Friday, March 13, 2009

Question What is a Message Driven Bean, What functions does a message

Question :Question What is a Message Driven Bean, What functions does a message
driven bean have and how do they work in collaboration with JMS? (EJB)

Answer :Message driven beans are the latest addition to the family of component
bean types defined by the EJB specification. The original bean types
include session beans, which contain business logic and maintain a state
associated with client sessions, and entity beans, which map objects to
persistent data.
Message driven beans will provide asynchrony to EJB based applications by
acting as JMS message consumers. A message bean is associated with a
JMS topic or queue and receives JMS messages sent by EJB clients or other
Unlike entity beans and session beans, message beans do not have home
or remote interfaces. Instead, message driven beans are instantiated by
the container as required. Like stateless session beans, message beans
maintain no client-specific state, allowing the container to optimally
manage a pool of message-bean instances.
Clients send JMS messages to message beans in exactly the same manner
as they would send messages to any other JMS destination. This similarity
is a fundamental design goal of the JMS capabilities of the new
To receive JMS messages, message driven beans implement the
javax.jms.MessageListener interface, which defines a single "onMessage()"
When a message arrives, the container ensures that a message bean
corresponding to the message topic/queue exists (instantiating it if
necessary), and calls its onMessage method passing the client's message
as the single argument. The message bean's implementation of this
method contains the business logic required to process the message.
Note that session beans and entity beans are not allowed to function as
message beans.

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