3.0
Copyright © 2004-2009 Rod Johnson, Juergen Hoeller, Keith Donald, Colin Sampaleanu, Rob Harrop, Alef Arendsen, Thomas Risberg, Darren Davison, Dmitriy Kopylenko, Mark Pollack, Thierry Templier, Erwin Vervaet, Portia Tung, Ben Hale, Adrian Colyer, John Lewis, Costin Leau, Mark Fisher, Sam Brannen, Ramnivas Laddad, Arjen Poutsma, Chris Beams, Tareq Abedrabbo, Andy Clement
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.springsource.org/.
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:
Make a Java method execute in a database transaction without having to deal with transaction APIs.
Make a local Java method a remote procedure without having to deal with remote APIs.
Make a local Java method a management operation without having to deal with JMX APIs.
Make a local Java method a message handler without having to deal with JMS APIs.
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, Hibernate, and iBatis. 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, Web-Struts, 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-Struts module contains the support classes for integrating a classic Struts web tier within a Spring application. Note that this support is now deprecated as of Spring 3.0. Consider migrating your application to Struts 2.0 and its Spring integration or to a Spring MVC solution.
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, JDO and iBatis;
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 WebWork, Struts, Tapestry, 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 shot name for the module (e.g. spring-core
,
spring-webmvc
, spring-jms
, etc.). The actual
jar file name that you use may be in this form (see below) or it may
not, and normally it also has a version number in the file name (e.g.
spring-core-3.0.0.RELEASE.jar
).
In general, Spring publishes its artifacts to four different places:
On the community download site http://www.springsource.org/downloads/community.
Here you find all the Spring jars bundled together into a zip file
for easy download. The names of the jars here since version 3.0
are in the form
org.springframework.*-<version>.jar
.
Maven Central, which is the default repository that Maven
queries, and does not require any special configuration to use.
Many of the common libraries that Spring depends on also are
available from Maven Central and a large section of the Spring
community uses Maven for dependency management, so this is
convenient for them. The names of the jars here are in the form
spring-*-<version>.jar
and the Maven groupId is
org.springframework
.
The Enterprise Bundle Repository (EBR), which is run by
SpringSource and also hosts all the libraries that integrate with
Spring. Both Maven and Ivy repositories are available here for all
Spring jars and their dependencies, plus a large number of other
common libraries that people use in applications with Spring. Both
full releases and also milestones and development snapshots are
deployed here. The names of the jar files are in the same form as
the community download
(org.springframework.*-<version>.jar
), and the
dependencies are also in this "long" form, with external libraries
(not from SpringSource) having the prefix
com.springsource
. See the FAQ
for more information.
In a public Maven repository hosted on Amazon S3 for development snapshots and milestone releases (a copy of the final releases is also held here). The jar file names are in the same form as Maven Central, so this is a useful place to get development versions of Spring to use with other libraries depoyed in Maven Central.
So the first thing you need to decide is how to manage your dependencies: most people use an automated system like Maven or Ivy, but you can also do it manually by downloading all the jars yourself. When obtaining Spring with Maven or Ivy you have then to decide which place you'll get it from. In general, if you care about OSGi, use the EBR, since it houses OSGi compatible artifacts for all of Spring's dependencies, such as Hibernate and Freemarker. If OSGi does not matter to you, either place works, though there are some pros and cons between them. In general, pick one place or the other for your project; do not mix them. This is particularly important since EBR artifacts necessarily use a different naming convention than Maven Central artifacts.
Table 1.1. Comparison of Maven Central and SpringSource EBR Repositories
Feature | Maven Central | EBR |
---|---|---|
OSGi Compatible | Not explicit | Yes |
Number of Artifacts | Tens of thousands; all kinds | Hundreds; those that Spring integrates with |
Consistent Naming Conventions | No | Yes |
Naming Convention: GroupId | Varies. Newer artifacts often use domain name, e.g. org.slf4j. Older ones often just use the artifact name, e.g. log4j. | Domain name of origin or main package root, e.g. org.springframework |
Naming Convention: ArtifactId | Varies. Generally the project or module name, using a hyphen "-" separator, e.g. spring-core, logj4. | Bundle Symbolic Name, derived from the main package root, e.g. org.springframework.beans. If the jar had to be patched to ensure OSGi compliance then com.springsource is appended, e.g. com.springsource.org.apache.log4j |
Naming Convention: Version | Varies. Many new artifacts use m.m.m or m.m.m.X (with m=digit, X=text). Older ones use m.m. Some neither. Ordering is defined but not often relied on, so not strictly reliable. | OSGi version number m.m.m.X, e.g. 3.0.0.RC3. The text qualifier imposes alphabetic ordering on versions with the same numeric values. |
Publishing | Usually automatic via rsync or source control updates. Project authors can upload individual jars to JIRA. | Manual (JIRA processed by SpringSource) |
Quality Assurance | By policy. Accuracy is responsibility of authors. | Extensive for OSGi manifest, Maven POM and Ivy metadata. QA performed by Spring team. |
Hosting | Contegix. Funded by Sonatype with several mirrors. | S3 funded by SpringSource. |
Search Utilities | Various | http://www.springsource.com/repository |
Integration with SpringSource Tools | Integration through STS with Maven dependency management | Extensive integration through STS with Maven, Roo, CloudFoundry |
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 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 Ivy to manage dependencies when it is building, and our samples mostly use 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>3.0.0.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.
We used the Maven Central naming conventions in the example above, so that works with Maven Central or the SpringSource S3 Maven repository. To use the S3 Maven repository (e.g. for milestones or developer snaphots), you need to specify the repository location in your Maven configuration. For full releases:
<repositories> <repository> <id>com.springsource.repository.maven.release</id> <url>http://maven.springframework.org/release/</url> <snapshots><enabled>false</enabled></snapshots> </repository> </repositories>
For milestones:
<repositories> <repository> <id>com.springsource.repository.maven.milestone</id> <url>http://maven.springframework.org/milestone/</url> <snapshots><enabled>false</enabled></snapshots> </repository> </repositories>
And for snapshots:
<repositories> <repository> <id>com.springsource.repository.maven.snapshot</id> <url>http://maven.springframework.org/snapshot/</url> <snapshots><enabled>true</enabled></snapshots> </repository> </repositories>
To use the SpringSource EBR you would need to use a different naming convention for the dependencies. The names are usually easy to guess, e.g. in this case it is:
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>org.springframework.context</artifactId> <version>3.0.0.RELEASE</version> <scope>runtime</scope> </dependency> </dependencies>
You also need to declare the location of the repository explicitly (only the URL is important):
<repositories> <repository> <id>com.springsource.repository.bundles.release</id> <url>http://repository.springsource.com/maven/bundles/release/</url> </repository> </repositories>
If you are managing your dependencies by hand, the URL in the repository declaration above is not browseable, but there is a user interface at http://www.springsource.com/repository that can be used to search for and download dependencies. It also has handy snippets of Maven and Ivy configuration that you can copy and paste if you are using those tools.
If you prefer to use Ivy to manage dependencies then there are similar names and configuration options.
To configure Ivy to point to the SpringSource EBR add the
following resolvers to your
ivysettings.xml
:
<resolvers> <url name="com.springsource.repository.bundles.release"> <ivy pattern="http://repository.springsource.com/ivy/bundles/release/ [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> <artifact pattern="http://repository.springsource.com/ivy/bundles/release/ [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> </url> <url name="com.springsource.repository.bundles.external"> <ivy pattern="http://repository.springsource.com/ivy/bundles/external/ [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> <artifact pattern="http://repository.springsource.com/ivy/bundles/external/ [organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> </url> </resolvers>
The XML above is not valid because the lines are too long - if you copy-paste then remove the extra line endings in the middle of the url patterns.
