Android开发之媒体扫描详细解析（上）

用过Android手机的同学都知道，每次开机的时候系统会先扫描sdcard，sdcard重新插拔(挂载)也会扫描一次sdcard。

为什么要扫描sdcard，其实是为了给系统的其他应用提供便利，比如，Gallary、Music、VideoPlayer等应用，进入Gallary后会显示sdcard中的所有图片，

如果进入Gallary后再去扫描，可想而知，你会厌恶这个应用，因为我们会觉得它反应太慢了。还有Music你看到播放列表的时候实际能看到这首歌曲的时长、演唱者、专辑

等信息，这个也不是你进入应用后一下子可以读出来的。

所以Android使用sdcard挂载后扫描的机制，先将这些媒体相关的信息扫描出来保存在数据库中，当打开应用的时候直接去数据库读取(或者所通过MediaProvider去从数据库读取)并show给用户，这样用户体验会好很多，下面我们分析这种扫描机制是如何实现的。

在源码目录的\packages\providers\MediaProvider下面是MediaProvider的源码，它就是完成扫描并将数据保存于数据库中的程序。

先看下它的AndroidManifest.xml文件

application android:process="android.process.media"，也就是应用程序名称为android.process.media，我们用adb 连接到android 设备，并且进入shell后输入ps可以看到的确有应用程序app_4     2796  2075  165192 19420 ffffffff 6fd0eb58 S android.process.media 在运行。另外此程序中有三个部分，分别是provider - MediaProvider 、receiver - MediaScannerReceiver、service - MediaScannerService，它并没有activity，说明它是一直运行于后台的程序。并且从receiver中的

                "android.intent.action.BOOT_COMPLETED" />

可以看出它是开机自启动的。下面从这个广播开始看代码。

public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        Uri uri = intent.getData();
        String externalStoragePath = Environment.getExternalStorageDirectory().getPath();

        if (action.equals(Intent.ACTION_BOOT_COMPLETED)) {
            // scan internal storage
            scan(context, MediaProvider.INTERNAL_VOLUME);
        } else {
     。 。 。
     }
  }
}

收到开机广播后，首先执行scan函数STEP 1，

private void scan(Context context, String volume) {
        Bundle args = new Bundle();
        args.putString("volume", volume);
        context.startService(
                new Intent(context, MediaScannerService.class).putExtras(args));
    }

scan函数主要传进来一个volume卷名，MediaProvider.INTERNAL_VOLUME实际就是内置存储卡"internal"，在此我们首先理解为开机后首先扫描内置存储卡。

然后启动services MediaScannerService，这也是此服务第一次被启动。

service的启动流程就不说了，onCreate肯定是首先被调用的

public void onCreate()
   {
       PowerManager pm = (PowerManager)getSystemService(Context.POWER_SERVICE);
       mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);

       // Start up the thread running the service.  Note that we create a
       // separate thread because the service normally runs in the process's
       // main thread, which we don't want to block.
       Thread thr = new Thread(null, this, "MediaScannerService");
       thr.start();
   }

此处前面是申请了一把wake lock ,主要是防止CPU休眠的，然后启动了一个线程实际就是 MediaScannerService自身的线程，它继承自Runnable，下面主要看Run函数

public void run()
    {
        // reduce priority below other background threads to avoid interfering
        // with other services at boot time.
        Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND +
                Process.THREAD_PRIORITY_LESS_FAVORABLE);
        Looper.prepare();

        mServiceLooper = Looper.myLooper();
        mServiceHandler = new ServiceHandler();

        Looper.loop();
    }

可以看出此线程的目的是为了处理hander消息ServiceHandler

执行完onCreate后就会执行onStartCommand

public int onStartCommand(Intent intent, int flags, int startId)
   {
       while (mServiceHandler == null) {
           synchronized (this) {
               try {
                   wait(100);
               } catch (InterruptedException e) {
               }
           }
       }

       if (intent == null) {
           Log.e(TAG, "Intent is null in onStartCommand: ",
               new NullPointerException());
           return Service.START_NOT_STICKY;
       }

       Message msg = mServiceHandler.obtainMessage();
       msg.arg1 = startId;
       msg.obj = intent.getExtras();
       mServiceHandler.sendMessage(msg);

       // Try again later if we are killed before we can finish scanning.
       return Service.START_REDELIVER_INTENT;
   }

