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Messenger 详解

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概念

让服务与远程进程通信，可使用 Messenger 为服务提供接口。通过 Messenger 发送消息（Message）来实现进程间的通信 (IPC)。

Messenger 实现双向通信及示例

通信流程

  • 双方实例化 Messenger
    Activity 和 Service 两边各自都要实例化 Messenger ，分别为：AMessenger 和 SMessenger。
  • Activity 到 Service 的通信
    Service 在 onBind 时，返回SMessenger 的 IBinder 。Activity 在 bindService 连接后，通过该 IBinder 得到 SMessenger 实例，并通过它来向 Service 发送消息。
  • Service 到 Activity 的通信（回调）
    Activity 组建消息 Message 并将 AMessenger 存放消息中，再通过 SMessenger 实例发送消息时传递给 Service，Service 通过拿到的 AMessenger 实例发送消息，形成对 Activity 的回调。

服务端

  • 定义各种 Message 的 ID 
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// 从客户端来的消息
public static final int MSG_FROM_CLIENT_REGISTER = 20001;
public static final int MSG_FROM_CLIENT_UNREGISTER = 20002;
public static final int MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK = 20003;

// 服务端反馈给客户端的消息  
public static final int MSG_FROM_SERVICE_RESULT = 30001;
  • 自定义 Handler 并实例化 SMessenger
    接受来自客户端的消息，后台处理：
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/** Keeps track of all current registered clients. */
private ArrayList<Messenger> mClients = new ArrayList<Messenger>();

private class IncomingFromClientsHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        Log.d(TAG, "IncomingFromClientsHandler:handleMessage: ");
        switch (msg.what) {
            case MSG_FROM_CLIENT_REGISTER:
                Log.d(TAG, "handleMessage: MSG_FROM_CLIENT_REGISTER");
                mClients.add(msg.replyTo);
                break;
            case MSG_FROM_CLIENT_UNREGISTER:
                Log.d(TAG, "handleMessage: MSG_FROM_CLIENT_UNREGISTER");
                mClients.remove(msg.replyTo);
                break;
            case MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK:
                Log.d(TAG, "handleMessage: MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK");
                doLongRunningWork(msg);
                break;
            default:
                super.handleMessage(msg);
        }
    }
}

final Messenger mSMessenger = new Messenger(new IncomingFromClientsHandler());
  • onBind 返回 Messenger 
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@Override
public IBinder onBind(Intent intent) {
    Log.d(TAG, "onBind: ");
    return mSMessenger.getBinder();
}
  • 服务端向客户端发送消息（回调）
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private void feedbackToClient(Messenger replyTo){
    Log.d(TAG, "feedbackToClient: ");
    Message msg = Message.obtain(null, MSG_FROM_SERVICE_RESULT);
    MyDataParcelable data = RandomData.genMyDataParcelable();
    Bundle bundle = new Bundle(MyDataParcelable.class.getClassLoader());
    bundle.putParcelable(IPC_PARCELABLE_DATA_KEY ,data);
    msg.setData(bundle);
    try {
        replyTo.send(msg);
    } catch (RemoteException e) {
        e.printStackTrace();
    }
}

客户端

  • 自定义 Handler 并实例化 AMessenger
    接受来自服务端的消息，响应回调：
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private class IncomingFromServiceHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        Log.d(TAG, "IncomingFromServiceHandler:handleMessage: ");
        switch (msg.what) {
            case MessengerService.MSG_FROM_SERVICE_RESULT:
                Log.d(TAG, "handleMessage: MSG_FROM_SERVICE_RESULT");
                showMsgFromServiceResult(msg);
                break;
            default:
                super.handleMessage(msg);
        }
    }
}

/**
 * Target we publish for clients to send messages to IncomingHandler.
 */
final Messenger mAMessenger = new Messenger(new IncomingFromServiceHandler());
  • 绑定服务后获取 SMessenger
    其中 mService 即从服务器端获取到的 SMessenger： 
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/** Flag indicating whether we have called bind on the service. */
private boolean mBound = false;
/** Messenger for communicating with service. */
private Messenger mService;
private ServiceConnection mMessengerServiceConnection = new ServiceConnection() {
    @Override
    public void onServiceConnected(ComponentName name, IBinder service) {
        Log.d(TAG, "onServiceConnected: Bound!");
        mBound = true;
        // 拿到服务器端的 SMessenger
        mService = new Messenger(service);
    }
    ...
};
  • 客户端向服务端发送消息
    也就是客户端有两个 Messenger 的实例，一个用于响应回调的 AMessenger，一个用于发送消息的 SMessenger： 
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private void doLongRunningWork(){
    Log.d(TAG, "doLongRunningWork: ");
    Message msg = Message.obtain(null, 
        MessengerService.MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK);
    Bundle bundle = new Bundle(MyDataParcelable.class.getClassLoader());
    MyDataParcelable data = RandomData.genMyDataParcelable();
    bundle.putParcelable(MessengerService.IPC_PARCELABLE_DATA_KEY, data);
    msg.setData(bundle);
    // 客户端的 AMessenger，传递给服务端后用于回调
    msg.replyTo = mAMessenger;
    if(mService != null){
        try {
            // SMessenger 发送消息给服务端
            mService.send(msg);
        } catch (RemoteException e){
            e.printStackTrace();
        }
    }
}

