Programming message services in Java

Asynchronous interactions seem to be the way of the future

By John Hunt • Get more from this author

Posted in Developer, 25th September 2007 17:40 GMT

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The consumer

In our case, our consumer will be a Message Driven Bean (MDB). This is a type of Enterprise Java Bean (or EJB) that can be deployed within a J2EE application server such as JBoss. However, unlike other EJBs it only has an implementation class. In addition, the lifecycle of a MDB is also simpler than that of either session beans or entity beans. This is because all communication with a MDB is via a JMS queue or topic, no direct synchronous interaction is allowed.

With the advent of EJB3, the creation and definition of all EJBS and of MDBs in particular has become a lot simpler. It is only now necessary to implement the MessageListener interface (a standard JMS interface) and to use some annotations to completely define your MDB. The MessageListener interface defines a single method, namely the onMessage method.

package com.regdeveloper.jms.mdb;

import javax.ejb.ActivationConfigProperty;
import javax.ejb.MessageDriven;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageListener;
import javax.jms.TextMessage;

@MessageDriven(name="NewsMessageDrivenBean", 
         activationConfig = {
            @ActivationConfigProperty(
                    propertyName="destinationType", 
                    propertyValue="javax.jms.Queue"),
            @ActivationConfigProperty(
                    propertyName="destination", 
                    propertyValue="queue/testQueue")
})
public class NewsMDB implements MessageListener {

   public void onMessage(Message message) {
      try {
         String text = ((TextMessage)message).getText();
         System.out.println("In the onMessage method(" + text + ")");  
      } catch (JMSException exp) {
         exp.printStackTrace();
      }
   }
   
}

The annotations on the class (the elements before the class definition starting with an “@” sign) are used to specify to the application server how to bind the message driven bean to an appropriate JMS queue. In this case, we give the MDB a name (“NewsMessageDrivenBean”) and a configuration to be used during deployment. This configuration indicates that the MDB should be bound to a queue and that the queue to bind it to has a JNDI name of “queue/testQueue”.

The MDB is packaged up in a jar file (news.jar) and placed within an EAR file using the following application.xml file:

<?xml version="1.0" encoding="UTF-8" ?> 
<application xmlns="http://java.sun.com/xml/ns/j2ee" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee 
http://java.sun.com/xml/ns/j2ee/application_1_4.xsd" 
        version="1.4"> 
        <display-name>News MDB App</display-name> 
        <module> 
                <ejb>news.jar</ejb> 
        </module> 
</application>

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