AN INTRODUCTION TO ENTERPRISE APPLICATION DEVELOPMENT USING EJBs by SHREERAM IYER 08/29/2001

1
AN INTRODUCTION TO ENTERPRISE APPLICATION
DEVELOPMENT USING EJBsbySHREERAM IYER08/29/2001
2
Why is Enterprise Application Development
different and complex?

• Done using components and component technology
• When a component fails, ensure faults are
  isolated and that it does not corrupt others
• In a distributed application, components can
  reside anywhere on the network
• Enterprises are essentially heterogeneous
  environments with presence of multiple Operating
  Systems

3
What is EJB?

• EJB (Enterprise Java Beans)
• EJB is a server component model for Java
• Specification for creating server side, scalable,
  transactional, multi-user and secure
  enterprise-level applications
• Provides a consistent component architecture for
  creating distributed n-tier middleware

4
Why EJBs?

• Frees application developer to concentrate on
  programming only the business logic
• Removes the need to write all the plumbing code
  required in any enterprise application
  development scenario
• Clients become thin as they have to worry only
  about presentation logic
• EJBs containing the business logic are platform
  independent
• Support for readymade components
• WORA for Server side components

5
Why EJBs?

• EJB establishes roles for Application development
• EJB takes care of Transaction Management
• Helps create portable and scalable solutions
• Provides for vendor-specific enhancements

6
JavaBeans and EJBs DeMystified

• In a nutshell, JavaBeans are Java classes with
  accessor methods
• Much smaller than EJBs
• Do not need a runtime environment like an
  Application Server
• A JavaBean can be a visual component
• JavaBeans are like ActiveX components

7
EJB Architecture
EJB Architecture
8
EJB Architecture

• Basic EJB Architecture consists of
• An EJB Server
• EJB Containers that run within the Server
• Home Objects, Remote EJBObjects and Enterprise
  Beans that run within Containers
• EJB Clients
• Auxiliary systems JNDI, JTS and security services
• The idea of the EJB architecture is the Server
  and the Container are responsible for providing
  the hard stuff (transactions, persistence,
  pooling, multiple instances, security, etc.)

9
EJB Servers

• Analogous to the CORBA ORB
• Provides Systems services like
• Raw execution environment
• Multiprocessing
• Load balancing
• Naming
• Transaction Services

10
EJB Containers

• Interface between EJB and outside world
• Client never accesses a bean directly
• Session containers contain transient,
  non-persistent EJBs whose states are not saved
• Entity containers contains persistent EJBs
  whose states are saved between invocations

11
Home Interface and Home Object

• Factory methods for locating, creating and
  removing instances of EJB classes
• Home Object is the implementation of the Home
  Interface
• EJB Developer defines the Home Interface for his
  Bean
• The EJB Container Vendor provide tools that
  automatically generate the implementation code
  for the Home Interface as defined by the EJB
  Developer

12
Remote Interface and EJBObject

• The Remote Interface lists the business methods
  available for the EJB
• The EJBObject is the Clients view of the EJB and
  implements the Remote Interface
• The EJB Developer defines the RI
• The Container Vendor provides tools to generate
  implementation code for EJBObject
• The Client invokes the EJBObjects methods
• The EJB Container first handles the request
  before delegating it to the EJBprovides features
  of EJB like transaction, pooling, security, etc.

13
Naming Services and JNDI

• A Naming Service is a dedicated piece of software
  that manages a naming system or namespace
• JNDI is a client API that provides naming and
  directory functionality
• Specified in JAVA
• Designed to provide a common interface for
  accessing existing services like DNS, NDS, LDAP,
  CORBA or RMI
• Provides an interface that hides the
  implementation details of different naming and
  directory services behind the JNDI API

14
Types of EJBs

• EJBs can be transient or persistent
• Two types of Enterprise Beans
• Entity Beans Generally used to model a business
  entity
• Session Beans General purpose server side beans

15
Entity Beans

• Represent data in a domain model
• Persistent Entity Bean
• Container managed persistence
• Bean managed persistence
• Entity beans can share access from multiple users
• Can survive EJB server crashes

16
Session Beans

• Created by a client and in most cases, exists
  only for the duration of a single session
• Stateless Session beans
• Stateful Session beans
• Execute on behalf of a single client
• Relatively short-lived
• May be destroyed when EJB Server crashes
• Do not represent data stored in DataBase

17
The Whole EJB Picture
18
Modeling using Session and Entity Beans

• Use Session beans for application logic
• Use Session beans as the only interface to the
  client
• Expect little reuse of Session beans
• Use Session beans to control the workflow of a
  group of entity beans eg. Credit Card Validation

19
Modeling using Session and Entity Beans

• Use Entity beans to develop a persistent object
  model
• Use Entity beans to enforce accuracy / integrity
  of your database
• Insist on reuse of Entity beans
• Use Entity beans to model domain objects with a
  unique identity shared by multiple clients eg.
  Employees

20
An Enterprise Application
21
Drawbacks of EJBs

• Developer needs to know JAVA
• EJBs are not allowed to access native libraries
  written in C/C
• EJBs cannot be used to develop any kind of GUI
• Learning curve and understanding of different
  technologies involved

22
Useful links

• http//www.ejbportal.com/
• http//www.c2.com/cgi/wiki?EjbRoadmap
• http//www.mgm-edv.de/ejbsig/ejbsig.html
• http//www.jdance.com/ejb.shtm
• http//www.sys-con.com/java/index2.html
• http//www.nordija.com/beanbuilder/
• http//www.ejbnow.com/
• http//www.theserverside.com
• http//www.beasys.com/support/newsgroup.html
• http//www.ejbean.com