O'Reilly - Java Message Service

Java Message Service
When the TopicSubscriber receives a message from its topic, it invokes the onMessage( )
method of its MessageListener objects. The Chat class itself implements the
MessageListener interface and implements the onMessage( ) method:
public class Chat implements javax.jms.MessageListener{
...
public void onMessage(Message message){
try{
TextMessage textMessage = (TextMessage)message;
String text = textMessage.getText( );
System.out.println(text);
} catch (JMSException jmse){jmse.printStackTrace( );}
}
...
}
The Chat class is a MessageListener type, and therefore registers itself with the
TopicSubscriber in its constructor:
TopicSubscriber subscriber = subSession.createSubscriber(chatTopic);
subscriber.setMessageListener(this);
When the message server pushes a message to the TopicSubscriber, the TopicSubscriber
invokes the Chat object's onMessage( ) method.
2.1.2.9 The Message
It's fairly easy to confuse the Java Message Service with its use of a
Java event model. JMS is an API for asynchronous distributed
enterprise messaging that spans processes and machines across a
network. The Java event model is used to synchronously deliver
events by invoking methods on one or more objects in the same
process that have registered as listeners. The JMS pub/sub model uses
the Java event model so that a TopicSubscriber can notify its
MessageListener object in the same process that a message has
arrived from the message server.
In the chat example, the TextMessage class is used to encapsulate the messages we send
and receive. A TextMessage contains a java.lang.String as its body and is the most
commonly used message type. The onMessage( ) method receives TextMessage objects
from the TopicSubscriber. Likewise, the writeMessage( ) method creates and publishes
TextMessage objects using the TopicPublisher:
public void onMessage(Message message){
try{
TextMessage textMessage = (TextMessage)message;
String text = textMessage.getText( );
System.out.println(text);
} catch (JMSException jmse){jmse.printStackTrace( );}
}
protected void writeMessage(String text) throws JMSException{
TextMessage message = pubSession.createTextMessage( );
message.setText(username+" : "+text);
publisher.publish(message);
}
30