Enterprise JavaBeans, Third Edition

The Stateful Session Bean

Each stateful session bean is dedicated to one client for the life of the bean instance; it acts on behalf of that client as its agent. Stateful session beans are not swapped among EJB objects or kept in an instance pool like entity and stateless session 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.^[43]

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 to the bean’s state in one method call can affect the results 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 cannot 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 like entity beans. Entity beans represent data in the database; their persistence fields ...