This chapter covers the Spring Framework implementation of the Inversion of Control (IoC) [1] principle. IoC is also known as dependency injection (DI). It is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies by using direct construction of classes, or a mechanism such as the Service Locator pattern.
The org.springframework.beans
and org.springframework.context
packages are the basis
for Spring Framework’s IoC container. The
BeanFactory
interface provides an advanced configuration mechanism capable of managing any type of
object.
ApplicationContext
is a sub-interface of BeanFactory
. It adds easier integration with Spring’s AOP
features; message resource handling (for use in internationalization), event
publication; and application-layer specific contexts such as the WebApplicationContext
for use in web applications.
In short, the BeanFactory
provides the configuration framework and basic
functionality, and the ApplicationContext
adds more enterprise-specific functionality.
The ApplicationContext
is a complete superset of the BeanFactory
, and is used
exclusively in this chapter in descriptions of Spring’s IoC container. For more
information on using the BeanFactory
instead of the ApplicationContext,
refer to
Section 4.17, “The BeanFactory”.
In Spring, the objects that form the backbone of your application and that are managed by the Spring IoC container are called beans. A bean is an object that is instantiated, assembled, and otherwise managed by a Spring IoC container. Otherwise, a bean is simply one of many objects in your application. Beans, and the dependencies among them, are reflected in the configuration metadata used by a container.
The interface org.springframework.context.ApplicationContext
represents the Spring IoC
container and is responsible for instantiating, configuring, and assembling the
aforementioned beans. The container gets its instructions on what objects to
instantiate, configure, and assemble by reading configuration metadata. The
configuration metadata is represented in XML, Java annotations, or Java code. It allows
you to express the objects that compose your application and the rich interdependencies
between such objects.
Several implementations of the ApplicationContext
interface are supplied
out-of-the-box with Spring. In standalone applications it is common to create an
instance of
ClassPathXmlApplicationContext
or FileSystemXmlApplicationContext
.
While XML has been the traditional format for defining configuration metadata you can
instruct the container to use Java annotations or code as the metadata format by
providing a small amount of XML configuration to declaratively enable support for these
additional metadata formats.
In most application scenarios, explicit user code is not required to instantiate one or
more instances of a Spring IoC container. For example, in a web application scenario, a
simple eight (or so) lines of boilerplate web descriptor XML in the web.xml
file
of the application will typically suffice (see Section 4.16.4, “Convenient ApplicationContext instantiation for web applications”). If you are using the
SpringSource Tool Suite Eclipse-powered development
environment this boilerplate configuration can be easily created with few mouse clicks or
keystrokes.
The following diagram is a high-level view of how Spring works. Your application classes
are combined with configuration metadata so that after the ApplicationContext
is
created and initialized, you have a fully configured and executable system or
application.
As the preceding diagram shows, the Spring IoC container consumes a form of configuration metadata; this configuration metadata represents how you as an application developer tell the Spring container to instantiate, configure, and assemble the objects in your application.
Configuration metadata is traditionally supplied in a simple and intuitive XML format, which is what most of this chapter uses to convey key concepts and features of the Spring IoC container.
Note | |
---|---|
XML-based metadata is not the only allowed form of configuration metadata. The Spring IoC container itself is totally decoupled from the format in which this configuration metadata is actually written. These days many developers choose Java-based configuration for their Spring applications. |
For information about using other forms of metadata with the Spring container, see:
@Configuration
, @Bean
, @Import
and @DependsOn
annotations.
Spring configuration consists of at least one and typically more than one bean
definition that the container must manage. XML-based configuration metadata shows these
beans configured as <bean/>
elements inside a top-level <beans/>
element. Java
configuration typically uses @Bean
annotated methods within a @Configuration
class.
These bean definitions correspond to the actual objects that make up your application.
Typically you define service layer objects, data access objects (DAOs), presentation
objects such as Struts Action
instances, infrastructure objects such as Hibernate
SessionFactories
, JMS Queues
, and so forth. Typically one does not configure
fine-grained domain objects in the container, because it is usually the responsibility
of DAOs and business logic to create and load domain objects. However, you can use
Spring’s integration with AspectJ to configure objects that have been created outside
the control of an IoC container. See Using AspectJ to
dependency-inject domain objects with Spring.
The following example shows the basic structure of XML-based configuration metadata:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <bean id="..." class="..."> <!-- collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions go here --> </beans>
The id
attribute is a string that you use to identify the individual bean definition.
The class
attribute defines the type of the bean and uses the fully qualified
classname. The value of the id attribute refers to collaborating objects. The XML for
referring to collaborating objects is not shown in this example; see
Dependencies for more information.
Instantiating a Spring IoC container is straightforward. The location path or paths
supplied to an ApplicationContext
constructor are actually resource strings that allow
the container to load configuration metadata from a variety of external resources such
as the local file system, from the Java CLASSPATH
, and so on.
ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"});
Note | |
---|---|
After you learn about Spring’s IoC container, you may want to know more about Spring’s
|
The following example shows the service layer objects (services.xml)
configuration file:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <!-- services --> <bean id="petStore" class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl"> <property name="accountDao" ref="accountDao"/> <property name="itemDao" ref="itemDao"/> <!-- additional collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions for services go here --> </beans>
The following example shows the data access objects daos.xml
file:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="accountDao" class="org.springframework.samples.jpetstore.dao.jpa.JpaAccountDao"> <!-- additional collaborators and configuration for this bean go here --> </bean> <bean id="itemDao" class="org.springframework.samples.jpetstore.dao.jpa.JapItemDao"> <!-- additional collaborators and configuration for this bean go here --> </bean> <!-- more bean definitions for data access objects go here --> </beans>
In the preceding example, the service layer consists of the class PetStoreServiceImpl
,
and two data access objects of the type JpaAccountDao
and JpaItemDao
(based
on the JPA Object/Relational mapping standard). The property name
element refers to the
name of the JavaBean property, and the ref
element refers to the name of another bean
definition. This linkage between id
and ref
elements expresses the dependency between
collaborating objects. For details of configuring an object’s dependencies, see
Dependencies.
It can be useful to have bean definitions span multiple XML files. Often each individual XML configuration file represents a logical layer or module in your architecture.
You can use the application context constructor to load bean definitions from all these
XML fragments. This constructor takes multiple Resource
locations, as was shown in the
previous section. Alternatively, use one or more occurrences of the <import/>
element
to load bean definitions from another file or files. For example:
<beans> <import resource="services.xml"/> <import resource="resources/messageSource.xml"/> <import resource="/resources/themeSource.xml"/> <bean id="bean1" class="..."/> <bean id="bean2" class="..."/> </beans>
In the preceding example, external bean definitions are loaded from three files,
services.xml
, messageSource.xml
, and themeSource.xml
. All location paths are
relative to the definition file doing the importing, so services.xml
must be in the
same directory or classpath location as the file doing the importing, while
messageSource.xml
and themeSource.xml
must be in a resources
location below the
location of the importing file. As you can see, a leading slash is ignored, but given
that these paths are relative, it is better form not to use the slash at all. The
contents of the files being imported, including the top level <beans/>
element, must
be valid XML bean definitions according to the Spring Schema.
Note | |
---|---|
It is possible, but not recommended, to reference files in parent directories using a relative "../" path. Doing so creates a dependency on a file that is outside the current application. In particular, this reference is not recommended for "classpath:" URLs (for example, "classpath:../services.xml"), where the runtime resolution process chooses the "nearest" classpath root and then looks into its parent directory. Classpath configuration changes may lead to the choice of a different, incorrect directory. You can always use fully qualified resource locations instead of relative paths: for example, "file:C:/config/services.xml" or "classpath:/config/services.xml". However, be aware that you are coupling your application’s configuration to specific absolute locations. It is generally preferable to keep an indirection for such absolute locations, for example, through "${…}" placeholders that are resolved against JVM system properties at runtime. |
The ApplicationContext
is the interface for an advanced factory capable of maintaining
a registry of different beans and their dependencies. Using the method T getBean(String
name, Class<T> requiredType)
you can retrieve instances of your beans.
The ApplicationContext
enables you to read bean definitions and access them as follows:
// create and configure beans ApplicationContext context = new ClassPathXmlApplicationContext(new String[] {"services.xml", "daos.xml"}); // retrieve configured instance PetStoreService service = context.getBean("petStore", PetStoreService.class); // use configured instance List<String> userList = service.getUsernameList();
You use getBean()
to retrieve instances of your beans. The ApplicationContext
interface has a few other methods for retrieving beans, but ideally your application
code should never use them. Indeed, your application code should have no calls to the
getBean()
method at all, and thus no dependency on Spring APIs at all. For example,
Spring’s integration with web frameworks provides for dependency injection for various
web framework classes such as controllers and JSF-managed beans.
A Spring IoC container manages one or more beans. These beans are created with the
configuration metadata that you supply to the container, for example, in the form of XML
<bean/>
definitions.
Within the container itself, these bean definitions are represented as BeanDefinition
objects, which contain (among other information) the following metadata:
This metadata translates to a set of properties that make up each bean definition.
Table 4.1. The bean definition
Property | Explained in… |
---|---|
class | |
name | |
scope | |
constructor arguments | |
properties | |
autowiring mode | |
lazy-initialization mode | |
initialization method | |
destruction method |
In addition to bean definitions that contain information on how to create a specific
bean, the ApplicationContext
implementations also permit the registration of existing
objects that are created outside the container, by users. This is done by accessing the
ApplicationContext’s BeanFactory via the method getBeanFactory()
which returns the
BeanFactory implementation DefaultListableBeanFactory
. DefaultListableBeanFactory
supports this registration through the methods registerSingleton(..)
and
registerBeanDefinition(..)
. However, typical applications work solely with beans
defined through metadata bean definitions.
Every bean has one or more identifiers. These identifiers must be unique within the container that hosts the bean. A bean usually has only one identifier, but if it requires more than one, the extra ones can be considered aliases.
In XML-based configuration metadata, you use the id
and/or name
attributes
to specify the bean identifier(s). The id
attribute allows you to specify
exactly one id. Conventionally these names are alphanumeric (myBean,
fooService, etc.), but may contain special characters as well. If you want to
introduce other aliases to the bean, you can also specify them in the name
attribute, separated by a comma (,
), semicolon (;
), or white space. As a
historical note, in versions prior to Spring 3.1, the id
attribute was
defined as an xsd:ID
type, which constrained possible characters. As of 3.1,
it is defined as an xsd:string
type. Note that bean id
uniqueness is still
enforced by the container, though no longer by XML parsers.
You are not required to supply a name or id for a bean. If no name or id is supplied
explicitly, the container generates a unique name for that bean. However, if you want to
refer to that bean by name, through the use of the ref
element or
Service Locator style lookup, you must provide a name.
Motivations for not supplying a name are related to using inner
beans and autowiring collaborators.
In a bean definition itself, you can supply more than one name for the bean, by using a
combination of up to one name specified by the id
attribute, and any number of other
names in the name
attribute. These names can be equivalent aliases to the same bean,
and are useful for some situations, such as allowing each component in an application to
refer to a common dependency by using a bean name that is specific to that component
itself.
Specifying all aliases where the bean is actually defined is not always adequate,
however. It is sometimes desirable to introduce an alias for a bean that is defined
elsewhere. This is commonly the case in large systems where configuration is split
amongst each subsystem, each subsystem having its own set of object definitions. In
XML-based configuration metadata, you can use the <alias/>
element to accomplish this.
<alias name="fromName" alias="toName"/>
In this case, a bean in the same container which is named fromName
, may also,
after the use of this alias definition, be referred to as toName
.
For example, the configuration metadata for subsystem A may refer to a DataSource via
the name subsystemA-dataSource
. The configuration metadata for subsystem B may refer to
a DataSource via the name subsystemB-dataSource
. When composing the main application
that uses both these subsystems the main application refers to the DataSource via the
name myApp-dataSource
. To have all three names refer to the same object you add to the
MyApp configuration metadata the following aliases definitions:
<alias name="subsystemA-dataSource" alias="subsystemB-dataSource"/> <alias name="subsystemA-dataSource" alias="myApp-dataSource" />
Now each component and the main application can refer to the dataSource through a name that is unique and guaranteed not to clash with any other definition (effectively creating a namespace), yet they refer to the same bean.
A bean definition essentially is a recipe for creating one or more objects. The container looks at the recipe for a named bean when asked, and uses the configuration metadata encapsulated by that bean definition to create (or acquire) an actual object.
If you use XML-based configuration metadata, you specify the type (or class) of object
that is to be instantiated in the class
attribute of the <bean/>
element. This
class
attribute, which internally is a Class
property on a BeanDefinition
instance, is usually mandatory. (For exceptions, see
the section called “Instantiation using an instance factory method” and Section 4.7, “Bean definition inheritance”.)
You use the Class
property in one of two ways:
new
operator.
static
factory method that will be
invoked to create the object, in the less common case where the container invokes a
static
, factory method on a class to create the bean. The object type returned
from the invocation of the static
factory method may be the same class or another
class entirely.
When you create a bean by the constructor approach, all normal classes are usable by and compatible with Spring. That is, the class being developed does not need to implement any specific interfaces or to be coded in a specific fashion. Simply specifying the bean class should suffice. However, depending on what type of IoC you use for that specific bean, you may need a default (empty) constructor.
The Spring IoC container can manage virtually any class you want it to manage; it is not limited to managing true JavaBeans. Most Spring users prefer actual JavaBeans with only a default (no-argument) constructor and appropriate setters and getters modeled after the properties in the container. You can also have more exotic non-bean-style classes in your container. If, for example, you need to use a legacy connection pool that absolutely does not adhere to the JavaBean specification, Spring can manage it as well.
With XML-based configuration metadata you can specify your bean class as follows:
<bean id="exampleBean" class="examples.ExampleBean"/> <bean name="anotherExample" class="examples.ExampleBeanTwo"/>
For details about the mechanism for supplying arguments to the constructor (if required) and setting object instance properties after the object is constructed, see Injecting Dependencies.
When defining a bean that you create with a static factory method, you use the class
attribute to specify the class containing the static
factory method and an attribute
named factory-method
to specify the name of the factory method itself. You should be
able to call this method (with optional arguments as described later) and return a live
object, which subsequently is treated as if it had been created through a constructor.
One use for such a bean definition is to call static
factories in legacy code.
The following bean definition specifies that the bean will be created by calling a
factory-method. The definition does not specify the type (class) of the returned object,
only the class containing the factory method. In this example, the createInstance()
method must be a static method.
<bean id="clientService" class="examples.ClientService" factory-method="createInstance"/>
public class ClientService { private static ClientService clientService = new ClientService(); private ClientService() {} public static ClientService createInstance() { return clientService; } }
For details about the mechanism for supplying (optional) arguments to the factory method and setting object instance properties after the object is returned from the factory, see Dependencies and configuration in detail.
Similar to instantiation through a static
factory method, instantiation with an instance factory method invokes a non-static
method of an existing bean from the container to create a new bean. To use this
mechanism, leave the class
attribute empty, and in the factory-bean
attribute,
specify the name of a bean in the current (or parent/ancestor) container that contains
the instance method that is to be invoked to create the object. Set the name of the
factory method itself with the factory-method
attribute.
<!-- the factory bean, which contains a method called createInstance() --> <bean id="serviceLocator" class="examples.DefaultServiceLocator"> <!-- inject any dependencies required by this locator bean --> </bean> <!-- the bean to be created via the factory bean --> <bean id="clientService" factory-bean="serviceLocator" factory-method="createClientServiceInstance"/>
public class DefaultServiceLocator { private static ClientService clientService = new ClientServiceImpl(); private DefaultServiceLocator() {} public ClientService createClientServiceInstance() { return clientService; } }
One factory class can also hold more than one factory method as shown here:
<bean id="serviceLocator" class="examples.DefaultServiceLocator"> <!-- inject any dependencies required by this locator bean --> </bean> <bean id="clientService" factory-bean="serviceLocator" factory-method="createClientServiceInstance"/> <bean id="accountService" factory-bean="serviceLocator" factory-method="createAccountServiceInstance"/>
public class DefaultServiceLocator { private static ClientService clientService = new ClientServiceImpl(); private static AccountService accountService = new AccountServiceImpl(); private DefaultServiceLocator() {} public ClientService createClientServiceInstance() { return clientService; } public AccountService createAccountServiceInstance() { return accountService; } }
This approach shows that the factory bean itself can be managed and configured through dependency injection (DI). See Dependencies and configuration in detail.
Note | |
---|---|
In Spring documentation, factory bean refers to a bean that is configured in the
Spring container that will create objects through an
instance or
static factory method. By contrast,
|
A typical enterprise application does not consist of a single object (or bean in the Spring parlance). Even the simplest application has a few objects that work together to present what the end-user sees as a coherent application. This next section explains how you go from defining a number of bean definitions that stand alone to a fully realized application where objects collaborate to achieve a goal.
Dependency injection (DI) is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean. This process is fundamentally the inverse, hence the name Inversion of Control (IoC), of the bean itself controlling the instantiation or location of its dependencies on its own by using direct construction of classes, or the Service Locator pattern.
Code is cleaner with the DI principle and decoupling is more effective when objects are provided with their dependencies. The object does not look up its dependencies, and does not know the location or class of the dependencies. As such, your classes become easier to test, in particular when the dependencies are on interfaces or abstract base classes, which allow for stub or mock implementations to be used in unit tests.
DI exists in two major variants, Constructor-based dependency injection and Setter-based dependency injection.
Constructor-based DI is accomplished by the container invoking a constructor with a
number of arguments, each representing a dependency. Calling a static
factory method
with specific arguments to construct the bean is nearly equivalent, and this discussion
treats arguments to a constructor and to a static
factory method similarly. The
following example shows a class that can only be dependency-injected with constructor
injection. Notice that there is nothing special about this class, it is a POJO that
has no dependencies on container specific interfaces, base classes or annotations.
public class SimpleMovieLister { // the SimpleMovieLister has a dependency on a MovieFinder private MovieFinder movieFinder; // a constructor so that the Spring container can inject a MovieFinder public SimpleMovieLister(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually uses the injected MovieFinder is omitted... }
Constructor argument resolution matching occurs using the argument’s type. If no potential ambiguity exists in the constructor arguments of a bean definition, then the order in which the constructor arguments are defined in a bean definition is the order in which those arguments are supplied to the appropriate constructor when the bean is being instantiated. Consider the following class:
package x.y; public class Foo { public Foo(Bar bar, Baz baz) { // ... } }
No potential ambiguity exists, assuming that Bar
and Baz
classes are not related by
inheritance. Thus the following configuration works fine, and you do not need to specify
the constructor argument indexes and/or types explicitly in the <constructor-arg/>
element.
<beans> <bean id="foo" class="x.y.Foo"> <constructor-arg ref="bar"/> <constructor-arg ref="baz"/> </bean> <bean id="bar" class="x.y.Bar"/> <bean id="baz" class="x.y.Baz"/> </beans>
When another bean is referenced, the type is known, and matching can occur (as was the
case with the preceding example). When a simple type is used, such as
<value>true</value>
, Spring cannot determine the type of the value, and so cannot match
by type without help. Consider the following class:
package examples; public class ExampleBean { // Number of years to calculate the Ultimate Answer private int years; // The Answer to Life, the Universe, and Everything private String ultimateAnswer; public ExampleBean(int years, String ultimateAnswer) { this.years = years; this.ultimateAnswer = ultimateAnswer; } }
In the preceding scenario, the container can use type matching with simple types if
you explicitly specify the type of the constructor argument using the type
attribute.
For example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg type="int" value="7500000"/> <constructor-arg type="java.lang.String" value="42"/> </bean>
Use the index
attribute to specify explicitly the index of constructor arguments. For
example:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg index="0" value="7500000"/> <constructor-arg index="1" value="42"/> </bean>
In addition to resolving the ambiguity of multiple simple values, specifying an index resolves ambiguity where a constructor has two arguments of the same type. Note that the index is 0 based.
