Firebase – Android Application Not Sending Notifications

I’m encountering an issue with sending notifications to my Android app despite having implemented all necessary additions.

I developed a WebView application and followed the common practices found on various internet sources for implementing notifications. Firstly, in MainActivity.java, within the onCreate method, I invoked:

FirebaseMessaging.getInstance().getToken()
        .addOnCompleteListener(new OnCompleteListener<String>() {
            @Override
            public void onComplete(@NonNull Task<String> task) {
                if (!task.isSuccessful()) {
                    System.out.println("Fetching FCM registration token failed");
                    return;
                }

                // Get new FCM registration token
                String token = task.getResult();

                // Log and toast
            }
        });

Furthermore, I made the necessary additions to the MyFirebaseMessagingService.java file as depicted in various online resources:

public class MyFirebaseMessagingService extends FirebaseMessagingService {
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        System.out.println("From: " + remoteMessage.getFrom());

        if (remoteMessage.getNotification() != null) {
            System.out.println("Message Notification Body: " + remoteMessage.getNotification().getBody());
        }

        sendNotification(remoteMessage.getFrom(), remoteMessage.getNotification().getBody());
    }

    private void sendNotification(String from,String body){
        new Handler(Looper.getMainLooper()).post(new Runnable() {
            @Override
            public void run() {
            }
        });
    }

    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 | PendingIntent.FLAG_IMMUTABLE);

        String channelId = "notification";
        Uri defaultSoundUri = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
        NotificationCompat.Builder notificationBuilder =
                new NotificationCompat.Builder(this, channelId)
                        .setSmallIcon(R.drawable.ic_stat_notification)
                        .setContentTitle("TeknoFore")
                        .setContentText(messageBody)
                        .setAutoCancel(true)
                        .setSound(defaultSoundUri)
                        .setContentIntent(pendingIntent);

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

        // Since android Oreo notification channel is needed.
        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 /* ID of notification */, notificationBuilder.build());
    }
}

I also declared the service in AndroidManifest.xml as follows:

<service
    android:name=".MyFirebaseMessagingService"
    android:exported="false">
    <intent-filter>
        <action android:name="com.google.firebase.MESSAGING_EVENT" />
    </intent-filter>
</service>

Additionally, I ensured that the POST_NOTIFICATIONS permission was granted and placed the google-services.json file in the app directory. However, despite these steps, neither the test notifications sent via token nor the notifications sent directly to the application are being received.

What could be causing this issue, and how can I resolve it? Any insights would be greatly appreciated. Thank you.