Once Ivy is configured to look in the EBR adding a dependency is
easy. Simply pull up the details page for the bundle in question in
the repository browser and you'll find an Ivy snippet ready for you to
include in your dependencies section. For example (in
ivy.xml
):
<dependency org="org.springframework" name="org.springframework.core" rev="3.0.0.RELEASE" conf="compile->runtime"/>
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.
Switching off commons-logging
is easy: just make
sure it isn't on the classpath at runtime. In Maven terms you exclude
the dependency, and because of the way that the Spring dependencies
are declared, you only have to do that once.
<dependencies> <dependency> <groupId>org.springframework</groupId> <artifactId>spring-context</artifactId> <version>3.0.0.RELEASE</version> <scope>runtime</scope> <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 prvided. 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-context</artifactId> <version>3.0.0.RELEASE</version> <scope>runtime</scope> <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> <scope>runtime</scope> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.5.8</version> <scope>runtime</scope> </dependency> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-log4j12</artifactId> <version>1.5.8</version> <scope>runtime</scope> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.14</version> <scope>runtime</scope> </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 libaries not four (jcl-over-slf4j
and
logback
). If you do that you might also need to exlude 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 have 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-context</artifactId> <version>3.0.0.RELEASE</version> <scope>runtime</scope> </dependency> <dependency> <groupId>log4j</groupId> <artifactId>log4j</artifactId> <version>1.2.14</version> <scope>runtime</scope> </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.
If you have been using the Spring Framework for some time, you will be aware that Spring has undergone two major revisions: Spring 2.0, released in October 2006, and Spring 2.5, released in November 2007. It is now time for a third overhaul resulting in Spring 3.0.
The entire framework code has been revised to take advantage of Java 5 features like generics, varargs and other language improvements. We have done our best to still keep the code backwards compatible. We now have consistent use of generic Collections and Maps, consistent use of generic FactoryBeans, and also consistent resolution of bridge methods in the Spring AOP API. Generic ApplicationListeners automatically receive specific event types only. All callback interfaces such as TransactionCallback and HibernateCallback declare a generic result value now. Overall, the Spring core codebase is now freshly revised and optimized for Java 5.
Spring's TaskExecutor abstraction has been updated for close integration with Java 5's java.util.concurrent facilities. We provide first-class support for Callables and Futures now, as well as ExecutorService adapters, ThreadFactory integration, etc. This has been aligned with JSR-236 (Concurrency Utilities for Java EE 6) as far as possible. Furthermore, we provide support for asynchronous method invocations through the use of the new @Async annotation (or EJB 3.1's @Asynchronous annotation).
The Spring reference documentation has also substantially been updated to reflect all of the changes and new features for Spring 3.0. While every effort has been made to ensure that there are no errors in this documentation, some errors may nevertheless have crept in. If you do spot any typos or even more serious errors, and you can spare a few cycles during lunch, please do bring the error to the attention of the Spring team by raising an issue.
There is now a new getting started tutorial for developing a basic Spring 3.0 MVC web application. This tutorial is a separate document that can be found at the Spring Documentation page.
The framework modules have been revised and are now managed separately with one source-tree per module jar:
org.springframework.aop
org.springframework.beans
org.springframework.context
org.springframework.context.support
org.springframework.expression
org.springframework.instrument
org.springframework.jdbc
org.springframework.jms
org.springframework.orm
org.springframework.oxm
org.springframework.test
org.springframework.transaction
org.springframework.web
org.springframework.web.portlet
org.springframework.web.servlet
org.springframework.web.struts
We are now using a new Spring build system as known from Spring Web Flow 2.0. This gives us:
Ivy-based "Spring Build" system
consistent deployment procedure
consistent dependency management
consistent generation of OSGi manifests
This is a list of new features for Spring 3.0. We will cover these features in more detail later in this section.
Spring Expression Language
IoC enhancements/Java based bean metadata
General-purpose type conversion system and field formatting system
Object to XML mapping functionality (OXM) moved from Spring Web Services project
Comprehensive REST support
@MVC additions
Declarative model validation
Early support for Java EE 6
Embedded database support
BeanFactory interface returns typed bean instances as far as possible:
T getBean(Class<T> requiredType)
T getBean(String name, Class<T> requiredType)
Map<String, T> getBeansOfType(Class<T> type)
Spring's TaskExecutor interface now extends
java.util.concurrent.Executor
:
extended AsyncTaskExecutor supports standard Callables with Futures
New Java 5 based converter API and SPI:
stateless ConversionService and Converters
superseding standard JDK PropertyEditors
Typed ApplicationListener<E>
Spring introduces an expression language which is similar to Unified EL in its syntax but offers significantly more features. The expression language can be used when defining XML and Annotation based bean definitions and also serves as the foundation for expression language support across the Spring portfolio. Details of this new functionality can be found in the chapter Spring Expression Language (SpEL).
The Spring Expression Language was created to provide the Spring community 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.
The following is an example of how the Expression Language can be used to configure some properties of a database setup
<bean class="mycompany.RewardsTestDatabase"> <property name="databaseName" value="#{systemProperties.databaseName}"/> <property name="keyGenerator" value="#{strategyBean.databaseKeyGenerator}"/> </bean>
This functionality is also available if you prefer to configure your components using annotations:
@Repository public class RewardsTestDatabase { @Value("#{systemProperties.databaseName}") public void setDatabaseName(String dbName) { … } @Value("#{strategyBean.databaseKeyGenerator}") public void setKeyGenerator(KeyGenerator kg) { … } }
Some core features from the JavaConfig project have been added to the Spring Framework now. This means that the following annotations are now directly supported:
@Configuration
@Bean
@DependsOn
@Primary
@Lazy
@Import
@ImportResource
@Value
Here is an example of a Java class providing basic configuration using the new JavaConfig features:
package org.example.config; @Configuration public class AppConfig { private @Value("#{jdbcProperties.url}") String jdbcUrl; private @Value("#{jdbcProperties.username}") String username; private @Value("#{jdbcProperties.password}") String password; @Bean public FooService fooService() { return new FooServiceImpl(fooRepository()); } @Bean public FooRepository fooRepository() { return new HibernateFooRepository(sessionFactory()); } @Bean public SessionFactory sessionFactory() { // wire up a session factory AnnotationSessionFactoryBean asFactoryBean = new AnnotationSessionFactoryBean(); asFactoryBean.setDataSource(dataSource()); // additional config return asFactoryBean.getObject(); } @Bean public DataSource dataSource() { return new DriverManagerDataSource(jdbcUrl, username, password); } }
To get this to work you need to add the following component scanning entry in your minimal application context XML file.
<context:component-scan base-package="org.example.config"/> <util:properties id="jdbcProperties" location="classpath:org/example/config/jdbc.properties"/>
Or you can bootstrap a @Configuration
class directly using
AnnotationConfigApplicationContext
:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class); FooService fooService = ctx.getBean(FooService.class); fooService.doStuff(); }
See Section 3.11.2, “Instantiating the Spring container using
AnnotationConfigApplicationContext” for full information on
AnnotationConfigApplicationContext
.
