Overview of Enterprise JavaBeans EJB

1
Overview of Enterprise JavaBeans (EJB)

• Source
• Enterprise JavaBeans, Richard Monson-Haefel

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Applications to Date

• Web Applications (Servlets/JSPs)
• Provided user interface
• access to database
• simple business logic
• no transactions except at the database level no
  distributed transactions across business logic
• Distributed Applications (RMI)
• Distributed access to business logic
• access to, but no direct support for database
• limited transactions
• We have to write a lot of plumbing code

3
What didnt we address ?

• Could someone else deploy your application?
• Component reusability
• Security
• Threading
• Resource Management
• Load-Balancing
• Fault Tolerance (a little with activation)

4
Paradigm Shift

• We need to start thinking of applications as a
  set of reusable components and some business
  logic that ties the components together
• Deployed in a re-usable environment that handles
  middleware and deployment requirements

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Components Vs. Objects

• Usually larger-grained
• Provide a complete capability
• Credit Card Verification
• Can be customized for deployment

6
Component Architectures

• Client-Side
• Applets
• Java Beans (development components)
• ActiveX
• Server Side
• Servlets
• Enterprise Java Beans (EJB)
• CORBA Components
• Microsoft Distributed interNet Applications
  Architecture (DNA)

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Distributed Objects

• Makes business objects more accessible
• permits more of an 3-tier architecture
• user interface at the first tier (Web
  Applications)
• business logic at the second tier (Business
  Applications)
• enterprise resources at the third tier
  (Databases)
• Key technologies include RMI, CORBA, and DCOM
• Provide for communication, but limited in
  server-side component support results in roll
  your own component models

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Server-Side Components

• Architecture for developing distributed business
  objects
• Separates the development from the assembly into
  specific applications
• allows for the sale of smaller/reusable
  components rather than end-to-end systems
• assembler determines actual transactional,
  security, persistence behavior, etc. for the
  component.

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Component Transaction Monitors (CTMs)

• Sophisticated distributed object Application
  Servers
• usually made up of web servers, ORBs, Messaging,
  Databases, Naming, etc.
• Hybrid of TP Monitors (e.g., CICS and Tuxedo)and
  ORBs (e.g., CORBA and RMI)
• Provide infrastructure for managing transactions,
  object distribution, concurrency, security,
  persistence, and resource management
• the developer isnt left rolling their own
• the developer complies with the model and
  basically implements a lot of callback event and
  declarative programming
• Analogy CD-player CTM, CD Server-side
  Component

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Enterprise JavaBeans (EJBs)

• Standard server-side component model for Java
  Enterprise Applications
• security
• resource pooling
• persistence
• concurrency
• transactional integrity
• Has nothing to do with JavaBeans
• JavaBeans designed for intra-process purposes
• GUIs
• non-visual widgets
• Enterprise Java Beans (EJB) designed for
  inter-process purposes

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EJBs (cont.)

• Java
• platform independence
• write once, run anywhere
• EJB components
• platform/implementation independence
• write once, run in any Application Server
  complying with the EJB spec
• J2EE reference implementation
• IBMs Websphere
• BEAs Weblogic Server and Weblogic Enterprise
• Gemstone/J
• ...

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EJB Architecture

• Interfaces and Classes
• beans
• entity bean
• session session
• Stateless session bean
• Statefull session bean
• primary key
• home interface
• remote interface (EJB Object)
• Container
• Application Server

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EJB Conceptual Class Overview
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Bean Class Instance Lifecycle
No State
1 newInstance() 2 setEJBContext()
Pooled State
3 bean associated to an EJB Object
5 bean disassociated from EJB Object
Ready State
4 service client requests
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Bean Types

• Entity beans
• models persistent state - this state is
  maintained through all method and server
  invocations
• nouns of the domain
• real world objects (e.g. Owner, Account,
  Transaction)
• Session beans
• models non-persistent state - this state will be
  lost between method invocations (stateless
  session) or server invocations (statefull
  session)
• manage tasks performed on behalf of a single
  client (e.g. Teller, Monthly Statement)
• contains the business processes in which to use
  entity beans
• manages actions that may cross entity beans or go
  outside the concern of an entity bean
• e.g. Teller may authenticate the user and
  transfer funds between accounts
• e.g. Statement may include transactions from
  multiple accounts

