源码位置

android.app.ActivityManagerNative.java

public abstract class ActivityManagerNative extends Binder 
implements IActivityManager{}

ActivityManagerNative是一个抽象类,继承了Binder类，实现了IActivityManager接口
ActivityManagerService是它的子类，也就实现了IActivityManager接口，
下面的ActivityManagerProxy类也实现了IActivityManager接口

getDefault()方法, 最终得到的是一个ActivityManagerProxy对象

static public IActivityManager getDefault() {
    return gDefault.get();
}

查看gDefault

private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
        protected IActivityManager create() {

           //获取到ActivityManagerService的Binder
            IBinder b = ServiceManager.getService("activity");

            if (false) {
                Log.v("ActivityManager", "default service binder = " + b);
            }

            //得到一个ActivityManagerProxy对象
            IActivityManager am = asInterface(b);
            if (false) {
                Log.v("ActivityManager", "default service = " + am);
            }
            return am;
        }
    };

查看asInterface方法,返回了一个ActivityManagerProxy对象

  /**
    * Cast a Binder object into an activity manager interface, generating
    * a proxy if needed.
    * 把一个Binder对象强转为一个activity manger 接口，生成一个代理对象
    */
static public IActivityManager asInterface(IBinder obj) {
       if (obj == null) {
           return null;
       }

        //String descriptor  = "android.app.IActivityManager";

       IActivityManager in =
           (IActivityManager)obj.queryLocalInterface(descriptor);

       if (in != null) {
           return in;
       }
       //返回了一个ActivityManagerProxy

       return new ActivityManagerProxy(obj);

   }

查看queryLocalInterface方法

/**
 * Attempt to retrieve a local implementation of an interface
 * for this Binder object.  If null is returned, you will need
 * to instantiate a proxy class to marshall calls through
 * the transact() method.
 *    尝试为这个binder对象取出一个本地接口实现。如果返回null,你需要通过transaction()实例化一个代理对象
 *    引领调用命令
 *
 */
public IInterface queryLocalInterface(String descriptor);

android.os.Binder.java,实现了IBinder接口

/**
  * Use information supplied to attachInterface() to return the
  * associated IInterface if it matches the requested
  * descriptor.
  */
 public IInterface queryLocalInterface(String descriptor) {
     if (mDescriptor.equals(descriptor)) {
         return mOwner;
     }
     return null;
 }

查看attachInterface()方法

/**
 * Convenience method for associating a specific interface with the Binder.
 * After calling, queryLocalInterface() will be implemented for you
 * to return the given owner IInterface when the corresponding
 * descriptor is requested.
 这个方法把一个特定的接口和Binder关联在一起。调用这个方法之后，queryLocalInterface() 会被实现，然后当相关的descriptor
 被请求的时候，你就可以把owner IInterface返回给他
 */
public void attachInterface(IInterface owner, String descriptor) {
    mOwner = owner;
    mDescriptor = descriptor;
}

最后查看ActivityManagerProxy类,这是ActivityManagerNative的内部类。它实现了IActivityManager接口，
通过Binder通信，最终会调用ActivityManagerService里面的类似startActivity等等的方法

class ActivityManagerProxy implements IActivityManager
{
    public ActivityManagerProxy(IBinder remote)
    {
        mRemote = remote;
    }

    public IBinder asBinder()
    {
        return mRemote;
    }

    public int startActivity(IApplicationThread caller, String callingPackage, Intent intent,
            String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle options) throws RemoteException {
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        data.writeString(callingPackage);
        intent.writeToParcel(data, 0);
        data.writeString(resolvedType);
        data.writeStrongBinder(resultTo);
        data.writeString(resultWho);
        data.writeInt(requestCode);
        data.writeInt(startFlags);
        if (profilerInfo != null) {
            data.writeInt(1);
            profilerInfo.writeToParcel(data, Parcelable.PARCELABLE_WRITE_RETURN_VALUE);
        } else {
            data.writeInt(0);
        }
        if (options != null) {
            data.writeInt(1);
            options.writeToParcel(data, 0);
        } else {
            data.writeInt(0);
        }
        mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
        reply.readException();
        int result = reply.readInt();
        reply.recycle();
        data.recycle();
        return result;
    }

    ...
}

###转到ActivityManagerService-startActivity