示例及 Log 分析

客户端向服务端下发耗时任务，并响应服务端的回调

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// 启动客户端 Activity
17:24:16.964 30355-30355/D:ShowIPCActivity:: onCreate: 
// 绑定服务
17:24:18.228 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = Bind Service
17:24:18.228 30355-30355/D:ShowIPCActivity:: bindMessengerService: 
17:24:18.229 30355-30355/D:ShowIPCActivity:: bindMessengerService: Binding...
// 服务端启动
17:24:18.246 30355-30355/D:MessengerService:: onCreate: 
17:24:18.246 30355-30355/D:MessengerService:: onBind: 
// 连接服务端
17:24:18.252 30355-30355/D:ShowIPCActivity:: onServiceConnected: Bound!
17:24:18.253 30355-30355/D:ShowIPCActivity:: registerClient: 
17:24:18.253 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
17:24:18.253 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_REGISTER
// 下发耗时任务
17:24:20.886 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = LongRunningWork
17:24:20.886 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
17:24:20.892 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
        data = name: bic, arrays: 1 2 3 , book.name: icfhd, book.price: 49
// 服务端接受到耗时任务消息
17:24:20.899 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
17:24:20.899 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK
17:24:20.899 30355-30355/D:MessengerService:: doLongRunningWork: 
17:24:20.899 30355-30355/D:MessengerService:: doLongRunningWork: 
        msg.data = name: bic, arrays: 1 2 3 , book.name: icfhd, book.price: 49
// 服务端新建线程 30467 执行耗时任务
17:24:20.905 30355-30476/D:MessengerService:: MyDataRunnable:run... 
17:24:23.908 30355-30476/D:MessengerService:: MyDataRunnable:run: done!
// 服务端执行完任务后，发送消息给客户端（回调）
17:24:23.908 30355-30476/D:MessengerService:: feedbackToClient: 
17:24:23.909 30355-30476/D:MessengerService:: feedbackToClient: 
        data = name: ahf, arrays: 1 2 3 , book.name: ifebd, book.price: 40
// 客户端收到服务端的回调消息并处理
17:24:23.909 30355-30355/D:ShowIPCActivity:: IncomingFromServiceHandler:handleMessage: 
17:24:23.909 30355-30355/D:ShowIPCActivity:: handleMessage: MSG_FROM_SERVICE_RESULT
17:24:23.909 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
17:24:23.909 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
        msg.data = name: ahf, arrays: 1 2 3 , book.name: ifebd, book.price: 40
// 解除服务绑定
17:24:31.812 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = Unbind Service
17:24:31.812 30355-30355/D:ShowIPCActivity:: unBindMessengerService: 
17:24:31.812 30355-30355/D:ShowIPCActivity:: unBindMessengerService: unBind!
17:24:31.812 30355-30355/D:ShowIPCActivity:: unregisterClient: 
17:24:31.816 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
17:24:31.816 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_UNREGISTER
// 服务销毁
17:24:31.817 30355-30355/D:MessengerService:: onDestroy:

客户端同时下发三条耗时任务

下发了三条耗时任务，服务端串行接收这些消息并处理。耗时任务是新建线程执行的，可以等待该线程执行完了再接受消息（即串行执行耗时任务），也可以接受消息时再开启新开线程执行（即并行执行耗时任务），示例是并行执行耗时任务： 

