Enterprise Java Beans

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Enterprise Java Beans

• Session Beans and Resource Management

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Topics

• Container and Enterprise Bean review
• Resource Management
• Session Beans
• Stateless
• Stateful
• Resources
• References

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Enterprise Bean

• Component that can be deployed in any
  EJB-compliant application server
• Remote interface for its operations
• extends EJBObject
• Home Interface for create,find, and remove
• Bean Implementation
• implement SessionBean or EntityBean
• Required container lifecycle callbacks
• implement business methods

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Bean Types

• Session Beans
• Represents business logic operations
• Teller and transfer()
• Stateless and Stateful
• Exist for lifetime of client session
• Entity Beans
• Represents persistent data in a database
• Exist indefinitely
• Bean-managed and Container-managed persistence

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Containers and Beans

• Provides implementation of Home and Remote
  interface
• Container manages bean lifecycle
• Creation, Pooling, Persistence
• Registers bean for remote access
• Intercepts all client calls to bean
• Manages Security, Transaction Context, Threading

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Server and Containers
Relational Database
Server
TopLink Container
CICS Container
CICS
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Container and Beans
Home Interface
Container
Pool
Home
Home Stub
create
Client
Remote Object Stub
EJBObject
Teller Bean
transfer
Remote Interface
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Key Point

• All method invocations are intercepted by the
  container/EJBObject and then delegated to your
  bean implementation
• Why ?
• Manage Concurrency (threading)
• Manage Security
• Manage Transactions
• Manage Resources
• Remote Access

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Resource Management

• As the number of clients increases, the number of
  server objects increases
• large systems need to manage resources
• EJB suggests two mechanisms
• Instance Pooling
• Activation

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Managing Beans

• Bean characteristics require different management
  strategies
• Responsibility of container
• will call back bean at prescribed points in the
  lifecycle

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Bean Characteristics
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Stateless Session Beans
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Stateless Session Beans

• A stateless session bean holds no state on behalf
  of a client
• Each bean instance is equivalent
• Sufficient client information must be passed with
  each method invocation

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Example

• interface Teller extends javax.ejb.EJBObject
• public void transfer( int from, int to, float
  amount )
• throws java.rmi.RemoteException,
  TransferException
• All required parameters are in method invocation
• Any teller object is as good as any other

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EJBObject
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Example (Cont)

• interface TellerHome extends javax.ejb.EJBHome
• public Teller create() throws RemoteException,
  CreateException
• Serves as factory for creating tellers
• Stateless session beans must have a create()
  method that takes no arguments
• Bean implementation needs ejbCreate()

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EJB Home
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Bean Implementation

• public class TellerBean extends
  javax.ejb.SessionBean
• public void ejbCreate()
• public void ejbRemove()
• public void ejbActivate()
• public void ejbPassivate()
• public void setSessionContext(SessionContext
  ctx)
• public void transfer( int from, int to, float
  amount )
• throws RemoteException, TransferException
• // Perform transfer. JDBC, etc.

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Deployment Descriptor(ejb-jar.xml)

• Inc.//DTD Enterprise JavaBeans 1.1//EN'
  'http//java.sun.com/j2ee/dtds/ejb-jar_1_1.dtd'
• This component is used manage the bank..
• Bank Component
• ...

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Deployment Descriptor

• Used to handle actions that span accounts.
• Teller Bean
• Teller
• TellerHome
• Teller
• TellerBean
• Stateless
• Container

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Deployment Descriptor

• Used to manage specific
  accounts
• ejb/AccountHome
• Entity
• AccountHome
• Account
• Account

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Weblogic Deployment Descriptor(weblogic-ejb-jar.x
ml)

• Teller
• ejb/AccountHome-name
• AccountHome
  
• TellerHome

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Client Interaction

• Looks up home interface in JNDI
• Invoke create() on Home stub
• Invoke transfer() on EJBObject
• Invoke remove() when done

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Pooling

• Stateless session beans can be very efficiently
  pooled to save resources
• Pooling technique is vendor-specific. Pool can be
  pre-created as well as dynamically sized
• Adding beans to the pool
• Class.newInstance() to create TellerBean
• setSessionContext()
• ejbCreate

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Why Pool ?

• Efficient memory utilization
• Larger systems can not afford to have a bean in
  memory dedicated to each client
• Significant delay between client calls is
  possible
• Just-In-Time paradigm. Bean is pulled
  dynamically from pool to handle method invocation
  and then returned to pool
• Pool size can be adjusted dynamically to deal
  with changing client loads

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Method Invocation

• Bean is selected from pool and associated with
  clients EJBObject
• EJBObject/Container manages transactions,
  security, threading, etc.
• Method is then delegated to bean implementation

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Stateless Session Behavior

• Lifecycle of the EJBObject and the bean
  implementation are separate.
• EJBHomecreate() creates an EJBObject for the
  client - not an instance of your bean
  implementation
• EJBHomeremove() removes the clients EJBObject
  - not an instance of your bean implementation

