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。

【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~!~!】

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:

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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)?

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//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

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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。

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/*
* 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:

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//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:

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/**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

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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:

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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:

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<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:

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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:

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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:

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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:

messenger-logcat.jpg

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~!~!

想阅读作者的更多文章,可以查看我 个人博客 和公共号:

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