Example: Event listener

The following code example demonstrates how to fire a listenerCountChanged event:

// imagine this snippet inside an EJB or servlet method.
//  Make an inner class implementing the required event interfaces.
EventSourceEvents listener = new Object() implements EventSourceEvents.class
{
  void listenerCountChanged(EventSource es, int old, int newCount)
  {
    try
    {
      InitialContext ic = new InitialContext();
      // Here, the asynchronous bean can access an environment variable of
      // the component which created it.
       int i = (Integer)ic.lookup("java:comp/env/countValue").intValue());
      if(newCount == i)
      {
        // do something interesting 
      }
      // call this event when the following code executes:
    }
    catch(NamingException e)
    {
    }
  }
  void listenerExceptionThrown(  EventSource es, Object listener, 
              String methodName, Throwable exception)
  {
  }
  void unexpectedException(EventSource es, Object runnable, Throwable exception)
  {
  }
}
// register it.
es.addListener(listener);

...

// now fire an event which the previous listener receives.
EventSourceEvents proxy = (EventSourceEvents)
      es.getEventTrigger(EventSourceEvents.class, false);

proxy.listenerCountChanged(es, 0, 1);

// now, fire another event, one can call any of the methods.
proxy.listenerCountChanged(es, 4, 5);

The output in this example is a proxy for the interface on which the method fires. Then, call the method corresponding to the event on the proxy. This action causes the same method with the same parameters to be called on any event listeners that implement the EventSourceEvents interface and that were previously registered with the EventSource "es". The same proxy can be used to send multiple events simultaneously.

The boolean parameter on the getEventTrigger() method is sameTransaction. When the sameTransaction parameter is false, a new transaction is started for each event listener invoked and these event listeners can be called in parallel to the caller. However, the event() method is blocked until all of the event listeners are notified. If the sameTransaction parameter is true, then the current transaction (if any) on the thread is used for all of the event listeners. The event listeners share the transaction of the method that fired the event. For that reason, all event listeners must run serially in an undetermined order. The order that listeners are called is undefined, and the order in which listeners are registered does not act as a guide for the order used at run time. The method on the proxy does not return until all of the event listeners are called, which means that this action is a synchronous operation.

The parameters that references and listeners pass do not interfere with the function of these references, unless you configure the method to do so. For example, event listeners can be used as collaborators and add data to a map, which was a parameter. Each event listener runs on its own transaction, independent of any transaction that is active on the thread. Extreme care must be taken when the sameTransaction parameter is false because the parameters can be accessed by multiple threads.

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Related Tasks

Developing event listeners

 

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