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// 绑定服务
18:45:32.446 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = Bind Service
18:45:32.446 30355-30355/D:ShowIPCActivity:: bindMessengerService: 
18:45:32.446 30355-30355/D:ShowIPCActivity:: bindMessengerService: Binding...
18:45:32.455 30355-30355/D:MessengerService:: onCreate: 
18:45:32.455 30355-30355/D:MessengerService:: onBind: 
18:45:32.457 30355-30355/D:ShowIPCActivity:: onServiceConnected: Bound!
18:45:32.457 30355-30355/D:ShowIPCActivity:: registerClient: 
18:45:32.457 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
18:45:32.457 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_REGISTER
// 下发耗时任务 1
18:45:35.196 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = LongRunningWork
18:45:35.196 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
18:45:35.196 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
        data = name: eha, arrays: 1 2 3 , book.name: ddaca, book.price: 99
// 服务端接受到耗时任务消息 1
18:45:35.197 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
18:45:35.197 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK
18:45:35.197 30355-30355/D:MessengerService:: doLongRunningWork: 
18:45:35.198 30355-30355/D:MessengerService:: doLongRunningWork: 
        msg.data = name: eha, arrays: 1 2 3 , book.name: ddaca, book.price: 99
// 新建线程 8198 执行耗时任务1
18:45:35.203 30355-8198/D:MessengerService:: MyDataRunnable:run... 
// 下发耗时任务2
18:45:35.418 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = LongRunningWork
18:45:35.418 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
18:45:35.418 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
        data = name: hhg, arrays: 1 2 3 , book.name: haace, book.price: 13
// 服务端接受到耗时任务消息2
18:45:35.418 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
18:45:35.418 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK
18:45:35.418 30355-30355/D:MessengerService:: doLongRunningWork: 
18:45:35.418 30355-30355/D:MessengerService:: doLongRunningWork: 
        msg.data = name: hhg, arrays: 1 2 3 , book.name: haace, book.price: 13
// 新建线程 8199 执行耗时任务2
18:45:35.423 30355-8199/D:MessengerService:: MyDataRunnable:run... 
// 下发耗时任务3
18:45:35.648 30355-30355/D:ShowIPCActivity:: onClick: button.onClick = LongRunningWork
18:45:35.648 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
18:45:35.648 30355-30355/D:ShowIPCActivity:: doLongRunningWork: 
        data = name: gid, arrays: 1 2 3 , book.name: hfbie, book.price: 40
// 服务端接受到耗时任务消息3
18:45:35.648 30355-30355/D:MessengerService:: IncomingFromClientsHandler:handleMessage: 
18:45:35.649 30355-30355/D:MessengerService:: handleMessage: MSG_FROM_CLIENT_DO_LONG_RUNNING_WORK
18:45:35.649 30355-30355/D:MessengerService:: doLongRunningWork: 
18:45:35.649 30355-30355/D:MessengerService:: doLongRunningWork: 
        msg.data = name: gid, arrays: 1 2 3 , book.name: hfbie, book.price: 40
// 新建线程 8209 执行耗时任务3
18:45:35.650 30355-8209/D:MessengerService:: MyDataRunnable:run... 
// 耗时任务1 线程 8198 执行完毕，并发送回调消息
18:45:38.203 30355-8198/D:MessengerService:: MyDataRunnable:run: done!
18:45:38.203 30355-8198/D:MessengerService:: feedbackToClient: 
18:45:38.203 30355-8198/D:MessengerService:: feedbackToClient: 
        data = name: afi, arrays: 1 2 3 , book.name: ibabe, book.price: 93
18:45:38.204 30355-30355/D:ShowIPCActivity:: IncomingFromServiceHandler:handleMessage: 
18:45:38.204 30355-30355/D:ShowIPCActivity:: handleMessage: MSG_FROM_SERVICE_RESULT
18:45:38.204 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
18:45:38.204 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
        msg.data = name: afi, arrays: 1 2 3 , book.name: ibabe, book.price: 93
// 耗时任务2 线程 8199 执行完毕，并发送回调消息
18:45:38.423 30355-8199/D:MessengerService:: MyDataRunnable:run: done!
18:45:38.423 30355-8199/D:MessengerService:: feedbackToClient: 
18:45:38.423 30355-8199/D:MessengerService:: feedbackToClient: 
        data = name: ffc, arrays: 1 2 3 , book.name: dcdfd, book.price: 65
18:45:38.424 30355-30355/D:ShowIPCActivity:: IncomingFromServiceHandler:handleMessage: 
18:45:38.424 30355-30355/D:ShowIPCActivity:: handleMessage: MSG_FROM_SERVICE_RESULT
18:45:38.424 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
18:45:38.424 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
        msg.data = name: ffc, arrays: 1 2 3 , book.name: dcdfd, book.price: 65
// 耗时任务3 线程 8209 执行完毕，并发送回调消息
18:45:38.650 30355-8209/D:MessengerService:: MyDataRunnable:run: done!
18:45:38.650 30355-8209/D:MessengerService:: feedbackToClient: 
18:45:38.650 30355-8209/D:MessengerService:: feedbackToClient: 
        data = name: icc, arrays: 1 2 3 , book.name: ibicd, book.price: 53
18:45:38.651 30355-30355/D:ShowIPCActivity:: IncomingFromServiceHandler:handleMessage: 
18:45:38.651 30355-30355/D:ShowIPCActivity:: handleMessage: MSG_FROM_SERVICE_RESULT
18:45:38.651 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
18:45:38.651 30355-30355/D:ShowIPCActivity:: showMsgFromServiceResult: 
        msg.data = name: icc, arrays: 1 2 3 , book.name: ibicd, book.price: 53

