@WebServlet(name = "CounterServlet", urlPatterns = "/licznik")
public class CounterServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        HttpSession session = request.getSession();
        Integer counter = (Integer) session.getAttribute("visitCounter");

        int numberOfVisits;
        if (counter != null) {
            numberOfVisits = counter + 1;
        } else {
            numberOfVisits = 1;
        }
        session.setAttribute("visitCounter", numberOfVisits);

        RequestDispatcher rd = request.getRequestDispatcher("counter.jsp");
        rd.forward(request, response);
    }
}