Question

I am a newbie servlet programmer. I am trying to do this right.

I wrote a filter to intercept a servlet request and check if the URL needs the user to be logged in. If so the user gets directed to the login page. This is working. But then I want to redirect the user back the page he wanted to go to in the first place. What is the correct way to keep this state? Do I just store the URL in a data structure indexed using the session id from the cookie?

Answer 1

I wrote a filter to intercept a servlet request and check if the URL needs the user to be logged in.

This doesn’t seem to sound right. You should rather map the Filter on the same url-pattern of the Servlet, or better yet, on the servlet-name of the Servlet. This way the Filter is only invoked whenever the Servlet is called.

Back to your actual problem: when the user is not logged in, you have two options:

1. Store the URL in session:

   if (session.getAttribute("user") == null) {
       session.setAttribute("back", httpRequest.getRequestURI());
       httpRequest.sendRedirect("login");
   } else {
       chain.doFilter(request, response);
   }
   

   which you use on login:

   User user = userDAO.find(username, password);
   if (user != null) {
       session.setAttribute("user", user);
       String back = (String) session.getAttribute("back");
       if (back != null) {
           session.removeAttribute("back");
           response.sendRedirect(back);
       } else {
           response.sendRedirect("home"); // Home page?
       }
   } else {
       // Show error?
       request.setAttribute("message", "Unknown user, please retry");
       request.getRequestDispatcher("login").forward(request, response);
   }
   

2. Pass the URL as request parameter:

   if (session.getAttribute("user") == null) {
       httpRequest.sendRedirect("login?back=" + httpRequest.getRequestURI());
   } else {
       chain.doFilter(request, response);
   }
   

   which you pass through to subsequent request as hidden input field:

   <input type="hidden" name="back" value="${param.back}">
   

   which you use on login:

   User user = userDAO.find(username, password);
   if (user != null) {
       session.setAttribute("user", user);
       String back = request.getParameter("back");
       if (back != null) {
           response.sendRedirect(back);
       } else {
           response.sendRedirect("home"); // Home page?
       }
   } else {
       // Show error?
       request.setAttribute("message", "Unknown user, please retry");
       request.getRequestDispatcher("login").forward(request, response);
   }
   

URL encoding as some suggest is not needed as the getRequestURI() won’t be decoded.