I think if you place an specifiec object for example a String into the session  first time a user  called your front servlet or your front/home JSP

by this snippet: (in the doGet method of your front servlet)

String userName = null;

userName = request.getParameter("user_name");

if (userName == null) {

    final HttpSession session = request.getSession();


    if (session.getAttribute("USER_LOGIN") != null) {
        /** do normal forwarding to other parts of system **/
        } else {
        /** forward to a page which asks the user name/password from user and then sends them to this front servlet again **/
        request.getRequestDipatcher("login.jsp").forward(req, res);
    }
return;
}


session.setAttribute("USER_LOGIN", userName);

and then:

Thread thread = new Thread() {
   public void run() {

        sleep(30*60*1000);
        session.invalidate();
    }
}

thread.start();