ActivityManagerNative.getDefault()
                .startActivity

public abstract class ActivityManagerNative extends Binder implements IActivityManager
{

//从类声明上，我们可以看到ActivityManagerNative是Binder的一个子类，而且实现了IActivityManager接口
 static public IActivityManager getDefault() {
        return gDefault.get();
    }

 //通过单例模式获取一个IActivityManager对象，这个对象通过asInterface(b)获得
 private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {
            IBinder b = ServiceManager.getService("activity");
            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };
}


//最终返回的还是一个ActivityManagerProxy对象
static public IActivityManager asInterface(IBinder obj) {
        if (obj == null) {
            return null;
        }
        IActivityManager in =
            (IActivityManager)obj.queryLocalInterface(descriptor);
        if (in != null) {
            return in;
        }

     //这里面的Binder类型的obj参数会作为ActivityManagerProxy的成员变量保存为mRemote成员变量，负责进行IPC通信
        return new ActivityManagerProxy(obj);
    }


}

private class ApplicationThread extends ApplicationThreadNative {}

  public abstract class ApplicationThreadNative extends Binder implements IApplicationThread{}

  class ApplicationThreadProxy implements IApplicationThread {}