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Enterprise JavaBeans (EJB)

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Title: Enterprise JavaBeans (EJB)


1
Enterprise JavaBeans (EJB)
  • Payam Barnaghi

2
Enterprise JavaBeans (EJB)
  • Enterprise JavaBeans provides a standard
    distributed computing model.
  • EJB component is a JavaBean component, but
    moreover EJB architecture provides a framework
    which consists transaction processing, security,
    persistence, and resource pooling facilities.

3
What is EJB?
  • EJB is a standard for building MIDDLEWARE in
    multi-tiered solutions.
  • EJB is a solution to provide services that are
    hard to implement for the users.
  • EJB architecture is a component architecture for
    the deployment and development of component-based
    distributed business applications.

4
EJB API
  • EJB API is a standard extension to java,
    available in the javax.ejb package.
  • EJB is a specification for how distributed
    components should work within the java
    environment.
  • To use EJB objects, you need to install an
    EJB-enabled server.

5
EJB Container
  • EJB-enabled application servers.
  • EJB Container and Deployment Tools.
  • Available from BEA, IBM, Sun Java System
    Application Server and many other vendors.
  • Open source Glassfish, JBoss,

6
EJB Container
EJB Container
EJB Client
Transaction management Persistence
Management Security Management Resource Pooling
EJB Context, JNDI
Bean
Business Methods
7
Where is EJB in a Big Picture of J2EE?
Source Sang Shin, Enterprise JavaBeans TM (EJB
TM Architecture Overview) Technology and XML
8
An EJB Environment
Source Java Enterprise in a Nutshell
9
EJB Client
  • An EJB client uses remote EJB objects to access
    data and perform the tasks.
  • EJB clients are application clients that interact
    with the J2EE EJB tier.
  • EJB clients are GUI programs that typically
    execute on a desktop computer or they are web
    applications they manage their own GUI and offer
    a user experience similar to that of native
    applications.

10
Types of EJB
  • There are three types of EJBs
  • Entity bean
  • corresponds to a record in a database
  • Session bean
  • handles business flow (stateless and stateful)
    (one per client, only in stateless then may be
    shared)
  • Message-Driven Beans (EJB version 2.0)

11
Session Beans
  • Does work on behalf of a single client.
  • Is not persistent and hence relatively short
    lived.
  • Is removed when the EJB server crashes
  • Does not represent data in data store, although
    can access/update such data.

Source Sang Shin, Enterprise JavaBeans
12
When to Use Session Beans?
  • to model process or control objects specific to a
    particular client.
  • to model workflow, processes or tasks, manage
    activities (make reservation, purchase...).
  • to coordinated processes between entity beans,
    control interactions of beans.
  • to move business application logic from Client to
    the Server Side.

Source Sang Shin, Enterprise JavaBeans
13
Types of Session Beans
  • Stateless execute a request and return a result
    without saving any client specific state
    information. It is shared among a number of
    clients.
  • transient
  • temporary piece of business logic needed by a
    specific client for a limited time span
  • Stateful is created for a specific client and
    maintains client specific state.

Source Sang Shin, Enterprise JavaBeans
14
Where to use different types of Session Beans
  • Stateless session beans
  • Catalog
  • No client specific state needs to be preserved
  • Interest calculator
  • No client specific state needs to be preserved
  • Business logic with no need for database access
  • Stateful session beans
  • Shopping cart
  • Client specific state needs to be preserved

Source Sang Shin, Enterprise JavaBeans
15
Entity Beans
  • Provides object view of data in data store
  • Its lifetime is not related to the duration of
    interaction with clients.
  • Lives as long as data exists in database (i.e.
    long lived).
  • In most cases, synchronised with relational
    databases.
  • Shared access among clients.

Source Sang Shin, Enterprise JavaBeans
16
Entity Beans (contd)
  • Clients normally look up (find) an existing
    entity EJB
  • Creation means adding a row to a database table.
  • Finding means finding a row in a existing
    database table.
  • Entity bean instance has unique identifier called
    primary key
  • Primary key can be any class

Source Sang Shin, Enterprise JavaBeans
17
Examples of where to use Entity Beans
  • Customer
  • Customer data has to persist, thus it is
    maintained in the database.
  • Customer data has to survive in the case of
    server crash.
  • Customer data is shared by many clients.
  • Each customer has unique identification such as
    customer number.

