EJB Architecture and Design

CS486

Global Knowledge Networks Instructor : Dr. V. Juggy

Presentation by:

Aravind Vinnakota

What is EJB?

An EJB is just a collection of Java classes and XML file, bundled into a single unit. The Java classes must follow certain rules and provide certain callback methods.

EJB is just a specification. It is not a product.

EJBs are reusable components.

What is EJB?

EJB is a widely-adopted server-side component architecture for J2EE.

EJB components are designed to encapsulate business logic, and to protect the application developer from having to worry about system level issues.

Contents

Services provided by EJB containerCircumstances of EJB component usageHow an EJB component looks like?View of an EJB component by client

programmer and EJB developerMechanisms by which EJB container

provides its servicesRules an EJB developer must follow and how

to use EJBs in a web architecture?

Key features of EJB technology

EJB components are server-side components written entirely in the Java programming language

EJB components contain business logic only - no System-level programming

System-level services (i.e. "plumbing") such as transactions, security, Life-cycle, threading, persistence, etc. are automatically managed for the EJB component by the EJB server

Key features of EJB technology

EJB architecture is inherently transactional, distributed, portable, multi-tier, scalable and secure

EJB components are fully portable across any EJB server and any OS, work with any client.

Components are declaratively customizedThere are four major parts to every bean: the

home interface, the remote interface, the implementation class and the XML deployment descriptor

EJB vs JavaBeans

The JavaBeans architecture is meant to provide a format for general-purpose components whereas the EJB architecture provides a format for encapsulation and management of business logic.

JavaBeans has tier of execution at Client and EJB has at Server (specifically business logic tier)

EJB vs JavaBeans

In JavaBeans the runtime execution environment provides services like Java libraries, Java application etc. The EJB runtime environment provides services of Persistence, declarative transactions and security, connection pooling and lifecycle services.

Varieties of Beans

Session Beans Stateful session bean Stateless session beanEntity Beans With container-managed persistence With bean-managed persistenceMessage-Driven Beans

Why use EJBs in your design?

EJB specification provides enterprise-level services, that is, it provides software services that are fundamental to an organization’s purpose.

EJB’s API was designed to keep the application programmer from having to provide systems-level services, so that they are free to concentrate on business logic.

Why use EJBs in your design?

A requirement of any of the services provided by an EJB container like transactions, scalability, persistence, security, future growth possibilities is an appropriate reason to use EJB in the design of the application.

EJB Architecture

J2EE Application Server

EJB Container

Application Logic DataClient

RDBMS

Corba

Mail

RM

I JDB

C JavaM

ail JMSJTA

Session Bean

Entity Bean

Client Application

Roles in EJB Development

EJB provider - a person who develops EJB Components

EJB Deployer - a person responsible for deploying EJB’s in EJB server

Application Server/ EJB Container Vendor - one who provides application server on which the application is deployed

Roles in EJB Development

Application assembler - one who combine the EJB components with other software to make a complete application

System administrator - one who manages the application after it has been deployed into a target environment.

Roles in EJB Development

EJBProvider

ApplicationAssembler

App Server/EJB Container

Provider

Deployer

SystemAdministrator

EJB Container and its Services

A container is an execution environment for a component. The component lives in the container and the container provides the services for the component.

Similarly, a container lives in an application server, which provides an execution environment for it and other containers.

Services provided by an EJB container

Persistence Ex: simple connection pooling,

automatic persistence, etc. EJBs created with application development tools will encapsulate data access in components.

Services provided by an EJB container

Declarative transactionsData cachingDeclarative SecurityError HandlingComponent Framework for Business LogicScalability and Fall-OverPortabilityManageability

How the Container Provides Services

There are three basic ideas: First, there are clearly defined responsibilities

between the various parts of an application using EJB component namely the client, 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 in such a way that they are orthogonal to the component. In other words, security, persistence, transactions are separate from the Java files that implement the business logic of the component.

How the Container Provides Services

Third, the container interposes on each and every call to an EJB component so that it can provide its services. In other words, the container puts itself between the client and the component on every single business method call.

Contracts

EJB Container/Application Server

Enterprise JavaBean

Client

Rules for the bean programmer

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

The bean provider must implement the ejbCreate(), ejbPostCreate(),ejbRemove() methods and the ejbFind<METHOD>() methods if the bean is an entity with bean managed persistence

The bean provider must define the enterprise bean’s home and remote interfaces

For session beans, the bean provider must implement the container callbacks defined in the javax.ejb.SessionBean interface

Rules for the bean programmer

For entity beans, the provider must implement the container callbacks defined in the javax.ejb.EntityBean interface

The bean provider must not use programming practices that would interfere with the container’s runtime management of the enterprise bean instances

Interposition : method call to an EJB Container from a remote client

First, the client makes a call on the RMI stub

This RMI stub interposes on the method call in order to marshal parameters and send the information across the network

A skeleton on the server side unmarshals the parameters and delivers them to the EJB Container