在这里我们通过STEP 1中传入进来的volume字符串就作为了msg.obj通过handler来处理了

private final class ServiceHandler extends Handler
   {
       @Override
       public void handleMessage(Message msg)
       {
           Bundle arguments = (Bundle) msg.obj;
           String filePath = arguments.getString("filepath");
           String folder = arguments.getString("folder");
           try {
               if (filePath != null) {
                   IBinder binder = arguments.getIBinder("listener");
                   IMediaScannerListener listener =
                           (binder == null ? null : IMediaScannerListener.Stub.asInterface(binder));
                   Uri uri = scanFile(filePath, arguments.getString("mimetype"));
                   if (listener != null) {
                       listener.scanCompleted(filePath, uri);
                   }
               } else if(folder != null) {
                   String volume = arguments.getString("volume");
                   String[] directories = null;
                   directories = new String[] {
                       new File(folder).getPath(),
                   };
                   if (directories != null) {
                       if (Config.LOGD) Log.d(TAG, "start scanning volume " + volume + " ; path = " + folder);
                       scan(directories, volume);
                       if (Config.LOGD) Log.d(TAG, "done scanning volume " + volume + " ; path = " + folder);
                   }
               }"color:#ff0000;">else {
                   String volume = arguments.getString("volume");
                   String[] directories = null;

                   if (MediaProvider.INTERNAL_VOLUME.equals(volume)) {
                       // scan internal media storage
                       directories = new String[] {
                               Environment.getRootDirectory() + "/media",
                       };
                   }
                   else if (MediaProvider.EXTERNAL_VOLUME.equals(volume)) {
                       // scan external storage
                       directories = new String[] {
                               Environment.getExternalStorageDirectory().getPath(),
                               };
                   }

                   if (directories != null) {
                       if (Config.LOGD) Log.d(TAG, "start scanning volume " + volume);
                       scan(directories, volume);
                       if (Config.LOGD) Log.d(TAG, "done scanning volume " + volume);
                   }
               }
           } catch (Exception e) {
               Log.e(TAG, "Exception in handleMessage", e);
           }

           stopSelf(msg.arg1);
       }
   };

很显然我们会执行到标记为红色的else中，我们是先扫描内置sdcard，很显然directories的值为/system/media ,然后调用 scan(directories, volume);函数，应该是内置sdcard中所有的媒体文件都几种存储在/system/media下面所以只需要扫描这一个路径就行了。STEP2

private void scan(String[] directories, String volumeName) {
        // don't sleep while scanning
        mWakeLock.acquire();

        ContentValues values = new ContentValues();
        values.put(MediaStore.MEDIA_SCANNER_VOLUME, volumeName);
        Uri scanUri = getContentResolver().insert(MediaStore.getMediaScannerUri(), values);

        Uri uri = Uri.parse("file://" + directories[0]);
        sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_STARTED, uri));

        try {
            if (volumeName.equals(MediaProvider.EXTERNAL_VOLUME)) {
                 openDatabase(volumeName);
            }

            MediaScanner scanner = createMediaScanner();
            scanner.scanDirectories(directories, volumeName);
        } catch (Exception e) {
            Log.e(TAG, "exception in MediaScanner.scan()", e);
        }

        getContentResolver().delete(scanUri, null, null);

        sendBroadcast(new Intent(Intent.ACTION_MEDIA_SCANNER_FINISHED, uri));
        mWakeLock.release();
    }

开始扫描和结束扫描时都会发送一个全局的广播，第三方应用程序也可以通过注册这两个广播来避开在media 扫描的时候往改扫描文件夹里面写入或删除文件，这个我在项目中就遇到过这种bug。在这一步骤中创建了MediaScanner并调用它的scanDirectories方法   STEP3

public void scanDirectories(String[] directories, String volumeName) {
      try {
          long start = System.currentTimeMillis();
          initialize(volumeName);
          prescan(null);
          long prescan = System.currentTimeMillis();

          for (int i = 0; i < directories.length; i++) {
              processDirectory(directories[i], MediaFile.sFileExtensions, mClient);
          }
          long scan = System.currentTimeMillis();
          postscan(directories);
          long end = System.currentTimeMillis();

          if (Config.LOGD) {
              Log.d(TAG, " prescan time: " + (prescan - start) + "ms\n");
              Log.d(TAG, "    scan time: " + (scan - prescan) + "ms\n");
              Log.d(TAG, "postscan time: " + (end - scan) + "ms\n");
              Log.d(TAG, "   total time: " + (end - start) + "ms\n");
          }
      } catch (SQLException e) {
          // this might happen if the SD card is removed while the media scanner is running
          Log.e(TAG, "SQLException in MediaScanner.scan()", e);
      } catch (UnsupportedOperationException e) {
          // this might happen if the SD card is removed while the media scanner is running
          Log.e(TAG, "UnsupportedOperationException in MediaScanner.scan()", e);
      } catch (RemoteException e) {
          Log.e(TAG, "RemoteException in MediaScanner.scan()", e);
      }
  }

