FCM HTTP Request returns message_id but no message is received on Android

Question

I'm trying to send notifications on my server to my Android device. I'm using Firebase Cloud Messaging for sending the notifications. I can send notifications via the Firebase Console and I receive the message on my phone. However, I'm trying to send a message via my server, which isn't working yet.

I'm getting the following response when I execute the code below:

"{\"message_id\":58934758934758936346437}"

When we look into the documentation of Firebase right here Firebase Documentation, we can see that receiving a message_id implies that the message has been send succesfully. I'm not receiving it on my phone though.

I did subscribe the app to the right topic.

I'm running the following code:

private void test(String topic) {
    try {
        //Setup request
        URL url = new URL("https://fcm.googleapis.com/fcm/send");
        HttpURLConnection hc = (HttpURLConnection) url.openConnection();
        hc.setDoOutput(true);
        hc.setDoInput(true);

        //Set request params
        String message = "{\"to\": \"/topics/" + topic + "\"}";

        hc.addRequestProperty("Content-Type", "application/json");
        hc.addRequestProperty("Authorization", "key=SECRET");

        DataOutputStream dos = new DataOutputStream(hc.getOutputStream());
        dos.writeBytes(message);
        dos.close();

        //Get Response  
        InputStream is = hc.getInputStream();
        BufferedReader rd = new BufferedReader(new InputStreamReader(is));
        StringBuilder response = new StringBuilder(); // or StringBuffer if Java version 5+
        String line;
        while ((line = rd.readLine()) != null) {
          response.append(line);
        }
        rd.close();

        label.setText("RESPONSE: " + response.toString());

    } catch (Exception ex) {
        label.setText("Er ging iets mis");
        ex.printStackTrace();
    }
}

Answer 1

Check the message format sent from the server.

For example, here is a JSON-formatted message in the same IM app as above, where the information is encapsulated in the data key and the client app is expected to interpret the content:

{
   "to" : "bk3RNwTe3H0:CI2k_HHwgIpoDKCIZvvDMExUdFQ3P1...",
   "data" : {
     "Nick" : "Mario",
     "body" : "great match!",
     "Room" : "PortugalVSDenmark"
   },
 }

For more query, please check this documentation: FCM Docs

Answer 2

You're payload does not contain any message.

String message = "{\"to\": \"/topics/" + topic + "\"}";

It only contains the recipient (to), but there is no actual message. Either send a notification or data message payload. Something like this:

String message = "{\"to\": \"/topics/" + topic + "\",
    \"notification\" : {
        \"title\" : \"sample title\",
        \"body\" : \"sample body\"
    }
}";

See the available parameters for notification and data here and note that those two message payloads are handled differently. See the Receiving Messages in Android docs for more details.