6/19/20151. 2 Enterprise Java Beans Introduction –Application ServerApplication Server –Java 2 Enterprise EditionJava 2 Enterprise Edition What is an.

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Enterprise Java Beans• Introduction

– Application Server– Java 2 Enterprise Edition

• What is an Enterprise Bean ?– EJB Properties– EJB Overview

– Deployment Phase

– Type of beans

• Client access with interfaces– Remote access– Local Access

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Enterprise Java Beans

• Contents of an Enterprise Bean• EJB Example• EJB vs MTS• A few EJB implementations • What’s new in EJB 2.0

• Bibliography

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Introduction

• Enterprise Java Beans ( EJB ) is – a middleware component model for Java and

CORBA

– a specification for creating server-side, scalable, transactional, multi-user and secure enterprise-level applications

• Presented by Sun in the 1999, they are easier than other technologies as RMI or Corba

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Introduction

• This is the three level structure for Application Server

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Applicaton Server

• Presentation– HTML Application – Java Application

• Business Logic

• Data Access

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Presentation

• HTML– Generated server-

side HTML– Runs on any Web

browser– Less client-side

power

• Java– Required Java virtual

Machine

– More client side power

– Runned on a page

– Launched from a browser or a standalone application

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Business Logic

• Implements the logic of the application defining all the function that may be used from a client– Change Business Rules Easily– Re-use components– Make complex applications manageable

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Data Access

• Utility to access external datas such as Database or other Web component

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J2EE Application Server

• Java 2 Enterprise Edition standardizes interfaces for Application Server components

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What is an Enterprise Bean ?

• Is a server side component written in Java Language

• Industry standard distribuited component model

• Incorporates the business logic of an application ( the code that implements the purpose of the application)

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EJB Properties

• Bean writers need not write– Remote access Protocols– Transactional Behaviour– Threads– Security– State Management– Object life cycle– Resource pooling– Persistence

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EJB Overview

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Deployment Phase

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Deployment Phase

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When to use Enterprise bean

• The application must be scalable.It will run on different machine and their location will remain transparent to the client

• Transaction requirement• The application will have lot of different type

of clients

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Type of beans

• Session Bean• Entity Bean• Message Driven Bean

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Session Bean

• Represents a single client inside the server• The client calls the session bean to invoke methods

of an application on the server • Perform works for its client, hiding the complexity of

interaction with other objects in the server• Is not shared• Is not persistent• When the client stops the session,the bean can be

assigned to another client from the server

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Session Bean

• Stateful session bean

• Stateless session bean

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Stateful Session Bean

• Contains the state of a single client session: – Information on the client – On method called – Return values

This state is called conversational state and is not retained when the session ends, also if the client not removes the bean

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Stateless Session Bean

• Not maintain a conversational state for a particular client

• Contains values only for the duration of the single invocation

• Except during method invocation, all instances of stateless session bean are equivalent

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Entity Bean

• Represents a business object in a persistent storage mechanism such as a relational database

• Usually is a table in the database and each instance of that entity bean is a row in that table

Properties:• Persistent• Allow shared access• Have primary key • H

ave relationship with other entity beans.

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Entity Bean persistent

• Bean managed persistence

• Container managed persistence

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Bean managed persistence

• Who write the bean’s code must access the database and save his own data

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Container managed persistence

• The container save the data• There is no code in the bean for access the

database• The container handles all database access

required for the bean• Links between beans are created using a

structure called abstract schema

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Entity bean’s shared access

• Entity beans can be used by different clients• It’s important that they work whithin transactions• The EJB container provides transaction

management • The transaction’s attribute are specified in the

bean’s deployment description

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Entity bean’s primary key

• Each entity bean has a unique object identifier like a key in a database table

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Entity bean’s relationship

• Container managed persistent – The container performs all the operation to create

relationship

• Bean managed persistent– The code to perform relations must be written in

the bean

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Message Driven bean

• Allows applications to process messages asynchronously

• The messages may be sent by :– An application client– Another enterprise bean– A Web component

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Message Driven bean

• Retain no data or conversational state for a specific client

• All instances are equivalent, allowing the EJB container to assign a message to any message-driven bean instance. The container can pool these instances to allow streams of messages to be processed concurrently

• Can process messages from multiple clients

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Message Driven bean

• A client can’t access directly to a message driven bean

• When a message arrive, the container gives it to a message driven bean

• The bean process the message

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Client access with interfaces

• A client may access a session or an entity bean only through the methods defined in the bean's interfaces

• They define the client's view of a bean • Types of access:

