Android Service之MountService源码分析

Android 存储设备管理框架

在android之VOLD进程启动源码分析一文中介绍了存储设备的管控中心Vold进程，Vold属于native后台进程，通过netlink方式接收kernel的uevent消息，并通过socket方式将uevent消息发送给MountService，同时实时接收MountService的命令消息，MountService，Vold，Kernel三者的关系如下图所示：

android之VOLD进程启动源码分析一文中介绍了NetlinkManager模块在启动过程中，创建了一个socket监听线程，用于监听kernel发送过来的uevent消息；CommandListener模块在启动时同样创建了一个socket监听线程，不同的是该线程用于监听MountServcie的连接，接收MountService向Vold发送的命令消息；MountService要接收来自kernel的uevent消息，必定也需要创建一个socket监听线程，在接下来将对该socket监听线程进行详细讲解。

Android MountService框架设计

MountService作为Android的Java服务之一，在SystemServer进程启动的第二阶段创建并注册到ServiceManager中，同时长驻于SystemServer进程中，MountService创建及注册过程如下：

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

MountService各个类关系图：

构造MountService对象实例：

public MountService(Context context) {
	mContext = context; 
	//从xml中读取存储设备列表
	readStorageList();

	if (mPrimaryVolume != null) {
		mExternalStoragePath = mPrimaryVolume.getPath();
		mEmulateExternalStorage = mPrimaryVolume.isEmulated();
		if (mEmulateExternalStorage) {
			Slog.d(TAG, "using emulated external storage");
			mVolumeStates.put(mExternalStoragePath, Environment.MEDIA_MOUNTED);
		}
	}
	
	//add mount state for inernal storage in NAND
	if (Environment.getSecondStorageType() == Environment.SECOND_STORAGE_TYPE_NAND) {
		mVolumeStates.put(Environment.getSecondStorageDirectory().getPath(), Environment.MEDIA_MOUNTED);
	}
	// 查询PackageManagerService服务
	mPms = (PackageManagerService) ServiceManager.getService("package");
	IntentFilter filter = new IntentFilter();
	filter.addAction(Intent.ACTION_BOOT_COMPLETED);
	// don't bother monitoring USB if mass storage is not supported on our primary volume
	if (mPrimaryVolume != null && mPrimaryVolume.allowMassStorage()) {
		filter.addAction(UsbManager.ACTION_USB_STATE);
	}
	//注册开机完成及USB状态变化广播接收器
	mContext.registerReceiver(mBroadcastReceiver, filter, null, null);
    //创建并启动一个带消息循环的MountService工作线程
	mHandlerThread = new HandlerThread("MountService");
	mHandlerThread.start();
	//为MountService工作线程创建一个Handler
	mHandler = new MountServiceHandler(mHandlerThread.getLooper());

	//为MountService工作线程创建一个ObbActionHandler
	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);
    //创建并启动一个socket连接监听线程
	Thread thread = new Thread(mConnector, VOLD_TAG);
	thread.start();

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

在开始构造MountService前，首先读取frameworks/base/core/res/res/xml/storage_list.xml文件，该文件以XML方式保存了所有存储设备的参数，文件内容如下所示：

<StorageList xmlns:android="http://schemas.android.com/apk/res/android">
    <!-- removable is not set in nosdcard product -->
    <storage android:mountPoint="/mnt/sdcard"
                 android:storageDescription="@string/storage_usb"
                 android:primary="true" />
</StorageList>

该文件的读取这里不在介绍，读者自行研究，使用XML解析器读取该XML的文件内容，根据读取到的存储设备参数来构造StorageVolume对象，并将构造的所有StorageVolume对象存放到列表mVolumes中。通过源码清晰地知道，在构造MountService时，注册了一个广播接收器，用于接收开机完成广播及USB状态广播，当开机完成时自动挂载存储设备，在大容量设备存储有效情况下，当USB状态变化也自动地挂载存储设备。创建了两个工作线程，MountService线程用于消息循环处理，为什么要开启一个异步消息处理线程呢？我们知道大量的Java Service驻留在SystemServer进程中，如果所有的服务消息都发送到SystemServer的主线程中处理的话，主线程的负荷很重，消息不能及时得到处理，因此需为每一个Service开启一个消息处理线程，专门处理本Service的消息。如下图所示：

