Ejb Fundamentals 5y3v1l

EJB Fundamentals

Contents     

Introduction Technology Overview EJB Architecture EJB Specification Sample Application

Sun’s Definition 

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EJB architecture is a component architecture for the development and deployment of component based distributed business applications. Applications written using EJB architecture are scalable, transactional, and multi- secure. These applications may be write once, and then deployed on any server platform that s the EJB specification.

Short Definition 

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EJB is a server-side component architecture that simplifies the process of building enterprise-class distributed component applications in Java. EJB provides standard for component architecture.

Enterprise Bean 

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Server side software components that can be deployed in distributed multi tier environment. They encapsulate business logic of an application. Consists of one or more java objects.

Types of Beans   

Session Bean Entity Bean Message driven Bean

Session Bean 

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Session beans model business processes. They are like ‘verbs’ because they are actions. E.g. billing engine, catalog engine etc:

Entity Bean  

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Entity Bean Models business data. They are like ‘nouns’ because they are data objects E.g. product, an order, an employee etc: Session beans typically calls entity beans to perform business goals.

Message Driven Bean 

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Message driven beans are similar to session beans. They are actions. They are called only when they receive some message. E.g. : stock trade message.

EJB Architecture Presentation Tier

Firewall

Messaging Client

C/C++ Client

Messaging

CORBA-IIOP

Java Applet Java Appl RMI-IIOP

HTML Client

Business Partner System

HTTP

Servlet

SOAP,UDDI WSDL,ebXML

JSP

Web Server

RMI-IIOP RMI-IIOP Application Server

EJB Message Driven Bean

EJB Session Bean

EJB Session Bean

EJB Session Bean

EJB Entity Bean

EJB Session Bean

Business Tier

EJB Foundation: Distributed Objects Distributed Object

Client

Remote Interface Remote Interface Skeleton

Stub Network

We take a monolithic application and break it into distributed system with multiple clients connecting to multiple servers and databases over network.

What do we need to worry about now?

Services 

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Remote Method Invocation Load Balancing Transparent Fail Over Back end integration. Clustering Dynamic Re deployment

Middleware

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Object life cycle Caching Security Resource Pooling System Management Message Oriented Middleware And many more…..

Explicit Middleware Transaction API

Distributed Object

Client Remote

Interface

Remote Interface

Skeleton

Stub Network

Transaction Services

Security API

Security Services Database API

Database drivers

Example – Bank obj Transfer(Acct acc1, Acct acc2,long amt) //1.Call middleware API to perform security check. //2.Call middleware API to start a trans //3.Call middleware API to load rows from db. //4.perform trans. //5.Call middleware API to store rows in db //6.Call middleware API to end the trans. 

Explicit Middleware   

Difficult to Write. Difficult to Maintain. Difficult to .

Implicit Middleware Distributed

Client

Object Remote Interface

Transaction API

Request Interceptor Remote

Interface

Remote Interface

Skeleton

Stub Network

Transaction Services

Security API

Security Services Database API

Database drivers

Example – Bank obj Transfer(Acct acc1,Acct acc2,long amt) //1. Perform trans.(subtract bal from one and add to other). 

Implicit Middleware   

Easy to Write. Easy to Maintain. Easy to .

EJB Container 

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House enterprise bean and makes them available to the client to invoke them remotely. It intercepts the client request and delegates them to corresponding bean class. It automatically performs implicit middleware that the distributed object needs. EJB object is the physical part of the container.

EJB Component Ingredients  

Enterprise Bean Class Interfaces 

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Remote and Home interface for remote access. Local and Local Home interface for local access.

Deployment Descriptor. Vendor Specific files.

Enterprise Bean Class 

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It conforms to a well defined interface. It contains business implementation details of our component. Each bean type has more specific interface that extends javax.ejb.EnterpriseBean interface. Bean class implements the interface corresponding to the

EJB Object  

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Client request interceptor. It duplicates all the business logic methods that the corresponding bean class exposes. Proprietary and specific to each EJB container.