自定义 Parcelable 数据传输

进程间通信时，Messenger 在传递数据时，需要通过 Bundle 将自定义的数据打包传输。但是另一个进程在取出 Bundle 时的 ClassLoader 为 null，会使用系统默认值 BootClassLoader 来加载自定义的 Parcelable 数据，所以会出现异常找不到该数据类。如果是在相同进程中使用 Messenger 则不会出现这个问题，不过如果是相同进程也不需要使用 Messenger 来通信了。

  • 解决方案
    在取出 Parcelable 数据前，设置该应用为 ClassLoader 的值： 
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if(msg != null && msg.getData() != null){
    Bundle bundle = msg.getData();
    if(bundle != null) {
        // must set class loader before getParcelable, ClassNotFoundException
        // 必须设置 ClassLoader，才能获取 Bundle 中的数据
        bundle.setClassLoader(getClassLoader());
        MyDataParcelable data = bundle.getParcelable(IPC_PARCELABLE_DATA_KEY);
    }
}
  • 异常
    ClassNotFoundException 异常 Log ： 
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14:50:33.803 14568-14568/:messenger D/AndroidRuntime: Shutting down VM
14:50:33.804 14568-14568/:messenger E/AndroidRuntime: FATAL EXCEPTION: main
   Process: :messenger, PID: 14568
   android.os.BadParcelableException: ClassNotFoundException when unmarshalling: .ipc.MyDataParcelable
       at android.os.Parcel.readParcelableCreator(Parcel.java:2295)
       at android.os.Parcel.readParcelable(Parcel.java:2245)
       at android.os.Parcel.readValue(Parcel.java:2152)
       at android.os.Parcel.readArrayMapInternal(Parcel.java:2485)
       at android.os.BaseBundle.unparcel(BaseBundle.java:221)
       at android.os.Bundle.getParcelable(Bundle.java:755)
       at .ipc.MessengerService.doLongRunningWork(MessengerService.java:129)
       at .ipc.MessengerService.access$100(MessengerService.java:18)
       at .ipc.MessengerService$IncomingFromClientsHandler.handleMessage(MessengerService.java:71)
       at android.os.Handler.dispatchMessage(Handler.java:102)
       at android.os.Looper.loop(Looper.java:135)
       at android.app.ActivityThread.main(ActivityThread.java:5344)
       at java.lang.reflect.Method.invoke(Native Method)
       at java.lang.reflect.Method.invoke(Method.java:372)
       at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:908)
       at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:703)

源码分析

从源码可以看出：Messenger 的本质还是使用的 AIDL，只是封装后更方便使用。 

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// Messenger.java
public final class Messenger implements Parcelable {
    // IMessenger.aidl 实例
    private final IMessenger mTarget;
    
    /* Create a new Messenger pointing to the given Handler. */
    public Messenger(Handler target) {
        mTarget = target.getIMessenger();
    }
    
    /* Create a Messenger from a raw IBinder */
    public Messenger(IBinder target) {
        mTarget = IMessenger.Stub.asInterface(target);
    }

    ...
}

// IMessenger.aidl的具体实现是在 Handler.java 中
final IMessenger getIMessenger() {
    synchronized (mQueue) {
        if (mMessenger != null) {
            return mMessenger;
        }
        mMessenger = new MessengerImpl();
        return mMessenger;
    }
}

// 具体实现，消息的发送是通过 Handler 实现的
private final class MessengerImpl extends IMessenger.Stub {
    public void send(Message msg) {
        msg.sendingUid = Binder.getCallingUid();
        Handler.this.sendMessage(msg);
    }
}

Messenger 和 Message 的区别和联系

从源码中可以看出，Message 是一个包含各种数据的容器，Messenger 可以作为 Parcelable 数据，通过 Message 装载后传递出去。而从 Messenger 的 AIDL 来看，它唯一的方法就是发送 Message，是一名信差。

Messenger 总结

  • 本质还是 AIDL
  • 只能异步通信，同步的话必须使用 AIDL
  • 因为使用的是 Handler 和 Message，所以所有的消息都是串行接受和处理的，处理的过程可以是多线程或单线程来处理

参考文档

  1. https://developer.android.com/guide/components/bound-services.html#Binding
  2. http://blog.csdn.net/lmj623565791/article/details/47017485
  3. http://blog.csdn.net/jiwangkailai02/article/details/48098087
  4. http://blog.csdn.net/hyhdcl/article/details/52680301