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Using TargetSource Implementations

Spring offers the concept of a TargetSource, expressed in the org.springframework.aop.TargetSource interface. This interface is responsible for returning the “target object” that implements the join point. The TargetSource implementation is asked for a target instance each time the AOP proxy handles a method invocation.

Developers who use Spring AOP do not normally need to work directly with TargetSource implementations, but this provides a powerful means of supporting pooling, hot swappable, and other sophisticated targets. For example, a pooling TargetSource can return a different target instance for each invocation, by using a pool to manage instances.

If you do not specify a TargetSource, a default implementation is used to wrap a local object. The same target is returned for each invocation (as you would expect).

The rest of this section describes the standard target sources provided with Spring and how you can use them.

When using a custom target source, your target will usually need to be a prototype rather than a singleton bean definition. This allows Spring to create a new target instance when required.

Hot-swappable Target Sources

The org.springframework.aop.target.HotSwappableTargetSource exists to let the target of an AOP proxy be switched while letting callers keep their references to it.

Changing the target source’s target takes effect immediately. The HotSwappableTargetSource is thread-safe.

You can change the target by using the swap() method on HotSwappableTargetSource, as the follow example shows:

  • Java

  • Kotlin

HotSwappableTargetSource swapper = (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);
val swapper = beanFactory.getBean("swapper") as HotSwappableTargetSource
val oldTarget = swapper.swap(newTarget)

The following example shows the required XML definitions:

<bean id="initialTarget" class="mycompany.OldTarget"/>

<bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource">
	<constructor-arg ref="initialTarget"/>
</bean>

<bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="targetSource" ref="swapper"/>
</bean>

The preceding swap() call changes the target of the swappable bean. Clients that hold a reference to that bean are unaware of the change but immediately start hitting the new target.

Although this example does not add any advice (it is not necessary to add advice to use a TargetSource), any TargetSource can be used in conjunction with arbitrary advice.

Pooling Target Sources

Using a pooling target source provides a similar programming model to stateless session EJBs, in which a pool of identical instances is maintained, with method invocations going to free objects in the pool.

A crucial difference between Spring pooling and SLSB pooling is that Spring pooling can be applied to any POJO. As with Spring in general, this service can be applied in a non-invasive way.

Spring provides support for Commons Pool 2.2, which provides a fairly efficient pooling implementation. You need the commons-pool Jar on your application’s classpath to use this feature. You can also subclass org.springframework.aop.target.AbstractPoolingTargetSource to support any other pooling API.

Commons Pool 1.5+ is also supported but is deprecated as of Spring Framework 4.2.

The following listing shows an example configuration:

<bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject"
		scope="prototype">
	... properties omitted
</bean>

<bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPool2TargetSource">
	<property name="targetBeanName" value="businessObjectTarget"/>
	<property name="maxSize" value="25"/>
</bean>

<bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean">
	<property name="targetSource" ref="poolTargetSource"/>
	<property name="interceptorNames" value="myInterceptor"/>
</bean>

Note that the target object (businessObjectTarget in the preceding example) must be a prototype. This lets the PoolingTargetSource implementation create new instances of the target to grow the pool as necessary. See the javadoc of AbstractPoolingTargetSource and the concrete subclass you wish to use for information about its properties. maxSize is the most basic and is always guaranteed to be present.

In this case, myInterceptor is the name of an interceptor that would need to be defined in the same IoC context. However, you need not specify interceptors to use pooling. If you want only pooling and no other advice, do not set the interceptorNames property at all.

You can configure Spring to be able to cast any pooled object to the org.springframework.aop.target.PoolingConfig interface, which exposes information about the configuration and current size of the pool through an introduction. You need to define an advisor similar to the following:

<bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
	<property name="targetObject" ref="poolTargetSource"/>
	<property name="targetMethod" value="getPoolingConfigMixin"/>
</bean>

This advisor is obtained by calling a convenience method on the AbstractPoolingTargetSource class, hence the use of MethodInvokingFactoryBean. This advisor’s name (poolConfigAdvisor, here) must be in the list of interceptors names in the ProxyFactoryBean that exposes the pooled object.

The cast is defined as follows:

  • Java

  • Kotlin

PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject");
System.out.println("Max pool size is " + conf.getMaxSize());
val conf = beanFactory.getBean("businessObject") as PoolingConfig
println("Max pool size is " + conf.maxSize)
Pooling stateless service objects is not usually necessary. We do not believe it should be the default choice, as most stateless objects are naturally thread-safe, and instance pooling is problematic if resources are cached.

Simpler pooling is available by using auto-proxying. You can set the TargetSource implementations used by any auto-proxy creator.

Prototype Target Sources

Setting up a “prototype” target source is similar to setting up a pooling TargetSource. In this case, a new instance of the target is created on every method invocation. Although the cost of creating a new object is not high in a modern JVM, the cost of wiring up the new object (satisfying its IoC dependencies) may be more expensive. Thus, you should not use this approach without very good reason.

To do this, you could modify the poolTargetSource definition shown earlier as follows (we also changed the name, for clarity):

<bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource">
	<property name="targetBeanName" ref="businessObjectTarget"/>
</bean>

The only property is the name of the target bean. Inheritance is used in the TargetSource implementations to ensure consistent naming. As with the pooling target source, the target bean must be a prototype bean definition.

ThreadLocal Target Sources

ThreadLocal target sources are useful if you need an object to be created for each incoming request (per thread that is). The concept of a ThreadLocal provides a JDK-wide facility to transparently store a resource alongside a thread. Setting up a ThreadLocalTargetSource is pretty much the same as was explained for the other types of target source, as the following example shows:

<bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource">
	<property name="targetBeanName" value="businessObjectTarget"/>
</bean>
ThreadLocal instances come with serious issues (potentially resulting in memory leaks) when incorrectly using them in multi-threaded and multi-classloader environments. You should always consider wrapping a ThreadLocal in some other class and never directly use the ThreadLocal itself (except in the wrapper class). Also, you should always remember to correctly set and unset (where the latter simply involves a call to ThreadLocal.set(null)) the resource local to the thread. Unsetting should be done in any case, since not unsetting it might result in problematic behavior. Spring’s ThreadLocal support does this for you and should always be considered in favor of using ThreadLocal instances without other proper handling code.