generic DefaultIntroductionAdvisor
:
<bean id="lockMixin" class="test.mixin.LockMixin" scope="prototype"/> <bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultIntroductionAdvisor" scope="prototype"> <constructor-arg ref="lockMixin"/> </bean>
Note that both lockMixin
and lockableAdvisor
are defined as prototypes.
Spring offers the concept of a TargetSource, expressed in the
org.springframework.aop.TargetSource
interface. This interface is responsible for
returning the "target object" implementing the join point. The TargetSource
implementation is asked for a target instance each time the AOP proxy handles a method
invocation.
Developers using Spring AOP don’t normally need to work directly with TargetSources, but this provides a powerful means of supporting pooling, hot swappable and other sophisticated targets. For example, a pooling TargetSource can return a different target instance for each invocation, using a pool to manage instances.
If you do not specify a TargetSource, a default implementation is used that wraps a local object. The same target is returned for each invocation (as you would expect).
Let’s look at the standard target sources provided with Spring, and how you can use them.
Tip | |
---|---|
When using a custom target source, your target will usually need to be a prototype rather than a singleton bean definition. This allows Spring to create a new target instance when required. |
The org.springframework.aop.target.HotSwappableTargetSource
exists to allow the target
of an AOP proxy to be switched while allowing callers to keep their references to it.
Changing the target source’s target takes effect immediately. The
HotSwappableTargetSource
is threadsafe.
You can change the target via the swap()
method on HotSwappableTargetSource as follows:
HotSwappableTargetSource swapper = (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);
The XML definitions required look as follows:
<bean id="initialTarget" class="mycompany.OldTarget"/> <bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource"> <constructor-arg ref="initialTarget"/> </bean> <bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="swapper"/> </bean>
The above swap()
call changes the target of the swappable bean. Clients who hold a
reference to that bean will be unaware of the change, but will immediately start hitting
the new target.
Although this example doesn’t add any advice - and it’s not necessary to add advice to
use a TargetSource
- of course any TargetSource
can be used in conjunction with
arbitrary advice.
Using a pooling target source provides a similar programming model to stateless session EJBs, in which a pool of identical instances is maintained, with method invocations going to free objects in the pool.
A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to any POJO. As with Spring in general, this service can be applied in a non-invasive way.
Spring provides out-of-the-box support for Jakarta Commons Pool 1.3, which provides a
fairly efficient pooling implementation. You’ll need the commons-pool Jar on your
application’s classpath to use this feature. It’s also possible to subclass
org.springframework.aop.target.AbstractPoolingTargetSource
to support any other
pooling API.
Sample configuration is shown below:
<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject" scope="prototype"> ... properties omitted </bean> <bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> <property name="maxSize" value="25"/> </bean> <bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="poolTargetSource"/> <property name="interceptorNames" value="myInterceptor"/> </bean>
Note that the target object - "businessObjectTarget" in the example - must be a
prototype. This allows the PoolingTargetSource
implementation to create new instances
of the target to grow the pool as necessary. See the javadoc for
AbstractPoolingTargetSource
and the concrete subclass you wish to use for information
about its properties: "maxSize" is the most basic, and always guaranteed to be present.
In this case, "myInterceptor" is the name of an interceptor that would need to be defined in the same IoC context. However, it isn’t necessary to specify interceptors to use pooling. If you want only pooling, and no other advice, don’t set the interceptorNames property at all.
It’s possible to configure Spring so as to be able to cast any pooled object to the
org.springframework.aop.target.PoolingConfig
interface, which exposes information
about the configuration and current size of the pool through an introduction. You’ll
need to define an advisor like this:
<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"> <property name="targetObject" ref="poolTargetSource"/> <property name="targetMethod" value="getPoolingConfigMixin"/> </bean>
This advisor is obtained by calling a convenience method on the
AbstractPoolingTargetSource
class, hence the use of MethodInvokingFactoryBean. This
advisor’s name ("poolConfigAdvisor" here) must be in the list of interceptors names in
the ProxyFactoryBean exposing the pooled object.
The cast will look as follows:
PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject"); System.out.println("Max pool size is " + conf.getMaxSize());
Note | |
---|---|
Pooling stateless service objects is not usually necessary. We don’t believe it should be the default choice, as most stateless objects are naturally thread safe, and instance pooling is problematic if resources are cached. |
Simpler pooling is available using auto-proxying. It’s possible to set the TargetSources used by any auto-proxy creator.
Setting up a "prototype" target source is similar to a pooling TargetSource. In this case, a new instance of the target will be created on every method invocation. Although the cost of creating a new object isn’t high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be more expensive. Thus you shouldn’t use this approach without very good reason.
To do this, you could modify the poolTargetSource
definition shown above as follows.
(I’ve also changed the name, for clarity.)
<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource"> <property name="targetBeanName" ref="businessObjectTarget"/> </bean>
There’s only one property: the name of the target bean. Inheritance is used in the TargetSource implementations to ensure consistent naming. As with the pooling target source, the target bean must be a prototype bean definition.
ThreadLocal
target sources are useful if you need an object to be created for each
incoming request (per thread that is). The concept of a ThreadLocal
provide a JDK-wide
facility to transparently store resource alongside a thread. Setting up a
ThreadLocalTargetSource
is pretty much the same as was explained for the other types
of target source:
<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> </bean>
Note | |
---|---|
ThreadLocals come with serious issues (potentially resulting in memory leaks) when
incorrectly using them in a multi-threaded and multi-classloader environments. One
should always consider wrapping a threadlocal in some other class and never directly use
the |
Spring AOP is designed to be extensible. While the interception implementation strategy is presently used internally, it is possible to support arbitrary advice types in addition to the out-of-the-box interception around advice, before, throws advice and after returning advice.
The org.springframework.aop.framework.adapter
package is an SPI package allowing
support for new custom advice types to be added without changing the core framework. The
only constraint on a custom Advice
type is that it must implement the
org.aopalliance.aop.Advice
tag interface.
Please refer to the org.springframework.aop.framework.adapter
package’s Javadocs for
further information.
Please refer to the Spring sample applications for further examples of Spring AOP:
TransactionProxyFactoryBean
for declarative transaction management.
/attributes
directory of the JPetStore illustrates the use of attribute-driven
declarative transaction management.
Testing is an integral part of enterprise software development. This chapter focuses on the value-add of the IoC principle to unit testing and on the benefits of the Spring Framework’s support for integration testing. (A thorough treatment of testing in the enterprise is beyond the scope of this reference manual.)
Dependency Injection should make your code less dependent on the container than it would
be with traditional Java EE development. The POJOs that make up your application should
be testable in JUnit or TestNG tests, with objects simply instantiated using the new
operator, without Spring or any other container. You can use mock
objects (in conjunction with other valuable testing techniques) to test your code in
isolation. If you follow the architecture recommendations for Spring, the resulting
clean layering and componentization of your codebase will facilitate easier unit
testing. For example, you can test service layer objects by stubbing or mocking DAO or
Repository interfaces, without needing to access persistent data while running unit
tests.
True unit tests typically run extremely quickly, as there is no runtime infrastructure to set up. Emphasizing true unit tests as part of your development methodology will boost your productivity. You may not need this section of the testing chapter to help you write effective unit tests for your IoC-based applications. For certain unit testing scenarios, however, the Spring Framework provides the following mock objects and testing support classes.
The org.springframework.mock.env
package contains mock implementations of the
Environment
and PropertySource
abstractions (see Section 4.13, “Bean definition profiles and environment abstraction”
and Section 4.14, “PropertySource Abstraction”). MockEnvironment
and
MockPropertySource
are useful for developing out-of-container tests for code that
depends on environment-specific properties.
The org.springframework.mock.jndi
package contains an implementation of the JNDI SPI,
which you can use to set up a simple JNDI environment for test suites or stand-alone
applications. If, for example, JDBC DataSource
s get bound to the same JNDI names in
test code as within a Java EE container, you can reuse both application code and
configuration in testing scenarios without modification.
The org.springframework.mock.web
package contains a comprehensive set of Servlet API
mock objects, targeted at usage with Spring’s Web MVC framework, which are useful for
testing web contexts and controllers. These mock objects are generally more convenient
to use than dynamic mock objects such as EasyMock or existing
Servlet API mock objects such as MockObjects.
The org.springframework.test.util
package contains ReflectionTestUtils
, which is a
collection of reflection-based utility methods. Developers use these methods in unit and
integration testing scenarios in which they need to set a non- public
field or invoke
a non- public
setter method when testing application code involving, for example:
private
or protected
field
access as opposed to public
setter methods for properties in a domain entity.
@Autowired
, @Inject
, and @Resource,
which provides dependency injection for private
or protected
fields, setter
methods, and configuration methods.
The org.springframework.test.web
package contains ModelAndViewAssert
, which you can
use in combination with JUnit, TestNG, or any other testing framework for unit tests
dealing with Spring MVC ModelAndView
objects.
Unit testing Spring MVC Controllers | |
---|---|
To test your Spring MVC Note: As of Spring 4.0, the set of mocks in the |
It is important to be able to perform some integration testing without requiring deployment to your application server or connecting to other enterprise infrastructure. This will enable you to test things such as:
The Spring Framework provides first-class support for integration testing in the
spring-test
module. The name of the actual JAR file might include the release version
and might also be in the long org.springframework.test
form, depending on where you
get it from (see the section on Dependency Management for an
explanation). This library includes the org.springframework.test
package, which
contains valuable classes for integration testing with a Spring container. This testing
does not rely on an application server or other deployment environment. Such tests are
slower to run than unit tests but much faster than the equivalent Selenium tests or remote
tests that rely on deployment to an application server.
In Spring 2.5 and later, unit and integration testing support is provided in the form of the annotation-driven Spring TestContext Framework. The TestContext framework is agnostic of the actual testing framework in use, thus allowing instrumentation of tests in various environments including JUnit, TestNG, and so on.
Spring’s integration testing support has the following primary goals:
The next few sections describe each goal and provide links to implementation and configuration details.
The Spring TestContext Framework provides consistent loading of Spring
ApplicationContext
s and WebApplicationContext
s as well as caching of those
contexts. Support for the caching of loaded contexts is important, because startup time
can become an issue — not because of the overhead of Spring itself, but because the
objects instantiated by the Spring container take time to instantiate. For example, a
project with 50 to 100 Hibernate mapping files might take 10 to 20 seconds to load the
mapping files, and incurring that cost before running every test in every test fixture
leads to slower overall test runs that reduce developer productivity.
Test classes typically declare either an array of resource locations for XML
configuration metadata — often in the classpath — or an array of annotated classes
that is used to configure the application. These locations or classes are the same as or
similar to those specified in web.xml
or other deployment configuration files.
By default, once loaded, the configured ApplicationContext
is reused for each test.
Thus the setup cost is incurred only once per test suite, and subsequent test execution
is much faster. In this context, the term test suite means all tests run in the same
JVM — for example, all tests run from an Ant, Maven, or Gradle build for a given
project or module. In the unlikely case that a test corrupts the application context and
requires reloading — for example, by modifying a bean definition or the state of an
application object — the TestContext framework can be configured to reload the
configuration and rebuild the application context before executing the next test.
See the section called “Context management” and the section called “Context caching” with the TestContext framework.
When the TestContext framework loads your application context, it can optionally
configure instances of your test classes via Dependency Injection. This provides a
convenient mechanism for setting up test fixtures using preconfigured beans from your
application context. A strong benefit here is that you can reuse application contexts
across various testing scenarios (e.g., for configuring Spring-managed object graphs,
transactional proxies, DataSource
s, etc.), thus avoiding the need to duplicate
complex test fixture setup for individual test cases.
As an example, consider the scenario where we have a class, HibernateTitleRepository
,
that implements data access logic for a Title
domain entity. We want to write
integration tests that test the following areas:
HibernateTitleRepository
bean correct and present?
HibernateTitleRepository
: does the configured instance of this
class perform as anticipated?
See dependency injection of test fixtures with the TestContext framework.
One common issue in tests that access a real database is their effect on the state of the persistence store. Even when you’re using a development database, changes to the state may affect future tests. Also, many operations — such as inserting or modifying persistent data — cannot be performed (or verified) outside a transaction.
The TestContext framework addresses this issue. By default, the framework will create
and roll back a transaction for each test. You simply write code that can assume the
existence of a transaction. If you call transactionally proxied objects in your tests,
they will behave correctly, according to their configured transactional semantics. In
addition, if a test method deletes the contents of selected tables while running within
the transaction managed for the test, the transaction will roll back by default, and the
database will return to its state prior to execution of the test. Transactional support
is provided to a test via a PlatformTransactionManager
bean defined in the test’s
application context.
If you want a transaction to commit — unusual, but occasionally useful when you want a
particular test to populate or modify the database — the TestContext framework can be
instructed to cause the transaction to commit instead of roll back via the
@TransactionConfiguration
and
@Rollback
annotations.
See transaction management with the TestContext framework.
The Spring TestContext Framework provides several abstract
support classes that
simplify the writing of integration tests. These base test classes provide well-defined
hooks into the testing framework as well as convenient instance variables and methods,
which enable you to access:
ApplicationContext
, for performing explicit bean lookups or testing the state of
the context as a whole.
JdbcTemplate
, for executing SQL statements to query the database. Such queries can
be used to confirm database state both prior to and after execution of
database-related application code, and Spring ensures that such queries run in the
scope of the same transaction as the application code. When used in conjunction with
an ORM tool, be sure to avoid false positives.
In addition, you may want to create your own custom, application-wide superclass with instance variables and methods specific to your project.
See support classes for the TestContext framework.
The org.springframework.test.jdbc
package contains JdbcTestUtils
, which is a
collection of JDBC related utility functions intended to simplify standard database
testing scenarios. Note that AbstractTransactionalJUnit4SpringContextTests
and
AbstractTransactionalTestNGSpringContextTests
provide convenience methods which delegate to JdbcTestUtils
internally.
The spring-jdbc
module provides support for configuring and launching an embedded
database which can be used in integration tests that interact with a database. For
details, see Section 13.8, “Embedded database support” and
Section 13.8.8, “Testing data access logic with an embedded database”.
The Spring Framework provides the following set of Spring-specific annotations that you can use in your unit and integration tests in conjunction with the TestContext framework. Refer to the respective Javadoc for further information, including default attribute values, attribute aliases, and so on.
@ContextConfiguration
Defines class-level metadata that is used to determine how to load and configure an
ApplicationContext
for integration tests. Specifically, @ContextConfiguration
declares either the application context resource locations
or the annotated
classes
that will be used to load the context.
Resource locations are typically XML configuration files located in the classpath;
whereas, annotated classes are typically @Configuration
classes. However, resource
locations can also refer to files in the file system, and annotated classes can be
component classes, etc.
@ContextConfiguration("/test-config.xml") public class XmlApplicationContextTests { // class body... }
@ContextConfiguration(classes = TestConfig.class) public class ConfigClassApplicationContextTests { // class body... }
As an alternative or in addition to declaring resource locations or annotated classes,
@ContextConfiguration
may be used to declare ApplicationContextInitializer
classes.
@ContextConfiguration(initializers = CustomContextIntializer.class) public class ContextInitializerTests { // class body... }
@ContextConfiguration
may optionally be used to declare the ContextLoader
strategy
as well. Note, however, that you typically do not need to explicitly configure the
loader since the default loader supports either resource locations
or annotated
classes
as well as initializers
.
@ContextConfiguration(locations = "/test-context.xml", loader = CustomContextLoader.class) public class CustomLoaderXmlApplicationContextTests { // class body... }
Note | |
---|---|
|
See the section called “Context management” and the Javadoc for @ContextConfiguration
for
further details.
@WebAppConfiguration
A class-level annotation that is used to declare that the ApplicationContext
loaded
for an integration test should be a WebApplicationContext
. The mere presence of
@WebAppConfiguration
on a test class ensures that a WebApplicationContext
will be
loaded for the test, using the default value of "file:src/main/webapp"
for the path to
the root of the web application (i.e., the resource base path). The resource base
path is used behind the scenes to create a MockServletContext
which serves as the
ServletContext
for the test’s WebApplicationContext
.
@ContextConfiguration @WebAppConfiguration public class WebAppTests { // class body... }
To override the default, specify a different base resource path via the implicit
value
attribute. Both classpath:
and file:
resource prefixes are supported. If no
resource prefix is supplied the path is assumed to be a file system resource.
@ContextConfiguration @WebAppConfiguration("classpath:test-web-resources") public class WebAppTests { // class body... }
Note that @WebAppConfiguration
must be used in conjunction with
@ContextConfiguration
, either within a single test class or within a test class
hierarchy. See the Javadoc for @WebAppConfiguration
for further details.
@ContextHierarchy
A class-level annotation that is used to define a hierarchy of ApplicationContext
s
for integration tests. @ContextHierarchy
should be declared with a list of one or more
@ContextConfiguration
instances, each of which defines a level in the context
hierarchy. The following examples demonstrate the use of @ContextHierarchy
within a
single test class; however, @ContextHierarchy
can also be used within a test class
hierarchy.
@ContextHierarchy({ @ContextConfiguration("/parent-config.xml"), @ContextConfiguration("/child-config.xml") }) public class ContextHierarchyTests { // class body... }
@WebAppConfiguration @ContextHierarchy({ @ContextConfiguration(classes = AppConfig.class), @ContextConfiguration(classes = WebConfig.class) }) public class WebIntegrationTests { // class body... }
If you need to merge or override the configuration for a given level of the context
hierarchy within a test class hierarchy, you must explicitly name that level by
supplying the same value to the name
attribute in @ContextConfiguration
at each
corresponding level in the class hierarchy. See
the section called “Context hierarchies” and the Javadoc for @ContextHierarchy
for further examples.
@ActiveProfiles
A class-level annotation that is used to declare which bean definition profiles
should be active when loading an ApplicationContext
for test classes.
@ContextConfiguration @ActiveProfiles("dev") public class DeveloperTests { // class body... }
@ContextConfiguration @ActiveProfiles({"dev", "integration"}) public class DeveloperIntegrationTests { // class body... }
Note | |
---|---|
|
See the section called “Context configuration with environment profiles” and the Javadoc for @ActiveProfiles
for examples and further details.
@DirtiesContext
Indicates that the underlying Spring ApplicationContext
has been dirtied during
the execution of a test (i.e., modified or corrupted in some manner — for example, by
changing the state of a singleton bean) and should be closed, regardless of whether the
test passed. When an application context is marked dirty, it is removed from the
testing framework’s cache and closed. As a consequence, the underlying Spring container
will be rebuilt for any subsequent test that requires a context with the same
configuration metadata.
@DirtiesContext
can be used as both a class-level and method-level annotation within
the same test class. In such scenarios, the ApplicationContext
is marked as dirty
after any such annotated method as well as after the entire class. If the ClassMode
is
set to AFTER_EACH_TEST_METHOD
, the context is marked dirty after each test method in
the class.
The following examples explain when the context would be dirtied for various configuration scenarios:
After the current test class, when declared on a class with class mode set to
AFTER_CLASS
(i.e., the default class mode).
@DirtiesContext public class ContextDirtyingTests { // some tests that result in the Spring container being dirtied }
After each test method in the current test class, when declared on a class with class
mode set to AFTER_EACH_TEST_METHOD.
@DirtiesContext(classMode = ClassMode.AFTER_EACH_TEST_METHOD) public class ContextDirtyingTests { // some tests that result in the Spring container being dirtied }
After the current test, when declared on a method.
@DirtiesContext @Test public void testProcessWhichDirtiesAppCtx() { // some logic that results in the Spring container being dirtied }
If @DirtiesContext
is used in a test whose context is configured as part of a context
hierarchy via @ContextHierarchy
, the hierarchyMode
flag can be used to control how
the context cache is cleared. By default an exhaustive algorithm will be used that
clears the context cache including not only the current level but also all other context
hierarchies that share an ancestor context common to the current test; all
ApplicationContext
s that reside in a sub-hierarchy of the common ancestor context
will be removed from the context cache and closed. If the exhaustive algorithm is
overkill for a particular use case, the simpler current level algorithm can be
specified instead, as seen below.
@ContextHierarchy({ @ContextConfiguration("/parent-config.xml"), @ContextConfiguration("/child-config.xml") }) public class BaseTests { // class body... } public class ExtendedTests extends BaseTests { @Test @DirtiesContext(hierarchyMode = HierarchyMode.CURRENT_LEVEL) public void test() { // some logic that results in the child context being dirtied } }
For further details regarding the EXHAUSTIVE
and CURRENT_LEVEL
algorithms see the
Javadoc for DirtiesContext.HierarchyMode
.
@TestExecutionListeners
Defines class-level metadata for configuring which TestExecutionListener
s should be
registered with the TestContextManager
. Typically, @TestExecutionListeners
is used
in conjunction with @ContextConfiguration
.
@ContextConfiguration @TestExecutionListeners({CustomTestExecutionListener.class, AnotherTestExecutionListener.class}) public class CustomTestExecutionListenerTests { // class body... }
@TestExecutionListeners
supports inherited listeners by default. See the Javadoc
for an example and further details.
@TransactionConfiguration
Defines class-level metadata for configuring transactional tests. Specifically, the bean
name of the PlatformTransactionManager
that should be used to drive transactions can
be explicitly specified if there are multiple beans of type PlatformTransactionManager
in the test’s ApplicationContext
and if the bean name of the desired
PlatformTransactionManager
is not "transactionManager". In addition, you can change
the defaultRollback
flag to false
. Typically, @TransactionConfiguration
is used in
conjunction with @ContextConfiguration
.
@ContextConfiguration @TransactionConfiguration(transactionManager = "txMgr", defaultRollback = false) public class CustomConfiguredTransactionalTests { // class body... }
Note | |
---|---|
If the default conventions are sufficient for your test configuration, you can avoid
using |
@Rollback
Indicates whether the transaction for the annotated test method should be rolled
back after the test method has completed. If true
, the transaction is rolled back;
otherwise, the transaction is committed. Use @Rollback
to override the default
rollback flag configured at the class level.
@Rollback(false) @Test public void testProcessWithoutRollback() { // ... }
@BeforeTransaction
Indicates that the annotated public void
method should be executed before a
transaction is started for test methods configured to run within a transaction via the
@Transactional
annotation.
@BeforeTransaction public void beforeTransaction() { // logic to be executed before a transaction is started }
@AfterTransaction
Indicates that the annotated public void
method should be executed after a
transaction has ended for test methods configured to run within a transaction via the
@Transactional
annotation.
@AfterTransaction public void afterTransaction() { // logic to be executed after a transaction has ended }
The following annotations are supported with standard semantics for all configurations of the Spring TestContext Framework. Note that these annotations are not specific to tests and can be used anywhere in the Spring Framework.
@Autowired
@Qualifier
@Resource
(javax.annotation) if JSR-250 is present
@Inject
(javax.inject) if JSR-330 is present
@Named
(javax.inject) if JSR-330 is present
@PersistenceContext
(javax.persistence) if JPA is present
@PersistenceUnit
(javax.persistence) if JPA is present
@Required
@Transactional
JSR-250 Lifecycle Annotations | |
---|---|
In the Spring TestContext Framework If a method within a test class is annotated with |
The following annotations are only supported when used in conjunction with the SpringJUnit4ClassRunner or the JUnit support classes.
@IfProfileValue
Indicates that the annotated test is enabled for a specific testing environment. If the
configured ProfileValueSource
returns a matching value
for the provided name
, the
test is enabled. This annotation can be applied to an entire class or to individual
methods. Class-level usage overrides method-level usage.
@IfProfileValue(name="java.vendor", value="Oracle Corporation") @Test public void testProcessWhichRunsOnlyOnOracleJvm() { // some logic that should run only on Java VMs from Oracle Corporation }
Alternatively, you can configure @IfProfileValue
with a list of values
(with OR
semantics) to achieve TestNG-like support for test groups in a JUnit environment.
Consider the following example:
@IfProfileValue(name="test-groups", values={"unit-tests", "integration-tests"}) @Test public void testProcessWhichRunsForUnitOrIntegrationTestGroups() { // some logic that should run only for unit and integration test groups }
@ProfileValueSourceConfiguration
Class-level annotation that specifies what type of ProfileValueSource
to use when
retrieving profile values configured through the @IfProfileValue
annotation. If
@ProfileValueSourceConfiguration
is not declared for a test,
SystemProfileValueSource
is used by default.
@ProfileValueSourceConfiguration(CustomProfileValueSource.class) public class CustomProfileValueSourceTests { // class body... }
@Timed
Indicates that the annotated test method must finish execution in a specified time period (in milliseconds). If the text execution time exceeds the specified time period, the test fails.
The time period includes execution of the test method itself, any repetitions of the
test (see @Repeat
), as well as any set up or tear down of the test fixture.
@Timed(millis=1000) public void testProcessWithOneSecondTimeout() { // some logic that should not take longer than 1 second to execute }
Spring’s @Timed
annotation has different semantics than JUnit’s @Test(timeout=...)
support. Specifically, due to the manner in which JUnit handles test execution timeouts
(that is, by executing the test method in a separate Thread
), @Test(timeout=...)
applies to each iteration in the case of repetitions and preemptively fails the test
if the test takes too long. Spring’s @Timed
, on the other hand, times the total
test execution time (including all repetitions) and does not preemptively fail the test
but rather waits for the test to complete before failing.
@Repeat
Indicates that the annotated test method must be executed repeatedly. The number of times that the test method is to be executed is specified in the annotation.
The scope of execution to be repeated includes execution of the test method itself as well as any set up or tear down of the test fixture.
@Repeat(10) @Test public void testProcessRepeatedly() { // ... }
As of Spring Framework 4.0, it is now possible to use test-related annotations as meta-annotations in order to create custom composed annotations and reduce configuration duplication across a test suite.
Each of the following may be used as meta-annotations in conjunction with the TestContext framework.
@ContextConfiguration
@ContextHierarchy
@ActiveProfiles
@DirtiesContext
@WebAppConfiguration
@TestExecutionListeners
@Transactional
@BeforeTransaction
@AfterTransaction
@TransactionConfiguration
@Rollback
@Repeat
@Timed
@IfProfileValue
@ProfileValueSourceConfiguration
For example, if we discover that we are repeating the following configuration across our JUnit-based test suite…
@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"}) @ActiveProfiles("dev") @Transactional public class OrderRepositoryTests { } @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"}) @ActiveProfiles("dev") @Transactional public class UserRepositoryTests { }
We can reduce the above duplication by introducing a custom composed annotation that centralizes the common test configuration like this:
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @ContextConfiguration({"/app-config.xml", "/test-data-access-config.xml"}) @ActiveProfiles("dev") @Transactional public @interface TransactionalDevTest { }
Then we can use our custom @TransactionalDevTest
annotation to simplify the
configuration of individual test classes as follows:
@RunWith(SpringJUnit4ClassRunner.class) @TransactionalDevTest public class OrderRepositoryTests { } @RunWith(SpringJUnit4ClassRunner.class) @TransactionalDevTest public class UserRepositoryTests { }
The Spring TestContext Framework (located in the
org.springframework.test.context
package) provides generic, annotation-driven unit and
integration testing support that is agnostic of the testing framework in use. The
TestContext framework also places a great deal of importance on convention over
configuration with reasonable defaults that can be overridden through annotation-based
configuration.
In addition to generic testing infrastructure, the TestContext framework provides
explicit support for JUnit and TestNG in the form of abstract
support classes. For
JUnit, Spring also provides a custom JUnit Runner
that allows one to write so-called
POJO test classes. POJO test classes are not required to extend a particular class
hierarchy.
The following section provides an overview of the internals of the TestContext framework. If you are only interested in using the framework and not necessarily interested in extending it with your own custom listeners or custom loaders, feel free to go directly to the configuration (context management, dependency injection, transaction management), support classes, and annotation support sections.
The core of the framework consists of the TestContext
and TestContextManager
classes
and the TestExecutionListener
, ContextLoader
, and SmartContextLoader
interfaces. A
TestContextManager
is created on a per-test basis (e.g., for the execution of a single
test method in JUnit). The TestContextManager
in turn manages a TestContext
that
holds the context of the current test. The TestContextManager
also updates the state
of the TestContext
as the test progresses and delegates to TestExecutionListener
s,
which instrument the actual test execution by providing dependency injection, managing
transactions, and so on. A ContextLoader
(or SmartContextLoader
) is responsible for
loading an ApplicationContext
for a given test class. Consult the Javadoc and the
Spring test suite for further information and examples of various implementations.
TestContext
: Encapsulates the context in which a test is executed, agnostic of the
actual testing framework in use, and provides context management and caching support
for the test instance for which it is responsible. The TestContext
also delegates to
a ContextLoader
(or SmartContextLoader
) to load an ApplicationContext
if
requested.
TestContextManager
: The main entry point into the Spring TestContext Framework,
which manages a single TestContext
and signals events to all registered
TestExecutionListener
s at well-defined test execution points:
TestExecutionListener
: Defines a listener API for reacting to test execution
events published by the TestContextManager
with which the listener is registered.
Spring provides four TestExecutionListener
implementations that are configured by
default: ServletTestExecutionListener
, DependencyInjectionTestExecutionListener
,
DirtiesContextTestExecutionListener
, and TransactionalTestExecutionListener
.
Respectively, they support Servlet API mocks for a WebApplicationContext
, dependency
injection of the test instance, handling of the @DirtiesContext
annotation, and
transactional test execution with default rollback semantics.
ContextLoader
: Strategy interface introduced in Spring 2.5 for loading an
ApplicationContext
for an integration test managed by the Spring TestContext
Framework.
As of Spring 3.1, implement SmartContextLoader
instead of this interface in order to
provide support for annotated classes and active bean definition profiles.
SmartContextLoader
: Extension of the ContextLoader
interface introduced in Spring
3.1.
The SmartContextLoader
SPI supersedes the ContextLoader
SPI that was introduced in
Spring 2.5. Specifically, a SmartContextLoader
can choose to process resource
locations
, annotated classes
, or context initializers
. Furthermore, a
SmartContextLoader
can set active bean definition profiles in the context that it
loads.
Spring provides the following implementations:
DelegatingSmartContextLoader
: one of two default loaders which delegates internally
to an AnnotationConfigContextLoader
or a GenericXmlContextLoader
depending either on
the configuration declared for the test class or on the presence of default locations or
default configuration classes.
WebDelegatingSmartContextLoader
: one of two default loaders which delegates
internally to an AnnotationConfigWebContextLoader
or a GenericXmlWebContextLoader
depending either on the configuration declared for the test class or on the presence of
default locations or default configuration classes. A web ContextLoader
will only be
used if @WebAppConfiguration
is present on the test class.
AnnotationConfigContextLoader
: loads a standard ApplicationContext
from
annotated classes.
AnnotationConfigWebContextLoader
: loads a WebApplicationContext
from annotated
classes.
GenericXmlContextLoader
: loads a standard ApplicationContext
from XML resource
locations.
GenericXmlWebContextLoader
: loads a WebApplicationContext
from XML resource
locations.
GenericPropertiesContextLoader
: loads a standard ApplicationContext
from Java
Properties files.
The following sections explain how to configure the TestContext framework through annotations and provide working examples of how to write unit and integration tests with the framework.
Each TestContext
provides context management and caching support for the test instance
it is responsible for. Test instances do not automatically receive access to the
configured ApplicationContext
. However, if a test class implements the
ApplicationContextAware
interface, a reference to the ApplicationContext
is supplied
to the test instance. Note that AbstractJUnit4SpringContextTests
and
AbstractTestNGSpringContextTests
implement ApplicationContextAware
and therefore
provide access to the ApplicationContext
automatically.
@Autowired ApplicationContext | |
---|---|
As an alternative to implementing the @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration public class MyTest { @Autowired private ApplicationContext applicationContext; // class body... } Similarly, if your test is configured to load a @RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextConfiguration public class MyWebAppTest { @Autowired private WebApplicationContext wac; // class body... } Dependency injection via |
Test classes that use the TestContext framework do not need to extend any particular
class or implement a specific interface to configure their application context. Instead,
configuration is achieved simply by declaring the @ContextConfiguration
annotation at
the class level. If your test class does not explicitly declare application context
resource locations
or annotated classes
, the configured ContextLoader
determines
how to load a context from a default location or default configuration classes. In
addition to context resource locations
and annotated classes
, an application context
can also be configured via application context initializers
.
The following sections explain how to configure an ApplicationContext
via XML
configuration files, annotated classes (typically @Configuration
classes), or context
initializers using Spring’s @ContextConfiguration
annotation. Alternatively, you can
implement and configure your own custom SmartContextLoader
for advanced use cases.
To load an ApplicationContext
for your tests using XML configuration files, annotate
your test class with @ContextConfiguration
and configure the locations
attribute
with an array that contains the resource locations of XML configuration metadata. A
plain or relative path — for example "context.xml"
— will be treated as a classpath
resource that is relative to the package in which the test class is defined. A path
starting with a slash is treated as an absolute classpath location, for example
"/org/example/config.xml"
. A path which represents a resource URL (i.e., a path
prefixed with classpath:
, file:
, http:
, etc.) will be used as is.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from "/app-config.xml" and // "/test-config.xml" in the root of the classpath @ContextConfiguration(locations={"/app-config.xml", "/test-config.xml"}) public class MyTest { // class body... }
@ContextConfiguration
supports an alias for the locations
attribute through the
standard Java value
attribute. Thus, if you do not need to declare additional
attributes in @ContextConfiguration
, you can omit the declaration of the locations
attribute name and declare the resource locations by using the shorthand format
demonstrated in the following example.
@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration({"/app-config.xml", "/test-config.xml"}) public class MyTest { // class body... }
If you omit both the locations
and value
attributes from the @ContextConfiguration
annotation, the TestContext framework will attempt to detect a default XML resource
location. Specifically, GenericXmlContextLoader
detects a default location based on
the name of the test class. If your class is named com.example.MyTest
,
GenericXmlContextLoader
loads your application context from
"classpath:/com/example/MyTest-context.xml"
.
package com.example; @RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from // "classpath:/com/example/MyTest-context.xml" @ContextConfiguration public class MyTest { // class body... }
To load an ApplicationContext
for your tests using annotated classes (see
Section 4.12, “Java-based container configuration”), annotate your test class with @ContextConfiguration
and configure the
classes
attribute with an array that contains references to annotated classes.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from AppConfig and TestConfig @ContextConfiguration(classes = {AppConfig.class, TestConfig.class}) public class MyTest { // class body... }
Annotated Classes | |
---|---|
The term annotated class can refer to any of the following.
Consult the Javadoc for |
If you omit the classes
attribute from the @ContextConfiguration
annotation, the
TestContext framework will attempt to detect the presence of default configuration
classes. Specifically, AnnotationConfigContextLoader
will detect all static inner
classes of the test class that meet the requirements for configuration class
implementations as specified in the Javadoc for @Configuration
. In the following
example, the OrderServiceTest
class declares a static inner configuration class named
Config
that will be automatically used to load the ApplicationContext
for the test
class. Note that the name of the configuration class is arbitrary. In addition, a test
class can contain more than one static inner configuration class if desired.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from the // static inner Config class @ContextConfiguration public class OrderServiceTest { @Configuration static class Config { // this bean will be injected into the OrderServiceTest class @Bean public OrderService orderService() { OrderService orderService = new OrderServiceImpl(); // set properties, etc. return orderService; } } @Autowired private OrderService orderService; @Test public void testOrderService() { // test the orderService } }
It may sometimes be desirable to mix XML resources and annotated classes (i.e.,
typically @Configuration
classes) to configure an ApplicationContext
for your tests.
For example, if you use XML configuration in production, you may decide that you want to
use @Configuration
classes to configure specific Spring-managed components for your
tests, or vice versa. As mentioned in the section called “Spring Testing Annotations” the
TestContext framework does not allow you to declare both via
@ContextConfiguration
, but this does not mean that you cannot use both.
If you want to use XML and @Configuration
classes to configure your tests, you
will have to pick one as the entry point, and that one will have to include or
import the other. For example, in XML you can include @Configuration
classes via
component scanning or define them as normal Spring beans in XML; whereas, in a
@Configuration
class you can use @ImportResource
to import XML configuration files.
Note that this behavior is semantically equivalent to how you configure your application
in production: in production configuration you will define either a set of XML resource
locations or a set of @Configuration
classes that your production ApplicationContext
will be loaded from, but you still have the freedom to include or import the other type
of configuration.
To configure an ApplicationContext
for your tests using context initializers, annotate
your test class with @ContextConfiguration
and configure the initializers
attribute
with an array that contains references to classes that implement
ApplicationContextInitializer
. The declared context initializers will then be used to
initialize the ConfigurableApplicationContext
that is loaded for your tests. Note that
the concrete ConfigurableApplicationContext
type supported by each declared
initializer must be compatible with the type of ApplicationContext
created by the
SmartContextLoader
in use (i.e., typically a GenericApplicationContext
).
Furthermore, the order in which the initializers are invoked depends on whether they
implement Spring’s Ordered
interface or are annotated with Spring’s @Order
annotation.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from TestConfig // and initialized by TestAppCtxInitializer @ContextConfiguration( classes = TestConfig.class, initializers = TestAppCtxInitializer.class) public class MyTest { // class body... }
It is also possible to omit the declaration of XML configuration files or annotated
classes in @ContextConfiguration
entirely and instead declare only
ApplicationContextInitializer
classes which are then responsible for registering beans
in the context — for example, by programmatically loading bean definitions from XML
files or configuration classes.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be initialized by EntireAppInitializer // which presumably registers beans in the context @ContextConfiguration(initializers = EntireAppInitializer.class) public class MyTest { // class body... }
@ContextConfiguration
supports boolean inheritLocations
and inheritInitializers
attributes that denote whether resource locations or annotated classes and context
initializers declared by superclasses should be inherited. The default value for
both flags is true
. This means that a test class inherits the resource locations or
annotated classes as well as the context initializers declared by any superclasses.
Specifically, the resource locations or annotated classes for a test class are appended
to the list of resource locations or annotated classes declared by superclasses.
Similarly, the initializers for a given test class will be added to the set of
initializers defined by test superclasses. Thus, subclasses have the option
of extending the resource locations, annotated classes, or context initializers.
If @ContextConfiguration
's inheritLocations
or inheritInitializers
attribute is
set to false
, the resource locations or annotated classes and the context
initializers, respectively, for the test class shadow and effectively replace the
configuration defined by superclasses.
In the following example that uses XML resource locations, the ApplicationContext
for
ExtendedTest
will be loaded from "base-config.xml" and
"extended-config.xml", in that order. Beans defined in "extended-config.xml" may
therefore override (i.e., replace) those defined in "base-config.xml".
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from "/base-config.xml" // in the root of the classpath @ContextConfiguration("/base-config.xml") public class BaseTest { // class body... } // ApplicationContext will be loaded from "/base-config.xml" and // "/extended-config.xml" in the root of the classpath @ContextConfiguration("/extended-config.xml") public class ExtendedTest extends BaseTest { // class body... }
Similarly, in the following example that uses annotated classes, the
ApplicationContext
for ExtendedTest
will be loaded from the BaseConfig
and
ExtendedConfig
classes, in that order. Beans defined in ExtendedConfig
may therefore
override (i.e., replace) those defined in BaseConfig
.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from BaseConfig @ContextConfiguration(classes = BaseConfig.class) public class BaseTest { // class body... } // ApplicationContext will be loaded from BaseConfig and ExtendedConfig @ContextConfiguration(classes = ExtendedConfig.class) public class ExtendedTest extends BaseTest { // class body... }
In the following example that uses context initializers, the ApplicationContext
for
ExtendedTest
will be initialized using BaseInitializer
and
ExtendedInitializer
. Note, however, that the order in which the initializers are
invoked depends on whether they implement Spring’s Ordered
interface or are annotated
with Spring’s @Order
annotation.
@RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be initialized by BaseInitializer @ContextConfiguration(initializers=BaseInitializer.class) public class BaseTest { // class body... } // ApplicationContext will be initialized by BaseInitializer // and ExtendedInitializer @ContextConfiguration(initializers=ExtendedInitializer.class) public class ExtendedTest extends BaseTest { // class body... }
Spring 3.1 introduced first-class support in the framework for the notion of
environments and profiles (a.k.a., bean definition profiles), and integration tests
can be configured to activate particular bean definition profiles for various testing
scenarios. This is achieved by annotating a test class with the @ActiveProfiles
annotation and supplying a list of profiles that should be activated when loading the
ApplicationContext
for the test.
Note | |
---|---|
|
Let’s take a look at some examples with XML configuration and @Configuration
classes.
<!-- app-config.xml --> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jdbc="http://www.springframework.org/schema/jdbc" xmlns:jee="http://www.springframework.org/schema/jee" xsi:schemaLocation="..."> <bean id="transferService" class="com.bank.service.internal.DefaultTransferService"> <constructor-arg ref="accountRepository"/> <constructor-arg ref="feePolicy"/> </bean> <bean id="accountRepository" class="com.bank.repository.internal.JdbcAccountRepository"> <constructor-arg ref="dataSource"/> </bean> <bean id="feePolicy" class="com.bank.service.internal.ZeroFeePolicy"/> <beans profile="dev"> <jdbc:embedded-database id="dataSource"> <jdbc:script location="classpath:com/bank/config/sql/schema.sql"/> <jdbc:script location="classpath:com/bank/config/sql/test-data.sql"/> </jdbc:embedded-database> </beans> <beans profile="production"> <jee:jndi-lookup id="dataSource" jndi-name="java:comp/env/jdbc/datasource"/> </beans> <beans profile="default"> <jdbc:embedded-database id="dataSource"> <jdbc:script location="classpath:com/bank/config/sql/schema.sql"/> </jdbc:embedded-database> </beans> </beans>
package com.bank.service; @RunWith(SpringJUnit4ClassRunner.class) // ApplicationContext will be loaded from "classpath:/app-config.xml" @ContextConfiguration("/app-config.xml") @ActiveProfiles("dev") public class TransferServiceTest { @Autowired private TransferService transferService; @Test public void testTransferService() { // test the transferService } }
When TransferServiceTest
is run, its ApplicationContext
will be loaded from the
app-config.xml
configuration file in the root of the classpath. If you inspect
app-config.xml
you’ll notice that the accountRepository
bean has a dependency on a
dataSource
bean; however, dataSource
is not defined as a top-level bean. Instead,
dataSource
is defined three times: in the production profile, the
dev profile, and the default profile.
By annotating TransferServiceTest
with @ActiveProfiles("dev")
we instruct the Spring
TestContext Framework to load the ApplicationContext
with the active profiles set to
{"dev"}
. As a result, an embedded database will be created and populated with test data,
and the accountRepository
bean will be wired with a reference to the development
DataSource
. And that’s likely what we want in an integration test.
It is sometimes useful to assign beans to a default
profile. Beans within the default profile
are only included when no other profile is specifically activated. This can be used to define
fallback beans to be used in the application’s default state. For example, you may
explicitly provide a data source for dev
and production
profiles, but define an in-memory
data source as a default when neither of these is active.
The following code listings demonstrate how to implement the same configuration and
integration test but using @Configuration
classes instead of XML.
@Configuration @Profile("dev") public class StandaloneDataConfig { @Bean public DataSource dataSource() { return new EmbeddedDatabaseBuilder() .setType(EmbeddedDatabaseType.HSQL) .addScript("classpath:com/bank/config/sql/schema.sql") .addScript("classpath:com/bank/config/sql/test-data.sql") .build(); } }
@Configuration @Profile("production") public class JndiDataConfig { @Bean public DataSource dataSource() throws Exception { Context ctx = new InitialContext(); return (DataSource) ctx.lookup("java:comp/env/jdbc/datasource"); } }
@Configuration @Profile("default") public class DefaultDataConfig { @Bean public DataSource dataSource() { return new EmbeddedDatabaseBuilder() .setType(EmbeddedDatabaseType.HSQL) .addScript("classpath:com/bank/config/sql/schema.sql") .build(); } }
@Configuration public class TransferServiceConfig { @Autowired DataSource dataSource; @Bean public TransferService transferService() { return new DefaultTransferService(accountRepository(), feePolicy()); } @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } @Bean public FeePolicy feePolicy() { return new ZeroFeePolicy(); } }
package com.bank.service; @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes = { TransferServiceConfig.class, StandaloneDataConfig.class, JndiDataConfig.class, DefaultDataConfig.class}) @ActiveProfiles("dev") public class TransferServiceTest { @Autowired private TransferService transferService; @Test public void testTransferService() { // test the transferService } }
In this variation, we have split the XML configuration into four independent
@Configuration
classes:
TransferServiceConfig
: acquires a dataSource
via dependency injection using
@Autowired
StandaloneDataConfig
: defines a dataSource
for an embedded database suitable for
developer tests
JndiDataConfig
: defines a dataSource
that is retrieved from JNDI in a production
environment
DefaultDataConfig
: defines a dataSource
for a default embedded database in case
no profile is active
As with the XML-based configuration example, we still annotate TransferServiceTest
with @ActiveProfiles("dev")
, but this time we specify all four configuration classes
via the @ContextConfiguration
annotation. The body of the test class itself remains
completely unchanged.
It is often the case that a single set of profiles is used across multiple test classes
within a given project. Thus, to avoid duplicate declarations of the @ActiveProfiles
annotation it is possible to declare @ActiveProfiles
once on a base class, and
subclasses will automatically inherit the @ActiveProfiles
configuration from the base
class. In the following example, the declaration of @ActiveProfiles
(as well as other
annotations) has been moved to an abstract superclass, AbstractIntegrationTest
.
package com.bank.service; @RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(classes = { TransferServiceConfig.class, StandaloneDataConfig.class, JndiDataConfig.class, DefaultDataConfig.class}) @ActiveProfiles("dev") public abstract class AbstractIntegrationTest { }
package com.bank.service; // "dev" profile inherited from superclass public class TransferServiceTest extends AbstractIntegrationTest { @Autowired private TransferService transferService; @Test public void testTransferService() { // test the transferService } }
@ActiveProfiles
also supports an inheritProfiles
attribute that can be used to
disable the inheritance of active profiles.
package com.bank.service; // "dev" profile overridden with "production" @ActiveProfiles(profiles = "production", inheritProfiles = false) public class ProductionTransferServiceTest extends AbstractIntegrationTest { // test body }
Furthermore, it is sometimes necessary to resolve active profiles for tests programmatically instead of declaratively — for example, based on:
To resolve active bean definition profiles programmatically, simply implement a custom
ActiveProfilesResolver
and register it via the resolver
attribute of
@ActiveProfiles
. The following example demonstrates how to implement and register a
custom OperatingSystemActiveProfilesResolver
. For further information, refer to the
respective Javadoc.
package com.bank.service; // "dev" profile overridden programmatically via a custom resolver @ActiveProfiles( resolver = OperatingSystemActiveProfilesResolver.class, inheritProfiles = false) public class TransferServiceTest extends AbstractIntegrationTest { // test body }
package com.bank.service.test; public class OperatingSystemActiveProfilesResolver implements ActiveProfilesResolver { @Override String[] resolve(Class<?> testClass) { String profile = ...; // determine the value of profile based on the operating system return new String[] {profile}; } }
Spring 3.2 introduced support for loading a WebApplicationContext
in integration
tests. To instruct the TestContext framework to load a WebApplicationContext
instead
of a standard ApplicationContext
, simply annotate the respective test class with
@WebAppConfiguration
.
The presence of @WebAppConfiguration
on your test class instructs the TestContext
framework (TCF) that a WebApplicationContext
(WAC) should be loaded for your
integration tests. In the background the TCF makes sure that a MockServletContext
is
created and supplied to your test’s WAC. By default the base resource path for your
MockServletContext
will be set to "src/main/webapp". This is interpreted as a path
relative to the root of your JVM (i.e., normally the path to your project). If you’re
familiar with the directory structure of a web application in a Maven project, you’ll
know that "src/main/webapp" is the default location for the root of your WAR. If you
need to override this default, simply provide an alternate path to the
@WebAppConfiguration
annotation (e.g., @WebAppConfiguration("src/test/webapp")
). If
you wish to reference a base resource path from the classpath instead of the file
system, just use Spring’s classpath: prefix.
Please note that Spring’s testing support for WebApplicationContexts
is on par with
its support for standard ApplicationContexts
. When testing with a
WebApplicationContext
you are free to declare either XML configuration files or
@Configuration
classes via @ContextConfiguration
. You are of course also free to use
any other test annotations such as @TestExecutionListeners
,
@TransactionConfiguration
, @ActiveProfiles
, etc.
The following examples demonstrate some of the various configuration options for loading
a WebApplicationContext
.
Conventions.
@RunWith(SpringJUnit4ClassRunner.class) // defaults to "file:src/main/webapp" @WebAppConfiguration // detects "WacTests-context.xml" in same package // or static nested @Configuration class @ContextConfiguration public class WacTests { //... }
The above example demonstrates the TestContext framework’s support for convention over
configuration. If you annotate a test class with @WebAppConfiguration
without
specifying a resource base path, the resource path will effectively default
to "file:src/main/webapp". Similarly, if you declare @ContextConfiguration
without
specifying resource locations
, annotated classes
, or context initializers
, Spring
will attempt to detect the presence of your configuration using conventions
(i.e., "WacTests-context.xml" in the same package as the WacTests
class or static
nested @Configuration
classes).
Default resource semantics.
@RunWith(SpringJUnit4ClassRunner.class) // file system resource @WebAppConfiguration("webapp") // classpath resource @ContextConfiguration("/spring/test-servlet-config.xml") public class WacTests { //... }
This example demonstrates how to explicitly declare a resource base path with
@WebAppConfiguration
and an XML resource location with @ContextConfiguration
. The
important thing to note here is the different semantics for paths with these two
annotations. By default, @WebAppConfiguration
resource paths are file system based;
whereas, @ContextConfiguration
resource locations are classpath based.
Explicit resource semantics.
@RunWith(SpringJUnit4ClassRunner.class) // classpath resource @WebAppConfiguration("classpath:test-web-resources") // file system resource @ContextConfiguration("file:src/main/webapp/WEB-INF/servlet-config.xml") public class WacTests { //... }
In this third example, we see that we can override the default resource semantics for both annotations by specifying a Spring resource prefix. Contrast the comments in this example with the previous example.
To provide comprehensive web testing support, Spring 3.2 introduced a
ServletTestExecutionListener
that is enabled by default. When testing against a
WebApplicationContext
this TestExecutionListener sets
up default thread-local state via Spring Web’s RequestContextHolder
before each test
method and creates a MockHttpServletRequest
, MockHttpServletResponse
, and
ServletWebRequest
based on the base resource path configured via
@WebAppConfiguration
. ServletTestExecutionListener
also ensures that the
MockHttpServletResponse
and ServletWebRequest
can be injected into the test
instance, and once the test is complete it cleans up thread-local state.
Once you have a WebApplicationContext
loaded for your test you might find that you
need to interact with the web mocks — for example, to set up your test fixture or to
perform assertions after invoking your web component. The following example demonstrates
which mocks can be autowired into your test instance. Note that the
WebApplicationContext
and MockServletContext
are both cached across the test suite;
whereas, the other mocks are managed per test method by the
ServletTestExecutionListener
.
Injecting mocks.
@WebAppConfiguration @ContextConfiguration public class WacTests { @Autowired WebApplicationContext wac; // cached @Autowired MockServletContext servletContext; // cached @Autowired MockHttpSession session; @Autowired MockHttpServletRequest request; @Autowired MockHttpServletResponse response; @Autowired ServletWebRequest webRequest; //... }
Once the TestContext framework loads an ApplicationContext
(or
WebApplicationContext
) for a test, that context will be cached and reused for all
subsequent tests that declare the same unique context configuration within the same test
suite. To understand how caching works, it is important to understand what is meant by
unique and test suite.
An ApplicationContext
can be uniquely identified by the combination of
configuration parameters that are used to load it. Consequently, the unique combination
of configuration parameters are used to generate a key under which the context is
cached. The TestContext framework uses the following configuration parameters to build
the context cache key:
locations
(from @ContextConfiguration)
classes
(from @ContextConfiguration)
contextInitializerClasses
(from @ContextConfiguration)
contextLoader
(from @ContextConfiguration)
activeProfiles
(from @ActiveProfiles)
resourceBasePath
(from @WebAppConfiguration)
For example, if TestClassA
specifies {"app-config.xml", "test-config.xml"}
for the
locations
(or value
) attribute of @ContextConfiguration
, the TestContext framework
will load the corresponding ApplicationContext
and store it in a static
context
cache under a key that is based solely on those locations. So if TestClassB
also
defines {"app-config.xml", "test-config.xml"}
for its locations (either explicitly or
implicitly through inheritance) but does not define @WebAppConfiguration
, a different
ContextLoader
, different active profiles, or different context initializers, then the
same ApplicationContext
will be shared by both test classes. This means that the setup
cost for loading an application context is incurred only once (per test suite), and
subsequent test execution is much faster.
Test suites and forked processes | |
---|---|
The Spring TestContext framework stores application contexts in a static cache. This
means that the context is literally stored in a To benefit from the caching mechanism, all tests must run within the same process or
test suite. This can be achieved by executing all tests as a group within an IDE.
Similarly, when executing tests with a build framework such as Ant, Maven, or Gradle it
is important to make sure that the build framework does not fork between tests. For
example, if the
forkMode
for the Maven Surefire plug-in is set to |
In the unlikely case that a test corrupts the application context and requires reloading — for example, by modifying a bean definition or the state of an application object — you can annotate your test class or test method with @DirtiesContext
(see the
discussion of @DirtiesContext
in the section called “Spring Testing Annotations”). This
instructs Spring to remove the context from the cache and rebuild the application
context before executing the next test. Note that support for the @DirtiesContext
annotation is provided by the DirtiesContextTestExecutionListener
which is enabled by
default.
When writing integration tests that rely on a loaded Spring ApplicationContext
, it is
often sufficient to test against a single context; however, there are times when it is
beneficial or even necessary to test against a hierarchy of ApplicationContext
s. For
example, if you are developing a Spring MVC web application you will typically have a
root WebApplicationContext
loaded via Spring’s ContextLoaderListener
and a child
WebApplicationContext
loaded via Spring’s DispatcherServlet
. This results in a
parent-child context hierarchy where shared components and infrastructure configuration
are declared in the root context and consumed in the child context by web-specific
components. Another use case can be found in Spring Batch applications where you often
have a parent context that provides configuration for shared batch infrastructure and a
child context for the configuration of a specific batch job.
As of Spring Framework 3.2.2, it is possible to write integration tests that use context
hierarchies by declaring context configuration via the @ContextHierarchy
annotation,
either on an individual test class or within a test class hierarchy. If a context
hierarchy is declared on multiple classes within a test class hierarchy it is also
possible to merge or override the context configuration for a specific, named level in
the context hierarchy. When merging configuration for a given level in the hierarchy the
configuration resource type (i.e., XML configuration files or annotated classes) must be
consistent; otherwise, it is perfectly acceptable to have different levels in a context
hierarchy configured using different resource types.
The following JUnit-based examples demonstrate common configuration scenarios for integration tests that require the use of context hierarchies.
ControllerIntegrationTests
represents a typical integration testing scenario for a
Spring MVC web application by declaring a context hierarchy consisting of two levels,
one for the root WebApplicationContext (loaded using the TestAppConfig
@Configuration
class) and one for the dispatcher servlet WebApplicationContext
(loaded using the WebConfig
@Configuration
class). The WebApplicationContext
that
is autowired into the test instance is the one for the child context (i.e., the
lowest context in the hierarchy).
@RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextHierarchy({ @ContextConfiguration(classes = TestAppConfig.class), @ContextConfiguration(classes = WebConfig.class) }) public class ControllerIntegrationTests { @Autowired private WebApplicationContext wac; // ... }
The following test classes define a context hierarchy within a test class hierarchy.
AbstractWebTests
declares the configuration for a root WebApplicationContext
in a
Spring-powered web application. Note, however, that AbstractWebTests
does not declare
@ContextHierarchy
; consequently, subclasses of AbstractWebTests
can optionally
participate in a context hierarchy or simply follow the standard semantics for
@ContextConfiguration
. SoapWebServiceTests
and RestWebServiceTests
both extend
AbstractWebTests
and define a context hierarchy via @ContextHierarchy
. The result is
that three application contexts will be loaded (one for each declaration of
@ContextConfiguration
), and the application context loaded based on the configuration
in AbstractWebTests
will be set as the parent context for each of the contexts loaded
for the concrete subclasses.
@RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextConfiguration("file:src/main/webapp/WEB-INF/applicationContext.xml") public abstract class AbstractWebTests {} @ContextHierarchy(@ContextConfiguration("/spring/soap-ws-config.xml") public class SoapWebServiceTests extends AbstractWebTests {} @ContextHierarchy(@ContextConfiguration("/spring/rest-ws-config.xml") public class RestWebServiceTests extends AbstractWebTests {}
The following classes demonstrate the use of named hierarchy levels in order to
merge the configuration for specific levels in a context hierarchy. BaseTests
defines two levels in the hierarchy, parent
and child
. ExtendedTests
extends
BaseTests
and instructs the Spring TestContext Framework to merge the context
configuration for the child
hierarchy level, simply by ensuring that the names
declared via ContextConfiguration
's name
attribute are both "child"
. The result is
that three application contexts will be loaded: one for "/app-config.xml"
, one for
"/user-config.xml"
, and one for {"/user-config.xml", "/order-config.xml"}
. As with
the previous example, the application context loaded from "/app-config.xml"
will be
set as the parent context for the contexts loaded from "/user-config.xml"
and
{"/user-config.xml", "/order-config.xml"}
.
@RunWith(SpringJUnit4ClassRunner.class) @ContextHierarchy({ @ContextConfiguration(name = "parent", locations = "/app-config.xml"), @ContextConfiguration(name = "child", locations = "/user-config.xml") }) public class BaseTests {} @ContextHierarchy( @ContextConfiguration(name = "child", locations = "/order-config.xml") ) public class ExtendedTests extends BaseTests {}
In contrast to the previous example, this example demonstrates how to override the
configuration for a given named level in a context hierarchy by setting
ContextConfiguration
's inheritLocations
flag to false
. Consequently, the
application context for ExtendedTests
will be loaded only from
"/test-user-config.xml"
and will have its parent set to the context loaded from
"/app-config.xml"
.
@RunWith(SpringJUnit4ClassRunner.class) @ContextHierarchy({ @ContextConfiguration(name = "parent", locations = "/app-config.xml"), @ContextConfiguration(name = "child", locations = "/user-config.xml") }) public class BaseTests {} @ContextHierarchy( @ContextConfiguration( name = "child", locations = "/test-user-config.xml", inheritLocations = false )) public class ExtendedTests extends BaseTests {}
Dirtying a context within a context hierarchy | |
---|---|
If |
When you use the DependencyInjectionTestExecutionListener
— which is configured by
default — the dependencies of your test instances are injected from beans in the
application context that you configured with @ContextConfiguration
. You may use setter
injection, field injection, or both, depending on which annotations you choose and
whether you place them on setter methods or fields. For consistency with the annotation
support introduced in Spring 2.5 and 3.0, you can use Spring’s @Autowired
annotation
or the @Inject
annotation from JSR 300.
Tip | |
---|---|
The TestContext framework does not instrument the manner in which a test instance is
instantiated. Thus the use of |
Because @Autowired
is used to perform autowiring by type, if you have multiple bean definitions of the same type, you cannot rely on this
approach for those particular beans. In that case, you can use @Autowired
in
conjunction with @Qualifier
. As of Spring 3.0 you may also choose to use @Inject
in
conjunction with @Named
. Alternatively, if your test class has access to its
ApplicationContext
, you can perform an explicit lookup by using (for example) a call
to applicationContext.getBean("titleRepository")
.
If you do not want dependency injection applied to your test instances, simply do not
annotate fields or setter methods with @Autowired
or @Inject
. Alternatively, you can
disable dependency injection altogether by explicitly configuring your class with
@TestExecutionListeners
and omitting DependencyInjectionTestExecutionListener.class
from the list of listeners.
Consider the scenario of testing a HibernateTitleRepository
class, as outlined in the
Goals section. The next two code listings demonstrate the
use of @Autowired
on fields and setter methods. The application context configuration
is presented after all sample code listings.
Note | |
---|---|
The dependency injection behavior in the following code listings is not specific to JUnit. The same DI techniques can be used in conjunction with any testing framework. The following examples make calls to static assertion methods such as |
The first code listing shows a JUnit-based implementation of the test class that uses
@Autowired
for field injection.
@RunWith(SpringJUnit4ClassRunner.class) // specifies the Spring configuration to load for this test fixture @ContextConfiguration("repository-config.xml") public class HibernateTitleRepositoryTests { // this instance will be dependency injected by type @Autowired private HibernateTitleRepository titleRepository; @Test public void findById() { Title title = titleRepository.findById(new Long(10)); assertNotNull(title); } }
Alternatively, you can configure the class to use @Autowired
for setter injection as
seen below.
@RunWith(SpringJUnit4ClassRunner.class) // specifies the Spring configuration to load for this test fixture @ContextConfiguration("repository-config.xml") public class HibernateTitleRepositoryTests { // this instance will be dependency injected by type private HibernateTitleRepository titleRepository; @Autowired public void setTitleRepository(HibernateTitleRepository titleRepository) { this.titleRepository = titleRepository; } @Test public void findById() { Title title = titleRepository.findById(new Long(10)); assertNotNull(title); } }
The preceding code listings use the same XML context file referenced by the
@ContextConfiguration
annotation (that is, repository-config.xml
), which looks like
this:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- this bean will be injected into the HibernateTitleRepositoryTests class --> <bean id="titleRepository" class="com.foo.repository.hibernate.HibernateTitleRepository"> <property name="sessionFactory" ref="sessionFactory"/> </bean> <bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <!-- configuration elided for brevity --> </bean> </beans>
Note | |
---|---|
If you are extending from a Spring-provided test base class that happens to use
// ... @Autowired @Override public void setDataSource(@Qualifier("myDataSource") DataSource dataSource) { super.setDataSource(dataSource); } // ... The specified qualifier value indicates the specific |
Request and session scoped beans have been supported by Spring for several years now, but it’s always been a bit non-trivial to test them. As of Spring 3.2 it’s a breeze to test your request-scoped and session-scoped beans by following these steps.
WebApplicationContext
is loaded for your test by annotating your test
class with @WebAppConfiguration
.
WebApplicationContext
(i.e., via dependency injection).
The following code snippet displays the XML configuration for a login use case. Note
that the userService
bean has a dependency on a request-scoped loginAction
bean.
Also, the LoginAction
is instantiated using SpEL expressions that
retrieve the username and password from the current HTTP request. In our test, we will
want to configure these request parameters via the mock managed by the TestContext
framework.
Request-scoped bean configuration.
<beans> <bean id="userService" class="com.example.SimpleUserService" c:loginAction-ref="loginAction" /> <bean id="loginAction" class="com.example.LoginAction" c:username="{request.getParameter(user)}" c:password="{request.getParameter(pswd)}" scope="request"> <aop:scoped-proxy /> </bean> </beans>
In RequestScopedBeanTests
we inject both the UserService
(i.e., the subject under
test) and the MockHttpServletRequest
into our test instance. Within our
requestScope()
test method we set up our test fixture by setting request parameters in
the provided MockHttpServletRequest
. When the loginUser()
method is invoked on our
userService
we are assured that the user service has access to the request-scoped
loginAction
for the current MockHttpServletRequest
(i.e., the one we just set
parameters in). We can then perform assertions against the results based on the known
inputs for the username and password.
Request-scoped bean test.
@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration @WebAppConfiguration public class RequestScopedBeanTests { @Autowired UserService userService; @Autowired MockHttpServletRequest request; @Test public void requestScope() { request.setParameter("user", "enigma"); request.setParameter("pswd", "$pr!ng"); LoginResults results = userService.loginUser(); // assert results } }
The following code snippet is similar to the one we saw above for a request-scoped bean;
however, this time the userService
bean has a dependency on a session-scoped
userPreferences
bean. Note that the UserPreferences
bean is instantiated using a
SpEL expression that retrieves the theme from the current HTTP session. In our test,
we will need to configure a theme in the mock session managed by the TestContext
framework.
Session-scoped bean configuration.
<beans> <bean id="userService" class="com.example.SimpleUserService" c:userPreferences-ref="userPreferences" /> <bean id="userPreferences" class="com.example.UserPreferences" c:theme="#{session.getAttribute(theme)}" scope="session"> <aop:scoped-proxy /> </bean> </beans>
In SessionScopedBeanTests
we inject the UserService
and the MockHttpSession
into
our test instance. Within our sessionScope()
test method we set up our test fixture by
setting the expected "theme" attribute in the provided MockHttpSession
. When the
processUserPreferences()
method is invoked on our userService
we are assured that
the user service has access to the session-scoped userPreferences
for the current
MockHttpSession
, and we can perform assertions against the results based on the
configured theme.
Session-scoped bean test.
@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration @WebAppConfiguration public class SessionScopedBeanTests { @Autowired UserService userService; @Autowired MockHttpSession session; @Test public void sessionScope() throws Exception { session.setAttribute("theme", "blue"); Results results = userService.processUserPreferences(); // assert results } }
In the TestContext framework, transactions are managed by the
TransactionalTestExecutionListener
. Note that TransactionalTestExecutionListener
is
configured by default, even if you do not explicitly declare @TestExecutionListeners
on your test class. To enable support for transactions, however, you must provide a
PlatformTransactionManager
bean in the application context loaded by
@ContextConfiguration
semantics. In addition, you must declare @Transactional
either
at the class or method level for your tests.
For class-level transaction configuration (i.e., setting an explicit bean name for the
transaction manager and the default rollback flag), see the @TransactionConfiguration
entry in the annotation support section.
If transactions are not enabled for the entire test class, you can annotate methods
explicitly with @Transactional
. To control whether a transaction should commit for a
particular test method, you can use the @Rollback
annotation to override the
class-level default rollback setting.
AbstractTransactionalJUnit4SpringContextTests
and
AbstractTransactionalTestNGSpringContextTests
are preconfigured for transactional support at the class level.
Occasionally you need to execute certain code before or after a transactional test
method but outside the transactional context, for example, to verify the initial
database state prior to execution of your test or to verify expected transactional
commit behavior after test execution (if the test was configured not to roll back the
transaction). TransactionalTestExecutionListener
supports the @BeforeTransaction
and
@AfterTransaction
annotations exactly for such scenarios. Simply annotate any public
void
method in your test class with one of these annotations, and the
TransactionalTestExecutionListener
ensures that your before transaction method or
after transaction method is executed at the appropriate time.
Tip | |
---|---|
Any before methods (such as methods annotated with JUnit’s |
The following JUnit-based example displays a fictitious integration testing scenario highlighting several transaction-related annotations. Consult the annotation support section for further information and configuration examples.
@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration @TransactionConfiguration(transactionManager="txMgr", defaultRollback=false) @Transactional public class FictitiousTransactionalTest { @BeforeTransaction public void verifyInitialDatabaseState() { // logic to verify the initial state before a transaction is started } @Before public void setUpTestDataWithinTransaction() { // set up test data within the transaction } @Test // overrides the class-level defaultRollback setting @Rollback(true) public void modifyDatabaseWithinTransaction() { // logic which uses the test data and modifies database state } @After public void tearDownWithinTransaction() { // execute "tear down" logic within the transaction } @AfterTransaction public void verifyFinalDatabaseState() { // logic to verify the final state after transaction has rolled back } }
Avoid false positives when testing ORM code | |
---|---|
When you test application code that manipulates the state of the Hibernate session, make sure to flush the underlying session within test methods that execute that code. Failing to flush the underlying session can produce false positives: your test may pass, but the same code throws an exception in a live, production environment. In the following Hibernate-based example test case, one method demonstrates a false positive, and the other method correctly exposes the results of flushing the session. Note that this applies to JPA and any other ORM frameworks that maintain an in-memory unit of work. // ... @Autowired private SessionFactory sessionFactory; @Test // no expected exception! public void falsePositive() { updateEntityInHibernateSession(); // False positive: an exception will be thrown once the session is // finally flushed (i.e., in production code) } @Test(expected = GenericJDBCException.class) public void updateWithSessionFlush() { updateEntityInHibernateSession(); // Manual flush is required to avoid false positive in test sessionFactory.getCurrentSession().flush(); } // ... |
The org.springframework.test.context.junit4
package provides support classes for JUnit
4.5+ based test cases.
AbstractJUnit4SpringContextTests
: Abstract base test class that integrates the
Spring TestContext Framework with explicit ApplicationContext
testing support in
a JUnit 4.5+ environment.
When you extend AbstractJUnit4SpringContextTests
, you can access the following
protected
instance variable:
applicationContext
: Use this variable to perform explicit bean lookups or to test
the state of the context as a whole.
AbstractTransactionalJUnit4SpringContextTests
: Abstract transactional extension
of AbstractJUnit4SpringContextTests
that also adds some convenience functionality
for JDBC access. Expects a javax.sql.DataSource
bean and a
PlatformTransactionManager
bean to be defined in the ApplicationContext
. When you
extend AbstractTransactionalJUnit4SpringContextTests
you can access the following
protected
instance variables:
applicationContext
: Inherited from the AbstractJUnit4SpringContextTests
superclass. Use this variable to perform explicit bean lookups or to test the state of
the context as a whole.
jdbcTemplate
: Use this variable to execute SQL statements to query the database.
Such queries can be used to confirm database state both prior to and after
execution of database-related application code, and Spring ensures that such queries run
in the scope of the same transaction as the application code. When used in conjunction
with an ORM tool, be sure to avoid false positives.
Tip | |
---|---|
These classes are a convenience for extension. If you do not want your test classes to
be tied to a Spring-specific class hierarchy — for example, if you want to directly
extend the class you are testing — you can configure your own custom test classes by
using |
The Spring TestContext Framework offers full integration with JUnit 4.5+ through a
custom runner (tested on JUnit 4.5 — 4.11). By annotating test classes with
@RunWith(SpringJUnit4ClassRunner.class)
, developers can implement standard JUnit-based
unit and integration tests and simultaneously reap the benefits of the TestContext
framework such as support for loading application contexts, dependency injection of test
instances, transactional test method execution, and so on. The following code listing
displays the minimal requirements for configuring a test class to run with the custom
Spring Runner. @TestExecutionListeners
is configured with an empty list in order to
disable the default listeners, which otherwise would require an ApplicationContext to be
configured through @ContextConfiguration
.
@RunWith(SpringJUnit4ClassRunner.class) @TestExecutionListeners({}) public class SimpleTest { @Test public void testMethod() { // execute test logic... } }
The org.springframework.test.context.testng
package provides support classes for
TestNG based test cases.
AbstractTestNGSpringContextTests
: Abstract base test class that integrates the
Spring TestContext Framework with explicit ApplicationContext
testing support in
a TestNG environment.
When you extend AbstractTestNGSpringContextTests
, you can access the following
protected
instance variable:
applicationContext
: Use this variable to perform explicit bean lookups or to test
the state of the context as a whole.
AbstractTransactionalTestNGSpringContextTests
: Abstract transactional extension
of AbstractTestNGSpringContextTests
that adds some convenience functionality for JDBC
access. Expects a javax.sql.DataSource
bean and a PlatformTransactionManager
bean to
be defined in the ApplicationContext
. When you extend
AbstractTransactionalTestNGSpringContextTests
, you can access the following
protected
instance variables:
applicationContext
: Inherited from the AbstractTestNGSpringContextTests
superclass. Use this variable to perform explicit bean lookups or to test the state of
the context as a whole.
jdbcTemplate
: Use this variable to execute SQL statements to query the database.
Such queries can be used to confirm database state both prior to and after
execution of database-related application code, and Spring ensures that such queries run
in the scope of the same transaction as the application code. When used in conjunction
with an ORM tool, be sure to avoid false positives.
Tip | |
---|---|
These classes are a convenience for extension. If you do not want your test classes to
be tied to a Spring-specific class hierarchy — for example, if you want to directly
extend the class you are testing — you can configure your own custom test classes by
using |
The Spring MVC Test framework provides first class JUnit support for testing client
and server-side Spring MVC code through a fluent API. Typically it loads the actual
Spring configuration through the TestContext framework and always uses the
DispatcherServlet
to process requests thus approximating full integration tests
without requiring a running Servlet container.
Client-side tests are RestTemplate
-based and allow tests for code that relies on the
RestTemplate
without requiring a running server to respond to the requests.
Before Spring Framework 3.2, the most likely way to test a Spring MVC controller was to
write a unit test that instantiates the controller, injects it with mock or stub
dependencies, and then calls its methods directly, using a MockHttpServletRequest
and
MockHttpServletResponse
where necessary.
Although this is pretty easy to do, controllers have many annotations, and much remains
untested. Request mappings, data binding, type conversion, and validation are just a few
examples of what isn’t tested. Furthermore, there are other types of annotated methods
such as @InitBinder
, @ModelAttribute
, and @ExceptionHandler
that get invoked as
part of request processing.
The idea behind Spring MVC Test is to be able to re-write those controller tests by
performing actual requests and generating responses, as they would be at runtime, along
the way invoking controllers through the Spring MVC DispatcherServlet
. Controllers can
still be injected with mock dependencies, so tests can remain focused on the web layer.
Spring MVC Test builds on the familiar "mock" implementations of the Servlet API
available in the spring-test
module. This allows performing requests and generating
responses without the need for running in a Servlet container. For the most part
everything should work as it does at runtime with the exception of JSP rendering, which
is not available outside a Servlet container. Furthermore, if you are familiar with how
the MockHttpServletResponse
works, you’ll know that forwards and redirects are not
actually executed. Instead "forwarded" and "redirected" URLs are saved and can be
asserted in tests. This means if you are using JSPs, you can verify the JSP page to
which the request was forwarded.
All other means of rendering including @ResponseBody
methods and View
types (besides
JSPs) such as Freemarker, Velocity, Thymeleaf, and others for rendering HTML, JSON, XML,
and so on should work as expected, and the response will contain the generated content.
Below is an example of a test requesting account information in JSON format:
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*; import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*; @RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextConfiguration("test-servlet-context.xml") public class ExampleTests { @Autowired private WebApplicationContext wac; private MockMvc mockMvc; @Before public void setup() { this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build(); } @Test public void getAccount() throws Exception { this.mockMvc.perform(get("/accounts/1").accept(MediaType.parseMediaType("application/json;charset=UTF-8"))) .andExpect(status().isOk()) .andExpect(content().contentType("application/json")) .andExpect(jsonPath("$.name").value("Lee")); } }
The test relies on the WebApplicationContext
support of the TestContext framework.
It loads Spring configuration from an XML configuration file located in the same package
as the test class (also supports JavaConfig) and injects the created
WebApplicationContext
into the test so a MockMvc
instance can be created with it.
The MockMvc
is then used to perform a request to "/accounts/1"
and verify the
resulting response status is 200, the response content type is "application/json"
, and
response content has a JSON property called "name" with the value "Lee". JSON content is
inspected with the help of Jayway’s JsonPath
project. There are lots of other options for verifying the result of the performed
request and those will be discussed later.
The fluent API in the example above requires a few static imports such as
MockMvcRequestBuilders.*
, MockMvcResultMatchers.*
, and MockMvcBuilders.*
. An easy
way to find these classes is to search for types matching "MockMvc*". If using
Eclipse, be sure to add them as "favorite static members" in the Eclipse preferences
underJava → Editor → Content Assist → Favorites. That will allow use of content
assist after typing the first character of the static method name. Other IDEs (e.g.
IntelliJ) may not require any additional configuration. Just check the support for code
completion on static members.
The goal of server-side test setup is to create an instance of MockMvc
that can be
used to perform requests. There are two main options.
The first option is to point to Spring MVC configuration through the TestContext
framework, which loads the Spring configuration and injects a WebApplicationContext
into the test to use to create a MockMvc
:
@RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextConfiguration("my-servlet-context.xml") public class MyWebTests { @Autowired private WebApplicationContext wac; private MockMvc mockMvc; @Before public void setup() { this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build(); } // ... }
The second option is to simply register a controller instance without loading any Spring configuration. Instead basic Spring MVC configuration suitable for testing annotated controllers is automatically created. The created configuration is comparable to that of the MVC JavaConfig (and the MVC namespace) and can be customized to a degree through builder-style methods:
public class MyWebTests { private MockMvc mockMvc; @Before public void setup() { this.mockMvc = MockMvcBuilders.standaloneSetup(new AccountController()).build(); } // ... }
Which option should you use?
The "webAppContextSetup" loads the actual Spring MVC configuration resulting in a more complete integration test. Since the TestContext framework caches the loaded Spring configuration, it helps to keep tests running fast even as more tests get added. Furthermore, you can inject mock services into controllers through Spring configuration, in order to remain focused on testing the web layer. Here is an example of declaring a mock service with Mockito:
<bean id="accountService" class="org.mockito.Mockito" factory-method="mock"> <constructor-arg value="org.example.AccountService"/> </bean>
Then you can inject the mock service into the test in order set up and verify expectations:
@RunWith(SpringJUnit4ClassRunner.class) @WebAppConfiguration @ContextConfiguration("test-servlet-context.xml") public class AccountTests { @Autowired private WebApplicationContext wac; private MockMvc mockMvc; @Autowired private AccountService accountService; // ... }
The "standaloneSetup" on the other hand is a little closer to a unit test. It tests one controller at a time, the controller can be injected with mock dependencies manually, and it doesn’t involve loading Spring configuration. Such tests are more focused in style and make it easier to see which controller is being tested, whether any specific Spring MVC configuration is required to work, and so on. The "standaloneSetup" is also a very convenient way to write ad-hoc tests to verify some behavior or to debug an issue.
Just like with integration vs unit testing, there is no right or wrong answer. Using the "standaloneSetup" does imply the need for some additional "webAppContextSetup" tests to verify the Spring MVC configuration. Alternatively, you can decide write all tests with "webAppContextSetup" and always test against actual Spring MVC configuration.
To perform requests, use the appropriate HTTP method and additional builder-style
methods corresponding to properties of MockHttpServletRequest
. For example:
mockMvc.perform(post("/hotels/{id}", 42).accept(MediaType.APPLICATION_JSON));
In addition to all the HTTP methods, you can also perform file upload requests, which
internally creates an instance of MockMultipartHttpServletRequest
:
mockMvc.perform(fileUpload("/doc").file("a1", "ABC".getBytes("UTF-8")));
Query string parameters can be specified in the URI template:
mockMvc.perform(get("/hotels?foo={foo}", "bar"));
Or by adding Servlet request parameters:
mockMvc.perform(get("/hotels").param("foo", "bar"));
If application code relies on Servlet request parameters, and doesn’t check the query
string, as is most often the case, then it doesn’t matter how parameters are added. Keep
in mind though that parameters provided in the URI template will be decoded while
parameters provided through the param(...)
method are expected to be decoded.
In most cases it’s preferable to leave out the context path and the Servlet path from
the request URI. If you must test with the full request URI, be sure to set the
contextPath
and servletPath
accordingly so that request mappings will work:
mockMvc.perform(get("/app/main/hotels/{id}").contextPath("/app").servletPath("/main"))
Looking at the above example, it would be cumbersome to set the contextPath and
servletPath with every performed request. That’s why you can define default request
properties when building the MockMvc
:
public class MyWebTests { private MockMvc mockMvc; @Before public void setup() { mockMvc = standaloneSetup(new AccountController()) .defaultRequest(get("/") .contextPath("/app").servletPath("/main") .accept(MediaType.APPLICATION_JSON).build(); }
The above properties will apply to every request performed through the MockMvc
. If the
same property is also specified on a given request, it will override the default value.
That is why, the HTTP method and URI don’t matter, when setting default request
properties, since they must be specified on every request.
Expectations can be defined by appending one or more .andExpect(..)
after call to
perform the request:
mockMvc.perform(get("/accounts/1")).andExpect(status().isOk());
MockMvcResultMatchers.*
defines a number of static members, some of which return types
with additional methods, for asserting the result of the performed request. The
assertions fall in two general categories.
The first category of assertions verify properties of the response, i.e the response status, headers, and content. Those are the most important things to test for.
The second category of assertions go beyond the response, and allow inspecting Spring MVC specific constructs such as which controller method processed the request, whether an exception was raised and handled, what the content of the model is, what view was selected, what flash attributes were added, and so on. It is also possible to verify Servlet specific constructs such as request and session attributes. The following test asserts that binding/validation failed:
mockMvc.perform(post("/persons")) .andExpect(status().isOk()) .andExpect(model().attributeHasErrors("person"));
Many times when writing tests, it’s useful to dump the result of the performed request.
This can be done as follows, where print()
is a static import from
MockMvcResultHandlers
:
mockMvc.perform(post("/persons")) .andDo(print()) .andExpect(status().isOk()) .andExpect(model().attributeHasErrors("person"));
As long as request processing causes an unhandled exception, the print()
method will
print all the available result data to System.out
.
In some cases, you may want to get direct access to the result and verify something that
cannot be verified otherwise. This can be done by appending .andReturn()
at the end
after all expectations:
MvcResult mvcResult = mockMvc.perform(post("/persons")).andExpect(status().isOk()).andReturn(); // ...
When all tests repeat the same expectations, you can define the common expectations once
when building the MockMvc
:
standaloneSetup(new SimpleController()) .alwaysExpect(status().isOk()) .alwaysExpect(content().contentType("application/json;charset=UTF-8")) .build()
Note that the expectation is always applied and cannot be overridden without
creating a separate MockMvc
instance.
When JSON response content contains hypermedia links created with Spring HATEOAS, the resulting links can be verified:
mockMvc.perform(get("/people").accept(MediaType.APPLICATION_JSON)) .andExpect(jsonPath("$.links[?(@.rel == self)].href").value("http://localhost:8080/people"));
When XML response content contains hypermedia links created with Spring HATEOAS, the resulting links can be verified:
Map<String, String> ns = Collections.singletonMap("ns", "http://www.w3.org/2005/Atom"); mockMvc.perform(get("/handle").accept(MediaType.APPLICATION_XML)) .andExpect(xpath("/person/ns:link[@rel=self]/@href", ns).string("http://localhost:8080/people"));
When setting up a MockMvc
, you can register one or more Filter
instances:
mockMvc = standaloneSetup(new PersonController()).addFilters(new CharacterEncodingFilter()).build();
Registered filters will be invoked through MockFilterChain
from spring-test
and the
last filter will delegates to the DispatcherServlet
.
The framework’s own tests include many sample tests intended to demonstrate how to use Spring MVC Test. Browse these examples for further ideas. Also the spring-mvc-showcase has full test coverage based on Spring MVC Test.
Client-side tests are for code using the RestTemplate
. The goal is to define expected
requests and provide "stub" responses:
RestTemplate restTemplate = new RestTemplate(); MockRestServiceServer mockServer = MockRestServiceServer.createServer(restTemplate); mockServer.expect(requestTo("/greeting")).andRespond(withSuccess("Hello world", "text/plain")); // use RestTemplate ... mockServer.verify();
In the above example, MockRestServiceServer
— the central class for client-side REST
tests — configures the RestTemplate
with a custom ClientHttpRequestFactory
that
asserts actual requests against expectations and returns "stub" responses. In this case
we expect a single request to "/greeting" and want to return a 200 response with
"text/plain" content. We could define as many additional requests and stub responses as
necessary.
Once expected requests and stub responses have been defined, the RestTemplate
can be
used in client-side code as usual. At the end of the tests mockServer.verify()
can be
used to verify that all expected requests were performed.
Just like with server-side tests, the fluent API for client-side tests requires a few
static imports. Those are easy to find by searching "MockRest*". Eclipse users
should add "MockRestRequestMatchers.*"
and "MockRestResponseCreators.*"
as "favorite
static members" in the Eclipse preferences under Java → Editor → Content Assist →
Favorites. That allows using content assist after typing the first character of the
static method name. Other IDEs (e.g. IntelliJ) may not require any additional
configuration. Just check the support for code completion on static members.
Spring MVC Test’s own tests include example tests of client-side REST tests.
The PetClinic application, available on
Github, illustrates several features
of the Spring TestContext Framework in a JUnit 4.5+ environment. Most test
functionality is included in the AbstractClinicTests
, for which a partial listing
is shown below:
import static org.junit.Assert.assertEquals; // import ... @ContextConfiguration public abstract class AbstractClinicTests extends AbstractTransactionalJUnit4SpringContextTests { @Autowired protected Clinic clinic; @Test public void getVets() { Collection<Vet> vets = this.clinic.getVets(); assertEquals("JDBC query must show the same number of vets", super.countRowsInTable("VETS"), vets.size()); Vet v1 = EntityUtils.getById(vets, Vet.class, 2); assertEquals("Leary", v1.getLastName()); assertEquals(1, v1.getNrOfSpecialties()); assertEquals("radiology", (v1.getSpecialties().get(0)).getName()); // ... } // ... }
Notes:
AbstractTransactionalJUnit4SpringContextTests
class, from
which it inherits configuration for Dependency Injection (through the
DependencyInjectionTestExecutionListener
) and transactional behavior (through the
TransactionalTestExecutionListener
).
clinic
instance variable — the application object being tested — is set by
Dependency Injection through @Autowired
semantics.
testGetVets()
method illustrates how you can use the inherited
countRowsInTable()
method to easily verify the number of rows in a given table, thus
verifying correct behavior of the application code being tested. This allows for
stronger tests and lessens dependency on the exact test data. For example, you can add
additional rows in the database without breaking tests.
AbstractClinicTests
depend on a minimum amount of data already in the database
before the test cases run. Alternatively, you might choose to populate the database
within the test fixture set up of your test cases — again, within the same
transaction as the tests.
The PetClinic application supports three data access technologies: JDBC, Hibernate, and
JPA. By declaring @ContextConfiguration
without any specific resource locations, the
AbstractClinicTests
class will have its application context loaded from the default
location, AbstractClinicTests-context.xml
, which declares a common DataSource
.
Subclasses specify additional context locations that must declare a
PlatformTransactionManager
and a concrete implementation of Clinic
.
For example, the Hibernate implementation of the PetClinic tests contains the following
implementation. For this example, HibernateClinicTests
does not contain a single line
of code: we only need to declare @ContextConfiguration
, and the tests are inherited
from AbstractClinicTests
. Because @ContextConfiguration
is declared without any
specific resource locations, the Spring TestContext Framework loads an application
context from all the beans defined in AbstractClinicTests-context.xml
(i.e., the
inherited locations) and HibernateClinicTests-context.xml
, with
HibernateClinicTests-context.xml
possibly overriding beans defined in
AbstractClinicTests-context.xml
.
@ContextConfiguration public class HibernateClinicTests extends AbstractClinicTests { }
In a large-scale application, the Spring configuration is often split across multiple
files. Consequently, configuration locations are typically specified in a common base
class for all application-specific integration tests. Such a base class may also add
useful instance variables — populated by Dependency Injection, naturally — such as a
SessionFactory
in the case of an application using Hibernate.
As far as possible, you should have exactly the same Spring configuration files in your
integration tests as in the deployed environment. One likely point of difference
concerns database connection pooling and transaction infrastructure. If you are
deploying to a full-blown application server, you will probably use its connection pool
(available through JNDI) and JTA implementation. Thus in production you will use a
JndiObjectFactoryBean
or <jee:jndi-lookup>
for the DataSource
and
JtaTransactionManager
. JNDI and JTA will not be available in out-of-container
integration tests, so you should use a combination like the Commons DBCP
BasicDataSource
and DataSourceTransactionManager
or HibernateTransactionManager
for them. You can factor out this variant behavior into a single XML file, having the
choice between application server and a local configuration separated from all other
configuration, which will not vary between the test and production environments. In
addition, it is advisable to use properties files for connection settings. See the
PetClinic application for an example.
Consult the following resources for more information about testing:
This part of the reference documentation is concerned with data access and the interaction between the data access layer and the business or service layer.
Spring’s comprehensive transaction management support is covered in some detail, followed by thorough coverage of the various data access frameworks and technologies that the Spring Framework integrates with.
Comprehensive transaction support is among the most compelling reasons to use the Spring Framework. The Spring Framework provides a consistent abstraction for transaction management that delivers the following benefits:
The following sections describe the Spring Framework’s transaction value-adds and technologies. (The chapter also includes discussions of best practices, application server integration, and solutions to common problems.)
DataSource
instances from a variety of sources.
Traditionally, Java EE developers have had two choices for transaction management: global or local transactions, both of which have profound limitations. Global and local transaction management is reviewed in the next two sections, followed by a discussion of how the Spring Framework’s transaction management support addresses the limitations of the global and local transaction models.
Global transactions enable you to work with multiple transactional resources, typically
relational databases and message queues. The application server manages global
transactions through the JTA, which is a cumbersome API to use (partly due to its
exception model). Furthermore, a JTA UserTransaction
normally needs to be sourced from
JNDI, meaning that you also need to use JNDI in order to use JTA. Obviously the use
of global transactions would limit any potential reuse of application code, as JTA is
normally only available in an application server environment.
Previously, the preferred way to use global transactions was via EJB CMT (Container Managed Transaction): CMT is a form of declarative transaction management (as distinguished from programmatic transaction management). EJB CMT removes the need for transaction-related JNDI lookups, although of course the use of EJB itself necessitates the use of JNDI. It removes most but not all of the need to write Java code to control transactions. The significant downside is that CMT is tied to JTA and an application server environment. Also, it is only available if one chooses to implement business logic in EJBs, or at least behind a transactional EJB facade. The negatives of EJB in general are so great that this is not an attractive proposition, especially in the face of compelling alternatives for declarative transaction management.
Local transactions are resource-specific, such as a transaction associated with a JDBC connection. Local transactions may be easier to use, but have significant disadvantages: they cannot work across multiple transactional resources. For example, code that manages transactions using a JDBC connection cannot run within a global JTA transaction. Because the application server is not involved in transaction management, it cannot help ensure correctness across multiple resources. (It is worth noting that most applications use a single transaction resource.) Another downside is that local transactions are invasive to the programming model.
Spring resolves the disadvantages of global and local transactions. It enables application developers to use a consistent programming model in any environment. You write your code once, and it can benefit from different transaction management strategies in different environments. The Spring Framework provides both declarative and programmatic transaction management. Most users prefer declarative transaction management, which is recommended in most cases.
With programmatic transaction management, developers work with the Spring Framework transaction abstraction, which can run over any underlying transaction infrastructure. With the preferred declarative model, developers typically write little or no code related to transaction management, and hence do not depend on the Spring Framework transaction API, or any other transaction API.
The key to the Spring transaction abstraction is the notion of a transaction
strategy. A transaction strategy is defined by the
org.springframework.transaction.PlatformTransactionManager
interface:
public interface PlatformTransactionManager { TransactionStatus getTransaction( TransactionDefinition definition) throws TransactionException; void commit(TransactionStatus status) throws TransactionException; void rollback(TransactionStatus status) throws TransactionException; }
This is primarily a service provider interface (SPI), although it can be used
programmatically from your application code. Because
PlatformTransactionManager
is an interface, it can be easily mocked or stubbed as
necessary. It is not tied to a lookup strategy such as JNDI.
PlatformTransactionManager
implementations are defined like any other object (or bean)
in the Spring Framework IoC container. This benefit alone makes Spring Framework
transactions a worthwhile abstraction even when you work with JTA. Transactional code
can be tested much more easily than if it used JTA directly.
Again in keeping with Spring’s philosophy, the TransactionException
that can be thrown
by any of the PlatformTransactionManager
interface’s methods is unchecked (that
is, it extends the java.lang.RuntimeException
class). Transaction infrastructure
failures are almost invariably fatal. In rare cases where application code can actually
recover from a transaction failure, the application developer can still choose to catch
and handle TransactionException
. The salient point is that developers are not
forced to do so.
The getTransaction(..)
method returns a TransactionStatus
object, depending on a
TransactionDefinition
parameter. The returned TransactionStatus
might represent a
new transaction, or can represent an existing transaction if a matching transaction
exists in the current call stack. The implication in this latter case is that, as with
Java EE transaction contexts, a TransactionStatus
is associated with a thread of
execution.
The TransactionDefinition
interface specifies:
These settings reflect standard transactional concepts. If necessary, refer to resources that discuss transaction isolation levels and other core transaction concepts. Understanding these concepts is essential to using the Spring Framework or any transaction management solution.
The TransactionStatus
interface provides a simple way for transactional code to
control transaction execution and query transaction status. The concepts should be
familiar, as they are common to all transaction APIs:
public interface TransactionStatus extends SavepointManager { boolean isNewTransaction(); boolean hasSavepoint(); void setRollbackOnly(); boolean isRollbackOnly(); void flush(); boolean isCompleted(); }
Regardless of whether you opt for declarative or programmatic transaction management in
Spring, defining the correct PlatformTransactionManager
implementation is absolutely
essential. You typically define this implementation through dependency injection.
PlatformTransactionManager
implementations normally require knowledge of the
environment in which they work: JDBC, JTA, Hibernate, and so on. The following examples
show how you can define a local PlatformTransactionManager
implementation. (This
example works with plain JDBC.)
You define a JDBC DataSource
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}" /> <property name="url" value="${jdbc.url}" /> <property name="username" value="${jdbc.username}" /> <property name="password" value="${jdbc.password}" /> </bean>
The related PlatformTransactionManager
bean definition will then have a reference to
the DataSource
definition. It will look like this:
<bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean>
If you use JTA in a Java EE container then you use a container DataSource
, obtained
through JNDI, in conjunction with Spring’s JtaTransactionManager
. This is what the JTA
and JNDI lookup version would look like:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jee="http://www.springframework.org/schema/jee" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee.xsd"> <jee:jndi-lookup id="dataSource" jndi-name="jdbc/jpetstore"/> <bean id="txManager" class="org.springframework.transaction.jta.JtaTransactionManager" /> <!-- other <bean/> definitions here --> </beans>
The JtaTransactionManager
does not need to know about the DataSource
, or any other
specific resources, because it uses the container’s global transaction management
infrastructure.
Note | |
---|---|
The above definition of the |
You can also use Hibernate local transactions easily, as shown in the following
examples. In this case, you need to define a Hibernate LocalSessionFactoryBean
, which
your application code will use to obtain Hibernate Session
instances.
The DataSource
bean definition will be similar to the local JDBC example shown
previously and thus is not shown in the following example.
Note | |
---|---|
If the |
The txManager
bean in this case is of the HibernateTransactionManager
type. In the
same way as the DataSourceTransactionManager
needs a reference to the DataSource
,
the HibernateTransactionManager
needs a reference to the SessionFactory
.
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="dataSource" /> <property name="mappingResources"> <list> <value>org/springframework/samples/petclinic/hibernate/petclinic.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=${hibernate.dialect} </value> </property> </bean> <bean id="txManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory" /> </bean>
If you are using Hibernate and Java EE container-managed JTA transactions, then you
should simply use the same JtaTransactionManager
as in the previous JTA example for
JDBC.
<bean id="txManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
Note | |
---|---|
If you use JTA , then your transaction manager definition will look the same regardless of what data access technology you use, be it JDBC, Hibernate JPA or any other supported technology. This is due to the fact that JTA transactions are global transactions, which can enlist any transactional resource. |
In all these cases, application code does not need to change. You can change how transactions are managed merely by changing configuration, even if that change means moving from local to global transactions or vice versa.
It should now be clear how you create different transaction managers, and how they are
linked to related resources that need to be synchronized to transactions (for example
DataSourceTransactionManager
to a JDBC DataSource
, HibernateTransactionManager
to
a Hibernate SessionFactory
, and so forth). This section describes how the application
code, directly or indirectly using a persistence API such as JDBC, Hibernate, or JDO,
ensures that these resources are created, reused, and cleaned up properly. The section
also discusses how transaction synchronization is triggered (optionally) through the
relevant PlatformTransactionManager
.
The preferred approach is to use Spring’s highest level template based persistence
integration APIs or to use native ORM APIs with transaction- aware factory beans or
proxies for managing the native resource factories. These transaction-aware solutions
internally handle resource creation and reuse, cleanup, optional transaction
synchronization of the resources, and exception mapping. Thus user data access code does
not have to address these tasks, but can be focused purely on non-boilerplate
persistence logic. Generally, you use the native ORM API or take a template approach
for JDBC access by using the JdbcTemplate
. These solutions are detailed in subsequent
chapters of this reference documentation.
Classes such as DataSourceUtils
(for JDBC), EntityManagerFactoryUtils
(for JPA),
SessionFactoryUtils
(for Hibernate), PersistenceManagerFactoryUtils
(for JDO), and
so on exist at a lower level. When you want the application code to deal directly with
the resource types of the native persistence APIs, you use these classes to ensure that
proper Spring Framework-managed instances are obtained, transactions are (optionally)
synchronized, and exceptions that occur in the process are properly mapped to a
consistent API.
For example, in the case of JDBC, instead of the traditional JDBC approach of calling
the getConnection()
method on the DataSource
, you instead use Spring’s
org.springframework.jdbc.datasource.DataSourceUtils
class as follows:
Connection conn = DataSourceUtils.getConnection(dataSource);
If an existing transaction already has a connection synchronized (linked) to it, that
instance is returned. Otherwise, the method call triggers the creation of a new
connection, which is (optionally) synchronized to any existing transaction, and made
available for subsequent reuse in that same transaction. As mentioned, any
SQLException
is wrapped in a Spring Framework CannotGetJdbcConnectionException
, one
of the Spring Framework’s hierarchy of unchecked DataAccessExceptions. This approach
gives you more information than can be obtained easily from the SQLException
, and
ensures portability across databases, even across different persistence technologies.
This approach also works without Spring transaction management (transaction synchronization is optional), so you can use it whether or not you are using Spring for transaction management.
Of course, once you have used Spring’s JDBC support, JPA support or Hibernate support,
you will generally prefer not to use DataSourceUtils
or the other helper classes,
because you will be much happier working through the Spring abstraction than directly
with the relevant APIs. For example, if you use the Spring JdbcTemplate
or
jdbc.object
package to simplify your use of JDBC, correct connection retrieval occurs
behind the scenes and you won’t need to write any special code.
At the very lowest level exists the TransactionAwareDataSourceProxy
class. This is a
proxy for a target DataSource
, which wraps the target DataSource
to add awareness of
Spring-managed transactions. In this respect, it is similar to a transactional JNDI
DataSource
as provided by a Java EE server.
It should almost never be necessary or desirable to use this class, except when existing
code must be called and passed a standard JDBC DataSource
interface implementation. In
that case, it is possible that this code is usable, but participating in Spring managed
transactions. It is preferable to write your new code by using the higher level
abstractions mentioned above.
Note | |
---|---|
Most Spring Framework users choose declarative transaction management. This option has the least impact on application code, and hence is most consistent with the ideals of a non-invasive lightweight container. |
The Spring Framework’s declarative transaction management is made possible with Spring aspect-oriented programming (AOP), although, as the transactional aspects code comes with the Spring Framework distribution and may be used in a boilerplate fashion, AOP concepts do not generally have to be understood to make effective use of this code.
The Spring Framework’s declarative transaction management is similar to EJB CMT in that
you can specify transaction behavior (or lack of it) down to individual method level. It
is possible to make a setRollbackOnly()
call within a transaction context if
necessary. The differences between the two types of transaction management are:
setRollbackOnly()
.
The concept of rollback rules is important: they enable you to specify which exceptions
(and throwables) should cause automatic rollback. You specify this declaratively, in
configuration, not in Java code. So, although you can still call setRollbackOnly()
on
the TransactionStatus
object to roll back the current transaction back, most often you
can specify a rule that MyApplicationException
must always result in rollback. The
significant advantage to this option is that business objects do not depend on the
transaction infrastructure. For example, they typically do not need to import Spring
transaction APIs or other Spring APIs.
Although EJB container default behavior automatically rolls back the transaction on a
system exception (usually a runtime exception), EJB CMT does not roll back the
transaction automatically on anapplication exception (that is, a checked exception
other than java.rmi.RemoteException
). While the Spring default behavior for
declarative transaction management follows EJB convention (roll back is automatic only
on unchecked exceptions), it is often useful to customize this behavior.
It is not sufficient to tell you simply to annotate your classes with the
@Transactional
annotation, add @EnableTransactionManagement
to your configuration,
and then expect you to understand how it all works. This section explains the inner
workings of the Spring Framework’s declarative transaction infrastructure in the event
of transaction-related issues.
The most important concepts to grasp with regard to the Spring Framework’s declarative
transaction support are that this support is enabled
via AOP proxies, and that the transactional advice
is driven by metadata (currently XML- or annotation-based). The combination of AOP
with transactional metadata yields an AOP proxy that uses a TransactionInterceptor
in
conjunction with an appropriate PlatformTransactionManager
implementation to drive
transactions around method invocations.
Note | |
---|---|
Spring AOP is covered in Chapter 8, Aspect Oriented Programming with Spring. |
Conceptually, calling a method on a transactional proxy looks like this…
Consider the following interface, and its attendant implementation. This example uses
Foo
and Bar
classes as placeholders so that you can concentrate on the transaction
usage without focusing on a particular domain model. For the purposes of this example,
the fact that the DefaultFooService
class throws UnsupportedOperationException
instances in the body of each implemented method is good; it allows you to see
transactions created and then rolled back in response to the
UnsupportedOperationException
instance.
// the service interface that we want to make transactional package x.y.service; public interface FooService { Foo getFoo(String fooName); Foo getFoo(String fooName, String barName); void insertFoo(Foo foo); void updateFoo(Foo foo); }
// an implementation of the above interface package x.y.service; public class DefaultFooService implements FooService { public Foo getFoo(String fooName) { throw new UnsupportedOperationException(); } public Foo getFoo(String fooName, String barName) { throw new UnsupportedOperationException(); } public void insertFoo(Foo foo) { throw new UnsupportedOperationException(); } public void updateFoo(Foo foo) { throw new UnsupportedOperationException(); } }
Assume that the first two methods of the FooService
interface, getFoo(String)
and
getFoo(String, String)
, must execute in the context of a transaction with read-only
semantics, and that the other methods, insertFoo(Foo)
and updateFoo(Foo)
, must
execute in the context of a transaction with read-write semantics. The following
configuration is explained in detail in the next few paragraphs.
<!-- from the file context.xml --> <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- this is the service object that we want to make transactional --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- the transactional advice (what happens; see the <aop:advisor/> bean below) --> <tx:advice id="txAdvice" transaction-manager="txManager"> <!-- the transactional semantics... --> <tx:attributes> <!-- all methods starting with get are read-only --> <tx:method name="get*" read-only="true"/> <!-- other methods use the default transaction settings (see below) --> <tx:method name=""/> </tx:attributes> </tx:advice> <!-- ensure that the above transactional advice runs for any execution of an operation defined by the FooService interface --> <aop:config> <aop:pointcut id="fooServiceOperation" expression="execution( x.y.service.FooService.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="fooServiceOperation"/> </aop:config> <!-- don't forget the DataSource --> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/> <property name="url" value="jdbc:oracle:thin:@rj-t42:1521:elvis"/> <property name="username" value="scott"/> <property name="password" value="tiger"/> </bean> <!-- similarly, don't forget the PlatformTransactionManager --> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean> <!-- other <bean/> definitions here --> </beans>
Examine the preceding configuration. You want to make a service object, the fooService
bean, transactional. The transaction semantics to apply are encapsulated in the
<tx:advice/>
definition. The <tx:advice/>
definition reads as "… all methods on
starting with 'get'
are to execute in the context of a read-only transaction, and all
other methods are to execute with the default transaction semantics". The
transaction-manager
attribute of the <tx:advice/>
tag is set to the name of the
PlatformTransactionManager
bean that is going to drive the transactions, in this
case, the txManager
bean.
Tip | |
---|---|
You can omit the |
The <aop:config/>
definition ensures that the transactional advice defined by the
txAdvice
bean executes at the appropriate points in the program. First you define a
pointcut that matches the execution of any operation defined in the FooService
interface ( fooServiceOperation
). Then you associate the pointcut with the txAdvice
using an advisor. The result indicates that at the execution of a fooServiceOperation
,
the advice defined by txAdvice
will be run.