@Bean
annotated methods are also supported
inside Spring components. They contribute a factory bean definition to
the container. See Defining bean metadata within
components for more information
A general purpose type conversion system has been introduced. The system is currently used by SpEL for type conversion, and may also be used by a Spring Container and DataBinder when binding bean property values.
In addition, a formatter SPI has been introduced for formatting field values. This SPI provides a simpler and more robust alternative to JavaBean PropertyEditors for use in client environments such as Spring MVC.
Object to XML mapping functionality (OXM) from the Spring Web
Services project has been moved to the core Spring Framework now. The
functionality is found in the org.springframework.oxm
package. More information on the use of the OXM
module can be found in the Marshalling XML using O/X
Mappers chapter.
The most exciting new feature for the Web Tier is the support for building RESTful web services and web applications. There are also some new annotations that can be used in any web application.
Server-side support for building RESTful applications has been
provided as an extension of the existing annotation driven MVC web
framework. Client-side support is provided by the
RestTemplate
class in the spirit of other
template classes such as JdbcTemplate
and
JmsTemplate
. Both server and client side REST
functionality make use of
HttpConverter
s to facilitate the
conversion between objects and their representation in HTTP requests
and responses.
The MarshallingHttpMessageConverter
uses
the Object to XML mapping functionality mentioned
earlier.
Refer to the sections on MVC and the RestTemplate for more information.
A mvc
namespace has been introduced that greatly simplifies Spring MVC configuration.
Additional annotations such as
@CookieValue
and
@RequestHeaders
have been added. See Mapping cookie values with the
@CookieValue annotation and Mapping request header attributes with
the @RequestHeader annotation for more information.
Several validation enhancements, including JSR 303 support that uses Hibernate Validator as the default provider.
We provide support for asynchronous method invocations through the use of the new @Async annotation (or EJB 3.1's @Asynchronous annotation).
JSR 303, JSF 2.0, JPA 2.0, etc
Convenient support for embedded Java database engines, including HSQL, H2, and Derby, is now provided.
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 3.14, “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 providng 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
J2EE web descriptor XML in the web.xml
file of the
application will typically suffice (see Section 3.13.4, “Convenient ApplicationContext
instantiation for web applications”). If you are using the SpringSource Tool
Suite Eclipse-powered development environment or Spring Roo 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. |
For information about using other forms of metadata with the Spring container, see:
Annotation-based configuration: Spring 2.5 introduced support for annotation-based configuration metadata.
Java-based
configuration: Starting with Spring 3.0, many features
provided by the Spring JavaConfig
project became part of the core Spring Framework. Thus you
can define beans external to your application classes by using Java
rather than XML files. To use these new features, see the
@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.
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-3.0.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-3.0.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-3.0.xsd"> <bean id="accountDao" class="org.springframework.samples.jpetstore.dao.ibatis.SqlMapAccountDao"> <!-- additional collaborators and configuration for this bean go here --> </bean> <bean id="itemDao" class="org.springframework.samples.jpetstore.dao.ibatis.SqlMapItemDao"> <!-- 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 SqlMapAccountDao
and SqlMapItemDao
are based on the iBatis
Object/Relational mapping framework. 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 or DTD.
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(Stringname, 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 PetStoreServiceImpl service = context.getBean("petStore", PetStoreServiceImpl.class); // use configured instance List 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:
A package-qualified class name: typically the actual implementation class of the bean being defined.
Bean behavioral configuration elements, which state how the bean should behave in the container (scope, lifecycle callbacks, and so forth).
References to other beans that are needed for the bean to do its work; these references are also called collaborators or dependencies.
Other configuration settings to set in the newly created object, for example, the number of connections to use in a bean that manages a connection pool, or the size limit of the pool.
This metadata translates to a set of properties that make up each bean definition.
Table 3.1. The bean definition
Property | Explained in... |
---|---|
class | |
name | |
scope | |
constructor arguments | |
properties | |
autowiring mode | |
dependency checking 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, and because it is a real XML
element ID attribute, the XML parser can do some extra validation when
other elements reference the id. As such, it is the preferred way to
specify a bean identifier. However, the XML specification does limit the
characters that are legal in XML ids. This is usually not a constraint,
but if you need to use one of these special XML characters, or 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.
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 Location 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 defintions. In XML-based
configuration metadata, you can use of 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 Section 3.3.2.3, “Instantiation using an instance factory method” and Section 3.7, “Bean definition inheritance”.) You use the
Class
property in one of two ways:
Typically, to specify the bean class to be constructed in
the case where the container itself directly creates the bean by
calling its constructor reflectively, somewhat equivalent to Java
code using the new
operator.
To specify the actual class containing the
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="exampleBean" class="examples.ExampleBean2" factory-method="createInstance"/>
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="com.foo.DefaultServiceLocator"> <!-- inject any dependencies required by this locator bean --> </bean> <!-- the bean to be created via the factory bean --> <bean id="exampleBean" factory-bean="serviceLocator" factory-method="createInstance"/>
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 { // No. of years to the 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.
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 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.
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:
The ApplicationContext
is
created an initialized with configuration metadata that describes
all the beans. Configuration metadata can be specified via XML,
Java code or annotations.
For each bean, its dependencies are expressed in the form of properties, constructor arguments, or arguments to the static-factory method if you are using that instead of a normal constructor. These dependencies are provided to the bean, when the bean is actually created.
Each property or constructor argument an actual definition of the value to set, or a reference to another bean in the container.
Each property or constructor argument which is a value is
converted from its specified format to the actual type of that
property or constructor argument. By default Spring can convert a
value supplied in string format to all built-in types, such as
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 3.5, “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 IntializingBean 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. As mentioned
previously, JavaBeans PropertyEditors
are
used to convert these string 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-3.0.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 local="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,
or
local,
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 local
attribute leverages the ability of the XML parser to validate XML id
references within the same file. The value of the
local
attribute must be the same as the
id
attribute of the target bean. The XML parser
issues an error if no matching element is found in the same file. As
such, using the local variant is the best choice (in order to know
about errors as early as possible) if the target bean is in the same
XML file.
<ref local="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>
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 scoped as prototypes. 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
As of Spring 2.0, the container 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. The merging feature is available
only in Spring 2.0 and later.
In Java 5 and later, you can use strongly typed collections
(using generic types). 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 2.0 and later 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-3.0.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 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-3.0.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 |
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:
Autowiring can significantly reduce the need to specify properties or constructor arguments. (Other mechanisms such as a bean template discussed elsewhere in this chapter are also valuable in this regard.)
Autowiring can update a configuration as your objects evolve. For example, if you need to add a dependency to a class, that dependency can be satisfied automatically your needing to modify the configuration. Thus autowiring can be especially useful during development, without negating the option of switching to explicit wiring when the code base becomes more stable.
[2] When using XML-based configuration metadata, 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 3.2. 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. If this is not desirable, setting the
|
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. |
autodetect | Chooses constructor or byType through introspection of the bean class. If a default constructor is found, the byType mode is applied. |
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:
Explicit dependencies in 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.
Autowiring is less exact than explicit wiring. Although, as noted in the above table, Spring is careful to avoid guessing in case of ambiguity that might have unexpected results, the relationships between your Spring-managed objects are no longer documented explicitly.
Wiring information may not be available to tools that may generate documentation from a Spring container.
Multiple bean definitions within the container may match the type specified by the setter method or constructor argument to be autowired. For arrays, collections, or Maps, this is not necessarily a problem. However for dependencies that expect a single value, this ambiguity is not arbitrarily resolved. If no unique bean definition is available, an exception is thrown.
