Android : Java/Servlet Notification API

I am making an API for sending notification to android mobile using Google cloud. I have done this on servlet,

MulticastResult result = null;
try {
    String share = "APA91bERgNUDKD80bGsF-10vnbCbZxLbE6yL1HHLqtJK7jbLvXTDYa0R3urM7cL3R-TqeWOEZTdzSl45oVsNcOHU-Ykt8RjgJsepOJ96wHtSjRhlkqM5vQMAFzGdeGjsYU3bVFwEbtWLcHQEkXXg6RTeyrKAael-_HM_1uJPzCQvW_Wu9K5Q05A";
    String userMessage = "Hello";
    String GOOGLE_SERVER_KEY = "AIzaSyBojZP8afz3MdUcHYoQeRsgvh3nI2wfv50";
    Sender sender = new Sender(GOOGLE_SERVER_KEY);
    String MESSAGE_KEY = "message";
    Message message = new Message.Builder().timeToLive(30).delayWhileIdle(true).addData(MESSAGE_KEY, userMessage).build();
    List<String> regIdList = new ArrayList<String>();
    regIdList.add(share);
    result = sender.send(message, regIdList, 1);
    request.setAttribute("pushStatus", result.toString());
}
catch(Exception ioe){
    ioe.printStackTrace();
}

But it gives error that, cannot retry due to server authentication, in streaming mode. I can't figure out what this error is. Is this the right method for notification or is there another method present for this?