@RequestMapping("/login")
 public String login(HttpServletRequest request, HttpServletResponse response, Map<String, Object> map) throws Exception{ 
  String userName = request.getParameter("userName");
  String password = request.getParameter("password");
  HttpSession session = request.getSession();
  ServletContext application = session.getServletContext(); //获取application
  
  User user = new User(userName, password);
  User currentUser = userService.login(user);
  if (currentUser == null) {
   request.setAttribute("error", "用户名或密码错误");
   return "login";
  }
  
  String userId = String.valueOf(currentUser.getId()); //获取userId
  map.put("currentUser", currentUser); //将user保存到session中，注意要使用@SessionAttributes注释
  if(application.getAttribute("userId") == null){ //说明还没有用户登录
   application.setAttribute(userId, session.getId()); //将sessionId保存到application
   application.setAttribute(session.getId(), session); //将session保存到application
  }else{ //说明已经有用户登录
   String sessionId = (String) application.getAttribute(userId); //根据userId获取上一个用户的sessionId
   HttpSession oldSession = (HttpSession) application.getAttribute(sessionId); //根据sessionId获取上一个用户的session
   oldSession.invalidate(); //使oldSession失效
   application.removeAttribute(oldSession.getId()); //将oldSession从application中移除
   application.setAttribute(session.getId(), session); //将新的session保存到application
   application.removeAttribute(userId); //将oldSession的id从application中移除
   application.setAttribute(userId, session.getId()); //将新的session的Id保存到application
  }
  return "main";
 }