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they are stateless, any instance of a matching type of message driven bean can

handle the message.

Implementing a message driven bean is straightforward. The bean developer

extends the 

javax.ejb.MessageDrivenBean

 interface and a message listener inter 

face for the bean the message listener interface corresponds to the enterprise's

messaging system. For enterprises using JMS, the developer extends the

javax.jms.MessageListener

 interface. A developer may embed business logic to

handle particular messages within the 

MessageListener

 methods. For example,

when a message driven bean consumes JMS messages, the developer codes the

message handling business logic within the 

onMessage

 method. When a message

appropriate for this bean arrives at a message destination, the EJB container

invokes the message driven bean's 

onMessage

 method. 

With message driven beans, you can make the invocation of the bean part of a

transaction. That is, you can ensure that the message delivery from the message

destination to the message driven bean is part of any subsequent transactional

work initiated by the bean's logic. If problems occur with the subsequent logic

causing the transaction to roll back, the message delivery also rolls back and the

message is redelivered to another message driven bean instance.

6.2.3 EIS Access Technologies

The J2EE 1.4 platform includes the J2EE Connector architecture, a technology

designed specifically for accessing enterprise information systems (EISs). The Con 

nector architecture simplifies integrating diverse EISs into the platform, since

each EIS can use the same, single resource adapter to integrate with all compliant

J2EE servers.

The J2EE Connector architecture provides a standard architecture for integrat 

ing J2EE applications with existing EISs and applications, and particularly for data

integration with non relational databases. The Connector architecture enables

adapters for external EISs to be plugged into the J2EE application server. Enter 

prise applications can use these adapters to support and manage secure, transac 

tional, and scalable, bi directional communication with EISs. The EIS vendor

knows that its adapter will work with all J2EE compliant application servers, and

the compliant J2EE server can connect to multiple EISs. See Figure 6.3.