In the latter scenario, you have several options:
Abandon autowiring in favor of explicit wiring.
Avoid autowiring for a bean definition by setting its
autowire-candidate
attributes to
false
as described in the next section.
Designate a single bean definition as the
primary candidate by setting the
primary
attribute of its
<bean/>
element to
true
.
If you are using Java 5 or later, implement the more fine-grained control available with annotation-based configuration, as described in Section 3.9, “Annotation-based container configuration”.
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.
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.
The Spring IoC container can check for unresolved dependencies of a bean deployed into the container. When enabling checking for unresolved dependencies all JavaBean properties of the bean must have explicit values set for them in the bean definition or have their values set via autowiring.
This feature is useful when you want to ensure that all properties
(or all properties of a certain type) are set on a bean. A bean class
often has default values for many properties, or some properties do not
apply to all usage scenarios, so this feature is of limited use. You can
enable dependency checking per bean, just as with the autowiring
functionality. The default is to not check
dependencies. In XML-based configuration metadata, you specify
dependency checking via the dependency-check
attribute in a bean definition, which can have the following
values.
Table 3.3. Dependency checking modes
Mode | Explanation |
---|---|
none | (Default) No dependency checking. Properties of the bean that have no value specified for them are not set. |
simple | Dependency checking for primitive types and collections (everything except collaborators). |
object | Dependency checking for collaborators only. |
all | Dependency checking for collaborators, primitive types, and collections. |
If you use Java 5 and thus have access to source-level
annotations, you may find Section 27.2.1, “@Required”
to be of
interest.
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, you must have the CGLIB
jar(s) in your classpath. 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 3.4. Bean scopes
Scope | Description |
---|---|
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 |
Thread-scoped beans | |
---|---|
As of Spring 3.0, a thread scope is available, but is not registered by default. For more information, see the documentation for SimpleThreadScope. For instructions on how to register this or any other custom scope, see Section 3.5.5.2, “Using a custom scope”. |
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:
<!-- using spring-beans-2.0.dtd --> <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 3.6.1, “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 3.4.7, “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 2.4+ web container, with requests processed
outside of Spring's DispatcherServlet (for example, when using JSF or
Struts), you need to add the following
javax.servlet.ServletRequestListener
to
the declarations in your web applications web.xml
file:
<web-app> ... <listener> <listener-class> org.springframework.web.context.request.RequestContextListener </listener-class> </listener> ... </web-app>
If you use an older web container (Servlet 2.3), use the
provided javax.servlet.Filter
implementation. The following snippet of XML configuration must be
included in the web.xml
file of your web
application if you want to access web-scoped beans in requests outside
of Spring's DispatcherServlet on a Servlet 2.3 container. (The filter
mapping depends on the surrounding web application configuration, so
you must 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-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.xsd"> <!-- an HTTP Session-scoped bean exposed as a proxy --> <bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <!-- this next element effects the proxying of 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. (If
you choose class-based proxying, you also need the CGLIB library in
your classpath. See the section called “Choosing the type of proxy to create” and Appendix C, 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. This means that you
need to have the CGLIB library in the classpath of your
application.
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 7.6, “Proxying mechanisms”.
As of Spring 2.0, 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 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-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.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. You can also achieve the same integration with the container
without coupling your classes to Spring interfaces though 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 3.8, “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, 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,
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 beans 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:
Methods annotated with
@PostConstruct
afterPropertiesSet()
as defined by the
InitializingBean
callback
interface
A custom configured init()
method
Destroy methods are called in the same order:
Methods annotated with
@PreDestroy
destroy()
as defined by the
DisposableBean
callback
interface
A custom configured 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.contxt.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 3.13, “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 3.4.5, “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
ApplicationFactory
is autowired into a
field, constructor argument, or method parameter that is expecting the
BeanFactory
type if the field,
constructor, or method in question carries the
@Autowired
annotation. For more
information, see Section 3.9.2, “@Autowired and @Inject”.
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
s
afterPropertiesSet or a custom init-method.
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 any
ApplicationContext
implementation classes.
You can extend The Spring IoC container infinitely 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 otherwise initializing a bean, you can
plug in one or more BeanPostProcessor
implementations.
You can configure multiple BeanPostProcessor
interfaces. You can control the order in which these
BeanPostProcessor
interfaces 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 more details, consult the Javadoc for the
BeanPostProcessor
and
Ordered
interfaces.
Note | |
---|---|
To change the actual bean definition (that is, the recipe 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 afterPropertiesSet
and any declared init method) are called, and also afterwards. 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 and
they do this proxy-wrapping logic.
An ApplicationContext
automatically detects any beans that are defined in
the configuration metadata it receives that implement the
BeanPostProcessor
interface. The
ApplicationContext
registers these beans
as post-processors, to be then called appropriately by the container
upon bean creation. You can then deploy the post-processors as you would
any bean.
BeanPostProcessors and AOP auto-proxying | |
---|---|
Classes that implement the
For any such bean, you should see an info log message: “Bean foo is not eligible for getting processed by all BeanPostProcessor interfaces (for example: not eligible for auto-proxying)”. |
The following examples show how to write, register, and use
BeanPostProcessors
in the context of 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-3.0.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-3.0.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 2.0 dynamic language support is detailed in the chapter
entitled Chapter 26, Dynamic language support.)
The following small driver script 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 execution 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
shown in Section 27.2.1, “@Required” which
demonstrates the usage of a custom
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 the
BeanPostProcessor
, with one major
difference: BeanFactoryPostProcessor
s operate on the
bean configuration metadata; that is, the Spring
IoC container allows BeanFactoryPostProcessors
to
read the configuration metadata and potentially change it
before the container instantiates any beans other
than BeanFactoryPostProcessors
.
You can configure multiple
BeanFactoryPostProcessors
. 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 (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 of an ApplicationContext,
in order to apply changes to the configuration metadata that defines a
container. Spring includes a number of pre-existing 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
detects any
beans that are deployed into it and that implement the
BeanFactoryPostProcessor
interface. It
automatically uses these beans as bean factory post-processors, at the
appropriate time. You can then 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 into another
separate file in 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 ${jdbc.username} is replaced at runtime with the value 'sa' and similarly for other placeholder values that match to keys in the property file. The PropertyPlaceholderConfigurer checks for placeholders in most locations of a bean definition and the placeholder prefix and suffix can be customized.
With the context
namespace introduced in
Spring 2.5, it is possible to configure property placeholders with a
dedicated configuration element. You can provide multiple locations as
a comma-separated list in the location
attribute.
<context:property-placeholder location="classpath:com/foo/jdbc.properties"/>
The PropertyPlaceholderConfigurer
does
not look for properties only in the Properties
file you specify, but also checks against the Java
System
properties if it cannot find a property
you are trying to use. You can customize this behavior by setting the
systemPropertiesMode
property of the configurer. It
has three values that specify configurer behavior: always override,
never override, and override only if the property
is not found in the properties file specified.
Consult the Javadoc for the
PropertyPlaceholderConfigurer
for more
information.
Class name substitution | |
---|---|
You can use the
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations"> <value>classpath:com/foo/strategy.properties</value> </property> <property name="properties"> <value>custom.strategy.class=com.foo.DefaultStrategy</value> </property> </bean> <bean id="serviceStrategy" class="${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 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 is usable against 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
... the 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"/>
You 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, not the bean
it produces, you preface the bean id with the ampersand symbol
&
(without quotes) 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
, and invoking
getBean("&myBean")
returns the
FactoryBean
instance itself.
