There are two types of enterprise beans defined in EJB 3.x:

1. 1. Session Bean – Performs a task for a client, encapsulates the business logic of the application. Session beans are of three types:

              (a). Stateless Session Bean: A stateless session bean does not maintain a conversational state with the client. When a client invokes the methods of a stateless bean, the bean’s instance variables may contain a state specific to that client but only for the duration of the invocation. When the method is finished, the client-specific state should not be retained.

             (b). Stateful Session Bean: In a stateful session bean, the instance variables represent the state of a unique client/bean session. If the client completes all the operations on the bean, the session ends and the state disappears.

             (c). Singleton Session Bean: A singleton session bean is instantiated once per application and exists for the lifecycle of the application. Singleton session beans are designed for circumstances in which a single enterprise bean instance is shared across and concurrently accessed by clients.

2. 2. Message Driven Bean – Acts as an asynchronous message consumer. This type of bean normally acts as a JMS (Java Message Service) message listener.