MountService服务的线程模型

从上图可以清晰地看出SystemServer主线程启动MountService服务，该服务启动时会创建一个MountService带有消息循环的工作线程，用于处理MountServiceHandle和ObbActionHandler分发过来的消息；同时创建一个用于连接Vold服务端socket的VoldConnector线程，该线程在进入闭环运行前会创建一个带有消息循环的VoldConnector.CallbackHandler线程，用于处理native层的Vold进程发送过来的uevent事件消息；然后向服务端Vold发送连接请求，得到socket连接后，从该socket中循环读取数据以接收来之服务端Vold的uevent消息，当读取的数据长度为0时，向服务端重新发起连接，如此循环，保证客户端MountService与服务端Vold一直保持正常连接。当成功连接到服务端Vold时，VoldConnector线程会创建一个MountService#onDaemonConnected线程，用于处理本次连接请求响应。

1.MountService线程创建

        mHandlerThread = new HandlerThread("MountService");
        mHandlerThread.start();
        mHandler = new MountServiceHandler(mHandlerThread.getLooper());

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

MountServiceHandler消息处理：

public void handleMessage(Message msg) {
    switch (msg.what) {
	case H_UNMOUNT_PM_UPDATE: {
	    UnmountCallBack ucb = (UnmountCallBack) msg.obj;
	    mForceUnmounts.add(ucb);
	    // Register only if needed.
	    if (!mUpdatingStatus) {
		if (DEBUG_UNMOUNT) Slog.i(TAG, "Updating external media status on PackageManager");
		mUpdatingStatus = true;
		mPms.updateExternalMediaStatus(false, true);
	    }
	    break;
	}
	case H_UNMOUNT_PM_DONE: {
	    mUpdatingStatus = false;
	    int size = mForceUnmounts.size();
	    int sizeArr[] = new int[size];
	    int sizeArrN = 0;
	    // Kill processes holding references first
	    ActivityManagerService ams = (ActivityManagerService)
	    ServiceManager.getService("activity");
	    for (int i = 0; i < size; i++) {
		UnmountCallBack ucb = mForceUnmounts.get(i);
		String path = ucb.path;
		boolean done = false;
		if (!ucb.force) {
		    done = true;
		} else {
		    int pids[] = getStorageUsers(path);
		    if (pids == null || pids.length == 0) {
			done = true;
		    } else {
			// Eliminate system process here?
			ams.killPids(pids, "unmount media", true);
			// Confirm if file references have been freed.
			pids = getStorageUsers(path);
			if (pids == null || pids.length == 0) {
			    done = true;
			}
		    }
		}
		if (!done && (ucb.retries < MAX_UNMOUNT_RETRIES)) {
		    // Retry again
		    Slog.i(TAG, "Retrying to kill storage users again");
		    mHandler.sendMessageDelayed(mHandler.obtainMessage(H_UNMOUNT_PM_DONE,ucb.retries++),RETRY_UNMOUNT_DELAY);
		} else {
		    if (ucb.retries >= MAX_UNMOUNT_RETRIES) {
			Slog.i(TAG, "Failed to unmount media inspite of " +
				MAX_UNMOUNT_RETRIES + " retries. Forcibly killing processes now");
		    }
		    sizeArr[sizeArrN++] = i;
		    mHandler.sendMessage(mHandler.obtainMessage(H_UNMOUNT_MS,ucb));
		}
	    }
	    // Remove already processed elements from list.
	    for (int i = (sizeArrN-1); i >= 0; i--) {
		mForceUnmounts.remove(sizeArr[i]);
	    }
	    break;
	}
	case H_UNMOUNT_MS: {
	    UnmountCallBack ucb = (UnmountCallBack) msg.obj;
	    ucb.handleFinished();
	    break;
	}
    }
}

