Session Beans

In the Java Platform, Enterprise Edition specifications, a Session Bean is a type of Enterprise Beans. In the J2EE architecture, the other two types are Entity Beans and Message-driven beans. However, in Java EE 5 entity beans have been replaced by Java Persistence API entities.

Contrary to Entity Beans which represent persistent data maintained in a database, a Session Bean implements a business task and is hosted by an EJB container.

A Session Bean is created by a client and usually exists only for the duration of a single client-server session. A session bean performs operations, such as calculations or database access, for the client. Although a session bean can be transactional, it is not recoverable should a system crash occur. Session bean objects either can be stateless or can maintain conversational state across methods and transactions. If a session bean maintains state, then the EJB container manages this state if the object must be removed from memory. However, the session bean object itself must manage its own persistent data.

tateless Session Beans

A stateless session bean is a distributed object that does not have an associated conversational state, thus allowing concurrent access to the bean. The contents of instance variables are not guaranteed to be preserved across method calls. All instances of a stateless session bean are identical.

Remote Stateless SessionBean Hello World example:

Java EE 5

import javax.ejb.Remote;

@Remotepublic interface HelloWorld { String getHello();}

import javax.ejb.Stateless;

@Statelesspublic class HelloWorldBean implements HelloWorld { public String getHello() { return "Hello World !";

import java.io.*;import javax.ejb.EJB;import javax.servlet.*;import javax.servlet.http.*;

public class TestServlet extends HttpServlet { @EJB private HelloWorld helloWorld;

public void service (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { resp.getWriter().println(helloWorld.getHello());

J2EE 1.4

Remote interface, declares methods clients can invoke on the EJB:

import javax.ejb.EJBObject;import java.rmi.RemoteException;

public interface HelloWorld extends EJBObject { public String getHello() throws RemoteException;}

Home interface, declares create, destroy and finder methods for the EJB depending on type:

import javax.ejb.EJBHome;import javax.ejb.CreateException;import java.rmi.*;

public interface HelloWorldHome extends EJBHome { // Create method used by the Container to create the EJB // must return remote interface of EJB public HelloWorld create() throws RemoteException, CreateException;}

The implementing EJB class:

import javax.ejb.SessionBean;import javax.ejb.SessionContext;import java.rmi.*;

public class HelloWorldEJB implements SessionBean { private SessionContext con;

// Implementation of method declared in remote interface public String getHello() throws RemoteException { return "Hello World!"; } // Used by the EJB Container public void setSessionContext (SessionContext con) { this.con = con; }

public void ejbCreate() { }

public void ejbRemove() { }

public void ejbActivate() { }

public void ejbPassivate() {

A simple client:

import javax.naming.InitialContext;import javax.rmi.PortableRemoteObject;

public class Client {

private HelloWorld hello = null;

public String sayHello() throws Exception { private InitialContext init = new InitialContext();

// Looking up the EJB based on its name in JNDI Object objref = init.lookup("HelloWorld"); HelloWorldHome home = (HelloWorldHome)PortableRemoteObject.narrow (objref, HelloWorldHome.class); hello = home.create();

return hello.getHello(); }

public static void main (String [] args) { Client client = new Client();

try { System.out.println(client.sayHello()); } catch (Exception e) { System.err.println("Error: " + e); }

tateful Session Beans

Stateful session beans are distributed objects having a conversational state. The state could be persisted, but access to the bean is limited to only one client.

External links

* [http://java.sun.com/j2ee/1.4/docs/tutorial/doc/EJBConcepts3.html What is a Session Bean ? (Sun's J2EE Tutorial)]
* [http://java.sun.com/j2ee/1.4/docs/tutorial/doc/EJBConcepts9.html Entreprise Beans lifecycle]
* [http://www.unix.org.ua/orelly/java-ent/ebeans/appb_02.htm Life cycle state diagram of session beans]