Praveen EJB Architecture

8/6/2019 Praveen EJB Architecture

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EJB Architecture and Design

R. Praveen.


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What is EJB?

An EJB is just a collection of Javaclasses and XML file, bundled into a

single unit. The Java classes mustfollow certain rules and providecertain callback methods.

EJB is just a specification. It is not aproduct.EJBs are reusable components.


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What is EJB?

EJB is a widely-adopted server-sidecomponent architecture for J2EE.

EJB components are designed toencapsulate business logic, and toprotect the application developer

from having to worry about systemlevel issues.


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Contents

Services provided by EJB containerCircumstances of EJB component usage

How an EJB component looks like?View of an EJB component by clientprogrammer and EJB developerMechanisms by which EJB container providesits servicesRules an EJB developer must follow and how to use EJBs in a web architecture?


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Key features of EJBtechnology

EJB architecture is inherently transactional, distributed,portable, multi-tier, scalable and secureEJB components are fully portable across any EJB

server and any OS, work with any client.Components are declaratively customized

There are four major parts to every bean: the homeinterface, the remote interface, the implementation

class and the XML deployment descriptor


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EJB vs JavaBeans

The JavaBeans architecture is meant toprovide a format for general-purpose

components whereas the EJBarchitecture provides a format forencapsulation and management of business logic.

JavaBeans has tier of execution at Clientand EJB has at Server (specificallybusiness logic tier)


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EJB vs JavaBeans

In JavaBeans the runtime executionenvironment provides services like

Java libraries, Java application etc. The EJB runtime environmentprovides services of Persistence,

declarative transactions andsecurity, connection pooling andlifecycle services.


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Varieties of Beans

Session Beans Stateful session bean

Stateless session beanEntity Beans

With container-managed persistence

With bean-managed persistenceMessage-Driven Beans


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Why use EJBs in your design?

EJB specification provides enterprise-levelservices, that is, it provides software servicesthat are fundamental to an organizationspurpose.EJBs API was designed to keep the applicationprogrammer from having to provide systems-level services, so that they are free toconcentrate on business logic.


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Why use EJBs in your design?

A requirement of any of the servicesprovided by an EJB container like

transactions, scalability, persistence,security, future growth possibilities isan appropriate reason to use EJB in

the design of the application.


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EJB Architecture

J2EE Application Server

EJB Container

Application Logic DataClient

RDBMS

Corba

Mail

RMI

J DB C

J a v a M a i l

J M S

J T ASession Bean

Entity Bean

Client

Application


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Roles in EJB Development

EJB provider - a person who developsEJB Components

EJB Deployer - a person responsiblefor deploying EJBs in EJB serverApplication Server/ EJB Container

Vendor - one who providesapplication server on which theapplication is deployed


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Application assembler - one whocombine the EJB components with

other software to make a completeapplicationSystem administrator - one who

manages the application after it hasbeen deployed into a targetenvironment.


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EJBProvider

ApplicationAssembler

App Server/EJB Container

Provider

Deployer

SystemAdministrator


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EJB Container and its Services

A container is an execution environmentfor a component. The component lives in

the container and the container providesthe services for the component.Similarly, a container lives in anapplication server, which provides anexecution environment for it and othercontainers.


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Services provided by an EJB container

Persistence Ex: simple connection pooling,

automatic persistence, etc. EJBscreated with applicationdevelopment tools will encapsulate

data access in components.


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Services provided by an EJB container

Declarative transactionsData caching

Declarative SecurityError HandlingComponent Framework for Business Logic

Scalability and Fall-OverPortabilityManageability


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How the Container Provides Services

There are three basic ideas:First, there are clearly defined responsibilities between thevarious parts of an application using EJB component namely theclient, the EJB container and the EJB component. The definition of

these responsibilities is formally known as a contract .Second, the services that the container provides are defined insuch a way that they are orthogonal to the component. In otherwords, security, persistence, transactions are separate from the

Java files that implement the business logic of the component.