MountServiceHandler分别对H_UNMOUNT_PM_UPDATE,H_UNMOUNT_PM_DONE,H_UNMOUNT_MS

public void handleMessage(Message msg) {
    switch (msg.what) {
	case OBB_RUN_ACTION: {
	    final ObbAction action = (ObbAction) msg.obj;
	    // If a bind was already initiated we don't really
	    // need to do anything. The pending install
	    // will be processed later on.
	    if (!mBound) {
		// If this is the only one pending we might
		// have to bind to the service again.
		if (!connectToService()) {
		    Slog.e(TAG, "Failed to bind to media container service");
		    action.handleError();
		    return;
		}
	    }
	    mActions.add(action);
	    break;
	}
	case OBB_MCS_BOUND: {
	    if (msg.obj != null) {
		mContainerService = (IMediaContainerService) msg.obj;
	    }
	    if (mContainerService == null) {
		for (ObbAction action : mActions) {
		    // Indicate service bind error
		    action.handleError();
		}
		mActions.clear();
	    } else if (mActions.size() > 0) {
		final ObbAction action = mActions.get(0);
		if (action != null) {
		    action.execute(this);
		}
	    } else {
		// Should never happen ideally.
		Slog.w(TAG, "Empty queue");
	    }
	    break;
	}
	case OBB_MCS_RECONNECT: {
	    if (mActions.size() > 0) {
		if (mBound) {
		    disconnectService();
		}
		if (!connectToService()) {
		    Slog.e(TAG, "Failed to bind to media container service");
		    for (ObbAction action : mActions) {
			// Indicate service bind error
			action.handleError();
		    }
		    mActions.clear();
		}
	    }
	    break;
	}
	case OBB_MCS_UNBIND: {
	    // Delete pending install
	    if (mActions.size() > 0) {
		mActions.remove(0);
	    }
	    if (mActions.size() == 0) {
		if (mBound) {
		    disconnectService();
		}
	    } else {
		// There are more pending requests in queue.
		// Just post MCS_BOUND message to trigger processing
		// of next pending install.
		mObbActionHandler.sendEmptyMessage(OBB_MCS_BOUND);
	    }
	    break;
	}
	case OBB_FLUSH_MOUNT_STATE: {
	    final String path = (String) msg.obj;
	    synchronized (mObbMounts) {
		final List<ObbState> obbStatesToRemove = new LinkedList<ObbState>();
		final Iterator<Entry<String, ObbState>> i =mObbPathToStateMap.entrySet().iterator();
		while (i.hasNext()) {
		    final Entry<String, ObbState> obbEntry = i.next();
		    if (obbEntry.getKey().startsWith(path)) {
			obbStatesToRemove.add(obbEntry.getValue());
		    }
		}

		for (final ObbState obbState : obbStatesToRemove) {
		    removeObbStateLocked(obbState);
		    try {
			obbState.token.onObbResult(obbState.filename, obbState.nonce,
				OnObbStateChangeListener.UNMOUNTED);
		    } catch (RemoteException e) {
			Slog.i(TAG, "Couldn't send unmount notification for  OBB: "+ obbState.filename);
		    }
		}
	    }
	    break;
	}
    }
}

MountService命令下发流程

Vold作为存储设备的管控中心，需要接收来自上层MountService的操作命令，MountService驻留在SystemServer进程中，和Vold作为两个不同的进程，它们之间的通信方式采用的是socket通信，在 android之VOLD进程启动源码分析一文中介绍了 CommandListener模块启动了一个socket监听线程，用于专门接收来之上层MountService的连接请求。而在MountService这端，同样启动了VoldConnector socket连接线程，用于循环连接服务端，保证连接不被中断，当成功连接Vold时，循环从服务端读取数据。MountService按照指定格式向Vold发送命令，由于发送的命令比较多，这里不做一一接收，只对其中的mount命令的发送流程进行介绍：

从以上的时序图可以看出，MountService对命令的发送首先是调用makeCommand来组合成指定格式的命令，然后直接写入到Vold socket即可，整个命令发送流程比较简单。

public int mountVolume(String path) {
        //权限检验
	validatePermission(android.Manifest.permission.MOUNT_UNMOUNT_FILESYSTEMS);
	waitForReady();

