Enterprise JavaBeans

Introduction and architecture

The beginning - Applets

• One reason the Java initially gained

popularity was its support for

downloadable Java programs known

as applets

• Java applets were first introduced in 1995

• Applet executes only on the "client" platform environment of a system

Java on the server

• Recognizing the potential for Java as a server language in Web environments, Sun Microsystems wrote the Java Servlet specification

• Java servlet specification was finalized in 1997

• Servlets are Java programs designed to run on Web server machines

Servlets

• Java servlets are best suited as a middle tier component connecting front-end Web requests with back-end data resources

• However, servlets alone do not provide a sufficient model for true enterprise computing

What is Enterprise Computing?

• What is an enterprise?• Large organization such as multinational corporation,

university, hospital, research laboratory, or government organization

• Requires special computing solutions because of its size

• Enterprise computing - use of computers in networks that encompass variety of operating systems, protocols, and network architectures

Three-tier architecture

• Emerged in the 1990s to overcome the limitations of the two-tier architecture

• The middle tier

supports application

server software, a

functional extension

of the Web server

Presentationtier

Business logic tier

Data tier

Advantages of multi-tier model

• Scalability (mērogojamība)

• Better re-use

• Improved data integrity

• Improved security

• Reduced distribution

• Improved availability

• Hidden database structure

Java Platform Enterprise Edition

• Java EE technology aims to extend the reach of the Java platform to large-scale server environments

• Industry standard for developing portable, robust, scalable, distributed and secure multi-tier server-side Java applications

• The Enterprise JavaBeans specification is one of the several Java APIs in the Java EE

Enterprise JavaBeans

• EJB is a server-side component that encapsulates the business logic of an application

• Standard way to implement the "business" code typically found in enterprise applications

Motivation

• Solutions to common problems are often repeatedly re-implemented by programmers

• EJB were intended to handle such common concerns as:• persistence • transactional integrity• security

in a standard way, leaving programmers free to concentrate on the particular problem

EJB goals

• To be the standard component architecture for building distributed object-oriented business applications

• To make it easy to write applications: developers will not have to understand • low-level transaction and state management details

• multi-threading

• resource pooling

• and other complex low-level APIs

EJB goals

• To follow the "Write Once, Run Anywhere" philosophy of the Java programming language• EJB can be developed once and then deployed on

multiple platforms without recompilation or source code modification

• To address the development, deployment, and run-time aspects of an enterprise application's life cycle

EJB goals

• To define the contracts that enable tools from multiple vendors to develop and deploy components that can interoperate at run time

• To provide interoperability between EJBs and non-Java programming language applications

• To be compatible with• existing server platforms• other Java programming language APIs• CORBA (Common Object Request Broker

Architecture)

Benefits of using EJBs

• EJBs make it simpler to write applications • EJB container is charged with the task of making system

services available to EJB components

• Component portability • A simple, elegant component container model• Java server components can be developed once and

deployed in any EJB-compliant server

• Architecture independence• Independent of any specific platform, proprietary

protocol, or middleware infrastructure

Benefits of using EJBs

• Built-in support for typical enterprise-level system services:• distributed objects• transactions• database• security• global naming

• Developer productivity • Standardization and automation of complex infrastructure

services • Developers can create complex applications by focusing on

business logic rather than environmental issues

EJB history

• The EJB specification was originally developed in 1997 by IBM and later adopted by Sun Microsystems

• EJB 1.1 ( J2EE 1.2 ), 1999• Session Beans (stateless & stateful), Entity Beans

• Remote interface

• EJB 2.0 ( J2EE 1.3 ), 2001• Message-Driven Beans

• Entity Beans 2.x and EJB QL

• Local and Remote interfaces

Adoption and criticism

• Enterprise JavaBeans were quickly adopted by large companies

• Problems were quick to appear and the reputation of EJBs began to suffer

• APIs of the standard were too complex and counter-intuitive• required interfaces• checked exceptions• deployment descriptors

Adoption and criticism

• Businesses found that using EJBs to encapsulate business logic brought a performance penalty• Original specification only allowed for remote

method invocation

• Long development cycle

• Tools made it easy to create and use EJBs by automating most of the repetitive tasks...

• But tools did not make it easier to learn how to use the technology!

EJB history

• EJB 2.1 ( J2EE 1.4 ) 2003• EJB Timer Service

• EJB Web Service Endpoints

• Minor EJB QL enhancements

• Reinventing EJBs• The functionality delivered by simpler frameworks

like Spring and Hibernate was more useful to enterprise applications

• EJB 3.0 specification was a radical departure from its predecessors

EJB history• EJB 3.0 ( Java EE 5.0 ) 2006

• Many improvements to its predecessor!

