What Is a Message-Driven Bean?
This section contains text from The Java Message Service Tutorial. Because message-driven beans rely on Java Message Service (JMS) technology, to fully understand how these beans work you should consult the tutorial at this URL: http://java.sun.com/products/jms/tutorial/index.html
A message-driven bean is an enterprise bean that allows J2EE applications to process messages asynchronously. It acts as a JMS message listener, which is similar to an event listener except that it receives messages instead of events. The messages may be sent by any J2EE componentan application client, another enterprise bean, or a Web componentor by a JMS application or system that does not use J2EE technology.
Message-driven beans currently process only JMS messages, but in the future they may be used to process other kinds of messages.
For a code sample, see Chapter 7.
What Makes Message-Driven Beans Different from Session and Entity Beans?
The most visible difference between message-driven beans and session and entity beans is that clients do not access message-driven beans through interfaces. Interfaces are described in the section Defining Client Access with Interfaces (page 58). Unlike a session or entity bean, a message-driven bean has only a bean class.
In several respects, a message-driven bean resembles a stateless session bean.
A message-driven bean's instances retain no data or conversational state for a specific client.
All instances of a message-driven bean are equivalent, allowing the EJB container to assign a message to any message-driven bean instance. The container can pool these instances to allow streams of messages to be processed concurrently.
A single message-driven bean can process messages from multiple clients.
The instance variables of the message-driven bean instance can contain some state across the handling of client messagesfor example, a JMS API connection, an open database connection, or an object reference to an enterprise bean object.
When a message arrives, the container calls the message-driven bean's onMessage method to process the message. The onMessage method normally casts the message to one of the five JMS message types and handles it in accordance with the application's business logic. The onMessage method may call helper methods, or it may invoke a session or entity bean to process the information in the message or to store it in a database.
A message may be delivered to a message-driven bean within a transaction context, so that all operations within the onMessage method are part of a single transaction. If message processing is rolled back, the message will be redelivered. For more information, see Chapter 7.
When to Use Message-Driven Beans
Session beans and entity beans allow you to send JMS messages and to receive them synchronously, but not asynchronously. To avoid tying up server resources, you may prefer not to use blocking synchronous receives in a server-side component. To receive messages asynchronously, use a message-driven bean.