Android源代码分析(三) MediaScanner源码分析(下)

MediaScanner位置在frameworks\base\media\下，包括jni和java文件，
在Android的SDK里面是看不到这个类的，因为被google隐藏了。通过Android的源码我们可以看到MediaScanner的类注解多了一个@hide的标注。所以对于一般应用开发者，此文意义不是很大，大家可以绕道。在前两篇文章中，最后我们都了解了Android的媒体文件的扫描是在MediaScannerService中调用MediaScanner的scanDirectories或者scanSingleFile完成最终的扫描的。那么MediaScanner是如何工作的呢？

先来看看google对该类的注释：

1: /* In summary:

   2: * Java MediaScannerService calls

   3: * Java MediaScanner scanDirectories, which calls

   4: * Java MediaScanner processDirectory (native method), which calls

   5: * native MediaScanner processDirectory, which calls

   6: * native MyMediaScannerClient scanFile, which calls

   7: * Java MyMediaScannerClient scanFile, which calls

   8: * Java MediaScannerClient doScanFile, which calls

   9: * Java MediaScanner processFile (native method), which calls

  10: * native MediaScanner processFile, which calls

  11: * native parseMP3, parseMP4, parseMidi, parseOgg or parseWMA, which calls

  12: * native MyMediaScanner handleStringTag, which calls

  13: * Java MyMediaScanner handleStringTag.

  14: * Once MediaScanner processFile returns, an entry is inserted in to the database.

  15: *

  16: * {@hide}

  17: */

下面为调用时序图，如下：

先看看java实现。
这个类巨复杂，而且和MediaProvider交互频繁。在分析的时候要时刻回到MediaProvider去
看看。
1. 初始化

public class MediaScanner{static {//libmedia_jni.so的加载是在MediaScanner类中完成的//这么重要的so为何放在如此不起眼的地方加载？？？System.loadLibrary("media_jni");native_init();}public MediaScanner(Context c) {native_setup();//调用jni层的初始化，暂时不用看了，无非就是一些//初始化工作，待会在再进去看看……..}

刚才MSS中是调用scanDirectories函数，我们看看这个。

2. scanDirectories

public void scanDirectories(String[] directories, String volumeName) {try {long start = System.currentTimeMillis();initialize(volumeName);//初始化prescan(null);//扫描前的预处理long prescan = System.currentTimeMillis();for (int i = 0; i < directories.length; i++) {//扫描文件夹，这里有一个很重要的参数 mClient// processDirectory是一个native函数processDirectory(directories[i], MediaFile.sFileExtensions, mClient);}long scan = System.currentTimeMillis();postscan(directories);//扫描后处理long end = System.currentTimeMillis();…..打印时间，异常处理…没了…

下面简单讲讲initialize ，preScan和postScan都干嘛了。

private void initialize(String volumeName) {//打开MediaProvider，获得它的一个实例mMediaProvider = mContext.getContentResolver().acquireProvider("media");//得到一些urimAudioUri = Audio.Media.getContentUri(volumeName);mVideoUri = Video.Media.getContentUri(volumeName);mImagesUri = Images.Media.getContentUri(volumeName);mThumbsUri = Images.Thumbnails.getContentUri(volumeName);//外部存储的话，可以支持播放列表之类的东西，搞了一些个缓存池之类的//如mGenreCache等if (!volumeName.equals("internal")) {// we only support playlists on external mediamProcessPlaylists = true;mGenreCache = new HashMap<String, Uri>();…

preScan，这个函数很复杂：
大概就是创建一个FileCache，用来缓存扫描文件的一些信息，例如last_modified等。这个
FileCache是从MediaProvider中已有信息构建出来的，也就是历史信息。后面根据扫描得到的新
信息来对应更新历史信息。
postScan,这个函数做一些清除工作，例如以前有video生成了一些缩略图，现在video文件
被干掉了，则对应的缩略图也要被干掉。
另外还有一个mClient，这个是从MediaScannerClient派生下来的一个东西，里边保存了一
个文件的一些信息。后续再分析。
刚才说到，具体扫描工作是在processDirectory函数中完成的。这个是一个native函数。
在frameworks\base\media\jni\android_media_MediaScanner.cpp中。

