You asked a very broad question. Go to java.sun.com/j2ee or www.ibm.com/developerworks and read about J2EE. Do a google search on introduction to J2EE and read some articles. When you are familiar with the platform, go through thr J2EE tutorial (I will let you find it on java.sun.com website).
In general, J2EE is 1. A platform with collection of technologies such as Servlets, JSP, EJB and lot more 2. Specification (PDF file) that describes what qualifies as J2EE application and how servers should run J2EE applications 3. Bunch of tools (eclipse, websphere, weblogic and many many others) and frameworks (spring, struts and many others) people created to ease J2EE development and deployment. The tools are not formally part of J2EE platform but your skillset will be incomplete without them
From job perspective, you need to at least know Servlets and JSPs and then go from there.
J2ee today is something between a vast ocean of knowledge and a big mess :-) The main problem with all the j2ee technologies is that they don�t exist only by themselves. To give you an example let�s say you will play with some examples and in couple of weeks you�ll be able to build a web-centric application that uses servlets/jsps and jdbc to access a database. You are also familiar with the transactional model and probably with jta/jts. Basic knowledge about the implicit/explicit security model is also required, but again let�s say that you�ve picked this up as well. Next day you�ll go to an interview and we�ll be able to answer to all questions relating the api you�ve used and explain your application architecture (which is not going to be trivial though). One of the possible questions you�ll get might be about testing. What kind of tools or framework did you uses for testing your app? You�ll go back home, reading more about jakarta cactus tests, testing inside and outside of the container, designing your classes to be fully testable etc. Lot of information though, only to perform a very simple task: testing your application. Next week you�ll be back to the interview again; now you know almost everything and the questions will be about your application design: did you use an MVC pattern? Did you use JSF? Aren�t you familiar with XML/XSL? Hence you have to go back home and chose between Jakarta Struts, JSF, Tapestry, or maybe all of them. Once you�re done with this task you�ll see that your interviewer needs you to tell him more about the data access. Are you familiar with any persistence framework, like Hibernate or JDO? I guess you got my point. As I said from the industry perspective it�s much more information than one can get in a lifetime. You must realize this and you should focus on defining the way you like to present yourself to employers. You better present yourself as a junior programmer that has only such and such experience, but is eager to learn and become a good developer. Good luck!
I think, therefore I exist -- Rene Descartes
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