need of EJB and where and when we use this EJB component?Why we cant do the same work

[pradeep singh]

Hi to all
I have just started preparation of this SCBCD exam.But one very basic question strikes my mind that is What is the need of EJB and where and when we use this EJB component?Why we cant do the same work by Web components(servlets/jsp)as EJB do ?In what situation and condition and why we use EJB ,inspite of web components.What is the necessity of using this EJB rather than web components.
Please explain me with an example.
[ June 30, 2008: Message edited by: pradeep singh ]

[Mirko Bonasorte]

Well,
for example, if you need mission critical applications which require a lot of scalability.
Or if you need a real transational application.
Or if you need asynchronous elaborations.
Or if you have to implement reusable business components.

Any other?

[pradeep singh]

Hi
Sorry ,unable to understand.Please clear me in detail.

[Mirko Bonasorte]

For example, EJB Container gives you the opportunity to manage your business logic in a transaction-aware environment. The transaction context where your business logic lives extends the transaction concept beyond the simple database transaction (for example, if you create a timer, a roll back invalidates that creation).
Moreover, you might need to execute asynchronous elaborations. This might be done sending messages and writing your business logic into your Message Driven Bean.
Consider, also, that the container manages pool of instances in a way that it scales over the number of requests. And if it is not enough, most commercial application servers allow you to clusterize your EJB container.

Finally, consider that EJB are not the solution for any application: for example, if you don't have real mission-critical environments, the web tier should be enough. But it would be a strict bond: one day, if your logic required a more affordable environment, or if you had to asynchronously execute elaborations, you would be in trouble.

[sanjeev mehra]

Best answer; read KS EJB 2x;

that would help to let you know containers role & importance; & EJB life cycle.

but for now, container (a box) in which EJB resides, provides transaction mgmt, security, bean pooling, ..... etc.
In other case, if you keep your business logic in (POJO) Java class, you would need to take care of all services which are provides by container.
hope this would help.