What is an enterprise Javabean?

Question

What is it??? And what can it do?

Answer 1

Enterprise Java Beans (EJBs) are used in server-side programming, most often for distributed environments. Most new java programmers, those that are just learning the J2SE API, don't really need to write one. However, if you are interested in doing client-server programming (or are already doing it without EJBs), you may want to use EJBs. EJBs are one of SUN's way of doing Remote Method Invocation (RMI). Let me further explain this. Lets say you write a client application which has its own Java Virtual Machine (JVM). Also, lets pretend that you have an application server, say Sybase's EAServer for example, and you write code on the server. The classes for the client are loaded into one JVM while the classes for the server are loaded into another JVM. How do you get a class on your client to execute a method in a class that lives in an entirely different JVM on the server? You could use an industry standard approach called CORBA. But its very complex to implement. So Sun came up with an easier way to do RMI (but it only supports java while CORBA is a specification that can be used by most languages). SUN also came up with the concept of EJB's that fully support RMI. So, you could write an EJB on the server side that accesses data from a database. This EJB will have methods that can be called from the client. Remember EJBs are loaded into their own JVM on the server. Using a protocol called IIOP, the client is capable of getting a handle (a reference) to this EJB and calling its methods to receive that data. This is cool stuff because you can write several different clients on different machines (different OS) in different languages and access the same EJB to retrieve the data.
There are different types of EJB's also. Session EJBs (Stateful and Stateless), Entity EJBs, and Message Driven EJBs. Of course, this is just a rudimentary explanation of EJB's. Don't even think about using them unless you are interested in doing client-server programming. EJB's are more complex than most java API's.
I hope this helped,
tajenkins