EJB Fundamentals

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EJB Fundamentals
Celsina Bignoli bignolic_at_smccd.net
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Enterprise Bean

• Server-side software component that can be
  deployed in a distributed multi-tier environment
• Must conform to the EJB specification
• Must expose specific methods
• Methods are used by the EJB container
  (Application Server) to manage the bean

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Types of Beans

• Session Beans
• Model business processes
• Ex pricing engine, catalog engine, credit card
  authorizer
• Entity Beans
• Model business data
• Ex product, order, employee
• Message-driven Beans
• Similar to session beans
• Can only be called implicitly via messages
• Ex beans that receive credit card authorization
  messages

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Distributed Objects

• EJB components are based on distributed objects
• A distributed object is an object that is
  callable from a remote system
• In-process client
• Out-process client
• Client located elsewhere on the network

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Distributed Objects
Client
Implementation Object
Remote Interface
Remote Interface
Stub
Skeleton
Network
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Distributed Objects(2)

• Stub client-side proxy object
• Masks network communications from the client
• Knows how to call over the network using sockets
• Knows how to convert parameters into their
  network representation
• Skeleton server-side proxy object
• Masks network communication from the distributed
  object
• Understands how to receive calls on a socket
• Knows how to convert parameters from their
  network representation to their Java
  representation

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Distribution Transparency

• The stub, skeleton and implementation object
  together constitute the abstraction known as
  distributed object
• The client has the impression it is calling the
  object directly instead, it calls an empty stub
  which knows how to go over the network
• The stub clones the implementation object
  signatures (both implement same interface)
• Can be achieved using several technologies
  (CORBA, DCOM, RMI-IIOP)

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Middleware

• Distributed objects allow to distribute an
  application across a network.
• They require middleware services
• Explicit Middleware
• Difficult to write
• Requires unnecessary large amounts of code
• Difficult to maintain
• Need to write new code to accomplish changes
• Difficult to support
• Code is not distributed to clients
• Clients cannot make changes

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Middleware(2)

• Implicit Middleware
• Easy to write
• No middleware code
• Declare things in a text file (descriptor file)
• Easy to maintain
• Separation between business logic and middleware
• Changes to middleware do not require changes to
  code
• Easy to support
• Changes can be accomplished by tweaking
  descriptor file

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Enterprise Bean Class

• Contains implementation details of the component
  (i.e. the logic)
• It is a Java class that conforms to a
  well-defined interface and obeys certain rules
• All beans implement the javax.ejb.EnterpriseBean

java.io.Serializable
javax.ejb.EnterpriseBean
javax.ejb.SessionBean
javax.ejb.MessageDrivenBean
javax.ejb.EntityBean
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The EJB Object

• Surrogate object that knows about networking,
  transactions, security etc
• It is request interceptor, or glue between the
  client and the bean
• Replicates and exposes every method that the bean
  itself exposes
• Delegates client requests to beans
• Each container generates automatically a vendor
  specific EJB class object

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The Remote Interface

• Special interface created by the Bean Provider
• Must comply with special rules defined by the EJB
  specification
• Must derive from javax.ejb.EJBObject
• Must duplicate the beans business methods
• Client code does not call the bean object
  directly but the EJBObject methods.
• When the client invokes a business object method
  the EJB object delegates the method to the bean,
  where the actual implementation of the method
  resides

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The Remote Interface

• public interface javax.ejb.EJBObject extends
  java.rmi.Remote
• public javax.ejb.EJBHome getEJBHome()
• throws java.rmi.RemoteException
• public java.jang.Object getPrimaryKey()
• throws java.rmi.RemoteException
• public void remove()
• throws java.rmi.RemoteException,
• javax.ejb.RemoveException
• public javax.ejb.Handle getHandle()
• throws java.rmi.RemoteException
• public boolean isIdentical(javax.ejb.EJBObject)
• throws java.rmi.RemoteException

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The Home Object

• Create EJB objects
• Clients cannot instantiate EJB objects directly
  because EJB objects might reside on a different
  machine
• Finds existing EJB objects
• Remove EJB objects
• Home objects are specific to the container
  therefore they are automatically generated by the
  container

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The Home Interface

• Define methods for creating, finding and
  destroying EJB objects
• The containers home Object implement the
  (user-defined) Home Interface
• javax.ejb.EJBHome define required methods all
  Home Interfaces must support

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Home Interface

• public interface javax.ejb.EJBHome extends
  java.rmi.Remote
• public EJBMetaData getEJBMetaData()
• throws java.rmi.RemoteException
• public javax.ejb.HomeHandle getHomeHandle()
• throws java.rmi.RemoteException
• public void remove(javax.ejb.Handle handle)
• throws java.rmi.RemoteException,
• javax.ejb.RemoveException
• public void remove(Object primaryKey)
• throws java.rmi.RemoteException,
• javax.ejb.RemoveException

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The Local Interfaces

• Used to improve performance.
• Similar to the remote interfaces and objects
  except they are local, therefore eliminate some
  tasks
• only work when calling beans in the same process.

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Deployment Descriptor File

• key to implicit middleware
• used by the container to determine what services
  to provide for the bean
• Used to specify bean requirements
• bean management and lifecycle
• persistence
• transaction
• security

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Ejb-jar File

• compressed file that follows the .zip compression
  format

Jar File Creator
EJB Jar File