1 Understanding Message-driven Beans

A message-driven bean implements loosely coupled or asynchronous business logic in which the response to a request need not be immediate. A message-driven bean receives messages from a JMS Queue or Topic, and performs business logic based on the message contents. It is an asynchronous interface between EJBs and JMS.

Throughout its life cycle, an MDB instance can process messages from multiple clients, although not simultaneously. It does not retain state for a specific client. All instances of a message-driven bean are equivalent—the EJB container can assign a message to any MDB instance. The container can pool these instances to allow streams of messages to be processed concurrently.

The EJB container interacts directly with a message-driven bean—creating bean instances and passing JMS messages to those instances as necessary. The container creates bean instances at deployment time, adding and removing instances during operation based on message traffic.

Example: In an on-line shopping application, where the process of taking an order from a customer results in a process that issues a purchase order to a supplier, the supplier ordering process could be implemented by a message-driven bean. While taking the customer order always results in placing a supplier order, the steps are loosely coupled because it is not necessary to generate the supplier order before confirming the customer order. It is acceptable or beneficial for customer orders to "stack up" before the associated supplier orders are issued.

A message-driven bean (MDB) is an enterprise bean that allows J2EE applications to process messages asynchronously. An MDB acts as a JMS or JCA 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 component—an application client, another enterprise bean, or a Web component—or by non-J2EE applications.

These are the key features of message-driven beans:

• Clients do not access message-driven beans through interfaces. A message-driven bean has only a bean class.

• 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.

• Throughout its life cycle, a message-driven bean instance can process messages from multiple clients, although not concurrently.

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 re-delivered.

For information about design alternatives for message-driven beans, see MDBs and Messaging Models.

For a description of the overall EJB development process, see Oracle Fusion Middleware Programming Enterprise JavaBeans, Version 3.0 for Oracle WebLogic Server

JCA-Based MDBs

MDBs can be configured to receive messages from JCA 1.5-compliant resource adapters, as defined by the JCA specification. To configure a MDB to use JCA, set the resource-adapter-jndi-name deployment descriptor.

For more information, see the JCA 1.5 specification and "resource-adapter-jndi-name" in Oracle Fusion Middleware Programming Enterprise JavaBeans for Oracle WebLogic Server.