Interposition diagram

Interposition class

ClientContainergenerated

classEJB

RMI Stub

RMIStub

Network

Interposition : from EJB Container to EJBs

The container will examine the security credentials of the caller of the method

It will start or join with any required transactions It will make any necessary calls to persistence functions It will trigger various callbacks to allow the EJB

Component to acquire resources Only after all this is done will the actual business

method be called Once it is called, the container will do some more work

with transactions, persistence, callbacks and returns data or exception to the remote client

Working with EJBs

The Enterprise JavaBeans specification is written for three audiences:

The Client developerThe EJB developerThe EJB container developer

EJB Clients

EJB Clients are applications that access EJB components in EJB containers. There are two possible types. The first category is application clients which are stand-alone applications accessing the EJB components using the RMI-IIOP protocol. The second category of application clients are components in the web container. They are java servlets and JSPs which also access the EJB components via the RMI-IIOP protocol.

The Client Developer’s View

The client has a smaller set of concerns then a bean 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 resourcesThe client need not worry about the

implementation of the EJB, callbacks that the EJB container will make on the EJB or nature of the services provided to the EJB.

EJB’s interface

Home Interface : It is primarily for the life cycle operations of the bean: creating, finding, and removing EJBs. The home interface is not associated with a particular bean, just with a type of bean.

Remote Interface : It is for business methods. Logically, it represents a particular bean on the server. The remote interface also provides some infrastructure methods associated with a bean instance, rather than a bean type.

Sample client application pseudo code

A client programmer will acquire an EJB’s home interface through JNDI, and they use this home interface to :

Create or findinstance of bean

Execute methods Reference(Handle)

Remove bean

Client.java

Package orderMgmt; import java.util.properties; import java.naming.Context; // for name-to-object findings import java.naming.InitialContext;// context for naming operations public class Client { try { Properties prop = new Properties(); // server dependent properties for InitialContext prop.put(Context.INITIAL_CONTEXT_FACTORY,

“org.jnp.interfaces.NamingContextFactory”); prop.put(Context.PROVIDER_URL, “localhost:1099”); Context ctx = new InitialContext(prop); Object objref = ctx.lookup(“OrderManagement”);

Client contd..

// casting home interface reference to the OrderManagementHome OrderManagementHome home = (OrderManagementHome)

javax.rmi.PortableRemoteObject.narrow(objref, OrderManagementHome.class);

// home interface to create an instance of the OrderManagement OrderManagement orderManagement = home.create(); // calling placeOrder() orderManagement.placeOrder("Dan OConnor", "Wrox books on programming", 1000); orderManagement.remove(); System.out.println("Order successfully placed."); } catch (Exception e) { e.printStackTrace(); } } }

The Bean Programmer’s view

Main responsibility is write business logic and structure the code in a particular structure. The structure has 4 files, the home interface, remote interface, business logic class file and the XML file. The XML file called the deployment descriptor, contains the structural information about the bean, declares the bean’s external dependencies and specifies certain information about how services such as transaction and security work.

Interface EJBObject

package javax.ejb; public interface javax.ejb.EJBObject extends

java.rmi.Remote { EJBHome getEJBHome() throws java.rmi.RemoteException; Handle getHandle() throws java.rmi.RemoteException; Object getPrimaryKey() throws java.rmi.RemoteException; boolean isIdentical(EJBObject obj) throws

java.rmi.RemoteException; void remove() throws java.rmi.RemoteException; }

OrderManagement code..

package orderMgmt; import javax.ejb.*;

public interface OrderManagement extends javax.ejb.EJBObject { public void placeOrder(String custName, String prodName, int

quantity) throws java.rmi.RemoteException; public void cancelOrder(String custName, String prodName) throws java.rmi.RemoteException;

public boolean isShipped(String custName, String prodName) throws java.rmi.RemoteException;

}

OrderManagementBean code..

package orderMgmt; import javax.ejb.*;

public class OrderManagementEJB implements javax.ejb.SessionBean

{

public void placeOrder(String custName, String prodName, int quantity)

{ // ... Business logic ...} public void cancelOrder(String custName, String prodName) { // ... Business logic ...} public boolean isShipped(String custName, String prodName) { // ... Business logic … return true; }

OrderManagementBean code..

public void ejbCreate() { // Can be empty } public void ejbRemove() { // Can be empty } public void ejbActivate() { // Can be empty} public void ejbPassivate() { // Can be empty} public void setSessionContext( SessionContext ctx ) { // Can be empty} }

Interface EJBHome

Package javax.ejb; public interface EJBHome extends java.rmi.Remote { EJBMetaData getEJBMetaData () throws

java.rmi.RemoteException; HomeHandle getHomeHandle() throws

java.rmi.RemoteException; void remove(Handle handle) throws

java.rmi.RemoteException, java.ejb.RemoveException; void remove(Object primary key) throws

java.rmi.RemoteException, java.ejb.RemoveException; }

OrderManagementHome code..