The expression defined within the <aop:pointcut/>
element is an AspectJ pointcut
expression; see Chapter 8, Aspect Oriented Programming with Spring for more details on pointcut expressions in Spring.
A common requirement is to make an entire service layer transactional. The best way to do this is simply to change the pointcut expression to match any operation in your service layer. For example:
<aop:config> <aop:pointcut id="fooServiceMethods" expression="execution(* x.y.service..(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="fooServiceMethods"/> </aop:config>
Note | |
---|---|
In this example it is assumed that all your service interfaces are defined in the
|
Now that we’ve analyzed the configuration, you may be asking yourself, "Okay… but what does all this configuration actually do?".
The above configuration will be used to create a transactional proxy around the object
that is created from the fooService
bean definition. The proxy will be configured with
the transactional advice, so that when an appropriate method is invoked on the
proxy, a transaction is started, suspended, marked as read-only, and so on, depending
on the transaction configuration associated with that method. Consider the following
program that test drives the above configuration:
public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("context.xml", Boot.class); FooService fooService = (FooService) ctx.getBean("fooService"); fooService.insertFoo (new Foo()); } }
The output from running the preceding program will resemble the following. (The Log4J output and the stack trace from the UnsupportedOperationException thrown by the insertFoo(..) method of the DefaultFooService class have been truncated for clarity.)
<!-- the Spring container is starting up... --> [AspectJInvocationContextExposingAdvisorAutoProxyCreator] - Creating implicit proxy for bean fooService with 0 common interceptors and 1 specific interceptors <!-- the DefaultFooService is actually proxied --> [JdkDynamicAopProxy] - Creating JDK dynamic proxy for [x.y.service.DefaultFooService] <!-- ... the insertFoo(..) method is now being invoked on the proxy --> [TransactionInterceptor] - Getting transaction for x.y.service.FooService.insertFoo <!-- the transactional advice kicks in here... --> [DataSourceTransactionManager] - Creating new transaction with name [x.y.service.FooService.insertFoo] [DataSourceTransactionManager] - Acquired Connection [org.apache.commons.dbcp.PoolableConnection@a53de4] for JDBC transaction <!-- the insertFoo(..) method from DefaultFooService throws an exception... --> [RuleBasedTransactionAttribute] - Applying rules to determine whether transaction should rollback on java.lang.UnsupportedOperationException [TransactionInterceptor] - Invoking rollback for transaction on x.y.service.FooService.insertFoo due to throwable [java.lang.UnsupportedOperationException] <!-- and the transaction is rolled back (by default, RuntimeException instances cause rollback) --> [DataSourceTransactionManager] - Rolling back JDBC transaction on Connection [org.apache.commons.dbcp.PoolableConnection@a53de4] [DataSourceTransactionManager] - Releasing JDBC Connection after transaction [DataSourceUtils] - Returning JDBC Connection to DataSource Exception in thread "main" java.lang.UnsupportedOperationException at x.y.service.DefaultFooService.insertFoo(DefaultFooService.java:14) <!-- AOP infrastructure stack trace elements removed for clarity --> at $Proxy0.insertFoo(Unknown Source) at Boot.main(Boot.java:11)
The previous section outlined the basics of how to specify transactional settings for classes, typically service layer classes, declaratively in your application. This section describes how you can control the rollback of transactions in a simple declarative fashion.
The recommended way to indicate to the Spring Framework’s transaction infrastructure
that a transaction’s work is to be rolled back is to throw an Exception
from code that
is currently executing in the context of a transaction. The Spring Framework’s
transaction infrastructure code will catch any unhandled Exception
as it bubbles up
the call stack, and make a determination whether to mark the transaction for rollback.
In its default configuration, the Spring Framework’s transaction infrastructure code
only marks a transaction for rollback in the case of runtime, unchecked exceptions;
that is, when the thrown exception is an instance or subclass of RuntimeException
. (
Error
s will also - by default - result in a rollback). Checked exceptions that are
thrown from a transactional method do not result in rollback in the default
configuration.
You can configure exactly which Exception
types mark a transaction for rollback,
including checked exceptions. The following XML snippet demonstrates how you configure
rollback for a checked, application-specific Exception
type.
<tx:advice id="txAdvice" transaction-manager="txManager"> <tx:attributes> <tx:method name="get*" read-only="true" rollback-for="NoProductInStockException"/> <tx:method name="*"/> </tx:attributes> </tx:advice>
You can also specify no rollback rules, if you do not want a transaction rolled
back when an exception is thrown. The following example tells the Spring Framework’s
transaction infrastructure to commit the attendant transaction even in the face of an
unhandled InstrumentNotFoundException
.
<tx:advice id="txAdvice"> <tx:attributes> <tx:method name="updateStock" no-rollback-for="InstrumentNotFoundException"/> <tx:method name="*"/> </tx:attributes> </tx:advice>
When the Spring Framework’s transaction infrastructure catches an exception and is
consults configured rollback rules to determine whether to mark the transaction for
rollback, the strongest matching rule wins. So in the case of the following
configuration, any exception other than an InstrumentNotFoundException
results in a
rollback of the attendant transaction.
<tx:advice id="txAdvice"> <tx:attributes> <tx:method name="*" rollback-for="Throwable" no-rollback-for="InstrumentNotFoundException"/> </tx:attributes> </tx:advice>
You can also indicate a required rollback programmatically. Although very simple, this process is quite invasive, and tightly couples your code to the Spring Framework’s transaction infrastructure:
public void resolvePosition() { try { // some business logic... } catch (NoProductInStockException ex) { // trigger rollback programmatically TransactionAspectSupport.currentTransactionStatus().setRollbackOnly(); } }
You are strongly encouraged to use the declarative approach to rollback if at all possible. Programmatic rollback is available should you absolutely need it, but its usage flies in the face of achieving a clean POJO-based architecture.
Consider the scenario where you have a number of service layer objects, and you want to
apply a totally different transactional configuration to each of them. You do this
by defining distinct <aop:advisor/>
elements with differing pointcut
and
advice-ref
attribute values.
As a point of comparison, first assume that all of your service layer classes are
defined in a root x.y.service
package. To make all beans that are instances of classes
defined in that package (or in subpackages) and that have names ending in Service
have
the default transactional configuration, you would write the following:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <aop:config> <aop:pointcut id="serviceOperation" expression="execution(* x.y.service..Service.(..))"/> <aop:advisor pointcut-ref="serviceOperation" advice-ref="txAdvice"/> </aop:config> <!-- these two beans will be transactional... --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <bean id="barService" class="x.y.service.extras.SimpleBarService"/> <!-- ... and these two beans won't --> <bean id="anotherService" class="org.xyz.SomeService"/> <!-- (not in the right package) --> <bean id="barManager" class="x.y.service.SimpleBarManager"/> <!-- (doesn't end in Service) --> <tx:advice id="txAdvice"> <tx:attributes> <tx:method name="get*" read-only="true"/> <tx:method name="*"/> </tx:attributes> </tx:advice> <!-- other transaction infrastructure beans such as a PlatformTransactionManager omitted... --> </beans>
The following example shows how to configure two distinct beans with totally different transactional settings.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <aop:config> <aop:pointcut id="defaultServiceOperation" expression="execution(* x.y.service.Service.(..))"/> <aop:pointcut id="noTxServiceOperation" expression="execution(* x.y.service.ddl.DefaultDdlManager.(..))"/> <aop:advisor pointcut-ref="defaultServiceOperation" advice-ref="defaultTxAdvice"/> <aop:advisor pointcut-ref="noTxServiceOperation" advice-ref="noTxAdvice"/> </aop:config> <!-- this bean will be transactional (see the defaultServiceOperation pointcut) --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this bean will also be transactional, but with totally different transactional settings --> <bean id="anotherFooService" class="x.y.service.ddl.DefaultDdlManager"/> <tx:advice id="defaultTxAdvice"> <tx:attributes> <tx:method name="get" read-only="true"/> <tx:method name=""/> </tx:attributes> </tx:advice> <tx:advice id="noTxAdvice"> <tx:attributes> <tx:method name="" propagation="NEVER"/> </tx:attributes> </tx:advice> <!-- other transaction infrastructure beans such as a PlatformTransactionManager omitted... --> </beans>
This section summarizes the various transactional settings that can be specified using
the <tx:advice/>
tag. The default <tx:advice/>
settings are:
REQUIRED.
DEFAULT.
RuntimeException
triggers rollback, and any checked Exception
does not.
You can change these default settings; the various attributes of the <tx:method/>
tags
that are nested within <tx:advice/>
and <tx:attributes/>
tags are summarized below:
Table 11.1. <tx:method/> settings
Attribute | Required? | Default | Description |
---|---|---|---|
| Yes | Method name(s) with which the transaction attributes are to be associated. The
wildcard (*) character can be used to associate the same transaction attribute
settings with a number of methods; for example, | |
| No | REQUIRED | Transaction propagation behavior. |
| No | DEFAULT | Transaction isolation level. |
| No | -1 | Transaction timeout value (in seconds). |
| No | false | Is this transaction read-only? |
| No |
| |
| No |
|
In addition to the XML-based declarative approach to transaction configuration, you can use an annotation-based approach. Declaring transaction semantics directly in the Java source code puts the declarations much closer to the affected code. There is not much danger of undue coupling, because code that is meant to be used transactionally is almost always deployed that way anyway.
The ease-of-use afforded by the use of the @Transactional
annotation is best
illustrated with an example, which is explained in the text that follows. Consider the
following class definition:
// the service class that we want to make transactional @Transactional public class DefaultFooService implements FooService { Foo getFoo(String fooName); Foo getFoo(String fooName, String barName); void insertFoo(Foo foo); void updateFoo(Foo foo); }
When the above POJO is defined as a bean in a Spring IoC container, the bean instance can be made transactional by adding merely one line of XML configuration:
<!-- from the file context.xml --> <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- this is the service object that we want to make transactional --> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- enable the configuration of transactional behavior based on annotations --> <tx:annotation-driven transaction-manager="txManager"/><!-- a PlatformTransactionManager is still required --> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <!-- (this dependency is defined somewhere else) --> <property name="dataSource" ref="dataSource"/> </bean> <!-- other <bean/> definitions here --> </beans>
Tip | |
---|---|
You can omit the |
Note | |
---|---|
The |
You can place the @Transactional
annotation before an interface definition, a method
on an interface, a class definition, or a public method on a class. However, the
mere presence of the @Transactional
annotation is not enough to activate the
transactional behavior. The @Transactional
annotation is simply metadata that can be
consumed by some runtime infrastructure that is @Transactional
-aware and that can use
the metadata to configure the appropriate beans with transactional behavior. In the
preceding example, the <tx:annotation-driven/>
element switches on the
transactional behavior.
Tip | |
---|---|
Spring recommends that you only annotate concrete classes (and methods of concrete
classes) with the |
Note | |
---|---|
In proxy mode (which is the default), only external method calls coming in through the
proxy are intercepted. This means that self-invocation, in effect, a method within the
target object calling another method of the target object, will not lead to an actual
transaction at runtime even if the invoked method is marked with |
Consider the use of AspectJ mode (see mode attribute in table below) if you expect
self-invocations to be wrapped with transactions as well. In this case, there will not
be a proxy in the first place; instead, the target class will be weaved (that is, its
byte code will be modified) in order to turn @Transactional
into runtime behavior on
any kind of method.
Table 11.2. Annotation driven transaction settings
XML Attribute | Annotation Attribute | Default | Description |
---|---|---|---|
| N/A (See | transactionManager | Name of transaction manager to use. Only required if the name of the transaction
manager is not |
|
| proxy | The default mode "proxy" processes annotated beans to be proxied using Spring’s AOP framework (following proxy semantics, as discussed above, applying to method calls coming in through the proxy only). The alternative mode "aspectj" instead weaves the affected classes with Spring’s AspectJ transaction aspect, modifying the target class byte code to apply to any kind of method call. AspectJ weaving requires spring-aspects.jar in the classpath as well as load-time weaving (or compile-time weaving) enabled. (See the section called “Spring configuration” for details on how to set up load-time weaving.) |
|
| false | Applies to proxy mode only. Controls what type of transactional proxies are created
for classes annotated with the |
|
| Ordered.LOWEST_PRECEDENCE | Defines the order of the transaction advice that is applied to beans annotated with
|
Note | |
---|---|
The |
Note | |
---|---|
|
The most derived location takes precedence when evaluating the transactional settings
for a method. In the case of the following example, the DefaultFooService
class is
annotated at the class level with the settings for a read-only transaction, but the
@Transactional
annotation on the updateFoo(Foo)
method in the same class takes
precedence over the transactional settings defined at the class level.
@Transactional(readOnly = true) public class DefaultFooService implements FooService { public Foo getFoo(String fooName) { // do something } // these settings have precedence for this method @Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW) public void updateFoo(Foo foo) { // do something } }
The @Transactional
annotation is metadata that specifies that an interface, class, or
method must have transactional semantics; for example, "start a brand new read-only
transaction when this method is invoked, suspending any existing transaction". The
default @Transactional
settings are as follows:
PROPAGATION_REQUIRED.
ISOLATION_DEFAULT.
RuntimeException
triggers rollback, and any checked Exception
does not.
These default settings can be changed; the various properties of the @Transactional
annotation are summarized in the following table:
Table 11.3. @
Property | Type | Description |
---|---|---|
String | Optional qualifier specifying the transaction manager to be used. | |
enum: | Optional propagation setting. | |
| enum: | Optional isolation level. |
| boolean | Read/write vs. read-only transaction |
| int (in seconds granularity) | Transaction timeout. |
| Array of | Optional array of exception classes that must cause rollback. |
| Array of class names. Classes must be derived from | Optional array of names of exception classes that must cause rollback. |
| Array of | Optional array of exception classes that must not cause rollback. |
| Array of | Optional array of names of exception classes that must not cause rollback. |
Currently you cannot have explicit control over the name of a transaction, where name
means the transaction name that will be shown in a transaction monitor, if applicable
(for example, WebLogic’s transaction monitor), and in logging output. For declarative
transactions, the transaction name is always the fully-qualified class name + "."
+ method name of the transactionally-advised class. For example, if the
handlePayment(..)
method of the BusinessService
class started a transaction, the
name of the transaction would be: com.foo.BusinessService.handlePayment
.
Most Spring applications only need a single transaction manager, but there may be
situations where you want multiple independent transaction managers in a single
application. The value attribute of the @Transactional
annotation can be used to
optionally specify the identity of the PlatformTransactionManager
to be used. This can
either be the bean name or the qualifier value of the transaction manager bean. For
example, using the qualifier notation, the following Java code
public class TransactionalService { @Transactional("order") public void setSomething(String name) { ... } @Transactional("account") public void doSomething() { ... } }
could be combined with the following transaction manager bean declarations in the application context.
<tx:annotation-driven/> <bean id="transactionManager1" class="org.springframework.jdbc.DataSourceTransactionManager"> ... <qualifier value="order"/> </bean> <bean id="transactionManager2" class="org.springframework.jdbc.DataSourceTransactionManager"> ... <qualifier value="account"/> </bean>
In this case, the two methods on TransactionalService
will run under separate
transaction managers, differentiated by the "order" and "account" qualifiers. The
default <tx:annotation-driven>
target bean name transactionManager
will still be
used if no specifically qualified PlatformTransactionManager bean is found.
If you find you are repeatedly using the same attributes with @Transactional
on many
different methods, then Spring’s meta-annotation support allows
you to define custom shortcut annotations for your specific use cases. For example,
defining the following annotations
@Target({ElementType.METHOD, ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Transactional("order") public @interface OrderTx { } @Target({ElementType.METHOD, ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Transactional("account") public @interface AccountTx { }
allows us to write the example from the previous section as
public class TransactionalService { @OrderTx public void setSomething(String name) { ... } @AccountTx public void doSomething() { ... } }
Here we have used the syntax to define the transaction manager qualifier, but could also have included propagation behavior, rollback rules, timeouts etc.
This section describes some semantics of transaction propagation in Spring. Please note that this section is not an introduction to transaction propagation proper; rather it details some of the semantics regarding transaction propagation in Spring.
In Spring-managed transactions, be aware of the difference between physical and logical transactions, and how the propagation setting applies to this difference.
PROPAGATION_REQUIRED
When the propagation setting is PROPAGATION_REQUIRED
, a logical transaction scope
is created for each method upon which the setting is applied. Each such logical
transaction scope can determine rollback-only status individually, with an outer
transaction scope being logically independent from the inner transaction scope. Of
course, in case of standard PROPAGATION_REQUIRED
behavior, all these scopes will be
mapped to the same physical transaction. So a rollback-only marker set in the inner
transaction scope does affect the outer transaction’s chance to actually commit (as you
would expect it to).
However, in the case where an inner transaction scope sets the rollback-only marker, the
outer transaction has not decided on the rollback itself, and so the rollback (silently
triggered by the inner transaction scope) is unexpected. A corresponding
UnexpectedRollbackException
is thrown at that point. This is expected behavior so
that the caller of a transaction can never be misled to assume that a commit was
performed when it really was not. So if an inner transaction (of which the outer caller
is not aware) silently marks a transaction as rollback-only, the outer caller still
calls commit. The outer caller needs to receive an UnexpectedRollbackException
to
indicate clearly that a rollback was performed instead.
PROPAGATION_REQUIRES_NEW
PROPAGATION_REQUIRES_NEW
, in contrast to PROPAGATION_REQUIRED
, uses a completely
independent transaction for each affected transaction scope. In that case, the
underlying physical transactions are different and hence can commit or roll back
independently, with an outer transaction not affected by an inner transaction’s rollback
status.
PROPAGATION_NESTED
uses a single physical transaction with multiple savepoints
that it can roll back to. Such partial rollbacks allow an inner transaction scope to
trigger a rollback for its scope, with the outer transaction being able to continue
the physical transaction despite some operations having been rolled back. This setting
is typically mapped onto JDBC savepoints, so will only work with JDBC resource
transactions. See Spring’s DataSourceTransactionManager
.
Suppose you want to execute both transactional and some basic profiling advice.
How do you effect this in the context of <tx:annotation-driven/>
?
When you invoke the updateFoo(Foo)
method, you want to see the following actions:
Note | |
---|---|
This chapter is not concerned with explaining AOP in any great detail (except as it applies to transactions). See Chapter 8, Aspect Oriented Programming with Spring for detailed coverage of the following AOP configuration and AOP in general. |
Here is the code for a simple profiling aspect discussed above. The ordering of advice
is controlled through the Ordered
interface. For full details on advice ordering, see
the section called “Advice ordering”.
.
package x.y; import org.aspectj.lang.ProceedingJoinPoint; import org.springframework.util.StopWatch; import org.springframework.core.Ordered; public class SimpleProfiler implements Ordered { private int order; // allows us to control the ordering of advice public int getOrder() { return this.order; } public void setOrder(int order) { this.order = order; } // this method is the around advice public Object profile(ProceedingJoinPoint call) throws Throwable { Object returnValue; StopWatch clock = new StopWatch(getClass().getName()); try { clock.start(call.toShortString()); returnValue = call.proceed(); } finally { clock.stop(); System.out.println(clock.prettyPrint()); } return returnValue; } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- this is the aspect --> <bean id="profiler" class="x.y.SimpleProfiler"> <!-- execute before the transactional advice (hence the lower order number) --> <property name="order" value="1"/> </bean> <tx:annotation-driven transaction-manager="txManager" order="200"/> <aop:config> <!-- this advice will execute around the transactional advice --> <aop:aspect id="profilingAspect" ref="profiler"> <aop:pointcut id="serviceMethodWithReturnValue" expression="execution(!void x.y..Service.(..))"/> <aop:around method="profile" pointcut-ref="serviceMethodWithReturnValue"/> </aop:aspect> </aop:config> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="oracle.jdbc.driver.OracleDriver"/> <property name="url" value="jdbc:oracle:thin:@rj-t42:1521:elvis"/> <property name="username" value="scott"/> <property name="password" value="tiger"/> </bean> <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource"/> </bean> </beans>
The result of the above configuration is a fooService
bean that has profiling and
transactional aspects applied to it in the desired order. You configure any number
of additional aspects in similar fashion.
The following example effects the same setup as above, but uses the purely XML declarative approach.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean id="fooService" class="x.y.service.DefaultFooService"/> <!-- the profiling advice --> <bean id="profiler" class="x.y.SimpleProfiler"> <!-- execute before the transactional advice (hence the lower order number) --> <property name="order" value="1"/> </bean> <aop:config> <aop:pointcut id="entryPointMethod" expression="execution(* x.y..Service.(..))"/> <!-- will execute after the profiling advice (c.f. the order attribute) --> <aop:advisor advice-ref="txAdvice" pointcut-ref="entryPointMethod" order="2"/> <!-- order value is higher than the profiling aspect --> <aop:aspect id="profilingAspect" ref="profiler"> <aop:pointcut id="serviceMethodWithReturnValue" expression="execution(!void x.y..Service.(..))"/> <aop:around method="profile" pointcut-ref="serviceMethodWithReturnValue"/> </aop:aspect> </aop:config> <tx:advice id="txAdvice" transaction-manager="txManager"> <tx:attributes> <tx:method name="get*" read-only="true"/> <tx:method name="*"/> </tx:attributes> </tx:advice> <!-- other <bean/> definitions such as a DataSource and a PlatformTransactionManager here --> </beans>
The result of the above configuration will be a fooService
bean that has profiling and
transactional aspects applied to it in that order. If you want the profiling advice
to execute after the transactional advice on the way in, and before the
transactional advice on the way out, then you simply swap the value of the profiling
aspect bean’s order
property so that it is higher than the transactional advice’s
order value.
You configure additional aspects in similar fashion.
It is also possible to use the Spring Framework’s @Transactional
support outside of a
Spring container by means of an AspectJ aspect. To do so, you first annotate your
classes (and optionally your classes' methods) with the @Transactional
annotation, and
then you link (weave) your application with the
org.springframework.transaction.aspectj.AnnotationTransactionAspect
defined in the
spring-aspects.jar
file. The aspect must also be configured with a transaction
manager. You can of course use the Spring Framework’s IoC container to take care of
dependency-injecting the aspect. The simplest way to configure the transaction
management aspect is to use the <tx:annotation-driven/>
element and specify the mode
attribute to aspectj
as described in Section 11.5.6, “Using @Transactional”. Because
we’re focusing here on applications running outside of a Spring container, we’ll show
you how to do it programmatically.
Note | |
---|---|
Prior to continuing, you may want to read Section 11.5.6, “Using @Transactional” and Chapter 8, Aspect Oriented Programming with Spring respectively. |
// construct an appropriate transaction manager DataSourceTransactionManager txManager = new DataSourceTransactionManager(getDataSource()); // configure the AnnotationTransactionAspect to use it; this must be done before executing any transactional methods AnnotationTransactionAspect.aspectOf().setTransactionManager(txManager);
Note | |
---|---|
When using this aspect, you must annotate the implementation class (and/or methods within that class), not the interface (if any) that the class implements. AspectJ follows Java’s rule that annotations on interfaces are not inherited. |
The @Transactional
annotation on a class specifies the default transaction semantics
for the execution of any method in the class.
The @Transactional
annotation on a method within the class overrides the default
transaction semantics given by the class annotation (if present). Any method may be
annotated, regardless of visibility.
To weave your applications with the AnnotationTransactionAspect
you must either build
your application with AspectJ (see the
AspectJ Development
Guide) or use load-time weaving. See Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework” for a discussion of load-time
weaving with AspectJ.
The Spring Framework provides two means of programmatic transaction management:
TransactionTemplate
.
PlatformTransactionManager
implementation directly.
The Spring team generally recommends the TransactionTemplate
for programmatic
transaction management. The second approach is similar to using the JTA
UserTransaction
API, although exception handling is less cumbersome.
The TransactionTemplate
adopts the same approach as other Spring templates such as
the JdbcTemplate
. It uses a callback approach, to free application code from having to
do the boilerplate acquisition and release of transactional resources, and results in
code that is intention driven, in that the code that is written focuses solely on what
the developer wants to do.
Note | |
---|---|
As you will see in the examples that follow, using the |
Application code that must execute in a transactional context, and that will use the
TransactionTemplate
explicitly, looks like the following. You, as an application
developer, write a TransactionCallback
implementation (typically expressed as an
anonymous inner class) that contains the code that you need to execute in the context of
a transaction. You then pass an instance of your custom TransactionCallback
to the
execute(..)
method exposed on the TransactionTemplate
.
public class SimpleService implements Service { // single TransactionTemplate shared amongst all methods in this instance private final TransactionTemplate transactionTemplate; // use constructor-injection to supply the PlatformTransactionManager public SimpleService(PlatformTransactionManager transactionManager) { Assert.notNull(transactionManager, "The transactionManager argument must not be null."); this.transactionTemplate = new TransactionTemplate(transactionManager); } public Object someServiceMethod() { return transactionTemplate.execute(new TransactionCallback() { // the code in this method executes in a transactional context public Object doInTransaction(TransactionStatus status) { updateOperation1(); return resultOfUpdateOperation2(); } }); } }
If there is no return value, use the convenient TransactionCallbackWithoutResult
class
with an anonymous class as follows:
transactionTemplate.execute(new TransactionCallbackWithoutResult() { protected void doInTransactionWithoutResult(TransactionStatus status) { updateOperation1(); updateOperation2(); } });
Code within the callback can roll the transaction back by calling the
setRollbackOnly()
method on the supplied TransactionStatus
object:
transactionTemplate.execute(new TransactionCallbackWithoutResult() { protected void doInTransactionWithoutResult(TransactionStatus status) { try { updateOperation1(); updateOperation2(); } catch (SomeBusinessExeption ex) { status.setRollbackOnly(); } } });
You can specify transaction settings such as the propagation mode, the isolation level,
the timeout, and so forth on the TransactionTemplate
either programmatically or in
configuration. TransactionTemplate
instances by default have the
default transactional settings. The
following example shows the programmatic customization of the transactional settings for
a specific TransactionTemplate:
public class SimpleService implements Service { private final TransactionTemplate transactionTemplate; public SimpleService(PlatformTransactionManager transactionManager) { Assert.notNull(transactionManager, "The transactionManager argument must not be null."); this.transactionTemplate = new TransactionTemplate(transactionManager); // the transaction settings can be set here explicitly if so desired this.transactionTemplate.setIsolationLevel(TransactionDefinition.ISOLATION_READ_UNCOMMITTED); this.transactionTemplate.setTimeout(30); // 30 seconds // and so forth... } }
The following example defines a TransactionTemplate
with some custom transactional
settings, using Spring XML configuration. The sharedTransactionTemplate
can then be
injected into as many services as are required.
<bean id="sharedTransactionTemplate" class="org.springframework.transaction.support.TransactionTemplate"> <property name="isolationLevelName" value="ISOLATION_READ_UNCOMMITTED"/> <property name="timeout" value="30"/> </bean>"
Finally, instances of the TransactionTemplate
class are threadsafe, in that instances
do not maintain any conversational state. TransactionTemplate
instances do however
maintain configuration state, so while a number of classes may share a single instance
of a TransactionTemplate
, if a class needs to use a TransactionTemplate
with
different settings (for example, a different isolation level), then you need to create
two distinct TransactionTemplate
instances.
You can also use the org.springframework.transaction.PlatformTransactionManager
directly to manage your transaction. Simply pass the implementation of the
PlatformTransactionManager
you are using to your bean through a bean reference. Then,
using the TransactionDefinition
and TransactionStatus
objects you can initiate
transactions, roll back, and commit.
DefaultTransactionDefinition def = new DefaultTransactionDefinition(); // explicitly setting the transaction name is something that can only be done programmatically def.setName("SomeTxName"); def.setPropagationBehavior(TransactionDefinition.PROPAGATION_REQUIRED); TransactionStatus status = txManager.getTransaction(def); try { // execute your business logic here } catch (MyException ex) { txManager.rollback(status); throw ex; } txManager.commit(status);
Programmatic transaction management is usually a good idea only if you have a small
number of transactional operations. For example, if you have a web application that
require transactions only for certain update operations, you may not want to set up
transactional proxies using Spring or any other technology. In this case, using the
TransactionTemplate
may be a good approach. Being able to set the transaction name
explicitly is also something that can only be done using the programmatic approach to
transaction management.
On the other hand, if your application has numerous transactional operations, declarative transaction management is usually worthwhile. It keeps transaction management out of business logic, and is not difficult to configure. When using the Spring Framework, rather than EJB CMT, the configuration cost of declarative transaction management is greatly reduced.
Spring’s transaction abstraction generally is application server agnostic. Additionally,
Spring’s JtaTransactionManager
class, which can optionally perform a JNDI lookup for
the JTA UserTransaction
and TransactionManager
objects, autodetects the location for
the latter object, which varies by application server. Having access to the JTA
TransactionManager
allows for enhanced transaction semantics, in particular supporting
transaction suspension. See the JtaTransactionManager
Javadocs for details.
Spring’s JtaTransactionManager
is the standard choice to run on Java EE application
servers, and is known to work on all common servers. Advanced functionality such as
transaction suspension works on many servers as well — including GlassFish, JBoss and
Geronimo — without any special configuration required. However, for fully supported
transaction suspension and further advanced integration, Spring ships special adapters
for WebLogic Server and WebSphere. These adapters are discussed in the following
sections.
For standard scenarios, including WebLogic Server and WebSphere, consider using the
convenient <tx:jta-transaction-manager/>
configuration element. When configured,
this element automatically detects the underlying server and chooses the best
transaction manager available for the platform. This means that you won’t have to
configure server-specific adapter classes (as discussed in the following sections)
explicitly; rather, they are chosen automatically, with the standard
JtaTransactionManager
as default fallback.
On WebSphere 6.1.0.9 and above, the recommended Spring JTA transaction manager to use is
WebSphereUowTransactionManager
. This special adapter leverages IBM’s UOWManager
API,
which is available in WebSphere Application Server 6.0.2.19 and later and 6.1.0.9 and
later. With this adapter, Spring-driven transaction suspension (suspend/resume as
initiated by PROPAGATION_REQUIRES_NEW
) is officially supported by IBM!
On WebLogic Server 9.0 or above, you typically would use the
WebLogicJtaTransactionManager
instead of the stock JtaTransactionManager
class. This
special WebLogic-specific subclass of the normal JtaTransactionManager
supports the
full power of Spring’s transaction definitions in a WebLogic-managed transaction
environment, beyond standard JTA semantics: Features include transaction names,
per-transaction isolation levels, and proper resuming of transactions in all cases.
Use the correct PlatformTransactionManager
implementation based on your choice of
transactional technologies and requirements. Used properly, the Spring Framework merely
provides a straightforward and portable abstraction. If you are using global
transactions, you must use the
org.springframework.transaction.jta.JtaTransactionManager
class (or an
application server-specific subclass of
it) for all your transactional operations. Otherwise the transaction infrastructure
attempts to perform local transactions on resources such as container DataSource
instances. Such local transactions do not make sense, and a good application server
treats them as errors.
For more information about the Spring Framework’s transaction support:
The Data Access Object (DAO) support in Spring is aimed at making it easy to work with data access technologies like JDBC, Hibernate, JPA or JDO in a consistent way. This allows one to switch between the aforementioned persistence technologies fairly easily and it also allows one to code without worrying about catching exceptions that are specific to each technology.
Spring provides a convenient translation from technology-specific exceptions like
SQLException
to its own exception class hierarchy with the DataAccessException
as
the root exception. These exceptions wrap the original exception so there is never any
risk that one might lose any information as to what might have gone wrong.
In addition to JDBC exceptions, Spring can also wrap Hibernate-specific exceptions, converting them from proprietary, checked exceptions (in the case of versions of Hibernate prior to Hibernate 3.0), to a set of focused runtime exceptions (the same is true for JDO and JPA exceptions). This allows one to handle most persistence exceptions, which are non-recoverable, only in the appropriate layers, without having annoying boilerplate catch-and-throw blocks and exception declarations in one’s DAOs. (One can still trap and handle exceptions anywhere one needs to though.) As mentioned above, JDBC exceptions (including database-specific dialects) are also converted to the same hierarchy, meaning that one can perform some operations with JDBC within a consistent programming model.
The above holds true for the various template classes in Springs support for various ORM
frameworks. If one uses the interceptor-based classes then the application must care
about handling HibernateExceptions
and JDOExceptions
itself, preferably via
delegating to SessionFactoryUtils
' convertHibernateAccessException(..)
or
convertJdoAccessException()
methods respectively. These methods convert the exceptions
to ones that are compatible with the exceptions in the org.springframework.dao
exception hierarchy. As JDOExceptions
are unchecked, they can simply get thrown too,
sacrificing generic DAO abstraction in terms of exceptions though.
The exception hierarchy that Spring provides can be seen below. (Please note that the
class hierarchy detailed in the image shows only a subset of the entire
DataAccessException
hierarchy.)
The best way to guarantee that your Data Access Objects (DAOs) or repositories provide
exception translation is to use the @Repository
annotation. This annotation also
allows the component scanning support to find and configure your DAOs and repositories
without having to provide XML configuration entries for them.
@Repository public class SomeMovieFinder implements MovieFinder { // ... }
Any DAO or repository implementation will need to access to a persistence resource,
depending on the persistence technology used; for example, a JDBC-based repository will
need access to a JDBC DataSource
; a JPA-based repository will need access to an
EntityManager
. The easiest way to accomplish this is to have this resource dependency
injected using one of the @Autowired,
, @Inject
, @Resource
or @PersistenceContext
annotations. Here is an example for a JPA repository:
@Repository public class JpaMovieFinder implements MovieFinder { @PersistenceContext private EntityManager entityManager; // ... }
If you are using the classic Hibernate APIs than you can inject the SessionFactory:
@Repository public class HibernateMovieFinder implements MovieFinder { private SessionFactory sessionFactory; @Autowired public void setSessionFactory(SessionFactory sessionFactory) { this.sessionFactory = sessionFactory; } // ... }
Last example we will show here is for typical JDBC support. You would have the
DataSource
injected into an initialization method where you would create a
JdbcTemplate
and other data access support classes like SimpleJdbcCall
etc using
this DataSource
.
@Repository public class JdbcMovieFinder implements MovieFinder { private JdbcTemplate jdbcTemplate; @Autowired public void init(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } // ... }
Note | |
---|---|
Please see the specific coverage of each persistence technology for details on how to configure the application context to take advantage of these annotations. |
The value-add provided by the Spring Framework JDBC abstraction is perhaps best shown by the sequence of actions outlined in the table below. The table shows what actions Spring will take care of and which actions are the responsibility of you, the application developer.
Table 13.1. Spring JDBC - who does what?
Action | Spring | You |
---|---|---|
Define connection parameters. | X | |
Open the connection. | X | |
Specify the SQL statement. | X | |
Declare parameters and provide parameter values | X | |
Prepare and execute the statement. | X | |
Set up the loop to iterate through the results (if any). | X | |
Do the work for each iteration. | X | |
Process any exception. | X | |
Handle transactions. | X | |
Close the connection, statement and resultset. | X |
The Spring Framework takes care of all the low-level details that can make JDBC such a tedious API to develop with.
You can choose among several approaches to form the basis for your JDBC database access. In addition to three flavors of the JdbcTemplate, a new SimpleJdbcInsert and SimplejdbcCall approach optimizes database metadata, and the RDBMS Object style takes a more object-oriented approach similar to that of JDO Query design. Once you start using one of these approaches, you can still mix and match to include a feature from a different approach. All approaches require a JDBC 2.0-compliant driver, and some advanced features require a JDBC 3.0 driver.
JdbcTemplate
to provide named parameters
instead of the traditional JDBC "?" placeholders. This approach provides better
documentation and ease of use when you have multiple parameters for an SQL statement.
The Spring Framework’s JDBC abstraction framework consists of four different packages,
namely core
, datasource
, object
, and support
.
The org.springframework.jdbc.core
package contains the JdbcTemplate
class and its
various callback interfaces, plus a variety of related classes. A subpackage named
org.springframework.jdbc.core.simple
contains the SimpleJdbcInsert
and
SimpleJdbcCall
classes. Another subpackage named
org.springframework.jdbc.core.namedparam
contains the NamedParameterJdbcTemplate
class and the related support classes. See Section 13.2, “Using the JDBC core classes to control basic JDBC processing and error handling”, Section 13.4, “JDBC batch operations”, and
Section 13.5, “Simplifying JDBC operations with the SimpleJdbc classes”
The org.springframework.jdbc.datasource
package contains a utility class for easy
DataSource
access, and various simple DataSource
implementations that can be used
for testing and running unmodified JDBC code outside of a Java EE container. A
subpackage named org.springfamework.jdbc.datasource.embedded
provides support for
creating in-memory database instances using Java database engines such as HSQL and H2.
See Section 13.3, “Controlling database connections” and Section 13.8, “Embedded database support”
The org.springframework.jdbc.object
package contains classes that represent RDBMS
queries, updates, and stored procedures as thread safe, reusable objects. See
Section 13.6, “Modeling JDBC operations as Java objects”.This approach is modeled by JDO, although of course objects returned by
queries are "disconnected" from the database. This higher level of JDBC abstraction
depends on the lower-level abstraction in the org.springframework.jdbc.core
package.
The org.springframework.jdbc.support
package provides SQLException
translation
functionality and some utility classes. Exceptions thrown during JDBC processing are
translated to exceptions defined in the org.springframework.dao
package. This means
that code using the Spring JDBC abstraction layer does not need to implement JDBC or
RDBMS-specific error handling. All translated exceptions are unchecked, which gives you
the option of catching the exceptions from which you can recover while allowing other
exceptions to be propagated to the caller. See Section 13.2.3, “SQLExceptionTranslator”.
The JdbcTemplate
class is the central class in the JDBC core package. It handles the
creation and release of resources, which helps you avoid common errors such as
forgetting to close the connection. It performs the basic tasks of the core JDBC
workflow such as statement creation and execution, leaving application code to provide
SQL and extract results. The JdbcTemplate
class executes SQL queries, update
statements and stored procedure calls, performs iteration over ResultSet
s and
extraction of returned parameter values. It also catches JDBC exceptions and translates
them to the generic, more informative, exception hierarchy defined in the
org.springframework.dao
package.
When you use the JdbcTemplate
for your code, you only need to implement callback
interfaces, giving them a clearly defined contract. The PreparedStatementCreator
callback interface creates a prepared statement given a Connection
provided by this
class, providing SQL and any necessary parameters. The same is true for the
CallableStatementCreator
interface, which creates callable statements. The
RowCallbackHandler
interface extracts values from each row of a ResultSet
.
The JdbcTemplate
can be used within a DAO implementation through direct instantiation
with a DataSource
reference, or be configured in a Spring IoC container and given to
DAOs as a bean reference.
Note | |
---|---|
The |
All SQL issued by this class is logged at the DEBUG
level under the category
corresponding to the fully qualified class name of the template instance (typically
JdbcTemplate
, but it may be different if you are using a custom subclass of the
JdbcTemplate
class).
This section provides some examples of JdbcTemplate
class usage. These examples are
not an exhaustive list of all of the functionality exposed by the JdbcTemplate
; see
the attendant Javadocs for that.
Here is a simple query for getting the number of rows in a relation:
int rowCount = this.jdbcTemplate.queryForObject("select count(*) from t_actor", Integer.class);
A simple query using a bind variable:
int countOfActorsNamedJoe = this.jdbcTemplate.queryForObject( "select count(*) from t_actor where first_name = ?", Integer.class, "Joe");
Querying for a String
:
String lastName = this.jdbcTemplate.queryForObject( "select last_name from t_actor where id = ?", new Object[]{1212L}, String.class);
Querying and populating a single domain object:
Actor actor = this.jdbcTemplate.queryForObject( "select first_name, last_name from t_actor where id = ?", new Object[]{1212L}, new RowMapper<Actor>() { public Actor mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setLastName(rs.getString("last_name")); return actor; } });
Querying and populating a number of domain objects:
List<Actor> actors = this.jdbcTemplate.query( "select first_name, last_name from t_actor", new RowMapper<Actor>() { public Actor mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setLastName(rs.getString("last_name")); return actor; } });
If the last two snippets of code actually existed in the same application, it would make
sense to remove the duplication present in the two RowMapper
anonymous inner classes,
and extract them out into a single class (typically a static
inner class) that can
then be referenced by DAO methods as needed. For example, it may be better to write the
last code snippet as follows:
public List<Actor> findAllActors() { return this.jdbcTemplate.query( "select first_name, last_name from t_actor", new ActorMapper()); } private static final class ActorMapper implements RowMapper<Actor> { public Actor mapRow(ResultSet rs, int rowNum) throws SQLException { Actor actor = new Actor(); actor.setFirstName(rs.getString("first_name")); actor.setLastName(rs.getString("last_name")); return actor; } }
You use the update(..)
method to perform insert, update and delete operations.
Parameter values are usually provided as var args or alternatively as an object array.
this.jdbcTemplate.update( "insert into t_actor (first_name, last_name) values (?, ?)", "Leonor", "Watling");
this.jdbcTemplate.update( "update t_actor set last_name = ? where id = ?", "Banjo", 5276L);
this.jdbcTemplate.update( "delete from actor where id = ?", Long.valueOf(actorId));
You can use the execute(..)
method to execute any arbitrary SQL, and as such the
method is often used for DDL statements. It is heavily overloaded with variants taking
callback interfaces, binding variable arrays, and so on.
this.jdbcTemplate.execute("create table mytable (id integer, name varchar(100))");
The following example invokes a simple stored procedure. More sophisticated stored procedure support is covered later.
this.jdbcTemplate.update( "call SUPPORT.REFRESH_ACTORS_SUMMARY(?)", Long.valueOf(unionId));
Instances of the JdbcTemplate
class are threadsafe once configured. This is
important because it means that you can configure a single instance of a JdbcTemplate
and then safely inject this shared reference into multiple DAOs (or repositories).
The JdbcTemplate
is stateful, in that it maintains a reference to a DataSource
, but
this state is not conversational state.
A common practice when using the JdbcTemplate
class (and the associated
NamedParameterJdbcTemplate
classes) is to
configure a DataSource
in your Spring configuration file, and then dependency-inject
that shared DataSource
bean into your DAO classes; the JdbcTemplate
is created in
the setter for the DataSource
. This leads to DAOs that look in part like the following:
public class JdbcCorporateEventDao implements CorporateEventDao { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } // JDBC-backed implementations of the methods on the CorporateEventDao follow... }
The corresponding configuration might look like this.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <bean id="corporateEventDao" class="com.example.JdbcCorporateEventDao"> <property name="dataSource" ref="dataSource"/> </bean> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <context:property-placeholder location="jdbc.properties"/> </beans>
An alternative to explicit configuration is to use component-scanning and annotation
support for dependency injection. In this case you annotate the class with @Repository
(which makes it a candidate for component-scanning) and annotate the DataSource
setter
method with @Autowired
.
@Repository public class JdbcCorporateEventDao implements CorporateEventDao { private JdbcTemplate jdbcTemplate; @Autowired public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } // JDBC-backed implementations of the methods on the CorporateEventDao follow... }
The corresponding XML configuration file would look like the following:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <!-- Scans within the base package of the application for @Components to configure as beans --> <context:component-scan base-package="org.springframework.docs.test" /> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <context:property-placeholder location="jdbc.properties"/> </beans>
If you are using Spring’s JdbcDaoSupport
class, and your various JDBC-backed DAO classes
extend from it, then your sub-class inherits a setDataSource(..)
method from the
JdbcDaoSupport
class. You can choose whether to inherit from this class. The
JdbcDaoSupport
class is provided as a convenience only.
Regardless of which of the above template initialization styles you choose to use (or
not), it is seldom necessary to create a new instance of a JdbcTemplate
class each
time you want to execute SQL. Once configured, a JdbcTemplate
instance is threadsafe.
You may want multiple JdbcTemplate
instances if your application accesses multiple
databases, which requires multiple DataSources
, and subsequently multiple differently
configured JdbcTemplates
.
The NamedParameterJdbcTemplate
class adds support for programming JDBC statements
using named parameters, as opposed to programming JDBC statements using only classic
placeholder ( '?'
) arguments. The NamedParameterJdbcTemplate
class wraps a
JdbcTemplate
, and delegates to the wrapped JdbcTemplate
to do much of its work. This
section describes only those areas of the NamedParameterJdbcTemplate
class that differ
from the JdbcTemplate
itself; namely, programming JDBC statements using named
parameters.
// some JDBC-backed DAO class... private NamedParameterJdbcTemplate namedParameterJdbcTemplate; public void setDataSource(DataSource dataSource) { this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource); } public int countOfActorsByFirstName(String firstName) { String sql = "select count(*) from T_ACTOR where first_name = :first_name"; SqlParameterSource namedParameters = new MapSqlParameterSource("first_name", firstName); return this.namedParameterJdbcTemplate.queryForObject(sql, Integer.class, namedParameters); }
Notice the use of the named parameter notation in the value assigned to the sql
variable, and the corresponding value that is plugged into the namedParameters
variable (of type MapSqlParameterSource
).
Alternatively, you can pass along named parameters and their corresponding values to a
NamedParameterJdbcTemplate
instance by using the Map
-based style.The remaining
methods exposed by the NamedParameterJdbcOperations
and implemented by the
NamedParameterJdbcTemplate
class follow a similar pattern and are not covered here.
The following example shows the use of the Map
-based style.
// some JDBC-backed DAO class... private NamedParameterJdbcTemplate namedParameterJdbcTemplate; public void setDataSource(DataSource dataSource) { this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource); } public int countOfActorsByFirstName(String firstName) { String sql = "select count(*) from T_ACTOR where first_name = :first_name"; Map<String, String> namedParameters = Collections.singletonMap("first_name", firstName); return this.namedParameterJdbcTemplate.queryForObject(sql, Integer.class, namedParameters); }
One nice feature related to the NamedParameterJdbcTemplate
(and existing in the same
Java package) is the SqlParameterSource
interface. You have already seen an example of
an implementation of this interface in one of the previous code snippet (the
MapSqlParameterSource
class). An SqlParameterSource
is a source of named parameter
values to a NamedParameterJdbcTemplate
. The MapSqlParameterSource
class is a very
simple implementation that is simply an adapter around a java.util.Map
, where the keys
are the parameter names and the values are the parameter values.
Another SqlParameterSource
implementation is the BeanPropertySqlParameterSource
class. This class wraps an arbitrary JavaBean (that is, an instance of a class that
adheres to the
JavaBean conventions), and uses the properties of the wrapped JavaBean as the source
of named parameter values.
public class Actor { private Long id; private String firstName; private String lastName; public String getFirstName() { return this.firstName; } public String getLastName() { return this.lastName; } public Long getId() { return this.id; } // setters omitted... }
// some JDBC-backed DAO class... private NamedParameterJdbcTemplate namedParameterJdbcTemplate; public void setDataSource(DataSource dataSource) { this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource); } public int countOfActors(Actor exampleActor) { // notice how the named parameters match the properties of the above Actor class String sql = "select count(*) from T_ACTOR where first_name = :firstName and last_name = :lastName"; SqlParameterSource namedParameters = new BeanPropertySqlParameterSource(exampleActor); return this.namedParameterJdbcTemplate.queryForObject(sql, Integer.class, namedParameters); }
Remember that the NamedParameterJdbcTemplate
class wraps a classic JdbcTemplate
template; if you need access to the wrapped JdbcTemplate
instance to access
functionality only present in the JdbcTemplate
class, you can use the
getJdbcOperations()
method to access the wrapped JdbcTemplate
through the
JdbcOperations
interface.
See also the section called “JdbcTemplate best practices” for guidelines on using the
NamedParameterJdbcTemplate
class in the context of an application.
SQLExceptionTranslator
is an interface to be implemented by classes that can translate
between SQLExceptions
and Spring’s own org.springframework.dao.DataAccessException
,
which is agnostic in regard to data access strategy. Implementations can be generic (for
example, using SQLState codes for JDBC) or proprietary (for example, using Oracle error
codes) for greater precision.
SQLErrorCodeSQLExceptionTranslator
is the implementation of SQLExceptionTranslator
that is used by default. This implementation uses specific vendor codes. It is more
precise than the SQLState
implementation. The error code translations are based on
codes held in a JavaBean type class called SQLErrorCodes
. This class is created and
populated by an SQLErrorCodesFactory
which as the name suggests is a factory for
creating SQLErrorCodes
based on the contents of a configuration file named
sql-error-codes.xml
. This file is populated with vendor codes and based on the
DatabaseProductName
taken from the DatabaseMetaData
. The codes for the actual
database you are using are used.
The SQLErrorCodeSQLExceptionTranslator
applies matching rules in the following sequence:
Note | |
---|---|
The |
SQLErrorCodeSQLExceptionTranslator
is used so this rule does not apply. It only
applies if you have actually provided a subclass implementation.
SQLExceptionTranslator
interface that is provided
as the customSqlExceptionTranslator
property of the SQLErrorCodes
class.
CustomSQLErrorCodesTranslation
class, provided for the
customTranslations
property of the SQLErrorCodes
class, are searched for a match.
SQLExceptionSubclassTranslator
is the default fallback
translator. If this translation is not available then the next fallback translator is
the SQLStateSQLExceptionTranslator
.
You can extend SQLErrorCodeSQLExceptionTranslator:
public class CustomSQLErrorCodesTranslator extends SQLErrorCodeSQLExceptionTranslator { protected DataAccessException customTranslate(String task, String sql, SQLException sqlex) { if (sqlex.getErrorCode() == -12345) { return new DeadlockLoserDataAccessException(task, sqlex); } return null; } }
In this example, the specific error code -12345
is translated and other errors are
left to be translated by the default translator implementation. To use this custom
translator, it is necessary to pass it to the JdbcTemplate
through the method
setExceptionTranslator
and to use this JdbcTemplate
for all of the data access
processing where this translator is needed. Here is an example of how this custom
translator can be used:
private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { // create a JdbcTemplate and set data source this.jdbcTemplate = new JdbcTemplate(); this.jdbcTemplate.setDataSource(dataSource); // create a custom translator and set the DataSource for the default translation lookup CustomSQLErrorCodesTranslator tr = new CustomSQLErrorCodesTranslator(); tr.setDataSource(dataSource); this.jdbcTemplate.setExceptionTranslator(tr); } public void updateShippingCharge(long orderId, long pct) { // use the prepared JdbcTemplate for this update this.jdbcTemplate.update("update orders" + " set shipping_charge = shipping_charge * ? / 100" + " where id = ?", pct, orderId); }
The custom translator is passed a data source in order to look up the error codes in
sql-error-codes.xml
.
Executing an SQL statement requires very little code. You need a DataSource
and a
JdbcTemplate
, including the convenience methods that are provided with the
JdbcTemplate
. The following example shows what you need to include for a minimal but
fully functional class that creates a new table:
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class ExecuteAStatement { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public void doExecute() { this.jdbcTemplate.execute("create table mytable (id integer, name varchar(100))"); } }
Some query methods return a single value. To retrieve a count or a specific value from
one row, use queryForObject(..)
. The latter converts the returned JDBC Type
to the
Java class that is passed in as an argument. If the type conversion is invalid, then an
InvalidDataAccessApiUsageException
is thrown. Here is an example that contains two
query methods, one for an int
and one that queries for a String
.
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class RunAQuery { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public int getCount() { return this.jdbcTemplate.queryForObject("select count(*) from mytable", Integer.class); } public String getName() { return this.jdbcTemplate.queryForObject("select name from mytable", String.class); } public void setDataSource(DataSource dataSource) { this.dataSource = dataSource; } }
In addition to the single result query methods, several methods return a list with an
entry for each row that the query returned. The most generic method is
queryForList(..)
which returns a List
where each entry is a Map
with each entry in
the map representing the column value for that row. If you add a method to the above
example to retrieve a list of all the rows, it would look like this:
private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public List<Map<String, Object>> getList() { return this.jdbcTemplate.queryForList("select * from mytable"); }
The list returned would look something like this:
[{name=Bob, id=1}, {name=Mary, id=2}]
The following example shows a column updated for a certain primary key. In this example, an SQL statement has placeholders for row parameters. The parameter values can be passed in as varargs or alternatively as an array of objects. Thus primitives should be wrapped in the primitive wrapper classes explicitly or using auto-boxing.
import javax.sql.DataSource; import org.springframework.jdbc.core.JdbcTemplate; public class ExecuteAnUpdate { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public void setName(int id, String name) { this.jdbcTemplate.update("update mytable set name = ? where id = ?", name, id); } }
An update()
convenience method supports the retrieval of primary keys generated by the
database. This support is part of the JDBC 3.0 standard; see Chapter 13.6 of the
specification for details. The method takes a PreparedStatementCreator
as its first
argument, and this is the way the required insert statement is specified. The other
argument is a KeyHolder
, which contains the generated key on successful return from
the update. There is not a standard single way to create an appropriate
PreparedStatement
(which explains why the method signature is the way it is). The
following example works on Oracle but may not work on other platforms:
final String INSERT_SQL = "insert into my_test (name) values(?)"; final String name = "Rob"; KeyHolder keyHolder = new GeneratedKeyHolder(); jdbcTemplate.update( new PreparedStatementCreator() { public PreparedStatement createPreparedStatement(Connection connection) throws SQLException { PreparedStatement ps = connection.prepareStatement(INSERT_SQL, new String[] {"id"}); ps.setString(1, name); return ps; } }, keyHolder); // keyHolder.getKey() now contains the generated key
Spring obtains a connection to the database through a DataSource
. A DataSource
is
part of the JDBC specification and is a generalized connection factory. It allows a
container or a framework to hide connection pooling and transaction management issues
from the application code. As a developer, you need not know details about how to
connect to the database; that is the responsibility of the administrator that sets up
the datasource. You most likely fill both roles as you develop and test code, but you do
not necessarily have to know how the production data source is configured.
When using Spring’s JDBC layer, you obtain a data source from JNDI or you configure your own with a connection pool implementation provided by a third party. Popular implementations are Apache Jakarta Commons DBCP and C3P0. Implementations in the Spring distribution are meant only for testing purposes and do not provide pooling.
This section uses Spring’s DriverManagerDataSource
implementation, and several
additional implementations are covered later.
Note | |
---|---|
Only use the |
You obtain a connection with DriverManagerDataSource
as you typically obtain a JDBC
connection. Specify the fully qualified classname of the JDBC driver so that the
DriverManager
can load the driver class. Next, provide a URL that varies between JDBC
drivers. (Consult the documentation for your driver for the correct value.) Then provide
a username and a password to connect to the database. Here is an example of how to
configure a DriverManagerDataSource
in Java code:
DriverManagerDataSource dataSource = new DriverManagerDataSource(); dataSource.setDriverClassName("org.hsqldb.jdbcDriver"); dataSource.setUrl("jdbc:hsqldb:hsql://localhost:"); dataSource.setUsername("sa"); dataSource.setPassword("");
Here is the corresponding XML configuration:
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <context:property-placeholder location="jdbc.properties"/>
The following examples show the basic connectivity and configuration for DBCP and C3P0. To learn about more options that help control the pooling features, see the product documentation for the respective connection pooling implementations.
DBCP configuration:
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <context:property-placeholder location="jdbc.properties"/>
C3P0 configuration:
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close"> <property name="driverClass" value="${jdbc.driverClassName}"/> <property name="jdbcUrl" value="${jdbc.url}"/> <property name="user" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <context:property-placeholder location="jdbc.properties"/>
The DataSourceUtils
class is a convenient and powerful helper class that provides
static
methods to obtain connections from JNDI and close connections if necessary. It
supports thread-bound connections with, for example, DataSourceTransactionManager
.
The SmartDataSource
interface should be implemented by classes that can provide a
connection to a relational database. It extends the DataSource
interface to allow
classes using it to query whether the connection should be closed after a given
operation. This usage is efficient when you know that you will reuse a connection.
AbstractDataSource
is an abstract
base class for Spring’s DataSource
implementations that implements code that is common to all DataSource
implementations.
You extend the AbstractDataSource
class if you are writing your own DataSource
implementation.
The SingleConnectionDataSource
class is an implementation of the SmartDataSource
interface that wraps a single Connection
that is not closed after each use.
Obviously, this is not multi-threading capable.
If any client code calls close
in the assumption of a pooled connection, as when using
persistence tools, set the suppressClose
property to true
. This setting returns a
close-suppressing proxy wrapping the physical connection. Be aware that you will not be
able to cast this to a native Oracle Connection
or the like anymore.
This is primarily a test class. For example, it enables easy testing of code outside an
application server, in conjunction with a simple JNDI environment. In contrast to
DriverManagerDataSource
, it reuses the same connection all the time, avoiding
excessive creation of physical connections.
The DriverManagerDataSource
class is an implementation of the standard DataSource
interface that configures a plain JDBC driver through bean properties, and returns a new
Connection
every time.
This implementation is useful for test and stand-alone environments outside of a Java EE
container, either as a DataSource
bean in a Spring IoC container, or in conjunction
with a simple JNDI environment. Pool-assuming Connection.close()
calls will simply
close the connection, so any DataSource
-aware persistence code should work. However,
using JavaBean-style connection pools such as commons-dbcp
is so easy, even in a test
environment, that it is almost always preferable to use such a connection pool over
DriverManagerDataSource
.
TransactionAwareDataSourceProxy
is a proxy for a target DataSource
, which wraps that
target DataSource
to add awareness of Spring-managed transactions. In this respect, it
is similar to a transactional JNDI DataSource
as provided by a Java EE server.
Note | |
---|---|
It is rarely desirable to use this class, except when already existing code that must be
called and passed a standard JDBC |
(See the TransactionAwareDataSourceProxy
Javadocs for more details.)
The DataSourceTransactionManager
class is a PlatformTransactionManager
implementation for single JDBC datasources. It binds a JDBC connection from the
specified data source to the currently executing thread, potentially allowing for one
thread connection per data source.
Application code is required to retrieve the JDBC connection through
DataSourceUtils.getConnection(DataSource)
instead of Java EE’s standard
DataSource.getConnection
. It throws unchecked org.springframework.dao
exceptions
instead of checked SQLExceptions
. All framework classes like JdbcTemplate
use this
strategy implicitly. If not used with this transaction manager, the lookup strategy
behaves exactly like the common one - it can thus be used in any case.
The DataSourceTransactionManager
class supports custom isolation levels, and timeouts
that get applied as appropriate JDBC statement query timeouts. To support the latter,
application code must either use JdbcTemplate
or call the
DataSourceUtils.applyTransactionTimeout(..)
method for each created statement.
This implementation can be used instead of JtaTransactionManager
in the single
resource case, as it does not require the container to support JTA. Switching between
both is just a matter of configuration, if you stick to the required connection lookup
pattern. JTA does not support custom isolation levels!
Sometimes you need to access vendor specific JDBC methods that differ from the standard
JDBC API. This can be problematic if you are running in an application server or with a
DataSource
that wraps the Connection
, Statement
and ResultSet
objects with its
own wrapper objects. To gain access to the native objects you can configure your
JdbcTemplate
or OracleLobHandler
with a NativeJdbcExtractor
.
The NativeJdbcExtractor
comes in a variety of flavors to match your execution
environment:
Usually the SimpleNativeJdbcExtractor
is sufficient for unwrapping a Connection
object in most environments. See the Javadocs for more details.
Most JDBC drivers provide improved performance if you batch multiple calls to the same prepared statement. By grouping updates into batches you limit the number of round trips to the database.
You accomplish JdbcTemplate
batch processing by implementing two methods of a special
interface, BatchPreparedStatementSetter
, and passing that in as the second parameter
in your batchUpdate
method call. Use the getBatchSize
method to provide the size of
the current batch. Use the setValues
method to set the values for the parameters of
the prepared statement. This method will be called the number of times that you
specified in the getBatchSize
call. The following example updates the actor table
based on entries in a list. The entire list is used as the batch in this example:
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public int[] batchUpdate(final List<Actor> actors) { int[] updateCounts = jdbcTemplate.batchUpdate("update t_actor set first_name = ?, " + "last_name = ? where id = ?", new BatchPreparedStatementSetter() { public void setValues(PreparedStatement ps, int i) throws SQLException { ps.setString(1, actors.get(i).getFirstName()); ps.setString(2, actors.get(i).getLastName()); ps.setLong(3, actors.get(i).getId().longValue()); } public int getBatchSize() { return actors.size(); } }); return updateCounts; } // ... additional methods }
If you are processing a stream of updates or reading from a file, then you might have a
preferred batch size, but the last batch might not have that number of entries. In this
case you can use the InterruptibleBatchPreparedStatementSetter
interface, which allows
you to interrupt a batch once the input source is exhausted. The isBatchExhausted
method
allows you to signal the end of the batch.
Both the JdbcTemplate
and the NamedParameterJdbcTemplate
provides an alternate way
of providing the batch update. Instead of implementing a special batch interface, you
provide all parameter values in the call as a list. The framework loops over these
values and uses an internal prepared statement setter. The API varies depending on
whether you use named parameters. For the named parameters you provide an array of
SqlParameterSource
, one entry for each member of the batch. You can use the
SqlParameterSource.createBatch
method to create this array, passing in either an array
of JavaBeans or an array of Maps containing the parameter values.
This example shows a batch update using named parameters:
public class JdbcActorDao implements ActorDao { private NamedParameterTemplate namedParameterJdbcTemplate; public void setDataSource(DataSource dataSource) { this.namedParameterJdbcTemplate = new NamedParameterJdbcTemplate(dataSource); } public int[] batchUpdate(final List<Actor> actors) { SqlParameterSource[] batch = SqlParameterSourceUtils.createBatch(actors.toArray()); int[] updateCounts = namedParameterJdbcTemplate.batchUpdate( "update t_actor set first_name = :firstName, last_name = :lastName where id = :id", batch); return updateCounts; } // ... additional methods }
For an SQL statement using the classic "?" placeholders, you pass in a list containing an object array with the update values. This object array must have one entry for each placeholder in the SQL statement, and they must be in the same order as they are defined in the SQL statement.
The same example using classic JDBC "?" placeholders:
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public int[] batchUpdate(final List<Actor> actors) { List<Object[]> batch = new ArrayList<Object[]>(); for (Actor actor : actors) { Object[] values = new Object[] { actor.getFirstName(), actor.getLastName(), actor.getId()}; batch.add(values); } int[] updateCounts = jdbcTemplate.batchUpdate( "update t_actor set first_name = ?, last_name = ? where id = ?", batch); return updateCounts; } // ... additional methods }
All of the above batch update methods return an int array containing the number of affected rows for each batch entry. This count is reported by the JDBC driver. If the count is not available, the JDBC driver returns a -2 value.
The last example of a batch update deals with batches that are so large that you want to
break them up into several smaller batches. You can of course do this with the methods
mentioned above by making multiple calls to the batchUpdate
method, but there is now a
more convenient method. This method takes, in addition to the SQL statement, a
Collection of objects containing the parameters, the number of updates to make for each
batch and a ParameterizedPreparedStatementSetter
to set the values for the parameters
of the prepared statement. The framework loops over the provided values and breaks the
update calls into batches of the size specified.
This example shows a batch update using a batch size of 100:
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); } public int[][] batchUpdate(final Collection<Actor> actors) { int[][] updateCounts = jdbcTemplate.batchUpdate( "update t_actor set first_name = ?, last_name = ? where id = ?", actors, 100, new ParameterizedPreparedStatementSetter<Actor>() { public void setValues(PreparedStatement ps, Actor argument) throws SQLException { ps.setString(1, argument.getFirstName()); ps.setString(2, argument.getLastName()); ps.setLong(3, argument.getId().longValue()); } }); return updateCounts; } // ... additional methods }
The batch update methods for this call returns an array of int arrays containing an array entry for each batch with an array of the number of affected rows for each update. The top level array’s length indicates the number of batches executed and the second level array’s length indicates the number of updates in that batch. The number of updates in each batch should be the the batch size provided for all batches except for the last one that might be less, depending on the total number of update objects provided. The update count for each update statement is the one reported by the JDBC driver. If the count is not available, the JDBC driver returns a -2 value.
The SimpleJdbcInsert
and SimpleJdbcCall
classes provide a simplified configuration
by taking advantage of database metadata that can be retrieved through the JDBC driver.
This means there is less to configure up front, although you can override or turn off
the metadata processing if you prefer to provide all the details in your code.
Let’s start by looking at the SimpleJdbcInsert
class with the minimal amount of
configuration options. You should instantiate the SimpleJdbcInsert
in the data access
layer’s initialization method. For this example, the initializing method is the
setDataSource
method. You do not need to subclass the SimpleJdbcInsert
class; simply
create a new instance and set the table name using the withTableName
method.
Configuration methods for this class follow the "fluid" style that returns the instance
of the SimpleJdbcInsert
, which allows you to chain all configuration methods. This
example uses only one configuration method; you will see examples of multiple ones later.
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcInsert insertActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.insertActor = new SimpleJdbcInsert(dataSource).withTableName("t_actor"); } public void add(Actor actor) { Map<String, Object> parameters = new HashMap<String, Object>(3); parameters.put("id", actor.getId()); parameters.put("first_name", actor.getFirstName()); parameters.put("last_name", actor.getLastName()); insertActor.execute(parameters); } // ... additional methods }
The execute method used here takes a plain java.utils.Map
as its only parameter. The
important thing to note here is that the keys used for the Map must match the column
names of the table as defined in the database. This is because we read the metadata in
order to construct the actual insert statement.
This example uses the same insert as the preceding, but instead of passing in the id it
retrieves the auto-generated key and sets it on the new Actor object. When you create
the SimpleJdbcInsert
, in addition to specifying the table name, you specify the name
of the generated key column with the usingGeneratedKeyColumns
method.
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcInsert insertActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.insertActor = new SimpleJdbcInsert(dataSource) .withTableName("t_actor") .usingGeneratedKeyColumns("id"); } public void add(Actor actor) { Map<String, Object> parameters = new HashMap<String, Object>(2); parameters.put("first_name", actor.getFirstName()); parameters.put("last_name", actor.getLastName()); Number newId = insertActor.executeAndReturnKey(parameters); actor.setId(newId.longValue()); } // ... additional methods }
The main difference when executing the insert by this second approach is that you do not
add the id to the Map and you call the executeReturningKey
method. This returns a
java.lang.Number
object with which you can create an instance of the numerical type that
is used in our domain class. You cannot rely on all databases to return a specific Java
class here; java.lang.Number
is the base class that you can rely on. If you have
multiple auto-generated columns, or the generated values are non-numeric, then you can
use a KeyHolder
that is returned from the executeReturningKeyHolder
method.
You can limit the columns for an insert by specifying a list of column names with the
usingColumns
method:
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcInsert insertActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.insertActor = new SimpleJdbcInsert(dataSource) .withTableName("t_actor") .usingColumns("first_name", "last_name") .usingGeneratedKeyColumns("id"); } public void add(Actor actor) { Map<String, Object> parameters = new HashMap<String, Object>(2); parameters.put("first_name", actor.getFirstName()); parameters.put("last_name", actor.getLastName()); Number newId = insertActor.executeAndReturnKey(parameters); actor.setId(newId.longValue()); } // ... additional methods }
The execution of the insert is the same as if you had relied on the metadata to determine which columns to use.
Using a Map
to provide parameter values works fine, but it’s not the most convenient
class to use. Spring provides a couple of implementations of the SqlParameterSource
interface that can be used instead.The first one is BeanPropertySqlParameterSource
,
which is a very convenient class if you have a JavaBean-compliant class that contains
your values. It will use the corresponding getter method to extract the parameter
values. Here is an example:
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcInsert insertActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.insertActor = new SimpleJdbcInsert(dataSource) .withTableName("t_actor") .usingGeneratedKeyColumns("id"); } public void add(Actor actor) { SqlParameterSource parameters = new BeanPropertySqlParameterSource(actor); Number newId = insertActor.executeAndReturnKey(parameters); actor.setId(newId.longValue()); } // ... additional methods }
Another option is the MapSqlParameterSource
that resembles a Map but provides a more
convenient addValue
method that can be chained.
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcInsert insertActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.insertActor = new SimpleJdbcInsert(dataSource) .withTableName("t_actor") .usingGeneratedKeyColumns("id"); } public void add(Actor actor) { SqlParameterSource parameters = new MapSqlParameterSource() .addValue("first_name", actor.getFirstName()) .addValue("last_name", actor.getLastName()); Number newId = insertActor.executeAndReturnKey(parameters); actor.setId(newId.longValue()); } // ... additional methods }
As you can see, the configuration is the same; only the executing code has to change to use these alternative input classes.
The SimpleJdbcCall
class leverages metadata in the database to look up names of in
and out
parameters, so that you do not have to declare them explicitly. You can
declare parameters if you prefer to do that, or if you have parameters such as ARRAY
or STRUCT
that do not have an automatic mapping to a Java class. The first example
shows a simple procedure that returns only scalar values in VARCHAR
and DATE
format
from a MySQL database. The example procedure reads a specified actor entry and returns
first_name
, last_name
, and birth_date
columns in the form of out
parameters.
CREATE PROCEDURE read_actor ( IN in_id INTEGER, OUT out_first_name VARCHAR(100), OUT out_last_name VARCHAR(100), OUT out_birth_date DATE) BEGIN SELECT first_name, last_name, birth_date INTO out_first_name, out_last_name, out_birth_date FROM t_actor where id = in_id; END;
The in_id
parameter contains the id
of the actor you are looking up. The out
parameters return the data read from the table.
The SimpleJdbcCall
is declared in a similar manner to the SimpleJdbcInsert
. You
should instantiate and configure the class in the initialization method of your data
access layer. Compared to the StoredProcedure class, you don’t have to create a subclass
and you don’t have to declare parameters that can be looked up in the database metadata.
Following is an example of a SimpleJdbcCall configuration using the above stored
procedure. The only configuration option, in addition to the DataSource
, is the name
of the stored procedure.
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcCall procReadActor; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); this.procReadActor = new SimpleJdbcCall(dataSource) .withProcedureName("read_actor"); } public Actor readActor(Long id) { SqlParameterSource in = new MapSqlParameterSource() .addValue("in_id", id); Map out = procReadActor.execute(in); Actor actor = new Actor(); actor.setId(id); actor.setFirstName((String) out.get("out_first_name")); actor.setLastName((String) out.get("out_last_name")); actor.setBirthDate((Date) out.get("out_birth_date")); return actor; } // ... additional methods }
The code you write for the execution of the call involves creating an SqlParameterSource
containing the IN parameter. It’s important to match the name provided for the input value
with that of the parameter name declared in the stored procedure. The case does not have
to match because you use metadata to determine how database objects should be referred to
in a stored procedure. What is specified in the source for the stored procedure is not
necessarily the way it is stored in the database. Some databases transform names to all
upper case while others use lower case or use the case as specified.
The execute
method takes the IN parameters and returns a Map containing any out
parameters keyed by the name as specified in the stored procedure. In this case they are
out_first_name, out_last_name
and out_birth_date
.
The last part of the execute
method creates an Actor instance to use to return the
data retrieved. Again, it is important to use the names of the out
parameters as they
are declared in the stored procedure. Also, the case in the names of the out
parameters stored in the results map matches that of the out
parameter names in the
database, which could vary between databases. To make your code more portable you should
do a case-insensitive lookup or instruct Spring to use a CaseInsensitiveMap
from the
Jakarta Commons project. To do the latter, you create your own JdbcTemplate
and set
the setResultsMapCaseInsensitive
property to true
. Then you pass this customized
JdbcTemplate
instance into the constructor of your SimpleJdbcCall
. You must include
the commons-collections.jar
in your classpath for this to work. Here is an example of
this configuration:
public class JdbcActorDao implements ActorDao { private SimpleJdbcCall procReadActor; public void setDataSource(DataSource dataSource) { JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); jdbcTemplate.setResultsMapCaseInsensitive(true); this.procReadActor = new SimpleJdbcCall(jdbcTemplate) .withProcedureName("read_actor"); } // ... additional methods }
By taking this action, you avoid conflicts in the case used for the names of your
returned out
parameters.
You have seen how the parameters are deduced based on metadata, but you can declare then
explicitly if you wish. You do this by creating and configuring SimpleJdbcCall
with
the declareParameters
method, which takes a variable number of SqlParameter
objects
as input. See the next section for details on how to define an SqlParameter
.
Note | |
---|---|
Explicit declarations are necessary if the database you use is not a Spring-supported database. Currently Spring supports metadata lookup of stored procedure calls for the following databases: Apache Derby, DB2, MySQL, Microsoft SQL Server, Oracle, and Sybase. We also support metadata lookup of stored functions for: MySQL, Microsoft SQL Server, and Oracle. |
You can opt to declare one, some, or all the parameters explicitly. The parameter
metadata is still used where you do not declare parameters explicitly. To bypass all
processing of metadata lookups for potential parameters and only use the declared
parameters, you call the method withoutProcedureColumnMetaDataAccess
as part of the
declaration. Suppose that you have two or more different call signatures declared for a
database function. In this case you call the useInParameterNames
to specify the list
of IN parameter names to include for a given signature.
The following example shows a fully declared procedure call, using the information from the preceding example.
public class JdbcActorDao implements ActorDao { private SimpleJdbcCall procReadActor; public void setDataSource(DataSource dataSource) { JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); jdbcTemplate.setResultsMapCaseInsensitive(true); this.procReadActor = new SimpleJdbcCall(jdbcTemplate) .withProcedureName("read_actor") .withoutProcedureColumnMetaDataAccess() .useInParameterNames("in_id") .declareParameters( new SqlParameter("in_id", Types.NUMERIC), new SqlOutParameter("out_first_name", Types.VARCHAR), new SqlOutParameter("out_last_name", Types.VARCHAR), new SqlOutParameter("out_birth_date", Types.DATE) ); } // ... additional methods }
The execution and end results of the two examples are the same; this one specifies all details explicitly rather than relying on metadata.
To define a parameter for the SimpleJdbc classes and also for the RDBMS operations
classes, covered in Section 13.6, “Modeling JDBC operations as Java objects”, you use an SqlParameter
or one of its subclasses.
You typically specify the parameter name and SQL type in the constructor. The SQL type
is specified using the java.sql.Types
constants. We have already seen declarations
like:
new SqlParameter("in_id", Types.NUMERIC), new SqlOutParameter("out_first_name", Types.VARCHAR),
The first line with the SqlParameter
declares an IN parameter. IN parameters can be
used for both stored procedure calls and for queries using the SqlQuery
and its
subclasses covered in the following section.
The second line with the SqlOutParameter
declares an out
parameter to be used in a
stored procedure call. There is also an SqlInOutParameter
for InOut
parameters,
parameters that provide an IN
value to the procedure and that also return a value.
Note | |
---|---|
Only parameters declared as |
For IN parameters, in addition to the name and the SQL type, you can specify a scale for
numeric data or a type name for custom database types. For out
parameters, you can
provide a RowMapper
to handle mapping of rows returned from a REF
cursor. Another
option is to specify an SqlReturnType
that provides an opportunity to define
customized handling of the return values.
You call a stored function in almost the same way as you call a stored procedure, except
that you provide a function name rather than a procedure name. You use the
withFunctionName
method as part of the configuration to indicate that we want to make
a call to a function, and the corresponding string for a function call is generated. A
specialized execute call, executeFunction,
is used to execute the function and it
returns the function return value as an object of a specified type, which means you do
not have to retrieve the return value from the results map. A similar convenience method
named executeObject
is also available for stored procedures that only have one out
parameter. The following example is based on a stored function named get_actor_name
that returns an actor’s full name. Here is the MySQL source for this function:
CREATE FUNCTION get_actor_name (in_id INTEGER) RETURNS VARCHAR(200) READS SQL DATA BEGIN DECLARE out_name VARCHAR(200); SELECT concat(first_name, ' ', last_name) INTO out_name FROM t_actor where id = in_id; RETURN out_name; END;
To call this function we again create a SimpleJdbcCall
in the initialization method.
public class JdbcActorDao implements ActorDao { private JdbcTemplate jdbcTemplate; private SimpleJdbcCall funcGetActorName; public void setDataSource(DataSource dataSource) { this.jdbcTemplate = new JdbcTemplate(dataSource); JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); jdbcTemplate.setResultsMapCaseInsensitive(true); this.funcGetActorName = new SimpleJdbcCall(jdbcTemplate) .withFunctionName("get_actor_name"); } public String getActorName(Long id) { SqlParameterSource in = new MapSqlParameterSource() .addValue("in_id", id); String name = funcGetActorName.executeFunction(String.class, in); return name; } // ... additional methods }
The execute method used returns a String
containing the return value from the function
call.
Calling a stored procedure or function that returns a result set is a bit tricky. Some
databases return result sets during the JDBC results processing while others require an
explicitly registered out
parameter of a specific type. Both approaches need
additional processing to loop over the result set and process the returned rows. With
the SimpleJdbcCall
you use the returningResultSet
method and declare a RowMapper
implementation to be used for a specific parameter. In the case where the result set is
returned during the results processing, there are no names defined, so the returned
results will have to match the order in which you declare the RowMapper
implementations. The name specified is still used to store the processed list of results
in the results map that is returned from the execute statement.
The next example uses a stored procedure that takes no IN parameters and returns all rows from the t_actor table. Here is the MySQL source for this procedure:
CREATE PROCEDURE read_all_actors() BEGIN SELECT a.id, a.first_name, a.last_name, a.birth_date FROM t_actor a; END;
To call this procedure you declare the RowMapper
. Because the class you want to map to
follows the JavaBean rules, you can use a ParameterizedBeanPropertyRowMapper
that is
created by passing in the required class to map to in the newInstance
method.
public class JdbcActorDao implements ActorDao { private SimpleJdbcCall procReadAllActors; public void setDataSource(DataSource dataSource) { JdbcTemplate jdbcTemplate = new JdbcTemplate(dataSource); jdbcTemplate.setResultsMapCaseInsensitive(true); this.procReadAllActors = new SimpleJdbcCall(jdbcTemplate) .withProcedureName("read_all_actors") .returningResultSet("actors", ParameterizedBeanPropertyRowMapper.newInstance(Actor.class)); } public List getActorsList() { Map m = procReadAllActors.execute(new HashMap<String, Object>(0)); return (List) m.get("actors"); } // ... additional methods }
The execute call passes in an empty Map because this call does not take any parameters. The list of Actors is then retrieved from the results map and returned to the caller.
The org.springframework.jdbc.object
package contains classes that allow you to access
the database in a more object-oriented manner. As an example, you can execute queries
and get the results back as a list containing business objects with the relational
column data mapped to the properties of the business object. You can also execute stored
procedures and run update, delete, and insert statements.
Note | |
---|---|
Many Spring developers believe that the various RDBMS operation classes described below
(with the exception of the However, if you are getting measurable value from using the RDBMS operation classes, continue using these classes. |
SqlQuery
is a reusable, threadsafe class that encapsulates an SQL query. Subclasses
must implement the newRowMapper(..)
method to provide a RowMapper
instance that can
create one object per row obtained from iterating over the ResultSet
that is created
during the execution of the query. The SqlQuery
class is rarely used directly because
the MappingSqlQuery
subclass provides a much more convenient implementation for
mapping rows to Java classes. Other implementations that extend SqlQuery
are
MappingSqlQueryWithParameters
and UpdatableSqlQuery
.
MappingSqlQuery
is a reusable query in which concrete subclasses must implement the
abstract mapRow(..)
method to convert each row of the supplied ResultSet
into an
object of the type specified. The following example shows a custom query that maps the
data from the t_actor
relation to an instance of the Actor
class.
public class ActorMappingQuery extends MappingSqlQuery<Actor> { public ActorMappingQuery(DataSource ds) { super(ds, "select id, first_name, last_name from t_actor where id = ?"); super.declareParameter(new SqlParameter("id", Types.INTEGER)); compile(); } @Override protected Actor mapRow(ResultSet rs, int rowNumber) throws SQLException { Actor actor = new Actor(); actor.setId(rs.getLong("id")); actor.setFirstName(rs.getString("first_name")); actor.setLastName(rs.getString("last_name")); return actor; } }
The class extends MappingSqlQuery
parameterized with the Actor
type. The constructor
for this customer query takes the DataSource
as the only parameter. In this
constructor you call the constructor on the superclass with the DataSource
and the SQL
that should be executed to retrieve the rows for this query. This SQL will be used to
create a PreparedStatement
so it may contain place holders for any parameters to be
passed in during execution.You must declare each parameter using the declareParameter
method passing in an SqlParameter
. The SqlParameter
takes a name and the JDBC type
as defined in java.sql.Types
. After you define all parameters, you call the
compile()
method so the statement can be prepared and later executed. This class is
thread-safe after it is compiled, so as long as these instances are created when the DAO
is initialized they can be kept as instance variables and be reused.
private ActorMappingQuery actorMappingQuery; @Autowired public void setDataSource(DataSource dataSource) { this.actorMappingQuery = new ActorMappingQuery(dataSource); } public Customer getCustomer(Long id) { return actorMappingQuery.findObject(id); }
The method in this example retrieves the customer with the id that is passed in as the
only parameter. Since we only want one object returned we simply call the convenience
method findObject
with the id as parameter. If we had instead a query that returned a
list of objects and took additional parameters then we would use one of the execute
methods that takes an array of parameter values passed in as varargs.
public List<Actor> searchForActors(int age, String namePattern) { List<Actor> actors = actorSearchMappingQuery.execute(age, namePattern); return actors; }
The SqlUpdate
class encapsulates an SQL update. Like a query, an update object is
reusable, and like all RdbmsOperation
classes, an update can have parameters and is
defined in SQL. This class provides a number of update(..)
methods analogous to the
execute(..)
methods of query objects. The SQLUpdate
class is concrete. It can be
subclassed, for example, to add a custom update method, as in the following snippet
where it’s simply called execute
. However, you don’t have to subclass the SqlUpdate
class since it can easily be parameterized by setting SQL and declaring parameters.
import java.sql.Types; import javax.sql.DataSource; import org.springframework.jdbc.core.SqlParameter; import org.springframework.jdbc.object.SqlUpdate; public class UpdateCreditRating extends SqlUpdate { public UpdateCreditRating(DataSource ds) { setDataSource(ds); setSql("update customer set credit_rating = ? where id = ?"); declareParameter(new SqlParameter("creditRating", Types.NUMERIC)); declareParameter(new SqlParameter("id", Types.NUMERIC)); compile(); } /** * @param id for the Customer to be updated * @param rating the new value for credit rating * @return number of rows updated */ public int execute(int id, int rating) { return update(rating, id); } }
The StoredProcedure
class is a superclass for object abstractions of RDBMS stored
procedures. This class is abstract
, and its various execute(..)
methods have
protected
access, preventing use other than through a subclass that offers tighter
typing.
The inherited sql
property will be the name of the stored procedure in the RDBMS.
To define a parameter for the StoredProcedure
class, you use an SqlParameter
or one
of its subclasses. You must specify the parameter name and SQL type in the constructor
like in the following code snippet. The SQL type is specified using the java.sql.Types
constants.
new SqlParameter("in_id", Types.NUMERIC), new SqlOutParameter("out_first_name", Types.VARCHAR),
The first line with the SqlParameter
declares an IN parameter. IN parameters can be
used for both stored procedure calls and for queries using the SqlQuery
and its
subclasses covered in the following section.
The second line with the SqlOutParameter
declares an out
parameter to be used in the
stored procedure call. There is also an SqlInOutParameter
for I
nOut
parameters,
parameters that provide an in
value to the procedure and that also return a value.
For i
n
parameters, in addition to the name and the SQL type, you can specify a
scale for numeric data or a type name for custom database types. For out
parameters
you can provide a RowMapper
to handle mapping of rows returned from a REF cursor.
Another option is to specify an SqlReturnType
that enables you to define customized
handling of the return values.
Here is an example of a simple DAO that uses a StoredProcedure
to call a function,
sysdate()
,which comes with any Oracle database. To use the stored procedure
functionality you have to create a class that extends StoredProcedure
. In this
example, the StoredProcedure
class is an inner class, but if you need to reuse the
StoredProcedure
you declare it as a top-level class. This example has no input
parameters, but an output parameter is declared as a date type using the class
SqlOutParameter
. The execute()
method executes the procedure and extracts the
returned date from the results Map
. The results Map
has an entry for each declared
output parameter, in this case only one, using the parameter name as the key.
import java.sql.Types; import java.util.Date; import java.util.HashMap; import java.util.Map; import javax.sql.DataSource; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.jdbc.core.SqlOutParameter; import org.springframework.jdbc.object.StoredProcedure; public class StoredProcedureDao { private GetSysdateProcedure getSysdate; @Autowired public void init(DataSource dataSource) { this.getSysdate = new GetSysdateProcedure(dataSource); } public Date getSysdate() { return getSysdate.execute(); } private class GetSysdateProcedure extends StoredProcedure { private static final String SQL = "sysdate"; public GetSysdateProcedure(DataSource dataSource) { setDataSource(dataSource); setFunction(true); setSql(SQL); declareParameter(new SqlOutParameter("date", Types.DATE)); compile(); } public Date execute() { // the sysdate sproc has no input parameters, so an empty Map is supplied... Map<String, Object> results = execute(new HashMap<String, Object>()); Date sysdate = (Date) results.get("date"); return sysdate; } } }
The following example of a StoredProcedure
has two output parameters (in this case,
Oracle REF cursors).
import oracle.jdbc.OracleTypes; import org.springframework.jdbc.core.SqlOutParameter; import org.springframework.jdbc.object.StoredProcedure; import javax.sql.DataSource; import java.util.HashMap; import java.util.Map; public class TitlesAndGenresStoredProcedure extends StoredProcedure { private static final String SPROC_NAME = "AllTitlesAndGenres"; public TitlesAndGenresStoredProcedure(DataSource dataSource) { super(dataSource, SPROC_NAME); declareParameter(new SqlOutParameter("titles", OracleTypes.CURSOR, new TitleMapper())); declareParameter(new SqlOutParameter("genres", OracleTypes.CURSOR, new GenreMapper())); compile(); } public Map<String, Object> execute() { // again, this sproc has no input parameters, so an empty Map is supplied return super.execute(new HashMap<String, Object>()); } }
Notice how the overloaded variants of the declareParameter(..)
method that have been
used in the TitlesAndGenresStoredProcedure
constructor are passed RowMapper
implementation instances; this is a very convenient and powerful way to reuse existing
functionality. The code for the two RowMapper
implementations is provided below.
The TitleMapper
class maps a ResultSet
to a Title
domain object for each row in
the supplied ResultSet
:
import org.springframework.jdbc.core.RowMapper; import java.sql.ResultSet; import java.sql.SQLException; import com.foo.domain.Title; public final class TitleMapper implements RowMapper<Title> { public Title mapRow(ResultSet rs, int rowNum) throws SQLException { Title title = new Title(); title.setId(rs.getLong("id")); title.setName(rs.getString("name")); return title; } }
The GenreMapper
class maps a ResultSet
to a Genre
domain object for each row in
the supplied ResultSet
.
import org.springframework.jdbc.core.RowMapper; import java.sql.ResultSet; import java.sql.SQLException; import com.foo.domain.Genre; public final class GenreMapper implements RowMapper<Genre> { public Genre mapRow(ResultSet rs, int rowNum) throws SQLException { return new Genre(rs.getString("name")); } }
To pass parameters to a stored procedure that has one or more input parameters in its
definition in the RDBMS, you can code a strongly typed execute(..)
method that would
delegate to the superclass' untyped execute(Map parameters)
method (which has
protected
access); for example:
import oracle.jdbc.OracleTypes; import org.springframework.jdbc.core.SqlOutParameter; import org.springframework.jdbc.core.SqlParameter; import org.springframework.jdbc.object.StoredProcedure; import javax.sql.DataSource; import java.sql.Types; import java.util.Date; import java.util.HashMap; import java.util.Map; public class TitlesAfterDateStoredProcedure extends StoredProcedure { private static final String SPROC_NAME = "TitlesAfterDate"; private static final String CUTOFF_DATE_PARAM = "cutoffDate"; public TitlesAfterDateStoredProcedure(DataSource dataSource) { super(dataSource, SPROC_NAME); declareParameter(new SqlParameter(CUTOFF_DATE_PARAM, Types.DATE); declareParameter(new SqlOutParameter("titles", OracleTypes.CURSOR, new TitleMapper())); compile(); } public Map<String, Object> execute(Date cutoffDate) { Map<String, Object> inputs = new HashMap<String, Object>(); inputs.put(CUTOFF_DATE_PARAM, cutoffDate); return super.execute(inputs); } }
Common problems with parameters and data values exist in the different approaches provided by the Spring Framework JDBC.
Usually Spring determines the SQL type of the parameters based on the type of parameter passed in. It is possible to explicitly provide the SQL type to be used when setting parameter values. This is sometimes necessary to correctly set NULL values.
You can provide SQL type information in several ways:
JdbcTemplate
take an additional parameter in
the form of an int
array. This array is used to indicate the SQL type of the
corresponding parameter using constant values from the java.sql.Types
class. Provide
one entry for each parameter.
SqlParameterValue
class to wrap the parameter value that needs this
additional information.Create a new instance for each value and pass in the SQL type
and parameter value in the constructor. You can also provide an optional scale
parameter for numeric values.
SqlParameterSource
classes
BeanPropertySqlParameterSource
or MapSqlParameterSource
. They both have methods
for registering the SQL type for any of the named parameter values.
You can store images, other binary objects, and large chunks of text. These large object
are called BLOB for binary data and CLOB for character data. In Spring you can handle
these large objects by using the JdbcTemplate directly and also when using the higher
abstractions provided by RDBMS Objects and the SimpleJdbc
classes. All of these
approaches use an implementation of the LobHandler
interface for the actual management
of the LOB data. The LobHandler
provides access to a LobCreator
class, through the
getLobCreator
method, used for creating new LOB objects to be inserted.
The LobCreator/LobHandler
provides the following support for LOB input and output:
The next example shows how to create and insert a BLOB. Later you will see how to read it back from the database.
This example uses a JdbcTemplate
and an implementation of the
AbstractLobCreatingPreparedStatementCallbac
k
. It implements one method,
setValues
. This method provides a LobCreator
that you use to set the values for the
LOB columns in your SQL insert statement.
For this example we assume that there is a variable, lobHandle
r
, that already is
set to an instance of a DefaultLobHandler
. You typically set this value through
dependency injection.
final File blobIn = new File("spring2004.jpg"); final InputStream blobIs = new FileInputStream(blobIn); final File clobIn = new File("large.txt"); final InputStream clobIs = new FileInputStream(clobIn); final InputStreamReader clobReader = new InputStreamReader(clobIs); jdbcTemplate.execute( "INSERT INTO lob_table (id, a_clob, a_blob) VALUES (?, ?, ?)", new AbstractLobCreatingPreparedStatementCallback(lobHandler) { <<1>> protected void setValues(PreparedStatement ps, LobCreator lobCreator) throws SQLException { ps.setLong(1, 1L); lobCreator.setClobAsCharacterStream(ps, 2, clobReader, (int)clobIn.length()); <<2>> lobCreator.setBlobAsBinaryStream(ps, 3, blobIs, (int)blobIn.length()); <<3>> } } ); blobIs.close(); clobReader.close();
???
Pass in the lobHandler that in this example is a plain DefaultLobHandler
???
Using the method setClobAsCharacterStream
, pass in the contents of the CLOB.
???
Using the method setBlobAsBinaryStream
, pass in the contents of the BLOB.
Now it’s time to read the LOB data from the database. Again, you use a JdbcTemplate
with the same instance variable l
obHandler
and a reference to a DefaultLobHandler
.
List<Map<String, Object>> l = jdbcTemplate.query("select id, a_clob, a_blob from lob_table", new RowMapper<Map<String, Object>>() { public Map<String, Object> mapRow(ResultSet rs, int i) throws SQLException { Map<String, Object> results = new HashMap<String, Object>(); String clobText = lobHandler.getClobAsString(rs, "a_clob"); <<1>> results.put("CLOB", clobText); byte[] blobBytes = lobHandler.getBlobAsBytes(rs, "a_blob"); <<2>> results.put("BLOB", blobBytes); return results; } });
???
Using the method getClobAsString
, retrieve the contents of the CLOB.
???
Using the method getBlobAsBytes
, retrieve the contents of the BLOB.
The SQL standard allows for selecting rows based on an expression that includes a
variable list of values. A typical example would be select * from T_ACTOR where id in
(1, 2, 3)
. This variable list is not directly supported for prepared statements by the
JDBC standard; you cannot declare a variable number of placeholders. You need a number
of variations with the desired number of placeholders prepared, or you need to generate
the SQL string dynamically once you know how many placeholders are required. The named
parameter support provided in the NamedParameterJdbcTemplate
and JdbcTemplate
takes
the latter approach. Pass in the values as a java.util.List
of primitive objects. This
list will be used to insert the required placeholders and pass in the values during the
statement execution.
Note | |
---|---|
Be careful when passing in many values. The JDBC standard does not guarantee that you
can use more than 100 values for an |
In addition to the primitive values in the value list, you can create a java.util.List
of object arrays. This list would support multiple expressions defined for the in
clause such as select * from T_ACTOR where (id, last_name) in ((1, 'Johnson'), (2,
'Harrop'))
. This of course requires that your database supports this syntax.
When you call stored procedures you can sometimes use complex types specific to the
database. To accommodate these types, Spring provides a SqlReturnType
for handling
them when they are returned from the stored procedure call and SqlTypeValue
when they
are passed in as a parameter to the stored procedure.
Here is an example of returning the value of an Oracle STRUCT
object of the user
declared type ITEM_TYPE
. The SqlReturnType
interface has a single method named
getTypeValue
that must be implemented. This interface is used as part of the
declaration of an SqlOutParameter
.
final TestItem = new TestItem(123L, "A test item", new SimpleDateFormat("yyyy-M-d").parse("2010-12-31")); declareParameter(new SqlOutParameter("item", OracleTypes.STRUCT, "ITEM_TYPE", new SqlReturnType() { public Object getTypeValue(CallableStatement cs, int colIndx, int sqlType, String typeName) throws SQLException { STRUCT struct = (STRUCT) cs.getObject(colIndx); Object[] attr = struct.getAttributes(); TestItem item = new TestItem(); item.setId(((Number) attr[0]).longValue()); item.setDescription((String) attr[1]); item.setExpirationDate((java.util.Date) attr[2]); return item; } }));
You use the SqlTypeValue
to pass in the value of a Java object like TestItem
into a
stored procedure. The SqlTypeValue
interface has a single method named
createTypeValue
that you must implement. The active connection is passed in, and you
can use it to create database-specific objects such as StructDescriptor
s, as shown in
the following example, or ArrayDescriptor
s.
final TestItem = new TestItem(123L, "A test item", new SimpleDateFormat("yyyy-M-d").parse("2010-12-31")); SqlTypeValue value = new AbstractSqlTypeValue() { protected Object createTypeValue(Connection conn, int sqlType, String typeName) throws SQLException { StructDescriptor itemDescriptor = new StructDescriptor(typeName, conn); Struct item = new STRUCT(itemDescriptor, conn, new Object[] { testItem.getId(), testItem.getDescription(), new java.sql.Date(testItem.getExpirationDate().getTime()) }); return item; } };
This SqlTypeValue
can now be added to the Map containing the input parameters for the
execute call of the stored procedure.
Another use for the SqlTypeValue
is passing in an array of values to an Oracle stored
procedure. Oracle has its own internal ARRAY
class that must be used in this case, and
you can use the SqlTypeValue
to create an instance of the Oracle ARRAY
and populate
it with values from the Java ARRAY
.
final Long[] ids = new Long[] {1L, 2L}; SqlTypeValue value = new AbstractSqlTypeValue() { protected Object createTypeValue(Connection conn, int sqlType, String typeName) throws SQLException { ArrayDescriptor arrayDescriptor = new ArrayDescriptor(typeName, conn); ARRAY idArray = new ARRAY(arrayDescriptor, conn, ids); return idArray; } };
The org.springframework.jdbc.datasource.embedded
package provides support for embedded
Java database engines. Support for HSQL,
H2, and Derby is provided
natively. You can also use an extensible API to plug in new embedded database types and
DataSource
implementations.
An embedded database is useful during the development phase of a project because of its lightweight nature. Benefits include ease of configuration, quick startup time, testability, and the ability to rapidly evolve SQL during development.
If you want to expose an embedded database instance as a bean in a Spring ApplicationContext, use the embedded-database tag in the spring-jdbc namespace:
<jdbc:embedded-database id="dataSource"> <jdbc:script location="classpath:schema.sql"/> <jdbc:script location="classpath:test-data.sql"/> </jdbc:embedded-database>
The preceding configuration creates an embedded HSQL database populated with SQL from
schema.sql and testdata.sql resources in the classpath. The database instance is made
available to the Spring container as a bean of type javax.sql.DataSource
. This bean
can then be injected into data access objects as needed.
The EmbeddedDatabaseBuilder
class provides a fluent API for constructing an embedded
database programmatically. Use this when you need to create an embedded database
instance in a standalone environment, such as a data access object unit test:
EmbeddedDatabaseBuilder builder = new EmbeddedDatabaseBuilder(); EmbeddedDatabase db = builder.setType(H2).addScript("my-schema.sql").addScript("my-test-data.sql").build(); // do stuff against the db (EmbeddedDatabase extends javax.sql.DataSource) db.shutdown()
Spring JDBC embedded database support can be extended in two ways:
EmbeddedDatabaseConfigurer
to support a new embedded database type, such
as Apache Derby.
DataSourceFactory
to support a new DataSource implementation, such as a
connection pool, to manage embedded database connections.
You are encouraged to contribute back extensions to the Spring community at jira.springframework.org[jira.springframework.org].
Spring supports HSQL 1.8.0 and above. HSQL is the default embedded database if no type
is specified explicitly. To specify HSQL explicitly, set the type
attribute of the
embedded-database
tag to HSQL
. If you are using the builder API, call the
setType(EmbeddedDatabaseType)
method with EmbeddedDatabaseType.HSQL
.
Spring supports the H2 database as well. To enable H2, set the type
attribute of the
embedded-database
tag to H2
. If you are using the builder API, call the
setType(EmbeddedDatabaseType)
method with EmbeddedDatabaseType.H2
.
Spring also supports Apache Derby 10.5 and above. To enable Derby, set the type
attribute of the embedded-database
tag to Derby
. If using the builder API, call the
setType(EmbeddedDatabaseType)
method with EmbeddedDatabaseType.Derby
.
Embedded databases provide a lightweight way to test data access code. The following is a data access unit test template that uses an embedded database:
public class DataAccessUnitTestTemplate { private EmbeddedDatabase db; @Before public void setUp() { // creates an HSQL in-memory database populated from default scripts // classpath:schema.sql and classpath:data.sql db = new EmbeddedDatabaseBuilder().addDefaultScripts().build(); } @Test public void testDataAccess() { JdbcTemplate template = new JdbcTemplate(db); template.query(...); } @After public void tearDown() { db.shutdown(); } }
The org.springframework.jdbc.datasource.init
package provides support for initializing
an existing DataSource
. The embedded database support provides one option for creating
and initializing a DataSource
for an application, but sometimes you need to initialize
an instance running on a server somewhere.
If you want to initialize a database and you can provide a reference to a DataSource
bean, use the initialize-database
tag in the spring-jdbc
namespace:
<jdbc:initialize-database data-source="dataSource"> <jdbc:script location="classpath:com/foo/sql/db-schema.sql"/> <jdbc:script location="classpath:com/foo/sql/db-test-data.sql"/> </jdbc:initialize-database>
The example above runs the two scripts specified against the database: the first script
is a schema creation, and the second is a test data set insert. The script locations can
also be patterns with wildcards in the usual ant style used for resources in Spring
(e.g. classpath*:/com/foo/**/sql/*-data.sql
). If a pattern is used the scripts are
executed in lexical order of their URL or filename.
The default behavior of the database initializer is to unconditionally execute the scripts provided. This will not always be what you want, for instance if running against an existing database that already has test data in it. The likelihood of accidentally deleting data is reduced by the commonest pattern (as shown above) that creates the tables first and then inserts the data - the first step will fail if the tables already exist.
However, to get more control over the creation and deletion of existing data, the XML namespace provides a couple more options. The first is flag to switch the initialization on and off. This can be set according to the environment (e.g. to pull a boolean value from system properties or an environment bean), e.g.
<jdbc:initialize-database data-source="dataSource" enabled="#{systemProperties.INITIALIZE_DATABASE}"> <jdbc:script location="..."/> </jdbc:initialize-database>
The second option to control what happens with existing data is to be more tolerant of failures. To this end you can control the ability of the initializer to ignore certain errors in the SQL it executes from the scripts, e.g.
<jdbc:initialize-database data-source="dataSource" ignore-failures="DROPS"> <jdbc:script location="..."/> </jdbc:initialize-database>
In this example we are saying we expect that sometimes the scripts will be run against
an empty database and there are some DROP statements in the scripts which would
therefore fail. So failed SQL DROP
statements will be ignored, but other failures will
cause an exception. This is useful if your SQL dialect doesn’t support DROP ... IF
EXISTS
(or similar) but you want to unconditionally remove all test data before
re-creating it. In that case the first script is usually a set of drops, followed by a
set of CREATE
statements.
The ignore-failures
option can be set to NONE
(the default), DROPS
(ignore failed
drops) or ALL
(ignore all failures).
If you need more control than you get from the XML namespace, you can simply use the
DataSourceInitializer
directly, and define it as a component in your application.
A large class of applications can just use the database initializer with no further complications: those that do not use the database until after the Spring context has started. If your application is not one of those then you might need to read the rest of this section.
The database initializer depends on a data source instance and runs the scripts provided
in its initialization callback (c.f. init-method
in an XML bean definition or
InitializingBean
). If other beans depend on the same data source and also use the data
source in an initialization callback then there might be a problem because the data has
not yet been initialized. A common example of this is a cache that initializes eagerly
and loads up data from the database on application startup.
To get round this issue you two options: change your cache initialization strategy to a later phase, or ensure that the database initializer is initialized first.
The first option might be easy if the application is in your control, and not otherwise. Some suggestions for how to implement this are
Lifecycle
or SmartLifecycle
. When the application context starts up a
SmartLifecycle
can be automatically started if its autoStartup
flag is set, and a
Lifecycle
can be started manually by calling
ConfigurableApplicationContext.start()
on the enclosing context.
ApplicationEvent
or similar custom observer mechanism to trigger the
cache initialization. ContextRefreshedEvent
is always published by the context when
it is ready for use (after all beans have been initialized), so that is often a useful
hook (this is how the SmartLifecycle
works by default).
The second option can also be easy. Some suggestions on how to implement this are
The Spring Framework supports integration with Hibernate, Java Persistence API (JPA) and Java Data Objects (JDO) for resource management, data access object (DAO) implementations, and transaction strategies. For example, for Hibernate there is first-class support with several convenient IoC features that address many typical Hibernate integration issues. You can configure all of the supported features for O/R (object relational) mapping tools through Dependency Injection. They can participate in Spring’s resource and transaction management, and they comply with Spring’s generic transaction and DAO exception hierarchies. The recommended integration style is to code DAOs against plain Hibernate, JPA, and JDO APIs. The older style of using Spring’s DAO templates is no longer recommended; however, coverage of this style can be found in the Section 31.1, “Classic ORM usage” in the appendices.
Spring adds significant enhancements to the ORM layer of your choice when you create data access applications. You can leverage as much of the integration support as you wish, and you should compare this integration effort with the cost and risk of building a similar infrastructure in-house. You can use much of the ORM support as you would a library, regardless of technology, because everything is designed as a set of reusable JavaBeans. ORM in a Spring IoC container facilitates configuration and deployment. Thus most examples in this section show configuration inside a Spring container.
Benefits of using the Spring Framework to create your ORM DAOs include:
SessionFactory
instances, JDBC DataSource
instances, transaction managers, and mapped object implementations (if needed). This
in turn makes it much easier to test each piece of persistence-related code in
isolation.