– Remote access– Local access

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Remote access

• A remote client of an enterprise bean has the following traits:– It may run on a different machine and a different

Java virtual machine than the enterprise bean it accesses (It is not required to run on a different JVM )

– It can be a Web component– It can be another enterprise bean

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Remote access

• To create an enterprise bean with remote access, you must :– Code a remote interface

• Business methods

– Code a home interface• Finder methods • Home methods

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Remote access example

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Local access

• A local client has these characteristics– It must run in the same JVM as the enterprise bean

it accesses– It may be a Web component or another enterprise

bean– To the local client, the location of the enterprise

bean it accesses is not transparent– It is often an entity bean that has a container-

managed relationship with another entity bean

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Local access

• To create an enterprise bean with local access, you must :– Code the local interface

• Bean's business methods

– Code the local home interface• Life cycle• Finder methods

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Local interfaces

• If an entity bean is the target of a container managed relationship it MUST have local interfaces

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Contents of an Enterprise Bean

• Deployment descriptor– Persistence type

– Transaction attribute

• Enterprise bean class• Interfaces• Helper classes

– Exception

– Utility classes

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EJB Example

• The OnLine Bank

We will take a not completed system to give an idea to how choose if a component is an entity, session or message driven bean.

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EJB ExampleVirtual Bank

Client

Security Accounts

Services

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EJB Example

The example has three component:– Services: what the client can do in the system such as see

the foreign currency , listed shares or make operations on his hown account.

– Accounts: a database containing the accounts of all the clients of the bank with information about credit,debit,access etc..

– Security: is a subsystem that receives all the alarm caused from wrong access and performs action about the situation

( calls police and stops operation of that client keeping information about him )

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EJB Example

• In this example is easy to create an EJB structure.– Client will have a web page at client side to insert values and

connect the system.This will be done using JSP ( Java Servlet Pages )

– Services will be a Statefull Session Bean and it will be different for each client connecting the system mantaining data about the client connected.

– Accounts will be formed by an Entity Bean for each account in the system with a code-account as primary key.

– Security will be a Message driven bean and will be called only from container if some operation are abnormal for result or the autentification for the same client fails too much times.

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EJB vs MTS

• Microsoft Transaction Server is based on the Component Object Model (COM) which is the middleware component model for Windows NT

• MTS can be defined as a component-based programming model

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EJB vs MTSAnalogies

• Implement business logic for Application Server components

• Have a Server and a container ( for MTS called MTS Executive )

• Similar architecture in both models• A client invokes wrapped method

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EJB vs MTSDifference

• EJB– Component instance

are pooled– Don’t support

heterogeneous transactions

– Portability accross multiple platforms using Java platform

• MTS– Component is not

created until the call from a client reaches the container

– Support heterogeneous transactions

– Portability only on Windows NT

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EJB vs MTSDifference

• EJB– Invoked by clients

using RMI– Has both

persistent and non-persistent components

• MTS– Invoked by clients

using DCOM or through local COM calls

– Components are not persistent, even though they may contain information

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A few EJB implementations

• WebLogic • Bluestone • Novera• Persistence • Oracle AS • Oracle8i

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What’s new in EJB 2.0

• Released On April 26, 2001• Integration with JavaTM Message Service (JMS) --

Asynchronous Capabilities Streamline Systems– Send asynchronous messages via the JMS API

• Container-Managed Persistence (CMP) -- Simplifying and Expediting Application Development– Used to isolate the application developer from the physical

database schema– Introduces for the first time a portable query language,

based on the abstract schema

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What’s new in EJB 2.0

• Local Interfaces -- Streamlining Calls Between Local Beans– The local interface may be defined for a bean during

development, to allow streamlined calls to the bean if a caller is in the same container

• Inter-Server Interoperability -- Enabling Heterogeneous Environments– Takes the benefit of cross-server application portability

– Able to deploy the EJB technology-based application across a heterogeneous environment mixing application servers from different vendors

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Bibliography

• The J2EE Tutorial– http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/EJBConcepts.h

tml

• Meeting about Application servers– http://www.phxjug.org/meetings/silverstream/sld001.htm

• Developing Enterprise components– http://spectral.mscs.mu.edu/EJB20001113/index.html

• A detailed Comparison of EJB & MTS models– http://members.tripod.com/gsraj/misc/ejbmts/ejbmtscomp.ht

ml

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Bibliography

• What's new in the Enterprise JavaBeansTM 2.0 Specification?– http://java.sun.com/products/ejb/2.0.html

• Introduction to Enterprise JavaBeans– http://cosmos.inesc.pt/~a01/javacourses/ejb/

