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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 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 messages--for 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.
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