使用列表属性避免Spring bean中的循环依赖

Question

Let's say that I have a Maven project using the Spring framework. The Maven project itself has three modules: common , component_a , and component_b . And let's say that common defines an event listener bean with a property listeners , which is a list of everything listening for events. Listeners are defined in the component_a and component_b modules.

To summarize:

applicationcontext-component_a.xml:

<beans>
    <bean id="someListener" class="com.whatever.component_a.SomeListener"/>
    <bean id="anotherListener" class="com.whatever.component_a.AnotherListener"/>
</beans>

applicationcontext-component_b.xml:

<beans>
    <bean id="yetAnotherListener" class="com.whatever.component_b.YetAnotherListener"/>
</beans>

applicationcontext-common.xml:

<beans>
    <bean id="eventListener" class="com.whatever.common.EventListenerImpl">
        <property name="listeners">
            <list>
                <ref bean="someListener"/>
                <ref bean="anotherListener"/>
                <ref bean="yetAnotherListener"/>
            </list>
        </property>
    </bean>
</beans>

(I think I got the format right here...but forgive me if I didn't)

My problem is that now common has a dependency on both of the component_* modules, instead of the other way around. My common module no longer builds properly, because my integration tests cannot properly create the container without those other modules.

So given the fact that my eventListener needs to have a list of all of its listeners, what's a good pattern to use that will keep my modules independent during building/testing?

I can think of a few patterns:

1. Using different versions of eventListener within different contexts
2. Using a new bean to facilitate the registration
3. Inside each setEventListener() call, remember to call eventListener.addListener(this);

But I'm curious if Spring has any built-in way of doing this better...

Answer 1

What you could do instead is autowiring a list of listeners in your main module. Each other module can then just provide implementations of this interface and a proper componentscan/xml declaration.

Mainmodule: (pseudocode)

@Service
ListenerService {

    @Autowire
    List<ListenerIntf> listeners;

    public handleListeners() {
        foreach listeners {
            listener.onEvent();
        }
    }
}

Module 1:

@Component
@Order // if desired
Listener1 implements ListenerIntf {
    public onEvent();
}