整个Vold机制应该算system层，与framwork层的交互在Android—— 4.2 Vold挂载管理_CommandListener (二)中有提到过，是通过一个"vold"的socket进行通信的，这里分析一下framework中负责与Vold通信的：MountService

                                                       撰写不易,转载请注明出处：http://blog.csdn.net/jscese/article/details/38978387

一.MountService启动:

在/frameworks/base/services/java/com/android/server/SystemServer.java中有：

            if (!"0".equals(SystemProperties.get("system_init.startmountservice"))) {
                try {
                    /*
                     * NotificationManagerService is dependant on MountService,
                     * (for media / usb notifications) so we must start MountService first.
                     */
                    Slog.i(TAG, "Mount Service");
                    mountService = new MountService(context);
                    ServiceManager.addService("mount", mountService);
                } catch (Throwable e) {
                    reportWtf("starting Mount Service", e);
                }
            }

public MountService(Context context) {
        mContext = context;

        synchronized (mVolumesLock) {
            readStorageListLocked(); // 解析/frameworks/base/core/res/res/xml/storage_list.xml保存volume到 MountService的list ：mVolumes中
        }

        // XXX: This will go away soon in favor of IMountServiceObserver
        mPms = (PackageManagerService) ServiceManager.getService("package");

        mHandlerThread = new HandlerThread("MountService");
        mHandlerThread.start();
        mHandler = new MountServiceHandler(mHandlerThread.getLooper());//新建消息处理handler

        // Watch for user changes
        final IntentFilter userFilter = new IntentFilter();
        userFilter.addAction(Intent.ACTION_USER_ADDED);
        userFilter.addAction(Intent.ACTION_USER_REMOVED);
        mContext.registerReceiver(mUserReceiver, userFilter, null, mHandler);//注册广播接收

        // Watch for USB changes on primary volume
        final StorageVolume primary = getPrimaryPhysicalVolume();
        if (primary != null && primary.allowMassStorage()) {
            mContext.registerReceiver(
                    mUsbReceiver, new IntentFilter(UsbManager.ACTION_USB_STATE), null, mHandler);
        }

        // Add OBB Action Handler to MountService thread.
        mObbActionHandler = new ObbActionHandler(mHandlerThread.getLooper());

        /*
         * Create the connection to vold with a maximum queue of twice the
         * amount of containers we'd ever expect to have. This keeps an
         * "asec list" from blocking a thread repeatedly.
         */
        mConnector = new NativeDaemonConnector(this, "vold", MAX_CONTAINERS * 2, VOLD_TAG, 25);//创建 vold 的监听接收，用于接收system中Vold的socket消息

        Thread thread = new Thread(mConnector, VOLD_TAG);
        thread.start();//启动线程，NativeDaemonConnector实现了Runnable接口，实现在 run中

        // Add ourself to the Watchdog monitors if enabled.
        if (WATCHDOG_ENABLE) {
            Watchdog.getInstance().addMonitor(this);
        }
    }

二.MountService接收Socket:

上面有看到构造了NativeDaemonConnector用来接收来自下层的socket消息，先看构造：

    NativeDaemonConnector(INativeDaemonConnectorCallbacks callbacks, String socket,
            int responseQueueSize, String logTag, int maxLogSize) {
        mCallbacks = callbacks; //回调
        mSocket = socket; // socket名称
        mResponseQueue = new ResponseQueue(responseQueueSize);//构建一个响应队列
        mSequenceNumber = new AtomicInteger(0);
        TAG = logTag != null ? logTag : "NativeDaemonConnector";
        mLocalLog = new LocalLog(maxLogSize);
    }

在上面开启监测线程的run方法：

    public void run() {
        HandlerThread thread = new HandlerThread(TAG + ".CallbackHandler"); //TAG 为 VoldConnector，这里新建一个名为VoldConnector.CallbackHandler的消息处理线程，用于下面接收到vold 的socket之后的处理
        thread.start();
        mCallbackHandler = new Handler(thread.getLooper(), this); //创建handler 用于分发消息

        while (true) {
            try {
                listenToSocket();// while 循环 监听socket
            } catch (Exception e) {
                loge("Error in NativeDaemonConnector: " + e);
                SystemClock.sleep(5000);
            }
        }
    }

    private void listenToSocket() throws IOException {
        LocalSocket socket = null;

        try {
            socket = new LocalSocket();  //创建本地socket
            LocalSocketAddress address = new LocalSocketAddress(mSocket,
                    LocalSocketAddress.Namespace.RESERVED);//获得服务端vold socket的地址

            socket.connect(address);//连接

            InputStream inputStream = socket.getInputStream();
            synchronized (mDaemonLock) {
                mOutputStream = socket.getOutputStream();
            }//获取输入输出流

            mCallbacks.onDaemonConnected();//回调，在MountService中执行，初始化一些Volume状态信息

            byte[] buffer = new byte[BUFFER_SIZE];
            int start = 0;

            while (true) {
                int count = inputStream.read(buffer, start, BUFFER_SIZE - start);//读取数据到buffer
                if (count < 0) {//连接断开，跳出当前while ，外部while循环 重新调用该函数连接
                    loge("got " + count + " reading with start = " + start);
                    break;
                }


                // Add our starting point to the count and reset the start.
                count += start;
                start = 0;

                for (int i = 0; i < count; i++) {
                    if (buffer[i] == 0) {
                        final String rawEvent = new String(
                                buffer, start, i - start, Charsets.UTF_8);
                        log("RCV <- {" + rawEvent + "}");

                        try {
                            final NativeDaemonEvent event = NativeDaemonEvent.parseRawEvent(
                                    rawEvent);   //解析成event 保存
                            if (event.isClassUnsolicited()) {    //判断event的code范围 code >= 600 && code < 700
                                // TODO: migrate to sending NativeDaemonEvent instances
                                mCallbackHandler.sendMessage(mCallbackHandler.obtainMessage( //发送消息，把event交给handle来分发处理
                                        event.getCode(), event.getRawEvent()));
                            } else {
                                mResponseQueue.add(event.getCmdNumber(), event);//加入到响应队列
                            }
                        } catch (IllegalArgumentException e) {
                            log("Problem parsing message: " + rawEvent + " - " + e);
                        }

                        start = i + 1;
                    }
                }

        ...