SessionFactory
instances, JPA EntityManagerFactory
instances, JDBC DataSource
instances, and other related resources. This makes these
values easy to manage and change. Spring offers efficient, easy, and safe handling of
persistence resources. For example, related code that uses Hibernate generally needs to
use the same Hibernate Session
to ensure efficiency and proper transaction handling.
Spring makes it easy to create and bind a Session
to the current thread transparently,
by exposing a current Session
through the Hibernate SessionFactory
. Thus Spring
solves many chronic problems of typical Hibernate usage, for any local or JTA
transaction environment.
@Transactional
annotation or by explicitly configuring the transaction AOP advice in
an XML configuration file. In both cases, transaction semantics and exception handling
(rollback, and so on) are handled for you. As discussed below, in
Resource and transaction management, you can also swap various
transaction managers, without affecting your ORM-related code. For example, you can
swap between local transactions and JTA, with the same full services (such as
declarative transactions) available in both scenarios. Additionally, JDBC-related code
can fully integrate transactionally with the code you use to do ORM. This is useful
for data access that is not suitable for ORM, such as batch processing and BLOB
streaming, which still need to share common transactions with ORM operations.
Tip | |
---|---|
For more comprehensive ORM support, including support for alternative database technologies such as MongoDB, you might want to check out the Spring Data suite of projects. If you are a JPA user, the Getting Started Accessing Data with JPA guide from http://spring.io provides a great introduction. |
This section highlights considerations that apply to all ORM technologies. The Section 14.3, “Hibernate” section provides more details and also show these features and configurations in a concrete context.
The major goal of Spring’s ORM integration is clear application layering, with any data access and transaction technology, and for loose coupling of application objects. No more business service dependencies on the data access or transaction strategy, no more hard-coded resource lookups, no more hard-to-replace singletons, no more custom service registries. One simple and consistent approach to wiring up application objects, keeping them as reusable and free from container dependencies as possible. All the individual data access features are usable on their own but integrate nicely with Spring’s application context concept, providing XML-based configuration and cross-referencing of plain JavaBean instances that need not be Spring-aware. In a typical Spring application, many important objects are JavaBeans: data access templates, data access objects, transaction managers, business services that use the data access objects and transaction managers, web view resolvers, web controllers that use the business services,and so on.
Typical business applications are cluttered with repetitive resource management code. Many projects try to invent their own solutions, sometimes sacrificing proper handling of failures for programming convenience. Spring advocates simple solutions for proper resource handling, namely IoC through templating in the case of JDBC and applying AOP interceptors for the ORM technologies.
The infrastructure provides proper resource handling and appropriate conversion of
specific API exceptions to an unchecked infrastructure exception hierarchy. Spring
introduces a DAO exception hierarchy, applicable to any data access strategy. For direct
JDBC, the JdbcTemplate
class mentioned in a previous section provides connection
handling and proper conversion of SQLException
to the DataAccessException
hierarchy,
including translation of database-specific SQL error codes to meaningful exception
classes. For ORM technologies, see the next section for how to get the same exception
translation benefits.
When it comes to transaction management, the JdbcTemplate
class hooks in to the Spring
transaction support and supports both JTA and JDBC transactions, through respective
Spring transaction managers. For the supported ORM technologies Spring offers Hibernate,
JPA and JDO support through the Hibernate, JPA, and JDO transaction managers as well as
JTA support. For details on transaction support, see the Chapter 11, Transaction Management chapter.
When you use Hibernate, JPA, or JDO in a DAO, you must decide how to handle the
persistence technology’s native exception classes. The DAO throws a subclass of a
HibernateException
, PersistenceException
or JDOException
depending on the
technology. These exceptions are all run-time exceptions and do not have to be declared
or caught. You may also have to deal with IllegalArgumentException
and
IllegalStateException
. This means that callers can only treat exceptions as generally
fatal, unless they want to depend on the persistence technology’s own exception
structure. Catching specific causes such as an optimistic locking failure is not
possible without tying the caller to the implementation strategy. This trade off might
be acceptable to applications that are strongly ORM-based and/or do not need any special
exception treatment. However, Spring enables exception translation to be applied
transparently through the @Repository
annotation:
@Repository public class ProductDaoImpl implements ProductDao { // class body here... }
<beans> <!-- Exception translation bean post processor --> <bean class="org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor"/> <bean id="myProductDao" class="product.ProductDaoImpl"/> </beans>
The postprocessor automatically looks for all exception translators (implementations of
the PersistenceExceptionTranslator
interface) and advises all beans marked with the
@Repository
annotation so that the discovered translators can intercept and apply the
appropriate translation on the thrown exceptions.
In summary: you can implement DAOs based on the plain persistence technology’s API and annotations, while still benefiting from Spring-managed transactions, dependency injection, and transparent exception conversion (if desired) to Spring’s custom exception hierarchies.
We will start with a coverage of Hibernate 3 in a Spring environment, using it to demonstrate the approach that Spring takes towards integrating O/R mappers. This section will cover many issues in detail and show different variations of DAO implementations and transaction demarcation. Most of these patterns can be directly translated to all other supported ORM tools. The following sections in this chapter will then cover the other ORM technologies, showing briefer examples there.
Note | |
---|---|
As of Spring 4.0, Spring requires Hibernate 3.6 or later. |
To avoid tying application objects to hard-coded resource lookups, you can define
resources such as a JDBC DataSource
or a Hibernate SessionFactory
as beans in the
Spring container. Application objects that need to access resources receive references
to such predefined instances through bean references, as illustrated in the DAO
definition in the next section.
The following excerpt from an XML application context definition shows how to set up a
JDBC DataSource
and a Hibernate SessionFactory
on top of it:
<beans> <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="org.hsqldb.jdbcDriver"/> <property name="url" value="jdbc:hsqldb:hsql://localhost:9001"/> <property name="username" value="sa"/> <property name="password" value=""/> </bean> <bean id="mySessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource"/> <property name="mappingResources"> <list> <value>product.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.HSQLDialect </value> </property> </bean> </beans>
Switching from a local Jakarta Commons DBCP BasicDataSource
to a JNDI-located
DataSource
(usually managed by an application server) is just a matter of
configuration:
<beans> <jee:jndi-lookup id="myDataSource" jndi-name="java:comp/env/jdbc/myds"/> </beans>
You can also access a JNDI-located SessionFactory
, using Spring’s
JndiObjectFactoryBean
/ <jee:jndi-lookup>
to retrieve and expose it. However, that
is typically not common outside of an EJB context.
Hibernate 3 has a feature called contextual sessions, wherein Hibernate itself manages
one current Session
per transaction. This is roughly equivalent to Spring’s
synchronization of one Hibernate Session
per transaction. A corresponding DAO
implementation resembles the following example, based on the plain Hibernate API:
public class ProductDaoImpl implements ProductDao { private SessionFactory sessionFactory; public void setSessionFactory(SessionFactory sessionFactory) { this.sessionFactory = sessionFactory; } public Collection loadProductsByCategory(String category) { return this.sessionFactory.getCurrentSession() .createQuery("from test.Product product where product.category=?") .setParameter(0, category) .list(); } }
This style is similar to that of the Hibernate reference documentation and examples,
except for holding the SessionFactory
in an instance variable. We strongly recommend
such an instance-based setup over the old-school static
HibernateUtil
class from
Hibernate’s CaveatEmptor sample application. (In general, do not keep any resources in
static
variables unless absolutely necessary.)
The above DAO follows the dependency injection pattern: it fits nicely into a Spring IoC
container, just as it would if coded against Spring’s HibernateTemplate
. Of course,
such a DAO can also be set up in plain Java (for example, in unit tests). Simply
instantiate it and call setSessionFactory(..)
with the desired factory reference. As a
Spring bean definition, the DAO would resemble the following:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> </beans>
The main advantage of this DAO style is that it depends on Hibernate API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and will no doubt feel more natural to Hibernate developers.
However, the DAO throws plain HibernateException
(which is unchecked, so does not have
to be declared or caught), which means that callers can only treat exceptions as
generally fatal - unless they want to depend on Hibernate’s own exception hierarchy.
Catching specific causes such as an optimistic locking failure is not possible without
tying the caller to the implementation strategy. This trade off might be acceptable to
applications that are strongly Hibernate-based and/or do not need any special exception
treatment.
Fortunately, Spring’s LocalSessionFactoryBean
supports Hibernate’s
SessionFactory.getCurrentSession()
method for any Spring transaction strategy,
returning the current Spring-managed transactional Session
even with
HibernateTransactionManager
. Of course, the standard behavior of that method remains
the return of the current Session
associated with the ongoing JTA transaction, if any.
This behavior applies regardless of whether you are using Spring’s
JtaTransactionManager
, EJB container managed transactions (CMTs), or JTA.
In summary: you can implement DAOs based on the plain Hibernate 3 API, while still being able to participate in Spring-managed transactions.
We recommend that you use Spring’s declarative transaction support, which enables you to replace explicit transaction demarcation API calls in your Java code with an AOP transaction interceptor. This transaction interceptor can be configured in a Spring container using either Java annotations or XML.This declarative transaction capability allows you to keep business services free of repetitive transaction demarcation code and to focus on adding business logic, which is the real value of your application.
Note | |
---|---|
Prior to continuing, you are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so. |
Furthermore, transaction semantics like propagation behavior and isolation level can be changed in a configuration file and do not affect the business service implementations.
The following example shows how you can configure an AOP transaction interceptor, using XML, for a simple service class:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- SessionFactory, DataSource, etc. omitted --> <bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory"/> </bean> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> <tx:advice id="txAdvice" transaction-manager="myTxManager"> <tx:attributes> <tx:method name="increasePrice" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="*" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> <bean id="myProductService" class="product.SimpleProductService"> <property name="productDao" ref="myProductDao"/> </bean> </beans>
This is the service class that is advised:
public class ProductServiceImpl implements ProductService { private ProductDao productDao; public void setProductDao(ProductDao productDao) { this.productDao = productDao; } // notice the absence of transaction demarcation code in this method // Spring's declarative transaction infrastructure will be demarcating // transactions on your behalf public void increasePriceOfAllProductsInCategory(final String category) { List productsToChange = this.productDao.loadProductsByCategory(category); // ... } }
We also show an attribute-support based configuration, in the following example. You annotate the service layer with @Transactional annotations and instruct the Spring container to find these annotations and provide transactional semantics for these annotated methods.
public class ProductServiceImpl implements ProductService { private ProductDao productDao; public void setProductDao(ProductDao productDao) { this.productDao = productDao; } @Transactional public void increasePriceOfAllProductsInCategory(final String category) { List productsToChange = this.productDao.loadProductsByCategory(category); // ... } @Transactional(readOnly = true) public List<Product> findAllProducts() { return this.productDao.findAllProducts(); } }
As you can see from the following configuration example, the configuration is much simplified, compared to the XML example above, while still providing the same functionality driven by the annotations in the service layer code. All you need to provide is the TransactionManager implementation and a "<tx:annotation-driven/>" entry.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- SessionFactory, DataSource, etc. omitted --> <bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory"/> </bean> <tx:annotation-driven/> <bean id="myProductService" class="product.SimpleProductService"> <property name="productDao" ref="myProductDao"/> </bean> </beans>
You can demarcate transactions in a higher level of the application, on top of such
lower-level data access services spanning any number of operations. Nor do restrictions
exist on the implementation of the surrounding business service; it just needs a Spring
PlatformTransactionManager
. Again, the latter can come from anywhere, but preferably
as a bean reference through a setTransactionManager(..)
method, just as the
productDAO
should be set by a setProductDao(..)
method. The following snippets show
a transaction manager and a business service definition in a Spring application context,
and an example for a business method implementation:
<beans> <bean id="myTxManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="transactionManager" ref="myTxManager"/> <property name="productDao" ref="myProductDao"/> </bean> </beans>
public class ProductServiceImpl implements ProductService { private TransactionTemplate transactionTemplate; private ProductDao productDao; public void setTransactionManager(PlatformTransactionManager transactionManager) { this.transactionTemplate = new TransactionTemplate(transactionManager); } public void setProductDao(ProductDao productDao) { this.productDao = productDao; } public void increasePriceOfAllProductsInCategory(final String category) { this.transactionTemplate.execute(new TransactionCallbackWithoutResult() { public void doInTransactionWithoutResult(TransactionStatus status) { List productsToChange = this.productDao.loadProductsByCategory(category); // do the price increase... } }); } }
Spring’s TransactionInterceptor
allows any checked application exception to be thrown
with the callback code, while TransactionTemplate
is restricted to unchecked
exceptions within the callback. TransactionTemplate
triggers a rollback in case of an
unchecked application exception, or if the transaction is marked rollback-only by the
application (via TransactionStatus
). TransactionInterceptor
behaves the same way by
default but allows configurable rollback policies per method.
Both TransactionTemplate
and TransactionInterceptor
delegate the actual transaction
handling to a PlatformTransactionManager
instance, which can be a
HibernateTransactionManager
(for a single Hibernate SessionFactory
, using a
ThreadLocal
Session
under the hood) or a JtaTransactionManager
(delegating to the
JTA subsystem of the container) for Hibernate applications. You can even use a custom
PlatformTransactionManager
implementation. Switching from native Hibernate transaction
management to JTA, such as when facing distributed transaction requirements for certain
deployments of your application, is just a matter of configuration. Simply replace
the Hibernate transaction manager with Spring’s JTA transaction implementation. Both
transaction demarcation and data access code will work without changes, because they
just use the generic transaction management APIs.
For distributed transactions across multiple Hibernate session factories, simply combine
JtaTransactionManager
as a transaction strategy with multiple
LocalSessionFactoryBean
definitions. Each DAO then gets one specific SessionFactory
reference passed into its corresponding bean property. If all underlying JDBC data
sources are transactional container ones, a business service can demarcate transactions
across any number of DAOs and any number of session factories without special regard, as
long as it is using JtaTransactionManager
as the strategy.
<beans> <jee:jndi-lookup id="dataSource1" jndi-name="java:comp/env/jdbc/myds1"/> <jee:jndi-lookup id="dataSource2" jndi-name="java:comp/env/jdbc/myds2"/> <bean id="mySessionFactory1" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource1"/> <property name="mappingResources"> <list> <value>product.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.MySQLDialect hibernate.show_sql=true </value> </property> </bean> <bean id="mySessionFactory2" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="myDataSource2"/> <property name="mappingResources"> <list> <value>inventory.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <value> hibernate.dialect=org.hibernate.dialect.OracleDialect </value> </property> </bean> <bean id="myTxManager" class="org.springframework.transaction.jta.JtaTransactionManager"/> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory1"/> </bean> <bean id="myInventoryDao" class="product.InventoryDaoImpl"> <property name="sessionFactory" ref="mySessionFactory2"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> <property name="inventoryDao" ref="myInventoryDao"/> </bean> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> <tx:advice id="txAdvice" transaction-manager="myTxManager"> <tx:attributes> <tx:method name="increasePrice" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="*" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> </beans>
Both HibernateTransactionManager
and JtaTransactionManager
allow for proper
JVM-level cache handling with Hibernate, without container-specific transaction manager
lookup or a JCA connector (if you are not using EJB to initiate transactions).
HibernateTransactionManager
can export the Hibernate JDBC Connection
to plain JDBC
access code, for a specific DataSource
. This capability allows for high-level
transaction demarcation with mixed Hibernate and JDBC data access completely without
JTA, if you are accessing only one database. HibernateTransactionManager
automatically
exposes the Hibernate transaction as a JDBC transaction if you have set up the passed-in
SessionFactory
with a DataSource
through the dataSource
property of the
LocalSessionFactoryBean
class. Alternatively, you can specify explicitly the
DataSource
for which the transactions are supposed to be exposed through the
dataSource
property of the HibernateTransactionManager
class.
You can switch between a container-managed JNDI SessionFactory
and a locally defined
one, without having to change a single line of application code. Whether to keep
resource definitions in the container or locally within the application is mainly a
matter of the transaction strategy that you use. Compared to a Spring-defined local
SessionFactory
, a manually registered JNDI SessionFactory
does not provide any
benefits. Deploying a SessionFactory
through Hibernate’s JCA connector provides the
added value of participating in the Java EE server’s management infrastructure, but does
not add actual value beyond that.
Spring’s transaction support is not bound to a container. Configured with any strategy
other than JTA, transaction support also works in a stand-alone or test environment.
Especially in the typical case of single-database transactions, Spring’s single-resource
local transaction support is a lightweight and powerful alternative to JTA. When you use
local EJB stateless session beans to drive transactions, you depend both on an EJB
container and JTA, even if you access only a single database, and only use stateless
session beans to provide declarative transactions through container-managed
transactions. Also, direct use of JTA programmatically requires a Java EE environment as
well. JTA does not involve only container dependencies in terms of JTA itself and of
JNDI DataSource
instances. For non-Spring, JTA-driven Hibernate transactions, you have
to use the Hibernate JCA connector, or extra Hibernate transaction code with the
TransactionManagerLookup
configured for proper JVM-level caching.
Spring-driven transactions can work as well with a locally defined Hibernate
SessionFactory
as they do with a local JDBC DataSource
if they are accessing a
single database. Thus you only have to use Spring’s JTA transaction strategy when you
have distributed transaction requirements. A JCA connector requires container-specific
deployment steps, and obviously JCA support in the first place. This configuration
requires more work than deploying a simple web application with local resource
definitions and Spring-driven transactions. Also, you often need the Enterprise Edition
of your container if you are using, for example, WebLogic Express, which does not
provide JCA. A Spring application with local resources and transactions spanning one
single database works in any Java EE web container (without JTA, JCA, or EJB) such as
Tomcat, Resin, or even plain Jetty. Additionally, you can easily reuse such a middle
tier in desktop applications or test suites.
All things considered, if you do not use EJBs, stick with local SessionFactory
setup
and Spring’s HibernateTransactionManager
or JtaTransactionManager
. You get all of
the benefits, including proper transactional JVM-level caching and distributed
transactions, without the inconvenience of container deployment. JNDI registration of a
Hibernate SessionFactory
through the JCA connector only adds value when used in
conjunction with EJBs.
In some JTA environments with very strict XADataSource
implementations — currently
only some WebLogic Server and WebSphere versions — when Hibernate is configured without
regard to the JTA PlatformTransactionManager
object for that environment, it is
possible for spurious warning or exceptions to show up in the application server log.
These warnings or exceptions indicate that the connection being accessed is no longer
valid, or JDBC access is no longer valid, possibly because the transaction is no longer
active. As an example, here is an actual exception from WebLogic:
java.sql.SQLException: The transaction is no longer active - status: Committed. No
further JDBC access is allowed within this transaction.
You resolve this warning by simply making Hibernate aware of the JTA
PlatformTransactionManager
instance, to which it will synchronize (along with Spring).
You have two options for doing this:
PlatformTransactionManager
object (presumably from JNDI through
JndiObjectFactoryBean
or <jee:jndi-lookup>
) and feeding it, for example, to
Spring’s JtaTransactionManager
, then the easiest way is to specify a reference to
the bean defining this JTA PlatformTransactionManager
instance as the value of the
jtaTransactionManager
property for LocalSessionFactoryBean.
Spring then makes the
object available to Hibernate.
PlatformTransactionManager
instance,
because Spring’s JtaTransactionManager
can find it itself. Thus you need to
configure Hibernate to look up JTA PlatformTransactionManager
directly. You do this
by configuring an application server- specific TransactionManagerLookup
class in the
Hibernate configuration, as described in the Hibernate manual.
The remainder of this section describes the sequence of events that occur with and
without Hibernate’s awareness of the JTA PlatformTransactionManager
.
When Hibernate is not configured with any awareness of the JTA
PlatformTransactionManager
, the following events occur when a JTA transaction commits:
JtaTransactionManager
is synchronized to the JTA transaction, so it is
called back through an afterCompletion callback by the JTA transaction manager.
afterTransactionCompletion
callback (used to clear
the Hibernate cache), followed by an explicit close()
call on the Hibernate Session,
which causes Hibernate to attempt to close()
the JDBC Connection.
Connection.close()
call then triggers the warning or
error, as the application server no longer considers the Connection
usable at all,
because the transaction has already been committed.
When Hibernate is configured with awareness of the JTA PlatformTransactionManager
, the
following events occur when a JTA transaction commits:
JtaTransactionManager
is synchronized to the JTA transaction, so the
transaction is called back through a beforeCompletion callback by the JTA
transaction manager.
Session
needs to be closed at all, Spring will close it now.
Spring supports the standard JDO 2.0 and 2.1 APIs as data access strategy, following the
same style as the Hibernate support. The corresponding integration classes reside in the
org.springframework.orm.jdo
package.
Spring provides a LocalPersistenceManagerFactoryBean
class that allows you to define a
local JDO PersistenceManagerFactory
within a Spring application context:
<beans> <bean id="myPmf" class="org.springframework.orm.jdo.LocalPersistenceManagerFactoryBean"> <property name="configLocation" value="classpath:kodo.properties"/> </bean> </beans>
Alternatively, you can set up a PersistenceManagerFactory
through direct instantiation
of a PersistenceManagerFactory
implementation class. A JDO PersistenceManagerFactory
implementation class follows the JavaBeans pattern, just like a JDBC DataSource
implementation class, which is a natural fit for a configuration that uses Spring. This
setup style usually supports a Spring-defined JDBC DataSource
, passed into the
connectionFactory
property. For example, for the open source JDO implementation
DataNucleus (formerly JPOX) ( http://www.datanucleus.org/),
this is the XML configuration of the PersistenceManagerFactory
implementation:
<beans> <bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> <bean id="myPmf" class="org.datanucleus.jdo.JDOPersistenceManagerFactory" destroy-method="close"> <property name="connectionFactory" ref="dataSource"/> <property name="nontransactionalRead" value="true"/> </bean> </beans>
You can also set up JDO PersistenceManagerFactory
in the JNDI environment of a Java EE
application server, usually through the JCA connector provided by the particular JDO
implementation. Spring’s standard JndiObjectFactoryBean
or <jee:jndi-lookup>
can be
used to retrieve and expose such a PersistenceManagerFactory
. However, outside an EJB
context, no real benefit exists in holding the PersistenceManagerFactory
in JNDI: only
choose such a setup for a good reason. See Section 14.3.6, “Comparing container-managed and locally defined resources” for a discussion;
the arguments there apply to JDO as well.
DAOs can also be written directly against plain JDO API, without any Spring
dependencies, by using an injected PersistenceManagerFactory
. The following is an
example of a corresponding DAO implementation:
public class ProductDaoImpl implements ProductDao { private PersistenceManagerFactory persistenceManagerFactory; public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) { this.persistenceManagerFactory = pmf; } public Collection loadProductsByCategory(String category) { PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager(); try { Query query = pm.newQuery(Product.class, "category = pCategory"); query.declareParameters("String pCategory"); return query.execute(category); } finally { pm.close(); } } }
Because the above DAO follows the dependency injection pattern, it fits nicely into a
Spring container, just as it would if coded against Spring’s JdoTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> </beans>
The main problem with such DAOs is that they always get a new PersistenceManager
from
the factory. To access a Spring-managed transactional PersistenceManager
, define a
TransactionAwarePersistenceManagerFactoryProxy
(as included in Spring) in front of
your target PersistenceManagerFactory
, then passing a reference to that proxy into
your DAOs as in the following example:
<beans> <bean id="myPmfProxy" class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy"> <property name="targetPersistenceManagerFactory" ref="myPmf"/> </bean> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmfProxy"/> </bean> </beans>
Your data access code will receive a transactional PersistenceManager
(if any) from
the PersistenceManagerFactory.getPersistenceManager()
method that it calls. The latter
method call goes through the proxy, which first checks for a current transactional
PersistenceManager
before getting a new one from the factory. Any close()
calls on
the PersistenceManager
are ignored in case of a transactional PersistenceManager
.
If your data access code always runs within an active transaction (or at least within
active transaction synchronization), it is safe to omit the PersistenceManager.close()
call and thus the entire finally
block, which you might do to keep your DAO
implementations concise:
public class ProductDaoImpl implements ProductDao { private PersistenceManagerFactory persistenceManagerFactory; public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) { this.persistenceManagerFactory = pmf; } public Collection loadProductsByCategory(String category) { PersistenceManager pm = this.persistenceManagerFactory.getPersistenceManager(); Query query = pm.newQuery(Product.class, "category = pCategory"); query.declareParameters("String pCategory"); return query.execute(category); } }
With such DAOs that rely on active transactions, it is recommended that you enforce
active transactions through turning off
TransactionAwarePersistenceManagerFactoryProxy
's allowCreate
flag:
<beans> <bean id="myPmfProxy" class="org.springframework.orm.jdo.TransactionAwarePersistenceManagerFactoryProxy"> <property name="targetPersistenceManagerFactory" ref="myPmf"/> <property name="allowCreate" value="false"/> </bean> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmfProxy"/> </bean> </beans>
The main advantage of this DAO style is that it depends on JDO API only; no import of any Spring class is required. This is of course appealing from a non-invasiveness perspective, and might feel more natural to JDO developers.
However, the DAO throws plain JDOException
(which is unchecked, so does not have to be
declared or caught), which means that callers can only treat exceptions as fatal, unless
you want to depend on JDO’s own exception structure. Catching specific causes such as an
optimistic locking failure is not possible without tying the caller to the
implementation strategy. This trade off might be acceptable to applications that are
strongly JDO-based and/or do not need any special exception treatment.
In summary, you can DAOs based on the plain JDO API, and they can still participate in
Spring-managed transactions. This strategy might appeal to you if you are already
familiar with JDO. However, such DAOs throw plain JDOException
, and you would have to
convert explicitly to Spring’s DataAccessException
(if desired).
Note | |
---|---|
You are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so, to get a more detailed coverage of Spring’s declarative transaction support. |
To execute service operations within transactions, you can use Spring’s common declarative transaction facilities. For example:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean id="myTxManager" class="org.springframework.orm.jdo.JdoTransactionManager"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> </bean> <tx:advice id="txAdvice" transaction-manager="txManager"> <tx:attributes> <tx:method name="increasePrice*" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution( product.ProductService.*(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> </beans>
JDO requires an active transaction to modify a persistent object. The non-transactional
flush concept does not exist in JDO, in contrast to Hibernate. For this reason, you need
to set up the chosen JDO implementation for a specific environment. Specifically, you
need to set it up explicitly for JTA synchronization, to detect an active JTA
transaction itself. This is not necessary for local transactions as performed by
Spring’s JdoTransactionManager
, but it is necessary to participate in JTA
transactions, whether driven by Spring’s JtaTransactionManager
or by EJB CMT and plain
JTA.
JdoTransactionManager
is capable of exposing a JDO transaction to JDBC access code
that accesses the same JDBC DataSource
, provided that the registered JdoDialect
supports retrieval of the underlying JDBC Connection
. This is the case for JDBC-based
JDO 2.0 implementations by default.
As an advanced feature, both JdoTemplate
and JdoTransactionManager
support a custom
JdoDialect
that can be passed into the jdoDialect
bean property. In this scenario,
the DAOs will not receive a PersistenceManagerFactory
reference but rather a full
JdoTemplate
instance (for example, passed into the jdoTemplate
property of
JdoDaoSupport
). Using a JdoDialect
implementation, you can enable advanced features
supported by Spring, usually in a vendor-specific manner:
Connection
for exposure to JDBC-based DAOs
PersistenceManager,
to make transactional changes visible to
JDBC-based data access code
JDOExceptions
to Spring DataAccessExceptions
See the JdoDialect
Javadoc for more details on its operations and how to use them
within Spring’s JDO support.
The Spring JPA, available under the org.springframework.orm.jpa
package, offers
comprehensive support for the
Java Persistence
API in a similar manner to the integration with Hibernate or JDO, while being aware of
the underlying implementation in order to provide additional features.
The Spring JPA support offers three ways of setting up the JPA EntityManagerFactory
that will be used by the application to obtain an entity manager.
Note | |
---|---|
Only use this option in simple deployment environments such as stand-alone applications and integration tests. |
The LocalEntityManagerFactoryBean
creates an EntityManagerFactory
suitable for
simple deployment environments where the application uses only JPA for data access. The
factory bean uses the JPA PersistenceProvider
autodetection mechanism (according to
JPA’s Java SE bootstrapping) and, in most cases, requires you to specify only the
persistence unit name:
<beans> <bean id="myEmf" class="org.springframework.orm.jpa.LocalEntityManagerFactoryBean"> <property name="persistenceUnitName" value="myPersistenceUnit"/> </bean> </beans>
This form of JPA deployment is the simplest and the most limited. You cannot refer to an
existing JDBC DataSource
bean definition and no support for global transactions
exists. Furthermore, weaving (byte-code transformation) of persistent classes is
provider-specific, often requiring a specific JVM agent to specified on startup. This
option is sufficient only for stand-alone applications and test environments, for which
the JPA specification is designed.
Note | |
---|---|
Use this option when deploying to a Java EE 5 server. Check your server’s documentation on how to deploy a custom JPA provider into your server, allowing for a different provider than the server’s default. |
Obtaining an EntityManagerFactory
from JNDI (for example in a Java EE 5 environment),
is simply a matter of changing the XML configuration:
<beans> <jee:jndi-lookup id="myEmf" jndi-name="persistence/myPersistenceUnit"/> </beans>
This action assumes standard Java EE 5 bootstrapping: the Java EE server autodetects
persistence units (in effect, META-INF/persistence.xml
files in application jars) and
persistence-unit-ref
entries in the Java EE deployment descriptor (for example,
web.xml
) and defines environment naming context locations for those persistence units.
In such a scenario, the entire persistence unit deployment, including the weaving
(byte-code transformation) of persistent classes, is up to the Java EE server. The JDBC
DataSource
is defined through a JNDI location in the META-INF/persistence.xml
file;
EntityManager transactions are integrated with the server’s JTA subsystem. Spring merely
uses the obtained EntityManagerFactory
, passing it on to application objects through
dependency injection, and managing transactions for the persistence unit, typically
through JtaTransactionManager
.
If multiple persistence units are used in the same application, the bean names of such
JNDI-retrieved persistence units should match the persistence unit names that the
application uses to refer to them, for example, in @PersistenceUnit
and
@PersistenceContext
annotations.
Note | |
---|---|
Use this option for full JPA capabilities in a Spring-based application environment. This includes web containers such as Tomcat as well as stand-alone applications and integration tests with sophisticated persistence requirements. |
The LocalContainerEntityManagerFactoryBean
gives full control over
EntityManagerFactory
configuration and is appropriate for environments where
fine-grained customization is required. The LocalContainerEntityManagerFactoryBean
creates a PersistenceUnitInfo
instance based on the persistence.xml
file, the
supplied dataSourceLookup
strategy, and the specified loadTimeWeaver
. It is thus
possible to work with custom data sources outside of JNDI and to control the weaving
process. The following example shows a typical bean definition for a
LocalContainerEntityManagerFactoryBean
:
<beans> <bean id="myEmf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="dataSource" ref="someDataSource"/> <property name="loadTimeWeaver"> <bean class="org.springframework.instrument.classloading.InstrumentationLoadTimeWeaver"/> </property> </bean> </beans>
The following example shows a typical persistence.xml
file:
<persistence xmlns="http://java.sun.com/xml/ns/persistence" version="1.0"> <persistence-unit name="myUnit" transaction-type="RESOURCE_LOCAL"> <mapping-file>META-INF/orm.xml</mapping-file> <exclude-unlisted-classes/> </persistence-unit> </persistence>
Note | |
---|---|
The |
Using the LocalContainerEntityManagerFactoryBean
is the most powerful JPA setup
option, allowing for flexible local configuration within the application. It supports
links to an existing JDBC DataSource
, supports both local and global transactions, and
so on. However, it also imposes requirements on the runtime environment, such as the
availability of a weaving-capable class loader if the persistence provider demands
byte-code transformation.
This option may conflict with the built-in JPA capabilities of a Java EE 5 server. In a
full Java EE 5 environment, consider obtaining your EntityManagerFactory
from JNDI.
Alternatively, specify a custom persistenceXmlLocation
on your
LocalContainerEntityManagerFactoryBean
definition, for example,
META-INF/my-persistence.xml, and only include a descriptor with that name in your
application jar files. Because the Java EE 5 server only looks for default
META-INF/persistence.xml
files, it ignores such custom persistence units and hence
avoid conflicts with a Spring-driven JPA setup upfront. (This applies to Resin 3.1, for
example.)
The LoadTimeWeaver
interface is a Spring-provided class that allows JPA
ClassTransformer
instances to be plugged in a specific manner, depending whether the
environment is a web container or application server. Hooking ClassTransformers
through an
agent
typically is not efficient. The agents work against the entire virtual machine and
inspect every class that is loaded, which is usually undesirable in a production
server environment.
Spring provides a number of LoadTimeWeaver
implementations for various environments,
allowing ClassTransformer
instances to be applied only per class loader and not
per VM.
Refer to the section called “Spring configuration” in the AOP chapter for more insight regarding the
LoadTimeWeaver
implementations and their setup, either generic or customized to
various platforms (such as Tomcat, WebLogic, GlassFish, Resin and JBoss).
As described in the aforementioned section, you can configure a context-wide
LoadTimeWeaver
using the @EnableLoadTimeWeaving
annotation of
context:load-time-weaver
XML element. Such a global weaver is picked up by all JPA
LocalContainerEntityManagerFactoryBeans
automatically. This is the preferred way of
setting up a load-time weaver, delivering autodetection of the platform (WebLogic,
GlassFish, Tomcat, Resin, JBoss or VM agent) and automatic propagation of the weaver to
all weaver-aware beans:
<context:load-time-weaver/> <bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> ... </bean>
However, if needed, one can manually specify a dedicated weaver through the
loadTimeWeaver
property:
<bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="loadTimeWeaver"> <bean class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/> </property> </bean>
No matter how the LTW is configured, using this technique, JPA applications relying on instrumentation can run in the target platform (ex: Tomcat) without needing an agent. This is important especially when the hosting applications rely on different JPA implementations because the JPA transformers are applied only at class loader level and thus are isolated from each other.
For applications that rely on multiple persistence units locations, stored in various
JARS in the classpath, for example, Spring offers the PersistenceUnitManager
to act as
a central repository and to avoid the persistence units discovery process, which can be
expensive. The default implementation allows multiple locations to be specified that are
parsed and later retrieved through the persistence unit name. (By default, the classpath
is searched for META-INF/persistence.xml
files.)
<bean id="pum" class="org.springframework.orm.jpa.persistenceunit.DefaultPersistenceUnitManager"> <property name="persistenceXmlLocations"> <list> <value>org/springframework/orm/jpa/domain/persistence-multi.xml</value> <value>classpath:/my/package/*/custom-persistence.xml</value> <value>classpath:META-INF/persistence.xml</value> </list> </property> <property name="dataSources"> <map> <entry key="localDataSource" value-ref="local-db"/> <entry key="remoteDataSource" value-ref="remote-db"/> </map> </property> <!-- if no datasource is specified, use this one --> <property name="defaultDataSource" ref="remoteDataSource"/> </bean> <bean id="emf" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean"> <property name="persistenceUnitManager" ref="pum"/> <property name="persistenceUnitName" value="myCustomUnit"/> </bean>
The default implementation allows customization of the PersistenceUnitInfo
instances,
before they are fed to the JPA provider, declaratively through its properties, which
affect all hosted units, or programmatically, through the
PersistenceUnitPostProcessor
, which allows persistence unit selection. If no
PersistenceUnitManager
is specified, one is created and used internally by
LocalContainerEntityManagerFactoryBean
.
Note | |
---|---|
Although |
It is possible to write code against the plain JPA without any Spring dependencies, by
using an injected EntityManagerFactory
or EntityManager
. Spring can understand
@PersistenceUnit
and @PersistenceContext
annotations both at field and method level
if a PersistenceAnnotationBeanPostProcessor
is enabled. A plain JPA DAO implementation
using the @PersistenceUnit
annotation might look like this:
public class ProductDaoImpl implements ProductDao { private EntityManagerFactory emf; @PersistenceUnit public void setEntityManagerFactory(EntityManagerFactory emf) { this.emf = emf; } public Collection loadProductsByCategory(String category) { EntityManager em = this.emf.createEntityManager(); try { Query query = em.createQuery("from Product as p where p.category = ?1"); query.setParameter(1, category); return query.getResultList(); } finally { if (em != null) { em.close(); } } } }
The DAO above has no dependency on Spring and still fits nicely into a Spring
application context. Moreover, the DAO takes advantage of annotations to require the
injection of the default EntityManagerFactory
:
<beans> <!-- bean post-processor for JPA annotations --> <bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor"/> <bean id="myProductDao" class="product.ProductDaoImpl"/> </beans>
As an alternative to defining a PersistenceAnnotationBeanPostProcessor
explicitly,
consider using the Spring context:annotation-config
XML element in your application
context configuration. Doing so automatically registers all Spring standard
post-processors for annotation-based configuration, including
CommonAnnotationBeanPostProcessor
and so on.
<beans> <!-- post-processors for all standard config annotations --> <context:annotation-config/> <bean id="myProductDao" class="product.ProductDaoImpl"/> </beans>
The main problem with such a DAO is that it always creates a new EntityManager
through
the factory. You can avoid this by requesting a transactional EntityManager
(also
called "shared EntityManager" because it is a shared, thread-safe proxy for the actual
transactional EntityManager) to be injected instead of the factory:
public class ProductDaoImpl implements ProductDao { @PersistenceContext private EntityManager em; public Collection loadProductsByCategory(String category) { Query query = em.createQuery("from Product as p where p.category = :category"); query.setParameter("category", category); return query.getResultList(); } }
The @PersistenceContext
annotation has an optional attribute type
, which defaults to
PersistenceContextType.TRANSACTION
. This default is what you need to receive a shared
EntityManager proxy. The alternative, PersistenceContextType.EXTENDED
, is a completely
different affair: This results in a so-called extended EntityManager, which is not
thread-safe and hence must not be used in a concurrently accessed component such as a
Spring-managed singleton bean. Extended EntityManagers are only supposed to be used in
stateful components that, for example, reside in a session, with the lifecycle of the
EntityManager not tied to a current transaction but rather being completely up to the
application.
The injected EntityManager
is Spring-managed (aware of the ongoing transaction). It is
important to note that even though the new DAO implementation uses method level
injection of an EntityManager
instead of an EntityManagerFactory
, no change is
required in the application context XML due to annotation usage.
The main advantage of this DAO style is that it only depends on Java Persistence API; no import of any Spring class is required. Moreover, as the JPA annotations are understood, the injections are applied automatically by the Spring container. This is appealing from a non-invasiveness perspective, and might feel more natural to JPA developers.
Note | |
---|---|
You are strongly encouraged to read Section 11.5, “Declarative transaction management” if you have not done so, to get a more detailed coverage of Spring’s declarative transaction support. |
To execute service operations within transactions, you can use Spring’s common declarative transaction facilities. For example:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean id="myTxManager" class="org.springframework.orm.jpa.JpaTransactionManager"> <property name="entityManagerFactory" ref="myEmf"/> </bean> <bean id="myProductService" class="product.ProductServiceImpl"> <property name="productDao" ref="myProductDao"/> </bean> <aop:config> <aop:pointcut id="productServiceMethods" expression="execution(* product.ProductService.(..))"/> <aop:advisor advice-ref="txAdvice" pointcut-ref="productServiceMethods"/> </aop:config> <tx:advice id="txAdvice" transaction-manager="myTxManager"> <tx:attributes> <tx:method name="increasePrice" propagation="REQUIRED"/> <tx:method name="someOtherBusinessMethod" propagation="REQUIRES_NEW"/> <tx:method name="*" propagation="SUPPORTS" read-only="true"/> </tx:attributes> </tx:advice> </beans>
Spring JPA allows a configured JpaTransactionManager
to expose a JPA transaction to
JDBC access code that accesses the same JDBC DataSource
, provided that the registered
JpaDialect
supports retrieval of the underlying JDBC Connection
. Out of the box,
Spring provides dialects for the Toplink, Hibernate and OpenJPA JPA implementations. See
the next section for details on the JpaDialect
mechanism.
As an advanced feature JpaTemplate
, JpaTransactionManager
and subclasses of
AbstractEntityManagerFactoryBean
support a custom JpaDialect
, to be passed into the
jpaDialect
bean property. In such a scenario, the DAOs do not receive an
EntityManagerFactory
reference but rather a full JpaTemplate
instance (for example,
passed into the jpaTemplate
property of JpaDaoSupport
). A JpaDialect
implementation can enable some advanced features supported by Spring, usually in a
vendor-specific manner:
Connection
for exposure to JDBC-based DAOs)
PersistenceExceptions
to Spring DataAccessExceptions
This is particularly valuable for special transaction semantics and for advanced
translation of exception. The default implementation used ( DefaultJpaDialect
) does
not provide any special capabilities and if the above features are required, you have to
specify the appropriate dialect.
See the JpaDialect
Javadoc for more details of its operations and how they are used
within Spring’s JPA support.
In this chapter, we will describe Spring’s Object/XML Mapping support. Object/XML Mapping, or O/X mapping for short, is the act of converting an XML document to and from an object. This conversion process is also known as XML Marshalling, or XML Serialization. This chapter uses these terms interchangeably.
Within the field of O/X mapping, a marshaller is responsible for serializing an object (graph) to XML. In similar fashion, an unmarshaller deserializes the XML to an object graph. This XML can take the form of a DOM document, an input or output stream, or a SAX handler.
Some of the benefits of using Spring for your O/X mapping needs are:
Spring’s bean factory makes it easy to configure marshallers, without needing to construct JAXB context, JiBX binding factories, etc. The marshallers can be configured as any other bean in your application context. Additionally, XML Schema-based configuration is available for a number of marshallers, making the configuration even simpler.
Spring’s O/X mapping operates through two global interfaces: the Marshaller
and
Unmarshaller
interface. These abstractions allow you to switch O/X mapping frameworks
with relative ease, with little or no changes required on the classes that do the
marshalling. This approach has the additional benefit of making it possible to do XML
marshalling with a mix-and-match approach (e.g. some marshalling performed using JAXB,
other using XMLBeans) in a non-intrusive fashion, leveraging the strength of each
technology.
Spring provides a conversion from exceptions from the underlying O/X mapping tool to its
own exception hierarchy with the XmlMappingException
as the root exception. As can be
expected, these runtime exceptions wrap the original exception so no information is lost.
As stated in the introduction, a marshaller serializes an object to XML, and an unmarshaller deserializes XML stream to an object. In this section, we will describe the two Spring interfaces used for this purpose.
Spring abstracts all marshalling operations behind the
org.springframework.oxm.Marshaller
interface, the main methods of which is listed
below.
public interface Marshaller { /** * Marshals the object graph with the given root into the provided Result. */ void marshal(Object graph, Result result) throws XmlMappingException, IOException; }
The Marshaller
interface has one main method, which marshals the given object to a
given javax.xml.transform.Result
. Result is a tagging interface that basically
represents an XML output abstraction: concrete implementations wrap various XML
representations, as indicated in the table below.
Result implementation | Wraps XML representation |
---|---|
|
|
|
|
|
|
Note | |
---|---|
Although the |
Similar to the Marshaller
, there is the org.springframework.oxm.Unmarshaller
interface.
public interface Unmarshaller { /** * Unmarshals the given provided Source into an object graph. */ Object unmarshal(Source source) throws XmlMappingException, IOException; }
This interface also has one method, which reads from the given
javax.xml.transform.Source
(an XML input abstraction), and returns the object read. As
with Result, Source is a tagging interface that has three concrete implementations. Each
wraps a different XML representation, as indicated in the table below.
Source implementation | Wraps XML representation |
---|---|
|
|
|
|
|
|
Even though there are two separate marshalling interfaces ( Marshaller
and
Unmarshaller
), all implementations found in Spring-WS implement both in one class.
This means that you can wire up one marshaller class and refer to it both as a
marshaller and an unmarshaller in your applicationContext.xml
.
Spring converts exceptions from the underlying O/X mapping tool to its own exception
hierarchy with the XmlMappingException
as the root exception. As can be expected,
these runtime exceptions wrap the original exception so no information will be lost.
Additionally, the MarshallingFailureException
and UnmarshallingFailureException
provide a distinction between marshalling and unmarshalling operations, even though the
underlying O/X mapping tool does not do so.
The O/X Mapping exception hierarchy is shown in the following figure: image::images/oxm-exceptions.png[width=400]
O/X Mapping exception hierarchy
Spring’s OXM can be used for a wide variety of situations. In the following example, we will use it to marshal the settings of a Spring-managed application as an XML file. We will use a simple JavaBean to represent the settings:
public class Settings { private boolean fooEnabled; public boolean isFooEnabled() { return fooEnabled; } public void setFooEnabled(boolean fooEnabled) { this.fooEnabled = fooEnabled; } }
The application class uses this bean to store its settings. Besides a main method, the
class has two methods: saveSettings()
saves the settings bean to a file named
settings.xml
, and loadSettings()
loads these settings again. A main()
method
constructs a Spring application context, and calls these two methods.
import java.io.FileInputStream; import java.io.FileOutputStream; import java.io.IOException; import javax.xml.transform.stream.StreamResult; import javax.xml.transform.stream.StreamSource; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.oxm.Marshaller; import org.springframework.oxm.Unmarshaller; public class Application { private static final String FILE_NAME = "settings.xml"; private Settings settings = new Settings(); private Marshaller marshaller; private Unmarshaller unmarshaller; public void setMarshaller(Marshaller marshaller) { this.marshaller = marshaller; } public void setUnmarshaller(Unmarshaller unmarshaller) { this.unmarshaller = unmarshaller; } public void saveSettings() throws IOException { FileOutputStream os = null; try { os = new FileOutputStream(FILE_NAME); this.marshaller.marshal(settings, new StreamResult(os)); } finally { if (os != null) { os.close(); } } } public void loadSettings() throws IOException { FileInputStream is = null; try { is = new FileInputStream(FILE_NAME); this.settings = (Settings) this.unmarshaller.unmarshal(new StreamSource(is)); } finally { if (is != null) { is.close(); } } } public static void main(String[] args) throws IOException { ApplicationContext appContext = new ClassPathXmlApplicationContext("applicationContext.xml"); Application application = (Application) appContext.getBean("application"); application.saveSettings(); application.loadSettings(); } }
The Application
requires both a marshaller
and unmarshaller
property to be set. We
can do so using the following applicationContext.xml
:
<beans> <bean id="application" class="Application"> <property name="marshaller" ref="castorMarshaller" /> <property name="unmarshaller" ref="castorMarshaller" /> </bean> <bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller"/> </beans>
This application context uses Castor, but we could have used any of the other marshaller
instances described later in this chapter. Note that Castor does not require any further
configuration by default, so the bean definition is rather simple. Also note that the
CastorMarshaller
implements both Marshaller
and Unmarshaller
, so we can refer to the
castorMarshaller
bean in both the marshaller
and unmarshaller
property of the
application.
This sample application produces the following settings.xml
file:
<?xml version="1.0" encoding="UTF-8"?> <settings foo-enabled="false"/>
Marshallers could be configured more concisely using tags from the OXM namespace. To make these tags available, the appropriate schema has to be referenced first in the preamble of the XML configuration file. Note the oxm related text below:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:oxm="http://www.springframework.org/schema/oxm" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/oxm http://www.springframework.org/schema/oxm/spring-oxm.xsd">
Currently, the following tags are available:
Each tag will be explained in its respective marshaller’s section. As an example though, here is how the configuration of a JAXB2 marshaller might look like:
<oxm:jaxb2-marshaller id="marshaller" contextPath="org.springframework.ws.samples.airline.schema"/>
The JAXB binding compiler translates a W3C XML Schema into one or more Java classes, a
jaxb.properties
file, and possibly some resource files. JAXB also offers a way to
generate a schema from annotated Java classes.
Spring supports the JAXB 2.0 API as XML marshalling strategies, following the
Marshaller
and Unmarshaller
interfaces described in Section 15.2, “Marshaller and Unmarshaller”.
The corresponding integration classes reside in the org.springframework.oxm.jaxb
package.
The Jaxb2Marshaller
class implements both the Spring Marshaller
and Unmarshaller
interface. It requires a context path to operate, which you can set using the
contextPath
property. The context path is a list of colon (:) separated Java package
names that contain schema derived classes. It also offers a classesToBeBound
property,
which allows you to set an array of classes to be supported by the marshaller. Schema
validation is performed by specifying one or more schema resource to the bean, like so:
<beans> <bean id="jaxb2Marshaller" class="org.springframework.oxm.jaxb.Jaxb2Marshaller"> <property name="classesToBeBound"> <list> <value>org.springframework.oxm.jaxb.Flight</value> <value>org.springframework.oxm.jaxb.Flights</value> </list> </property> <property name="schema" value="classpath:org/springframework/oxm/schema.xsd"/> </bean> ... </beans>
The jaxb2-marshaller
tag configures a org.springframework.oxm.jaxb.Jaxb2Marshaller
.
Here is an example:
<oxm:jaxb2-marshaller id="marshaller" contextPath="org.springframework.ws.samples.airline.schema"/>
Alternatively, the list of classes to bind can be provided to the marshaller via the
class-to-be-bound
child tag:
<oxm:jaxb2-marshaller id="marshaller"> <oxm:class-to-be-bound name="org.springframework.ws.samples.airline.schema.Airport"/> <oxm:class-to-be-bound name="org.springframework.ws.samples.airline.schema.Flight"/> ... </oxm:jaxb2-marshaller>
Available attributes are:
Attribute | Description | Required |
---|---|---|
| the id of the marshaller | no |
| the JAXB Context path | no |
Castor XML mapping is an open source XML binding framework. It allows you to transform the data contained in a java object model into/from an XML document. By default, it does not require any further configuration, though a mapping file can be used to have more control over the behavior of Castor.
For more information on Castor, refer to the
Castor web site. The Spring
integration classes reside in the org.springframework.oxm.castor
package.
As with JAXB, the CastorMarshaller
implements both the Marshaller
and Unmarshaller
interface. It can be wired up as follows:
<beans> <bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller" /> ... </beans>
Although it is possible to rely on Castor’s default marshalling behavior, it might be necessary to have more control over it. This can be accomplished using a Castor mapping file. For more information, refer to Castor XML Mapping.
The mapping can be set using the mappingLocation
resource property, indicated below
with a classpath resource.
<beans> <bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller" > <property name="mappingLocation" value="classpath:mapping.xml" /> </bean> </beans>
The castor-marshaller
tag configures a
org.springframework.oxm.castor.CastorMarshaller
. Here is an example:
<oxm:castor-marshaller id="marshaller" mapping-location="classpath:org/springframework/oxm/castor/mapping.xml"/>
The marshaller instance can be configured in two ways, by specifying either the location
of a mapping file (through the mapping-location
property), or by identifying Java
POJOs (through the target-class
or target-package
properties) for which there exist
corresponding XML descriptor classes. The latter way is usually used in conjunction with
XML code generation from XML schemas.
Available attributes are:
Attribute | Description | Required |
---|---|---|
| the id of the marshaller | no |
| the encoding to use for unmarshalling from XML | no |
| a Java class name for a POJO for which an XML class descriptor is available (as generated through code generation) | no |
| a Java package name that identifies a package that contains POJOs and their corresponding Castor XML descriptor classes (as generated through code generation from XML schemas) | no |
| location of a Castor XML mapping file | no |
XMLBeans is an XML binding tool that has full XML Schema support, and offers full XML
Infoset fidelity. It takes a different approach to that of most other O/X mapping
frameworks, in that all classes that are generated from an XML Schema are all derived
from XmlObject
, and contain XML binding information in them.
For more information on XMLBeans, refer to the XMLBeans
web site . The Spring-WS integration classes reside in the
org.springframework.oxm.xmlbeans
package.
The XmlBeansMarshaller
implements both the Marshaller
and Unmarshaller
interfaces.
It can be configured as follows:
<beans> <bean id="xmlBeansMarshaller" class="org.springframework.oxm.xmlbeans.XmlBeansMarshaller" /> ... </beans>
Note | |
---|---|
Note that the |
The xmlbeans-marshaller
tag configures a
org.springframework.oxm.xmlbeans.XmlBeansMarshaller
. Here is an example:
<oxm:xmlbeans-marshaller id="marshaller"/>
Available attributes are:
Attribute | Description | Required |
---|---|---|
| the id of the marshaller | no |
| the bean name of the XmlOptions that is to be used for this marshaller. Typically a
| no |
The JiBX framework offers a solution similar to that which JDO provides for ORM: a binding definition defines the rules for how your Java objects are converted to or from XML. After preparing the binding and compiling the classes, a JiBX binding compiler enhances the class files, and adds code to handle converting instances of the classes from or to XML.
For more information on JiBX, refer to the JiBX web
site. The Spring integration classes reside in the org.springframework.oxm.jibx
package.
The JibxMarshaller
class implements both the Marshaller
and Unmarshaller
interface. To operate, it requires the name of the class to marshal in, which you can
set using the targetClass
property. Optionally, you can set the binding name using the
bindingName
property. In the next sample, we bind the Flights
class:
<beans> <bean id="jibxFlightsMarshaller" class="org.springframework.oxm.jibx.JibxMarshaller"> <property name="targetClass">org.springframework.oxm.jibx.Flights</property> </bean> ... </beans>
A JibxMarshaller
is configured for a single class. If you want to marshal multiple
classes, you have to configure multiple JibxMarshaller
s with different targetClass
property values.
The jibx-marshaller
tag configures a org.springframework.oxm.jibx.JibxMarshaller
.
Here is an example:
<oxm:jibx-marshaller id="marshaller" target-class="org.springframework.ws.samples.airline.schema.Flight"/>
Available attributes are:
Attribute | Description | Required |
---|---|---|
| the id of the marshaller | no |
| the target class for this marshaller | yes |
| the binding name used by this marshaller | no |
XStream is a simple library to serialize objects to XML and back again. It does not require any mapping, and generates clean XML.
For more information on XStream, refer to the XStream
web site. The Spring integration classes reside in the
org.springframework.oxm.xstream
package.
The XStreamMarshaller
does not require any configuration, and can be configured in an
application context directly. To further customize the XML, you can set analias map,
which consists of string aliases mapped to classes:
<beans> <bean id="xstreamMarshaller" class="org.springframework.oxm.xstream.XStreamMarshaller"> <property name="aliases"> <props> <prop key="Flight">org.springframework.oxm.xstream.Flight</prop> </props> </property> </bean> ... </beans>
Warning | |
---|---|
By default, XStream allows for arbitrary classes to be unmarshalled, which can result in
security vulnerabilities. As such, it is not recommended to use the
<bean id="xstreamMarshaller" class="org.springframework.oxm.xstream.XStreamMarshaller"> <property name="supportedClasses" value="org.springframework.oxm.xstream.Flight"/> ... </bean> This will make sure that only the registered classes are eligible for unmarshalling. Additionally, you can register
custom
converters to make sure that only your supported classes can be unmarshalled. You might
want to add a |
Note | |
---|---|
Note that XStream is an XML serialization library, not a data binding library. Therefore, it has limited namespace support. As such, it is rather unsuitable for usage within Web services. |
This part of the reference documentation covers Spring Framework’s support for the presentation tier (and specifically web-based presentation tiers) including support for WebSocket-style messaging in web applications.
Spring Framework’s own web framework, Spring Web MVC, is covered in the first couple of chapters. Subsequent chapters are concerned with Spring Framework’s integration with other web technologies, such as Struts JSF and others.
Following that is coverage of Spring Framework’s MVC portlet framework.
The section then concludes with comprehensive coverage of the Spring Framework Chapter 20, WebSocket Support (including Section 20.4, “STOMP Messaging”).
The Spring Web model-view-controller (MVC) framework is designed around a
DispatcherServlet
that dispatches requests to handlers, with configurable handler
mappings, view resolution, locale, time zone and theme resolution as well as support for
uploading files. The default handler is based on the @Controller
and @RequestMapping
annotations, offering a wide range of flexible handling methods. With the introduction
of Spring 3.0, the @Controller
mechanism also allows you to create RESTful Web sites
and applications, through the @PathVariable
annotation and other features.
In Spring Web MVC you can use any object as a command or form-backing object; you do not need to implement a framework-specific interface or base class. Spring’s data binding is highly flexible: for example, it treats type mismatches as validation errors that can be evaluated by the application, not as system errors. Thus you need not duplicate your business objects' properties as simple, untyped strings in your form objects simply to handle invalid submissions, or to convert the Strings properly. Instead, it is often preferable to bind directly to your business objects.
Spring’s view resolution is extremely flexible. A Controller
is typically responsible
for preparing a model Map
with data and selecting a view name but it can also write
directly to the response stream and complete the request. View name resolution is highly
configurable through file extension or Accept header content type negotiation, through
bean names, a properties file, or even a custom ViewResolver
implementation. The model
(the M in MVC) is a Map
interface, which allows for the complete abstraction of the
view technology. You can integrate directly with template based rendering technologies
such as JSP, Velocity and Freemarker, or directly generate XML, JSON, Atom, and many
other types of content. The model Map
is simply transformed into an appropriate
format, such as JSP request attributes, a Velocity template model.
Spring’s web module includes many unique web support features:
DispatcherServlet
, handler mapping, view resolver, and so
on — can be fulfilled by a specialized object.
Map
supports easy
integration with any view technology.
Session
.
This is not a specific feature of Spring MVC itself, but rather of the
WebApplicationContext
container(s) that Spring MVC uses. These bean scopes are
described in Section 4.5.4, “Request, session, and global session scopes”
Non-Spring MVC implementations are preferable for some projects. Many teams expect to leverage their existing investment in skills and tools. A large body of knowledge and experience exist for the Struts framework. If you can abide Struts' architectural flaws, it can be a viable choice for the web layer; the same applies to JSF and other web MVC frameworks.
If you do not want to use Spring’s web MVC, but intend to leverage other solutions that
Spring offers, you can integrate the web MVC framework of your choice with Spring
easily. Simply start up a Spring root application context through its
ContextLoaderListener
, and access it through its ServletContext
attribute (or
Spring’s respective helper method) from within a Struts action. No "plug-ins"
are involved, so no dedicated integration is necessary. From the web layer’s point of
view, you simply use Spring as a library, with the root application context instance as
the entry point.
Your registered beans and Spring’s services can be at your fingertips even without Spring’s Web MVC. Spring does not compete with Struts in this scenario. It simply addresses the many areas that the pure web MVC frameworks do not, from bean configuration to data access and transaction handling. So you can enrich your application with a Spring middle tier and/or data access tier, even if you just want to use, for example, the transaction abstraction with JDBC or Hibernate.
Spring’s web MVC framework is, like many other web MVC frameworks, request-driven,
designed around a central Servlet that dispatches requests to controllers and offers
other functionality that facilitates the development of web applications. Spring’s
DispatcherServlet
however, does more than just that. It is completely integrated with
the Spring IoC container and as such allows you to use every other feature that Spring
has.
The request processing workflow of the Spring Web MVC DispatcherServlet
is illustrated
in the following diagram. The pattern-savvy reader will recognize that the
DispatcherServlet
is an expression of the "Front Controller" design pattern (this is a
pattern that Spring Web MVC shares with many other leading web frameworks).
The request processing workflow in Spring Web MVC (high level)
The DispatcherServlet
is an actual Servlet
(it inherits from the HttpServlet
base
class), and as such is declared in the web.xml
of your web application. You need to
map requests that you want the DispatcherServlet
to handle, by using a URL mapping in
the same web.xml
file. This is standard Java EE Servlet configuration; the following
example shows such a DispatcherServlet
declaration and mapping:
<web-app> <servlet> <servlet-name>example</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>example</servlet-name> <url-pattern>/example/*</url-pattern> </servlet-mapping> </web-app>
In the preceding example, all requests starting with /example
will be handled by the
DispatcherServlet
instance named example
. In a Servlet 3.0+ environment, you also
have the option of configuring the Servlet container programmatically. Below is the code
based equivalent of the above web.xml
example:
public class MyWebApplicationInitializer implements WebApplicationInitializer { @Override public void onStartup(ServletContext container) { ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet()); registration.setLoadOnStartup(1); registration.addMapping("/example/*"); } }
WebApplicationInitializer
is an interface provided by Spring MVC that ensures your
code-based configuration is detected and automatically used to initialize any Servlet 3
container. An abstract base class implementation of this interace named
AbstractDispatcherServletInitializer
makes it even easier to register the
DispatcherServlet
by simply specifying its servlet mapping.
See Code-based Servlet container initialization for more details.
The above is only the first step in setting up Spring Web MVC. You now need to configure
the various beans used by the Spring Web MVC framework (over and above the
DispatcherServlet
itself).
As detailed in Section 4.16, “Additional Capabilities of the ApplicationContext”, ApplicationContext
instances in Spring can be
scoped. In the Web MVC framework, each DispatcherServlet
has its own
WebApplicationContext
, which inherits all the beans already defined in the root
WebApplicationContext
. These inherited beans can be overridden in the servlet-specific
scope, and you can define new scope-specific beans local to a given Servlet instance.
Upon initialization of a DispatcherServlet
, Spring MVC looks for a file named
[servlet-name]-servlet.xml in the WEB-INF
directory of your web application and
creates the beans defined there, overriding the definitions of any beans defined with
the same name in the global scope.
Consider the following DispatcherServlet
Servlet configuration (in the web.xml
file):
<web-app> <servlet> <servlet-name>golfing</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>golfing</servlet-name> <url-pattern>/golfing/*</url-pattern> </servlet-mapping> </web-app>
With the above Servlet configuration in place, you will need to have a file called
/WEB-INF/golfing-servlet.xml
in your application; this file will contain all of your
Spring Web MVC-specific components (beans). You can change the exact location of this
configuration file through a Servlet initialization parameter (see below for details).
The WebApplicationContext
is an extension of the plain ApplicationContext
that has
some extra features necessary for web applications. It differs from a normal
ApplicationContext
in that it is capable of resolving themes (see
Section 16.10, “Using themes”), and that it knows which Servlet it is associated with (by having
a link to the ServletContext
). The WebApplicationContext
is bound in the
ServletContext
, and by using static methods on the RequestContextUtils
class you can
always look up the WebApplicationContext
if you need access to it.
The Spring DispatcherServlet
uses special beans to process requests and render the
appropriate views. These beans are part of Spring MVC. You can choose which special
beans to use by simply configuring one or more of them in the WebApplicationContext
.
However, you don’t need to do that initially since Spring MVC maintains a list of
default beans to use if you don’t configure any. More on that in the next section. First
see the table below listing the special bean types the DispatcherServlet
relies on.
Table 16.1. Special bean types in the WebApplicationContext
Bean type | Explanation |
---|---|
Maps incoming requests to handlers and a list of pre- and post-processors (handler
interceptors) based on some criteria the details of which vary by | |
HandlerAdapter | Helps the |
Maps exceptions to views also allowing for more complex exception handling code. | |
Resolves logical String-based view names to actual | |
Resolves the locale a client is using and possibly their time zone, in order to be able to offer internationalized views | |
Resolves themes your web application can use, for example, to offer personalized layouts | |
Parses multi-part requests for example to support processing file uploads from HTML forms. | |
Stores and retrieves the "input" and the "output" |
As mentioned in the previous section for each special bean the DispatcherServlet
maintains a list of implementations to use by default. This information is kept in the
file DispatcherServlet.properties
in the package org.springframework.web.servlet
.
All special beans have some reasonable defaults of their own. Sooner or later though
you’ll need to customize one or more of the properties these beans provide. For example
it’s quite common to configure an InternalResourceViewResolver
settings its prefix
property to the parent location of view files.
Regardless of the details, the important concept to understand here is that once
you configure a special bean such as an InternalResourceViewResolver
in your
WebApplicationContext
, you effectively override the list of default implementations
that would have been used otherwise for that special bean type. For example if you
configure an InternalResourceViewResolver
, the default list of ViewResolver
implementations is ignored.
In Section 16.16, “Configuring Spring MVC” you’ll learn about other options for configuring Spring MVC including MVC Java config and the MVC XML namespace both of which provide a simple starting point and assume little knowledge of how Spring MVC works. Regardless of how you choose to configure your application, the concepts explained in this section are fundamental should be of help to you.
After you set up a DispatcherServlet
, and a request comes in for that specific
DispatcherServlet
, the DispatcherServlet
starts processing the request as follows:
WebApplicationContext
is searched for and bound in the request as an attribute
that the controller and other elements in the process can use. It is bound by default
under the key DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE
.
MultipartHttpServletRequest
for
further processing by other elements in the process. See Section 16.11, “Spring’s multipart (file upload) support” for further
information about multipart handling.
Handler exception resolvers that are declared in the WebApplicationContext
pick up
exceptions that are thrown during processing of the request. Using these exception
resolvers allows you to define custom behaviors to address exceptions.
The Spring DispatcherServlet
also supports the return of the
last-modification-date, as specified by the Servlet API. The process of determining
the last modification date for a specific request is straightforward: the
DispatcherServlet
looks up an appropriate handler mapping and tests whether the
handler that is found implements the LastModified interface. If so, the value of the
long getLastModified(request)
method of the LastModified
interface is returned to
the client.
You can customize individual DispatcherServlet
instances by adding Servlet
initialization parameters ( init-param
elements) to the Servlet declaration in the
web.xml
file. See the following table for the list of supported parameters.
Table 16.2. DispatcherServlet initialization parameters
Parameter | Explanation |
---|---|
| Class that implements |
| String that is passed to the context instance (specified by |
| Namespace of the |
Controllers provide access to the application behavior that you typically define through a service interface. Controllers interpret user input and transform it into a model that is represented to the user by the view. Spring implements a controller in a very abstract way, which enables you to create a wide variety of controllers.
Spring 2.5 introduced an annotation-based programming model for MVC controllers that
uses annotations such as @RequestMapping
, @RequestParam
, @ModelAttribute
, and so
on. This annotation support is available for both Servlet MVC and Portlet MVC.
Controllers implemented in this style do not have to extend specific base classes or
implement specific interfaces. Furthermore, they do not usually have direct dependencies
on Servlet or Portlet APIs, although you can easily configure access to Servlet or
Portlet facilities.
Tip | |
---|---|
Available in the spring-projects Org on Github, a number of web applications leverage the annotation support described in this section including MvcShowcase, MvcAjax, MvcBasic, PetClinic, PetCare, and others. |
@Controller public class HelloWorldController { @RequestMapping("/helloWorld") public String helloWorld(Model model) { model.addAttribute("message", "Hello World!"); return "helloWorld"; } }
As you can see, the @Controller
and @RequestMapping
annotations allow flexible
method names and signatures. In this particular example the method accepts a Model
and
returns a view name as a String
, but various other method parameters and return values
can be used as explained later in this section. @Controller
and @RequestMapping
and
a number of other annotations form the basis for the Spring MVC implementation. This
section documents these annotations and how they are most commonly used in a Servlet
environment.
The @Controller
annotation indicates that a particular class serves the role of
a controller. Spring does not require you to extend any controller base class or
reference the Servlet API. However, you can still reference Servlet-specific features if
you need to.
The @Controller
annotation acts as a stereotype for the annotated class, indicating
its role. The dispatcher scans such annotated classes for mapped methods and detects
@RequestMapping
annotations (see the next section).
You can define annotated controller beans explicitly, using a standard Spring bean
definition in the dispatcher’s context. However, the @Controller
stereotype also
allows for autodetection, aligned with Spring general support for detecting component
classes in the classpath and auto-registering bean definitions for them.
To enable autodetection of such annotated controllers, you add component scanning to your configuration. Use the spring-context schema as shown in the following XML snippet:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="org.springframework.samples.petclinic.web"/> <!-- ... --> </beans>
You use the @RequestMapping
annotation to map URLs such as /appointments
onto an
entire class or a particular handler method. Typically the class-level annotation maps a
specific request path (or path pattern) onto a form controller, with additional
method-level annotations narrowing the primary mapping for a specific HTTP method
request method ("GET", "POST", etc.) or an HTTP request parameter condition.
The following example from the Petcare sample shows a controller in a Spring MVC application that uses this annotation:
@Controller @RequestMapping("/appointments") public class AppointmentsController { private final AppointmentBook appointmentBook; @Autowired public AppointmentsController(AppointmentBook appointmentBook) { this.appointmentBook = appointmentBook; } @RequestMapping(method = RequestMethod.GET) public Map<String, Appointment> get() { return appointmentBook.getAppointmentsForToday(); } @RequestMapping(value="/{day}", method = RequestMethod.GET) public Map<String, Appointment> getForDay(@PathVariable @DateTimeFormat(iso=ISO.DATE) Date day, Model model) { return appointmentBook.getAppointmentsForDay(day); } @RequestMapping(value="/new", method = RequestMethod.GET) public AppointmentForm getNewForm() { return new AppointmentForm(); } @RequestMapping(method = RequestMethod.POST) public String add(@Valid AppointmentForm appointment, BindingResult result) { if (result.hasErrors()) { return "appointments/new"; } appointmentBook.addAppointment(appointment); return "redirect:/appointments"; } }
In the example, the @RequestMapping
is used in a number of places. The first usage is
on the type (class) level, which indicates that all handling methods on this controller
are relative to the /appointments
path. The get()
method has a further
@RequestMapping
refinement: it only accepts GET requests, meaning that an HTTP GET for
/appointments
invokes this method. The post()
has a similar refinement, and the
getNewForm()
combines the definition of HTTP method and path into one, so that GET
requests for appointments/new
are handled by that method.
The getForDay()
method shows another usage of @RequestMapping
: URI templates. (See
the next section).
A @RequestMapping
on the class level is not required. Without it, all paths are simply
absolute, and not relative. The following example from the PetClinic sample
application shows a multi-action controller using @RequestMapping
:
@Controller public class ClinicController { private final Clinic clinic; @Autowired public ClinicController(Clinic clinic) { this.clinic = clinic; } @RequestMapping("/") public void welcomeHandler() { } @RequestMapping("/vets") public ModelMap vetsHandler() { return new ModelMap(this.clinic.getVets()); } }
@RequestMapping On Interface Methods | |
---|---|
A common pitfall when working with annotated controller classes happens when applying
functionality that requires creating a proxy for the controller object (e.g.
Note however that method argument annotations, e.g. |
Spring 3.1 introduced a new set of support classes for @RequestMapping
methods called
RequestMappingHandlerMapping
and RequestMappingHandlerAdapter
respectively. They are
recommended for use and even required to take advantage of new features in Spring MVC
3.1 and going forward. The new support classes are enabled by default by the MVC
namespace and the MVC Java config but must be configured explicitly if using neither.
This section describes a few important differences between the old and the new support
classes.
Prior to Spring 3.1, type and method-level request mappings were examined in two
separate stages — a controller was selected first by the
DefaultAnnotationHandlerMapping
and the actual method to invoke was narrowed down
second by the AnnotationMethodHandlerAdapter
.
With the new support classes in Spring 3.1, the RequestMappingHandlerMapping
is the
only place where a decision is made about which method should process the request. Think
of controller methods as a collection of unique endpoints with mappings for each method
derived from type and method-level @RequestMapping
information.
This enables some new possibilities. For once a HandlerInterceptor
or a
HandlerExceptionResolver
can now expect the Object-based handler to be a
HandlerMethod
, which allows them to examine the exact method, its parameters and
associated annotations. The processing for a URL no longer needs to be split across
different controllers.
There are also several things no longer possible:
SimpleUrlHandlerMapping
or
BeanNameUrlHandlerMapping
and then narrow the method based on @RequestMapping
annotations.
@RequestMapping
methods that don’t have an explicit path mapping URL path but
otherwise match equally, e.g. by HTTP method. In the new support classes
@RequestMapping
methods have to be mapped uniquely.
The above features are still supported with the existing support classes. However to take advantage of new Spring MVC 3.1 features you’ll need to use the new support classes.
URI templates can be used for convenient access to selected parts of a URL in a
@RequestMapping
method.
A URI Template is a URI-like string, containing one or more variable names. When you
substitute values for these variables, the template becomes a URI. The
proposed RFC for URI Templates defines
how a URI is parameterized. For example, the URI Template
http://www.example.com/users/{userId}
contains the variable userId. Assigning the
value fred to the variable yields http://www.example.com/users/fred
.
In Spring MVC you can use the @PathVariable
annotation on a method argument to bind it
to the value of a URI template variable:
@RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET) public String findOwner(@PathVariable String ownerId, Model model) { Owner owner = ownerService.findOwner(ownerId); model.addAttribute("owner", owner); return "displayOwner"; }
The URI Template " /owners/{ownerId}
" specifies the variable name ownerId
. When the
controller handles this request, the value of ownerId
is set to the value found in the
appropriate part of the URI. For example, when a request comes in for /owners/fred
,
the value of ownerId
is fred
.
Tip | |
---|---|
To process the @PathVariable annotation, Spring MVC needs to find the matching URI template variable by name. You can specify it in the annotation: @RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET) public String findOwner(@PathVariable("ownerId") String theOwner, Model model) { // implementation omitted } Or if the URI template variable name matches the method argument name you can omit that detail. As long as your code is not compiled without debugging information, Spring MVC will match the method argument name to the URI template variable name: @RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET) public String findOwner(@PathVariable String ownerId, Model model) { // implementation omitted } |
A method can have any number of @PathVariable
annotations:
@RequestMapping(value="/owners/{ownerId}/pets/{petId}", method=RequestMethod.GET) public String findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) { Owner owner = ownerService.findOwner(ownerId); Pet pet = owner.getPet(petId); model.addAttribute("pet", pet); return "displayPet"; }
When a @PathVariable
annotation is used on a Map<String, String>
argument, the map
is populated with all URI template variables.
A URI template can be assembled from type and path level @RequestMapping
annotations. As a result the findPet()
method can be invoked with a URL such as
/owners/42/pets/21
.
@Controller @RequestMapping("/owners/{ownerId}") public class RelativePathUriTemplateController { @RequestMapping("/pets/{petId}") public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) { // implementation omitted } }
A @PathVariable
argument can be of any simple type such as int, long, Date, etc.
Spring automatically converts to the appropriate type or throws a
TypeMismatchException
if it fails to do so. You can also register support for parsing
additional data types. See the section called “Method Parameters And Type Conversion” and the section called “Customizing WebDataBinder initialization”.
Sometimes you need more precision in defining URI template variables. Consider the URL
"/spring-web/spring-web-3.0.5.jar"
. How do you break it down into multiple parts?
The @RequestMapping
annotation supports the use of regular expressions in URI template
variables. The syntax is {varName:regex}
where the first part defines the variable
name and the second - the regular expression.For example:
@RequestMapping("/spring-web/{symbolicName:[a-z-]}-{version:\\d\\.\\d\\.\\d}{extension:\\.[a-z]
}")
public void handle(@PathVariable String version, @PathVariable String extension) {
// ...
}
}
In addition to URI templates, the @RequestMapping
annotation also supports Ant-style
path patterns (for example, /myPath/*.do
). A combination of URI templates and
Ant-style globs is also supported (for example, /owners/*/pets/{petId}
).
Patterns in @RequestMapping
annotations support ${…} placeholders against local
properties and/or system properties and environment variables. This may be useful in
cases where the path a controller is mapped to may need to be customized through
configuration. For more information on placeholders see the Javadoc for
PropertyPlaceholderConfigurer
.
The URI specification RFC 3986 defines the possibility of including name-value pairs within path segments. There is no specific term used in the spec. The general "URI path parameters" could be applied although the more unique "Matrix URIs", originating from an old post by Tim Berners-Lee, is also frequently used and fairly well known. Within Spring MVC these are referred to as matrix variables.
Matrix variables can appear in any path segment, each matrix variable separated with a
";" (semicolon). For example: "/cars;color=red;year=2012"
. Multiple values may be
either "," (comma) separated "color=red,green,blue"
or the variable name may be
repeated "color=red;color=green;color=blue"
.
If a URL is expected to contain matrix variables, the request mapping pattern must represent them with a URI template. This ensures the request can be matched correctly regardless of whether matrix variables are present or not and in what order they are provided.
Below is an example of extracting the matrix variable "q":
// GET /pets/42;q=11;r=22 @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET) public void findPet(@PathVariable String petId, @MatrixVariable int q) { // petId == 42 // q == 11 }
Since all path segments may contain matrix variables, in some cases you need to be more specific to identify where the variable is expected to be:
// GET /owners/42;q=11/pets/21;q=22 @RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET) public void findPet( @MatrixVariable(value="q", pathVar="ownerId") int q1, @MatrixVariable(value="q", pathVar="petId") int q2) { // q1 == 11 // q2 == 22 }
A matrix variable may be defined as optional and a default value specified:
// GET /pets/42 @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET) public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) { // q == 1 }
All matrix variables may be obtained in a Map:
// GET /owners/42;q=11;r=12/pets/21;q=22;s=23 @RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET) public void findPet( @MatrixVariable Map<String, String> matrixVars, @MatrixVariable(pathVar="petId"") Map<String, String> petMatrixVars) { // matrixVars: ["q" : [11,22], "r" : 12, "s" : 23] // petMatrixVars: ["q" : 11, "s" : 23] }
Note that to enable the use of matrix variables, you must set the
removeSemicolonContent
property of RequestMappingHandlerMapping
to false
. By
default it is set to true
.
Tip | |
---|---|
The MVC Java config and the MVC namespace both provide options for enabling the use of matrix variables. If you are using Java config, The Advanced Customizations
with MVC Java Config section describes how the In the MVC namespace, the <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <mvc:annotation-driven enable-matrix-variables="true"/> </beans> |
You can narrow the primary mapping by specifying a list of consumable media types. The request will be matched only if the Content-Type request header matches the specified media type. For example:
@Controller @RequestMapping(value = "/pets", method = RequestMethod.POST, consumes="application/json") public void addPet(@RequestBody Pet pet, Model model) { // implementation omitted }
Consumable media type expressions can also be negated as in !text/plain to match to all requests other than those with Content-Type of text/plain.
Tip | |
---|---|
The consumes condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level consumable types override rather than extend type-level consumable types. |
You can narrow the primary mapping by specifying a list of producible media types. The request will be matched only if the Accept request header matches one of these values. Furthermore, use of the produces condition ensures the actual content type used to generate the response respects the media types specified in the produces condition. For example:
@Controller @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET, produces="application/json") @ResponseBody public Pet getPet(@PathVariable String petId, Model model) { // implementation omitted }
Just like with consumes, producible media type expressions can be negated as in !text/plain to match to all requests other than those with an Accept header value of text/plain.
Tip | |
---|---|
The produces condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level producible types override rather than extend type-level producible types. |
You can narrow request matching through request parameter conditions such as
"myParam"
, "!myParam"
, or "myParam=myValue"
. The first two test for request
parameter presence/absence and the third for a specific parameter value. Here is an
example with a request parameter value condition:
@Controller @RequestMapping("/owners/{ownerId}") public class RelativePathUriTemplateController { @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET, params="myParam=myValue") public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) { // implementation omitted } }
The same can be done to test for request header presence/absence or to match based on a specific request header value:
@Controller @RequestMapping("/owners/{ownerId}") public class RelativePathUriTemplateController { @RequestMapping(value = "/pets", method = RequestMethod.GET, headers="myHeader=myValue") public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) { // implementation omitted } }
Tip | |
---|---|
Although you can match to Content-Type and Accept header values using media type wild cards (for example "content-type=text/*" will match to "text/plain" and "text/html"), it is recommended to use the consumes and produces conditions respectively instead. They are intended specifically for that purpose. |
An @RequestMapping
handler method can have a very flexible signatures. The supported
method arguments and return values are described in the following section. Most
arguments can be used in arbitrary order with the only exception of BindingResult
arguments. This is described in the next section.
Note | |
---|---|
Spring 3.1 introduced a new set of support classes for |
The following are the supported method arguments:
ServletRequest
or HttpServletRequest
.
HttpSession
. An argument of this type enforces
the presence of a corresponding session. As a consequence, such an argument is never
null
.
Note | |
---|---|
Session access may not be thread-safe, in particular in a Servlet environment. Consider
setting the |
org.springframework.web.context.request.WebRequest
or
org.springframework.web.context.request.NativeWebRequest
. Allows for generic request
parameter access as well as request/session attribute access, without ties to the
native Servlet/Portlet API.
java.util.Locale
for the current request locale, determined by the most specific
locale resolver available, in effect, the configured LocaleResolver
in a Servlet
environment.
java.io.InputStream
/ java.io.Reader
for access to the request’s content. This
value is the raw InputStream/Reader as exposed by the Servlet API.
java.io.OutputStream
/ java.io.Writer
for generating the response’s content. This
value is the raw OutputStream/Writer as exposed by the Servlet API.
java.security.Principal
containing the currently authenticated user.
@PathVariable
annotated parameters for access to URI template variables. See
the section called “URI Template Patterns”.
@MatrixVariable
annotated parameters for access to name-value pairs located in URI
path segments. See the section called “Matrix Variables”.
@RequestParam
annotated parameters for access to specific Servlet request
parameters. Parameter values are converted to the declared method argument type. See
the section called “Binding request parameters to method parameters with @RequestParam”.
@RequestHeader
annotated parameters for access to specific Servlet request HTTP
headers. Parameter values are converted to the declared method argument type.
@RequestBody
annotated parameters for access to the HTTP request body. Parameter
values are converted to the declared method argument type using
HttpMessageConverter
s. See the section called “Mapping the request body with the @RequestBody annotation”.
@RequestPart
annotated parameters for access to the content of a
"multipart/form-data" request part. See Section 16.11.5, “Handling a file upload request from programmatic clients” and
Section 16.11, “Spring’s multipart (file upload) support”.
HttpEntity<?>
parameters for access to the Servlet request HTTP headers and
contents. The request stream will be converted to the entity body using
HttpMessageConverter
s. See the section called “Using HttpEntity”.
java.util.Map
/ org.springframework.ui.Model
/ org.springframework.ui.ModelMap
for enriching the implicit model that is exposed to the web view.
org.springframework.web.servlet.mvc.support.RedirectAttributes
to specify the exact
set of attributes to use in case of a redirect and also to add flash attributes
(attributes stored temporarily on the server-side to make them available to the
request after the redirect). RedirectAttributes
is used instead of the implicit
model if the method returns a "redirect:" prefixed view name or RedirectView
.
@InitBinder
methods and/or the HandlerAdapter configuration. See the webBindingInitializer
property on RequestMappingHandlerAdapter
. Such command objects along with their
validation results will be exposed as model attributes by default, using the command
class class name - e.g. model attribute "orderAddress" for a command object of type
"some.package.OrderAddress". The ModelAttribute
annotation can be used on a method
argument to customize the model attribute name used.
org.springframework.validation.Errors
/
org.springframework.validation.BindingResult
validation results for a preceding
command or form object (the immediately preceding method argument).
org.springframework.web.bind.support.SessionStatus
status handle for marking form
processing as complete, which triggers the cleanup of session attributes that have
been indicated by the @SessionAttributes
annotation at the handler type level.
org.springframework.web.util.UriComponentsBuilder
a builder for preparing a URL
relative to the current request’s host, port, scheme, context path, and the literal
part of the servlet mapping.
The Errors
or BindingResult
parameters have to follow the model object that is being
bound immediately as the method signature might have more that one model object and
Spring will create a separate BindingResult
instance for each of them so the following
sample won’t work:
Invalid ordering of BindingResult and @ModelAttribute.
@RequestMapping(method = RequestMethod.POST) public String processSubmit(@ModelAttribute("pet") Pet pet, Model model, BindingResult result) { ... }
Note, that there is a Model
parameter in between Pet
and BindingResult
. To get
this working you have to reorder the parameters as follows:
@RequestMapping(method = RequestMethod.POST) public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result, Model model) { ... }
The following are the supported return types:
ModelAndView
object, with the model implicitly enriched with command objects and
the results of @ModelAttribute
annotated reference data accessor methods.
Model
object, with the view name implicitly determined through a
RequestToViewNameTranslator
and the model implicitly enriched with command objects
and the results of @ModelAttribute
annotated reference data accessor methods.
Map
object for exposing a model, with the view name implicitly determined through
a RequestToViewNameTranslator
and the model implicitly enriched with command objects
and the results of @ModelAttribute
annotated reference data accessor methods.
View
object, with the model implicitly determined through command objects and
@ModelAttribute
annotated reference data accessor methods. The handler method may
also programmatically enrich the model by declaring a Model
argument (see above).
String
value that is interpreted as the logical view name, with the model
implicitly determined through command objects and @ModelAttribute
annotated
reference data accessor methods. The handler method may also programmatically enrich
the model by declaring a Model
argument (see above).
void
if the method handles the response itself (by writing the response content
directly, declaring an argument of type ServletResponse
/ HttpServletResponse
for
that purpose) or if the view name is supposed to be implicitly determined through a
RequestToViewNameTranslator
(not declaring a response argument in the handler method
signature).
@ResponseBody
, the return type is written to the
response HTTP body. The return value will be converted to the declared method argument
type using HttpMessageConverter
s. See the section called “Mapping the response body with the @ResponseBody annotation”.
HttpEntity<?>
or ResponseEntity<?>
object to provide access to the Servlet
response HTTP headers and contents. The entity body will be converted to the response
stream using HttpMessageConverter
s. See the section called “Using HttpEntity”.
HttpHeaders
object to return a response with no body.
Callable<?>
can be returned when the application wants to produce the return value
asynchronously in a thread managed by Spring MVC.
DeferredResult<?>
can be returned when the application wants to produce the return
value from a thread of its own choosing.
@ModelAttribute
at the method
level (or the default attribute name based on the return type class name). The model
is implicitly enriched with command objects and the results of @ModelAttribute
annotated reference data accessor methods.
Use the @RequestParam
annotation to bind request parameters to a method parameter in
your controller.
The following code snippet shows the usage:
@Controller @RequestMapping("/pets") @SessionAttributes("pet") public class EditPetForm { // ... @RequestMapping(method = RequestMethod.GET) public String setupForm(@RequestParam("petId") int petId, ModelMap model) { Pet pet = this.clinic.loadPet(petId); model.addAttribute("pet", pet); return "petForm"; } // ... }
Parameters using this annotation are required by default, but you can specify that a
parameter is optional by setting @RequestParam
's required
attribute to false
(e.g., @RequestParam(value="id", required=false)
).
Type conversion is applied automatically if the target method parameter type is not
String
. See the section called “Method Parameters And Type Conversion”.
The @RequestBody
method parameter annotation indicates that a method parameter should
be bound to the value of the HTTP request body. For example:
@RequestMapping(value = "/something", method = RequestMethod.PUT) public void handle(@RequestBody String body, Writer writer) throws IOException { writer.write(body); }
You convert the request body to the method argument by using an HttpMessageConverter
.
HttpMessageConverter
is responsible for converting from the HTTP request message to an
object and converting from an object to the HTTP response body. The
RequestMappingHandlerAdapter
supports the @RequestBody
annotation with the following
default HttpMessageConverters
:
ByteArrayHttpMessageConverter
converts byte arrays.
StringHttpMessageConverter
converts strings.
FormHttpMessageConverter
converts form data to/from a MultiValueMap<String, String>.
SourceHttpMessageConverter
converts to/from a javax.xml.transform.Source.
For more information on these converters, see Message Converters. Also note that if using the MVC namespace or the MVC Java config, a wider range of message converters are registered by default. See Section 16.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” for more information.
If you intend to read and write XML, you will need to configure the
MarshallingHttpMessageConverter
with a specific Marshaller
and an Unmarshaller
implementation from the org.springframework.oxm
package. The example below shows how
to do that directly in your configuration but if your application is configured through
the MVC namespace or the MVC Java config see Section 16.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” instead.
<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"> <property name="messageConverters"> <util:list id="beanList"> <ref bean="stringHttpMessageConverter"/> <ref bean="marshallingHttpMessageConverter"/> </util:list> </property </bean> <bean id="stringHttpMessageConverter" class="org.springframework.http.converter.StringHttpMessageConverter"/> <bean id="marshallingHttpMessageConverter" class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter"> <property name="marshaller" ref="castorMarshaller" /> <property name="unmarshaller" ref="castorMarshaller" /> </bean> <bean id="castorMarshaller" class="org.springframework.oxm.castor.CastorMarshaller"/>
An @RequestBody
method parameter can be annotated with @Valid
, in which case it will
be validated using the configured Validator
instance. When using the MVC namespace or
the MVC Java config, a JSR-303 validator is configured automatically assuming a JSR-303
implementation is available on the classpath.
Just like with @ModelAttribute
parameters, an Errors
argument can be used to examine
the errors. If such an argument is not declared, a MethodArgumentNotValidException
will be raised. The exception is handled in the DefaultHandlerExceptionResolver
, which
sends a 400
error back to the client.
Note | |
---|---|
Also see Section 16.16.1, “Enabling the MVC Java Config or the MVC XML Namespace” for information on configuring message converters and a validator through the MVC namespace or the MVC Java config. |
The @ResponseBody
annotation is similar to @RequestBody
. This annotation can be put
on a method and indicates that the return type should be written straight to the HTTP
response body (and not placed in a Model, or interpreted as a view name). For example:
@RequestMapping(value = "/something", method = RequestMethod.PUT) @ResponseBody public String helloWorld() { return "Hello World"; }
The above example will result in the text Hello World
being written to the HTTP
response stream.
As with @RequestBody
, Spring converts the returned object to a response body by using
an HttpMessageConverter
. For more information on these converters, see the previous
section and Message Converters.
It’s a very common use case to have Controllers implement a REST API, thus serving only
JSON, XML or custom MediaType content. For convenience, instead of annotating all your
@RequestMapping
methods with @ResponseBody
, you can annotate your Controller Class
with @RestController
.
@RestController
is a stereotype annotation that combines @ResponseBody
and @Controller
. More than
that, it gives more meaning to your Controller and also may carry additional semantics
in future releases of the framework.
As with regular @Controllers
, a @RestController
may be assisted by a
@ControllerAdvice
Bean. See the the section called “Assisting Controllers with the @ControllerAdvice annotation” section for more details.
The HttpEntity
is similar to @RequestBody
and @ResponseBody
. Besides getting
access to the request and response body, HttpEntity
(and the response-specific
subclass ResponseEntity
) also allows access to the request and response headers, like
so:
@RequestMapping("/something") public ResponseEntity<String> handle(HttpEntity<byte[]> requestEntity) throws UnsupportedEncodingException { String requestHeader = requestEntity.getHeaders().getFirst("MyRequestHeader")); byte[] requestBody = requestEntity.getBody(); // do something with request header and body HttpHeaders responseHeaders = new HttpHeaders(); responseHeaders.set("MyResponseHeader", "MyValue"); return new ResponseEntity<String>("Hello World", responseHeaders, HttpStatus.CREATED); }
The above example gets the value of the MyRequestHeader
request header, and reads the
body as a byte array. It adds the MyResponseHeader
to the response, writes Hello
World
to the response stream, and sets the response status code to 201 (Created).
As with @RequestBody
and @ResponseBody
, Spring uses HttpMessageConverter
to
convert from and to the request and response streams. For more information on these
converters, see the previous section and Message Converters.
The @ModelAttribute
annotation can be used on methods or on method arguments. This
section explains its usage on methods while the next section explains its usage on
method arguments.
An @ModelAttribute
on a method indicates the purpose of that method is to add one or
more model attributes. Such methods support the same argument types as @RequestMapping
methods but cannot be mapped directly to requests. Instead @ModelAttribute
methods in
a controller are invoked before @RequestMapping
methods, within the same controller. A
couple of examples:
// Add one attribute // The return value of the method is added to the model under the name "account" // You can customize the name via @ModelAttribute("myAccount") @ModelAttribute public Account addAccount(@RequestParam String number) { return accountManager.findAccount(number); } // Add multiple attributes @ModelAttribute public void populateModel(@RequestParam String number, Model model) { model.addAttribute(accountManager.findAccount(number)); // add more ... }
@ModelAttribute
methods are used to populate the model with commonly needed attributes
for example to fill a drop-down with states or with pet types, or to retrieve a command
object like Account in order to use it to represent the data on an HTML form. The latter
case is further discussed in the next section.
Note the two styles of @ModelAttribute
methods. In the first, the method adds an
attribute implicitly by returning it. In the second, the method accepts a Model
and
adds any number of model attributes to it. You can choose between the two styles
depending on your needs.
A controller can have any number of @ModelAttribute
methods. All such methods are
invoked before @RequestMapping
methods of the same controller.
@ModelAttribute
methods can also be defined in an @ControllerAdvice
-annotated class
and such methods apply to many controllers. See the the section called “Assisting Controllers with the @ControllerAdvice annotation” section
for more details.
Tip | |
---|---|
What happens when a model attribute name is not explicitly specified? In such cases a
default name is assigned to the model attribute based on its type. For example if the
method returns an object of type |
The @ModelAttribute
annotation can be used on @RequestMapping
methods as well. In
that case the return value of the @RequestMapping
method is interpreted as a model
attribute rather than as a view name. The view name is derived from view name
conventions instead much like for methods returning void — see Section 16.13.3, “The View - RequestToViewNameTranslator”.
As explained in the previous section @ModelAttribute
can be used on methods or on
method arguments. This section explains its usage on method arguments.
An @ModelAttribute
on a method argument indicates the argument should be retrieved
from the model. If not present in the model, the argument should be instantiated first
and then added to the model. Once present in the model, the argument’s fields should be
populated from all request parameters that have matching names. This is known as data
binding in Spring MVC, a very useful mechanism that saves you from having to parse each
form field individually.
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST) public String processSubmit(@ModelAttribute Pet pet) { }
Given the above example where can the Pet instance come from? There are several options:
@SessionAttributes
— see
the section called “Using @SessionAttributes to store model attributes in the HTTP session between requests”.
@ModelAttribute
method in the same
controller — as explained in the previous section.
An @ModelAttribute
method is a common way to to retrieve an attribute from the
database, which may optionally be stored between requests through the use of
@SessionAttributes
. In some cases it may be convenient to retrieve the attribute by
using an URI template variable and a type converter. Here is an example:
@RequestMapping(value="/accounts/{account}", method = RequestMethod.PUT) public String save(@ModelAttribute("account") Account account) { }
In this example the name of the model attribute (i.e. "account") matches the name of a
URI template variable. If you register Converter<String, Account>
that can turn the
String
account value into an Account
instance, then the above example will work
without the need for an @ModelAttribute
method.
The next step is data binding. The WebDataBinder
class matches request parameter names — including query string parameters and form fields — to model attribute fields by
name. Matching fields are populated after type conversion (from String to the target
field type) has been applied where necessary. Data binding and validation are covered in
Chapter 6, Validation, Data Binding, and Type Conversion. Customizing the data binding process for a controller level is covered
in the section called “Customizing WebDataBinder initialization”.
As a result of data binding there may be errors such as missing required fields or type
conversion errors. To check for such errors add a BindingResult
argument immediately
following the @ModelAttribute
argument:
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST) public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) { if (result.hasErrors()) { return "petForm"; } // ... }
With a BindingResult
you can check if errors were found in which case it’s common to
render the same form where the errors can be shown with the help of Spring’s <errors>
form tag.
In addition to data binding you can also invoke validation using your own custom
validator passing the same BindingResult
that was used to record data binding errors.
That allows for data binding and validation errors to be accumulated in one place and
subsequently reported back to the user:
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST) public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) { new PetValidator().validate(pet, result); if (result.hasErrors()) { return "petForm"; } // ... }
Or you can have validation invoked automatically by adding the JSR-303 @Valid
annotation:
@RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST) public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) { if (result.hasErrors()) { return "petForm"; } // ... }
See Section 6.8, “Spring Validation” and Chapter 6, Validation, Data Binding, and Type Conversion for details on how to configure and use validation.
The type-level @SessionAttributes
annotation declares session attributes used by a
specific handler. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session or some
conversational storage, serving as form-backing beans between subsequent requests.
The following code snippet shows the usage of this annotation, specifying the model attribute name:
@Controller @RequestMapping("/editPet.do") @SessionAttributes("pet") public class EditPetForm { // ... }
Note | |
---|---|
When using controller interfaces (e.g., for AOP proxying), make sure to consistently put
all your mapping annotations - such as |
By default all model attributes are considered to be exposed as URI template variables in the redirect URL. Of the remaining attributes those that are primitive types or collections/arrays of primitive types are automatically appended as query parameters.
In annotated controllers however the model may contain additional attributes originally
added for rendering purposes (e.g. drop-down field values). To gain precise control over
the attributes used in a redirect scenario, an @RequestMapping
method can declare an
argument of type RedirectAttributes
and use it to add attributes for use in
RedirectView
. If the controller method does redirect, the content of
RedirectAttributes
is used. Otherwise the content of the default Model
is used.
The RequestMappingHandlerAdapter
provides a flag called
"ignoreDefaultModelOnRedirect"
that can be used to indicate the content of the default
Model
should never be used if a controller method redirects. Instead the controller
method should declare an attribute of type RedirectAttributes
or if it doesn’t do so
no attributes should be passed on to RedirectView
. Both the MVC namespace and the MVC
Java config keep this flag set to false
in order to maintain backwards compatibility.
However, for new applications we recommend setting it to true
The RedirectAttributes
interface can also be used to add flash attributes. Unlike
other redirect attributes, which end up in the target redirect URL, flash attributes are
saved in the HTTP session (and hence do not appear in the URL). The model of the
controller serving the target redirect URL automatically receives these flash attributes
after which they are removed from the session. See Section 16.6, “Using flash attributes” for an
overview of the general support for flash attributes in Spring MVC.
The previous sections covered use of @ModelAttribute
to support form submission
requests from browser clients. The same annotation is recommended for use with requests
from non-browser clients as well. However there is one notable difference when it comes
to working with HTTP PUT requests. Browsers can submit form data via HTTP GET or HTTP
POST. Non-browser clients can also submit forms via HTTP PUT. This presents a challenge
because the Servlet specification requires the ServletRequest.getParameter*()
family
of methods to support form field access only for HTTP POST, not for HTTP PUT.
To support HTTP PUT and PATCH requests, the spring-web
module provides the filter
HttpPutFormContentFilter
, which can be configured in web.xml
:
<filter> <filter-name>httpPutFormFilter</filter-name> <filter-class>org.springframework.web.filter.HttpPutFormContentFilter</filter-class> </filter> <filter-mapping> <filter-name>httpPutFormFilter</filter-name> <servlet-name>dispatcherServlet</servlet-name> </filter-mapping> <servlet> <servlet-name>dispatcherServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> </servlet>
The above filter intercepts HTTP PUT and PATCH requests with content type
application/x-www-form-urlencoded
, reads the form data from the body of the request,
and wraps the ServletRequest
in order to make the form data available through the
ServletRequest.getParameter*()
family of methods.
Note | |
---|---|
As |
The @CookieValue
annotation allows a method parameter to be bound to the value of an
HTTP cookie.
Let us consider that the following cookie has been received with an http request:
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
The following code sample demonstrates how to get the value of the JSESSIONID
cookie:
@RequestMapping("/displayHeaderInfo.do") public void displayHeaderInfo(@CookieValue("JSESSIONID") String cookie) { //... }
Type conversion is applied automatically if the target method parameter type is not
String
. See the section called “Method Parameters And Type Conversion”.
This annotation is supported for annotated handler methods in Servlet and Portlet environments.
The @RequestHeader
annotation allows a method parameter to be bound to a request header.
Here is a sample request header:
Host localhost:8080 Accept text/html,application/xhtml+xml,application/xml;q=0.9 Accept-Language fr,en-gb;q=0.7,en;q=0.3 Accept-Encoding gzip,deflate Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive 300
The following code sample demonstrates how to get the value of the Accept-Encoding
and
Keep-Alive
headers:
@RequestMapping("/displayHeaderInfo.do") public void displayHeaderInfo(@RequestHeader("Accept-Encoding") String encoding, @RequestHeader("Keep-Alive") long keepAlive) { //... }
Type conversion is applied automatically if the method parameter is not String
. See
the section called “Method Parameters And Type Conversion”.
Tip | |
---|---|
Built-in support is available for converting a comma-separated string into an
array/collection of strings or other types known to the type conversion system. For
example a method parameter annotated with |
This annotation is supported for annotated handler methods in Servlet and Portlet environments.
String-based values extracted from the request including request parameters, path
variables, request headers, and cookie values may need to be converted to the target
type of the method parameter or field (e.g., binding a request parameter to a field in
an @ModelAttribute
parameter) they’re bound to. If the target type is not String
,
Spring automatically converts to the appropriate type. All simple types such as int,
long, Date, etc. are supported. You can further customize the conversion process through
a WebDataBinder
(see the section called “Customizing WebDataBinder initialization”) or by registering Formatters
with
the FormattingConversionService
(see Section 6.6, “Spring Field Formatting”).
To customize request parameter binding with PropertyEditors through Spring’s
WebDataBinder
, you can use @InitBinder
-annotated methods within your controller,
@InitBinder
methods within an @ControllerAdvice
class, or provide a custom
WebBindingInitializer
. See the the section called “Assisting Controllers with the @ControllerAdvice annotation” section for more details.
Annotating controller methods with @InitBinder
allows you to configure web data
binding directly within your controller class. @InitBinder
identifies methods that
initialize the WebDataBinder
that will be used to populate command and form object
arguments of annotated handler methods.
Such init-binder methods support all arguments that @RequestMapping
supports, except
for command/form objects and corresponding validation result objects. Init-binder
methods must not have a return value. Thus, they are usually declared as void
. Typical
arguments include WebDataBinder
in combination with WebRequest
or
java.util.Locale
, allowing code to register context-specific editors.
The following example demonstrates the use of @InitBinder
to configure a
CustomDateEditor
for all java.util.Date
form properties.
@Controller public class MyFormController { @InitBinder public void initBinder(WebDataBinder binder) { SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd"); dateFormat.setLenient(false); binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false)); } // ... }
To externalize data binding initialization, you can provide a custom implementation of
the WebBindingInitializer
interface, which you then enable by supplying a custom bean
configuration for an AnnotationMethodHandlerAdapter
, thus overriding the default
configuration.
The following example from the PetClinic application shows a configuration using a
custom implementation of the WebBindingInitializer
interface,
org.springframework.samples.petclinic.web.ClinicBindingInitializer
, which configures
PropertyEditors required by several of the PetClinic controllers.
<bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"> <property name="cacheSeconds" value="0" /> <property name="webBindingInitializer"> <bean class="org.springframework.samples.petclinic.web.ClinicBindingInitializer" /> </property> </bean>
@InitBinder
methods can also be defined in an @ControllerAdvice
-annotated class in
which case they apply to matching controllers. This provides an alternative to using a
WebBindingInitializer
. See the the section called “Assisting Controllers with the @ControllerAdvice annotation” section for more details.
An @RequestMapping
method may wish to support 'Last-Modified'
HTTP requests, as
defined in the contract for the Servlet API’s getLastModified
method, to facilitate
content caching. This involves calculating a lastModified long
value for a given
request, comparing it against the 'If-Modified-Since'
request header value, and
potentially returning a response with status code 304 (Not Modified). An annotated
controller method can achieve that as follows:
@RequestMapping public String myHandleMethod(WebRequest webRequest, Model model) { long lastModified = // 1. application-specific calculation if (request.checkNotModified(lastModified)) { // 2. shortcut exit - no further processing necessary return null; } // 3. or otherwise further request processing, actually preparing content model.addAttribute(...); return "myViewName"; }
There are two key elements to note: calling request.checkNotModified(lastModified)
and
returning null
. The former sets the response status to 304 before it returns true
.
The latter, in combination with the former, causes Spring MVC to do no further
processing of the request.
The @ControllerAdvice
annotation is a component annotation allowing implementation
classes to be autodetected through classpath scanning. It is automatically enabled when
using the MVC namespace or the MVC Java config.
Classes annotated with @ControllerAdvice
can contain @ExceptionHandler
,
@InitBinder
, and @ModelAttribute
annotated methods and those will apply to
@RequestMapping
methods across controller hierarchies as opposed to the controller
hierarchy within which they are declared.
