深入解析AIDL的实现：Messenger

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#getIMessengerfinal IMessenger getIMessenger() {    synchronized (mQueue) {        if (mMessenger != null) {            return mMessenger;        }        mMessenger = new MessengerImpl();        return mMessenger;    }}//Handler#MessengerImpl//重点MessengerImp是继承IMessenger.Stubprivate 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#Messengerpublic Messenger(Handler target) {        mTarget = target.getIMessenger();    }//Handler#getIMessengerfinal IMessenger getIMessenger() {    synchronized (mQueue) {        if (mMessenger != null) {            return mMessenger;        }        mMessenger = new MessengerImpl();        return mMessenger;    }}//Handler#MessengerImpl//重点MessengerImp是继承IMessenger.Stubprivate 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.asInterfacepublic 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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