Android foreground service notification not showing

It’s because of Android O bg services restrictions.

So now you need to call startForeground() only for services that were started with startForegroundService() and call it in first 5 seconds after service has been started.

Here is the guide – https://developer.android.com/about/versions/oreo/background#services

Like this:

//Start service:
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
  startForegroundService(new Intent(this, YourService.class));
} else {
  startService(new Intent(this, YourService.class));
}

Then create and show notification (with channel as supposed earlier):

private void createAndShowForegroundNotification(Service yourService, int notificationId) {

    final NotificationCompat.Builder builder = getNotificationBuilder(yourService,
         "com.example.your_app.notification.CHANNEL_ID_FOREGROUND", // Channel id
    NotificationManagerCompat.IMPORTANCE_LOW); //Low importance prevent visual appearance for this notification channel on top 
    builder.setOngoing(true)
    .setSmallIcon(R.drawable.small_icon)
    .setContentTitle(yourService.getString(R.string.title))
    .setContentText(yourService.getString(R.string.content));

    Notification notification = builder.build();

    yourService.startForeground(notificationId, notification);

    if (notificationId != lastShownNotificationId) {
          // Cancel previous notification
          final NotificationManager nm = (NotificationManager) yourService.getSystemService(Activity.NOTIFICATION_SERVICE);
          nm.cancel(lastShownNotificationId);
    }
    lastShownNotificationId = notificationId;
}

public static NotificationCompat.Builder getNotificationBuilder(Context context, String channelId, int importance) {
    NotificationCompat.Builder builder;
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        prepareChannel(context, channelId, importance);
        builder = new NotificationCompat.Builder(context, channelId);
    } else {
        builder = new NotificationCompat.Builder(context);
    }
    return builder;
}

@TargetApi(26)
private static void prepareChannel(Context context, String id, int importance) {
    final String appName = context.getString(R.string.app_name);
    String description = context.getString(R.string.notifications_channel_description);
    final NotificationManager nm = (NotificationManager) context.getSystemService(Activity.NOTIFICATION_SERVICE);

    if(nm != null) {
        NotificationChannel nChannel = nm.getNotificationChannel(id);

        if (nChannel == null) {
            nChannel = new NotificationChannel(id, appName, importance);
            nChannel.setDescription(description);
            nm.createNotificationChannel(nChannel);
        }
    }
}

Remember that your foreground notification will have the same state as your other notifications even if you’ll use different channel ids, so it might be hidden as a group with others. Use different groups to avoid it.