18
Differences Between the Session and Entity Beans
  • Entity Beans
  • Represent persistent data
  • Long-lived
  • Session Beans
  • Interact with clients
  • Model business Logic
  • Short-lived

Session Bean
Entity Bean
Enterprise JavaBeans Container
Adapted from Dave Ellimans G53ELC slides,
http//www.cs.nott.ac.uk/dge/G53ELC/
19
Differences Between the Session and Entity Beans
  • Session Beans
  • Represent a business process
  • One instance per client
  • Short-lived Life of client is life of bean
  • Transient
  • Doesnt survive server crashes
  • May be transactional
  • Entity Beans
  • Represent business data
  • Shared instance for multiple clients
  • Long-lived as long as data in database
  • Persistent
  • Survive server crashes
  • Always transactional

Source Sang Shin, Enterprise JavaBeans
20
EJB Architecture
  • In this section we look at both EJB2 and EJB3
    architectures.
  • Complexity has been a major factor holding back
    EJB adaption.
  • EJB 3.0 is an attempt to simplify enterprise java
    development
  • EJB 2.0 component development and requirements
  • EJB 3.0 component development

21
Anatomy of an EJB2 Component
  • A EJB2 module contains
  • Interface classes (must be present)
  • Home interface
  • Remote interface
  • Bean class (must be present)
  • Deployment descriptor (must be present)
  • Helper classes (present only when needed by Bean
    class)

Source Sang Shin, Enterprise JavaBeans
22
Anatomy of EJB 2.0 Module (EJB-JAR file)
Source Sang Shin, Enterprise JavaBeans
23
An Example of ejb-jar File Layout in EJB 2.0
CatalogServer
CatalogServer.class
CatalogServerLocal.class
CatalogServerHome.class
CatalogServerLocalHome.class
CatalogServerBean.class
META-INF
ejb-jar.xml
24
Deploying an EJB2 Component
  • You need to know how to deploy the EJB to a
    server.
  • You need to know how to find and use the bean
    from a client.
  • EJB-JAR Deployment Descriptor
  • Defines contract between producer and consumer of
    ejb-jar file
  • It is an XML document that must be well formed
    in XML sense and must be valid with respect to
    the DTD given in EJB specification
  • stored with the name META-INF/ejb-jar.xml in the
    ejb-jar file
  • Captures two basic kinds of information
  • Structural and Application assembly information.

source Sang Shin, Enterprise JavaBeans,
http//www.jvapassion.com
25
EJB2 - Home Interface
  • EJB home interfaces are located by clients using
    JNDI.
  • An EJB server publishes the home interface for a
    particular EJB object under a particular name in
    JNDI namespace.
  • The EJB client connects to JNDI server and looks
    up the EJB home interface.

26
Fundamental Steps an EJB Client Performs (in EJB2
model)
  • Gets a JNDI context from the EJB/J2EE Server.
  • Uses this context to look up a home interface for
    the bean.
  • Uses the home interface to create (or find) a
    reference to an EJB.
  • Calls methods on the bean.

27
Example of EJB2 Client
  • import javax.ejb.
  • import javax.naming.
  • import java.rmi.
  • import java.util.Properties
  • public class PersonClient
  • public static void main(String args)
  • String name args0
  • try
  • // Get a JNDI context for our EJB server
    (EJBHome, in this case)
  • Properties p new Properties()
  • p.put(Context.INITIAL_CONTEXT_FACTORY,
    "com.ejbhome.naming.spi.rmi.RMIInitCtxFactory")
  • // Add URL, host or port options, if needed...
  • Context context new InitialContext(p)
  • // Get the home interface for Person beans
  • PersonHome pHome
    (PersonHome)context.lookup("People")
  • // Create a named person

Source Java Enterprise in a Nutshell by David
Flanagan et al
28
Example of EJB2 Client (contd)
  • Person person pHome.create(name)
  • // Use the remote stub interface to access the
    person's data . . .
  • catch (NoSuchPersonException nspe)
  • System.out.println("Invalid person " name)
  • catch (Exception e)
  • System.out.println("Error while creating/using
    person.")

