Session Beans - AbsoluteAstronomy.com

In the Java Platform, Enterprise Edition

Java Platform, Enterprise Edition

Java Platform, Enterprise Edition or Java EE is widely used platform for server programming in the Java programming language. The Java platform differs from the Java Standard Edition Platform in that it adds libraries which provide functionality to deploy fault-tolerant, distributed, multi-tier...

specifications, a Session Bean is a type of Enterprise Bean

Enterprise JavaBean

Enterprise JavaBeans is a managed, server-side component architecture for modular construction of enterprise applications.The EJB specification is one of several Java APIs in the Java EE specification. EJB is a server-side model that encapsulates the business logic of an application...

. The only other type is the Message-driven bean. Legacy EJB versions from before 2006 (EJB3) had a third type of bean, the Entity Bean

Entity Bean

An Entity Bean is a type of Enterprise JavaBean, a server-side J2EE component, that represents persistent data maintained in a database. An entity bean can manage its own persistence or can delegate this function to its EJB Container . An entity bean is identified by a primary key...

. In EJB 3.0 (Java EE 5) those Entity Beans have been replaced by Java Persistence API

Java Persistence API

The Java Persistence API, sometimes referred to as JPA, is a Java programming language framework managing relational data in applications using Java Platform, Standard Edition and Java Platform, Enterprise Edition....

entities.

Contrary to JPA Entities, 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 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.

Stateless Session Beans

A stateless session bean is an object that does not have an associated conversational state, but may have instance state. It does not allow 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 should be considered identical by the client.

Local Stateless SessionBean Hello World

Hello world program

A "Hello world" program is a computer program that outputs "Hello world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to...

example:

Java EE 6

import javax.ejb.Stateless;

@Stateless
public class HelloWorldBean {
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 HelloWorldBean helloWorld;

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

Remote Stateless SessionBean Hello World

Hello world program

example:

Java EE 5

import javax.ejb.Remote;

@Remote
public interface HelloWorld {
String getHello;
}

import javax.ejb.Stateless;

@Stateless
public 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 {
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);
}
}
}

Stateful Session Beans

The state of an object consists of its instance variables. In a stateful session bean, the instance variables represent the state of unique client-bean sessions. The interaction of the client with bean is called as conversational state.

External links

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