I've messing around searching for someone who explains me clearly the intrinsics of the configuration of the web.xml file and finally I've found something in a manual from oracle:

http://download.oracle.com/docs/cd/E13222_01/wls/docs70/webapp/webappdeployment.htm

I attach here for the sake of not loose it:

webapp.pdf

And I don't want to loose the opportunity to to comment one thing: the issue of the context parameters.

Context Parameters are this: parameters that the application –or a library, also known these days as framework– can read from the web.xml file.

Here is the syntax to define a context parameter:

  <context-param>
 
     <param-name>user-defined parameter name</param-name>
     <param-value>user-defined parameter value</param-value>
     <description>optional description, just text</description>
 
  </context-param>
 

How can them be read:

javax.servlet.ServletContext.getInitParameter()
javax.servlet.ServletContext.getInitParameterNames()