Enterprise%20Java%20Beans%20-%20(EJB)

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Enterprise Java Beans - (EJB)
By Bharath Reddy Allam
By Bharath Reddy Allam
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EJB Overview
Enterprise Java Beans Technology is part of
larger Framework - The J2EE (Java 2 Enterprise
Edition) Component Architecture for Distributed
Systems Framework for creating middleware
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Why EJB?

• EJB Architecture simplifies the development of
  distributed enterprise applications.
• EJB Specification is a solution that is
  operating-system independent.
• Middleware Independence
• EJB defines a solution that eases the development
  and deployment of distributed applications

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Why EJB?

• EJB enables the development of reusable and
  portable components.
• EJB has Broad Industry Support Server Vendors
  and End Users.

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EJB Vs JavaBeans

• EJB component model logically extends the
  JavaBeans component model to support server
  components.
• EJB components cannot be manipulated by a visual
  Java IDE in the same way that JavaBeans can, They
  are prepackaged pieces of application
  functionality that are designed to run in an
  application server.

• JavaBeans component model defines a standard
  mechanism to develop portable, Java components,
  such as widgets or controls.
• JavaBeans technology can be used in any visual
  Java technology integrated development
  environment (IDE)

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EJB Architecture

• EJB Server
• EJB Container
• EJB Client
• Enterprise Java Beans
• Auxiliary Systems
• - JNDI, JTA,
• JavaMail

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Roles in EJB Development
EJB Developer - Person who provides
implementations of EJB classes EJB Deployer -
Responsible for Deploying EJBs in EJB Server EJB
Container Vendor - Provides software to
install an EJB into an EJB Server EJB Server
Vendor - Provides an Application Framework in
which to run EJB Containers Application
Developer - Who writes client applications
that uses EJB.
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EJB Server
Is a High Level Process or Application
that Manages and makes EJB Containers
Visible Provides Access to System
Services Provides Naming and Transaction Services
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EJB Container
Interface between Enterprise Java Bean and
outside world Accessing of EJB is done through
Container generated methods, which in turn call
the EJB methods
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EJB Clients

• Is an Application, Servlet, JSP or Applet that
• Find EJB Containers via JNDI
• Uses EJBHome to Create or Remove EJB from
  Container
• Uses EJBObject to Invoke Business Methods of EJB

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Enterprise Java Beans
Entity 1. Bean Managed 2. Container Managed
Session Bean 1. Stateless 2. Stateful
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Session Vs Entity Beans
Session 1. Performs a task for a Client 2.
Associated with a particular client 3. Non
Persistent and do not survive System Shutdown
Entity 1. Represents a business entity Object
that exists in a persistent Storage 2. Shared
by multiple clients 3. Persistent across
multiple Invocations and survives System Shutdown
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Passivation and Activation
Passivation - Saving the state of a Bean to a
persistent Storage Device and swapping it
out Activation - Restoring the state of Bean from
a persistent Storage Device and swapping it
in Depends upon Container Provider Applies to
both Session and Entity Beans
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Stateless Vs Stateful Session Beans
Stateful 1. Posses State during Client-Bean
Session 2. Is passivated 3. One Per Client
Stateless 1. No State during Client-Bean
Session 2. Is not passivated 3. Can be
shared by multiple clients
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SessionBean Interface
public abstract interface SessionBean extends
EnterpriseBean public void setSessionContext(Ses
sionContext ctx) throws EJBException,
java.rmi.RemoteException public void
ejbRemove() throws EJBException,
java.rmi.RemoteException public void
ejbActivate() throws EJBException,
java.rmi.RemoteException public void
ejbPassivate() throws EJBException,
java.rmi.RemoteException
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Writing a Session Bean-Step 1
Create a Remote Interface - Should extend
EJBObject - Provide the Business methods -
Arguments and Return types must be valid RMI
types - The throws Clause of the Business
methods must include java.rmi.RemoteException
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Writing a Session Bean-Step 2
Create Home Interface - Should extend EJBHome
- The argument and return types of create
method(s) must be valid RMI types - create
method(s) should return the Remote Interface -
Throws clause of create method(s) must include
java.rmi.RemoteException and javax.ejb.CreateExcep
tion
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Writing a Session Bean-Step 3
Writing the Bean Class (SessionBean) - should
implement SessionBean interface - Should be a
public class, and cannot be abstract or final
- contains public constructor with no Arguments
- should implement ejbCreate methods whose
arguments should correspond to create methods of
Home Interface - Return type of ejbCreate
methods should be void - throws clause of
ejbCreate methods should implement
javax.ejb.CreateException - implements
Business Methods of Remote Interface
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Writing an EJB Client
Locate the Home Interface Create an Enterprise
Bean Instance Invoke a Business Method upon the
Enterprise Bean Remove the Bean
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Life Cycle of Stateful SessionBean
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Life Cycle of Stateless SessionBean
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Container Vs Bean Managed EntityBean
Bean-Managed EntityBean 1. Bean is
responsible for saving its own state 2.
Entity Bean consists Code for DB calls 3.
Some times the code Contains DB specific calls
which makes non portable
Container-Managed EntityBean 1. Container is
responsible for Saving the state of Bean 2. In
DD, Specify the Container managed fields 3. Is
Independent of Data Store, Such as a Relational
Database
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EntityBean Interface
public abstract interface EntityBean extends
EnterpriseBean public void
setEntityContext(EntityContext ctx) throws
EJBException,
java.rmi.RemoteException public void
unsetEntityContext() throws EJBException,
java.rmi.RemoteException
public void ejbRemove() throws RemoveException,
EJBException,
java.rmi.RemoteException public void
ejbActivate() throws EJBException,
java.rmi.RemoteException public void
ejbPassivate() throws EJBException,
java.rmi.RemoteException public void
ejbLoad() throws EJBException, java.rmi.RemoteExce
ption public void ejbStore() throws
EJBException, java.rmi.RemoteException
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Writing a Entity Bean - Step 1
Create a Remote Interface - Should extend
EJBObject - Provide the Business methods -
Arguments and Return types must be valid RMI
types - The throws Clause of the Business
methods must include java.rmi.RemoteException
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Writing a Entity Bean - Step 2
Create Home Interface - Should extend EJBHome
- The argument and return types of create
method(s) must be valid RMI types - create
method(s) should return the Remote Interface -
Throws clause of create method(s) must include
java.rmi.RemoteException and javax.ejb.CreateExcep
tion
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Writing a Entity Bean - Step 3
Writing the Bean Class (EntityBean) - should
implement EntityBean interface - Should be a
public class, and cannot be abstract or final
- contains public constructor with no Arguments
- should implement ejbCreate and ejbPostCreate
method(s) whose arguments should correspond to
create method(s) of Home Interface - Return
type of ejbCreate method(s) is Primarykey -
Return type of ejbPostCreate method(s) is void
- throws clause of ejbCreate methods should
implement javax.ejb.CreateException -
implements Business Methods of Remote Interface
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Life Cycle of Entity Bean
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Deployment of EJBs in J2EE Server
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Transactions

