Where can i find an example (or tutorial) of a java Web Service that receives and returns data ?
A web service is an integration technology, not an implementation technology. It is used for connecting applications only. There is no "logic" besides what is needed to enable the applications to communicate.
There is much more than XML involved and there are different technologies for implementing web services.
For example, a SOAP-based web service uses a client SOAP Engine and a server SOAP Engine.
Application A wants to get data.
Application B has direct connection to database.
There is a web service which allows app A to get data using app B functionality.
The purpose of the web service is to allow two applications to communicate with each other. That is all.
There is a good tutorial in:
http://www.amazon.com/Building-Web-Services-Java-Making/dp/0672326418/ref=sr_1_1?ie=UTF8&s=books&qid=1263050542&sr=1-1