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Map of Enums and dependency injection in Spring 2.5

Solutons:


What would be wrong with doing something like the following:

Defining a ‘Maintainer’ interface with the addListener(Listener, Enum) method.

Create a DefaultMaintainer class (as above) which implements Maintainer.

Then, in each Listener class, ‘inject’ the Maintainer interface (constructor injection might be a good choice). The listener can then register itself with the Maintainer.

other than that, I’m not 100% clear on exactly what your difficulty is with Spring at the moment! 🙂

Slightly offtopic (as this is not about Spring) but there is a race condition in your implementation of AddListener:

  if( ( listeners = map.get( eventType ) ) == null ) {
     listeners = new java.util.concurrent.CopyOnWriteArrayList<Listener>();
     map.put( eventType, listeners );
  }
  listeners.add( listener );

If two threads call this method at the same time (for an event type that previously had no listeners), map.get( eventType ) will return null in both threads, each thread will create its own CopyOnWriteArrayList (each containing a single listener), one thread will replace the list created by the other, and the first listener will be forgotten.

To fix this, change:

private Map<Enum, List<Listener>> map;

...

map.put( eventType, listeners );

to:

private ConcurrentMap<Enum, List<Listener>> map;

...

map.putIfAbsent( eventType, listeners );
listeners = map.get( eventType );

1) Define Maintainer class as a Spring bean.

Standard Spring syntax applies:

<bean id="maintainer" class="com.example.Maintainer"/>

2) Make it so that all sorts of listeners would be able to register themselves to Maintainer via XML by using addListener method. Spring doc nor Google are very generous in examples.

This is trickier. You could use MethodInvokingFactoryBean to individually call maintainer#addListener, like so:

<bean id="listener" class="com.example.Listener"/>

<bean id="maintainer.addListener" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetObject" ref="maintainer"/>
  <property name="targetMethod" value="addListener"/>
  <property name="arguments">
    <list>
      <ref>listener</ref>
      <value>com.example.MyEnum</value>
   </list>
 </property>
</bean>

However, this is unwieldy, and potentially error-prone. I attempted something similar on a project, and created a Spring utility class to help out instead. I don’t have the source code available at the moment, so I’ll describe how to implement what I did.

1) Refactor the event types listened to into a MyListener interface

public interface MyListener extends Listener {
  public Enum[] getEventTypes()
}

Which changes the registration method to

public void addListener(MyListener listener)

2) Create Spring helper class that finds all relevant listeners in the context, and calls maintainer#addListener for each listener found. I would start with BeanFilteringSupport, and also implement BeanPostProcessor (or ApplicationListener) to register the beans after all beans have been instantiated.

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