@WebServlet(name="SessionTimeoutCheck", urlPatterns={"/SessionTimeoutCheck"})
public class SessionTimeoutCheckServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        response.setContentType("text/html");

        PrintWriter out = response.getWriter();
        out.write("Current session ID: " + session.getId());
        out.write("<br />\n");
        out.write("Requested session ID " + request.getRequestedSessionId());
        out.write("<br />\n");
        if (request.getRequestedSessionId() != null) {
            if (request.isRequestedSessionIdValid()) {
                out.write("Valid session");
            } else {
                out.write("Session expired");
            }
        } else {
            out.write("No sessionId specified => new session");
        }
    }
}