• Programming WebLogic Enterprise JavaBeans– http://e-docs.bea.com/wls/docs61/ejb/index.html

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The MTS architecture

• Is made up of:– The MTS Executive (mtxex.dll) – The Factory Wrappers and Context Wrappers for

each component – The MTS Server Component – MTS clients – Auxiliary systems like

• COM runtime services, • Service Control Manager (SCM)• The Microsoft Distributed Transaction Coordinator (MS-

DTC) • The Microsoft Message Queue (MSMQ) • The COM-Transaction Integrator (COM-TI)

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The MTS architecture

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The EJB architecture

• Consists of:– An EJB server – EJB containers that run within the server – Home objects– Remote EJBObjects – Enterprise Beans– EJB clients – Auxiliary systems like

• Java Naming and Directory Interface (JNDI)• Java Transaction Service (JTS) • Security services

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The EJB architecture

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Stateful session bean’s life cycle

• The client invoke the create method• The EJB container :

– Instantiates the bean – Invokes the setSessionContext – Invokes ejbCreate

• The bean is ready

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Stateful session bean’s life cycle

• While in the ready state– EJB container may passivate the bean moving it

from memory to secondary storage – A client may invoke a business method – EJB container may activate a bean,moving it back

to the ready stage, and then calls the bean's ejbActivate method

– A client may invoke the remove method and the container calls the bean's ejbRemove method

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Stateful session bean’s life cycle

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Stateless session bean’s life cycle

• The client invoke the create method• The EJB container :

– Instantiates the bean – Invokes the setSessionContext – Invokes ejbCreate

• The bean is ready

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Stateless session bean’s life cycle

• While in the ready state– A client may invoke a business method– A client may invoke the remove method and the

container calls the bean's ejbRemove method– It’s never passivate

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Stateless session bean’s life cycle

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Entity bean’s life cycle

• The EJB container :– Creates the instance– Calls the setEntityContext

• The entity bean moves to a pool of available instances

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Entity bean’s life cycle

• While in the pool :– Instance is not associated with any particular object identity – All instances in the pool are identical – EJB container may assign an identity to an instance when

moving it to the ready stage invoking the ejbActivate method

– A client may invoke the create method• EJB container calls ejbCreate and ejbPostCreate

– EJB container may remove the instance invoking unsetEntityContext

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Entity bean’s life cycle

• While in the ready state : – A client may invoke entity bean's business

methods– A client may invoke the remove method

• EJB container calls the ejbRemove method

– EJB container may invoke the ejbPassivate method

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Entity bean’s life cycle

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Message driven bean’s life cycle

• EJB container creates a pool of message-driven bean instances

• For each instance, the EJB container instantiates the bean :– It calls the setMessageDrivenContext – It calls the instance's ejbCreate

• Like a stateless session bean,it’s never passivated, It has only two states: – Nonexistent – Ready to receive messages.

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Message driven bean’s life cycle

• While in the ready state : – EJB container may call onMessage – EJB container may call the ejbRemove

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Message driven bean’s life cycle

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Abstract schema

• Part of an entity bean's deployment descriptor

• Defines the bean's persistent fields and relationships.The term abstract distinguishes this schema from the physical schema of the underlying data store

• You specify the name of an abstract schema in the deployment descriptor

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Example of Abstract schema

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Abstract schema

• Persistent fields– Are stored in the underlying data store– Constitute the state of the bean. At runtime, the

EJB container automatically synchronizes this state with the database

– During deployment, the container • Maps the entity bean to a database table• Maps the persistent fields to the table's columns

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Abstract schema

• Relationship fields– It’s like a foreign key in a database table.It

identifies a related bean – Like a persistent field, a relationship field is virtual

and is defined in the enterprise bean class with access methods

– Unlike a persistent field, a relationship field does not represent the bean's state

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Abstract schema

• Multiplicity in Container-Managed Relationships– One-to-one: Each entity bean instance is related

to a single instance of another entity bean – One-to-many: An entity bean instance may be

related to multiple instances of the other entity bean

– Many-to-one: Multiple instances of an entity bean may be related to a single instance of the other entity bean

– Many-to-many: The entity bean instances may be related to multiple instances of each other

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Abstract schema

• Direction in Container-Managed Relationships – Bidirectional relationship: each entity bean has a

relationship field that refers to the other bean. Through the relationship field, an entity bean's code can access its related object

– Unidirectional relationship: only one entity bean has a relationship field that refers to the other

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EJB 3.0

This section based on

Mastering EJB 3.0 4th Ed

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EJB 3.0

• Key issues for distributed systems– Remote Method Invocation– Load Balancing– Transparent failover– Back-end integration to legacy systems– Transactions– Clustering (state replication across servers)– Dynamic redeployment (upgrade while running)

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EJB 3.0

• More key issues– Clean shutdown– Logging & auditing– Systems management– Threading– Message-oriented middleware– Component lifecycle– Resource pooling

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EJB 3.0

• Yet more issues– Security– Caching– Etc., etc.