package orderMgmt; import javax.ejb.*;

public interface OrderManagementHome extends javax.ejb.EJBHome

{ public OrderManagement create() throws java.rmi.RemoteException,

javax.ejb.CreateException; }

The xml file : ejb-jar.xml

<?xml version=“1.0”?> <ejb-jar> <enterprise-beans> <session> <ejb-name>OrderManagement</ejb-name> <home>orderMgmt.OrderManagementHome</home> <remote>orderMgmt.OrderManagement</remote> <ejb-class>orderMgmt.OrderManagementBean</ejb-class> <session-type>Stateless</session-type> <transaction-type>Container</transaction-type> </session> </enterprise-beans>

The xml file : ejb-jar.xml

<assembly-descriptor> <container-transaction> <method> <ejb-name>OrderManagement</ejb-name> <method-name>*</method-name> </method> <trans-attribute>Required</trans-attribute> </container-transaction> </assembly-descriptor> </ejb-jar>

Structure of JAR file

META -INF\ ejb-jar.xml orderMgmt\ OrderManagement.class OrderManagementHome.class

OrderManagementBean.class

What you can’t do in an EJB component?

You cannot use Reflection API to access information inaccessible to you.

You cannot create a class loader or replace a security manager.

You cannot set the socket factory used by ServerSocket or Socket

You cannot use the object substitution features of the serialization protocol

What you can’t do in an EJB component?

use Threads or the Threading APIuse the AWTAct as a Network Serveruse Read/Write static fieldsuse java.io packageLoad a native libraryuse “this” as an Argument or Return valueuse Loopback Calls

EJB Components on the Web

Three classes of objects in MVC architecture:Model : This is the data and business-logic

component. It can serve multiple views.View : This is the presentation component or

the user-interface component. There can be different presentations of a single model.

Controller : This is the component that responds to user input. Translates user-interface events into changes to the model and defines the way the user-interface reacts to those events.

Implementation of MVC in a web site

Controller

Model

Browser Client

view1.jsp

view2.jsp

view3.jsp

Main.jsp

Views 1

2

3

4

5

Design of the EJB Tier

UML use cases: UML is the Unified Modeling Language, the standard language for expressing the model of the software system that we intend to build.

Use cases are subset of UML that expresses the functionality of the software to be delivered. Use cases describe what to do, but not how to do it.

Analysis Objects

Interface Objects : The interface object is responsible for controlling access to the EJB tier from any client. An interface object should always be represented by a session bean in the implementation.

Ex : controller servlet for the web application’s model-view-controller architecture.

Control Objects

Control objects provide services to the application. They model functionality that is not naturally associated with a particular entity or interface. Control objects should be represented by session beans in the implementation.

Entity Objects

Entity objects model those business objects that should maintain their state after the use case completes. This means they represent data in the database. Entity beans are often represented by entity beans in the implementation model.

An Example of EJB Design

Consider the case of a company that develops products, takes orders for those products, and then manufactures and ships them.

Actors in the company : An engineer, a web customer, a phone operator who takes orders from a catalog, floor manager who manages the manufacturing process, a crew member that actually builds the product ordered and a manager who tracks overdue orders.

Use Cases

Create a ProductPlace an OrderCancel an OrderSelect an Order for ManufactureBuild a ProductShip an OrderList Overdue Orders

Use case diagram from analysis

Engineer

Customer

Operator

Manager

Crew

M’ment

Create Product

Place Order

Cancel Order

Select Order

Build Product

Ship an Order

Overdue Orders

Stereotype icons in UML

Interface Object :

Entity Object :

Control Object :

Translation of analysis model into implementation

Actor User Interface Type Interface Object Impl’ation

Engineer Visual Basic Session Bean (RMI/IIOP) Customer Web Application JavaBean proxy / S Bean Operator Swing GUI Session Bean Manager Web Application JavaBean proxy / S Bean Crew Palm Pilot XHTML Servelet to Session Bean Management Web Application JavaBean proxy / S Bean

View of use case actors and their respective interface objects

Engineer

Customer

Operator

Manager

Crew

Manage--ment

VB App

Web App

Swing app

Web App

Web App

Palm App

View of interaction of interface and control objects

Shiporder

BuildProduct

Select

Cancel

Place

Create

ListOverdue

VBApp

WebApp

WebApp

Web App

SwingApp

PalmApp

View of interaction between control objects and entity objects

RoutingProduct

Order

Shipment

Account

Supplier

ShippingCompany

Customer

CreateProduct

PlaceOrderCancelOrder

Select forManufactureBuildProduct

Ship Order

ListOverdue

Summary

EJBs are intended for transactional systemsEJBs are portable, reusable server-side

components that execute in a containerAssist developer productivity, extend

application capability, and improve system stability

Are accessible from many different types of clients

There are three types of beans : stateful session, stateless session, and entity

Summary

There are four major parts to every bean: the home interface, the remote interface, the implementation class, and the XML deployment descriptor

The enterprise bean developer must follow certain rules to get the benefits of EJB technology

The roles of EJBs can be understood by analyzing a model of your enterprise in terms of interface, control and entity objects