@RequestMapping(value = "/main_page")
 public String mainPage(Model model) {
   if (!userService.isAuth()) {
     model.addAttribute("message", "Access denied.\n");
     return "login";
   }
   return "main_page";
 }
 @RequestMapping(value = "/logout")
 public String logout(Model model) {
   if (userService.isAuth()) userService.unsetAuth();
   return "redirect:/";
 }