java为什么startService（）和bindService（）仅在活动的onStart（）方法中使用时才起作用？

Question

最初，我尝试使用MainActivity中的函数启动和绑定我的服务，如下所示：

public void startTimerService(){
    Intent intent = new Intent(this, TimerService.class);
    startService(intent);

    ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            Log.d("MainActivity", "onServiceConnected");
            TimerService.LocalBinder binder = (TimerService.LocalBinder) iBinder;
            timerService = binder.getService();
            boundToTimer = true;

        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {
            boundToTimer = false;
            Log.d("MainActivity", "onServiceDisconnected");
        }
    };
    Intent intent2 = new Intent(this, TimerService.class);

    bindService(intent2, mConnection, Context.BIND_AUTO_CREATE);
    timerService.startTimer();
}

当我试图访问timerService.startTimer();时，这给了我一个nullPointerException，但是，当我将代码移动到onStart()方法时，如下所示：

@Override
public void onStart(){
    super.onStart();
    Intent intent = new Intent(this, TimerService.class);
    startService(intent);

    ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName componentName, IBinder iBinder) {
            Log.d("MainActivity", "onServiceConnected");
            TimerService.LocalBinder binder = (TimerService.LocalBinder) iBinder;
            timerService = binder.getService();
            boundToTimer = true;

        }

        @Override
        public void onServiceDisconnected(ComponentName componentName) {
            boundToTimer = false;
            Log.d("MainActivity", "onServiceDisconnected");
        }
    };
    Intent intent2 = new Intent(this, TimerService.class);

    bindService(intent2, mConnection, Context.BIND_AUTO_CREATE);
}

并且只在startTimerService()方法中调用timerService.startTimer();方法，一切都很好。唯一的问题是我不知道为什么会这样，也许有人能启发我