Session Beans: Overview

• Session
  • A client’s conversation with a server
  • Client has server carry out business operations
• Session Bean
  • A bean that represents a client’s session with server
  • Encapsulates a business process
  • Usually transient
  • Client can be end user, another bean or anything else
  • Examples: stock broker, bank teller, shopping cart
• Session beans can be stateless or stateful
  • A given design may use either or both
  • Container handles them differently
  • Details follow...

Stateless Session Beans...

• Stateless session beans hold no session data
  • Hence usually have no instance variables
• For example, a stock broker:
  • Acts as gateway to your portfolio
  • Does not hold your money; puts it into an account
  • A broker bean would be stateless
  • Portfolio and account beans would have state

...Stateless Session Beans

• Stateless session beans are shared
  • Container creates a pool of identical instances
  • Any instance can serve any method call from any client
  • Idle instances remain in pool
  • When necessary, container can expand or shrink pool

Stateful Session Beans...

• Stateful session beans hold session data:
  • They create & maintain data as session proceeds
  • Data is held in instance variables
• For example, a customer's shopping cart:
  • Cart grows during a session
  • When customer "leaves the store", session ends

...Stateful Session Beans

• Stateful session beans are not shared:
  • One instance is allocated per client session
  • Instance is released when session ends
  • Each client gets its own
  • Idle instances can be swapped out of / into memory

Session Beans & Persistence

• State and persistence are related but different:
  • Stateless session beans are always transient
  • Stateful session beans can be persistent if desired
  • Persistent state usually relates to entity beans
• Why make a stateful session bean persistent?
  • To let client suspend and resume a session
  • E.g., to hold a shopping cart pending customer's return
• If a stateful bean's state must persist:
  • Handle persistence explicitly (More on this later...)
  • Or use an entity bean instead

Session Bean Life Cycle...

• Every session bean has a home object
  • A "factory" for that bean
  • EJB client uses bean's home to create bean instances
  • All clients share one home object for given bean type.
• Creating a session:
  • Establishes client’s connection to a session
  • Creates a skeleton
  • To create a session, a client:
    • Gets a reference to home object using JNDI
    • Uses home object's methods to connect to a session bean

...Session Bean Life Cycle

• Removing a session:
  • Severs client's connection to the bean
  • Destroys skeleton
    • Client retains stub but can not use it
    • Client must create a new session again through home object
  • To do this, client uses a method of remote reference
    • More on this shortly...
• Stateless and stateful beans are handled differently
  • But they look identical to the client
  • Details follow...

Stateless Session Beans

• Clients share a pool of identical instances
• "Creating" simply connects to a skeleton
• "Removing" simply disconnects from skeleton
• State diagram for a stateless session bean
  • See notes for details...

Stateful Session Beans

• Each client gets its own dedicated instance
• "Creating" creates an instance
• "Removing" destroys it, losing state
• State diagram for a stateful session bean

Bean Use Examples

• Two examples illustrate use of session beans:
  • Code snippets from client applications
  • The StockBroker example:
    • Stateless bean is shared by all
    • create( ) connects client to session
    • Removal severs connection
  • The ShoppingCart example:
    • Session is created using Customer ID
    • Bean accumulates state
    • Removal kills session, loses state
• See notes for code...

Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

Writing a Session Bean...

• There are 3 parts of a session bean you must write:
  • The remote interface:
    • Java interface describing bean's remote functions
    • Like CORBA IDL or RMI remote interface
  • The home interface:
    • Describes the bean factory
  • The bean class:
    • Provides code for remote and home interfaces
• A note on class names:
  • Conventional names: Xyz, XyzHome, XyzBean
  • Handy, but not required
• See diagram on next page; details follow...

...Writing a Session Bean

The Remote Interface

• Defines the bean's remote "business methods"
• Must be a public interface
• Must extend javax.ejb.EJBObject
  • EJBObject defines all EJB remote interfaces
  • EJBObject extends java.rmi.Remote
• Must meet RMI remote interface requirements:
  • Methods are public & throw java.rmi.RemoteException
  • Arguments & return values are valid remote types:
    • Primitives, Serializable classes or Remote classes
• See examples in notes...
  • A stateless bean: StockBroker for an online brokerage
  • A stateful bean: ShoppingCart for an online bookstore

Continuation of Notes...

• This is a full page of notes.
• The slide is hidden.

...Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

EJBObject

• All remote interfaces extend javax.ejb.EJBObject
  • Defines all stubs and skeletons
• This interface defines methods to:
  • Remove the bean (discussed here)
  • Do various other things (discussed later...)
• Removing the bean:
  • Client calls this method to sever connection
  • RemoveException thrown if removal fails
    • Checked exception; must be trapped
    • Container may trigger the failure
  • java.rmi.RemoteException thrown if system failure occurs
    • More on exceptions later...