	return doMountVolume(path);
}

调用doMountVolume函数来挂载存储设备：

private int doMountVolume(String path) {
	int rc = StorageResultCode.OperationSucceeded;
	try {
	    //命令交给NativeDaemonConnector去发送
	    mConnector.execute("volume", "mount", path);
	} catch (NativeDaemonConnectorException e) {
	    //捕获命令发送的异常，根据异常码来决定发送失败的原因
	    String action = null;
	    int code = e.getCode();
	    if (code == VoldResponseCode.OpFailedNoMedia) {
		/*
		 * Attempt to mount but no media inserted
		 */
		rc = StorageResultCode.OperationFailedNoMedia;
	    } else if (code == VoldResponseCode.OpFailedMediaBlank) {
		if (DEBUG_EVENTS) Slog.i(TAG, " updating volume state :: media nofs");
		/*
		 * Media is blank or does not contain a supported filesystem
		 */
		updatePublicVolumeState(path, Environment.MEDIA_NOFS);
		action = Intent.ACTION_MEDIA_NOFS;
		rc = StorageResultCode.OperationFailedMediaBlank;
	    } else if (code == VoldResponseCode.OpFailedMediaCorrupt) {
		if (DEBUG_EVENTS) Slog.i(TAG, "updating volume state media corrupt");
		/*
		 * Volume consistency check failed
		 */
		updatePublicVolumeState(path, Environment.MEDIA_UNMOUNTABLE);
		action = Intent.ACTION_MEDIA_UNMOUNTABLE;
		rc = StorageResultCode.OperationFailedMediaCorrupt;
	    } else {
		rc = StorageResultCode.OperationFailedInternalError;
	    }
	    /*
	     * Send broadcast intent (if required for the failure)
	     */
	    if (action != null) {
		sendStorageIntent(action, path);
	    }
	}
	return rc;
}

NativeDaemonConnector命令发送：

public NativeDaemonEvent execute(String cmd, Object... args)
    throws NativeDaemonConnectorException {
        //使用executeForList函数来发送命令和命令参数，并返回一组NativeDaemonEvent事件
	final NativeDaemonEvent[] events = executeForList(cmd, args);
	if (events.length != 1) {
	    throw new NativeDaemonConnectorException("Expected exactly one response, but received " + events.length);
	}
	return events[0];
}

调用executeForList来发送命令和命令参数，并在这里设置超时时间：

public NativeDaemonEvent[] executeForList(String cmd, Object... args)
    throws NativeDaemonConnectorException {
    //设置超时时间：DEFAULT_TIMEOUT = 1 * 60 * 1000
    return execute(DEFAULT_TIMEOUT, cmd, args);
}

真正命令发送：

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(' ');
	//发送起始时间
	final long startTime = SystemClock.elapsedRealtime();
	//命令组合
	makeCommand(cmdBuilder, cmd, args);
	final String logCmd = cmdBuilder.toString(); /* includes cmdNum, cmd, args */
	log("SND -> {" + logCmd + "}"); //SND -> {8 volume mount /storage/sdcard1}
	cmdBuilder.append('\0');
	final String sentCmd = cmdBuilder.toString(); /* logCmd + \0 */
	synchronized (mDaemonLock) {
	    if (mOutputStream == null) {
		throw new NativeDaemonConnectorException("missing output stream");
	    } else {
		try {
		    //向socket中写入命令
		    mOutputStream.write(sentCmd.getBytes(Charsets.UTF_8));
		} catch (IOException e) {
		    throw new NativeDaemonConnectorException("problem sending command", e);
		}
	    }
	}
	NativeDaemonEvent event = null;
	do {
	    event = mResponseQueue.remove(sequenceNumber, timeout, sentCmd);
	    if (event == null) {
		loge("timed-out waiting for response to " + logCmd);
		throw new NativeDaemonFailureException(logCmd, event);
	    }
	    log("RMV <- {" + event + "}");
	    events.add(event);
	} while (event.isClassContinue());
        //发送结束时间
	final long endTime = SystemClock.elapsedRealtime();
	if (endTime - startTime > WARN_EXECUTE_DELAY_MS) {
	    loge("NDC Command {" + logCmd + "} took too long (" + (endTime - startTime) + "ms)");
	}
	if (event.isClassClientError()) {
	    throw new NativeDaemonArgumentException(logCmd, event);
	}
	if (event.isClassServerError()) {
	    throw new NativeDaemonFailureException(logCmd, event);
	}
	return events.toArray(new NativeDaemonEvent[events.size()]);
}