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Bean Usages

• Entity beans
• model state maintained across all client
  interactions
• represent a row of data in a database
• Session beans
• model business process being performed by a
  single client involving one or more entity beans
• it extends the actions of the client into the
  server
• simplifies the actions programmed by the client
• limits the number of distributed calls required
  between the client and the entity beans
• limits the number of stubs that have to be loaded
  by the client
• are not persisted to a database

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Stateful and Stateless session beans

• Stateful session bean
• maintain the conversational state between a
  client and the session bean
• may be serialized out and passivated to conserve
  system resources
• will be serialized in and activated when needed
  in the future
• e.g. Teller session bean who is logged into an
  transfers funds between accounts
• Stateless session bean
• do not maintain conversational state
• each method is independent of another and the
  only information needed is is supplied in the
  call parameters
• e.g. Statement that is given a list of accounts
  or an owner to generate a textual report for
• consumes the least amount of resources among all
  the bean types

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EJB Classes and Interfaces
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Remote Interface

• Called the EJB Object
• Defines the business methods that will be
  available to clients in a distributed call
• e.g. Account.debit(transactionRec)
• e.g. Teller.transfer(sourceAccount,
  targetAccount)
• Gets compiled by the ejb compiler to creates RMI
  stubs and skeletons
• clients are used by RMI to translate a method
  invocation to wire format
• skeletons are used by RMI to translate wire
  format to a method invocation

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Remote Interface (cont.)

• Defined as a Java interface
• extends javax.ejb.EJBObject
• extends java.rmi.Remote
• getEJBHome() - returns the Home object for the
  bean
• getPrimaryKey() - returns the primary key for the
  object
• getHandle() - returns a handle to object that may
  be used to re-establish communications at a later
  time possibly in another server
• remove() - removes this EJBObject object (prior
  to eviction)
• isIdentical(EJBObject) - returns if both objects
  are same

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Remote Interface ExampleCabin

• package com.titan.cabin
• import java.rmi.RemoteException
• public interface Cabin extends javax.ejb.EJBObject
  
• public String getName() throws
  RemoteException
• public void setName(String str) throws
  RemoteException
• public int getDeckLevel() throws
  RemoteException
• public void setDeckLevel(int level)
  throws RemoteException
• public int getShip() throws RemoteException
• public void setShip(int sp) throws
  RemoteException
• public int getBedCount() throws
  RemoteException
• public void setBedCount(int bc) throws
  RemoteException

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Home Interface

• Defines the lifecycle methods that will be
  available to clients in a distributed call
• create new bean objects, locate or remove
  existing bean objects
• e.g. AccountHome.create(owner, initialBalance)
• e.g. AccountHome.findByPrimaryKey(accountId)
• e.g. TellerHome.create(login, pin)
• e.g. MonthlyStatementHome.create()
• Gets compiled by the ejb compiler to creates RMI
  stubs and skeletons

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Home Interface (cont.)

• Defined as a Java interface
• extends javax.ejb.EJBHome
• extends java.rmi.Remote
• getEJBMetaData() - returns metadata about the
  bean
• remove(primaryKey) - removes object identified by
  primary key
• remove(handle) - removes object identified by its
  EJBHandle

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Home Interface ExampleCabinHome

• package com.titan.cabin
• import java.rmi.RemoteException
• import javax.ejb.CreateException
• import javax.ejb.FinderException
• public interface CabinHome extends
  javax.ejb.EJBHome
• public Cabin create(int id)
• throws CreateException, RemoteException
• public Cabin findByPrimaryKey(CabinPK pk)
• throws FinderException, RemoteException

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Bean Class

• Implements the business methods defined in the
  Object interface
• does not inherit from the Object or Home
  interfaces
• must have methods that match signatures supplied
  in all of the Object interface and portions of
  the Home interface
• Account.deposit(transactionRec) -
  AccountBean.deposit(transarctionRec)
• AccountHome.create(owner) - AccountBean.ejbCreate(
  owner)
• the connection between the Object/Home interface
  calls and the Bean implementation is done by the
  EJB compiler that creates the container-specific
  skeletal code
• uses the Deployment Descriptor to help generate
  glue code
• similar in functionality to a C template or a
  C CORBA TIE class

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Bean Class (cont.)

