CISH6960: Developing Enterprise Applications

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CISH-6960 Developing Enterprise Applications

• Lecture 7
• July 10, 2004

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Enterprise JavaBeans

• Fundamentals and Session Beans

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Introduction

• Web-based Java applications
• Many companies currently use
• Java classes
• Model
• JavaServer Pages
• View
• Java servlets
• Controller
• JDBC to talk to databases

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J2EE Services

• This is often good enough, but what about
• Security
• Persistence
• Lifecycle management
• Transactions
• Distributed programs
• Remote access
• Concurrency
• These are collectively known as services

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Container Managed Services

• Some services
• Concurrency
• Thousands of connections per hour, accessing
  limited resources
• Handles threading and resource issues
• Security
• XML files declare authentication and
  authorization info
• Transactions
• XML files declare which methods must be in
  transactions
• Method-by-method control available

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Container Managed Services

• Other services
• Location transparency
• Objects accessed through a searchable registry
• Object locations can move without affecting the
  client
• Distributed systems
• Server operates across systems using RMI
• Persistence
• Container can provide object/relational mappings
  and update them as necessary
• Can even ensure referential integrity
• Can be controversial, but is getting better

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Container Managed Services

• How do we make all this happen?
• Create components, instead of classes
• Install components inside application servers
• Configure components, servers, and services using
  deployment descriptors
• Its no longer simple to do simple things
• Its relative easy to do complicated things
• If you dont want or need these services, you
  dont need EJBs

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Containers
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The EJB Architecture

• Existing technologies predating EJBs
• Transaction managers
• Distribution services
• Persistence mechanisms
• Common Object Request Broker Architecture (CORBA)
• Specifications governed by the Object Management
  Group (OMG)
• EJBs borrow heavily from this technology, along
  with RMI

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The EJB Architecture

• CORBA services incorporated into EJBs
• Naming
• Lifecycle management
• Security
• Persistence
• Distribution structures
• CORBA is language neutral, using interfaces
  written in Interface Definition Language (IDL)
• CORBA uses Object Request Brokers (ORBs) for
  networking and data marshalling
• Data marshalling between languages could be
  difficult

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The EJB Architecture

• Java has Remote Method Invocations (RMI)
• Relatively easy to work with
• Requires Java on both ends
• Supports communication with the JVM
• Does not offer many enterprise services
• EJB architecture combines features of CORBA and
  RMI
• Tries to eliminate any weaknesses they may have
• Loses language independence
• Maintains the services
• Data marshalling handled using RMI and Java

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The EJB Architecture

• Developers must provide a way to access objects
  in a remote, managed environment
• The EJBs must be written in a manner the
  environment understands
• EJB container provides services to the beans
• Must be a well-defined relationship between the
  EJB component and the EJB container
• Developers take advantage of container services

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The EJB Architecture

• Object interface
• Provides client access to the bean
• Method calls execute business logic on the bean
• Specific to a given EJB
• Ensures the container is involved in every
  request sent to the EJB
• Home interface
• Provides lifecycle methods for the EJB
• Can create, remove, and/or find beans
• The home and object interfaces are client views
  on the EJB

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The EJB Architecture

• Each of the interfaces can have two types
• Remote
• Used for accessing beans outside the JVM managing
  the bean class
• Restricts access to pass by value, and parameters
  must be Serializable
• Local
• Used by clients that share a JVM with the bean
• Do not involve networking
• Use pass by reference semantics
• Allow association with other EJBs

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Architecture
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Enterprise JavaBeans

• Session beans
• Stateless
• Stateful
• Entity beans
• CMP
• BMP
• Message-driven beans (EJB 2.0)

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EJBs
Enterprise JavaBeans
Message Driven Beans
Session Beans
Entity Beans
Stateless
Stateful
Container Managed
Bean Managed
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EJBs

• Session beans represent processes
• Travel Agent
• Credit card validator
• Entity beans represent business objects
• Ship
• Cruise line
• Cabin
• Message-driven beans are consumers in a
  message-driven application

