J2EE Part 2:J2EE Part 2:Enterprise JavaBeansEnterprise JavaBeans

CSCI 4300CSCI 4300Images and code samples from jGuru EJB tutorial,Images and code samples from jGuru EJB tutorial,http://java.sun.com/developer/onlineTraining/EJBIntro/EJBIntro.htmlhttp://java.sun.com/developer/onlineTraining/EJBIntro/EJBIntro.html

EJB containerEJB container

• Created by an application server program

• We use SUN Application Server on zion.cs.uga.edu

• Must write EJB to specified interfaces

EJB InterfacesEJB Interfaces

• Client: a Java program (servlet, bean)

• Home interface: local object

• Remote interface: access the actual EJB, possibly across a network

EJB Interface exampleEJB Interface example

• CustomerHome home = // ... • // Use the home interface to

create a new instance of the Customer bean.

• Customer customer = home.create(customerID);

• // using a business method on the Customer.

• customer.setName(someName);

Entity beans represent data Entity beans represent data objects:objects:

import javax.ejb.EJBObject; import java.rmi.RemoteException;

public interface Customer extends EJBObject { public Name getName() throws RemoteException; public void setName(Name name) throws

RemoteException; public Address getAddress() throws RemoteException;public void setAddress(Address address) throws

RemoteException; }

Session Beans represent business Session Beans represent business processes:processes:

public interface HotelClerk extends EJBObject { public void reserveRoom(Customer cust, RoomInfo ri, Date from, Date to)

throws RemoteException;

public RoomInfo availableRooms( Location loc, Date from, Date to)

throws RemoteException; }

An Entity Bean classAn Entity Bean class

• The Bean class actually implements the EntityBean interface (business logic)

• For example, maintains a database connection for customer info

• Client reads and writes the bean, and does not need to do DB access

EJB lifecycle methods (home EJB lifecycle methods (home interface)interface)

public interface CustomerHome extends EJBHome {

public Customer create(Integer customerNumber)

throws RemoteException, CreateException;

public Customer findByPrimaryKey(

Integer customerNumber)

throws RemoteException, FinderException;

public Enumeration findByZipCode(int zipCode)

throws RemoteException, FinderException;

}

-- these return instances of Customer, the remote interface

Stub and Skeleton MethodsStub and Skeleton Methods

• Stub method on local system represents the remote object

• Skeleton method on remote system represents the local client

Container-Managed PersistenceContainer-Managed Persistence

• Persistence: arranging that data stored will be available in a later session

• Container-Managed Persistence: EJB container automatically saves EntityBean contents to a database and restores on demand

• Developer writes no DB code!• Easiest for developer, hardest for EJB

container

EntityBean creation codeEntityBean creation code

EntityBean business logicEntityBean business logic

EntityBean Callback methodsEntityBean Callback methods

Customer Data MembersCustomer Data Members

Serializable objectsSerializable objects

• Serializable: an object can be saved to a string and correctly restored from the string

• A class whose data members are primitive values is automatically serializable

• Classes that use trees, etc. can be made serializable:

Automatically generated SQL for Automatically generated SQL for this EntityBean:this EntityBean:

Bean-Managed persistenceBean-Managed persistence

The entity bean itself must contain the DB code to:

• Store the bean data contents into the database

• Restore the bean from its stored contents

These methods invoked as callbacks from the EJB container

Stateless Session BeansStateless Session Beans

• Represent business processes (transactions)• Contain no persistent data• Shared among multiple clients

Acme SessionBean creationAcme SessionBean creation

• JNDI: Java Directory and Naming Interface (find objects in a networked environment)

• InitialContext: provided by EJB container, gives JNDI access

Acme SessionBean operationsAcme SessionBean operations

• Normally, we would open an output stream and write the request to the server!

EJB DescriptorEJB Descriptor

Assembly descriptorAssembly descriptor

EJB DeploymentEJB Deployment

The EJB jar file contains:

• Home interface class

• Remote interface class

• Bean implementation class

• META-INF/ejb-jar.xml

Then, drop it into the JBOSS deploy directory!

WAR and EAR filesWAR and EAR files

Buildfile source:

http://www.developer.com/java/data/article.php/10932_3405781_1