• Clients never interact with bean classes
  themselves
• always interact with the bean through home
  (create, find, remove) and remote interfaces
  (business methods)
• beans that interact with other beans are simply
  clients of the other bean
• stubs and skeletons are created from the
  interfaces that glue the client, the database,
  and the bean code together

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Bean Class (cont.)

• implements javax.ejb.EntitySessionBean
• extends empty javax.ejb.EnterpriseBean extends
  java.io.Serializable
• set/unsetEntity/SessionContext()
• provides callback into container to find caller
  id, transaction information, etc.
• ejbCreate()
• container calls this when the bean gets
  associated with an object
• ejbPostCreate() Entity Beans Only
• container calls this after persisting the
  objects state

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Bean Class (cont.)

• ejbPassivate()
• container calls this method when the instance is
  being dis-associated with a particular EJB Object
  (pre serialization)
• ejbActivate()
• container calls this method when the instance is
  taken fro a pool of available instances and
  associated with a particular EJB Object (post
  de-serialization)
• ejbRemove()
• container calls this method of an instance before
  removing an EJB Object from the system

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Bean Class (cont.)

• ejbLoad() Entity Beans Only
• container calls this method to instruct the
  instance to synchronize its state with the state
  stored in the database
• command (bean managed), completion event
  (container managed)
• ejbStore() Entity Beans Only
• container calls this method to instruct the
  instance to synchronize the state store in the
  database with its state
• command (bean managed), completion event
  (container managed)

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Bean Class ExampleCabinBean

• package com.titan.cabin
• import javax.ejb.EntityContext
• public class CabinBean implements
  javax.ejb.EntityBean
• public int id
• public String name
• public int deckLevel
• public int ship
• public int bedCount

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Bean Class ExampleCabinBean (cont.)

• public void ejbCreate(int id)
• this.id id
• public void ejbPostCreate(int id)
• // Do nothing. Required.
• public String getName()
• return name
• public void setName(String str)
• name str
• public int getShip()
• return ship

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Bean Class ExampleCabinBean (cont.)

• public void setShip(int sp)
• ship sp
• public int getBedCount()
• return bedCount
• public void setBedCount(int bc)
• bedCount bc
• public int getDeckLevel()
• return deckLevel

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Bean Class ExampleCabinBean (cont.)

• public void setDeckLevel(int level )
• deckLevel level
• public void setEntityContext(EntityContext
  ctx)
• // Not implemented.
• public void unsetEntityContext()
• // Not implemented.
• public void ejbActivate()
• // Not implemented.
• public void ejbPassivate()
• // Not implemented.

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Bean Class ExampleCabinBean (cont.)

• public void ejbLoad()
• // Not implemented.
• public void ejbStore()
• // Not implemented.
• public void ejbRemove()
• // Not implemented.

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Primary Key Class

• Value that uniquely identifies the object in the
  database
• Implements java.io.Serializable
• Implemenets
• hashCode()
• equals()
• toString() helpful
• Contains identical public attributes from the
  Bean Class for those that represent the beans
  primary key value(s).

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Primary Key Class ExampleCabinPK

• package com.titan.cabin
• public class CabinPK implements
  java.io.Serializable
• public int id
• public int hashCode( )
• return id
• public boolean equals(Object obj)
• if(obj instanceof CabinPK)
• return (id ((CabinPK)obj).id)
• return false
• public String toString()
• return String.valueOf(id)

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Containers

• Manages interaction between the bean and its
  server
• Provides a uniform interface to the bean and to
  the server

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Containers (cont.)

