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What Is a Session Bean?

Asession bean represents a single client inside the Application Server. To access anapplication that is deployed on the server, the client invokes the session bean’smethods. The session bean performs work for its client, shielding the client fromcomplexity by executing business tasks inside the server.

As its name suggests, a session bean is similar to an interactive session.A session bean is not shared; it can have only one client, inthe same way that an interactive session can have only one user. Likean interactive session, a session bean is not persistent. (That is, its datais not saved to a database.) When the client terminates, its session beanappears to terminate and is no longer associated with the client.

For code samples, seeChapter 22, Session Bean Examples.

State Management Modes

There are two types of session beans: stateful and stateless.

Stateful Session Beans

The state of an object consists of the values of its instance variables.In astateful session bean, the instance variables represent the state of aunique client-bean session. Because the client interacts (“talks”) with its bean, this stateis often called the conversational state.

The state is retained for the duration of the client-bean session. If theclient removes the bean or terminates, the session ends and the state disappears.This transient nature of the state is not a problem, however, because whenthe conversation between the client and the bean ends there is no needto retain the state.

Stateless Session Beans

Astateless session bean does not maintain a conversational state with the client.When a client invokes the methods of a stateless bean, the bean’s instancevariables may contain a state specific to that client, but only for theduration of the invocation. When the method is finished, the client-specific state shouldnot be retained. Clients may, however, change the state of instance variables inpooled stateless beans, and this state is held over to the next invocationof the pooled stateless bean. Except during method invocation, all instances of astateless bean are equivalent, allowing the EJB container to assign an instance toany client. That is, the state of a stateless session bean should applyaccross all clients.

Because stateless session beans can support multiple clients, they can offer better scalabilityfor applications that require large numbers of clients. Typically, an application requires fewerstateless session beans than stateful session beans to support the same number ofclients.

A stateless session bean can implement a web service, but other types ofenterprise beans cannot.

When to Use Session Beans

In general, you should use a session bean if the following circumstances hold:

At any given time, only one client has access to the bean instance.
The state of the bean is not persistent, existing only for a short period (perhaps a few hours).
The bean implements a web service.

Stateful session beans are appropriate if any of the following conditions are true:

The bean’s state represents the interaction between the bean and a specific client.
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. For an example, see theAccountControllerBean session bean inChapter 37, The Duke's Bank Application.

To improve performance, you might choose a stateless session bean if it hasany of these traits:

The bean’s state has no data for a specific client.
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 email that confirms an online order.