Messenger可以翻译为信使，顾名思义，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，就可以轻松地实现数据的进程间传递了。Messenger是一种轻量级的IPC方案，它是AIDL在Android中的一种经典实践。
文章主要讲述Messenger利用AIDL进行进程间通信，其中不免会涉及到AIDL的知识点。对ADIL不熟悉的同学可以阅读我之前写过的一篇Android：IPC之AIDL的学习和总结。
【本篇文章中讲述的都是跨进程通信，在相同进程中使用Messenger文章不做讲述！！】
本来想先通过Demo来引出Messenger，然后再根据Demo一步一步分析源码。但是最后还是觉得本次应该先讲述Messenger的基础知识，结合aidl的知识分析源码，然后再通过讲述Demo深入一些分析Android当时设计Messenger的心情。 先来看看Messenger的主要成员变量和成员方法：
public final class Messenger implements Parcelable { private final IMessenger mTarget; public Messenger(Handler target) { mTarget = target.getIMessenger(); } public void send(Message message) throws RemoteException { mTarget.send(message); } public IBinder getBinder() { return mTarget.asBinder(); } /**其他代码省略**/ //重点mTarget为IMessenger.Stub.asInterface public Messenger(IBinder target) { mTarget = IMessenger.Stub.asInterface(target); } }Messenger一共有两个构造函数，一个通过IBinder构造，一个是通过Handler构造。 两种实现方式意义相同么（这是一个非常重要的问题）？
//Handler#getIMessenger final IMessenger getIMessenger() { synchronized (mQueue) { if (mMessenger != null) { return mMessenger; } mMessenger = new MessengerImpl(); return mMessenger; } } //Handler#MessengerImpl //重点MessengerImp是继承IMessenger.Stub private final class MessengerImpl extends IMessenger.Stub { public void send(Message msg) { msg.sendingUid = Binder.getCallingUid(); Handler.this.sendMessage(msg); } }IMessenger是一个aidl接口，MessengerImpl为它的实现类
package android.os; import android.os.Message; /** @hide */ oneway interface IMessenger { void send(in Message msg); }之前介绍的Android：IPC之AIDL的学习和总结文章中对aidl接口在编译后生成的Java类文件做了详细的分析和讲解。 这次我们再看一遍生成的IMessenger.java文件，对此熟悉的同学可以略过了。
/* * This file is auto-generated. DO NOT MODIFY. * Original file: frameworks/base/core/java/android/os/IMessenger.aidl */ package android.os; /** @hide */ public interface IMessenger extends android.os.IInterface { public void send(android.os.Message msg) throws android.os.RemoteException; /** Local-side IPC implementation stub class. */ public static abstract class Stub extends android.os.Binder implements android.os.IMessenger { private static final java.lang.String DESCRIPTOR = "android.os.IMessenger"; static final int TRANSACTION_send = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0); /** Construct the stub at attach it to the interface. */ public Stub() { this.attachInterface(this, DESCRIPTOR); } /** * Cast an IBinder object into an android.os.IMessenger interface, * generating a proxy if needed. */ public static android.os.IMessenger asInterface(android.os.IBinder obj) { if ((obj == null)) { return null; } android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR); if (((iin != null) && (iin instanceof android.os.IMessenger))) { return ((android.os.IMessenger) iin); } return new android.os.IMessenger.Stub.Proxy(obj); } @Override public android.os.IBinder asBinder() { return this; } @Override public boolean onTransact(int code, android.os.Parcel data, android.os.Parcel reply, int flags) throws android.os.RemoteException { switch (code) { case INTERFACE_TRANSACTION: { reply.writeString(DESCRIPTOR); return true; } case TRANSACTION_send: { data.enforceInterface(DESCRIPTOR); android.os.Message _arg0; if ((0 != data.readInt())) { _arg0 = android.os.Message.CREATOR.createFromParcel(data); } else { _arg0 = null; } this.send(_arg0); return true; } } return super.onTransact(code, data, reply, flags); } //远端代理类 private static class Proxy implements android.os.IMessenger { private android.os.IBinder mRemote; Proxy(android.os.IBinder remote) { mRemote = remote; } @Override public android.os.IBinder asBinder() { return mRemote; } public java.lang.String getInterfaceDescriptor() { return DESCRIPTOR; } @Override public void send(android.os.Message msg) throws android.os.RemoteException { android.os.Parcel _data = android.os.Parcel.obtain(); try { _data.writeInterfaceToken(DESCRIPTOR); if ((msg != null)) { _data.writeInt(1); msg.writeToParcel(_data, 0); } else { _data.writeInt(0); } mRemote.transact(Stub.TRANSACTION_send, _data, null, android.os.IBinder.FLAG_ONEWAY); } finally { _data.recycle(); } } } } }看完这个IMessenger.java这个类，我们再回一下刚才Messenger的两个构造方法。
使用Handler构造：
//Messenger#Messenger public Messenger(Handler target) { mTarget = target.getIMessenger(); } //Handler#getIMessenger final IMessenger getIMessenger() { synchronized (mQueue) { if (mMessenger != null) { return mMessenger; } mMessenger = new MessengerImpl(); return mMessenger; } } //Handler#MessengerImpl //重点MessengerImp是继承IMessenger.Stub private final class MessengerImpl extends IMessenger.Stub { public void send(Message msg) { msg.sendingUid = Binder.getCallingUid(); Handler.this.sendMessage(msg); } }使用Handler构造出来的Messenger的mTarget成员变量类型为IMessenger.Stub即Binder的实现类。
使用IBinder构造：
/**其他代码省略**/ //重点mTarget为IMessenger.Stub.asInterface public Messenger(IBinder target) { mTarget = IMessenger.Stub.asInterface(target); }IMessenger.java类中我们可以看到IMessenger.Stub.asInterface方法获得是IMessenger.Proxy类型对象，为Binder的代理类。这样构造出来的Messenger即为Binder的代理类。
通过源码的详细比较我们得出了结论：
使用Handler构造出来的Messenger为Binder的实现类；使用使用IBinder构造构造出来的Messenger为Binder的代理类；到这里我们队Messenger代码的研究就差不多结束了，接下来我们现在实现一个简单的利用Messenger在两个不同的进程进行简单通信的例子，希望通过这个Demo的讲解能将我们前面的知识消化吸收。
现在看看服务端的代码
public class MessengerService extends Service { private static final String TAG = "MessengerService"; private static class MessengerHandler extends Handler{ @Override public void handleMessage(Message msg) { super.handleMessage(msg); String msgString = msg.getData().getString("msg"); if (!TextUtils.isEmpty(msgString)) { Log.d(TAG, "handleMessage: msg = [" + msgString + "]"); } } } //创建service端处理消息的Messenger private final Messenger mMessenger = new Messenger(new MessengerHandler()); @Nullable @Override public IBinder onBind(Intent intent) { return mMessenger.getBinder(); } }客户端代码：
public class MessengerActivity extends AppCompatActivity { private static final String TAG = "MessengerActivity"; private Messenger mService; private ServiceConnection mConnection = new ServiceConnection() { @Override public void onServiceConnected(ComponentName name, IBinder service) { //构造客户端的Messenger往服务端发消息 mService = new Messenger(service); Message msg = Message.obtain(); Bundle data = new Bundle(); String msgString = "hello ,this is client."; data.putString("msg", msgString); msg.setData(data); try { mService.send(msg); } catch (RemoteException e) { e.printStackTrace(); } } @Override public void onServiceDisconnected(ComponentName name) { } }; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_messenger); bindService(new Intent(this, MessengerService.class),mConnection, Context.BIND_AUTO_CREATE); } @Override protected void onDestroy() { super.onDestroy(); unbindService(mConnection); } }将MessengerService在AndroidManifest.xml注册在另个一remote进程中：
<service android:name=".MessengerService" android:enabled="true" android:exported="true" android:process=":remote" />在com.example.ybb.aidlexample:remote进程中的Logcat输出结果：
06-28 14:55:29.793 22206-22206/com.example.ybb.aidlexample:remote D/MessengerService: handleMessage() : msg = [hello ,this is client.]是不是感觉很简单呢？那么如果在这个代码上修改一下：让客户端和服务端能互相发送和接受消息。
首相我们回想一下：
发送消息必须要得到远端的Binder对象来构造Messenger；处理消息必须新建一个Handler来构造Messenger；就以上这两点我们来重新写一下service端和Client端的代码：
Service端代码：
public class MessengerService extends Service { private static final String TAG = "MessengerService"; private static class MessengerHandler extends Handler{ boolean received = false; @Override public void handleMessage(Message msg) { super.handleMessage(msg); String msgString = msg.getData().getString("msg"); if (!TextUtils.isEmpty(msgString)) { Log.d(TAG, "handleMessage: msg = [" + msgString + "]"); } if (!received) { //利用Client端接受处理消息的Messenger来发送Message Messenger client = msg.replyTo; Message data = Message.obtain(); Bundle bundle = new Bundle(); bundle.putString("reply", "Your messages had received~!"); data.setData(bundle); try { client.send(data); } catch (RemoteException e) { e.printStackTrace(); } received = true; } } } private final Messenger mMessenger = new Messenger(new MessengerHandler()); @Nullable @Override public IBinder onBind(Intent intent) { return mMessenger.getBinder(); } }Client端代码：
public class MessengerActivity extends AppCompatActivity { private static final String TAG = "MessengerActivity"; private static class ClientHandler extends Handler { @Override public void handleMessage(Message msg) { super.handleMessage(msg); String replyString = msg.getData().getString("reply"); if (!TextUtils.isEmpty(replyString)) { Log.d(TAG, "handleMessage() : reply = [" + replyString + "]"); } } } private Messenger clientHandler = new Messenger(new ClientHandler()); private Messenger mService; private ServiceConnection mConnection = new ServiceConnection() { @Override public void onServiceConnected(ComponentName name, IBinder service) { mService = new Messenger(service); Message msg = Message.obtain(); Bundle data = new Bundle(); String msgString = "hello ,this is client."; data.putString("msg", msgString); msg.setData(data); //将处理消息的Messenger绑定到消息上带到服务端 msg.replyTo = clientHandler; try { mService.send(msg); } catch (RemoteException e) { e.printStackTrace(); } } @Override public void onServiceDisconnected(ComponentName name) { } }; @Override protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_messenger); bindService(new Intent(this, MessengerService.class),mConnection, Context.BIND_AUTO_CREATE); } @Override protected void onDestroy() { super.onDestroy(); unbindService(mConnection); } }运行结果：
文章到这里就全部讲述完啦，若有其他需要交流的可以留言哦！！
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静默加载 认证博客专家 gradle Android Studio Java 优秀的判断力来自于经验，但经验来自于错误的判断。在看着别人走过的路时，思考怎么走自己的路。无私奉献不是天方夜谈，有时候，我们也可以做到。