EJB Session

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.

As its name suggests, a session bean is similar to an interactive session. A session bean is not shared--it may have just one client, in the same way that an interactive session may have just one user. Like an interactive session, a session bean is not persistent. (That is, its data is not saved to a database.) When the client terminates, its session bean appears to terminate and is no longer associated with the client.

Session Bean Class

The session bean class for this example is called CartBean. Like any session bean, the CartBean class must meet these requirements:

It implements the SessionBean interface.
The class is defined as public.
The class cannot be defined as abstract or final.
It implements one or more ejbCreate methods.
It implements the business methods.
It contains a public constructor with no parameters.
It must not define the finalize method.

The source code for the CartBean class follows.

import java.util.*;
import javax.ejb.*;

public class CartBean implements SessionBean {

   String customerName;
   String customerId;
   Vector contents;

   public void ejbCreate(String person)
      throws CreateException {

      if (person == null) {
         throw new CreateException("Null person not allowed.");
      }
      else {
         customerName = person;
      }

      customerId = "0";
      contents = new Vector();
   }

   public void ejbCreate(String person, String id)
      throws CreateException {

      if (person == null) {
         throw new CreateException("Null person not allowed.");
      }
      else {
         customerName = person;
      }

      IdVerifier idChecker = new IdVerifier();
      if (idChecker.validate(id)) {
         customerId = id;
      }
      else {
         throw new CreateException("Invalid id: "+ id);
      }

      contents = new Vector();
   }

   public void addBook(String title) {
      contents.addElement(title);
   }

   public void removeBook(String title) throws BookException {

      boolean result = contents.removeElement(title);
      if (result == false) {
         throw new BookException(title + "not in cart.");
      }
   }

   public Vector getContents() {
      return contents;
   }

   public CartBean() {}
   public void ejbRemove() {}
   public void ejbActivate() {}
   public void ejbPassivate() {}
   public void setSessionContext(SessionContext sc) {}

}

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 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.

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. 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.

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. Except during method invocation, all instances of a stateless bean are equivalent, allowing the EJB container to assign an instance to any client.

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.

At times, the EJB container may 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.

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 of time (perhaps a few hours).

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.

To improve performance, you might choose a stateless session bean if it has any 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 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. Such a bean, for example, could retrieve the table rows that represent the products that are on sale this month.

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