官方文档解释：它引用了一个Handler对象，以便others能够向它发送消息(使用mMessenger.send(Message msg)方法)。该类允许跨进程间基于Message的通信(即两个进程间可以通过Message进行通信)，在服务端使用Handler创建一个Messenger，客户端持有这个Messenger就可以与服务端通信了。
以前我们使用Handler+Message的方式进行通信，都是在同一个进程中，从线程持有一个主线程的Handler对象，并向主线程发送消息。
而Android既然可以使用bindler机制进行跨进行通信，所以我们当然可以将Handler与bindler结合起来进行跨进程发送消息。
查看API就可以发现，Messenger就是这种方式的实现。
一般使用方法如下：
１。远程通过
mMessenger = new Messenger(mHandler)
创建一个信使对象
２。客户端使用bindlerService请求连接远程
３。远程onBind方法返回一个bindler
return mMessenger.getBinder();
4.客户端使用远程返回的bindler得到一个信使（即得到远程信使）
public void onServiceConnected(ComponentName name, IBinder service) {
rMessenger = new Messenger(service);
......
}
这里虽然是new了一个Messenger，但我们查看它的实现
public Messenger(IBinder target) { mTarget = IMessenger.Stub.asInterface(target); }
发现它的mTarget是通过Aidl得到的，实际上就是远程创建的那个。
５。客户端可以使用这个远程信使对象向远程发送消息：rMessenger.send(msg);
这样远程服务端的Handler对象就能收到消息了，然后可以在其handlerMessage(Message msg)方法中进行处理。（该Handler对象就是第一步服务端创建Messenger时使用的参数mHandler）.
经过这５个步骤貌似只有客户端向服务端发送消息，这样的消息传递是单向的，那么如何实现双向传递呢？
首先需要在第５步稍加修改，在send(msg)前通过msm.replyTo = mMessenger将自己的信使设置到消息中，这样服务端接收到消息时同时也得到了客户端的信使对象了，然后服务端可以通过/得到客户端的信使对象，并向它发送消息 cMessenger = msg.replyTo; cMessenger.send(message);
即完成了从服务端向客户端发送消息的功能，这样客服端可以在自己的Handler对象的handlerMessage方法中接收服务端发送来的message进行处理。
双向通信宣告完成。
以下代码来自ApiDemo
Service code:
[java]
public class MessengerService extends Service { /** For showing and hiding our notification. */ NotificationManager mNM; /** Keeps track of all current registered clients. */ ArrayList<Messenger> mClients = new ArrayList<Messenger>(); /** Holds last value set by a client. */ int mValue = 0; /** * Command to the service to register a client, receiving callbacks * from the service. The Message's replyTo field must be a Messenger of * the client where callbacks should be sent. */ static final int MSG_REGISTER_CLIENT = 1; /** * Command to the service to unregister a client, ot stop receiving callbacks * from the service. The Message's replyTo field must be a Messenger of * the client as previously given with MSG_REGISTER_CLIENT. */ static final int MSG_UNREGISTER_CLIENT = 2; /** * Command to service to set a new value. This can be sent to the * service to supply a new value, and will be sent by the service to * any registered clients with the new value. */ static final int MSG_SET_VALUE = 3; /** * Handler of incoming messages from clients. */ class IncomingHandler extends Handler { @Override public void handleMessage(Message msg) { switch (msg.what) { case MSG_REGISTER_CLIENT: mClients.add(msg.replyTo); break; case MSG_UNREGISTER_CLIENT: mClients.remove(msg.replyTo); break; case MSG_SET_VALUE: mValue = msg.arg1; for (int i = mClients.size() - 1; i >= 0; i --) { try { mClients.get(i).send(Message.obtain(null, MSG_SET_VALUE, mValue, 0)); } catch (RemoteException e) { // The client is dead. Remove it from the list; // we are going through the list from back to front // so this is safe to do inside the loop. mClients.remove(i); } } break; default: super.handleMessage(msg); } } } /** * Target we publish for clients to send messages to IncomingHandler. */ final Messenger mMessenger = new Messenger(new IncomingHandler()); @Override public void onCreate() { mNM = (NotificationManager)getSystemService(NOTIFICATION_SERVICE); // Display a notification about us starting. showNotification(); } @Override public void onDestroy() { // Cancel the persistent notification. mNM.cancel(R.string.remote_service_started); // Tell the user we stopped. Toast.makeText(this, R.string.remote_service_stopped, Toast.LENGTH_SHORT).show(); } /** * When binding to the service, we return an interface to our messenger * for sending messages to the service. */ @Override public IBinder onBind(Intent intent) { return mMessenger.getBinder(); } /** * Show a notification while this service is running. */ private void showNotification() { // In this sample, we'll use the same text for the ticker and the expanded notification CharSequence text = getText(R.string.remote_service_started); // Set the icon, scrolling text and timestamp Notification notification = new Notification(R.drawable.stat_sample, text, System.currentTimeMillis()); // The PendingIntent to launch our activity if the user selects this notification PendingIntent contentIntent = PendingIntent.getActivity(this, 0, new Intent(this, Controller.class), 0); // Set the info for the views that show in the notification panel. notification.setLatestEventInfo(this, getText(R.string.remote_service_label), text, contentIntent); // Send the notification. // We use a string id because it is a unique number. We use it later to cancel. mNM.notify(R.string.remote_service_started, notification); } }