Why does not show notification on Android 9

Question

Does not show notification on Android 9.0 and works great on 8 and below. I had trouble with old code and I read that is needed to pass Channel on new android version, I got this code from internet but still is not working on Android 9.0. I don't know why. Any help will be appreciated.

As I read through internet everything looks good. But notification does not show.

    private void showSmallNotification( int icon, String title, String message, String timeStamp, PendingIntent resultPendingIntent){
        String CHANNEL_ID = Constants.ChannelID;
        String CHANNEL_NAME = "Notification";

        NotificationManager notificationManager = (NotificationManager) this.getSystemService(Context.NOTIFICATION_SERVICE);;
        NotificationCompat.Builder notification;
        NotificationCompat.InboxStyle inboxStyle = new NotificationCompat.InboxStyle();

        inboxStyle.addLine(message);

        if (Build.VERSION.SDK_INT >= 26) {
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, CHANNEL_NAME, NotificationManager.IMPORTANCE_HIGH);
            channel.enableVibration(true);
            channel.setLightColor(Color.BLUE);
            channel.enableLights(true);
            channel.setSound(Uri.parse(ContentResolver.SCHEME_ANDROID_RESOURCE + "://" + getApplicationContext().getPackageName() + "/" + R.raw.notification),
                    new AudioAttributes.Builder()
                            .setContentType(AudioAttributes.CONTENT_TYPE_SONIFICATION)
                            .setUsage(AudioAttributes.USAGE_NOTIFICATION)
                            .build());
            channel.canShowBadge();
            notificationManager.createNotificationChannel(channel);
            notification = new NotificationCompat.Builder(getApplicationContext(), CHANNEL_ID);
        } else {
            notification = new NotificationCompat.Builder(getApplicationContext(),CHANNEL_ID);
        }
        notification
                .setVibrate(new long[]{0, 100})
                .setPriority(Notification.PRIORITY_MAX)
                .setLights(Color.BLUE, 3000, 3000)
                .setAutoCancel(true)
                .setContentTitle(title)
                .setContentIntent(resultPendingIntent)
                .setWhen(getTimeMilliSec(timeStamp))
                .setSmallIcon(R.mipmap.ic_launcher)
                .setStyle(inboxStyle)
                .setLargeIcon(BitmapFactory.decodeResource(this.getResources(), icon))
                .setContentText(message)
                .build();

        notificationManager.notify(CHANNEL_ID, 1, notification.build());
    }

Answer 1

Okay this code now works for me, and also teseted until android 12 First I create Channel

// Create Channel For Notifications (if does not exist)
private void createNotificationChannel() {
    // Create the NotificationChannel, but only on API 26+ because
    // the NotificationChannel class is new and not in the support library
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationManager mNotificationManager = getSystemService(NotificationManager.class);
        NotificationChannel existingChannel = mNotificationManager.getNotificationChannel(Constants.notification_channel_ID);
        // If channel does not exist then create.
        if (existingChannel == null) {
            CharSequence name = getResources().getString(R.string.app_name);
            String description = getResources().getString(R.string.notification_description);
            int importance = NotificationManager.IMPORTANCE_DEFAULT;
            NotificationChannel notificationChannel = new NotificationChannel( Constants.notification_channel_ID , name , importance) ;
            notificationChannel.setDescription(description);
            notificationChannel.enableLights(true) ;
            notificationChannel.setLightColor(Color.BLUE) ;
            NotificationManager notificationManager = getSystemService(NotificationManager.class);
            notificationManager.createNotificationChannel(notificationChannel);
        }
    }
}

Maybe is not needed to check if channel exists but I check. Then when I need to show notification I call this method:

private void notification(){
     Intent intent = new Intent(this, MainActivity.class);
     intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
     intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
     PendingIntent pendingIntent = Build.VERSION.SDK_INT >= Build.VERSION_CODES.M ? PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE) : PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
    
     NotificationCompat.Builder builder = new NotificationCompat.Builder(this, Constants.notification_channel_ID)
                                .setChannelId(Constants.notification_channel_ID)
                                .setSmallIcon(R.mipmap.ic_launcher)
                                .setContentTitle(this.getResources().getString(R.string.app_name))
                                .setContentText(this.getResources().getString(R.string.description))
                                .setContentIntent(pendingIntent)
                                .setAutoCancel(true);
      NotificationManagerCompat notificationManager = NotificationManagerCompat.from(this);
      notificationManager.notify(Constants.NOTIFICATION_ID, builder.build());
}