Timer Task stops after hibernation

public class Service {
private static ScheduledFuture<?> mainTask;
public static void startService() {
    ScheduledExecutorService st = Executors.newSingleThreadScheduledExecutor();
        //Start checking for new notification
        mainTask = st.scheduleAtFixedRate(new NotificationTask(), 0, 5, TimeUnit.SECONDS);
//        Timer t = new Timer();
//        t.schedule(new NotificationTask(), 0, 5000);
}

static class NotificationTask extends TimerTask implements Runnable {

        private JSONObject notifications;
        private JSONObject old;

        public NotificationTask() {
            old = new JSONObject();
        }

        @Override
        public void run() {
            try {
                //Read new notification
                notifications = WebConnection.getInstance().readNotifications();
                if (!notifications.equals(old)) {
                //
                    processOrderNotification();
                }

            } catch (IOException | ParseException ex) {
                //probably Network issues
//                log for debugging purposes
                Utility.writeLog(ex.toString());
            } catch (Exception e) {
//                Every other error should fall in here
                Utility.writeLog(e);
            }
        }

    }
}