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EJB Overview

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Agenda

What is and why EJB?

EJB Architecture

Component and Container ArchitectureTypes of Beans

Roles

Anatomy of EJB Module

How client invokes methods of EJB

RMI Communication model

Deployment Descriptor & Packaging

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What is EJB(From Specification)

The Enterprise Java Beans architecture is a componentarchitecture for the development and deployment of

component-based distributed business applications.

Applications written using Enterprise Java Beans

Architecture are scalable, transactional, and multi-usersecure.

These Applications may be written once, then deployedon any server platform that supports the Enterprise JavaBeans Specification.

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EJB Design Principles

EJB applications are loosely coupled

EJB behavior is specified by interfaces

EJB applications do not manage resourcesThe container supports the application developer

EJB applications are tiered

The session tier is the API to the applicationThe entity tier is the API to the data sources

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EJB applications are Loosely

CoupledSupports the integration of components from different

vendors

EJBs refer to other components and services to whichthey have access by arbitrary names

EJB can be authored without a detailed knowledge of

the environmentApplications can be assembled from separatecomponents

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EJB Architecture

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EJB behavior is specified

by Java Interfaces

An EJBs interaction with its clients is specified entirelyin terms of java interfaces

- Interfaces expose the methods that clients cancall, and thereby set out a contract between client

and EJB- Implementation is hidden from client

- supports portability and modularity

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EJB applications do not

manage Resources

EJBs get access to external resources (databases,legacy systems) through their container

- Programmer does not have to worry aboutresource allocation and de-allocation

It is the containers job to manage these resources, andmake the access as efficient as possible

- Container is configured by System Admin not

through programming APIs

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Container supports

application developer

Container provides System services

- Persistence- Security

- Transaction

- Connection Pooling- Component Life Cycle Management

- Threading

Application developer and deployer specifies his/herrequirements in declarative fashion (in deploymentdescriptor)

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EJB in a

Big picture of J2EE

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Where is EJB?

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EJB, EJB Server, EJB Client

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Why EJB Technology?

Leverages the benefits component-model on the serverside

Separates business logic from System codeProvides framework for portable components

- Over different J2EE-compliant servers

- Over different operational environmentsEnables deployment time configuration

- Deployment Descriptor

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Enterprise Java Beans

Defined as business Logic ONLY

No low level plumbing

Reusable across multiple EJB Servers

Implements interfaces that allow container to managethem

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EJB Architecture

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EJB Architecture Contracts

Contracts are specified in EJB specification

Client View Contract- Contract between client and container

Component Contract- Contract between an Enterprise Bean and itsContainer

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Client View Contract

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Client View Contract

Client of an EJB can be- Web tier components: Servlet and JSP

- Standalone Java application or Applet

- Another EJB in same or different container- Web services client (in EJB 2.1)

Provides development model for clients using EJB

ServicesClient view contract is comprised of

- Home Interface

- Logic (also called Remote) Interface- Object Identity

- Metadata Interface and Handle

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Component Contract : What

container does(for Beans)

Enables EJB method invocations from EJB instances

Manages the life cycle of EJB Bean InstancesImplements Home and Remote Interfaces

Provide persistence for CMP entity beans

Provide runtime context information to beansManage transactions, security, exceptions etc..

Implements callbacks

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EJB Contracts

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J2EE

Component & Container

Architecture

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J2EE containers & components

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Containers & Components

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What the EJB Container does

Generate concrete class for

- Remote (or Local) Home Interface- Remote (or Local) Logic Interface

Binds home object to Naming service

- clients can lookup the home object using JNDICreates free beans pool

Caches recently accessed beans

Provides JDBC connection poolingBinds external resources to Naming service

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Types of Beans

Session Beans

Stateful session beans

Stateless session beans

Entity Beans

Bean Managed Persistence (BMP) Container Managed Persistence (CMP)

Message Driven Beans

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Roles

Distinct Roles in the application development anddeployment Life Cycle

- EJB Bean Provider

- Application Assembler- Deployer

- EJB Server Provider

- System Administrator- Tool Provider

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Life Cycle Illustration

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Anatomy of EJB Module

(EJB-JAR file)

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Home Interface

Defines methods for creating, finding, and removingbeans

- Factory Interface

Implemented by container- EJB home object

Client gets reference to stub object of the EJB home

object via JNDICan be remote or local

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Remote Interface

Defines business methodsImplemented by container

- EJB Object

Client gets reference to stub object of the EJB objectthrough create() or find() of home interface

Can be remote or local

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Steps for client to invoke

methods of EJB

Get EJB home object (actually a stub object of the EJB

home object) via JNDI- Get Initial Context

- Perform Lookup

- Perform narrowingFrom EJB home object, get a EJB object (actually a stubobject of the EJB object)

Invoke business methods from EJB object

Clean up

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Example

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RMI CommunicationModel

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Distributed Objects

Collaborating objects reside in different computers(different VMs)

- client objects invoke methods of server objects

There have to be some mechanisms

- passing method signature from client to server

- marshalling parameters from client to server

- unmarshalling received parameters at the server- marshalling return values from server to client

- unmarshalling received results at the client

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RMI Communication Model

l l

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RMI Control Flow

Client Object

1.Calls methods on

stub

2.Receives results

from stub

Stub implements Interface

2.Marshalls arguments

3.passes call to remote

object

8.receives and unmarshall

results

Remote Object(Server)

Implements interface

4.receives calls and

arguments

5.processes call

6.returns results

3

7

Interface Definition

Declares

Methods

Where does RMI Communication

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Where does RMI Communication

Occur

RMI Over IIOP

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RMI Over IIOP

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RMI over IIOP

RMI mechanism used between EJB client and EJB server- Before EJB 2.0, RMI over IIOP has to be usedeven if EJB client and EJB server reside in the

same VM, which results in unnecessary overhead- Vendor products provide some optimizedcommunication if client and server reside in singleVM

RMI operations in general are expensive

- Reason why local interface are introduced in EJB2.0 (J2EE 1.3)

EJB-JAR Deployment

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EJB JAR Deployment

DescriptorGives the container instructions on how to manage theenterprise bean

Allows declarative customization

Control behaviors for:- Transactions

- Security

- Life Cycle- State Management

- Persistence

Deployment Descriptor

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Deployment Descriptor

Example

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EJB 2.0 New Features

Integrated support for JMS

Support for local interfaces

Improved Architecture for CMP

- Support for container managed relationships forentity beans

Enterprise Java Beans Query Language (EJB QL)

Home methods for entity BeansNetwork interoperability

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Thank You..

R f c s

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References

J2EE Specifications by SUN

J2EE Passion Web Site j2eepassion.comA Practical Introduction to EJBs

http://www.comptechdoc.org/docs/kanti/ejb/index.html

Enterprise Java Beans - http://my.execpc.com/~gopalanhttp://www.theserverside.com/articles/article.tss