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EJBs

• Session beans
• Perform specific tasks
• Are not persistent
• Stateful session beans use lightweight
  persistence, i.e., they are saved to secondary
  storage
• Stateless session beans are pooled by the
  container

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EJBs

• Entity beans
• Represent data in a database
• Are shared among clients
• Creates, stores, and refreshes all handled by the
  container
• Represent business objects

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EJBs

• Entity beans
• Container managed
• Container generates all the required SQL
• Developer writes abstract class with only
  abstract get and set methods
• Container infers what attributes are required
• Bean managed
• Developer provides SQL calls as callback methods
• In either case
• The container calls load() and store()

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EJB Components
Home Interface
EJB Container
EJB Home Object
lifecycle methods
Bean Class
Client
EJB Object
callback methods
Object Interface
objects createdby the container
business methods
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EJB Components

• All EJBs require
• A Home interface
• Contains lifecycle methods, like create() or
  find()
• Accessible to client through JNDI
• A Remote interface
• Contains business logic
• Acquired via the home interface
• Client uses an RMI stub to access
• A bean class
• Contains callback methods
• A deployment descriptor
• An EJB Container in which to run

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Stateless Session Bean

• Home interface
• Required create() method

package beans import java.rmi.RemoteException i
mport javax.ejb.EJBHome import
javax.ejb.CreateException public interface
SimpleSessionHome extends EJBHome // The
create method for the SimpleSession bean public
SimpleSession create() throws CreateException,
RemoteException
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Stateless Session Bean

• Remote interface
• Business logic

package beans import java.rmi.RemoteException i
mport javax.ejb.EJBObject public interface
SimpleSession extends EJBObject // The public
business method on the SimpleSession bean public
String getEchoString(String clientString) throws
RemoteException
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Stateless Session Bean

• Bean class

package beans import javax.ejb.SessionBean impo
rt javax.ejb.SessionContext public class
SimpleSessionBean implements SessionBean
public String getEchoString(String
clientString) return clientString " - from
session bean" public void ejbActivate()
public void ejbPassivate() public void
ejbRemove() public void ejbCreate()
public void setSessionContext(SessionContext
context)
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Stateless Session Bean
lt?xml version"1.0" encoding"UTF-8"?gt lt!DOCTYPE
ejb-jar PUBLIC '-//Sun Microsystems, Inc.//DTD
Enterprise JavaBeans 2.0//EN' 'http//java.sun.com
/dtd/ejb-jar_2_0.dtd'gt ltejb-jargt
ltdisplay-namegtSimpleSessionlt/display-namegt
ltenterprise-beansgt ltsessiongt
ltdisplay-namegtSimpleSessionEjblt/display-namegt
ltejb-namegtSimpleSessionEjblt/ejb-namegt
lthomegtSimpleSessionHomelt/homegt
ltremotegtSimpleSessionlt/remotegt
ltejb-classgtSimpleSessionBeanlt/ejb-classgt
ltsession-typegtStatelesslt/session-typegt
lttransaction-typegtBeanlt/transaction-typegt
ltsecurity-identitygt ltdescriptiongtlt/descrip
tiongt ltuse-caller-identitygtlt/use-caller-id
entitygt lt/security-identitygt
lt/sessiongt lt/enterprise-beansgtlt/ejb-jargt
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Session Beans

• Components that provide a transactional,
  distributed, managed resource without persistence
• Session beans may have state, but the values will
  not be persisted by the container
• Session beans are frequently task processors or
  flow controllers
• Session beans represent the ability to perform a
  task
• Drive-up teller at a bank
• Travel agent
• Session beans use other, persistent objects to
  get the job done, but are not persistent
  themselves

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The Role of Session Beans

• Usually serves as a gateway, or process handler
• Encapsulates actions with the business logic
• Serves as transaction boundaries
• Groups operations