You can also use the constructor parameter name for value disambiguation:
<bean id="exampleBean" class="examples.ExampleBean"> <constructor-arg name="years" value="7500000"/> <constructor-arg name="ultimateanswer" value="42"/> </bean>
Keep in mind that to make this work out of the box your code must be compiled with the debug flag enabled so that Spring can look up the parameter name from the constructor. If you can’t compile your code with debug flag (or don’t want to) you can use @ConstructorProperties JDK annotation to explicitly name your constructor arguments. The sample class would then have to look as follows:
package examples; public class ExampleBean { // Fields omitted @ConstructorProperties({"years", "ultimateAnswer"}) public ExampleBean(int years, String ultimateAnswer) { this.years = years; this.ultimateAnswer = ultimateAnswer; } }
Setter-based DI is accomplished by the container calling setter methods on your
beans after invoking a no-argument constructor or no-argument static
factory method to
instantiate your bean.
The following example shows a class that can only be dependency-injected using pure setter injection. This class is conventional Java. It is a POJO that has no dependencies on container specific interfaces, base classes or annotations.
public class SimpleMovieLister { // the SimpleMovieLister has a dependency on the MovieFinder private MovieFinder movieFinder; // a setter method so that the Spring container can inject a MovieFinder public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // business logic that actually uses the injected MovieFinder is omitted... }
The ApplicationContext
supports constructor-based and setter-based DI for the beans it
manages. It also supports setter-based DI after some dependencies have already been
injected through the constructor approach. You configure the dependencies in the form of
a BeanDefinition
, which you use in conjunction with PropertyEditor
instances to
convert properties from one format to another. However, most Spring users do not work
with these classes directly (i.e., programmatically) but rather with XML bean
definitions, annotated components (i.e., classes annotated with @Component
,
@Controller
, etc.), or @Bean
methods in Java-based @Configuration
classes. These
sources are then converted internally into instances of BeanDefinition
and used to
load an entire Spring IoC container instance.
The container performs bean dependency resolution as follows:
ApplicationContext
is created and initialized with configuration metadata that
describes all the beans. Configuration metadata can be specified via XML, Java code, or
annotations.
int
,
long
, String
, boolean
, etc.
The Spring container validates the configuration of each bean as the container is created. However, the bean properties themselves are not set until the bean is actually created. Beans that are singleton-scoped and set to be pre-instantiated (the default) are created when the container is created. Scopes are defined in Section 4.5, “Bean scopes”. Otherwise, the bean is created only when it is requested. Creation of a bean potentially causes a graph of beans to be created, as the bean’s dependencies and its dependencies' dependencies (and so on) are created and assigned. Note that resolution mismatches among those dependencies may show up late, i.e. on first creation of the affected bean.
You can generally trust Spring to do the right thing. It detects configuration problems,
such as references to non-existent beans and circular dependencies, at container
load-time. Spring sets properties and resolves dependencies as late as possible, when
the bean is actually created. This means that a Spring container which has loaded
correctly can later generate an exception when you request an object if there is a
problem creating that object or one of its dependencies. For example, the bean throws an
exception as a result of a missing or invalid property. This potentially delayed
visibility of some configuration issues is why ApplicationContext
implementations by
default pre-instantiate singleton beans. At the cost of some upfront time and memory to
create these beans before they are actually needed, you discover configuration issues
when the ApplicationContext
is created, not later. You can still override this default
behavior so that singleton beans will lazy-initialize, rather than be pre-instantiated.
If no circular dependencies exist, when one or more collaborating beans are being injected into a dependent bean, each collaborating bean is totally configured prior to being injected into the dependent bean. This means that if bean A has a dependency on bean B, the Spring IoC container completely configures bean B prior to invoking the setter method on bean A. In other words, the bean is instantiated (if not a pre-instantiated singleton), its dependencies are set, and the relevant lifecycle methods (such as a configured init method or the InitializingBean callback method) are invoked.
The following example uses XML-based configuration metadata for setter-based DI. A small part of a Spring XML configuration file specifies some bean definitions:
<bean id="exampleBean" class="examples.ExampleBean"> <!-- setter injection using the nested <ref/> element --> <property name="beanOne"> <ref bean="anotherExampleBean"/> </property> <!-- setter injection using the neater ref attribute --> <property name="beanTwo" ref="yetAnotherBean"/> <property name="integerProperty" value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public void setBeanOne(AnotherBean beanOne) { this.beanOne = beanOne; } public void setBeanTwo(YetAnotherBean beanTwo) { this.beanTwo = beanTwo; } public void setIntegerProperty(int i) { this.i = i; } }
In the preceding example, setters are declared to match against the properties specified in the XML file. The following example uses constructor-based DI:
<bean id="exampleBean" class="examples.ExampleBean"> <!-- constructor injection using the nested <ref/> element --> <constructor-arg> <ref bean="anotherExampleBean"/> </constructor-arg> <!-- constructor injection using the neater ref attribute --> <constructor-arg ref="yetAnotherBean"/> <constructor-arg type="int" value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { private AnotherBean beanOne; private YetAnotherBean beanTwo; private int i; public ExampleBean( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { this.beanOne = anotherBean; this.beanTwo = yetAnotherBean; this.i = i; } }
The constructor arguments specified in the bean definition will be used as arguments to
the constructor of the ExampleBean
.
Now consider a variant of this example, where instead of using a constructor, Spring is
told to call a static
factory method to return an instance of the object:
<bean id="exampleBean" class="examples.ExampleBean" factory-method="createInstance"> <constructor-arg ref="anotherExampleBean"/> <constructor-arg ref="yetAnotherBean"/> <constructor-arg value="1"/> </bean> <bean id="anotherExampleBean" class="examples.AnotherBean"/> <bean id="yetAnotherBean" class="examples.YetAnotherBean"/>
public class ExampleBean { // a private constructor private ExampleBean(...) { ... } // a static factory method; the arguments to this method can be // considered the dependencies of the bean that is returned, // regardless of how those arguments are actually used. public static ExampleBean createInstance ( AnotherBean anotherBean, YetAnotherBean yetAnotherBean, int i) { ExampleBean eb = new ExampleBean (...); // some other operations... return eb; } }
Arguments to the static
factory method are supplied via <constructor-arg/>
elements,
exactly the same as if a constructor had actually been used. The type of the class being
returned by the factory method does not have to be of the same type as the class that
contains the static
factory method, although in this example it is. An instance
(non-static) factory method would be used in an essentially identical fashion (aside
from the use of the factory-bean
attribute instead of the class
attribute), so
details will not be discussed here.
As mentioned in the previous section, you can define bean properties and constructor
arguments as references to other managed beans (collaborators), or as values defined
inline. Spring’s XML-based configuration metadata supports sub-element types within its
<property/>
and <constructor-arg/>
elements for this purpose.
The value
attribute of the <property/>
element specifies a property or constructor
argument as a human-readable string representation. Spring’s
conversion service is used to convert these
values from a String
to the actual type of the property or argument.
<bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close"> <!-- results in a setDriverClassName(String) call --> <property name="driverClassName" value="com.mysql.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/mydb"/> <property name="username" value="root"/> <property name="password" value="masterkaoli"/> </bean>
The following example uses the p-namespace for even more succinct XML configuration.
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="myDataSource" class="org.apache.commons.dbcp.BasicDataSource" destroy-method="close" p:driverClassName="com.mysql.jdbc.Driver" p:url="jdbc:mysql://localhost:3306/mydb" p:username="root" p:password="masterkaoli"/> </beans>
The preceding XML is more succinct; however, typos are discovered at runtime rather than design time, unless you use an IDE such as IntelliJ IDEA or the SpringSource Tool Suite (STS) that support automatic property completion when you create bean definitions. Such IDE assistance is highly recommended.
You can also configure a java.util.Properties
instance as:
<bean id="mappings" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <!-- typed as a java.util.Properties --> <property name="properties"> <value> jdbc.driver.className=com.mysql.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/mydb </value> </property> </bean>
The Spring container converts the text inside the <value/>
element into a
java.util.Properties
instance by using the JavaBeans PropertyEditor
mechanism. This
is a nice shortcut, and is one of a few places where the Spring team do favor the use of
the nested <value/>
element over the value
attribute style.
The idref
element is simply an error-proof way to pass the id (string value - not
a reference) of another bean in the container to a <constructor-arg/>
or <property/>
element.
<bean id="theTargetBean" class="..."/> <bean id="theClientBean" class="..."> <property name="targetName"> <idref bean="theTargetBean" /> </property> </bean>
The above bean definition snippet is exactly equivalent (at runtime) to the following snippet:
<bean id="theTargetBean" class="..." /> <bean id="client" class="..."> <property name="targetName" value="theTargetBean" /> </bean>
The first form is preferable to the second, because using the idref
tag allows the
container to validate at deployment time that the referenced, named bean actually
exists. In the second variation, no validation is performed on the value that is passed
to the targetName
property of the client
bean. Typos are only discovered (with most
likely fatal results) when the client
bean is actually instantiated. If the client
bean is a prototype bean, this typo and the resulting exception
may only be discovered long after the container is deployed.
Additionally, if the referenced bean is in the same XML unit, and the bean name is the
bean id, you can use the local
attribute, which allows the XML parser itself to
validate the bean id earlier, at XML document parse time.
<property name="targetName"> <!-- a bean with id theTargetBean must exist; otherwise an exception will be thrown --> <idref bean="theTargetBean"/> </property>
A common place (at least in versions earlier than Spring 2.0) where the <idref/>
element
brings value is in the configuration of AOP interceptors in a
ProxyFactoryBean
bean definition. Using <idref/>
elements when you specify the
interceptor names prevents you from misspelling an interceptor id.
The ref
element is the final element inside a <constructor-arg/>
or <property/>
definition element. Here you set the value of the specified property of a bean to be a
reference to another bean (a collaborator) managed by the container. The referenced bean
is a dependency of the bean whose property will be set, and it is initialized on demand
as needed before the property is set. (If the collaborator is a singleton bean, it may
be initialized already by the container.) All references are ultimately a reference to
another object. Scoping and validation depend on whether you specify the id/name of the
other object through the bean
, local,
or parent
attributes.
Specifying the target bean through the bean
attribute of the <ref/>
tag is the most
general form, and allows creation of a reference to any bean in the same container or
parent container, regardless of whether it is in the same XML file. The value of the
bean
attribute may be the same as the id
attribute of the target bean, or as one of
the values in the name
attribute of the target bean.
<ref bean="someBean"/>
Specifying the target bean through the parent
attribute creates a reference to a bean
that is in a parent container of the current container. The value of the parent
attribute may be the same as either the id
attribute of the target bean, or one of the
values in the name
attribute of the target bean, and the target bean must be in a
parent container of the current one. You use this bean reference variant mainly when you
have a hierarchy of containers and you want to wrap an existing bean in a parent
container with a proxy that will have the same name as the parent bean.
<!-- in the parent context --> <bean id="accountService" class="com.foo.SimpleAccountService"> <!-- insert dependencies as required as here --> </bean>
<!-- in the child (descendant) context --> <bean id="accountService" <-- bean name is the same as the parent bean --> class="org.springframework.aop.framework.ProxyFactoryBean"> <property name="target"> <ref parent="accountService"/> <!-- notice how we refer to the parent bean --> </property> <!-- insert other configuration and dependencies as required here --> </bean>
Note | |
---|---|
The |
A <bean/>
element inside the <property/>
or <constructor-arg/>
elements defines a
so-called inner bean.
<bean id="outer" class="..."> <!-- instead of using a reference to a target bean, simply define the target bean inline --> <property name="target"> <bean class="com.example.Person"> <!-- this is the inner bean --> <property name="name" value="Fiona Apple"/> <property name="age" value="25"/> </bean> </property> </bean>
An inner bean definition does not require a defined id or name; the container ignores
these values. It also ignores the scope
flag. Inner beans are always anonymous and
they are always created with the outer bean. It is not possible to inject inner
beans into collaborating beans other than into the enclosing bean.
In the <list/>
, <set/>
, <map/>
, and <props/>
elements, you set the properties
and arguments of the Java Collection
types List
, Set
, Map
, and Properties
,
respectively.
<bean id="moreComplexObject" class="example.ComplexObject"> <!-- results in a setAdminEmails(java.util.Properties) call --> <property name="adminEmails"> <props> <prop key="administrator">[email protected]</prop> <prop key="support">[email protected]</prop> <prop key="development">[email protected]</prop> </props> </property> <!-- results in a setSomeList(java.util.List) call --> <property name="someList"> <list> <value>a list element followed by a reference</value> <ref bean="myDataSource" /> </list> </property> <!-- results in a setSomeMap(java.util.Map) call --> <property name="someMap"> <map> <entry key="an entry" value="just some string"/> <entry key ="a ref" value-ref="myDataSource"/> </map> </property> <!-- results in a setSomeSet(java.util.Set) call --> <property name="someSet"> <set> <value>just some string</value> <ref bean="myDataSource" /> </set> </property> </bean>
The value of a map key or value, or a set value, can also again be any of the following elements:
bean | ref | idref | list | set | map | props | value | null
The Spring container also supports the merging of collections. An application
developer can define a parent-style <list/>
, <map/>
, <set/>
or <props/>
element,
and have child-style <list/>
, <map/>
, <set/>
or <props/>
elements inherit and
override values from the parent collection. That is, the child collection’s values are
the result of merging the elements of the parent and child collections, with the child’s
collection elements overriding values specified in the parent collection.
This section on merging discusses the parent-child bean mechanism. Readers unfamiliar with parent and child bean definitions may wish to read the relevant section before continuing.
The following example demonstrates collection merging:
<beans> <bean id="parent" abstract="true" class="example.ComplexObject"> <property name="adminEmails"> <props> <prop key="administrator">[email protected]</prop> <prop key="support">[email protected]</prop> </props> </property> </bean> <bean id="child" parent="parent"> <property name="adminEmails"> <!-- the merge is specified on the child collection definition --> <props merge="true"> <prop key="sales">[email protected]</prop> <prop key="support">[email protected]</prop> </props> </property> </bean> <beans>
Notice the use of the merge=true
attribute on the <props/>
element of the
adminEmails
property of the child
bean definition. When the child
bean is resolved
and instantiated by the container, the resulting instance has an adminEmails
Properties
collection that contains the result of the merging of the child’s
adminEmails
collection with the parent’s adminEmails
collection.
[email protected] [email protected] [email protected]
The child Properties
collection’s value set inherits all property elements from the
parent <props/>
, and the child’s value for the support
value overrides the value in
the parent collection.
This merging behavior applies similarly to the <list/>
, <map/>
, and <set/>
collection types. In the specific case of the <list/>
element, the semantics
associated with the List
collection type, that is, the notion of an ordered
collection of values, is maintained; the parent’s values precede all of the child list’s
values. In the case of the Map
, Set
, and Properties
collection types, no ordering
exists. Hence no ordering semantics are in effect for the collection types that underlie
the associated Map
, Set
, and Properties
implementation types that the container
uses internally.
You cannot merge different collection types (such as a Map
and a List
), and if you
do attempt to do so an appropriate Exception
is thrown. The merge
attribute must be
specified on the lower, inherited, child definition; specifying the merge
attribute on
a parent collection definition is redundant and will not result in the desired merging.
With the introduction of generic types in Java 5, you can use strongly typed collections.
That is, it is possible to declare a Collection
type such that it can only contain
String
elements (for example). If you are using Spring to dependency-inject a
strongly-typed Collection
into a bean, you can take advantage of Spring’s
type-conversion support such that the elements of your strongly-typed Collection
instances are converted to the appropriate type prior to being added to the Collection
.
public class Foo { private Map<String, Float> accounts; public void setAccounts(Map<String, Float> accounts) { this.accounts = accounts; } }
<beans> <bean id="foo" class="x.y.Foo"> <property name="accounts"> <map> <entry key="one" value="9.99"/> <entry key="two" value="2.75"/> <entry key="six" value="3.99"/> </map> </property> </bean> </beans>
When the accounts
property of the foo
bean is prepared for injection, the generics
information about the element type of the strongly-typed Map<String, Float>
is
available by reflection. Thus Spring’s type conversion infrastructure recognizes the
various value elements as being of type Float
, and the string values 9.99, 2.75
, and
3.99
are converted into an actual Float
type.
Spring treats empty arguments for properties and the like as empty Strings
. The
following XML-based configuration metadata snippet sets the email property to the empty
String
value ("").
<bean class="ExampleBean"> <property name="email" value=""/> </bean>
The preceding example is equivalent to the following Java code:
exampleBean.setEmail("")
. The <null/>
element handles null
values. For example:
<bean class="ExampleBean"> <property name="email"> <null/> </property> </bean>
The above configuration is equivalent to the following Java code:
exampleBean.setEmail(null)
.
The p-namespace enables you to use the bean
element’s attributes, instead of nested
<property/>
elements, to describe your property values and/or collaborating beans.
Spring supports extensible configuration formats with namespaces, which are
based on an XML Schema definition. The beans
configuration format discussed in this
chapter is defined in an XML Schema document. However, the p-namespace is not defined in
an XSD file and exists only in the core of Spring.
The following example shows two XML snippets that resolve to the same result: The first uses standard XML format and the second uses the p-namespace.
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean name="classic" class="com.example.ExampleBean"> <property name="email" value="[email protected]"/> </bean> <bean name="p-namespace" class="com.example.ExampleBean" p:email="[email protected]"/> </beans>
The example shows an attribute in the p-namespace called email in the bean definition. This tells Spring to include a property declaration. As previously mentioned, the p-namespace does not have a schema definition, so you can set the name of the attribute to the property name.
This next example includes two more bean definitions that both have a reference to another bean:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean name="john-classic" class="com.example.Person"> <property name="name" value="John Doe"/> <property name="spouse" ref="jane"/> </bean> <bean name="john-modern" class="com.example.Person" p:name="John Doe" p:spouse-ref="jane"/> <bean name="jane" class="com.example.Person"> <property name="name" value="Jane Doe"/> </bean> </beans>
As you can see, this example includes not only a property value using the p-namespace,
but also uses a special format to declare property references. Whereas the first bean
definition uses <property name="spouse" ref="jane"/>
to create a reference from bean
john
to bean jane
, the second bean definition uses p:spouse-ref="jane"
as an
attribute to do the exact same thing. In this case spouse
is the property name,
whereas the -ref
part indicates that this is not a straight value but rather a
reference to another bean.
Note | |
---|---|
The p-namespace is not as flexible as the standard XML format. For example, the format
for declaring property references clashes with properties that end in |
Similar to the the section called “XML shortcut with the p-namespace”, the c-namespace, newly introduced in Spring
3.1, allows usage of inlined attributes for configuring the constructor arguments rather
then nested constructor-arg
elements.
Let’s review the examples from the section called “Constructor-based dependency injection” with the c:
namespace:
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:c="http://www.springframework.org/schema/c" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="bar" class="x.y.Bar"/> <bean id="baz" class="x.y.Baz"/> <!-- traditional declaration --> <bean id="foo" class="x.y.Foo"> <constructor-arg ref="bar"/> <constructor-arg ref="baz"/> <constructor-arg value="[email protected]"/> </bean> <!-- c-namespace declaration --> <bean id="foo" class="x.y.Foo" c:bar-ref="bar" c:baz-ref="baz" c:email="[email protected]"/> </beans>
The c:
namespace uses the same conventions as the p:
one (trailing -ref
for bean
references) for setting the constructor arguments by their names. And just as well, it
needs to be declared even though it is not defined in an XSD schema (but it exists
inside the Spring core).
For the rare cases where the constructor argument names are not available (usually if the bytecode was compiled without debugging information), one can use fallback to the argument indexes:
<!-- c-namespace index declaration --> <bean id="foo" class="x.y.Foo" c:_0-ref="bar" c:_1-ref="baz"/>
Note | |
---|---|
Due to the XML grammar, the index notation requires the presence of the leading |
In practice, the constructor resolution mechanism is quite efficient in matching arguments so unless one really needs to, we recommend using the name notation through-out your configuration.
You can use compound or nested property names when you set bean properties, as long as
all components of the path except the final property name are not null
. Consider the
following bean definition.
