Session Tracking is one of the most powerful features of 
Servlets and JSP. Basically, the servlet engine takes care 
of using Cookies in the right way to preserve state across 
successive requests by the same user. Your servlet just 
needs to call a single method (getSession) and it has 
access to a persistent hashtable of data that's unique to 
the current user

public void doPost(HttpServletRequest request, 
HttpServletResponse response){
...
//lets take the session obj
HttpSession session = request.getSession(true);
//now let us take the user name assosiated with this session
String currUserLogin = (String) session.getAttribute
("currentUserLogin")
...
//now we do the work according to the user name
doSomeWork(currUserLogin);
...

}