W R I T E   A N D   D E P L O Y   J 2 E E   A P P L I C A T I O N S  

Stateful Session Beans 

Stateful Session Beans (SFSB for short) is either in a ready pool (unused) or assigned to a 

particular client. Thus a client call to a SFSB will always reach the same SFSB with the 

exceptions when a SFSB times out or when the client removes and creates a new SFSB. 

In the transaction example the Teller Session Bean is s SLSB meaning that when 

information like to which bank the teller belongs has to be delivered by the client on every 

call. With a SFSB it can contain these informations and free the client from doing so. 

The disadvantage is that for every client using this Session Bean there is a SFSB copy 

reserved. So when you have 10,000 concurrent users could have 10,000 instances of this 

SFSB even when they are rarely used. 

Another feature of the SFSB is that a user transaction (BMT) started in a method does not 

have to be finished within this method call. This allows you to have a method creating an 

order and then several method calls to add items to this order. At the end the client places 

the order and thus finishes the transaction. Important to note that the client has to know 

what starts and what ends a transaction because a transaction cannot be finished when not 

started in the first place and when a transaction is never finished it will be rolled back by 

the server anyway and therefore everything is lost. 

Message Driven Beans 

Message Driven Beans (or MDB for short) are like SLSB but with the exception that they do 

not have a Home interface and therefore cannot be called by a client directly. Instead they 

are listening for JMS messages from a particular Destination. Thus any client able to send a 

message to the JMS provider will invoke the  onMessage()  on the MDB. This client must 

not be a Java (JMS) client but it could be a mainframe computer sending a message. 

To write a MDB it is necessary to add the MessageListener interface to the implements list 

even this looks superfluous. The idea behind is to be able to add other interfaces than the 

MessageListener in the future. The Template project already contains an example of a MDB 

that listens on the  queue/testQueue  queue. 

Unfortunately XDoclet 1.1.2 does not support some advanced features of JBoss MDB 

settings. The DTD description for MDBs looks like: