一、概念

Messenger，可翻译为信使，通过它可以在不同进程中传递Message对象，在Message中放入我们需要传递的数据，就可以实现数据在进程间传递了。Messenger的底层实现是AIDL，它封装了Handler跟Binder。Messenger的构造方法如下：

//参数为Handler
public Messenger(Handler target) {
    mTarget = target.getIMessenger();
}

//参数为IBinder
public Messenger(IBinder target) {
    mTarget = IMessenger.Stub.asInterface(target);
}

二、使用

1.服务端进程

在服务端创建一个Service来处理客户端的连接请求，同时创建一个Handler并通过它来创建一个Messenger对象，然后在Service的onBind中返回这个Messenger对象底层的Binder即可。

2.客户端进程

绑定服务端的Service，绑定成功后用服务端返回的IBinder对象创建一个Messenger，通过这个Messenger就可以向服务端发送消息了，发消息类型为Message对象。如果需要服务端能够回应客户端，就和服务端一样，还需要创建一个Handler并创建一个新的Messenger，并把这个Messenger对象通过Message的replyTo参数传递给服务端，服务端通过这个replyTo参数就可以回应客户端。

三、例子

//服务端
public class MessengerService extends Service {
    private static final String TAG = MessengerService.class.getSimpleName();
    private static final int MSG_FROM_CLIENT = 0;
    private static final int MSG_FROM_SERVER = 1;

    private static class ServerHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_FROM_CLIENT:
                    Log.d(TAG, "zwm, MSG_FROM_CLIENT: " + msg.getData().getString("msg"));
                    Messenger clientMessenger = msg.replyTo;
                    Message replyMsg = Message.obtain(null, MSG_FROM_SERVER);
                    Bundle bundle = new Bundle();
                    bundle.putString("reply", "来自服务端");
                    replyMsg.setData(bundle);
                    try {
                        clientMessenger.send(replyMsg);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    private final Messenger mServerMessenger = new Messenger(new ServerHandler());

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        Log.d(TAG, "zwm, onBind");
        return mServerMessenger.getBinder();
    }
}

//客户端
public class MainActivity extends AppCompatActivity {
    private static final String TAG = MainActivity.class.getSimpleName();
    private static final int MSG_FROM_CLIENT = 0;
    private static final int MSG_FROM_SERVER = 1;
    private Messenger mServerMessenger;
    private Messenger mClientMessenger = new Messenger(new ClientHandler());

    private static class ClientHandler extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case MSG_FROM_SERVER:
                    Log.d(TAG, "zwm, MSG_FROM_SERVER: " + msg.getData().getString("reply"));
                    break;
                default:
                    super.handleMessage(msg);
            }
        }
    }

    private ServiceConnection mConnection = new ServiceConnection() {
        @Override
        public void onServiceConnected(ComponentName name, IBinder service) {
            Log.d(TAG, "zwm, onServiceConnected");
            mServerMessenger = new Messenger(service);
            Message msg = Message.obtain(null, MSG_FROM_CLIENT);
            Bundle data = new Bundle();
            data.putString("msg", "来自客户端");
            msg.setData(data);
            msg.replyTo = mClientMessenger;
            try {
                mServerMessenger.send(msg);
            } catch (RemoteException e) {
                e.printStackTrace();
            }
        }

        @Override
        public void onServiceDisconnected(ComponentName name) {
            Log.d(TAG, "zwm, onServiceDisconnected");
        }
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        Log.d(TAG, "zwm, onCreate");
        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() {
        Log.d(TAG, "zwm, onDestroy");
        unbindService(mConnection);
        super.onDestroy();
    }
}

//AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.tomorrow.messengertest">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <service android:name="MessengerService"
            android:process=":MessengerService"/>
    </application>

</manifest>