How do I send notification to a specific user in Android using FCM token?

Question

I'm asking how I can send a notification to a specific user device by using the FCM token. The token is stored in the RealtimeDatabase in Firebase which is structured like this:

project-name: {
   users: {
      username: {
         name: "..."
         token: "..."
      }
   }
}

This is the code I use to store the token

    FirebaseInstanceId.getInstance().getInstanceId().addOnCompleteListener(new OnCompleteListener<InstanceIdResult>() {
    @Override
    public void onComplete(@NonNull Task<InstanceIdResult> task) {
       if (task.isSuccessful()) {
          String token = task.getResult().getToken();
          saveToken(token);
       }
    }
});
                                                    
private void saveToken(String token) {
   reference.setValue(token);
}

where "reference" is the correct pointer to the db.. this works properly. I want to use the token stored to send a push-notification to the user targeted. I also have implemented the class MyFirebaseMessagingService but I don't know how to use it to send notification to a specific user using his FCM token that I stored as I posted above.

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        Log.d(TAG, "From: " + remoteMessage.getFrom());

    }

    @Override
    public void onNewToken(String token) {
        Log.d(TAG, "Refreshed token: " + token);

        sendRegistrationToServer(token);
    }

    private void sendRegistrationToServer(String token) {
        //here I have code that store the token correctly
    }

    private void sendNotification(String messageBody) {
        Intent intent = new Intent(this, MainActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0 /* Request code */, intent,
                PendingIntent.FLAG_ONE_SHOT);

        String channelId = getString(R.string.default_notification_channel_id);
        Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
        NotificationCompat.Builder notificationBuilder =
                new NotificationCompat.Builder(this, channelId)
                        .setSmallIcon(R.drawable.ic_default)
                        .setContentTitle(getString(R.string.fcm_message))
                        .setContentText(messageBody)
                        .setAutoCancel(true)
                        .setSound(defaultSoundUri)
                        .setContentIntent(pendingIntent);

        NotificationManager notificationManager =
                (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(channelId,
                    "Channel human readable title",
                    NotificationManager.IMPORTANCE_DEFAULT);
            notificationManager.createNotificationChannel(channel);
        }

        notificationManager.notify(0, notificationBuilder.build());
    }
}

So I want to target a specific user by his FCM token and sending him a notification but I can't find a way to do this. Please help me.

Answer 1

To send the notification to a specific user you have to call this API:

https://fcm.googleapis.com/fcm/send

with Authorization:"key=YOUR_FCM_KEY" and Content-Type:"application/json" in header and the request body should be like:

{ 
  "to": "FCM Token",
  "priority": "high",
  "notification": {
    "title": "Your Title",
    "text": "Your Text"
  },
  "data": {
    "customId": "02",
    "badge": 1,
    "sound": "",
    "alert": "Alert"
  }
}

You should call this api from backend (Recommended). You can also call it from your android device, but

Be aware of hijacking your API Key

For android you can use okhttp for calling API

implementation("com.squareup.okhttp3:okhttp:4.9.0")

and the sample code will be like

public static void senPushdNotification(final String body, final String title, final String fcmToken) {
    new AsyncTask<Void, Void, Void>() {
        @Override
        protected Void doInBackground(Void... params) {
            try {
                OkHttpClient client = new OkHttpClient();
                JSONObject json = new JSONObject();
                JSONObject notificationJson = new JSONObject();
                JSONObject dataJson = new JSONObject();
                notificationJson.put("text", body);
                notificationJson.put("title", title);
                notificationJson.put("priority", "high");
                dataJson.put("customId", "02");
                dataJson.put("badge", 1);
                dataJson.put("alert", "Alert");
                json.put("notification", notificationJson);
                json.put("data", dataJson);
                json.put("to", fcmToken);
                RequestBody body = RequestBody.create(MediaType.parse("application/json; charset=utf-8"), json.toString());
                Request request = new Request.Builder()
                        .header("Authorization", "key=YOUR_FCM_KEY")
                        .url("https://fcm.googleapis.com/fcm/send")
                        .post(body)
                        .build();
                Response response = client.newCall(request).execute();
                String finalResponse = response.body().string();
                Log.i("TAG", finalResponse);
            } catch (Exception e) {

                Log.i("TAG", e.getMessage());
            }
            return null;
        }
    }.execute();
}

Answer 2

You have to create a backend server to send notifications for specific actions: https://firebase.google.com/docs/cloud-messaging/send-message

Although you can use Firebase Realtime Database with Cloud Functions to achieve that also:

https://medium.com/@97preveenraj/firebase-cloud-messaging-fcm-with-firebase-realtime-database-7388493cb869