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Stateless Session Bean Summary

• Models a business process that consists of one
  operation
• transfer(), logError(), Math.sin()
• Has no state between method invocations
• Can be pooled efficiently
• Usually the most efficient bean type

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Stateful Session Beans
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Stateful Session Beans

• Represents a conversation between client and
  server
• shopping cart
• Exists for the clients lifetime (or timeout
  period)
• Each client method invocation accesses the same
  bean implementation

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Home Interface

• public interface CartHome extends EJBHome
• public Cart create()
• throws RemoteException, CreateException
• public Cart create(String product)
• throws RemoteException, CreateException

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Bean Implementation

• Implements all ejb Create methods
• public class CartBean implements SessionBean
• // lifecycle methods omitted
• public void ejbCreate() throws
  CreateException
• public void ejbCreate(String product) throws
  CreateException
• addProduct( product)
• public void addProduct( String product )
  // put product in Vector
• public Vector products_ new Vector()

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Resource Management

• Can not be pooled the same way as stateless
  session beans
• bean instance is for a specific client
• Activation and Passivation are used
• Beans are passivated (swapped to disk) when
  resources run low in server
• object serialization typically used

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Deployment Descriptor (ejb-jar.xml)

• Holds items while shopping
  
• Shopping Cart Bean
• ShoppingCart
• CartHome
• Cart
• CartBean
• Stateful
• Container

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Weblogic Deployment Descriptor(weblogic-ejb-jar.x
ml)

• ShoppingCart
• /tmp
• CartHome

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Bean Attributes

• Must be Serializable to permit activation and
  passivation
• Bean itself is already serializable
  (EnterpriseBean)
• Object Serialization semantics are used
• Exception transient fields are not guaranteed to
  be initialized

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Lifecycle Behavior

• Client create() on home interface results in
  creation of bean
• contrast with stateless session bean
• Client remove() of EJBObject results in removal
  of bean
• contrast with stateless session bean
• Creation/Deletion controlled by client
• Activation/Passivation controlled by container

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Actions Required

• ejbPassivate()
• Release resources held
• Prepare fields for Serialization
• ejbActivate()
• Acquire client-specific resources
• Prepare object for access
• may set transient, calculated fields

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Consequences of Statefulness

• Session Beans do not (typically) survive failures
• Loss of a long conversation could have a large
  impact
• short conversations
• checkpoint state to database
• advanced container capability

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Stateful Session Bean Summary

• Models a complete conversation with a client
  where state needs to be preserved between method
  invocations
• Be prepared to lose state !
• Session beans do not survive failure
• code clients to re-establish state upon failure
  (if possible)
• some products checkpoint session state (BEA)

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Bean Characteristics
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Misc. Topics

• Obtaining Container Services
• Concurrency
• Exceptions
• The Business Interface
• Client APIs

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Obtaining Container Services

• Context object passed to bean implementation
• setSessionContext()
• setEntityContext()
• Allows bean to participate in
• security
• transactions
• Obtain this reference

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Context Operations (Cont)

• Save off reference to context for later use
• public class TellerBean implements SessionBean
• ..
• public void setSessionContext( SessionContext
  ctx )
• ctx_ ctx
• SessionContext ctx

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Threading

• Concurrency
• Access to a bean instance is serialized
• Bean methods may not be synchronized
• Reentrance
• bean instances are non-reentrant by default
• session beans can never be reentrant
• entity beans can be configured to be reentrant
  (discouraged)
• allows multithreaded access to bean !

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Exceptions

• Client sees exceptions as specified in Home and
  Remote Interfaces
• All methods can throw Remote Exception
• Server Exception categories
• system
• application
• unchecked

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Exception Types

• System (Remote)
• EJB container/server can potentially handle
  system exceptions
• Ex. database failure
• Application
• Always passed to client
• Unchecked Bean Exceptions
• Must be caught by container

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Business Interface

• Our bean must have the same methods that we
  specify in the Remote/EJBObject interface.
• Dont want to implement the Remote interface
• its Remote !
• Has additional EJBObject methods
• could accidentally pass this when we wanted to
  pass a reference to an EJBObject
• Develop an intermediate Business Interface

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The Business Interface
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Client Programming

• Typically invokes home create() method and then
  invokes business methods. Invokes remove() when
  done.
• isIdentical()
• getHandle()
• portable reference to an EJBObject
• similar to stringified CORBA object reference
• HandlegetEJBObject() to re-create object

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Resources

• Main EJB Page
• http//java.sun.com/products/ejb/

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References

• Developing Java Enterprise Applications. Asbury,
  Weiner. Wiley. 1999
• Java Enterprise in a Nutshell. Flanagan et al.
  OReilly 1999
• Mastering Enterprise JavaBeans and the Java 2
  Platform, Enterprise Edition. Roman. Wiley. 1999
• Java Report. Whats new in EJB 1.1?,
  Monson-Haefel. August 1999.