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Enterprise Java Beans

Exploring different types of EJB

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Are of two types: Entity & Session

Entity bean:

Are enterprise beans that persist across multiplesessions and multiple clients

Are of two types:

Bean-managed persistence

Container-managed persistence

Enterprise JavaBeans Components

Enterprise Bean:

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Session bean:

Perform business tasks without having a persistent storage

mechanism

Are of two types:

Stateful session bean

Stateless session bean

Enterprise JavaBeans Component Architecture (Contd.)

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EJB Types

Session Beans

Stateless Session Beans

Dont maintain state between calls

Statefull Session Beans

Store state between calls

Entity Beans Used for persistence

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Problem

Richard is creating an application for a Super Mall. He needsto create an application that would accept the price of Itemand would calculate the tax. Find out the type of enterprise

bean to be created.

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Life Cycle of a Stateless Session Bean

Life cycle of stateless session bean:

Does Not Exist

Method - Ready

newInstance()

setSessionContext()

ejbCreate()

ejbRemove()

Session Beans

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RBS Bank wants an Currency Converter tool to convert Dollars to

Rupees. Identify the choice of EJB to create the component and

specify the code.

Situation

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EJB is the appropriate technology to solve the problem:

EJB components automatically handle system level services

Enterprise bean implements the business logic only

The enterprise bean consists of:

Remote interface

Home interface

EJB class

Create a stateless session bean for the above said converter

application

Solution

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Defines all the business methods of the enterprise bean

Steps to write the remote interface:

Import the javax.ejb.EJBObject and

java.rmi.RemoteException interfaces

Then, create a remote interface by extending the EJBObject

interface

Finally, define all the business methods that will be implemented in

the EJB class

Remote Interface:

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Defines method that allow EJB clients to create and find EJB

components

Steps to write the home interface:

Import the following interfaces:

java.io.Serializable java.rmi.RemoteException

javax.ejb.CreateException javax.ejb.EJBHome

Then, create a home interface by extending the EJBHome interface

define the create() method to create an instance of a particular

EJB object

Then, create a home interface by extending the EJBHome interfaceAnd finally define the create() method to create an instance of a particularEJB object

Home Interface:

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Implements all the business methods declared in the remote

interface

Steps to write the EJB class:

Import the following interfaces:

java.rmi.RemoteException javax.ejb.SessionBean

javax.ejb.SessionContext

Then, create the EJB class by implementing the SessionBean interface.Then, implement the business method defined in the remote interface.write the ejbCreate(), ejbRemove(), ejbActivate(), ejbPassivate(),setSessionContext(), and the default implementation for the constructormethods

EJB Class

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To create the enterprise bean called CurrencyConverter, create the

following files:

Converter.java containing code for the remote interface

ConverterHome.java containing code for the home interface

ConverterEJB.java containing code for the EJB class

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//The remote interface

import javax.ejb.EJBObject;

import java.rmi.RemoteException;

public interface myRemote extends EJBObject

{

public int methodOne(int i);

}

Find out the mistake in the above code?

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Start the J2EE server

Start the deploytool

Create a J2EE application

Package the enterprise bean

Deploy the J2EE application

Deployment process

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Start the J2EE server

Type the command:

J2ee verbose

Type the command:

deploytool

Package the enterprise bean

Use the New Enterprise Bean Wizard of the Application Deployment

Tool

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A J2EE application is assembled from three

components:

J2EE application clients

Web components

Enterprise beans

J2EE Application Components

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Is an XML file

Contains the following information:

The access control list (ACL)

Name of the EJB class

Name of the home interface

Name of the remote interface

A list of container-managed fields if the enterprisebean is an entity bean

A value specifying if the enterprise bean is stateful orstateless, in case of a session bean

Deployment Descriptor

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Is a naming service called Java Naming and DirectoryInterface

Is used to access the enterprise bean methods

Is a standard extension to the Java platform that provides

multiple naming and directory services

Is used to locate and search for distributed objects

JNDI

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Steps to locate the home interface:

First, create a JNDI Naming Context

Next, use the InitialContext class to locate the JNDI name

specified when the J2EE application was deployed. The lookup()method is used to lookup for the JNDI name

Finally, the object returned by the lookup() method must be cast to a

variable by using the PortableRemoteObject.narrow() method

Client Program

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To create an instance of enterprise bean:

The EJB client must invoke the create() method of the home

interface

The remote interface defines the business methods implemented in

the EJB class

The client invokes these methods by using the remote interface object

returned by the create() method

Set the classpath to the remote application client.Save and compile the application client

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A J2EE application is assembled from three components: J2EEapplication clients, enterprise beans, and Web components

The Web component files, which are .war files and the enterprisebean and application client files, which are .jar files are assembledinto a J2EE application, which has an .ear extension

Deployment descriptor is an XML file that contains informationabout deployment of enterprise bean

JNDI is a standard Java extension that provides multiple namingand directory services

A naming service provides a mechanism for locating distributed

objects

Summary

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A directory service organizes distributed objects and otherresources, such as files into hierarchical structures

Client uses JNDI to initiate a connection to an EJB server and tolocate a specific EJB Home

The steps to deploy an enterprise bean are:

Start the J2EE server

Start the deployment tool

Create a J2EE application and assemble the remote and homeinterfaces and enterprise bean class files into a .jar file

Deploy the J2EE application to the J2EE server

Summary (Contd.)

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The target server is the name of the host where the J2EE server is

running

The Return Client Jar file is used by the client program to locate the

server

The lookup() method of the InitialContext class is used to

locate the JNDI name

The PortableRemoteObject.narrow() method is used to cast

the object returned by the lookup() method to the home interface

type

Summary (Contd.)