EJB
Rabie A. Ramadan
What is EJB?
 Specification
defines an architecture for
the development and deployment of
transactional,
distributed
object
applications-based, server-side software
components
2
What is the difference between Two-tier
and three-tier environments?

two-tier applications access database services or transaction
services directly from the client.

Three-tier client-server applications employ an intermediary, or
middle-tier, application server, which operates between client
applications and the back-end databases. The middle tier houses
the business logic of the system and coordinates the interaction of
the presentation on the client with the back-end databases.
3
What is a Fat – Client?

the application logic resides on the client,
making the clients large and complex.
4
What are the fundamental motivations
for using a three-tier architecture?

Improved scalability, availability, and
performance

Improved flexibility and extensibility of business
systems
5
Differentiate between Pooling and
Clustering in three tier architecture?

Pooling makes three-tier systems more effective by allowing many
clients to share scarce resources like database connections, which
reduces the workload on back-end servers.

Clustering makes three-tier systems more available and scalable
because multiple servers and resources can support fail-over and
balance the loads of a growing client population.
6
What is the difference between Javabeans
and Enterprise JavaBeans ?

An enterprise bean is not the same as a JavaBean. A JavaBean is
developed using the java.beans package, which is part of the Java
2 Standard Edition. JavaBeans are components that run on one
machine, within a single address space. JavaBeans are process
components. An enterprise bean is developed using the javax.ejb
package, a standard JDK extension, which is a part of the Java 2
Enterprise Edition. Enterprise beans are components that run on
multiple machines, across several address spaces. Enterprise beans
are thus interprocess components. JavaBeans are typically used as
GUI widgets, while enterprise beans are used as distributed
business objects.
7
What is EJB Container and why it is
needed ?

Enterprise beans are software components that run in a
special environment called an EJB container. The
container hosts and manages an enterprise bean in the
same manner that the Java Web server hosts a servlet or
an HTML browser hosts a Java applet.

The container isolates the enterprise bean from direct
access by client applications.
8
Describe a possible scenario of bean call
from the client?
 Pages
7 and 8
9
The enterprise bean interacts with its
container through one of three mechanisms
which are :
10
The enterprise bean interacts with its
container through one of three mechanisms
which are :
11
What is the meaning of portability of EJB
?
 Page
8
12
What is Entity Bean?
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Page 11
13
What is a session bean ?
 Page
11
14
EJB Architecture and Design
What is EJB?


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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
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Services provided by EJB container
Circumstances of EJB component usage
How an EJB component looks like?
View of an EJB component by client
programmer and EJB developer
Mechanisms by which EJB container
provides its services
Rules 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 customized

There 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 bean
 Entity Beans
 With container-managed persistence
 With bean-managed persistence
 Message-Driven Beans

Why use EJBs in your design?


EJB specification provides enterpriselevel 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
Client
Application Logic
Data
J2EE Application Server
RDBMS
EJB Container
Corba
Client
Application
JMS
JTA
Java
Mail
JDBC
RMI
Session Bean
Mail
Entity Bean
Roles in EJB Development

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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
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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
EJB
Provider
Application
Assembler
Deployer
App Server/
EJB Container
Provider
System
Administrator
EJB Container and its Services

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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.
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Services provided by an EJB container
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Declarative transactions
Data caching
Declarative Security
Error Handling
Component Framework for Business
Logic
Scalability and Fall-Over
Portability
Manageability
How the Container Provides Services
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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
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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

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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
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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
Client
RMI
Stub
Network
RMI
Stub
Container
generated
class
Interposition class
EJB
Interposition : from EJB Container to
EJBs
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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
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The Enterprise JavaBeans specification
is written for three audiences:
The Client developer
The EJB developer
The 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
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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 resources
The 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

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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 find
Execute methods
instance of bean
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..
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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;
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public void cancelOrder(String custName, String prodName)
throws java.rmi.RemoteException;
public boolean isShipped(String custName, String prodName)
throws java.rmi.RemoteException;
}
OrderManagementBean code..
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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..
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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..
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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</ejbclass>
<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?
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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?
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use Threads or the Threading API
use the AWT
Act as a Network Server
use Read/Write static fields
use java.io package
Load a native library
use “this” as an Argument or Return
value
use Loopback Calls
EJB Components on the Web
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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 userinterface events into changes to the model
and defines the way the user-interface reacts
to those events.
Implementation of MVC in a web site
Model
4
2
view1.jsp
3
Main.jsp
view2.jsp
Controller
view3.jsp
Views
5
Browser
Client
1