Example: Creating Callable and Runnable tasks

You can create a task that dynamically subscribes to a topic and any component can add a message listener on demand.

The following is an example of a task that dynamically subscribes to a topic:
class SampleTask implements Callable<Object>
{
   Set<MessageListener> listeners =
       Collections.newSetFromMap(new ConcurrentHashMap<MessageListener, Boolean>);
   Topic targetTopic;
   TopicConnectionFactory tcf;

   public SampleTask(TopicConnectionFactory tcf, Topic targetTopic)
   {
      this.targetTopic = targetTopic;
      this.tcf = tcf;
   }

   public void addMessageListener(MessageListener listener)
   {
      listeners.add(listener);
   }

   public Object call() throws JMSException
   {
      // Set up JMS.
      TopicConnection tc = tcf.createConnection();
      try
      {
         TopicSession sess = tc.createSession(false, Session.AUTOACK);
         tc.start();

         while( !Thread.currentThread().isInterrupted() )
         {
            // Block for up to 5 seconds.
            Message msg = sess.receiveMessage(5000);
            if(msg != null)
            {
               for (MessageListener listener : listeners)
                  // Fire an event when we get a message.
                  listener.onMessage(msg);
            }
         }
         tc.close();
      }
      finally
      {
         if (tc != null) tc.close();
      }
      return null;
   }
}

As a result, any component can add a message listener on demand, which enables components to subscribe to a topic in a more scalable way than by simply giving each client subscriber its own thread.


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