背景
这几天在学习安卓的进程间通信方式，方学AIDL，又看Messenger。
Messenger可以和Handler结合，从而使IPC就像线程间通信一样
而Messenger的实现原理，其实也是用的AIDL，如果对AIDL不甚熟悉，可以参见我的文章安卓开发学习之AIDL的使用
使用步骤
1、在服务端建立Service
内部构造一个messenger，做为接收客户端来信的信使，代码如下
public class MessengerService extends Service {
private Messenger messenger = new Messenger(new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 0:
try {
Bundle content = (Bundle) msg.obj;
String info = content.getString("info", "nothing");
System.out.println(info);
Message replyMesg = Message.obtain();
replyMesg.what = 0;
content.clear();
content.putString("info", "Got it，service pid:" + Process.myPid());
replyMesg.obj = content;
msg.replyTo.send(replyMesg); // replyTo也是一个Messenger
} catch (Exception e) {
e.printStackTrace();
}
break;
}
}
});
@Override
public IBinder onBind(Intent intent) {
return messenger.getBinder(); // 一会儿将看到，getBinder()返回的是一个Stub对象
}
}
在清单文件中注册，把name填成:remote，让service做为新的进程，代码如下
<service android:name=".MessengerService"
android:exported="true"
android:process=":remote">
<intent-filter>
<action android:name="com.example.songzeceng.Messenger"></action>
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
</service>
2、客户端建立连接
建立两个Messenger，ServiceMessenger用来向服务端发送数据，ClientMessenger用来从服务端接收数据，代码如下
public class MainActivity extends Activity {
public static final String TAG = "MainActivity";
private boolean isConnected = false;
private Messenger serviceMessenger;
private Messenger clientMessenger = new Messenger(new Handler() {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case 0:
Bundle content = (Bundle) msg.obj;
String info = content.getString("info", "nothing");
logger(info);
break;
}
}
});
private ServiceConnection connection = new ServiceConnection() {
@Override
public void onServiceConnected(ComponentName name, IBinder service) {
try {
System.out.println(service.getClass().getCanonicalName());
serviceMessenger = new Messenger(service); // 此时ServiceMessenger内部负责传递的则是Proxy类
Message msg = Message.obtain();
msg.what = 0;
// msg.obj = "客户端来信，pid:" + Process.myPid(); // IPC时传递的数据类型必须实现Parcelable接口，然而String没有实现
Bundle bundle = new Bundle();
bundle.putString("info", "客户端来信，pid:" + Process.myPid());
msg.obj = bundle;
msg.replyTo = clientMessenger;
serviceMessenger.send(msg);
isConnected = true;
} catch (Exception e) {
e.printStackTrace();
}
}
@Override
public void onServiceDisconnected(ComponentName name) {
isConnected = false;
}
};
private void logger(String s) {
Log.i(TAG, s);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
@Override
protected void onStart() {
super.onStart();
if (!isConnected) {
Intent intent = new Intent(this, MessengerService.class);
intent.setAction("com.example.songzeceng.Messenger");
bindService(intent, connection, BIND_AUTO_CREATE);
}
}
@Override
protected void onStop() {
super.onStop();
if (isConnected) {
unbindService(connection);
isConnected = false;
}
}
}
运行结果
服务端截图
客户端截图
可见，实现了进程间的通信
源码分析
1、服务端
从服务端开始，进行源码分析
我们先是构造了一个Messenger对象，给构造方法传入一个Handler对象，Messenger的这个构造方法代码如下
public Messenger(Handler target) {
mTarget = target.getIMessenger();
}
mTarget是IMessenger类型的，这个IMessenger就是用aidl生成的接口类，而后面调用了Handler的getIMessenger()方法，这个方法源码如下
final IMessenger getIMessenger() {
synchronized (mQueue) {
if (mMessenger != null) {
return mMessenger;
}
mMessenger = new MessengerImpl();
return mMessenger;
}
}
返回的是一个MessengerImpl对象，MessengerImpl是Handler的内部类，源码如下
private final class MessengerImpl extends IMessenger.Stub {
public void send(Message msg) {
msg.sendingUid = Binder.getCallingUid();
Handler.this.sendMessage(msg);
}
}
相当于IMessenger.Stub对象，这似乎也是Stub的唯一子类，所以我也认为Stub的send()方法就是最终调用了handler的sendMessage()方法
好，返回Messenger的构造方法，服务端的Messenger里的mTarget就是一个MessengerImpl对象，也是一个Stub对象
之后客户端连接service，服务端调用onBind()，代码如下
@Override
public IBinder onBind(Intent intent) {
return messenger.getBinder();
}
调用了messenger.getBinder()，此方法代码如下
public IBinder getBinder() {
return mTarget.asBinder();
}
调用的是mTarget.asBinder()，我方才说过，mTarget是一个IMessenger.Stub，这个类的源码如下
public interface IMessenger extends android.os.IInterface {
/**
* 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";
/**
* 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 = (android.os.IInterface) obj
.queryLocalInterface(DESCRIPTOR);
if (((iin != null) && (iin instanceof android.os.IMessenger))) {
return ((android.os.IMessenger) iin);
}
return new android.os.IMessenger.Stub.Proxy(obj);
}
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);
}
static final int TRANSACTION_send = (android.os.IBinder.FIRST_CALL_TRANSACTION + 0);
}
public void send(android.os.Message msg) throws android.os.RemoteException;
}
asBinder()方法返回的是Stub自己，所以messenger.getBinder()方法，返回的是handler里面的IMessengerImpl对象，也就是Stub对象
2、客户端
客户端直接看onServiceConnected()里的内容，上来根据服务端onBind()方法返回的IBinder对象，构造serviceMessenger，这个构造方法代码如下
public Messenger(IBinder target) {
mTarget = IMessenger.Stub.asInterface(target);
}
翻翻上面的Stub的asInterface()方法，会发现如果客户端本地没有IMessenger接口对象，就返回Proxy对象，而Proxy对象里的mRemote，就是服务端返回的IBinder，也就是一个Stub；但如果已经有了IMessenger实现类对象，就返回那个对象
现在，虽然之前在客户端我们实例化了一个ClientMessenger，但它里面的handler的IMessengerImpl和服务端返回的IMessengerImpl是两个对象，所以asInterface()还是返回的是Proxy对象
通过反射获取serviceMessenger.mTarget类型的结果如下
所以当我们通过serviceMessenger发送消息时，调用的是Proxy的send()方法，通过clientMessenger接收消息时，调用的是Stub的send()方法
Proxy的源码如下
private static class Proxy implements android.os.IMessenger {
private android.os.IBinder mRemote;
Proxy(android.os.IBinder remote) {
mRemote = remote;
}
public android.os.IBinder asBinder() {
return mRemote;
}
public java.lang.String getInterfaceDescriptor() {
return DESCRIPTOR;
}
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); // Message实现了Parcelable接口
} else {
_data.writeInt(0);
}
mRemote.transact(Stub.TRANSACTION_send, _data, null,
android.os.IBinder.FLAG_ONEWAY);
} finally {
_data.recycle();
}
}
}
所以，综上所述，serviceMessenger.send()调用的是Proxy.send()，然后经过onTransact调用的是Service端的messenger.handler.IMessengerImpl.send()方法；服务端的message.reply.send()则是调用了客户端clientMessenger.handler.IMessengerImpl.send()方法。本质都是Stub在发送，都是AIDL的封装