As mentioned in Section 3.8.1.2, “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. As of
Spring 2.5, it is now 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 3.4.5, “Autowiring collaborators” but
with more fine-grained control and wider applicability. Spring 2.5 also
adds support for JSR-250 annotations such as
@Resource
,
@PostConstruct
, and
@PreDestroy
. Spring 3.0 adds support for
JSR-330 (Dependency Injection for Java) annotations contained in the
javax.inject package such as @Inject
,
@Qualifier, @Named, and @Provider
if the JSR330 jar is
present on the classpath. Use of these annotations also requires that
certain BeanPostProcessors
be registered
within the Spring container. 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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.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:
Note | |
---|---|
JSR 330's @Inject annotation can be used in place of Spring's
|
public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
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; } // ... }
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
,
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() { } // ... }
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:
Note | |
---|---|
JSR 330's |
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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.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:
Note | |
---|---|
You can use JSR 330's |
@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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.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 3.10, “Classpath scanning and managed components”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Section 3.10.7, “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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.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>
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 particular implementation of
AutowireCandidateResolver
that is
activated for the application context depends on the Java version. In
versions earlier than Java 5, the qualifier annotations are not
supported, and therefore autowire candidates are solely determined by
the autowire-candidate
value of each bean definition
as well as by any default-autowire-candidates
pattern(s) available on the <beans/>
element.
In Java 5 or later, the presence of
@Qualifier
annotations and any custom
annotations registered with the
CustomAutowireConfigurer
will also play a
role.
Regardless of the Java version, when multiple beans qualify as
autowire candidates, the determination of a "primary" candidate is the
same: 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 Java
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 Section 3.6.1.4, “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 3.9, “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 |
In Spring 2.0 and later, 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 13.2.2, “Exception translation”.
Spring 2.5 introduces 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.
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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd"> <context:component-scan base-package="org.example"/> </beans>
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. |
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
|
Note | |
---|---|
In Spring 3.0 RC1 you can use JSR 330's
|
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 3.5. Filter Types
Filter Type | Example Expression | Description |
---|---|---|
annotation | org.example.SomeAnnotation | An annotation to be present at the type level in target components. |
assignable | org.example.SomeClass | A class (or interface) that the target components are assignable to (extend/implement). |
aspectj | org.example..*Service+ | An AspectJ type expression to be matched by the target components. |
regex | org\.example\.Default.* | A regex expression to be matched by the target components class names. |
custom | org.example.MyTypeFilter | A custom implementation of the
org.springframework.core.type
.TypeFilter interface. |
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. 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 @BeanAge(1) 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.
Note | |
---|---|
JSR 330's @Named annotation can be used as a means to both detect components and to provide them with a name. This behavior is enabled automatically if you have the JSR 330 JAR on the classpath. |
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(StandardScopes.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 Section 3.5.4.5, “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 3.9.3, “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. |
The central artifact in Spring's new Java-configuration support is
the @Configuration
-annotated class. These
classes consist principally of
@Bean
-annotated methods that define
instantiation, configuration, and initialization logic for objects to
be managed by the Spring IoC container.
Annotating a class with the
@Configuration
indicates that the class
can be used by the Spring IoC container as a source of bean definitions.
The simplest possible @Configuration
class would read as follows:
@Configuration public class AppConfig { @Bean public void MyService myService() { return new MyServiceImpl(); } }
For those more familiar with Spring <beans/>
XML, the AppConfig
class above would be equivalent to:
<beans> <bean id="myService" class="com.acme.services.MyServiceImpl"/> </beans>
As you can see, the @Bean
annotation plays the same role
as the <bean/>
element. The @Bean
annotation 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) { ApplicationContext 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) { ApplicationContext 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 --> <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 JavaConfigWebApplicationContext 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 /main/* to the dispatcher servlet --> <servlet-mapping> <servlet-name>dispatcher</servlet-name> <url-pattern>/main/*</url-pattern> </servlet-mapping> </web-app>
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 { public @Bean A a() { return new A(); } } @Configuration @Import(ConfigA.class) public class ConfigB { public @Bean 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 { private @Autowired AccountRepository accountRepository; public @Bean TransferService transferService() { return new TransferServiceImpl(accountRepository); } } @Configuration public class RepositoryConfig { private @Autowired DataSource dataSource; public @Bean AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } } @Configuration @Import({ServiceConfig.class, RepositoryConfig.class}) public class SystemTestConfig { public @Bean 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 { private @Autowired RepositoryConfig repositoryConfig; public @Bean 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 { private @Autowired RepositoryConfig repositoryConfig; public @Bean TransferService transferService() { return new TransferServiceImpl(repositoryConfig.accountRepository()); } } @Configuration public interface RepositoryConfig { @Bean AccountRepository accountRepository(); } @Configuration public class DefaultRepositoryConfig implements RepositoryConfig { public @Bean AccountRepository accountRepository() { return new JdbcAccountRepository(...); } } @Configuration @Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config! public class SystemTestConfig { public @Bean 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.
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 { private @Autowired DataSource dataSource; public @Bean AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } public @Bean 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 { private @Value("${jdbcProperties.url}") String url; private @Value("${jdbcProperties.username}") String username; private @Value("${jdbcProperties.password}") String password; public @Bean 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
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
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.
Beans declared in a
@Configuration
-annotated class support
the regular lifecycle callbacks. Any classes defined with the
@Bean
annotation 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; }
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(); } } }
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 3.3.1, “Naming beans”, it is sometimes desirable
to give a single bean multiple names, otherwise known as bean 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... } }
The context
namespace introduced in Spring 2.5
provides a load-time-weaver
element.
<beans> <context:load-time-weaver/> </beans>
Adding this element to an XML-based Spring configuration file
activates a Spring LoadTimeWeaver
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 7.8.4, “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 implements 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:
Access to messages in i18n-style, through
the MessageSource
interface.
Access to resources, such as URLs and
files, through the ResourceLoader
interface.
Event publication to beans implementing the
ApplicationListener
interface, through
the use of the
ApplicationEventPublisher
interface.
Loading of multiple (hierarchical)
contexts, allowing each to be focused on one particular
layer, such as the web layer of an application, through the
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="test-messages"/> </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 3.6. Built-in Events
Event | Explanation |
---|---|
ContextRefreshedEvent | Published when the
ApplicationContext is initialized
or refreshed, for example, using the
refresh() method on the
ConfigurableApplicationContext
interface. "Initialized" here means that all beans are loaded,
post-processor beans are detected and activated, singletons are
pre-instantiated, and the
ApplicationContext object is
ready for use. As long as the context has not been closed, a
refresh can be triggered multiple times, provided that the
chosen ApplicationContext
actually supports such "hot" refreshes. For example,
XmlWebApplicationContext supports hot
refreshes, but GenericApplicationContext
does not. |
ContextStartedEvent | Published when the
ApplicationContext is started,
using the start() method on the
ConfigurableApplicationContext
interface. "Started" here means that all
Lifecycle beans receive an
explicit start signal. Typically this signal is used to restart
beans after an explicit stop, but it may also be used to start
components that have not been configured for autostart , for
example, components that have not already started on
initialization. |
ContextStoppedEvent | Published when the
ApplicationContext is stopped,
using the stop() method on the
ConfigurableApplicationContext
interface. "Stopped" here means that all
Lifecycle beans receive an
explicit stop signal. A stopped context may be restarted through
a start() call. |
ContextClosedEvent | Published when the
ApplicationContext is closed,
using the close() method on the
ConfigurableApplicationContext
interface. "Closed" here means that all singleton beans are
destroyed. A closed context reaches its end of life; it cannot
be refreshed or restarted. |
RequestHandledEvent | 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
DispatcherServlet . |
You can also implement custom events. Simply call the
publishEvent()
method on the
ApplicationContext
, specifying a
parameter that is an instance of your custom event class that implements
ApplicationEvent
. Event listeners receive events
synchronously. This means the publishEvent()
method blocks until all listeners have finished processing the event.
