application作用域实现用户登录挤掉之前登录用户代码

@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"; }