A session bean encapsulates business logic that can be invoked programmatically by a client over local, remote, or web service client views. 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.
A session bean is not persistent. (That is, its data is not saved to a database.)
For code samples, see Chapter 16, Running the Enterprise Bean Examples.
There are three types of session beans: stateful, stateless, and singleton.
The state of an object consists of the values of its instance variables. In a stateful session bean, the instance variables represent the state of a unique client-bean session. Because the client interacts (“talks”) with its bean, this state is often called the conversational state.
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, in the same way that an interactive session can have only one user. When the client terminates, its session bean appears to terminate and is no longer associated with the client.
The state is retained for the duration of the client-bean session. If the client removes the bean, the session ends and the state disappears. This transient nature of the state is not a problem, however, because when the conversation between the client and the bean ends there is no need to retain the state.
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. Clients may, however, change the state of instance variables in pooled stateless beans, and this state is held over to the next invocation of the pooled stateless bean. Except during method invocation, all instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client. That is, the state of a stateless session bean should apply across all clients.
Because stateless session beans can support multiple clients, they can 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.
A stateless session bean can implement a web service, but a stateful session bean cannot.
A singleton session bean is instantiated once per application, and exists for the lifecycle of the application. Singleton session beans are designed for circumstances where a single enterprise bean instance is shared across and concurrently accessed by clients.
Singleton session beans offer similar functionality to stateless session beans, but differ from stateless session beans in that there is only one singleton session bean per application, as opposed to a pool of stateless session beans, any of which may respond to a client request. Like stateless session beans, singleton session beans can implement web service endpoints.
Singleton session beans maintain their state between client invocations, but are not required to maintain their state across server crashes or shutdowns.
Applications that use a singleton session bean may specify that the singleton should be instantiated upon application startup, which allows the singleton to perform initialization tasks for the application. The singleton may perform cleanup tasks on application shutdown as well, because the singleton will operate throughout the lifecycle of the application.
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.
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.
The bean implements a web service.
Singleton session beans are appropriate in the following circumstances:
State needs to be shared across the application.
A single enterprise bean needs to be accessed by multiple threads concurrently.
The application needs an enterprise bean to perform tasks upon application startup and shutdown.
The bean implements a web service.