EJB Basics

EJB --- OVERVIEW
Service -- A group of related components that carry out a given business process function
SOA - a Service Oriented Architecture is a process of focusing on the development of services rather than piecemeal components such that these services provide a higher level of abstraction from the fuctional point

One Characteristic of SOA is they are autonomous in nature. These independent entities can interact with others inspite of differences in the way they have been implemented and the platform they have been deployed.

EJB is a standard for building server side components in JAVA. It defines an agreement b/w components and application server that enables any component to run in any application server.

EJB components can perform the following tasks

Perform Business Logic, Access the Database, Access another system (like cobol - cics, erp -- using J2ee connector Architecture -- JCA)

Types of Beans :

Session Beans : Session bean models business processes.
Entity Beans : Entity bean models business data.
Message Driven Beans : MDB are similar to session beans in that they model business process
the difference is that you can call a MDB by explicitly sending a message to the bean.

In Java any object the implements java.rmi.remote is a remote object that is callable from a different JVM.

The Java files needed for a EJB program are
1. Remote Interface
2. Home Interface
3. Bean Business Logic
4. A Client that calls the bean using JNDI

In EJB2.0, Local Objects (stubs) implement a local interface rather than a remote interface as done in EJB 1.0
Thus local objects are fast and make high performance enterprise beans.

Steps in EJB 1.0
1. The Client calls a local stub
2. The stub marshalls the parameters into a form suitable for a network.
3. The stub then sends the marshalled data to the skeletons
4. The skeleton demarshalls the parameters
5. the skeleton sends the data to the EjbObject
6. The EjbOject performs the middleware functions as connection-pooling, transaction, security...
7. EjbObject calls the enterprise beans instance and it does its work ( Business Logic )


Steps in EJB 2.0
1. The Client calls the local Object
2. Local Object performs needed middleware functions
3. Once enterprise bean does its work, it returns control to local Object

In EJB2.0 when we write local Object, we extend javax.ejb.EJBLocalObject and when we extend a Local home interface you extend javax.ejb.EjbLocalHome

Drawbacks of Local Interfaces
1. They only work when you are calling beans in the same process. -- You cannot call a bean remotely if your code relies on a local interface. If you decide to switch b/w a local and a remote call, you must change your code from using the local interface to using the remote interface.
2. They marshall parameters by reference rather than by value. whille this may speed up our application since parameters are not copied, it also changes the symantics of the application.


Writing EJB starters ---------- ;)

1. Remote Interface --- The remote interface supports every method that our bean expose.
a) Extend javax.ejb.EjbObject
b) We declare all the business methods

2. Local Interface -- Local Clients will use our local Interface.to call bean methods
If the Client is Local then we can use local interface instead of Remote Interface ( Supported in EJB 2.0)

3. Home Interface -- The Home Interface had methods to create and destory EJB Objects.
a) the create() method is a Factory method that clients use to get a reference to the EJB Object. The create method is also used to initialize the bean.
b) the create() method throws two exception java.rmi.RemoteException and javax.ejb.CreateException
c) Home Interface extends javax.ejb.EJBHome

4. Local Home Interface --- The performance of local-home interface is more and is used by local Clients
a) It extends EJBLocalHome interface instead of EJBHome interface
b) The EJBLocalHome interface does not extend java.rmi.Remote Exception

There are two different types of Clients
a) Java RMI-IIOP based Clients -- - These Clients use a JNDI to look up objects over the network
b) CORBA Clients --- This enables EJB components to be invoked from C++, or other platform.