The @ControllerAdvice
annotation can also target a subset of controllers with its
attributes:
// Target all Controllers annotated with @RestController @ControllerAdvice(annotations = RestController.class) public class AnnotationAdvice {} // Target all Controllers within specific packages @ControllerAdvice("org.example.controllers") public class BasePackageAdvice {} // Target all Controllers assignable to specific classes @ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class}) public class AssignableTypesAdvice {}
Check out the
@ControllerAdvice
documentation for more details.
Spring MVC 3.2 introduced Servlet 3 based asynchronous request processing. Instead of
returning a value, as usual, a controller method can now return a
java.util.concurrent.Callable
and produce the return value from a separate thread.
Meanwhile the main Servlet container thread is released and allowed to process other
requests. Spring MVC invokes the Callable
in a separate thread with the help of a
TaskExecutor
and when the Callable
returns, the request is dispatched back to the
Servlet container to resume processing with the value returned by the Callable
. Here
is an example controller method:
@RequestMapping(method=RequestMethod.POST) public Callable<String> processUpload(final MultipartFile file) { return new Callable<String>() { public String call() throws Exception { // ... return "someView"; } }; }
A second option is for the controller to return an instance of DeferredResult
. In this
case the return value will also be produced from a separate thread. However, that thread
is not known to Spring MVC. For example the result may be produced in response to some
external event such as a JMS message, a scheduled task, etc. Here is an example
controller method:
@RequestMapping("/quotes") @ResponseBody public DeferredResult<String> quotes() { DeferredResult<String> deferredResult = new DeferredResult<String>(); // Save the deferredResult in in-memory queue ... return deferredResult; } // In some other thread... deferredResult.setResult(data);
This may be difficult to understand without any knowledge of the Servlet 3 async processing feature. It would certainly help to read up on it. At a very minimum consider the following basic facts:
ServletRequest
can be put in asynchronous mode by calling request.startAsync()
.
The main effect of doing so is that the Servlet, as well as any Filters, can exit but
the response will remain open allowing some other thread to complete processing.
request.startAsync()
returns an AsyncContext
, which can be used for
further control over async processing. For example it provides the method dispatch
,
which can be called from an application thread in order to "dispatch" the request back
to the Servlet container. An async dispatch is similar to a forward except it is made
from one (application) thread to another (Servlet container) thread whereas a forward
occurs synchronously in the same (Servlet container) thread.
ServletRequest
provides access to the current DispatcherType
, which can be used to
distinguish if a Servlet
or a Filter
is processing on the initial request
processing thread and when it is processing in an async dispatch.
With the above in mind, the following is the sequence of events for async request
processing with a Callable
: (1) Controller returns a Callable
, (2) Spring MVC starts
async processing and submits the Callable
to a TaskExecutor
for processing in a
separate thread, (3) the DispatcherServlet
and all Filter’s exit the request
processing thread but the response remains open, (4) the Callable
produces a result
and Spring MVC dispatches the request back to the Servlet container, (5) the
DispatcherServlet
is invoked again and processing resumes with the asynchronously
produced result from the Callable
. The exact sequencing of (2), (3), and (4) may vary
depending on the speed of execution of the concurrent threads.
The sequence of events for async request processing with a DeferredResult
is the same
in principal except it’s up to the application to produce the asynchronous result from
some thread: (1) Controller returns a DeferredResult
and saves it in some in-memory
queue or list where it can be accessed, (2) Spring MVC starts async processing, (3) the
DispatcherServlet
and all configured Filter’s exit the request processing thread but
the response remains open, (4) the application sets the DeferredResult
from some
thread and Spring MVC dispatches the request back to the Servlet container, (5) the
DispatcherServlet
is invoked again and processing resumes with the asynchronously
produced result.
Explaining the motivation for async request processing and when or why to use it are beyond the scope of this document. For further information you may wish to read this blog post series.
What happens if a Callable
returned from a controller method raises an Exception while
being executed? The effect is similar to what happens when any controller method raises
an exception. It is handled by a matching @ExceptionHandler
method in the same
controller or by one of the configured HandlerExceptionResolver
instances.
Note | |
---|---|
Under the covers, when a |
When using a DeferredResult
, you have a choice of calling its setErrorResult(Object)
method and provide an Exception
or any other Object you’d like to use as the result.
If the result is an Exception
, it will be processed with a matching
@ExceptionHandler
method in the same controller or with any configured
HandlerExceptionResolver
instance.
An existing HandlerInterceptor
can implement AsyncHandlerInterceptor
, which provides
one additional method afterConcurrentHandlingStarted
. It is invoked after async
processing starts and when the initial request processing thread is being exited. See
the Javadoc of AsyncHandlerInterceptor
for more details on that.
Further options for async request lifecycle callbacks are provided directly on
DeferredResult
, which has the methods onTimeout(Runnable)
and
onCompletion(Runnable)
. Those are called when the async request is about to time out
or has completed respectively. The timeout event can be handled by setting the
DeferredResult
to some value. The completion callback however is final and the result
can no longer be set.
Similar callbacks are also available with a Callable
. However, you will need to wrap
the Callable
in an instance of WebAsyncTask
and then use that to register the
timeout and completion callbacks. Just like with DeferredResult
, the timeout event can
be handled and a value can be returned while the completion event is final.
You can also register a CallableProcessingInterceptor
or a
DeferredResultProcessingInterceptor
globally through the MVC Java config or the MVC
namespace. Those interceptors provide a full set of callbacks and apply every time a
Callable
or a DeferredResult
is used.
To use Servlet 3 async request processing, you need to update web.xml
to version 3.0:
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0"> ... </web-app>
The DispatcherServlet
and any Filter
configuration need to have the
<async-supported>true</async-supported>
sub-element. Additionally, any Filter
that
also needs to get involved in async dispatches should also be configured to support the
ASYNC dispatcher type. Note that it is safe to enable the ASYNC dispatcher type for all
filters provided with the Spring Framework since they will not get involved in async
dispatches unless needed.
If using Servlet 3, Java based configuration, e.g. via WebApplicationInitializer
,
you’ll also need to set the "asyncSupported" flag as well as the ASYNC dispatcher type
just like with web.xml
. To simplify all this configuration, consider extending
AbstractDispatcherServletInitializer
or
AbstractAnnotationConfigDispatcherServletInitializer
, which automatically set those
options and make it very easy to register Filter
instances.
The MVC Java config and the MVC namespace both provide options for configuring async
request processing. WebMvcConfigurer
has the method configureAsyncSupport
while
<mvc:annotation-driven> has an <async-support> sub-element.
Those allow you to configure the default timeout value to use for async requests, which
if not set depends on the underlying Servlet container (e.g. 10 seconds on Tomcat). You
can also configure an AsyncTaskExecutor
to use for executing Callable
instances
returned from controller methods. It is highly recommended to configure this property
since by default Spring MVC uses SimpleAsyncTaskExecutor
. The MVC Java config and the
MVC namespace also allow you to register CallableProcessingInterceptor
and
DeferredResultProcessingInterceptor
instances.
If you need to override the default timeout value for a specific DeferredResult
, you
can do so by using the appropriate class constructor. Similarly, for a Callable
, you
can wrap it in a WebAsyncTask
and use the appropriate class constructor to customize
the timeout value. The class constructor of WebAsyncTask
also allows providing an
AsyncTaskExecutor
.
The spring-test
module offers first class support for testing annotated controllers.
See Section 10.3.6, “Spring MVC Test Framework”.
In previous versions of Spring, users were required to define one or more
HandlerMapping
beans in the web application context to map incoming web requests to
appropriate handlers. With the introduction of annotated controllers, you generally
don’t need to do that because the RequestMappingHandlerMapping
automatically looks for
@RequestMapping
annotations on all @Controller
beans. However, do keep in mind that
all HandlerMapping
classes extending from AbstractHandlerMapping
have the following
properties that you can use to customize their behavior:
interceptors
List of interceptors to use. HandlerInterceptor
s are discussed in
Section 16.4.1, “Intercepting requests with a HandlerInterceptor”.
defaultHandler
Default handler to use, when this handler mapping does not result in
a matching handler.
order
Based on the value of the order property (see the
org.springframework.core.Ordered
interface), Spring sorts all handler mappings
available in the context and applies the first matching handler.
alwaysUseFullPath
If true
, Spring uses the full path within the current Servlet
context to find an appropriate handler. If false
(the default), the path within the
current Servlet mapping is used. For example, if a Servlet is mapped using
/testing/*
and the alwaysUseFullPath
property is set to true,
/testing/viewPage.html
is used, whereas if the property is set to false,
/viewPage.html
is used.
urlDecode
Defaults to true
, as of Spring 2.5. If you prefer to compare encoded
paths, set this flag to false
. However, the HttpServletRequest
always exposes the
Servlet path in decoded form. Be aware that the Servlet path will not match when
compared with encoded paths.
The following example shows how to configure an interceptor:
<beans> <bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"> <property name="interceptors"> <bean class="example.MyInterceptor"/> </property> </bean> <beans>
Spring’s handler mapping mechanism includes handler interceptors, which are useful when you want to apply specific functionality to certain requests, for example, checking for a principal.
Interceptors located in the handler mapping must implement HandlerInterceptor
from the
org.springframework.web.servlet
package. This interface defines three methods:
preHandle(..)
is called before the actual handler is executed; postHandle(..)
is
called after the handler is executed; and afterCompletion(..)
is called after
the complete request has finished. These three methods should provide enough
flexibility to do all kinds of preprocessing and postprocessing.
The preHandle(..)
method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns true
, the
handler execution chain will continue; when it returns false, the DispatcherServlet
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.
Interceptors can be configured using the interceptors
property, which is present on
all HandlerMapping
classes extending from AbstractHandlerMapping
. This is shown in
the example below:
<beans> <bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"> <property name="interceptors"> <list> <ref bean="officeHoursInterceptor"/> </list> </property> </bean> <bean id="officeHoursInterceptor" class="samples.TimeBasedAccessInterceptor"> <property name="openingTime" value="9"/> <property name="closingTime" value="18"/> </bean> <beans>
package samples; public class TimeBasedAccessInterceptor extends HandlerInterceptorAdapter { private int openingTime; private int closingTime; public void setOpeningTime(int openingTime) { this.openingTime = openingTime; } public void setClosingTime(int closingTime) { this.closingTime = closingTime; } public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { Calendar cal = Calendar.getInstance(); int hour = cal.get(HOUR_OF_DAY); if (openingTime <= hour && hour < closingTime) { return true; } response.sendRedirect("http://host.com/outsideOfficeHours.html"); return false; } }
Any request handled by this mapping is intercepted by the TimeBasedAccessInterceptor
.
If the current time is outside office hours, the user is redirected to a static HTML
file that says, for example, you can only access the website during office hours.
Note | |
---|---|
When using the |
As you can see, the Spring adapter class HandlerInterceptorAdapter
makes it easier to
extend the HandlerInterceptor
interface.
Tip | |
---|---|
In the example above, the configured interceptor will apply to all requests handled with
annotated controller methods. If you want to narrow down the URL paths to which an
interceptor applies, you can use the MVC namespace or the MVC Java config, or declare
bean instances of type |
All MVC frameworks for web applications provide a way to address views. Spring provides view resolvers, which enable you to render models in a browser without tying you to a specific view technology. Out of the box, Spring enables you to use JSPs, Velocity templates and XSLT views, for example. See Chapter 17, View technologies for a discussion of how to integrate and use a number of disparate view technologies.
The two interfaces that are important to the way Spring handles views are ViewResolver
and View
. The ViewResolver
provides a mapping between view names and actual views.
The View
interface addresses the preparation of the request and hands the request over
to one of the view technologies.
As discussed in Section 16.3, “Implementing Controllers”, all handler methods in the Spring Web MVC
controllers must resolve to a logical view name, either explicitly (e.g., by returning a
String
, View
, or ModelAndView
) or implicitly (i.e., based on conventions). Views
in Spring are addressed by a logical view name and are resolved by a view resolver.
Spring comes with quite a few view resolvers. This table lists most of them; a couple of
examples follow.
Table 16.3. View resolvers
ViewResolver | Description |
---|---|
| Abstract view resolver that caches views. Often views need preparation before they can be used; extending this view resolver provides caching. |
| Implementation of |
| Implementation of |
| Simple implementation of the |
| Convenient subclass of |
| Convenient subclass of |
| Implementation of the |
As an example, with JSP as a view technology, you can use the UrlBasedViewResolver
.
This view resolver translates a view name to a URL and hands the request over to the
RequestDispatcher to render the view.
<bean id="viewResolver" class="org.springframework.web.servlet.view.UrlBasedViewResolver"> <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/> <property name="prefix" value="/WEB-INF/jsp/"/> <property name="suffix" value=".jsp"/> </bean>
When returning test
as a logical view name, this view resolver forwards the request to
the RequestDispatcher
that will send the request to /WEB-INF/jsp/test.jsp
.
When you combine different view technologies in a web application, you can use the
ResourceBundleViewResolver
:
<bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver"> <property name="basename" value="views"/> <property name="defaultParentView" value="parentView"/> </bean>
The ResourceBundleViewResolver
inspects the ResourceBundle
identified by the
basename, and for each view it is supposed to resolve, it uses the value of the property
[viewname].(class)
as the view class and the value of the property [viewname].url
as
the view url. Examples can be found in the next chapter which covers view technologies.
As you can see, you can identify a parent view, from which all views in the properties
file "extend". This way you can specify a default view class, for example.
Note | |
---|---|
Subclasses of |
Spring supports multiple view resolvers. Thus you can chain resolvers and, for example,
override specific views in certain circumstances. You chain view resolvers by adding
more than one resolver to your application context and, if necessary, by setting the
order
property to specify ordering. Remember, the higher the order property, the later
the view resolver is positioned in the chain.
In the following example, the chain of view resolvers consists of two resolvers, an
InternalResourceViewResolver
, which is always automatically positioned as the last
resolver in the chain, and an XmlViewResolver
for specifying Excel views. Excel views
are not supported by the InternalResourceViewResolver
.
<bean id="jspViewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver"> <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/> <property name="prefix" value="/WEB-INF/jsp/"/> <property name="suffix" value=".jsp"/> </bean> <bean id="excelViewResolver" class="org.springframework.web.servlet.view.XmlViewResolver"> <property name="order" value="1"/> <property name="location" value="/WEB-INF/views.xml"/> </bean> <!-- in views.xml --> <beans> <bean name="report" class="org.springframework.example.ReportExcelView"/> </beans>
If a specific view resolver does not result in a view, Spring examines the context for
other view resolvers. If additional view resolvers exist, Spring continues to inspect
them until a view is resolved. If no view resolver returns a view, Spring throws a
ServletException
.
The contract of a view resolver specifies that a view resolver can return null to
indicate the view could not be found. Not all view resolvers do this, however, because
in some cases, the resolver simply cannot detect whether or not the view exists. For
example, the InternalResourceViewResolver
uses the RequestDispatcher
internally, and
dispatching is the only way to figure out if a JSP exists, but this action can only
execute once. The same holds for the VelocityViewResolver
and some others. Check the
Javadoc for the view resolver to see whether it reports non-existing views. Thus,
putting an InternalResourceViewResolver
in the chain in a place other than the last,
results in the chain not being fully inspected, because the
InternalResourceViewResolver
will always return a view!
As mentioned previously, a controller typically returns a logical view name, which a
view resolver resolves to a particular view technology. For view technologies such as
JSPs that are processed through the Servlet or JSP engine, this resolution is usually
handled through the combination of InternalResourceViewResolver
and
InternalResourceView
, which issues an internal forward or include via the Servlet
API’s RequestDispatcher.forward(..)
method or RequestDispatcher.include()
method.
For other view technologies, such as Velocity, XSLT, and so on, the view itself writes
the content directly to the response stream.
It is sometimes desirable to issue an HTTP redirect back to the client, before the view
is rendered. This is desirable, for example, when one controller has been called with
POST
data, and the response is actually a delegation to another controller (for
example on a successful form submission). In this case, a normal internal forward will
mean that the other controller will also see the same POST
data, which is potentially
problematic if it can confuse it with other expected data. Another reason to perform a
redirect before displaying the result is to eliminate the possibility of the user
submitting the form data multiple times. In this scenario, the browser will first send
an initial POST
; it will then receive a response to redirect to a different URL; and
finally the browser will perform a subsequent GET
for the URL named in the redirect
response. Thus, from the perspective of the browser, the current page does not reflect
the result of a POST
but rather of a GET
. The end effect is that there is no way the
user can accidentally re- POST
the same data by performing a refresh. The refresh
forces a GET
of the result page, not a resend of the initial POST
data.
One way to force a redirect as the result of a controller response is for the controller
to create and return an instance of Spring’s RedirectView
. In this case,
DispatcherServlet
does not use the normal view resolution mechanism. Rather because it
has been given the (redirect) view already, the DispatcherServlet
simply instructs the
view to do its work.
The RedirectView
issues an HttpServletResponse.sendRedirect()
call that returns to
the client browser as an HTTP redirect. By default all model attributes are considered
to be exposed as URI template variables in the redirect URL. Of the remaining attributes
those that are primitive types or collections/arrays of primitive types are
automatically appended as query parameters.
Appending primitive type attributes as query parameters may be the desired result if a
model instance was prepared specifically for the redirect. However, in annotated
controllers the model may contain additional attributes added for rendering purposes
(e.g. drop-down field values). To avoid the possibility of having such attributes appear
in the URL an annotated controller can declare an argument of type RedirectAttributes
and use it to specify the exact attributes to make available to RedirectView
. If the
controller method decides to redirect, the content of RedirectAttributes
is used.
Otherwise the content of the model is used.
Note that URI template variables from the present request are automatically made
available when expanding a redirect URL and do not need to be added explicitly neither
through Model
nor RedirectAttributes
. For example:
@RequestMapping(value = "/files/{path}", method = RequestMethod.POST) public String upload(...) { // ... return "redirect:files/{path}"; }
If you use RedirectView
and the view is created by the controller itself, it is
recommended that you configure the redirect URL to be injected into the controller so
that it is not baked into the controller but configured in the context along with the
view names. The next section discusses this process.
While the use of RedirectView
works fine, if the controller itself creates the
RedirectView
, there is no avoiding the fact that the controller is aware that a
redirection is happening. This is really suboptimal and couples things too tightly. The
controller should not really care about how the response gets handled. In general it
should operate only in terms of view names that have been injected into it.
The special redirect:
prefix allows you to accomplish this. If a view name is returned
that has the prefix redirect:
, the UrlBasedViewResolver
(and all subclasses) will
recognize this as a special indication that a redirect is needed. The rest of the view
name will be treated as the redirect URL.
The net effect is the same as if the controller had returned a RedirectView
, but now
the controller itself can simply operate in terms of logical view names. A logical view
name such as redirect:/myapp/some/resource
will redirect relative to the current
Servlet context, while a name such as redirect:http://myhost.com/some/arbitrary/path
will redirect to an absolute URL.
It is also possible to use a special forward:
prefix for view names that are
ultimately resolved by UrlBasedViewResolver
and subclasses. This creates an
InternalResourceView
(which ultimately does a RequestDispatcher.forward()
) around
the rest of the view name, which is considered a URL. Therefore, this prefix is not
useful with InternalResourceViewResolver
and InternalResourceView
(for JSPs for
example). But the prefix can be helpful when you are primarily using another view
technology, but still want to force a forward of a resource to be handled by the
Servlet/JSP engine. (Note that you may also chain multiple view resolvers, instead.)
As with the redirect:
prefix, if the view name with the forward:
prefix is injected
into the controller, the controller does not detect that anything special is happening
in terms of handling the response.
The ContentNegotiatingViewResolver
does not resolve views itself but rather delegates
to other view resolvers, selecting the view that resembles the representation requested
by the client. Two strategies exist for a client to request a representation from the
server:
http://www.example.com/users/fred.pdf
requests a PDF
representation of the user fred, and http://www.example.com/users/fred.xml
requests
an XML representation.
Accept
HTTP
request header to list the media
types that it understands. For example, an HTTP request for
http://www.example.com/users/fred
with an Accept
header set to application/pdf
requests a PDF representation of the user fred, while
http://www.example.com/users/fred
with an Accept
header set to text/xml
requests
an XML representation. This strategy is known as
content negotiation.
Note | |
---|---|
One issue with the Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
For this reason it is common to see the use of a distinct URI for each representation when developing browser based web applications. |
To support multiple representations of a resource, Spring provides the
ContentNegotiatingViewResolver
to resolve a view based on the file extension or
Accept
header of the HTTP request. ContentNegotiatingViewResolver
does not perform
the view resolution itself but instead delegates to a list of view resolvers that you
specify through the bean property ViewResolvers
.
The ContentNegotiatingViewResolver
selects an appropriate View
to handle the request
by comparing the request media type(s) with the media type (also known as
Content-Type
) supported by the View
associated with each of its ViewResolvers
. The
first View
in the list that has a compatible Content-Type
returns the representation
to the client. If a compatible view cannot be supplied by the ViewResolver
chain, then
the list of views specified through the DefaultViews
property will be consulted. This
latter option is appropriate for singleton Views
that can render an appropriate
representation of the current resource regardless of the logical view name. The Accept
header may include wild cards, for example text/*
, in which case a View
whose
Content-Type was text/xml
is a compatible match.
To support the resolution of a view based on a file extension, use the
ContentNegotiatingViewResolver
bean property mediaTypes
to specify a mapping of file
extensions to media types. For more information on the algorithm used to determine the
request media type, refer to the API documentation for ContentNegotiatingViewResolver
.
Here is an example configuration of a ContentNegotiatingViewResolver:
<bean class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver"> <property name="mediaTypes"> <map> <entry key="atom" value="application/atom+xml"/> <entry key="html" value="text/html"/> <entry key="json" value="application/json"/> </map> </property> <property name="viewResolvers"> <list> <bean class="org.springframework.web.servlet.view.BeanNameViewResolver"/> <bean class="org.springframework.web.servlet.view.InternalResourceViewResolver"> <property name="prefix" value="/WEB-INF/jsp/"/> <property name="suffix" value=".jsp"/> </bean> </list> </property> <property name="defaultViews"> <list> <bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView" /> </list> </property> </bean> <bean id="content" class="com.springsource.samples.rest.SampleContentAtomView"/>
The InternalResourceViewResolver
handles the translation of view names and JSP pages,
while the BeanNameViewResolver
returns a view based on the name of a bean. (See
"Resolving views with the ViewResolver interface" for more
details on how Spring looks up and instantiates a view.) In this example, the content
bean is a class that inherits from AbstractAtomFeedView
, which returns an Atom RSS
feed. For more information on creating an Atom Feed representation, see the section Atom
Views.
In the above configuration, if a request is made with an .html
extension, the view
resolver looks for a view that matches the text/html
media type. The
InternalResourceViewResolver
provides the matching view for text/html
. If the
request is made with the file extension .atom
, the view resolver looks for a view that
matches the application/atom+xml
media type. This view is provided by the
BeanNameViewResolver
that maps to the SampleContentAtomView
if the view name
returned is content
. If the request is made with the file extension .json
, the
MappingJackson2JsonView
instance from the DefaultViews
list will be selected
regardless of the view name. Alternatively, client requests can be made without a file
extension but with the Accept
header set to the preferred media-type, and the same
resolution of request to views would occur.
Note | |
---|---|
If |
The corresponding controller code that returns an Atom RSS feed for a URI of the form
http://localhost/content.atom
or http://localhost/content
with an Accept
header of
application/atom+xml is shown below.
@Controller public class ContentController { private List<SampleContent> contentList = new ArrayList<SampleContent>(); @RequestMapping(value="/content", method=RequestMethod.GET) public ModelAndView getContent() { ModelAndView mav = new ModelAndView(); mav.setViewName("content"); mav.addObject("sampleContentList", contentList); return mav; } }
Flash attributes provide a way for one request to store attributes intended for use in another. This is most commonly needed when redirecting — for example, the Post/Redirect/Get pattern. Flash attributes are saved temporarily before the redirect (typically in the session) to be made available to the request after the redirect and removed immediately.
Spring MVC has two main abstractions in support of flash attributes. FlashMap
is used
to hold flash attributes while FlashMapManager
is used to store, retrieve, and manage
FlashMap
instances.
Flash attribute support is always "on" and does not need to enabled explicitly although
if not used, it never causes HTTP session creation. On each request there is an "input"
FlashMap
with attributes passed from a previous request (if any) and an "output"
FlashMap
with attributes to save for a subsequent request. Both FlashMap
instances
are accessible from anywhere in Spring MVC through static methods in
RequestContextUtils
.
Annotated controllers typically do not need to work with FlashMap
directly. Instead an
@RequestMapping
method can accept an argument of type RedirectAttributes
and use it
to add flash attributes for a redirect scenario. Flash attributes added via
RedirectAttributes
are automatically propagated to the "output" FlashMap. Similarly
after the redirect attributes from the "input" FlashMap
are automatically added to the
Model
of the controller serving the target URL.
Spring MVC provides a mechanism for building and encoding a URI using
UriComponentsBuilder
and UriComponents
.
For example you can expand and encode a URI template string:
UriComponents uriComponents = UriComponentsBuilder.fromUriString( "http://example.com/hotels/{hotel}/bookings/{booking}").build(); URI uri = uriComponents.expand("42", "21").encode().toUri();
Note that UriComponents
is immutable and the expand()
and encode()
operations
return new instances if necessary.
You can also expand and encode using individual URI components:
UriComponents uriComponents = UriComponentsBuilder.newInstance() .scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build() .expand("42", "21") .encode();
In a Servlet environment the ServletUriComponentsBuilder
sub-class provides static
factory methods to copy available URL information from a Servlet requests:
HttpServletRequest request = ... // Re-use host, scheme, port, path and query string // Replace the "accountId" query param ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromRequest(request) .replaceQueryParam("accountId", "{id}").build() .expand("123") .encode();
Alternatively, you may choose to copy a subset of the available information up to and including the context path:
// Re-use host, port and context path // Append "/accounts" to the path ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromContextPath(request) .path("/accounts").build()
Or in cases where the DispatcherServlet
is mapped by name (e.g. /main/*
), you can
also have the literal part of the servlet mapping included:
// Re-use host, port, context path // Append the literal part of the servlet mapping to the path // Append "/accounts" to the path ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromServletMapping(request) .path("/accounts").build()
Spring MVC provides another mechanism for building and encoding URIs that link to
Controllers and methods defined within an application.
MvcUriComponentsBuilder
extends UriComponentsBuilder
and provides such possibilities.
Given this Controller:
@Controller @RequestMapping("/hotels/{hotel}") public class BookingController { @RequestMapping("/bookings/{booking}") public String getBooking(@PathVariable Long booking) { // ... }
and using the MvcUriComponentsBuilder
, the previous example is now:
UriComponents uriComponents = MvcUriComponentsBuilder .fromMethodName(BookingController.class, "getBooking",21).buildAndExpand(42); URI uri = uriComponents.encode().toUri();
The MvcUriComponentsBuilder
can also create "mock Controllers", thus enabling to create
URIs by coding against the actual Controller’s API:
UriComponents uriComponents = MvcUriComponentsBuilder .fromMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42); URI uri = uriComponents.encode().toUri();
Most parts of Spring’s architecture support internationalization, just as the Spring web
MVC framework does. DispatcherServlet
enables you to automatically resolve messages
using the client’s locale. This is done with LocaleResolver
objects.
When a request comes in, the DispatcherServlet
looks for a locale resolver, and if it
finds one it tries to use it to set the locale. Using the RequestContext.getLocale()
method, you can always retrieve the locale that was resolved by the locale resolver.
In addition to automatic locale resolution, you can also attach an interceptor to the handler mapping (see Section 16.4.1, “Intercepting requests with a HandlerInterceptor” for more information on handler mapping interceptors) to change the locale under specific circumstances, for example, based on a parameter in the request.
Locale resolvers and interceptors are defined in the
org.springframework.web.servlet.i18n
package and are configured in your application
context in the normal way. Here is a selection of the locale resolvers included in
Spring.
In addition to obtaining the client’s locale, it is often useful to know their time zone.
The LocaleContextResolver
interface offers an extension to LocaleResolver
that allows
resolvers to provide a richer LocaleContext
, which may include time zone information.
When available, the user’s TimeZone
can be obtained using the
RequestContext.getTimeZone()
method. Time zone information will automatically be used
by Date/Time Converter
and Formatter
objects registered with Spring’s
ConversionService
.
This locale resolver inspects the accept-language
header in the request that was sent
by the client (e.g., a web browser). Usually this header field contains the locale of
the client’s operating system. Note that this resolver does not support time zone
information.
This locale resolver inspects a Cookie
that might exist on the client to see if a
Locale
or TimeZone
is specified. If so, it uses the specified details. Using the
properties of this locale resolver, you can specify the name of the cookie as well as the
maximum age. Find below an example of defining a CookieLocaleResolver
.
<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver"> <property name="cookieName" value="clientlanguage"/> <!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) --> <property name="cookieMaxAge" value="100000"> </bean>
Table 16.4. CookieLocaleResolver properties
Property | Default | Description |
---|---|---|
cookieName | classname + LOCALE | The name of the cookie |
cookieMaxAge | Integer.MAX_INT | The maximum time a cookie will stay persistent on the client. If -1 is specified, the cookie will not be persisted; it will only be available until the client shuts down their browser. |
cookiePath | / | Limits the visibility of the cookie to a certain part of your site. When cookiePath is specified, the cookie will only be visible to that path and the paths below it. |
The SessionLocaleResolver
allows you to retrieve Locale
and TimeZone
from the
session that might be associated with the user’s request.
You can enable changing of locales by adding the LocaleChangeInterceptor
to one of the
handler mappings (see Section 16.4, “Handler mappings”). It will detect a parameter in the request
and change the locale. It calls setLocale()
on the LocaleResolver
that also exists
in the context. The following example shows that calls to all *.view
resources
containing a parameter named siteLanguage
will now change the locale. So, for example,
a request for the following URL, http://www.sf.net/home.view?siteLanguage=nl
will
change the site language to Dutch.
<bean id="localeChangeInterceptor" class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor"> <property name="paramName" value="siteLanguage"/> </bean> <bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/> <bean id="urlMapping" class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping"> <property name="interceptors"> <list> <ref bean="localeChangeInterceptor"/> </list> </property> <property name="mappings"> <value>/*/.view=someController</value> </property> </bean>
You can apply Spring Web MVC framework themes to set the overall look-and-feel of your application, thereby enhancing user experience. A theme is a collection of static resources, typically style sheets and images, that affect the visual style of the application.
To use themes in your web application, you must set up an implementation of the
org.springframework.ui.context.ThemeSource
interface. The WebApplicationContext
interface extends ThemeSource
but delegates its responsibilities to a dedicated
implementation. By default the delegate will be an
org.springframework.ui.context.support.ResourceBundleThemeSource
implementation that
loads properties files from the root of the classpath. To use a custom ThemeSource
implementation or to configure the base name prefix of the ResourceBundleThemeSource
,
you can register a bean in the application context with the reserved name themeSource
.
The web application context automatically detects a bean with that name and uses it.
When using the ResourceBundleThemeSource
, a theme is defined in a simple properties
file. The properties file lists the resources that make up the theme. Here is an example:
styleSheet=/themes/cool/style.css background=/themes/cool/img/coolBg.jpg
The keys of the properties are the names that refer to the themed elements from view
code. For a JSP, you typically do this using the spring:theme
custom tag, which is
very similar to the spring:message
tag. The following JSP fragment uses the theme
defined in the previous example to customize the look and feel:
<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%> <html> <head> <link rel="stylesheet" href="<spring:theme code=styleSheet/>" type="text/css"/> </head> <body style="background=<spring:theme code=background/>"> ... </body> </html>
By default, the ResourceBundleThemeSource
uses an empty base name prefix. As a result,
the properties files are loaded from the root of the classpath. Thus you would put the
cool.properties
theme definition in a directory at the root of the classpath, for
example, in /WEB-INF/classes
. The ResourceBundleThemeSource
uses the standard Java
resource bundle loading mechanism, allowing for full internationalization of themes. For
example, we could have a /WEB-INF/classes/cool_nl.properties
that references a special
background image with Dutch text on it.
After you define themes, as in the preceding section, you decide which theme to use. The
DispatcherServlet
will look for a bean named themeResolver
to find out which
ThemeResolver
implementation to use. A theme resolver works in much the same way as a
LocaleResolver
. It detects the theme to use for a particular request and can also
alter the request’s theme. The following theme resolvers are provided by Spring:
Table 16.5. ThemeResolver implementations
Class | Description |
---|---|
| Selects a fixed theme, set using the |
| The theme is maintained in the user’s HTTP session. It only needs to be set once for each session, but is not persisted between sessions. |
| The selected theme is stored in a cookie on the client. |
Spring also provides a ThemeChangeInterceptor
that allows theme changes on every
request with a simple request parameter.
Spring’s built-in multipart support handles file uploads in web applications. You enable
this multipart support with pluggable MultipartResolver
objects, defined in the
org.springframework.web.multipart
package. Spring provides one MultipartResolver
implementation for use with Commons
FileUpload and another for use with Servlet 3.0 multipart request parsing.
By default, Spring does no multipart handling, because some developers want to handle
multiparts themselves. You enable Spring multipart handling by adding a multipart
resolver to the web application’s context. Each request is inspected to see if it
contains a multipart. If no multipart is found, the request continues as expected. If a
multipart is found in the request, the MultipartResolver
that has been declared in
your context is used. After that, the multipart attribute in your request is treated
like any other attribute.
The following example shows how to use the CommonsMultipartResolver
:
<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver"> <!-- one of the properties available; the maximum file size in bytes --> <property name="maxUploadSize" value="100000"/> </bean>
Of course you also need to put the appropriate jars in your classpath for the multipart
resolver to work. In the case of the CommonsMultipartResolver
, you need to use
commons-fileupload.jar
.
When the Spring DispatcherServlet
detects a multi-part request, it activates the
resolver that has been declared in your context and hands over the request. The resolver
then wraps the current HttpServletRequest
into a MultipartHttpServletRequest
that
supports multipart file uploads. Using the MultipartHttpServletRequest
, you can get
information about the multiparts contained by this request and actually get access to
the multipart files themselves in your controllers.
In order to use Servlet 3.0 based multipart parsing, you need to mark the
DispatcherServlet
with a "multipart-config"
section in web.xml
, or with a
javax.servlet.MultipartConfigElement
in programmatic Servlet registration, or in case
of a custom Servlet class possibly with a javax.servlet.annotation.MultipartConfig
annotation on your Servlet class. Configuration settings such as maximum sizes or
storage locations need to be applied at that Servlet registration level as Servlet 3.0
does not allow for those settings to be done from the MultipartResolver.
Once Servlet 3.0 multipart parsing has been enabled in one of the above mentioned ways
you can add the StandardServletMultipartResolver
to your Spring configuration:
<bean id="multipartResolver" class="org.springframework.web.multipart.support.StandardServletMultipartResolver"> </bean>
After the MultipartResolver
completes its job, the request is processed like any
other. First, create a form with a file input that will allow the user to upload a form.
The encoding attribute ( enctype="multipart/form-data"
) lets the browser know how to
encode the form as multipart request:
<html> <head> <title>Upload a file please</title> </head> <body> <h1>Please upload a file</h1> <form method="post" action="/form" enctype="multipart/form-data"> <input type="text" name="name"/> <input type="file" name="file"/> <input type="submit"/> </form> </body> </html>
The next step is to create a controller that handles the file upload. This controller is
very similar to a normal annotated @Controller
, except that we
use MultipartHttpServletRequest
or MultipartFile
in the method parameters:
@Controller public class FileUploadController { @RequestMapping(value = "/form", method = RequestMethod.POST) public String handleFormUpload(@RequestParam("name") String name, @RequestParam("file") MultipartFile file) { if (!file.isEmpty()) { byte[] bytes = file.getBytes(); // store the bytes somewhere return "redirect:uploadSuccess"; } return "redirect:uploadFailure"; } }
Note how the @RequestParam
method parameters map to the input elements declared in the
form. In this example, nothing is done with the byte[]
, but in practice you can save
it in a database, store it on the file system, and so on.
When using Servlet 3.0 multipart parsing you can also use javax.servlet.http.Part
for
the method parameter:
@Controller public class FileUploadController { @RequestMapping(value = "/form", method = RequestMethod.POST) public String handleFormUpload(@RequestParam("name") String name, @RequestParam("file") Part file) { InputStream inputStream = file.getInputStream(); // store bytes from uploaded file somewhere return "redirect:uploadSuccess"; } }
Multipart requests can also be submitted from non-browser clients in a RESTful service scenario. All of the above examples and configuration apply here as well. However, unlike browsers that typically submit files and simple form fields, a programmatic client can also send more complex data of a specific content type — for example a multipart request with a file and second part with JSON formatted data:
POST /someUrl Content-Type: multipart/mixed --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="meta-data" Content-Type: application/json; charset=UTF-8 Content-Transfer-Encoding: 8bit { "name": "value" } --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="file-data"; filename="file.properties" Content-Type: text/xml Content-Transfer-Encoding: 8bit ... File Data ...
You could access the part named "meta-data" with a @RequestParam("meta-data") String
metadata
controller method argument. However, you would probably prefer to accept a
strongly typed object initialized from the JSON formatted data in the body of the
request part, very similar to the way @RequestBody
converts the body of a
non-multipart request to a target object with the help of an HttpMessageConverter
.
You can use the @RequestPart
annotation instead of the @RequestParam
annotation for
this purpose. It allows you to have the content of a specific multipart passed through
an HttpMessageConverter
taking into consideration the 'Content-Type'
header of the
multipart:
@RequestMapping(value="/someUrl", method = RequestMethod.POST) public String onSubmit(@RequestPart("meta-data") MetaData metadata, @RequestPart("file-data") MultipartFile file) { // ... }
Notice how MultipartFile
method arguments can be accessed with @RequestParam
or with
@RequestPart
interchangeably. However, the @RequestPart("meta-data") MetaData
method
argument in this case is read as JSON content based on its 'Content-Type'
header and
converted with the help of the MappingJackson2HttpMessageConverter
.
Spring HandlerExceptionResolver
implementations deal with unexpected exceptions that
occur during controller execution. A HandlerExceptionResolver
somewhat resembles the
exception mappings you can define in the web application descriptor web.xml
. However,
they provide a more flexible way to do so. For example they provide information about
which handler was executing when the exception was thrown. Furthermore, a programmatic
way of handling exceptions gives you more options for responding appropriately before
the request is forwarded to another URL (the same end result as when you use the Servlet
specific exception mappings).
Besides implementing the HandlerExceptionResolver
interface, which is only a matter of
implementing the resolveException(Exception, Handler)
method and returning a
ModelAndView
, you may also use the provided SimpleMappingExceptionResolver
or create
@ExceptionHandler
methods. The SimpleMappingExceptionResolver
enables you to take
the class name of any exception that might be thrown and map it to a view name. This is
functionally equivalent to the exception mapping feature from the Servlet API, but it is
also possible to implement more finely grained mappings of exceptions from different
handlers. The @ExceptionHandler
annotation on the other hand can be used on methods
that should be invoked to handle an exception. Such methods may be defined locally
within an @Controller
or may apply to many @Controller
classes when defined within an
@ControllerAdvice
class. The following sections explain this in more detail.
The HandlerExceptionResolver
interface and the SimpleMappingExceptionResolver
implementations allow you to map Exceptions to specific views declaratively along with
some optional Java logic before forwarding to those views. However, in some cases,
especially when relying on @ResponseBody
methods rather than on view resolution, it
may be more convenient to directly set the status of the response and optionally write
error content to the body of the response.
You can do that with @ExceptionHandler
methods. When declared within a controller such
methods apply to exceptions raised by @RequestMapping
methods of that contoroller (or
any of its sub-classes). You can also declare an @ExceptionHandler
method within an
@ControllerAdvice
class in which case it handles exceptions from @RequestMapping
methods from many controllers. Below is an example of a controller-local
@ExceptionHandler
method:
@Controller public class SimpleController { // @RequestMapping methods omitted ... @ExceptionHandler(IOException.class) public ResponseEntity<String> handleIOException(IOException ex) { // prepare responseEntity return responseEntity; } }
The @ExceptionHandler
value can be set to an array of Exception types. If an exception
is thrown that matches one of the types in the list, then the method annotated with the
matching @ExceptionHandler
will be invoked. If the annotation value is not set then
the exception types listed as method arguments are used.
Much like standard controller methods annotated with a @RequestMapping
annotation, the
method arguments and return values of @ExceptionHandler
methods can be flexible. For
example, the HttpServletRequest
can be accessed in Servlet environments and the
PortletRequest
in Portlet environments. The return type can be a String
, which is
interpreted as a view name, a ModelAndView
object, a ResponseEntity
, or you can also
add the @ResponseBody
to have the method return value converted with message
converters and written to the response stream.
Spring MVC may raise a number of exceptions while processing a request. The
SimpleMappingExceptionResolver
can easily map any exception to a default error view as
needed. However, when working with clients that interpret responses in an automated way
you will want to set specific status code on the response. Depending on the exception
raised the status code may indicate a client error (4xx) or a server error (5xx).
The DefaultHandlerExceptionResolver
translates Spring MVC exceptions to specific error
status codes. It is registered by default with the MVC namespace, the MVC Java config,
and also by the the DispatcherServlet
(i.e. when not using the MVC namespace or Java
config). Listed below are some of the exceptions handled by this resolver and the
corresponding status codes:
Exception | HTTP Status Code |
---|---|
| 400 (Bad Request) |
| 500 (Internal Server Error) |
| 406 (Not Acceptable) |
| 415 (Unsupported Media Type) |
| 400 (Bad Request) |
| 500 (Internal Server Error) |
| 405 (Method Not Allowed) |
| 400 (Bad Request) |
| 400 (Bad Request) |
| 400 (Bad Request) |
| 404 (Not Found) |
| 404 (Not Found) |
| 400 (Bad Request) |
The DefaultHandlerExceptionResolver
works transparently by setting the status of the
response. However, it stops short of writing any error content to the body of the
response while your application may need to add developer-friendly content to every
error response for example when providing a REST API. You can prepare a ModelAndView
and render error content through view resolution — i.e. by configuring a
ContentNegotiatingViewResolver
, MappingJacksonJsonView
, and so on. However, you may
prefer to use @ExceptionHandler
methods instead.
If you prefer to write error content via @ExceptionHandler
methods you can extend
ResponseEntityExceptionHandler
instead. This is a convenient base for
@ControllerAdvice
classes providing an @ExceptionHandler
method to handle standard
Spring MVC exceptions and return ResponseEntity
. That allows you to customize the
response and write error content with message converters. See the Javadoc of
ResponseEntityExceptionHandler
for more details.
A business exception can be annotated with @ResponseStatus
. When the exception is
raised, the ResponseStatusExceptionResolver
handles it by setting the status of the
response accordingly. By default the DispatcherServlet
registers the
ResponseStatusExceptionResolver
and it is available for use.
When the status of the response is set to an error status code and the body of the
response is empty, Servlet containers commonly render an HTML formatted error page. To
customize the default error page of the container, you can declare an <error-page>
element in web.xml
. Up until Servlet 3, that element had to be mapped to a specific
status code or exception type. Starting with Servlet 3 an error page does not need to be
mapped, which effectively means the specified location customizes the default Servlet
container error page.
<error-page> <location>/error</location> </error-page>
Note that the actual location for the error page can be a JSP page or some other URL
within the container including one handled through an @Controller
method:
When writing error information, the status code and the error message set on the
HttpServletResponse
can be accessed through request attributes in a controller:
@Controller public class ErrorController { @RequestMapping(value="/error", produces="application/json") @ResponseBody public Map<String, Object> handle(HttpServletRequest request) { Map<String, Object> map = new HashMap<String, Object>(); map.put("status", request.getAttribute("javax.servlet.error.status_code")); map.put("reason", request.getAttribute("javax.servlet.error.message")); return map; } }
or in a JSP:
<%@ page contentType="application/json" pageEncoding="UTF-8"%> { status:<%=request.getAttribute("javax.servlet.error.status_code") %>, reason:<%=request.getAttribute("javax.servlet.error.message") %> }
For a lot of projects, sticking to established conventions and having reasonable
defaults is just what they (the projects) need, and Spring Web MVC now has explicit
support for convention over configuration. What this means is that if you establish
a set of naming conventions and suchlike, you can substantially cut down on the
amount of configuration that is required to set up handler mappings, view resolvers,
ModelAndView
instances, etc. This is a great boon with regards to rapid prototyping,
and can also lend a degree of (always good-to-have) consistency across a codebase should
you choose to move forward with it into production.
Convention-over-configuration support addresses the three core areas of MVC: models, views, and controllers.
The ControllerClassNameHandlerMapping
class is a HandlerMapping
implementation that
uses a convention to determine the mapping between request URLs and the Controller
instances that are to handle those requests.
Consider the following simple Controller
implementation. Take special notice of the
name of the class.
public class ViewShoppingCartController implements Controller { public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) { // the implementation is not hugely important for this example... } }
Here is a snippet from the corresponding Spring Web MVC configuration file:
<bean class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"/> <bean id="viewShoppingCart" class="x.y.z.ViewShoppingCartController"> <!-- inject dependencies as required... --> </bean>
The ControllerClassNameHandlerMapping
finds all of the various handler (or
Controller
) beans defined in its application context and strips Controller
off the
name to define its handler mappings. Thus, ViewShoppingCartController
maps to the
/viewshoppingcart*
request URL.
Let’s look at some more examples so that the central idea becomes immediately familiar.
(Notice all lowercase in the URLs, in contrast to camel-cased Controller
class names.)
WelcomeController
maps to the /welcome*
request URL
HomeController
maps to the /home*
request URL
IndexController
maps to the /index*
request URL
RegisterController
maps to the /register*
request URL
In the case of MultiActionController
handler classes, the mappings generated are
slightly more complex. The Controller
names in the following examples are assumed to
be MultiActionController
implementations:
AdminController
maps to the /admin/*
request URL
CatalogController
maps to the /catalog/*
request URL
If you follow the convention of naming your Controller
implementations as
xxxController
, the ControllerClassNameHandlerMapping
saves you the tedium of
defining and maintaining a potentially looooong SimpleUrlHandlerMapping
(or
suchlike).
The ControllerClassNameHandlerMapping
class extends the AbstractHandlerMapping
base
class so you can define HandlerInterceptor
instances and everything else just as you
would with many other HandlerMapping
implementations.
The ModelMap
class is essentially a glorified Map
that can make adding objects that
are to be displayed in (or on) a View
adhere to a common naming convention. Consider
the following Controller
implementation; notice that objects are added to the
ModelAndView
without any associated name specified.
public class DisplayShoppingCartController implements Controller { public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) { List cartItems = // get a List of CartItem objects User user = // get the User doing the shopping ModelAndView mav = new ModelAndView("displayShoppingCart"); <-- the logical view name mav.addObject(cartItems); <-- look ma, no name, just the object mav.addObject(user); <-- and again ma! return mav; } }
The ModelAndView
class uses a ModelMap
class that is a custom Map
implementation
that automatically generates a key for an object when an object is added to it. The
strategy for determining the name for an added object is, in the case of a scalar object
such as User
, to use the short class name of the object’s class. The following
examples are names that are generated for scalar objects put into a ModelMap
instance.
x.y.User
instance added will have the name user
generated.
x.y.Registration
instance added will have the name registration
generated.
x.y.Foo
instance added will have the name foo
generated.
java.util.HashMap
instance added will have the name hashMap
generated. You
probably want to be explicit about the name in this case because hashMap
is less
than intuitive.
null
will result in an IllegalArgumentException
being thrown. If the object
(or objects) that you are adding could be null
, then you will also want to be
explicit about the name.
The strategy for generating a name after adding a Set
or a List
is to peek into the
collection, take the short class name of the first object in the collection, and use
that with List
appended to the name. The same applies to arrays although with arrays
it is not necessary to peek into the array contents. A few examples will make the
semantics of name generation for collections clearer:
x.y.User[]
array with zero or more x.y.User
elements added will have the name
userList
generated.
x.y.Foo[]
array with zero or more x.y.User
elements added will have the name
fooList
generated.
java.util.ArrayList
with one or more x.y.User
elements added will have the name
userList
generated.
java.util.HashSet
with one or more x.y.Foo
elements added will have the name
fooList
generated.
java.util.ArrayList
will not be added at all (in effect, the
addObject(..)
call will essentially be a no-op).
The RequestToViewNameTranslator
interface determines a logical View
name when no
such logical view name is explicitly supplied. It has just one implementation, the
DefaultRequestToViewNameTranslator
class.
The DefaultRequestToViewNameTranslator
maps request URLs to logical view names, as
with this example:
public class RegistrationController implements Controller { public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) { // process the request... ModelAndView mav = new ModelAndView(); // add data as necessary to the model... return mav; // notice that no View or logical view name has been set } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- this bean with the well known name generates view names for us --> <bean id="viewNameTranslator" class="org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator"/> <bean class="x.y.RegistrationController"> <!-- inject dependencies as necessary --> </bean> <!-- maps request URLs to Controller names --> <bean class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"/> <bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver"> <property name="prefix" value="/WEB-INF/jsp/"/> <property name="suffix" value=".jsp"/> </bean> </beans>
Notice how in the implementation of the handleRequest(..)
method no View
or logical
view name is ever set on the ModelAndView
that is returned. The
DefaultRequestToViewNameTranslator
is tasked with generating a logical view name
from the URL of the request. In the case of the above RegistrationController
, which is
used in conjunction with the ControllerClassNameHandlerMapping
, a request URL of
http://localhost/registration.html
results in a logical view name of registration
being generated by the DefaultRequestToViewNameTranslator
. This logical view name is
then resolved into the /WEB-INF/jsp/registration.jsp
view by the
InternalResourceViewResolver
bean.
Tip | |
---|---|
You do not need to define a |
Of course, if you need to change the default settings, then you do need to configure
your own DefaultRequestToViewNameTranslator
bean explicitly. Consult the comprehensive
Javadoc for the DefaultRequestToViewNameTranslator
class for details of the various
properties that can be configured.
An ETag (entity tag) is an HTTP response header
returned by an HTTP/1.1 compliant web server used to determine change in content at a
given URL. It can be considered to be the more sophisticated successor to the
Last-Modified
header. When a server returns a representation with an ETag header, the
client can use this header in subsequent GETs, in an If-None-Match
header. If the
content has not changed, the server returns 304: Not Modified
.
Support for ETags is provided by the Servlet filter ShallowEtagHeaderFilter
. It is a
plain Servlet Filter, and thus can be used in combination with any web framework. The
ShallowEtagHeaderFilter
filter creates so-called shallow ETags (as opposed to deep
ETags, more about that later).The filter caches the content of the rendered JSP (or
other content), generates an MD5 hash over that, and returns that as an ETag header in
the response. The next time a client sends a request for the same resource, it uses that
hash as the If-None-Match
value. The filter detects this, renders the view again, and
compares the two hashes. If they are equal, a 304
is returned. This filter will not
save processing power, as the view is still rendered. The only thing it saves is
bandwidth, as the rendered response is not sent back over the wire.
You configure the ShallowEtagHeaderFilter
in web.xml
:
<filter> <filter-name>etagFilter</filter-name> <filter-class>org.springframework.web.filter.ShallowEtagHeaderFilter</filter-class> </filter> <filter-mapping> <filter-name>etagFilter</filter-name> <servlet-name>petclinic</servlet-name> </filter-mapping>
In a Servlet 3.0+ environment, you have the option of configuring the Servlet container
programmatically as an alternative or in combination with a web.xml
file. Below is an
example of registering a DispatcherServlet
:
import org.springframework.web.WebApplicationInitializer; public class MyWebApplicationInitializer implements WebApplicationInitializer { @Override public void onStartup(ServletContext container) { XmlWebApplicationContext appContext = new XmlWebApplicationContext(); appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml"); ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext)); registration.setLoadOnStartup(1); registration.addMapping("/"); } }
WebApplicationInitializer
is an interface provided by Spring MVC that ensures your
implementation is detected and automatically used to initialize any Servlet 3 container.
An abstract base class implementation of WebApplicationInitializer
named
AbstractDispatcherServletInitializer
makes it even easier to register the
DispatcherServlet
by simply overriding methods to specify the servlet mapping and the
location of the DispatcherServlet
configuration:
public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer { @Override protected Class<?>[] getRootConfigClasses() { return null; } @Override protected Class<?>[] getServletConfigClasses() { return new Class[] { MyWebConfig.class }; } @Override protected String[] getServletMappings() { return new String[] { "/" }; } }
The above example is for an application that uses Java-based Spring configuration. If
using XML-based Spring configuration, extend directly from
AbstractDispatcherServletInitializer
:
public class MyWebAppInitializer extends AbstractDispatcherServletInitializer { @Override protected WebApplicationContext createRootApplicationContext() { return null; } @Override protected WebApplicationContext createServletApplicationContext() { XmlWebApplicationContext cxt = new XmlWebApplicationContext(); cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml"); return cxt; } @Override protected String[] getServletMappings() { return new String[] { "/" }; } }
AbstractDispatcherServletInitializer
also provides a convenient way to add Filter
instances and have them automatically mapped to the DispatcherServlet
:
public class MyWebAppInitializer extends AbstractDispatcherServletInitializer { // ... @Override protected Filter[] getServletFilters() { return new Filter[] { new HiddenHttpMethodFilter(), new CharacterEncodingFilter() }; } }
Each filter is added with a default name based on its concrete type and automatically
mapped to the DispatcherServlet
.
The isAsyncSupported
protected method of AbstractDispatcherServletInitializer
provides a single place to enable async support on the DispatcherServlet
and all
filters mapped to it. By default this flag is set to true
.
Section 16.2.1, “Special Bean Types In the WebApplicationContext” and Section 16.2.2, “Default DispatcherServlet Configuration” explained about Spring
MVC’s special beans and the default implementations used by the DispatcherServlet
. In
this section you’ll learn about two additional ways of configuring Spring MVC. Namely
the MVC Java config and the MVC XML namespace.
The MVC Java config and the MVC namespace provide similar default configuration that
overrides the DispatcherServlet
defaults. The goal is to spare most applications from
having to having to create the same configuration and also to provide higher-level
constructs for configuring Spring MVC that serve as a simple starting point and require
little or no prior knowledge of the underlying configuration.
You can choose either the MVC Java config or the MVC namespace depending on your preference. Also as you will see further below, with the MVC Java config it is easier to see the underlying configuration as well as to make fine-grained customizations directly to the created Spring MVC beans. But let’s start from the beginning.
To enable MVC Java config add the annotation @EnableWebMvc
to one of your
@Configuration
classes:
@Configuration @EnableWebMvc public class WebConfig { }
To achieve the same in XML use the mvc:annotation-driven
element:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:mvc="http://www.springframework.org/schema/mvc" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd"> <mvc:annotation-driven /> </beans>
The above registers a RequestMappingHandlerMapping
, a RequestMappingHandlerAdapter
,
and an ExceptionHandlerExceptionResolver
(among others) in support of processing
requests with annotated controller methods using annotations such as @RequestMapping
,
@ExceptionHandler
, and others.
It also enables the following:
@NumberFormat
annotation
through the ConversionService
.
@DateTimeFormat
annotation.
@Valid
, if
a JSR-303 Provider is present on the classpath.
HttpMessageConverter support for @RequestBody
method parameters and @ResponseBody
method return values from @RequestMapping
or @ExceptionHandler
methods.
This is the complete list of HttpMessageConverters set up by mvc:annotation-driven:
ByteArrayHttpMessageConverter
converts byte arrays.
StringHttpMessageConverter
converts strings.
ResourceHttpMessageConverter
converts to/from
org.springframework.core.io.Resource
for all media types.
SourceHttpMessageConverter
converts to/from a javax.xml.transform.Source
.
FormHttpMessageConverter
converts form data to/from a MultiValueMap<String,
String>
.
Jaxb2RootElementHttpMessageConverter
converts Java objects to/from XML — added if
JAXB2 is present on the classpath.
MappingJackson2HttpMessageConverter
(or MappingJacksonHttpMessageConverter
)
converts to/from JSON — added if Jackson 2 (or Jackson) is present on the classpath.
AtomFeedHttpMessageConverter
converts Atom feeds — added if Rome is present on the
classpath.
RssChannelHttpMessageConverter
converts RSS feeds — added if Rome is present on
the classpath.
To customize the default configuration in Java you simply implement the
WebMvcConfigurer
interface or more likely extend the class WebMvcConfigurerAdapter
and override the methods you need. Below is an example of some of the available methods
to override. See
WebMvcConfigurer
for a list of all methods and the Javadoc for further details:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override protected void addFormatters(FormatterRegistry registry) { // Add formatters and/or converters } @Override public void configureMessageConverters(List<HttpMessageConverter<?>> converters) { // Configure the list of HttpMessageConverters to use } }
To customize the default configuration of <mvc:annotation-driven />
check what
attributes and sub-elements it supports. You can view the
Spring MVC XML schema or use the code
completion feature of your IDE to discover what attributes and sub-elements are
available. The sample below shows a subset of what is available:
<mvc:annotation-driven conversion-service="conversionService"> <mvc:message-converters> <bean class="org.example.MyHttpMessageConverter"/> <bean class="org.example.MyOtherHttpMessageConverter"/> </mvc:message-converters> </mvc:annotation-driven> <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean"> <property name="formatters"> <list> <bean class="org.example.MyFormatter"/> <bean class="org.example.MyOtherFormatter"/> </list> </property> </bean>
You can configure HandlerInterceptors
or WebRequestInterceptors
to be applied to all
incoming requests or restricted to specific URL path patterns.
An example of registering interceptors in Java:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void addInterceptors(InterceptorRegistry registry) { registry.addInterceptor(new LocaleInterceptor()); registry.addInterceptor(new ThemeInterceptor()).addPathPatterns("/").excludePathPatterns("/admin/"); registry.addInterceptor(new SecurityInterceptor()).addPathPatterns("/secure/*"); } }
And in XML use the <mvc:interceptors>
element:
<mvc:interceptors> <bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor" /> <mvc:interceptor> <mvc:mapping path="/"/> <mvc:exclude-mapping path="/admin/"/> <bean class="org.springframework.web.servlet.theme.ThemeChangeInterceptor" /> </mvc:interceptor> <mvc:interceptor> <mvc:mapping path="/secure/*"/> <bean class="org.example.SecurityInterceptor" /> </mvc:interceptor> </mvc:interceptors>
You can configure how Spring MVC determines the requested media types from the client for request mapping as well as for content negotiation purposes. The available options are to check the file extension in the request URI, the "Accept" header, a request parameter, as well as to fall back on a default content type. By default, file extension in the request URI is checked first and the "Accept" header is checked next.
For file extensions in the request URI, the MVC Java config and the MVC namespace,
automatically register extensions such as .json
, .xml
, .rss
, and .atom
if the
corresponding dependencies such as Jackson, JAXB2, or Rome are present on the classpath.
Additional extensions may be not need to be registered explicitly if they can be
discovered via ServletContext.getMimeType(String)
or the Java Activation Framework
(see javax.activation.MimetypesFileTypeMap
). You can register more extensions with the
setUseRegisteredSuffixPatternMatch
method.
The introduction of ContentNegotiationManger
also enables selective suffix pattern
matching for incoming requests. For more details, see the Javadoc of
Below is an example of customizing content negotiation options through the MVC Java config:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void configureContentNegotiation(ContentNegotiationConfigurer configurer) { configurer.favorPathExtension(false).favorParameter(true); } }
In the MVC namespace, the <mvc:annotation-driven>
element has a
content-negotiation-manager
attribute, which expects a ContentNegotiationManager
that in turn can be created with a ContentNegotiationManagerFactoryBean
:
<mvc:annotation-driven content-negotiation-manager="contentNegotiationManager" /> <bean id="contentNegotiationManager" class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean"> <property name="favorPathExtension" value="false" /> <property name="favorParameter" value="true" /> <property name="mediaTypes" > <value> json=application/json xml=application/xml </value> </property> </bean>
If not using the MVC Java config or the MVC namespace, you’ll need to create an instance
of ContentNegotiationManager
and use it to configure RequestMappingHandlerMapping
for request mapping purposes, and RequestMappingHandlerAdapter
and
ExceptionHandlerExceptionResolver
for content negotiation purposes.
Note that ContentNegotiatingViewResolver
now can also be configured with a
ContentNegotiatingViewResolver
, so you can use one instance throughout Spring MVC.
In more advanced cases, it may be useful to configure multiple
ContentNegotiationManager
instances that in turn may contain custom
ContentNegotiationStrategy
implementations. For example you could configure
ExceptionHandlerExceptionResolver
with a ContentNegotiationManager
that always
resolves the requested media type to "application/json"
. Or you may want to plug a
custom strategy that has some logic to select a default content type (e.g. either XML or
JSON) if no content types were requested.
This is a shortcut for defining a ParameterizableViewController
that immediately
forwards to a view when invoked. Use it in static cases when there is no Java controller
logic to execute before the view generates the response.
An example of forwarding a request for "/"
to a view called "home"
in Java:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void addViewControllers(ViewControllerRegistry registry) { registry.addViewController("/").setViewName("home"); } }
And the same in XML use the <mvc:view-controller>
element:
<mvc:view-controller path="/" view-name="home"/>
This option allows static resource requests following a particular URL pattern to be
served by a ResourceHttpRequestHandler
from any of a list of Resource
locations.
This provides a convenient way to serve static resources from locations other than the
web application root, including locations on the classpath. The cache-period
property
may be used to set far future expiration headers (1 year is the recommendation of
optimization tools such as Page Speed and YSlow) so that they will be more efficiently
utilized by the client. The handler also properly evaluates the Last-Modified
header
(if present) so that a 304
status code will be returned as appropriate, avoiding
unnecessary overhead for resources that are already cached by the client. For example,
to serve resource requests with a URL pattern of /resources/**
from a
public-resources
directory within the web application root you would use:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/"); } }
And the same in XML:
<mvc:resources mapping="/resources/**" location="/public-resources/"/>
To serve these resources with a 1-year future expiration to ensure maximum use of the browser cache and a reduction in HTTP requests made by the browser:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/").setCachePeriod(31556926); } }
And in XML:
<mvc:resources mapping="/resources/**" location="/public-resources/" cache-period="31556926"/>
The mapping
attribute must be an Ant pattern that can be used by
SimpleUrlHandlerMapping
, and the location
attribute must specify one or more valid
resource directory locations. Multiple resource locations may be specified using a
comma-separated list of values. The locations specified will be checked in the specified
order for the presence of the resource for any given request. For example, to enable the
serving of resources from both the web application root and from a known path of
/META-INF/public-web-resources/
in any jar on the classpath use:
@EnableWebMvc @Configuration public class WebConfig extends WebMvcConfigurerAdapter { @Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler("/resources/**") .addResourceLocations("/", "classpath:/META-INF/public-web-resources/"); } }
And in XML:
<mvc:resources mapping="/resources/**" location="/, classpath:/META-INF/public-web-resources/"/>
When serving resources that may change when a new version of the application is deployed, it is recommended that you incorporate a version string into the mapping pattern used to request the resources, so that you may force clients to request the newly deployed version of your application’s resources. Such a version string can be parameterized and accessed using SpEL so that it may be easily managed in a single place when deploying new versions.
As an example, let’s consider an application that uses a performance-optimized custom
build (as recommended) of the Dojo JavaScript library in production, and that the build
is generally deployed within the web application at a path of
/public-resources/dojo/dojo.js
. Since different parts of Dojo may be incorporated into
the custom build for each new version of the application, the client web browsers need
to be forced to re-download that custom-built dojo.js
resource any time a new version
of the application is deployed. A simple way to achieve this would be to manage the
version of the application in a properties file, such as:
application.version=1.0.0
and then to make the properties file’s values accessible to SpEL as a bean using the
util:properties
tag:
<util:properties id="applicationProps" location="/WEB-INF/spring/application.properties"/>
With the application version now accessible via SpEL, we can incorporate this into the
use of the resources
tag:
<mvc:resources mapping="/resources-#{applicationProps[application.version]}/**" location="/public-resources/"/>
In Java, you can use the @PropertySouce
annotation and then inject the Environment
abstraction for access to all defined properties:
@Configuration @EnableWebMvc @PropertySource("/WEB-INF/spring/application.properties") public class WebConfig extends WebMvcConfigurerAdapter { @Inject Environment env; @Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler( "/resources-" + env.getProperty("application.version") + "/**") .addResourceLocations("/public-resources/"); } }
and finally, to request the resource with the proper URL, we can take advantage of the Spring JSP tags:
<spring:eval expression="@applicationProps[application.version]" var="applicationVersion"/> <spring:url value="/resources-{applicationVersion}" var="resourceUrl"> <spring:param name="applicationVersion" value="${applicationVersion}"/> </spring:url> <script src="${resourceUrl}/dojo/dojo.js" type="text/javascript"> </script>
This tag allows for mapping the DispatcherServlet
to "/" (thus overriding the mapping
of the container’s default Servlet), while still allowing static resource requests to be
handled by the container’s default Servlet. It configures a
DefaultServletHttpRequestHandler
with a URL mapping of "/**" and the lowest priority
relative to other URL mappings.
This handler will forward all requests to the default Servlet. Therefore it is important
that it remains last in the order of all other URL HandlerMappings
. That will be the
case if you use <mvc:annotation-driven>
or alternatively if you are setting up your
own customized HandlerMapping
instance be sure to set its order
property to a value
lower than that of the DefaultServletHttpRequestHandler
, which is Integer.MAX_VALUE
.
To enable the feature using the default setup use:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) { configurer.enable(); } }
Or in XML:
<mvc:default-servlet-handler/>
The caveat to overriding the "/" Servlet mapping is that the RequestDispatcher
for the
default Servlet must be retrieved by name rather than by path. The
DefaultServletHttpRequestHandler
will attempt to auto-detect the default Servlet for
the container at startup time, using a list of known names for most of the major Servlet
containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere).
If the default Servlet has been custom configured with a different name, or if a
different Servlet container is being used where the default Servlet name is unknown,
then the default Servlet’s name must be explicitly provided as in the following example:
@Configuration @EnableWebMvc public class WebConfig extends WebMvcConfigurerAdapter { @Override public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) { configurer.enable("myCustomDefaultServlet"); } }
Or in XML:
<mvc:default-servlet-handler default-servlet-name="myCustomDefaultServlet"/>
See the following links and pointers for more resources about Spring Web MVC:
As you can see from the above examples, MVC Java config and the MVC namespace provide higher level constructs that do not require deep knowledge of the underlying beans created for you. Instead it helps you to focus on your application needs. However, at some point you may need more fine-grained control or you may simply wish to understand the underlying configuration.
The first step towards more fine-grained control is to see the underlying beans created
for you. In MVC Java config you can see the Javadoc and the @Bean
methods in
WebMvcConfigurationSupport
. The configuration in this class is automatically imported
through the @EnableWebMvc
annotation. In fact if you open @EnableWebMvc
you can see
the @Import
statement.
The next step towards more fine-grained control is to customize a property on one of the
beans created in WebMvcConfigurationSupport
or perhaps to provide your own instance.
This requires two things — remove the @EnableWebMvc
annotation in order to prevent
the import and then extend from DelegatingWebMvcConfiguration
, a subclass of
WebMvcConfigurationSupport
.
Here is an example:
@Configuration public class WebConfig extends DelegatingWebMvcConfiguration { @Override public void addInterceptors(InterceptorRegistry registry){ // ... } @Override @Bean public RequestMappingHandlerAdapter requestMappingHandlerAdapter() { // Create or let "super" create the adapter // Then customize one of its properties } }
Note | |
---|---|
An application should have only one configuration extending Modifying beans in this way does not prevent you from using any of the higher-level
constructs shown earlier in this section. |
Fine-grained control over the configuration created for you is a bit harder with the MVC namespace.
If you do need to do that, rather than replicating the configuration it provides,
consider configuring a BeanPostProcessor
that detects the bean you want to customize
by type and then modifying its properties as necessary. For example:
@Component public class MyPostProcessor implements BeanPostProcessor { public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException { if (bean instanceof RequestMappingHandlerAdapter) { // Modify properties of the adapter } } }
Note that MyPostProcessor
needs to be included in an <component scan />
in order for
it to be detected or if you prefer you can declare it explicitly with an XML bean
declaration.
One of the areas in which Spring excels is in the separation of view technologies from the rest of the MVC framework. For example, deciding to use Velocity or XSLT in place of an existing JSP is primarily a matter of configuration. This chapter covers the major view technologies that work with Spring and touches briefly on how to add new ones. This chapter assumes you are already familiar with Section 16.5, “Resolving views” which covers the basics of how views in general are coupled to the MVC framework.
Spring provides a couple of out-of-the-box solutions for JSP and JSTL views. Using JSP
or JSTL is done using a normal view resolver defined in the WebApplicationContext
.
Furthermore, of course you need to write some JSPs that will actually render the view.
Note | |
---|---|
Setting up your application to use JSTL is a common source of error, mainly caused by confusion over the different servlet spec., JSP and JSTL version numbers, what they mean and how to declare the taglibs correctly. The article How to Reference and Use JSTL in your Web Application provides a useful guide to the common pitfalls and how to avoid them. Note that as of Spring 3.0, the minimum supported servlet version is 2.4 (JSP 2.0 and JSTL 1.1), which reduces the scope for confusion somewhat. |
Just as with any other view technology you’re integrating with Spring, for JSPs you’ll
need a view resolver that will resolve your views. The most commonly used view resolvers
when developing with JSPs are the InternalResourceViewResolver
and the
ResourceBundleViewResolver
. Both are declared in the WebApplicationContext
:
<!-- the ResourceBundleViewResolver --> <bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver"> <property name="basename" value="views"/> </bean> # And a sample properties file is uses (views.properties in WEB-INF/classes): welcome.(class)=org.springframework.web.servlet.view.JstlView welcome.url=/WEB-INF/jsp/welcome.jsp productList.(class)=org.springframework.web.servlet.view.JstlView productList.url=/WEB-INF/jsp/productlist.jsp
As you can see, the ResourceBundleViewResolver
needs a properties file defining the
view names mapped to 1) a class and 2) a URL. With a ResourceBundleViewResolver
you
can mix different types of views using only one resolver.
<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver"> <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/> <property name="prefix" value="/WEB-INF/jsp/"/> <property name="suffix" value=".jsp"/> </bean>
The InternalResourceBundleViewResolver
can be configured for using JSPs as described
above. As a best practice, we strongly encourage placing your JSP files in a directory
under the 'WEB-INF'
directory, so there can be no direct access by clients.
When using the Java Standard Tag Library you must use a special view class, the
JstlView
, as JSTL needs some preparation before things such as the I18N features will
work.
Spring provides data binding of request parameters to command objects as described in earlier chapters. To facilitate the development of JSP pages in combination with those data binding features, Spring provides a few tags that make things even easier. All Spring tags haveHTML escaping features to enable or disable escaping of characters.
The tag library descriptor (TLD) is included in the spring-webmvc.jar
. Further
information about the individual tags can be found in the appendix entitled
Chapter 35, spring.tld.
As of version 2.0, Spring provides a comprehensive set of data binding-aware tags for handling form elements when using JSP and Spring Web MVC. Each tag provides support for the set of attributes of its corresponding HTML tag counterpart, making the tags familiar and intuitive to use. The tag-generated HTML is HTML 4.01/XHTML 1.0 compliant.
Unlike other form/input tag libraries, Spring’s form tag library is integrated with Spring Web MVC, giving the tags access to the command object and reference data your controller deals with. As you will see in the following examples, the form tags make JSPs easier to develop, read and maintain.
Let’s go through the form tags and look at an example of how each tag is used. We have included generated HTML snippets where certain tags require further commentary.
The form tag library comes bundled in spring-webmvc.jar
. The library descriptor is
called spring-form.tld
.
To use the tags from this library, add the following directive to the top of your JSP page:
<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %>
where form
is the tag name prefix you want to use for the tags from this library.
This tag renders an HTML form tag and exposes a binding path to inner tags for
binding. It puts the command object in the PageContext
so that the command object can
be accessed by inner tags. All the other tags in this library are nested tags of the
form
tag.
Let’s assume we have a domain object called User
. It is a JavaBean with properties
such as firstName
and lastName
. We will use it as the form backing object of our
form controller which returns form.jsp
. Below is an example of what form.jsp
would
look like:
<form:form> <table> <tr> <td>First Name:</td> <td><form:input path="firstName" /></td> </tr> <tr> <td>Last Name:</td> <td><form:input path="lastName" /></td> </tr> <tr> <td colspan="2"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form:form>
The firstName
and lastName
values are retrieved from the command object placed in
the PageContext
by the page controller. Keep reading to see more complex examples of
how inner tags are used with the form
tag.
The generated HTML looks like a standard form:
<form method="POST"> <table> <tr> <td>First Name:</td> <td><input name="firstName" type="text" value="Harry"/></td> </tr> <tr> <td>Last Name:</td> <td><input name="lastName" type="text" value="Potter"/></td> </tr> <tr> <td colspan="2"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form>
The preceding JSP assumes that the variable name of the form backing object is
'command'
. If you have put the form backing object into the model under another name
(definitely a best practice), then you can bind the form to the named variable like so:
<form:form commandName="user"> <table> <tr> <td>First Name:</td> <td><form:input path="firstName" /></td> </tr> <tr> <td>Last Name:</td> <td><form:input path="lastName" /></td> </tr> <tr> <td colspan="2"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form:form>
This tag renders an HTML input tag using the bound value and type=text by default. For an example of this tag, see the section called “The form tag”. Starting with Spring 3.1 you can use other types such HTML5-specific types like email, tel, date, and others.
This tag renders an HTML input tag with type checkbox.
Let’s assume our User
has preferences such as newsletter subscription and a list of
hobbies. Below is an example of the Preferences
class:
public class Preferences { private boolean receiveNewsletter; private String[] interests; private String favouriteWord; public boolean isReceiveNewsletter() { return receiveNewsletter; } public void setReceiveNewsletter(boolean receiveNewsletter) { this.receiveNewsletter = receiveNewsletter; } public String[] getInterests() { return interests; } public void setInterests(String[] interests) { this.interests = interests; } public String getFavouriteWord() { return favouriteWord; } public void setFavouriteWord(String favouriteWord) { this.favouriteWord = favouriteWord; } }
The form.jsp
would look like:
<form:form> <table> <tr> <td>Subscribe to newsletter?:</td> <%-- Approach 1: Property is of type java.lang.Boolean --%> <td><form:checkbox path="preferences.receiveNewsletter"/></td> </tr> <tr> <td>Interests:</td> <%-- Approach 2: Property is of an array or of type java.util.Collection --%> <td> Quidditch: <form:checkbox path="preferences.interests" value="Quidditch"/> Herbology: <form:checkbox path="preferences.interests" value="Herbology"/> Defence Against the Dark Arts: <form:checkbox path="preferences.interests" value="Defence Against the Dark Arts"/> </td> </tr> <tr> <td>Favourite Word:</td> <%-- Approach 3: Property is of type java.lang.Object --%> <td> Magic: <form:checkbox path="preferences.favouriteWord" value="Magic"/> </td> </tr> </table> </form:form>
There are 3 approaches to the checkbox
tag which should meet all your checkbox needs.
java.lang.Boolean
, the
input(checkbox)
is marked as checked if the bound value is true
. The value
attribute corresponds to the resolved value of the setValue(Object)
value property.
array
or java.util.Collection
, the
input(checkbox)
is marked as checked if the configured setValue(Object)
value is
present in the bound Collection
.
input(checkbox)
is marked as
checked if the configured setValue(Object)
is equal to the bound value.
Note that regardless of the approach, the same HTML structure is generated. Below is an HTML snippet of some checkboxes:
<tr> <td>Interests:</td> <td> Quidditch: <input name="preferences.interests" type="checkbox" value="Quidditch"/> <input type="hidden" value="1" name="_preferences.interests"/> Herbology: <input name="preferences.interests" type="checkbox" value="Herbology"/> <input type="hidden" value="1" name="_preferences.interests"/> Defence Against the Dark Arts: <input name="preferences.interests" type="checkbox" value="Defence Against the Dark Arts"/> <input type="hidden" value="1" name="_preferences.interests"/> </td> </tr>
What you might not expect to see is the additional hidden field after each checkbox.
When a checkbox in an HTML page is not checked, its value will not be sent to the
server as part of the HTTP request parameters once the form is submitted, so we need a
workaround for this quirk in HTML in order for Spring form data binding to work. The
checkbox
tag follows the existing Spring convention of including a hidden parameter
prefixed by an underscore ("_") for each checkbox. By doing this, you are effectively
telling Spring that "the checkbox was visible in the form and I want my object to
which the form data will be bound to reflect the state of the checkbox no matter what".
This tag renders multiple HTML input tags with type checkbox.
Building on the example from the previous checkbox
tag section. Sometimes you prefer
not to have to list all the possible hobbies in your JSP page. You would rather provide
a list at runtime of the available options and pass that in to the tag. That is the
purpose of the checkboxes
tag. You pass in an Array
, a List
or a Map
containing
the available options in the "items" property. Typically the bound property is a
collection so it can hold multiple values selected by the user. Below is an example of
the JSP using this tag:
<form:form> <table> <tr> <td>Interests:</td> <td> <%-- Property is of an array or of type java.util.Collection --%> <form:checkboxes path="preferences.interests" items="${interestList}"/> </td> </tr> </table> </form:form>
This example assumes that the "interestList" is a List
available as a model attribute
containing strings of the values to be selected from. In the case where you use a Map,
the map entry key will be used as the value and the map entry’s value will be used as
the label to be displayed. You can also use a custom object where you can provide the
property names for the value using "itemValue" and the label using "itemLabel".
This tag renders an HTML input tag with type radio.
A typical usage pattern will involve multiple tag instances bound to the same property but with different values.
<tr> <td>Sex:</td> <td> Male: <form:radiobutton path="sex" value="M"/> <br/> Female: <form:radiobutton path="sex" value="F"/> </td> </tr>
This tag renders multiple HTML input tags with type radio.
Just like the checkboxes
tag above, you might want to pass in the available options as
a runtime variable. For this usage you would use the radiobuttons
tag. You pass in an
Array
, a List
or a Map
containing the available options in the "items" property.
In the case where you use a Map, the map entry key will be used as the value and the map
entry’s value will be used as the label to be displayed. You can also use a custom
object where you can provide the property names for the value using "itemValue" and the
label using "itemLabel".
<tr> <td>Sex:</td> <td><form:radiobuttons path="sex" items="${sexOptions}"/></td> </tr>
This tag renders an HTML input tag with type password using the bound value.
<tr> <td>Password:</td> <td> <form:password path="password" /> </td> </tr>
Please note that by default, the password value is not shown. If you do want the
password value to be shown, then set the value of the 'showPassword'
attribute to
true, like so.
<tr> <td>Password:</td> <td> <form:password path="password" value="^76525bvHGq" showPassword="true" /> </td> </tr>
This tag renders an HTML select element. It supports data binding to the selected
option as well as the use of nested option
and options
tags.
Let’s assume a User
has a list of skills.
<tr> <td>Skills:</td> <td><form:select path="skills" items="${skills}"/></td> </tr>
If the User's
skill were in Herbology, the HTML source of the Skills row would look
like:
<tr> <td>Skills:</td> <td> <select name="skills" multiple="true"> <option value="Potions">Potions</option> <option value="Herbology" selected="selected">Herbology</option> <option value="Quidditch">Quidditch</option> </select> </td> </tr>
This tag renders an HTML option. It sets selected as appropriate based on the bound value.
<tr> <td>House:</td> <td> <form:select path="house"> <form:option value="Gryffindor"/> <form:option value="Hufflepuff"/> <form:option value="Ravenclaw"/> <form:option value="Slytherin"/> </form:select> </td> </tr>
If the User's
house was in Gryffindor, the HTML source of the House row would look
like:
<tr> <td>House:</td> <td> <select name="house"> <option value="Gryffindor" selected="selected">Gryffindor</option> <option value="Hufflepuff">Hufflepuff</option> <option value="Ravenclaw">Ravenclaw</option> <option value="Slytherin">Slytherin</option> </select> </td> </tr>
This tag renders a list of HTML option tags. It sets the selected attribute as appropriate based on the bound value.
<tr> <td>Country:</td> <td> <form:select path="country"> <form:option value="-" label="--Please Select"/> <form:options items="${countryList}" itemValue="code" itemLabel="name"/> </form:select> </td> </tr>
If the User
lived in the UK, the HTML source of the Country row would look like:
<tr> <td>Country:</td> <td> <select name="country"> <option value="-">--Please Select</option> <option value="AT">Austria</option> <option value="UK" selected="selected">United Kingdom</option> <option value="US">United States</option> </select> </td> </tr>
As the example shows, the combined usage of an option
tag with the options
tag
generates the same standard HTML, but allows you to explicitly specify a value in the
JSP that is for display only (where it belongs) such as the default string in the
example: "-- Please Select".
The items
attribute is typically populated with a collection or array of item objects.
itemValue
and itemLabel
simply refer to bean properties of those item objects, if
specified; otherwise, the item objects themselves will be stringified. Alternatively,
you may specify a Map
of items, in which case the map keys are interpreted as option
values and the map values correspond to option labels. If itemValue
and/or itemLabel
happen to be specified as well, the item value property will apply to the map key and
the item label property will apply to the map value.
This tag renders an HTML textarea.
<tr> <td>Notes:</td> <td><form:textarea path="notes" rows="3" cols="20" /></td> <td><form:errors path="notes" /></td> </tr>
This tag renders an HTML input tag with type hidden using the bound value. To submit
an unbound hidden value, use the HTML input
tag with type hidden.
<form:hidden path="house" />
If we choose to submit the house value as a hidden one, the HTML would look like:
<input name="house" type="hidden" value="Gryffindor"/>
This tag renders field errors in an HTML span tag. It provides access to the errors created in your controller or those that were created by any validators associated with your controller.
Let’s assume we want to display all error messages for the firstName
and lastName
fields once we submit the form. We have a validator for instances of the User
class
called UserValidator
.
public class UserValidator implements Validator { public boolean supports(Class candidate) { return User.class.isAssignableFrom(candidate); } public void validate(Object obj, Errors errors) { ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "required", "Field is required."); ValidationUtils.rejectIfEmptyOrWhitespace(errors, "lastName", "required", "Field is required."); } }
The form.jsp
would look like:
<form:form> <table> <tr> <td>First Name:</td> <td><form:input path="firstName" /></td> <%-- Show errors for firstName field --%> <td><form:errors path="firstName" /></td> </tr> <tr> <td>Last Name:</td> <td><form:input path="lastName" /></td> <%-- Show errors for lastName field --%> <td><form:errors path="lastName" /></td> </tr> <tr> <td colspan="3"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form:form>
If we submit a form with empty values in the firstName
and lastName
fields, this is
what the HTML would look like:
<form method="POST"> <table> <tr> <td>First Name:</td> <td><input name="firstName" type="text" value=""/></td> <%-- Associated errors to firstName field displayed --%> <td><span name="firstName.errors">Field is required.</span></td> </tr> <tr> <td>Last Name:</td> <td><input name="lastName" type="text" value=""/></td> <%-- Associated errors to lastName field displayed --%> <td><span name="lastName.errors">Field is required.</span></td> </tr> <tr> <td colspan="3"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form>
What if we want to display the entire list of errors for a given page? The example below
shows that the errors
tag also supports some basic wildcarding functionality.
path="*"
- displays all errors
path="lastName"
- displays all errors associated with the lastName
field
path
is omitted - object errors only are displayed
The example below will display a list of errors at the top of the page, followed by field-specific errors next to the fields:
<form:form> <form:errors path="*" cssClass="errorBox" /> <table> <tr> <td>First Name:</td> <td><form:input path="firstName" /></td> <td><form:errors path="firstName" /></td> </tr> <tr> <td>Last Name:</td> <td><form:input path="lastName" /></td> <td><form:errors path="lastName" /></td> </tr> <tr> <td colspan="3"> <input type="submit" value="Save Changes" /> </td> </tr> </table> </form:form>
The HTML would look like:
<form method="POST"> <span name="*.errors" class="errorBox">Field is required.<br/>Field is required.</span> <table> <tr> <td>First Name:</td> <td><input name="firstName" type="text" value=""/></td> <td><span name="firstName.errors">Field is required.</span></td> </tr> <tr> <td>Last Name:</td> <td><input name="lastName" type="text" value=""/></td> <td><span name="lastName.errors">Field is required.</span></td> </tr> <tr> <td colspan="3"> <input type="submit" value="Save Changes" /> </td> </tr> </form>
A key principle of REST is the use of the Uniform Interface. This means that all
resources (URLs) can be manipulated using the same four HTTP methods: GET, PUT, POST,
and DELETE. For each method, the HTTP specification defines the exact semantics. For
instance, a GET should always be a safe operation, meaning that is has no side effects,
and a PUT or DELETE should be idempotent, meaning that you can repeat these operations
over and over again, but the end result should be the same. While HTTP defines these
four methods, HTML only supports two: GET and POST. Fortunately, there are two possible
workarounds: you can either use JavaScript to do your PUT or DELETE, or simply do a POST
with the real method as an additional parameter (modeled as a hidden input field in an
HTML form). This latter trick is what Spring’s HiddenHttpMethodFilter
does. This
filter is a plain Servlet Filter and therefore it can be used in combination with any
web framework (not just Spring MVC). Simply add this filter to your web.xml, and a POST
with a hidden _method parameter will be converted into the corresponding HTTP method
request.
To support HTTP method conversion the Spring MVC form tag was updated to support setting the HTTP method. For example, the following snippet taken from the updated Petclinic sample
<form:form method="delete"> <p class="submit"><input type="submit" value="Delete Pet"/></p> </form:form>
This will actually perform an HTTP POST, with the real DELETE method hidden behind a
request parameter, to be picked up by the HiddenHttpMethodFilter
, as defined in
web.xml:
<filter> <filter-name>httpMethodFilter</filter-name> <filter-class>org.springframework.web.filter.HiddenHttpMethodFilter</filter-class> </filter> <filter-mapping> <filter-name>httpMethodFilter</filter-name> <servlet-name>petclinic</servlet-name> </filter-mapping>
The corresponding @Controller method is shown below:
@RequestMapping(method = RequestMethod.DELETE) public String deletePet(@PathVariable int ownerId, @PathVariable int petId) { this.clinic.deletePet(petId); return "redirect:/owners/" + ownerId; }
Starting with Spring 3, the Spring form tag library allows entering dynamic attributes, which means you can enter any HTML5 specific attributes.
In Spring 3.1, the form input tag supports entering a type attribute other than text. This is intended to allow rendering new HTML5 specific input types such as email, date, range, and others. Note that entering type=text is not required since text is the default type.
It is possible to integrate Tiles - just as any other view technology - in web applications using Spring. The following describes in a broad way how to do this.
Note | |
---|---|
This section focuses on Spring’s support for Tiles v3 in the
|
To be able to use Tiles you have to have a couple of additional dependencies included in your project. The following is the list of dependencies you need.
Tiles version 2.1.2 or higher
Commons BeanUtils
Commons Digester
Commons Logging
To be able to use Tiles, you have to configure it using files containing definitions
(for basic information on definitions and other Tiles concepts, please have a look at
http://tiles.apache.org). In Spring this is done using the TilesConfigurer
. Have a
look at the following piece of example ApplicationContext configuration:
<bean id="tilesConfigurer" class="org.springframework.web.servlet.view.tiles2.TilesConfigurer"> <property name="definitions"> <list> <value>/WEB-INF/defs/general.xml</value> <value>/WEB-INF/defs/widgets.xml</value> <value>/WEB-INF/defs/administrator.xml</value> <value>/WEB-INF/defs/customer.xml</value> <value>/WEB-INF/defs/templates.xml</value> </list> </property> </bean>
As you can see, there are five files containing definitions, which are all located in
the 'WEB-INF/defs'
directory. At initialization of the WebApplicationContext
, the
files will be loaded and the definitions factory will be initialized. After that has
been done, the Tiles includes in the definition files can be used as views within your
Spring web application. To be able to use the views you have to have a ViewResolver
just as with any other view technology used with Spring. Below you can find two
possibilities, the UrlBasedViewResolver
and the ResourceBundleViewResolver
.
The UrlBasedViewResolver
instantiates the given viewClass
for each view it has to
resolve.
<bean id="viewResolver" class="org.springframework.web.servlet.view.UrlBasedViewResolver"> <property name="viewClass" value="org.springframework.web.servlet.view.tiles2.TilesView"/> </bean>
The ResourceBundleViewResolver
has to be provided with a property file containing
viewnames and viewclasses the resolver can use:
<bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver"> <property name="basename" value="views"/> </bean>
... welcomeView.(class)=org.springframework.web.servlet.view.tiles2.TilesView welcomeView.url=welcome (this is the name of a Tiles definition) vetsView.(class)=org.springframework.web.servlet.view.tiles2.TilesView vetsView.url=vetsView (again, this is the name of a Tiles definition) findOwnersForm.(class)=org.springframework.web.servlet.view.JstlView findOwnersForm.url=/WEB-INF/jsp/findOwners.jsp ...
As you can see, when using the ResourceBundleViewResolver
, you can easily mix
different view technologies.
Note that the TilesView
class supports JSTL (the JSP Standard Tag Library) out of the
box.
As an advanced feature, Spring also supports two special Tiles PreparerFactory
implementations. Check out the Tiles documentation for details on how to use
ViewPreparer
references in your Tiles definition files.
Specify SimpleSpringPreparerFactory
to autowire ViewPreparer instances based on
specified preparer classes, applying Spring’s container callbacks as well as applying
configured Spring BeanPostProcessors. If Spring’s context-wide annotation-config has
been activated, annotations in ViewPreparer classes will be automatically detected and
applied. Note that this expects preparer classes in the Tiles definition files, just
like the default PreparerFactory
does.
Specify SpringBeanPreparerFactory
to operate on specified preparer names instead
of classes, obtaining the corresponding Spring bean from the DispatcherServlet’s
application context. The full bean creation process will be in the control of the Spring
application context in this case, allowing for the use of explicit dependency injection
configuration, scoped beans etc. Note that you need to define one Spring bean definition
per preparer name (as used in your Tiles definitions).
<bean id="tilesConfigurer" class="org.springframework.web.servlet.view.tiles2.TilesConfigurer"> <property name="definitions"> <list> <value>/WEB-INF/defs/general.xml</value> <value>/WEB-INF/defs/widgets.xml</value> <value>/WEB-INF/defs/administrator.xml</value> <value>/WEB-INF/defs/customer.xml</value> <value>/WEB-INF/defs/templates.xml</value> </list> </property> <!-- resolving preparer names as Spring bean definition names --> <property name="preparerFactoryClass" value="org.springframework.web.servlet.view.tiles2.SpringBeanPreparerFactory"/> </bean>
Velocity and FreeMarker are two templating languages that can be used as view technologies within Spring MVC applications. The languages are quite similar and serve similar needs and so are considered together in this section. For semantic and syntactic differences between the two languages, see the FreeMarker web site.
Your web application will need to include velocity-1.x.x.jar
or freemarker-2.x.jar
in order to work with Velocity or FreeMarker respectively and commons-collections.jar
is required for Velocity. Typically they are included in the WEB-INF/lib
folder where
they are guaranteed to be found by a Java EE server and added to the classpath for your
application. It is of course assumed that you already have the spring-webmvc.jar
in
your 'WEB-INF/lib'
directory too! If you make use of Spring’s dateToolAttribute or
numberToolAttribute in your Velocity views, you will also need to include the
velocity-tools-generic-1.x.jar
A suitable configuration is initialized by adding the relevant configurer bean
definition to your '*-servlet.xml'
as shown below:
<!-- This bean sets up the Velocity environment for us based on a root path for templates. Optionally, a properties file can be specified for more control over the Velocity environment, but the defaults are pretty sane for file based template loading. --> <bean id="velocityConfig" class="org.springframework.web.servlet.view.velocity.VelocityConfigurer"> <property name="resourceLoaderPath" value="/WEB-INF/velocity/"/> </bean> <!-- View resolvers can also be configured with ResourceBundles or XML files. If you need different view resolving based on Locale, you have to use the resource bundle resolver. --> <bean id="viewResolver" class="org.springframework.web.servlet.view.velocity.VelocityViewResolver"> <property name="cache" value="true"/> <property name="prefix" value=""/> <property name="suffix" value=".vm"/> </bean>
<!-- freemarker config --> <bean id="freemarkerConfig" class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer"> <property name="templateLoaderPath" value="/WEB-INF/freemarker/"/> </bean> <!-- View resolvers can also be configured with ResourceBundles or XML files. If you need different view resolving based on Locale, you have to use the resource bundle resolver. --> <bean id="viewResolver" class="org.springframework.web.servlet.view.freemarker.FreeMarkerViewResolver"> <property name="cache" value="true"/> <property name="prefix" value=""/> <property name="suffix" value=".ftl"/> </bean>
Note | |
---|---|
For non web-apps add a |
Your templates need to be stored in the directory specified by the *Configurer
bean
shown above. This document does not cover details of creating templates for the two
languages - please see their relevant websites for information. If you use the view
resolvers highlighted, then the logical view names relate to the template file names in
similar fashion to InternalResourceViewResolver
for JSP’s. So if your controller
returns a ModelAndView object containing a view name of "welcome" then the resolvers
will look for the /WEB-INF/freemarker/welcome.ftl
or /WEB-INF/velocity/welcome.vm
template as appropriate.
The basic configurations highlighted above will be suitable for most application requirements, however additional configuration options are available for when unusual or advanced requirements dictate.
This file is completely optional, but if specified, contains the values that are passed
to the Velocity runtime in order to configure velocity itself. Only required for
advanced configurations, if you need this file, specify its location on the
VelocityConfigurer
bean definition above.
<bean id="velocityConfig" class="org.springframework.web.servlet.view.velocity.VelocityConfigurer"> <property name="configLocation" value="/WEB-INF/velocity.properties"/> </bean>
Alternatively, you can specify velocity properties directly in the bean definition for the Velocity config bean by replacing the "configLocation" property with the following inline properties.
<bean id="velocityConfig" class="org.springframework.web.servlet.view.velocity.VelocityConfigurer"> <property name="velocityProperties"> <props> <prop key="resource.loader">file</prop> <prop key="file.resource.loader.class"> org.apache.velocity.runtime.resource.loader.FileResourceLoader </prop> <prop key="file.resource.loader.path">${webapp.root}/WEB-INF/velocity</prop> <prop key="file.resource.loader.cache">false</prop> </props> </property> </bean>
Refer to the
API
documentation for Spring configuration of Velocity, or the Velocity documentation for
examples and definitions of the 'velocity.properties'
file itself.
FreeMarker Settings and SharedVariables can be passed directly to the FreeMarker
Configuration
object managed by Spring by setting the appropriate bean properties on
the FreeMarkerConfigurer
bean. The freemarkerSettings
property requires a
java.util.Properties
object and the freemarkerVariables
property requires a
java.util.Map
.
<bean id="freemarkerConfig" class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer"> <property name="templateLoaderPath" value="/WEB-INF/freemarker/"/> <property name="freemarkerVariables"> <map> <entry key="xml_escape" value-ref="fmXmlEscape"/> </map> </property> </bean> <bean id="fmXmlEscape" class="freemarker.template.utility.XmlEscape"/>
See the FreeMarker documentation for details of settings and variables as they apply to
the Configuration
object.
Spring provides a tag library for use in JSP’s that contains (amongst other things) a
<spring:bind/>
tag. This tag primarily enables forms to display values from form
backing objects and to show the results of failed validations from a Validator
in the
web or business tier. From version 1.1, Spring now has support for the same
functionality in both Velocity and FreeMarker, with additional convenience macros for
generating form input elements themselves.
A standard set of macros are maintained within the spring-webmvc.jar
file for both
languages, so they are always available to a suitably configured application.
Some of the macros defined in the Spring libraries are considered internal (private) but
no such scoping exists in the macro definitions making all macros visible to calling
code and user templates. The following sections concentrate only on the macros you need
to be directly calling from within your templates. If you wish to view the macro code
directly, the files are called spring.vm / spring.ftl and are in the packages
org.springframework.web.servlet.view.velocity
or
org.springframework.web.servlet.view.freemarker
respectively.
In your html forms (vm / ftl templates) that act as the formView for a Spring form
controller, you can use code similar to the following to bind to field values and
display error messages for each input field in similar fashion to the JSP equivalent.
Note that the name of the command object is "command" by default, but can be overridden
in your MVC configuration by setting the commandName bean property on your form
controller. Example code is shown below for the personFormV
and personFormF
views
configured earlier;
<!-- velocity macros are automatically available --> <html> ... <form action="" method="POST"> Name: #springBind( "command.name" ) <input type="text" name="${status.expression}" value="$!status.value" /><br> #foreach($error in $status.errorMessages) <b>$error</b> <br> #end <br> ... <input type="submit" value="submit"/> </form> ... </html>
<!-- freemarker macros have to be imported into a namespace. We strongly recommend sticking to spring --> <#import "/spring.ftl" as spring /> <html> ... <form action="" method="POST"> Name: <@spring.bind "command.name" /> <input type="text" name="${spring.status.expression}" value="${spring.status.value?default("")}" /><br> <#list spring.status.errorMessages as error> <b>${error}</b> <br> </#list> <br> ... <input type="submit" value="submit"/> </form> ... </html>
#springBind
/ <@spring.bind>
requires a path argument which consists of the name
of your command object (it will be command unless you changed it in your
FormController properties) followed by a period and the name of the field on the command
object you wish to bind to. Nested fields can be used too such as
"command.address.street". The bind
macro assumes the default HTML escaping behavior
specified by the ServletContext parameter defaultHtmlEscape
in web.xml
The optional form of the macro called #springBindEscaped
/ <@spring.bindEscaped>
takes a second argument and explicitly specifies whether HTML escaping should be used in
the status error messages or values. Set to true or false as required. Additional form
handling macros simplify the use of HTML escaping and these macros should be used
wherever possible. They are explained in the next section.
Additional convenience macros for both languages simplify both binding and form generation (including validation error display). It is never necessary to use these macros to generate form input fields, and they can be mixed and matched with simple HTML or calls direct to the spring bind macros highlighted previously.
The following table of available macros show the VTL and FTL definitions and the parameter list that each takes.
Table 17.1. Table of macro definitions
macro | VTL definition | FTL definition |
---|---|---|
message (output a string from a resource bundle based on the code parameter) | #springMessage($code) | <@spring.message code/> |
messageText (output a string from a resource bundle based on the code parameter, falling back to the value of the default parameter) | #springMessageText($code $text) | <@spring.messageText code, text/> |
url (prefix a relative URL with the application’s context root) | #springUrl($relativeUrl) | <@spring.url relativeUrl/> |
formInput (standard input field for gathering user input) | #springFormInput($path $attributes) | <@spring.formInput path, attributes, fieldType/> |
formHiddenInput * (hidden input field for submitting non-user input) | #springFormHiddenInput($path $attributes) | <@spring.formHiddenInput path, attributes/> |
formPasswordInput * (standard input field for gathering passwords. Note that no value will ever be populated in fields of this type) | #springFormPasswordInput($path $attributes) | <@spring.formPasswordInput path, attributes/> |
formTextarea (large text field for gathering long, freeform text input) | #springFormTextarea($path $attributes) | <@spring.formTextarea path, attributes/> |
formSingleSelect (drop down box of options allowing a single required value to be selected) | #springFormSingleSelect( $path $options $attributes) | <@spring.formSingleSelect path, options, attributes/> |
formMultiSelect (a list box of options allowing the user to select 0 or more values) | #springFormMultiSelect($path $options $attributes) | <@spring.formMultiSelect path, options, attributes/> |
formRadioButtons (a set of radio buttons allowing a single selection to be made from the available choices) | #springFormRadioButtons($path $options $separator $attributes) | <@spring.formRadioButtons path, options separator, attributes/> |
formCheckboxes (a set of checkboxes allowing 0 or more values to be selected) | #springFormCheckboxes($path $options $separator $attributes) | <@spring.formCheckboxes path, options, separator, attributes/> |
formCheckbox (a single checkbox) | #springFormCheckbox($path $attributes) | <@spring.formCheckbox path, attributes/> |
showErrors (simplify display of validation errors for the bound field) | #springShowErrors($separator $classOrStyle) | <@spring.showErrors separator, classOrStyle/> |
formInput
macro, specifying ' hidden
' or ' password
' as the value for the
fieldType
parameter.
The parameters to any of the above macros have consistent meanings:
SortedMap
such as a TreeMap
with a suitable Comparator may
be used and for arbitrary Maps that should return values in insertion order, use a
LinkedHashMap
or a LinkedMap
from commons-collections.
Examples of the macros are outlined below some in FTL and some in VTL. Where usage differences exist between the two languages, they are explained in the notes.
<!-- the Name field example from above using form macros in VTL --> ... Name: #springFormInput("command.name" "")<br> #springShowErrors("<br>" "")<br>
The formInput macro takes the path parameter (command.name) and an additional attributes parameter which is empty in the example above. The macro, along with all other form generation macros, performs an implicit spring bind on the path parameter. The binding remains valid until a new bind occurs so the showErrors macro doesn’t need to pass the path parameter again - it simply operates on whichever field a bind was last created for.
The showErrors macro takes a separator parameter (the characters that will be used to separate multiple errors on a given field) and also accepts a second parameter, this time a class name or style attribute. Note that FreeMarker is able to specify default values for the attributes parameter, unlike Velocity, and the two macro calls above could be expressed as follows in FTL:
<@spring.formInput "command.name"/> <@spring.showErrors "<br>"/>
Output is shown below of the form fragment generating the name field, and displaying a validation error after the form was submitted with no value in the field. Validation occurs through Spring’s Validation framework.
The generated HTML looks like this:
Name: <input type="text" name="name" value=""> <br> <b>required</b> <br> <br>
The formTextarea macro works the same way as the formInput macro and accepts the same parameter list. Commonly, the second parameter (attributes) will be used to pass style information or rows and cols attributes for the textarea.
Four selection field macros can be used to generate common UI value selection inputs in your HTML forms.
Each of the four macros accepts a Map of options containing the value for the form field, and the label corresponding to that value. The value and the label can be the same.
An example of radio buttons in FTL is below. The form backing object specifies a default value of London for this field and so no validation is necessary. When the form is rendered, the entire list of cities to choose from is supplied as reference data in the model under the name cityMap.
... Town: <@spring.formRadioButtons "command.address.town", cityMap, "" /><br><br>
This renders a line of radio buttons, one for each value in cityMap
using the
separator "". No additional attributes are supplied (the last parameter to the macro is
missing). The cityMap uses the same String for each key-value pair in the map. The map’s
keys are what the form actually submits as POSTed request parameters, map values are the
labels that the user sees. In the example above, given a list of three well known cities
and a default value in the form backing object, the HTML would be
Town: <input type="radio" name="address.town" value="London">London</input> <input type="radio" name="address.town" value="Paris" checked="checked">Paris</input> <input type="radio" name="address.town" value="New York">New York</input>
If your application expects to handle cities by internal codes for example, the map of codes would be created with suitable keys like the example below.
protected Map referenceData(HttpServletRequest request) throws Exception { Map cityMap = new LinkedHashMap(); cityMap.put("LDN", "London"); cityMap.put("PRS", "Paris"); cityMap.put("NYC", "New York"); Map m = new HashMap(); m.put("cityMap", cityMap); return m; }
The code would now produce output where the radio values are the relevant codes but the user still sees the more user friendly city names.
Town: <input type="radio" name="address.town" value="LDN">London</input> <input type="radio" name="address.town" value="PRS" checked="checked">Paris</input> <input type="radio" name="address.town" value="NYC">New York</input>
Default usage of the form macros above will result in HTML tags that are HTML 4.01 compliant and that use the default value for HTML escaping defined in your web.xml as used by Spring’s bind support. In order to make the tags XHTML compliant or to override the default HTML escaping value, you can specify two variables in your template (or in your model where they will be visible to your templates). The advantage of specifying them in the templates is that they can be changed to different values later in the template processing to provide different behavior for different fields in your form.
