EJB

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Enterprise Java Beans

·         Sun Microsystems introduced the J2EE application server and the Enterprise JavaBean (EJB) specifications as a venture into the multitier server side component architecture market.

·         It is important to note that though EJB and JavaBeans are both component models, they are not same. EJBs are interprocess components and JavaBeans are intraprocess components.

·          EJB is a specification for creating server side components that enables and simplifies the task of creating distributed objects.

The Key features of EJB are as follows

Ø EJB components are server side components written using Java

Ø EJB components implement the business logic only. We need not write code for system level services, such as managing transactions and security.

Ø EJB components provides services, such as transaction and security management and can be customized during deployment.

Ø EJB can maintain state information across various method calls.

Enterprise JavaBeans Component Architecture

EJB Server which contains the EJB container.

           

EJB container, which contains enterprise beans

Enterprise bean, which contains methods that implement the business logic.

EJB Server

EJB Server provides some low level services, such as network connectivity to the container. It also provides the following services:

1. Instance Passivation – If a container needs resource, it can decide to temporarily swap out a bean from the memory storage.]
2. Instance Pooling – If an instance of the requested bean exists in the memory, the bean is reassigned to another client.
3. Database Connection Pooling – When an Enterprise bean wants the access a database, it does not create a new database connection of the database connection already exists in the pool.

4. Precached Instances – The EJB server maintains a cache. This cache information about the state of the enterpirse bean.

sarayoa

EJB Container

• The EJB container acts as an Interface Between and Enterprise bean and the clients. Clients communicate with the enterprise bean through the Remote and Home Interfaces provide by the container.

• The client communicating to the enterprise bean through the container enables the container  to service the client’s request with great flexibility. For Instance, the container manages a pool of enterprise beans and uses them as the need arises, instead of creating a new instance of the bean for each client request.

The container also provides the following services:

1. Security
2. Transaction Management
3. Persistance -> Permanent Storage
4. Life Cycle Management

Enterprise Bean

Enterprise JavaBeans are write once, run anywhere, middle tier components that consists of methods that implements the business rule. The enterprise bean encapsulates the business logic. There are two types of enterprise bean.

1. Entity Bean
2. Session Bean

Entity Bean

            Entity Beans are enterprise beans that persist across multiple sessions and multiple clients. There are two types of Entity bean:

1. Bean Managed Persistance
2. Container Managed Persistance

            In a Bean managed persistance, the programmer has to write the code for database calls. On the other hand, in container managed persistance, the container takes care of database calls.

Session Bean

            Session bean perform business tasks without having a persistent storage mechanism, such as a database and can use the shared data. There are two types of session beans:

1. Stateful Session Bean
2. Stateless Session Bean

            A Stateful session bean can store information in an instance variable to be used across various method calls. Some of the application require information to be stored across various method calls.

            A Stateless session bean do not have instance variables to store information. Hence, stateless session beans can be used in situations where information need not to be used across method calls.

Life Cycle of a Stateless Session Bean

Does not Exists -> when the bean has not been instantiated.

Method Ready -> When the container requires it.

Class.newInstance() -> Create a new instance of the stateless session bean and allocates the required memory

SessionBean.setSessionContext(SessionContext ct) -> Sets the bean reference to the session context. A session context enables the enterprise bean to interact with the container. The enterprise bean can use the session context to query the container for information, such as transactional and security state.

ejbCreate() -> It is similar to the constructor of EJB class. It is invoked only once in the life cycle of the stateless session bean, when the client invokes the create() method of the home interface. The ejbCreate() method must not take any argument, as stateless session bean do not store any information in the instance variable.

ejbRemove() -> ends the life cycle of the stateless session bean. This method closes any open resource and frees the memory space.

EJB Servers

            Sun Java Server (J2EE Server)

            Web Logic (BEA)

            Websphere (IBM)

STATELESS BEAN

Home Interface

Remote Interface

Bean Class

Client Application

Remote Interface

            A remote interface defines all the business methods of the enterprise bean that the EJB client would invoke. The remove interface does not include system level operations, such as persistence, security and transactions.

Home Interface

            The home interface defines methods that allow EJB clients to create and find EJB components.

Bean Class

            The EJB class implements all the business methods declared in the remote interface.

Eg:

//CalculatorHome.java

// Home Interface

import java.io.Serializable;

import java.rmi.RemoteException;

import javax.ejb.CreateException;

import javax.ejb.EJBHome;

public interface CalculatorHome extends EJBHome

{

 public Calculator create() throws RemoteException, CreateException;

}

//Calculator.java

//Remote Interface

import javax.ejb.*;

import java.rmi.RemoteException;

public interface Calculator extends EJBObject

{

        public double dollarToRs(double dollars) throws RemoteException;

}

//CalculatorEJB.java

//Enterprise Bean class

import java.rmi.RemoteException;

import javax.ejb.SessionBean;

import javax.ejb.SessionContext;

public class CalculatorEJB implements SessionBean

{

        public double dollarToRs(double dollars)

        {

                return (dollars * 47.20);

        }

        public void ejbCreate() {}

        public void ejbRemove() {}

        public void ejbActivate() {}

        public void ejbPassivate() {}

        public void setSessionContext(SessionContext sc) {}

}

client program:

//CalculatorClient.java

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.swing.event.*;

import javax.naming.Context;

import javax.naming.InitialContext;

import javax.rmi.PortableRemoteObject;

import java.util.*;

import javax.naming.*;

import javax.ejb.*;

import java.rmi.*;

public class CalculatorClient extends JFrame implements ActionListener

{

        public static int w = 500;

        public static int h = 95;

        public static String str = "Earnest Bank Welcomes You";

        Container c;

        JLabel l,result;

        JTextField t;

        JButton b;

        public static String value;

        public static double dbl;

        public static double amt;

        public CalculatorClient()

        {

                super(str);

                c = getContentPane();

                c.setLayout(new GridLayout(2,2,2,2));

                l = new JLabel("Enter the amount in Dollars($)");

                c.add(l);

                t = new JTextField(10);

                c.add(t);

                b = new JButton("Calculator");

                c.add(b);

                result = new JLabel();

                c.add(result);

               // value = t.getText();

                b.addActionListener(this);

                setSize(w,h);

                show();

        }

     

                public void actionPerformed(ActionEvent e)

                {

                        dbl = Double.parseDouble(t.getText());

                    

try

                        {

                            

 Properties p=new Properties();

                p.put(Context.INITIAL_CONTEXT_FACTORY,"weblogic.jndi.WLInitialContextFactory");

                p.put(Context.PROVIDER_URL,"t3://localhost:7001");

                Context ic= new InitialContext(p);

                 CalculatorHome h=(CalculatorHome)ic.lookup("rose");

                Calculator r=h.create();

               

                                amt = r.dollarToRs(dbl);

                                result.setText("Result(Rs.): " + String.valueOf(amt));

                        }catch(Exception ex)

                        {

                                System.err.println("Caught an unexpected exception!");

                                ex.printStackTrace();

                        }

               

        }

        public static void main(String args[])

        {

                CalculatorClient m = new CalculatorClient();

        }

}

//<applet code="CalculatorClient" height=500 width=500></applet>

[this is for j2ee server]

{

Compile the Source files

s.bat

path=%path%;d:\jdk1.2\bin;d:\j2sdkee1.2\bin

set classpath=d:\j2sdkee1.2\lib\j2ee.jar;d:\demo\ejb;

set java_home=d:\jdk1.2

set j2ee_home=d:\j2sdkee1.2

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