• Creates new instances of beans and manages their
  persistence
• provides mapping between bean and containers
  underlying database
• provides the skeletal class code for the home and
  remote interfaces
• e.g. AccountHome.findByPrimaryKey(accountPK)
• container supplies code to create row in
  database, instantiate an EJB Object to represent
  that instance of an account
• e.g. AccountHome.remove(accountPK)
• container supplies code to remove row in database
  and remove any existing EJB Objects
• e.g. Account.debit(transactionRec)
• container supplies code obtain the EJB Object
  from storage, locate a bean to take on its state,
  and invoke behavior on the bean

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Deployment Descriptor

• Instructs the server on the type of bean (session
  or entity)
• Instructs the server how to apply services to the
  bean
• Described in XML
• Created from IDE or text source
• Supplied with other bean components in a jar
  (Java archive) file
• bean class
• remote interface
• home interface
• primary key (for entity beans)
• deployment descriptor
• rmi stub/skeleton

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Deployment Descriptor Exampleejb-jar.xml (Cabin)

• Inc.//DTD Enterprise JavaBeans 1.1//EN"
  "http//java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd"
• This Cabin enterprise bean
  entity represents a cabin on a cruise
  ship.
• CabinBean
• com.titan.cabin.CabinHome
• com.titan.cabin.Cabin
• com.titan.cabin.CabinBean-class

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Deployment Descriptor Example (cont.)

• Containertype
• com.titan.cabin.CabinPKprim-key-class
• False
• id
  
• - name
  
• - deckLevelname
• - ship
  
• - bedCountame
• ...

42
EJB Object class

• Written by the container-specific compiler after
  processing the deployment descriptor
• Implements methods definedin remote
  interfaceby delegating them toan instance of
  the beanclass

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EJB home class

• Written by the container-specific compiler after
  processing the deployment descriptor
• Implements methods defined in home interface
• locate
• create
• remove
• Handles interactions with resources pools,
  persistence mechanism, and resource managers

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Example Client

• Context jndiContext getInitialContext()
• Object obj jndiContext.lookup("ejb/CabinHome")
• System.out.println("found it! " obj)
• CabinHome home (CabinHome)javax.rmi.PortableRemo
  teObject.narrow(obj, CabinHome.class)
• System.out.println("narrowed it! " home)
• Cabin cabin_1 home.create(1)
• System.out.println("created it! " cabin_1)
• cabin_1.setName("Master Suite")
• cabin_1.setDeckLevel(1)
• cabin_1.setShip(1)
• cabin_1.setBedCount(3)

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Example Client

• CabinPK pk new CabinPK()
• pk.id 1
• System.out.println("keyed it! " pk)
• Cabin cabin_2 home.findByPrimaryKey(pk)
• System.out.println("found by key! " cabin_2)
• System.out.println(cabin_2.getName())
• System.out.println(cabin_2.getDeckLevel())
• System.out.println(cabin_2.getShip())
• System.out.println(cabin_2.getBedCount())

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Usage Scenarios
47
Creating an Account Object
AccountBean
AccountBean
3 select instance 8 return instance
AccountBean
5 insert
bean pool
4 ejbCreate(10.00) 6 ejbPostCreate(10.00) 7
ejbPassivate()
1 createAccount(10.00)
AccountHome_EJBHome
remote interface
2 new Account_EJBObject
Client
Account_EJBObject
AccountBean
48
Accessing an Account Object
AccountBean
AccountBean
3 select instance 10 return instance
AccountBean
2 select 8 update
bean pool
4 populate state 5 ejbLoad() 6 debit(10.00) 7
ejbStore()
Client
Account_EJBObject
AccountBean
1 debit(10.00)
49
Deleting an Account Object
AccountBean
AccountBean
3 select instance 10 return instance
AccountBean
2 select 7 delete
bean pool
4 populate state 5 ejbLoad() 6 ejbRemove()
1 remove(pKey)
AccountHome_EJBHome
8 remove()
Client
Account_EJBObject
AccountBean
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Questions to Ponder...

• Entity bean, stateful session bean, stateless
  session bean?
• object which requires access by multiple clients
  over its lifetime
• object whose work accepts and returns all
  information in the call
• object that must exist after server crash
• object that must exist between method invocations
• object that encapsulates a set of client actions
  to multiple beans
• Why is there not a ratio of 11 between instanced
  of beans (EJB Objects) and instances of the bean
  class?
• What GoF Pattern does this depict?