Enterprise JavaBeans (EJB)

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Enterprise JavaBeans (EJB)

• Payam Barnaghi

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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.

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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.

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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.

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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,

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EJB Container
EJB Container
EJB Client
Transaction management Persistence
Management Security Management Resource Pooling
EJB Context, JNDI
Bean
Business Methods
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Where is EJB in a Big Picture of J2EE?
Source Sang Shin, Enterprise JavaBeans TM (EJB
TM Architecture Overview) Technology and XML
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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.

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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)

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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.

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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/
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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

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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
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Anatomy of EJB 2.0 Module(EJB-JAR file)
Source Sang Shin, Enterprise JavaBeans
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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
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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
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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.

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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.

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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
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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.")

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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
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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

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Example

• A shopping cart EJB2 Component

The example is adapted from Bill Roth, An
Introduction to Enterprise JavaBeans Technology
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EJB 2.0 example - EJBHome

• public interface CartHome extends
  javax.ejb.EJBHome
• Cart create(String customerName, String account)
• throws RemoteException, BadAccountException

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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
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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
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EJB 2.0 example - Client-side, Locating the
EJBHome object
Context initialContext new InitialContext()
CartHome cartHome (CartHome)
initialContext.lookup( "applications/mall/shoppi
ng-carts")
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EJB 2.0 example - Client-side, using the
EJBHome
ShoppingCartEJB cart cartHome.create("Emma",
"0507") cart.addItem(100) cart.addItem(251)
cart.purchase()
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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.

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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

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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.

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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
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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
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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
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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.

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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.

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EJB 3.0 Session
import javax.ejb._at_Stateless_at_Remotepublic
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
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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
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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
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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

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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.