private String login(String username,String password,String randCode) throws IOException{
        URL url = new URL("https://kyfw.12306.cn/otn/login/loginAysnSuggest?loginUserDTO.user_name="+username+"&userDTO.password="+password+"&randCode="+randCode);
        HttpsURLConnection con = (HttpsURLConnection) url.openConnection();
        con.setSSLSocketFactory(Utils.getSsf());
        try {
            InputStream is = con.getInputStream();
            BufferedReader br = new BufferedReader(new InputStreamReader(is,"UTF-8"));
            String res = br.readLine();
            if(res==null)return null;
            String tmp = res.split("\"messages\":")[1];
            String message = tmp.substring(0,tmp.lastIndexOf(",\"validateMessages\":"));
            if(message!=null && !message.equals("")){
                return "0:"+message;
            }
            return getSessionId(con);
        } finally{
            con.disconnect();
        }
    }