Enterprise JavaBeans, Fourth 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.^[31]

Stateful session beans maintain conversational state, which means that the instance variables of the bean class can maintain data specific 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. Therefore, every method call from a client must be serviced by the same instance (at least conceptually), so the bean instance’s state can be predicted from one method invocation to the next. In contrast, stateless session beans don’t maintain client-specific data from one method call to the next, so any instance can be used to service any method call from any client.

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 are written directly to the database. Stateful session beans can access the database but do not represent data in the database. In addition, stateful beans ...