Remote Interface  

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Interface to request interceptor. Informs EJB Object auto generator which methods to clone. All remote interfaces must derive from javax.ejb.EJBObject. EJB remote interfaces must confirm to RMI rules.

Home Object  

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EJB object factory. Creates,finds and destroys EJB objects. Proprietary and specific to each EJB container. Home objects implements Home Interface.

Home Interface  

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EJB Object factory interface. They define methods for creating,destroying and finding EJB Objects. All home interfaces must extend javax.ejb.EJBHome. EJB remote interfaces must confirm to RMI rules.

Local Access  Local

objects make enterprise bean calls fast and efficient.  Local objects implements Local Interface.  Local home objects creates beans fast.  Local home object implemnts Local Home interface.

Deployment Descriptor 

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Declare how the container should perform middleware services for the EJB component. In EJB 2.0 deployment descriptor is a XML file. Key to implicit middleware.

Vendor Specific Files 

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All vendors have proprietary value added features. Include files specific to that vendor.

Package Remote Interface

Local Interface

Jar file creator Enterprise Bean

Home Interface

Deployment Descriptor

Vendor Specific

EJB jar file

Sample Application My First Bean!

Session Bean : FirstBean package example; public class FirstBean implements javax.ejb.SessionBean { private SessionContext ctx; public void ejbCreate() { System.out.println(“ejbCreate()”); } public void ejbRemove() { System.out.println(“ejbRemove()”); } public void ejbActivate() { System.out.println(“ejbActivate()”); }

public void ejbivate() { System.out.println(“ejbivate()”); } public void setSessionContext(javax.ejb.SessionContext ctx) { this.ctx=ctx; } public String first() { System.out.println(“first()”); return “My First Bean”; } }

Remote Interface : First.java package example; public interface First extends javax.ejb.EJBObject { public String first() throws java.rmi.RemoteException; }

Home Interface : FirstHome package example; public interface FirstHome extends javax.ejb.EJBHome { First create() throws java.rmi.RemoteException, javax.ejb.CreateException; }

Deployment Descriptor <ejb-jar> <enterprise-bean> <session> <ejb-name>First example.FirstHome example.First <ejb-class>example.FirstBean<ejb-class> <session-type>Stateless Container

Client Application package example; import javax.naming.*; Import java.util.*; public class FirstClient { public static void main(String[] arg) throws Exception { Properties props=System.getProperties(); Context ctx=new InitialContext(props); Object obj=ctx.llokup(“FirstHome”); FisrtHome home=(FirstHome) javax.rmi.RemotePortableObject.narrow (obj,FirstHome.class);

First first=home.create(); System.out.println(first.first()); first.remove(); } }

EJB Object Model < > java.rmi.Remote

< > java.io.Serializable

Comes with Java2 platform < > javax.ejb.EJBObject

< > Javax.ejb.EJBHome

< > javax.ejb.EnterpriseBean

< > javax.ejb.SessionBean

Comes with EJB Distribution < > Remote Interface

< > Home Interface

Bean Implement Class

Written by developer EJB Object

Home Object

Generated by Componet Vendor Tool

EJB Container 3: Create a new Home EJB Object Interface

Client

1 : Retrieve Home object Reference

5: Return EJB Object Reference

2 : Return Home object Reference

JNDI Naming Service

6: Invoke Business Method Remote Interface

Home Object 4: Create a new EJB Object

EJB Object

Enterprise Bean

7: Delegate Request to Bean

Advantages of EJB 

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Helps to write scalable,reliable and secure applications. Provides distributed component framework and hence s rapid application development. s application portablility and reusability across any vendor’s enterprise middleware services. It is agreed upon by industry.

References   

Mastering Enterprise Java Beans J2EE 1.4 Tutorials Professional Java Server Programming, J2EE Edition