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Enterprise Java Beans - (EJB)
By Bharath Reddy Allam
By Bharath Reddy Allam
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
Why EJB?
EJB Architecture simplifies the development of distributed enterprise applications.
EJB Specification is a solution that is operating-system independent.
Middleware IndependenceEJB defines a solution that eases the
development and deployment of distributed applications
Why EJB?
EJB enables the development of reusable and portable components.
EJB has Broad Industry Support: Server Vendors and End Users.
EJB Vs JavaBeans
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)
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.
EJB Architecture
•EJB Server
•EJB Container
•EJB Client
•Enterprise Java Beans
•Auxiliary Systems - JNDI, JTA, JavaMail
Roles in EJB Development
EJB Developer - Person who provides implementations of EJB classesEJB Deployer - Responsible for Deploying EJB’s in EJB ServerEJB Container Vendor - Provides software to install an EJB into an EJB ServerEJB Server Vendor - Provides an Application Framework in which to run EJB ContainersApplication Developer - Who writes client applications that uses EJB.
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
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
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
Enterprise Java Beans
Session Bean 1. Stateless 2. Stateful
Entity 1. Bean Managed 2. Container Managed
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
Passivation and Activation
Passivation - Saving the state of a Bean to a persistent Storage Device and swapping it outActivation - Restoring the state of Bean from a persistent Storage Device and swapping it inDepends upon Container ProviderApplies to both Session and Entity Beans
Stateless Vs Stateful Session Beans
Stateless 1. No State during Client-Bean Session 2. Is not passivated 3. Can be shared by multiple clients
Stateful 1. Posses State during Client-Bean Session 2. Is passivated 3. One Per Client
SessionBean Interface
public abstract interface SessionBean extends EnterpriseBean{
public void setSessionContext(SessionContext 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
}
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
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.CreateException
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
Writing an EJB Client
Locate the Home Interface
Create an Enterprise Bean Instance
Invoke a Business Method upon the Enterprise Bean
Remove the Bean
Life Cycle of Stateful SessionBean
Life Cycle of Stateless SessionBean
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
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.RemoteException public void ejbStore() throws EJBException, java.rmi.RemoteException}
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
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.CreateException
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
Life Cycle of Entity Bean
Deployment of EJB’s in J2EE Server
Transactions
Transactions are units of work that can maintain a reliable data source that is accessed by several clients.
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 transaction’s effects are
isolated from other transactions’ effects- Durability: The result of a transaction
persists
Java Transaction Service (JTS)
JTS is an implementation of an underlying transaction service
javax.transaction.TransactionService allows the application server to manage transaction boundaries.
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.
UserTransaction Interface
javax.transaction.UserTransaction begin commit rollback setRollbackOnly setTransactionTimeOut getStatus
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.
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
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(); }}
Bean-Managed Isolation
Use the following methods: UserTransaction.getStatus method
obtains status of a global transaction UserTransaction.rollback method rolls
back a global transactionDo not use the following methods:
setRollbackOnly() getRollbackOnly()
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.
Vendors Supporting EJB
•SUN
•IBM
•WebLogic
•Oracle
•Inprise
•BEA
•Novell
•Netscape
•IONA
•Gemstone
•Informix
•Allaire
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