EJB. What is, why it exist and HOW it works?

Guys, I HAVE tried reading tons of stuff about EJB. And I don't get it. It seems that most of the authors have a superficial knowledge on it. They basically say it's the business-logic 'stuff'. They don't show it how it interacts with the AppServer 开发者_JAVA技巧and so on, what it does, how, and why?

It is a huge question, but not that huge. It is not like asking what is physics. You basically run your business code inside container which is handling all the connections, lookup, transactions etc. There are alternatives to ejb, e.g. spring.

The question is huge indeed. EJBs in a general sense try to enforce a design pattern that encapsulates all of your reusable code or "business logic" into a specific tier in your architecture. By doing this you can reuse this code for your web/presentation layer and web services for example. EJBs provide a way of persisting your data to a DB.

The trend in java development now a days is POJO driven architectures that leverage dependency injection. Spring is a popular tool to facilitate this design pattern and I would encourage you to explore this instead of EJB.

an enterprise bean is a server-side component that encapsulates the business logic of an application. The business logic is the code that fulfills the purpose of the application. In an inventory control application, for example, the enterprise beans might implement the business logic in methods called checkInventoryLevel and orderProduct. By invoking these methods, clients can access the inventory services provided by the application.