<bean id="foo" class="foo.Bar"> <property name="fred.bob.sammy" value="123" /> </bean>
The foo
bean has a fred
property, which has a bob
property, which has a sammy
property, and that final sammy
property is being set to the value 123
. In order for
this to work, the fred
property of foo
, and the bob
property of fred
must not be
null
after the bean is constructed, or a NullPointerException
is thrown.
If a bean is a dependency of another that usually means that one bean is set as a
property of another. Typically you accomplish this with the <ref/>
element in XML-based configuration metadata. However, sometimes dependencies between
beans are less direct; for example, a static initializer in a class needs to be
triggered, such as database driver registration. The depends-on
attribute can
explicitly force one or more beans to be initialized before the bean using this element
is initialized. The following example uses the depends-on
attribute to express a
dependency on a single bean:
<bean id="beanOne" class="ExampleBean" depends-on="manager"/> <bean id="manager" class="ManagerBean" />
To express a dependency on multiple beans, supply a list of bean names as the value of
the depends-on
attribute, with commas, whitespace and semicolons, used as valid
delimiters:
<bean id="beanOne" class="ExampleBean" depends-on="manager,accountDao"> <property name="manager" ref="manager" /> </bean> <bean id="manager" class="ManagerBean" /> <bean id="accountDao" class="x.y.jdbc.JdbcAccountDao" />
Note | |
---|---|
The |
By default, ApplicationContext
implementations eagerly create and configure all
singleton beans as part of the initialization
process. Generally, this pre-instantiation is desirable, because errors in the
configuration or surrounding environment are discovered immediately, as opposed to hours
or even days later. When this behavior is not desirable, you can prevent
pre-instantiation of a singleton bean by marking the bean definition as
lazy-initialized. A lazy-initialized bean tells the IoC container to create a bean
instance when it is first requested, rather than at startup.
In XML, this behavior is controlled by the lazy-init
attribute on the <bean/>
element; for example:
<bean id="lazy" class="com.foo.ExpensiveToCreateBean" lazy-init="true"/> <bean name="not.lazy" class="com.foo.AnotherBean"/>
When the preceding configuration is consumed by an ApplicationContext
, the bean named
lazy
is not eagerly pre-instantiated when the ApplicationContext
is starting up,
whereas the not.lazy
bean is eagerly pre-instantiated.
However, when a lazy-initialized bean is a dependency of a singleton bean that is
not lazy-initialized, the ApplicationContext
creates the lazy-initialized bean at
startup, because it must satisfy the singleton’s dependencies. The lazy-initialized bean
is injected into a singleton bean elsewhere that is not lazy-initialized.
You can also control lazy-initialization at the container level by using the
default-lazy-init
attribute on the <beans/>
element; for example:
<beans default-lazy-init="true"> <!-- no beans will be pre-instantiated... --> </beans>
The Spring container can autowire relationships between collaborating beans. You can
allow Spring to resolve collaborators (other beans) automatically for your bean by
inspecting the contents of the ApplicationContext
. Autowiring has the following
advantages:
When using XML-based configuration metadata [2], you specify autowire
mode for a bean definition with the autowire
attribute of the <bean/>
element. The
autowiring functionality has five modes. You specify autowiring per bean and thus
can choose which ones to autowire.
Table 4.2. Autowiring modes
Mode | Explanation |
---|---|
no | (Default) No autowiring. Bean references must be defined via a |
byName | Autowiring by property name. Spring looks for a bean with the same name as the
property that needs to be autowired. For example, if a bean definition is set to
autowire by name, and it contains a master property (that is, it has a
setMaster(..) method), Spring looks for a bean definition named |
byType | Allows a property to be autowired if exactly one bean of the property type exists in the container. If more than one exists, a fatal exception is thrown, which indicates that you may not use byType autowiring for that bean. If there are no matching beans, nothing happens; the property is not set. |
constructor | Analogous to byType, but applies to constructor arguments. If there is not exactly one bean of the constructor argument type in the container, a fatal error is raised. |
With byType or constructor autowiring mode, you can wire arrays and
typed-collections. In such cases all autowire candidates within the container that
match the expected type are provided to satisfy the dependency. You can autowire
strongly-typed Maps if the expected key type is String
. An autowired Maps values will
consist of all bean instances that match the expected type, and the Maps keys will
contain the corresponding bean names.
You can combine autowire behavior with dependency checking, which is performed after autowiring completes.
Autowiring works best when it is used consistently across a project. If autowiring is not used in general, it might be confusing to developers to use it to wire only one or two bean definitions.
Consider the limitations and disadvantages of autowiring:
property
and constructor-arg
settings always override
autowiring. You cannot autowire so-called simple properties such as primitives,
Strings
, and Classes
(and arrays of such simple properties). This limitation is
by-design.
In the latter scenario, you have several options:
autowire-candidate
attributes
to false
as described in the next section.
primary
attribute of its <bean/>
element to true
.
On a per-bean basis, you can exclude a bean from autowiring. In Spring’s XML format, set
the autowire-candidate
attribute of the <bean/>
element to false
; the container
makes that specific bean definition unavailable to the autowiring infrastructure
(including annotation style configurations such as @Autowired
).
You can also limit autowire candidates based on pattern-matching against bean names. The
top-level <beans/>
element accepts one or more patterns within its
default-autowire-candidates
attribute. For example, to limit autowire candidate status
to any bean whose name ends with Repository, provide a value of *Repository. To
provide multiple patterns, define them in a comma-separated list. An explicit value of
true
or false
for a bean definitions autowire-candidate
attribute always takes
precedence, and for such beans, the pattern matching rules do not apply.
These techniques are useful for beans that you never want to be injected into other beans by autowiring. It does not mean that an excluded bean cannot itself be configured using autowiring. Rather, the bean itself is not a candidate for autowiring other beans.
In most application scenarios, most beans in the container are singletons. When a singleton bean needs to collaborate with another singleton bean, or a non-singleton bean needs to collaborate with another non-singleton bean, you typically handle the dependency by defining one bean as a property of the other. A problem arises when the bean lifecycles are different. Suppose singleton bean A needs to use non-singleton (prototype) bean B, perhaps on each method invocation on A. The container only creates the singleton bean A once, and thus only gets one opportunity to set the properties. The container cannot provide bean A with a new instance of bean B every time one is needed.
A solution is to forego some inversion of control. You can make
bean A aware of the container by implementing the ApplicationContextAware
interface,
and by making a getBean("B") call to the container ask for (a
typically new) bean B instance every time bean A needs it. The following is an example
of this approach:
// a class that uses a stateful Command-style class to perform some processing package fiona.apple; // Spring-API imports import org.springframework.beans.BeansException; import org.springframework.context.ApplicationContext; import org.springframework.context.ApplicationContextAware; public class CommandManager implements ApplicationContextAware { private ApplicationContext applicationContext; public Object process(Map commandState) { // grab a new instance of the appropriate Command Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } protected Command createCommand() { // notice the Spring API dependency! return this.applicationContext.getBean("command", Command.class); } public void setApplicationContext( ApplicationContext applicationContext) throws BeansException { this.applicationContext = applicationContext; } }
The preceding is not desirable, because the business code is aware of and coupled to the Spring Framework. Method Injection, a somewhat advanced feature of the Spring IoC container, allows this use case to be handled in a clean fashion.
Lookup method injection is the ability of the container to override methods on container managed beans, to return the lookup result for another named bean in the container. The lookup typically involves a prototype bean as in the scenario described in the preceding section. The Spring Framework implements this method injection by using bytecode generation from the CGLIB library to generate dynamically a subclass that overrides the method.
Note | |
---|---|
For this dynamic subclassing to work, the class that the Spring container will subclass
cannot be |
Looking at the CommandManager
class in the previous code snippet, you see that the
Spring container will dynamically override the implementation of the createCommand()
method. Your CommandManager
class will not have any Spring dependencies, as can be
seen in the reworked example:
package fiona.apple; // no more Spring imports! public abstract class CommandManager { public Object process(Object commandState) { // grab a new instance of the appropriate Command interface Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } // okay... but where is the implementation of this method? protected abstract Command createCommand(); }
In the client class containing the method to be injected (the CommandManager
in this
case), the method to be injected requires a signature of the following form:
<public|protected> [abstract] <return-type> theMethodName(no-arguments);
If the method is abstract
, the dynamically-generated subclass implements the method.
Otherwise, the dynamically-generated subclass overrides the concrete method defined in
the original class. For example:
<!-- a stateful bean deployed as a prototype (non-singleton) --> <bean id="command" class="fiona.apple.AsyncCommand" scope="prototype"> <!-- inject dependencies here as required --> </bean> <!-- commandProcessor uses statefulCommandHelper --> <bean id="commandManager" class="fiona.apple.CommandManager"> <lookup-method name="createCommand" bean="command"/> </bean>
The bean identified as commandManager calls its own method createCommand()
whenever it needs a new instance of the command bean. You must be careful to deploy
the command
bean as a prototype, if that is actually what is needed. If it is deployed
as a singleton, the same instance of the command
bean is returned each time.
Tip | |
---|---|
The interested reader may also find the |
A less useful form of method injection than lookup method Injection is the ability to replace arbitrary methods in a managed bean with another method implementation. Users may safely skip the rest of this section until the functionality is actually needed.
With XML-based configuration metadata, you can use the replaced-method
element to
replace an existing method implementation with another, for a deployed bean. Consider
the following class, with a method computeValue, which we want to override:
public class MyValueCalculator { public String computeValue(String input) { // some real code... } // some other methods... }
A class implementing the org.springframework.beans.factory.support.MethodReplacer
interface provides the new method definition.
/** * meant to be used to override the existing computeValue(String) * implementation in MyValueCalculator */ public class ReplacementComputeValue implements MethodReplacer { public Object reimplement(Object o, Method m, Object[] args) throws Throwable { // get the input value, work with it, and return a computed result String input = (String) args[0]; ... return ...; } }
The bean definition to deploy the original class and specify the method override would look like this:
<bean id="myValueCalculator" class="x.y.z.MyValueCalculator"> <!-- arbitrary method replacement --> <replaced-method name="computeValue" replacer="replacementComputeValue"> <arg-type>String</arg-type> </replaced-method> </bean> <bean id="replacementComputeValue" class="a.b.c.ReplacementComputeValue"/>
You can use one or more contained <arg-type/>
elements within the <replaced-method/>
element to indicate the method signature of the method being overridden. The signature
for the arguments is necessary only if the method is overloaded and multiple variants
exist within the class. For convenience, the type string for an argument may be a
substring of the fully qualified type name. For example, the following all match
java.lang.String
:
java.lang.String String Str
Because the number of arguments is often enough to distinguish between each possible choice, this shortcut can save a lot of typing, by allowing you to type only the shortest string that will match an argument type.
When you create a bean definition, you create a recipe for creating actual instances of the class defined by that bean definition. The idea that a bean definition is a recipe is important, because it means that, as with a class, you can create many object instances from a single recipe.
You can control not only the various dependencies and configuration values that are to
be plugged into an object that is created from a particular bean definition, but also
the scope of the objects created from a particular bean definition. This approach is
powerful and flexible in that you can choose the scope of the objects you create
through configuration instead of having to bake in the scope of an object at the Java
class level. Beans can be defined to be deployed in one of a number of scopes: out of
the box, the Spring Framework supports five scopes, three of which are available only if
you use a web-aware ApplicationContext
.
The following scopes are supported out of the box. You can also create a custom scope.
Table 4.3. Bean scopesThread-scoped beans
Scope | Description |
---|---|
(Default) Scopes a single bean definition to a single object instance per Spring IoC container. | |
Scopes a single bean definition to any number of object instances. | |
Scopes a single bean definition to the lifecycle of a single HTTP request; that is,
each HTTP request has its own instance of a bean created off the back of a single bean
definition. Only valid in the context of a web-aware Spring | |
Scopes a single bean definition to the lifecycle of an HTTP | |
Scopes a single bean definition to the lifecycle of a global HTTP |
Note | |
---|---|
As of Spring 3.0, a thread scope is available, but is not registered by default. For
more information, see the documentation for
|
Only one shared instance of a singleton bean is managed, and all requests for beans with an id or ids matching that bean definition result in that one specific bean instance being returned by the Spring container.
To put it another way, when you define a bean definition and it is scoped as a singleton, the Spring IoC container creates exactly one instance of the object defined by that bean definition. This single instance is stored in a cache of such singleton beans, and all subsequent requests and references for that named bean return the cached object.
Spring’s concept of a singleton bean differs from the Singleton pattern as defined in the Gang of Four (GoF) patterns book. The GoF Singleton hard-codes the scope of an object such that one and only one instance of a particular class is created per ClassLoader. The scope of the Spring singleton is best described as per container and per bean. This means that if you define one bean for a particular class in a single Spring container, then the Spring container creates one and only one instance of the class defined by that bean definition. The singleton scope is the default scope in Spring. To define a bean as a singleton in XML, you would write, for example:
<bean id="accountService" class="com.foo.DefaultAccountService"/> <!-- the following is equivalent, though redundant (singleton scope is the default) --> <bean id="accountService" class="com.foo.DefaultAccountService" scope="singleton"/>
The non-singleton, prototype scope of bean deployment results in the creation of a new
bean instance every time a request for that specific bean is made. That is, the bean
is injected into another bean or you request it through a getBean()
method call on the
container. As a rule, use the prototype scope for all stateful beans and the singleton
scope for stateless beans.
The following diagram illustrates the Spring prototype scope. A data access object (DAO) is not typically configured as a prototype, because a typical DAO does not hold any conversational state; it was just easier for this author to reuse the core of the singleton diagram.
The following example defines a bean as a prototype in XML:
<bean id="accountService" class="com.foo.DefaultAccountService" scope="prototype"/>
In contrast to the other scopes, Spring does not manage the complete lifecycle of a prototype bean: the container instantiates, configures, and otherwise assembles a prototype object, and hands it to the client, with no further record of that prototype instance. Thus, although initialization lifecycle callback methods are called on all objects regardless of scope, in the case of prototypes, configured destruction lifecycle callbacks are not called. The client code must clean up prototype-scoped objects and release expensive resources that the prototype bean(s) are holding. To get the Spring container to release resources held by prototype-scoped beans, try using a custom bean post-processor, which holds a reference to beans that need to be cleaned up.
In some respects, the Spring container’s role in regard to a prototype-scoped bean is a
replacement for the Java new
operator. All lifecycle management past that point must
be handled by the client. (For details on the lifecycle of a bean in the Spring
container, see Section 4.6.1, “Lifecycle callbacks”.)
When you use singleton-scoped beans with dependencies on prototype beans, be aware that dependencies are resolved at instantiation time. Thus if you dependency-inject a prototype-scoped bean into a singleton-scoped bean, a new prototype bean is instantiated and then dependency-injected into the singleton bean. The prototype instance is the sole instance that is ever supplied to the singleton-scoped bean.
However, suppose you want the singleton-scoped bean to acquire a new instance of the prototype-scoped bean repeatedly at runtime. You cannot dependency-inject a prototype-scoped bean into your singleton bean, because that injection occurs only once, when the Spring container is instantiating the singleton bean and resolving and injecting its dependencies. If you need a new instance of a prototype bean at runtime more than once, see Section 4.4.6, “Method injection”
The request
, session
, and global session
scopes are only available if you use
a web-aware Spring ApplicationContext
implementation (such as
XmlWebApplicationContext
). If you use these scopes with regular Spring IoC containers
such as the ClassPathXmlApplicationContext
, you get an IllegalStateException
complaining about an unknown bean scope.
To support the scoping of beans at the request
, session
, and global session
levels
(web-scoped beans), some minor initial configuration is required before you define your
beans. (This initial setup is not required for the standard scopes, singleton and
prototype.)
How you accomplish this initial setup depends on your particular Servlet environment..
If you access scoped beans within Spring Web MVC, in effect, within a request that is
processed by the Spring DispatcherServlet
, or DispatcherPortlet
, then no special
setup is necessary: DispatcherServlet
and DispatcherPortlet
already expose all
relevant state.
If you use a Servlet 2.5 web container, with requests processed outside of Spring’s
DispatcherServlet (for example, when using JSF or Struts), you need to register the
org.springframework.web.context.request.RequestContextListener
ServletRequestListener
.
For Servlet 3.0+, this can done programmatically via the WebApplicationInitializer
interface. Alternatively, or for older containers, add the following declaration to
your web application’s web.xml
file:
<web-app> ... <listener> <listener-class> org.springframework.web.context.request.RequestContextListener </listener-class> </listener> ... </web-app>
Alternatively, if there are issues with your listener setup, consider the provided
RequestContextFilter
. The filter mapping depends on the surrounding web
application configuration, so you have to change it as appropriate.
<web-app> ... <filter> <filter-name>requestContextFilter</filter-name> <filter-class>org.springframework.web.filter.RequestContextFilter</filter-class> </filter> <filter-mapping> <filter-name>requestContextFilter</filter-name> <url-pattern>/*</url-pattern> </filter-mapping> ... </web-app>
DispatcherServlet
, RequestContextListener
and RequestContextFilter
all do exactly
the same thing, namely bind the HTTP request object to the Thread
that is servicing
that request. This makes beans that are request- and session-scoped available further
down the call chain.
Consider the following bean definition:
<bean id="loginAction" class="com.foo.LoginAction" scope="request"/>
The Spring container creates a new instance of the LoginAction
bean by using the
loginAction
bean definition for each and every HTTP request. That is, the
loginAction
bean is scoped at the HTTP request level. You can change the internal
state of the instance that is created as much as you want, because other instances
created from the same loginAction
bean definition will not see these changes in state;
they are particular to an individual request. When the request completes processing, the
bean that is scoped to the request is discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/>
The Spring container creates a new instance of the UserPreferences
bean by using the
userPreferences
bean definition for the lifetime of a single HTTP Session
. In other
words, the userPreferences
bean is effectively scoped at the HTTP Session
level. As
with request-scoped
beans, you can change the internal state of the instance that is
created as much as you want, knowing that other HTTP Session
instances that are also
using instances created from the same userPreferences
bean definition do not see these
changes in state, because they are particular to an individual HTTP Session
. When the
HTTP Session
is eventually discarded, the bean that is scoped to that particular HTTP
Session
is also discarded.
Consider the following bean definition:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="globalSession"/>
The global session
scope is similar to the standard HTTP Session
scope
(described above), and applies only in the context of
portlet-based web applications. The portlet specification defines the notion of a global
Session
that is shared among all portlets that make up a single portlet web
application. Beans defined at the global session
scope are scoped (or bound) to the
lifetime of the global portlet Session
.
If you write a standard Servlet-based web application and you define one or more beans
as having global session
scope, the standard HTTP Session
scope is used, and no
error is raised.
The Spring IoC container manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject (for example) an HTTP request scoped bean into another bean, you must inject an AOP proxy in place of the scoped bean. That is, you need to inject a proxy object that exposes the same public interface as the scoped object but that can also retrieve the real, target object from the relevant scope (for example, an HTTP request) and delegate method calls onto the real object.
Note | |
---|---|
You do not need to use the |
The configuration in the following example is only one line, but it is important to understand the "why" as well as the "how" behind it.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <!-- an HTTP Session-scoped bean exposed as a proxy --> <bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <!-- instructs the container to proxy the surrounding bean --> <aop:scoped-proxy/> </bean> <!-- a singleton-scoped bean injected with a proxy to the above bean --> <bean id="userService" class="com.foo.SimpleUserService"> <!-- a reference to the proxied userPreferences bean --> <property name="userPreferences" ref="userPreferences"/> </bean> </beans>
To create such a proxy, you insert a child <aop:scoped-proxy/>
element into a scoped
bean definition. See the section called “Choosing the type of proxy to create” and
Chapter 33, XML Schema-based configuration.) Why do definitions of beans scoped at the request
, session
,
globalSession
and custom-scope levels require the <aop:scoped-proxy/>
element ?
Let’s examine the following singleton bean definition and contrast it with what you need
to define for the aforementioned scopes. (The following userPreferences
bean
definition as it stands is incomplete.)