(It is possible to supply an alternate event publishing strategy through
an ApplicationEventMulticaster
implementation). Furthermore, when a listener receives an event, it
operates inside the transaction context of the publisher, if a
transaction context is available.
This example shows the bean definitions used to configure an
ApplicationContext
:
<bean id="emailer" class="example.EmailBean"> <property name="blackList"> <list> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </list> </property> </bean> <bean id="blackListListener" class="example.BlackListNotifier"> <property name="notificationAddress" value="[email protected]"/> </bean>
This example shows the implementation of the classes refered to in the previous bean definitions:
public class EmailBean implements ApplicationContextAware { private List blackList; private ApplicationContext ctx; public void setBlackList(List blackList) { this.blackList = blackList; } public void setApplicationContext(ApplicationContext ctx) { this.ctx = ctx; } public void sendEmail(String address, String text) { if (blackList.contains(address)) { BlackListEvent event = new BlackListEvent(address, text); ctx.publishEvent(event); return; } // send email... } }
public class BlackListNotifier implements ApplicationListener { private String notificationAddress; public void setNotificationAddress(String notificationAddress) { this.notificationAddress = notificationAddress; } public void onApplicationEvent(ApplicationEvent event) { if (event instanceof BlackListEvent) { // notify appropriate person... } } }
When the sendEmail method is called, if there are any emails that should be blacklisted, a custom event of the type BlackListEvent is published to the application context. The BlackListNotifier class which implements the interface ApplicationListener is registered as a subscriber to the application context and will receive the BlackListEvent. In order to access properties specific to BlackListEvent, the listener must perform a downcast.
For optimal usage and understanding of application contexts, users
should generally familiarize themselves with Spring's
Resource
abstraction, as described in the
chapter Chapter 4, 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.
The ContextLoader
mechanism comes in two
flavors: the ContextLoaderListener
and the
ContextLoaderServlet
. They have the same
functionality but differ in that the listener version is not reliable in
Servlet 2.3 containers. In the Servlet 2.4 specification, Servlet
context listeners must execute immediately after the Servlet context for
the web application is created and is available to service the first
request (and also when the Servlet context is about to be shut down). As
such a Servlet context listener is an ideal place to initialize the
Spring ApplicationContext
. All things
being equal, you should probably prefer
ContextLoaderListener
; for more information on
compatibility, have a look at the Javadoc for the
ContextLoaderServlet
.
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.
In Spring 2.5 and later, 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 2.0 and later 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 3.7. 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 4.1. 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 4.7.3, “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 3.4.5, “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 3.9.2, “@Autowired and @Inject”.
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");
... will actually load its bean definitions from the classpath.
However, it is still a 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
... the resolver follows a more complex but defined procedure to
try to resolve the wildcard. It produces a Resource for the path up to
the last non-wildcard segment and obtains a URL from it. If this URL
is not a "jar:" URL or container-specific variant (e.g.
"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.
Classpath*: portability | |
---|---|
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 resoltion
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 and 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's 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 recourse to copy-n-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 Sun. 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 Sun's website ( java.sun.com/products/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 5.1. Examples of properties
Expression | Explanation |
---|---|
name | Indicates the property name
corresponding to the methods getName() or
isName() and
setName(..) |
account.name | Indicates the nested property name
of the property account corresponding e.g.
to the methods getAccount().setName() or
getAccount().getName() |
account[2] | Indicates the third element of the
indexed property account . Indexed
properties can be of type array ,
list or other naturally
ordered collection |
account[COMPANYNAME] | Indicates the value of the map entry indexed by the key
COMPANYNAME of the Map property
account |
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 Sun.
A couple of examples where property editing is used in Spring:
setting properties on beans is done
using 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.
parsing HTTP request parameters in
Spring's MVC framework is done using all kinds of 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 5.2. Built-in PropertyEditors
Class | Explanation |
---|---|
ByteArrayPropertyEditor | Editor for byte arrays. Strings will simply be
converted to their corresponding byte representations.
Registered by default by BeanWrapperImpl . |
ClassEditor | Parses Strings representing classes to actual classes
and the other way around. When a class is not found, an
IllegalArgumentException is thrown. Registered by default by
BeanWrapperImpl . |
CustomBooleanEditor | Customizable property editor for Boolean properties.
Registered by default by BeanWrapperImpl , but, can be
overridden by registering custom instance of it as custom
editor. |
CustomCollectionEditor | Property editor for Collections, converting any source
Collection to a given target Collection type. |
CustomDateEditor | Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user registered as needed with appropriate format. |
CustomNumberEditor | Customizable property editor for any Number subclass
like Integer , Long ,
Float , Double . Registered
by default by BeanWrapperImpl , but can be
overridden by registering custom instance of it as a custom editor. |
FileEditor | Capable of resolving Strings to
java.io.File objects. Registered by default by
BeanWrapperImpl . |
InputStreamEditor | One-way property editor, capable of taking a text
string and producing (via an intermediate ResourceEditor and
Resource ) an
InputStream , so InputStream
properties may be directly set as Strings. Note that the default usage
will not close the InputStream for
you! Registered by default by BeanWrapperImpl . |
LocaleEditor | Capable of resolving Strings to
Locale objects and vice versa (the String
format is [language]_[country]_[variant], which is the same
thing the toString() method of Locale provides). Registered by
default by BeanWrapperImpl . |
PatternEditor | Capable of resolving Strings to JDK 1.5
Pattern objects and vice versa. |
PropertiesEditor | Capable of converting Strings (formatted using the
format as defined in the Javadoc for the java.lang.Properties
class) to Properties objects. Registered by
default by BeanWrapperImpl . |
StringTrimmerEditor | Property editor that trims Strings. Optionally allows
transforming an empty string into a null value. NOT
registered by default; must be user registered as needed. |
URLEditor | Capable of resolving a String representation of a URL
to an actual URL object. Registered by
default by BeanWrapperImpl . |
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 { private String format; public void setFormat(String format) { this.format = format; } public void setAsText(String text) { if (format != null && format.equals("upperCase")) { text = text.toUpperCase(); } ExoticType type = new ExoticType(text); setValue(type); } }
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"> <bean class="example.ExoticTypeEditor"> <property name="format" value="upperCase"/> </bean> </entry> </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 type you are converting from and R to be 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 Class<?>[][] 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 suppliment 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"> <list> <bean class="example.MyCustomConverter" /> </list> </property> </bean>
To work with a ConversionService instance programatically, 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; }
At runtime, Formatters are registered in a FormatterRegistry. 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.
Review the FormatterRegistry SPI below:
package org.springframework.format; public interface FormatterRegistry { void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser); void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter); void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory); }
As shown above, Formatters can be registered by fieldType or annotation.
FormattingConversionService
is the implementation of FormatterRegistry
suitable for most environments.
This implementation may be configured programatically, or declaratively as a Spring bean using FormattingConversionServiceFactoryBean
.