MountService消息接收流程

MountService需要接收两种类型的消息：

1）当外部存储设备发生热插拔时，kernel将通过netlink方式通知Vold，Vold进程经过一系列处理后最终还是要叫uevent事件消息发送给MountService,Vold发送uevent的过程已经在 android之VOLD进程启动源码分析一文中详细介绍了；

2）当MountService向Vold发送命令后，将接收到Vold的响应消息；

无论是何种类型的消息，MountService都是通过VoldConnector线程来循环接收Vold的请求，整个消息接收流程如下：

1）socket连接

public void run() {
	//创建并启动VoldConnector.CallbackHandler线程，用于处理native层的Vold进程发送过来的uevent事件消息
	HandlerThread thread = new HandlerThread(TAG + ".CallbackHandler");
	thread.start();
	//为VoldConnector.CallbackHandler线程创建一个Handler，用于向该线程分发消息
	mCallbackHandler = new Handler(thread.getLooper(), this);
        //进入闭环socket连接模式
	while (true) {
		try {
			listenToSocket();
		} catch (Exception e) {
			loge("Error in NativeDaemonConnector: " + e);
			SystemClock.sleep(5000);
		}
	}
}

连接服务端Socket，并读取数据：

private void listenToSocket() throws IOException {
	LocalSocket socket = null;
	try {
		//创建Vold socket
		socket = new LocalSocket();
		LocalSocketAddress address = new LocalSocketAddress(mSocket,LocalSocketAddress.Namespace.RESERVED);
                //向服务端发起连接请求
		socket.connect(address);
                //从连接的socket中得到输入输出流
		InputStream inputStream = socket.getInputStream();
		synchronized (mDaemonLock) {
			mOutputStream = socket.getOutputStream();
		}
                //对本次连接请求做一些回调处理
		mCallbacks.onDaemonConnected();
                //定义buffer
		byte[] buffer = new byte[BUFFER_SIZE];
		int start = 0;
                //进入闭环数据读取模式
		while (true) {
			int count = inputStream.read(buffer, start, BUFFER_SIZE - start);
			//当读取的数据长度小于0时，表示连接已断开，跳出循环，重新向服务端发起新的连接请求
			if (count < 0) {
				loge("got " + count + " reading with start = " + start);
				break;
			}
			// Add our starting point to the count and reset the start.
			count += start;
			start = 0;
                        //解析读取到的数据，得到NativeDaemonEvent
			for (int i = 0; i < count; i++) {
				if (buffer[i] == 0) {
					final String rawEvent = new String(buffer, start, i - start, Charsets.UTF_8);
					//RCV <- {632 Volume sdcard /storage/sdcard1 bad removal (179:1)}
					log("RCV <- {" + rawEvent + "}");

					try {
						final NativeDaemonEvent event = NativeDaemonEvent.parseRawEvent(rawEvent);
						//如果命令码code >= 600 && code < 700
						if (event.isClassUnsolicited()) {
							//将读取到的事件发送到VoldConnector.CallbackHandler线程中处理
							mCallbackHandler.sendMessage(mCallbackHandler.obtainMessage(
									event.getCode(), event.getRawEvent()));
						//否则将改事件添加到响应队列中
						} else {
							mResponseQueue.add(event.getCmdNumber(), event);
						}
					} catch (IllegalArgumentException e) {
						log("Problem parsing message: " + rawEvent + " - " + e);
					}
					start = i + 1;
				}
			}
			if (start == 0) {
				final String rawEvent = new String(buffer, start, count, Charsets.UTF_8);
				log("RCV incomplete <- {" + rawEvent + "}");
			}
			// We should end at the amount we read. If not, compact then
			// buffer and read again.
			if (start != count) {
				final int remaining = BUFFER_SIZE - start;
				System.arraycopy(buffer, start, buffer, 0, remaining);
				start = remaining;
			} else {
				start = 0;
			}
		}
	} catch (IOException ex) {
		loge("Communications error: " + ex);
		throw ex;
	} finally {
		synchronized (mDaemonLock) {
			if (mOutputStream != null) {
				try {
					loge("closing stream for " + mSocket);
					mOutputStream.close();
				} catch (IOException e) {
					loge("Failed closing output stream: " + e);
				}
				mOutputStream = null;
			}
		}
		try {
			if (socket != null) {
				socket.close();
			}
		} catch (IOException ex) {
			loge("Failed closing socket: " + ex);
		}
	}
}

