android usb挂载分析---MountService启动

在android usb挂载分析----vold启动，我们的vold模块已经启动了，通信的机制也已经建立起来了，接下来我们分析一下MountService的启动，也就是我们FrameWork层的启动，首先看下其大概流程：

MountService的启动在SystemServer.java中，有如下代码：

try {
               /*
                * NotificationManagerService is dependant on MountService,
                * (for media / usb notifications) so we must start MountService first.
                */
               Slog.i(TAG, "Mount Service");
               ServiceManager.addService("mount", new MountService(context));
           } catch (Throwable e) {
               Slog.e(TAG, "Failure starting Mount Service", e);
           }

这里new 了一个 MountService，并把service添加到了 ServiceManager,我们看下MountService的构造函数：

/**
 * Constructs a new MountService instance
 *
 * @param context  Binder context for this service
 */
public MountService(Context context) {
    mContext = context;

    // XXX: This will go away soon in favor of IMountServiceObserver
    mPms = (PackageManagerService) ServiceManager.getService("package");//获取包管理服务

    mContext.registerReceiver(mBroadcastReceiver,
            new IntentFilter(Intent.ACTION_BOOT_COMPLETED), null, null);//注册广播接收器

    mHandlerThread = new HandlerThread("MountService");//处理消息
    mHandlerThread.start();
    mHandler = new MountServiceHandler(mHandlerThread.getLooper());

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

    /*
     * Vold does not run in the simulator, so pretend the connector thread
     * ran and did its thing.
     */
    if ("simulator".equals(SystemProperties.get("ro.product.device"))) {
        mReady = true;
        mUmsEnabling = true;
        return;
    }

    /*
     * 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",
            PackageManagerService.MAX_CONTAINERS * 2, VOLD_TAG);
    mReady = false;
    Thread thread = new Thread(mConnector, VOLD_TAG);
    thread.start();
}

后面new 了一个NativeDaemonConnector，注意这里传递了一个"vold"字符串，跟我们在vold启动的时候传给CommandListener是一样的。

接下来调用 thread.start()启动线程，我们看下它的run函数

public void run() {

        while (true) {
            try {
                listenToSocket();
            } catch (Exception e) {
                Slog.e(TAG, "Error in NativeDaemonConnector", e);
                SystemClock.sleep(5000);
            }
        }
    }

在循环中调用listenToSocket函数，看下这个函数

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

    try {
        socket = new LocalSocket();
        LocalSocketAddress address = new LocalSocketAddress(mSocket,   //这里mSocket=“vold"
                LocalSocketAddress.Namespace.RESERVED);              //注意这里的RESERVED

        socket.connect(address);              //连接到vold模块监听的套接字处
        mCallbacks.onDaemonConnected();       //实现在MountService中

        InputStream inputStream = socket.getInputStream();
        mOutputStream = socket.getOutputStream();

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

        while (true) {
            int count = inputStream.read(buffer, start, BUFFER_SIZE - start); //读取消息
            if (count < 0) 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) {
                    String event = new String(buffer, start, i - start);
                    if (LOCAL_LOGD) Slog.d(TAG, String.format("RCV <- {%s}", event));

                    String[] tokens = event.split(" ");
                    try {
                        int code = Integer.parseInt(tokens[0]);

                        if (code >= ResponseCode.UnsolicitedInformational) {
                            try {
                                if (!mCallbacks.onEvent(code, event, tokens)) {//实现在MountService中
                                    Slog.w(TAG, String.format(
                                            "Unhandled event (%s)", event));
                                }
                            } catch (Exception ex) {
                                Slog.e(TAG, String.format(
                                        "Error handling '%s'", event), ex);
                            }
                        }
                        try {
                            mResponseQueue.put(event);
                        } catch (InterruptedException ex) {
                            Slog.e(TAG, "Failed to put response onto queue", ex);
                        }
                    } catch (NumberFormatException nfe) {
                        Slog.w(TAG, String.format("Bad msg (%s)", event));
                    }
                    start = i + 1;
                }
            }

            // 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) {
        Slog.e(TAG, "Communications error", ex);
        throw ex;
    } finally {
        synchronized (this) {
            if (mOutputStream != null) {
                try {
                    mOutputStream.close();
                } catch (IOException e) {
                    Slog.w(TAG, "Failed closing output stream", e);
                }
                mOutputStream = null;
            }
        }

        try {
            if (socket != null) {
                socket.close();
            }
        } catch (IOException ex) {
            Slog.w(TAG, "Failed closing socket", ex);
        }
    }
}

onDaemonConnected的实现在MountServices吕，将向下下发volume list消息 获取到了磁盘的标签，挂载点与状态，调用connect函数连接到vold模块，connetc最终调用native函数connectLocal进行连接工作，我们看下他的jni层代码，最后调用的：

int socket_local_client_connect(int fd, const char *name, int namespaceId,
        int type)
{
    struct sockaddr_un addr;
    socklen_t alen;
    size_t namelen;
    int err;

    err = socket_make_sockaddr_un(name, namespaceId, &addr, &alen);

    if (err < 0) {
        goto error;
    }

    if(connect(fd, (struct sockaddr *) &addr, alen) < 0) {
        goto error;
    }

    return fd;

error:
    return -1;
}

/**
 * connect to peer named "name"
 * returns fd or -1 on error
 */

我们再跟进socket_make_sockaddr_un函数，这时namespaceId传的ANDROID_SOCKET_NAMESPACE_RESERVED,所以会执行下面几句：

case ANDROID_SOCKET_NAMESPACE_RESERVED:
           namelen = strlen(name) + strlen(ANDROID_RESERVED_SOCKET_PREFIX);
           /* unix_path_max appears to be missing on linux */
           if (namelen > sizeof(*p_addr)
                   - offsetof(struct sockaddr_un, sun_path) - 1) {
               goto error;
           }

           strcpy(p_addr->sun_path, ANDROID_RESERVED_SOCKET_PREFIX);  //  ANDROID_RESERVED_SOCKET_PREFIX="/dev/socket/"
           strcat(p_addr->sun_path, name);
       break;

注意在前面 connect  函数中的套接字的构造，使用了AF_LOCAL：

int socket_local_client(const char *name, int namespaceId, int type)
{
    int s;

    s = socket("color:#ff0000;">AF_LOCAL, type, 0);
    if(s < 0) return -1;

    if ( 0 > socket_local_client_connect(s, name, namespaceId, type)) {
        close(s);
        return -1;
    }

    return s;
}

这样，就建立了一条从FrameWork层到vold层的通信链路，后面FrameWork层就等待Vold发送消息过来了。。。FrameWork层的通信也ok了，就可以等待U盘挂载了。。

转自：http://blog.csdn.net/new_abc/article/details/7400740