SCBCD Notes

Client can NOT call EJB direct, but through mediator

JMS in case of MDB(Message Driven Bean)

 

Container provides class implementing bean's interface:  component interface &  home interface

Instance of class implementing component interface is called  EJBObject

 

client call EJB through two player:  EJBObject &  container

 

• statless bean is put in a pool serving many client
• statful bean serves one client only and destroyed after one session ends

 

Things to remember - Inside a Bean

• Do not directly read/write File Descriptor
• Never try to obstain security policy of source code
• Never use object substitute or subclass feature of java serializable protocol
• Never pass  this as parametor or return 
• Do not implement Bean as Server-like activity e.g. listener, multicasting on a socket
• Never create/set/access classloader
• Never set/create security manager
• Never try to stop JVM
• Never try to change input/output , error stream
• Never try to set factory that is used by Server socket, and stream handle factory that is used by URL
• Never
  • start, stop, resume a thread
  • change priority of a thread
  • change name of a thread

 

• Must not define a class
• Not try to acess security configuration object such as identifier, Signer, provider, security

 

Same Bean can not define two env entries with same name

Different Bean can defined same env entries with same name, as when they access from bean it get its own variable

 

EJB communicate with container through SessionContext & SessionBean Interface

 

Home interface has create() and remove() method, create() needs to be defined, but remove() is inherited from parent

 

Create method in EJB

	StateFul	StateLess
declare	mandatory, no argument method	at least one create() is mandatory, e.g. createABC(...) createBBB(...)
Bean instance creation	create a new bean	get bean from pooling
Common property	argument must be serializable, primitive	argument must be serializable，primitive

Remove method in EJB

	Entity	StateFul	StateLess
1. Remove(PrimaryKey) 2. Remove(Handle)	1	2. will remove instance	2. no effect, will go back to pool
Common		Contaner event, client can not call	Contaner event, client can not call

 

There is exactly one Object for each Session Bean Type, Multi EJBObject

e.g. 100 clients have 1 HomeInterface, but 100 EJbObject

 

method starting with ejbMethodA...() is container callback method

method can not throw RunTimeException(...)

 

isdential() method of EJBObject

• stateful bean never been true
• stateless bean return true, if referenced the client's handled by the same homeInterface
• EjbObject are compared by isIdentical(), NOT equals()

EJBObject's remove() doesnt have argument, remove itself

HomeInterface's remove(...) has argument, as it needs argument to determine which one to remove

HomeLocalInterface does NOT have method for Handler & metaDate( in save JVM, information of method can be abtained from reflection)

Local HomeInterface doesn't have remove(handler), remove can be done directly call the LocalEJBOBject to remove itself

Remove sessionbean client can remove via RemoteHomeInterface & remote EJBObject

LocalHomeInterface can NOT be passed to remove call

In Bean method, dont declare throw RemoteException

setEntityContext and setSessionContext are used to setup sessionBean to interact with container

For stateful bean, when user call create(), createABC() in home Interface, EJB container will call ejbCreate() or ejbCreateABC() defined in session Bean respectively

EntityContext & SessionContext

• One difference is that the entityContext has getPrimaryKey()
• setSessionContext() can call two method of container: get Home Interface, and JNDI access
• following method ejbCreate().

remove method call for statelss bean is ignored

Container will not passivate if session is in Transaction

remove is not call under:

1. time out in passivate state
2. bean throw exception
3. server crashes

 Entity Bean

• If can not find single entity, throw ObjectNotFoundException
• If can not find group entity, return empty collection

For CMP, don't need write find method

For BMP, each findXXX method in Home Interface, must have a match ejbFindXXX in Bean Class

 Home Interface can implement business logic which is not entity related

 for each method, you are required to write ejbHomeXXX method in Bean

 Each Bean Type has its own pool

 Entity bean can only use ONLY CMT demartation, can NEVER use getUserTransaction method

 Only difference between EntityContext & SessionContext is getPrimaryKey() defined in EntityContext

 

EJB Remote interface is closely related to RMI-IIOP protocol

Parameter passed into EJB method must be RMI-IIOP conpatible

 

EJB Object - Container generated Remote Interface Implementation

Local Object - Container generated local interface implementation

Home Object - Container generated home interface implementation

Loca home Object - Container generated local home interface implementation

 

System exception - Container will process it

Application excepton - should throw back to client/ the only exception for remote EJB thrown back is REMOTE or its subClass

 

EJB bean is called via EJB Object, thus if your bean calls another bean, you must pass reference to your bean's EJB Object rather than reference of your bean; Just like another client, everything must go through EJB Object

 

When bean is passivated, it might hold reference to container-implemented object, the following object will be passivated as well

1. EJB Object reference
2. Home Object reference
3. EJB context reference
4. JNDI naming context

 

Stateless can define a few overloaded ejbCreate(...)

Stateful can only define one empty ejbCreate()

 

The biggest difference between the stateful session bean and the other bean types is that stateful session beans don't use  instance pooling.