用戶登錄後，session超時

1.前臺
//判斷用戶是否登錄
function checkUser(){
	$.ajax({
		url:"/getUser",
		type:"post",
		success: function(result){
			if(result==null||result==""){
				alert("登錄超時~~");
				location = "/login";
			}else{
				$(".label").text("xxx，歡迎使用");
			}
		}
	});
}

2.後臺
	//登錄
	@RequestMapping("check")
	@ResponseBody
	public synchronized int check(String username,String pwd,HttpServletRequest request){
		int result = 0;
		if(username.equals("admin")&&pwd.equals("pwd123456")){
			result = 1;
			request.getSession().setAttribute("username", "admin");
		}
		return result;
	}

	//判斷登錄用戶session爲空
	@RequestMapping("getUser")
	@ResponseBody
	public synchronized String getUser(HttpServletRequest request){
		String user = "";
		if(request.getSession().getAttribute("username")==null){
			//獲取session爲空，表示已經超時(需要用戶重新登錄)
		}else{
			user = "admin";
		}
		return user;
	}