Enterprise JavaBeans, Second Edition

The Stateful Session Bean

Stateful session beans offer an alternative that lies between entity beans and stateless session beans. Stateful session beans are dedicated to one client for the life of the bean instance; a stateful session bean acts on behalf of a client as its agent. They are not swapped among EJB objects or kept in an instance pool like entity and stateless bean instances. Once a stateful session bean is instantiated and assigned to an EJB object, it is dedicated to that EJB object for its entire life cycle.^[30]

Stateful session beans maintain conversational state, which means that the instance variables of the bean class can cache data relative to the client between method invocations. This makes it possible for methods to be interdependent, so that changes made by methods to the bean’s state can affect the result of subsequent method invocations. In contrast, the stateless session beans we have been talking about do not maintain conversational state. Although stateless beans may have instance variables, these fields are not specific to one client. A stateless instance is swapped among many EJB objects, so you can’t predict which instance will service a method call. With stateful session beans, every method call from a client is serviced by the same instance (at least conceptually), so the bean instance’s state can be predicted from one method invocation to the next.

Although stateful session beans maintain conversational state, they are not themselves persistent ...