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Stateless Session Bean

• Everything needs to be installed
• jar file to contain classes
• war file to contain JSPs, servlets,
• Presentation logic
• Often use servlet test clients
• ear file to contain everything else
• Add XML deployment descriptors
• Usually contains app server specific info as well

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EJBs

• Enterprise JavaBeans require an act of faith on
  the part of the developer
• Developer provides interfaces
• Developer provides some callback methods
• Developer trusts that the container will
  automatically create the actual bean classes and
  call everything at the right time

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Stateless Session Beans

• No state
• Beans may have attributes
• Values are not assumed to be preserved between
  calls
• Beans are pooled

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Stateless Session Bean
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Stateless Session Bean
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Stateless Session Beans

• Most common and most simple type
• Managed by the container as if they have no state
• Class can declare variables, but container
  assumes that all stateless session beans are
  identical
• Allows instance pooling
• Any instance can be used to satisfy a request

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Stateful Session Beans

• Additional callback methods
• ejbActivate()
• Called by the container when restoring a session
  bean from temporary storage
• ejbPassivate()
• Called by the container when saving a session
  bean to temporary storage

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Stateful Session Beans
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Stateful Session Beans
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Stateful Session Beans

• State is assumed to be client specific, even
  though it isnt persisted
• Maintains a conversation with the client
• Each client has its own instance
• Cannot be pooled
• Application server can manage the lifecycle to
  help efficiency

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Session Bean Lifecycle

• Does Not Exist
• Bean unavailable for service
• The ejbCreate() method instantiates it, ties it
  to the container, and initializes it
• Pooled
• Available for service
• For stateless session beans, all method calls are
  matched to available instances
• Home interface has a create() method
• Container may use available instance rather than
  instantiate a new one

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Session Bean Lifecycle

• For stateful session beans
• Instances will be passivated and activated as
  necessary
• Essentially switches clients state in and out of
  instances on demand
• Instances are serialized to a disk location
• Known a lightweight persistence
• Object references are unique and can be saved by
  the client

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SessionContext

• Context classes are used by the beans to
  communicate with the container
• SessionContext
• EntityContext
• MessageDrivenContext

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SessionBean Interface
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Session Beans

• Developer must create a session bean that
  implements the javax.ejb.SessionBean interface
• Defines most of the lifecycle methods
• Provided methods are called by the container
• Known as callback methods, can do additional work
  to the standard lifecycle management provided by
  the container
• Note absence of a creation method
• Stateless session beans are pooled and identical
• Stateful session beans define their own create
  methods

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Creating Session Beans

• All lifecycle methods begin with the prefix ejb
• The promoted methods will automatically remove it
• Stateless session beans have a single create
  method
• public void ejbCreate() throws javax.ejb.CreateExc
  eption
• Promoting this to the home interface generates
  the appropriate create() method there
• Stateful session beans may have multiple creation
  methods, using parameters to set attributes
• public void createAtValue( String value ) throws
  CreateException

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Proxy Pattern
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The Proxy Design Pattern

• The Proxy design pattern described in the
  so-called Gang of Four book
• Gamma, Helm, Johnson, and Vlissides, Design
  Patterns Elements of Reusable, Object Oriented
  Software, 1995
• EJB implementation of Proxy pattern
• Two classes implement the same interface
• Proxy class does networking to reach Subject
  class, along with marshalling arguments and
  unmarshalling return values
• Subject class does actual behavior

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The Proxy Design Pattern
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The Proxy Design Pattern

• Developers create interfaces
• Container generates Proxy/Subject pairs
• Subject has a reference to developers
  implementation class
• Proxy instances retrieved via JNDI registry
• Enables the container to intercept all EJB method
  calls and apply required services

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Issues

• jar and war files are platform and server
  independent, ear files are not
• Assembling an ear file is usually done through a
  wizard
• Creating the deployment descriptor is usually
  done through a wizard
• Deployment descriptor settings affect application
  server
• Declarative programming
• Nothing needs to be recompiled