Java 如何从服务回调调用服务

我有一个java后台服务a和服务B。我已经在Mainactivity类的onStart方法中实例化了serviceA。如果服务A无法启动，我需要启动服务B。我已经编写了一个回调方法，它在MainActivity中被调用，但是当我试图调用serviceB时，它并没有被创建

我在MainActivity中编写了一个回调方法，该方法在serviceA中被调用，但当我尝试调用serviceB时，它没有被创建

    public class MainActivity extends Activity implements ServiceCallbacks{
         private static final String TAG = "Main Activity";
         private Context myContext = null;

         Intent myTestService;
         private MyTestService serviceA;
         private boolean bound = false;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        myContext = this;
        setContentView(R.layout.activity_main);
        Log.d(TAG,"onCreate called.");       
    }

    @Override
    protected void onResume() {
        super.onResume();

        if(serviceA == null) {
            serviceA = new Intent(this, MyTestService.class);
            Log.d(TAG,"onResume called.");
            Log.d(TAG,"myTestService instance created.");
        }
    }

    @Override
    protected void onPause() {
        super.onPause();

        Log.d(TAG,"onPause called.");
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();

        Log.d(TAG,"OnDestory called");
    }  

    @Override
    protected void onStart() {
        super.onStart();
        // bind to Service
        Intent intent = new Intent(this, MyTestService.class);
        bindService(intent, serviceConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onStop() {
        super.onStop();
        // Unbind from service
        if (bound) {
            myService.setCallbacks(null); // unregister
            unbindService(serviceConnection);
            bound = false;
        }
    }

    /** Callbacks for service binding, passed to bindService() */
    private ServiceConnection serviceConnection = new ServiceConnection() {

        @Override
        public void onServiceConnected(ComponentName className, IBinder service) {
            // cast the IBinder and get MyService instance
            LocalBinder binder = (LocalBinder) service;
            myService = binder.getService();
            bound = true;
            myService.setCallbacks(MainActivity.this); // register
        }

        @Override
        public void onServiceDisconnected(ComponentName arg0) {
            bound = false;
        }
    };

    @Override
    public void callBackToActivity() {
        unbindService(serviceConnection);
        StartNewService();
    }

    private void StartNewService(){
        Intent intent = new Intent(this, serviceB.class);
        startService(intent);
    }
}

如果serviceA无法启动，我想启动serviceB。

---

我是c#开发人员，不熟悉java，需要您的帮助。

我想解除已启动的现有服务的绑定并启动新服务。我的要求是有一个java服务安装程序，并且应该能够在第一个失败时启动备份服务。serviceA和serviceB使用不同的JAR，因此我想将它们分开。我不确定，但是你有没有试过短暂的？like

private transient boolean bound=false