• Transactions are units of work that can maintain
  a reliable data source that is accessed by
  several clients.

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Transactions

• Transactions have ACID Properties
• Atomicity A transaction either commits or aborts
• Consistency A transaction correctly manages the
  changing state of a system
• Isolation A transactions effects are isolated
  from other transactions effects
• Durability The result of a transaction persists

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Java Transaction Service (JTS)

• JTS is an implementation of an underlying
  transaction service
• javax.transaction.TransactionService allows the
  application server to manage transaction
  boundaries.

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EJB Transactions

• EJB Architecture
• Supports flat transactions
• Supports the two-phase commit protocol
• Prepare-ready
• Commit
• Supports access to transaction service using the
  JTS interface.

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UserTransaction Interface

• javax.transaction.UserTransaction
• begin
• commit
• rollback
• setRollbackOnly
• setTransactionTimeOut
• getStatus

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Container-Managed Isolation

• Enterprise Bean can still participate via the
  EJBContext
• The setRollbackOnly method is used by a Bean to
  force a rollback (usually due to an application
  exception).
• The getRollbackOnly method is used by a Bean to
  test if the transaction is marked for a rollback.

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Bean-Managed Isolation

• Session Bean Can
• Gain Access to the transaction service
• Define transaction type as Bean in the deployment
  descriptor
• Ensure methods do not declare attributes that
  conflict with Bean-Managed demarcation.
• Use the javax.transaction.UserTransaction
  interface.
• Start a new transaction only after others complete

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Bean-Managed Isolation

• import javax.transaction.UserTransaction
• public class CartBean implements SessionBean
• EJBContext ic null // initialize some
  where else
• void someMethod()
• UserTransaction ex
  ic.getUserTransaction()
• tx.begin()
• // do work
• tx.commit()

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Bean-Managed Isolation

• Use the following methods
• UserTransaction.getStatus method obtains status
  of a global transaction
• UserTransaction.rollback method rolls back a
  global transaction
• Do not use the following methods
• setRollbackOnly()
• getRollbackOnly()

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Bean-Managed Demarcation

• Stateful Session Beans
• Methods can return in the middle of a transaction
  without closing the transaction.
• Subsequent calls can go to any Bean method
• Stateless and Entity Beans
• Methods must complete a transaction before
  returning.

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Vendors Supporting EJB

• SUN
• IBM
• WebLogic
• Oracle
• Inprise
• BEA

• Novell
• Netscape
• IONA
• Gemstone
• Informix
• Allaire