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Introducing the Java Message Service

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Willy Farrell is a senior software engineer in the IBM Developer Skills Program. As part of the developerWorks team, he provides relevant technical information and insight to developers on the latest e-business and industry trends through Web content, articles, speaking engagements, and consulting to faculty at IBM Scholars Program member universities. He has been programming computers for a living since 1981, began using Java in 1996, and joined IBM in 1998.

Summary:  This tutorial provides an introductory overview of the Java Message Service, its functionality, and its capabilities. You will learn the basic programming techniques for creating JMS programs and work with sample code that demonstrates these techniques. Note: This tutorial has been updated to include changes to JMS version 1.1.

Date:  08 Jun 2004
Level:  Introductory PDF:  A4 and Letter (157 KB | 44 pages)Get Adobe® Reader®

Comments:  

Point-to-point programming

Point-to-point programming introduction

In this section, we'll walk through two programs that do point-to-point messaging -- QSender.java and QReceiver.java.

We'll look at the code in small sections and describe what each section does. You can see the complete listings in the Appendix: Code listing for QSender.java and Code listing for QReceiver.java.

QSender: Prompt for JNDI names

All of the sample programs are command-line programs that use System.in for input and System.out for output.

The QSender class has two methods: main(String[]) and send(). The main(String[]) method merely instantiates a QSender and calls its send() method.

The first section of the send() method prompts for the JNDI names of the administered objects that will be used to send messages.

import java.io.*;
import javax.jms.*;
import javax.naming.*;

public class QSender {

    public static void main(String[] args) {

        new QSender().send();
    }

    public void send() {

        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        try {
            //Prompt for JNDI names
            System.out.println("Enter QueueConnectionFactory name:");
            String factoryName = reader.readLine();
            System.out.println("Enter Queue name:");
            String queueName = reader.readLine();
    . . .


QSender: Look up administered objects

The next section of the send() method looks up the administered objects in JNDI, using the names input earlier.

JNDI is accessed by instantiating an InitialContext object; the administered objects are retrieved by calling the lookup(String) method, passing in the name of the object to be retrieved. Note that the lookup(String) method returns Object, so a typecast must be performed on the returned object.

    . . .
            //Look up administered objects
            InitialContext initContext = new InitialContext();
            QueueConnectionFactory factory =
                (QueueConnectionFactory) initContext.lookup(factoryName);
            Queue queue = (Queue) initContext.lookup(queueName);
            initContext.close();
            . . .


QSender: Create JMS objects

Now, we create the JMS objects we need to send messages. Note that we don't directly instantiate these objects using new. All of the objects are created by calling a method on another object.

First, we use the QueueConnectionFactory to create a QueueConnection. We then use that QueueConnection to create a QueueSession.

The QueueSession is not transacted ( false ) and will use automatic acknowledgment ( Session.AUTO_ACKNOWLEDGE ).

Finally, we create the QueueSender to send messages to the Queue we retrieved from JNDI.

    . . .
            //Create JMS objects
            QueueConnection connection = factory.createQueueConnection();
            QueueSession session =
                connection.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
            QueueSender sender = session.createSender(queue);
            . . .


QSender: Send messages

Now we're ready to send messages. In this section, we enter a loop where we prompt for the text of a message to send. If the user types quit, the loop exits.

Otherwise, we build a TextMessage from the entered text and use the QueueSender to send the message, then return to the top of the loop.

    . . .
            //Send messages
            String messageText = null;
            while (true) {
                System.out.println("Enter message to send or 'quit':");
                messageText = reader.readLine();
                if ("quit".equals(messageText))
                    break;
                TextMessage message = session.createTextMessage(messageText);
                sender.send(message);
            }
            . . .


QSender: Exit

Once the loop exits, we close the QueueConnection. Closing the QueueConnection automatically closes the QueueSession and QueueSender.

    . . .
            //Exit
            System.out.println("Exiting...");
            reader.close();
            connection.close();
            System.out.println("Goodbye!");

        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }
}


QReceiver: Prompt for JNDI names and look up administered objects

The QReceiver class, like the QSender class, has a main(String[]) method that simply instantiates a QReceiver and calls its primary method, receive().

The code for prompting for JNDI names and doing the lookup of administered objects is identical to that in QSender.

There are two differences in this class, however:

  • The boolean stop instance variable is used to indicate that the program should exit.

  • QReceiver implements the MessageListener interface in order to receive messages asynchronously.

import java.io.*;
import javax.jms.*;
import javax.naming.*;

public class QReceiver implements MessageListener {

    private boolean stop = false;

    public static void main(String[] args) {

        new QReceiver().receive();
    }

    public void receive() {

        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        try {
            //Prompt for JNDI names
            System.out.println("Enter QueueConnectionFactory name:");
            String factoryName = reader.readLine();
            System.out.println("Enter Queue name:");
            String queueName = reader.readLine();
            reader.close();

            //Look up administered objects
            InitialContext initContext = new InitialContext();
            QueueConnectionFactory factory =
                (QueueConnectionFactory) initContext.lookup(factoryName);
            Queue queue = (Queue) initContext.lookup(queueName);
            initContext.close();
            . . .


QReceiver: Create JMS objects

The QueueConnection and QueueSession are created as they are in QSender, and then a QueueReceiver is created.

Next, setMessageListener() is called, passing in this -- the local instance of QReceiver, which you will recall implements the MessageListener interface.

Finally, the QueueConnection is started to allow messages to be received.

    . . .
            //Create JMS objects
            QueueConnection connection = factory.createQueueConnection();
            QueueSession session =
                connection.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
            QueueReceiver receiver = session.createReceiver(queue);
            receiver.setMessageListener(this);
            connection.start();
            . . .


QReceiver: Wait for stop and exit

Next, the program goes into a loop that will exit when the stop variable becomes true. In the loop, the thread sleeps for one second. Once the loop has exited, the QueueConnection is closed and the program terminates.

    . . .
            //Wait for stop
            while (!stop) {
                Thread.sleep(1000);
            }

            //Exit
            System.out.println("Exiting...");
            connection.close();
            System.out.println("Goodbye!");

        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }
    . . .


QReceiver: onMessage(Message) method

The presence of the QReceiver class's onMessage(Message) method is required because QReceiver implements the MessageListener interface.

When a message is received, this method is called with the Message passed as the parameter.

In our implementation, we get the text content of the message and print it to System.out. We then check to see if the message equals stop, and if it does, the stop variable is set to true; this allows the loop in the receive() method to terminate.

    . . .
    public void onMessage(Message message) {

        try {
            String msgText = ((TextMessage) message).getText();
            System.out.println(msgText);
            if ("stop".equals(msgText))
                stop = true;
        } catch (JMSException e) {
            e.printStackTrace();
            stop = true;
        }
    }
}

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