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"/> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
In the preceding example, the singleton bean userManager
is injected with a reference
to the HTTP Session
-scoped bean userPreferences
. The salient point here is that the
userManager
bean is a singleton: it will be instantiated exactly once per
container, and its dependencies (in this case only one, the userPreferences
bean) are
also injected only once. This means that the userManager
bean will only operate on the
exact same userPreferences
object, that is, the one that it was originally injected
with.
This is not the behavior you want when injecting a shorter-lived scoped bean into a
longer-lived scoped bean, for example injecting an HTTP Session
-scoped collaborating
bean as a dependency into singleton bean. Rather, you need a single userManager
object, and for the lifetime of an HTTP Session
, you need a userPreferences
object
that is specific to said HTTP Session
. Thus the container creates an object that
exposes the exact same public interface as the UserPreferences
class (ideally an
object that is a UserPreferences
instance) which can fetch the real
UserPreferences
object from the scoping mechanism (HTTP request, Session
, etc.). The
container injects this proxy object into the userManager
bean, which is unaware that
this UserPreferences
reference is a proxy. In this example, when a UserManager
instance invokes a method on the dependency-injected UserPreferences
object, it
actually is invoking a method on the proxy. The proxy then fetches the real
UserPreferences
object from (in this case) the HTTP Session
, and delegates the
method invocation onto the retrieved real UserPreferences
object.
Thus you need the following, correct and complete, configuration when injecting
request-
, session-
, and globalSession-scoped
beans into collaborating objects:
<bean id="userPreferences" class="com.foo.UserPreferences" scope="session"> <aop:scoped-proxy/> </bean> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
By default, when the Spring container creates a proxy for a bean that is marked up with
the <aop:scoped-proxy/>
element, a CGLIB-based class proxy is created.
Note | |
---|---|
CGLIB proxies only intercept public method calls! Do not call non-public methods on such a proxy; they will not be delegated to the scoped target object. |
Alternatively, you can configure the Spring container to create standard JDK
interface-based proxies for such scoped beans, by specifying false
for the value of
the proxy-target-class
attribute of the <aop:scoped-proxy/>
element. Using JDK
interface-based proxies means that you do not need additional libraries in your
application classpath to effect such proxying. However, it also means that the class of
the scoped bean must implement at least one interface, and that all collaborators
into which the scoped bean is injected must reference the bean through one of its
interfaces.
<!-- DefaultUserPreferences implements the UserPreferences interface --> <bean id="userPreferences" class="com.foo.DefaultUserPreferences" scope="session"> <aop:scoped-proxy proxy-target-class="false"/> </bean> <bean id="userManager" class="com.foo.UserManager"> <property name="userPreferences" ref="userPreferences"/> </bean>
For more detailed information about choosing class-based or interface-based proxying, see Section 8.6, “Proxying mechanisms”.
The bean scoping mechanism is extensible; You can define your own
scopes, or even redefine existing scopes, although the latter is considered bad practice
and you cannot override the built-in singleton
and prototype
scopes.
To integrate your custom scope(s) into the Spring container, you need to implement the
org.springframework.beans.factory.config.Scope
interface, which is described in this
section. For an idea of how to implement your own scopes, see the Scope
implementations that are supplied with the Spring Framework itself and the
Scope
javadocs,
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 javadocs or a Spring scope implementation for more information on destruction callbacks.
void registerDestructionCallback(String name, Runnable destructionCallback)
The following method obtains the conversation identifier for the underlying scope. This identifier is different for each scope. For a session scoped implementation, this identifier can be the session identifier.
String getConversationId()
After you write and test one or more custom Scope
implementations, you need to make
the Spring container aware of your new scope(s). The following method is the central
method to register a new Scope
with the Spring container:
void registerScope(String scopeName, Scope scope);
This method is declared on the ConfigurableBeanFactory
interface, which is available
on most of the concrete ApplicationContext
implementations that ship with Spring via
the BeanFactory property.
The first argument to the registerScope(..)
method is the unique name associated with
a scope; examples of such names in the Spring container itself are singleton
and
prototype
. The second argument to the registerScope(..)
method is an actual instance
of the custom Scope
implementation that you wish to register and use.
Suppose that you write your custom Scope
implementation, and then register it as below.
Note | |
---|---|
The example below uses |
Scope threadScope = new SimpleThreadScope(); beanFactory.registerScope("thread", threadScope);
You then create bean definitions that adhere to the scoping rules of your custom Scope
:
<bean id="..." class="..." scope="thread">
With a custom Scope
implementation, you are not limited to programmatic registration
of the scope. You can also do the Scope
registration declaratively, using the
CustomScopeConfigurer
class:
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"> <bean class="org.springframework.beans.factory.config.CustomScopeConfigurer"> <property name="scopes"> <map> <entry key="thread"> <bean class="org.springframework.context.support.SimpleThreadScope"/> </entry> </map> </property> </bean> <bean id="bar" class="x.y.Bar" scope="thread"> <property name="name" value="Rick"/> <aop:scoped-proxy/> </bean> <bean id="foo" class="x.y.Foo"> <property name="bar" ref="bar"/> </bean> </beans>
Note | |
---|---|
When you place <aop:scoped-proxy/> in a |
To interact with the container’s management of the bean lifecycle, you can implement the
Spring InitializingBean
and DisposableBean
interfaces. The container calls
afterPropertiesSet()
for the former and destroy()
for the latter to allow the bean
to perform certain actions upon initialization and destruction of your beans.
Tip | |
---|---|
The JSR-250 If you don’t want to use the JSR-250 annotations but you are still looking to remove coupling consider the use of init-method and destroy-method object definition metadata. |
Internally, the Spring Framework uses BeanPostProcessor
implementations to process any
callback interfaces it can find and call the appropriate methods. If you need custom
features or other lifecycle behavior Spring does not offer out-of-the-box, you can
implement a BeanPostProcessor
yourself. For more information, see
Section 4.8, “Container Extension Points”.
In addition to the initialization and destruction callbacks, Spring-managed objects may
also implement the Lifecycle
interface so that those objects can participate in the
startup and shutdown process as driven by the container’s own lifecycle.
The lifecycle callback interfaces are described in this section.
The org.springframework.beans.factory.InitializingBean
interface allows a bean to
perform initialization work after all necessary properties on the bean have been set by
the container. The InitializingBean
interface specifies a single method:
void afterPropertiesSet() throws Exception;
It is recommended that you do not use the InitializingBean
interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PostConstruct
annotation or
specify a POJO initialization method. In the case of XML-based configuration metadata,
you use the init-method
attribute to specify the name of the method that has a void
no-argument signature. For example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" init-method="init"/>
public class ExampleBean { public void init() { // do some initialization work } }
…is exactly the same as…
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements InitializingBean { public void afterPropertiesSet() { // do some initialization work } }
but does not couple the code to Spring.
Implementing the org.springframework.beans.factory.DisposableBean
interface allows a
bean to get a callback when the container containing it is destroyed. The
DisposableBean
interface specifies a single method:
void destroy() throws Exception;
It is recommended that you do not use the DisposableBean
callback interface because it
unnecessarily couples the code to Spring. Alternatively, use
the @PreDestroy
annotation or
specify a generic method that is supported by bean definitions. With XML-based
configuration metadata, you use the destroy-method
attribute on the <bean/>
. For
example, the following definition:
<bean id="exampleInitBean" class="examples.ExampleBean" destroy-method="cleanup"/>
public class ExampleBean { public void cleanup() { // do some destruction work (like releasing pooled connections) } }
is exactly the same as:
<bean id="exampleInitBean" class="examples.AnotherExampleBean"/>
public class AnotherExampleBean implements DisposableBean { public void destroy() { // do some destruction work (like releasing pooled connections) } }
but does not couple the code to Spring.
When you write initialization and destroy method callbacks that do not use the
Spring-specific InitializingBean
and DisposableBean
callback interfaces, you
typically write methods with names such as init()
, initialize()
, dispose()
, and so
on. Ideally, the names of such lifecycle callback methods are standardized across a
project so that all developers use the same method names and ensure consistency.
You can configure the Spring container to look
for named initialization and destroy
callback method names on every bean. This means that you, as an application
developer, can write your application classes and use an initialization callback called
init()
, without having to configure an init-method="init"
attribute with each bean
definition. The Spring IoC container calls that method when the bean is created (and in
accordance with the standard lifecycle callback contract described previously). This
feature also enforces a consistent naming convention for initialization and destroy
method callbacks.
Suppose that your initialization callback methods are named init()
and destroy
callback methods are named destroy()
. Your class will resemble the class in the
following example.
public class DefaultBlogService implements BlogService { private BlogDao blogDao; public void setBlogDao(BlogDao blogDao) { this.blogDao = blogDao; } // this is (unsurprisingly) the initialization callback method public void init() { if (this.blogDao == null) { throw new IllegalStateException("The [blogDao] property must be set."); } } }
<beans default-init-method="init"> <bean id="blogService" class="com.foo.DefaultBlogService"> <property name="blogDao" ref="blogDao" /> </bean> </beans>
The presence of the default-init-method
attribute on the top-level <beans/>
element
attribute causes the Spring IoC container to recognize a method called init
on beans
as the initialization method callback. When a bean is created and assembled, if the bean
class has such a method, it is invoked at the appropriate time.
You configure destroy method callbacks similarly (in XML, that is) by using the
default-destroy-method
attribute on the top-level <beans/>
element.
Where existing bean classes already have callback methods that are named at variance
with the convention, you can override the default by specifying (in XML, that is) the
method name using the init-method
and destroy-method
attributes of the <bean/>
itself.
The Spring container guarantees that a configured initialization callback is called immediately after a bean is supplied with all dependencies. Thus the initialization callback is called on the raw bean reference, which means that AOP interceptors and so forth are not yet applied to the bean. A target bean is fully created first, then an AOP proxy (for example) with its interceptor chain is applied. If the target bean and the proxy are defined separately, your code can even interact with the raw target bean, bypassing the proxy. Hence, it would be inconsistent to apply the interceptors to the init method, because doing so would couple the lifecycle of the target bean with its proxy/interceptors and leave strange semantics when your code interacts directly to the raw target bean.
As of Spring 2.5, you have three options for controlling bean lifecycle behavior: the
InitializingBean
and
DisposableBean
callback interfaces; custom
init()
and destroy()
methods; and the
@PostConstruct
and @PreDestroy
annotations. You can combine these mechanisms to control a given bean.
Note | |
---|---|
If multiple lifecycle mechanisms are configured for a bean, and each mechanism is
configured with a different method name, then each configured method is executed in the
order listed below. However, if the same method name is configured - for example,
|
Multiple lifecycle mechanisms configured for the same bean, with different initialization methods, are called as follows:
@PostConstruct
afterPropertiesSet()
as defined by the InitializingBean
callback interface
init()
method
Destroy methods are called in the same order:
@PreDestroy
destroy()
as defined by the DisposableBean
callback interface
destroy()
method
The Lifecycle
interface defines the essential methods for any object that has its own
lifecycle requirements (e.g. starts and stops some background process):
public interface Lifecycle { void start(); void stop(); boolean isRunning(); }
Any Spring-managed object may implement that interface. Then, when the
ApplicationContext itself starts and stops, it will cascade those calls to all Lifecycle
implementations defined within that context. It does this by delegating to a
LifecycleProcessor
:
public interface LifecycleProcessor extends Lifecycle { void onRefresh(); void onClose(); }
Notice that the LifecycleProcessor
is itself an extension of the Lifecycle
interface. It also adds two other methods for reacting to the context being refreshed
and closed.
The order of startup and shutdown invocations can be important. If a "depends-on"
relationship exists between any two objects, the dependent side will start after its
dependency, and it will stop before its dependency. However, at times the direct
dependencies are unknown. You may only know that objects of a certain type should start
prior to objects of another type. In those cases, the SmartLifecycle
interface defines
another option, namely the getPhase()
method as defined on its super-interface,
Phased
.
public interface Phased { int getPhase(); }
public interface SmartLifecycle extends Lifecycle, Phased { boolean isAutoStartup(); void stop(Runnable callback); }
When starting, the objects with the lowest phase start first, and when stopping, the
reverse order is followed. Therefore, an object that implements SmartLifecycle
and
whose getPhase() method returns Integer.MIN_VALUE
would be among the first to start
and the last to stop. At the other end of the spectrum, a phase value of
Integer.MAX_VALUE
would indicate that the object should be started last and stopped
first (likely because it depends on other processes to be running). When considering the
phase value, it’s also important to know that the default phase for any "normal"
Lifecycle
object that does not implement SmartLifecycle
would be 0. Therefore, any
negative phase value would indicate that an object should start before those standard
components (and stop after them), and vice versa for any positive phase value.
As you can see the stop method defined by SmartLifecycle
accepts a callback. Any
implementation must invoke that callback’s run() method after that implementation’s
shutdown process is complete. That enables asynchronous shutdown where necessary since
the default implementation of the LifecycleProcessor
interface,
DefaultLifecycleProcessor
, will wait up to its timeout value for the group of objects
within each phase to invoke that callback. The default per-phase timeout is 30 seconds.
You can override the default lifecycle processor instance by defining a bean named
"lifecycleProcessor" within the context. If you only want to modify the timeout, then
defining the following would be sufficient:
<bean id="lifecycleProcessor" class="org.springframework.context.support.DefaultLifecycleProcessor"> <!-- timeout value in milliseconds --> <property name="timeoutPerShutdownPhase" value="10000"/> </bean>
As mentioned, the LifecycleProcessor
interface defines callback methods for the
refreshing and closing of the context as well. The latter will simply drive the shutdown
process as if stop() had been called explicitly, but it will happen when the context is
closing. The refresh callback on the other hand enables another feature of
SmartLifecycle
beans. When the context is refreshed (after all objects have been
instantiated and initialized), that callback will be invoked, and at that point the
default lifecycle processor will check the boolean value returned by each
SmartLifecycle
object’s isAutoStartup()
method. If "true", then that object will be
started at that point rather than waiting for an explicit invocation of the context’s or
its own start() method (unlike the context refresh, the context start does not happen
automatically for a standard context implementation). The "phase" value as well as any
"depends-on" relationships will determine the startup order in the same way as described
above.
Note | |
---|---|
This section applies only to non-web applications. Spring’s web-based
|
If you are using Spring’s IoC container in a non-web application environment; for example, in a rich client desktop environment; you register a shutdown hook with the JVM. Doing so ensures a graceful shutdown and calls the relevant destroy methods on your singleton beans so that all resources are released. Of course, you must still configure and implement these destroy callbacks correctly.
To register a shutdown hook, you call the registerShutdownHook()
method that is
declared on the AbstractApplicationContext
class:
import org.springframework.context.support.AbstractApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public final class Boot { public static void main(final String[] args) throws Exception { AbstractApplicationContext ctx = new ClassPathXmlApplicationContext( new String []{"beans.xml"}); // add a shutdown hook for the above context... ctx.registerShutdownHook(); // app runs here... // main method exits, hook is called prior to the app shutting down... } }
When an ApplicationContext
creates a class that implements the
org.springframework.context.ApplicationContextAware
interface, the class is provided
with a reference to that ApplicationContext
.
public interface ApplicationContextAware { void setApplicationContext(ApplicationContext applicationContext) throws BeansException; }
Thus beans can manipulate programmatically the ApplicationContext
that created them,
through the ApplicationContext
interface, or by casting the reference to a known
subclass of this interface, such as ConfigurableApplicationContext
, which exposes
additional functionality. One use would be the programmatic retrieval of other beans.
Sometimes this capability is useful; however, in general you should avoid it, because it
couples the code to Spring and does not follow the Inversion of Control style, where
collaborators are provided to beans as properties. Other methods of the
ApplicationContext provide access to file resources, publishing application events, and
accessing a MessageSource. These additional features are described in
Section 4.16, “Additional Capabilities of the ApplicationContext”
As of Spring 2.5, autowiring is another alternative to obtain reference to the
ApplicationContext
. The "traditional" constructor
and byType
autowiring modes (as
described in Section 4.4.5, “Autowiring collaborators”) can provide a dependency of type
ApplicationContext
for a constructor argument or setter method parameter,
respectively. For more flexibility, including the ability to autowire fields and
multiple parameter methods, use the new annotation-based autowiring features. If you do,
the ApplicationContext
is autowired into a field, constructor argument, or method
parameter that is expecting the ApplicationContext
type if the field, constructor, or
method in question carries the @Autowired
annotation. For more information, see
Section 4.9.2, “@Autowired”.
When an ApplicationContext creates a class that implements the
org.springframework.beans.factory.BeanNameAware
interface, the class is provided with
a reference to the name defined in its associated object definition.
public interface BeanNameAware { void setBeanName(string name) throws BeansException; }
The callback is invoked after population of normal bean properties but before an
initialization callback such as InitializingBean
afterPropertiesSet or a custom
init-method.
Besides ApplicationContextAware
and BeanNameAware
discussed above, Spring offers a
range of Aware
interfaces that allow beans to indicate to the container that they
require a certain infrastructure dependency. The most important Aware
interfaces
are summarized below - as a general rule, the name is a good indication of the
dependency type:
Table 4.4. Aware interfaces
Name | Injected Dependency | Explained in… |
---|---|---|
| Declaring | |
| Event publisher of the enclosing | Section 4.16, “Additional Capabilities of the ApplicationContext” |
| Class loader used to load the bean classes. | |
| Declaring | |
| Name of the declaring bean | |
| Resource adapter | |
| Defined weaver for processing class definition at load time | Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework” |
| Configured strategy for resolving messages (with support for parametrization and internationalization) | Section 4.16, “Additional Capabilities of the ApplicationContext” |
| Spring JMX notification publisher | |
| Current | |
| Current | |
| Configured loader for low-level access to resources | |
| Current | |
| Current |
Note again that usage of these interfaces ties your code to the Spring API and does not follow the Inversion of Control style. As such, they are recommended for infrastructure beans that require programmatic access to the container.
A bean definition can contain a lot of configuration information, including constructor arguments, property values, and container-specific information such as initialization method, static factory method name, and so on. A child bean definition inherits configuration data from a parent definition. The child definition can override some values, or add others, as needed. Using parent and child bean definitions can save a lot of typing. Effectively, this is a form of templating.
If you work with an ApplicationContext
interface programmatically, child bean
definitions are represented by the ChildBeanDefinition
class. Most users do not work
with them on this level, instead configuring bean definitions declaratively in something
like the ClassPathXmlApplicationContext
. When you use XML-based configuration
metadata, you indicate a child bean definition by using the parent
attribute,
specifying the parent bean as the value of this attribute.
<bean id="inheritedTestBean" abstract="true" class="org.springframework.beans.TestBean"> <property name="name" value="parent"/> <property name="age" value="1"/> </bean> <bean id="inheritsWithDifferentClass" class="org.springframework.beans.DerivedTestBean" parent="inheritedTestBean" init-method="initialize"> <property name="name" value="override"/> <!-- the age property value of 1 will be inherited from parent --> </bean>
A child bean definition uses the bean class from the parent definition if none is specified, but can also override it. In the latter case, the child bean class must be compatible with the parent, that is, it must accept the parent’s property values.
A child bean definition inherits constructor argument values, property values, and
method overrides from the parent, with the option to add new values. Any initialization
method, destroy method, and/or static
factory method settings that you specify will
override the corresponding parent settings.
The remaining settings are always taken from the child definition: depends on, autowire mode, dependency check, singleton, scope, lazy init.
The preceding example explicitly marks the parent bean definition as abstract by using
the abstract
attribute. If the parent definition does not specify a class, explicitly
marking the parent bean definition as abstract
is required, as follows:
<bean id="inheritedTestBeanWithoutClass" abstract="true"> <property name="name" value="parent"/> <property name="age" value="1"/> </bean> <bean id="inheritsWithClass" class="org.springframework.beans.DerivedTestBean" parent="inheritedTestBeanWithoutClass" init-method="initialize"> <property name="name" value="override"/> <!-- age will inherit the value of 1 from the parent bean definition--> </bean>
The parent bean cannot be instantiated on its own because it is incomplete, and it is
also explicitly marked as abstract
. When a definition is abstract
like this, it is
usable only as a pure template bean definition that serves as a parent definition for
child definitions. Trying to use such an abstract
parent bean on its own, by referring
to it as a ref property of another bean or doing an explicit getBean()
call with the
parent bean id, returns an error. Similarly, the container’s internal
preInstantiateSingletons()
method ignores bean definitions that are defined as
abstract.