3，mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);

   1: status_t MediaScanner::processDirectory(const char *path, const char* extensions,

   2:         MediaScannerClient& client, ExceptionCheck exceptionCheck, void* exceptionEnv)

   3: {//这方法不知道干吗的，估计跟线程有关

   4:     InitializeForThread();

5:

   6:     int pathLength = strlen(path);

   7:     if (pathLength >= PATH_MAX) {

   8:         return PVMFFailure;

   9:     }

  10:     char* pathBuffer = (char *)malloc(PATH_MAX + 1);

  11:     if (!pathBuffer) {

  12:         return PVMFFailure;

  13:     }

14:

  15:     int pathRemaining = PATH_MAX - pathLength;

  16:     strcpy(pathBuffer, path);

  17:     if (pathBuffer[pathLength - 1] != '/') {

  18:         pathBuffer[pathLength] = '/';

  19:         pathBuffer[pathLength + 1] = 0;

  20:         --pathRemaining;

  21:     }

22:

  23:     client.setLocale(mLocale);

  24:     //有是一个关键点

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

  26:     //释放内存

  27:     free(pathBuffer);

  28:     return result;

  29: }

4，doProcessDirectory

   1: status_t MediaScanner::doProcessDirectory(char *path, int pathRemaining, const char* extensions,

   2:         MediaScannerClient& client, ExceptionCheck exceptionCheck, void* exceptionEnv)

   3: {

   4:     ……

   5:      ……

   6:         if (type == DT_REG || type == DT_DIR) {

   7:             int nameLength = strlen(name);

   8:             bool isDirectory = (type == DT_DIR);

9:

  10:             if (nameLength > pathRemaining || (isDirectory && nameLength + 1 > pathRemaining)) {

  11:                 // path too long!

  12:                 continue;

  13:             }

14:

  15:             strcpy(fileSpot, name);

  16:             if (isDirectory) {

  17:                 // ignore directories with a name that starts with '.'

  18:                 // for example, the Mac ".Trashes" directory

  19:                 if (name[0] == '.') continue;

20:

  21:                 strcat(fileSpot, "/");

  22:                 //文件夹，递归调用

  23:                 int err = doProcessDirectory(path, pathRemaining - nameLength - 1, extensions, client, exceptionCheck, exceptionEnv);

  24:                 if (err) {

  25:                     // pass exceptions up - ignore other errors

  26:                     if (exceptionCheck && exceptionCheck(exceptionEnv)) goto failure;

  27:                     LOGE("Error processing '%s' - skipping\n", path);

  28:                     continue;

  29:                 }

  30:             } else if (fileMatchesExtension(path, extensions)) {

  31:                 //文件，扩展名符合

  32:                 struct stat statbuf;

  33:                 stat(path, &statbuf);

  34:                 if (statbuf.st_size > 0) {

  35:                     //调用client的scanFile方法

  36:                     client.scanFile(path, statbuf.st_mtime, statbuf.st_size);

  37:                 }

  38:                 if (exceptionCheck && exceptionCheck(exceptionEnv)) goto failure;

  39:             }

  40:         }

  41: ……

  42: ……

5，client.scanFile

   1: // returns true if it succeeded, false if an exception occured in the Java code

   2:  virtual bool scanFile(const char* path, long long lastModified, long long fileSize)

   3:  {

   4:      jstring pathStr;

   5:      if ((pathStr = mEnv->NewStringUTF(path)) == NULL) return false;

   6:      //有点反射的感觉，调用java里面mClient中的scanFile方法

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

8:

   9:      mEnv->DeleteLocalRef(pathStr);

  10:      return (!mEnv->ExceptionCheck());

  11:  }

6，mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified, fileSize);让我们回到Java

在android.media.MediaScanner.MyMediaScannerClient中的scanFile方法是直接调用doScanFile的，来看看doScanFile

1:

