Session Beans A session bean represents a single client inside the J2EE server. To access an application that is deployed on the server, the client invokes the session bean's methods. The session bean performs work for its client, shielding the client from complexity by executing business tasks inside the server. At any given time, only one client has access to the bean instance. The state (i.e. the values of the instance variables) of the bean is not persistent, existing only for a short period of time. A session bean can be stateful or stateless.
Stateful session Beans In a stateful session bean, the instance variables represent the state of a unique client-bean session. This state is often called the conversational state. The state is retained for the duration of the client-bean session. If the client removes the bean or terminates, the session ends and the state disappears.
Stateless session Beans A stateless session bean does not maintain a conversational state for a particular client. When a client invokes the method of a stateless bean, the bean's instance variables may contain a state, but only for the duration of the invocation. When the method is finished, the state is no longer retained.
Stateless vs. stateful session Beans All instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client. => Stateless session beans can support multiple clients, and offer better scalability for applications that require large numbers of clients. Typically, an application requires fewer stateless session beans than stateful session beans to support the same number of clients.
Stateless vs. stateful session Beans The EJB container can write a stateful session bean to secondary storage. However, stateless session beans are never written to secondary storage. Therefore, stateless beans may offer better performance than stateful beans.
Stateful session Beans – examples of use • 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.
Stateless session Beans – examples of use • 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 e-mail that confirms an online order. • The bean fetches from a database a set of read-only data that is often used by clients.
Message Driven Beans A message-driven bean is an enterprise bean that allows J2EE applications to process messages asynchronously. It acts as a JMS message listener, which is similar to an event listener except that it receives messages instead of events. The messages may be sent by any J2EE component--an application client, another enterprise bean, or a Web component--or by a JMS application or system that does not use J2EE technology.
Message Driven Beans A message-driven bean's instances retain no data or conversational state for a specific client. All instances of a message-driven bean are equivalent, allowing the EJB container to assign a message to any message-driven bean instance. The container can pool these instances to allow streams of messages to be processed concurrently. A single message-driven bean can process messages from multiple clients.
KEY CONCEPTS • Entity beans: synchronous, shared access, concurrency-safe • CMP • BMP • Session Beans: synchronous • Stateful single access, conversational state • Stateless sharable, no state • Message-Drive Beans: asynchronous, sharable, no state