To switch to XHTML compliance for your tags, specify a value of true for a model/context variable named xhtmlCompliant:
# for Velocity.. #set($springXhtmlCompliant = true) <-- for FreeMarker --> <#assign xhtmlCompliant = true in spring>
Any tags generated by the Spring macros will now be XHTML compliant after processing this directive.
In similar fashion, HTML escaping can be specified per field:
<#-- until this point, default HTML escaping is used --> <#assign htmlEscape = true in spring> <#-- next field will use HTML escaping --> <@spring.formInput "command.name" /> <#assign htmlEscape = false in spring> <#-- all future fields will be bound with HTML escaping off -->
XSLT is a transformation language for XML and is popular as a view technology within web applications. XSLT can be a good choice as a view technology if your application naturally deals with XML, or if your model can easily be converted to XML. The following section shows how to produce an XML document as model data and have it transformed with XSLT in a Spring Web MVC application.
This example is a trivial Spring application that creates a list of words in the
Controller
and adds them to the model map. The map is returned along with the view
name of our XSLT view. See Section 16.3, “Implementing Controllers” for details of Spring Web MVC’s
Controller
interface. The XSLT view will turn the list of words into a simple XML
document ready for transformation.
Configuration is standard for a simple Spring application. The dispatcher servlet config
file contains a reference to a ViewResolver
, URL mappings and a single controller
bean…
<bean id="homeController"class="xslt.HomeController"/>
The controller logic is encapsulated in a subclass of AbstractController
, with the
handler method being defined like so…
protected ModelAndView handleRequestInternal(HttpServletRequest request, HttpServletResponse response) throws Exception { Map map = new HashMap(); List wordList = new ArrayList(); wordList.add("hello"); wordList.add("world"); map.put("wordList", wordList); return new ModelAndView("home", map); }
So far we’ve done nothing that’s XSLT specific. The model data has been created in the
same way as you would for any other Spring MVC application. Depending on the
configuration of the application now, that list of words could be rendered by JSP/JSTL
by having them added as request attributes, or they could be handled by Velocity by
adding the object to the VelocityContext
. In order to have XSLT render them, they of
course have to be converted into an XML document somehow. There are software packages
available that will automatically domify an object graph, but within Spring, you have
complete flexibility to create the DOM from your model in any way you choose. This
prevents the transformation of XML playing too great a part in the structure of your
model data which is a danger when using tools to manage the domification process.
In order to create a DOM document from our list of words or any other model data, we
must subclass the (provided)
org.springframework.web.servlet.view.xslt.AbstractXsltView
class. In doing so, we must
also typically implement the abstract method createXsltSource(..)
method. The first
parameter passed to this method is our model map. Here’s the complete listing of the
HomePage
class in our trivial word application:
package xslt; // imports omitted for brevity public class HomePage extends AbstractXsltView { protected Source createXsltSource(Map model, String rootName, HttpServletRequest request, HttpServletResponse response) throws Exception { Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument(); Element root = document.createElement(rootName); List words = (List) model.get("wordList"); for (Iterator it = words.iterator(); it.hasNext();) { String nextWord = (String) it.next(); Element wordNode = document.createElement("word"); Text textNode = document.createTextNode(nextWord); wordNode.appendChild(textNode); root.appendChild(wordNode); } return new DOMSource(root); } }
A series of parameter name/value pairs can optionally be defined by your subclass which
will be added to the transformation object. The parameter names must match those defined
in your XSLT template declared with <xsl:param
name="myParam">defaultValue</xsl:param>
. To specify the parameters, override the
getParameters()
method of the AbstractXsltView
class and return a Map
of the
name/value pairs. If your parameters need to derive information from the current
request, you can override the getParameters(HttpServletRequest request)
method instead.
The views.properties file (or equivalent xml definition if you’re using an XML based view resolver as we did in the Velocity examples above) looks like this for the one-view application that is My First Words:
home.(class)=xslt.HomePage home.stylesheetLocation=/WEB-INF/xsl/home.xslt home.root=words
Here, you can see how the view is tied in with the HomePage
class just written which
handles the model domification in the first property '.(class)'
. The
'stylesheetLocation'
property points to the XSLT file which will handle the XML
transformation into HTML for us and the final property '.root'
is the name that will
be used as the root of the XML document. This gets passed to the HomePage
class above
in the second parameter to the createXsltSource(..)
method(s).
Finally, we have the XSLT code used for transforming the above document. As shown in the
above 'views.properties'
file, the stylesheet is called 'home.xslt'
and it lives in
the war file in the 'WEB-INF/xsl'
directory.
<?xml version="1.0" encoding="utf-8"?> <xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"> <xsl:output method="html" omit-xml-declaration="yes"/> <xsl:template match="/"> <html> <head><title>Hello!</title></head> <body> <h1>My First Words</h1> <xsl:apply-templates/> </body> </html> </xsl:template> <xsl:template match="word"> <xsl:value-of select="."/><br/> </xsl:template> </xsl:stylesheet>
A summary of the files discussed and their location in the WAR file is shown in the simplified WAR structure below.
ProjectRoot | +- WebContent | +- WEB-INF | +- classes | | | +- xslt | | | | | +- HomePageController.class | | +- HomePage.class | | | +- views.properties | +- lib | | | +- spring-*.jar | +- xsl | | | +- home.xslt | +- frontcontroller-servlet.xml
You will also need to ensure that an XML parser and an XSLT engine are available on the classpath. JDK 1.4 provides them by default, and most Java EE containers will also make them available by default, but it’s a possible source of errors to be aware of.
Returning an HTML page isn’t always the best way for the user to view the model output, and Spring makes it simple to generate a PDF document or an Excel spreadsheet dynamically from the model data. The document is the view and will be streamed from the server with the correct content type to (hopefully) enable the client PC to run their spreadsheet or PDF viewer application in response.
In order to use Excel views, you need to add the poi library to your classpath, and for PDF generation, the iText library.
Document based views are handled in an almost identical fashion to XSLT views, and the following sections build upon the previous one by demonstrating how the same controller used in the XSLT example is invoked to render the same model as both a PDF document and an Excel spreadsheet (which can also be viewed or manipulated in Open Office).
First, let’s amend the views.properties file (or xml equivalent) and add a simple view definition for both document types. The entire file now looks like this with the XSLT view shown from earlier:
home.(class)=xslt.HomePage home.stylesheetLocation=/WEB-INF/xsl/home.xslt home.root=words xl.(class)=excel.HomePage pdf.(class)=pdf.HomePage
If you want to start with a template spreadsheet or a fillable PDF form to add your model data to, specify the location as the url property in the view definition
The controller code we’ll use remains exactly the same from the XSLT example earlier other than to change the name of the view to use. Of course, you could be clever and have this selected based on a URL parameter or some other logic - proof that Spring really is very good at decoupling the views from the controllers!
Exactly as we did for the XSLT example, we’ll subclass suitable abstract classes in
order to implement custom behavior in generating our output documents. For Excel, this
involves writing a subclass of
org.springframework.web.servlet.view.document.AbstractExcelView
(for Excel files
generated by POI) or org.springframework.web.servlet.view.document.AbstractJExcelView
(for JExcelApi-generated Excel files) and implementing the buildExcelDocument()
method.
Here’s the complete listing for our POI Excel view which displays the word list from the model map in consecutive rows of the first column of a new spreadsheet:
package excel; // imports omitted for brevity public class HomePage extends AbstractExcelView { protected void buildExcelDocument(Map model, HSSFWorkbook wb, HttpServletRequest req, HttpServletResponse resp) throws Exception { HSSFSheet sheet; HSSFRow sheetRow; HSSFCell cell; // Go to the first sheet // getSheetAt: only if wb is created from an existing document // sheet = wb.getSheetAt(0); sheet = wb.createSheet("Spring"); sheet.setDefaultColumnWidth((short) 12); // write a text at A1 cell = getCell(sheet, 0, 0); setText(cell, "Spring-Excel test"); List words = (List) model.get("wordList"); for (int i=0; i < words.size(); i++) { cell = getCell(sheet, 2+i, 0); setText(cell, (String) words.get(i)); } } }
And the following is a view generating the same Excel file, now using JExcelApi:
package excel; // imports omitted for brevity public class HomePage extends AbstractJExcelView { protected void buildExcelDocument(Map model, WritableWorkbook wb, HttpServletRequest request, HttpServletResponse response) throws Exception { WritableSheet sheet = wb.createSheet("Spring", 0); sheet.addCell(new Label(0, 0, "Spring-Excel test")); List words = (List) model.get("wordList"); for (int i = 0; i < words.size(); i++) { sheet.addCell(new Label(2+i, 0, (String) words.get(i))); } } }
Note the differences between the APIs. We’ve found that the JExcelApi is somewhat more intuitive, and furthermore, JExcelApi has slightly better image-handling capabilities. There have been memory problems with large Excel files when using JExcelApi however.
If you now amend the controller such that it returns xl
as the name of the view (
return new ModelAndView("xl", map);
) and run your application again, you should find
that the Excel spreadsheet is created and downloaded automatically when you request the
same page as before.
The PDF version of the word list is even simpler. This time, the class extends
org.springframework.web.servlet.view.document.AbstractPdfView
and implements the
buildPdfDocument()
method as follows:
package pdf; // imports omitted for brevity public class PDFPage extends AbstractPdfView { protected void buildPdfDocument(Map model, Document doc, PdfWriter writer, HttpServletRequest req, HttpServletResponse resp) throws Exception { List words = (List) model.get("wordList"); for (int i=0; i<words.size(); i++) { doc.add( new Paragraph((String) words.get(i))); } } }
Once again, amend the controller to return the pdf
view with return new
ModelAndView("pdf", map);
, and reload the URL in your application. This time a PDF
document should appear listing each of the words in the model map.
JasperReports ( http://jasperreports.sourceforge.net) is a powerful open-source reporting engine that supports the creation of report designs using an easily understood XML file format. JasperReports is capable of rendering reports in four different formats: CSV, Excel, HTML and PDF.
Your application will need to include the latest release of JasperReports, which at the time of writing was 0.6.1. JasperReports itself depends on the following projects:
JasperReports also requires a JAXP compliant XML parser.
To configure JasperReports views in your Spring container configuration you need to
define a ViewResolver
to map view names to the appropriate view class depending on
which format you want your report rendered in.
Typically, you will use the ResourceBundleViewResolver
to map view names to view
classes and files in a properties file.
<bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver"> <property name="basename" value="views"/> </bean>
Here we’ve configured an instance of the ResourceBundleViewResolver
class that will
look for view mappings in the resource bundle with base name views
. (The content of
this file is described in the next section.)
The Spring Framework contains five different View
implementations for JasperReports,
four of which correspond to one of the four output formats supported by JasperReports,
and one that allows for the format to be determined at runtime:
Table 17.2. JasperReports View classes
Class Name | Render Format |
---|---|
| CSV |
| HTML |
| |
| Microsoft Excel |
| The view is decided upon at runtime |
Mapping one of these classes to a view name and a report file is a matter of adding the appropriate entries in the resource bundle configured in the previous section as shown here:
simpleReport.(class)=org.springframework.web.servlet.view.jasperreports.JasperReportsPdfView simpleReport.url=/WEB-INF/reports/DataSourceReport.jasper
Here you can see that the view with name simpleReport
is mapped to the
JasperReportsPdfView
class, causing the output of this report to be rendered in PDF
format. The url
property of the view is set to the location of the underlying report
file.
JasperReports has two distinct types of report file: the design file, which has a
.jrxml
extension, and the compiled report file, which has a .jasper
extension.
Typically, you use the JasperReports Ant task to compile your .jrxml
design file into
a .jasper
file before deploying it into your application. With the Spring Framework
you can map either of these files to your report file and the framework will take care
of compiling the .jrxml
file on the fly for you. You should note that after a .jrxml
file is compiled by the Spring Framework, the compiled report is cached for the lifetime
of the application. Thus, to make changes to the file you will need to restart your
application.
The JasperReportsMultiFormatView
allows for the report format to be specified at
runtime. The actual rendering of the report is delegated to one of the other
JasperReports view classes - the JasperReportsMultiFormatView
class simply adds a
wrapper layer that allows for the exact implementation to be specified at runtime.
The JasperReportsMultiFormatView
class introduces two concepts: the format key and the
discriminator key. The JasperReportsMultiFormatView
class uses the mapping key to look
up the actual view implementation class, and it uses the format key to lookup up the
mapping key. From a coding perspective you add an entry to your model with the format
key as the key and the mapping key as the value, for example:
public ModelAndView handleSimpleReportMulti(HttpServletRequest request, HttpServletResponse response) throws Exception { String uri = request.getRequestURI(); String format = uri.substring(uri.lastIndexOf(".") + 1); Map model = getModel(); model.put("format", format); return new ModelAndView("simpleReportMulti", model); }
In this example, the mapping key is determined from the extension of the request URI and
is added to the model under the default format key: format
. If you wish to use a
different format key then you can configure this using the formatKey
property of the
JasperReportsMultiFormatView
class.
By default the following mapping key mappings are configured in
JasperReportsMultiFormatView
:
Table 17.3. JasperReportsMultiFormatView Default Mapping Key Mappings
Mapping Key | View Class |
---|---|
csv |
|
html |
|
| |
xls |
|
So in the example above a request to URI /foo/myReport.pdf would be mapped to the
JasperReportsPdfView
class. You can override the mapping key to view class mappings
using the formatMappings
property of JasperReportsMultiFormatView
.
In order to render your report correctly in the format you have chosen, you must supply
Spring with all of the data needed to populate your report. For JasperReports this means
you must pass in all report parameters along with the report datasource. Report
parameters are simple name/value pairs and can be added to the Map
for your model as
you would add any name/value pair.
When adding the datasource to the model you have two approaches to choose from. The
first approach is to add an instance of JRDataSource
or a Collection
type to the
model Map
under any arbitrary key. Spring will then locate this object in the model
and treat it as the report datasource. For example, you may populate your model like so:
private Map getModel() { Map model = new HashMap(); Collection beanData = getBeanData(); model.put("myBeanData", beanData); return model; }
The second approach is to add the instance of JRDataSource
or Collection
under a
specific key and then configure this key using the reportDataKey
property of the view
class. In both cases Spring will wrap instances of Collection
in a
JRBeanCollectionDataSource
instance. For example:
private Map getModel() { Map model = new HashMap(); Collection beanData = getBeanData(); Collection someData = getSomeData(); model.put("myBeanData", beanData); model.put("someData", someData); return model; }
Here you can see that two Collection
instances are being added to the model. To ensure
that the correct one is used, we simply modify our view configuration as appropriate:
simpleReport.(class)=org.springframework.web.servlet.view.jasperreports.JasperReportsPdfView simpleReport.url=/WEB-INF/reports/DataSourceReport.jasper simpleReport.reportDataKey=myBeanData
Be aware that when using the first approach, Spring will use the first instance of
JRDataSource
or Collection
that it encounters. If you need to place multiple
instances of JRDataSource
or Collection
into the model you need to use the second
approach.
JasperReports provides support for embedded sub-reports within your master report files. There are a wide variety of mechanisms for including sub-reports in your report files. The easiest way is to hard code the report path and the SQL query for the sub report into your design files. The drawback of this approach is obvious: the values are hard-coded into your report files reducing reusability and making it harder to modify and update report designs. To overcome this you can configure sub-reports declaratively, and you can include additional data for these sub-reports directly from your controllers.
To control which sub-report files are included in a master report using Spring, your report file must be configured to accept sub-reports from an external source. To do this you declare a parameter in your report file like so:
<parameter name="ProductsSubReport" class="net.sf.jasperreports.engine.JasperReport"/>
Then, you define your sub-report to use this sub-report parameter:
<subreport> <reportElement isPrintRepeatedValues="false" x="5" y="25" width="325" height="20" isRemoveLineWhenBlank="true" backcolor="#ffcc99"/> <subreportParameter name="City"> <subreportParameterExpression><![CDATA[$F{city}]]></subreportParameterExpression> </subreportParameter> <dataSourceExpression><![CDATA[$P{SubReportData}]]></dataSourceExpression> <subreportExpression class="net.sf.jasperreports.engine.JasperReport"> <![CDATA[$P{ProductsSubReport}]]></subreportExpression> </subreport>
This defines a master report file that expects the sub-report to be passed in as an
instance of net.sf.jasperreports.engine.JasperReports
under the parameter
ProductsSubReport
. When configuring your Jasper view class, you can instruct Spring to
load a report file and pass it into the JasperReports engine as a sub-report using the
subReportUrls
property:
<property name="subReportUrls"> <map> <entry key="ProductsSubReport" value="/WEB-INF/reports/subReportChild.jrxml"/> </map> </property>
Here, the key of the Map
corresponds to the name of the sub-report parameter in the
report design file, and the entry is the URL of the report file. Spring will load this
report file, compiling it if necessary, and pass it into the JasperReports engine under
the given key.
This step is entirely optional when using Spring to configure your sub-reports. If you
wish, you can still configure the data source for your sub-reports using static queries.
However, if you want Spring to convert data returned in your ModelAndView
into
instances of JRDataSource
then you need to specify which of the parameters in your
ModelAndView
Spring should convert. To do this, configure the list of parameter names
using the subReportDataKeys
property of your chosen view class:
<property name="subReportDataKeys" value="SubReportData"/>
Here, the key you supply must correspond to both the key used in your ModelAndView
and the key used in your report design file.
If you have special requirements for exporter configuration — perhaps you want a
specific page size for your PDF report — you can configure these exporter parameters
declaratively in your Spring configuration file using the exporterParameters
property
of the view class. The exporterParameters
property is typed as a Map
. In your
configuration the key of an entry should be the fully-qualified name of a static field
that contains the exporter parameter definition, and the value of an entry should be the
value you want to assign to the parameter. An example of this is shown below:
<bean id="htmlReport" class="org.springframework.web.servlet.view.jasperreports.JasperReportsHtmlView"> <property name="url" value="/WEB-INF/reports/simpleReport.jrxml"/> <property name="exporterParameters"> <map> <entry key="net.sf.jasperreports.engine.export.JRHtmlExporterParameter.HTML_FOOTER"> <value>Footer by Spring! </td><td width="50%">&nbsp; </td></tr> </table></body></html> </value> </entry> </map> </property> </bean>
Here you can see that the JasperReportsHtmlView
is configured with an exporter
parameter for net.sf.jasperreports.engine.export.JRHtmlExporterParameter.HTML_FOOTER
which will output a footer in the resulting HTML.
Both AbstractAtomFeedView
and AbstractRssFeedView
inherit from the base class
AbstractFeedView
and are used to provide Atom and RSS Feed views respectfully. They
are based on java.net’s ROME project and are located in the
package org.springframework.web.servlet.view.feed
.
AbstractAtomFeedView
requires you to implement the buildFeedEntries()
method and
optionally override the buildFeedMetadata()
method (the default implementation is
empty), as shown below.
public class SampleContentAtomView extends AbstractAtomFeedView { @Override protected void buildFeedMetadata(Map<String, Object> model, Feed feed, HttpServletRequest request) { // implementation omitted } @Override protected List<Entry> buildFeedEntries(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception { // implementation omitted } }
Similar requirements apply for implementing AbstractRssFeedView
, as shown below.
public class SampleContentAtomView extends AbstractRssFeedView { @Override protected void buildFeedMetadata(Map<String, Object> model, Channel feed, HttpServletRequest request) { // implementation omitted } @Override protected List<Item> buildFeedItems(Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception { // implementation omitted } }
The buildFeedItems()
and buildFeedEntires()
methods pass in the HTTP request in case
you need to access the Locale. The HTTP response is passed in only for the setting of
cookies or other HTTP headers. The feed will automatically be written to the response
object after the method returns.
For an example of creating an Atom view please refer to Alef Arendsen’s SpringSource Team Blog entry.
The MarhsallingView
uses an XML Marshaller
defined in the org.springframework.oxm
package to render the response content as XML. The object to be marshalled can be set
explicitly using MarhsallingView
's modelKey
bean property. Alternatively, the view
will iterate over all model properties and marshal only those types that are supported
by the Marshaller
. For more information on the functionality in the
org.springframework.oxm
package refer to the chapter Marshalling XML using O/X
Mappers.
The MappingJackson2JsonView
(or MappingJacksonJsonView
depending on the the Jackson
version you have) uses the Jackson library’s ObjectMapper
to render the response
content as JSON. By default, the entire contents of the model map (with the exception of
framework-specific classes) will be encoded as JSON. For cases where the contents of the
map need to be filtered, users may specify a specific set of model attributes to encode
via the RenderedAttributes
property. The extractValueFromSingleKeyModel
property may
also be used to have the value in single-key models extracted and serialized directly
rather than as a map of model attributes.
JSON mapping can be customized as needed through the use of Jackson’s provided
annotations. When further control is needed, a custom ObjectMapper
can be injected
through the ObjectMapper
property for cases where custom JSON
serializers/deserializers need to be provided for specific types.
This chapter details Spring’s integration with third party web frameworks such as JSF, Struts, and Tapestry.
One of the core value propositions of the Spring Framework is that of enabling choice. In a general sense, Spring does not force one to use or buy into any particular architecture, technology, or methodology (although it certainly recommends some over others). This freedom to pick and choose the architecture, technology, or methodology that is most relevant to a developer and their development team is arguably most evident in the web area, where Spring provides its own web framework (Spring MVC), while at the same time providing integration with a number of popular third party web frameworks. This allows one to continue to leverage any and all of the skills one may have acquired in a particular web framework such as Struts, while at the same time being able to enjoy the benefits afforded by Spring in other areas such as data access, declarative transaction management, and flexible configuration and application assembly.
Having dispensed with the woolly sales patter (c.f. the previous paragraph), the remainder of this chapter will concentrate upon the meaty details of integrating your favorite web framework with Spring. One thing that is often commented upon by developers coming to Java from other languages is the seeming super-abundance of web frameworks available in Java. There are indeed a great number of web frameworks in the Java space; in fact there are far too many to cover with any semblance of detail in a single chapter. This chapter thus picks four of the more popular web frameworks in Java, starting with the Spring configuration that is common to all of the supported web frameworks, and then detailing the specific integration options for each supported web framework.
Note | |
---|---|
Please note that this chapter does not attempt to explain how to use any of the supported web frameworks. For example, if you want to use Struts for the presentation layer of your web application, the assumption is that you are already familiar with Struts. If you need further details about any of the supported web frameworks themselves, please do consult Section 18.6, “Further Resources” at the end of this chapter. |
Before diving into the integration specifics of each supported web framework, let us first take a look at the Spring configuration that is not specific to any one web framework. (This section is equally applicable to Spring’s own web framework, Spring MVC.)
One of the concepts (for want of a better word) espoused by (Spring’s) lightweight
application model is that of a layered architecture. Remember that in a classic
layered architecture, the web layer is but one of many layers; it serves as one of the
entry points into a server side application and it delegates to service objects
(facades) defined in a service layer to satisfy business specific (and
presentation-technology agnostic) use cases. In Spring, these service objects, any other
business-specific objects, data access objects, etc. exist in a distinct business
context, which contains no web or presentation layer objects (presentation objects
such as Spring MVC controllers are typically configured in a distinct presentation
context). This section details how one configures a Spring container (a
WebApplicationContext
) that contains all of the business beans in one’s application.
On to specifics: all that one need do is to declare a
ContextLoaderListener
in the standard Java EE servlet web.xml
file of one’s web application, and add a
contextConfigLocation
<context-param/> section (in the same file) that defines which
set of Spring XML configuration files to load.
Find below the <listener/> configuration:
<listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener>
Find below the <context-param/> configuration:
<context-param> <param-name>contextConfigLocation</param-name> <param-value>/WEB-INF/applicationContext*.xml</param-value> </context-param>
If you don’t specify the contextConfigLocation
context parameter, the
ContextLoaderListener
will look for a file called /WEB-INF/applicationContext.xml
to
load. Once the context files are loaded, Spring creates a
WebApplicationContext
object based on the bean definitions and stores it in the ServletContext
of the web
application.
All Java web frameworks are built on top of the Servlet API, and so one can use the
following code snippet to get access to this business context ApplicationContext
created by the ContextLoaderListener
.
WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(servletContext);
The
WebApplicationContextUtils
class is for convenience, so you don’t have to remember the name of the ServletContext
attribute. Its getWebApplicationContext() method will return null
if an object
doesn’t exist under the WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
key. Rather than risk getting NullPointerExceptions
in your application, it’s better
to use the getRequiredWebApplicationContext()
method. This method throws an exception
when the ApplicationContext
is missing.
Once you have a reference to the WebApplicationContext
, you can retrieve beans by
their name or type. Most developers retrieve beans by name and then cast them to one of
their implemented interfaces.
Fortunately, most of the frameworks in this section have simpler ways of looking up beans. Not only do they make it easy to get beans from a Spring container, but they also allow you to use dependency injection on their controllers. Each web framework section has more detail on its specific integration strategies.
JavaServer Faces (JSF) is the JCP’s standard component-based, event-driven web user interface framework. As of Java EE 5, it is an official part of the Java EE umbrella.
For a popular JSF runtime as well as for popular JSF component libraries, check out the Apache MyFaces project. The MyFaces project also provides common JSF extensions such as MyFaces Orchestra: a Spring-based JSF extension that provides rich conversation scope support.
Note | |
---|---|
Spring Web Flow 2.0 provides rich JSF support through its newly established Spring Faces module, both for JSF-centric usage (as described in this section) and for Spring-centric usage (using JSF views within a Spring MVC dispatcher). Check out the Spring Web Flow website for details! |
The key element in Spring’s JSF integration is the JSF ELResolver
mechanism.
SpringBeanFacesELResolver
is a JSF 1.2 compliant ELResolver
implementation,
integrating with the standard Unified EL as used by JSF 1.2 and JSP 2.1. Like
SpringBeanVariableResolver
, it delegates to the Spring’s business context
WebApplicationContext
first, then to the default resolver of the underlying JSF
implementation.
Configuration-wise, simply define SpringBeanFacesELResolver
in your JSF 1.2
faces-context.xml file:
<faces-config> <application> <el-resolver>org.springframework.web.jsf.el.SpringBeanFacesELResolver</el-resolver> ... </application> </faces-config>
A custom VariableResolver
works well when mapping one’s properties to beans
in faces-config.xml, but at times one may need to grab a bean explicitly. The
FacesContextUtils
class makes this easy. It is similar to WebApplicationContextUtils
, except that it
takes a FacesContext
parameter rather than a ServletContext
parameter.
ApplicationContext ctx = FacesContextUtils.getWebApplicationContext(FacesContext.getCurrentInstance());
Invented by Craig McClanahan, Struts is an open source project hosted by the Apache Software Foundation. At the time, it greatly simplified the JSP/Servlet programming paradigm and won over many developers who were using proprietary frameworks. It simplified the programming model, it was open source (and thus free as in beer), and it had a large community, which allowed the project to grow and become popular among Java web developers.
Check out the Struts Spring Plugin for the built-in Spring integration shipped with Struts.
From the Tapestry homepage:
Tapestry is a "Component oriented framework for creating dynamic, robust, highly scalable web applications in Java."
While Spring has its own powerful web layer, there are a number of unique advantages to building an enterprise Java application using a combination of Tapestry for the web user interface and the Spring container for the lower layers.
For more information, check out Tapestry’s dedicated integration module for Spring.
In addition to supporting conventional (servlet-based) Web development, Spring also supports JSR-168 Portlet development. As much as possible, the Portlet MVC framework is a mirror image of the Web MVC framework, and also uses the same underlying view abstractions and integration technology. So, be sure to review the chapters entitled Chapter 16, Web MVC framework and Chapter 17, View technologies before continuing with this chapter.
Note | |
---|---|
Bear in mind that while the concepts of Spring MVC are the same in Spring Portlet MVC, there are some notable differences created by the unique workflow of JSR-168 portlets. |
The main way in which portlet workflow differs from servlet workflow is that the request to the portlet can have two distinct phases: the action phase and the render phase. The action phase is executed only once and is where any backend changes or actions occur, such as making changes in a database. The render phase then produces what is displayed to the user each time the display is refreshed. The critical point here is that for a single overall request, the action phase is executed only once, but the render phase may be executed multiple times. This provides (and requires) a clean separation between the activities that modify the persistent state of your system and the activities that generate what is displayed to the user.
The dual phases of portlet requests are one of the real strengths of the JSR-168
specification. For example, dynamic search results can be updated routinely on the
display without the user explicitly rerunning the search. Most other portlet MVC
frameworks attempt to completely hide the two phases from the developer and make it look
as much like traditional servlet development as possible - we think this approach
removes one of the main benefits of using portlets. So, the separation of the two phases
is preserved throughout the Spring Portlet MVC framework. The primary manifestation of
this approach is that where the servlet version of the MVC classes will have one method
that deals with the request, the portlet version of the MVC classes will have two
methods that deal with the request: one for the action phase and one for the render
phase. For example, where the servlet version of AbstractController
has the
handleRequestInternal(..)
method, the portlet version of AbstractController
has
handleActionRequestInternal(..)
and handleRenderRequestInternal(..)
methods.
The framework is designed around a DispatcherPortlet
that dispatches requests to
handlers, with configurable handler mappings and view resolution, just as the
DispatcherServlet
in the web framework does. File upload is also supported in the same
way.
Locale resolution and theme resolution are not supported in Portlet MVC - these areas
are in the purview of the portal/portlet container and are not appropriate at the Spring
level. However, all mechanisms in Spring that depend on the locale (such as
internationalization of messages) will still function properly because
DispatcherPortlet
exposes the current locale in the same way as DispatcherServlet
.
The default handler is still a very simple Controller
interface, offering just two
methods:
void handleActionRequest(request,response)
ModelAndView handleRenderRequest(request,response)
The framework also includes most of the same controller implementation hierarchy, such
as AbstractController
, SimpleFormController
, and so on. Data binding, command object
usage, model handling, and view resolution are all the same as in the servlet framework.
All the view rendering capabilities of the servlet framework are used directly via a
special bridge servlet named ViewRendererServlet
. By using this servlet, the portlet
request is converted into a servlet request and the view can be rendered using the
entire normal servlet infrastructure. This means all the existing renderers, such as
JSP, Velocity, etc., can still be used within the portlet.
Spring Portlet MVC supports beans whose lifecycle is scoped to the current HTTP request
or HTTP Session
(both normal and global). This is not a specific feature of Spring
Portlet MVC itself, but rather of the WebApplicationContext
container(s) that Spring
Portlet MVC uses. These bean scopes are described in detail in
Section 4.5.4, “Request, session, and global session scopes”
Portlet MVC is a request-driven web MVC framework, designed around a portlet that
dispatches requests to controllers and offers other functionality facilitating the
development of portlet applications. Spring’s DispatcherPortlet
however, does more
than just that. It is completely integrated with the Spring ApplicationContext
and
allows you to use every other feature Spring has.
Like ordinary portlets, the DispatcherPortlet
is declared in the portlet.xml
file of
your web application:
<portlet> <portlet-name>sample</portlet-name> <portlet-class>org.springframework.web.portlet.DispatcherPortlet</portlet-class> <supports> <mime-type>text/html</mime-type> <portlet-mode>view</portlet-mode> </supports> <portlet-info> <title>Sample Portlet</title> </portlet-info> </portlet>
The DispatcherPortlet
now needs to be configured.
In the Portlet MVC framework, each DispatcherPortlet
has its own
WebApplicationContext
, which inherits all the beans already defined in the Root
WebApplicationContext
. These inherited beans can be overridden in the portlet-specific
scope, and new scope-specific beans can be defined local to a given portlet instance.
The framework will, on initialization of a DispatcherPortlet
, look for a file named
[portlet-name]-portlet.xml
in the WEB-INF
directory of your web application and
create the beans defined there (overriding the definitions of any beans defined with the
same name in the global scope).
The config location used by the DispatcherPortlet
can be modified through a portlet
initialization parameter (see below for details).
The Spring DispatcherPortlet
has a few special beans it uses, in order to be able to
process requests and render the appropriate views. These beans are included in the
Spring framework and can be configured in the WebApplicationContext
, just as any other
bean would be configured. Each of those beans is described in more detail below. Right
now, we’ll just mention them, just to let you know they exist and to enable us to go on
talking about the DispatcherPortlet
. For most of the beans, defaults are provided so
you don’t have to worry about configuring them.
Table 19.1. Special beans in the WebApplicationContext
Expression | Explanation |
---|---|
handler mapping(s) | (Section 19.5, “Handler mappings”) a list of pre- and post-processors and controllers that will be executed if they match certain criteria (for instance a matching portlet mode specified with the controller) |
controller(s) | (Section 19.4, “Controllers”) the beans providing the actual functionality (or at least, access to the functionality) as part of the MVC triad |
view resolver | (Section 19.6, “Views and resolving them”) capable of resolving view names to view definitions |
multipart resolver | (Section 19.7, “Multipart (file upload) support”) offers functionality to process file uploads from HTML forms |
handler exception resolver | (Section 19.8, “Handling exceptions”) offers functionality to map exceptions to views or implement other more complex exception handling code |
When a DispatcherPortlet
is setup for use and a request comes in for that specific
DispatcherPortlet
, it starts processing the request. The list below describes the
complete process a request goes through if handled by a DispatcherPortlet
:
PortletRequest.getLocale()
is bound to the request to let
elements in the process resolve the locale to use when processing the request (rendering
the view, preparing data, etc.).
ActionRequest
, the request is
inspected for multiparts and if they are found, it is wrapped in a
MultipartActionRequest
for further processing by other elements in the process. (See
Section 19.7, “Multipart (file upload) support” for further information about multipart handling).
WebApplicationContext
. If no model is returned (which could be due
to a pre- or post-processor intercepting the request, for example, for security
reasons), no view is rendered, since the request could already have been fulfilled.
Exceptions that are thrown during processing of the request get picked up by any of the
handler exception resolvers that are declared in the WebApplicationContext
. Using
these exception resolvers you can define custom behavior in case such exceptions get
thrown.
You can customize Spring’s DispatcherPortlet
by adding context parameters in the
portlet.xml
file or portlet init-parameters. The possibilities are listed below.
Table 19.2. DispatcherPortlet initialization parameters
Parameter | Explanation |
---|---|
| Class that implements |
| String which is passed to the context instance (specified by |
| The namespace of the |
| The URL at which |
The rendering process in Portlet MVC is a bit more complex than in Web MVC. In order to
reuse all the view technologies from Spring Web MVC, we must convert the
PortletRequest
/ PortletResponse
to HttpServletRequest
/ HttpServletResponse
and
then call the render
method of the View
. To do this, DispatcherPortlet
uses a
special servlet that exists for just this purpose: the ViewRendererServlet
.
In order for DispatcherPortlet
rendering to work, you must declare an instance of the
ViewRendererServlet
in the web.xml
file for your web application as follows:
<servlet> <servlet-name>ViewRendererServlet</servlet-name> <servlet-class>org.springframework.web.servlet.ViewRendererServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>ViewRendererServlet</servlet-name> <url-pattern>/WEB-INF/servlet/view</url-pattern> </servlet-mapping>
To perform the actual rendering, DispatcherPortlet
does the following:
WebApplicationContext
to the request as an attribute under the same
WEB_APPLICATION_CONTEXT_ATTRIBUTE
key that DispatcherServlet
uses.
Model
and View
objects to the request to make them available to the
ViewRendererServlet
.
PortletRequestDispatcher
and performs an include
using the /WEB-
INF/servlet/view
URL that is mapped to the ViewRendererServlet
.
The ViewRendererServlet
is then able to call the render
method on the View
with
the appropriate arguments.
The actual URL for the ViewRendererServlet
can be changed using DispatcherPortlet
's
viewRendererUrl
configuration parameter.
The controllers in Portlet MVC are very similar to the Web MVC Controllers, and porting code from one to the other should be simple.
The basis for the Portlet MVC controller architecture is the
org.springframework.web.portlet.mvc.Controller
interface, which is listed below.
public interface Controller { /** * Process the render request and return a ModelAndView object which the * DispatcherPortlet will render. */ ModelAndView handleRenderRequest(RenderRequest request, RenderResponse response) throws Exception; /** * Process the action request. There is nothing to return. */ void handleActionRequest(ActionRequest request, ActionResponse response) throws Exception; }
As you can see, the Portlet Controller
interface requires two methods that handle the
two phases of a portlet request: the action request and the render request. The action
phase should be capable of handling an action request, and the render phase should be
capable of handling a render request and returning an appropriate model and view. While
the Controller
interface is quite abstract, Spring Portlet MVC offers several
controllers that already contain a lot of the functionality you might need; most of
these are very similar to controllers from Spring Web MVC. The Controller
interface
just defines the most common functionality required of every controller: handling an
action request, handling a render request, and returning a model and a view.
Of course, just a Controller
interface isn’t enough. To provide a basic
infrastructure, all of Spring Portlet MVC’s Controller
s inherit from
AbstractController
, a class offering access to Spring’s ApplicationContext
and
control over caching.
Table 19.3. Features offered by the AbstractController
Parameter | Explanation |
---|---|
| Indicates whether or not this |
| Use this if you want handling by this controller to be synchronized on the user’s
session. To be more specific, the extending controller will override the
|
| If you want your controller to actually render the view when the portlet is in a minimized state, set this to true. By default, this is set to false so that portlets that are in a minimized state don’t display any content. |
| When you want a controller to override the default cache expiration defined for the
portlet, specify a positive integer here. By default it is set to |
The requireSession
and cacheSeconds
properties are declared on the
PortletContentGenerator
class, which is the superclass of AbstractController
) but
are included here for completeness.
When using the AbstractController
as a base class for your controllers (which is not
recommended since there are a lot of other controllers that might already do the job for
you) you only have to override either the handleActionRequestInternal(ActionRequest,
ActionResponse)
method or the handleRenderRequestInternal(RenderRequest,
RenderResponse)
method (or both), implement your logic, and return a ModelAndView
object (in the case of handleRenderRequestInternal
).
The default implementations of both handleActionRequestInternal(..)
and
handleRenderRequestInternal(..)
throw a PortletException
. This is consistent with
the behavior of GenericPortlet
from the JSR- 168 Specification API. So you only need
to override the method that your controller is intended to handle.
Here is short example consisting of a class and a declaration in the web application context.
package samples; import javax.portlet.RenderRequest; import javax.portlet.RenderResponse; import org.springframework.web.portlet.mvc.AbstractController; import org.springframework.web.portlet.ModelAndView; public class SampleController extends AbstractController { public ModelAndView handleRenderRequestInternal(RenderRequest request, RenderResponse response) { ModelAndView mav = new ModelAndView("foo"); mav.addObject("message", "Hello World!"); return mav; } }
<bean id="sampleController" class="samples.SampleController"> <property name="cacheSeconds" value="120"/> </bean>
The class above and the declaration in the web application context is all you need besides setting up a handler mapping (see Section 19.5, “Handler mappings”) to get this very simple controller working.
Although you can extend AbstractController
, Spring Portlet MVC provides a number of
concrete implementations which offer functionality that is commonly used in simple MVC
applications.
The ParameterizableViewController
is basically the same as the example above, except
for the fact that you can specify the view name that it will return in the web
application context (no need to hard-code the view name).
The PortletModeNameViewController
uses the current mode of the portlet as the view
name. So, if your portlet is in View mode (i.e. PortletMode.VIEW
) then it uses "view"
as the view name.
Spring Portlet MVC has the exact same hierarchy of command controllers as Spring Web
MVC. They provide a way to interact with data objects and dynamically bind parameters
from the PortletRequest
to the data object specified. Your data objects don’t have to
implement a framework-specific interface, so you can directly manipulate your persistent
objects if you desire. Let’s examine what command controllers are available, to get an
overview of what you can do with them:
AbstractCommandController
- a command controller you can use to create your own
command controller, capable of binding request parameters to a data object you
specify. This class does not offer form functionality, it does however offer
validation features and lets you specify in the controller itself what to do with the
command object that has been filled with the parameters from the request.
AbstractFormController
- an abstract controller offering form submission support.
Using this controller you can model forms and populate them using a command object you
retrieve in the controller. After a user has filled the form, AbstractFormController
binds the fields, validates, and hands the object back to the controller to take
appropriate action. Supported features are: invalid form submission (resubmission),
validation, and normal form workflow. You implement methods to determine which views
are used for form presentation and success. Use this controller if you need forms, but
don’t want to specify what views you’re going to show the user in the application
context.
SimpleFormController
- a concrete AbstractFormController
that provides even more
support when creating a form with a corresponding command object. The
SimpleFormController
lets you specify a command object, a viewname for the form, a
viewname for the page you want to show the user when form submission has succeeded,
and more.
AbstractWizardFormController
— a concrete AbstractFormController
that provides a
wizard-style interface for editing the contents of a command object across multiple
display pages. Supports multiple user actions: finish, cancel, or page change, all of
which are easily specified in request parameters from the view.
These command controllers are quite powerful, but they do require a detailed understanding of how they operate in order to use them efficiently. Carefully review the Javadocs for this entire hierarchy and then look at some sample implementations before you start using them.
Instead of developing new controllers, it is possible to use existing portlets and map
requests to them from a DispatcherPortlet
. Using the PortletWrappingController
, you
can instantiate an existing Portlet
as a Controller
as follows:
<bean id="myPortlet" class="org.springframework.web.portlet.mvc.PortletWrappingController"> <property name="portletClass" value="sample.MyPortlet"/> <property name="portletName" value="my-portlet"/> <property name="initParameters"> <value>config=/WEB-INF/my-portlet-config.xml</value> </property> </bean>
This can be very valuable since you can then use interceptors to pre-process and
post-process requests going to these portlets. Since JSR-168 does not support any kind
of filter mechanism, this is quite handy. For example, this can be used to wrap the
Hibernate OpenSessionInViewInterceptor
around a MyFaces JSF Portlet.
Using a handler mapping you can map incoming portlet requests to appropriate handlers.
There are some handler mappings you can use out of the box, for example, the
PortletModeHandlerMapping
, but let’s first examine the general concept of a
HandlerMapping
.
Note: We are intentionally using the term "Handler" here instead of "Controller".
DispatcherPortlet
is designed to be used with other ways to process requests than just
Spring Portlet MVC’s own Controllers. A Handler is any Object that can handle portlet
requests. Controllers are an example of Handlers, and they are of course the default. To
use some other framework with DispatcherPortlet
, a corresponding implementation of
HandlerAdapter
is all that is needed.
The functionality a basic HandlerMapping
provides is the delivering of a
HandlerExecutionChain
, which must contain the handler that matches the incoming
request, and may also contain a list of handler interceptors that are applied to the
request. When a request comes in, the DispatcherPortlet
will hand it over to the
handler mapping to let it inspect the request and come up with an appropriate
HandlerExecutionChain
. Then the DispatcherPortlet
will execute the handler and
interceptors in the chain (if any). These concepts are all exactly the same as in Spring
Web MVC.
The concept of configurable handler mappings that can optionally contain interceptors
(executed before or after the actual handler was executed, or both) is extremely
powerful. A lot of supporting functionality can be built into a custom HandlerMapping
.
Think of a custom handler mapping that chooses a handler not only based on the portlet
mode of the request coming in, but also on a specific state of the session associated
with the request.
In Spring Web MVC, handler mappings are commonly based on URLs. Since there is really no such thing as a URL within a Portlet, we must use other mechanisms to control mappings. The two most common are the portlet mode and a request parameter, but anything available to the portlet request can be used in a custom handler mapping.
The rest of this section describes three of Spring Portlet MVC’s most commonly used
handler mappings. They all extend AbstractHandlerMapping
and share the following
properties:
interceptors
: The list of interceptors to use. HandlerInterceptor
s are discussed
in Section 19.5.4, “Adding HandlerInterceptors”.
defaultHandler
: The default handler to use, when this handler mapping does not
result in a matching handler.
order
: Based on the value of the order property (see the
org.springframework.core.Ordered
interface), Spring will sort all handler mappings
available in the context and apply the first matching handler.
lazyInitHandlers
: Allows for lazy initialization of singleton handlers (prototype
handlers are always lazily initialized). Default value is false. This property is
directly implemented in the three concrete Handlers.
This is a simple handler mapping that maps incoming requests based on the current mode of the portlet (e.g. view, edit, help). An example:
<bean class="org.springframework.web.portlet.handler.PortletModeHandlerMapping"> <property name="portletModeMap"> <map> <entry key="view" value-ref="viewHandler"/> <entry key="edit" value-ref="editHandler"/> <entry key="help" value-ref="helpHandler"/> </map> </property> </bean>
If we need to navigate around to multiple controllers without changing portlet mode, the simplest way to do this is with a request parameter that is used as the key to control the mapping.
ParameterHandlerMapping
uses the value of a specific request parameter to control the
mapping. The default name of the parameter is 'action'
, but can be changed using the
'parameterName'
property.
The bean configuration for this mapping will look something like this:
<bean class="org.springframework.web.portlet.handler.ParameterHandlerMapping"> <property name="parameterMap"> <map> <entry key="add" value-ref="addItemHandler"/> <entry key="edit" value-ref="editItemHandler"/> <entry key="delete" value-ref="deleteItemHandler"/> </map> </property> </bean>
The most powerful built-in handler mapping, PortletModeParameterHandlerMapping
combines the capabilities of the two previous ones to allow different navigation within
each portlet mode.
Again the default name of the parameter is "action", but can be changed using the
parameterName
property.
By default, the same parameter value may not be used in two different portlet modes.
This is so that if the portal itself changes the portlet mode, the request will no
longer be valid in the mapping. This behavior can be changed by setting the
allowDupParameters
property to true. However, this is not recommended.
The bean configuration for this mapping will look something like this:
<bean class="org.springframework.web.portlet.handler.PortletModeParameterHandlerMapping"> <property name="portletModeParameterMap"> <map> <entry key="view"> <!-- view portlet mode --> <map> <entry key="add" value-ref="addItemHandler"/> <entry key="edit" value-ref="editItemHandler"/> <entry key="delete" value-ref="deleteItemHandler"/> </map> </entry> <entry key="edit"> <!-- edit portlet mode --> <map> <entry key="prefs" value-ref="prefsHandler"/> <entry key="resetPrefs" value-ref="resetPrefsHandler"/> </map> </entry> </map> </property> </bean>
This mapping can be chained ahead of a PortletModeHandlerMapping
, which can then
provide defaults for each mode and an overall default as well.
Spring’s handler mapping mechanism has a notion of handler interceptors, which can be extremely useful when you want to apply specific functionality to certain requests, for example, checking for a principal. Again Spring Portlet MVC implements these concepts in the same way as Web MVC.
Interceptors located in the handler mapping must implement HandlerInterceptor
from the
org.springframework.web.portlet
package. Just like the servlet version, this interface
defines three methods: one that will be called before the actual handler will be
executed ( preHandle
), one that will be called after the handler is executed (
postHandle
), and one that is called after the complete request has finished (
afterCompletion
). These three methods should provide enough flexibility to do all
kinds of pre- and post- processing.
The preHandle
method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns true
, the
handler execution chain will continue. When it returns false
, the DispatcherPortlet
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.
The postHandle
method is only called on a RenderRequest
. The preHandle
and
afterCompletion
methods are called on both an ActionRequest
and a RenderRequest
.
If you need to execute logic in these methods for just one type of request, be sure to
check what kind of request it is before processing it.
As with the servlet package, the portlet package has a concrete implementation of
HandlerInterceptor
called HandlerInterceptorAdapter
. This class has empty versions
of all the methods so that you can inherit from this class and implement just one or two
methods when that is all you need.
The portlet package also has a concrete interceptor named ParameterMappingInterceptor
that is meant to be used directly with ParameterHandlerMapping
and
PortletModeParameterHandlerMapping
. This interceptor will cause the parameter that is
being used to control the mapping to be forwarded from an ActionRequest
to the
subsequent RenderRequest
. This will help ensure that the RenderRequest
is mapped to
the same Handler as the ActionRequest
. This is done in the preHandle
method of the
interceptor, so you can still modify the parameter value in your handler to change where
the RenderRequest
will be mapped.
Be aware that this interceptor is calling setRenderParameter
on the ActionResponse
,
which means that you cannot call sendRedirect
in your handler when using this
interceptor. If you need to do external redirects then you will either need to forward
the mapping parameter manually or write a different interceptor to handle this for you.
As mentioned previously, Spring Portlet MVC directly reuses all the view technologies
from Spring Web MVC. This includes not only the various View
implementations
themselves, but also the ViewResolver
implementations. For more information, refer to
Chapter 17, View technologies and Section 16.5, “Resolving views” respectively.
A few items on using the existing View
and ViewResolver
implementations are worth
mentioning:
sendRedirect(..)
method of ActionResponse
cannot be used to stay within the
portal). So, RedirectView
and use of the 'redirect:'
prefix will not work
correctly from within Portlet MVC.
'forward:'
prefix from within Portlet MVC. However,
remember that since you are in a portlet, you have no idea what the current URL looks
like. This means you cannot use a relative URL to access other resources in your web
application and that you will have to use an absolute URL.
Also, for JSP development, the new Spring Taglib and the new Spring Form Taglib both work in portlet views in exactly the same way that they work in servlet views.
Spring Portlet MVC has built-in multipart support to handle file uploads in portlet
applications, just like Web MVC does. The design for the multipart support is done with
pluggable PortletMultipartResolver
objects, defined in the
org.springframework.web.portlet.multipart
package. Spring provides a
PortletMultipartResolver
for use with
Commons FileUpload. How uploading files is
supported will be described in the rest of this section.
By default, no multipart handling will be done by Spring Portlet MVC, as some developers
will want to handle multiparts themselves. You will have to enable it yourself by adding
a multipart resolver to the web application’s context. After you have done that,
DispatcherPortlet
will inspect each request to see if it contains a multipart. If no
multipart is found, the request will continue as expected. However, if a multipart is
found in the request, the PortletMultipartResolver
that has been declared in your
context will be used. After that, the multipart attribute in your request will be
treated like any other attribute.
Note | |
---|---|
Any configured |
The following example shows how to use the CommonsPortletMultipartResolver
:
<bean id="portletMultipartResolver" class="org.springframework.web.portlet.multipart.CommonsPortletMultipartResolver"> <!-- one of the properties available; the maximum file size in bytes --> <property name="maxUploadSize" value="100000"/> </bean>
Of course you also need to put the appropriate jars in your classpath for the multipart
resolver to work. In the case of the CommonsMultipartResolver
, you need to use
commons-fileupload.jar
. Be sure to use at least version 1.1 of Commons FileUpload as
previous versions do not support JSR-168 Portlet applications.
Now that you have seen how to set Portlet MVC up to handle multipart requests, let’s
talk about how to actually use it. When DispatcherPortlet
detects a multipart request,
it activates the resolver that has been declared in your context and hands over the
request. What the resolver then does is wrap the current ActionRequest
in a
MultipartActionRequest
that has support for multipart file uploads. Using the
MultipartActionRequest
you can get information about the multiparts contained by this
request and actually get access to the multipart files themselves in your controllers.
Note that you can only receive multipart file uploads as part of an ActionRequest
, not
as part of a RenderRequest
.
After the PortletMultipartResolver
has finished doing its job, the request will be
processed like any other. To use the PortletMultipartResolver
, create a form with an
upload field (see example below), then let Spring bind the file onto your form (backing
object). To actually let the user upload a file, we have to create a (JSP/HTML) form:
<h1>Please upload a file</h1> <form method="post" action="<portlet:actionURL/>" enctype="multipart/form-data"> <input type="file" name="file"/> <input type="submit"/> </form>
As you can see, we’ve created a field named "file" that matches the property of the bean
that holds the byte[]
array. Furthermore we’ve added the encoding attribute (
enctype="multipart/form-data"
), which is necessary to let the browser know how to
encode the multipart fields (do not forget this!).
Just as with any other property that’s not automagically convertible to a string or
primitive type, to be able to put binary data in your objects you have to register a
custom editor with the PortletRequestDataBinder
. There are a couple of editors
available for handling files and setting the results on an object. There’s a
StringMultipartFileEditor
capable of converting files to Strings (using a user-defined
character set), and there is a ByteArrayMultipartFileEditor
which converts files to
byte arrays. They function analogous to the CustomDateEditor
.
So, to be able to upload files using a form, declare the resolver, a mapping to a controller that will process the bean, and the controller itself.
<bean id="portletMultipartResolver" class="org.springframework.web.portlet.multipart.CommonsPortletMultipartResolver"/> <bean class="org.springframework.web.portlet.handler.PortletModeHandlerMapping"> <property name="portletModeMap"> <map> <entry key="view" value-ref="fileUploadController"/> </map> </property> </bean> <bean id="fileUploadController" class="examples.FileUploadController"> <property name="commandClass" value="examples.FileUploadBean"/> <property name="formView" value="fileuploadform"/> <property name="successView" value="confirmation"/> </bean>
After that, create the controller and the actual class to hold the file property.
public class FileUploadController extends SimpleFormController { public void onSubmitAction(ActionRequest request, ActionResponse response, Object command, BindException errors) throws Exception { // cast the bean FileUploadBean bean = (FileUploadBean) command; // let's see if there's content there byte[] file = bean.getFile(); if (file == null) { // hmm, that's strange, the user did not upload anything } // do something with the file here } protected void initBinder(PortletRequest request, PortletRequestDataBinder binder) throws Exception { // to actually be able to convert Multipart instance to byte[] // we have to register a custom editor binder.registerCustomEditor(byte[].class, new ByteArrayMultipartFileEditor()); // now Spring knows how to handle multipart object and convert } } public class FileUploadBean { private byte[] file; public void setFile(byte[] file) { this.file = file; } public byte[] getFile() { return file; } }
As you can see, the FileUploadBean
has a property of type byte[]
that holds the
file. The controller registers a custom editor to let Spring know how to actually
convert the multipart objects the resolver has found to properties specified by the
bean. In this example, nothing is done with the byte[]
property of the bean itself,
but in practice you can do whatever you want (save it in a database, mail it to
somebody, etc).
An equivalent example in which a file is bound straight to a String-typed property on a form backing object might look like this:
public class FileUploadController extends SimpleFormController { public void onSubmitAction(ActionRequest request, ActionResponse response, Object command, BindException errors) throws Exception { // cast the bean FileUploadBean bean = (FileUploadBean) command; // let's see if there's content there String file = bean.getFile(); if (file == null) { // hmm, that's strange, the user did not upload anything } // do something with the file here } protected void initBinder(PortletRequest request, PortletRequestDataBinder binder) throws Exception { // to actually be able to convert Multipart instance to a String // we have to register a custom editor binder.registerCustomEditor(String.class, new StringMultipartFileEditor()); // now Spring knows how to handle multipart objects and convert } } public class FileUploadBean { private String file; public void setFile(String file) { this.file = file; } public String getFile() { return file; } }
Of course, this last example only makes (logical) sense in the context of uploading a plain text file (it wouldn’t work so well in the case of uploading an image file).
The third (and final) option is where one binds directly to a MultipartFile
property
declared on the (form backing) object’s class. In this case one does not need to
register any custom property editor because there is no type conversion to be performed.
public class FileUploadController extends SimpleFormController { public void onSubmitAction(ActionRequest request, ActionResponse response, Object command, BindException errors) throws Exception { // cast the bean FileUploadBean bean = (FileUploadBean) command; // let's see if there's content there MultipartFile file = bean.getFile(); if (file == null) { // hmm, that's strange, the user did not upload anything } // do something with the file here } } public class FileUploadBean { private MultipartFile file; public void setFile(MultipartFile file) { this.file = file; } public MultipartFile getFile() { return file; } }
Just like Servlet MVC, Portlet MVC provides HandlerExceptionResolver
s to ease the
pain of unexpected exceptions that occur while your request is being processed by a
handler that matched the request. Portlet MVC also provides a portlet-specific, concrete
SimpleMappingExceptionResolver
that enables you to take the class name of any
exception that might be thrown and map it to a view name.
Spring 2.5 introduced an annotation-based programming model for MVC controllers, using
annotations such as @RequestMapping
, @RequestParam
, @ModelAttribute
, etc. This
annotation support is available for both Servlet MVC and Portlet MVC. Controllers
implemented in this style do not have to extend specific base classes or implement
specific interfaces. Furthermore, they do not usually have direct dependencies on
Servlet or Portlet API’s, although they can easily get access to Servlet or Portlet
facilities if desired.
The following sections document these annotations and how they are most commonly used in a Portlet environment.
@RequestMapping
will only be processed if a corresponding HandlerMapping
(for
type level annotations) and/or HandlerAdapter
(for method level annotations) is
present in the dispatcher. This is the case by default in both DispatcherServlet
and
DispatcherPortlet
.
However, if you are defining custom HandlerMappings
or HandlerAdapters
, then you
need to make sure that a corresponding custom DefaultAnnotationHandlerMapping
and/or
AnnotationMethodHandlerAdapter
is defined as well - provided that you intend to use
@RequestMapping
.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean class="org.springframework.web.portlet.mvc.annotation.DefaultAnnotationHandlerMapping"/> <bean class="org.springframework.web.portlet.mvc.annotation.AnnotationMethodHandlerAdapter"/> // ... (controller bean definitions) ... </beans>
Defining a DefaultAnnotationHandlerMapping
and/or AnnotationMethodHandlerAdapter
explicitly also makes sense if you would like to customize the mapping strategy, e.g.
specifying a custom WebBindingInitializer
(see below).
The @Controller
annotation indicates that a particular class serves the role of a
controller. There is no need to extend any controller base class or reference the
Portlet API. You are of course still able to reference Portlet-specific features if you
need to.
The basic purpose of the @Controller
annotation is to act as a stereotype for the
annotated class, indicating its role. The dispatcher will scan such annotated classes
for mapped methods, detecting @RequestMapping
annotations (see the next section).
Annotated controller beans may be defined explicitly, using a standard Spring bean
definition in the dispatcher’s context. However, the @Controller
stereotype also
allows for autodetection, aligned with Spring 2.5’s general support for detecting
component classes in the classpath and auto-registering bean definitions for them.
To enable autodetection of such annotated controllers, you have to add component scanning to your configuration. This is easily achieved by using the spring-context schema as shown in the following XML snippet:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="org.springframework.samples.petportal.portlet"/> // ... </beans>
The @RequestMapping
annotation is used to map portlet modes like VIEW/EDIT onto an
entire class or a particular handler method. Typically the type-level annotation maps a
specific mode (or mode plus parameter condition) onto a form controller, with additional
method-level annotations narrowing the primary mapping for specific portlet request
parameters.
Tip | |
---|---|
In the following discussion, we’ll focus on controllers that are based on annotated handler methods. |
The following is an example of a form controller from the PetPortal sample application using this annotation:
@Controller @RequestMapping("EDIT") @SessionAttributes("site") public class PetSitesEditController { private Properties petSites; public void setPetSites(Properties petSites) { this.petSites = petSites; } @ModelAttribute("petSites") public Properties getPetSites() { return this.petSites; } @RequestMapping // default (action=list) public String showPetSites() { return "petSitesEdit"; } @RequestMapping(params = "action=add") // render phase public String showSiteForm(Model model) { // Used for the initial form as well as for redisplaying with errors. if (!model.containsAttribute("site")) { model.addAttribute("site", new PetSite()); } return "petSitesAdd"; } @RequestMapping(params = "action=add") // action phase public void populateSite(@ModelAttribute("site") PetSite petSite, BindingResult result, SessionStatus status, ActionResponse response) { new PetSiteValidator().validate(petSite, result); if (!result.hasErrors()) { this.petSites.put(petSite.getName(), petSite.getUrl()); status.setComplete(); response.setRenderParameter("action", "list"); } } @RequestMapping(params = "action=delete") public void removeSite(@RequestParam("site") String site, ActionResponse response) { this.petSites.remove(site); response.setRenderParameter("action", "list"); } }
Handler methods which are annotated with @RequestMapping
are allowed to have very
flexible signatures. They may have arguments of the following types, in arbitrary order
(except for validation results, which need to follow right after the corresponding
command object, if desired):
null
.
org.springframework.web.context.request.WebRequest
or
org.springframework.web.context.request.NativeWebRequest
. Allows for generic request
parameter access as well as request/session attribute access, without ties to the
native Servlet/Portlet API.
java.util.Locale
for the current request locale (the portal locale in a Portlet
environment).
java.util.TimeZone
/ java.time.ZoneId
for the current request time zone.
java.io.InputStream
/ java.io.Reader
for access to the request’s content. This
will be the raw InputStream/Reader as exposed by the Portlet API.
java.io.OutputStream
/ java.io.Writer
for generating the response’s content. This
will be the raw OutputStream/Writer as exposed by the Portlet API.
@RequestParam
annotated parameters for access to specific Portlet request
parameters. Parameter values will be converted to the declared method argument type.
java.util.Map
/ org.springframework.ui.Model
/ org.springframework.ui.ModelMap
for enriching the implicit model that will be exposed to the web view.
@InitBinder
methods and/or the
HandlerAdapter configuration - see the " webBindingInitializer
" property on
AnnotationMethodHandlerAdapter
. Such command objects along with their validation
results will be exposed as model attributes, by default using the non-qualified
command class name in property notation (e.g. "orderAddress" for type
"mypackage.OrderAddress"). Specify a parameter-level ModelAttribute
annotation for
declaring a specific model attribute name.
org.springframework.validation.Errors
/
org.springframework.validation.BindingResult
validation results for a preceding
command/form object (the immediate preceding argument).
org.springframework.web.bind.support.SessionStatus
status handle for marking form
processing as complete (triggering the cleanup of session attributes that have been
indicated by the @SessionAttributes
annotation at the handler type level).
The following return types are supported for handler methods:
ModelAndView
object, with the model implicitly enriched with command objects and
the results of @ModelAttribute
annotated reference data accessor methods.
Model
object, with the view name implicitly determined through a
RequestToViewNameTranslator
and the model implicitly enriched with command objects
and the results of @ModelAttribute
annotated reference data accessor methods.
Map
object for exposing a model, with the view name implicitly determined through
a RequestToViewNameTranslator
and the model implicitly enriched with command objects
and the results of @ModelAttribute
annotated reference data accessor methods.
View
object, with the model implicitly determined through command objects and
@ModelAttribute
annotated reference data accessor methods. The handler method may
also programmatically enrich the model by declaring a Model
argument (see above).
String
value which is interpreted as view name, with the model implicitly
determined through command objects and @ModelAttribute
annotated reference data
accessor methods. The handler method may also programmatically enrich the model by
declaring a Model
argument (see above).
void
if the method handles the response itself (e.g. by writing the response content
directly).
@ModelAttribute
at the method level
(or the default attribute name based on the return type’s class name otherwise). The
model will be implicitly enriched with command objects and the results of
@ModelAttribute
annotated reference data accessor methods.
The @RequestParam
annotation is used to bind request parameters to a method parameter
in your controller.
The following code snippet from the PetPortal sample application shows the usage:
@Controller @RequestMapping("EDIT") @SessionAttributes("site") public class PetSitesEditController { // ... public void removeSite(@RequestParam("site") String site, ActionResponse response) { this.petSites.remove(site); response.setRenderParameter("action", "list"); } // ... }
Parameters using this annotation are required by default, but you can specify that a
parameter is optional by setting @RequestParam
's required
attribute to false
(e.g., @RequestParam(value="id", required=false)
).
@ModelAttribute
has two usage scenarios in controllers. When placed on a method
parameter, @ModelAttribute
is used to map a model attribute to the specific, annotated
method parameter (see the populateSite()
method below). This is how the controller
gets a reference to the object holding the data entered in the form. In addition, the
parameter can be declared as the specific type of the form backing object rather than as
a generic java.lang.Object
, thus increasing type safety.
@ModelAttribute
is also used at the method level to provide reference data for the
model (see the getPetSites()
method below). For this usage the method signature can
contain the same types as documented above for the @RequestMapping
annotation.
Note | |
---|---|
|
The following code snippet shows these two usages of this annotation:
@Controller @RequestMapping("EDIT") @SessionAttributes("site") public class PetSitesEditController { // ... @ModelAttribute("petSites") public Properties getPetSites() { return this.petSites; } @RequestMapping(params = "action=add") // action phase public void populateSite( @ModelAttribute("site") PetSite petSite, BindingResult result, SessionStatus status, ActionResponse response) { new PetSiteValidator().validate(petSite, result); if (!result.hasErrors()) { this.petSites.put(petSite.getName(), petSite.getUrl()); status.setComplete(); response.setRenderParameter("action", "list"); } } }
The type-level @SessionAttributes
annotation declares session attributes used by a
specific handler. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session or some
conversational storage, serving as form-backing beans between subsequent requests.
The following code snippet shows the usage of this annotation:
@Controller @RequestMapping("EDIT") @SessionAttributes("site") public class PetSitesEditController { // ... }
To customize request parameter binding with PropertyEditors, etc. via Spring’s
WebDataBinder
, you can either use @InitBinder
-annotated methods within your
controller or externalize your configuration by providing a custom
WebBindingInitializer
.
Annotating controller methods with @InitBinder
allows you to configure web data
binding directly within your controller class. @InitBinder
identifies methods which
initialize the WebDataBinder
which will be used for populating command and form object
arguments of annotated handler methods.
Such init-binder methods support all arguments that @RequestMapping
supports, except
for command/form objects and corresponding validation result objects. Init-binder
methods must not have a return value. Thus, they are usually declared as void
. Typical
arguments include WebDataBinder
in combination with WebRequest
or
java.util.Locale
, allowing code to register context-specific editors.
The following example demonstrates the use of @InitBinder
for configuring a
CustomDateEditor
for all java.util.Date
form properties.
@Controller public class MyFormController { @InitBinder public void initBinder(WebDataBinder binder) { SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd"); dateFormat.setLenient(false); binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false)); } // ... }
To externalize data binding initialization, you can provide a custom implementation of
the WebBindingInitializer
interface, which you then enable by supplying a custom bean
configuration for an AnnotationMethodHandlerAdapter
, thus overriding the default
configuration.
The process of deploying a Spring Portlet MVC application is no different than deploying any JSR-168 Portlet application. However, this area is confusing enough in general that it is worth talking about here briefly.
Generally, the portal/portlet container runs in one webapp in your servlet container and
your portlets run in another webapp in your servlet container. In order for the portlet
container webapp to make calls into your portlet webapp it must make cross-context calls
to a well-known servlet that provides access to the portlet services defined in your
portlet.xml
file.
The JSR-168 specification does not specify exactly how this should happen, so each portlet container has its own mechanism for this, which usually involves some kind of "deployment process" that makes changes to the portlet webapp itself and then registers the portlets within the portlet container.
At a minimum, the web.xml
file in your portlet webapp is modified to inject the
well-known servlet that the portlet container will call. In some cases a single servlet
will service all portlets in the webapp, in other cases there will be an instance of the
servlet for each portlet.
Some portlet containers will also inject libraries and/or configuration files into the webapp as well. The portlet container must also make its implementation of the Portlet JSP Tag Library available to your webapp.
The bottom line is that it is important to understand the deployment needs of your target portal and make sure they are met (usually by following the automated deployment process it provides). Be sure to carefully review the documentation from your portal for this process.
Once you have deployed your portlet, review the resulting web.xml
file for sanity.
Some older portals have been known to corrupt the definition of the
ViewRendererServlet
, thus breaking the rendering of your portlets.
This part of the reference documentation covers Spring Framework’s support for WebSocket-style messaging in web applications including use of STOMP as an application level WebSocket sub-protocol.
Section 20.1, “Introduction” establishes a frame of mind in which to think about WebSocket, covering adoption challenges, design considerations, and thoughts on when it is a good fit.
Section 20.2, “WebSocket Server” reviews the Spring WebSocket API on the server-side while Section 20.3, “Fallback Options” explains the SockJS protocol and shows how to configure and use it.
Section 20.4.1, “Overview of STOMP” introduces the STOMP messaging protocol. Section 20.4.2, “Enable STOMP (over WebSocket)” demonstrates how to configure STOMP support in Spring. Section 20.4.3, “Overview of STOMP Message Handling” explains how to use it including writing annotated message handling methods, sending messages, choosing message broker options, as well as working with the special "user" destinations. Finally the section called “Testing Message Handling Controllers” lists three approaches to testing STOMP/WebSocket applications.
The WebSocket protocol RFC 6455 defines an important new capability for web applications: full-duplex, two-way communication between client and server. It is an exciting new capability on the heels of a long history of techniques to make the web more interactive including Java applets, XMLHttpRequest, Adobe Flash, ActiveXObject, various Comet techniques, server-sent events, and others.
A proper introduction of the WebSocket protocol is beyond the scope of this document. At a minimum however it’s important to understand that HTTP is used only for the initial handshake, which relies on a mechanism built into HTTP to request a protocol upgrade (or in this case a protocol switch) to which the server can respond with HTTP status 101 (switching protocols) if it agrees. Assuming the handshake succeeds the TCP socket underlying the HTTP upgrade request remains open and both client and server can use it to send messages to each other.
Spring Framework 4 includes a new spring-websocket
module with comprehensive
WebSocket support. It is compatible with the Java WebSocket API standard
(JSR-356)
and also provides additional value-add as explained in the rest of the introduction.
An important challenge to adoption is the lack of support for WebSocket in some browsers. Notably the first Internet Explorer version to support WebSocket is version 10 (see http://caniuse.com/websockets for support by browser versions). Furthermore, some restrictive proxies may be configured in ways that either preclude the attempt to do HTTP upgrade or otherwise break connection after some time because it has remained opened for too long. A good overview on this topic from Peter Lubbers is available in the InfoQ article "How HTML5 Web Sockets Interact With Proxy Servers".
Therefore to build a WebSocket application today, fallback options are required to simulate the WebSocket API where necessary. Spring Framework provides such transparent fallback options based on the SockJS protocol. These options can be enabled through configuration and do not require modifying the application otherwise.
Aside from short-to-midterm adoption challenges, using WebSocket brings up important design considerations that are important to recognize early on, especially in contrast to what we know about building web applications today.
Today REST is a widely accepted, understood, and supported architecture for building web applications. It is an architecture that relies on having many URLs (nouns), a handful of HTTP methods (verbs), and other principles such as using hypermedia (links), remaining stateless, etc.
By contrast a WebSocket application may use a single URL only for the initial HTTP handshake. All messages thereafter share and flow on the same TCP connection. This points to an entirely different, asynchronous, event-driven, messaging architecture. One that is much closer to traditional messaging applications (e.g. JMS, AMQP).
Spring Framework 4 includes a new spring-messaging
module with key
abstractions from the
Spring Integration project
such as Message
, MessageChannel
, MessageHandler
and others that can serve as
a foundation for such a messaging architecture. The module also includes a
set of annotations for mapping messages to methods, similar to the Spring MVC
annotation based programming model.
WebSocket does imply a messaging architecture but does not mandate the use of any specific messaging protocol. It is a very thin layer over TCP that transforms a stream of bytes into a stream of messages (either text or binary) and not much more. It is up to applications to interpret the meaning of a message.
Unlike HTTP, which is an application-level protocol, in the WebSocket protocol there is simply not enough information in an incoming message for a framework or container to know how to route it or process it. Therefore WebSocket is arguably too low level for anything but a very trivial application. It can be done, but it will likely lead to creating a framework on top. This is comparable to how most web applications today are written using a web framework rather than the Servlet API alone.
For this reason the WebSocket RFC defines the use of
sub-protocols.
During the handshake, client and server can use the header
Sec-WebSocket-Protocol
to agree on a sub-protocol, i.e. a higher, application-level
protocol to use. The use of a sub-protocol is not required, but
even if not used, applications will still need to choose a message
format that both client and server can understand. That format can be custom,
framework-specific, or a standard messaging protocol.
Spring Framework provides support for using STOMP — a simple, messaging protocol originally created for use in scripting languages with frames inspired by HTTP. STOMP is widely support and well suited for use over WebSocket and over the web.
With all the design considerations surrounding the use of WebSocket, it is reasonable to ask when is it appropriate to use?
The best fit for WebSocket is in web applications where client and server need to exchange events at high frequency and at low latency. Prime candidates include but are not limited to applications in finance, games, collaboration, and others. Such applications are both very sensitive to time delays and also need to exchange a wide variety of messages at high frequency.
For other application types, however, this may not be the case. For example, a news or social feed that shows breaking news as they become available may be perfectly okay with simple polling once every few minutes. Here latency is important, but it is acceptable if the news takes a few minutes to appear.
Even in cases where latency is crucial, if the volume of messages is relatively low (e.g. monitoring network failures) the use of long polling should be considered as a relatively simple alternative that works reliably and is comparable by efficiency (again assuming the volume of messages is relatively low).
It is the combination of both low latency and high frequency of messages that can make the use of the WebSocket protocol critical. Even in such applications, the choice remains whether all client-server communication should be done through WebSocket messages as opposed to using HTTP and REST? The answer is going to vary by application, however, it is likely that some functionality may be exposed over both WebSocket and as a REST API in order to provide clients with alternatives. Furthermore, a REST API call may need to broadcast a message to interested clients connected via WebSocket.
Spring Framework allows @Controller
classes to have both
HTTP request handling and WebSocket message handling methods.
Furthermore, a Spring MVC request handling method, or any application
method for that matter, can easily broadcast a message to all interested
WebSocket clients or to a specific user.
The Spring Framework provides a WebSocket API designed to adapt to various WebSocket engines. For example, it runs on JSR-356 runtimes such as Tomcat (7.0.47+) and GlassFish (4.0+) but can also adapt to other WebSocket runtimes such as the Jetty (9.0+) native WebSocket support.
Note | |
---|---|
As explained in the introduction, direct use of a WebSocket API is too low level for applications — until assumptions are made about the format of a message there is little a framework can do to interpret messages or route them via annotations. This is why applications should consider using a sub-protocol and Spring’s STOMP over WebSocket support. When using a higher level protocol, the details of the WebSocket API become less relevant, much like the details of TCP communication are not exposed to applications when using HTTP. Nevertheless this section covers the details of using WebSocket directly. |
Creating a WebSocket server is as simple as implementing WebSocketHandler
or more
likely extending either TextWebSocketHandler
or BinaryWebSocketHandler
:
import org.springframework.web.socket.WebSocketHandler; import org.springframework.web.socket.WebSocketSession; import org.springframework.web.socket.TextMessage; public class MyHandler extends TextWebSocketHandler { @Override public void handleTextMessage(WebSocketSession session, TextMessage message) { // ... } }
There is dedicated WebSocket Java-config and XML namespace support for mapping the above WebSocket handler at a specific URL:
import org.springframework.web.socket.config.annotation.EnableWebSocket; import org.springframework.web.socket.config.annotation.WebSocketConfigurer; import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry; @Configuration @EnableWebSocket public class WebSocketConfig implements WebSocketConfigurer { @Override public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) { registry.addHandler(myHandler(), "/myHandler"); } @Bean public WebSocketHandler myHandler() { return new MyHandler(); } }
XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:handlers> <websocket:mapping path="/myHandler" handler="myHandler"/> </websocket:handlers> <bean id="myHandler" class="org.springframework.samples.MyHandler"/> </beans>
The above is for use in Spring MVC applications and should be included in the
configuration of a DispatcherServlet. However, Spring’s WebSocket
support does not depend on Spring MVC. It is relatively simple to integrate a WebSocketHandler
into other HTTP serving environments with the help of
WebSocketHttpRequestHandler.
The easiest way to customize the initial HTTP WebSocket handshake request is through
a HandshakeInterceptor
, which exposes "before" and "after" the handshake methods.
Such an interceptor can be used to preclude the handshake or to make any attributes
available to the WebSocketSession
. For example, there is a built-in interceptor
for passing HTTP session attributes to the WebSocket session:
@Configuration @EnableWebSocket public class WebSocketConfig implements WebSocketConfigurer { @Override public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) { registry.addHandler(new MyHandler(), "/myHandler") .addInterceptors(new HttpSessionHandshakeInterceptor()); } }
And the XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:handlers> <websocket:mapping path="/myHandler" handler="myHandler"/> <websocket:handshake-interceptors> <bean class="org.springframework.web.socket.server.support.HttpSessionHandshakeInterceptor"/> </websocket:handshake-interceptors> </websocket:handlers> <bean id="myHandler" class="org.springframework.samples.MyHandler"/> </beans>
A more advanced option is to extend the DefaultHandshakeHandler
that performs
the steps of the WebSocket handshake, including validating the client origin,
negotiating a sub-protocol, and others. An application may also need to use this
option if it needs to configure a custom RequestUpgradeStrategy
in order to
adapt to a WebSocket server engine and version that is not yet supported
(also see Section 20.2.4, “Deployment Considerations” for more on this subject).
Both the Java-config and XML namespace make it possible to configure a custom
HandshakeHandler
.
Spring provides a WebSocketHandlerDecorator
base class that can be used to decorate
a WebSocketHandler
with additional behavior. Logging and exception handling
implementations are provided and added by default when using the WebSocket Java-config
or XML namespace. The ExceptionWebSocketHandlerDecorator
catches all uncaught
exceptions arising from any WebSocketHandler method and closes the WebSocket
session with status 1011
that indicates a server error.
The Spring WebSocket API is easy to integrate into a Spring MVC application where
the DispatcherServlet
serves both HTTP WebSocket handshake as well as other
HTTP requests. It is also easy to integrate into other HTTP processing scenarios
by invoking WebSocketHttpRequestHandler
. This is convenient and easy to
understand. However, special considerations apply with regards to JSR-356 runtimes.
The Java WebSocket API (JSR-356) provides two deployment mechanisms. The first
involves a Servlet container classpath scan (Servlet 3 feature) at startup; and
the other is a registration API to use at Servlet container initialization.
Neither of these mechanism make it possible to use a single "front controller"
for all HTTP processing — including WebSocket handshake and all other HTTP
requests — such as Spring MVC’s DispatcherServlet
.
This is a significant limitation of JSR-356 that Spring’s WebSocket support
addresses by providing a server-specific RequestUpgradeStrategy
even when
running in a JSR-356 runtime. At present such support is available on
Tomcat 7.0.47+, Jetty 9.0+, and GlassFish 4.0+. Additional support will be
added as more WebSocket runtimes become available.
Note | |
---|---|
A request to overcome the above limitation in the Java WebSocket API has been created and can be followed at WEBSOCKET_SPEC-211. Also note that Tomcat and Jetty already provide native API alternatives that makes it easy to overcome the limitation. We are hopeful that more servers will follow their example regardless of when it is addressed in the Java WebSocket API. |
A secondary consideration is that Servlet containers with JSR-356 support
are expected to perform an SCI scan that can slow down application startup,
in some cases dramatically. If a significant impact is observed after an
upgrade to a Servlet container version with JSR-356 support, it should
be possible to selectively enable or disable web fragments (and SCI scanning)
through the use of an <absolute-ordering />
element in web.xml
:
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0"> <absolute-ordering/> </web-app>
You can then selectively enable web fragments by name, such as Spring’s own
SpringServletContainerInitializer
that provides support for the Servlet 3
Java initialization API, if required:
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0"> <absolute-ordering> <name>spring_web</name> </absolute-ordering> </web-app>
Each underlying WebSocket engine exposes configuration properties that control runtime characteristics such as the size of message buffer sizes, idle timeout, and others.
For Tomcat, WildFly, and Glassfish add a WebSocketContainerFactoryBean
to your
WebSocket Java config:
@Configuration @EnableWebSocket public class WebSocketConfig implements WebSocketConfigurer { @Bean public WebSocketContainerFactoryBean createWebSocketContainer() { WebSocketContainerFactoryBean container = new WebSocketContainerFactoryBean(); container.setMaxTextMessageBufferSize(8192); container.setMaxBinaryMessageBufferSize(8192); return container; } }
or WebSocket XML namespace:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket.xsd"> <bean class="org.springframework...WebSocketContainerFactoryBean"> <property name="maxTextMessageBufferSize" value="8192"/> <property name="maxBinaryMessageBufferSize" value="8192"/> </bean> </beans>
For Jetty, you’ll need to supply a pre-configured Jetty WebSocketServerFactory
and plug
that into Spring’s DefaultHandshakeHandler
through your WebSocket Java config:
@Configuration @EnableWebSocket public class WebSocketConfig implements WebSocketConfigurer { @Override public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) { registry.addHandler(echoWebSocketHandler(), "/echo").setHandshakeHandler(handshakeHandler()); } @Bean public DefaultHandshakeHandler handshakeHandler() { WebSocketPolicy policy = new WebSocketPolicy(WebSocketBehavior.SERVER); policy.setInputBufferSize(8192); policy.setIdleTimeout(600000); return new DefaultHandshakeHandler( new JettyRequestUpgradeStrategy(new WebSocketServerFactory(policy))); } }
or WebSocket XML namespace:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket.xsd"> <websocket:handlers> <websocket:mapping path="/echo" handler="echoHandler"/> <websocket:handshake-handler ref="handshakeHandler"/> </websocket:handlers> <bean id="handshakeHandler" class="org.springframework...DefaultHandshakeHandler"> <constructor-arg ref="upgradeStrategy"/> </bean> <bean id="upgradeStrategy" class="org.springframework...JettyRequestUpgradeStrategy"> <constructor-arg ref="serverFactory"/> </bean> <bean id="serverFactory" class="org.eclipse.jetty...WebSocketServerFactory"> <constructor-arg> <bean class="org.eclipse.jetty...WebSocketPolicy"> <constructor-arg value="SERVER"/> <property name="inputBufferSize" value="8092"/> <property name="idleTimeout" value="600000"/> </bean> </constructor-arg> </bean> </beans>
As explained in the introduction, WebSocket is not supported in all browsers yet and may be precluded by restrictive network proxies. This is why Spring provides fallback options that emulate the WebSocket API as close as possible based on the SockJS protocol.
SockJS is easy to enable through a configuration:
@Configuration @EnableWebSocket public class WebSocketConfig implements WebSocketConfigurer { @Override public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) { registry.addHandler(myHandler(), "/myHandler").withSockJS(); } @Bean public WebSocketHandler myHandler() { return new MyHandler(); } }
and the XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:handlers> <websocket:mapping path="/myHandler" handler="myHandler"/> <websocket:sockjs/> </websocket:handlers> <bean id="myHandler" class="org.springframework.samples.MyHandler"/> </beans>
The above is for use in Spring MVC applications and should be included in the configuration of a DispatcherServlet. However, Spring’s WebSocket and SockJS support does not depend on Spring MVC. It is relatively simple to integrate into other HTTP serving environments with the help of SockJsHttpRequestHandler.
On the browser side, applications can use the sockjs-client that emulates the W3C WebSocket API and communicates with the server to select the best transport option depending on the browser it’s running in. Review the sockjs-client page and the list of transport types supported by browser. The client also provides several configuration options, for example, to specify which transports to include.
An in-depth description of how SockJS works is beyond the scope of this document. This section summarizes a few key facts to aid with understanding. The SockJS protocol itself is defined in a test suite, with comments explaining the protocol. There is also an HTML version of that test showing comments on the right and code on the left.
The SockJS client issues HTTP requests with this URL structure:
SockJS URL.
http://host:port/{path-to-sockjs-endpoint}/{server-id}/{session-id}/{transport}
The WebSocket transport type uses a single HTTP connection to perform a WebSocket handshake and establish an actual WebSocket session. HTTP-based transports on the other hand must simulate the WebSocket API and at any time may use two HTTP connections — one for server-to-client messages, via HTTP streaming or long polling, and another for sending client messages to the server via HTTP POST.
The session id is useful with HTTP transports to associate individual HTTP requests that belong to the same SockJS session. The server id is not used in the protocol but is added to help in clustered environments.
SockJS adds a minimal amount of message framing. For example, the server can send
an "open frame" (the letter o
), a "heartbeat frame" (the letter h
), or a
"close frame" (the letter c
); while the client sends messages as a JSON-encoded
array prepended with the letter a
(e.g. a["message1","message2"]
).
By default the server sends a heartbeat frame every 25 seconds to keep proxies and loadbalancers from timing out the connection.
In the Spring Framework, server-side support for the SockJS protocol is provided through a
hierarchy of classes that implement the SockJsService
interface, while
SockJsHttpRequestHandler
integrates the service into HTTP request processing.
To implement HTTP streaming and long polling in Servlet containers (both of which require an HTTP connection to remain open longer than usual), Spring’s SockJS support relies on Servlet 3 async support.
Warning | |
---|---|
Servlet 3 async support does not expose a notification when a client disconnects, e.g. a browser tab is closed, page is refreshed, etc (see SERVLET_SPEC-44). However, the Serlvet container will usually raise an IOException on the next attempt to write to the response; at which point the SockJS session will be closed. Since the SockJsService sends a heartbeat every 25 seconds, typically a disconnected client will be detected within that time period. |
The WebSocket protocol defines two main types of messages — text and binary — but leaves their content undefined. Instead it’s expected that client and server may agree on using a sub-protocol, i.e. a higher-level protocol that defines the message content. Using a sub-protocol is optional but either way client and server both need to understand how to interpret messages.
STOMP is a simple messaging protocol originally created for scripting languages (such as Ruby, Python and Perl) to connect to enterprise message brokers. It is designed to address a subset of commonly used patterns in messaging protocols. STOMP can be used over any reliable 2-way streaming network protocol such as TCP and WebSocket.
STOMP is a frame based protocol with frames modelled on HTTP. This is the structure of a frame:
COMMAND header1:value1 header2:value2 Body^@
For example, a client can use the SEND
command to send a message or the
SUBSCRIBE
command to express interest in receiving messages. Both of these commands
require a "destination"
header that indicates where to send a message to, or likewise
what to subscribe to.
Here is an example of a client sending a request to buy stock shares:
SEND destination:/queue/trade content-type:application/json content-length:44 {"action":"BUY","ticker":"MMM","shares",44}^@
Here is an example of a client subscribing to receive stock quotes:
SUBSCRIBE id:sub-1 destination:/topic/price.stock.* ^@
Note | |
---|---|
The meaning of a destination is intentionally left opaque in the STOMP spec. It can
be any string and it’s entirely up to STOMP servers to define the semantics and
the syntax of the destinations that they support. It is very common however, for
destinations to be path-like strings where |
STOMP servers can use the MESSAGE
command to broadcast messages to all subscribers.
Here is an example of a server sending a stock quote to a subscribed client:
MESSAGE message-id:nxahklf6-1 subscription:sub-1 destination:/topic/price.stock.MMM {"ticker":"MMM","price":129.45}^@
Note | |
---|---|
It’s important to know that a server cannot send unsolicited messages.
All messages from a server must be in response to a specific client subscription
and the |
The above overview is intended to provide the most basic understanding of the STOMP protocol. It is recommended to review the protocol specification, which is easy to follow and manageable in terms of size.
The following summarizes the benefits for an application from using STOMP over WebSocket:
Most importantly the use of STOMP (vs plain WebSocket) enables the Spring Framework to provide a programming model for application-level use in the same way that Spring MVC provides a programming model based on HTTP.
The Spring Framework provides support for using STOMP over WebSocket through
the spring-messaging
and spring-websocket
modules. It’s easy to enable it.
Here is an example of configuring a STOMP WebSocket endpoint with SockJS fallback
options. The endpoint is available for clients to connect to at URL path /portfolio
:
import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker; import org.springframework.web.socket.config.annotation.StompEndpointRegistry; @Configuration @EnableWebSocketMessageBroker public class WebSocketConfig implements WebSocketMessageBrokerConfigurer { @Override public void registerStompEndpoints(StompEndpointRegistry registry) { registry.addEndpoint("/portfolio").withSockJS(); } // ... }
XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:message-broker> <websocket:stomp-endpoint path="/portfolio"> <websocket:sockjs/> </websocket:stomp-endpoint> ... </websocket:message-broker> </beans>
On the browser side, a client might connect as follows using stomp.js and the sockjs-client:
var socket = new SockJS("/spring-websocket-portfolio/portfolio"); var stompClient = Stomp.over(socket);
Or if connecting via WebSocket (without SockJS):
var socket = new WebSocket("/spring-websocket-portfolio/portfolio"); var stompClient = Stomp.over(socket);
When a STOMP endpoint is configured, the Spring application effectively becomes the broker to connected clients, handling incoming messages and broadcasting messages back to them. This part of the documentation describes how STOMP messages are handled within the application.
As mentioned in the introduction the
spring-messaging
module contains key abstractions from the
Spring Integration project, including
Message
, MessageChannel
, MessageHandler
and a few others.
Note | |
---|---|
Spring Integration 4 will be the first version to start using the abstractions
from the package structure of the |
MessageChannel
is a simple contract for passing messages between components without
creating tight coupling among them.
SubscribableChannel extends
it, with the ability to register subscribers; and
ExecutorSubscribableChannel
is an implementation that passes messages to subscribers in
the same thread or a different thread depending on whether it has been provided with
a java.util.concurrent.Executor
. This enables assembling message
handling flows from various components and modifying them through configuration.
The provided Java-config @EnableWebSocketMessageBroker
and XML namespace
<websocket:message-broker>
each put together a default message handling
flow for applications to use, as explained next. This flow can be modified,
customized, or extended. For example, an application can add a
ChannelInterceptor
to any message channel in order to intercept messages passing through it;
it can register additional message handling components, alternate between
synchronous and asynchronous message passing, and so on.
Incoming client STOMP messages are passed to a message channel with the name
"clientInboundChannel"
. By default the messages are routed to annotated
methods as well as to a "simple" message broker. This simple message broker
automatically records subscriptions, in-memory, and broadcasts messages as
necessary. As explained later, you can also use a full-featured message broker
(e.g. RabbitMQ, ActiveMQ, and any other broker that supports STOMP) to manage
subscriptions and broadcast messages.
Below is example configuration:
@Configuration @EnableWebSocketMessageBroker public class WebSocketConfig implements WebSocketMessageBrokerConfigurer { @Override public void registerStompEndpoints(StompEndpointRegistry registry) { registry.addEndpoint("/portfolio").withSockJS(); } @Override public void configureMessageBroker(MessageBrokerRegistry registry) { registry.enableSimpleBroker("/topic/"); registry.setApplicationDestinationPrefixes("/app"); } }
XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:message-broker application-destination-prefix="/app"> <websocket:stomp-endpoint path="/portfolio" /> <websocket:sockjs/> </websocket:stomp-endpoint> <websocket:simple-broker prefix="/topic"/> </websocket:message-broker> </beans>
The configuration example assigns destination prefixes — /app
for filtering
messages to annotated methods and /topic
for messages to the broker. The
examples below demonstrate how this can be used.
The destination prefix should not be included in annotation mappings. For
example, this method handles messages to destination /app/greetings
:
@Controller public class GreetingController { @MessageMapping("/greetings") { public void handle(String greeting) { // ... } }
The method accepts a String extracted from the payload of the message,
possibly converted based on its content type. The method can also return a
value, which is wrapped as the payload of a new message and sent to a message
channel named "brokerChannel"
(which is used for sending messages to the broker
from within the application). The new message is sent to the same destination
as that of the client message, but with the default prefix /topic
(you can also use @SendTo
for any other target destination):
@Controller public class GreetingController { @MessageMapping("/greetings") { public String handle(String greeting) { return "[" + getTimestamp() + ": " + greeting; } }
As a result, to put it all together, a client sends a greeting message to
destination /app/greetings
. The message is routed to GreetingController
,
which enriches the greeting with a timestamp and sends a new message to the
broker with destination /topic/greetings
. The broker then broadcasts the
message to all subscribed, connected clients.
The @MessageMapping
annotation is supported on methods of @Controller
-annotated classes.
It can be used for mapping methods to path-like message destinations. It is also
possible to combine with a type-level @MessageMapping
for expressing shared
mappings across all annotated methods within a controller.
Destination mappings can contain Ant-style patterns (e.g. "/foo*", "/foo/**")
and template variables (e.g. "/foo/{id}"), which can then be accessed via
@DestinationVariable
method arguments. This should be familiar to Spring MVC
users, in fact the same AntPathMatcher
is used for matching destinations based
on patterns and for extracting template variables.
The following method arguments are supported for @MessageMapping
methods:
Message
method argument to get access to the complete message being processed.
@Payload
-annotated argument for access to the payload of a message, converted with
a org.springframework.messaging.converter.MessageConverter
.
The presence of the annotation is not required since it is assumed by default.
Payload method arguments annotated with Validation annotations (like @Validated
) will
be subject to JSR-303 validation.
@Header
-annotated arguments for access to a specific header value along with
type conversion using an org.springframework.core.convert.converter.Converter
if necessary.
@Headers
-annotated method argument that must also be assignable to java.util.Map
for access to all headers in the message.
MessageHeaders
method argument for getting access to a map of all headers.
MessageHeaderAccessor
, SimpMessageHeaderAccessor
, or StompHeaderAccessor
for access to headers via typed accessor methods.
@DestinationVariable
-annotated arguments for access to template
variables extracted from the message destination. Values will be converted to
the declared method argument type as necessary.
java.security.Principal
method arguments reflecting the user logged in at
the time of the WebSocket HTTP handshake.
The return value from an @MessageMapping
method is converted with a
org.springframework.messaging.converter.MessageConverter
and used as the body
of a new message that is then sent, by default, to the "brokerChannel"
with
the same destination as the client message but using the prefix "/topic" by
default. An @SendTo
message level annotation can be used to specify any
other destination instead.
An @SubscribeMapping
annotation can also be used to map subscription requests
to @Controller
methods. It is supported on the method level, but can also be
combined with a type level @MessageMapping
annotation that expresses shared
mappings across all message handling methods within the same controller.
By default the return value from an @SubscribeMapping
method is sent as a
message directly back to the connected client and does not pass through the
broker. This is useful for implementing request-reply message interactions; for
example, to fetch application data when the application UI is being initialized.
Or alternatively an @SubscribeMapping
method can be annotated with @SendTo
in which case the resulting message is sent to the "brokerChannel"
using
the specified target destination.
What if you wanted to send messages to connected clients from any part of the
application? Any application component can send messages to the "brokerChannel"
.
The easist way to do that is to have a SimpMessagingTemplate
injected, and
use it to send messages. Typically it should be easy to have it injected by
type, for example:
@Controller public class GreetingController { private SimpMessagingTemplate template; @Autowired public GreetingController(SimpMessagingTemplate template) { this.template = template; } @RequestMapping(value="/greetings", method=POST) public void greet(String greeting) { String text = "[" + getTimestamp() + "]:" + greeting; this.template.convertAndSend("/topic/greetings", text); } }
But it can also be qualified by its name "brokerMessagingTemplate" if another bean of the same type exists.
The built-in, simple, message broker handles subscription requests from clients, stores them in memory, and broadcasts messages to connected clients with matching destinations. The broker supports path-like destinations, including subscriptions to Ant-style destination patterns.
The simple broker is great for getting started but supports only a subset of STOMP commands (e.g. no acks, receipts, etc), relies on a simple message sending loop, and is not suitable for clustering.
Instead, applications can use a full-featured message broker and use it for managing client subscriptions and broadcasting messages; while annotated methods can still be used for application processing. In other words, all else remains the same, except a full-featured broker replaces the simple broker.
Check your message broker STOMP documentation (e.g. RabbitMQ, ActiveMQ), install and run the broker with STOMP support enabled. Then enable the STOMP broker relay in the Spring configuration as an alternative to the simple broker.
Below is example configuration that enables use of a full-featured broker:
@Configuration @EnableWebSocketMessageBroker public class WebSocketConfig implements WebSocketMessageBrokerConfigurer { @Override public void registerStompEndpoints(StompEndpointRegistry registry) { registry.addEndpoint("/portfolio").withSockJS(); } @Override public void configureMessageBroker(MessageBrokerRegistry registry) { registry.enableStompBrokerRelay("/topic/", "/queue/"); registry.setApplicationDestinationPrefixes("/app"); } }
XML configuration equivalent:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:websocket="http://www.springframework.org/schema/websocket" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/websocket http://www.springframework.org/schema/websocket/spring-websocket-4.0.xsd"> <websocket:message-broker application-destination-prefix="/app"> <websocket:stomp-endpoint path="/portfolio" /> <websocket:sockjs/> </websocket:stomp-endpoint> <websocket:stomp-broker-relay prefix="/topic,/queue" /> </websocket:message-broker> </beans>
The STOMP "broker relay" from the above configuration manages TCP connections to the external broker, and forwards messages with matching destination prefixes to it. Likewise, any messages received from the external broker are matched and routed to connected clients.
In effect, messages are now broadcast through a full-featured, robust and scalable message broker.
An application can also send messages targeting a specific user.
To do so a user must first be authenticated. Although the STOMP CONNECT
frame
has authentication headers when used over WebSocket, it makes more sense to use
the same HTTP-based authentication already used to secure the application.
For example, an application can use Spring Security as usual to protect HTTP URLs,
including paths to STOMP WebSocket endpoint(s). The authenticanted user, based
on the value returned from HttpServletRequest.getUserPrincipal()
, will be
saved in the WebSocket session and subsequently added as a header named user
to all incoming messages for that STOMP/WebSocket session.
Spring’s STOMP support recognizes destinations prefixed with /user/
.
For example, a client can subscribe to destination /user/position-updates
.
This destination will be handled by the UserDestinationMessageHandler
and
transformed into a destination unique to the user’s session,
e.g. /user/position-updates-123
. This provides the convenience of subscribing
to a generically named destination, while also ensuring that it doesn’t "collide"
with any other user that also subscribes to /user/position-updates
in order to receive stock position updates unique to them.
On the sending side, messages can be sent to a destination such as
/user/{username}/position-updates
, which in turn will be translated
by the UserDestinationMessageHandler
into the same unique destination
belonging to the specified user name.
This allows any component within the application to send messages to a specific user without necessarily knowing anything more than their name and a generic destination.
When this is used with an external message broker, check the broker documentation
on how to manage inactive queues, so that when the user session is over, all
unique user queues are removed. For example, RabbitMQ creates auto-delete queues
when destinations like /exchange/amq.direct/position-updates
are used.
So in that case the client could subscribe to /user/exchange/amq.direct/position-updates
.
ActiveMQ has configuration options
for purging inactive destinations.
There are two main approaches to testing applications using Spring’s STOMP over WebSocket support. The first is to write server-side tests verifying the functionality of controllers and their annotated message handling methods. The second is to write full end-to-end tests that involve running a client and a server.
The two approaches are not mutually exclusive. On the contrary each has a place in an overall test strategy. Server-side tests are more focused and easier to write and maintain. End-to-end integration tests on the other hand are more complete and test much more but they’re also more involved to write and maintain.
The simplest form of server-side tests is to write controller unit tests. However this is not useful enough since much of what a controller does depends on its annotations. Pure unit tests simply can’t test that.
Ideally controllers under test should be invoked as they are at runtime, much like the approach to testing controllers handling HTTP requests using the Spring MVC Test framework. i.e. without running a Servlet container but relying on the Spring Framework to invoke the annotated controllers. Just like with Spring MVC Test here there are two two possible alternatives, either using a "context-based" or "standalone" setup:
SimpAnnotationMethodMessageHandler
) and pass messages for
controllers directly to it.
Both of these setup scenarios are demonstrated in the tests for the stock portfolio sample application.
The second approach is to create end-to-end integration tests. For that you will need to run a WebSocket server in embedded mode and connect to it as a WebSocket client sending WebSocket messages containing STOMP frames. The tests for the stock portfolio sample application also demonstrate this approach using Tomcat as the embedded WebSocket server and a simple STOMP client for test purposes.
This part of the reference documentation covers the Spring Framework’s integration with a number of Java EE (and related) technologies.
Spring features integration classes for remoting support using various technologies. The remoting support eases the development of remote-enabled services, implemented by your usual (Spring) POJOs. Currently, Spring supports the following remoting technologies:
RmiProxyFactoryBean
and
the RmiServiceExporter
Spring supports both traditional RMI (with java.rmi.Remote
interfaces and java.rmi.RemoteException
) and transparent remoting via RMI invokers
(with any Java interface).
HttpInvokerProxyFactoryBean
and
HttpInvokerServiceExporter
.
HessianProxyFactoryBean
and the
HessianServiceExporter
you can transparently expose your services using the
lightweight binary HTTP-based protocol provided by Caucho.
BurlapProxyFactoryBean
and BurlapServiceExporter
.
JmsInvokerServiceExporter
and JmsInvokerProxyFactoryBean
classes.
While discussing the remoting capabilities of Spring, we’ll use the following domain model and corresponding services:
public class Account implements Serializable{ private String name; public String getName(){ return name; } public void setName(String name) { this.name = name; } }
public interface AccountService { public void insertAccount(Account account); public List<Account> getAccounts(String name); }
public interface RemoteAccountService extends Remote { public void insertAccount(Account account) throws RemoteException; public List<Account> getAccounts(String name) throws RemoteException; }
// the implementation doing nothing at the moment public class AccountServiceImpl implements AccountService { public void insertAccount(Account acc) { // do something... } public List<Account> getAccounts(String name) { // do something... } }
We will start exposing the service to a remote client by using RMI and talk a bit about the drawbacks of using RMI. We’ll then continue to show an example using Hessian as the protocol.
Using Spring’s support for RMI, you can transparently expose your services through the RMI infrastructure. After having this set up, you basically have a configuration similar to remote EJBs, except for the fact that there is no standard support for security context propagation or remote transaction propagation. Spring does provide hooks for such additional invocation context when using the RMI invoker, so you can for example plug in security frameworks or custom security credentials here.
Using the RmiServiceExporter
, we can expose the interface of our AccountService object
as RMI object. The interface can be accessed by using RmiProxyFactoryBean
, or via
plain RMI in case of a traditional RMI service. The RmiServiceExporter
explicitly
supports the exposing of any non-RMI services via RMI invokers.
Of course, we first have to set up our service in the Spring container:
<bean id="accountService" class="example.AccountServiceImpl"> <!-- any additional properties, maybe a DAO? --> </bean>
Next we’ll have to expose our service using the RmiServiceExporter
:
<bean class="org.springframework.remoting.rmi.RmiServiceExporter"> <!-- does not necessarily have to be the same name as the bean to be exported --> <property name="serviceName" value="AccountService"/> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> <!-- defaults to 1099 --> <property name="registryPort" value="1199"/> </bean>
As you can see, we’re overriding the port for the RMI registry. Often, your application
server also maintains an RMI registry and it is wise to not interfere with that one.
Furthermore, the service name is used to bind the service under. So right now, the
service will be bound at 'rmi://HOST:1199/AccountService'
. We’ll use the URL later on
to link in the service at the client side.
Note | |
---|---|
The |
Our client is a simple object using the AccountService
to manage accounts:
public class SimpleObject { private AccountService accountService; public void setAccountService(AccountService accountService) { this.accountService = accountService; } // additional methods using the accountService }
To link in the service on the client, we’ll create a separate Spring container, containing the simple object and the service linking configuration bits:
<bean class="example.SimpleObject"> <property name="accountService" ref="accountService"/> </bean> <bean id="accountService" class="org.springframework.remoting.rmi.RmiProxyFactoryBean"> <property name="serviceUrl" value="rmi://HOST:1199/AccountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
That’s all we need to do to support the remote account service on the client. Spring
will transparently create an invoker and remotely enable the account service through the
RmiServiceExporter
. At the client we’re linking it in using the RmiProxyFactoryBean
.
Hessian offers a binary HTTP-based remoting protocol. It is developed by Caucho and more information about Hessian itself can be found at http://www.caucho.com.
Hessian communicates via HTTP and does so using a custom servlet. Using Spring’s
DispatcherServlet
principles, as known from Spring Web MVC usage, you can easily wire
up such a servlet exposing your services. First we’ll have to create a new servlet in
your application (this is an excerpt from 'web.xml'
):
<servlet> <servlet-name>remoting</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>remoting</servlet-name> <url-pattern>/remoting/*</url-pattern> </servlet-mapping>
You’re probably familiar with Spring’s DispatcherServlet
principles and if so, you
know that now you’ll have to create a Spring container configuration resource named
'remoting-servlet.xml'
(after the name of your servlet) in the 'WEB-INF'
directory.