29
EJB2 example Remote Client Interface
  • import javax.ejb.
  • import java.rmi.Remote
  • import java.rmi.RemoteException
  • public interface Person extends EJBObject
  • public String getName() throws RemoteExeption
  • public String setName() throws RemoteException

Source Java Enterprise in a Nutshell by David
Flanagan et al
30
The Enterprise JavaBeans Object - EJB2
  • To develop your own EJB object, you typically
    need to provide three java interface/classes in
    order to fully describe your EJB object
  • A bean interface (remote and/or local version)
  • A home interface (remote and/or local version)
  • An enterprise bean implementation

31
Example
  • A shopping cart EJB2 Component

The example is adapted from Bill Roth, An
Introduction to Enterprise JavaBeans Technology
32
EJB 2.0 example - EJBHome
  • public interface CartHome extends
    javax.ejb.EJBHome
  • Cart create(String customerName, String account)
  • throws RemoteException, BadAccountException

33
EJB 2.0 example - the remote interface for the
Bean
public interface ShoppingCart extends
javax.ejb.EJBObject boolean addItem(int
itemNumber) throws java.rmi.RemoteException
boolean purchase() throws java.rmi.RemoteExcept
ion
34
EJB 2.0 example - Implementation
public class ShoppingCartEJB implements
SessionBean Public boolean addItem(int
itemNumber) // the code for adding items to
the cart // may include JDBC code.
public boolean purchase () // the code for
purchases public ejbCreate(String
accountName, String account) // object
initialization code
35
EJB 2.0 example - Client-side, Locating the
EJBHome object
Context initialContext new InitialContext()
CartHome cartHome (CartHome)
initialContext.lookup( "applications/mall/shoppi
ng-carts")
36
EJB 2.0 example - Client-side, using the
EJBHome
ShoppingCartEJB cart cartHome.create("Emma",
"0507") cart.addItem(100) cart.addItem(251)
cart.purchase()
37
EJBs component model
  • Platform Independence
  • EJB technology is a part of java technology.
  • Middleware Independence
  • Components designed to the EJB specification run
    on all middleware solutions that implement the
    EJB specification.

38
EJB component model (contd)
  • Preservation of Investment
  • enterprise developers can use the benefits of
    developing EJB technology-based applications and
    also continue to use their existing middleware
    solutions.
  • Ease of Development and Deployment

39
EJBs component model (contd)
  • Reusability and Portability
  • EJB provides reusable and portable components
  • EJBs server-side component architecture provides
    an environment that hide the system level details
    necessary to provide transactions, distributed
    computing, persistence, and more from the eyes of
    programmers, and allowed the programmers to
    quickly build and deploy middleware-independent
    applications.

40
EJB 3.0
  • Simplified programming model
  • EJB 2.1 is too noisy
  • Inherit javax.ejb
  • Interface/implementation
  • Strange combination of checked/unchecked
    exceptions
  • Callback methods
  • JNDI lookups
  • Metadata XML hell
  • Verbosity
  • Lack of useful defaults
  • Cant see code and metadata at the same time

Entity Beans in EJB3, Gavin King, JBoss Inc
41
EJB 3.0 Goals
  • Simplify Development
  • EJB Plain Old Java Object (POJO)
  • Use metadata annotations
  • Reduce number of artifacts (Class, interface)
  • Attract broad range of developers
  • Standardize persistence API
  • O-R Mapping similar to Oracle TopLink, JBoss,
    Hibernate

EJB 3.0 and JSF Update, Sean Yeh, Oracle Taiwan
Corporation
42
EJB3 Simplification
  • POJO and POJI
  • Removes complexity of earlier versions using
    simple and familiar Java artifacts.
  • EJB Class will be a Plain Java class.
  • EJB Interface does not have to implement
    EJBObject.
  • No need for home interface.
  • Annotation for type of EJB and interface.