2）连接成功回调处理

public void onDaemonConnected() {
    //创建一个工作线程
	new Thread("MountService#onDaemonConnected") {
		@Override
		public void run() {
			/**
			 * Determine media state and UMS detection status
			 */
			try {
				//向vold查询所有的存储设备
				final String[] vols = NativeDaemonEvent.filterMessageList(
						mConnector.executeForList("volume", "list"),
						VoldResponseCode.VolumeListResult);
			        //判断存储设备状态
				for (String volstr : vols) {
					String[] tok = volstr.split(" ");
					// FMT: <label> <mountpoint> <state>
					String path = tok[1];
					String state = Environment.MEDIA_REMOVED;

					int st = Integer.parseInt(tok[2]);
					if (st == VolumeState.NoMedia) {
						state = Environment.MEDIA_REMOVED;
					} else if (st == VolumeState.Idle) {
						state = Environment.MEDIA_UNMOUNTED;
					} else if (st == VolumeState.Mounted) {
						state = Environment.MEDIA_MOUNTED;
						Slog.i(TAG, "Media already mounted on daemon connection");
					} else if (st == VolumeState.Shared) {
						state = Environment.MEDIA_SHARED;
						Slog.i(TAG, "Media shared on daemon connection");
					} else {
						throw new Exception(String.format("Unexpected state %d", st));
					}

					if (state != null) {
						if (DEBUG_EVENTS) Slog.i(TAG, "Updating valid state " + state);
						//更新Volume状态
						updatePublicVolumeState(path, state);
					}
				}
			} catch (Exception e) {
				Slog.e(TAG, "Error processing initial volume state", e);
				updatePublicVolumeState(mExternalStoragePath, Environment.MEDIA_REMOVED);
			}

			/*
			 * Now that we've done our initialization, release
			 * the hounds!
			 */
			mConnectedSignal.countDown();
			mConnectedSignal = null;

			// 使用PackageManagerService扫描外边存储设备上的APK信息
			mPms.scanAvailableAsecs();

			// Notify people waiting for ASECs to be scanned that it's done.
			mAsecsScanned.countDown();
			mAsecsScanned = null;
		}
	}.start();
}

存储设备状态更新：

private boolean updatePublicVolumeState(String path, String state) {
	String oldState;
	synchronized(mVolumeStates) {
		oldState = mVolumeStates.put(path, state);
	}
	if (state.equals(oldState)) {
		Slog.w(TAG, String.format("Duplicate state transition (%s -> %s) for %s",state, state, path));
		return false;
	}
	Slog.d(TAG, "volume state changed for " + path + " (" + oldState + " -> " + state + ")");
	if (path.equals(mExternalStoragePath)) {
		// Update state on PackageManager, but only of real events
		if (!mEmulateExternalStorage) {
			if (Environment.MEDIA_UNMOUNTED.equals(state)) {
				mPms.updateExternalMediaStatus(false, false);

				/*
				 * Some OBBs might have been unmounted when this volume was
				 * unmounted, so send a message to the handler to let it know to
				 * remove those from the list of mounted OBBS.
				 */
				mObbActionHandler.sendMessage(mObbActionHandler.obtainMessage(OBB_FLUSH_MOUNT_STATE, path));
			} else if (Environment.MEDIA_MOUNTED.equals(state)) {
				mPms.updateExternalMediaStatus(true, false);
			}
		}
	}
	synchronized (mListeners) {
		for (int i = mListeners.size() -1; i >= 0; i--) {
			MountServiceBinderListener bl = mListeners.get(i);
			try {
				//调用已注册的MountServiceBinderListener来通知存储设备状态改变
				bl.mListener.onStorageStateChanged(path, oldState, state);
			} catch (RemoteException rex) {
				Slog.e(TAG, "Listener dead");
				mListeners.remove(i);
			} catch (Exception ex) {
				Slog.e(TAG, "Listener failed", ex);
			}
		}
	}
	return true;
}