Note | |
---|---|
|
Typically, an application developer does not need to subclass ApplicationContext
implementation classes. Instead, the Spring IoC container can be extended by plugging in
implementations of special integration interfaces. The next few sections describe these
integration interfaces.
The BeanPostProcessor
interface defines callback methods that you can implement to
provide your own (or override the container’s default) instantiation logic,
dependency-resolution logic, and so forth. If you want to implement some custom logic
after the Spring container finishes instantiating, configuring, and initializing a bean,
you can plug in one or more BeanPostProcessor
implementations.
You can configure multiple BeanPostProcessor
instances, and you can control the order
in which these BeanPostProcessor
s execute by setting the order
property. You can
set this property only if the BeanPostProcessor
implements the Ordered
interface; if
you write your own BeanPostProcessor
you should consider implementing the Ordered
interface too. For further details, consult the javadocs of the BeanPostProcessor
and
Ordered
interfaces. See also the note below on
programmatic
registration of BeanPostProcessors
Note | |
---|---|
To change the actual bean definition (i.e., the blueprint that defines the bean),
you instead need to use a |
The org.springframework.beans.factory.config.BeanPostProcessor
interface consists of
exactly two callback methods. When such a class is registered as a post-processor with
the container, for each bean instance that is created by the container, the
post-processor gets a callback from the container both before container
initialization methods (such as InitializingBean’s afterPropertiesSet() and any
declared init method) are called as well as after any bean initialization callbacks.
The post-processor can take any action with the bean instance, including ignoring the
callback completely. A bean post-processor typically checks for callback interfaces or
may wrap a bean with a proxy. Some Spring AOP infrastructure classes are implemented as
bean post-processors in order to provide proxy-wrapping logic.
An ApplicationContext
automatically detects any beans that are defined in the
configuration metadata which implement the BeanPostProcessor
interface. The
ApplicationContext
registers these beans as post-processors so that they can be called
later upon bean creation. Bean post-processors can be deployed in the container just
like any other beans.
Note | |
---|---|
Programmatically registering BeanPostProcessors While the recommended approach for |
Note | |
---|---|
BeanPostProcessors and AOP auto-proxying Classes that implement the For any such bean, you should see an informational log message: "Bean foo is not eligible for getting processed by all BeanPostProcessor interfaces (for example: not eligible for auto-proxying)". Note that if you have beans wired into your |
The following examples show how to write, register, and use BeanPostProcessors
in an
ApplicationContext
.
This first example illustrates basic usage. The example shows a custom
BeanPostProcessor
implementation that invokes the toString()
method of each bean as
it is created by the container and prints the resulting string to the system console.
Find below the custom BeanPostProcessor
implementation class definition:
package scripting; import org.springframework.beans.factory.config.BeanPostProcessor; import org.springframework.beans.BeansException; public class InstantiationTracingBeanPostProcessor implements BeanPostProcessor { // simply return the instantiated bean as-is public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException { return bean; // we could potentially return any object reference here... } public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException { System.out.println("Bean " + beanName + " created : " + bean.toString()); return bean; } }
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:lang="http://www.springframework.org/schema/lang" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang.xsd"> <lang:groovy id="messenger" script-source="classpath:org/springframework/scripting/groovy/Messenger.groovy"> <lang:property name="message" value="Fiona Apple Is Just So Dreamy."/> </lang:groovy> <!-- when the above bean (messenger) is instantiated, this custom BeanPostProcessor implementation will output the fact to the system console --> <bean class="scripting.InstantiationTracingBeanPostProcessor"/> </beans>
Notice how the InstantiationTracingBeanPostProcessor
is simply defined. It does not
even have a name, and because it is a bean it can be dependency-injected just like any
other bean. (The preceding configuration also defines a bean that is backed by a Groovy
script. The Spring dynamic language support is detailed in the chapter entitled
Chapter 28, Dynamic language support.)
The following simple Java application executes the preceding code and configuration:
import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; import org.springframework.scripting.Messenger; public final class Boot { public static void main(final String[] args) throws Exception { ApplicationContext ctx = new ClassPathXmlApplicationContext("scripting/beans.xml"); Messenger messenger = (Messenger) ctx.getBean("messenger"); System.out.println(messenger); } }
The output of the preceding application resembles the following:
Bean messenger created : org.springframework.scripting.groovy.GroovyMessenger@272961
org.springframework.scripting.groovy.GroovyMessenger@272961
Using callback interfaces or annotations in conjunction with a custom
BeanPostProcessor
implementation is a common means of extending the Spring IoC
container. An example is Spring’s RequiredAnnotationBeanPostProcessor
- a
BeanPostProcessor
implementation that ships with the Spring distribution which ensures
that JavaBean properties on beans that are marked with an (arbitrary) annotation are
actually (configured to be) dependency-injected with a value.
The next extension point that we will look at is the
org.springframework.beans.factory.config.BeanFactoryPostProcessor
. The semantics of
this interface are similar to those of the BeanPostProcessor
, with one major
difference: BeanFactoryPostProcessor
operates on the bean configuration metadata;
that is, the Spring IoC container allows a BeanFactoryPostProcessor
to read the
configuration metadata and potentially change it before the container instantiates
any beans other than BeanFactoryPostProcessors
.
You can configure multiple BeanFactoryPostProcessors
, and you can control the order in
which these BeanFactoryPostProcessors
execute by setting the order
property.
However, you can only set this property if the BeanFactoryPostProcessor
implements the
Ordered
interface. If you write your own BeanFactoryPostProcessor
, you should
consider implementing the Ordered
interface too. Consult the javadocs of the
BeanFactoryPostProcessor
and Ordered
interfaces for more details.
Note | |
---|---|
If you want to change the actual bean instances (i.e., the objects that are created
from the configuration metadata), then you instead need to use a Also, |
A bean factory post-processor is executed automatically when it is declared inside an
ApplicationContext
, in order to apply changes to the configuration metadata that
define the container. Spring includes a number of predefined bean factory
post-processors, such as PropertyOverrideConfigurer
and
PropertyPlaceholderConfigurer
. A custom BeanFactoryPostProcessor
can also be used,
for example, to register custom property editors.
An ApplicationContext
automatically detects any beans that are deployed into it that
implement the BeanFactoryPostProcessor
interface. It uses these beans as bean factory
post-processors, at the appropriate time. You can deploy these post-processor beans as
you would any other bean.
Note | |
---|---|
As with |
You use the PropertyPlaceholderConfigurer
to externalize property values from a bean
definition in a separate file using the standard Java Properties
format. Doing so
enables the person deploying an application to customize environment-specific properties
such as database URLs and passwords, without the complexity or risk of modifying the
main XML definition file or files for the container.
Consider the following XML-based configuration metadata fragment, where a DataSource
with placeholder values is defined. The example shows properties configured from an
external Properties
file. At runtime, a PropertyPlaceholderConfigurer
is applied to
the metadata that will replace some properties of the DataSource. The values to replace
are specified as placeholders of the form ${property-name}
which follows the Ant /
log4j / JSP EL style.
<bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations" value="classpath:com/foo/jdbc.properties"/> </bean> <bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource"> <property name="driverClassName" value="${jdbc.driverClassName}"/> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean>
The actual values come from another file in the standard Java Properties
format:
jdbc.driverClassName=org.hsqldb.jdbcDriver jdbc.url=jdbc:hsqldb:hsql://production:9002 jdbc.username=sa jdbc.password=root
Therefore, the string ${jdbc.username}
is replaced at runtime with the value sa, and
the same applies for other placeholder values that match keys in the properties file.
The PropertyPlaceholderConfigurer
checks for placeholders in most properties and
attributes of a bean definition. Furthermore, the placeholder prefix and suffix can be
customized.
With the context
namespace introduced in Spring 2.5, it is possible to configure
property placeholders with a dedicated configuration element. One or more locations can
be provided as a comma-separated list in the location
attribute.
<context:property-placeholder location="classpath:com/foo/jdbc.properties"/>
The PropertyPlaceholderConfigurer
not only looks for properties in the Properties
file you specify. By default it also checks against the Java System
properties if it
cannot find a property in the specified properties files. You can customize this
behavior by setting the systemPropertiesMode
property of the configurer with one of
the following three supported integer values:
Consult the PropertyPlaceholderConfigurer
javadocs for more information.
Tip | |
---|---|
You can use the <bean class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer"> <property name="locations"> <value>classpath:com/foo/strategy.properties</value> </property> <property name="properties"> <value>custom.strategy.class=com.foo.DefaultStrategy</value> </property> </bean> <bean id="serviceStrategy" class="${custom.strategy.class}"/> If the class cannot be resolved at runtime to a valid class, resolution of the bean
fails when it is about to be created, which is during the |
The PropertyOverrideConfigurer
, another bean factory post-processor, resembles the
PropertyPlaceholderConfigurer
, but unlike the latter, the original definitions can
have default values or no values at all for bean properties. If an overriding
Properties
file does not have an entry for a certain bean property, the default
context definition is used.
Note that the bean definition is not aware of being overridden, so it is not
immediately obvious from the XML definition file that the override configurer is being
used. In case of multiple PropertyOverrideConfigurer
instances that define different
values for the same bean property, the last one wins, due to the overriding mechanism.
Properties file configuration lines take this format:
beanName.property=value
For example:
dataSource.driverClassName=com.mysql.jdbc.Driver dataSource.url=jdbc:mysql:mydb
This example file can be used with a container definition that contains a bean called dataSource, which has driver and url properties.
Compound property names are also supported, as long as every component of the path except the final property being overridden is already non-null (presumably initialized by the constructors). In this example…
foo.fred.bob.sammy=123
sammy
property of the bob
property of the fred
property of the foo
bean
is set to the scalar value 123
.
Note | |
---|---|
Specified override values are always literal values; they are not translated into bean references. This convention also applies when the original value in the XML bean definition specifies a bean reference. |
With the context
namespace introduced in Spring 2.5, it is possible to configure
property overriding with a dedicated configuration element:
<context:property-override location="classpath:override.properties"/>
Implement the org.springframework.beans.factory.FactoryBean
interface for objects that
are themselves factories.
The FactoryBean
interface is a point of pluggability into the Spring IoC container’s
instantiation logic. If you have complex initialization code that is better expressed in
Java as opposed to a (potentially) verbose amount of XML, you can create your own
FactoryBean
, write the complex initialization inside that class, and then plug your
custom FactoryBean
into the container.
The FactoryBean
interface provides three methods:
Object getObject()
: returns an instance of the object this factory creates. The
instance can possibly be shared, depending on whether this factory returns singletons
or prototypes.
boolean isSingleton()
: returns true
if this FactoryBean
returns singletons,
false
otherwise.
Class getObjectType()
: returns the object type returned by the getObject()
method
or null
if the type is not known in advance.
The FactoryBean
concept and interface is used in a number of places within the Spring
Framework; more than 50 implementations of the FactoryBean
interface ship with Spring
itself.
When you need to ask a container for an actual FactoryBean
instance itself instead of
the bean it produces, preface the bean’s id with the ampersand symbol ( &
) when
calling the getBean()
method of the ApplicationContext
. So for a given FactoryBean
with an id of myBean
, invoking getBean("myBean")
on the container returns the
product of the FactoryBean
; whereas, invoking getBean("&myBean")
returns the
FactoryBean
instance itself.
An alternative to XML setups is provided by annotation-based configuration which rely on
the bytecode metadata for wiring up components instead of angle-bracket declarations.
Instead of using XML to describe a bean wiring, the developer moves the configuration
into the component class itself by using annotations on the relevant class, method, or
field declaration. As mentioned in the section called “Example: The RequiredAnnotationBeanPostProcessor”, using
a BeanPostProcessor
in conjunction with annotations is a common means of extending the
Spring IoC container. For example, Spring 2.0 introduced the possibility of enforcing
required properties with the @Required annotation. Spring
2.5 made it possible to follow that same general approach to drive Spring’s dependency
injection. Essentially, the @Autowired
annotation provides the same capabilities as
described in Section 4.4.5, “Autowiring collaborators” but with more fine-grained control and wider
applicability. Spring 2.5 also added support for JSR-250 annotations such as
@PostConstruct
, and @PreDestroy
. Spring 3.0 added support for JSR-330 (Dependency
Injection for Java) annotations contained in the javax.inject package such as @Inject
and @Named
. Details about those annotations can be found in the
relevant section.
Note | |
---|---|
Annotation injection is performed before XML injection, thus the latter configuration will override the former for properties wired through both approaches. |
As always, you can register them as individual bean definitions, but they can also be
implicitly registered by including the following tag in an XML-based Spring
configuration (notice the inclusion of the context
namespace):
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> </beans>
(The implicitly registered post-processors include
AutowiredAnnotationBeanPostProcessor
,
CommonAnnotationBeanPostProcessor
,
PersistenceAnnotationBeanPostProcessor
,
as well as the aforementioned
RequiredAnnotationBeanPostProcessor
.)
Note | |
---|---|
|
The @Required
annotation applies to bean property setter methods, as in the following
example:
public class SimpleMovieLister { private MovieFinder movieFinder; @Required public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
This annotation simply indicates that the affected bean property must be populated at
configuration time, through an explicit property value in a bean definition or through
autowiring. The container throws an exception if the affected bean property has not been
populated; this allows for eager and explicit failure, avoiding NullPointerException
s
or the like later on. It is still recommended that you put assertions into the bean
class itself, for example, into an init method. Doing so enforces those required
references and values even when you use the class outside of a container.
As expected, you can apply the @Autowired
annotation to "traditional" setter methods:
public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Note | |
---|---|
JSR 330’s @Inject annotation can be used in place of Spring’s |
You can also apply the annotation to methods with arbitrary names and/or multiple arguments:
public class MovieRecommender { private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public void prepare(MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) { this.movieCatalog = movieCatalog; this.customerPreferenceDao = customerPreferenceDao; } // ... }
You can apply @Autowired
to constructors and fields:
public class MovieRecommender { @Autowired private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public MovieRecommender(CustomerPreferenceDao customerPreferenceDao) { this.customerPreferenceDao = customerPreferenceDao; } // ... }
It is also possible to provide all beans of a particular type from the
ApplicationContext
by adding the annotation to a field or method that expects an array
of that type:
public class MovieRecommender { @Autowired private MovieCatalog[] movieCatalogs; // ... }
The same applies for typed collections:
public class MovieRecommender { private Set<MovieCatalog> movieCatalogs; @Autowired public void setMovieCatalogs(Set<MovieCatalog> movieCatalogs) { this.movieCatalogs = movieCatalogs; } // ... }
Tip | |
---|---|
Your beans can implement the |
Even typed Maps can be autowired as long as the expected key type is String
. The Map
values will contain all beans of the expected type, and the keys will contain the
corresponding bean names:
public class MovieRecommender { private Map<String, MovieCatalog> movieCatalogs; @Autowired public void setMovieCatalogs(Map<String, MovieCatalog> movieCatalogs) { this.movieCatalogs = movieCatalogs; } // ... }
By default, the autowiring fails whenever zero candidate beans are available; the default behavior is to treat annotated methods, constructors, and fields as indicating required dependencies. This behavior can be changed as demonstrated below.
public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired(required=false) public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Note | |
---|---|
Only one annotated constructor per-class can be marked as required, but multiple non-required constructors can be annotated. In that case, each is considered among the candidates and Spring uses the greediest constructor whose dependencies can be satisfied, that is the constructor that has the largest number of arguments.
|
You can also use @Autowired
for interfaces that are well-known resolvable
dependencies: BeanFactory
, ApplicationContext
, Environment
, ResourceLoader
,
ApplicationEventPublisher
, and MessageSource
. These interfaces and their extended
interfaces, such as ConfigurableApplicationContext
or ResourcePatternResolver
, are
automatically resolved, with no special setup necessary.
public class MovieRecommender { @Autowired private ApplicationContext context; public MovieRecommender() { } // ... }
Note | |
---|---|
|
Because autowiring by type may lead to multiple candidates, it is often necessary to
have more control over the selection process. One way to accomplish this is with
Spring’s @Qualifier
annotation. You can associate qualifier values with specific
arguments, narrowing the set of type matches so that a specific bean is chosen for each
argument. In the simplest case, this can be a plain descriptive value:
public class MovieRecommender { @Autowired @Qualifier("main") private MovieCatalog movieCatalog; // ... }
The @Qualifier
annotation can also be specified on individual constructor arguments or
method parameters:
public class MovieRecommender { private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public void prepare(@Qualifier("main")MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) { this.movieCatalog = movieCatalog; this.customerPreferenceDao = customerPreferenceDao; } // ... }
The corresponding bean definitions appear as follows. The bean with qualifier value "main" is wired with the constructor argument that is qualified with the same value.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier value="main"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <qualifier value="action"/> <!-- inject any dependencies required by this bean --> </bean> <bean id="movieRecommender" class="example.MovieRecommender"/> </beans>
For a fallback match, the bean name is considered a default qualifier value. Thus you
can define the bean with an id "main" instead of the nested qualifier element, leading
to the same matching result. However, although you can use this convention to refer to
specific beans by name, @Autowired
is fundamentally about type-driven injection with
optional semantic qualifiers. This means that qualifier values, even with the bean name
fallback, always have narrowing semantics within the set of type matches; they do not
semantically express a reference to a unique bean id. Good qualifier values are "main"
or "EMEA" or "persistent", expressing characteristics of a specific component that are
independent from the bean id, which may be auto-generated in case of an anonymous bean
definition like the one in the preceding example.
Qualifiers also apply to typed collections, as discussed above, for example, to
Set<MovieCatalog>
. In this case, all matching beans according to the declared
qualifiers are injected as a collection. This implies that qualifiers do not have to be
unique; they rather simply constitute filtering criteria. For example, you can define
multiple MovieCatalog
beans with the same qualifier value "action"; all of which would
be injected into a Set<MovieCatalog>
annotated with @Qualifier("action")
.
Tip | |
---|---|
If you intend to express annotation-driven injection by name, do not primarily use
As a specific consequence of this semantic difference, beans that are themselves defined
as a collection or map type cannot be injected through
|
You can create your own custom qualifier annotations. Simply define an annotation and
provide the @Qualifier
annotation within your definition:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Genre { String value(); }
Then you can provide the custom qualifier on autowired fields and parameters:
public class MovieRecommender { @Autowired @Genre("Action") private MovieCatalog actionCatalog; private MovieCatalog comedyCatalog; @Autowired public void setComedyCatalog(@Genre("Comedy") MovieCatalog comedyCatalog) { this.comedyCatalog = comedyCatalog; } // ... }
Next, provide the information for the candidate bean definitions. You can add
<qualifier/>
tags as sub-elements of the <bean/>
tag and then specify the type
and
value
to match your custom qualifier annotations. The type is matched against the
fully-qualified class name of the annotation. Or, as a convenience if no risk of
conflicting names exists, you can use the short class name. Both approaches are
demonstrated in the following example.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier type="Genre" value="Action"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> _<qualifier type="example.Genre" value="Comedy"/> <!-- inject any dependencies required by this bean --> </bean> <bean id="movieRecommender" class="example.MovieRecommender"/> </beans>
In Section 4.10, “Classpath scanning and managed components”, you will see an annotation-based alternative to providing the qualifier metadata in XML. Specifically, see Section 4.10.8, “Providing qualifier metadata with annotations”.
