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Tags/Keywords : How-to-Tutorial - JMS-Topic-and-Publish-Subscribe - J2EE-Technology Example
Author : Sameer
Date (Year/Month/Date): 2009-02-03
JMS Topic and Publish Subscribe using Java

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There are many aspects of
messaging using JMS.

1. Message delivery system
and it can be synchronous 
and asynchronous.

2. Messaging model, and it
can be queue (that is point
to point) and publish subscribe.

3. Durability - storing messages
in persistence in case of messaging
 server fail over conditions.

For showing/demonstrating or 
understanding all these aspects
of JMS I have used OpenJMS from 
http://openjms.sourceforge.net/
As it is Open Source implementation
of JMS, I found this easy to use
for debugging purposes.




By using OpenJMS v0.7.7-alpha, I am dealing with implementation of JMS
specification of Java Message Service 1.1.

Basic steps of using either Topic or Queue are as follows:
1. set properties related to the JMS server/provider URL and
 the initial context factory implementation class file.
 1.1. java.naming.provider.url=tcp://localhost:3035

 "tcp://localhost:3035" is for the OpenJMS JMS implementation
 and can vary based on JMS server.

 1.2. java.naming.factory.initial=org.exolab.jms.jndi.InitialContextFactory

 Again this class file "org.exolab.jms.jndi.InitialContextFactory" is
 dependent on the JMS server used.

2. Using JNDI initial context, look up for the
   JMS Connection Factory.
   Type cast any particular required connection
   factory like QueueConnectionFactory in case 
   of JMS Queue implementation.
   and TopicConnectionFactory in case of JMS 
   Topic implementation.

3. Using specific connection factory, a specific
   QueueConnection or TopicConnection can be created.

4. Based on these connections like, 
   QuestionConnection or TopicConnection specific
   Sessions line QueueSession or TopicSession are created,
   respectively.

5. Again using JNDI context, proper queue or topic (as defined in JMS server
   configuration) are to be looked up.

6. Using Session (QueueSession or TopicSession) 
   either a Sender (in case of queue)
   or a Publisher (in case of topic) is created by 
   passing either queue or topic
   as parameter to method createSender(Queue) or
   createPublisher(Topic).

7. using Session (TopicSession or QueueSession),
   create a specific type of Message (here I have
   used TextMessage) and send this message to either
   
   a queue or a topic by using either Sender or Publisher.

8. Closing connections (either TopicConnection or 
   QueueConnection) could be done in finally block.

Portion of Sample code based on the above mentioned steps:
For Queue Message sender
 
try{
  Properties prop = new Properties();
  prop.put(Context.INITIAL_CONTEXT_FACTORY,
        "org.exolab.jms.jndi.InitialContextFactory");
  prop.put(Context.PROVIDER_URL,"tcp://localhost:3035");

  Context context = new InitialContext(prop);

  QueueConnectionFactory factory = (QueueConnectionFactory)
  context.lookup("ConnectionFactory");

  queueConn = factory.createQueueConnection();
  QueueSession queueSession = queueConn
             .createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
  Queue queue = (Queue) context.lookup("queue1");

  QueueSender queueSender = queueSession.createSender(queue);
  TextMessage txtMsg = queueSession.createTextMessage();
  txtMsg.setText("Sample Message");
  queueSender.send(txtMsg);
 } catch(Exception ex) {
 } finally {
   try{
     queueConn.close();
   } catch(JMSException jmsEx) {
   //TODO
   }
 }


For Queue Message receiver, and this Receiver class implements
javax.jms.MessageListener. This class handles any incoming message
by the onMessage method handler.
 
 try{
   Properties prop = new Properties();
   prop.put(Context.INITIAL_CONTEXT_FACTORY,
          "org.exolab.jms.jndi.InitialContextFactory");
   prop.put(Context.PROVIDER_URL,"tcp://localhost:3035");

   Context context = new InitialContext(prop);

   QueueConnectionFactory factory = (QueueConnectionFactory)
   context.lookup("ConnectionFactory");

   queueConn = factory.createQueueConnection();
   QueueSession queueSession = queueConn
              .createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
   Queue queue = (Queue) context.lookup("queue1");

   QueueReceiver receiver = queueSession.createReceiver(queue);
   receiver.setMessageListener(this);

   queueConn.start();
  } catch(Exception ex) {
   //TODO
  }

 public void onMessage(Message msg) {
   try{
     TextMessage txtMsg = (TextMessage) msg;
     String message = txtMsg.getText();
     System.out.println(message);
   } catch(Exception ex) {
     //TODO
   } finally {
     try{
        queueConn.close();
     } catch (JMSException ex) {
     //TODO
   }
 }
}


Incase of two receivers are pointing or referring to a single queue
and on message arrival to this queue, who gets the message?

To my understanding, in this case, message delivery happens
automatically and no way it can be determined that which receiver
gets the message, after the message is delivered to one out of two
listeners, message gets deleted from the queue and other listener
won't get any message. So it is advisable to configure one only
receiver to any queue.

If you want to contradict this understanding of mine, please do so.
and add your suggestions or note by using Reply Box, after log in.



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