The application context will be used in the next section.
Alternatively, consider the use of Spring’s simpler HttpRequestHandlerServlet
. This
allows you to embed the remote exporter definitions in your root application context (by
default in 'WEB-INF/applicationContext.xml'
), with individual servlet definitions
pointing to specific exporter beans. Each servlet name needs to match the bean name of
its target exporter in this case.
In the newly created application context called remoting-servlet.xml
, we’ll create a
HessianServiceExporter
exporting your services:
<bean id="accountService" class="example.AccountServiceImpl"> <!-- any additional properties, maybe a DAO? --> </bean> <bean name="/AccountService" class="org.springframework.remoting.caucho.HessianServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
Now we’re ready to link in the service at the client. No explicit handler mapping is
specified, mapping request URLs onto services, so BeanNameUrlHandlerMapping
will be
used: Hence, the service will be exported at the URL indicated through its bean name
within the containing DispatcherServlet
's mapping (as defined above):
'http://HOST:8080/remoting/AccountService'
.
Alternatively, create a HessianServiceExporter
in your root application context (e.g.
in 'WEB-INF/applicationContext.xml'
):
<bean name="accountExporter" class="org.springframework.remoting.caucho.HessianServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
In the latter case, define a corresponding servlet for this exporter in 'web.xml'
,
with the same end result: The exporter getting mapped to the request path
/remoting/AccountService
. Note that the servlet name needs to match the bean name of
the target exporter.
<servlet> <servlet-name>accountExporter</servlet-name> <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>accountExporter</servlet-name> <url-pattern>/remoting/AccountService</url-pattern> </servlet-mapping>
Using the HessianProxyFactoryBean
we can link in the service at the client. The same
principles apply as with the RMI example. We’ll create a separate bean factory or
application context and mention the following beans where the SimpleObject
is using
the AccountService
to manage accounts:
<bean class="example.SimpleObject"> <property name="accountService" ref="accountService"/> </bean> <bean id="accountService" class="org.springframework.remoting.caucho.HessianProxyFactoryBean"> <property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
We won’t discuss Burlap, the XML-based equivalent of Hessian, in detail here, since it
is configured and set up in exactly the same way as the Hessian variant explained above.
Just replace the word Hessian
with Burlap
and you’re all set to go.
One of the advantages of Hessian and Burlap is that we can easily apply HTTP basic
authentication, because both protocols are HTTP-based. Your normal HTTP server security
mechanism can easily be applied through using the web.xml
security features, for
example. Usually, you don’t use per-user security credentials here, but rather shared
credentials defined at the Hessian/BurlapProxyFactoryBean
level (similar to a JDBC
DataSource
).
<bean class="org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping"> <property name="interceptors" ref="authorizationInterceptor"/> </bean> <bean id="authorizationInterceptor" class="org.springframework.web.servlet.handler.UserRoleAuthorizationInterceptor"> <property name="authorizedRoles" value="administrator,operator"/> </bean>
This is an example where we explicitly mention the BeanNameUrlHandlerMapping
and set
an interceptor allowing only administrators and operators to call the beans mentioned in
this application context.
Note | |
---|---|
Of course, this example doesn’t show a flexible kind of security infrastructure. For more options as far as security is concerned, have a look at the Spring Security project at http://projects.spring.io/spring-security/. |
As opposed to Burlap and Hessian, which are both lightweight protocols using their own slim serialization mechanisms, Spring HTTP invokers use the standard Java serialization mechanism to expose services through HTTP. This has a huge advantage if your arguments and return types are complex types that cannot be serialized using the serialization mechanisms Hessian and Burlap use (refer to the next section for more considerations when choosing a remoting technology).
Under the hood, Spring uses either the standard facilities provided by J2SE to perform
HTTP calls or Commons HttpClient
. Use the latter if you need more advanced and
easy-to-use functionality. Refer to
jakarta.apache.org/commons/httpclient for
more info.
Setting up the HTTP invoker infrastructure for a service object resembles closely the
way you would do the same using Hessian or Burlap. Just as Hessian support provides the
HessianServiceExporter
, Spring’s HttpInvoker support provides the
org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter
.
To expose the AccountService
(mentioned above) within a Spring Web MVC
DispatcherServlet
, the following configuration needs to be in place in the
dispatcher’s application context:
<bean name="/AccountService" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
Such an exporter definition will be exposed through the DispatcherServlet
's standard
mapping facilities, as explained in the section on Hessian.
Alternatively, create an HttpInvokerServiceExporter
in your root application context
(e.g. in 'WEB-INF/applicationContext.xml'
):
<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.HttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
In addition, define a corresponding servlet for this exporter in 'web.xml'
, with the
servlet name matching the bean name of the target exporter:
<servlet> <servlet-name>accountExporter</servlet-name> <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>accountExporter</servlet-name> <url-pattern>/remoting/AccountService</url-pattern> </servlet-mapping>
If you are running outside of a servlet container and are using Oracle’s Java 6, then you
can use the built-in HTTP server implementation. You can configure the
SimpleHttpServerFactoryBean
together with a SimpleHttpInvokerServiceExporter
as is
shown in this example:
<bean name="accountExporter" class="org.springframework.remoting.httpinvoker.SimpleHttpInvokerServiceExporter"> <property name="service" ref="accountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean> <bean id="httpServer" class="org.springframework.remoting.support.SimpleHttpServerFactoryBean"> <property name="contexts"> <util:map> <entry key="/remoting/AccountService" value-ref="accountExporter"/> </util:map> </property> <property name="port" value="8080" /> </bean>
Again, linking in the service from the client much resembles the way you would do it when using Hessian or Burlap. Using a proxy, Spring will be able to translate your calls to HTTP POST requests to the URL pointing to the exported service.
<bean id="httpInvokerProxy" class="org.springframework.remoting.httpinvoker.HttpInvokerProxyFactoryBean"> <property name="serviceUrl" value="http://remotehost:8080/remoting/AccountService"/> <property name="serviceInterface" value="example.AccountService"/> </bean>
As mentioned before, you can choose what HTTP client you want to use. By default, the
HttpInvokerProxy
uses the J2SE HTTP functionality, but you can also use the Commons
HttpClient
by setting the httpInvokerRequestExecutor
property:
<property name="httpInvokerRequestExecutor"> <bean class="org.springframework.remoting.httpinvoker.CommonsHttpInvokerRequestExecutor"/> </property>
Spring provides full support for standard Java web services APIs:
In addition to stock support for JAX-WS in Spring Core, the Spring portfolio also features Spring Web Services, a solution for contract-first, document-driven web services - highly recommended for building modern, future-proof web services.
Spring provides a convenient base class for JAX-WS servlet endpoint implementations -
SpringBeanAutowiringSupport
. To expose our AccountService
we extend Spring’s
SpringBeanAutowiringSupport
class and implement our business logic here, usually
delegating the call to the business layer. We’ll simply use Spring’s @Autowired
annotation for expressing such dependencies on Spring-managed beans.
/** * JAX-WS compliant AccountService implementation that simply delegates * to the AccountService implementation in the root web application context. * * This wrapper class is necessary because JAX-WS requires working with dedicated * endpoint classes. If an existing service needs to be exported, a wrapper that * extends SpringBeanAutowiringSupport for simple Spring bean autowiring (through * the @Autowired annotation) is the simplest JAX-WS compliant way. * * This is the class registered with the server-side JAX-WS implementation. * In the case of a Java EE 5 server, this would simply be defined as a servlet * in web.xml, with the server detecting that this is a JAX-WS endpoint and reacting * accordingly. The servlet name usually needs to match the specified WS service name. * * The web service engine manages the lifecycle of instances of this class. * Spring bean references will just be wired in here. */ import org.springframework.web.context.support.SpringBeanAutowiringSupport; @WebService(serviceName="AccountService") public class AccountServiceEndpoint extends SpringBeanAutowiringSupport { @Autowired private AccountService biz; @WebMethod public void insertAccount(Account acc) { biz.insertAccount(acc); } @WebMethod public Account[] getAccounts(String name) { return biz.getAccounts(name); } }
Our AccountServletEndpoint
needs to run in the same web application as the Spring
context to allow for access to Spring’s facilities. This is the case by default in Java
EE 5 environments, using the standard contract for JAX-WS servlet endpoint deployment.
See Java EE 5 web service tutorials for details.
The built-in JAX-WS provider that comes with Oracle’s JDK 1.6 supports exposure of web
services using the built-in HTTP server that’s included in JDK 1.6 as well. Spring’s
SimpleJaxWsServiceExporter
detects all @WebService
annotated beans in the Spring
application context, exporting them through the default JAX-WS server (the JDK 1.6 HTTP
server).
In this scenario, the endpoint instances are defined and managed as Spring beans
themselves; they will be registered with the JAX-WS engine but their lifecycle will be
up to the Spring application context. This means that Spring functionality like explicit
dependency injection may be applied to the endpoint instances. Of course,
annotation-driven injection through @Autowired
will work as well.
<bean class="org.springframework.remoting.jaxws.SimpleJaxWsServiceExporter"> <property name="baseAddress" value="http://localhost:8080/"/> </bean> <bean id="accountServiceEndpoint" class="example.AccountServiceEndpoint"> ... </bean> ...
The AccountServiceEndpoint
may derive from Spring’s SpringBeanAutowiringSupport
but
doesn’t have to since the endpoint is a fully Spring-managed bean here. This means that
the endpoint implementation may look like as follows, without any superclass declared -
and Spring’s @Autowired
configuration annotation still being honored:
@WebService(serviceName="AccountService") public class AccountServiceEndpoint { @Autowired private AccountService biz; @WebMethod public void insertAccount(Account acc) { biz.insertAccount(acc); } @WebMethod public List<Account> getAccounts(String name) { return biz.getAccounts(name); } }
Oracle’s JAX-WS RI, developed as part of the GlassFish project, ships Spring support as part of its JAX-WS Commons project. This allows for defining JAX-WS endpoints as Spring-managed beans, similar to the standalone mode discussed in the previous section - but this time in a Servlet environment. Note that this is not portable in a Java EE 5 environment; it is mainly intended for non-EE environments such as Tomcat, embedding the JAX-WS RI as part of the web application.
The difference to the standard style of exporting servlet-based endpoints is that the
lifecycle of the endpoint instances themselves will be managed by Spring here, and that
there will be only one JAX-WS servlet defined in web.xml
. With the standard Java EE 5
style (as illustrated above), you’ll have one servlet definition per service endpoint,
with each endpoint typically delegating to Spring beans (through the use of
@Autowired
, as shown above).
Check out https://jax-ws-commons.dev.java.net/spring/ for the details on setup and usage style.
Spring provides two factory beans to create JAX-WS web service proxies, namely
LocalJaxWsServiceFactoryBean
and JaxWsPortProxyFactoryBean
. The former can only
return a JAX-WS service class for us to work with. The latter is the full-fledged
version that can return a proxy that implements our business service interface. In this
example we use the latter to create a proxy for the AccountService
endpoint (again):
<bean id="accountWebService" class="org.springframework.remoting.jaxws.JaxWsPortProxyFactoryBean"> <property name="serviceInterface" value="example.AccountService"/> <property name="wsdlDocumentUrl" value="http://localhost:8888/AccountServiceEndpoint?WSDL"/> <property name="namespaceUri" value="http://example/"/> <property name="serviceName" value="AccountService"/> <property name="portName" value="AccountServiceEndpointPort"/> </bean>
Where serviceInterface
is our business interface the clients will use.
wsdlDocumentUrl
is the URL for the WSDL file. Spring needs this a startup time to
create the JAX-WS Service. namespaceUri
corresponds to the targetNamespace in the
.wsdl file. serviceName
corresponds to the service name in the .wsdl file. portName
corresponds to the port name in the .wsdl file.
Accessing the web service is now very easy as we have a bean factory for it that will
expose it as AccountService
interface. We can wire this up in Spring:
<bean id="client" class="example.AccountClientImpl"> ... <property name="service" ref="accountWebService"/> </bean>
From the client code we can access the web service just as if it was a normal class:
public class AccountClientImpl { private AccountService service; public void setService(AccountService service) { this.service = service; } public void foo() { service.insertAccount(...); } }
Note | |
---|---|
The above is slightly simplified in that JAX-WS requires endpoint interfaces
and implementation classes to be annotated with |
It is also possible to expose services transparently using JMS as the underlying
communication protocol. The JMS remoting support in the Spring Framework is pretty basic
- it sends and receives on the same thread
and in the same non-transactional
Session
, and as such throughput will be very implementation dependent. Note that these
single-threaded and non-transactional constraints apply only to Spring’s JMS
remoting support. See Chapter 23, JMS (Java Message Service) for information on Spring’s rich support for JMS-based
messaging.
The following interface is used on both the server and the client side.
package com.foo; public interface CheckingAccountService { public void cancelAccount(Long accountId); }
The following simple implementation of the above interface is used on the server-side.
package com.foo; public class SimpleCheckingAccountService implements CheckingAccountService { public void cancelAccount(Long accountId) { System.out.println("Cancelling account [" + accountId + "]"); } }
This configuration file contains the JMS-infrastructure beans that are shared on both the client and server.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="connectionFactory" class="org.apache.activemq.ActiveMQConnectionFactory"> <property name="brokerURL" value="tcp://ep-t43:61616"/> </bean> <bean id="queue" class="org.apache.activemq.command.ActiveMQQueue"> <constructor-arg value="mmm"/> </bean> </beans>
On the server, you just need to expose the service object using the
JmsInvokerServiceExporter
.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="checkingAccountService" class="org.springframework.jms.remoting.JmsInvokerServiceExporter"> <property name="serviceInterface" value="com.foo.CheckingAccountService"/> <property name="service"> <bean class="com.foo.SimpleCheckingAccountService"/> </property> </bean> <bean class="org.springframework.jms.listener.SimpleMessageListenerContainer"> <property name="connectionFactory" ref="connectionFactory"/> <property name="destination" ref="queue"/> <property name="concurrentConsumers" value="3"/> <property name="messageListener" ref="checkingAccountService"/> </bean> </beans>
package com.foo; import org.springframework.context.support.ClassPathXmlApplicationContext; public class Server { public static void main(String[] args) throws Exception { new ClassPathXmlApplicationContext(new String[]{"com/foo/server.xml", "com/foo/jms.xml"}); } }
The client merely needs to create a client-side proxy that will implement the agreed
upon interface ( CheckingAccountService
). The resulting object created off the back of
the following bean definition can be injected into other client side objects, and the
proxy will take care of forwarding the call to the server-side object via JMS.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="checkingAccountService" class="org.springframework.jms.remoting.JmsInvokerProxyFactoryBean"> <property name="serviceInterface" value="com.foo.CheckingAccountService"/> <property name="connectionFactory" ref="connectionFactory"/> <property name="queue" ref="queue"/> </bean> </beans>
package com.foo; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class Client { public static void main(String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext( new String[] {"com/foo/client.xml", "com/foo/jms.xml"}); CheckingAccountService service = (CheckingAccountService) ctx.getBean("checkingAccountService"); service.cancelAccount(new Long(10)); } }
You may also wish to investigate the support provided by the Lingo project, which (to quote the homepage blurb) "… is a lightweight POJO based remoting and messaging library based on the Spring Framework’s remoting libraries which extends it to support JMS."
Refer to the Spring AMQP Reference Document Spring Remoting with AMQP section for more information.
The main reason why auto-detection of implemented interfaces does not occur for remote
interfaces is to avoid opening too many doors to remote callers. The target object might
implement internal callback interfaces like InitializingBean
or DisposableBean
which
one would not want to expose to callers.
Offering a proxy with all interfaces implemented by the target usually does not matter in the local case. But when exporting a remote service, you should expose a specific service interface, with specific operations intended for remote usage. Besides internal callback interfaces, the target might implement multiple business interfaces, with just one of them intended for remote exposure. For these reasons, we require such a service interface to be specified.
This is a trade-off between configuration convenience and the risk of accidental exposure of internal methods. Always specifying a service interface is not too much effort, and puts you on the safe side regarding controlled exposure of specific methods.
Each and every technology presented here has its drawbacks. You should carefully consider your needs, the services you are exposing and the objects you’ll be sending over the wire when choosing a technology.
When using RMI, it’s not possible to access the objects through the HTTP protocol, unless you’re tunneling the RMI traffic. RMI is a fairly heavy-weight protocol in that it supports full-object serialization which is important when using a complex data model that needs serialization over the wire. However, RMI-JRMP is tied to Java clients: It is a Java-to-Java remoting solution.
Spring’s HTTP invoker is a good choice if you need HTTP-based remoting but also rely on Java serialization. It shares the basic infrastructure with RMI invokers, just using HTTP as transport. Note that HTTP invokers are not only limited to Java-to-Java remoting but also to Spring on both the client and server side. (The latter also applies to Spring’s RMI invoker for non-RMI interfaces.)
Hessian and/or Burlap might provide significant value when operating in a heterogeneous environment, because they explicitly allow for non-Java clients. However, non-Java support is still limited. Known issues include the serialization of Hibernate objects in combination with lazily-initialized collections. If you have such a data model, consider using RMI or HTTP invokers instead of Hessian.
JMS can be useful for providing clusters of services and allowing the JMS broker to take care of load balancing, discovery and auto-failover. By default: Java serialization is used when using JMS remoting but the JMS provider could use a different mechanism for the wire formatting, such as XStream to allow servers to be implemented in other technologies.
Last but not least, EJB has an advantage over RMI in that it supports standard role-based authentication and authorization and remote transaction propagation. It is possible to get RMI invokers or HTTP invokers to support security context propagation as well, although this is not provided by core Spring: There are just appropriate hooks for plugging in third-party or custom solutions here.
The RestTemplate
is the core class for client-side access to RESTful services. It is
conceptually similar to other template classes in Spring, such as JdbcTemplate
and
JmsTemplate
and other template classes found in other Spring portfolio projects.
RestTemplate
's behavior is customized by providing callback methods and configuring
the HttpMessageConverter
used to marshal objects into the HTTP request body and to
unmarshal any response back into an object. As it is common to use XML as a message
format, Spring provides a MarshallingHttpMessageConverter
that uses the Object-to-XML
framework that is part of the org.springframework.oxm
package. This gives you a wide
range of choices of XML to Object mapping technologies to choose from.
This section describes how to use the RestTemplate
and its associated
HttpMessageConverters
.
Invoking RESTful services in Java is typically done using a helper class such as Apache
HttpComponents HttpClient
. For common REST operations this approach is too low level as
shown below.
String uri = "http://example.com/hotels/1/bookings"; PostMethod post = new PostMethod(uri); String request = // create booking request content post.setRequestEntity(new StringRequestEntity(request)); httpClient.executeMethod(post); if (HttpStatus.SC_CREATED == post.getStatusCode()) { Header location = post.getRequestHeader("Location"); if (location != null) { System.out.println("Created new booking at :" + location.getValue()); } }
RestTemplate provides higher level methods that correspond to each of the six main HTTP methods that make invoking many RESTful services a one-liner and enforce REST best practices.
Note | |
---|---|
RestTemplate has an asynchronous counter-part: see Section 21.10.3, “Async RestTemplate”. |
Table 21.1. Overview of RestTemplate methods
HTTP Method | RestTemplate Method |
---|---|
DELETE | |
GET | |
HEAD | |
OPTIONS | |
POST | postForLocation(String url, Object request, String… urlVariables) postForObject(String url, Object request, Class<T> responseType, String… uriVariables) |
PUT | |
PATCH and others |
The names of RestTemplate
methods follow a naming convention, the first part indicates
what HTTP method is being invoked and the second part indicates what is returned. For
example, the method getForObject()
will perform a GET, convert the HTTP response into
an object type of your choice and return that object. The method postForLocation()
will do a POST, converting the given object into a HTTP request and return the response
HTTP Location header where the newly created object can be found. In case of an
exception processing the HTTP request, an exception of the type RestClientException
will be thrown; this behavior can be changed by plugging in another
ResponseErrorHandler
implementation into the RestTemplate
.
The exchange
and execute
methods are generalized versions of the more
specific methods listed above them and can support additional combinations and methods,
like HTTP PATCH. However, note that the underlying HTTP library must also support the
desired combination. The JDK HttpURLConnection
does not support the PATCH
method, but
Apache HttpComponents HttpClient version 4.2 or later does. They also enable
RestTemplate
to read an HTTP response to a generic type (e.g. List<Account>
), using a
ParameterizedTypeReference
, a new class that enables capturing and passing generic
type info.
Objects passed to and returned from these methods are converted to and from HTTP
messages by HttpMessageConverter
instances. Converters for the main mime types are
registered by default, but you can also write your own converter and register it via the
messageConverters()
bean property. The default converter instances registered with the
template are ByteArrayHttpMessageConverter
, StringHttpMessageConverter
,
FormHttpMessageConverter
and SourceHttpMessageConverter
. You can override these
defaults using the messageConverters()
bean property as would be required if using the
MarshallingHttpMessageConverter
or MappingJackson2HttpMessageConverter
.
Each method takes URI template arguments in two forms, either as a String
variable
length argument or a Map<String,String>
. For example,
String result = restTemplate.getForObject( "http://example.com/hotels/{hotel}/bookings/{booking}", String.class,"42", "21");
using variable length arguments and
Map<String, String> vars = Collections.singletonMap("hotel", "42"); String result = restTemplate.getForObject( "http://example.com/hotels/{hotel}/rooms/{hotel}", String.class, vars);
using a Map<String,String>
.
To create an instance of RestTemplate
you can simply call the default no-arg
constructor. This will use standard Java classes from the java.net
package as the
underlying implementation to create HTTP requests. This can be overridden by specifying
an implementation of ClientHttpRequestFactory
. Spring provides the implementation
HttpComponentsClientHttpRequestFactory
that uses the Apache HttpComponents
HttpClient
to create requests. HttpComponentsClientHttpRequestFactory
is configured
using an instance of org.apache.http.client.HttpClient
which can in turn be configured
with credentials information or connection pooling functionality.
Tip | |
---|---|
Note that the |
The previous example using Apache HttpComponents HttpClient
directly rewritten to use
the RestTemplate
is shown below
uri = "http://example.com/hotels/{id}/bookings"; RestTemplate template = new RestTemplate(); Booking booking = // create booking object URI location = template.postForLocation(uri, booking, "1");
To use Apache HttpComponents instead of the native java.net
functionality, construct
the RestTemplate
as follows:
RestTemplate template = new RestTemplate(new HttpComponentsClientHttpRequestFactory());
Tip | |
---|---|
Apache HttpClient supports gzip encoding via the HttpClient httpClient = new DecompressingHttpClient(new DefaultHttpClient()); ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient); RestTemplate template = new RestTemplate(requestFactory); |
The general callback interface is RequestCallback
and is called when the execute
method is invoked.
public <T> T execute(String url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor<T> responseExtractor, String... urlVariables) // also has an overload with urlVariables as a Map<String, String>.
The RequestCallback
interface is defined as
public interface RequestCallback { void doWithRequest(ClientHttpRequest request) throws IOException; }
and allows you to manipulate the request headers and write to the request body. When using the execute method you do not have to worry about any resource management, the template will always close the request and handle any errors. Refer to the API documentation for more information on using the execute method and the meaning of its other method arguments.
For each of the main HTTP methods, the RestTemplate
provides variants that either take
a String URI or java.net.URI
as the first argument.
The String URI variants accept template arguments as a String variable length argument
or as a Map<String,String>
. They also assume the URL String is not encoded and needs
to be encoded. For example the following:
restTemplate.getForObject("http://example.com/hotel list", String.class);
will perform a GET on http://example.com/hotel%20list
. That means if the input URL
String is already encoded, it will be encoded twice — i.e.
http://example.com/hotel%20list
will become http://example.com/hotel%2520list
. If
this is not the intended effect, use the java.net.URI
method variant, which assumes
the URL is already encoded is also generally useful if you want to reuse a single (fully
expanded) URI
multiple times.
The UriComponentsBuilder
class can be used to build and encode the URI
including
support for URI templates. For example you can start with a URL String:
UriComponents uriComponents = UriComponentsBuilder.fromUriString( "http://example.com/hotels/{hotel}/bookings/{booking}").build() .expand("42", "21") .encode(); URI uri = uriComponents.toUri();
Or specify each URI component individually:
UriComponents uriComponents = UriComponentsBuilder.newInstance() .scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build() .expand("42", "21") .encode(); URI uri = uriComponents.toUri();
Besides the methods described above, the RestTemplate
also has the exchange()
method, which can be used for arbitrary HTTP method execution based on the HttpEntity
class.
Perhaps most importantly, the exchange()
method can be used to add request headers and
read response headers. For example:
HttpHeaders requestHeaders = new HttpHeaders(); requestHeaders.set("MyRequestHeader", "MyValue"); HttpEntity<?> requestEntity = new HttpEntity(requestHeaders); HttpEntity<String> response = template.exchange( "http://example.com/hotels/{hotel}", HttpMethod.GET, requestEntity, String.class, "42"); String responseHeader = response.getHeaders().getFirst("MyResponseHeader"); String body = response.getBody();
In the above example, we first prepare a request entity that contains the
MyRequestHeader
header. We then retrieve the response, and read the MyResponseHeader
and body.
Objects passed to and returned from the methods getForObject()
, postForLocation()
,
and put()
are converted to HTTP requests and from HTTP responses by
HttpMessageConverters
. The HttpMessageConverter
interface is shown below to give you
a better feel for its functionality
public interface HttpMessageConverter<T> { // Indicate whether the given class and media type can be read by this converter. boolean canRead(Class<?> clazz, MediaType mediaType); // Indicate whether the given class and media type can be written by this converter. boolean canWrite(Class<?> clazz, MediaType mediaType); // Return the list of MediaType objects supported by this converter. List<MediaType> getSupportedMediaTypes(); // Read an object of the given type from the given input message, and returns it. T read(Class<T> clazz, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException; // Write an given object to the given output message. void write(T t, HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException; }
Concrete implementations for the main media (mime) types are provided in the framework
and are registered by default with the RestTemplate
on the client-side and with
AnnotationMethodHandlerAdapter
on the server-side.
The implementations of HttpMessageConverter
s are described in the following sections.
For all converters a default media type is used but can be overridden by setting the
supportedMediaTypes
bean property
An HttpMessageConverter
implementation that can read and write Strings from the HTTP
request and response. By default, this converter supports all text media types (
text/*
), and writes with a Content-Type
of text/plain
.
An HttpMessageConverter
implementation that can read and write form data from the HTTP
request and response. By default, this converter reads and writes the media type
application/x-www-form-urlencoded
. Form data is read from and written into a
MultiValueMap<String, String>
.
An HttpMessageConverter
implementation that can read and write byte arrays from the
HTTP request and response. By default, this converter supports all media types ( */*
),
and writes with a Content-Type
of application/octet-stream
. This can be overridden
by setting the supportedMediaTypes
property, and overriding getContentType(byte[])
.
An HttpMessageConverter
implementation that can read and write XML using Spring’s
Marshaller
and Unmarshaller
abstractions from the org.springframework.oxm
package.
This converter requires a Marshaller
and Unmarshaller
before it can be used. These
can be injected via constructor or bean properties. By default this converter supports (
text/xml
) and ( application/xml
).
An HttpMessageConverter
implementation that can read and write JSON using Jackson’s
ObjectMapper
. JSON mapping can be customized as needed through the use of Jackson’s
provided annotations. When further control is needed, a custom ObjectMapper
can be
injected through the ObjectMapper
property for cases where custom JSON
serializers/deserializers need to be provided for specific types. By default this
converter supports ( application/json
).
An HttpMessageConverter
implementation that can read and write
javax.xml.transform.Source
from the HTTP request and response. Only DOMSource
,
SAXSource
, and StreamSource
are supported. By default, this converter supports (
text/xml
) and ( application/xml
).
Web applications often need to query external REST services those days. The very nature of HTTP and synchronous calls can lead up to challenges when scaling applications for those needs: multiple threads may be blocked, waiting for remote HTTP responses.
AsyncRestTemplate
and Section 21.10.1, “RestTemplate”'s APIs are very similar; see
Table 21.1, “Overview of RestTemplate methods”. The main difference between those APIs is
that AsyncRestTemplate
returns
ListenableFuture
wrappers as opposed to concrete results.
The previous RestTemplate
example translates to:
// async call Future<ResponseEntity<String>> futureEntity = template.getForEntity( "http://example.com/hotels/{hotel}/bookings/{booking}", String.class, "42", "21"); // get the concrete result - synchronous call ResponseEntity<String> entity = futureEntity.get();
ListenableFuture
accepts completion callbacks:
ListenableFuture<ResponseEntity<String>> futureEntity = template.getForEntity( "http://example.com/hotels/{hotel}/bookings/{booking}", String.class, "42", "21"); // register a callback futureEntity.addCallback(new ListenableFutureCallback<ResponseEntity<String>>() { @Override public void onSuccess(ResponseEntity<String> entity) { //... } @Override public void onFailure(Throwable t) { //... } });
Note | |
---|---|
The default |
See ListenableFuture
Javadoc and
AsyncRestTemplate
Javadoc for more details.
As a lightweight container, Spring is often considered an EJB replacement. We do believe that for many if not most applications and use cases, Spring as a container, combined with its rich supporting functionality in the area of transactions, ORM and JDBC access, is a better choice than implementing equivalent functionality via an EJB container and EJBs.
However, it is important to note that using Spring does not prevent you from using EJBs. In fact, Spring makes it much easier to access EJBs and implement EJBs and functionality within them. Additionally, using Spring to access services provided by EJBs allows the implementation of those services to later transparently be switched between local EJB, remote EJB, or POJO (plain old Java object) variants, without the client code having to be changed.
In this chapter, we look at how Spring can help you access and implement EJBs. Spring provides particular value when accessing stateless session beans (SLSBs), so we’ll begin by discussing this.
To invoke a method on a local or remote stateless session bean, client code must normally perform a JNDI lookup to obtain the (local or remote) EJB Home object, then use a create method call on that object to obtain the actual (local or remote) EJB object. One or more methods are then invoked on the EJB.
To avoid repeated low-level code, many EJB applications use the Service Locator and Business Delegate patterns. These are better than spraying JNDI lookups throughout client code, but their usual implementations have significant disadvantages. For example:
The Spring approach is to allow the creation and use of proxy objects, normally configured inside a Spring container, which act as codeless business delegates. You do not need to write another Service Locator, another JNDI lookup, or duplicate methods in a hand-coded Business Delegate unless you are actually adding real value in such code.
Assume that we have a web controller that needs to use a local EJB. We’ll follow best
practice and use the EJB Business Methods Interface pattern, so that the EJB’s local
interface extends a non EJB-specific business methods interface. Let’s call this
business methods interface MyComponent
.
public interface MyComponent { ... }
One of the main reasons to use the Business Methods Interface pattern is to ensure that
synchronization between method signatures in local interface and bean implementation
class is automatic. Another reason is that it later makes it much easier for us to
switch to a POJO (plain old Java object) implementation of the service if it makes sense
to do so. Of course we’ll also need to implement the local home interface and provide an
implementation class that implements SessionBean
and the MyComponent
business
methods interface. Now the only Java coding we’ll need to do to hook up our web tier
controller to the EJB implementation is to expose a setter method of type MyComponent
on the controller. This will save the reference as an instance variable in the
controller:
private MyComponent myComponent; public void setMyComponent(MyComponent myComponent) { this.myComponent = myComponent; }
We can subsequently use this instance variable in any business method in the controller.
Now assuming we are obtaining our controller object out of a Spring container, we can
(in the same context) configure a LocalStatelessSessionProxyFactoryBean
instance,
which will be the EJB proxy object. The configuration of the proxy, and setting of the
myComponent
property of the controller is done with a configuration entry such as:
<bean id="myComponent" class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean"> <property name="jndiName" value="ejb/myBean"/> <property name="businessInterface" value="com.mycom.MyComponent"/> </bean> <bean id="myController" class="com.mycom.myController"> <property name="myComponent" ref="myComponent"/> </bean>
There’s a lot of work happening behind the scenes, courtesy of the Spring AOP framework,
although you aren’t forced to work with AOP concepts to enjoy the results. The
myComponent
bean definition creates a proxy for the EJB, which implements the business
method interface. The EJB local home is cached on startup, so there’s only a single JNDI
lookup. Each time the EJB is invoked, the proxy invokes the classname
method on the
local EJB and invokes the corresponding business method on the EJB.
The myController
bean definition sets the myComponent
property of the controller
class to the EJB proxy.
Alternatively (and preferably in case of many such proxy definitions), consider using
the <jee:local-slsb>
configuration element in Spring’s "jee" namespace:
<jee:local-slsb id="myComponent" jndi-name="ejb/myBean" business-interface="com.mycom.MyComponent"/> <bean id="myController" class="com.mycom.myController"> <property name="myComponent" ref="myComponent"/> </bean>
This EJB access mechanism delivers huge simplification of application code: the web tier
code (or other EJB client code) has no dependence on the use of EJB. If we want to
replace this EJB reference with a POJO or a mock object or other test stub, we could
simply change the myComponent
bean definition without changing a line of Java code.
Additionally, we haven’t had to write a single line of JNDI lookup or other EJB plumbing
code as part of our application.
Benchmarks and experience in real applications indicate that the performance overhead of this approach (which involves reflective invocation of the target EJB) is minimal, and is typically undetectable in typical use. Remember that we don’t want to make fine-grained calls to EJBs anyway, as there’s a cost associated with the EJB infrastructure in the application server.
There is one caveat with regards to the JNDI lookup. In a bean container, this class is
normally best used as a singleton (there simply is no reason to make it a prototype).
However, if that bean container pre-instantiates singletons (as do the various XML
ApplicationContext
variants) you may have a problem if the bean container is loaded
before the EJB container loads the target EJB. That is because the JNDI lookup will be
performed in the init()
method of this class and then cached, but the EJB will not
have been bound at the target location yet. The solution is to not pre-instantiate this
factory object, but allow it to be created on first use. In the XML containers, this is
controlled via the lazy-init
attribute.
Although this will not be of interest to the majority of Spring users, those doing
programmatic AOP work with EJBs may want to look at LocalSlsbInvokerInterceptor
.
Accessing remote EJBs is essentially identical to accessing local EJBs, except that the
SimpleRemoteStatelessSessionProxyFactoryBean
or <jee:remote-slsb>
configuration
element is used. Of course, with or without Spring, remote invocation semantics apply; a
call to a method on an object in another VM in another computer does sometimes have to
be treated differently in terms of usage scenarios and failure handling.
Spring’s EJB client support adds one more advantage over the non-Spring approach.
Normally it is problematic for EJB client code to be easily switched back and forth
between calling EJBs locally or remotely. This is because the remote interface methods
must declare that they throw RemoteException
, and client code must deal with this,
while the local interface methods don’t. Client code written for local EJBs which needs
to be moved to remote EJBs typically has to be modified to add handling for the remote
exceptions, and client code written for remote EJBs which needs to be moved to local
EJBs, can either stay the same but do a lot of unnecessary handling of remote
exceptions, or needs to be modified to remove that code. With the Spring remote EJB
proxy, you can instead not declare any thrown RemoteException
in your Business Method
Interface and implementing EJB code, have a remote interface which is identical except
that it does throw RemoteException
, and rely on the proxy to dynamically treat the two
interfaces as if they were the same. That is, client code does not have to deal with the
checked RemoteException
class. Any actual RemoteException
that is thrown during the
EJB invocation will be re-thrown as the non-checked RemoteAccessException
class, which
is a subclass of RuntimeException
. The target service can then be switched at will
between a local EJB or remote EJB (or even plain Java object) implementation, without
the client code knowing or caring. Of course, this is optional; there is nothing
stopping you from declaring RemoteExceptions
in your business interface.
Accessing EJB 2.x Session Beans and EJB 3 Session Beans via Spring is largely
transparent. Spring’s EJB accessors, including the <jee:local-slsb>
and
<jee:remote-slsb>
facilities, transparently adapt to the actual component at runtime.
They handle a home interface if found (EJB 2.x style), or perform straight component
invocations if no home interface is available (EJB 3 style).
Note: For EJB 3 Session Beans, you could effectively use a JndiObjectFactoryBean
/
<jee:jndi-lookup>
as well, since fully usable component references are exposed for
plain JNDI lookups there. Defining explicit <jee:local-slsb>
/ <jee:remote-slsb>
lookups simply provides consistent and more explicit EJB access configuration.
For EJB 3 Session Beans and Message-Driven Beans, Spring provides a convenient
interceptor that resolves Spring’s @Autowired
annotation in the EJB component
class: org.springframework.ejb.interceptor.SpringBeanAutowiringInterceptor
. This
interceptor can be applied through an @Interceptors
annotation in the EJB component
class, or through an interceptor-binding
XML element in the EJB deployment descriptor.
@Stateless @Interceptors(SpringBeanAutowiringInterceptor.class) public class MyFacadeEJB implements MyFacadeLocal { // automatically injected with a matching Spring bean @Autowired private MyComponent myComp; // for business method, delegate to POJO service impl. public String myFacadeMethod(...) { return myComp.myMethod(...); } ... }
SpringBeanAutowiringInterceptor
by default obtains target beans from a
ContextSingletonBeanFactoryLocator
, with the context defined in a bean definition file
named beanRefContext.xml
. By default, a single context definition is expected, which
is obtained by type rather than by name. However, if you need to choose between multiple
context definitions, a specific locator key is required. The locator key (i.e. the name
of the context definition in beanRefContext.xml
) can be explicitly specified either
through overriding the getBeanFactoryLocatorKey
method in a custom
SpringBeanAutowiringInterceptor
subclass.
Alternatively, consider overriding SpringBeanAutowiringInterceptor
's getBeanFactory
method, e.g. obtaining a shared ApplicationContext
from a custom holder class.
Spring provides a JMS integration framework that simplifies the use of the JMS API much like Spring’s integration does for the JDBC API.
JMS can be roughly divided into two areas of functionality, namely the production and
consumption of messages. The JmsTemplate
class is used for message production and
synchronous message reception. For asynchronous reception similar to Java EE’s
message-driven bean style, Spring provides a number of message listener containers that
are used to create Message-Driven POJOs (MDPs).
The package org.springframework.jms.core
provides the core functionality for using
JMS. It contains JMS template classes that simplify the use of the JMS by handling the
creation and release of resources, much like the JdbcTemplate
does for JDBC. The
design principle common to Spring template classes is to provide helper methods to
perform common operations and for more sophisticated usage, delegate the essence of the
processing task to user implemented callback interfaces. The JMS template follows the
same design. The classes offer various convenience methods for the sending of messages,
consuming a message synchronously, and exposing the JMS session and message producer to
the user.
The package org.springframework.jms.support
provides JMSException
translation
functionality. The translation converts the checked JMSException
hierarchy to a
mirrored hierarchy of unchecked exceptions. If there are any provider specific
subclasses of the checked javax.jms.JMSException
, this exception is wrapped in the
unchecked UncategorizedJmsException
.
The package org.springframework.jms.support.converter
provides a MessageConverter
abstraction to convert between Java objects and JMS messages.
The package org.springframework.jms.support.destination
provides various strategies
for managing JMS destinations, such as providing a service locator for destinations
stored in JNDI.
Finally, the package org.springframework.jms.connection
provides an implementation of
the ConnectionFactory
suitable for use in standalone applications. It also contains an
implementation of Spring’s PlatformTransactionManager
for JMS (the cunningly named
JmsTransactionManager
). This allows for seamless integration of JMS as a transactional
resource into Spring’s transaction management mechanisms.
The JmsTemplate
class is the central class in the JMS core package. It simplifies the
use of JMS since it handles the creation and release of resources when sending or
synchronously receiving messages.
Code that uses the JmsTemplate
only needs to implement callback interfaces giving them
a clearly defined high level contract. The MessageCreator
callback interface creates a
message given a Session
provided by the calling code in JmsTemplate
. In order to
allow for more complex usage of the JMS API, the callback SessionCallback
provides the
user with the JMS session and the callback ProducerCallback
exposes a Session
and
MessageProducer
pair.
The JMS API exposes two types of send methods, one that takes delivery mode, priority,
and time-to-live as Quality of Service (QOS) parameters and one that takes no QOS
parameters which uses default values. Since there are many send methods in
JmsTemplate
, the setting of the QOS parameters have been exposed as bean properties to
avoid duplication in the number of send methods. Similarly, the timeout value for
synchronous receive calls is set using the property setReceiveTimeout
.
Some JMS providers allow the setting of default QOS values administratively through the
configuration of the ConnectionFactory. This has the effect that a call to
MessageProducer
's send method send(Destination destination, Message message)
will
use different QOS default values than those specified in the JMS specification. In order
to provide consistent management of QOS values, the JmsTemplate
must therefore be
specifically enabled to use its own QOS values by setting the boolean property
isExplicitQosEnabled
to true
.
Note | |
---|---|
Instances of the |
The JmsTemplate
requires a reference to a ConnectionFactory
. The ConnectionFactory
is part of the JMS specification and serves as the entry point for working with JMS. It
is used by the client application as a factory to create connections with the JMS
provider and encapsulates various configuration parameters, many of which are vendor
specific such as SSL configuration options.
When using JMS inside an EJB, the vendor provides implementations of the JMS interfaces
so that they can participate in declarative transaction management and perform pooling
of connections and sessions. In order to use this implementation, Java EE containers
typically require that you declare a JMS connection factory as a resource-ref
inside
the EJB or servlet deployment descriptors. To ensure the use of these features with the
JmsTemplate
inside an EJB, the client application should ensure that it references the
managed implementation of the ConnectionFactory
.
The standard API involves creating many intermediate objects. To send a message the following API walk is performed
ConnectionFactory->Connection->Session->MessageProducer->send
Between the ConnectionFactory and the Send operation there are three intermediate objects that are created and destroyed. To optimise the resource usage and increase performance two implementations of IConnectionFactory are provided.
Spring provides an implementation of the ConnectionFactory
interface,
SingleConnectionFactory
, that will return the same Connection
on all
createConnection()
calls and ignore calls to close()
. This is useful for testing and
standalone environments so that the same connection can be used for multiple
JmsTemplate
calls that may span any number of transactions. SingleConnectionFactory
takes a reference to a standard ConnectionFactory
that would typically come from JNDI.
The CachingConnectionFactory
extends the functionality of SingleConnectionFactory
and adds the caching of Sessions, MessageProducers, and MessageConsumers. The initial
cache size is set to 1, use the property SessionCacheSize
to increase the number of
cached sessions. Note that the number of actual cached sessions will be more than that
number as sessions are cached based on their acknowledgment mode, so there can be up to
4 cached session instances when SessionCacheSize
is set to one, one for each
AcknowledgementMode. MessageProducers and MessageConsumers are cached within their
owning session and also take into account the unique properties of the producers and
consumers when caching. MessageProducers are cached based on their destination.
MessageConsumers are cached based on a key composed of the destination, selector,
noLocal delivery flag, and the durable subscription name (if creating durable consumers).
Destinations, like ConnectionFactories, are JMS administered objects that can be stored
and retrieved in JNDI. When configuring a Spring application context you can use the
JNDI factory class JndiObjectFactoryBean
/ <jee:jndi-lookup>
to perform dependency
injection on your object’s references to JMS destinations. However, often this strategy
is cumbersome if there are a large number of destinations in the application or if there
are advanced destination management features unique to the JMS provider. Examples of
such advanced destination management would be the creation of dynamic destinations or
support for a hierarchical namespace of destinations. The JmsTemplate
delegates the
resolution of a destination name to a JMS destination object to an implementation of the
interface DestinationResolver
. DynamicDestinationResolver
is the default
implementation used by JmsTemplate
and accommodates resolving dynamic destinations. A
JndiDestinationResolver
is also provided that acts as a service locator for
destinations contained in JNDI and optionally falls back to the behavior contained in
DynamicDestinationResolver
.
Quite often the destinations used in a JMS application are only known at runtime and
therefore cannot be administratively created when the application is deployed. This is
often because there is shared application logic between interacting system components
that create destinations at runtime according to a well-known naming convention. Even
though the creation of dynamic destinations is not part of the JMS specification, most
vendors have provided this functionality. Dynamic destinations are created with a name
defined by the user which differentiates them from temporary destinations and are often
not registered in JNDI. The API used to create dynamic destinations varies from provider
to provider since the properties associated with the destination are vendor specific.
However, a simple implementation choice that is sometimes made by vendors is to
disregard the warnings in the JMS specification and to use the TopicSession
method
createTopic(String topicName)
or the QueueSession
method createQueue(String
queueName)
to create a new destination with default destination properties. Depending
on the vendor implementation, DynamicDestinationResolver
may then also create a
physical destination instead of only resolving one.
The boolean property pubSubDomain
is used to configure the JmsTemplate
with
knowledge of what JMS domain is being used. By default the value of this property is
false, indicating that the point-to-point domain, Queues, will be used. This property
used by JmsTemplate
determines the behavior of dynamic destination resolution via
implementations of the DestinationResolver
interface.
You can also configure the JmsTemplate
with a default destination via the property
defaultDestination
. The default destination will be used with send and receive
operations that do not refer to a specific destination.
One of the most common uses of JMS messages in the EJB world is to drive message-driven beans (MDBs). Spring offers a solution to create message-driven POJOs (MDPs) in a way that does not tie a user to an EJB container. (See Section 23.4.2, “Asynchronous Reception - Message-Driven POJOs” for detailed coverage of Spring’s MDP support.)
A message listener container is used to receive messages from a JMS message queue and drive the MessageListener that is injected into it. The listener container is responsible for all threading of message reception and dispatches into the listener for processing. A message listener container is the intermediary between an MDP and a messaging provider, and takes care of registering to receive messages, participating in transactions, resource acquisition and release, exception conversion and suchlike. This allows you as an application developer to write the (possibly complex) business logic associated with receiving a message (and possibly responding to it), and delegates boilerplate JMS infrastructure concerns to the framework.
There are two standard JMS message listener containers packaged with Spring, each with its specialised feature set.
This message listener container is the simpler of the two standard flavors. It creates a
fixed number of JMS sessions and consumers at startup, registers the listener using the
standard JMS MessageConsumer.setMessageListener()
method, and leaves it up the JMS
provider to perform listener callbacks. This variant does not allow for dynamic adaption
to runtime demands or for participation in externally managed transactions.
Compatibility-wise, it stays very close to the spirit of the standalone JMS
specification - but is generally not compatible with Java EE’s JMS restrictions.
This message listener container is the one used in most cases. In contrast to
SimpleMessageListenerContainer
, this container variant does allow for dynamic adaption
to runtime demands and is able to participate in externally managed transactions. Each
received message is registered with an XA transaction when configured with a
JtaTransactionManager
; so processing may take advantage of XA transaction semantics.
This listener container strikes a good balance between low requirements on the JMS
provider, advanced functionality such as transaction participation, and compatibility
with Java EE environments.
Spring provides a JmsTransactionManager
that manages transactions for a single JMS
ConnectionFactory
. This allows JMS applications to leverage the managed transaction
features of Spring as described in Chapter 11, Transaction Management. The JmsTransactionManager
performs
local resource transactions, binding a JMS Connection/Session pair from the specified
ConnectionFactory
to the thread. JmsTemplate
automatically detects such
transactional resources and operates on them accordingly.
In a Java EE environment, the ConnectionFactory
will pool Connections and Sessions, so
those resources are efficiently reused across transactions. In a standalone environment,
using Spring’s SingleConnectionFactory
will result in a shared JMS Connection
, with
each transaction having its own independent Session
. Alternatively, consider the use
of a provider-specific pooling adapter such as ActiveMQ’s PooledConnectionFactory
class.
JmsTemplate
can also be used with the JtaTransactionManager
and an XA-capable JMS
ConnectionFactory
for performing distributed transactions. Note that this requires the
use of a JTA transaction manager as well as a properly XA-configured ConnectionFactory!
(Check your Java EE server’s / JMS provider’s documentation.)
Reusing code across a managed and unmanaged transactional environment can be confusing
when using the JMS API to create a Session
from a Connection
. This is because the
JMS API has only one factory method to create a Session
and it requires values for the
transaction and acknowledgement modes. In a managed environment, setting these values is
the responsibility of the environment’s transactional infrastructure, so these values
are ignored by the vendor’s wrapper to the JMS Connection. When using the JmsTemplate
in an unmanaged environment you can specify these values through the use of the
properties sessionTransacted
and sessionAcknowledgeMode
. When using a
PlatformTransactionManager
with JmsTemplate
, the template will always be given a
transactional JMS Session
.
The JmsTemplate
contains many convenience methods to send a message. There are send
methods that specify the destination using a javax.jms.Destination
object and those
that specify the destination using a string for use in a JNDI lookup. The send method
that takes no destination argument uses the default destination. Here is an example that
sends a message to a queue using the 1.0.2 implementation.
import javax.jms.ConnectionFactory; import javax.jms.JMSException; import javax.jms.Message; import javax.jms.Queue; import javax.jms.Session; import org.springframework.jms.core.MessageCreator; import org.springframework.jms.core.JmsTemplate; public class JmsQueueSender { private JmsTemplate jmsTemplate; private Queue queue; public void setConnectionFactory(ConnectionFactory cf) { this.jmsTemplate = new JmsTemplate(cf); } public void setQueue(Queue queue) { this.queue = queue; } public void simpleSend() { this.jmsTemplate.send(this.queue, new MessageCreator() { public Message createMessage(Session session) throws JMSException { return session.createTextMessage("hello queue world"); } }); } }
This example uses the MessageCreator
callback to create a text message from the
supplied Session
object. The JmsTemplate
is constructed by passing a reference to a
ConnectionFactory
. As an alternative, a zero argument constructor and
connectionFactory
is provided and can be used for constructing the instance in
JavaBean style (using a BeanFactory or plain Java code). Alternatively, consider
deriving from Spring’s JmsGatewaySupport
convenience base class, which provides
pre-built bean properties for JMS configuration.
The method send(String destinationName, MessageCreator creator)
lets you send a
message using the string name of the destination. If these names are registered in JNDI,
you should set the destinationResolver
property of the template to an instance of
JndiDestinationResolver
.
If you created the JmsTemplate
and specified a default destination, the
send(MessageCreator c)
sends a message to that destination.
In order to facilitate the sending of domain model objects, the JmsTemplate
has
various send methods that take a Java object as an argument for a message’s data
content. The overloaded methods convertAndSend()
and receiveAndConvert()
in
JmsTemplate
delegate the conversion process to an instance of the MessageConverter
interface. This interface defines a simple contract to convert between Java objects and
JMS messages. The default implementation SimpleMessageConverter
supports conversion
between String
and TextMessage
, byte[]
and BytesMesssage
, and java.util.Map
and MapMessage
. By using the converter, you and your application code can focus on the
business object that is being sent or received via JMS and not be concerned with the
details of how it is represented as a JMS message.
The sandbox currently includes a MapMessageConverter
which uses reflection to convert
between a JavaBean and a MapMessage
. Other popular implementation choices you might
implement yourself are Converters that use an existing XML marshalling package, such as
JAXB, Castor, XMLBeans, or XStream, to create a TextMessage
representing the object.
To accommodate the setting of a message’s properties, headers, and body that can not be
generically encapsulated inside a converter class, the MessagePostProcessor
interface
gives you access to the message after it has been converted, but before it is sent. The
example below demonstrates how to modify a message header and a property after a
java.util.Map
is converted to a message.
public void sendWithConversion() { Map map = new HashMap(); map.put("Name", "Mark"); map.put("Age", new Integer(47)); jmsTemplate.convertAndSend("testQueue", map, new MessagePostProcessor() { public Message postProcessMessage(Message message) throws JMSException { message.setIntProperty("AccountID", 1234); message.setJMSCorrelationID("123-00001"); return message; } }); }
This results in a message of the form:
MapMessage={ Header={ ... standard headers ... CorrelationID={123-00001} } Properties={ AccountID={Integer:1234} } Fields={ Name={String:Mark} Age={Integer:47} } }
While the send operations cover many common usage scenarios, there are cases when you
want to perform multiple operations on a JMS Session
or MessageProducer
. The
SessionCallback
and ProducerCallback
expose the JMS Session
and Session
/
MessageProducer
pair respectively. The execute()
methods on JmsTemplate
execute
these callback methods.
While JMS is typically associated with asynchronous processing, it is possible to
consume messages synchronously. The overloaded receive(..)
methods provide this
functionality. During a synchronous receive, the calling thread blocks until a message
becomes available. This can be a dangerous operation since the calling thread can
potentially be blocked indefinitely. The property receiveTimeout
specifies how long
the receiver should wait before giving up waiting for a message.
In a fashion similar to a Message-Driven Bean (MDB) in the EJB world, the Message-Driven
POJO (MDP) acts as a receiver for JMS messages. The one restriction (but see also below
for the discussion of the MessageListenerAdapter
class) on an MDP is that it must
implement the javax.jms.MessageListener
interface. Please also be aware that in the
case where your POJO will be receiving messages on multiple threads, it is important to
ensure that your implementation is thread-safe.
Below is a simple implementation of an MDP:
import javax.jms.JMSException; import javax.jms.Message; import javax.jms.MessageListener; import javax.jms.TextMessage; public class ExampleListener implements MessageListener { public void onMessage(Message message) { if (message instanceof TextMessage) { try { System.out.println(((TextMessage) message).getText()); } catch (JMSException ex) { throw new RuntimeException(ex); } } else { throw new IllegalArgumentException("Message must be of type TextMessage"); } } }
Once you’ve implemented your MessageListener
, it’s time to create a message listener
container.
Find below an example of how to define and configure one of the message listener
containers that ships with Spring (in this case the DefaultMessageListenerContainer
).
<!-- this is the Message Driven POJO (MDP) --> <bean id="messageListener" class="jmsexample.ExampleListener" /> <!-- and this is the message listener container --> <bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer"> <property name="connectionFactory" ref="connectionFactory"/> <property name="destination" ref="destination"/> <property name="messageListener" ref="messageListener" /> </bean>
Please refer to the Spring Javadoc of the various message listener containers for a full description of the features supported by each implementation.
The SessionAwareMessageListener
interface is a Spring-specific interface that provides
a similar contract to the JMS MessageListener
interface, but also provides the message
handling method with access to the JMS Session
from which the Message
was received.
package org.springframework.jms.listener; public interface SessionAwareMessageListener { void onMessage(Message message, Session session) throws JMSException; }
You can choose to have your MDPs implement this interface (in preference to the standard
JMS MessageListener
interface) if you want your MDPs to be able to respond to any
received messages (using the Session
supplied in the onMessage(Message, Session)
method). All of the message listener container implementations that ship with Spring
have support for MDPs that implement either the MessageListener
or
SessionAwareMessageListener
interface. Classes that implement the
SessionAwareMessageListener
come with the caveat that they are then tied to Spring
through the interface. The choice of whether or not to use it is left entirely up to you
as an application developer or architect.
Please note that the 'onMessage(..)'
method of the SessionAwareMessageListener
interface throws JMSException
. In contrast to the standard JMS MessageListener
interface, when using the SessionAwareMessageListener
interface, it is the
responsibility of the client code to handle any exceptions thrown.
The MessageListenerAdapter
class is the final component in Spring’s asynchronous
messaging support: in a nutshell, it allows you to expose almost any class as a MDP
(there are of course some constraints).
Consider the following interface definition. Notice that although the interface extends
neither the MessageListener
nor SessionAwareMessageListener
interfaces, it can still
be used as a MDP via the use of the MessageListenerAdapter
class. Notice also how the
various message handling methods are strongly typed according to the contents of the
various Message
types that they can receive and handle.
public interface MessageDelegate { void handleMessage(String message); void handleMessage(Map message); void handleMessage(byte[] message); void handleMessage(Serializable message); }
public class DefaultMessageDelegate implements MessageDelegate { // implementation elided for clarity... }
In particular, note how the above implementation of the MessageDelegate
interface (the
above DefaultMessageDelegate
class) has no JMS dependencies at all. It truly is a
POJO that we will make into an MDP via the following configuration.
<!-- this is the Message Driven POJO (MDP) --> <bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter"> <constructor-arg> <bean class="jmsexample.DefaultMessageDelegate"/> </constructor-arg> </bean> <!-- and this is the message listener container... --> <bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer"> <property name="connectionFactory" ref="connectionFactory"/> <property name="destination" ref="destination"/> <property name="messageListener" ref="messageListener" /> </bean>
Below is an example of another MDP that can only handle the receiving of JMS
TextMessage
messages. Notice how the message handling method is actually called
'receive'
(the name of the message handling method in a MessageListenerAdapter
defaults to 'handleMessage'
), but it is configurable (as you will see below). Notice
also how the 'receive(..)'
method is strongly typed to receive and respond only to JMS
TextMessage
messages.
public interface TextMessageDelegate { void receive(TextMessage message); }
public class DefaultTextMessageDelegate implements TextMessageDelegate { // implementation elided for clarity... }
The configuration of the attendant MessageListenerAdapter
would look like this:
<bean id="messageListener" class="org.springframework.jms.listener.adapter.MessageListenerAdapter"> <constructor-arg> <bean class="jmsexample.DefaultTextMessageDelegate"/> </constructor-arg> <property name="defaultListenerMethod" value="receive"/> <!-- we don't want automatic message context extraction --> <property name="messageConverter"> <null/> </property> </bean>
Please note that if the above 'messageListener'
receives a JMS Message
of a type
other than TextMessage
, an IllegalStateException
will be thrown (and subsequently
swallowed). Another of the capabilities of the MessageListenerAdapter
class is the
ability to automatically send back a response Message
if a handler method returns a
non-void value. Consider the interface and class:
public interface ResponsiveTextMessageDelegate { // notice the return type... String receive(TextMessage message); }
public class DefaultResponsiveTextMessageDelegate implements ResponsiveTextMessageDelegate { // implementation elided for clarity... }
If the above DefaultResponsiveTextMessageDelegate
is used in conjunction with a
MessageListenerAdapter
then any non-null value that is returned from the execution of
the 'receive(..)'
method will (in the default configuration) be converted into a
TextMessage
. The resulting TextMessage
will then be sent to the Destination
(if
one exists) defined in the JMS Reply-To property of the original Message
, or the
default Destination
set on the MessageListenerAdapter
(if one has been configured);
if no Destination
is found then an InvalidDestinationException
will be thrown (and
please note that this exception will not be swallowed and will propagate up the
call stack).
Invoking a message listener within a transaction only requires reconfiguration of the listener container.
Local resource transactions can simply be activated through the sessionTransacted
flag
on the listener container definition. Each message listener invocation will then operate
within an active JMS transaction, with message reception rolled back in case of listener
execution failure. Sending a response message (via SessionAwareMessageListener
) will
be part of the same local transaction, but any other resource operations (such as
database access) will operate independently. This usually requires duplicate message
detection in the listener implementation, covering the case where database processing
has committed but message processing failed to commit.
<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer"> <property name="connectionFactory" ref="connectionFactory"/> <property name="destination" ref="destination"/> <property name="messageListener" ref="messageListener"/> <property name="sessionTransacted" value="true"/> </bean>
For participating in an externally managed transaction, you will need to configure a
transaction manager and use a listener container which supports externally managed
transactions: typically DefaultMessageListenerContainer
.
To configure a message listener container for XA transaction participation, you’ll want
to configure a JtaTransactionManager
(which, by default, delegates to the Java EE
server’s transaction subsystem). Note that the underlying JMS ConnectionFactory needs to
be XA-capable and properly registered with your JTA transaction coordinator! (Check your
Java EE server’s configuration of JNDI resources.) This allows message reception as well
as e.g. database access to be part of the same transaction (with unified commit
semantics, at the expense of XA transaction log overhead).
<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
Then you just need to add it to our earlier container configuration. The container will take care of the rest.
<bean id="jmsContainer" class="org.springframework.jms.listener.DefaultMessageListenerContainer"> <property name="connectionFactory" ref="connectionFactory"/> <property name="destination" ref="destination"/> <property name="messageListener" ref="messageListener"/> <property name="transactionManager" ref="transactionManager"/> </bean>
Beginning with version 2.5, Spring also provides support for a JCA-based
MessageListener
container. The JmsMessageEndpointManager
will attempt to
automatically determine the ActivationSpec
class name from the provider’s
ResourceAdapter
class name. Therefore, it is typically possible to just provide
Spring’s generic JmsActivationSpecConfig
as shown in the following example.
<bean class="org.springframework.jms.listener.endpoint.JmsMessageEndpointManager"> <property name="resourceAdapter" ref="resourceAdapter"/> <property name="activationSpecConfig"> <bean class="org.springframework.jms.listener.endpoint.JmsActivationSpecConfig"> <property name="destinationName" value="myQueue"/> </bean> </property> <property name="messageListener" ref="myMessageListener"/> </bean>
Alternatively, you may set up a JmsMessageEndpointManager
with a given
ActivationSpec
object. The ActivationSpec
object may also come from a JNDI lookup
(using <jee:jndi-lookup>
).
<bean class="org.springframework.jms.listener.endpoint.JmsMessageEndpointManager"> <property name="resourceAdapter" ref="resourceAdapter"/> <property name="activationSpec"> <bean class="org.apache.activemq.ra.ActiveMQActivationSpec"> <property name="destination" value="myQueue"/> <property name="destinationType" value="javax.jms.Queue"/> </bean> </property> <property name="messageListener" ref="myMessageListener"/> </bean>
Using Spring’s ResourceAdapterFactoryBean
, the target ResourceAdapter
may be
configured locally as depicted in the following example.
<bean id="resourceAdapter" class="org.springframework.jca.support.ResourceAdapterFactoryBean"> <property name="resourceAdapter"> <bean class="org.apache.activemq.ra.ActiveMQResourceAdapter"> <property name="serverUrl" value="tcp://localhost:61616"/> </bean> </property> <property name="workManager"> <bean class="org.springframework.jca.work.SimpleTaskWorkManager"/> </property> </bean>
The specified WorkManager
may also point to an environment-specific thread pool -
typically through SimpleTaskWorkManager's
"asyncTaskExecutor" property. Consider
defining a shared thread pool for all your ResourceAdapter
instances if you happen to
use multiple adapters.
In some environments (e.g. WebLogic 9 or above), the entire ResourceAdapter
object may
be obtained from JNDI instead (using <jee:jndi-lookup>
). The Spring-based message
listeners can then interact with the server-hosted ResourceAdapter
, also using the
server’s built-in WorkManager
.
Please consult the JavaDoc for JmsMessageEndpointManager
, JmsActivationSpecConfig
,
and ResourceAdapterFactoryBean
for more details.
Spring also provides a generic JCA message endpoint manager which is not tied to JMS:
org.springframework.jca.endpoint.GenericMessageEndpointManager
. This component allows
for using any message listener type (e.g. a CCI MessageListener) and any
provider-specific ActivationSpec object. Check out your JCA provider’s documentation to
find out about the actual capabilities of your connector, and consult
GenericMessageEndpointManager
's JavaDoc for the Spring-specific configuration details.
Note | |
---|---|
JCA-based message endpoint management is very analogous to EJB 2.1 Message-Driven Beans; it uses the same underlying resource provider contract. Like with EJB 2.1 MDBs, any message listener interface supported by your JCA provider can be used in the Spring context as well. Spring nevertheless provides explicit convenience support for JMS, simply because JMS is the most common endpoint API used with the JCA endpoint management contract. |
Spring 2.5 introduces an XML namespace for simplifying JMS configuration. To use the JMS namespace elements you will need to reference the JMS schema:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jms="http://www.springframework.org/schema/jms" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/jms http://www.springframework.org/schema/jms/spring-jms.xsd"> <!-- bean definitions here --> </beans>
The namespace consists of two top-level elements: <listener-container/>
and
<jca-listener-container/>
both of which may contain one or more <listener/>
child
elements. Here is an example of a basic configuration for two listeners.
<jms:listener-container> <jms:listener destination="queue.orders" ref="orderService" method="placeOrder"/> <jms:listener destination="queue.confirmations" ref="confirmationLogger" method="log"/> </jms:listener-container>
The example above is equivalent to creating two distinct listener container bean
definitions and two distinct MessageListenerAdapter
bean definitions as demonstrated
in Section 23.4.4, “the MessageListenerAdapter”. In addition to the attributes shown
above, the listener
element may contain several optional ones. The following table
describes all available attributes:
Table 23.1. Attributes of the JMS <listener> element
Attribute | Description |
---|---|
id | A bean name for the hosting listener container. If not specified, a bean name will be automatically generated. |
destination (required) | The destination name for this listener, resolved through the |
ref (required) | The bean name of the handler object. |
method | The name of the handler method to invoke. If the |
response-destination | The name of the default response destination to send response messages to. This will be applied in case of a request message that does not carry a "JMSReplyTo" field. The type of this destination will be determined by the listener-container’s "destination-type" attribute. Note: This only applies to a listener method with a return value, for which each result object will be converted into a response message. |
subscription | The name of the durable subscription, if any. |
selector | An optional message selector for this listener. |
The <listener-container/>
element also accepts several optional attributes. This
allows for customization of the various strategies (for example, taskExecutor
and
destinationResolver
) as well as basic JMS settings and resource references. Using
these attributes, it is possible to define highly-customized listener containers while
still benefiting from the convenience of the namespace.
<jms:listener-container connection-factory="myConnectionFactory" task-executor="myTaskExecutor" destination-resolver="myDestinationResolver" transaction-manager="myTransactionManager" concurrency="10"> <jms:listener destination="queue.orders" ref="orderService" method="placeOrder"/> <jms:listener destination="queue.confirmations" ref="confirmationLogger" method="log"/> </jms:listener-container>
The following table describes all available attributes. Consult the class-level Javadoc
of the AbstractMessageListenerContainer
and its concrete subclasses for more details
on the individual properties. The Javadoc also provides a discussion of transaction
choices and message redelivery scenarios.
Table 23.2. Attributes of the JMS <listener-container> element
Attribute | Description |
---|---|
container-type | The type of this listener container. Available options are: |
container-class | A custom listener container implementation class as fully qualified class name.
Default is Spring’s standard |
connection-factory | A reference to the JMS |
task-executor | A reference to the Spring |
destination-resolver | A reference to the |
message-converter | A reference to the |
error-handler | A reference to an |
destination-type | The JMS destination type for this listener: |
client-id | The JMS client id for this listener container. Needs to be specified when using durable subscriptions. |
cache | The cache level for JMS resources: |
acknowledge | The native JMS acknowledge mode: |
transaction-manager | A reference to an external |
concurrency | The number of concurrent sessions/consumers to start for each listener. Can either be a simple number indicating the maximum number (e.g. "5") or a range indicating the lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint and might be ignored at runtime. Default is 1; keep concurrency limited to 1 in case of a topic listener or if queue ordering is important; consider raising it for general queues. |
prefetch | The maximum number of messages to load into a single session. Note that raising this number might lead to starvation of concurrent consumers! |
receive-timeout | The timeout to use for receive calls (in milliseconds). The default is |
recovery-interval | Specify the interval between recovery attempts, in milliseconds. The default is |
phase | The lifecycle phase within which this container should start and stop. The lower the
value the earlier this container will start and the later it will stop. The default is
|
Configuring a JCA-based listener container with the "jms" schema support is very similar.
<jms:jca-listener-container resource-adapter="myResourceAdapter" destination-resolver="myDestinationResolver" transaction-manager="myTransactionManager" concurrency="10"> <jms:listener destination="queue.orders" ref="myMessageListener"/> </jms:jca-listener-container>
The available configuration options for the JCA variant are described in the following table:
Table 23.3. Attributes of the JMS <jca-listener-container/> element
Attribute | Description |
---|---|
resource-adapter | A reference to the JCA |
activation-spec-factory | A reference to the |
destination-resolver | A reference to the |
message-converter | A reference to the |
destination-type | The JMS destination type for this listener: |
client-id | The JMS client id for this listener container. Needs to be specified when using durable subscriptions. |
acknowledge | The native JMS acknowledge mode: |
transaction-manager | A reference to a Spring |
concurrency | The number of concurrent sessions/consumers to start for each listener. Can either be a simple number indicating the maximum number (e.g. "5") or a range indicating the lower as well as the upper limit (e.g. "3-5"). Note that a specified minimum is just a hint and will typically be ignored at runtime when using a JCA listener container. Default is 1. |
prefetch | The maximum number of messages to load into a single session. Note that raising this number might lead to starvation of concurrent consumers! |
The JMX support in Spring provides you with the features to easily and transparently integrate your Spring application into a JMX infrastructure.
Specifically, Spring’s JMX support provides four core features:
These features are designed to work without coupling your application components to either Spring or JMX interfaces and classes. Indeed, for the most part your application classes need not be aware of either Spring or JMX in order to take advantage of the Spring JMX features.
The core class in Spring’s JMX framework is the MBeanExporter
. This class is
responsible for taking your Spring beans and registering them with a JMX MBeanServer
.
For example, consider the following class:
package org.springframework.jmx; public class JmxTestBean implements IJmxTestBean { private String name; private int age; private boolean isSuperman; public int getAge() { return age; } public void setAge(int age) { this.age = age; } public void setName(String name) { this.name = name; } public String getName() { return name; } public int add(int x, int y) { return x + y; } public void dontExposeMe() { throw new RuntimeException(); } }
To expose the properties and methods of this bean as attributes and operations of an
MBean you simply configure an instance of the MBeanExporter
class in your
configuration file and pass in the bean as shown below:
<beans> <!-- this bean must not be lazily initialized if the exporting is to happen --> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter" lazy-init="false"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
The pertinent bean definition from the above configuration snippet is the exporter
bean. The beans
property tells the MBeanExporter
exactly which of your beans must be
exported to the JMX MBeanServer
. In the default configuration, the key of each entry
in the beans
Map
is used as the ObjectName
for the bean referenced by the
corresponding entry value. This behavior can be changed as described in Section 24.4, “Controlling the ObjectNames for your beans”.
With this configuration the testBean
bean is exposed as an MBean under the
ObjectName
bean:name=testBean1
. By default, all public properties of the bean
are exposed as attributes and all public methods (bar those inherited from the
Object
class) are exposed as operations.
The above configuration assumes that the application is running in an environment that
has one (and only one) MBeanServer
already running. In this case, Spring will attempt
to locate the running MBeanServer
and register your beans with that server (if any).
This behavior is useful when your application is running inside a container such as
Tomcat or IBM WebSphere that has its own MBeanServer
.
However, this approach is of no use in a standalone environment, or when running inside
a container that does not provide an MBeanServer
. To address this you can create an
MBeanServer
instance declaratively by adding an instance of the
org.springframework.jmx.support.MBeanServerFactoryBean
class to your configuration.
You can also ensure that a specific MBeanServer
is used by setting the value of the
MBeanExporter
's server
property to the MBeanServer
value returned by an
MBeanServerFactoryBean
; for example:
<beans> <bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean"/> <!-- this bean needs to be eagerly pre-instantiated in order for the exporting to occur; this means that it must not be marked as lazily initialized --> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> <property name="server" ref="mbeanServer"/> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
Here an instance of MBeanServer
is created by the MBeanServerFactoryBean
and is
supplied to the MBeanExporter
via the server property. When you supply your own
MBeanServer
instance, the MBeanExporter
will not attempt to locate a running
MBeanServer
and will use the supplied MBeanServer
instance. For this to work
correctly, you must (of course) have a JMX implementation on your classpath.
If no server is specified, the MBeanExporter
tries to automatically detect a running
MBeanServer
. This works in most environment where only one MBeanServer
instance is
used, however when multiple instances exist, the exporter might pick the wrong server.
In such cases, one should use the MBeanServer
agentId
to indicate which instance to
be used:
<beans> <bean id="mbeanServer" class="org.springframework.jmx.support.MBeanServerFactoryBean"> <!-- indicate to first look for a server --> <property name="locateExistingServerIfPossible" value="true"/> <!-- search for the MBeanServer instance with the given agentId --> <property name="agentId" value="MBeanServer_instance_agentId>"/> </bean> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="server" ref="mbeanServer"/> ... </bean> </beans>
For platforms/cases where the existing MBeanServer
has a dynamic (or unknown)
agentId
which is retrieved through lookup methods, one should use
factory-method:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="server"> <!-- Custom MBeanServerLocator --> <bean class="platform.package.MBeanServerLocator" factory-method="locateMBeanServer"/> </property> </bean> <!-- other beans here --> </beans>
If you configure a bean with the MBeanExporter
that is also configured for lazy
initialization, then the MBeanExporter
will not break this contract and will avoid
instantiating the bean. Instead, it will register a proxy with the MBeanServer
and
will defer obtaining the bean from the container until the first invocation on the proxy
occurs.
Any beans that are exported through the MBeanExporter
and are already valid MBeans are
registered as-is with the MBeanServer
without further intervention from Spring. MBeans
can be automatically detected by the MBeanExporter
by setting the autodetect
property to true
:
<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="autodetect" value="true"/> </bean> <bean name="spring:mbean=true" class="org.springframework.jmx.export.TestDynamicMBean"/>
Here, the bean called spring:mbean=true
is already a valid JMX MBean and will be
automatically registered by Spring. By default, beans that are autodetected for JMX
registration have their bean name used as the ObjectName
. This behavior can be
overridden as detailed in Section 24.4, “Controlling the ObjectNames for your beans”.
Consider the scenario where a Spring MBeanExporter
attempts to register an MBean
with an MBeanServer
using the ObjectName
'bean:name=testBean1'
. If an MBean
instance has already been registered under that same ObjectName
, the default behavior
is to fail (and throw an InstanceAlreadyExistsException
).
It is possible to control the behavior of exactly what happens when an MBean
is
registered with an MBeanServer
. Spring’s JMX support allows for three different
registration behaviors to control the registration behavior when the registration
process finds that an MBean
has already been registered under the same ObjectName
;
these registration behaviors are summarized on the following table:
Table 24.1. Registration Behaviors
Registration behavior | Explanation |
---|---|
| This is the default registration behavior. If an |
| If an |
| If an |
The above values are defined as constants on the MBeanRegistrationSupport
class (the
MBeanExporter
class derives from this superclass). If you want to change the default
registration behavior, you simply need to set the value of the
registrationBehaviorName
property on your MBeanExporter
definition to one of those
values.
The following example illustrates how to effect a change from the default registration
behavior to the REGISTRATION_REPLACE_EXISTING
behavior:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> <property name="registrationBehaviorName" value="REGISTRATION_REPLACE_EXISTING"/> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
In the previous example, you had little control over the management interface of your bean; all of the public properties and methods of each exported bean was exposed as JMX attributes and operations respectively. To exercise finer-grained control over exactly which properties and methods of your exported beans are actually exposed as JMX attributes and operations, Spring JMX provides a comprehensive and extensible mechanism for controlling the management interfaces of your beans.
Behind the scenes, the MBeanExporter
delegates to an implementation of the
org.springframework.jmx.export.assembler.MBeanInfoAssembler
interface which is
responsible for defining the management interface of each bean that is being exposed.
The default implementation,
org.springframework.jmx.export.assembler.SimpleReflectiveMBeanInfoAssembler
, simply
defines a management interface that exposes all public properties and methods (as you
saw in the previous examples). Spring provides two additional implementations of the
MBeanInfoAssembler
interface that allow you to control the generated management
interface using either source-level metadata or any arbitrary interface.
Using the MetadataMBeanInfoAssembler
you can define the management interfaces for your
beans using source level metadata. The reading of metadata is encapsulated by the
org.springframework.jmx.export.metadata.JmxAttributeSource
interface. Spring JMX
provides a default implementation which uses JDK 5.0 annotations, namely
org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource
. The
MetadataMBeanInfoAssembler
must be configured with an implementation instance of
the JmxAttributeSource
interface for it to function correctly (there is no
default).
To mark a bean for export to JMX, you should annotate the bean class with the
ManagedResource
annotation. Each method you wish to expose as an operation must be
marked with the ManagedOperation
annotation and each property you wish to expose must
be marked with the ManagedAttribute
annotation. When marking properties you can omit
either the annotation of the getter or the setter to create a write-only or read-only
attribute respectively.
The example below shows the annotated version of the JmxTestBean
class that you saw
earlier:
package org.springframework.jmx; import org.springframework.jmx.export.annotation.ManagedResource; import org.springframework.jmx.export.annotation.ManagedOperation; import org.springframework.jmx.export.annotation.ManagedAttribute; @ManagedResource( objectName="bean:name=testBean4", description="My Managed Bean", log=true, logFile="jmx.log", currencyTimeLimit=15, persistPolicy="OnUpdate", persistPeriod=200, persistLocation="foo", persistName="bar") public class AnnotationTestBean implements IJmxTestBean { private String name; private int age; @ManagedAttribute(description="The Age Attribute", currencyTimeLimit=15) public int getAge() { return age; } public void setAge(int age) { this.age = age; } @ManagedAttribute(description="The Name Attribute", currencyTimeLimit=20, defaultValue="bar", persistPolicy="OnUpdate") public void setName(String name) { this.name = name; } @ManagedAttribute(defaultValue="foo", persistPeriod=300) public String getName() { return name; } @ManagedOperation(description="Add two numbers") @ManagedOperationParameters({ @ManagedOperationParameter(name = "x", description = "The first number"), @ManagedOperationParameter(name = "y", description = "The second number")}) public int add(int x, int y) { return x + y; } public void dontExposeMe() { throw new RuntimeException(); } }
Here you can see that the JmxTestBean
class is marked with the ManagedResource
annotation and that this ManagedResource
annotation is configured with a set of
properties. These properties can be used to configure various aspects of the MBean that
is generated by the MBeanExporter
, and are explained in greater detail later in
section entitled Section 24.3.3, “Source-Level Metadata Types”.
You will also notice that both the age
and name
properties are annotated with the
ManagedAttribute
annotation, but in the case of the age
property, only the getter is
marked. This will cause both of these properties to be included in the management
interface as attributes, but the age
attribute will be read-only.
Finally, you will notice that the add(int, int)
method is marked with the
ManagedOperation
attribute whereas the dontExposeMe()
method is not. This will cause
the management interface to contain only one operation, add(int, int)
, when using the
MetadataMBeanInfoAssembler
.
The configuration below shows how you configure the MBeanExporter
to use the
MetadataMBeanInfoAssembler
:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="assembler" ref="assembler"/> <property name="namingStrategy" ref="namingStrategy"/> <property name="autodetect" value="true"/> </bean> <bean id="jmxAttributeSource" class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/> <!-- will create management interface using annotation metadata --> <bean id="assembler" class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler"> <property name="attributeSource" ref="jmxAttributeSource"/> </bean> <!-- will pick up the ObjectName from the annotation --> <bean id="namingStrategy" class="org.springframework.jmx.export.naming.MetadataNamingStrategy"> <property name="attributeSource" ref="jmxAttributeSource"/> </bean> <bean id="testBean" class="org.springframework.jmx.AnnotationTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
Here you can see that an MetadataMBeanInfoAssembler
bean has been configured with an
instance of the AnnotationJmxAttributeSource
class and passed to the MBeanExporter
through the assembler property. This is all that is required to take advantage of
metadata-driven management interfaces for your Spring-exposed MBeans.
The following source level metadata types are available for use in Spring JMX:
Table 24.2. Source-Level Metadata Types
Purpose | Annotation | Annotation Type |
---|---|---|
Mark all instances of a |
| Class |
Mark a method as a JMX operation |
| Method |
Mark a getter or setter as one half of a JMX attribute |
| Method (only getters and setters) |
Define descriptions for operation parameters |
| Method |
The following configuration parameters are available for use on these source-level metadata types:
Table 24.3. Source-Level Metadata Parameters
Parameter | Description | Applies to |
---|---|---|
| Used by |
|
| Sets the friendly description of the resource, attribute or operation |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the value of the |
|
| Sets the display name of an operation parameter |
|
| Sets the index of an operation parameter |
|
To simplify configuration even further, Spring introduces the
AutodetectCapableMBeanInfoAssembler
interface which extends the MBeanInfoAssembler
interface to add support for autodetection of MBean resources. If you configure the
MBeanExporter
with an instance of AutodetectCapableMBeanInfoAssembler
then it is
allowed to "vote" on the inclusion of beans for exposure to JMX.
Out of the box, the only implementation of the AutodetectCapableMBeanInfo
interface is
the MetadataMBeanInfoAssembler
which will vote to include any bean which is marked
with the ManagedResource
attribute. The default approach in this case is to use the
bean name as the ObjectName
which results in a configuration like this:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <!-- notice how no beans are explicitly configured here --> <property name="autodetect" value="true"/> <property name="assembler" ref="assembler"/> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> <bean id="assembler" class="org.springframework.jmx.export.assembler.MetadataMBeanInfoAssembler"> <property name="attributeSource"> <bean class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/> </property> </bean> </beans>
Notice that in this configuration no beans are passed to the MBeanExporter
; however,
the JmxTestBean
will still be registered since it is marked with the ManagedResource
attribute and the MetadataMBeanInfoAssembler
detects this and votes to include it. The
only problem with this approach is that the name of the JmxTestBean
now has business
meaning. You can address this issue by changing the default behavior for ObjectName
creation as defined in Section 24.4, “Controlling the ObjectNames for your beans”.
In addition to the MetadataMBeanInfoAssembler
, Spring also includes the
InterfaceBasedMBeanInfoAssembler
which allows you to constrain the methods and
properties that are exposed based on the set of methods defined in a collection of
interfaces.
Although the standard mechanism for exposing MBeans is to use interfaces and a simple
naming scheme, the InterfaceBasedMBeanInfoAssembler
extends this functionality by
removing the need for naming conventions, allowing you to use more than one interface
and removing the need for your beans to implement the MBean interfaces.
Consider this interface that is used to define a management interface for the
JmxTestBean
class that you saw earlier:
public interface IJmxTestBean { public int add(int x, int y); public long myOperation(); public int getAge(); public void setAge(int age); public void setName(String name); public String getName(); }
This interface defines the methods and properties that will be exposed as operations and attributes on the JMX MBean. The code below shows how to configure Spring JMX to use this interface as the definition for the management interface:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean5" value-ref="testBean"/> </map> </property> <property name="assembler"> <bean class="org.springframework.jmx.export.assembler.InterfaceBasedMBeanInfoAssembler"> <property name="managedInterfaces"> <value>org.springframework.jmx.IJmxTestBean</value> </property> </bean> </property> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
Here you can see that the InterfaceBasedMBeanInfoAssembler
is configured to use the
IJmxTestBean
interface when constructing the management interface for any bean. It is
important to understand that beans processed by the InterfaceBasedMBeanInfoAssembler
are not required to implement the interface used to generate the JMX management
interface.
In the case above, the IJmxTestBean
interface is used to construct all management
interfaces for all beans. In many cases this is not the desired behavior and you may
want to use different interfaces for different beans. In this case, you can pass
InterfaceBasedMBeanInfoAssembler
a Properties
instance via the interfaceMappings
property, where the key of each entry is the bean name and the value of each entry is a
comma-separated list of interface names to use for that bean.
If no management interface is specified through either the managedInterfaces
or
interfaceMappings
properties, then the InterfaceBasedMBeanInfoAssembler
will reflect
on the bean and use all of the interfaces implemented by that bean to create the
management interface.
The MethodNameBasedMBeanInfoAssembler
allows you to specify a list of method names
that will be exposed to JMX as attributes and operations. The code below shows a sample
configuration for this:
<bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean5" value-ref="testBean"/> </map> </property> <property name="assembler"> <bean class="org.springframework.jmx.export.assembler.MethodNameBasedMBeanInfoAssembler"> <property name="managedMethods"> <value>add,myOperation,getName,setName,getAge</value> </property> </bean> </property> </bean>
Here you can see that the methods add
and myOperation
will be exposed as JMX
operations and getName()
, setName(String)
and getAge()
will be exposed as the
appropriate half of a JMX attribute. In the code above, the method mappings apply to
beans that are exposed to JMX. To control method exposure on a bean-by-bean basis, use
the methodMappings
property of MethodNameMBeanInfoAssembler
to map bean names to
lists of method names.
Behind the scenes, the MBeanExporter
delegates to an implementation of the
ObjectNamingStrategy
to obtain ObjectName
s for each of the beans it is registering.
The default implementation, KeyNamingStrategy
, will, by default, use the key of the
beans
Map
as the ObjectName
. In addition, the KeyNamingStrategy
can map the key
of the beans
Map
to an entry in a Properties
file (or files) to resolve the
ObjectName
. In addition to the KeyNamingStrategy
, Spring provides two additional
ObjectNamingStrategy
implementations: the IdentityNamingStrategy
that builds an
ObjectName
based on the JVM identity of the bean and the MetadataNamingStrategy
that
uses source level metadata to obtain the ObjectName
.
You can configure your own KeyNamingStrategy
instance and configure it to read
ObjectName
s from a Properties
instance rather than use bean key. The
KeyNamingStrategy
will attempt to locate an entry in the Properties
with a key
corresponding to the bean key. If no entry is found or if the Properties
instance is
null
then the bean key itself is used.
The code below shows a sample configuration for the KeyNamingStrategy
:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="testBean" value-ref="testBean"/> </map> </property> <property name="namingStrategy" ref="namingStrategy"/> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> <bean id="namingStrategy" class="org.springframework.jmx.export.naming.KeyNamingStrategy"> <property name="mappings"> <props> <prop key="testBean">bean:name=testBean1</prop> </props> </property> <property name="mappingLocations"> <value>names1.properties,names2.properties</value> </property> </bean> </beans>
Here an instance of KeyNamingStrategy
is configured with a Properties
instance that
is merged from the Properties
instance defined by the mapping property and the
properties files located in the paths defined by the mappings property. In this
configuration, the testBean
bean will be given the ObjectName
bean:name=testBean1
since this is the entry in the Properties
instance that has a key corresponding to the
bean key.
If no entry in the Properties
instance can be found then the bean key name is used as
the ObjectName
.
The MetadataNamingStrategy
uses the objectName
property of the ManagedResource
attribute on each bean to create the ObjectName
. The code below shows the
configuration for the MetadataNamingStrategy
:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="testBean" value-ref="testBean"/> </map> </property> <property name="namingStrategy" ref="namingStrategy"/> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> <bean id="namingStrategy" class="org.springframework.jmx.export.naming.MetadataNamingStrategy"> <property name="attributeSource" ref="attributeSource"/> </bean> <bean id="attributeSource" class="org.springframework.jmx.export.annotation.AnnotationJmxAttributeSource"/> </beans>
If no objectName
has been provided for the ManagedResource
attribute, then an
ObjectName
will be created with the following
format:[fully-qualified-package-name]:type=[short-classname],name=[bean-name]. For
example, the generated ObjectName
for the following bean would be:
com.foo:type=MyClass,name=myBean.
<bean id="myBean" class="com.foo.MyClass"/>
If you prefer using the annotation based approach to define
your management interfaces, then a convenience subclass of MBeanExporter
is available:
AnnotationMBeanExporter
. When defining an instance of this subclass, the
namingStrategy
, assembler
, and attributeSource
configuration is no longer needed,
since it will always use standard Java annotation-based metadata (autodetection is
always enabled as well). In fact, rather than defining an MBeanExporter
bean, an even
simpler syntax is supported by the @EnableMBeanExport
@Configuration
annotation.
@Configuration @EnableMBeanExport public class AppConfig { }
If you prefer XML based configuration the ' context:mbean-export'
element serves the
same purpose.
<context:mbean-export/>
You can provide a reference to a particular MBean server
if necessary, and the
defaultDomain
attribute (a property of AnnotationMBeanExporter
) accepts an alternate
value for the generated MBean ObjectNames
' domains. This would be used in place of the
fully qualified package name as described in the previous section on
MetadataNamingStrategy
.
@EnableMBeanExport(server="myMBeanServer", defaultDomain="myDomain") @Configuration ContextConfiguration { }
<context:mbean-export server="myMBeanServer" default-domain="myDomain"/>
Note | |
---|---|
Do not use interface-based AOP proxies in combination with autodetection of JMX
annotations in your bean classes. Interface-based proxies hide the target class, which
also hides the JMX managed resource annotations. Hence, use target-class proxies in that
case: through setting the proxy-target-class flag on |
For remote access, Spring JMX module offers two FactoryBean
implementations inside the
org.springframework.jmx.support
package for creating both server- and client-side
connectors.
To have Spring JMX create, start and expose a JSR-160 JMXConnectorServer
use the
following configuration:
<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean"/>
By default ConnectorServerFactoryBean
creates a JMXConnectorServer
bound to
"service:jmx:jmxmp://localhost:9875"
. The serverConnector
bean thus exposes the
local MBeanServer
to clients through the JMXMP protocol on localhost, port 9875. Note
that the JMXMP protocol is marked as optional by the JSR 160 specification: currently,
the main open-source JMX implementation, MX4J, and the one provided with J2SE 5.0
do not support JMXMP.
To specify another URL and register the JMXConnectorServer
itself with the
MBeanServer
use the serviceUrl
and ObjectName
properties respectively:
<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean"> <property name="objectName" value="connector:name=rmi"/> <property name="serviceUrl" value="service:jmx:rmi://localhost/jndi/rmi://localhost:1099/myconnector"/> </bean>
If the ObjectName
property is set Spring will automatically register your connector
with the MBeanServer
under that ObjectName
. The example below shows the full set of
parameters which you can pass to the ConnectorServerFactoryBean
when creating a
JMXConnector:
<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean"> <property name="objectName" value="connector:name=iiop"/> <property name="serviceUrl" value="service:jmx:iiop://localhost/jndi/iiop://localhost:900/myconnector"/> <property name="threaded" value="true"/> <property name="daemon" value="true"/> <property name="environment"> <map> <entry key="someKey" value="someValue"/> </map> </property> </bean>
Note that when using a RMI-based connector you need the lookup service (tnameserv or rmiregistry) to be started in order for the name registration to complete. If you are using Spring to export remote services for you via RMI, then Spring will already have constructed an RMI registry. If not, you can easily start a registry using the following snippet of configuration:
<bean id="registry" class="org.springframework.remoting.rmi.RmiRegistryFactoryBean"> <property name="port" value="1099"/> </bean>
To create an MBeanServerConnection
to a remote JSR-160 enabled MBeanServer
use the
MBeanServerConnectionFactoryBean
as shown below:
<bean id="clientConnector" class="org.springframework.jmx.support.MBeanServerConnectionFactoryBean"> <property name="serviceUrl" value="service:jmx:rmi://localhost/jndi/rmi://localhost:1099/jmxrmi"/> </bean>
JSR-160 permits extensions to the way in which communication is done between the client and the server. The examples above are using the mandatory RMI-based implementation required by the JSR-160 specification (IIOP and JRMP) and the (optional) JMXMP. By using other providers or JMX implementations (such as MX4J) you can take advantage of protocols like SOAP, Hessian, Burlap over simple HTTP or SSL and others:
<bean id="serverConnector" class="org.springframework.jmx.support.ConnectorServerFactoryBean"> <property name="objectName" value="connector:name=burlap"/> <property name="serviceUrl" value="service:jmx:burlap://localhost:9874"/> </bean>
In the case of the above example, MX4J 3.0.0 was used; see the official MX4J documentation for more information.
Spring JMX allows you to create proxies that re-route calls to MBeans registered in a
local or remote MBeanServer
. These proxies provide you with a standard Java interface
through which you can interact with your MBeans. The code below shows how to configure a
proxy for an MBean running in a local MBeanServer
:
<bean id="proxy" class="org.springframework.jmx.access.MBeanProxyFactoryBean"> <property name="objectName" value="bean:name=testBean"/> <property name="proxyInterface" value="org.springframework.jmx.IJmxTestBean"/> </bean>
Here you can see that a proxy is created for the MBean registered under the
ObjectName
: bean:name=testBean
. The set of interfaces that the proxy will implement
is controlled by the proxyInterfaces
property and the rules for mapping methods and
properties on these interfaces to operations and attributes on the MBean are the same
rules used by the InterfaceBasedMBeanInfoAssembler
.
The MBeanProxyFactoryBean
can create a proxy to any MBean that is accessible via an
MBeanServerConnection
. By default, the local MBeanServer
is located and used, but
you can override this and provide an MBeanServerConnection
pointing to a remote
MBeanServer
to cater for proxies pointing to remote MBeans:
<bean id="clientConnector" class="org.springframework.jmx.support.MBeanServerConnectionFactoryBean"> <property name="serviceUrl" value="service:jmx:rmi://remotehost:9875"/> </bean> <bean id="proxy" class="org.springframework.jmx.access.MBeanProxyFactoryBean"> <property name="objectName" value="bean:name=testBean"/> <property name="proxyInterface" value="org.springframework.jmx.IJmxTestBean"/> <property name="server" ref="clientConnector"/> </bean>
Here you can see that we create an MBeanServerConnection
pointing to a remote machine
using the MBeanServerConnectionFactoryBean
. This MBeanServerConnection
is then
passed to the MBeanProxyFactoryBean
via the server
property. The proxy that is
created will forward all invocations to the MBeanServer
via this
MBeanServerConnection
.
Spring’s JMX offering includes comprehensive support for JMX notifications.
Spring’s JMX support makes it very easy to register any number of
NotificationListeners
with any number of MBeans (this includes MBeans exported by
Spring’s MBeanExporter
and MBeans registered via some other mechanism). By way of an
example, consider the scenario where one would like to be informed (via a
Notification
) each and every time an attribute of a target MBean changes.
package com.example; import javax.management.AttributeChangeNotification; import javax.management.Notification; import javax.management.NotificationFilter; import javax.management.NotificationListener; public class ConsoleLoggingNotificationListener implements NotificationListener, NotificationFilter { public void handleNotification(Notification notification, Object handback) { System.out.println(notification); System.out.println(handback); } public boolean isNotificationEnabled(Notification notification) { return AttributeChangeNotification.class.isAssignableFrom(notification.getClass()); } }
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> <property name="notificationListenerMappings"> <map> <entry key="bean:name=testBean1"> <bean class="com.example.ConsoleLoggingNotificationListener"/> </entry> </map> </property> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
With the above configuration in place, every time a JMX Notification
is broadcast from
the target MBean ( bean:name=testBean1
), the ConsoleLoggingNotificationListener
bean
that was registered as a listener via the notificationListenerMappings
property will
be notified. The ConsoleLoggingNotificationListener
bean can then take whatever action
it deems appropriate in response to the Notification
.
You can also use straight bean names as the link between exported beans and listeners:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> <property name="notificationListenerMappings"> <map> <entry key="testBean"> <bean class="com.example.ConsoleLoggingNotificationListener"/> </entry> </map> </property> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
If one wants to register a single NotificationListener
instance for all of the beans
that the enclosing MBeanExporter
is exporting, one can use the special wildcard '*'
(sans quotes) as the key for an entry in the notificationListenerMappings
property
map; for example:
<property name="notificationListenerMappings"> <map> <entry key="*"> <bean class="com.example.ConsoleLoggingNotificationListener"/> </entry> </map> </property>
If one needs to do the inverse (that is, register a number of distinct listeners against
an MBean), then one has to use the notificationListeners
list property instead (and in
preference to the notificationListenerMappings
property). This time, instead of
configuring simply a NotificationListener
for a single MBean, one configures
NotificationListenerBean
instances… a NotificationListenerBean
encapsulates a
NotificationListener
and the ObjectName
(or ObjectNames
) that it is to be
registered against in an MBeanServer
. The NotificationListenerBean
also encapsulates
a number of other properties such as a NotificationFilter
and an arbitrary handback
object that can be used in advanced JMX notification scenarios.
The configuration when using NotificationListenerBean
instances is not wildly
different to what was presented previously:
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean"/> </map> </property> <property name="notificationListeners"> <list> <bean class="org.springframework.jmx.export.NotificationListenerBean"> <constructor-arg> <bean class="com.example.ConsoleLoggingNotificationListener"/> </constructor-arg> <property name="mappedObjectNames"> <list> <value>bean:name=testBean1</value> </list> </property> </bean> </list> </property> </bean> <bean id="testBean" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> </beans>
The above example is equivalent to the first notification example. Lets assume then that
we want to be given a handback object every time a Notification
is raised, and that
additionally we want to filter out extraneous Notifications
by supplying a
NotificationFilter
. (For a full discussion of just what a handback object is, and
indeed what a NotificationFilter
is, please do consult that section of the JMX
specification (1.2) entitled The JMX Notification Model.)
<beans> <bean id="exporter" class="org.springframework.jmx.export.MBeanExporter"> <property name="beans"> <map> <entry key="bean:name=testBean1" value-ref="testBean1"/> <entry key="bean:name=testBean2" value-ref="testBean2"/> </map> </property> <property name="notificationListeners"> <list> <bean class="org.springframework.jmx.export.NotificationListenerBean"> <constructor-arg ref="customerNotificationListener"/> <property name="mappedObjectNames"> <list> <!-- handles notifications from two distinct MBeans --> <value>bean:name=testBean1</value> <value>bean:name=testBean2</value> </list> </property> <property name="handback"> <bean class="java.lang.String"> <constructor-arg value="This could be anything..."/> </bean> </property> <property name="notificationFilter" ref="customerNotificationListener"/> </bean> </list> </property> </bean> <!-- implements both the NotificationListener and NotificationFilter interfaces --> <bean id="customerNotificationListener" class="com.example.ConsoleLoggingNotificationListener"/> <bean id="testBean1" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="TEST"/> <property name="age" value="100"/> </bean> <bean id="testBean2" class="org.springframework.jmx.JmxTestBean"> <property name="name" value="ANOTHER TEST"/> <property name="age" value="200"/> </bean> </beans>
Spring provides support not just for registering to receive Notifications
, but also
for publishing Notifications
.
Note | |
---|---|
Please note that this section is really only relevant to Spring managed beans that have
been exposed as MBeans via an |
The key interface in Spring’s JMX notification publication support is the
NotificationPublisher
interface (defined in the
org.springframework.jmx.export.notification
package). Any bean that is going to be
exported as an MBean via an MBeanExporter
instance can implement the related
NotificationPublisherAware
interface to gain access to a NotificationPublisher
instance. The NotificationPublisherAware
interface simply supplies an instance of a
NotificationPublisher
to the implementing bean via a simple setter method, which the
bean can then use to publish Notifications
.
As stated in the Javadoc for the NotificationPublisher
class, managed beans that are
publishing events via the NotificationPublisher
mechanism are not responsible for
the state management of any notification listeners and the like … Spring’s JMX support
will take care of handling all the JMX infrastructure issues. All one need do as an
application developer is implement the NotificationPublisherAware
interface and start
publishing events using the supplied NotificationPublisher
instance. Note that the
NotificationPublisher
will be set after the managed bean has been registered with
an MBeanServer
.
Using a NotificationPublisher
instance is quite straightforward… one simply creates
a JMX Notification
instance (or an instance of an appropriate Notification
subclass), populates the notification with the data pertinent to the event that is to be
published, and one then invokes the sendNotification(Notification)
on the
NotificationPublisher
instance, passing in the Notification
.
Find below a simple example… in this scenario, exported instances of the JmxTestBean
are going to publish a NotificationEvent
every time the add(int, int)
operation is
invoked.
package org.springframework.jmx; import org.springframework.jmx.export.notification.NotificationPublisherAware; import org.springframework.jmx.export.notification.NotificationPublisher; import javax.management.Notification; public class JmxTestBean implements IJmxTestBean, NotificationPublisherAware { private String name; private int age; private boolean isSuperman; private NotificationPublisher publisher; // other getters and setters omitted for clarity public int add(int x, int y) { int answer = x + y; this.publisher.sendNotification(new Notification("add", this, 0)); return answer; } public void dontExposeMe() { throw new RuntimeException(); } public void setNotificationPublisher(NotificationPublisher notificationPublisher) { this.publisher = notificationPublisher; } }
The NotificationPublisher
interface and the machinery to get it all working is one of
the nicer features of Spring’s JMX support. It does however come with the price tag of
coupling your classes to both Spring and JMX; as always, the advice here is to be
pragmatic… if you need the functionality offered by the NotificationPublisher
and
you can accept the coupling to both Spring and JMX, then do so.
This section contains links to further resources about JMX.
Java EE provides a specification to standardize access to enterprise information systems (EIS): the JCA (J2EE Connector Architecture). This specification is divided into several different parts:
The aim of the Spring CCI support is to provide classes to access a CCI connector in typical Spring style, leveraging the Spring Framework’s general resource and transaction management facilities.
Note | |
---|---|
The client side of connectors doesn’t alway use CCI. Some connectors expose their own APIs, only providing JCA resource adapter to use the system contracts of a Java EE container (connection pooling, global transactions, security). Spring does not offer special support for such connector-specific APIs. |
The base resource to use JCA CCI is the ConnectionFactory
interface. The connector
used must provide an implementation of this interface.
To use your connector, you can deploy it on your application server and fetch the
ConnectionFactory
from the server’s JNDI environment (managed mode). The connector
must be packaged as a RAR file (resource adapter archive) and contain a ra.xml
file to
describe its deployment characteristics. The actual name of the resource is specified
when you deploy it. To access it within Spring, simply use Spring’s
JndiObjectFactoryBean
/ <jee:jndi-lookup>
fetch the factory by its JNDI name.
Another way to use a connector is to embed it in your application (non-managed mode),
not using an application server to deploy and configure it. Spring offers the
possibility to configure a connector as a bean, through a provided FactoryBean
(
LocalConnectionFactoryBean
). In this manner, you only need the connector library in
the classpath (no RAR file and no ra.xml
descriptor needed). The library must be
extracted from the connector’s RAR file, if necessary.
Once you have got access to your ConnectionFactory
instance, you can inject it into
your components. These components can either be coded against the plain CCI API or
leverage Spring’s support classes for CCI access (e.g. CciTemplate
).
Note | |
---|---|
When you use a connector in non-managed mode, you can’t use global transactions because the resource is never enlisted / delisted in the current global transaction of the current thread. The resource is simply not aware of any global Java EE transactions that might be running. |
In order to make connections to the EIS, you need to obtain a ConnectionFactory
from
the application server if you are in a managed mode, or directly from Spring if you are
in a non-managed mode.
In a managed mode, you access a ConnectionFactory
from JNDI; its properties will be
configured in the application server.
<jee:jndi-lookup id="eciConnectionFactory" jndi-name="eis/cicseci"/>
In non-managed mode, you must configure the ConnectionFactory
you want to use in the
configuration of Spring as a JavaBean. The LocalConnectionFactoryBean
class offers
this setup style, passing in the ManagedConnectionFactory
implementation of your
connector, exposing the application-level CCI ConnectionFactory
.
<bean id="eciManagedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory"> <property name="serverName" value="TXSERIES"/> <property name="connectionURL" value="tcp://localhost/"/> <property name="portNumber" value="2006"/> </bean> <bean id="eciConnectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="eciManagedConnectionFactory"/> </bean>
Note | |
---|---|
You can’t directly instantiate a specific |
JCA CCI allow the developer to configure the connections to the EIS using the
ConnectionSpec
implementation of your connector. In order to configure its properties,
you need to wrap the target connection factory with a dedicated adapter,
ConnectionSpecConnectionFactoryAdapter
. So, the dedicated ConnectionSpec
can be
configured with the property connectionSpec
(as an inner bean).
This property is not mandatory because the CCI ConnectionFactory
interface defines two
different methods to obtain a CCI connection. Some of the ConnectionSpec
properties
can often be configured in the application server (in managed mode) or on the
corresponding local ManagedConnectionFactory
implementation.
public interface ConnectionFactory implements Serializable, Referenceable { ... Connection getConnection() throws ResourceException; Connection getConnection(ConnectionSpec connectionSpec) throws ResourceException; ... }
Spring provides a ConnectionSpecConnectionFactoryAdapter
that allows for specifying a
ConnectionSpec
instance to use for all operations on a given factory. If the adapter’s
connectionSpec
property is specified, the adapter uses the getConnection
variant
with the ConnectionSpec
argument, otherwise the variant without argument.