In some cases, it may be sufficient to use an annotation without a value. This may be useful when the annotation serves a more generic purpose and can be applied across several different types of dependencies. For example, you may provide an offline catalog that would be searched when no Internet connection is available. First define the simple annotation:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface Offline { }
Then add the annotation to the field or property to be autowired:
public class MovieRecommender { @Autowired @Offline private MovieCatalog offlineCatalog; // ... }
Now the bean definition only needs a qualifier type
:
<bean class="example.SimpleMovieCatalog"> <qualifier type="Offline"/> <!-- inject any dependencies required by this bean --> </bean>
You can also define custom qualifier annotations that accept named attributes in
addition to or instead of the simple value
attribute. If multiple attribute values are
then specified on a field or parameter to be autowired, a bean definition must match
all such attribute values to be considered an autowire candidate. As an example,
consider the following annotation definition:
@Target({ElementType.FIELD, ElementType.PARAMETER}) @Retention(RetentionPolicy.RUNTIME) @Qualifier public @interface MovieQualifier { String genre(); Format format(); }
In this case Format
is an enum:
public enum Format {
VHS, DVD, BLURAY
}
The fields to be autowired are annotated with the custom qualifier and include values
for both attributes: genre
and format
.
public class MovieRecommender { @Autowired @MovieQualifier(format=Format.VHS, genre="Action") private MovieCatalog actionVhsCatalog; @Autowired @MovieQualifier(format=Format.VHS, genre="Comedy") private MovieCatalog comedyVhsCatalog; @Autowired @MovieQualifier(format=Format.DVD, genre="Action") private MovieCatalog actionDvdCatalog; @Autowired @MovieQualifier(format=Format.BLURAY, genre="Comedy") private MovieCatalog comedyBluRayCatalog; // ... }
Finally, the bean definitions should contain matching qualifier values. This example
also demonstrates that bean meta attributes may be used instead of the
<qualifier/>
sub-elements. If available, the <qualifier/>
and its attributes take
precedence, but the autowiring mechanism falls back on the values provided within the
<meta/>
tags if no such qualifier is present, as in the last two bean definitions in
the following example.
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:annotation-config/> <bean class="example.SimpleMovieCatalog"> <qualifier type="MovieQualifier"> <attribute key="format" value="VHS"/> <attribute key="genre" value="Action"/> </qualifier> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <qualifier type="MovieQualifier"> <attribute key="format" value="VHS"/> <attribute key="genre" value="Comedy"/> </qualifier> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <meta key="format" value="DVD"/> <meta key="genre" value="Action"/> <!-- inject any dependencies required by this bean --> </bean> <bean class="example.SimpleMovieCatalog"> <meta key="format" value="BLURAY"/> <meta key="genre" value="Comedy"/> <!-- inject any dependencies required by this bean --> </bean> </beans>
In addition to the @Qualifier
annotation, it is also possible to use Java generic types
as an implicit form of qualification. For example, suppose you have the following
configuration:
@Configuration public class MyConfiguration { @Bean public StringStore stringStore() { return new StringStore(); } @Bean public IntegerStore integerStore() { return new IntegerStore(); } }
Assuming that beans above implement a generic interface, i.e. Store<String>
and
Store<Integer>
, you can @Autowire
the Store
interface and the generic will
be used as a qualifier:
@Autowired private Store<String> s1; // <String> qualifier, injects the stringStore bean @Autowired private Store<Integer> s2; // <Integer> qualifier, injects the integerStore bean
Generic qualifiers also apply when autowiring Lists, Maps and Arrays:
// Inject all Store beans as long as they have an <Integer> generic // Store<String> beans will not appear in this list @Autowired private List<Store<Integer>> s;
The
CustomAutowireConfigurer
is a BeanFactoryPostProcessor
that enables you to register your own custom qualifier
annotation types even if they are not annotated with Spring’s @Qualifier
annotation.
<bean id="customAutowireConfigurer" class="org.springframework.beans.factory.annotation.CustomAutowireConfigurer"> <property name="customQualifierTypes"> <set> <value>example.CustomQualifier</value> </set> </property> </bean>
The AutowireCandidateResolver
determines autowire candidates by:
autowire-candidate
value of each bean definition
default-autowire-candidates
pattern(s) available on the <beans/>
element
@Qualifier
annotations and any custom annotations registered
with the CustomAutowireConfigurer
When multiple beans qualify as autowire candidates, the determination of a "primary" is
the following: if exactly one bean definition among the candidates has a primary
attribute set to true
, it will be selected.
Spring also supports injection using the JSR-250 @Resource
annotation on fields or
bean property setter methods. This is a common pattern in Java EE 5 and 6, for example
in JSF 1.2 managed beans or JAX-WS 2.0 endpoints. Spring supports this pattern for
Spring-managed objects as well.
@Resource
takes a name attribute, and by default Spring interprets that value as the
bean name to be injected. In other words, it follows by-name semantics, as
demonstrated in this example:
public class SimpleMovieLister { private MovieFinder movieFinder; @Resource(name="myMovieFinder") public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
If no name is specified explicitly, the default name is derived from the field name or setter method. In case of a field, it takes the field name; in case of a setter method, it takes the bean property name. So the following example is going to have the bean with name "movieFinder" injected into its setter method:
public class SimpleMovieLister { private MovieFinder movieFinder; @Resource public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
Note | |
---|---|
The name provided with the annotation is resolved as a bean name by the
|
In the exclusive case of @Resource
usage with no explicit name specified, and similar
to @Autowired
, @Resource
finds a primary type match instead of a specific named bean
and resolves well-known resolvable dependencies: the BeanFactory
,
ApplicationContext
, ResourceLoader
, ApplicationEventPublisher
, and MessageSource
interfaces.
Thus in the following example, the customerPreferenceDao
field first looks for a bean
named customerPreferenceDao, then falls back to a primary type match for the type
CustomerPreferenceDao
. The "context" field is injected based on the known resolvable
dependency type ApplicationContext
.
public class MovieRecommender { @Resource private CustomerPreferenceDao customerPreferenceDao; @Resource private ApplicationContext context; public MovieRecommender() { } // ... }
The CommonAnnotationBeanPostProcessor
not only recognizes the @Resource
annotation
but also the JSR-250 lifecycle annotations. Introduced in Spring 2.5, the support
for these annotations offers yet another alternative to those described in
initialization callbacks and
destruction callbacks. Provided that the
CommonAnnotationBeanPostProcessor
is registered within the Spring
ApplicationContext
, a method carrying one of these annotations is invoked at the same
point in the lifecycle as the corresponding Spring lifecycle interface method or
explicitly declared callback method. In the example below, the cache will be
pre-populated upon initialization and cleared upon destruction.
public class CachingMovieLister { @PostConstruct public void populateMovieCache() { // populates the movie cache upon initialization... } @PreDestroy public void clearMovieCache() { // clears the movie cache upon destruction... } }
Note | |
---|---|
For details about the effects of combining various lifecycle mechanisms, see the section called “Combining lifecycle mechanisms”. |
Most examples in this chapter use XML to specify the configuration metadata that
produces each BeanDefinition
within the Spring container. The previous section
(Section 4.9, “Annotation-based container configuration”) demonstrates how to provide a lot of the configuration
metadata through source-level annotations. Even in those examples, however, the "base"
bean definitions are explicitly defined in the XML file, while the annotations only
drive the dependency injection. This section describes an option for implicitly
detecting the candidate components by scanning the classpath. Candidate components
are classes that match against a filter criteria and have a corresponding bean
definition registered with the container. This removes the need to use XML to perform
bean registration, instead you can use annotations (for example @Component), AspectJ
type expressions, or your own custom filter criteria to select which classes will have
bean definitions registered with the container.
Note | |
---|---|
Starting with Spring 3.0, many features provided by the Spring JavaConfig project are
part of the core Spring Framework. This allows you to define beans using Java rather
than using the traditional XML files. Take a look at the |
The @Repository
annotation is a marker for any class that fulfills the role or
stereotype (also known as Data Access Object or DAO) of a repository. Among the uses
of this marker is the automatic translation of exceptions as described in
Section 14.2.2, “Exception translation”.
Spring provides further stereotype annotations: @Component
, @Service
, and
@Controller
. @Component
is a generic stereotype for any Spring-managed component.
@Repository
, @Service
, and @Controller
are specializations of @Component
for
more specific use cases, for example, in the persistence, service, and presentation
layers, respectively. Therefore, you can annotate your component classes with
@Component
, but by annotating them with @Repository
, @Service
, or @Controller
instead, your classes are more properly suited for processing by tools or associating
with aspects. For example, these stereotype annotations make ideal targets for
pointcuts. It is also possible that @Repository
, @Service
, and @Controller
may
carry additional semantics in future releases of the Spring Framework. Thus, if you are
choosing between using @Component
or @Service
for your service layer, @Service
is
clearly the better choice. Similarly, as stated above, @Repository
is already
supported as a marker for automatic exception translation in your persistence layer.
Many of the annotations provided by Spring can be used as "meta-annotations" in
your own code. A meta-annotation is simply an annotation, that can be applied to another
annotation. For example, The @Service
annotation mentioned above is meta-annotated with
with @Component
:
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Component // Spring will see this and treat @Service in the same way as @Component public @interface Service { // .... }
Meta-annotations can also be combined together to create composed annotations. For
example, the @RestController
annotation from Spring MVC is composed of
@Controller
and @ResponseBody
.
With the exception of value()
, meta-annotated types may redeclare attributes from the
source annotation to allow user customization. This can be particularly useful when you
want to only expose a subset of the source annotation attributes. For example, here is a
custom @Scope
annotation that defines session
scope, but still allows customization
of the proxyMode
.
@Target({ElementType.TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented @Scope("session") public @interface SessionScope { ScopedProxyMode proxyMode() default ScopedProxyMode.DEFAULT }
Spring can automatically detect stereotyped classes and register corresponding
BeanDefinition
s with the ApplicationContext
. For example, the following two classes
are eligible for such autodetection:
@Service public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public SimpleMovieLister(MovieFinder movieFinder) { this.movieFinder = movieFinder; } }
@Repository public class JpaMovieFinder implements MovieFinder { // implementation elided for clarity }
To autodetect these classes and register the corresponding beans, you need to include the following element in XML, where the base-package element is a common parent package for the two classes. (Alternatively, you can specify a comma-separated list that includes the parent package of each class.)
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> <context:component-scan base-package="org.example"/> </beans>
Tip | |
---|---|
The use of |
Note | |
---|---|
The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, make sure that you do not activate the files-only switch of the JAR task. Also, classpath directories may not get exposed based on security policies in some environments, e.g. standalone apps on JDK 1.7.0_45 and higher (which requires Trusted-Library setup in your manifests; see http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources). |
Furthermore, the AutowiredAnnotationBeanPostProcessor
and
CommonAnnotationBeanPostProcessor
are both included implicitly when you use the
component-scan element. That means that the two components are autodetected and
wired together - all without any bean configuration metadata provided in XML.
Note | |
---|---|
You can disable the registration of |
By default, classes annotated with @Component
, @Repository
, @Service
,
@Controller
, or a custom annotation that itself is annotated with @Component
are the
only detected candidate components. However, you can modify and extend this behavior
simply by applying custom filters. Add them as include-filter or exclude-filter
sub-elements of the component-scan
element. Each filter element requires the type
and expression
attributes. The following table describes the filtering options.
Table 4.5. Filter Types
Filter Type | Example Expression | Description |
---|---|---|
annotation |
| An annotation to be present at the type level in target components. |
assignable |
| A class (or interface) that the target components are assignable to (extend/implement). |
aspectj |
| An AspectJ type expression to be matched by the target components. |
regex |
| A regex expression to be matched by the target components class names. |
custom |
| A custom implementation of the |
The following example shows the XML configuration ignoring all @Repository
annotations
and using "stub" repositories instead.
<beans> <context:component-scan base-package="org.example"> <context:include-filter type="regex" expression=".*Stub.*Repository"/> <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Repository"/> </context:component-scan> </beans>
Note | |
---|---|
You can also disable the default filters by providing use-default-filters="false" as
an attribute of the <component-scan/> element. This will in effect disable automatic
detection of classes annotated with |
Spring components can also contribute bean definition metadata to the container. You do
this with the same @Bean
annotation used to define bean metadata within
@Configuration
annotated classes. Here is a simple example:
@Component public class FactoryMethodComponent { @Bean @Qualifier("public") public TestBean publicInstance() { return new TestBean("publicInstance"); } public void doWork() { // Component method implementation omitted } }
This class is a Spring component that has application-specific code contained in its
doWork()
method. However, it also contributes a bean definition that has a factory
method referring to the method publicInstance()
. The @Bean
annotation identifies the
factory method and other bean definition properties, such as a qualifier value through
the @Qualifier
annotation. Other method level annotations that can be specified are
@Scope
, @Lazy
, and custom qualifier annotations.
Tip | |
---|---|
In addition to its role for component initialization, the |
Autowired fields and methods are supported as previously discussed, with additional
support for autowiring of @Bean
methods:
@Component public class FactoryMethodComponent { private static int i; @Bean @Qualifier("public") public TestBean publicInstance() { return new TestBean("publicInstance"); } // use of a custom qualifier and autowiring of method parameters @Bean protected TestBean protectedInstance( @Qualifier("public") TestBean spouse, @Value("#{privateInstance.age}") String country) { TestBean tb = new TestBean("protectedInstance", 1); tb.setSpouse(tb); tb.setCountry(country); return tb; } @Bean @Scope(BeanDefinition.SCOPE_SINGLETON) private TestBean privateInstance() { return new TestBean("privateInstance", i++); } @Bean @Scope(value = WebApplicationContext.SCOPE_SESSION, proxyMode = ScopedProxyMode.TARGET_CLASS) public TestBean requestScopedInstance() { return new TestBean("requestScopedInstance", 3); } }
The example autowires the String
method parameter country
to the value of the Age
property on another bean named privateInstance
. A Spring Expression Language element
defines the value of the property through the notation #{ <expression> }
. For @Value
annotations, an expression resolver is preconfigured to look for bean names when
resolving expression text.
The @Bean
methods in a Spring component are processed differently than their
counterparts inside a Spring @Configuration
class. The difference is that @Component
classes are not enhanced with CGLIB to intercept the invocation of methods and fields.
CGLIB proxying is the means by which invoking methods or fields within @Bean
methods
in @Configuration
classes creates bean metadata references to collaborating objects;
such methods are not invoked with normal Java semantics. In contrast, invoking a
method or field in an @Bean
method within a @Component
class has standard Java
semantics.
When a component is autodetected as part of the scanning process, its bean name is
generated by the BeanNameGenerator
strategy known to that scanner. By default, any
Spring stereotype annotation ( @Component
, @Repository
, @Service
, and
@Controller
) that contains a name
value will thereby provide that name to the
corresponding bean definition.
If such an annotation contains no name
value or for any other detected component (such
as those discovered by custom filters), the default bean name generator returns the
uncapitalized non-qualified class name. For example, if the following two components
were detected, the names would be myMovieLister and movieFinderImpl:
@Service("myMovieLister") public class SimpleMovieLister { // ... }
@Repository public class MovieFinderImpl implements MovieFinder { // ... }
Note | |
---|---|
If you do not want to rely on the default bean-naming strategy, you can provide a custom
bean-naming strategy. First, implement the
|
<beans> <context:component-scan base-package="org.example" name-generator="org.example.MyNameGenerator" /> </beans>
As a general rule, consider specifying the name with the annotation whenever other components may be making explicit references to it. On the other hand, the auto-generated names are adequate whenever the container is responsible for wiring.
As with Spring-managed components in general, the default and most common scope for
autodetected components is singleton. However, sometimes you need other scopes, which
Spring 2.5 provides with a new @Scope
annotation. Simply provide the name of the scope
within the annotation:
@Scope("prototype") @Repository public class MovieFinderImpl implements MovieFinder { // ... }
Note | |
---|---|
To provide a custom strategy for scope resolution rather than relying on the
annotation-based approach, implement the
|
<beans> <context:component-scan base-package="org.example" scope-resolver="org.example.MyScopeResolver" /> </beans>
When using certain non-singleton scopes, it may be necessary to generate proxies for the scoped objects. The reasoning is described in the section called “Scoped beans as dependencies”. For this purpose, a scoped-proxy attribute is available on the component-scan element. The three possible values are: no, interfaces, and targetClass. For example, the following configuration will result in standard JDK dynamic proxies:
<beans> <context:component-scan base-package="org.example" scoped-proxy="interfaces" /> </beans>
The @Qualifier
annotation is discussed in Section 4.9.3, “Fine-tuning annotation-based autowiring with qualifiers”.
The examples in that section demonstrate the use of the @Qualifier
annotation and
custom qualifier annotations to provide fine-grained control when you resolve autowire
candidates. Because those examples were based on XML bean definitions, the qualifier
metadata was provided on the candidate bean definitions using the qualifier
or meta
sub-elements of the bean
element in the XML. When relying upon classpath scanning for
autodetection of components, you provide the qualifier metadata with type-level
annotations on the candidate class. The following three examples demonstrate this
technique:
@Component @Qualifier("Action") public class ActionMovieCatalog implements MovieCatalog { // ... }
@Component @Genre("Action") public class ActionMovieCatalog implements MovieCatalog { // ... }
@Component @Offline public class CachingMovieCatalog implements MovieCatalog { // ... }
Note | |
---|---|
As with most annotation-based alternatives, keep in mind that the annotation metadata is bound to the class definition itself, while the use of XML allows for multiple beans of the same type to provide variations in their qualifier metadata, because that metadata is provided per-instance rather than per-class. |
Starting with Spring 3.0, Spring offers support for JSR-330 standard annotations (Dependency Injection). Those annotations are scanned in the same way as the Spring annotations. You just need to have the relevant jars in your classpath.
Note | |
---|---|
If you are using Maven, the <dependency> <groupId>javax.inject</groupId> <artifactId>javax.inject</artifactId> <version>1</version> </dependency> |
Instead of @Autowired
, @javax.inject.Inject
may be used as follows:
import javax.inject.Inject; public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
As with @Autowired
, it is possible to use @Inject
at the class-level, field-level,
method-level and constructor-argument level. If you would like to use a qualified name
for the dependency that should be injected, you should use the @Named
annotation as
follows:
import javax.inject.Inject; import javax.inject.Named; public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(@Named("main") MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
Instead of @Component
, @javax.inject.Named
may be used as follows:
import javax.inject.Inject; import javax.inject.Named; @Named("movieListener") public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
It is very common to use @Component
without
specifying a name for the component. @Named
can be used in a similar fashion:
import javax.inject.Inject; import javax.inject.Named; @Named public class SimpleMovieLister { private MovieFinder movieFinder; @Inject public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... }
When using @Named
, it is possible to use
component-scanning in the exact same way as when using Spring annotations:
<beans> <context:component-scan base-package="org.example"/> </beans>
When working with standard annotations, it is important to know that some significant features are not available as shown in the table below:
Table 4.6. Spring annotations vs. standard annotations
Spring | javax.inject.* | javax.inject restrictions / comments |
---|---|---|
@Autowired | @Inject | @Inject has no required attribute |
@Component | @Named | - |
@Scope("singleton") | @Singleton | The JSR-330 default scope is like Spring’s
|
@Qualifier | @Named | - |
@Value | - | no equivalent |
@Required | - | no equivalent |
@Lazy | - | no equivalent |
The central artifacts in Spring’s new Java-configuration support are
@Configuration
-annotated classes and @Bean
-annotated methods.
The @Bean
annotation is used to indicate that a method instantiates, configures and
initializes a new object to be managed by the Spring IoC container. For those familiar
with Spring’s <beans/>
XML configuration the @Bean
annotation plays the same role as
the <bean/>
element. You can use @Bean
annotated methods with any Spring
@Component
, however, they are most often used with @Configuration
beans.
Annotating a class with @Configuration
indicates that its primary purpose is as a
source of bean definitions. Furthermore, @Configuration
classes allow inter-bean
dependencies to be defined by simply calling other @Bean
methods in the same class.
