Transcript An Introduction to Enterprise Application Development using EJBs

AN INTRODUCTION TO
ENTERPRISE APPLICATION
DEVELOPMENT USING EJBs
by
SHREERAM IYER
08/29/2001
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
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
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
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
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
EJB Architecture…
EJB Architecture
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.)
EJB Servers
Analogous to the CORBA ORB
Provides Systems services like
Raw execution environment
Multiprocessing
Load balancing
Naming
Transaction Services
EJB Containers
Interface between EJB and outside world
Client never accesses a bean directly
Session containers – contain transient, nonpersistent EJBs whose states are not saved
Entity containers – contains persistent EJBs
whose states are saved between invocations
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
Remote Interface and
EJBObject
The Remote Interface lists the business
methods available for the EJB
The EJBObject is the Client’s 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 EJBObject’s methods
The EJB Container first handles the request
before delegating it to the EJB…provides
features of EJB like transaction, pooling,
security, etc.
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
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
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
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
The Whole EJB Picture
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
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
An Enterprise Application
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
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