<bean id="managedConnectionFactory" class="com.sun.connector.cciblackbox.CciLocalTxManagedConnectionFactory"> <property name="connectionURL" value="jdbc:hsqldb:hsql://localhost:9001"/> <property name="driverName" value="org.hsqldb.jdbcDriver"/> </bean> <bean id="targetConnectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="managedConnectionFactory"/> </bean> <bean id="connectionFactory" class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter"> <property name="targetConnectionFactory" ref="targetConnectionFactory"/> <property name="connectionSpec"> <bean class="com.sun.connector.cciblackbox.CciConnectionSpec"> <property name="user" value="sa"/> <property name="password" value=""/> </bean> </property> </bean>
If you want to use a single CCI connection, Spring provides a further
ConnectionFactory
adapter to manage this. The SingleConnectionFactory
adapter class
will open a single connection lazily and close it when this bean is destroyed at
application shutdown. This class will expose special Connection
proxies that behave
accordingly, all sharing the same underlying physical connection.
<bean id="eciManagedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory"> <property name="serverName" value="TEST"/> <property name="connectionURL" value="tcp://localhost/"/> <property name="portNumber" value="2006"/> </bean> <bean id="targetEciConnectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="eciManagedConnectionFactory"/> </bean> <bean id="eciConnectionFactory" class="org.springframework.jca.cci.connection.SingleConnectionFactory"> <property name="targetConnectionFactory" ref="targetEciConnectionFactory"/> </bean>
Note | |
---|---|
This |
One of the aims of the JCA CCI support is to provide convenient facilities for
manipulating CCI records. The developer can specify the strategy to create records and
extract datas from records, for use with Spring’s CciTemplate
. The following
interfaces will configure the strategy to use input and output records if you don’t want
to work with records directly in your application.
In order to create an input Record
, the developer can use a dedicated implementation
of the RecordCreator
interface.
public interface RecordCreator { Record createRecord(RecordFactory recordFactory) throws ResourceException, DataAccessException; }
As you can see, the createRecord(..)
method receives a RecordFactory
instance as
parameter, which corresponds to the RecordFactory
of the ConnectionFactory
used.
This reference can be used to create IndexedRecord
or MappedRecord
instances. The
following sample shows how to use the RecordCreator
interface and indexed/mapped
records.
public class MyRecordCreator implements RecordCreator { public Record createRecord(RecordFactory recordFactory) throws ResourceException { IndexedRecord input = recordFactory.createIndexedRecord("input"); input.add(new Integer(id)); return input; } }
An output Record
can be used to receive data back from the EIS. Hence, a specific
implementation of the RecordExtractor
interface can be passed to Spring’s
CciTemplate
for extracting data from the output Record
.
public interface RecordExtractor { Object extractData(Record record) throws ResourceException, SQLException, DataAccessException; }
The following sample shows how to use the RecordExtractor
interface.
public class MyRecordExtractor implements RecordExtractor { public Object extractData(Record record) throws ResourceException { CommAreaRecord commAreaRecord = (CommAreaRecord) record; String str = new String(commAreaRecord.toByteArray()); String field1 = string.substring(0,6); String field2 = string.substring(6,1); return new OutputObject(Long.parseLong(field1), field2); } }
The CciTemplate
is the central class of the core CCI support package (
org.springframework.jca.cci.core
). It simplifies the use of CCI since it handles the
creation and release of resources. This helps to avoid common errors like forgetting to
always close the connection. It cares for the lifecycle of connection and interaction
objects, letting application code focus on generating input records from application
data and extracting application data from output records.
The JCA CCI specification defines two distinct methods to call operations on an EIS. The
CCI Interaction
interface provides two execute method signatures:
public interface javax.resource.cci.Interaction { ... boolean execute(InteractionSpec spec, Record input, Record output) throws ResourceException; Record execute(InteractionSpec spec, Record input) throws ResourceException; ... }
Depending on the template method called, CciTemplate
will know which execute
method
to call on the interaction. In any case, a correctly initialized InteractionSpec
instance is mandatory.
CciTemplate.execute(..)
can be used in two ways:
Record
arguments. In this case, you simply need to pass the CCI input
record in, and the returned object be the corresponding CCI output record.
RecordCreator
and RecordExtractor
instances.
With the first approach, the following methods of the template will be used. These
methods directly correspond to those on the Interaction
interface.
public class CciTemplate implements CciOperations { public Record execute(InteractionSpec spec, Record inputRecord) throws DataAccessException { ... } public void execute(InteractionSpec spec, Record inputRecord, Record outputRecord) throws DataAccessException { ... } }
With the second approach, we need to specify the record creation and record extraction
strategies as arguments. The interfaces used are those describe in the previous section
on record conversion. The corresponding CciTemplate
methods are the following:
public class CciTemplate implements CciOperations { public Record execute(InteractionSpec spec, RecordCreator inputCreator) throws DataAccessException { // ... } public Object execute(InteractionSpec spec, Record inputRecord, RecordExtractor outputExtractor) throws DataAccessException { // ... } public Object execute(InteractionSpec spec, RecordCreator creator, RecordExtractor extractor) throws DataAccessException { // ... } }
Unless the outputRecordCreator
property is set on the template (see the following
section), every method will call the corresponding execute
method of the CCI
Interaction
with two parameters: InteractionSpec
and input Record
, receiving an
output Record
as return value.
CciTemplate
also provides methods to create IndexRecord
and MappedRecord
outside a
RecordCreator
implementation, through its createIndexRecord(..)
and
createMappedRecord(..)
methods. This can be used within DAO implementations to create
Record
instances to pass into corresponding CciTemplate.execute(..)
methods.
public class CciTemplate implements CciOperations { public IndexedRecord createIndexedRecord(String name) throws DataAccessException { ... } public MappedRecord createMappedRecord(String name) throws DataAccessException { ... } }
Spring’s CCI support provides a abstract class for DAOs, supporting injection of a
ConnectionFactory
or a CciTemplate
instances. The name of the class is
CciDaoSupport
: It provides simple setConnectionFactory
and setCciTemplate
methods.
Internally, this class will create a CciTemplate
instance for a passed-in
ConnectionFactory
, exposing it to concrete data access implementations in subclasses.
public abstract class CciDaoSupport { public void setConnectionFactory(ConnectionFactory connectionFactory) { // ... } public ConnectionFactory getConnectionFactory() { // ... } public void setCciTemplate(CciTemplate cciTemplate) { // ... } public CciTemplate getCciTemplate() { // ... } }
If the connector used only supports the Interaction.execute(..)
method with input and
output records as parameters (that is, it requires the desired output record to be
passed in instead of returning an appropriate output record), you can set the
outputRecordCreator
property of the CciTemplate
to automatically generate an output
record to be filled by the JCA connector when the response is received. This record will
be then returned to the caller of the template.
This property simply holds an implementation of the RecordCreator
interface, used for
that purpose. The RecordCreator
interface has already been discussed in
Section 25.3.1, “Record conversion”. The outputRecordCreator
property must be directly specified on
the CciTemplate
. This could be done in the application code like so:
cciTemplate.setOutputRecordCreator(new EciOutputRecordCreator());
Or (recommended) in the Spring configuration, if the CciTemplate
is configured as a
dedicated bean instance:
<bean id="eciOutputRecordCreator" class="eci.EciOutputRecordCreator"/> <bean id="cciTemplate" class="org.springframework.jca.cci.core.CciTemplate"> <property name="connectionFactory" ref="eciConnectionFactory"/> <property name="outputRecordCreator" ref="eciOutputRecordCreator"/> </bean>
Note | |
---|---|
As the |
The following table summarizes the mechanisms of the CciTemplate
class and the
corresponding methods called on the CCI Interaction
interface:
Table 25.1. Usage of Interaction execute methods
CciTemplate method signature | CciTemplate outputRecordCreator property | execute method called on the CCI Interaction |
---|---|---|
Record execute(InteractionSpec, Record) | not set | Record execute(InteractionSpec, Record) |
Record execute(InteractionSpec, Record) | set | boolean execute(InteractionSpec, Record, Record) |
void execute(InteractionSpec, Record, Record) | not set | void execute(InteractionSpec, Record, Record) |
void execute(InteractionSpec, Record, Record) | set | void execute(InteractionSpec, Record, Record) |
Record execute(InteractionSpec, RecordCreator) | not set | Record execute(InteractionSpec, Record) |
Record execute(InteractionSpec, RecordCreator) | set | void execute(InteractionSpec, Record, Record) |
Record execute(InteractionSpec, Record, RecordExtractor) | not set | Record execute(InteractionSpec, Record) |
Record execute(InteractionSpec, Record, RecordExtractor) | set | void execute(InteractionSpec, Record, Record) |
Record execute(InteractionSpec, RecordCreator, RecordExtractor) | not set | Record execute(InteractionSpec, Record) |
Record execute(InteractionSpec, RecordCreator, RecordExtractor) | set | void execute(InteractionSpec, Record, Record) |
CciTemplate
also offers the possibility to work directly with CCI connections and
interactions, in the same manner as JdbcTemplate
and JmsTemplate
. This is useful
when you want to perform multiple operations on a CCI connection or interaction, for
example.
The interface ConnectionCallback
provides a CCI Connection
as argument, in order to
perform custom operations on it, plus the CCI ConnectionFactory
which the Connection
was created with. The latter can be useful for example to get an associated
RecordFactory
instance and create indexed/mapped records, for example.
public interface ConnectionCallback { Object doInConnection(Connection connection, ConnectionFactory connectionFactory) throws ResourceException, SQLException, DataAccessException; }
The interface InteractionCallback
provides the CCI Interaction
, in order to perform
custom operations on it, plus the corresponding CCI ConnectionFactory
.
public interface InteractionCallback { Object doInInteraction(Interaction interaction, ConnectionFactory connectionFactory) throws ResourceException, SQLException, DataAccessException; }
Note | |
---|---|
|
In this section, the usage of the CciTemplate
will be shown to acces to a CICS with
ECI mode, with the IBM CICS ECI connector.
Firstly, some initializations on the CCI InteractionSpec
must be done to specify which
CICS program to access and how to interact with it.
ECIInteractionSpec interactionSpec = new ECIInteractionSpec(); interactionSpec.setFunctionName("MYPROG"); interactionSpec.setInteractionVerb(ECIInteractionSpec.SYNC_SEND_RECEIVE);
Then the program can use CCI via Spring’s template and specify mappings between custom
objects and CCI Records
.
public class MyDaoImpl extends CciDaoSupport implements MyDao { public OutputObject getData(InputObject input) { ECIInteractionSpec interactionSpec = ...; OutputObject output = (ObjectOutput) getCciTemplate().execute(interactionSpec, new RecordCreator() { public Record createRecord(RecordFactory recordFactory) throws ResourceException { return new CommAreaRecord(input.toString().getBytes()); } }, new RecordExtractor() { public Object extractData(Record record) throws ResourceException { CommAreaRecord commAreaRecord = (CommAreaRecord)record; String str = new String(commAreaRecord.toByteArray()); String field1 = string.substring(0,6); String field2 = string.substring(6,1); return new OutputObject(Long.parseLong(field1), field2); } }); return output; } }
As discussed previously, callbacks can be used to work directly on CCI connections or interactions.
public class MyDaoImpl extends CciDaoSupport implements MyDao { public OutputObject getData(InputObject input) { ObjectOutput output = (ObjectOutput) getCciTemplate().execute( new ConnectionCallback() { public Object doInConnection(Connection connection, ConnectionFactory factory) throws ResourceException { // do something... } }); } return output; } }
Note | |
---|---|
With a |
For a more specific callback, you can implement an InteractionCallback
. The passed-in
Interaction
will be managed and closed by the CciTemplate
in this case.
public class MyDaoImpl extends CciDaoSupport implements MyDao { public String getData(String input) { ECIInteractionSpec interactionSpec = ...; String output = (String) getCciTemplate().execute(interactionSpec, new InteractionCallback() { public Object doInInteraction(Interaction interaction, ConnectionFactory factory) throws ResourceException { Record input = new CommAreaRecord(inputString.getBytes()); Record output = new CommAreaRecord(); interaction.execute(holder.getInteractionSpec(), input, output); return new String(output.toByteArray()); } }); return output; } }
For the examples above, the corresponding configuration of the involved Spring beans could look like this in non-managed mode:
<bean id="managedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory"> <property name="serverName" value="TXSERIES"/> <property name="connectionURL" value="local:"/> <property name="userName" value="CICSUSER"/> <property name="password" value="CICS"/> </bean> <bean id="connectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="managedConnectionFactory"/> </bean> <bean id="component" class="mypackage.MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
In managed mode (that is, in a Java EE environment), the configuration could look as follows:
<jee:jndi-lookup id="connectionFactory" jndi-name="eis/cicseci"/> <bean id="component" class="MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
The org.springframework.jca.cci.object
package contains support classes that allow you
to access the EIS in a different style: through reusable operation objects, analogous to
Spring’s JDBC operation objects (see JDBC chapter). This will usually encapsulate the
CCI API: an application-level input object will be passed to the operation object, so it
can construct the input record and then convert the received record data to an
application-level output object and return it.
Note | |
---|---|
This approach is internally based on the |
MappingRecordOperation
essentially performs the same work as CciTemplate
, but
represents a specific, pre-configured operation as an object. It provides two template
methods to specify how to convert an input object to a input record, and how to convert
an output record to an output object (record mapping):
createInputRecord(..)
to specify how to convert an input object to an input Record
extractOutputData(..)
to specify how to extract an output object from an output
Record
Here are the signatures of these methods:
public abstract class MappingRecordOperation extends EisOperation { ... protected abstract Record createInputRecord(RecordFactory recordFactory, Object inputObject) throws ResourceException, DataAccessException { // ... } protected abstract Object extractOutputData(Record outputRecord) throws ResourceException, SQLException, DataAccessException { // ... } ... }
Thereafter, in order to execute an EIS operation, you need to use a single execute method, passing in an application-level input object and receiving an application-level output object as result:
public abstract class MappingRecordOperation extends EisOperation { ... public Object execute(Object inputObject) throws DataAccessException { } ... }
As you can see, contrary to the CciTemplate
class, this execute(..)
method does not
have an InteractionSpec
as argument. Instead, the InteractionSpec
is global to the
operation. The following constructor must be used to instantiate an operation object
with a specific InteractionSpec
:
InteractionSpec spec = ...;
MyMappingRecordOperation eisOperation = new MyMappingRecordOperation(getConnectionFactory(), spec);
...
Some connectors use records based on a COMMAREA which represents an array of bytes
containing parameters to send to the EIS and data returned by it. Spring provides a
special operation class for working directly on COMMAREA rather than on records. The
MappingCommAreaOperation
class extends the MappingRecordOperation
class to provide
such special COMMAREA support. It implicitly uses the CommAreaRecord
class as input
and output record type, and provides two new methods to convert an input object into an
input COMMAREA and the output COMMAREA into an output object.
public abstract class MappingCommAreaOperation extends MappingRecordOperation { ... protected abstract byte[] objectToBytes(Object inObject) throws IOException, DataAccessException; protected abstract Object bytesToObject(byte[] bytes) throws IOException, DataAccessException; ... }
As every MappingRecordOperation
subclass is based on CciTemplate internally, the same
way to automatically generate output records as with CciTemplate
is available. Every
operation object provides a corresponding setOutputRecordCreator(..)
method. For
further information, see Section 25.3.4, “Automatic output record generation”.
The operation object approach uses records in the same manner as the CciTemplate
class.
Table 25.2. Usage of Interaction execute methods
MappingRecordOperation method signature | MappingRecordOperation outputRecordCreator property | execute method called on the CCI Interaction |
---|---|---|
Object execute(Object) | not set | Record execute(InteractionSpec, Record) |
Object execute(Object) | set | boolean execute(InteractionSpec, Record, Record) |
In this section, the usage of the MappingRecordOperation
will be shown to access a
database with the Blackbox CCI connector.
Note | |
---|---|
The original version of this connector is provided by the Java EE SDK (version 1.3), available from Oracle. |
Firstly, some initializations on the CCI InteractionSpec
must be done to specify which
SQL request to execute. In this sample, we directly define the way to convert the
parameters of the request to a CCI record and the way to convert the CCI result record
to an instance of the Person
class.
public class PersonMappingOperation extends MappingRecordOperation { public PersonMappingOperation(ConnectionFactory connectionFactory) { setConnectionFactory(connectionFactory); CciInteractionSpec interactionSpec = new CciConnectionSpec(); interactionSpec.setSql("select * from person where person_id=?"); setInteractionSpec(interactionSpec); } protected Record createInputRecord(RecordFactory recordFactory, Object inputObject) throws ResourceException { Integer id = (Integer) inputObject; IndexedRecord input = recordFactory.createIndexedRecord("input"); input.add(new Integer(id)); return input; } protected Object extractOutputData(Record outputRecord) throws ResourceException, SQLException { ResultSet rs = (ResultSet) outputRecord; Person person = null; if (rs.next()) { Person person = new Person(); person.setId(rs.getInt("person_id")); person.setLastName(rs.getString("person_last_name")); person.setFirstName(rs.getString("person_first_name")); } return person; } }
Then the application can execute the operation object, with the person identifier as argument. Note that operation object could be set up as shared instance, as it is thread-safe.
public class MyDaoImpl extends CciDaoSupport implements MyDao { public Person getPerson(int id) { PersonMappingOperation query = new PersonMappingOperation(getConnectionFactory()); Person person = (Person) query.execute(new Integer(id)); return person; } }
The corresponding configuration of Spring beans could look as follows in non-managed mode:
<bean id="managedConnectionFactory" class="com.sun.connector.cciblackbox.CciLocalTxManagedConnectionFactory"> <property name="connectionURL" value="jdbc:hsqldb:hsql://localhost:9001"/> <property name="driverName" value="org.hsqldb.jdbcDriver"/> </bean> <bean id="targetConnectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="managedConnectionFactory"/> </bean> <bean id="connectionFactory" class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter"> <property name="targetConnectionFactory" ref="targetConnectionFactory"/> <property name="connectionSpec"> <bean class="com.sun.connector.cciblackbox.CciConnectionSpec"> <property name="user" value="sa"/> <property name="password" value=""/> </bean> </property> </bean> <bean id="component" class="MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
In managed mode (that is, in a Java EE environment), the configuration could look as follows:
<jee:jndi-lookup id="targetConnectionFactory" jndi-name="eis/blackbox"/> <bean id="connectionFactory" class="org.springframework.jca.cci.connection.ConnectionSpecConnectionFactoryAdapter"> <property name="targetConnectionFactory" ref="targetConnectionFactory"/> <property name="connectionSpec"> <bean class="com.sun.connector.cciblackbox.CciConnectionSpec"> <property name="user" value="sa"/> <property name="password" value=""/> </bean> </property> </bean> <bean id="component" class="MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
In this section, the usage of the MappingCommAreaOperation
will be shown: accessing a
CICS with ECI mode with the IBM CICS ECI connector.
Firstly, the CCI InteractionSpec
needs to be initialized to specify which CICS program
to access and how to interact with it.
public abstract class EciMappingOperation extends MappingCommAreaOperation { public EciMappingOperation(ConnectionFactory connectionFactory, String programName) { setConnectionFactory(connectionFactory); ECIInteractionSpec interactionSpec = new ECIInteractionSpec(), interactionSpec.setFunctionName(programName); interactionSpec.setInteractionVerb(ECIInteractionSpec.SYNC_SEND_RECEIVE); interactionSpec.setCommareaLength(30); setInteractionSpec(interactionSpec); setOutputRecordCreator(new EciOutputRecordCreator()); } private static class EciOutputRecordCreator implements RecordCreator { public Record createRecord(RecordFactory recordFactory) throws ResourceException { return new CommAreaRecord(); } } }
The abstract EciMappingOperation
class can then be subclassed to specify mappings
between custom objects and Records
.
public class MyDaoImpl extends CciDaoSupport implements MyDao { public OutputObject getData(Integer id) { EciMappingOperation query = new EciMappingOperation(getConnectionFactory(), "MYPROG") { protected abstract byte[] objectToBytes(Object inObject) throws IOException { Integer id = (Integer) inObject; return String.valueOf(id); } protected abstract Object bytesToObject(byte[] bytes) throws IOException; String str = new String(bytes); String field1 = str.substring(0,6); String field2 = str.substring(6,1); String field3 = str.substring(7,1); return new OutputObject(field1, field2, field3); } }); return (OutputObject) query.execute(new Integer(id)); } }
The corresponding configuration of Spring beans could look as follows in non-managed mode:
<bean id="managedConnectionFactory" class="com.ibm.connector2.cics.ECIManagedConnectionFactory"> <property name="serverName" value="TXSERIES"/> <property name="connectionURL" value="local:"/> <property name="userName" value="CICSUSER"/> <property name="password" value="CICS"/> </bean> <bean id="connectionFactory" class="org.springframework.jca.support.LocalConnectionFactoryBean"> <property name="managedConnectionFactory" ref="managedConnectionFactory"/> </bean> <bean id="component" class="MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
In managed mode (that is, in a Java EE environment), the configuration could look as follows:
<jee:jndi-lookup id="connectionFactory" jndi-name="eis/cicseci"/> <bean id="component" class="MyDaoImpl"> <property name="connectionFactory" ref="connectionFactory"/> </bean>
JCA specifies several levels of transaction support for resource adapters. The kind of
transactions that your resource adapter supports is specified in its ra.xml
file.
There are essentially three options: none (for example with CICS EPI connector), local
transactions (for example with a CICS ECI connector), global transactions (for example
with an IMS connector).
<connector> <resourceadapter> <!-- <transaction-support>NoTransaction</transaction-support> --> <!-- <transaction-support>LocalTransaction</transaction-support> --> <transaction-support>XATransaction</transaction-support> <resourceadapter> <connector>
For global transactions, you can use Spring’s generic transaction infrastructure to
demarcate transactions, with JtaTransactionManager
as backend (delegating to the Java
EE server’s distributed transaction coordinator underneath).
For local transactions on a single CCI ConnectionFactory
, Spring provides a specific
transaction management strategy for CCI, analogous to the DataSourceTransactionManager
for JDBC. The CCI API defines a local transaction object and corresponding local
transaction demarcation methods. Spring’s CciLocalTransactionManager
executes such
local CCI transactions, fully compliant with Spring’s generic
PlatformTransactionManager
abstraction.
<jee:jndi-lookup id="eciConnectionFactory" jndi-name="eis/cicseci"/> <bean id="eciTransactionManager" class="org.springframework.jca.cci.connection.CciLocalTransactionManager"> <property name="connectionFactory" ref="eciConnectionFactory"/> </bean>
Both transaction strategies can be used with any of Spring’s transaction demarcation
facilities, be it declarative or programmatic. This is a consequence of Spring’s generic
PlatformTransactionManager
abstraction, which decouples transaction demarcation from
the actual execution strategy. Simply switch between JtaTransactionManager
and
CciLocalTransactionManager
as needed, keeping your transaction demarcation as-is.
For more information on Spring’s transaction facilities, see the chapter entitled Chapter 11, Transaction Management.
The Spring Framework provides a helpful utility library for sending email that shields the user from the specifics of the underlying mailing system and is responsible for low level resource handling on behalf of the client.
The org.springframework.mail
package is the root level package for the Spring
Framework’s email support. The central interface for sending emails is the MailSender
interface; a simple value object encapsulating the properties of a simple mail such as
from and to (plus many others) is the SimpleMailMessage
class. This package
also contains a hierarchy of checked exceptions which provide a higher level of
abstraction over the lower level mail system exceptions with the root exception being
MailException
. Please refer to the JavaDocs for more information on the rich mail
exception hierarchy.
The org.springframework.mail.javamail.JavaMailSender
interface adds specialized
JavaMail features such as MIME message support to the MailSender
interface (from
which it inherits). JavaMailSender
also provides a callback interface for preparation
of JavaMail MIME messages, called
org.springframework.mail.javamail.MimeMessagePreparator
Let’s assume there is a business interface called OrderManager
:
public interface OrderManager { void placeOrder(Order order); }
Let us also assume that there is a requirement stating that an email message with an order number needs to be generated and sent to a customer placing the relevant order.
import org.springframework.mail.MailException; import org.springframework.mail.MailSender; import org.springframework.mail.SimpleMailMessage; public class SimpleOrderManager implements OrderManager { private MailSender mailSender; private SimpleMailMessage templateMessage; public void setMailSender(MailSender mailSender) { this.mailSender = mailSender; } public void setTemplateMessage(SimpleMailMessage templateMessage) { this.templateMessage = templateMessage; } public void placeOrder(Order order) { // Do the business calculations... // Call the collaborators to persist the order... // Create a thread safe "copy" of the template message and customize it SimpleMailMessage msg = new SimpleMailMessage(this.templateMessage); msg.setTo(order.getCustomer().getEmailAddress()); msg.setText( "Dear " + order.getCustomer().getFirstName() + order.getCustomer().getLastName() + ", thank you for placing order. Your order number is " + order.getOrderNumber()); try{ this.mailSender.send(msg); } catch (MailException ex) { // simply log it and go on... System.err.println(ex.getMessage()); } } }
Find below the bean definitions for the above code:
<bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl"> <property name="host" value="mail.mycompany.com"/> </bean> <!-- this is a template message that we can pre-load with default state --> <bean id="templateMessage" class="org.springframework.mail.SimpleMailMessage"> <property name="from" value="[email protected]"/> <property name="subject" value="Your order"/> </bean> <bean id="orderManager" class="com.mycompany.businessapp.support.SimpleOrderManager"> <property name="mailSender" ref="mailSender"/> <property name="templateMessage" ref="templateMessage"/> </bean>
Here is another implementation of OrderManager
using the MimeMessagePreparator
callback interface. Please note in this case that the mailSender
property is of type
JavaMailSender
so that we are able to use the JavaMail MimeMessage
class:
import javax.mail.Message; import javax.mail.MessagingException; import javax.mail.internet.InternetAddress; import javax.mail.internet.MimeMessage; import javax.mail.internet.MimeMessage; import org.springframework.mail.MailException; import org.springframework.mail.javamail.JavaMailSender; import org.springframework.mail.javamail.MimeMessagePreparator; public class SimpleOrderManager implements OrderManager { private JavaMailSender mailSender; public void setMailSender(JavaMailSender mailSender) { this.mailSender = mailSender; } public void placeOrder(final Order order) { // Do the business calculations... // Call the collaborators to persist the order... MimeMessagePreparator preparator = new MimeMessagePreparator() { public void prepare(MimeMessage mimeMessage) throws Exception { mimeMessage.setRecipient(Message.RecipientType.TO, new InternetAddress(order.getCustomer().getEmailAddress())); mimeMessage.setFrom(new InternetAddress("[email protected]")); mimeMessage.setText( "Dear " + order.getCustomer().getFirstName() + " " + order.getCustomer().getLastName() + ", thank you for placing order. Your order number is " + order.getOrderNumber()); } }; try { this.mailSender.send(preparator); } catch (MailException ex) { // simply log it and go on... System.err.println(ex.getMessage()); } } }
Note | |
---|---|
The mail code is a crosscutting concern and could well be a candidate for refactoring
into a custom Spring AOP aspect, which then could be executed at appropriate
joinpoints on the |
The Spring Framework’s mail support ships with the standard JavaMail implementation. Please refer to the relevant JavaDocs for more information.
A class that comes in pretty handy when dealing with JavaMail messages is the
org.springframework.mail.javamail.MimeMessageHelper
class, which shields you from
having to use the verbose JavaMail API. Using the MimeMessageHelper
it is pretty easy
to create a MimeMessage
:
// of course you would use DI in any real-world cases JavaMailSenderImpl sender = new JavaMailSenderImpl(); sender.setHost("mail.host.com"); MimeMessage message = sender.createMimeMessage(); MimeMessageHelper helper = new MimeMessageHelper(message); helper.setTo("[email protected]"); helper.setText("Thank you for ordering!"); sender.send(message);
Multipart email messages allow for both attachments and inline resources. Examples of inline resources would be images or a stylesheet you want to use in your message, but that you don’t want displayed as an attachment.
The following example shows you how to use the MimeMessageHelper
to send an email
along with a single JPEG image attachment.
JavaMailSenderImpl sender = new JavaMailSenderImpl(); sender.setHost("mail.host.com"); MimeMessage message = sender.createMimeMessage(); // use the true flag to indicate you need a multipart message MimeMessageHelper helper = new MimeMessageHelper(message, true); helper.setTo("[email protected]"); helper.setText("Check out this image!"); // let's attach the infamous windows Sample file (this time copied to c:/) FileSystemResource file = new FileSystemResource(new File("c:/Sample.jpg")); helper.addAttachment("CoolImage.jpg", file); sender.send(message);
The following example shows you how to use the MimeMessageHelper
to send an email
along with an inline image.
JavaMailSenderImpl sender = new JavaMailSenderImpl(); sender.setHost("mail.host.com"); MimeMessage message = sender.createMimeMessage(); // use the true flag to indicate you need a multipart message MimeMessageHelper helper = new MimeMessageHelper(message, true); helper.setTo("[email protected]"); // use the true flag to indicate the text included is HTML helper.setText("<html><body><img src=cid:identifier1234></body></html>", true); // let's include the infamous windows Sample file (this time copied to c:/) FileSystemResource res = new FileSystemResource(new File("c:/Sample.jpg")); helper.addInline("identifier1234", res); sender.send(message);
Warning | |
---|---|
Inline resources are added to the mime message using the specified |
The code in the previous examples explicitly created the content of the email message,
using methods calls such as message.setText(..)
. This is fine for simple cases, and it
is okay in the context of the aforementioned examples, where the intent was to show you
the very basics of the API.
In your typical enterprise application though, you are not going to create the content of your emails using the above approach for a number of reasons.
Typically the approach taken to address these issues is to use a template library such as FreeMarker or Velocity to define the display structure of email content. This leaves your code tasked only with creating the data that is to be rendered in the email template and sending the email. It is definitely a best practice for when the content of your emails becomes even moderately complex, and with the Spring Framework’s support classes for FreeMarker and Velocity becomes quite easy to do. Find below an example of using the Velocity template library to create email content.
To use Velocity to create your email template(s), you will need to have the Velocity libraries available on your classpath. You will also need to create one or more Velocity templates for the email content that your application needs. Find below the Velocity template that this example will be using. As you can see it is HTML-based, and since it is plain text it can be created using your favorite HTML or text editor.
# in the com/foo/package <html> <body> <h3>Hi ${user.userName}, welcome to the Chipping Sodbury On-the-Hill message boards!</h3> <div> Your email address is <a href="mailto:${user.emailAddress}">${user.emailAddress}</a>. </div> </body> </html>
Find below some simple code and Spring XML configuration that makes use of the above Velocity template to create email content and send email(s).
package com.foo; import org.apache.velocity.app.VelocityEngine; import org.springframework.mail.javamail.JavaMailSender; import org.springframework.mail.javamail.MimeMessageHelper; import org.springframework.mail.javamail.MimeMessagePreparator; import org.springframework.ui.velocity.VelocityEngineUtils; import javax.mail.internet.MimeMessage; import java.util.HashMap; import java.util.Map; public class SimpleRegistrationService implements RegistrationService { private JavaMailSender mailSender; private VelocityEngine velocityEngine; public void setMailSender(JavaMailSender mailSender) { this.mailSender = mailSender; } public void setVelocityEngine(VelocityEngine velocityEngine) { this.velocityEngine = velocityEngine; } public void register(User user) { // Do the registration logic... sendConfirmationEmail(user); } private void sendConfirmationEmail(final User user) { MimeMessagePreparator preparator = new MimeMessagePreparator() { public void prepare(MimeMessage mimeMessage) throws Exception { MimeMessageHelper message = new MimeMessageHelper(mimeMessage); message.setTo(user.getEmailAddress()); message.setFrom("[email protected]"); // could be parameterized... Map model = new HashMap(); model.put("user", user); String text = VelocityEngineUtils.mergeTemplateIntoString( velocityEngine, "com/dns/registration-confirmation.vm", model); message.setText(text, true); } }; this.mailSender.send(preparator); } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="mailSender" class="org.springframework.mail.javamail.JavaMailSenderImpl"> <property name="host" value="mail.csonth.gov.uk"/> </bean> <bean id="registrationService" class="com.foo.SimpleRegistrationService"> <property name="mailSender" ref="mailSender"/> <property name="velocityEngine" ref="velocityEngine"/> </bean> <bean id="velocityEngine" class="org.springframework.ui.velocity.VelocityEngineFactoryBean"> <property name="velocityProperties"> <value> resource.loader=class class.resource.loader.class=org.apache.velocity.runtime.resource.loader.ClasspathResourceLoader </value> </property> </bean> </beans>
The Spring Framework provides abstractions for asynchronous execution and scheduling of
tasks with the TaskExecutor
and TaskScheduler
interfaces, respectively. Spring also
features implementations of those interfaces that support thread pools or delegation to
CommonJ within an application server environment. Ultimately the use of these
implementations behind the common interfaces abstracts away the differences between Java
SE 5, Java SE 6 and Java EE environments.
Spring also features integration classes for supporting scheduling with the Timer
,
part of the JDK since 1.3, and the Quartz Scheduler ( http://quartz-scheduler.org).
Both of those schedulers are set up using a FactoryBean
with optional references to
Timer
or Trigger
instances, respectively. Furthermore, a convenience class for both
the Quartz Scheduler and the Timer
is available that allows you to invoke a method of
an existing target object (analogous to the normal MethodInvokingFactoryBean
operation).
Spring 2.0 introduces a new abstraction for dealing with executors. Executors are the Java 5 name for the concept of thread pools. The "executor" naming is due to the fact that there is no guarantee that the underlying implementation is actually a pool; an executor may be single-threaded or even synchronous. Spring’s abstraction hides implementation details between Java SE 1.4, Java SE 5 and Java EE environments.
Spring’s TaskExecutor
interface is identical to the java.util.concurrent.Executor
interface. In fact, its primary reason for existence is to abstract away the need for
Java 5 when using thread pools. The interface has a single method execute(Runnable
task)
that accepts a task for execution based on the semantics and configuration of the
thread pool.
The TaskExecutor
was originally created to give other Spring components an abstraction
for thread pooling where needed. Components such as the ApplicationEventMulticaster
,
JMS’s AbstractMessageListenerContainer
, and Quartz integration all use the
TaskExecutor
abstraction to pool threads. However, if your beans need thread pooling
behavior, it is possible to use this abstraction for your own needs.
There are a number of pre-built implementations of TaskExecutor
included with the
Spring distribution. In all likelihood, you shouldn’t ever need to implement your own.
SimpleAsyncTaskExecutor
This implementation does not reuse any threads, rather it
starts up a new thread for each invocation. However, it does support a concurrency
limit which will block any invocations that are over the limit until a slot has been
freed up. If you’re looking for true pooling, keep scrolling further down the page.
SyncTaskExecutor
This implementation doesn’t execute invocations asynchronously.
Instead, each invocation takes place in the calling thread. It is primarily used in
situations where multithreading isn’t necessary such as simple test cases.
ConcurrentTaskExecutor
This implementation is a wrapper for a Java 5
java.util.concurrent.Executor
. There is an alternative, ThreadPoolTaskExecutor
,
that exposes the Executor
configuration parameters as bean properties. It is rare to
need to use the ConcurrentTaskExecutor
but if the ThreadPoolTaskExecutor
isn’t robust enough for your needs, the
ConcurrentTaskExecutor
is an alternative.
SimpleThreadPoolTaskExecutor
This implementation is actually a subclass of Quartz’s
SimpleThreadPool
which listens to Spring’s lifecycle callbacks. This is typically
used when you have a thread pool that may need to be shared by both Quartz and
non-Quartz components.
ThreadPoolTaskExecutor
This implementation can only be used in a Java 5 environment but is also the most
commonly used one in that environment. It exposes bean properties for configuring a
java.util.concurrent.ThreadPoolExecutor
and wraps it in a TaskExecutor
. If you need
something advanced such as a ScheduledThreadPoolExecutor
, it is recommended that you
use a ConcurrentTaskExecutor
instead.
TimerTaskExecutor
This implementation uses a single TimerTask
as its backing
implementation. It’s different from the SyncTaskExecutor
in
that the method invocations are executed in a separate thread, although they are
synchronous in that thread.
WorkManagerTaskExecutor
This implementation uses the CommonJ WorkManager as its backing implementation and is
the central convenience class for setting up a CommonJ WorkManager reference in a Spring
context. Similar to the SimpleThreadPoolTaskExecutor
, this class implements the WorkManager interface and
therefore can be used directly as a WorkManager as well.
Spring’s TaskExecutor
implementations are used as simple JavaBeans. In the example
below, we define a bean that uses the ThreadPoolTaskExecutor
to asynchronously print
out a set of messages.
import org.springframework.core.task.TaskExecutor; public class TaskExecutorExample { private class MessagePrinterTask implements Runnable { private String message; public MessagePrinterTask(String message) { this.message = message; } public void run() { System.out.println(message); } } private TaskExecutor taskExecutor; public TaskExecutorExample(TaskExecutor taskExecutor) { this.taskExecutor = taskExecutor; } public void printMessages() { for(int i = 0; i < 25; i++) { taskExecutor.execute(new MessagePrinterTask("Message" + i)); } } }
As you can see, rather than retrieving a thread from the pool and executing yourself,
you add your Runnable
to the queue and the TaskExecutor
uses its internal rules to
decide when the task gets executed.
To configure the rules that the TaskExecutor
will use, simple bean properties have
been exposed.
<bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor"> <property name="corePoolSize" value="5" /> <property name="maxPoolSize" value="10" /> <property name="queueCapacity" value="25" /> </bean> <bean id="taskExecutorExample" class="TaskExecutorExample"> <constructor-arg ref="taskExecutor" /> </bean>
In addition to the TaskExecutor
abstraction, Spring 3.0 introduces a TaskScheduler
with a variety of methods for scheduling tasks to run at some point in the future.
public interface TaskScheduler { ScheduledFuture schedule(Runnable task, Trigger trigger); ScheduledFuture schedule(Runnable task, Date startTime); ScheduledFuture scheduleAtFixedRate(Runnable task, Date startTime, long period); ScheduledFuture scheduleAtFixedRate(Runnable task, long period); ScheduledFuture scheduleWithFixedDelay(Runnable task, Date startTime, long delay); ScheduledFuture scheduleWithFixedDelay(Runnable task, long delay); }
The simplest method is the one named schedule that takes a Runnable
and Date
only.
That will cause the task to run once after the specified time. All of the other methods
are capable of scheduling tasks to run repeatedly. The fixed-rate and fixed-delay
methods are for simple, periodic execution, but the method that accepts a Trigger is
much more flexible.
The Trigger
interface is essentially inspired by JSR-236, which, as of Spring 3.0, has
not yet been officially implemented. The basic idea of the Trigger
is that execution
times may be determined based on past execution outcomes or even arbitrary conditions.
If these determinations do take into account the outcome of the preceding execution,
that information is available within a TriggerContext
. The Trigger
interface itself
is quite simple:
public interface Trigger { Date nextExecutionTime(TriggerContext triggerContext); }
As you can see, the TriggerContext
is the most important part. It encapsulates all of
the relevant data, and is open for extension in the future if necessary. The
TriggerContext
is an interface (a SimpleTriggerContext
implementation is used by
default). Here you can see what methods are available for Trigger
implementations.
public interface TriggerContext { Date lastScheduledExecutionTime(); Date lastActualExecutionTime(); Date lastCompletionTime(); }
Spring provides two implementations of the Trigger
interface. The most interesting one
is the CronTrigger
. It enables the scheduling of tasks based on cron expressions. For
example the following task is being scheduled to run 15 minutes past each hour but only
during the 9-to-5 "business hours" on weekdays.
scheduler.schedule(task, new CronTrigger("* 15 9-17 * * MON-FRI"));
The other out-of-the-box implementation is a PeriodicTrigger
that accepts a fixed
period, an optional initial delay value, and a boolean to indicate whether the period
should be interpreted as a fixed-rate or a fixed-delay. Since the TaskScheduler
interface already defines methods for scheduling tasks at a fixed-rate or with a
fixed-delay, those methods should be used directly whenever possible. The value of the
PeriodicTrigger
implementation is that it can be used within components that rely on
the Trigger
abstraction. For example, it may be convenient to allow periodic triggers,
cron-based triggers, and even custom trigger implementations to be used interchangeably.
Such a component could take advantage of dependency injection so that such Triggers
could be configured externally.
As with Spring’s TaskExecutor
abstraction, the primary benefit of the TaskScheduler
is that code relying on scheduling behavior need not be coupled to a particular
scheduler implementation. The flexibility this provides is particularly relevant when
running within Application Server environments where threads should not be created
directly by the application itself. For such cases, Spring provides a
TimerManagerTaskScheduler
that delegates to a CommonJ TimerManager instance, typically
configured with a JNDI-lookup.
A simpler alternative, the ThreadPoolTaskScheduler
, can be used whenever external
thread management is not a requirement. Internally, it delegates to a
ScheduledExecutorService
instance. ThreadPoolTaskScheduler
actually implements
Spring’s TaskExecutor
interface as well, so that a single instance can be used for
asynchronous execution as soon as possible as well as scheduled, and potentially
recurring, executions.
Spring provides annotation support for both task scheduling and asynchronous method execution.
To enable support for @Scheduled
and @Async
annotations add @EnableScheduling
and
@EnableAsync
to one of your @Configuration
classes:
@Configuration @EnableAsync @EnableScheduling public class AppConfig { }
You are free to pick and choose the relevant annotations for your application. For
example, if you only need support for @Scheduled
, simply omit @EnableAsync
. For more
fine-grained control you can additionally implement the SchedulingConfigurer
and/or
AsyncConfigurer
interfaces. See the Javadoc for full details.
If you prefer XML configuration use the <task:annotation-driven>
element.
<task:annotation-driven executor="myExecutor" scheduler="myScheduler"/> <task:executor id="myExecutor" pool-size="5"/> <task:scheduler id="myScheduler" pool-size="10"/>
Notice with the above XML that an executor reference is provided for handling those
tasks that correspond to methods with the @Async
annotation, and the scheduler
reference is provided for managing those methods annotated with @Scheduled
.
The @Scheduled annotation can be added to a method along with trigger metadata. For example, the following method would be invoked every 5 seconds with a fixed delay, meaning that the period will be measured from the completion time of each preceding invocation.
@Scheduled(fixedDelay=5000) public void doSomething() { // something that should execute periodically }
If a fixed rate execution is desired, simply change the property name specified within the annotation. The following would be executed every 5 seconds measured between the successive start times of each invocation.
@Scheduled(fixedRate=5000) public void doSomething() { // something that should execute periodically }
For fixed-delay and fixed-rate tasks, an initial delay may be specified indicating the number of milliseconds to wait before the first execution of the method.
@Scheduled(initialDelay=1000, fixedRate=5000) public void doSomething() { // something that should execute periodically }
If simple periodic scheduling is not expressive enough, then a cron expression may be provided. For example, the following will only execute on weekdays.
@Scheduled(cron="*/5 * * * * MON-FRI") public void doSomething() { // something that should execute on weekdays only }
Tip | |
---|---|
You can additionally use the |
Notice that the methods to be scheduled must have void returns and must not expect any arguments. If the method needs to interact with other objects from the Application Context, then those would typically have been provided through dependency injection.
Note | |
---|---|
Make sure that you are not initializing multiple instances of the same @Scheduled annotation class at runtime, unless you do want to schedule callbacks to each such instance. Related to this, make sure that you do not use @Configurable on bean classes which are annotated with @Scheduled and registered as regular Spring beans with the container: You would get double initialization otherwise, once through the container and once through the @Configurable aspect, with the consequence of each @Scheduled method being invoked twice. |
The @Async
annotation can be provided on a method so that invocation of that method
will occur asynchronously. In other words, the caller will return immediately upon
invocation and the actual execution of the method will occur in a task that has been
submitted to a Spring TaskExecutor
. In the simplest case, the annotation may be
applied to a void
-returning method.
@Async void doSomething() { // this will be executed asynchronously }
Unlike the methods annotated with the @Scheduled
annotation, these methods can expect
arguments, because they will be invoked in the "normal" way by callers at runtime rather
than from a scheduled task being managed by the container. For example, the following is
a legitimate application of the @Async
annotation.
@Async void doSomething(String s) { // this will be executed asynchronously }
Even methods that return a value can be invoked asynchronously. However, such methods
are required to have a Future
typed return value. This still provides the benefit of
asynchronous execution so that the caller can perform other tasks prior to calling
get()
on that Future.
@Async Future<String> returnSomething(int i) { // this will be executed asynchronously }
@Async
can not be used in conjunction with lifecycle callbacks such as
@PostConstruct
. To asynchronously initialize Spring beans you currently have to use a
separate initializing Spring bean that invokes the @Async
annotated method on the
target then.
public class SampleBeanImpl implements SampleBean { @Async void doSomething() { // ... } } public class SampleBeanInititalizer { private final SampleBean bean; public SampleBeanInitializer(SampleBean bean) { this.bean = bean; } @PostConstruct public void initialize() { bean.doSomething(); } }
By default when specifying @Async
on a method, the executor that will be used is the
one supplied to the annotation-driven element as described above. However, the value
attribute of the @Async
annotation can be used when needing to indicate that an
executor other than the default should be used when executing a given method.
@Async("otherExecutor") void doSomething(String s) { // this will be executed asynchronously by "otherExecutor" }
In this case, "otherExecutor" may be the name of any Executor
bean in the Spring
container, or may be the name of a qualifier associated with any Executor
, e.g. as
specified with the <qualifier>
element or Spring’s @Qualifier
annotation.
Beginning with Spring 3.0, there is an XML namespace for configuring TaskExecutor
and
TaskScheduler
instances. It also provides a convenient way to configure tasks to be
scheduled with a trigger.
The following element will create a ThreadPoolTaskScheduler
instance with the
specified thread pool size.
<task:scheduler id="scheduler" pool-size="10"/>
The value provided for the id attribute will be used as the prefix for thread names within the pool. The scheduler element is relatively straightforward. If you do not provide a pool-size attribute, the default thread pool will only have a single thread. There are no other configuration options for the scheduler.
The following will create a ThreadPoolTaskExecutor
instance:
<task:executor id="executor" pool-size="10"/>
As with the scheduler above, the value provided for the id attribute will be used as
the prefix for thread names within the pool. As far as the pool size is concerned, the
executor element supports more configuration options than the scheduler element. For
one thing, the thread pool for a ThreadPoolTaskExecutor
is itself more configurable.
Rather than just a single size, an executor’s thread pool may have different values for
the core and the max size. If a single value is provided then the executor will
have a fixed-size thread pool (the core and max sizes are the same). However, the
executor element’s pool-size attribute also accepts a range in the form of "min-max".
<task:executor id="executorWithPoolSizeRange" pool-size="5-25" queue-capacity="100"/>
As you can see from that configuration, a queue-capacity value has also been provided. The configuration of the thread pool should also be considered in light of the executor’s queue capacity. For the full description of the relationship between pool size and queue capacity, consult the documentation for ThreadPoolExecutor. The main idea is that when a task is submitted, the executor will first try to use a free thread if the number of active threads is currently less than the core size. If the core size has been reached, then the task will be added to the queue as long as its capacity has not yet been reached. Only then, if the queue’s capacity has been reached, will the executor create a new thread beyond the core size. If the max size has also been reached, then the executor will reject the task.
By default, the queue is unbounded, but this is rarely the desired configuration,
because it can lead to OutOfMemoryErrors
if enough tasks are added to that queue while
all pool threads are busy. Furthermore, if the queue is unbounded, then the max size has
no effect at all. Since the executor will always try the queue before creating a new
thread beyond the core size, a queue must have a finite capacity for the thread pool to
grow beyond the core size (this is why a fixed size pool is the only sensible case
when using an unbounded queue).
In a moment, we will review the effects of the keep-alive setting which adds yet another
factor to consider when providing a pool size configuration. First, let’s consider the
case, as mentioned above, when a task is rejected. By default, when a task is rejected,
a thread pool executor will throw a TaskRejectedException
. However, the rejection
policy is actually configurable. The exception is thrown when using the default
rejection policy which is the AbortPolicy
implementation. For applications where some
tasks can be skipped under heavy load, either the DiscardPolicy
or
DiscardOldestPolicy
may be configured instead. Another option that works well for
applications that need to throttle the submitted tasks under heavy load is the
CallerRunsPolicy
. Instead of throwing an exception or discarding tasks, that policy
will simply force the thread that is calling the submit method to run the task itself.
The idea is that such a caller will be busy while running that task and not able to
submit other tasks immediately. Therefore it provides a simple way to throttle the
incoming load while maintaining the limits of the thread pool and queue. Typically this
allows the executor to "catch up" on the tasks it is handling and thereby frees up some
capacity on the queue, in the pool, or both. Any of these options can be chosen from an
enumeration of values available for the rejection-policy attribute on the executor
element.
<task:executor id="executorWithCallerRunsPolicy" pool-size="5-25" queue-capacity="100" rejection-policy="CALLER_RUNS"/>
The most powerful feature of Spring’s task namespace is the support for configuring tasks to be scheduled within a Spring Application Context. This follows an approach similar to other "method-invokers" in Spring, such as that provided by the JMS namespace for configuring Message-driven POJOs. Basically a "ref" attribute can point to any Spring-managed object, and the "method" attribute provides the name of a method to be invoked on that object. Here is a simple example.
<task:scheduled-tasks scheduler="myScheduler"> <task:scheduled ref="beanA" method="methodA" fixed-delay="5000"/> </task:scheduled-tasks> <task:scheduler id="myScheduler" pool-size="10"/>
As you can see, the scheduler is referenced by the outer element, and each individual task includes the configuration of its trigger metadata. In the preceding example, that metadata defines a periodic trigger with a fixed delay indicating the number of milliseconds to wait after each task execution has completed. Another option is fixed-rate, indicating how often the method should be executed regardless of how long any previous execution takes. Additionally, for both fixed-delay and fixed-rate tasks an initial-delay parameter may be specified indicating the number of milliseconds to wait before the first execution of the method. For more control, a "cron" attribute may be provided instead. Here is an example demonstrating these other options.
<task:scheduled-tasks scheduler="myScheduler"> <task:scheduled ref="beanA" method="methodA" fixed-delay="5000" initial-delay="1000"/> <task:scheduled ref="beanB" method="methodB" fixed-rate="5000"/> <task:scheduled ref="beanC" method="methodC" cron="*/5 * * * * MON-FRI"/> </task:scheduled-tasks> <task:scheduler id="myScheduler" pool-size="10"/>
Quartz uses Trigger
, Job
and JobDetail
objects to realize scheduling of all kinds
of jobs. For the basic concepts behind Quartz, have a look at
http://quartz-scheduler.org. For convenience purposes, Spring offers a couple of
classes that simplify the usage of Quartz within Spring-based applications.
JobDetail
objects contain all information needed to run a job. The Spring Framework
provides a JobDetailBean
that makes the JobDetail
more of an actual JavaBean with
sensible defaults. Let’s have a look at an example:
<bean name="exampleJob" class="org.springframework.scheduling.quartz.JobDetailBean"> <property name="jobClass" value="example.ExampleJob" /> <property name="jobDataAsMap"> <map> <entry key="timeout" value="5" /> </map> </property> </bean>
The job detail bean has all information it needs to run the job ( ExampleJob
). The
timeout is specified in the job data map. The job data map is available through the
JobExecutionContext
(passed to you at execution time), but the JobDetailBean
also
maps the properties from the job data map to properties of the actual job. So in this
case, if the ExampleJob
contains a property named timeout
, the JobDetailBean
will
automatically apply it:
package example; public class ExampleJob extends QuartzJobBean { private int timeout; /** * Setter called after the ExampleJob is instantiated * with the value from the JobDetailBean (5) */ public void setTimeout(int timeout) { this.timeout = timeout; } protected void executeInternal(JobExecutionContext ctx) throws JobExecutionException { // do the actual work } }
All additional settings from the job detail bean are of course available to you as well.
Note | |
---|---|
Using the |
Often you just need to invoke a method on a specific object. Using the
MethodInvokingJobDetailFactoryBean
you can do exactly this:
<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"> <property name="targetObject" ref="exampleBusinessObject" /> <property name="targetMethod" value="doIt" /> </bean>
The above example will result in the doIt
method being called on the
exampleBusinessObject
method (see below):
public class ExampleBusinessObject { // properties and collaborators public void doIt() { // do the actual work } }
<bean id="exampleBusinessObject" class="examples.ExampleBusinessObject"/>
Using the MethodInvokingJobDetailFactoryBean
, you don’t need to create one-line jobs
that just invoke a method, and you only need to create the actual business object and
wire up the detail object.
By default, Quartz Jobs are stateless, resulting in the possibility of jobs interfering
with each other. If you specify two triggers for the same JobDetail
, it might be
possible that before the first job has finished, the second one will start. If
JobDetail
classes implement the Stateful
interface, this won’t happen. The second
job will not start before the first one has finished. To make jobs resulting from the
MethodInvokingJobDetailFactoryBean
non-concurrent, set the concurrent
flag to
false
.
<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"> <property name="targetObject" ref="exampleBusinessObject" /> <property name="targetMethod" value="doIt" /> <property name="concurrent" value="false" /> </bean>
Note | |
---|---|
By default, jobs will run in a concurrent fashion. |
We’ve created job details and jobs. We’ve also reviewed the convenience bean that allows
you to invoke a method on a specific object. Of course, we still need to schedule the
jobs themselves. This is done using triggers and a SchedulerFactoryBean
. Several
triggers are available within Quartz and Spring offers two Quartz FactoryBean
implementations with convenient defaults: CronTriggerFactoryBean
and
SimpleTriggerFactoryBean
.
Triggers need to be scheduled. Spring offers a SchedulerFactoryBean
that exposes
triggers to be set as properties. SchedulerFactoryBean
schedules the actual jobs with
those triggers.
Find below a couple of examples:
<bean id="simpleTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerFactoryBean"> <!-- see the example of method invoking job above --> <property name="jobDetail" ref="jobDetail" /> <!-- 10 seconds --> <property name="startDelay" value="10000" /> <!-- repeat every 50 seconds --> <property name="repeatInterval" value="50000" /> </bean> <bean id="cronTrigger" class="org.springframework.scheduling.quartz.CronTriggerFactoryBean"> <property name="jobDetail" ref="exampleJob" /> <!-- run every morning at 6 AM --> <property name="cronExpression" value="0 0 6 * * ?" /> </bean>
Now we’ve set up two triggers, one running every 50 seconds with a starting delay of 10
seconds and one every morning at 6 AM. To finalize everything, we need to set up the
SchedulerFactoryBean
:
<bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean"> <property name="triggers"> <list> <ref bean="cronTrigger" /> <ref bean="simpleTrigger" /> </list> </property> </bean>
More properties are available for the SchedulerFactoryBean
for you to set, such as the
calendars used by the job details, properties to customize Quartz with, etc. Have a look
at the
SchedulerFactoryBean
Javadoc
for more information.
Spring 2.0 introduces comprehensive support for using classes and objects that have been defined using a dynamic language (such as JRuby) with Spring. This support allows you to write any number of classes in a supported dynamic language, and have the Spring container transparently instantiate, configure and dependency inject the resulting objects.
The dynamic languages currently supported are:
Fully working examples of where this dynamic language support can be immediately useful are described in Section 28.4, “Scenarios”.
This bulk of this chapter is concerned with describing the dynamic language support in detail. Before diving into all of the ins and outs of the dynamic language support, let’s look at a quick example of a bean defined in a dynamic language. The dynamic language for this first bean is Groovy (the basis of this example was taken from the Spring test suite, so if you want to see equivalent examples in any of the other supported languages, take a look at the source code).
Find below the Messenger
interface that the Groovy bean is going to be implementing,
and note that this interface is defined in plain Java. Dependent objects that are
injected with a reference to the Messenger
won’t know that the underlying
implementation is a Groovy script.
package org.springframework.scripting; public interface Messenger { String getMessage(); }
Here is the definition of a class that has a dependency on the Messenger
interface.
package org.springframework.scripting; public class DefaultBookingService implements BookingService { private Messenger messenger; public void setMessenger(Messenger messenger) { this.messenger = messenger; } public void processBooking() { // use the injected Messenger object... } }
Here is an implementation of the Messenger
interface in Groovy.
// from the file Messenger.groovy package org.springframework.scripting.groovy; // import the Messenger interface (written in Java) that is to be implemented import org.springframework.scripting.Messenger // define the implementation in Groovy class GroovyMessenger implements Messenger { String message }
Finally, here are the bean definitions that will effect the injection of the
Groovy-defined Messenger
implementation into an instance of the
DefaultBookingService
class.
Note | |
---|---|
To use the custom dynamic language tags to define dynamic-language-backed beans, you
need to have the XML Schema preamble at the top of your Spring XML configuration file.
You also need to be using a Spring For more information on schema-based configuration, see Chapter 33, XML Schema-based configuration. |
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:lang="http://www.springframework.org/schema/lang" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd"> <!-- this is the bean definition for the Groovy-backed Messenger implementation --> <lang:groovy id="messenger" script-source="classpath:Messenger.groovy"> <lang:property name="message" value="I Can Do The Frug" /> </lang:groovy> <!-- an otherwise normal bean that will be injected by the Groovy-backed Messenger --> <bean id="bookingService" class="x.y.DefaultBookingService"> <property name="messenger" ref="messenger" /> </bean> </beans>
The bookingService
bean (a DefaultBookingService
) can now use its private
messenger
member variable as normal because the Messenger
instance that was injected
into it is a Messenger
instance. There is nothing special going on here, just
plain Java and plain Groovy.
Hopefully the above XML snippet is self-explanatory, but don’t worry unduly if it isn’t. Keep reading for the in-depth detail on the whys and wherefores of the above configuration.
This section describes exactly how you define Spring managed beans in any of the supported dynamic languages.
Please note that this chapter does not attempt to explain the syntax and idioms of the supported dynamic languages. For example, if you want to use Groovy to write certain of the classes in your application, then the assumption is that you already know Groovy. If you need further details about the dynamic languages themselves, please consult Section 28.6, “Further Resources” at the end of this chapter.
The steps involved in using dynamic-language-backed beans are as follows:
<lang:language/>
element in the XML configuration (you can of course define such beans programmatically
using the Spring API - although you will have to consult the source code for
directions on how to do this as this type of advanced configuration is not covered in
this chapter). Note this is an iterative step. You will need at least one bean
definition per dynamic language source file (although the same dynamic language source
file can of course be referenced by multiple bean definitions).
The first two steps (testing and writing your dynamic language source files) are beyond the scope of this chapter. Refer to the language specification and / or reference manual for your chosen dynamic language and crack on with developing your dynamic language source files. You will first want to read the rest of this chapter though, as Spring’s dynamic language support does make some (small) assumptions about the contents of your dynamic language source files.
The final step involves defining dynamic-language-backed bean definitions, one for each
bean that you want to configure (this is no different from normal JavaBean
configuration). However, instead of specifying the fully qualified classname of the
class that is to be instantiated and configured by the container, you use the
<lang:language/>
element to define the dynamic language-backed bean.
Each of the supported languages has a corresponding <lang:language/>
element:
<lang:jruby/>
(JRuby)
<lang:groovy/>
(Groovy)
<lang:bsh/>
(BeanShell)
The exact attributes and child elements that are available for configuration depends on exactly which language the bean has been defined in (the language-specific sections below provide the full lowdown on this).
One of the (if not the) most compelling value adds of the dynamic language support in Spring is the'refreshable bean' feature.
A refreshable bean is a dynamic-language-backed bean that with a small amount of configuration, a dynamic-language-backed bean can monitor changes in its underlying source file resource, and then reload itself when the dynamic language source file is changed (for example when a developer edits and saves changes to the file on the filesystem).
This allows a developer to deploy any number of dynamic language source files as part of an application, configure the Spring container to create beans backed by dynamic language source files (using the mechanisms described in this chapter), and then later, as requirements change or some other external factor comes into play, simply edit a dynamic language source file and have any change they make reflected in the bean that is backed by the changed dynamic language source file. There is no need to shut down a running application (or redeploy in the case of a web application). The dynamic-language-backed bean so amended will pick up the new state and logic from the changed dynamic language source file.
Note | |
---|---|
Please note that this feature is off by default. |
Let’s take a look at an example to see just how easy it is to start using refreshable
beans. To turn on the refreshable beans feature, you simply have to specify exactly
one additional attribute on the <lang:language/>
element of your bean definition.
So if we stick with the example from earlier in this
chapter, here’s what we would change in the Spring XML configuration to effect
refreshable beans:
<beans> <!-- this bean is now refreshable due to the presence of the refresh-check-delay attribute --> <lang:groovy id="messenger" refresh-check-delay="5000" <!-- switches refreshing on with 5 seconds between checks --> script-source="classpath:Messenger.groovy"> <lang:property name="message" value="I Can Do The Frug" /> </lang:groovy> <bean id="bookingService" class="x.y.DefaultBookingService"> <property name="messenger" ref="messenger" /> </bean> </beans>
That really is all you have to do. The 'refresh-check-delay'
attribute defined on the
'messenger'
bean definition is the number of milliseconds after which the bean will be
refreshed with any changes made to the underlying dynamic language source file. You can
turn off the refresh behavior by assigning a negative value to the
'refresh-check-delay'
attribute. Remember that, by default, the refresh behavior is
disabled. If you don’t want the refresh behavior, then simply don’t define the attribute.
If we then run the following application we can exercise the refreshable feature; please
do excuse the 'jumping-through-hoops-to-pause-the-execution' shenanigans in this
next slice of code. The System.in.read()
call is only there so that the execution of
the program pauses while I (the author) go off and edit the underlying dynamic language
source file so that the refresh will trigger on the dynamic-language-backed bean when
the program resumes execution.
import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.scripting.Messenger; public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml"); Messenger messenger = (Messenger) ctx.getBean("messenger"); System.out.println(messenger.getMessage()); // pause execution while I go off and make changes to the source file... System.in.read(); System.out.println(messenger.getMessage()); } }
Let’s assume then, for the purposes of this example, that all calls to the
getMessage()
method of Messenger
implementations have to be changed such that the
message is surrounded by quotes. Below are the changes that I (the author) make to the
Messenger.groovy
source file when the execution of the program is paused.
package org.springframework.scripting class GroovyMessenger implements Messenger { private String message = "Bingo" public String getMessage() { // change the implementation to surround the message in quotes return "" + this.message + "" } public void setMessage(String message) { this.message = message } }
When the program executes, the output before the input pause will be I Can Do The
Frug. After the change to the source file is made and saved, and the program resumes
execution, the result of calling the getMessage()
method on the
dynamic-language-backed Messenger
implementation will be 'I Can Do The Frug'
(notice the inclusion of the additional quotes).
It is important to understand that changes to a script will not trigger a refresh if
the changes occur within the window of the 'refresh-check-delay'
value. It is equally
important to understand that changes to the script are not actually picked up until
a method is called on the dynamic-language-backed bean. It is only when a method is
called on a dynamic-language-backed bean that it checks to see if its underlying script
source has changed. Any exceptions relating to refreshing the script (such as
encountering a compilation error, or finding that the script file has been deleted) will
result in a fatal exception being propagated to the calling code.
The refreshable bean behavior described above does not apply to dynamic language
source files defined using the <lang:inline-script/>
element notation (see
the section called “Inline dynamic language source files”). Additionally, it only applies to beans where
changes to the underlying source file can actually be detected; for example, by code
that checks the last modified date of a dynamic language source file that exists on the
filesystem.
The dynamic language support can also cater for dynamic language source files that are
embedded directly in Spring bean definitions. More specifically, the
<lang:inline-script/>
element allows you to define dynamic language source immediately
inside a Spring configuration file. An example will perhaps make the inline script
feature crystal clear:
<lang:groovy id="messenger"> <lang:inline-script> package org.springframework.scripting.groovy; import org.springframework.scripting.Messenger class GroovyMessenger implements Messenger { String message } </lang:inline-script> <lang:property name="message" value="I Can Do The Frug" /> </lang:groovy>
If we put to one side the issues surrounding whether it is good practice to define
dynamic language source inside a Spring configuration file, the <lang:inline-script/>
element can be useful in some scenarios. For instance, we might want to quickly add a
Spring Validator
implementation to a Spring MVC Controller
. This is but a moment’s
work using inline source. (See Section 28.4.2, “Scripted Validators” for such an
example.)
Find below an example of defining the source for a JRuby-based bean directly in a Spring
XML configuration file using the inline:
notation. (Notice the use of the <
characters to denote a '<'
character. In such a case surrounding the inline source in
a <![CDATA[]]>
region might be better.)
<lang:jruby id="messenger" script-interfaces="org.springframework.scripting.Messenger"> <lang:inline-script> require java include_class org.springframework.scripting.Messenger class RubyMessenger < Messenger def setMessage(message) @@message = message end def getMessage @@message end end </lang:inline-script> <lang:property name="message" value="Hello World!" /> </lang:jruby>
There is one very important thing to be aware of with regard to Spring’s dynamic language support. Namely, it is not (currently) possible to supply constructor arguments to dynamic-language-backed beans (and hence constructor-injection is not available for dynamic-language-backed beans). In the interests of making this special handling of constructors and properties 100% clear, the following mixture of code and configuration will not work.
// from the file Messenger.groovy package org.springframework.scripting.groovy; import org.springframework.scripting.Messenger class GroovyMessenger implements Messenger { GroovyMessenger() {} // this constructor is not available for Constructor Injection GroovyMessenger(String message) { this.message = message; } String message String anotherMessage }
<lang:groovy id="badMessenger" script-source="classpath:Messenger.groovy"> <!-- this next constructor argument will not be injected into the GroovyMessenger --> <!-- in fact, this isn't even allowed according to the schema --> <constructor-arg value="This will not work" /> <!-- only property values are injected into the dynamic-language-backed object --> <lang:property name="anotherMessage" value="Passed straight through to the dynamic-language-backed object" /> </lang>
In practice this limitation is not as significant as it first appears since setter injection is the injection style favored by the overwhelming majority of developers anyway (let’s leave the discussion as to whether that is a good thing to another day).
From the JRuby homepage…
"JRuby is an 100% pure-Java implementation of the Ruby programming language."
In keeping with the Spring philosophy of offering choice, Spring’s dynamic language support also supports beans defined in the JRuby language. The JRuby language is based on the quite intuitive Ruby language, and has support for inline regular expressions, blocks (closures), and a whole host of other features that do make solutions for some domain problems a whole lot easier to develop.
The implementation of the JRuby dynamic language support in Spring is interesting in
that what happens is this: Spring creates a JDK dynamic proxy implementing all of the
interfaces that are specified in the 'script-interfaces'
attribute value of the
<lang:ruby>
element (this is why you must supply at least one interface in the
value of the attribute, and (accordingly) program to interfaces when using JRuby-backed
beans).
Let us look at a fully working example of using a JRuby-based bean. Here is the JRuby
implementation of the Messenger
interface that was defined earlier in this chapter
(for your convenience it is repeated below).
package org.springframework.scripting; public interface Messenger { String getMessage(); }
require java class RubyMessenger include org.springframework.scripting.Messenger def setMessage(message) @@message = message end def getMessage @@message end end # this last line is not essential (but see below) RubyMessenger.new
And here is the Spring XML that defines an instance of the RubyMessenger
JRuby bean.
<lang:jruby id="messageService" script-interfaces="org.springframework.scripting.Messenger" script-source="classpath:RubyMessenger.rb"> <lang:property name="message" value="Hello World!" /> </lang:jruby>
Take note of the last line of that JRuby source ( 'RubyMessenger.new'
). When using
JRuby in the context of Spring’s dynamic language support, you are encouraged to
instantiate and return a new instance of the JRuby class that you want to use as a
dynamic-language-backed bean as the result of the execution of your JRuby source. You
can achieve this by simply instantiating a new instance of your JRuby class on the last
line of the source file like so:
require java include_class org.springframework.scripting.Messenger # class definition same as above... # instantiate and return a new instance of the RubyMessenger class RubyMessenger.new
If you forget to do this, it is not the end of the world; this will however result in
Spring having to trawl (reflectively) through the type representation of your JRuby
class looking for a class to instantiate. In the grand scheme of things this will be so
fast that you’ll never notice it, but it is something that can be avoided by simply
having a line such as the one above as the last line of your JRuby script. If you don’t
supply such a line, or if Spring cannot find a JRuby class in your script to instantiate
then an opaque ScriptCompilationException
will be thrown immediately after the source
is executed by the JRuby interpreter. The key text that identifies this as the root
cause of an exception can be found immediately below (so if your Spring container throws
the following exception when creating your dynamic-language-backed bean and the
following text is there in the corresponding stacktrace, this will hopefully allow you
to identify and then easily rectify the issue):
org.springframework.scripting.ScriptCompilationException: Compilation of JRuby script returned ''
To rectify this, simply instantiate a new instance of whichever class you want to expose as a JRuby-dynamic-language-backed bean (as shown above). Please also note that you can actually define as many classes and objects as you want in your JRuby script; what is important is that the source file as a whole must return an object (for Spring to configure).
See Section 28.4, “Scenarios” for some scenarios where you might want to use JRuby-based beans.
From the Groovy homepage…
"Groovy is an agile dynamic language for the Java 2 Platform that has many of the features that people like so much in languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax. "
If you have read this chapter straight from the top, you will already have seen an example of a Groovy-dynamic-language-backed bean. Let’s look at another example (again using an example from the Spring test suite).
package org.springframework.scripting; public interface Calculator { int add(int x, int y); }
Here is an implementation of the Calculator
interface in Groovy.
// from the file calculator.groovy
package org.springframework.scripting.groovy
class GroovyCalculator implements Calculator {
int add(int x, int y) {
x + y
}
}
<-- from the file beans.xml --> <beans> <lang:groovy id="calculator" script-source="classpath:calculator.groovy"/> </beans>
Lastly, here is a small application to exercise the above configuration.
package org.springframework.scripting; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class Main { public static void Main(String[] args) { ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml"); Calculator calc = (Calculator) ctx.getBean("calculator"); System.out.println(calc.add(2, 8)); } }
The resulting output from running the above program will be (unsurprisingly) 10. (Exciting example, huh? Remember that the intent is to illustrate the concept. Please consult the dynamic language showcase project for a more complex example, or indeed Section 28.4, “Scenarios” later in this chapter).
It is important that you do not define more than one class per Groovy source file. While this is perfectly legal in Groovy, it is (arguably) a bad practice: in the interests of a consistent approach, you should (in the opinion of this author) respect the standard Java conventions of one (public) class per source file.
The GroovyObjectCustomizer
interface is a callback that allows you to hook additional
creation logic into the process of creating a Groovy-backed bean. For example,
implementations of this interface could invoke any required initialization method(s), or
set some default property values, or specify a custom MetaClass
.
public interface GroovyObjectCustomizer { void customize(GroovyObject goo); }
The Spring Framework will instantiate an instance of your Groovy-backed bean, and will
then pass the created GroovyObject
to the specified GroovyObjectCustomizer
if one
has been defined. You can do whatever you like with the supplied GroovyObject
reference: it is expected that the setting of a custom MetaClass
is what most folks
will want to do with this callback, and you can see an example of doing that below.
public final class SimpleMethodTracingCustomizer implements GroovyObjectCustomizer { public void customize(GroovyObject goo) { DelegatingMetaClass metaClass = new DelegatingMetaClass(goo.getMetaClass()) { public Object invokeMethod(Object object, String methodName, Object[] arguments) { System.out.println("Invoking " + methodName + "."); return super.invokeMethod(object, methodName, arguments); } }; metaClass.initialize(); goo.setMetaClass(metaClass); } }
A full discussion of meta-programming in Groovy is beyond the scope of the Spring
reference manual. Consult the relevant section of the Groovy reference manual, or do a
search online: there are plenty of articles concerning this topic. Actually making use
of a GroovyObjectCustomizer
is easy if you are using the Spring namespace support.
<!-- define the GroovyObjectCustomizer just like any other bean --> <bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer" /> <!-- ... and plug it into the desired Groovy bean via the customizer-ref attribute --> <lang:groovy id="calculator" script-source="classpath:org/springframework/scripting/groovy/Calculator.groovy" customizer-ref="tracingCustomizer" />
If you are not using the Spring namespace support, you can still use the
GroovyObjectCustomizer
functionality.
<bean id="calculator" class="org.springframework.scripting.groovy.GroovyScriptFactory"> <constructor-arg value="classpath:org/springframework/scripting/groovy/Calculator.groovy"/> <!-- define the GroovyObjectCustomizer (as an inner bean) --> <constructor-arg> <bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer" /> </constructor-arg> </bean> <bean class="org.springframework.scripting.support.ScriptFactoryPostProcessor"/>
From the BeanShell homepage…
"BeanShell is a small, free, embeddable Java source interpreter with dynamic language features, written in Java. BeanShell dynamically executes standard Java syntax and extends it with common scripting conveniences such as loose types, commands, and method closures like those in Perl and JavaScript."
In contrast to Groovy, BeanShell-backed bean definitions require some (small) additional
configuration. The implementation of the BeanShell dynamic language support in Spring is
interesting in that what happens is this: Spring creates a JDK dynamic proxy
implementing all of the interfaces that are specified in the 'script-interfaces'
attribute value of the <lang:bsh>
element (this is why you must supply at least
one interface in the value of the attribute, and (accordingly) program to interfaces
when using BeanShell-backed beans). This means that every method call on a
BeanShell-backed object is going through the JDK dynamic proxy invocation mechanism.
Let’s look at a fully working example of using a BeanShell-based bean that implements
the Messenger
interface that was defined earlier in this chapter (repeated below for
your convenience).
package org.springframework.scripting; public interface Messenger { String getMessage(); }
Here is the BeanShell implementation (the term is used loosely here) of the
Messenger
interface.
String message; String getMessage() { return message; } void setMessage(String aMessage) { message = aMessage; }
And here is the Spring XML that defines an instance of the above class (again, the term is used very loosely here).
<lang:bsh id="messageService" script-source="classpath:BshMessenger.bsh" script-interfaces="org.springframework.scripting.Messenger"> <lang:property name="message" value="Hello World!" /> </lang:bsh>
See Section 28.4, “Scenarios” for some scenarios where you might want to use BeanShell-based beans.
The possible scenarios where defining Spring managed beans in a scripting language would be beneficial are, of course, many and varied. This section describes two possible use cases for the dynamic language support in Spring.
One group of classes that may benefit from using dynamic-language-backed beans is that of Spring MVC controllers. In pure Spring MVC applications, the navigational flow through a web application is to a large extent determined by code encapsulated within your Spring MVC controllers. As the navigational flow and other presentation layer logic of a web application needs to be updated to respond to support issues or changing business requirements, it may well be easier to effect any such required changes by editing one or more dynamic language source files and seeing those changes being immediately reflected in the state of a running application.
Remember that in the lightweight architectural model espoused by projects such as Spring, you are typically aiming to have a really thin presentation layer, with all the meaty business logic of an application being contained in the domain and service layer classes. Developing Spring MVC controllers as dynamic-language-backed beans allows you to change presentation layer logic by simply editing and saving text files; any changes to such dynamic language source files will (depending on the configuration) automatically be reflected in the beans that are backed by dynamic language source files.
Note | |
---|---|
In order to effect this automatic pickup of any changes to dynamic-language-backed beans, you will have had to enable the refreshable beans functionality. See the section called “Refreshable beans” for a full treatment of this feature. |
Find below an example of an org.springframework.web.servlet.mvc.Controller
implemented
using the Groovy dynamic language.
// from the file /WEB-INF/groovy/FortuneController.groovy package org.springframework.showcase.fortune.web import org.springframework.showcase.fortune.service.FortuneService import org.springframework.showcase.fortune.domain.Fortune import org.springframework.web.servlet.ModelAndView import org.springframework.web.servlet.mvc.Controller import javax.servlet.http.HttpServletRequest import javax.servlet.http.HttpServletResponse class FortuneController implements Controller { @Property FortuneService fortuneService ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse httpServletResponse) { return new ModelAndView("tell", "fortune", this.fortuneService.tellFortune()) } }
<lang:groovy id="fortune" refresh-check-delay="3000" script-source="/WEB-INF/groovy/FortuneController.groovy"> <lang:property name="fortuneService" ref="fortuneService"/> </lang:groovy>
Another area of application development with Spring that may benefit from the flexibility afforded by dynamic-language-backed beans is that of validation. It may be easier to express complex validation logic using a loosely typed dynamic language (that may also have support for inline regular expressions) as opposed to regular Java.
Again, developing validators as dynamic-language-backed beans allows you to change validation logic by simply editing and saving a simple text file; any such changes will (depending on the configuration) automatically be reflected in the execution of a running application and would not require the restart of an application.
Note | |
---|---|
Please note that in order to effect the automatic pickup of any changes to dynamic-language-backed beans, you will have had to enable the refreshable beans feature. See the section called “Refreshable beans” for a full and detailed treatment of this feature. |
Find below an example of a Spring org.springframework.validation.Validator
implemented
using the Groovy dynamic language. (See Section 6.2, “Validation using Spring’s Validator interface” for a discussion of the
Validator
interface.)
import org.springframework.validation.Validator import org.springframework.validation.Errors import org.springframework.beans.TestBean class TestBeanValidator implements Validator { boolean supports(Class clazz) { return TestBean.class.isAssignableFrom(clazz) } void validate(Object bean, Errors errors) { if(bean.name?.trim()?.size() > 0) { return } errors.reject("whitespace", "Cannot be composed wholly of whitespace.") } }
This last section contains some bits and bobs related to the dynamic language support.
It is possible to use the Spring AOP framework to advise scripted beans. The Spring AOP framework actually is unaware that a bean that is being advised might be a scripted bean, so all of the AOP use cases and functionality that you may be using or aim to use will work with scripted beans. There is just one (small) thing that you need to be aware of when advising scripted beans… you cannot use class-based proxies, you must use interface-based proxies.
You are of course not just limited to advising scripted beans… you can also write aspects themselves in a supported dynamic language and use such beans to advise other Spring beans. This really would be an advanced use of the dynamic language support though.
In case it is not immediately obvious, scripted beans can of course be scoped just like
any other bean. The scope
attribute on the various <lang:language/>
elements allows
you to control the scope of the underlying scripted bean, just as it does with a regular
bean. (The default scope is singleton, just as it is
with regular beans.)
Find below an example of using the scope
attribute to define a Groovy bean scoped as
a prototype.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:lang="http://www.springframework.org/schema/lang" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd"> <lang:groovy id="messenger" script-source="classpath:Messenger.groovy" scope="prototype"> <lang:property name="message" value="I Can Do The RoboCop" /> </lang:groovy> <bean id="bookingService" class="x.y.DefaultBookingService"> <property name="messenger" ref="messenger" /> </bean> </beans>
See Section 4.5, “Bean scopes” in Chapter 4, The IoC container for a fuller discussion of the scoping support in the Spring Framework.
Find below links to further resources about the various dynamic languages described in this chapter.
Some of the more active members of the Spring community have also added support for a number of additional dynamic languages above and beyond the ones covered in this chapter. While it is possible that such third party contributions may be added to the list of languages supported by the main Spring distribution, your best bet for seeing if your favourite scripting language is supported is the Spring Modules project.
Since version 3.1, Spring Framework provides support for transparently adding caching into an existing Spring application. Similar to the transaction support, the caching abstraction allows consistent use of various caching solutions with minimal impact on the code.
At its core, the abstraction applies caching to Java methods, reducing thus the number of executions based on the information available in the cache. That is, each time a targeted method is invoked, the abstraction will apply a caching behavior checking whether the method has been already executed for the given arguments. If it has, then the cached result is returned without having to execute the actual method; if it has not, then method is executed, the result cached and returned to the user so that, the next time the method is invoked, the cached result is returned. This way, expensive methods (whether CPU or IO bound) can be executed only once for a given set of parameters and the result reused without having to actually execute the method again. The caching logic is applied transparently without any interference to the invoker.
Important | |
---|---|
Obviously this approach works only for methods that are guaranteed to return the same output (result) for a given input (or arguments) no matter how many times it is being executed. |
To use the cache abstraction, the developer needs to take care of two aspects:
Note that just like other services in Spring Framework, the caching service is an
abstraction (not a cache implementation) and requires the use of an actual storage to
store the cache data - that is, the abstraction frees the developer from having to write
the caching logic but does not provide the actual stores. There are two integrations
available out of the box, for JDK java.util.concurrent.ConcurrentMap
and
EhCache - see Section 29.6, “Plugging-in different back-end caches” for more information on plugging in
other cache stores/providers.
For caching declaration, the abstraction provides two Java annotations: @Cacheable
and
@CacheEvict
which allow methods to trigger cache population or cache eviction. Let us
take a closer look at each annotation:
As the name implies, @Cacheable
is used to demarcate methods that are cacheable - that
is, methods for whom the result is stored into the cache so on subsequent invocations
(with the same arguments), the value in the cache is returned without having to actually
execute the method. In its simplest form, the annotation declaration requires the name
of the cache associated with the annotated method:
@Cacheable("books") public Book findBook(ISBN isbn) {...}
In the snippet above, the method findBook
is associated with the cache named books
.
Each time the method is called, the cache is checked to see whether the invocation has
been already executed and does not have to be repeated. While in most cases, only one
cache is declared, the annotation allows multiple names to be specified so that more
then one cache are being used. In this case, each of the caches will be checked before
executing the method - if at least one cache is hit, then the associated value will be
returned:
Note | |
---|---|
All the other caches that do not contain the method will be updated as well even though the cached method was not actually executed. |
@Cacheable({ "books", "isbns" }) public Book findBook(ISBN isbn) {...}
Since caches are essentially key-value stores, each invocation of a cached method needs
to be translated into a suitable key for cache access. Out of the box, the caching
abstraction uses a simple KeyGenerator
based on the following algorithm:
SimpleKey.EMPTY
.
SimpleKey
containing all parameters.
This approach works well for most use-cases; As long as parameters have natural keys
and implement valid hashCode()
and equals()
methods. If that is not the case the the
strategy needs to be changed.
To provide a different default key generator, one needs to implement the
org.springframework.cache.KeyGenerator
interface. Once configured, the generator will
be used for each declaration that does not specify its own key generation strategy (see
below).
Note | |
---|---|
The default key generation strategy changed with the release of Spring 4.0. Earlier
versions of Spring used a key generation strategy that, for multiple key parameters,
only considered the If you want to keep using the previous key strategy, you can configure the deprecated
|
Since caching is generic, it is quite likely the target methods have various signatures that cannot be simply mapped on top of the cache structure. This tends to become obvious when the target method has multiple arguments out of which only some are suitable for caching (while the rest are used only by the method logic). For example:
@Cacheable("books") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)
At first glance, while the two boolean
arguments influence the way the book is found,
they are no use for the cache. Further more what if only one of the two is important
while the other is not?
For such cases, the @Cacheable
annotation allows the user to specify how the key is
generated through its key
attribute. The developer can use SpEL to
pick the arguments of interest (or their nested properties), perform operations or even
invoke arbitrary methods without having to write any code or implement any interface.
This is the recommended approach over the
default generator since methods tend to be
quite different in signatures as the code base grows; while the default strategy might
work for some methods, it rarely does for all methods.
Below are some examples of various SpEL declarations - if you are not familiar with it, do yourself a favour and read Chapter 7, Spring Expression Language (SpEL):
@Cacheable(value="books", key="#isbn") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed) @Cacheable(value="books", key="#isbn.rawNumber") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed) @Cacheable(value="books", key="T(someType).hash(#isbn)") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)
The snippets above show how easy it is to select a certain argument, one of its properties or even an arbitrary (static) method.
Sometimes, a method might not be suitable for caching all the time (for example, it
might depend on the given arguments). The cache annotations support such functionality
through the conditional
parameter which takes a SpEL
expression that is evaluated to
either true
or false
. If true
, the method is cached - if not, it behaves as if the
method is not cached, that is executed every since time no matter what values are in the
cache or what arguments are used. A quick example - the following method will be cached
only if the argument name
has a length shorter than 32:
@Cacheable(value="book", condition="#name.length < 32") public Book findBook(String name)
In addition the conditional
parameter, the unless
parameter can be used to veto the
adding of a value to the cache. Unlike conditional
, unless
expressions are evaluated
after the method has been called. Expanding on the previous example - perhaps we
only want to cache paperback books:
@Cacheable(value="book", condition="#name.length < 32", unless="#result.hardback") public Book findBook(String name)
Each SpEL
expression evaluates again a dedicated
context
. In addition to the build in parameters, the
framework provides dedicated caching related metadata such as the argument names. The
next table lists the items made available to the context so one can use them for key and
conditional (see next section) computations:
Table 29.1. Cache SpEL available metadata
Name | Location | Description | Example |
---|---|---|---|
methodName | root object | The name of the method being invoked |
|
method | root object | The method being invoked |
|
target | root object | The target object being invoked |
|
targetClass | root object | The class of the target being invoked |
|
args | root object | The arguments (as array) used for invoking the target |
|
caches | root object | Collection of caches against which the current method is executed |
|
argument name | evaluation context | Name of any of the method argument. If for some reason the names are not available
(ex: no debug information), the argument names are also available under the |
|
result | evaluation context | The result of the method call (the value to be cached). Only available in ' |
|
For cases where the cache needs to be updated without interfering with the method
execution, one can use the @CachePut
annotation. That is, the method will always be
executed and its result placed into the cache (according to the @CachePut
options). It
supports the same options as @Cacheable
and should be used for cache population rather
then method flow optimization.
Note that using @CachePut
and @Cacheable
annotations on the same method is generally
discouraged because they have different behaviors. While the latter causes the method
execution to be skipped by using the cache, the former forces the execution in order to
execute a cache update. This leads to unexpected behavior and with the exception of
specific corner-cases (such as annotations having conditions that exclude them from each
other), such declarations should be avoided.
The cache abstraction allows not just population of a cache store but also eviction.
This process is useful for removing stale or unused data from the cache. Opposed to
@Cacheable
, annotation @CacheEvict
demarcates methods that perform cache
eviction, that is methods that act as triggers for removing data from the cache.
Just like its sibling, @CacheEvict
requires specifying one (or multiple) caches
that are affected by the action, allows a key or a condition to be specified but in
addition, features an extra parameter allEntries
which indicates whether a cache-wide
eviction needs to be performed rather then just an entry one (based on the key):
@CacheEvict(value = "books", allEntries=true) public void loadBooks(InputStream batch)
This option comes in handy when an entire cache region needs to be cleared out - rather then evicting each entry (which would take a long time since it is inefficient), all the entires are removed in one operation as shown above. Note that the framework will ignore any key specified in this scenario as it does not apply (the entire cache is evicted not just one entry).
One can also indicate whether the eviction should occur after (the default) or before
the method executes through the beforeInvocation
attribute. The former provides the
same semantics as the rest of the annotations - once the method completes successfully,
an action (in this case eviction) on the cache is executed. If the method does not
execute (as it might be cached) or an exception is thrown, the eviction does not occur.
The latter ( beforeInvocation=true
) causes the eviction to occur always, before the
method is invoked - this is useful in cases where the eviction does not need to be tied
to the method outcome.
It is important to note that void methods can be used with @CacheEvict
- as the
methods act as triggers, the return values are ignored (as they don’t interact with the
cache) - this is not the case with @Cacheable
which adds/updates data into the cache
and thus requires a result.
There are cases when multiple annotations of the same type, such as @CacheEvict
or
@CachePut
need to be specified, for example because the condition or the key
expression is different between different caches. Unfortunately Java does not support
such declarations however there is a workaround - using an enclosing annotation, in
this case, @Caching
. @Caching
allows multiple nested @Cacheable
, @CachePut
and
@CacheEvict
to be used on the same method:
@Caching(evict = { @CacheEvict("primary"), @CacheEvict(value = "secondary", key = "#p0") }) public Book importBooks(String deposit, Date date)
It is important to note that even though declaring the cache annotations does not automatically trigger their actions - like many things in Spring, the feature has to be declaratively enabled (which means if you ever suspect caching is to blame, you can disable it by removing only one configuration line rather then all the annotations in your code).
To enable caching annotations add the annotation @EnableCaching
to one of your
@Configuration
classes:
@Configuration @EnableCaching public class AppConfig { }
Alternatively for XML configuration use the cache:annotation-driven
element:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:cache="http://www.springframework.org/schema/cache" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/cache http://www.springframework.org/schema/cache/spring-cache.xsd"> <cache:annotation-driven /> </beans>
Both the cache:annotation-driven
element and @EnableCaching
annotation allow various
options to be specified that influence the way the caching behavior is added to the
application through AOP. The configuration is intentionally similar with that of
@Transactional
:
Table 29.2. Cache annotation settings
XML Attribute | Annotation Attribute | Default | Description |
---|---|---|---|
| N/A (See | cacheManager | Name of cache manager to use. Only required if the name of the cache manager is not
|
|
| proxy | The default mode "proxy" processes annotated beans to be proxied using Spring’s AOP framework (following proxy semantics, as discussed above, applying to method calls coming in through the proxy only). The alternative mode "aspectj" instead weaves the affected classes with Spring’s AspectJ caching aspect, modifying the target class byte code to apply to any kind of method call. AspectJ weaving requires spring-aspects.jar in the classpath as well as load-time weaving (or compile-time weaving) enabled. (See the section called “Spring configuration” for details on how to set up load-time weaving.) |
|
| false | Applies to proxy mode only. Controls what type of caching proxies are created for
classes annotated with the |
|
| Ordered.LOWEST_PRECEDENCE | Defines the order of the cache advice that is applied to beans annotated with
|
Note | |
---|---|
|
Tip | |
---|---|
Spring recommends that you only annotate concrete classes (and methods of concrete
classes) with the |
Note | |
---|---|
In proxy mode (which is the default), only external method calls coming in through the
proxy are intercepted. This means that self-invocation, in effect, a method within the
target object calling another method of the target object, will not lead to an actual
caching at runtime even if the invoked method is marked with |
The caching abstraction allows you to use your own annotations to identify what method
trigger cache population or eviction. This is quite handy as a template mechanism as it
eliminates the need to duplicate cache annotation declarations (especially useful if the
key or condition are specified) or if the foreign imports (org.springframework
) are
not allowed in your code base. Similar to the rest of the
stereotype annotations, both @Cacheable
and
@CacheEvict
can be used as meta-annotations, that is
annotations that can annotate other annotations. To wit, let us replace a common
@Cacheable
declaration with our own, custom annotation:
@Retention(RetentionPolicy.RUNTIME) @Target({ElementType.METHOD}) @Cacheable(value="books", key="#isbn") public @interface SlowService { }
Above, we have defined our own SlowService
annotation which itself is annotated with
@Cacheable
- now we can replace the following code:
@Cacheable(value="books", key="#isbn") public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)
with:
@SlowService public Book findBook(ISBN isbn, boolean checkWarehouse, boolean includeUsed)
Even though @SlowService
is not a Spring annotation, the container automatically picks
up its declaration at runtime and understands its meaning. Note that as mentioned
above, the annotation-driven behavior needs to be enabled.
If annotations are not an option (no access to the sources or no external code), one can use XML for declarative caching. So instead of annotating the methods for caching, one specifies the target method and the caching directives externally (similar to the declarative transaction management advice). The previous example can be translated into:
<!-- the service we want to make cacheable --> <bean id="bookService" class="x.y.service.DefaultBookService"/> <!-- cache definitions --> <cache:advice id="cacheAdvice" cache-manager="cacheManager"> <cache:caching cache="books"> <cache:cacheable method="findBook" key="#isbn"/> <cache:cache-evict method="loadBooks" all-entries="true"/> </cache:caching> </cache:advice> <!-- apply the cacheable behavior to all BookService interfaces --> <aop:config> <aop:advisor advice-ref="cacheAdvice" pointcut="execution(* x.y.BookService.*(..))"/> </aop:config> <!-- cache manager definition omitted -->
In the configuration above, the bookService
is made cacheable. The caching semantics
to apply are encapsulated in the cache:advice
definition which instructs method
findBooks
to be used for putting data into the cache while method loadBooks
for
evicting data. Both definitions are working against the books
cache.
The aop:config
definition applies the cache advice to the appropriate points in the
program by using the AspectJ pointcut expression (more information is available in
Chapter 8, Aspect Oriented Programming with Spring). In the example above, all methods from the BookService
are considered and
the cache advice applied to them.
The declarative XML caching supports all of the annotation-based model so moving between
the two should be fairly easy - further more both can be used inside the same
application. The XML based approach does not touch the target code however it is
inherently more verbose; when dealing with classes with overloaded methods that are
targeted for caching, identifying the proper methods does take an extra effort since the
method
argument is not a good discriminator - in these cases, the AspectJ pointcut can
be used to cherry pick the target methods and apply the appropriate caching
functionality. However through XML, it is easier to apply a package/group/interface-wide
caching (again due to the AspectJ pointcut) and to create template-like definitions (as
we did in the example above by defining the target cache through the cache:definitions
cache
attribute).
Out of the box, the cache abstraction provides integration with two storages - one on
top of the JDK ConcurrentMap
and one for EhCache library. To use
them, one needs to simply declare an appropriate CacheManager
- an entity that
controls and manages Cache
s and can be used to retrieve these for storage.
The JDK-based Cache
implementation resides under
org.springframework.cache.concurrent
package. It allows one to use ConcurrentHashMap
as a backing Cache
store.
<!-- generic cache manager --> <bean id="cacheManager" class="org.springframework.cache.support.SimpleCacheManager"> <property name="caches"> <set> <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="default"/> <bean class="org.springframework.cache.concurrent.ConcurrentMapCacheFactoryBean" p:name="books"/> </set> </property> </bean>
The snippet above uses the SimpleCacheManager
to create a CacheManager
for the two
nested ConcurrentMapCache
instances named default and books. Note that the
names are configured directly for each cache.
As the cache is created by the application, it is bound to its lifecycle, making it suitable for basic use cases, tests or simple applications. The cache scales well and is very fast but it does not provide any management or persistence capabilities nor eviction contracts.
The EhCache implementation is located under org.springframework.cache.ehcache
package.
Again, to use it, one simply needs to declare the appropriate CacheManager
:
<bean id="cacheManager" class="org.springframework.cache.ehcache.EhCacheCacheManager" p:cache-manager-ref="ehcache"/> <!-- EhCache library setup --> <bean id="ehcache" class="org.springframework.cache.ehcache.EhCacheManagerFactoryBean" p:config-location="ehcache.xml"/>
This setup bootstraps ehcache library inside Spring IoC (through bean ehcache
) which
is then wired into the dedicated CacheManager
implementation. Note the entire
ehcache-specific configuration is read from the resource ehcache.xml
.
GemFire is a memory-oriented/disk-backed, elastically scalable, continuously available, active (with built-in pattern-based subscription notifications), globally replicated database and provides fully-featured edge caching. For further information on how to use GemFire as a CacheManager (and more), please refer to the Spring GemFire reference documentation.
Sometimes when switching environments or doing testing, one might have cache declarations without an actual backing cache configured. As this is an invalid configuration, at runtime an exception will be thrown since the caching infrastructure is unable to find a suitable store. In situations like this, rather then removing the cache declarations (which can prove tedious), one can wire in a simple, dummy cache that performs no caching - that is, forces the cached methods to be executed every time:
<bean id="cacheManager" class="org.springframework.cache.support.CompositeCacheManager"> <property name="cacheManagers"> <list> <ref bean="jdkCache"/> <ref bean="gemfireCache"/> </list> </property> <property name="fallbackToNoOpCache" value="true"/> </bean>
The CompositeCacheManager
above chains multiple CacheManager
s and additionally,
through the fallbackToNoOpCache
flag, adds a no op cache that for all the
definitions not handled by the configured cache managers. That is, every cache
definition not found in either jdkCache
or gemfireCache
(configured above) will be
handled by the no op cache, which will not store any information causing the target
method to be executed every time.
Clearly there are plenty of caching products out there that can be used as a backing
store. To plug them in, one needs to provide a CacheManager
and Cache
implementation
since unfortunately there is no available standard that we can use instead. This may
sound harder then it is since in practice, the classes tend to be simple
adapters that map the caching abstraction
framework on top of the storage API as the ehcache
classes can show. Most
CacheManager
classes can use the classes in org.springframework.cache.support
package, such as AbstractCacheManager
which takes care of the boiler-plate code
leaving only the actual mapping to be completed. We hope that in time, the libraries
that provide integration with Spring can fill in this small configuration gap.
Directly through your cache provider. The cache abstraction is… well, an abstraction
not a cache implementation. The solution you are using might support various data
policies and different topologies which other solutions do not (take for example the JDK
ConcurrentHashMap
) - exposing that in the cache abstraction would be useless simply
because there would no backing support. Such functionality should be controlled directly
through the backing cache, when configuring it or through its native API.
Migration guides for upgrading from previous releases of the Spring Framework are now provided as a Wiki page. For details please refer to https://github.com/spring-projects/spring-framework/wiki/Migrating-from-earlier-versions-of-the-spring-framework
This appendix discusses some classic Spring usage patterns as a reference for developers maintaining legacy Spring applications. These usage patterns no longer reflect the recommended way of using these features and the current recommended usage is covered in the respective sections of the reference manual.
This section documents the classic usage patterns that you might encounter in a legacy Spring application. For the currently recommended usage patterns, please refer to the Chapter 14, Object Relational Mapping (ORM) Data Access chapter.
For the currently recommended usage patterns for Hibernate see Section 14.3, “Hibernate”
The basic programming model for templating looks as follows, for methods that can be
part of any custom data access object or business service. There are no restrictions on
the implementation of the surrounding object at all, it just needs to provide a
Hibernate SessionFactory
. It can get the latter from anywhere, but preferably as bean
reference from a Spring IoC container - via a simple setSessionFactory(..)
bean
property setter. The following snippets show a DAO definition in a Spring container,
referencing the above defined SessionFactory
, and an example for a DAO method
implementation.
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="sessionFactory" ref="mySessionFactory"/> </bean> </beans>
public class ProductDaoImpl implements ProductDao { private HibernateTemplate hibernateTemplate; public void setSessionFactory(SessionFactory sessionFactory) { this.hibernateTemplate = new HibernateTemplate(sessionFactory); } public Collection loadProductsByCategory(String category) throws DataAccessException { return this.hibernateTemplate.find("from test.Product product where product.category=?", category); } }
The HibernateTemplate
class provides many methods that mirror the methods exposed on
the Hibernate Session
interface, in addition to a number of convenience methods such
as the one shown above. If you need access to the Session
to invoke methods that are
not exposed on the HibernateTemplate
, you can always drop down to a callback-based
approach like so.
public class ProductDaoImpl implements ProductDao { private HibernateTemplate hibernateTemplate; public void setSessionFactory(SessionFactory sessionFactory) { this.hibernateTemplate = new HibernateTemplate(sessionFactory); } public Collection loadProductsByCategory(final String category) throws DataAccessException { return this.hibernateTemplate.execute(new HibernateCallback() { public Object doInHibernate(Session session) { Criteria criteria = session.createCriteria(Product.class); criteria.add(Expression.eq("category", category)); criteria.setMaxResults(6); return criteria.list(); } }; } }
A callback implementation effectively can be used for any Hibernate data access.
HibernateTemplate
will ensure that Session
instances are properly opened and closed,
and automatically participate in transactions. The template instances are thread-safe
and reusable, they can thus be kept as instance variables of the surrounding class. For
simple single step actions like a single find, load, saveOrUpdate, or delete call,
HibernateTemplate
offers alternative convenience methods that can replace such one
line callback implementations. Furthermore, Spring provides a convenient
HibernateDaoSupport
base class that provides a setSessionFactory(..)
method for
receiving a SessionFactory
, and getSessionFactory()
and getHibernateTemplate()
for
use by subclasses. In combination, this allows for very simple DAO implementations for
typical requirements:
public class ProductDaoImpl extends HibernateDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { return this.getHibernateTemplate().find( "from test.Product product where product.category=?", category); } }
As alternative to using Spring’s HibernateTemplate
to implement DAOs, data access code
can also be written in a more traditional fashion, without wrapping the Hibernate access
code in a callback, while still respecting and participating in Spring’s generic
DataAccessException
hierarchy. The HibernateDaoSupport
base class offers methods to
access the current transactional Session
and to convert exceptions in such a scenario;
similar methods are also available as static helpers on the SessionFactoryUtils
class.
Note that such code will usually pass ' false
' as the value of the getSession(..)
methods ' allowCreate
' argument, to enforce running within a transaction (which avoids
the need to close the returned Session
, as its lifecycle is managed by the
transaction).
public class HibernateProductDao extends HibernateDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException, MyException { Session session = getSession(false); try { Query query = session.createQuery("from test.Product product where product.category=?"); query.setString(0, category); List result = query.list(); if (result == null) { throw new MyException("No search results."); } return result; } catch (HibernateException ex) { throw convertHibernateAccessException(ex); } } }
The advantage of such direct Hibernate access code is that it allows any checked
application exception to be thrown within the data access code; contrast this to the
HibernateTemplate
class which is restricted to throwing only unchecked exceptions
within the callback. Note that you can often defer the corresponding checks and the
throwing of application exceptions to after the callback, which still allows working
with HibernateTemplate
. In general, the HibernateTemplate
class' convenience methods
are simpler and more convenient for many scenarios.
For the currently recommended usage patterns for JDO see Section 14.4, “JDO”
Each JDO-based DAO will then receive the PersistenceManagerFactory
through dependency
injection. Such a DAO could be coded against plain JDO API, working with the given
PersistenceManagerFactory
, but will usually rather be used with the Spring Framework’s
JdoTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="persistenceManagerFactory" ref="myPmf"/> </bean> </beans>
public class ProductDaoImpl implements ProductDao { private JdoTemplate jdoTemplate; public void setPersistenceManagerFactory(PersistenceManagerFactory pmf) { this.jdoTemplate = new JdoTemplate(pmf); } public Collection loadProductsByCategory(final String category) throws DataAccessException { return (Collection) this.jdoTemplate.execute(new JdoCallback() { public Object doInJdo(PersistenceManager pm) throws JDOException { Query query = pm.newQuery(Product.class, "category = pCategory"); query.declareParameters("String pCategory"); List result = query.execute(category); // do some further stuff with the result list return result; } }); } }
A callback implementation can effectively be used for any JDO data access. JdoTemplate
will ensure that PersistenceManager
s are properly opened and closed, and
automatically participate in transactions. The template instances are thread-safe and
reusable, they can thus be kept as instance variables of the surrounding class. For
simple single-step actions such as a single find
, load
, makePersistent
, or
delete
call, JdoTemplate
offers alternative convenience methods that can replace
such one line callback implementations. Furthermore, Spring provides a convenient
JdoDaoSupport
base class that provides a setPersistenceManagerFactory(..)
method for
receiving a PersistenceManagerFactory
, and getPersistenceManagerFactory()
and
getJdoTemplate()
for use by subclasses. In combination, this allows for very simple
DAO implementations for typical requirements:
public class ProductDaoImpl extends JdoDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { return getJdoTemplate().find(Product.class, "category = pCategory", "String category", new Object[] {category}); } }
As alternative to working with Spring’s JdoTemplate
, you can also code Spring-based
DAOs at the JDO API level, explicitly opening and closing a PersistenceManager
. As
elaborated in the corresponding Hibernate section, the main advantage of this approach
is that your data access code is able to throw checked exceptions. JdoDaoSupport
offers a variety of support methods for this scenario, for fetching and releasing a
transactional PersistenceManager
as well as for converting exceptions.
For the currently recommended usage patterns for JPA see Section 14.5, “JPA”
Each JPA-based DAO will then receive a EntityManagerFactory
via dependency injection.
