android notification not showing from service

Question

I have this service that will connect to the server and fetch notifications but unfortunately it doesn't show any notification this is the service class :

public class NotificationService extends Service {
    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
    public void startNotificationListener()
    {
      //start's a new thread
    new Thread(new Runnable() {
        @Override
        public void run() {
          //fetching notifications from server
         //if there is notifications then call this method
        ShowNotification();
        }
    }).start();
    @Override
    public void onCreate()
    {
        startNotificationListener();
        super.onCreate();
    }
    @Override
    public int onStartCommand(Intent intent,int flags,int startId)
    {
        return super.onStartCommand(intent,flags,startId);
    }
    @Override
    public void onDestroy()
    {
        super.onDestroy();
    }
    public void ShowNotification()
    {
      NotificationManager notificationManager =
                    (NotificationManager) getSystemService(Service.NOTIFICATION_SERVICE);
            Notification notification = new NotificationCompat.Builder(getBaseContext(),"notification_id")
                    .setSmallIcon(R.drawable.icon)
                    .setContentTitle("title")
                    .setContentText("content")
                    .setDefaults(NotificationCompat.DEFAULT_SOUND)
                    .build();
            notificationManager.notify(0, notification);
       //the notification is not showing

    }
}

and the notification is not showing when calling ShowNotification, I've tried pasting ShowNotification's code inside the main activity's oncreate like that

@Override
    protected void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
          NotificationManager notificationManager =
                (NotificationManager) getSystemService(Service.NOTIFICATION_SERVICE);
        Notification notification = new NotificationCompat.Builder(getBaseContext(),"n")
                .setSmallIcon(R.drawable.icon)
                .setContentTitle("Title")
                .setContentText("Content Text"))
                .setDefaults(NotificationCompat.DEFAULT_SOUND)
                .build();
        notificationManager.notify(0, notification);
    }

and it works.

Do I always need to create a notification from an activity ? if not, how to create a notification from a service ?

P.S: the service will run even if the app is not running

Answer 1

Yes you can create notification from service.but as per your code the close bracket of startNotificationListener() is missing after below code

new Thread(new Runnable() { @Override public void run() { //fetching notifications from server //if there is notifications then call this method ShowNotification(); } }).start();

and you have to register service in AndroidManifest.Xml file before </application>.

<service android:name=".NotificationService"/>

after that, you have to start service from your activity as per below.

startService(new Intent(this,NotificationService.class));

there is the code of your service:

public class NotificationService extends Service {

public void startNotificationListener() {
    //start's a new thread
    new Thread(new Runnable() {
        @Override
        public void run() {
            //fetching notifications from server
            //if there is notifications then call this method
            ShowNotification();
        }
    }).start();
}
@Override
public void onCreate()
{
    startNotificationListener();
    super.onCreate();
}
@Override
public int onStartCommand(Intent intent,int flags,int startId)
{
    return super.onStartCommand(intent,flags,startId);
}
@Override
public void onDestroy()
{
    super.onDestroy();
}

@Nullable
@Override
public IBinder onBind(Intent intent) {
    return null;
}

public void ShowNotification()
{
    NotificationManager notificationManager =
            (NotificationManager) getSystemService(Service.NOTIFICATION_SERVICE);
    Notification notification = new NotificationCompat.Builder(getBaseContext(),"notification_id")
            .setSmallIcon(R.mipmap.ic_launcher)
            .setContentTitle("title")
            .setContentText("content")
            .setDefaults(NotificationCompat.DEFAULT_SOUND)
            .build();
    notificationManager.notify(0, notification);
    //the notification is not showing

}

}

you will get the notification.please check screen shot