Using the Remote Interface

• Client uses a bean through the remote interface:
  • This applies to remote as well as local clients
  • Client code looks the same
• See notes for examples...
  • A client getting stock information.
  • A client buying books

Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

The Home Interface

• The home interface defines a bean's home object
  • Defines a "factory" for that bean
• Home interface requirements:
  • Interface is public
  • Extends javax.ejb.EJBHome
  • Methods meet RMI remote requirements
  • Has at least one create( ) method
• See examples in notes...
  • A StockBrokerHome interface
  • A ShoppingCartHome interface
  • Details follow...

Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

create(*)...

• “Creating” a session bean:
  • Represents creating a client session
  • Triggered by calling Home#create( … )
• create( ) is a factory method
  • Like a constructor, defines how to initialize a session
  • Returns the remote bean type
  • May be overloaded
• Syntax:

...create(*)

• Home must have at least one create( ) method:
  • Otherwise client cannot create a session
• What create( ) methods should a home have?
  • Arguments are used to initialize session state
  • For stateless beans: no-args version only
  • For stateful beans: any versions warranted by the design
• The CreateException:
  • javax.ejb.CreateException
  • A checked exception
  • Thrown in server code if bean instance can not be created
  • Comes back to client as failure of Home#create( * )

Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

The Session Bean Class

• The "bean class" implements the session bean
  • Provides functionality defined in remote interface
  • Responds to create( ) methods in home interface
  • This is the only class you must write for a session bean
• Requirements for a session bean class:
  • Class is public
  • Has a public default constructor
  • Has all methods of remote interface (“business methods”)
  • Has an ejbCreate( ) for each create( ) of home interface
  • Implements javax.ejb.SessionBean interface
• Details follow...

Business Methods...

• Bean class must provide business methods
  • Methods defined in remote interface
• But bean class does not implement this interface!
  • Container associates these methods with skeleton
  • Hence Java compiler will not detect errors/omissions
  • These will be flagged by server at deployment time
• Business methods & exceptions:
  • Remote interface methods must throw RemoteException
  • Bean class methods need not
    • They need only throw application-specific exceptions
    • And then, only when actually thrown in method body
  • More on exceptions later...

...Business Methods

• Q: Why doesn’t bean class implement remote interface?
  • Why not this?
• A: It would create an invalid “is a” relationship.
  • Remote interface defines an EJBObject
  • Bean class is not the EJBObject, just a servant
• A: EJBObject defines 5 methods.
  • Your bean class would have to implement them
• Only the stub & skeleton implement remote interface
  • These are generated by server’s deployment tool

ejbCreate( )

• The bean class will have ejbCreate( ) methods:
  • One to match each create( ) in home interface
  • Called by container when it creates bean instance
• What do these methods do?
  • Generally used to initialize state data
  • No-arguments version may be empty
• To abort session creation:
  • Throw javax.ejb.CreateException from ejbCreate()
    • Checked exception; must be declared if used!
  • Client sees failure of Home#create( )
  • See example in notes...

Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

The SessionBean Interface

• Bean class must implement SessionBean interface
  • javax.ejb.SessionBean
  • Extends javax.ejb.EnterpriseBean (a "marker" interface)
• SessionBean interface has life cycle methods:
  • "Event methods", called by container during bean's life cycle
  • Bean can respond or do nothing
  • See notes for complete listing…
• We will cover SessionBean in detail later...
  • For now, we will leave the methods empty

Continuation of Notes

Bean Class Examples

• Putting it together, here are 2 complete bean classes:
• A StockBrokerBean class:
  • Implements methods of StockBroker interface
  • Has single ejbCreate( ) for StockBrokerHome interface
• A ShoppingCartBean class:
  • Implements methods of ShoppingCart interface
  • Has two ejbCreate( ) for ShoppingCartHome interface
• In both examples:
  • Methods of SessionBean are empty
• See code in notes...

Continuation of Notes...

• This is a full page of notes.
• The slide is hidden.

...Continuation of Notes...

• This is a full page of notes.
• The slide is hidden.

...Continuation of Notes...

• This is a full page of notes.
• The slide is hidden.

...Continuation of Notes

• This is a full page of notes.
• The slide is hidden.

Writing a Bean: Recap

• The remote interface defines bean's remote methods
  • Extends EJBObject
  • Its methods must meet requirements of RMI methods
• The home interface defines home object:
  • Extends EJBHome
  • Will have one or more create( ) methods
• The bean class provides implementation of both
  • Implements remote interface methods
  • Has an ejbCreate( ) to match each home#create( )
  • Must implement SessionBean interface

Where We've Been

• Session beans:
  • Represent a business process
  • Are usually transient
• Stateless session beans:
  • Pooled, shared, recycled
• Stateful session beans:
  • Dedicated to client, not recycled
• The parts of a bean:
  • Remote interface
  • Home interface
  • Bean class

Session Bean Workshop

• Writing a Session Bean

...Continuation of Notes

• This is a full page of notes.
• The slide is hidden.