The simplest possible @Configuration
class would read as follows:
@Configuration public class AppConfig { @Bean public MyService myService() { return new MyServiceImpl(); } }
The AppConfig
class above would be equivalent to the following Spring <beans/>
XML:
<beans> <bean id="myService" class="com.acme.services.MyServiceImpl"/> </beans>
The @Bean
and @Configuration
annotations will be discussed in depth in the sections
below. First, however, we’ll cover the various ways of creating a spring container using
Java-based configuration.
The sections below document Spring’s AnnotationConfigApplicationContext
, new in Spring
3.0. This versatile ApplicationContext
implementation is capable of accepting not only
@Configuration
classes as input, but also plain @Component
classes and classes
annotated with JSR-330 metadata.
When @Configuration
classes are provided as input, the @Configuration
class itself
is registered as a bean definition, and all declared @Bean
methods within the class
are also registered as bean definitions.
When @Component
and JSR-330 classes are provided, they are registered as bean
definitions, and it is assumed that DI metadata such as @Autowired
or @Inject
are
used within those classes where necessary.
In much the same way that Spring XML files are used as input when instantiating a
ClassPathXmlApplicationContext
, @Configuration
classes may be used as input when
instantiating an AnnotationConfigApplicationContext
. This allows for completely
XML-free usage of the Spring container:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
As mentioned above, AnnotationConfigApplicationContext
is not limited to working only
with @Configuration
classes. Any @Component
or JSR-330 annotated class may be supplied
as input to the constructor. For example:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(MyServiceImpl.class, Dependency1.class, Dependency2.class); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
The above assumes that MyServiceImpl
, Dependency1
and Dependency2
use Spring
dependency injection annotations such as @Autowired
.
An AnnotationConfigApplicationContext
may be instantiated using a no-arg constructor
and then configured using the register()
method. This approach is particularly useful
when programmatically building an AnnotationConfigApplicationContext
.
public static void main(String[] args) { AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(); ctx.register(AppConfig.class, OtherConfig.class); ctx.register(AdditionalConfig.class); ctx.refresh(); MyService myService = ctx.getBean(MyService.class); myService.doStuff(); }
Experienced Spring users will be familiar with the following commonly-used XML
declaration from Spring’s context:
namespace
<beans> <context:component-scan base-package="com.acme"/> </beans>
In the example above, the com.acme
package will be scanned, looking for any
@Component
-annotated classes, and those classes will be registered as Spring bean
definitions within the container. AnnotationConfigApplicationContext
exposes the
scan(String...)
method to allow for the same component-scanning functionality:
public static void main(String[] args) { AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(); ctx.scan("com.acme"); ctx.refresh(); MyService myService = ctx.getBean(MyService.class); }
Note | |
---|---|
Remember that |
A WebApplicationContext
variant of AnnotationConfigApplicationContext
is available
with AnnotationConfigWebApplicationContext
. This implementation may be used when
configuring the Spring ContextLoaderListener
servlet listener, Spring MVC
DispatcherServlet
, etc. What follows is a web.xml
snippet that configures a typical
Spring MVC web application. Note the use of the contextClass
context-param and
init-param:
<web-app> <!-- Configure ContextLoaderListener to use AnnotationConfigWebApplicationContext instead of the default XmlWebApplicationContext --> <context-param> <param-name>contextClass</param-name> <param-value> org.springframework.web.context.support.AnnotationConfigWebApplicationContext </param-value> </context-param> <!-- Configuration locations must consist of one or more comma- or space-delimited fully-qualified @Configuration classes. Fully-qualified packages may also be specified for component-scanning --> <context-param> <param-name>contextConfigLocation</param-name> <param-value>com.acme.AppConfig</param-value> </context-param> <!-- Bootstrap the root application context as usual using ContextLoaderListener --> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <!-- Declare a Spring MVC DispatcherServlet as usual --> <servlet> <servlet-name>dispatcher</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <!-- Configure DispatcherServlet to use AnnotationConfigWebApplicationContext instead of the default XmlWebApplicationContext --> <init-param> <param-name>contextClass</param-name> <param-value> org.springframework.web.context.support.AnnotationConfigWebApplicationContext </param-value> </init-param> <!-- Again, config locations must consist of one or more comma- or space-delimited and fully-qualified @Configuration classes --> <init-param> <param-name>contextConfigLocation</param-name> <param-value>com.acme.web.MvcConfig</param-value> </init-param> </servlet> <!-- map all requests for /app/* to the dispatcher servlet --> <servlet-mapping> <servlet-name>dispatcher</servlet-name> <url-pattern>/app/*</url-pattern> </servlet-mapping> </web-app>
@Bean
is a method-level annotation and a direct analog of the XML <bean/>
element.
The annotation supports some of the attributes offered by <bean/>
, such as:
init-method,
destroy-method,
autowiring and name
.
You can use the @Bean
annotation in a @Configuration
-annotated or in a
@Component
-annotated class.
To declare a bean, simply annotate a method with the @Bean
annotation. You use this
method to register a bean definition within an ApplicationContext
of the type
specified as the method’s return value. By default, the bean name will be the same as
the method name. The following is a simple example of a @Bean
method declaration:
@Configuration public class AppConfig { @Bean public TransferService transferService() { return new TransferServiceImpl(); } }
The preceding configuration is exactly equivalent to the following Spring XML:
<beans> <bean id="transferService" class="com.acme.TransferServiceImpl"/> </beans>
Both declarations make a bean named transferService
available in the
ApplicationContext
, bound to an object instance of type TransferServiceImpl
:
transferService -> com.acme.TransferServiceImpl
Any classes defined with the @Bean
annotation support the regular lifecycle callbacks
and can use the @PostConstruct
and @PreDestroy
annotations from JSR-250, see
JSR-250 annotations for further
details.
The regular Spring lifecycle callbacks are fully supported as
well. If a bean implements InitializingBean
, DisposableBean
, or Lifecycle
, their
respective methods are called by the container.
The standard set of *Aware
interfaces such as BeanFactoryAware,
BeanNameAware,
MessageSourceAware,
ApplicationContextAware, and so on are also fully supported.
The @Bean
annotation supports specifying arbitrary initialization and destruction
callback methods, much like Spring XML’s init-method
and destroy-method
attributes
on the bean
element:
public class Foo { public void init() { // initialization logic } } public class Bar { public void cleanup() { // destruction logic } } @Configuration public class AppConfig { @Bean(initMethod = "init") public Foo foo() { return new Foo(); } @Bean(destroyMethod = "cleanup") public Bar bar() { return new Bar(); } }
Of course, in the case of Foo
above, it would be equally as valid to call the init()
method directly during construction:
@Configuration public class AppConfig { @Bean public Foo foo() { Foo foo = new Foo(); foo.init(); return foo; } // ... }
Tip | |
---|---|
When you work directly in Java, you can do anything you like with your objects and do not always need to rely on the container lifecycle! |
You can specify that your beans defined with the @Bean
annotation should have a
specific scope. You can use any of the standard scopes specified in the
Bean Scopes section.
The default scope is singleton
, but you can override this with the @Scope
annotation:
@Configuration public class MyConfiguration { @Bean @Scope("prototype") public Encryptor encryptor() { // ... } }
Spring offers a convenient way of working with scoped dependencies through
scoped proxies. The easiest way to create such
a proxy when using the XML configuration is the <aop:scoped-proxy/>
element.
Configuring your beans in Java with a @Scope annotation offers equivalent support with
the proxyMode attribute. The default is no proxy ( ScopedProxyMode.NO
), but you can
specify ScopedProxyMode.TARGET_CLASS
or ScopedProxyMode.INTERFACES
.
If you port the scoped proxy example from the XML reference documentation (see preceding
link) to our @Bean
using Java, it would look like the following:
// an HTTP Session-scoped bean exposed as a proxy @Bean @Scope(value = "session", proxyMode = ScopedProxyMode.TARGET_CLASS) public UserPreferences userPreferences() { return new UserPreferences(); } @Bean public Service userService() { UserService service = new SimpleUserService(); // a reference to the proxied userPreferences bean service.setUserPreferences(userPreferences()); return service; }
By default, configuration classes use a @Bean
method’s name as the name of the
resulting bean. This functionality can be overridden, however, with the name
attribute.
@Configuration public class AppConfig { @Bean(name = "myFoo") public Foo foo() { return new Foo(); } }
As discussed in Section 4.3.1, “Naming beans”, it is sometimes desirable to give a single bean
multiple names, otherwise known asbean aliasing. The name
attribute of the @Bean
annotation accepts a String array for this purpose.
@Configuration public class AppConfig { @Bean(name = { "dataSource", "subsystemA-dataSource", "subsystemB-dataSource" }) public DataSource dataSource() { // instantiate, configure and return DataSource bean... } }
Sometimes it is helpful to provide a more detailed textual description of a bean. This can be particularly useful when beans are exposed (perhaps via JMX) for monitoring purposes.
To add a description to a @Bean
the
@Description
annotation can be used:
@Configuration public class AppConfig { @Bean @Desciption("Provides a basic example of a bean") public Foo foo() { return new Foo(); } }
@Configuration
is a class-level annotation indicating that an object is a source of
bean definitions. @Configuration
classes declare beans via public @Bean
annotated
methods. Calls to @Bean
methods on @Configuration
classes can also be used to define
inter-bean dependencies. See Section 4.12.1, “Basic concepts: @Bean and @Configuration” for a general introduction.
When @Bean
s have dependencies on one another, expressing that dependency is as simple
as having one bean method call another:
@Configuration public class AppConfig { @Bean public Foo foo() { return new Foo(bar()); } @Bean public Bar bar() { return new Bar(); } }
In the example above, the foo
bean receives a reference to bar
via constructor
injection.
Note | |
---|---|
This method of declaring inter-bean dependencies only works when the |
As noted earlier, lookup method injection is an advanced feature that you should use rarely. It is useful in cases where a singleton-scoped bean has a dependency on a prototype-scoped bean. Using Java for this type of configuration provides a natural means for implementing this pattern.
public abstract class CommandManager { public Object process(Object commandState) { // grab a new instance of the appropriate Command interface Command command = createCommand(); // set the state on the (hopefully brand new) Command instance command.setState(commandState); return command.execute(); } // okay... but where is the implementation of this method? protected abstract Command createCommand(); }
Using Java-configuration support , you can create a subclass of CommandManager
where
the abstract createCommand()
method is overridden in such a way that it looks up a new
(prototype) command object:
@Bean @Scope("prototype") public AsyncCommand asyncCommand() { AsyncCommand command = new AsyncCommand(); // inject dependencies here as required return command; } @Bean public CommandManager commandManager() { // return new anonymous implementation of CommandManager with command() overridden // to return a new prototype Command object return new CommandManager() { protected Command createCommand() { return asyncCommand(); } } }
The following example shows a @Bean
annotated method being called twice:
@Configuration public class AppConfig { @Bean public ClientService clientService1() { ClientServiceImpl clientService = new ClientServiceImpl(); clientService.setClientDao(clientDao()); return clientService; } @Bean public ClientService clientService2() { ClientServiceImpl clientService = new ClientServiceImpl(); clientService.setClientDao(clientDao()); return clientService; } @Bean public ClientDao clientDao() { return new ClientDaoImpl(); } }
clientDao()
has been called once in clientService1()
and once in clientService2()
.
Since this method creates a new instance of ClientDaoImpl
and returns it, you would
normally expect having 2 instances (one for each service). That definitely would be
problematic: in Spring, instantiated beans have a singleton
scope by default. This is
where the magic comes in: All @Configuration
classes are subclassed at startup-time
with CGLIB
. In the subclass, the child method checks the container first for any
cached (scoped) beans before it calls the parent method and creates a new instance. Note
that as of Spring 3.2, it is no longer necessary to add CGLIB to your classpath because
CGLIB classes have been repackaged under org.springframework and included directly
within the spring-core JAR.
Note | |
---|---|
The behavior could be different according to the scope of your bean. We are talking about singletons here. |
Note | |
---|---|
There are a few restrictions due to the fact that CGLIB dynamically adds features at startup-time:
|
Much as the <import/>
element is used within Spring XML files to aid in modularizing
configurations, the @Import
annotation allows for loading @Bean
definitions from
another configuration class:
@Configuration public class ConfigA { @Bean public A a() { return new A(); } } @Configuration @Import(ConfigA.class) public class ConfigB { @Bean public B b() { return new B(); } }
Now, rather than needing to specify both ConfigA.class
and ConfigB.class
when
instantiating the context, only ConfigB
needs to be supplied explicitly:
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(ConfigB.class); // now both beans A and B will be available... A a = ctx.getBean(A.class); B b = ctx.getBean(B.class); }
This approach simplifies container instantiation, as only one class needs to be dealt
with, rather than requiring the developer to remember a potentially large number of
@Configuration
classes during construction.
The example above works, but is simplistic. In most practical scenarios, beans will have
dependencies on one another across configuration classes. When using XML, this is not an
issue, per se, because there is no compiler involved, and one can simply declare
ref="someBean"
and trust that Spring will work it out during container initialization.
Of course, when using @Configuration
classes, the Java compiler places constraints on
the configuration model, in that references to other beans must be valid Java syntax.
Fortunately, solving this problem is simple. Remember that @Configuration
classes are
ultimately just another bean in the container - this means that they can take advantage
of @Autowired
injection metadata just like any other bean!
Let’s consider a more real-world scenario with several @Configuration
classes, each
depending on beans declared in the others:
@Configuration public class ServiceConfig { @Autowired private AccountRepository accountRepository; @Bean public TransferService transferService() { return new TransferServiceImpl(accountRepository); } } @Configuration public class RepositoryConfig { @Autowired private DataSource dataSource; @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } } @Configuration @Import({ServiceConfig.class, RepositoryConfig.class}) public class SystemTestConfig { @Bean public DataSource dataSource() { // return new DataSource } } public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class); // everything wires up across configuration classes... TransferService transferService = ctx.getBean(TransferService.class); transferService.transfer(100.00, "A123", "C456"); }
In the scenario above, using @Autowired
works well and provides the desired
modularity, but determining exactly where the autowired bean definitions are declared is
still somewhat ambiguous. For example, as a developer looking at ServiceConfig
, how do
you know exactly where the @Autowired AccountRepository
bean is declared? It’s not
explicit in the code, and this may be just fine. Remember that the
SpringSource Tool Suite provides tooling that
can render graphs showing how everything is wired up - that may be all you need. Also,
your Java IDE can easily find all declarations and uses of the AccountRepository
type,
and will quickly show you the location of @Bean
methods that return that type.
In cases where this ambiguity is not acceptable and you wish to have direct navigation
from within your IDE from one @Configuration
class to another, consider autowiring the
configuration classes themselves:
@Configuration public class ServiceConfig { @Autowired private RepositoryConfig repositoryConfig; @Bean public TransferService transferService() { // navigate through the config class to the @Bean method! return new TransferServiceImpl(repositoryConfig.accountRepository()); } }
In the situation above, it is completely explicit where AccountRepository
is defined.
However, ServiceConfig
is now tightly coupled to RepositoryConfig
; that’s the
tradeoff. This tight coupling can be somewhat mitigated by using interface-based or
abstract class-based @Configuration
classes. Consider the following:
@Configuration public class ServiceConfig { @Autowired private RepositoryConfig repositoryConfig; @Bean public TransferService transferService() { return new TransferServiceImpl(repositoryConfig.accountRepository()); } } @Configuration public interface RepositoryConfig { @Bean AccountRepository accountRepository(); } @Configuration public class DefaultRepositoryConfig implements RepositoryConfig { @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(...); } } @Configuration @Import({ServiceConfig.class, DefaultRepositoryConfig.class}) // import the concrete config! public class SystemTestConfig { @Bean public DataSource dataSource() { // return DataSource } } public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(SystemTestConfig.class); TransferService transferService = ctx.getBean(TransferService.class); transferService.transfer(100.00, "A123", "C456"); }
Now ServiceConfig
is loosely coupled with respect to the concrete
DefaultRepositoryConfig
, and built-in IDE tooling is still useful: it will be easy for
the developer to get a type hierarchy of RepositoryConfig
implementations. In this
way, navigating @Configuration
classes and their dependencies becomes no different
than the usual process of navigating interface-based code.
It is often useful to conditionally enable to disable a complete @Configuration
class,
or even individual @Bean
methods, based on some arbitrary system state. One common
example of this it to use the @Profile
annotation to active beans only when a specific
profile has been enabled in the Spring Environment
(see Section 4.13, “Bean definition profiles and environment abstraction”
for details).
The @Profile
annotation is actually implemented using a much more flexible annotation
called @Conditional
.
The @Conditional
annotation indicates specific
org.springframework.context.annotation.Condition
implementations that should be
consulted before a @Bean
is registered.
Implementations of the Condition
interface simply provide a matches(...)
method that returns true
or false
. For example, here is the actual
Condition
implementation used for @Profile
:
@Override public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { if (context.getEnvironment() != null) { // Read the @Profile annotation attributes MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName()); if (attrs != null) { for (Object value : attrs.get("value")) { if (context.getEnvironment().acceptsProfiles(((String[]) value))) { return true; } } return false; } } return true; }
See the
@Conditional
javadocs for more detail.
Spring’s @Configuration
class support does not aim to be a 100% complete replacement
for Spring XML. Some facilities such as Spring XML namespaces remain an ideal way to
configure the container. In cases where XML is convenient or necessary, you have a
choice: either instantiate the container in an "XML-centric" way using, for example,
ClassPathXmlApplicationContext
, or in a "Java-centric" fashion using
AnnotationConfigApplicationContext
and the @ImportResource
annotation to import XML
as needed.
It may be preferable to bootstrap the Spring container from XML and include
@Configuration
classes in an ad-hoc fashion. For example, in a large existing codebase
that uses Spring XML, it will be easier to create @Configuration
classes on an
as-needed basis and include them from the existing XML files. Below you’ll find the
options for using @Configuration
classes in this kind of "XML-centric" situation.
Remember that @Configuration
classes are ultimately just bean definitions in the
container. In this example, we create a @Configuration
class named AppConfig
and
include it within system-test-config.xml
as a <bean/>
definition. Because
<context:annotation-config/>
is switched on, the container will recognize the
@Configuration
annotation, and process the @Bean
methods declared in AppConfig
properly.
@Configuration public class AppConfig { @Autowired private DataSource dataSource; @Bean public AccountRepository accountRepository() { return new JdbcAccountRepository(dataSource); } @Bean public TransferService transferService() { return new TransferService(accountRepository()); } }
system-test-config.xml <beans> <!-- enable processing of annotations such as @Autowired and @Configuration --> <context:annotation-config/> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> <bean class="com.acme.AppConfig"/> <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> </beans>
jdbc.properties jdbc.url=jdbc:hsqldb:hsql://localhost/xdb jdbc.username=sa jdbc.password=
public static void main(String[] args) { ApplicationContext ctx = new ClassPathXmlApplicationContext("classpath:/com/acme/system-test-config.xml"); TransferService transferService = ctx.getBean(TransferService.class); // ... }
Note | |
---|---|
In |
Because @Configuration
is meta-annotated with @Component
, @Configuration
-annotated
classes are automatically candidates for component scanning. Using the same scenario as
above, we can redefine system-test-config.xml
to take advantage of component-scanning.
Note that in this case, we don’t need to explicitly declare
<context:annotation-config/>
, because <context:component-scan/>
enables all the same
functionality.
system-test-config.xml <beans> <!-- picks up and registers AppConfig as a bean definition --> <context:component-scan base-package="com.acme"/> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> <bean class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="url" value="${jdbc.url}"/> <property name="username" value="${jdbc.username}"/> <property name="password" value="${jdbc.password}"/> </bean> </beans>
In applications where @Configuration
classes are the primary mechanism for configuring
the container, it will still likely be necessary to use at least some XML. In these
scenarios, simply use @ImportResource
and define only as much XML as is needed. Doing
so achieves a "Java-centric" approach to configuring the container and keeps XML to a
bare minimum.