其中initialize   prescan   processDirectory  postscan这四个函数都比较重要。STEP 4

private void initialize(String volumeName) {
       mMediaProvider = mContext.getContentResolver().acquireProvider("media");

       mAudioUri = Audio.Media.getContentUri(volumeName);
       mVideoUri = Video.Media.getContentUri(volumeName);
       mImagesUri = Images.Media.getContentUri(volumeName);
       mThumbsUri = Images.Thumbnails.getContentUri(volumeName);

       if (!volumeName.equals("internal")) {
           // we only support playlists on external media
           mProcessPlaylists = true;
           mProcessGenres = true;
           mGenreCache = new HashMap();
           mGenresUri = Genres.getContentUri(volumeName);
           mPlaylistsUri = Playlists.getContentUri(volumeName);
           // assuming external storage is FAT (case insensitive), except on the simulator.
           if ( Process.supportsProcesses()) {
               mCaseInsensitivePaths = true;
           }
       }
   }

做一些初始化的动作，得到MediaProvider和一些URI实际也就是操作数据库的一些表名。Audio.Media.getContentUri可以在MediaStore.java中找到，此类保存了所有的媒体格式URI等信息，此处获得的mAudioUri的值为“content://media/internal//audio/media"

STEP5

  private void prescan(String filePath) throws RemoteException {
    。 。 。
}

此函数比较长，在此省略代码，有兴趣的可以看源码，这里所做的操作是对于之前有扫描过的，就将数据库中现有的媒体信息放到几个数据结构中临时存储起来。

然后最重要的STEP 6 processDirectory是一个native函数，先注意几个传入参数directories[i]为STEP2中传入的路径/system/media ,MediaFile.sFileExtensions 这个你可以跟到MediaFile中看看这个是如何赋值的，实际就是所有支持的媒体格式后缀以‘，’的方式串在一起的字符串”MP3，M4A，3GA，WAV。。。“最重要的mClient是MyMediaScannerClient的一个实例，此对象将是native层回调函数的接口，所有扫描完后的媒体都会通过此对象来存储到数据库中。

下面进入Native层对应文件是android_media_MediaScanner.cpp       STEP 6

static void
android_media_MediaScanner_processDirectory(JNIEnv *env, jobject thiz, jstring path, jstring extensions, jobject client)
{
    MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);

    if (path == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }
    if (extensions == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }

    const char *pathStr = env->GetStringUTFChars(path, NULL);
    if (pathStr == NULL) {  // Out of memory
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }
    const char *extensionsStr = env->GetStringUTFChars(extensions, NULL);
    if (extensionsStr == NULL) {  // Out of memory
        env->ReleaseStringUTFChars(path, pathStr);
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    MyMediaScannerClient myClient(env, client);
    mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);
    env->ReleaseStringUTFChars(path, pathStr);
    env->ReleaseStringUTFChars(extensions, extensionsStr);
}

这里比较重要的一个点时MyMediaScannerClient myClient(env, client);定义了一个客户端，并将java层的client传入进去，很显然，是想通过MyMediaScannerClient 再来回调client。

STEP 7

status_t MediaScanner::processDirectory(
        const char *path, const char *extensions,
        MediaScannerClient &client,
        ExceptionCheck exceptionCheck, void *exceptionEnv) {
    int pathLength = strlen(path);
    if (pathLength >= PATH_MAX) {
        return UNKNOWN_ERROR;
    }
    char* pathBuffer = (char *)malloc(PATH_MAX + 1);
    if (!pathBuffer) {
        return UNKNOWN_ERROR;
    }

    int pathRemaining = PATH_MAX - pathLength;
    strcpy(pathBuffer, path);
    if (pathLength > 0 && pathBuffer[pathLength - 1] != '/') {
        pathBuffer[pathLength] = '/';
        pathBuffer[pathLength + 1] = 0;
        --pathRemaining;
    }

    client.setLocale(locale());

    status_t result =
        doProcessDirectory(
                pathBuffer, pathRemaining, extensions, client,
                exceptionCheck, exceptionEnv);

    free(pathBuffer);

    return result;
}

此函数没干什么事，具体工作是在doProcessDirectory中做的 STEP 8

status_t MediaScanner::doProcessDirectory(
        char *path, int pathRemaining, const char *extensions,
        MediaScannerClient &client, ExceptionCheck exceptionCheck,
        void *exceptionEnv) {
     . . .
}

此函数太长，在此不粘出来了，这里首先要解释下这些参数，path - 要扫描文件夹路径以'/'结尾，pathRemaining为路径长度与路径最大长度之间的差值，也就是防止扫描时路径超出范围,extensions 前面已经解释过是后缀，client是是STEP6中实例化的MyMediaScannerClient对象，后面两个参数是一些异常处理不用关心。