Because this implemementation also implements ConversionService
, it can be directly configured for use with Spring's DataBinder and the Spring Expression Language (SpEL).
In a Spring MVC application, you may configure a custom ConversionService instance explicity 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-3.0.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> <mvc:annotation-driven/> </beans>
With this one-line of configuation, 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:
<?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-3.0.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> <mvc:annotation-driven conversion-service="conversionService" /> <bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean" /> </beans>
A custom ConversionService instance is often constructed by a FactoryBean that internally registers custom Formatters and Converters programatically before the ConversionService is returned. See FormattingConversionServiceFactoryBean for an example.
Spring 3 introduces several enhancements to its validation support. First, the JSR-303 Bean Validation API is now fully supported. Second, when used programatically, 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 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, see the Bean Validation Specification. For information on the specific capabilities of the default reference implementation, see the Hibernate Validator documentation. To learn how to setup a JSR-303 implementation as a Spring bean, keep reading.
Spring provides full support for the JSR-303 Bean Validation API.
This includes convenient support for bootstrapping a JSR-303 implementation as a Spring bean.
This allows a javax.validation.Validator
to be injected wherever validation is needed in your application.
Use the LocalValidatorFactoryBean
to configure a default JSR-303 Validator as a Spring bean:
<bean id="validator" class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean" />
The basic configuration above will trigger JSR-303 to initialize using its default bootstrap mechanism. A JSR-303 provider, such as Hibernate Validator, is expected to be present in the classpath and will be detected automatically.
LocalValidatorFactoryBean
implements both javax.validation.Validator
and org.springframework.validation.Validator
.
You may inject a reference to one of these two interfaces into beans that need to invoke validation logic.
Inject a reference to javax.validation.Validator
if you prefer to work with the JSR-303 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 JSR-303 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 other configuration options for various JSR-303 constructs, from message interpolation to traversal resolution.
See the JavaDocs of LocalValidatorFactoryBean
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 programatically, 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();
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-3.0.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> <mvc:annotation-driven validator="globalValidator"/> </beans>
With JSR-303, a single javax.validation.Validator
instance typically validates all model objects that declare validation constraints.
To configure a JSR-303-backed Validator with Spring MVC, simply add a JSR-303 Provider, such as Hibernate Validator, to your classpath.
Spring MVC will detect it and automatically enable JSR-303 support across all Controllers.
The Spring MVC configuration required to enable JSR-303 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-3.0.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd"> <!-- JSR-303 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 JSR-303 provider. JSR-303, in turn, will enforce any constraints declared against the input. Any ConstaintViolations 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
Literal expressions
Boolean and relational operators
Regular expressions
Class expressions
Accessing properties, arrays, lists, maps
Method invocation
Relational operators
Assignment
Calling constructors
Ternary operator
Variables
User defined functions
Collection projection
Collection selection
Templated expressions
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(); context.setRootObject(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 the backslash character. 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");
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 Einstien')"; 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 numbers, strings and dates. Subtraction can be used on numbers and dates. 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);
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); // Mike 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!!!
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 TemplatedParserContext()).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 ${}
delimiter, in this case the result of calling that random() method. The
second argument to the method parseExpression()
is of
the type ParserContext
. The
ParserContext
interface is used to
influence how the expression is parsed in order to support the
expression templating functionality. The definition of
TemplatedParserContext
is shown below.
public class TemplatedParserContext implements ParserContext { public String getExpressionPrefix() { return "${"; } 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) { boolean found = false; for (Inventor inventor : members) { if (inventor.getName().equals(name)) { found = true; break; } } return found; } }
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...
... provide declarative enterprise services, especially as a replacement for EJB declarative services. The most important such service is declarative transaction management.
... allow users to implement custom aspects, complementing their use of OOP with AOP.
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: a modularization of a concern
that cuts across multiple classes. Transaction management is a good
example of a crosscutting concern in Java EE applications. In Spring
AOP, aspects are implemented using regular classes (the schema-based approach) or regular
classes annotated with the @Aspect
annotation (the @AspectJ
style).
Join point: a point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.
Advice: action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. (Advice types are discussed below.) Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors around the join point.
Pointcut: a predicate that matches join points. Advice is associated with a pointcut expression and runs at any join point matched by the pointcut (for example, the execution of a method with a certain name). The concept of join points as matched by pointcut expressions is central to AOP, and Spring uses the AspectJ pointcut expression language by default.
Introduction: declaring additional
methods or fields on behalf of a type. Spring AOP allows you to
introduce new interfaces (and a corresponding implementation) to any
advised object. For example, you could use an introduction to make a
bean implement an IsModified
interface, to simplify caching. (An introduction is known as an
inter-type declaration in the AspectJ community.)
Target object: object being advised by one or more aspects. Also referred to as the advised object. Since Spring AOP is implemented using runtime proxies, this object will always be a proxied object.
AOP proxy: an object created by the AOP framework in order to implement the aspect contracts (advise method executions and so on). In the Spring Framework, an AOP proxy will be a JDK dynamic proxy or a CGLIB proxy.
Weaving: linking aspects with other application types or objects to create an advised object. This can be done at compile time (using the AspectJ compiler, for example), load time, or at runtime. Spring AOP, like other pure Java AOP frameworks, performs weaving at runtime.
Types of advice:
Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).
After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.
After throwing advice: Advice to be executed if a method exits by throwing an exception.
After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).
Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception.
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 Java EE web 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 Java EE 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 2.0 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 | |
---|---|
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 7.4, “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 7.6.1, “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 Java 5 annotations. The @AspectJ style was introduced by the AspectJ project as part of the AspectJ 5 release. Spring 2.0 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.
Using the AspectJ compiler and weaver enables use of the
full AspectJ language, and is discussed in Section 7.8, “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 is enabled by including the following element inside your spring configuration:
<aop:aspectj-autoproxy/>
This assumes that you are using schema support as described in Appendix C, XML Schema-based configuration. See Section C.2.7, “The aop schema” for how to import the tags in the aop namespace.
If you are using the DTD, it is still possible to enable @AspectJ support by adding the following definition to your application context:
<bean class="org.springframework.aop.aspectj.annotation.AnnotationAwareAspectJAutoProxyCreator" />
You will also need two AspectJ libraries on the classpath of your
application: aspectjweaver.jar
and aspectjrt.jar
. These
libraries are available in the 'lib'
directory of an AspectJ installation
(version 1.5.1 or later required), or in the 'lib/aspectj'
directory of the
Spring-with-dependencies distribution.
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.
