Enterprise Java BeansYe Zhou

CS6704 PresentationVirginia Tech

Entity Beans An in-memory Java representation of

persistent data -- a view into a database Long-lived – as long as data lives Shared access from multiple users Transactional Can survive from the server crash

Entity Bean Model Entity Bean class

ejbLoad() – load state from underlying DB ejbStore() – synchronize the state regularly ejbRemove() – remove object when client calls ejbActivate() – bean instance out of pool ejbPassivate() – bean instance disassociated setEntityContext() – called by container on creation unsetEntityContext() – called by container before being

removed Primary Key class

Unique Identifier

Entity Bean usage

BMP & CMP BMP – Bean Managed Persistence

an entity bean that synchronizes its state with the database manually

CMP – Container Managed Persistence EJB container will do it instead

BMP & CMP

Entity bean example

Entity bean exampleimport javax.ejb.*;import java.rmi.remoteException;public interface AccountHome extends EJBHome{

Account create(String accountID, String ownerName) throws CreateException, AccountException;

public Account findByPrimaryKey(AccountPK key) throws FinderException, RemoteException;

public Collection findByOwnerName(String name) throws FinderException, RemoteException;

public double getTotalBankValue() throws RemoteException, AccountException;

}

Entity bean exampleimport java.io.Serializable;

public class AccountPK implements java.io.Serializable{public String accountID;public AccountPK(String id){

this.accountID = id;}public String toString() {

return accountID:}public boolean equals(Object account){

return ((AccountPK)account).accountID.equals(accountID);}

}

Entity bean examplepublic class AccountBean implements EntityBean{

protected EntityBeanContext ctx;

private String accountID; //primary keyprivate String ownerName;private double balance;

//getter & setter on fieldspublic setXXX(String value)public String getXXX()…

Entity bean example//Business Logic methodspublic void Deposit(double amt) throws AccountException {

balance+=amt;}

public void WithDraw(double amt) throws AccountException {if(amt > balance) {

throw new AccountException(accountID+“Limit reached”);}else balance-=amt;

}

Entity bean example//EJB required methods

public void ejbLoad() {AccountPK account = (AccountPK) ctx.getPrimaryKey();String id = account.toString();PreparedStatement ps =null; Connection con=null;try{con=getConnection();ps = con.preparedStatement(“select owner, balance from accounts where id = ?”);ps.setString(1,id);ResultSet rs = ps.executeQuery();rs.next();ownerName = rs.getString(1);balance = rs.getDouble(2);}catch(Exception e){throw new EJBException(“Fail to load from DB”);}}

Message-Driven Bean Motivation

Performance: RMI-IIOP is a blocking protocol

Reliability: RMI-IIOP can’t survive from the server failure

Multiple senders and receivers: RMI-IIOP is connection-oriented, which is not possible

for clients to broadcast the events to servers.

Message-Driven Bean Definition

A special EJB component that can receive JMS messages.

Message-Driven Bean Life cycle

Message-Driven Bean Characteristics

No home interface, local home interface and remote interface

It has a single, weakly-couple typed business method “OnMessage”

No return values Stateless Do not send Exception back to clients Subscription can be durable or non-durable

Message-Driven Bean exampleimport javax.ejb.*;import javax.jms.*;

public sampleBean implements MessageDrivenBean, MessageListener{

protected MessageDrivenContext ctx;

public void setMessageDrivenContext(MessageDrivenContext ctx){this.ctx=ctx;}

public void ejbCreate(){System.err.println(“bean initialized”);}

Message-Driven Bean examplepublic void onMessage(Message msg){

if(msg instanceOf TextMessage) {TextMessage tm=(TextMessage)msg;try{

String text = tm.getText();System.err.println(“Text is”+text);

}catch(JMSException e) {e.printStackTrace(); }

}}public void ejbRemove() {System.err.println(“bean removed”);}

}

Transaction Features

ACID Types

Programmatic Declarative Client-initiated

Package Javax.transaction.*

Reference Ed Roman, “Mastering Enterprise

Javabeans ”, John Wiley & Sons Inc., 2002

Linda DeMichiel, “Enterprise Javabeans Spec V2.0”, Sun Microsystem, 2001

Gopalan Raj,http://www.execpc.com/~gopalan/java/ejb.html