大家仔细看这个函数的代码就可以知道，它完成的是遍历文件夹并找到有相应extensions 里面后缀的文件fileMatchesExtension(path, extensions)，如果文件大小大于0就调用client.scanFile(path, statbuf.st_mtime, statbuf.st_size);来进行文件读取扫描  注意这里才会读文件的实际内容。

STEP 9

virtual bool scanFile(const char* path, long long lastModified, long long fileSize)
   {
       jstring pathStr;
       if ((pathStr = mEnv->NewStringUTF(path)) == NULL) return false;

       mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified, fileSize);

       mEnv->DeleteLocalRef(pathStr);
       return (!mEnv->ExceptionCheck());
   }

看看，终于用到了mClient,java层传进来的client ,这就是回调到了java 类MyMediaScannerClient里面的STEP 10

public void scanFile(String path, long lastModified, long fileSize) {
           // This is the callback funtion from native codes.
           // Log.v(TAG, "scanFile: "+path);
           doScanFile(path, null, lastModified, fileSize, false);
       }

主要看doScanFile     STEP 11

  public Uri doScanFile(String path, String mimeType, long lastModified, long fileSize, boolean scanAlways) {
            Uri result = null;
//            long t1 = System.currentTimeMillis();
            try {
                FileCacheEntry entry = beginFile(path, mimeType, lastModified, fileSize);
                // rescan for metadata if file was modified since last scan
                if (entry != null && (entry.mLastModifiedChanged || scanAlways)) {
                    String lowpath = path.toLowerCase();
                    boolean ringtones = (lowpath.indexOf(RINGTONES_DIR) > 0);
                    boolean notifications = (lowpath.indexOf(NOTIFICATIONS_DIR) > 0);
                    boolean alarms = (lowpath.indexOf(ALARMS_DIR) > 0);
                    boolean podcasts = (lowpath.indexOf(PODCAST_DIR) > 0);
                    boolean music = (lowpath.indexOf(MUSIC_DIR) > 0) ||
                        (!ringtones && !notifications && !alarms && !podcasts);

                    if (!MediaFile.isImageFileType(mFileType)) {
                        processFile(path, mimeType, this);
                    }

                    result = endFile(entry, ringtones, notifications, alarms, music, podcasts);
                }
            } catch (RemoteException e) {
                Log.e(TAG, "RemoteException in MediaScanner.scanFile()", e);
            }
//            long t2 = System.currentTimeMillis();
//            Log.v(TAG, "scanFile: " + path + " took " + (t2-t1));
            return result;
        }

此函数里面又有三个比较重要的函数beginFile   processFile   endFile
先看beginFile    STEP 12

  public FileCacheEntry beginFile(String path, String mimeType, long lastModified, long fileSize) {
     . . .
}

构建一个FileCacheEntry对象，存储文件的一些基本信息，并且放入mFileCache HashMap中。

根据此文件是否修改来觉得是否processFile   ,又进入到native中

在此插入一段代码

static void
android_media_MediaScanner_native_setup(JNIEnv *env, jobject thiz)
{
    MediaScanner *mp = NULL;
    char value[PROPERTY_VALUE_MAX];
    if (property_get("media.framework.option", value, NULL) && (!strcmp(value, "1"))){
#ifndef NO_OPENCORE
        mp = new PVMediaScanner();
#else
        mp = new StagefrightMediaScanner;
#endif
    }else{
        mp = new StagefrightMediaScanner;
    }

    if (mp == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    env->SetIntField(thiz, fields.context, (int)mp);
}

android2.2以上mediascanner使用StagefrightMediaScanner

STEP 13

static void
android_media_MediaScanner_processFile(JNIEnv *env, jobject thiz, jstring path, jstring mimeType, jobject client)
{
    MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);

    if (path == NULL) {
        jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
        return;
    }

    const char *pathStr = env->GetStringUTFChars(path, NULL);
    if (pathStr == NULL) {  // Out of memory
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }
    const char *mimeTypeStr = (mimeType ? env->GetStringUTFChars(mimeType, NULL) : NULL);
    if (mimeType && mimeTypeStr == NULL) {  // Out of memory
        env->ReleaseStringUTFChars(path, pathStr);
        jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
        return;
    }

    MyMediaScannerClient myClient(env, client);
    mp->processFile(pathStr, mimeTypeStr, myClient);
    env->ReleaseStringUTFChars(path, pathStr);
    if (mimeType) {
        env->ReleaseStringUTFChars(mimeType, mimeTypeStr);
    }
}

不再累述，直接进入 STEP 14

status_t StagefrightMediaScanner::processFile(
        const char *path, const char *mimeType,
        MediaScannerClient &client) {
     . . .
}

由于StagefrightMediaScanner又进入到了stagefright 框架，比较复杂，鉴于篇幅限制，在下一篇blog中继续分析STEP 14