Advising 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 Java 5 based 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:
execution - for matching method execution join points, this is the primary pointcut designator you will use when working with Spring AOP
within - limits matching to join points within certain types (simply the execution of a method declared within a matching type when using Spring AOP)
this - limits matching to join points (the execution of methods when using Spring AOP) where the bean reference (Spring AOP proxy) is an instance of the given type
target - limits matching to join points (the execution of methods when using Spring AOP) where the target object (application object being proxied) is an instance of the given type
args - limits matching to join points (the execution of methods when using Spring AOP) where the arguments are instances of the given types
@target
- limits matching to join points (the execution of methods when
using Spring AOP) where the class of the executing object has an
annotation of the given type
@args
-
limits matching to join points (the execution of methods when
using Spring AOP) where the runtime type of the actual arguments
passed have annotations of the given type(s)
@within
- limits matching to join points within types that have the given
annotation (the execution of methods declared in types with the
given annotation when using Spring AOP)
@annotation - limits matching to join points where the subject of the join point (method being executed in Spring AOP) has the given annotation
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 7.3, “Schema-based AOP support”. The transaction elements are discussed in
Chapter 10, 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.
the execution of any public method:
execution(public * *(..))
the execution of any method with a name beginning with "set":
execution(* set*(..))
the execution of any method defined by the
AccountService
interface:
execution(* com.xyz.service.AccountService.*(..))
the execution of any method defined in the service package:
execution(* com.xyz.service.*.*(..))
the execution of any method defined in the service package or a sub-package:
execution(* com.xyz.service..*.*(..))
any join point (method execution only in Spring AOP) within the service package:
within(com.xyz.service.*)
any join point (method execution only in Spring AOP) within the service package or a sub-package:
within(com.xyz.service..*)
any join point (method execution only in Spring AOP) where
the proxy implements the
AccountService
interface:
this(com.xyz.service.AccountService)
'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.
any join point (method execution only in Spring AOP) where
the target object implements the
AccountService
interface:
target(com.xyz.service.AccountService)
'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.
any join point (method execution only in Spring AOP) which
takes a single parameter, and where the argument passed at runtime
is Serializable
:
args(java.io.Serializable)
'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
.
any join point (method execution only in Spring AOP) where
the target object has an
@Transactional
annotation:
@target(org.springframework.transaction.annotation.Transactional)
'@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.
any join point (method execution only in Spring AOP) where
the declared type of the target object has an
@Transactional
annotation:
@within(org.springframework.transaction.annotation.Transactional)
'@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.
any join point (method execution only in Spring AOP) where
the executing method has an
@Transactional
annotation:
@annotation(org.springframework.transaction.annotation.Transactional)
'@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.
any join point (method execution only in Spring AOP) which
takes a single parameter, and where the runtime type of the
argument passed has the @Classified
annotation:
@args(com.xyz.security.Classified)
'@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.
any join point (method execution only in Spring AOP) on a
Spring bean named 'tradeService
':
bean(tradeService)
any join point (method execution only in Spring AOP) on
Spring beans having names that match the wildcard expression
'*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:
Kinded designators are those which select a particular kind of join point. For example: execution, get, set, call, handler
Scoping designators are those which select a group of join points of interest (of probably many kinds). For example: within, withincode
Contextual designators are those that match (and optionally bind) based on context. For example: this, target, @annotation
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: 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.
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 2.0 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(); // ... }
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:
If the parameter names have been specified by the user explicitly, then the specified parameter names are used: both the advice and the pointcut annotations have an optional "argNames" attribute which can be used to specify the argument names of the annotated method - these argument names are available at runtime. For example:
@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 }
Using the '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.
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.
If the code has been compiled without the necessary debug
information, then Spring AOP will attempt to deduce the pairing
of binding variables to parameters (for example, if only one
variable is bound in the pointcut expression, and the advice
method only takes one parameter, the pairing is obvious!). If
the binding of variables is ambiguous given the available
information, then an
AmbiguousBindingException
will be
thrown.
If all of the above strategies fail then an
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");
(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 are unable to use Java 5, or simply prefer an XML-based format, then Spring 2.0 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 7.2, “@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 Appendix C, XML Schema-based configuration. See Section C.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 7.2, “@AspectJ support”. If you are using the schema based declaration style with Java 5, you can refer to named pointcuts defined in types (@Aspects) within the pointcut expression, but this feature is not available on JDK 1.4 and below (it relies on the Java 5 specific AspectJ reflection APIs). On JDK 1.5 therefore, 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 Section 7.2.3.3, “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 Section 7.2.4.5, “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
Section 7.2.4.6, “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-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.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 Section 7.2.4.7, “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 8.3.2, “Advice types in Spring”. Advisors can take advantage of AspectJ pointcut expressions though.
Spring 2.0 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 2.0. 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 7.2.7, “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.
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. Clearly if you are not running on Java 5+, then the XML style is the appropriate choice; for Java 5 projects there are various tradeoffs to consider.
The XML style will be most familiar to existing Spring users. It can be used with any JDK level (referring to named pointcuts from within pointcut expressions does still require Java 5+ though) and 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 overriden.
You will need the CGLIB 2 binaries on your classpath, whereas dynamic proxies are available with the JDK. Spring will automatically warn you when it needs CGLIB and the CGLIB library classes are not found on the classpath.
The constructor of your proxied object will be called twice. This is a natural consequence of the CGLIB proxy model whereby a subclass is generated for each proxied object. For each proxied instance, two objects are created: the actual proxied object and an instance of the subclass that implements the advice. This behavior is not exhibited when using JDK proxies. Usually, calling the constructor of the proxied type twice, is not an issue, as there are usually only assignments taking place and no real logic is implemented in the constructor.
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 7.8.1, “Using AspectJ to dependency inject domain objects with
Spring” and Section 7.8.2, “Other Spring aspects for AspectJ”
discuss the content of this library and how you can use it. Section 7.8.3, “Configuring AspectJ aspects using Spring IoC” discusses how to dependency inject AspectJ
aspects that are woven using the AspectJ compiler. Finally, Section 7.8.4, “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 prototype-scoped bean definition 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
prototype-scoped 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
and
@Resource
at the field or method level (see
Section 3.9, “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 7.8.4, “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). The Spring context
namespace defines a convenient tag for doing this: just include
the following in your application context configuration:
<context:spring-configured/>
If you are using the DTD instead of schema, the equivalent definition is:
<bean class="org.springframework.beans.factory.aspectj.AnnotationBeanConfigurerAspect" factory-method="aspectOf"/>
Instances of @Configurable
objects
created before the aspect has been configured will
result in a warning being issued to the 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>
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
<context:spring-configured/>
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 <context:spring-configured/>
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/spring-instrument.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.
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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.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 Java 5+ 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. The latter option is of course only an option if you are using Java 5+, but it does mean 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)
aspectjrt.jar
(version 1.5 or later)
aspectjweaver.jar
(version 1.5 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
using XML 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
<context:load-time-weaver/>
element. Find
below a valid <context:load-time-weaver/>
definition that uses default 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:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd"> <context:load-time-weaver/> </beans>
The above <context:load-time-weaver/>
bean definition will define and register a number of LTW-specific
infrastructure beans for you automatically, such as a
LoadTimeWeaver
and an
AspectJWeavingEnabler
. Notice how the
<context:load-time-weaver/>
is defined in the
'context
' namespace; note also that the referenced
XML Schema file is only available in versions of Spring 2.5 and
later.
What the above configuration does is define and register a
default LoadTimeWeaver
bean for you.
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
(summarised in the following table).
Table 7.1. DefaultContextLoadTimeWeaver
LoadTimeWeavers
Runtime Environment | LoadTimeWeaver implementation |
---|---|
Running in BEA's Weblogic 10 |
|
Running in Oracle's OC4J |
|
Running in GlassFish |
|
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 by specifying the fully-qualified classname as the value
of the 'weaver-class
' attribute of the
<context:load-time-weaver/>
element. Find
below an example of doing just that:
<?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-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd"> <context:load-time-weaver weaver-class="org.springframework.instrument.classloading.ReflectiveLoadTimeWeaver"/> </beans>
The LoadTimeWeaver
that is
defined and registered by the
<context:load-time-weaver/>
element 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
<context:load-time-weaver/>
left to discuss:
the 'aspectj-weaving
' attribute. 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, summarised below,
with the default value if the attribute is not present being '
autodetect
'
Table 7.2. 'aspectj-weaving
' attribute values
Attribute 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.