EJB 3.0 and JSF Update, Sean Yeh, Oracle Taiwan
Corporation
43
Session Beans in EJB3
  • Plain java object plain java interface
  • No home interface required
  • Callback methods are optional
  • Dependency injection
  • Metadata as annotations
  • Bean type, _at_Session, _at_Stateful, _at_MessageDriven
  • Transaction attributes, _at_Transaction
  • Dependency injection _at_Inject, _at_EJB, _at_Resource
  • Remoteness _at_Remote, _at_Local, _at_WebService
  • Remove method for stateful beans _at_Remove

Entity Beans in EJB3, Gavin King, JBoss Inc
44
Annotation in Java 5
  • Java 5 has introduced a program annotation
    facility.
  • You can define custom annotations and then
    annotate fields, methods, classes, etc.
  • Annotations do not directly affect program
    semantics.
  • Tools can inspect the annotations to generated
    additional structures (similar to a deployment
    descriptor) or enforce specific runtime behaviour.

45
Stateless Beans in EJB3
  • It is very easy to create a Stateless Bean with
    EJB 3.0.
  • All bean types do not implement any home
    interface in EJB 3.0.
  • The class will be tagged as _at_Stateless. This
    marks the class as a stateless bean and the
    deployer will deploy that class as a stateless
    bean EJB container.

46
EJB 3.0 Session
import javax.ejb. _at_Stateless _at_Remote public
class HelloWorldBean    public String
sayHello()       return "Hello World!!!"   
EJB 3.0 and JSF Update, Sean Yeh, Oracle Taiwan
Corporation
47
Another example Stateless Session Bean in EJB3
  • import javax.ejb.Stateless
  • _at_Stateless public class CalculatorBean implements
    CalculatorRemote, CalculatorLocal
  • public int add(int x, int y)
  • return x y
  • public int subtract(int x, int y)
  • return x - y

Source http//docs.jboss.org/ejb3/app-server/tuto
rial/stateless/stateless.html
48
Stateless Session Bean in EJB3 -
CalculatorRemote.java
  • import javax.ejb.Remote
  • _at_Remote public interface CalculatorRemote extends
    Calculator

Source http//docs.jboss.org/ejb3/app-server/tuto
rial/stateless/stateless.html
49
Stateless Session Bean in EJB3-
CalculatorLocal.java
  • import javax.ejb.Local
  • _at_Local public interface CalculatorLocal extends
    Calculator

Source http//docs.jboss.org/ejb3/app-server/tuto
rial/stateless/stateless.html
50
Stateless Session Bean in EJB3 - the Client
  • import javax.naming.InitialContext
  • public class Client
  • public static void main(String args) throws
    Exception
  • InitialContext ctx new InitialContext()
  • Calculator calculator (Calculator)
    ctx.lookup("CalculatorBean/remote")
  • System.out.println("1 1 "
    calculator.add(1, 1))
  • System.out.println("1 - 1 "
    calculator.subtract(1, 1))

Source http//docs.jboss.org/ejb3/app-server/tuto
rial/stateless/stateless.html
51
Entity Beans in EJB3
  • Plain Java Objects
  • No required interfaces/inheritance
  • All public methods nonfinal, all instance
    variables private
  • Non-abstract persistent properties
  • Public/protected get/set pair
  • Or private instance variable
  • Public/protected constructor with no arguments
  • Identifier Version properties
  • No home interface required
  • Support for inheritance / polymorphism!

Entity Beans in EJB3, Gavin King, JBoss Inc
52
Declaring an Entity Bean
  • _at_Entity public class Flight implements
    Serializable
  • Long id
  • _at_Id public Long getId()
  • return id
  • public void setId(Long id)
  • this.id id

53
Do I Need an EJB Tier?
  • Yes, if you want to leverage middleware features
    provided by container
  • Resource management, instance life-cycle
    management,
  • concurrency control and threading
  • Persistence, transaction and security management
  • Messaging, scalability, availability
  • Yes, if you want to build portable and reusable
    business components
  • Maybe not, for a simple application whose main
    function is reading database tables.
  • Maybe not, if your application does not require
    components to be distributed amongst different
    servers.

Sang Shin, Enterprise JavaBeans, Sun
54
Resources
  • Sang Shin, EJB Overview, Sun Microsystems,
    Inc. http//www.javapassion.com/
  • Bill Roth, An Introduction to Enterprise
    JavaBeans Technology.
  • Monica Pawlan, Writing Enterprise Applications
    with Java 2 SDK, Enterprise Edition, Sun
    Microsystems.
  • Java Enterprise in a Nutshell, Farley et al,
    Chapter 8, OReilly.
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