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EJB 3.0

• All of these can be thought of as middleware services

• Developer of distributed app shouldn’t have to worry about them– Should concentrate on business logic– Much too expensive to build from scratch– Distributed app should be portable across

middleware service providers

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EJB 3.0

• EJB spec defines standard interfaces between distributed components & their containers

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EJB 3.0

• Advantages– Industry standard

• Vendors design containers to EJB specs

– Portability possible across containers– RAD capable because services provided

by container

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EJB 3.0

• What’s an EJB?– Server-side component encapsulating application functionality

• Business logic• DB access• Integrate with other legacy systems• Serve multiple clients

– Browser

– Desktop apps

– Other EJBs

– Web app components (e.g., Servlets)

– Web Service clients

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EJB 3.0

• How do they work?– EJB can be located anywhere, usually on another

machine– Use RMI-IIOP for remote invocation of methods

(P. 15)– Location transparency

• Client thinks remot EJB is local

– Run inside EJB container• Application servers

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EJB 3.0

• Who makes containers?– Most of the usual enterprise suspects

• BEA, IBM, Red Hat (JBOSS), Sun, Oracle

• Closely related to web services– P. 29

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EJB 3.0

• EJB 2.1 was NASTY!!– HelloWorld EJB 2.1 (pp. 40-49) – Many disappointed enterprise developers

• Vastly simplified in EJB 3.0– HelloWorld EJB 3.0 (pp. 83-85)– Many ‘once burned twice shy’ developers

• Sun’s dilemma:– ‘If we simplify it, will they resume coding EJBs?’

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EJB 3.0

• EJB best practices

• For appropriate use only – Remoting is required– Distributed transactions required– Component security required– Persistence required– Legacy system integration– Scalability required

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EJB 3.0

• If your application components don’t need to be distributed across servers, avoid using distributed components like EJBs

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EJB 3.0

This section based on:

J2EE Development Without EJB

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EJB 3.0

• Written by a ‘once burned twice shy’ EJB developer

• Observations apply to EJB 2.1 (and earlier) experiences – EJB 3.0 spec in early stages when book written

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EJB 3.0

• Basic thesis is EJBs….– Rarely necessary for web-based applications– Commonly overused– Symptom of over-engineering – Can obtain most of needed EJB services via other

programming techniques using lightweight containers (e.g., Spring)

• Java EE & Java SE• Aspect-oriented programming• Dynamic proxies

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EJB 3.0

• He’s not alone• Many other enterprise architects and

developers have critized EJBs since early 2000s

• Martin Fowler’s Law of Distributed Objects

‘Don’t do it!!’

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EJB 3.0

• Some criticisms– Too complex

• Specifications getting more complex• EJB 3.0 tries to address this

– Unit testing hard to do • Have to isolate app from container• If something broken, is it container related or app

related?

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EJB 3.0

– Other technologies (AOP) get the job done more easily

– Entity beans are a flop• EJB 3.0 doesn’t upgrade entity beans• Left to wither on the Java EE vine• Persistence API tries to address this

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EJB 3.0

• Metadata (a la .NET) are better than descriptor files– Annotations designed to address this

• Only truly unique service provided by EJB is remote access to a component

• Java EE provides other services already– JNDI, JTA, JCA, resource pooling, etc.

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EJB 3.0

• EJBs remain best choice for apps that genuinely need object distribution or use IIOP protocol

• EJBs also pretty good for apps based around asynchronous messaging– Message-drive beans pretty simple

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EJB 3.0

• Financial services middleware likely good place for EJB-driven apps

• Argues that EJBs are declining technology & will be legacy within 3 years

• Doesn’t think EJB 3.0 will make much of a difference

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EJB 3.0

• Advocates use of lightweight containers like Spring….

• Oh btw, author is co-founder of Spring framework!

• Bias is always an issue!

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EJB 3.0

• What’s an enterprise developer to do?

• Distributed systems a holy grail of system architecture & deployment since early 90s– CORBA initiative began in 91– Web Services are current frontier

• IT profession still learning when & how to do distributed systems

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EJB 3.0

References

Master Enterprise JavaBeans 3.0, 4th Ed, Sriganesh et. Al., 2006

J2EE Development Without EJB, Johnson, 2004