Such a DAO can be coded against plain JPA and work with the given EntityManagerFactory
or through Spring’s JpaTemplate
:
<beans> <bean id="myProductDao" class="product.ProductDaoImpl"> <property name="entityManagerFactory" ref="myEmf"/> </bean> </beans>
public class JpaProductDao implements ProductDao { private JpaTemplate jpaTemplate; public void setEntityManagerFactory(EntityManagerFactory emf) { this.jpaTemplate = new JpaTemplate(emf); } public Collection loadProductsByCategory(final String category) throws DataAccessException { return (Collection) this.jpaTemplate.execute(new JpaCallback() { public Object doInJpa(EntityManager em) throws PersistenceException { Query query = em.createQuery("from Product as p where p.category = :category"); query.setParameter("category", category); List result = query.getResultList(); // do some further processing with the result list return result; } }); } }
The JpaCallback
implementation allows any type of JPA data access. The JpaTemplate
will ensure that EntityManager
s are properly opened and closed and automatically
participate in transactions. Moreover, the JpaTemplate
properly handles exceptions,
making sure resources are cleaned up and the appropriate transactions rolled back. The
template instances are thread-safe and reusable and they can be kept as instance
variable of the enclosing class. Note that JpaTemplate
offers single-step actions such
as find, load, merge, etc along with alternative convenience methods that can replace
one line callback implementations.
Furthermore, Spring provides a convenient JpaDaoSupport
base class that provides the
get/setEntityManagerFactory
and getJpaTemplate()
to be used by subclasses:
public class ProductDaoImpl extends JpaDaoSupport implements ProductDao { public Collection loadProductsByCategory(String category) throws DataAccessException { Map<String, String> params = new HashMap<String, String>(); params.put("category", category); return getJpaTemplate().findByNamedParams("from Product as p where p.category = :category", params); } }
Besides working with Spring’s JpaTemplate
, one can also code Spring-based DAOs against
the JPA, doing one’s own explicit EntityManager
handling. As also elaborated in the
corresponding Hibernate section, the main advantage of this approach is that your data
access code is able to throw checked exceptions. JpaDaoSupport
offers a variety of
support methods for this scenario, for retrieving and releasing a transaction
EntityManager
, as well as for converting exceptions.
JpaTemplate mainly exists as a sibling of JdoTemplate and HibernateTemplate, offering the same style for people used to it.
One of the benefits of Spring’s JMS support is to shield the user from differences between the JMS 1.0.2 and 1.1 APIs. (For a description of the differences between the two APIs see sidebar on Domain Unification). Since it is now common to encounter only the JMS 1.1 API the use of classes that are based on the JMS 1.0.2 API has been deprecated in Spring 3.0. This section describes Spring JMS support for the JMS 1.0.2 deprecated classes.
Located in the package org.springframework.jms.core
the class JmsTemplate102
provides all of the features of the JmsTemplate
described the JMS chapter, but is
based on the JMS 1.0.2 API instead of the JMS 1.1 API. As a consequence, if you are
using JmsTemplate102 you need to set the boolean property pubSubDomain
to configure
the JmsTemplate
with knowledge of what JMS domain is being used. By default the value
of this property is false, indicating that the point-to-point domain, Queues, will be
used.
MessageListenerAdapter’s are used in
conjunction with Spring’s message listener containers to support
asynchronous message reception by exposing almost any class as a Message-driven POJO. If
you are using the JMS 1.0.2 API, you will want to use the 1.0.2 specific classes such as
MessageListenerAdapter102
, SimpleMessageListenerContainer102
, and
DefaultMessageListenerContainer102
. These classes provide the same functionality as
the JMS 1.1 based counterparts but rely only on the JMS 1.0.2 API.
The ConnectionFactory
interface is part of the JMS specification and serves as the
entry point for working with JMS. Spring provides an implementation of the
ConnectionFactory
interface, SingleConnectionFactory102
, based on the JMS 1.0.2 API
that will return the same Connection
on all createConnection()
calls and ignore
calls to close()
. You will need to set the boolean property pubSubDomain
to indicate
which messaging domain is used as SingleConnectionFactory102
will always explicitly
differentiate between a javax.jms.QueueConnection
and a javax.jmsTopicConnection
.
In a JMS 1.0.2 environment the class JmsTransactionManager102
provides support for
managing JMS transactions for a single Connection Factory. Please refer to the reference
documentation on JMS Transaction Management for more information on this
functionality.
In this appendix we discuss the lower-level Spring AOP APIs and the AOP support used in Spring 1.2 applications. For new applications, we recommend the use of the Spring 2.0 AOP support described in the AOP chapter, but when working with existing applications, or when reading books and articles, you may come across Spring 1.2 style examples. Spring 2.0 is fully backwards compatible with Spring 1.2 and everything described in this appendix is fully supported in Spring 2.0.
Let’s look at how Spring handles the crucial pointcut concept.
Spring’s pointcut model enables pointcut reuse independent of advice types. It’s possible to target different advice using the same pointcut.
The org.springframework.aop.Pointcut
interface is the central interface, used to
target advices to particular classes and methods. The complete interface is shown below:
public interface Pointcut { ClassFilter getClassFilter(); MethodMatcher getMethodMatcher(); }
Splitting the Pointcut
interface into two parts allows reuse of class and method
matching parts, and fine-grained composition operations (such as performing a "union"
with another method matcher).
The ClassFilter
interface is used to restrict the pointcut to a given set of target
classes. If the matches()
method always returns true, all target classes will be
matched:
public interface ClassFilter { boolean matches(Class clazz); }
The MethodMatcher
interface is normally more important. The complete interface is
shown below:
public interface MethodMatcher { boolean matches(Method m, Class targetClass); boolean isRuntime(); boolean matches(Method m, Class targetClass, Object[] args); }
The matches(Method, Class)
method is used to test whether this pointcut will ever
match a given method on a target class. This evaluation can be performed when an AOP
proxy is created, to avoid the need for a test on every method invocation. If the
2-argument matches method returns true for a given method, and the isRuntime()
method
for the MethodMatcher returns true, the 3-argument matches method will be invoked on
every method invocation. This enables a pointcut to look at the arguments passed to the
method invocation immediately before the target advice is to execute.
Most MethodMatchers are static, meaning that their isRuntime()
method returns false.
In this case, the 3-argument matches method will never be invoked.
Tip | |
---|---|
If possible, try to make pointcuts static, allowing the AOP framework to cache the results of pointcut evaluation when an AOP proxy is created. |
Spring supports operations on pointcuts: notably, union and intersection.
Since 2.0, the most important type of pointcut used by Spring is
org.springframework.aop.aspectj.AspectJExpressionPointcut
. This is a pointcut that
uses an AspectJ supplied library to parse an AspectJ pointcut expression string.
See the previous chapter for a discussion of supported AspectJ pointcut primitives.
Spring provides several convenient pointcut implementations. Some can be used out of the box; others are intended to be subclassed in application-specific pointcuts.
Static pointcuts are based on method and target class, and cannot take into account the method’s arguments. Static pointcuts are sufficient - and best - for most usages. It’s possible for Spring to evaluate a static pointcut only once, when a method is first invoked: after that, there is no need to evaluate the pointcut again with each method invocation.
Let’s consider some static pointcut implementations included with Spring.
One obvious way to specify static pointcuts is regular expressions. Several AOP
frameworks besides Spring make this possible.
org.springframework.aop.support.Perl5RegexpMethodPointcut
is a generic regular
expression pointcut, using Perl 5 regular expression syntax. The
Perl5RegexpMethodPointcut
class depends on Jakarta ORO for regular expression
matching. Spring also provides the JdkRegexpMethodPointcut
class that uses the regular
expression support in JDK 1.4+.
Using the Perl5RegexpMethodPointcut
class, you can provide a list of pattern Strings.
If any of these is a match, the pointcut will evaluate to true. (So the result is
effectively the union of these pointcuts.)
The usage is shown below:
<bean id="settersAndAbsquatulatePointcut" class="org.springframework.aop.support.Perl5RegexpMethodPointcut"> <property name="patterns"> <list> <value>.set.</value> <value>.*absquatulate</value> </list> </property> </bean>
Spring provides a convenience class, RegexpMethodPointcutAdvisor
, that allows us to
also reference an Advice (remember that an Advice can be an interceptor, before advice,
throws advice etc.). Behind the scenes, Spring will use a JdkRegexpMethodPointcut
.
Using RegexpMethodPointcutAdvisor
simplifies wiring, as the one bean encapsulates both
pointcut and advice, as shown below:
<bean id="settersAndAbsquatulateAdvisor" class="org.springframework.aop.support.RegexpMethodPointcutAdvisor"> <property name="advice"> <ref bean="beanNameOfAopAllianceInterceptor"/> </property> <property name="patterns"> <list> <value>.set.</value> <value>.*absquatulate</value> </list> </property> </bean>
RegexpMethodPointcutAdvisor can be used with any Advice type.
Dynamic pointcuts are costlier to evaluate than static pointcuts. They take into account methodarguments, as well as static information. This means that they must be evaluated with every method invocation; the result cannot be cached, as arguments will vary.
The main example is the control flow
pointcut.
Spring control flow pointcuts are conceptually similar to AspectJ cflow pointcuts,
although less powerful. (There is currently no way to specify that a pointcut executes
below a join point matched by another pointcut.) A control flow pointcut matches the
current call stack. For example, it might fire if the join point was invoked by a method
in the com.mycompany.web
package, or by the SomeCaller
class. Control flow pointcuts
are specified using the org.springframework.aop.support.ControlFlowPointcut
class.
Note | |
---|---|
Control flow pointcuts are significantly more expensive to evaluate at runtime than even other dynamic pointcuts. In Java 1.4, the cost is about 5 times that of other dynamic pointcuts. |
Spring provides useful pointcut superclasses to help you to implement your own pointcuts.
Because static pointcuts are most useful, you’ll probably subclass StaticMethodMatcherPointcut, as shown below. This requires implementing just one abstract method (although it’s possible to override other methods to customize behavior):
class TestStaticPointcut extends StaticMethodMatcherPointcut { public boolean matches(Method m, Class targetClass) { // return true if custom criteria match } }
There are also superclasses for dynamic pointcuts.
You can use custom pointcuts with any advice type in Spring 1.0 RC2 and above.
Because pointcuts in Spring AOP are Java classes, rather than language features (as in AspectJ) it’s possible to declare custom pointcuts, whether static or dynamic. Custom pointcuts in Spring can be arbitrarily complex. However, using the AspectJ pointcut expression language is recommended if possible.
Note | |
---|---|
Later versions of Spring may offer support for "semantic pointcuts" as offered by JAC: for example, "all methods that change instance variables in the target object." |
Let’s now look at how Spring AOP handles advice.
Each advice is a Spring bean. An advice instance can be shared across all advised objects, or unique to each advised object. This corresponds to per-class or per-instance advice.
Per-class advice is used most often. It is appropriate for generic advice such as transaction advisors. These do not depend on the state of the proxied object or add new state; they merely act on the method and arguments.
Per-instance advice is appropriate for introductions, to support mixins. In this case, the advice adds state to the proxied object.
It’s possible to use a mix of shared and per-instance advice in the same AOP proxy.
Spring provides several advice types out of the box, and is extensible to support arbitrary advice types. Let us look at the basic concepts and standard advice types.
The most fundamental advice type in Spring is interception around advice.
Spring is compliant with the AOP Alliance interface for around advice using method interception. MethodInterceptors implementing around advice should implement the following interface:
public interface MethodInterceptor extends Interceptor { Object invoke(MethodInvocation invocation) throws Throwable; }
The MethodInvocation
argument to the invoke()
method exposes the method being
invoked; the target join point; the AOP proxy; and the arguments to the method. The
invoke()
method should return the invocation’s result: the return value of the join
point.
A simple MethodInterceptor
implementation looks as follows:
public class DebugInterceptor implements MethodInterceptor { public Object invoke(MethodInvocation invocation) throws Throwable { System.out.println("Before: invocation=[" + invocation + "]"); Object rval = invocation.proceed(); System.out.println("Invocation returned"); return rval; } }
Note the call to the MethodInvocation’s proceed()
method. This proceeds down the
interceptor chain towards the join point. Most interceptors will invoke this method, and
return its return value. However, a MethodInterceptor, like any around advice, can
return a different value or throw an exception rather than invoke the proceed method.
However, you don’t want to do this without good reason!
Note | |
---|---|
MethodInterceptors offer interoperability with other AOP Alliance-compliant AOP implementations. The other advice types discussed in the remainder of this section implement common AOP concepts, but in a Spring-specific way. While there is an advantage in using the most specific advice type, stick with MethodInterceptor around advice if you are likely to want to run the aspect in another AOP framework. Note that pointcuts are not currently interoperable between frameworks, and the AOP Alliance does not currently define pointcut interfaces. |
A simpler advice type is a before advice. This does not need a MethodInvocation
object, since it will only be called before entering the method.
The main advantage of a before advice is that there is no need to invoke the proceed()
method, and therefore no possibility of inadvertently failing to proceed down the
interceptor chain.
The MethodBeforeAdvice
interface is shown below. (Spring’s API design would allow for
field before advice, although the usual objects apply to field interception and it’s
unlikely that Spring will ever implement it).
public interface MethodBeforeAdvice extends BeforeAdvice { void before(Method m, Object[] args, Object target) throws Throwable; }
Note the return type is void
. Before advice can insert custom behavior before the join
point executes, but cannot change the return value. If a before advice throws an
exception, this will abort further execution of the interceptor chain. The exception
will propagate back up the interceptor chain. If it is unchecked, or on the signature of
the invoked method, it will be passed directly to the client; otherwise it will be
wrapped in an unchecked exception by the AOP proxy.
An example of a before advice in Spring, which counts all method invocations:
public class CountingBeforeAdvice implements MethodBeforeAdvice { private int count; public void before(Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
Tip | |
---|---|
Before advice can be used with any pointcut. |
Throws advice is invoked after the return of the join point if the join point threw
an exception. Spring offers typed throws advice. Note that this means that the
org.springframework.aop.ThrowsAdvice
interface does not contain any methods: It is a
tag interface identifying that the given object implements one or more typed throws
advice methods. These should be in the form of:
afterThrowing([Method, args, target], subclassOfThrowable)
Only the last argument is required. The method signatures may have either one or four arguments, depending on whether the advice method is interested in the method and arguments. The following classes are examples of throws advice.
The advice below is invoked if a RemoteException
is thrown (including subclasses):
public class RemoteThrowsAdvice implements ThrowsAdvice { public void afterThrowing(RemoteException ex) throws Throwable { // Do something with remote exception } }
The following advice is invoked if a ServletException
is thrown. Unlike the above
advice, it declares 4 arguments, so that it has access to the invoked method, method
arguments and target object:
public class ServletThrowsAdviceWithArguments implements ThrowsAdvice { public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) { // Do something with all arguments } }
The final example illustrates how these two methods could be used in a single class,
which handles both RemoteException
and ServletException
. Any number of throws advice
methods can be combined in a single class.
public static class CombinedThrowsAdvice implements ThrowsAdvice { public void afterThrowing(RemoteException ex) throws Throwable { // Do something with remote exception } public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) { // Do something with all arguments } }
Note: If a throws-advice method throws an exception itself, it will override the original exception (i.e. change the exception thrown to the user). The overriding exception will typically be a RuntimeException; this is compatible with any method signature. However, if a throws-advice method throws a checked exception, it will have to match the declared exceptions of the target method and is hence to some degree coupled to specific target method signatures. Do not throw an undeclared checked exception that is incompatible with the target method’s signature!
Tip | |
---|---|
Throws advice can be used with any pointcut. |
An after returning advice in Spring must implement the org.springframework.aop.AfterReturningAdvice interface, shown below:
public interface AfterReturningAdvice extends Advice { void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable; }
An after returning advice has access to the return value (which it cannot modify), invoked method, methods arguments and target.
The following after returning advice counts all successful method invocations that have not thrown exceptions:
public class CountingAfterReturningAdvice implements AfterReturningAdvice { private int count; public void afterReturning(Object returnValue, Method m, Object[] args, Object target) throws Throwable { ++count; } public int getCount() { return count; } }
This advice doesn’t change the execution path. If it throws an exception, this will be thrown up the interceptor chain instead of the return value.
Tip | |
---|---|
After returning advice can be used with any pointcut. |
Spring treats introduction advice as a special kind of interception advice.
Introduction requires an IntroductionAdvisor
, and an IntroductionInterceptor
,
implementing the following interface:
public interface IntroductionInterceptor extends MethodInterceptor { boolean implementsInterface(Class intf); }
The invoke()
method inherited from the AOP Alliance MethodInterceptor
interface must
implement the introduction: that is, if the invoked method is on an introduced
interface, the introduction interceptor is responsible for handling the method call - it
cannot invoke proceed()
.
Introduction advice cannot be used with any pointcut, as it applies only at class,
rather than method, level. You can only use introduction advice with the
IntroductionAdvisor
, which has the following methods:
public interface IntroductionAdvisor extends Advisor, IntroductionInfo { ClassFilter getClassFilter(); void validateInterfaces() throws IllegalArgumentException; } public interface IntroductionInfo { Class[] getInterfaces(); }
There is no MethodMatcher
, and hence no Pointcut
, associated with introduction
advice. Only class filtering is logical.
The getInterfaces()
method returns the interfaces introduced by this advisor.
The validateInterfaces()
method is used internally to see whether or not the
introduced interfaces can be implemented by the configured IntroductionInterceptor
.
Let’s look at a simple example from the Spring test suite. Let’s suppose we want to introduce the following interface to one or more objects:
public interface Lockable { void lock(); void unlock(); boolean locked(); }
This illustrates a mixin. We want to be able to cast advised objects to Lockable,
whatever their type, and call lock and unlock methods. If we call the lock() method, we
want all setter methods to throw a LockedException
. Thus we can add an aspect that
provides the ability to make objects immutable, without them having any knowledge of it:
a good example of AOP.
Firstly, we’ll need an IntroductionInterceptor
that does the heavy lifting. In this
case, we extend the org.springframework.aop.support.DelegatingIntroductionInterceptor
convenience class. We could implement IntroductionInterceptor directly, but using
DelegatingIntroductionInterceptor
is best for most cases.
The DelegatingIntroductionInterceptor
is designed to delegate an introduction to an
actual implementation of the introduced interface(s), concealing the use of interception
to do so. The delegate can be set to any object using a constructor argument; the
default delegate (when the no-arg constructor is used) is this. Thus in the example
below, the delegate is the LockMixin
subclass of DelegatingIntroductionInterceptor
.
Given a delegate (by default itself), a DelegatingIntroductionInterceptor
instance
looks for all interfaces implemented by the delegate (other than
IntroductionInterceptor), and will support introductions against any of them. It’s
possible for subclasses such as LockMixin
to call the suppressInterface(Class intf)
method to suppress interfaces that should not be exposed. However, no matter how many
interfaces an IntroductionInterceptor
is prepared to support, the
IntroductionAdvisor
used will control which interfaces are actually exposed. An
introduced interface will conceal any implementation of the same interface by the target.
Thus LockMixin subclasses DelegatingIntroductionInterceptor
and implements Lockable
itself. The superclass automatically picks up that Lockable can be supported for
introduction, so we don’t need to specify that. We could introduce any number of
interfaces in this way.
Note the use of the locked
instance variable. This effectively adds additional state
to that held in the target object.
public class LockMixin extends DelegatingIntroductionInterceptor implements Lockable { private boolean locked; public void lock() { this.locked = true; } public void unlock() { this.locked = false; } public boolean locked() { return this.locked; } public Object invoke(MethodInvocation invocation) throws Throwable { if (locked() && invocation.getMethod().getName().indexOf("set") == 0) { throw new LockedException(); } return super.invoke(invocation); } }
Often it isn’t necessary to override the invoke()
method: the
DelegatingIntroductionInterceptor
implementation - which calls the delegate method if
the method is introduced, otherwise proceeds towards the join point - is usually
sufficient. In the present case, we need to add a check: no setter method can be invoked
if in locked mode.
The introduction advisor required is simple. All it needs to do is hold a distinct
LockMixin
instance, and specify the introduced interfaces - in this case, just
Lockable
. A more complex example might take a reference to the introduction
interceptor (which would be defined as a prototype): in this case, there’s no
configuration relevant for a LockMixin
, so we simply create it using new
.
public class LockMixinAdvisor extends DefaultIntroductionAdvisor { public LockMixinAdvisor() { super(new LockMixin(), Lockable.class); } }
We can apply this advisor very simply: it requires no configuration. (However, it is
necessary: It’s impossible to use an IntroductionInterceptor
without an
IntroductionAdvisor.) As usual with introductions, the advisor must be per-instance,
as it is stateful. We need a different instance of LockMixinAdvisor
, and hence
LockMixin
, for each advised object. The advisor comprises part of the advised object’s
state.
We can apply this advisor programmatically, using the Advised.addAdvisor()
method, or
(the recommended way) in XML configuration, like any other advisor. All proxy creation
choices discussed below, including "auto proxy creators," correctly handle introductions
and stateful mixins.
In Spring, an Advisor is an aspect that contains just a single advice object associated with a pointcut expression.
Apart from the special case of introductions, any advisor can be used with any advice.
org.springframework.aop.support.DefaultPointcutAdvisor
is the most commonly used
advisor class. For example, it can be used with a MethodInterceptor
, BeforeAdvice
or
ThrowsAdvice
.
It is possible to mix advisor and advice types in Spring in the same AOP proxy. For example, you could use a interception around advice, throws advice and before advice in one proxy configuration: Spring will automatically create the necessary interceptor chain.
If you’re using the Spring IoC container (an ApplicationContext or BeanFactory) for your business objects - and you should be! - you will want to use one of Spring’s AOP FactoryBeans. (Remember that a factory bean introduces a layer of indirection, enabling it to create objects of a different type.)
Note | |
---|---|
The Spring 2.0 AOP support also uses factory beans under the covers. |
The basic way to create an AOP proxy in Spring is to use the org.springframework.aop.framework.ProxyFactoryBean. This gives complete control over the pointcuts and advice that will apply, and their ordering. However, there are simpler options that are preferable if you don’t need such control.
The ProxyFactoryBean
, like other Spring FactoryBean
implementations, introduces a
level of indirection. If you define a ProxyFactoryBean
with name foo
, what objects
referencing foo
see is not the ProxyFactoryBean
instance itself, but an object
created by the ProxyFactoryBean
's implementation of the getObject()
method. This
method will create an AOP proxy wrapping a target object.
One of the most important benefits of using a ProxyFactoryBean
or another IoC-aware
class to create AOP proxies, is that it means that advices and pointcuts can also be
managed by IoC. This is a powerful feature, enabling certain approaches that are hard to
achieve with other AOP frameworks. For example, an advice may itself reference
application objects (besides the target, which should be available in any AOP
framework), benefiting from all the pluggability provided by Dependency Injection.
In common with most FactoryBean
implementations provided with Spring, the
ProxyFactoryBean
class is itself a JavaBean. Its properties are used to:
Some key properties are inherited from org.springframework.aop.framework.ProxyConfig
(the superclass for all AOP proxy factories in Spring). These key properties include:
proxyTargetClass
: true
if the target class is to be proxied, rather than the
target class' interfaces. If this property value is set to true
, then CGLIB proxies
will be created (but see also below Section 9.5.3, “JDK- and CGLIB-based proxies”).
optimize
: controls whether or not aggressive optimizations are applied to proxies
created via CGLIB. One should not blithely use this setting unless one fully
understands how the relevant AOP proxy handles optimization. This is currently used
only for CGLIB proxies; it has no effect with JDK dynamic proxies.
frozen
: if a proxy configuration is frozen
, then changes to the configuration are
no longer allowed. This is useful both as a slight optimization and for those cases
when you don’t want callers to be able to manipulate the proxy (via the Advised
interface) after the proxy has been created. The default value of this property is
false
, so changes such as adding additional advice are allowed.
exposeProxy
: determines whether or not the current proxy should be exposed in a
ThreadLocal
so that it can be accessed by the target. If a target needs to obtain
the proxy and the exposeProxy
property is set to true
, the target can use the
AopContext.currentProxy()
method.
aopProxyFactory
: the implementation of AopProxyFactory
to use. Offers a way of
customizing whether to use dynamic proxies, CGLIB or any other proxy strategy. The
default implementation will choose dynamic proxies or CGLIB appropriately. There
should be no need to use this property; it is intended to allow the addition of new
proxy types in Spring 1.1.
Other properties specific to ProxyFactoryBean
include:
proxyInterfaces
: array of String interface names. If this isn’t supplied, a CGLIB
proxy for the target class will be used (but see also below Section 9.5.3, “JDK- and CGLIB-based proxies”).
interceptorNames
: String array of Advisor
, interceptor or other advice names to
apply. Ordering is significant, on a first come-first served basis. That is to say
that the first interceptor in the list will be the first to be able to intercept the
invocation.
The names are bean names in the current factory, including bean names from ancestor
factories. You can’t mention bean references here since doing so would result in the
ProxyFactoryBean
ignoring the singleton setting of the advice.
You can append an interceptor name with an asterisk ( *
). This will result in the
application of all advisor beans with names starting with the part before the asterisk
to be applied. An example of using this feature can be found in Section 9.5.6, “Using global advisors”.
getObject()
method is called. Several FactoryBean
implementations offer
such a method. The default value is true
. If you want to use stateful advice - for
example, for stateful mixins - use prototype advices along with a singleton value of
false
.
This section serves as the definitive documentation on how the ProxyFactoryBean
chooses to create one of either a JDK- and CGLIB-based proxy for a particular target
object (that is to be proxied).
Note | |
---|---|
The behavior of the |
If the class of a target object that is to be proxied (hereafter simply referred to as
the target class) doesn’t implement any interfaces, then a CGLIB-based proxy will be
created. This is the easiest scenario, because JDK proxies are interface based, and no
interfaces means JDK proxying isn’t even possible. One simply plugs in the target bean,
and specifies the list of interceptors via the interceptorNames
property. Note that a
CGLIB-based proxy will be created even if the proxyTargetClass
property of the
ProxyFactoryBean
has been set to false
. (Obviously this makes no sense, and is best
removed from the bean definition because it is at best redundant, and at worst
confusing.)
If the target class implements one (or more) interfaces, then the type of proxy that is
created depends on the configuration of the ProxyFactoryBean
.
If the proxyTargetClass
property of the ProxyFactoryBean
has been set to true
,
then a CGLIB-based proxy will be created. This makes sense, and is in keeping with the
principle of least surprise. Even if the proxyInterfaces
property of the
ProxyFactoryBean
has been set to one or more fully qualified interface names, the fact
that the proxyTargetClass
property is set to true
will cause CGLIB-based
proxying to be in effect.
If the proxyInterfaces
property of the ProxyFactoryBean
has been set to one or more
fully qualified interface names, then a JDK-based proxy will be created. The created
proxy will implement all of the interfaces that were specified in the proxyInterfaces
property; if the target class happens to implement a whole lot more interfaces than
those specified in the proxyInterfaces
property, that is all well and good but those
additional interfaces will not be implemented by the returned proxy.
If the proxyInterfaces
property of the ProxyFactoryBean
has not been set, but
the target class does implement one (or more) interfaces, then the
ProxyFactoryBean
will auto-detect the fact that the target class does actually
implement at least one interface, and a JDK-based proxy will be created. The interfaces
that are actually proxied will be all of the interfaces that the target class
implements; in effect, this is the same as simply supplying a list of each and every
interface that the target class implements to the proxyInterfaces
property. However,
it is significantly less work, and less prone to typos.
Let’s look at a simple example of ProxyFactoryBean
in action. This example involves:
<bean id="personTarget" class="com.mycompany.PersonImpl"> <property name="name"><value>Tony</value></property> <property name="age"><value>51</value></property> </bean> <bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty"><value>Custom string property value</value></property> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"> </bean> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces"><value>com.mycompany.Person</value></property> <property name="target"><ref bean="personTarget"/></property> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
Note that the interceptorNames
property takes a list of String: the bean names of the
interceptor or advisors in the current factory. Advisors, interceptors, before, after
returning and throws advice objects can be used. The ordering of advisors is significant.
Note | |
---|---|
You might be wondering why the list doesn’t hold bean references. The reason for this is that if the ProxyFactoryBean’s singleton property is set to false, it must be able to return independent proxy instances. If any of the advisors is itself a prototype, an independent instance would need to be returned, so it’s necessary to be able to obtain an instance of the prototype from the factory; holding a reference isn’t sufficient. |
The "person" bean definition above can be used in place of a Person implementation, as follows:
Person person = (Person) factory.getBean("person");
Other beans in the same IoC context can express a strongly typed dependency on it, as with an ordinary Java object:
<bean id="personUser" class="com.mycompany.PersonUser"> <property name="person"><ref bean="person" /></property> </bean>
The PersonUser
class in this example would expose a property of type Person. As far as
it’s concerned, the AOP proxy can be used transparently in place of a "real" person
implementation. However, its class would be a dynamic proxy class. It would be possible
to cast it to the Advised
interface (discussed below).
It’s possible to conceal the distinction between target and proxy using an anonymous
inner bean, as follows. Only the ProxyFactoryBean
definition is different; the
advice is included only for completeness:
<bean id="myAdvisor" class="com.mycompany.MyAdvisor"> <property name="someProperty"><value>Custom string property value</value></property> </bean> <bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="proxyInterfaces"><value>com.mycompany.Person</value></property> <!-- Use inner bean, not local reference to target --> <property name="target"> <bean class="com.mycompany.PersonImpl"> <property name="name"><value>Tony</value></property> <property name="age"><value>51</value></property> </bean> </property> <property name="interceptorNames"> <list> <value>myAdvisor</value> <value>debugInterceptor</value> </list> </property> </bean>
This has the advantage that there’s only one object of type Person
: useful if we want
to prevent users of the application context from obtaining a reference to the un-advised
object, or need to avoid any ambiguity with Spring IoC autowiring. There’s also
arguably an advantage in that the ProxyFactoryBean definition is self-contained.
However, there are times when being able to obtain the un-advised target from the
factory might actually be an advantage: for example, in certain test scenarios.
What if you need to proxy a class, rather than one or more interfaces?
Imagine that in our example above, there was no Person
interface: we needed to advise
a class called Person
that didn’t implement any business interface. In this case, you
can configure Spring to use CGLIB proxying, rather than dynamic proxies. Simply set the
proxyTargetClass
property on the ProxyFactoryBean above to true. While it’s best to
program to interfaces, rather than classes, the ability to advise classes that don’t
implement interfaces can be useful when working with legacy code. (In general, Spring
isn’t prescriptive. While it makes it easy to apply good practices, it avoids forcing a
particular approach.)
If you want to, you can force the use of CGLIB in any case, even if you do have interfaces.
CGLIB proxying works by generating a subclass of the target class at runtime. Spring configures this generated subclass to delegate method calls to the original target: the subclass is used to implement the Decorator pattern, weaving in the advice.
CGLIB proxying should generally be transparent to users. However, there are some issues to consider:
Final
methods can’t be advised, as they can’t be overridden.
There’s little performance difference between CGLIB proxying and dynamic proxies. As of Spring 1.0, dynamic proxies are slightly faster. However, this may change in the future. Performance should not be a decisive consideration in this case.
By appending an asterisk to an interceptor name, all advisors with bean names matching the part before the asterisk, will be added to the advisor chain. This can come in handy if you need to add a standard set of global advisors:
<bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="target" ref="service"/> <property name="interceptorNames"> <list> <value>global*</value> </list> </property> </bean> <bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/> <bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/>
Especially when defining transactional proxies, you may end up with many similar proxy definitions. The use of parent and child bean definitions, along with inner bean definitions, can result in much cleaner and more concise proxy definitions.
First a parent, template, bean definition is created for the proxy:
<bean id="txProxyTemplate" abstract="true" class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributes"> <props> <prop key="*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
This will never be instantiated itself, so may actually be incomplete. Then each proxy which needs to be created is just a child bean definition, which wraps the target of the proxy as an inner bean definition, since the target will never be used on its own anyway.
<bean id="myService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MyServiceImpl"> </bean> </property> </bean>
It is of course possible to override properties from the parent template, such as in this case, the transaction propagation settings:
<bean id="mySpecialService" parent="txProxyTemplate"> <property name="target"> <bean class="org.springframework.samples.MySpecialServiceImpl"> </bean> </property> <property name="transactionAttributes"> <props> <prop key="get*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="find*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="load*">PROPAGATION_REQUIRED,readOnly</prop> <prop key="store*">PROPAGATION_REQUIRED</prop> </props> </property> </bean>
Note that in the example above, we have explicitly marked the parent bean definition as abstract by using the abstract attribute, as described previously, so that it may not actually ever be instantiated. Application contexts (but not simple bean factories) will by default pre-instantiate all singletons. It is therefore important (at least for singleton beans) that if you have a (parent) bean definition which you intend to use only as a template, and this definition specifies a class, you must make sure to set theabstract attribute to true, otherwise the application context will actually try to pre-instantiate it.
It’s easy to create AOP proxies programmatically using Spring. This enables you to use Spring AOP without dependency on Spring IoC.
The following listing shows creation of a proxy for a target object, with one interceptor and one advisor. The interfaces implemented by the target object will automatically be proxied:
ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl);
factory.addInterceptor(myMethodInterceptor);
factory.addAdvisor(myAdvisor);
MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();
The first step is to construct an object of type
org.springframework.aop.framework.ProxyFactory
. You can create this with a target
object, as in the above example, or specify the interfaces to be proxied in an alternate
constructor.
You can add interceptors or advisors, and manipulate them for the life of the ProxyFactory. If you add an IntroductionInterceptionAroundAdvisor you can cause the proxy to implement additional interfaces.
There are also convenience methods on ProxyFactory (inherited from AdvisedSupport
)
which allow you to add other advice types such as before and throws advice.
AdvisedSupport is the superclass of both ProxyFactory and ProxyFactoryBean.
Tip | |
---|---|
Integrating AOP proxy creation with the IoC framework is best practice in most applications. We recommend that you externalize configuration from Java code with AOP, as in general. |
However you create AOP proxies, you can manipulate them using the
org.springframework.aop.framework.Advised
interface. Any AOP proxy can be cast to this
interface, whichever other interfaces it implements. This interface includes the
following methods:
Advisor[] getAdvisors(); void addAdvice(Advice advice) throws AopConfigException; void addAdvice(int pos, Advice advice) throws AopConfigException; void addAdvisor(Advisor advisor) throws AopConfigException; void addAdvisor(int pos, Advisor advisor) throws AopConfigException; int indexOf(Advisor advisor); boolean removeAdvisor(Advisor advisor) throws AopConfigException; void removeAdvisor(int index) throws AopConfigException; boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException; boolean isFrozen();
The getAdvisors()
method will return an Advisor for every advisor, interceptor or
other advice type that has been added to the factory. If you added an Advisor, the
returned advisor at this index will be the object that you added. If you added an
interceptor or other advice type, Spring will have wrapped this in an advisor with a
pointcut that always returns true. Thus if you added a MethodInterceptor
, the advisor
returned for this index will be an DefaultPointcutAdvisor
returning your
MethodInterceptor
and a pointcut that matches all classes and methods.
The addAdvisor()
methods can be used to add any Advisor. Usually the advisor holding
pointcut and advice will be the generic DefaultPointcutAdvisor
, which can be used with
any advice or pointcut (but not for introductions).
By default, it’s possible to add or remove advisors or interceptors even once a proxy has been created. The only restriction is that it’s impossible to add or remove an introduction advisor, as existing proxies from the factory will not show the interface change. (You can obtain a new proxy from the factory to avoid this problem.)
A simple example of casting an AOP proxy to the Advised
interface and examining and
manipulating its advice:
Advised advised = (Advised) myObject; Advisor[] advisors = advised.getAdvisors(); int oldAdvisorCount = advisors.length; System.out.println(oldAdvisorCount + " advisors"); // Add an advice like an interceptor without a pointcut // Will match all proxied methods // Can use for interceptors, before, after returning or throws advice advised.addAdvice(new DebugInterceptor()); // Add selective advice using a pointcut advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice)); assertEquals("Added two advisors", oldAdvisorCount + 2, advised.getAdvisors().length);
Note | |
---|---|
It’s questionable whether it’s advisable (no pun intended) to modify advice on a business object in production, although there are no doubt legitimate usage cases. However, it can be very useful in development: for example, in tests. I have sometimes found it very useful to be able to add test code in the form of an interceptor or other advice, getting inside a method invocation I want to test. (For example, the advice can get inside a transaction created for that method: for example, to run SQL to check that a database was correctly updated, before marking the transaction for roll back.) |
Depending on how you created the proxy, you can usually set a frozen
flag, in which
case the Advised
isFrozen()
method will return true, and any attempts to modify
advice through addition or removal will result in an AopConfigException
. The ability
to freeze the state of an advised object is useful in some cases, for example, to
prevent calling code removing a security interceptor. It may also be used in Spring 1.1
to allow aggressive optimization if runtime advice modification is known not to be
required.
So far we’ve considered explicit creation of AOP proxies using a ProxyFactoryBean
or
similar factory bean.
Spring also allows us to use "autoproxy" bean definitions, which can automatically proxy selected bean definitions. This is built on Spring "bean post processor" infrastructure, which enables modification of any bean definition as the container loads.
In this model, you set up some special bean definitions in your XML bean definition file
to configure the auto proxy infrastructure. This allows you just to declare the targets
eligible for autoproxying: you don’t need to use ProxyFactoryBean
.
There are two ways to do this:
The org.springframework.aop.framework.autoproxy
package provides the following
standard autoproxy creators.
The BeanNameAutoProxyCreator
class is a BeanPostProcessor
that automatically creates
AOP proxies for beans with names matching literal values or wildcards.
<bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator"> <property name="beanNames"><value>jdk*,onlyJdk</value></property> <property name="interceptorNames"> <list> <value>myInterceptor</value> </list> </property> </bean>
As with ProxyFactoryBean
, there is an interceptorNames
property rather than a list
of interceptors, to allow correct behavior for prototype advisors. Named "interceptors"
can be advisors or any advice type.
As with auto proxying in general, the main point of using BeanNameAutoProxyCreator
is
to apply the same configuration consistently to multiple objects, with minimal volume of
configuration. It is a popular choice for applying declarative transactions to multiple
objects.
Bean definitions whose names match, such as "jdkMyBean" and "onlyJdk" in the above
example, are plain old bean definitions with the target class. An AOP proxy will be
created automatically by the BeanNameAutoProxyCreator
. The same advice will be applied
to all matching beans. Note that if advisors are used (rather than the interceptor in
the above example), the pointcuts may apply differently to different beans.
A more general and extremely powerful auto proxy creator is
DefaultAdvisorAutoProxyCreator
. This will automagically apply eligible advisors in the
current context, without the need to include specific bean names in the autoproxy
advisor’s bean definition. It offers the same merit of consistent configuration and
avoidance of duplication as BeanNameAutoProxyCreator
.
Using this mechanism involves:
DefaultAdvisorAutoProxyCreator
bean definition.
The DefaultAdvisorAutoProxyCreator
will automatically evaluate the pointcut contained
in each advisor, to see what (if any) advice it should apply to each business object
(such as "businessObject1" and "businessObject2" in the example).
This means that any number of advisors can be applied automatically to each business object. If no pointcut in any of the advisors matches any method in a business object, the object will not be proxied. As bean definitions are added for new business objects, they will automatically be proxied if necessary.
Autoproxying in general has the advantage of making it impossible for callers or dependencies to obtain an un-advised object. Calling getBean("businessObject1") on this ApplicationContext will return an AOP proxy, not the target business object. (The "inner bean" idiom shown earlier also offers this benefit.)
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="customAdvisor" class="com.mycompany.MyAdvisor"/> <bean id="businessObject1" class="com.mycompany.BusinessObject1"> <!-- Properties omitted --> </bean> <bean id="businessObject2" class="com.mycompany.BusinessObject2"/>
The DefaultAdvisorAutoProxyCreator
is very useful if you want to apply the same advice
consistently to many business objects. Once the infrastructure definitions are in place,
you can simply add new business objects without including specific proxy configuration.
You can also drop in additional aspects very easily - for example, tracing or
performance monitoring aspects - with minimal change to configuration.
The DefaultAdvisorAutoProxyCreator offers support for filtering (using a naming
convention so that only certain advisors are evaluated, allowing use of multiple,
differently configured, AdvisorAutoProxyCreators in the same factory) and ordering.
Advisors can implement the org.springframework.core.Ordered
interface to ensure
correct ordering if this is an issue. The TransactionAttributeSourceAdvisor used in the
above example has a configurable order value; the default setting is unordered.
This is the superclass of DefaultAdvisorAutoProxyCreator. You can create your own
autoproxy creators by subclassing this class, in the unlikely event that advisor
definitions offer insufficient customization to the behavior of the framework
DefaultAdvisorAutoProxyCreator
.
A particularly important type of autoproxying is driven by metadata. This produces a
similar programming model to .NET ServicedComponents
. Instead of using XML deployment
descriptors as in EJB, configuration for transaction management and other enterprise
services is held in source-level attributes.
In this case, you use the DefaultAdvisorAutoProxyCreator
, in combination with Advisors
that understand metadata attributes. The metadata specifics are held in the pointcut
part of the candidate advisors, rather than in the autoproxy creation class itself.
This is really a special case of the DefaultAdvisorAutoProxyCreator
, but deserves
consideration on its own. (The metadata-aware code is in the pointcuts contained in the
advisors, not the AOP framework itself.)
The /attributes
directory of the JPetStore sample application shows the use of
attribute-driven autoproxying. In this case, there’s no need to use the
TransactionProxyFactoryBean
. Simply defining transactional attributes on business
objects is sufficient, because of the use of metadata-aware pointcuts. The bean
definitions include the following code, in /WEB-INF/declarativeServices.xml
. Note that
this is generic, and can be used outside the JPetStore:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource"> <property name="attributes" ref="attributes"/> </bean> </property> </bean> <bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/>
The DefaultAdvisorAutoProxyCreator
bean definition (the name is not significant, hence
it can even be omitted) will pick up all eligible pointcuts in the current application
context. In this case, the "transactionAdvisor" bean definition, of type
TransactionAttributeSourceAdvisor
, will apply to classes or methods carrying a
transaction attribute. The TransactionAttributeSourceAdvisor depends on a
TransactionInterceptor, via constructor dependency. The example resolves this via
autowiring. The AttributesTransactionAttributeSource
depends on an implementation of
the org.springframework.metadata.Attributes
interface. In this fragment, the
"attributes" bean satisfies this, using the Jakarta Commons Attributes API to obtain
attribute information. (The application code must have been compiled using the Commons
Attributes compilation task.)
The /annotation
directory of the JPetStore sample application contains an analogous
example for auto-proxying driven by JDK 1.5+ annotations. The following configuration
enables automatic detection of Spring’s Transactional
annotation, leading to implicit
proxies for beans containing that annotation:
<bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/> <bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"> <property name="transactionInterceptor" ref="transactionInterceptor"/> </bean> <bean id="transactionInterceptor" class="org.springframework.transaction.interceptor.TransactionInterceptor"> <property name="transactionManager" ref="transactionManager"/> <property name="transactionAttributeSource"> <bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/> </property> </bean>
The TransactionInterceptor
defined here depends on a PlatformTransactionManager
definition, which is not included in this generic file (although it could be) because it
will be specific to the application’s transaction requirements (typically JTA, as in
this example, or Hibernate, JDO or JDBC):
<bean id="transactionManager" class="org.springframework.transaction.jta.JtaTransactionManager"/>
Tip | |
---|---|
If you require only declarative transaction management, using these generic XML definitions will result in Spring automatically proxying all classes or methods with transaction attributes. You won’t need to work directly with AOP, and the programming model is similar to that of .NET ServicedComponents. |
This mechanism is extensible. It’s possible to do autoproxying based on custom attributes. You need to:
It’s possible for such advisors to be unique to each advised class (for example,
mixins): they simply need to be defined as prototype, rather than singleton, bean
definitions. For example, the LockMixin
introduction interceptor from the Spring test
suite, shown above, could be used in conjunction with an attribute-driven pointcut to
target a mixin, as shown here. We use the generic DefaultPointcutAdvisor
, configured
using JavaBean properties:
<bean id="lockMixin" class="org.springframework.aop.LockMixin" scope="prototype"/> <bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor" scope="prototype"> <property name="pointcut" ref="myAttributeAwarePointcut"/> <property name="advice" ref="lockMixin"/> </bean> <bean id="anyBean" class="anyclass" ...
If the attribute aware pointcut matches any methods in the anyBean
or other bean
definitions, the mixin will be applied. Note that both lockMixin
and lockableAdvisor
definitions are prototypes. The myAttributeAwarePointcut
pointcut can be a singleton
definition, as it doesn’t hold state for individual advised objects.
Spring offers the concept of a TargetSource, expressed in the
org.springframework.aop.TargetSource
interface. This interface is responsible for
returning the "target object" implementing the join point. The TargetSource
implementation is asked for a target instance each time the AOP proxy handles a method
invocation.
Developers using Spring AOP don’t normally need to work directly with TargetSources, but this provides a powerful means of supporting pooling, hot swappable and other sophisticated targets. For example, a pooling TargetSource can return a different target instance for each invocation, using a pool to manage instances.
If you do not specify a TargetSource, a default implementation is used that wraps a local object. The same target is returned for each invocation (as you would expect).
Let’s look at the standard target sources provided with Spring, and how you can use them.
Tip | |
---|---|
When using a custom target source, your target will usually need to be a prototype rather than a singleton bean definition. This allows Spring to create a new target instance when required. |
The org.springframework.aop.target.HotSwappableTargetSource
exists to allow the target
of an AOP proxy to be switched while allowing callers to keep their references to it.
Changing the target source’s target takes effect immediately. The
HotSwappableTargetSource
is threadsafe.
You can change the target via the swap()
method on HotSwappableTargetSource as follows:
HotSwappableTargetSource swapper = (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);
The XML definitions required look as follows:
<bean id="initialTarget" class="mycompany.OldTarget"/> <bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource"> <constructor-arg ref="initialTarget"/> </bean> <bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="swapper"/> </bean>
The above swap()
call changes the target of the swappable bean. Clients who hold a
reference to that bean will be unaware of the change, but will immediately start hitting
the new target.
Although this example doesn’t add any advice - and it’s not necessary to add advice to
use a TargetSource
- of course any TargetSource
can be used in conjunction with
arbitrary advice.
Using a pooling target source provides a similar programming model to stateless session EJBs, in which a pool of identical instances is maintained, with method invocations going to free objects in the pool.
A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to any POJO. As with Spring in general, this service can be applied in a non-invasive way.
Spring provides out-of-the-box support for Jakarta Commons Pool 1.3, which provides a
fairly efficient pooling implementation. You’ll need the commons-pool Jar on your
application’s classpath to use this feature. It’s also possible to subclass
org.springframework.aop.target.AbstractPoolingTargetSource
to support any other
pooling API.
Sample configuration is shown below:
<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject" scope="prototype"> ... properties omitted </bean> <bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> <property name="maxSize" value="25"/> </bean> <bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="targetSource" ref="poolTargetSource"/> <property name="interceptorNames" value="myInterceptor"/> </bean>
Note that the target object - "businessObjectTarget" in the example - must be a
prototype. This allows the PoolingTargetSource
implementation to create new instances
of the target to grow the pool as necessary. See the Javadoc for
AbstractPoolingTargetSource
and the concrete subclass you wish to use for information
about its properties: "maxSize" is the most basic, and always guaranteed to be present.
In this case, "myInterceptor" is the name of an interceptor that would need to be defined in the same IoC context. However, it isn’t necessary to specify interceptors to use pooling. If you want only pooling, and no other advice, don’t set the interceptorNames property at all.
It’s possible to configure Spring so as to be able to cast any pooled object to the
org.springframework.aop.target.PoolingConfig
interface, which exposes information
about the configuration and current size of the pool through an introduction. You’ll
need to define an advisor like this:
<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"> <property name="targetObject" ref="poolTargetSource"/> <property name="targetMethod" value="getPoolingConfigMixin"/> </bean>
This advisor is obtained by calling a convenience method on the
AbstractPoolingTargetSource
class, hence the use of MethodInvokingFactoryBean. This
advisor’s name ("poolConfigAdvisor" here) must be in the list of interceptors names in
the ProxyFactoryBean exposing the pooled object.
The cast will look as follows:
PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject"); System.out.println("Max pool size is " + conf.getMaxSize());
Note | |
---|---|
Pooling stateless service objects is not usually necessary. We don’t believe it should be the default choice, as most stateless objects are naturally thread safe, and instance pooling is problematic if resources are cached. |
Simpler pooling is available using autoproxying. It’s possible to set the TargetSources used by any autoproxy creator.
Setting up a "prototype" target source is similar to a pooling TargetSource. In this case, a new instance of the target will be created on every method invocation. Although the cost of creating a new object isn’t high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be more expensive. Thus you shouldn’t use this approach without very good reason.
To do this, you could modify the poolTargetSource
definition shown above as follows.
(I’ve also changed the name, for clarity.)
<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource"> <property name="targetBeanName" ref="businessObjectTarget"/> </bean>
There’s only one property: the name of the target bean. Inheritance is used in the TargetSource implementations to ensure consistent naming. As with the pooling target source, the target bean must be a prototype bean definition.
ThreadLocal
target sources are useful if you need an object to be created for each
incoming request (per thread that is). The concept of a ThreadLocal
provide a JDK-wide
facility to transparently store resource alongside a thread. Setting up a
ThreadLocalTargetSource
is pretty much the same as was explained for the other types
of target source:
<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource"> <property name="targetBeanName" value="businessObjectTarget"/> </bean>
Note | |
---|---|
ThreadLocals come with serious issues (potentially resulting in memory leaks) when
incorrectly using them in a multi-threaded and multi-classloader environments. One
should always consider wrapping a threadlocal in some other class and never directly use
the |
Spring AOP is designed to be extensible. While the interception implementation strategy is presently used internally, it is possible to support arbitrary advice types in addition to the out-of-the-box interception around advice, before, throws advice and after returning advice.
The org.springframework.aop.framework.adapter
package is an SPI package allowing
support for new custom advice types to be added without changing the core framework. The
only constraint on a custom Advice
type is that it must implement the
org.aopalliance.aop.Advice
tag interface.
Please refer to the org.springframework.aop.framework.adapter
package’s Javadocs for
further information.
Please refer to the Spring sample applications for further examples of Spring AOP:
TransactionProxyFactoryBean
for declarative transaction management.
/attributes
directory of the JPetStore illustrates the use of attribute-driven
declarative transaction management.
This appendix details the XML Schema-based configuration introduced in Spring 2.0 and enhanced and extended in Spring 2.5 and 3.0.
The central motivation for moving to XML Schema based configuration files was to make
Spring XML configuration easier. The 'classic' <bean/>
-based approach is good, but
its generic-nature comes with a price in terms of configuration overhead.
From the Spring IoC containers point-of-view, everything is a bean. That’s great news for the Spring IoC container, because if everything is a bean then everything can be treated in the exact same fashion. The same, however, is not true from a developer’s point-of-view. The objects defined in a Spring XML configuration file are not all generic, vanilla beans. Usually, each bean requires some degree of specific configuration.
Spring 2.0’s new XML Schema-based configuration addresses this issue. The <bean/>
element is still present, and if you wanted to, you could continue to write the exact
same style of Spring XML configuration using only <bean/>
elements. The new XML
Schema-based configuration does, however, make Spring XML configuration files
substantially clearer to read. In addition, it allows you to express the intent of a
bean definition.
The key thing to remember is that the new custom tags work best for infrastructure or integration beans: for example, AOP, collections, transactions, integration with 3rd-party frameworks such as Mule, etc., while the existing bean tags are best suited to application-specific beans, such as DAOs, service layer objects, validators, etc.
The examples included below will hopefully convince you that the inclusion of XML Schema support in Spring 2.0 was a good idea. The reception in the community has been encouraging; also, please note the fact that this new configuration mechanism is totally customisable and extensible. This means you can write your own domain-specific configuration tags that would better represent your application’s domain; the process involved in doing so is covered in the appendix entitled Chapter 34, Extensible XML authoring.
To switch over from the DTD-style to the new XML Schema-style, you need to make the following change.
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "http://www.springframework.org/dtd/spring-beans-2.0.dtd"> <beans> <!-- bean definitions here --> </beans>
The equivalent file in the XML Schema-style would be…
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- bean definitions here --> </beans>
Note | |
---|---|
The |
The above Spring XML configuration fragment is boilerplate that you can copy and paste
(!) and then plug <bean/>
definitions into like you have always done. However, the
entire point of switching over is to take advantage of the new Spring 2.0 XML tags since
they make configuration easier. The section entitled Section 33.2.2, “the util schema”
demonstrates how you can start immediately by using some of the more common utility tags.
The rest of this chapter is devoted to showing examples of the new Spring XML Schema based configuration, with at least one example for every new tag. The format follows a before and after style, with a before snippet of XML showing the old (but still 100% legal and supported) style, followed immediately by an after example showing the equivalent in the new XML Schema-based style.
First up is coverage of the util
tags. As the name implies, the util
tags deal with
common, utility configuration issues, such as configuring collections, referencing
constants, and suchlike.
To use the tags in the util
schema, you need to have the following preamble at the top
of your Spring XML configuration file; the text in the snippet below references the
correct schema so that the tags in the util
namespace are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:util="http://www.springframework.org/schema/util" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util.xsd"> <!-- bean definitions here --> </beans>
Before…
<bean id="..." class="..."> <property name="isolation"> <bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" /> </property> </bean>
The above configuration uses a Spring FactoryBean
implementation, the
FieldRetrievingFactoryBean
, to set the value of the 'isolation'
property on a bean
to the value of the 'java.sql.Connection.TRANSACTION_SERIALIZABLE'
constant. This is
all well and good, but it is a tad verbose and (unnecessarily) exposes Spring’s internal
plumbing to the end user.
The following XML Schema-based version is more concise and clearly expresses the developer’s intent ('inject this constant value'), and it just reads better.
<bean id="..." class="..."> <property name="isolation"> <util:constant static-field="java.sql.Connection.TRANSACTION_SERIALIZABLE"/> </property> </bean>
FieldRetrievingFactoryBean
is a FactoryBean
which retrieves a static
or non-static field value. It is typically
used for retrieving public
static
final
constants, which may then be used to set a
property value or constructor arg for another bean.
Find below an example which shows how a static
field is exposed, by using the
staticField
property:
<bean id="myField" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"> <property name="staticField" value="java.sql.Connection.TRANSACTION_SERIALIZABLE"/> </bean>
There is also a convenience usage form where the static
field is specified as the bean
name:
<bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean"/>
This does mean that there is no longer any choice in what the bean id is (so any other bean that refers to it will also have to use this longer name), but this form is very concise to define, and very convenient to use as an inner bean since the id doesn’t have to be specified for the bean reference:
<bean id="..." class="..."> <property name="isolation"> <bean id="java.sql.Connection.TRANSACTION_SERIALIZABLE" class="org.springframework.beans.factory.config.FieldRetrievingFactoryBean" /> </property> </bean>
It is also possible to access a non-static (instance) field of another bean, as
described in the API documentation for the
FieldRetrievingFactoryBean
class.
Injecting enum values into beans as either property or constructor arguments is very
easy to do in Spring, in that you don’t actually have to do anything or know
anything about the Spring internals (or even about classes such as the
FieldRetrievingFactoryBean
). Let’s look at an example to see how easy injecting an
enum value is; consider this JDK 5 enum:
package javax.persistence; public enum PersistenceContextType { TRANSACTION, EXTENDED }
Now consider a setter of type PersistenceContextType
:
package example; public class Client { private PersistenceContextType persistenceContextType; public void setPersistenceContextType(PersistenceContextType type) { this.persistenceContextType = type; } }
<bean class="example.Client"> <property name="persistenceContextType" value="TRANSACTION" /> </bean>
This works for classic type-safe emulated enums (on JDK 1.4 and JDK 1.3) as well; Spring will automatically attempt to match the string property value to a constant on the enum class.
Before…
<!-- target bean to be referenced by name --> <bean id="testBean" class="org.springframework.beans.TestBean" scope="prototype"> <property name="age" value="10"/> <property name="spouse"> <bean class="org.springframework.beans.TestBean"> <property name="age" value="11"/> </bean> </property> </bean> <!-- will result in 10, which is the value of property age of bean testBean --> <bean id="testBean.age" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
The above configuration uses a Spring FactoryBean
implementation, the
PropertyPathFactoryBean
, to create a bean (of type int
) called 'testBean.age'
that
has a value equal to the 'age'
property of the 'testBean'
bean.
After…
<!-- target bean to be referenced by name --> <bean id="testBean" class="org.springframework.beans.TestBean" scope="prototype"> <property name="age" value="10"/> <property name="spouse"> <bean class="org.springframework.beans.TestBean"> <property name="age" value="11"/> </bean> </property> </bean> <!-- will result in 10, which is the value of property age of bean testBean --> <util:property-path id="name" path="testBean.age"/>
The value of the 'path'
attribute of the <property-path/>
tag follows the form
'beanName.beanProperty'
.
PropertyPathFactoryBean
is a FactoryBean
that evaluates a property path on a given
target object. The target object can be specified directly or via a bean name. This
value may then be used in another bean definition as a property value or constructor
argument.
Here’s an example where a path is used against another bean, by name:
// target bean to be referenced by name <bean id="person" class="org.springframework.beans.TestBean" scope="prototype"> <property name="age" value="10"/> <property name="spouse"> <bean class="org.springframework.beans.TestBean"> <property name="age" value="11"/> </bean> </property> </bean> // will result in 11, which is the value of property spouse.age of bean person <bean id="theAge" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"> <property name="targetBeanName" value="person"/> <property name="propertyPath" value="spouse.age"/> </bean>
In this example, a path is evaluated against an inner bean:
<!-- will result in 12, which is the value of property age of the inner bean --> <bean id="theAge" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"> <property name="targetObject"> <bean class="org.springframework.beans.TestBean"> <property name="age" value="12"/> </bean> </property> <property name="propertyPath" value="age"/> </bean>
There is also a shortcut form, where the bean name is the property path.
<!-- will result in 10, which is the value of property age of bean person --> <bean id="person.age" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/>
This form does mean that there is no choice in the name of the bean. Any reference to it will also have to use the same id, which is the path. Of course, if used as an inner bean, there is no need to refer to it at all:
<bean id="..." class="..."> <property name="age"> <bean id="person.age" class="org.springframework.beans.factory.config.PropertyPathFactoryBean"/> </property> </bean>
The result type may be specifically set in the actual definition. This is not necessary for most use cases, but can be of use for some. Please see the Javadocs for more info on this feature.
Before…
<!-- creates a java.util.Properties instance with values loaded from the supplied location --> <bean id="jdbcConfiguration" class="org.springframework.beans.factory.config.PropertiesFactoryBean"> <property name="location" value="classpath:com/foo/jdbc-production.properties"/> </bean>
The above configuration uses a Spring FactoryBean
implementation, the
PropertiesFactoryBean
, to instantiate a java.util.Properties
instance with values
loaded from the supplied Resource
location).
After…
<!-- creates a java.util.Properties instance with values loaded from the supplied location --> <util:properties id="jdbcConfiguration" location="classpath:com/foo/jdbc-production.properties"/>
Before…
<!-- creates a java.util.List instance with values loaded from the supplied sourceList --> <bean id="emails" class="org.springframework.beans.factory.config.ListFactoryBean"> <property name="sourceList"> <list> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </list> </property> </bean>
The above configuration uses a Spring FactoryBean
implementation, the
ListFactoryBean
, to create a java.util.List
instance initialized with values taken
from the supplied 'sourceList'
.
After…
<!-- creates a java.util.List instance with the supplied values --> <util:list id="emails"> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </util:list>
You can also explicitly control the exact type of List
that will be instantiated and
populated via the use of the 'list-class'
attribute on the <util:list/>
element. For
example, if we really need a java.util.LinkedList
to be instantiated, we could use the
following configuration:
<util:list id="emails" list-class="java.util.LinkedList"> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>d'[email protected]</value> </util:list>
If no 'list-class'
attribute is supplied, a List
implementation will be chosen by
the container.
Before…
<!-- creates a java.util.Map instance with values loaded from the supplied sourceMap --> <bean id="emails" class="org.springframework.beans.factory.config.MapFactoryBean"> <property name="sourceMap"> <map> <entry key="pechorin" value="[email protected]"/> <entry key="raskolnikov" value="[email protected]"/> <entry key="stavrogin" value="[email protected]"/> <entry key="porfiry" value="[email protected]"/> </map> </property> </bean>
The above configuration uses a Spring FactoryBean
implementation, the
MapFactoryBean
, to create a java.util.Map
instance initialized with key-value pairs
taken from the supplied 'sourceMap'
.
After…
<!-- creates a java.util.Map instance with the supplied key-value pairs --> <util:map id="emails"> <entry key="pechorin" value="[email protected]"/> <entry key="raskolnikov" value="[email protected]"/> <entry key="stavrogin" value="[email protected]"/> <entry key="porfiry" value="[email protected]"/> </util:map>
You can also explicitly control the exact type of Map
that will be instantiated and
populated via the use of the 'map-class'
attribute on the <util:map/>
element. For
example, if we really need a java.util.TreeMap
to be instantiated, we could use the
following configuration:
<util:map id="emails" map-class="java.util.TreeMap"> <entry key="pechorin" value="[email protected]"/> <entry key="raskolnikov" value="[email protected]"/> <entry key="stavrogin" value="[email protected]"/> <entry key="porfiry" value="[email protected]"/> </util:map>
If no 'map-class'
attribute is supplied, a Map
implementation will be chosen by the
container.
Before…
<!-- creates a java.util.Set instance with values loaded from the supplied sourceSet --> <bean id="emails" class="org.springframework.beans.factory.config.SetFactoryBean"> <property name="sourceSet"> <set> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </set> </property> </bean>
The above configuration uses a Spring FactoryBean
implementation, the
SetFactoryBean
, to create a java.util.Set
instance initialized with values taken
from the supplied 'sourceSet'
.
After…
<!-- creates a java.util.Set instance with the supplied values --> <util:set id="emails"> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </util:set>
You can also explicitly control the exact type of Set
that will be instantiated and
populated via the use of the 'set-class'
attribute on the <util:set/>
element. For
example, if we really need a java.util.TreeSet
to be instantiated, we could use the
following configuration:
<util:set id="emails" set-class="java.util.TreeSet"> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </util:set>
If no 'set-class'
attribute is supplied, a Set
implementation will be chosen by the
container.
The jee
tags deal with Java EE (Java Enterprise Edition)-related configuration issues,
such as looking up a JNDI object and defining EJB references.
To use the tags in the jee
schema, you need to have the following preamble at the top
of your Spring XML configuration file; the text in the following snippet references the
correct schema so that the tags in the jee
namespace are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jee="http://www.springframework.org/schema/jee" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/jee http://www.springframework.org/schema/jee/spring-jee.xsd"> <!-- bean definitions here --> </beans>
Before…
<bean id="dataSource" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="jdbc/MyDataSource"/> </bean> <bean id="userDao" class="com.foo.JdbcUserDao"> <!-- Spring will do the cast automatically (as usual) --> <property name="dataSource" ref="dataSource"/> </bean>
After…
<jee:jndi-lookup id="dataSource" jndi-name="jdbc/MyDataSource"/> <bean id="userDao" class="com.foo.JdbcUserDao"> <!-- Spring will do the cast automatically (as usual) --> <property name="dataSource" ref="dataSource"/> </bean>
Before…
<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="jdbc/MyDataSource"/> <property name="jndiEnvironment"> <props> <prop key="foo">bar</prop> </props> </property> </bean>
After…
<jee:jndi-lookup id="simple" jndi-name="jdbc/MyDataSource"> <jee:environment>foo=bar</jee:environment> </jee:jndi-lookup>
Before…
<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="jdbc/MyDataSource"/> <property name="jndiEnvironment"> <props> <prop key="foo">bar</prop> <prop key="ping">pong</prop> </props> </property> </bean>
After…
<jee:jndi-lookup id="simple" jndi-name="jdbc/MyDataSource"> <!-- newline-separated, key-value pairs for the environment (standard Properties format) --> <jee:environment> foo=bar ping=pong </jee:environment> </jee:jndi-lookup>
Before…
<bean id="simple" class="org.springframework.jndi.JndiObjectFactoryBean"> <property name="jndiName" value="jdbc/MyDataSource"/> <property name="cache" value="true"/> <property name="resourceRef" value="true"/> <property name="lookupOnStartup" value="false"/> <property name="expectedType" value="com.myapp.DefaultFoo"/> <property name="proxyInterface" value="com.myapp.Foo"/> </bean>
After…
<jee:jndi-lookup id="simple" jndi-name="jdbc/MyDataSource" cache="true" resource-ref="true" lookup-on-startup="false" expected-type="com.myapp.DefaultFoo" proxy-interface="com.myapp.Foo"/>
The <jee:local-slsb/>
tag configures a reference to an EJB Stateless SessionBean.
Before…
<bean id="simple" class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean"> <property name="jndiName" value="ejb/RentalServiceBean"/> <property name="businessInterface" value="com.foo.service.RentalService"/> </bean>
After…
<jee:local-slsb id="simpleSlsb" jndi-name="ejb/RentalServiceBean" business-interface="com.foo.service.RentalService"/>
<bean id="complexLocalEjb" class="org.springframework.ejb.access.LocalStatelessSessionProxyFactoryBean"> <property name="jndiName" value="ejb/RentalServiceBean"/> <property name="businessInterface" value="com.foo.service.RentalService"/> <property name="cacheHome" value="true"/> <property name="lookupHomeOnStartup" value="true"/> <property name="resourceRef" value="true"/> </bean>
After…
<jee:local-slsb id="complexLocalEjb" jndi-name="ejb/RentalServiceBean" business-interface="com.foo.service.RentalService" cache-home="true" lookup-home-on-startup="true" resource-ref="true">
The <jee:remote-slsb/>
tag configures a reference to a remote
EJB Stateless
SessionBean.
Before…
<bean id="complexRemoteEjb" class="org.springframework.ejb.access.SimpleRemoteStatelessSessionProxyFactoryBean"> <property name="jndiName" value="ejb/MyRemoteBean"/> <property name="businessInterface" value="com.foo.service.RentalService"/> <property name="cacheHome" value="true"/> <property name="lookupHomeOnStartup" value="true"/> <property name="resourceRef" value="true"/> <property name="homeInterface" value="com.foo.service.RentalService"/> <property name="refreshHomeOnConnectFailure" value="true"/> </bean>
After…
<jee:remote-slsb id="complexRemoteEjb" jndi-name="ejb/MyRemoteBean" business-interface="com.foo.service.RentalService" cache-home="true" lookup-home-on-startup="true" resource-ref="true" home-interface="com.foo.service.RentalService" refresh-home-on-connect-failure="true">
The lang
tags deal with exposing objects that have been written in a dynamic language
such as JRuby or Groovy as beans in the Spring container.
These tags (and the dynamic language support) are comprehensively covered in the chapter
entitled Chapter 28, Dynamic language support. Please do consult that chapter for full details on this
support and the lang
tags themselves.
In the interest of completeness, to use the tags in the lang
schema, you need to have
the following preamble at the top of your Spring XML configuration file; the text in the
following snippet references the correct schema so that the tags in the lang
namespace
are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:lang="http://www.springframework.org/schema/lang" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd"> <!-- bean definitions here --> </beans>
The jms
tags deal with configuring JMS-related beans such as Spring’s
MessageListenerContainers. These tags are detailed in the section of the
JMS chapter entitled Section 23.6, “JMS Namespace Support”. Please do consult that chapter for full
details on this support and the jms
tags themselves.
In the interest of completeness, to use the tags in the jms
schema, you need to have
the following preamble at the top of your Spring XML configuration file; the text in the
following snippet references the correct schema so that the tags in the jms
namespace
are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jms="http://www.springframework.org/schema/jms" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/jms http://www.springframework.org/schema/jms/spring-jms.xsd"> <!-- bean definitions here --> </beans>
The tx
tags deal with configuring all of those beans in Spring’s comprehensive support
for transactions. These tags are covered in the chapter entitled Chapter 11, Transaction Management.
Tip | |
---|---|
You are strongly encouraged to look at the |
In the interest of completeness, to use the tags in the tx
schema, you need to have
the following preamble at the top of your Spring XML configuration file; the text in the
following snippet references the correct schema so that the tags in the tx
namespace
are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- bean definitions here --> </beans>
Note | |
---|---|
Often when using the tags in the |
The aop
tags deal with configuring all things AOP in Spring: this includes Spring’s
own proxy-based AOP framework and Spring’s integration with the AspectJ AOP framework.
These tags are comprehensively covered in the chapter entitled Chapter 8, Aspect Oriented Programming with Spring.
In the interest of completeness, to use the tags in the aop
schema, you need to have
the following preamble at the top of your Spring XML configuration file; the text in the
following snippet references the correct schema so that the tags in the aop
namespace
are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- bean definitions here --> </beans>
The context
tags deal with ApplicationContext
configuration that relates to plumbing
- that is, not usually beans that are important to an end-user but rather beans that do
a lot of grunt work in Spring, such as BeanfactoryPostProcessors
. The following
snippet references the correct schema so that the tags in the context
namespace are
available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <!-- bean definitions here --> </beans>
Note | |
---|---|
The |
This element activates the replacement of ${...}
placeholders, resolved against the
specified properties file (as a Spring resource location). This element is
a convenience mechanism that sets up aPropertyPlaceholderConfigurer
for you; if you need more control over the
PropertyPlaceholderConfigurer
, just define one yourself explicitly.
Activates the Spring infrastructure for various annotations to be detected in bean
classes: Spring’s @Required
and
@Autowired
, as well as JSR 250’s @PostConstruct
,
@PreDestroy
and @Resource
(if available), and JPA’s @PersistenceContext
and
@PersistenceUnit
(if available). Alternatively, you can choose to activate the
individual BeanPostProcessors
for those annotations explicitly.
Note | |
---|---|
This element does not activate processing of Spring’s
|
This element is detailed in Section 4.9, “Annotation-based container configuration”.
This element is detailed in Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”.
This element is detailed in Section 8.8.1, “Using AspectJ to dependency inject domain objects with Spring”.
This element is detailed in Section 24.4.3, “Configuring annotation based MBean export”.
The tool
tags are for use when you want to add tooling-specific metadata to your
custom configuration elements. This metadata can then be consumed by tools that are
aware of this metadata, and the tools can then do pretty much whatever they want with it
(validation, etc.).
The tool
tags are not documented in this release of Spring as they are currently
undergoing review. If you are a third party tool vendor and you would like to contribute
to this review process, then do mail the Spring mailing list. The currently supported
tool
tags can be found in the file 'spring-tool.xsd'
in the
'src/org/springframework/beans/factory/xml'
directory of the Spring source
distribution.
The jdbc
tags allow you to quickly configure an embedded database or initialize an
existing data source. These tags are documented in Section 13.8, “Embedded database support”
and Section 13.9, “Initializing a DataSource” respectively.
To use the tags in the jdbc
schema, you need to have the following preamble at the top
of your Spring XML configuration file; the text in the following snippet references the
correct schema so that the tags in the jdbc
namespace are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jdbc="http://www.springframework.org/schema/jdbc" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc.xsd"> <!-- bean definitions here --> </beans>
The cache
tags can be used to enable support for Spring’s @CacheEvict
, @CachePut
and @Caching
annotations. It it also supports declarative XML-based caching. See
Section 29.3.5, “Enable caching annotations” and Section 29.4, “Declarative XML-based caching” for details.
To use the tags in the cache
schema, you need to have the following preamble at the
top of your Spring XML configuration file; the text in the following snippet references
the correct schema so that the tags in the cache
namespace are available to you.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:jdbc="http://www.springframework.org/schema/cache" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/cache http://www.springframework.org/schema/jdbc/spring-cache.xsd"> <!-- bean definitions here --> </beans>
Last but not least we have the tags in the beans
schema. These are the same tags that
have been in Spring since the very dawn of the framework. Examples of the various tags
in the beans
schema are not shown here because they are quite comprehensively covered
in Section 4.4.2, “Dependencies and configuration in detail” (and indeed in that entire chapter).
One thing that is new to the beans tags themselves in Spring 2.0 is the idea of
arbitrary bean metadata. In Spring 2.0 it is now possible to add zero or more key /
value pairs to <bean/>
XML definitions. What, if anything, is done with this extra
metadata is totally up to your own custom logic (and so is typically only of use if you
are writing your own custom tags as described in the appendix entitled
Chapter 34, Extensible XML authoring).
Find below an example of the <meta/>
tag in the context of a surrounding <bean/>
(please note that without any logic to interpret it the metadata is effectively useless
as-is).
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="foo" class="x.y.Foo"> <meta key="cacheName" value="foo"/> <property name="name" value="Rick"/> </bean> </beans>
In the case of the above example, you would assume that there is some logic that will consume the bean definition and set up some caching infrastructure using the supplied metadata.
Since version 2.0, Spring has featured a mechanism for schema-based extensions to the basic Spring XML format for defining and configuring beans. This section is devoted to detailing how you would go about writing your own custom XML bean definition parsers and integrating such parsers into the Spring IoC container.
To facilitate the authoring of configuration files using a schema-aware XML editor, Spring’s extensible XML configuration mechanism is based on XML Schema. If you are not familiar with Spring’s current XML configuration extensions that come with the standard Spring distribution, please first read the appendix entitledChapter 33, XML Schema-based configuration.
Creating new XML configuration extensions can be done by following these (relatively) simple steps:
NamespaceHandler
implementation
(this is an easy step, don’t worry).
BeanDefinitionParser
implementations
(this is where the real work is done).
What follows is a description of each of these steps. For the example, we will create an
XML extension (a custom XML element) that allows us to configure objects of the type
SimpleDateFormat
(from the java.text
package) in an easy manner. When we are done,
we will be able to define bean definitions of type SimpleDateFormat
like this:
<myns:dateformat id="dateFormat" pattern="yyyy-MM-dd HH:mm" lenient="true"/>
(Don’t worry about the fact that this example is very simple; much more detailed examples follow afterwards. The intent in this first simple example is to walk you through the basic steps involved.)
Creating an XML configuration extension for use with Spring’s IoC container starts with
authoring an XML Schema to describe the extension. What follows is the schema we’ll use
to configure SimpleDateFormat
objects.
<!-- myns.xsd (inside package org/springframework/samples/xml) --> <?xml version="1.0" encoding="UTF-8"?> <xsd:schema xmlns="http://www.mycompany.com/schema/myns" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:beans="http://www.springframework.org/schema/beans" targetNamespace="http://www.mycompany.com/schema/myns" elementFormDefault="qualified" attributeFormDefault="unqualified"> <xsd:import namespace="http://www.springframework.org/schema/beans"/> <xsd:element name="dateformat"> <xsd:complexType> <xsd:complexContent> <xsd:extension base="beans:identifiedType"> <xsd:attribute name="lenient" type="xsd:boolean"/> <xsd:attribute name="pattern" type="xsd:string" use="required"/> </xsd:extension> </xsd:complexContent> </xsd:complexType> </xsd:element> </xsd:schema>
(The emphasized line contains an extension base for all tags that will be identifiable
(meaning they have an id
attribute that will be used as the bean identifier in the
container). We are able to use this attribute because we imported the Spring-provided
'beans'
namespace.)
The above schema will be used to configure SimpleDateFormat
objects, directly in an
XML application context file using the <myns:dateformat/>
element.
<myns:dateformat id="dateFormat" pattern="yyyy-MM-dd HH:mm" lenient="true"/>
Note that after we’ve created the infrastructure classes, the above snippet of XML will
essentially be exactly the same as the following XML snippet. In other words, we’re just
creating a bean in the container, identified by the name 'dateFormat'
of type
SimpleDateFormat
, with a couple of properties set.
<bean id="dateFormat" class="java.text.SimpleDateFormat"> <constructor-arg value="yyyy-HH-dd HH:mm"/> <property name="lenient" value="true"/> </bean>
Note | |
---|---|
The schema-based approach to creating configuration format allows for tight integration with an IDE that has a schema-aware XML editor. Using a properly authored schema, you can use autocompletion to have a user choose between several configuration options defined in the enumeration. |
In addition to the schema, we need a NamespaceHandler
that will parse all elements of
this specific namespace Spring encounters while parsing configuration files. The
NamespaceHandler
should in our case take care of the parsing of the myns:dateformat
element.
The NamespaceHandler
interface is pretty simple in that it features just three methods:
init()
- allows for initialization of the NamespaceHandler
and will be called by
Spring before the handler is used
BeanDefinition parse(Element, ParserContext)
- called when Spring encounters a
top-level element (not nested inside a bean definition or a different namespace). This
method can register bean definitions itself and/or return a bean definition.
BeanDefinitionHolder decorate(Node, BeanDefinitionHolder, ParserContext)
- called
when Spring encounters an attribute or nested element of a different namespace. The
decoration of one or more bean definitions is used for example with
theout-of-the-box scopes Spring 2.0 supports. We’ll start by
highlighting a simple example, without using decoration, after which we will show
decoration in a somewhat more advanced example.
Although it is perfectly possible to code your own NamespaceHandler
for the entire
namespace (and hence provide code that parses each and every element in the namespace),
it is often the case that each top-level XML element in a Spring XML configuration file
results in a single bean definition (as in our case, where a single <myns:dateformat/>
element results in a single SimpleDateFormat
bean definition). Spring features a
number of convenience classes that support this scenario. In this example, we’ll make
use the NamespaceHandlerSupport
class:
package org.springframework.samples.xml; import org.springframework.beans.factory.xml.NamespaceHandlerSupport; public class MyNamespaceHandler extends NamespaceHandlerSupport { public void init() { registerBeanDefinitionParser("dateformat", new SimpleDateFormatBeanDefinitionParser()); } }
The observant reader will notice that there isn’t actually a whole lot of parsing logic
in this class. Indeed… the NamespaceHandlerSupport
class has a built in notion of
delegation. It supports the registration of any number of BeanDefinitionParser
instances, to which it will delegate to when it needs to parse an element in its
namespace. This clean separation of concerns allows a NamespaceHandler
to handle the
orchestration of the parsing of all of the custom elements in its namespace, while
delegating to BeanDefinitionParsers
to do the grunt work of the XML parsing; this
means that each BeanDefinitionParser
will contain just the logic for parsing a single
custom element, as we can see in the next step
A BeanDefinitionParser
will be used if the NamespaceHandler
encounters an XML
element of the type that has been mapped to the specific bean definition parser (which
is 'dateformat'
in this case). In other words, the BeanDefinitionParser
is
responsible for parsing one distinct top-level XML element defined in the schema. In
the parser, we’ll have access to the XML element (and thus its subelements too) so that
we can parse our custom XML content, as can be seen in the following example:
package org.springframework.samples.xml; import org.springframework.beans.factory.support.BeanDefinitionBuilder; import org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser; import org.springframework.util.StringUtils; import org.w3c.dom.Element; import java.text.SimpleDateFormat; public class SimpleDateFormatBeanDefinitionParser extends AbstractSingleBeanDefinitionParser { protected Class getBeanClass(Element element) { return SimpleDateFormat.class; } protected void doParse(Element element, BeanDefinitionBuilder bean) { // this will never be null since the schema explicitly requires that a value be supplied String pattern = element.getAttribute("pattern"); bean.addConstructorArg(pattern); // this however is an optional property String lenient = element.getAttribute("lenient"); if (StringUtils.hasText(lenient)) { bean.addPropertyValue("lenient", Boolean.valueOf(lenient)); } } }
We use the Spring-provided | |
We supply the |
In this simple case, this is all that we need to do. The creation of our single
BeanDefinition
is handled by the AbstractSingleBeanDefinitionParser
superclass, as
is the extraction and setting of the bean definition’s unique identifier.
The coding is finished! All that remains to be done is to somehow make the Spring XML
parsing infrastructure aware of our custom element; we do this by registering our custom
namespaceHandler
and custom XSD file in two special purpose properties files. These
properties files are both placed in a 'META-INF'
directory in your application, and
can, for example, be distributed alongside your binary classes in a JAR file. The Spring
XML parsing infrastructure will automatically pick up your new extension by consuming
these special properties files, the formats of which are detailed below.
The properties file called 'spring.handlers'
contains a mapping of XML Schema URIs to
namespace handler classes. So for our example, we need to write the following:
http\://www.mycompany.com/schema/myns=org.springframework.samples.xml.MyNamespaceHandler
(The ':'
character is a valid delimiter in the Java properties format, and so the
':'
character in the URI needs to be escaped with a backslash.)
The first part (the key) of the key-value pair is the URI associated with your custom
namespace extension, and needs to match exactly the value of the 'targetNamespace'
attribute as specified in your custom XSD schema.
The properties file called 'spring.schemas'
contains a mapping of XML Schema locations
(referred to along with the schema declaration in XML files that use the schema as part
of the 'xsi:schemaLocation'
attribute) to classpath resources. This file is needed
to prevent Spring from absolutely having to use a default EntityResolver
that requires
Internet access to retrieve the schema file. If you specify the mapping in this
properties file, Spring will search for the schema on the classpath (in this case
'myns.xsd'
in the 'org.springframework.samples.xml'
package):
http\://www.mycompany.com/schema/myns/myns.xsd=org/springframework/samples/xml/myns.xsd
The upshot of this is that you are encouraged to deploy your XSD file(s) right alongside
the NamespaceHandler
and BeanDefinitionParser
classes on the classpath.
Using a custom extension that you yourself have implemented is no different from using
one of the custom extensions that Spring provides straight out of the box. Find below
an example of using the custom <dateformat/>
element developed in the previous steps
in a Spring XML configuration file.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:myns="http://www.mycompany.com/schema/myns" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.mycompany.com/schema/myns http://www.mycompany.com/schema/myns/myns.xsd"> <!-- as a top-level bean --> <myns:dateformat id="defaultDateFormat" pattern="yyyy-MM-dd HH:mm" lenient="true"/> <bean id="jobDetailTemplate" abstract="true"> <property name="dateFormat"> <!-- as an inner bean --> <myns:dateformat pattern="HH:mm MM-dd-yyyy"/> </property> </bean> </beans>
Find below some much meatier examples of custom XML extensions.
This example illustrates how you might go about writing the various artifacts required to satisfy a target of the following configuration:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:foo="http://www.foo.com/schema/component" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.foo.com/schema/component http://www.foo.com/schema/component/component.xsd"> <foo:component id="bionic-family" name="Bionic-1"> <foo:component name="Mother-1"> <foo:component name="Karate-1"/> <foo:component name="Sport-1"/> </foo:component> <foo:component name="Rock-1"/> </foo:component> </beans>
The above configuration actually nests custom extensions within each other. The class
that is actually configured by the above <foo:component/>
element is the Component
class (shown directly below). Notice how the Component
class does not expose a
setter method for the 'components'
property; this makes it hard (or rather impossible)
to configure a bean definition for the Component
class using setter injection.
package com.foo; import java.util.ArrayList; import java.util.List; public class Component { private String name; private List<Component> components = new ArrayList<Component> (); // mmm, there is no setter method for the components public void addComponent(Component component) { this.components.add(component); } public List<Component> getComponents() { return components; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
The typical solution to this issue is to create a custom FactoryBean
that exposes a
setter property for the 'components'
property.
package com.foo; import org.springframework.beans.factory.FactoryBean; import java.util.List; public class ComponentFactoryBean implements FactoryBean<Component> { private Component parent; private List<Component> children; public void setParent(Component parent) { this.parent = parent; } public void setChildren(List<Component> children) { this.children = children; } public Component getObject() throws Exception { if (this.children != null && this.children.size() > 0) { for (Component child : children) { this.parent.addComponent(child); } } return this.parent; } public Class<Component> getObjectType() { return Component.class; } public boolean isSingleton() { return true; } }
This is all very well, and does work nicely, but exposes a lot of Spring plumbing to the end user. What we are going to do is write a custom extension that hides away all of this Spring plumbing. If we stick to the steps described previously, we’ll start off by creating the XSD schema to define the structure of our custom tag.
<?xml version="1.0" encoding="UTF-8" standalone="no"?> <xsd:schema xmlns="http://www.foo.com/schema/component" xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.foo.com/schema/component" elementFormDefault="qualified" attributeFormDefault="unqualified"> <xsd:element name="component"> <xsd:complexType> <xsd:choice minOccurs="0" maxOccurs="unbounded"> <xsd:element ref="component"/> </xsd:choice> <xsd:attribute name="id" type="xsd:ID"/> <xsd:attribute name="name" use="required" type="xsd:string"/> </xsd:complexType> </xsd:element> </xsd:schema>
We’ll then create a custom NamespaceHandler
.
package com.foo; import org.springframework.beans.factory.xml.NamespaceHandlerSupport; public class ComponentNamespaceHandler extends NamespaceHandlerSupport { public void init() { registerBeanDefinitionParser("component", new ComponentBeanDefinitionParser()); } }
Next up is the custom BeanDefinitionParser
. Remember that what we are creating is a
BeanDefinition
describing a ComponentFactoryBean
.
package com.foo; import org.springframework.beans.factory.config.BeanDefinition; import org.springframework.beans.factory.support.AbstractBeanDefinition; import org.springframework.beans.factory.support.BeanDefinitionBuilder; import org.springframework.beans.factory.support.ManagedList; import org.springframework.beans.factory.xml.AbstractBeanDefinitionParser; import org.springframework.beans.factory.xml.ParserContext; import org.springframework.util.xml.DomUtils; import org.w3c.dom.Element; import java.util.List; public class ComponentBeanDefinitionParser extends AbstractBeanDefinitionParser { protected AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) { return parseComponentElement(element); } private static AbstractBeanDefinition parseComponentElement(Element element) { BeanDefinitionBuilder factory = BeanDefinitionBuilder.rootBeanDefinition(ComponentFactoryBean.class); factory.addPropertyValue("parent", parseComponent(element)); List<Element> childElements = DomUtils.getChildElementsByTagName(element, "component"); if (childElements != null && childElements.size() > 0) { parseChildComponents(childElements, factory); } return factory.getBeanDefinition(); } private static BeanDefinition parseComponent(Element element) { BeanDefinitionBuilder component = BeanDefinitionBuilder.rootBeanDefinition(Component.class); component.addPropertyValue("name", element.getAttribute("name")); return component.getBeanDefinition(); } private static void parseChildComponents(List<Element> childElements, BeanDefinitionBuilder factory) { ManagedList<BeanDefinition> children = new ManagedList<BeanDefinition>(childElements.size()); for (Element element : childElements) { children.add(parseComponentElement(element)); } factory.addPropertyValue("children", children); } }
Lastly, the various artifacts need to be registered with the Spring XML infrastructure.
# in META-INF/spring.handlers
http\://www.foo.com/schema/component=com.foo.ComponentNamespaceHandler
# in META-INF/spring.schemas
http\://www.foo.com/schema/component/component.xsd=com/foo/component.xsd
Writing your own custom parser and the associated artifacts isn’t hard, but sometimes it is not the right thing to do. Consider the scenario where you need to add metadata to already existing bean definitions. In this case you certainly don’t want to have to go off and write your own entire custom extension; rather you just want to add an additional attribute to the existing bean definition element.
By way of another example, let’s say that the service class that you are defining a bean definition for a service object that will (unknown to it) be accessing a clustered JCache, and you want to ensure that the named JCache instance is eagerly started within the surrounding cluster:
<bean id="checkingAccountService" class="com.foo.DefaultCheckingAccountService" jcache:cache-name="checking.account"> <!-- other dependencies here... --> </bean>
What we are going to do here is create another BeanDefinition
when the
'jcache:cache-name'
attribute is parsed; this BeanDefinition
will then initialize
the named JCache for us. We will also modify the existing BeanDefinition
for the
'checkingAccountService'
so that it will have a dependency on this new
JCache-initializing BeanDefinition
.
package com.foo; public class JCacheInitializer { private String name; public JCacheInitializer(String name) { this.name = name; } public void initialize() { // lots of JCache API calls to initialize the named cache... } }
Now onto the custom extension. Firstly, the authoring of the XSD schema describing the custom attribute (quite easy in this case).
<?xml version="1.0" encoding="UTF-8" standalone="no"?> <xsd:schema xmlns="http://www.foo.com/schema/jcache" xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.foo.com/schema/jcache" elementFormDefault="qualified"> <xsd:attribute name="cache-name" type="xsd:string"/> </xsd:schema>
Next, the associated NamespaceHandler
.
package com.foo; import org.springframework.beans.factory.xml.NamespaceHandlerSupport; public class JCacheNamespaceHandler extends NamespaceHandlerSupport { public void init() { super.registerBeanDefinitionDecoratorForAttribute("cache-name", new JCacheInitializingBeanDefinitionDecorator()); } }
Next, the parser. Note that in this case, because we are going to be parsing an XML
attribute, we write a BeanDefinitionDecorator
rather than a BeanDefinitionParser
.
package com.foo; import org.springframework.beans.factory.config.BeanDefinitionHolder; import org.springframework.beans.factory.support.AbstractBeanDefinition; import org.springframework.beans.factory.support.BeanDefinitionBuilder; import org.springframework.beans.factory.xml.BeanDefinitionDecorator; import org.springframework.beans.factory.xml.ParserContext; import org.w3c.dom.Attr; import org.w3c.dom.Node; import java.util.ArrayList; import java.util.Arrays; import java.util.List; public class JCacheInitializingBeanDefinitionDecorator implements BeanDefinitionDecorator { private static final String[] EMPTY_STRING_ARRAY = new String[0]; public BeanDefinitionHolder decorate(Node source, BeanDefinitionHolder holder, ParserContext ctx) { String initializerBeanName = registerJCacheInitializer(source, ctx); createDependencyOnJCacheInitializer(holder, initializerBeanName); return holder; } private void createDependencyOnJCacheInitializer(BeanDefinitionHolder holder, String initializerBeanName) { AbstractBeanDefinition definition = ((AbstractBeanDefinition) holder.getBeanDefinition()); String[] dependsOn = definition.getDependsOn(); if (dependsOn == null) { dependsOn = new String[]{initializerBeanName}; } else { List dependencies = new ArrayList(Arrays.asList(dependsOn)); dependencies.add(initializerBeanName); dependsOn = (String[]) dependencies.toArray(EMPTY_STRING_ARRAY); } definition.setDependsOn(dependsOn); } private String registerJCacheInitializer(Node source, ParserContext ctx) { String cacheName = ((Attr) source).getValue(); String beanName = cacheName + "-initializer"; if (!ctx.getRegistry().containsBeanDefinition(beanName)) { BeanDefinitionBuilder initializer = BeanDefinitionBuilder.rootBeanDefinition(JCacheInitializer.class); initializer.addConstructorArg(cacheName); ctx.getRegistry().registerBeanDefinition(beanName, initializer.getBeanDefinition()); } return beanName; } }
Lastly, the various artifacts need to be registered with the Spring XML infrastructure.
# in META-INF/spring.handlers
http\://www.foo.com/schema/jcache=com.foo.JCacheNamespaceHandler
# in META-INF/spring.schemas
http\://www.foo.com/schema/jcache/jcache.xsd=com/foo/jcache.xsd
Find below links to further resources concerning XML Schema and the extensible XML support described in this chapter.
One of the view technologies you can use with the Spring Framework is Java Server Pages (JSPs). To help you implement views using Java Server Pages the Spring Framework provides you with some tags for evaluating errors, setting themes and outputting internationalized messages.
Please note that the various tags generated by this form tag library are compliant with the XHTML-1.0-Strict specification and attendant DTD.
This appendix describes the spring.tld
tag library.
Provides BindStatus object for the given bind path. The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a "defaultHtmlEscape" context-param in web.xml).
Table 35.1. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
ignoreNestedPath | false | true | Set whether to ignore a nested path, if any. Default is to not ignore. |
path | true | true | The path to the bean or bean property to bind status information for. For instance account.name, company.address.zipCode or just employee. The status object will exported to the page scope, specifically for this bean or bean property |
Escapes its enclosed body content, applying HTML escaping and/or JavaScript escaping. The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a "defaultHtmlEscape" context-param in web.xml).
Table 35.2. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
javaScriptEscape | false | true | Set JavaScript escaping for this tag, as boolean value. Default is false. |
Provides Errors instance in case of bind errors. The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a "defaultHtmlEscape" context-param in web.xml).
Table 35.3. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
name | true | true | The name of the bean in the request, that needs to be inspected for errors. If errors are available for this bean, they will be bound under the errors key. |
Sets default HTML escape value for the current page. Overrides a "defaultHtmlEscape" context-param in web.xml, if any.
Table 35.4. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
defaultHtmlEscape | true | true | Set the default value for HTML escaping, to be put into the current PageContext. |
Retrieves the message with the given code, or text if code isn’t resolvable. The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a "defaultHtmlEscape" context-param in web.xml).
Table 35.5. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
arguments | false | true | Set optional message arguments for this tag, as a (comma-)delimited String (each String argument can contain JSP EL), an Object array (used as argument array), or a single Object (used as single argument). |
argumentSeparator | false | true | The separator character to be used for splitting the arguments string value; defaults to a comma (,). |
code | false | true | The code (key) to use when looking up the message. If code is not provided, the text attribute will be used. |
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
javaScriptEscape | false | true | Set JavaScript escaping for this tag, as boolean value. Default is false. |
message | false | true | A MessageSourceResolvable argument (direct or through JSP EL). Fits nicely when used in conjunction with Spring’s own validation error classes which all implement the MessageSourceResolvable interface. For example, this allows you to iterate over all of the errors in a form, passing each error (using a runtime expression) as the value of this message attribute, thus effecting the easy display of such error messages. |
scope | false | true | The scope to use when exporting the result to a variable. This attribute is only used when var is also set. Possible values are page, request, session and application. |
text | false | true | Default text to output when a message for the given code could not be found. If both text and code are not set, the tag will output null. |
var | false | true | The string to use when binding the result to the page, request, session or application scope. If not specified, the result gets outputted to the writer (i.e. typically directly to the JSP). |
Sets a nested path to be used by the bind tag’s path.
Table 35.6. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
path | true | true | Set the path that this tag should apply. E.g. customer to allow bind paths like address.street rather than customer.address.street. |
Retrieves the theme message with the given code, or text if code isn’t resolvable. The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a "defaultHtmlEscape" context-param in web.xml).
Table 35.7. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
arguments | false | true | Set optional message arguments for this tag, as a (comma-)delimited String (each String argument can contain JSP EL), an Object array (used as argument array), or a single Object (used as single argument). |
argumentSeparator | false | true | The separator character to be used for splitting the arguments string value; defaults to a comma (,). |
code | false | true | The code (key) to use when looking up the message. If code is not provided, the text attribute will be used. |
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
javaScriptEscape | false | true | Set JavaScript escaping for this tag, as boolean value. Default is false. |
message | false | true | A MessageSourceResolvable argument (direct or through JSP EL). |
scope | false | true | The scope to use when exporting the result to a variable. This attribute is only used when var is also set. Possible values are page, request, session and application. |
text | false | true | Default text to output when a message for the given code could not be found. If both text and code are not set, the tag will output null. |
var | false | true | The string to use when binding the result to the page, request, session or application scope. If not specified, the result gets outputted to the writer (i.e. typically directly to the JSP). |
Provides transformation of variables to Strings, using an appropriate custom PropertyEditor from BindTag (can only be used inside BindTag). The HTML escaping flag participates in a page-wide or application-wide setting (i.e. by HtmlEscapeTag or a defaultHtmlEscape context-param in web.xml).
Table 35.8. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
htmlEscape | false | true | Set HTML escaping for this tag, as boolean value. Overrides the default HTML escaping setting for the current page. |
scope | false | true | The scope to use when exported the result to a variable. This attribute is only used when var is also set. Possible values are page, request, session and application. |
value | true | true | The value to transform. This is the actual object you want to have transformed (for instance a Date). Using the PropertyEditor that is currently in use by the spring:bind tag. |
var | false | true | The string to use when binding the result to the page, request, session or application scope. If not specified, the result gets outputted to the writer (i.e. typically directly to the JSP). |
Creates URLs with support for URI template variables, HTML/XML escaping, and Javascript escaping. Modeled after the JSTL c:url tag with backwards compatibility in mind.
Table 35.9. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
url | true | true | The URL to build. This value can include template {placeholders} that are replaced with the URL encoded value of the named parameter. Parameters must be defined using the param tag inside the body of this tag. |
context | false | true | Specifies a remote application context path. The default is the current application context path. |
var | false | true | The name of the variable to export the URL value to. If not specified the URL is written as output. |
scope | false | true | The scope for the var. application, session, request and page scopes are supported. Defaults to page scope. This attribute has no effect unless the var attribute is also defined. |
htmlEscape | false | true | Set HTML escaping for this tag, as a boolean value. Overrides the default HTML escaping setting for the current page. |
javaScriptEscape | false | true | Set JavaScript escaping for this tag, as a boolean value. Default is false. |
Evaluates a Spring expression (SpEL) and either prints the result or assigns it to a variable.
Table 35.10. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
expression | true | true | The expression to evaluate. |
var | false | true | The name of the variable to export the evaluation result to. If not specified the evaluation result is converted to a String and written as output. |
scope | false | true | The scope for the var. application, session, request and page scopes are supported. Defaults to page scope. This attribute has no effect unless the var attribute is also defined. |
htmlEscape | false | true | Set HTML escaping for this tag, as a boolean value. Overrides the default HTML escaping setting for the current page. |
javaScriptEscape | false | true | Set JavaScript escaping for this tag, as a boolean value. Default is false. |
One of the view technologies you can use with the Spring Framework is Java Server Pages (JSPs). To help you implement views using Java Server Pages the Spring Framework provides you with some tags for evaluating errors, setting themes and outputting internationalized messages.
Please note that the various tags generated by this form tag library are compliant with the XHTML-1.0-Strict specification and attendant DTD.
This appendix describes the spring-form.tld
tag library.
Renders an HTML input tag with type checkbox.
Table 36.1. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
label | false | true | Value to be displayed as part of the tag |
lang | false | true | HTML Standard Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
value | false | true | HTML Optional Attribute |
Renders multiple HTML input tags with type checkbox.
Table 36.2. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
delimiter | false | true | Delimiter to use between each input tag with type checkbox. There is no delimiter by default. |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
element | false | true | Specifies the HTML element that is used to enclose each input tag with type checkbox. Defaults to span. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
itemLabel | false | true | Value to be displayed as part of the input tags with type checkbox |
items | true | true | The Collection, Map or array of objects used to generate the input tags with type checkbox |
itemValue | false | true | Name of the property mapped to value attribute of the input tags with type checkbox |
lang | false | true | HTML Standard Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders field errors in an HTML span tag.
Table 36.3. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
delimiter | false | true | Delimiter for displaying multiple error messages. Defaults to the br tag. |
dir | false | true | HTML Standard Attribute |
element | false | true | Specifies the HTML element that is used to render the enclosing errors. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | false | true | Path to errors object for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML form tag and exposes a binding path to inner tags for binding.
Table 36.4. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
acceptCharset | false | true | Specifies the list of character encodings for input data that is accepted by the server processing this form. The value is a space- and/or comma-delimited list of charset values. The client must interpret this list as an exclusive-or list, i.e., the server is able to accept any single character encoding per entity received. |
action | false | true | HTML Required Attribute |
commandName | false | true | Name of the model attribute under which the form object is exposed. Defaults to command. |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
enctype | false | true | HTML Optional Attribute |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
method | false | true | HTML Optional Attribute |
modelAttribute | false | true | Name of the model attribute under which the form object is exposed. Defaults to command. |
name | false | true | HTML Standard Attribute - added for backwards compatibility cases |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
onreset | false | true | HTML Event Attribute |
onsubmit | false | true | HTML Event Attribute |
target | false | true | HTML Optional Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML input tag with type hidden using the bound value.
Table 36.5. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
path | true | true | Path to property for data binding |
Renders an HTML input tag with type text using the bound value.
Table 36.6. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
alt | false | true | HTML Optional Attribute |
autocomplete | false | true | Common Optional Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
maxlength | false | true | HTML Optional Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
onselect | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
readonly | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will make the HTML element readonly. |
size | false | true | HTML Optional Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders a form field label in an HTML label tag.
Table 36.7. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute. |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used only when errors are present. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
for | false | true | HTML Standard Attribute |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to errors object for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders a single HTML option. Sets selected as appropriate based on bound value.
Table 36.8. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
label | false | true | HTML Optional Attribute |
lang | false | true | HTML Standard Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
value | true | true | HTML Optional Attribute |
Renders a list of HTML option tags. Sets selected as appropriate based on bound value.
Table 36.9. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
itemLabel | false | true | Name of the property mapped to the inner text of the option tag |
items | true | true | The Collection, Map or array of objects used to generate the inner option tags |
itemValue | false | true | Name of the property mapped to value attribute of the option tag |
lang | false | true | HTML Standard Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML input tag with type password using the bound value.
Table 36.10. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
alt | false | true | HTML Optional Attribute |
autocomplete | false | true | Common Optional Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
maxlength | false | true | HTML Optional Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
onselect | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
readonly | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will make the HTML element readonly. |
showPassword | false | true | Is the password value to be shown? Defaults to false. |
size | false | true | HTML Optional Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML input tag with type radio.
Table 36.11. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
label | false | true | Value to be displayed as part of the tag |
lang | false | true | HTML Standard Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
value | false | true | HTML Optional Attribute |
Renders multiple HTML input tags with type radio.
Table 36.12. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
delimiter | false | true | Delimiter to use between each input tag with type radio. There is no delimiter by default. |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
element | false | true | Specifies the HTML element that is used to enclose each input tag with type radio. Defaults to span. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
itemLabel | false | true | Value to be displayed as part of the input tags with type radio |
items | true | true | The Collection, Map or array of objects used to generate the input tags with type radio |
itemValue | false | true | Name of the property mapped to value attribute of the input tags with type radio |
lang | false | true | HTML Standard Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML select element. Supports databinding to the selected option.
Table 36.13. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
itemLabel | false | true | Name of the property mapped to the inner text of the option tag |
items | false | true | The Collection, Map or array of objects used to generate the inner option tags |
itemValue | false | true | Name of the property mapped to value attribute of the option tag |
lang | false | true | HTML Standard Attribute |
multiple | false | true | HTML Optional Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
size | false | true | HTML Optional Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |
Renders an HTML textarea.
Table 36.14. Attributes
Attribute | Required? | Runtime Expression? | Description |
---|---|---|---|
accesskey | false | true | HTML Standard Attribute |
cols | false | true | HTML Required Attribute |
cssClass | false | true | Equivalent to "class" - HTML Optional Attribute |
cssErrorClass | false | true | Equivalent to "class" - HTML Optional Attribute. Used when the bound field has errors. |
cssStyle | false | true | Equivalent to "style" - HTML Optional Attribute |
dir | false | true | HTML Standard Attribute |
disabled | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will disable the HTML element. |
htmlEscape | false | true | Enable/disable HTML escaping of rendered values. |
id | false | true | HTML Standard Attribute |
lang | false | true | HTML Standard Attribute |
onblur | false | true | HTML Event Attribute |
onchange | false | true | HTML Event Attribute |
onclick | false | true | HTML Event Attribute |
ondblclick | false | true | HTML Event Attribute |
onfocus | false | true | HTML Event Attribute |
onkeydown | false | true | HTML Event Attribute |
onkeypress | false | true | HTML Event Attribute |
onkeyup | false | true | HTML Event Attribute |
onmousedown | false | true | HTML Event Attribute |
onmousemove | false | true | HTML Event Attribute |
onmouseout | false | true | HTML Event Attribute |
onmouseover | false | true | HTML Event Attribute |
onmouseup | false | true | HTML Event Attribute |
onselect | false | true | HTML Event Attribute |
path | true | true | Path to property for data binding |
readonly | false | true | HTML Optional Attribute. Setting the value of this attribute to true (without the quotes) will make the HTML element readonly. |
rows | false | true | HTML Required Attribute |
tabindex | false | true | HTML Standard Attribute |
title | false | true | HTML Standard Attribute |