Enterprise Java Beans

Tom Valesky

tvalesky@patriot.net

What is Enterprise JavaBeans?

Not really JavaBeans

Component architecture for distributed systems

Framework for creating middleware

Roles in EJB

Enterprise Bean provider

creates and sells EJBs

Application assembler

uses EJBs to build an application

EJB Server provider

creates and sells EJB server

EJB Container provider

creates and sells EJB containers

server provider will likely provide containers

The EJB Architecture

EJB servers

EJB containers

EJB clients

Enterprise Java Beans

Auxiliary systems

JNDI

JTS

EJB Architecture

EJB Servers

Analogous to CORBA ORB

server software

provides naming and transaction services

makes containers visible

EJB Containers

Interface between EJB Bean and outside world

Client never accesses bean directly; access is done through container-generated methods, which in turn call the bean’s methods

EJB Clients

Find EJB containers via JNDI

Make use of EJB beans

Enterprise Java Beans

Session beans

Entity beans

Session vs. Entity Beans

Session

associated with a particular client
created and destroyed by a client
do not survive system shutdown

Entity

shared by multiple clients
persist across multiple invocations
survive system shutdown

Passivation/Activation

EJB server has the right to manage its working set

Passivation -- saves state of a bean to persistent storage, then swaps it out.

Activation -- restores state of a bean from persistent storage, then swaps it in.

Applies to both session and entity beans.

Stateless vs. Stateful Session Beans

Stateless

no internal state
do not need to be passivated
can be pooled to service multiple clients

Stateful

possess internal state
need to handle passivation/activation
one per client

Persistent session beans

Session Beans can be saved and restored across client sessions.

To save, call the session bean’s getHandle() method; returns a Handle object

To restore, call the Handle object’s getEJBObject() method.

Entity Bean persistence

Container-managed

Container is responsible for saving state
In deployment descriptor, specify container-managed fields
persistence independent of data source

Bean-managed

Bean is responsible for saving its own state
Container doesn’t need to generate DB calls
Less adaptable; persistence is hard-coded

Writing an EJB Client

Locate the bean container

Allocate a bean, if necessary

Use the bean

Dispose of the bean

Client Example

Writing a session bean

Create remote interface

must extend javax.ejb.EJBObject interface

must give prototypes for business methods

class needn’t say "implements"; this is handled by the container

Create home interface

must extend javax.ejb.EJBHome interface

create() methods, remove() methods

Implement create methods

called by container at creation time

Implement the SessionBean interface

ejbActivate() - called when bean is activated

ejbPassivate() - called when bean is passivated

ejbRemove() - called when bean is destroyed

setSessionContext(SessionContext ctx) - called by container to give bean a context

Session Bean Example

Writing an Entity Bean

Implement the EntityBean interface

ejbActivate() -- called on activation

ejbPassivate() -- called on passivation

ejbLoad() -- tells bean to load state from database

ejbStore() -- tells bean to store state in database

ejbRemove() -- called when client calls remove()

setEntityContext() -- called by container when instance has been created

unsetEntityContext() -- called by container before removing the instance

must also implement ejbFind() -- allows client to look up EJB objects

Optionally implement create() methods

Create remote interface

must extend javax.ejb.EJBObject interface

Create home interface

Entity Bean example

Deploying EJBs

EJBs deployed as .SER files

serialized instance

Manifest file used to list EJBs

Must also provide a "deployment descriptor"

serialized instance of EntityDescriptor or SessionDescriptor

EJB-Jar manifest

Sample entry

Name: tom/RestaurantDeployment.ser

Enterprise-Bean: True

"Name" line

describes a serialized deployment descriptor

"Enterprise-Bean" line

indicates whether the entry should be treated as an EJB (not all entries need to be EJBs)

Deployment Descriptors

Serialized instances of a class

Used to pass info to container about bean’s deployment needs and preferences

Created by EJB provider

Auxiliary Systems

JNDI

Java Naming and Directory Interface
Used to allow clients to find EJB beans

JTS

Java Transaction Service
Used to provide transaction support in EJB

EJB-to-CORBA bindings

Uses COS naming for object lookup

Uses OTS (object transaction service) for transaction support

Specifies IDL mappings for EJB objects

Who’s announced EJB support?

WebLogic

IBM

Oracle

GemStone

BEA

Borland

Netscape

Lotus

Forte

Progress

Novell

Novera

Borland

Informix

IONA

More...

For further information

Sun’s Enterprise Homepage

http://java.sun.com/products/ejb/index.html

My EJB resources page:

http://www.patriot.net/users/tvalesky/ejb.html

WebLogic’s Tengah pages

http://www.weblogic.com