Messenger可以在不同进程中传递Message对象，是一种轻量级的IPC方案，底层实现是AIDL，Messenger不适合大量的并发请求。

Activity和Service使用Messenger通信：

service:

public class MessengerService extends Service {

    private static final String TAG = "MessengerService";

    private static class MessengerHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case MyConstants.MSG_FROM_CLIENT:
                    Log.i(TAG,"receive msg from Client:" + msg.getData().getString("msg"));
                    Messenger client = msg.replyTo;
                    Message relpyMessage = Message.obtain(null,MyConstants.MSG_FROM_SERVICE);
                    Bundle bundle = new Bundle();
                    bundle.putString("reply","消息已经收到，稍后回复你");
                    relpyMessage.setData(bundle);
                    try {
                        client.send(relpyMessage);
                    }catch (RemoteException e){
                        e.printStackTrace();
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    private final Messenger messenger = new Messenger(new MessengerHandler());

    @Override
    public IBinder onBind(Intent intent) {
        return messenger.getBinder();
    }
}

<service android:name=".MessengerService" android:process=":remote"></service>

activity:

public class MainActivity extends AppCompatActivity {

    private static final String TAG = "MessengerActivity";

    private Messenger mService;

    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            mService = new Messenger(service);
            Message msg = Message.obtain(null,MyConstants.MSG_FROM_CLIENT);
            Bundle date = new Bundle();
            date.putString("msg","hello, this is client.");
            msg.setData(date);
            msg.replyTo = mGetReplyMessenger;
            try {
                mService.send(msg);
            }catch (RemoteException e){
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
        }
    };
    private Messenger mGetReplyMessenger = new Messenger(new MessengerHandler());

    private static class MessengerHandler extends Handler{
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what){
                case MyConstants.MSG_FROM_SERVICE:
                    Log.i(TAG,"receive msg from Service:" + msg.getData().getString("reply"));
                    break;
                default:
                    super.handleMessage(msg);
            }

        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Intent intent = new Intent(this,MessengerService.class);
        bindService(intent,mConnection, Context.BIND_AUTO_CREATE);
    }

    @Override
    protected void onDestroy() {
        super.onDestroy();
        unbindService(mConnection);
    }
}