           }

     }

}

在NativeDaemonConnector中的handle处理为：

    @Override
    public boolean handleMessage(Message msg) {
        String event = (String) msg.obj;
        try {
            if (!mCallbacks.onEvent(msg.what, event, NativeDaemonEvent.unescapeArgs(event))) { //回调到MountService 的onEent函数
                log(String.format("Unhandled event '%s'", event));
            }
        } catch (Exception e) {
            loge("Error handling '" + event + "': " + e);
        }
        return true;
    }

到MountService中onEvent：

    public boolean onEvent(int code, String raw, String[] cooked) {
        if (DEBUG_EVENTS) {
            StringBuilder builder = new StringBuilder();
            builder.append("onEvent::");
            builder.append(" raw= " + raw);
            if (cooked != null) {
                builder.append(" cooked = " );
                for (String str : cooked) {
                    builder.append(" " + str);
                }
            }
            Slog.i(TAG, builder.toString());
        }
        if (code == VoldResponseCode.VolumeStateChange) { //根据 Vold的Code 执行
            /*
             * One of the volumes we're managing has changed state.
             * Format: "NNN Volume <label> <path> state changed
             * from <old_#> (<old_str>) to <new_#> (<new_str>)"
             */
            notifyVolumeStateChange(
                    cooked[2], cooked[3], Integer.parseInt(cooked[7]),
                            Integer.parseInt(cooked[10])); //更新状态
        } else if ((code == VoldResponseCode.VolumeDiskInserted) ||
                   (code == VoldResponseCode.VolumeDiskRemoved) ||
                   (code == VoldResponseCode.VolumeBadRemoval))
    ...

                  if (code == VoldResponseCode.VolumeDiskInserted) { //如果接收到的是插入disk的消息，则执行挂载操作
                new Thread() {
                    @Override
                    public void run() {
                        try {
                            int rc;
                            if ((rc = doMountVolume(path)) != StorageResultCode.OperationSucceeded) {
                                Slog.w(TAG, String.format("Insertion mount failed (%d)", rc));
                            }
                        } catch (Exception ex) {
                            Slog.w(TAG, "Failed to mount media on insertion", ex);
                        }
                    }
                }.start();
            }

    ...

}

可以看到根据code值执行相对应的操作，在Android—— 4.2 Vold挂载管理_DirectVolume/Volume (五)中的DirectVolume中DirectVolume::handleBlockEvent中就发送了ResponseCode::VolumeDiskInserted

MountService中对Vold的socket接收大体就是这样！

三.MountService下发Command:

就从上面doMountVolume来解析，MountService中对Volume的各种操作都是需要转换成符合Vold中socket command，这样Vold中才能正确的解析识别调用！

    private int doMountVolume(String path) {
        int rc = StorageResultCode.OperationSucceeded;

        final StorageVolume volume;
        synchronized (mVolumesLock) {
            volume = mVolumesByPath.get(path);
        }

        if (DEBUG_EVENTS) Slog.i(TAG, "doMountVolume: Mouting " + path);
        try {
            mConnector.execute("volume", "mount", path);// 调用到NativeDaemonConnector中的execute
        }

     ...

}

NativeDaemonConnector中：

    public NativeDaemonEvent execute(String cmd, Object... args)
            throws NativeDaemonConnectorException {
        final NativeDaemonEvent[] events = executeForList(cmd, args);//由executeForList发送，返回NativeDaemonEent事件

        if (events.length != 1) {
            throw new NativeDaemonConnectorException(
                    "Expected exactly one response, but received " + events.length);
        }
        return events[0];
    }

    public NativeDaemonEvent[] execute(int timeout, String cmd, Object... args)
            throws NativeDaemonConnectorException {
        final ArrayList<NativeDaemonEvent> events = Lists.newArrayList();

        final int sequenceNumber = mSequenceNumber.incrementAndGet();
        final StringBuilder cmdBuilder =
                new StringBuilder(Integer.toString(sequenceNumber)).append(' ');
        final long startTime = SystemClock.elapsedRealtime();

        makeCommand(cmdBuilder, cmd, args); //转换制作成标准的Command

        final String logCmd = cmdBuilder.toString(); /* includes cmdNum, cmd, args */
        log("SND -> {" + logCmd + "}");

        cmdBuilder.append('\0');
        final String sentCmd = cmdBuilder.toString(); /* logCmd + \0 */

        synchronized (mDaemonLock) {
            if (mOutputStream == null) {
                throw new NativeDaemonConnectorException("missing output stream");
            } else {
                try {
                    mOutputStream.write(sentCmd.getBytes(Charsets.UTF_8)); //通过在listenToSocket中获取的输出流，写入转换好的sentCmd
                } catch (IOException e) {
                    throw new NativeDaemonConnectorException("problem sending command", e);
                }
            }
        }

     ...

}

MountService 中往下发command的流程大体就是这样！

简单流程图：

至此，framework与Vold的分析就到这里！

Android—— 4.2 Vold挂载管理_MountService (六)