Spring request/session scope bean

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] 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.

<!-- 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>