Here are my answers..
3. Yes your understanding is right.
4. doXXX() methods are declared protected so that only child/same objects can call those methods. It makes sure that only servlet object calls the doXXX()
5.HttpServlet is made abstract to makesure that no one creates an instance of HTTPServlet object directly without creating its child classes.
6.YES when HttpServlet receives a HTTP request it calls Service(HttpReq,HttpRes). It also contains the service(ServletRequest,ServletResponse) to provide functionality for service method of GenericSErvlet method.
7. The servlet in include will use the passed req/response pair for its processing and will not generate any error/exception unless for other reasons.
10.Will display exception on browser.
11. Root-cause exception is the exception at the base of exception trace. ususally thrown by user coded methods.
any corrections need to be made ???