问Firebase服务在后台运行时无法创建新通知

EN

public class FirebaseNotifications extends FirebaseMessagingService {

String lastChange = "";
String currentChange = "";

@Override
public void onCreate() {
    super.onCreate();
    Log.i("firebase", "it started");
    SharedPreferences settings = getSharedPreferences(PREFS, 0);
    lastChange = settings.getString("lastFbInput", "");
    Log.i("lastChange", lastChange);
}

@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
    super.onMessageReceived(remoteMessage);

    currentChange = remoteMessage.getNotification().getBody();
    Log.i("from: ", remoteMessage.getFrom() + "");
    if(remoteMessage.getData().size()>0)
        Log.i("payload: ", remoteMessage.getData().toString());

    if(remoteMessage.getNotification() != null)
        Log.i("message body: ",  remoteMessage.getNotification().getBody());

    if(!lastChange.equals("")) {
        Log.i("equals", "1");
        if (!lastChange.equals(currentChange)) {
            showNotification(remoteMessage.getNotification().getBody());
            Log.i("equals", "2");
        }
    }
    else
    {
        showNotification(remoteMessage.getNotification().getBody());
        Log.i("equals", "3");
    }

}