@Configuration @ImportResource("classpath:/com/acme/properties-config.xml") public class AppConfig { @Value("${jdbc.url}") private String url; @Value("${jdbc.username}") private String username; @Value("${jdbc.password}") private String password; @Bean public DataSource dataSource() { return new DriverManagerDataSource(url, username, password); } }
properties-config.xml <beans> <context:property-placeholder location="classpath:/com/acme/jdbc.properties"/> </beans>
jdbc.properties jdbc.url=jdbc:hsqldb:hsql://localhost/xdb jdbc.username=sa jdbc.password=
public static void main(String[] args) { ApplicationContext ctx = new AnnotationConfigApplicationContext(AppConfig.class); TransferService transferService = ctx.getBean(TransferService.class); // ... }
Bean definition profiles is a mechanism in the core container that allows for registration of different beans in different environments. This feature can help with many use cases, including:
Find out more about Environment, XML Profiles and the @Profile annotation.
Spring’s Environment abstraction provides search operations over a configurable hierarchy of property sources.
You can find out more about
Unified
Property Management, the
PropertySource
class
and the @PropertySource
annotation.
The LoadTimeWeaver
is used by Spring to dynamically transform classes as they are
loaded into the Java virtual machine (JVM).
To enable load-time weaving add the @EnableLoadTimeWeaving
to one of your
@Configuration
classes:
@Configuration @EnableLoadTimeWeaving public class AppConfig { }
Alternatively for XML configuration use the context:load-time-weaver
element:
<beans> <context:load-time-weaver/> </beans>
Once configured for the ApplicationContext
. Any bean within that ApplicationContext
may implement LoadTimeWeaverAware
, thereby receiving a reference to the load-time
weaver instance. This is particularly useful in combination with Spring’s JPA
support where load-time weaving may be necessary for JPA class transformation. Consult
the LocalContainerEntityManagerFactoryBean
javadocs for more detail. For more on
AspectJ load-time weaving, see Section 8.8.4, “Load-time weaving with AspectJ in the Spring Framework”.
As was discussed in the chapter introduction, the org.springframework.beans.factory
package provides basic functionality for managing and manipulating beans, including in a
programmatic way. The org.springframework.context
package adds the
ApplicationContext
interface, which extends the BeanFactory
interface, in addition to extending other
interfaces to provide additional functionality in a more application
framework-oriented style. Many people use the ApplicationContext
in a completely
declarative fashion, not even creating it programmatically, but instead relying on
support classes such as ContextLoader
to automatically instantiate an
ApplicationContext
as part of the normal startup process of a J2EE web application.
To enhance BeanFactory
functionality in a more framework-oriented style the context
package also provides the following functionality:
MessageSource
interface.
ResourceLoader
interface.
ApplicationListener
interface,
through the use of the ApplicationEventPublisher
interface.
HierarchicalBeanFactory
interface.
The ApplicationContext
interface extends an interface called MessageSource
, and
therefore provides internationalization (i18n) functionality. Spring also provides the
interface HierarchicalMessageSource
, which can resolve messages hierarchically.
Together these interfaces provide the foundation upon which Spring effects message
resolution. The methods defined on these interfaces include:
String getMessage(String code, Object[] args, String default, Locale loc)
: The basic
method used to retrieve a message from the MessageSource
. When no message is found
for the specified locale, the default message is used. Any arguments passed in become
replacement values, using the MessageFormat
functionality provided by the standard
library.
String getMessage(String code, Object[] args, Locale loc)
: Essentially the same as
the previous method, but with one difference: no default message can be specified; if
the message cannot be found, a NoSuchMessageException
is thrown.
String getMessage(MessageSourceResolvable resolvable, Locale locale)
: All properties
used in the preceding methods are also wrapped in a class named
MessageSourceResolvable
, which you can use with this method.
When an ApplicationContext
is loaded, it automatically searches for a MessageSource
bean defined in the context. The bean must have the name messageSource
. If such a bean
is found, all calls to the preceding methods are delegated to the message source. If no
message source is found, the ApplicationContext
attempts to find a parent containing a
bean with the same name. If it does, it uses that bean as the MessageSource
. If the
ApplicationContext
cannot find any source for messages, an empty
DelegatingMessageSource
is instantiated in order to be able to accept calls to the
methods defined above.
Spring provides two MessageSource
implementations, ResourceBundleMessageSource
and
StaticMessageSource
. Both implement HierarchicalMessageSource
in order to do nested
messaging. The StaticMessageSource
is rarely used but provides programmatic ways to
add messages to the source. The ResourceBundleMessageSource
is shown in the following
example:
<beans> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="basenames"> <list> <value>format</value> <value>exceptions</value> <value>windows</value> </list> </property> </bean> </beans>
In the example it is assumed you have three resource bundles defined in your classpath
called format
, exceptions
and windows
. Any request to resolve a message will be
handled in the JDK standard way of resolving messages through ResourceBundles. For the
purposes of the example, assume the contents of two of the above resource bundle files
are…
# in format.properties message=Alligators rock!
# in exceptions.properties
argument.required=The {0} argument is required.
A program to execute the MessageSource
functionality is shown in the next example.
Remember that all ApplicationContext
implementations are also MessageSource
implementations and so can be cast to the MessageSource
interface.
public static void main(String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("message", null, "Default", null); System.out.println(message); }
The resulting output from the above program will be…
Alligators rock!
So to summarize, the MessageSource
is defined in a file called beans.xml
, which
exists at the root of your classpath. The messageSource
bean definition refers to a
number of resource bundles through its basenames
property. The three files that are
passed in the list to the basenames
property exist as files at the root of your
classpath and are called format.properties
, exceptions.properties
, and
windows.properties
respectively.
The next example shows arguments passed to the message lookup; these arguments will be converted into Strings and inserted into placeholders in the lookup message.
<beans> <!-- this MessageSource is being used in a web application --> <bean id="messageSource" class="org.springframework.context.support.ResourceBundleMessageSource"> <property name="basename" value="exceptions"/> </bean> <!-- lets inject the above MessageSource into this POJO --> <bean id="example" class="com.foo.Example"> <property name="messages" ref="messageSource"/> </bean> </beans>
public class Example { private MessageSource messages; public void setMessages(MessageSource messages) { this.messages = messages; } public void execute() { String message = this.messages.getMessage("argument.required", new Object [] {"userDao"}, "Required", null); System.out.println(message); } }
The resulting output from the invocation of the execute()
method will be…
The userDao argument is required.
With regard to internationalization (i18n), Spring’s various MessageResource
implementations follow the same locale resolution and fallback rules as the standard JDK
ResourceBundle
. In short, and continuing with the example messageSource
defined
previously, if you want to resolve messages against the British (en-GB
) locale, you
would create files called format_en_GB.properties
, exceptions_en_GB.properties
, and
windows_en_GB.properties
respectively.
Typically, locale resolution is managed by the surrounding environment of the application. In this example, the locale against which (British) messages will be resolved is specified manually.
# in exceptions_en_GB.properties
argument.required=Ebagum lad, the {0} argument is required, I say, required.
public static void main(final String[] args) { MessageSource resources = new ClassPathXmlApplicationContext("beans.xml"); String message = resources.getMessage("argument.required", new Object [] {"userDao"}, "Required", Locale.UK); System.out.println(message); }
The resulting output from the running of the above program will be…
Ebagum lad, the userDao argument is required, I say, required.
You can also use the MessageSourceAware
interface to acquire a reference to any
MessageSource
that has been defined. Any bean that is defined in an
ApplicationContext
that implements the MessageSourceAware
interface is injected with
the application context’s MessageSource
when the bean is created and configured.
Note | |
---|---|
As an alternative to |
Event handling in the ApplicationContext
is provided through the ApplicationEvent
class and ApplicationListener
interface. If a bean that implements the
ApplicationListener
interface is deployed into the context, every time an
ApplicationEvent
gets published to the ApplicationContext
, that bean is notified.
Essentially, this is the standard Observer design pattern. Spring provides the
following standard events:
Table 4.7. Built-in Events
Event | Explanation |
---|---|
| Published when the |
| Published when the |
| Published when the |
| Published when the |
| A web-specific event telling all beans that an HTTP request has been serviced. This
event is published after the request is complete. This event is only applicable to
web applications using Spring’s |
You can also create and publish your own custom events. This example demonstrates a
simple class that extends Spring’s ApplicationEvent
base class:
public class BlackListEvent extends ApplicationEvent { private final String address; private final String test; public BlackListEvent(Object source, String address, String test) { super(source); this.address = address; this.test = test; } // accessor and other methods... }
To publish a custom ApplicationEvent
, call the publishEvent()
method on an
ApplicationEventPublisher
. Typically this is done by creating a class that implements
ApplicationEventPublisherAware
and registering it as a Spring bean. The following
example demonstrates such a class:
public class EmailService implements ApplicationEventPublisherAware { private List<String> blackList; private ApplicationEventPublisher publisher; public void setBlackList(List<String> blackList) { this.blackList = blackList; } public void setApplicationEventPublisher(ApplicationEventPublisher publisher) { this.publisher = publisher; } public void sendEmail(String address, String text) { if (blackList.contains(address)) { BlackListEvent event = new BlackListEvent(this, address, text); publisher.publishEvent(event); return; } // send email... } }
At configuration time, the Spring container will detect that EmailService
implements
ApplicationEventPublisherAware
and will automatically call
setApplicationEventPublisher()
. In reality, the parameter passed in will be the Spring
container itself; you’re simply interacting with the application context via its
ApplicationEventPublisher
interface.
To receive the custom ApplicationEvent
, create a class that implements
ApplicationListener
and register it as a Spring bean. The following example
demonstrates such a class:
public class BlackListNotifier implements ApplicationListener<BlackListEvent> { private String notificationAddress; public void setNotificationAddress(String notificationAddress) { this.notificationAddress = notificationAddress; } public void onApplicationEvent(BlackListEvent event) { // notify appropriate parties via notificationAddress... } }
Notice that ApplicationListener
is generically parameterized with the type of your
custom event, BlackListEvent
. This means that the onApplicationEvent()
method can
remain type-safe, avoiding any need for downcasting. You may register as many event
listeners as you wish, but note that by default event listeners receive events
synchronously. This means the publishEvent()
method blocks until all listeners have
finished processing the event. One advantage of this synchronous and single-threaded
approach is that when a listener receives an event, it operates inside the transaction
context of the publisher if a transaction context is available. If another strategy for
event publication becomes necessary, refer to the JavaDoc for Spring’s
ApplicationEventMulticaster
interface.
The following example shows the bean definitions used to register and configure each of the classes above:
<bean id="emailService" class="example.EmailService"> <property name="blackList"> <list> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </list> </property> </bean> <bean id="blackListNotifier" class="example.BlackListNotifier"> <property name="notificationAddress" value="[email protected]"/> </bean>
Putting it all together, when the sendEmail()
method of the emailService
bean is
called, if there are any emails that should be blacklisted, a custom event of type
BlackListEvent
is published. The blackListNotifier
bean is registered as an
ApplicationListener
and thus receives the BlackListEvent
, at which point it can
notify appropriate parties.
Note | |
---|---|
Spring’s eventing mechanism is designed for simple communication between Spring beans within the same application context. However, for more sophisticated enterprise integration needs, the separately-maintained Spring Integration project provides complete support for building lightweight, pattern-oriented, event-driven architectures that build upon the well-known Spring programming model. |
For optimal usage and understanding of application contexts, users should generally
familiarize themselves with Spring’s Resource
abstraction, as described in the chapter
Chapter 5, Resources.
An application context is a ResourceLoader
, which can be used to load Resource
s. A
Resource
is essentially a more feature rich version of the JDK class java.net.URL
,
in fact, the implementations of the Resource
wrap an instance of java.net.URL
where
appropriate. A Resource
can obtain low-level resources from almost any location in a
transparent fashion, including from the classpath, a filesystem location, anywhere
describable with a standard URL, and some other variations. If the resource location
string is a simple path without any special prefixes, where those resources come from is
specific and appropriate to the actual application context type.
You can configure a bean deployed into the application context to implement the special
callback interface, ResourceLoaderAware
, to be automatically called back at
initialization time with the application context itself passed in as the
ResourceLoader
. You can also expose properties of type Resource
, to be used to
access static resources; they will be injected into it like any other properties. You
can specify those Resource
properties as simple String paths, and rely on a special
JavaBean PropertyEditor
that is automatically registered by the context, to convert
those text strings to actual Resource
objects when the bean is deployed.
The location path or paths supplied to an ApplicationContext
constructor are actually
resource strings, and in simple form are treated appropriately to the specific context
implementation. ClassPathXmlApplicationContext
treats a simple location path as a
classpath location. You can also use location paths (resource strings) with special
prefixes to force loading of definitions from the classpath or a URL, regardless of the
actual context type.
You can create ApplicationContext
instances declaratively by using, for example, a
ContextLoader
. Of course you can also create ApplicationContext
instances
programmatically by using one of the ApplicationContext
implementations.
You can register an ApplicationContext
using the ContextLoaderListener
as follows:
<context-param> <param-name>contextConfigLocation</param-name> <param-value>/WEB-INF/daoContext.xml /WEB-INF/applicationContext.xml</param-value> </context-param> <listener> <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class> </listener> <!-- or use the ContextLoaderServlet instead of the above listener <servlet> <servlet-name>context</servlet-name> <servlet-class>org.springframework.web.context.ContextLoaderServlet</servlet-class> <load-on-startup>1</load-on-startup> </servlet> -->
The listener inspects the contextConfigLocation
parameter. If the parameter does not
exist, the listener uses /WEB-INF/applicationContext.xml
as a default. When the
parameter does exist, the listener separates the String by using predefined
delimiters (comma, semicolon and whitespace) and uses the values as locations where
application contexts will be searched. Ant-style path patterns are supported as well.
Examples are /WEB-INF/*Context.xml
for all files with names ending with "Context.xml",
residing in the "WEB-INF" directory, and /WEB-INF/**/*Context.xml
, for all such files
in any subdirectory of "WEB-INF".
You can use ContextLoaderServlet
instead of ContextLoaderListener
. The Servlet uses
the contextConfigLocation
parameter just as the listener does.
It is possible to deploy a Spring ApplicationContext as a RAR file, encapsulating the context and all of its required bean classes and library JARs in a J2EE RAR deployment unit. This is the equivalent of bootstrapping a standalone ApplicationContext, just hosted in J2EE environment, being able to access the J2EE servers facilities. RAR deployment is a more natural alternative to scenario of deploying a headless WAR file, in effect, a WAR file without any HTTP entry points that is used only for bootstrapping a Spring ApplicationContext in a J2EE environment.
RAR deployment is ideal for application contexts that do not need HTTP entry points but
rather consist only of message endpoints and scheduled jobs. Beans in such a context can
use application server resources such as the JTA transaction manager and JNDI-bound JDBC
DataSources and JMS ConnectionFactory instances, and may also register with the
platform’s JMX server - all through Spring’s standard transaction management and JNDI
and JMX support facilities. Application components can also interact with the
application server’s JCA WorkManager through Spring’s TaskExecutor
abstraction.
Check out the JavaDoc of the
SpringContextResourceAdapter
class for the configuration details involved in RAR deployment.
For a simple deployment of a Spring ApplicationContext as a J2EE RAR file: package
all application classes into a RAR file, which is a standard JAR file with a different
file extension. Add all required library JARs into the root of the RAR archive. Add a
"META-INF/ra.xml" deployment descriptor (as shown in SpringContextResourceAdapter
's
JavaDoc) and the corresponding Spring XML bean definition file(s) (typically
"META-INF/applicationContext.xml"), and drop the resulting RAR file into your
application server’s deployment directory.
Note | |
---|---|
Such RAR deployment units are usually self-contained; they do not expose components to the outside world, not even to other modules of the same application. Interaction with a RAR-based ApplicationContext usually occurs through JMS destinations that it shares with other modules. A RAR-based ApplicationContext may also, for example, schedule some jobs, reacting to new files in the file system (or the like). If it needs to allow synchronous access from the outside, it could for example export RMI endpoints, which of course may be used by other application modules on the same machine. |
The BeanFactory
provides the underlying basis for Spring’s IoC functionality but it is
only used directly in integration with other third-party frameworks and is now largely
historical in nature for most users of Spring. The BeanFactory
and related interfaces,
such as BeanFactoryAware
, InitializingBean
, DisposableBean
, are still present in
Spring for the purposes of backward compatibility with the large number of third-party
frameworks that integrate with Spring. Often third-party components that can not use
more modern equivalents such as @PostConstruct
or @PreDestroy
in order to remain
compatible with JDK 1.4 or to avoid a dependency on JSR-250.
This section provides additional background into the differences between the
BeanFactory
and ApplicationContext
and how one might access the IoC container
directly through a classic singleton lookup.
Use an ApplicationContext
unless you have a good reason for not doing so.
Because the ApplicationContext
includes all functionality of the BeanFactory
, it is
generally recommended over the BeanFactory
, except for a few situations such as in an
Applet
where memory consumption might be critical and a few extra kilobytes might make
a difference. However, for most typical enterprise applications and systems, the
ApplicationContext
is what you will want to use. Spring makes heavy
use of the BeanPostProcessor
extension point (to
effect proxying and so on). If you use only a plain BeanFactory
, a fair amount of
support such as transactions and AOP will not take effect, at least not without some
extra steps on your part. This situation could be confusing because nothing is actually
wrong with the configuration.
The following table lists features provided by the BeanFactory
and
ApplicationContext
interfaces and implementations.
Table 4.8. Feature Matrix
Feature | BeanFactory | ApplicationContext |
---|---|---|
Bean instantiation/wiring | Yes | Yes |
Automatic | No | Yes |
Automatic | No | Yes |
Convenient | No | Yes |
| No | Yes |
To explicitly register a bean post-processor with a BeanFactory
implementation, you
must write code like this:
ConfigurableBeanFactory factory = new XmlBeanFactory(...); // now register any needed BeanPostProcessor instances MyBeanPostProcessor postProcessor = new MyBeanPostProcessor(); factory.addBeanPostProcessor(postProcessor); // now start using the factory
To explicitly register a BeanFactoryPostProcessor
when using a BeanFactory
implementation, you must write code like this:
XmlBeanFactory factory = new XmlBeanFactory(new FileSystemResource("beans.xml")); // bring in some property values from a Properties file PropertyPlaceholderConfigurer cfg = new PropertyPlaceholderConfigurer(); cfg.setLocation(new FileSystemResource("jdbc.properties")); // now actually do the replacement cfg.postProcessBeanFactory(factory);
In both cases, the explicit registration step is inconvenient, which is one reason why
the various ApplicationContext
implementations are preferred above plain BeanFactory
implementations in the vast majority of Spring-backed applications, especially when
using BeanFactoryPostProcessors
and BeanPostProcessors
. These mechanisms implement
important functionality such as property placeholder replacement and AOP.
It is best to write most application code in a dependency-injection (DI) style, where
that code is served out of a Spring IoC container, has its own dependencies supplied by
the container when it is created, and is completely unaware of the container. However,
for the small glue layers of code that are sometimes needed to tie other code together,
you sometimes need a singleton (or quasi-singleton) style access to a Spring IoC
container. For example, third-party code may try to construct new objects directly (
Class.forName()
style), without the ability to get these objects out of a Spring IoC
container.If the object constructed by the third-party code is a small stub or proxy,
which then uses a singleton style access to a Spring IoC container to get a real object
to delegate to, then inversion of control has still been achieved for the majority of
the code (the object coming out of the container). Thus most code is still unaware of
the container or how it is accessed, and remains decoupled from other code, with all
ensuing benefits. EJBs may also use this stub/proxy approach to delegate to a plain Java
implementation object, retrieved from a Spring IoC container. While the Spring IoC
container itself ideally does not have to be a singleton, it may be unrealistic in terms
of memory usage or initialization times (when using beans in the Spring IoC container
such as a Hibernate SessionFactory
) for each bean to use its own, non-singleton Spring
IoC container.
Looking up the application context in a service locator style is sometimes the only
option for accessing shared Spring-managed components, such as in an EJB 2.1
environment, or when you want to share a single ApplicationContext as a parent to
WebApplicationContexts across WAR files. In this case you should look into using the
utility class
ContextSingletonBeanFactoryLocator
locator that is described in this
SpringSource
team blog entry.