要保持一个Android服务长时间运行，可以使用以下方法：

1. 使用前台服务：将服务提升为前台服务，使其在系统通知栏显示一个持续运行的通知。这样可以防止系统在资源紧张时终止服务。以下是一个示例代码：

public class MyService extends Service {

    private static final int NOTIFICATION_ID = 1;
    private static final String CHANNEL_ID = "MyServiceChannel";

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        createNotificationChannel();
        Intent notificationIntent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0);

        Notification notification = new NotificationCompat.Builder(this, CHANNEL_ID)
                .setContentTitle("My Service")
                .setContentText("Service is running")
                .setSmallIcon(R.drawable.ic_notification)
                .setContentIntent(pendingIntent)
                .build();

        startForeground(NOTIFICATION_ID, notification);

        // 进行其他的服务逻辑

        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        stopForeground(true);
    }

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

    private void createNotificationChannel() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(CHANNEL_ID, "My Service Channel", NotificationManager.IMPORTANCE_DEFAULT);
            NotificationManager manager = getSystemService(NotificationManager.class);
            manager.createNotificationChannel(channel);
        }
    }
}

2. 使用AlarmManager：通过设置定时器，定期唤醒服务，以确保其持续运行。以下是一个示例代码：

public class MyService extends Service {

    private static final long INTERVAL = 60 * 1000; // 1分钟

    private AlarmManager alarmManager;
    private PendingIntent alarmIntent;

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
        Intent alarmIntent = new Intent(this, MyService.class);
        this.alarmIntent = PendingIntent.getService(this, 0, alarmIntent, 0);

        startAlarm();

        // 进行其他的服务逻辑

        return START_STICKY;
    }

    @Override
    public void onDestroy() {
        super.onDestroy();
        cancelAlarm();
    }

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

    private void startAlarm() {
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            alarmManager.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + INTERVAL, alarmIntent);
        } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            alarmManager.setExact(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + INTERVAL, alarmIntent);
        } else {
            alarmManager.set(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + INTERVAL, alarmIntent);
        }
    }

    private void cancelAlarm() {
        alarmManager.cancel(alarmIntent);
    }
}

以上是两种常用的保持Android服务长时间运行的方法，你可以根据自己的需求选择适合的方法。