Java – Unable to receive messages through firebase after server reset

I have a mobile app that gets notifications from a server through Firebase Cloud Messaging.

When I launch the mobile app and subscribe, the mobile app receives a notification from the server.

The problem is when the server shuts down and restarts. After the reset, the server sends a notification to the mobile application (using the same program as before), but the mobile application does not receive any information. Keep in mind that the mobile app remains unchanged (registered) at the same time (the server resets very quickly).

The IP and port of the server remain the same…

Here is the phone code to subscribe to the correct channel:

public class MainActivity extends AppCompatActivity  {

    private static final String TAG = "MainActivity";

    private static final String NEW_CONTACTS_TOPIC = "new_contacts";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            // Create channel to show notifications.
            String channelId  = getString(R.string.default_notification_channel_id);
            String channelName = getString(R.string.default_notification_channel_name);
            NotificationManager notificationManager =
                    getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(new NotificationChannel(channelId,
                    channelName, NotificationManager.IMPORTANCE_LOW));
        }

        if (getIntent().getExtras() != null) {
            for (String key : getIntent().getExtras().keySet()) {
                Object value = getIntent().getExtras().get(key);
                Log.d(TAG, "Key: " + key + " Value: " + value);
            }
        }

        Button subscribeButton = findViewById(R.id.subscribeButton);
        subscribeButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                FirebaseMessaging.getInstance().subscribeToTopic(NEW_CONTACTS_TOPIC);

                // Log and toast
                String msg = getString(R.string.msg_subscribed);
                Log.d(TAG, msg);
                Toast.makeText(MainActivity.this, msg, Toast.LENGTH_SHORT).show();
            }
        });

        Button logTokenButton = findViewById(R.id.logTokenButton);
        logTokenButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                // Get token
                String token = FirebaseInstanceId.getInstance().getToken();

                // Log and toast
                String msg = getString(R.string.msg_token_fmt, token);
                Log.d(TAG, msg);
                Toast.makeText(MainActivity.this, msg, Toast.LENGTH_SHORT).show();
            }
        });
    }
}

public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = "MyFirebaseMsgService";

    /**
     * Called when message is received.
     *
     * @param remoteMessage Object representing the message received from Firebase Cloud Messaging.
     */
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {

        Log.d(TAG, "From: " + remoteMessage.getFrom());

        // Check if message contains a data payload.
        if (remoteMessage.getData().size() > 0) {
            Log.d(TAG, "Message data payload: " + remoteMessage.getData());
            try {
                handleNow(remoteMessage);
            } catch (IOException e) {
                Log.e(TAG, e.getMessage());
            } catch (InterruptedException e) {
                Log.e(TAG, e.getMessage());
            }
        }

        // Check if message contains a notification payload.
        if (remoteMessage.getNotification() != null) {
            Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
        }
    }
}

Here’s how my server sends notifications:

@Service
public class AndroidPushNotificationsService {

private static final String FIREBASE_SERVER_KEY = "XXXX";
private static final String FIREBASE_API_URL = "https://fcm.googleapis.com/fcm/send";

@Async
public CompletableFuture<String> send(HttpEntity<String> entity) {

    RestTemplate restTemplate = new RestTemplate();

    ArrayList<ClientHttpRequestInterceptor> interceptors = new ArrayList<>();
    interceptors.add(new HeaderRequestInterceptor("Authorization", "key=" + FIREBASE_SERVER_KEY));
    interceptors.add(new HeaderRequestInterceptor("Content-Type", "application/json"));
    restTemplate.setInterceptors(interceptors);

    String firebaseResponse = restTemplate.postForObject(FIREBASE_API_URL, entity, String.class);

    return CompletableFuture.completedFuture(firebaseResponse);
}

Important: When the server starts, the communication between the server and the mobile app can last a long time…

How do I fix this?

In some cases, FCM might not deliver messages. This occurs when there are too many messages (>100).

In the Firebase documentation it says:
onDeletedMessages() is called when the FCM server deletes pending messages.

This may be due to:

1. Too many messages are stored on the FCM server. This occurs when the application’s server sends a bunch of non-collapsible messages to the FCM server while the device is offline.
2. The device hasn’t connected for a long time, and the app server recently sent a message (within the last 4 weeks) to the app on that device.