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system (through the API) to create messages and to communicate asynchronously

with one or more peer applications.

For the J2EE 1.4 platform, JMS includes some enhancements. In particular,

the addition of common interfaces enables you to use the JMS API so that it is not

specific to either a point to point or publish subscribe domain. A JMS provider

may also use the J2EE Connector architecture to integrate more closely with an

application server. (The section  EIS Access Technologies  on page 259 discusses

the J2EE Connector architecture.)

In many ways, JMS is to messaging systems what JDBC is to database sys 

tems. Just as JDBC provides a standard interface to many database systems, JMS

provides a standard API for MOM systems. In fact, JMS changed the proprietary

nature of MOM systems by providing a standard Java API that interfaces with any

MOM system. The developer now writes to this standard API rather than to indi 

vidual, proprietary APIs. The J2EE platform further simplified and made more

portable the integration of a MOM system with a J2EE application server. 

Similar to its support for JDBC, the J2EE platform has added support to JMS

for a connection oriented operational style: Developers can look up a factory and

a connection in the same way. Like JDBC, JMS supports transactions and can

continue JTA transactions started by either a Web or EJB component.

6.2.2.3

Message Driven Beans

Message driven beans, which are EJB components that receive incoming enterprise

messages from a messaging provider, contain the logic for processing these mes 

sages. The business logic of a message driven bean which may include initiating a

workflow, performing a computation, or sending a message may be driven by the

contents of the message itself or merely the receipt of the message. 

Message driven beans are particularly useful in situations where messages

need to be automatically delivered and asynchronous messaging is desired. They

enable applications to be integrated in a loosely coupled, but still reliable, fashion.

Message driven beans are also useful when the delivery of a message should be

the event initiating a workflow process or when a specific message must trigger a

subsequent action. 

When an application produces and sends a message to a particular message

destination queue or topic, the EJB container activates the corresponding

message driven bean (from the pool of available message driven beans). The acti 

vated bean instance consumes the message from the message destination. Since