In general, you should use a session bean if the following circumstances hold:
At any given time, only one client has access to the bean instance.
The state of the bean is not persistent, existing only for a short period (perhaps a few hours).
The bean implements a web service.
The bean’s state represents the interaction between the bean and a specific client.
The bean needs to hold information about the client across method invocations.
The bean mediates between the client and the other components of the application, presenting a simplified view to the client.
Behind the scenes, the bean manages the work flow of several enterprise beans. For an example, see the AccountControllerBean session bean in Chapter 37, The Duke’s Bank Application.
The bean’s state has no data for a specific client.
In a single method invocation, the bean performs a generic task for all clients. For example, you might use a stateless session bean to send an email that confirms an online order.