• Metadata annotations configuration by exception

• A higher degree of control over bean persistence

• Much more simplified programming model for developing EJBs

EJB 2 vs. EJB 3

EJB 3.1

• Status: In Progress

• Now in “Final Approval Ballot” stage:• Start: 17 Nov, 2009

• Finish: 30 Nov, 2009

• The purpose of the EJB 3.1 specification is to further simplify the EJB architecture by reducing its complexity from the developer's point of view, while also adding new functionality in response to the needs of the community

EJB specification

JSR-000220 Enterprise JavaBeans 3.0 Final Release:http://jcp.org/aboutJava/communityprocess/final/jsr220/index.html

Consists of three documents:

• EJB 3.0 Simplified API (59 pages)

• Java Persistence API (256 pages)

• EJB Core Contracts and Requirements (562 pages)

Overview – EJB in Java EE

http://java.sun.com/javaee/5/docs/tutorial/doc/bnabo.html

Types of EJBs

• There are actually three kinds of EJBs: • session beans

• entity beans

• message-driven beans

• Session beans• Implement the business logic of an application

• Can be Stateless or Stateful

• Entity beans• Are persisted in some data store

Session and Entity beans

In a typical scenario, the UI calls the methods of the session beans. Session beans can call other session beans and entity beans.

EJB 2.x: classes and interfaces

To implement an EJB, one needs to define:

• Remote interface• Defines the business methods a bean presents to the

outside world

• Home interface• Defines the bean's life cycle methods: create,

remove, find

• Bean class• Actually implements the bean's business methods

Example: EJB 2.x implementation

public interface TestSessionBean extends javax.ejb.EJBObject{

public String sayHello()

throws java.rmi.RemoteException;

}

public interface TestSessionBeanHome extends

javax.ejb.EJBHome{

public TestSessionBean create()

throws javax.ejb.CreateException, java.rmi.RemoteException;

}

Example: EJB 2.x implementation

public class MyTestSessionBean implements SessionBean{

public void ejbCreate() throws CreateException { } public void setSessionContext(SessionContext

aContext) throws EJBException { }

public void ejbActivate() throws EJBException { } public void ejbPassivate() throws EJBException { } public void ejbRemove() throws EJBException { }

public String sayHello(){ String msg="Hello! I am Session Bean"; System.out.println(msg); return msg; }}

Example: EJB 3 implementation

@Remote

public interface TestSessionBean {

public String sayHello();

}

@Stateless

public class MyTestSessionBean implements TestSessionBean {

public String sayHello(){

String msg="Hello! I am Session Bean";

System.out.println(msg);

return msg;

}

}

EJB server and container

• The EJB server provides an environment that supports the execution of applications developed using EJBs

• An EJB server manages and coordinates the allocation of resources to the applications

• The EJB server must provide one or more EJB containers

• An EJB container manages the enterprise beans contained within it

EJB container

For each enterprise bean, the container is responsible for • registering the object

• providing a remote interface for the object

• creating and destroying object instances

• checking security for the object

• managing the active state for the object

• coordinating distributed transactions

• (optionally) manage all persistent data within the object

Packaging Java EE applications

• A Java EE application is delivered in an Enterprise Archive (EAR) file

• An EAR file contains Java EE modules and deployment descriptors

EJB module

• An EJB module is used to assemble one or more EJBs into a single deployable unit

• An EJB module is stored in a standard Java archive (JAR) file

• Directory structure:

Creating directory structure with Maven

• Execute a command:

• Develop EJBs in /ejbs/ folder

mvn archetype:create -DgroupId=com.web.ejb

-DartifactId=ejb-jboss-demo

-DarchetypeArtifactId=maven-archetype-j2ee-simple

Bug related to “site” module

• There is a known bug regarding Maven’s J2EE archetype:

http://jira.codehaus.org/browse/ARCHETYPE-67

• If you inspect such an error when trying to build your application, then comment out “site” module from root pom.xml <modules>

<module>projects</module> <module>primary-source</module> <module>servlets</module> <module>ejbs</module> <module>ear</module> <!--module>site</module--> </modules>

Developing and packaging EJBs

• To work with \ejbs\ module in Eclipse separately do the following:• From project root execute “mvn install”

• From \ejbs\ folder execute “mvn eclipse:eclipse”

• Import “ejbs” project into Eclipse

• To package EJBs execute “mvn package” from \ejbs\ folder

• Result: EJB module is created:

\ejbs\target\ejbs-1.0.jar

Deploying EJBs

• You can deploy JAR file with EJBs as a standalone module

OR

• Deploy the full EAR

\my_project\ear\target\ear-1.0.ear

References

• Presentation about Enterprise Computing

http://joung.im.ntu.edu.tw/teaching/bcc/2004/Chapter14EnterpriseComputing_s.pdf

• Enterprise JavaBeans Technology http://java.sun.com/products/ejb/

• EJB 3.0 in a nutshell http://www.javaworld.com/javaworld/jw-08-2004/jw-0809-ejb.html

References

• Spring Vs. EJB 3.0 http://www.onjava.com/pub/a/onjava/2005/06/29/spring-ejb3.html

• Creating a simple J2EE Project with Maven

http://maven.apache.org/plugins/maven-archetype-plugin/examples/j2ee-simple.html

• Getting Started with EJB 3.0http://www.devx.com/Java/Article/30045/0/page/1