spring scoped proxy bean

Ref:- http://docs.spring.io/spring/docs/2.5.x/reference/beans.html#beans-factory-scopes-other-injection

3.4.4.5. Scoped beans as dependencies

Being able to define a bean scoped to a HTTP request or Session (or indeed a custom scope of your own devising) is all very well, but one of the main value-adds of the Spring IoC container is that it manages not only the instantiation of your objects (beans), but also the wiring up of collaborators (or dependencies). If you want to inject a (for example) HTTP request scoped bean into another bean, you will need to 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 is smart enough to be able to retrieve the real, target object from the relevant scope (for example a HTTP request) and delegate method calls onto the real object.

	Note
	You do not need to use the <aop:scoped-proxy/> in conjunction with beans that are scoped as singletons or prototypes. It is an error to try to create a scoped proxy for a singleton bean (and the resulting BeanCreationException will certainly set you straight in this regard).

Let's look at the configuration that is required to effect this; the configuration is not hugely complex (it takes just 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-2.5.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd">

    <!-- a HTTP Session-scoped bean exposed as a proxy -->
    <bean id="userPreferences" class="com.foo.UserPreferences" scope="session">
          
          <!-- this next element effects the proxying of the surrounding bean -->
          <aop:scoped-proxy/>
    </bean>
    
    <!-- a singleton-scoped bean injected with a proxy to the above bean -->
    <bean id="userService" class="com.foo.SimpleUserService">
    
        <!-- a reference to the proxied 'userPreferences' bean -->
        <property name="userPreferences" ref="userPreferences"/>

    </bean>
</beans>

To create such a proxy, you need only to insert a child <aop:scoped-proxy/> element into a scoped bean definition (you may also need the CGLIB library on your classpath so that the container can effect class-based proxying; you will also need to be using Appendix A, XML Schema-based configuration). So, just why do you need this <aop:scoped-proxy/> element in the definition of beans scoped at therequest, session, globalSession and 'insert your custom scope here' level? The reason is best explained by picking apart the following bean definition (please note that 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>

From the above configuration it is evident that the singleton bean 'userManager' is being 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) will also only be injected (once!). This means that the 'userManager' will (conceptually) only ever operate on the exact same 'userPreferences' object, that is the one that it was originally injected with. This is not what you want when you inject a HTTP Session-scoped bean as a dependency into a collaborating object (typically). Rather, what we do want is a single 'userManager' object, and then, for the lifetime of a HTTP Session, we want to see and use a 'userPreferences' object that is specific to said HTTP Session.

Rather what you need then is to inject some sort of object that exposes the exact same public interface as the UserPreferences class (ideally an object that is a UserPreferences instance) and that is smart enough to be able to go off and fetch the real UserPreferences object from whatever underlying scoping mechanism we have chosen (HTTP request, Session, etc.). We can then safely inject this proxy object into the 'userManager' bean, which will be blissfully unaware that the UserPreferences reference that it is holding onto is a proxy. In the case of this example, when aUserManager instance invokes a method on the dependency-injected UserPreferences object, it is really invoking a method on the proxy... the proxy will then go off and fetch the realUserPreferences object from (in this case) the HTTP Session, and delegate the method invocation onto the retrieved real UserPreferences object.

That is why 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>

3.4.4.5.1. Choosing the type of proxy created

By default, when the Spring container is creating a proxy for a bean that is marked up with the <aop:scoped-proxy/> element, a CGLIB-based class proxy will be created. This means that you need to have the CGLIB library on the classpath of your application.

Note: CGLIB proxies will 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.

You can choose to have the Spring container 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 does mean that you don't need any additional libraries on your application's classpath to effect such proxying, but it does mean that the class of the scoped bean must implement at least one interface, and all of the collaborators into which the scoped bean is injected must be referencing the bean via 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>