********* WHY EJB? WHEN EJB?? WHICH EJB? ************

Question

Hi
Can anyone tell me clearly when do we really think of using ejb in our project ?? why should we use ejb??
if there is a reason to use ejb's when should i think of session beans and entity beans.
I would be very thankful if anyone gives me a detail explanation.
i have gone thru the material but iam really confused.
tks in advance.

Answer 1

Following is a brief introduction to EJB that may clear some of your questions.
Enterprise JavaBeans
1.     Entity and Session Beans
Entity Beans represent the data items that are the core of applications.
     Eg Customer, Order, orderItem or product
Session Beans represent logical operations and usually operate on the entity beans.
     Eg OrderEntry, PaymentAuthorisation
A session bean is typically the main bean that an application uses although the application might manipulate the entity beans directly. For example an application might create an Order entity bean and an OrderItem entity bean and initialize its values. It would then use the OrderEntry session bean to store the order.
2.     Containers
An EJB server may support several virtual servers, and these are referred to as containers. The term container in this context essentially means server.
3.     Persistence
Saving and retrieving entity beans is extremely important in an EJB application. EJB persistence comes in two flavors.
Bean Managed persistence
With Bean Managed Persistence each entity bean is responsible for saving itself and retrieving itself from the database. It must have methods to insert itself, update itself, read itself and other housekeeping methods.
Container Managed Persistence
With Container Managed Persistence the EJB container itself handles the beans persistence. This means that entity and session beans can be written to do business logic and the container handles the database operations.
4.     Transactions
Operations in a session bean take place within the scope of a transaction that is managed by the EJB container. An EJB container can be instructed what kind of transaction scope is needed for each session bean method, for example it can be indicated that a particular method is always begins a new transaction or that a method can only be called after a transaction has been started. Transaction housekeeping is left to the EJB container again letting the entity and session beans concentrate on the business logic.
5.     Connection Pooling.
The EJB container handles database connection pooling. The container need to be configured so it knows how to create a database connection and it will grab a connection automatically. If a CMP bean is used then you may never have to use the database connection directly. Even with BMP you don�t need to worry about getting a connection and returning it when your done. The EJB container handles this for you.
6.     Object Pooling
An EJB server can pool Enterprise JavaBeans. EJB setup can take a lot of time depending on the bean. For beans that are used frequently you can tell the EJB server to keep a pool of the beans handy because you wil need them often.
7.     How it all fits together
Enterprise JavaBeans, servlets and Java Server Pages are the three technologies that represent the core of J2EE because they function as servers. The other API�s JDBC, JNDI, RMI, CORBA, JTS, JCA, JAAS and xml help support EJB, servlets and jsp servers.
With WEB application JSP and servlets interact with the browser and EJB concentrate on back end processing.