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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
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
Presentation
tier
Business
logic tier
Data tier
Advantages of multi-tier model
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Scalability
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
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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
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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
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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   
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EJB container is charged with the task of making system
services available to EJB components
• Component portability
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A simple, elegant component container model
Java server components can be developed once and
deployed in any EJB-compliant server
• Architecture independence
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Independent of any specific platform, proprietary
protocol, or middleware infrastructure
Benefits of using EJBs
• Built-in support for typical enterprise-level system
services:
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distributed objects
transactions
database
security
global naming
• Developer productivity
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Standardization and automation of complex infrastructure
services
Developers can create complex applications by focusing on
business logic rather than environmental issues
EJB history
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The EJB specification was originally developed in
1997 by IBM and later adopted by Sun Microsystems
• EJB 1.1 ( J2EE 1.2 ), 1999
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Session beans (stateless & stateful), Entity Beans
Remote interface
• EJB 2.0 ( J2EE 1.3 ), 2001
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Message-Driven Beans
Entity 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
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required interfaces
checked exceptions
deployment descriptors
Adoption and criticism
• Businesses found that using EJBs to encapsulate
business logic brought a performance penalty
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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
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EJB Timer Service
EJB Web Service Endpoints
Minor EJB QL enhancements
• Reinventing EJBs
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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 ( JavaEE 5.0 ) 2006
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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
Overview – EJB in JavaEE
http://java.sun.com/javaee/5/docs/tutorial/doc/bnabo.html
Types of EJBs
• There are actually three kinds of EJBs:
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session beans
entity beans
message-driven beans
• Session beans
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Implement the business logic of an application
Can be Stateless or Stateful
• Entity beans
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Are persisted in some data store
Session and Entity beans
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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: 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 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 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
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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:
mvn archetype:create
-DgroupId=lv.webkursi.ejb
-DartifactId
=java-eim-demo-ejb-jboss
-DarchetypeArtifactId
=maven-archetype-j2ee-simple
• Develop EJBs in /ejbs/ folder
References
• Enterprise JavaBeans Technology
http://java.sun.com/products/ejb/
• The history and goals of EJB architecture
http://www.ibm.com/developerworks/java/library
/j-what-are-ejbs/part1/
• EJB 3.0 in a nutshell
http://www.javaworld.com/javaworld/jw-082004/jw-0809-ejb.html
References
• Spring Vs. EJB 3.0
http://www.onjava.com/pub/a/onjava/2005/06/29/springejb3.html
• Getting Started with EJB 3.0
http://www.devx.com/Java/Article/30045/0/page/1