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How the Container Provides Services

Third, the container interposes oneach and every call to an EJB

component so that it can provide itsservices. In other words, thecontainer puts itself between the

client and the component on everysingle business method call.


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Contracts

EJB Container/Application

Server

Enterprise JavaBean

Client


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Rules for the bean programmer

The developer of the EJB component must implementthe business methods in the implementation class

The bean provider must implement the ejbCreate(),

ejbPostCreate(),ejbRemove() methods and theejbFind() methods if the bean is an entitywith bean managed persistence

The bean provider must define the enterprise beanshome and remote interfaces

For session beans, the bean provider must implementthe container callbacks defined in the

javax.ejb.SessionBean interface


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EJB Container from a remoteclient

First, the client makes a call on the RMIstub

This RMI stub interposes on the methodcall in order to marshal parameters andsend the information across the networkA skeleton on the server side unmarshalsthe parameters and delivers them to theEJB Container


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Interposition diagram

Interposition class

ClientContainer generated

classEJB

RMIStub

RMIStub

Network


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Interposition : from EJBContainer to EJBs

The container will examine the security credentials of thecaller of the methodIt will start or join with any required transactions

It will make any necessary calls to persistence functionsIt will trigger various callbacks to allow the EJB Componentto acquire resourcesOnly after all this is done will the actual business methodbe called

Once it is called, the container will do some more workwith transactions, persistence, callbacks and returns dataor exception to the remote client


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Working with EJBs

The Enterprise JavaBeansspecification is written for three

audiences: The Client developer The EJB developer

The EJB container developer


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EJB Clients

EJB Clients are applications that access EJBcomponents in EJB containers. There are twopossible types. The first category is application

clients which are stand-alone applicationsaccessing the EJB components using the RMI-IIOPprotocol. The second category of applicationclients are components in the web container.

They are java servlets and JSPs which also accessthe EJB components via the RMI-IIOP protocol.


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The Client DevelopersView

The client has a smaller set of concerns then abean developer with regard to using EJBs.Basically, he need to know :

how to find or create a bean, how to use its methods and how to release its resources

The client need not worry about theimplementation of the EJB, callbacks that the EJBcontainer will make on the EJB or nature of theservices provided to the EJB.


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EJBs interface

Home Interface : It is primarily for the life cycleoperations of the bean: creating, finding, andremoving EJBs. The home interface is not

associated with a particular bean, just with a typeof bean.Remote Interface : It is for business methods.Logically, it represents a particular bean on the

server. The remote interface also provides someinfrastructure methods associated with a beaninstance, rather than a bean type.


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Sample client applicationpseudo code

A client programmer will acquire an EJBshome interface through JNDI, and they usethis home interface to :

Create or findinstance of bean

Execute methods Reference(Handle)

Remove bean


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The Bean Programmersview

Main responsibility is write business logic and structurethe code in a particular structure. The structure has 4files, the home interface, remote interface, business

logic class file and the XML file. The XML file called thedeployment descriptor, contains the structuralinformation about the bean, declares the beansexternal dependencies and specifies certaininformation about how services such as transaction and

security work.


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Structure of JAR file

META -INF\ ejb-jar.xml

orderMgmt\ OrderManagement.class OrderManagementHome.class

OrderManagementBean.class


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What you cant do in an EJBcomponent?

You cannot use Reflection API to accessinformation inaccessible to you.

You cannot create a class loader orreplace a security manager. You cannot set the socket factory used byServerSocket or Socket

You cannot use the object substitutionfeatures of the serialization protocol


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What you cant do in an EJBcomponent?

use Threads or the Threading APIuse the AWTAct as a Network Server

use Read/Write static fieldsuse java.io packageLoad a native library

use this as an Argument or Return valueuse Loopback Calls


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EJB Components on theWeb

Three classes of objects in MVC architecture:Model : This is the data and business-logiccomponent. It can serve multiple views.View : This is the presentation component orthe user-interface component. There can bedifferent presentations of a single model.Controller : This is the component that respondsto user input. Translates user-interface eventsinto changes to the model and defines the waythe user-interface reacts to those events.

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Praveen EJB Architecture