MountServiceBinderListener的注册过程：

public void registerListener(IMountServiceListener listener) {
	synchronized (mListeners) {
		MountServiceBinderListener bl = new MountServiceBinderListener(listener);
		try {
			listener.asBinder().linkToDeath(bl, 0);
			mListeners.add(bl);
		} catch (RemoteException rex) {
			Slog.e(TAG, "Failed to link to listener death");
		}
	}
}

使用StorageManager的内部类MountServiceBinderListener对象来构造MountService的内部类MountServiceBinderListener对象，并添加到MountService的成员变量mListeners列表中。StorageManager的内部类MountServiceBinderListener定义如下：

private class MountServiceBinderListener extends IMountServiceListener.Stub {
	public void onUsbMassStorageConnectionChanged(boolean available) {
		final int size = mListeners.size();
		for (int i = 0; i < size; i++) {
			mListeners.get(i).sendShareAvailabilityChanged(available);
		}
	}

	public void onStorageStateChanged(String path, String oldState, String newState) {
		final int size = mListeners.size();
		for (int i = 0; i < size; i++) {
			mListeners.get(i).sendStorageStateChanged(path, oldState, newState);
		}
	}
}

最后调用ListenerDelegate的sendStorageStateChanged来实现

3）事件处理

mCallbackHandler.sendMessage(mCallbackHandler.obtainMessage(event.getCode(), event.getRawEvent())); 将事件已消息的方式发送到VoldConnector.CallbackHandler线程中处理：

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

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) {
		/*
		 * 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]));
	} else if ((code == VoldResponseCode.VolumeDiskInserted) ||
			   (code == VoldResponseCode.VolumeDiskRemoved) ||
			   (code == VoldResponseCode.VolumeBadRemoval)) {
		// FMT: NNN Volume <label> <mountpoint> disk inserted (<major>:<minor>)
		// FMT: NNN Volume <label> <mountpoint> disk removed (<major>:<minor>)
		// FMT: NNN Volume <label> <mountpoint> bad removal (<major>:<minor>)
		String action = null;
		final String label = cooked[2];
		final String path = cooked[3];
		int major = -1;
		int minor = -1;

		try {
			String devComp = cooked[6].substring(1, cooked[6].length() -1);
			String[] devTok = devComp.split(":");
			major = Integer.parseInt(devTok[0]);
			minor = Integer.parseInt(devTok[1]);
		} catch (Exception ex) {
			Slog.e(TAG, "Failed to parse major/minor", ex);
		}

		if (code == VoldResponseCode.VolumeDiskInserted) {
			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();
		} else if (code == VoldResponseCode.VolumeDiskRemoved) {
			/*
			 * This event gets trumped if we're already in BAD_REMOVAL state
			 */
			if (getVolumeState(path).equals(Environment.MEDIA_BAD_REMOVAL)) {
				return true;
			}
			/* Send the media unmounted event first */
			if (DEBUG_EVENTS) Slog.i(TAG, "Sending unmounted event first");
			updatePublicVolumeState(path, Environment.MEDIA_UNMOUNTED);
			sendStorageIntent(Environment.MEDIA_UNMOUNTED, path);

			if (DEBUG_EVENTS) Slog.i(TAG, "Sending media removed");
			updatePublicVolumeState(path, Environment.MEDIA_REMOVED);
			action = Intent.ACTION_MEDIA_REMOVED;
		} else if (code == VoldResponseCode.VolumeBadRemoval) {
			if (DEBUG_EVENTS) Slog.i(TAG, "Sending unmounted event first");
			/* Send the media unmounted event first */
			updatePublicVolumeState(path, Environment.MEDIA_UNMOUNTED);
			action = Intent.ACTION_MEDIA_UNMOUNTED;

			if (DEBUG_EVENTS) Slog.i(TAG, "Sending media bad removal");
			updatePublicVolumeState(path, Environment.MEDIA_BAD_REMOVAL);
			action = Intent.ACTION_MEDIA_BAD_REMOVAL;
		} else {
			Slog.e(TAG, String.format("Unknown code {%d}", code));
		}

		if (action != null) {
			sendStorageIntent(action, path);
		}
	} else {
		return false;
	}
	return true;
}

整个MountService与Vold的通信到此就介绍完了。