java components

JAVA Components

Seminar of “Component Based Software Engineering” course

By : Z.VaraminiUniversity of Science and Technology of Mazandaran,Babol

January 7,2010

Outline

Introduction to Java From Java to Java 2 Java 2 platforms Java Components

Applet,JavaBeans,Enterprise JavaBeans

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JAVA

Released in alpha-form in 1995 by Sun Originality:Concepts of Applets Java become common used by 1996

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Java 2

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Introduced in late 1998 Relegates applets to a marginal side role Introduced the notion of platform editions

Java 2 Platforms

J2SE:Java 2 Platform , Standard Edition Use for Standard Programming

J2EE:Java 2 Platform ,Enterprise Edition Released end 1999 The Flagship platform edition The most Successful Programming for Enterprise Systems

J2ME:Java 2 Platform , Micro Edition Use for small and embeded device It was Successful in the mobile phone sector

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Java-Components

Java component model :o Appleto JavaBeanso Enterprise JavaBeanso Servletso Application client components

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J2SE

J2EE

Applet-Policy

Applet ,Is a software component that runs in the context of another program, for example a web browser

With Goal of adding Animation to Web Pages

Sun's Hot Java browser,the first browser that supports Applet

Netscape and Microsoft Internet Explorer,Then supports Applet when included a Java virtual machine

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Problems In Netscape and IE

They didn't keep up with more modern versions of Javao To overcome this problem, Sun released a tool called the

"Java Plug-in" o it seamlessly plugs in to both Netscape and Internet

Explorer and allows both browsers to execute Java applets by using an external Java runtime environment that Sun supplies

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Life Cycle of an Applet Four Method Controls An Applet

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Init()

Start()

Stop()

destroy()

Browser,Load its page

user moves off the page

User returns to the page

Browser shuts down normally

JavaBeans- What is ? JavaBeans are reusable software components

for Java that can be manipulated visually in a builder tool.

Practically, they are classes written in the Java programming language conforming to a particular convention.

They are used to encapsulate many objects into a single object (the bean)

A JavaBean is a Java Object that is serializable, has a nullaryconstructor, and allows access to properties using getter and setter methods.

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JavaBeans- Solution to .. Enables Software developers to design

and create reusable pieces of software that easily integrateo with each other o with applications o with development tools

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JavaBeans – Main aspects

Events Properties Introspection Customization Persistence

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Enterprise JavaBeanse (EJB)

Provide a framework for component that may be “plugged in” to a server

Enterprise JavaBeans is totally different of JavaBeans,Except that it uses some similar concepts


EJB Vs JavaBean

Components built in the JavaBeansspecification are intraprocess components that stays in one address space and are typically used for Graphical User Interface (GUI) as visual widgets like buttons, tables, HTML viewers, etc

Components built in the EJB specification are interprocess components that stays in multipleaddress spaces as distributed object.

EJBs are not GUI components rather they sit behind the GUI's and perform all the hardcore business logic. GUIs like thick Clients, web-based Clients and web services are some of the Clients that can make connection to EJBs JAVA Components

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Enterprise JavaBeanse-Goals

Easy for developers to creat applications

Standard way for client/server applications

Compatible with and uses other java APIS

EJB can interoperate with non-java apps

EJB is Compatible with CORBA

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What constitutes an EJB? EJB Object

Client never invokes the method on an actual bean instance. All invocations go through the EJB Object

Remote Interface This interface duplicates all the methods that

the corresponding bean class exposes


What constitutes an EJB?(cont.)

Home Object How do clients acquire references to EJB objects?

The client asks for the EJB object fron the EJB Object factory

Home Interface How does a Home object know how you would like to intialize your EJB object?

This info is provided to the containerthrough Home interface


What constitutes an EJB?(cont.)

Local Interfaces Creating the beans through the home interface and then calling the beans though the remote interface is very slow.Local Interfaces makes beans' call faster

Deployment Descriptor You state your middleware needs in the deployment descriptor. This file is an XML based file

EJB-jar files EJB-JAR files are essentially zip files that contain Java classes and other files needed for deployment. You can generate these files by hand or with tools, such as Apache Ant.


What Specify in Deployment Descriptor ?

• Bean management and lifecycle requirements• Transaction requirements• Security requirements, including access

control entries


Enterprise JavaBeans Component


EJB Server

EJB Container

EJB Home

EJB Object

EJB Bean

EJB Homestub

EJB Object stub

Home

Remot or Local

Home

Remot or Local

Client

Disadvantages of EJB They have a large and complicated Specification

EJBs take longer to develop, and when things go wrong, they can be more difficult to debug, because there can be a possibility that the bug may not be in your code but in the application server/container itself.

No sooner have you deployed your EJB application, you see a new specification with newer features, rendering your application obsolete before it gets online. This situation, however, is unavoidable.


Reference

Clemens Szyperski, Dominik Gruntz and Stephan Murer, "Component Software - Beyond Object-Oriented Programming",Second Edition, Addison-Wesley/ACM Press, 2002 (589 pages), ISBN 0-201-74572-0en.wikipedia

Cay S. Horstmann, Gary Cornell,” Core Java™ 2 Volume I - Fundamentals”, Seventh Edition, Prentice Hall PTR, August 17 ,2004(784 pages) ISBN 0-13-148202-5


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