要创建一个始终运行的服务来监控用户的位置，你可以使用Android的后台服务（Foreground Service）和位置服务（Location Service）来实现。以下是一个基本的示例代码：

1. 创建一个继承自Service的类，用于实现后台服务：

public class LocationService extends Service {
    private static final String CHANNEL_ID = "LocationServiceChannel";
    private LocationManager locationManager;

    @Override
    public void onCreate() {
        super.onCreate();
        locationManager = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
    }

    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        createNotificationChannel();
        startForeground(1, createNotification());
        startLocationUpdates();
        return START_STICKY;
    }

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

    private void startLocationUpdates() {
        LocationListener locationListener = new LocationListener() {
            @Override
            public void onLocationChanged(Location location) {
                // 处理位置更新
            }

            @Override
            public void onStatusChanged(String provider, int status, Bundle extras) {}

            @Override
            public void onProviderEnabled(String provider) {}

            @Override
            public void onProviderDisabled(String provider) {}
        };

        locationManager.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1000, 0, locationListener);
    }

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

    private Notification createNotification() {
        Intent notificationIntent = new Intent(this, MainActivity.class);
        PendingIntent pendingIntent = PendingIntent.getActivity(
                this,
                0,
                notificationIntent,
                0
        );

        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, CHANNEL_ID)
                .setContentTitle("Location Service")
                .setContentText("Monitoring user location...")
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentIntent(pendingIntent);

        return builder.build();
    }
}

2. 在AndroidManifest.xml文件中声明服务：

3. 在你的Activity或Application中启动该服务：

Intent serviceIntent = new Intent(this, LocationService.class);
startService(serviceIntent);

这将启动一个后台服务，并通过GPS定位提供程序每秒更新一次用户的位置。你可以根据需要对位置更新进行处理。注意，在使用此代码之前，请确保已经在AndroidManifest.xml文件中添加了相应的权限：

这些权限用于访问设备的位置信息。