O'Reilly - Java Message Service

Java Message Service
line is passed to the Chat class's writeMessage( ) method. This method uses the
TopicPublisher to deliver a message to the topic:
protected void writeMessage(String text) throws JMSException{
TextMessage message = pubSession.createTextMessage( );
message.setText(username+" : "+text);
publisher.publish(message);
}
The TopicPublisher objects deliver messages to the topic asynchronously. Asynchronous
delivery and consumption of messages is a key characteristic of Message-Oriented
Middleware; the TopicPublisher doesn't block or wait until all the subscribers receive the
message. Instead, it returns from the publish( ) method as soon as the message server
receives the message. It's up to the message server to deliver the message to all the
subscribers for that topic.
2.1.2.8 The TopicSubscriber
The TopicSubscriber is created using the subSession and the chatTopic:
// Look up a JMS topic
Topic chatTopic = (Topic)jndi.lookup(topicName);
// Create a JMS publisher and subscriber
TopicPublisher publisher =
pubSession.createPublisher(chatTopic);
TopicSubscriber subscriber =
subSession.createSubscriber(chatTopic);
A TopicSubscriber receives messages from a specific topic. The Topic object argument
used in the createSubscriber( ) method identifies the topic from which the
TopicSubscriber will receive messages.
The TopicSubscriber receives messages from the message server one at a time (serially).
These messages are pushed from the message server to the TopicSubscriber
asynchronously, which means that the TopicSubscriber does not have to poll the message
server for messages. In our example, each chat client will receive any message published
by any of the other chat clients. When a user enters text at the command line, the text
message is delivered to all other chat clients that subscribe to the same topic.
The pub/sub messaging model in JMS includes an in-process Java event model for
handling incoming messages. This is similar to the event-driven model used by Java
beans. [3] An object simply implements the listener interface, in this case the
MessageListener, and then is registered with the TopicSubscriber. A TopicSubscriber may
have only one MessageListener object. Here is the definition of the MessageListener
interface used in JMS:
[3] Although the in-process event model used by TopicSubscriber is similar to the one used in
Java beans, JMS itself is an API and the interfaces it defines are not Java beans.
package javax.jms;
public interface MessageListener {
public void onMessage(Message message);
}
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