The most visible difference between message-driven beans and session beans is that clients do not access message-driven beans through interfaces. Interfaces are described in the section Defining Client Access with Interfaces. Unlike a session 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 messages (for example, a JMS API connection, an open database connection, or an object reference to an enterprise bean object).
Client components do not locate message-driven beans and invoke methods directly on them. Instead, a client accesses a message-driven bean through, for example, JMS by sending messages to the message destination for which the message-driven bean class is the MessageListener. You assign a message-driven bean’s destination during deployment by using Application Server resources.
Message-driven beans have the following characteristics:
They execute upon receipt of a single client message.
They are invoked asynchronously.
They are relatively short-lived.
They do not represent directly shared data in the database, but they can access and update this data.
They can be transaction-aware.
They are stateless.
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 can call helper methods, or it can invoke a session bean to process the information in the message or to store it in a database.
A message can be delivered to a message-driven bean within a transaction context, so 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 23, A Message-Driven Bean Example and Chapter 33, Transactions.