code: public void doUnlink(HttpServletRequest request, HttpServletResponse response)
              throws IOException, ServletException {
          if (!common.checkPost(request, response)) return;
          User user = common.requireLoggedInUser(request, response);
          if (user == null) return;
  
          // Remove the access token from the database.
          user.dropboxAccessToken = null;
          common.saveUserDb();
  
          response.sendRedirect("/");
      }
docs: -------------------------------------------------------------------------------------------