   2:         public Uri doScanFile(String path, String mimeType, long lastModified, long fileSize,

   3:                 boolean scanAlways) {

   4:             Uri result = null;

   5:             // long t1 = System.currentTimeMillis();

   6:             try {

   7:                 FileCacheEntry entry = beginFile(path, mimeType, lastModified, fileSize);

   8:                 // rescan for metadata if file was modified since last scan

   9:                 if (entry != null && (entry.mLastModifiedChanged || scanAlways)) {

  10:                     String lowpath = path.toLowerCase();

  11:                     boolean ringtones = (lowpath.indexOf(RINGTONES_DIR) > 0);

  12:                     boolean notifications = (lowpath.indexOf(NOTIFICATIONS_DIR) > 0);

  13:                     boolean alarms = (lowpath.indexOf(ALARMS_DIR) > 0);

  14:                     boolean podcasts = (lowpath.indexOf(PODCAST_DIR) > 0);

  15:                     boolean music = (lowpath.indexOf(MUSIC_DIR) > 0)

  16:                             || (!ringtones && !notifications && !alarms && !podcasts);

17:

  18:                     if (isMetadataSupported(mFileType)) {

  19:                         // 调用jni方法

  20:                         processFile(path, mimeType, this);

  21:                     } else if (MediaFile.isImageFileType(mFileType)) {

  22:                         // we used to compute the width and height but it's not

  23:                         // worth it

  24:                     }

25:

  26:                     result = endFile(entry, ringtones, notifications, alarms, music, podcasts);

  27:                 }

  28:             } catch (RemoteException e) {

  29:                 Log.e(TAG, "RemoteException in MediaScanner.scanFile()", e);

  30:             }

  31:             // long t2 = System.currentTimeMillis();

  32:             // Log.v(TAG, "scanFile: " + path + " took " + (t2-t1));

  33:             return result;

  34:         }

补充：result = endFile(entry, ringtones, notifications, alarms, music, podcasts);就是在这里将媒体数据信息存放到数据库的

7，接着是native的 processFile

   1: static void

   2: android_media_MediaScanner_processFile(JNIEnv *env, jobject thiz, jstring path, jstring mimeType, jobject client)

   3: {

   4:     MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);

5:

   6:     if (path == NULL) {

   7:         jniThrowException(env, "java/lang/IllegalArgumentException", NULL);

   8:         return;

   9:     }

10:

  11:     const char *pathStr = env->GetStringUTFChars(path, NULL);

  12:     if (pathStr == NULL) {  // Out of memory

  13:         jniThrowException(env, "java/lang/RuntimeException", "Out of memory");

  14:         return;

  15:     }

  16:     const char *mimeTypeStr = (mimeType ? env->GetStringUTFChars(mimeType, NULL) : NULL);

  17:     if (mimeType && mimeTypeStr == NULL) {  // Out of memory

  18:         env->ReleaseStringUTFChars(path, pathStr);

  19:         jniThrowException(env, "java/lang/RuntimeException", "Out of memory");

  20:         return;

  21:     }

22:

  23:     MyMediaScannerClient myClient(env, client);

  24:     //调用MediaScanner的processFile

  25:     mp->processFile(pathStr, mimeTypeStr, myClient);

  26:     env->ReleaseStringUTFChars(path, pathStr);

  27:     if (mimeType) {

  28:         env->ReleaseStringUTFChars(mimeType, mimeTypeStr);

  29:     }

  30: }

8，mp->processFile(pathStr, mimeTypeStr, myClient); 在此方法中根据不同的文件扩展名调用更加底层的解析方法，我想主要是ID3信息解析

   1: status_t MediaScanner::processFile(const char *path, const char* mimeType, MediaScannerClient& client)

   2: {

   3:     status_t result;

   4:     InitializeForThread();

   5:     //初始化client

   6:     client.setLocale(mLocale);

   7:     client.beginFile();

8:

   9:     //LOGD("processFile %s mimeType: %s\n", path, mimeType);

  10:     const char* extension = strrchr(path, '.');

  11:     //根据扩展名调用不同的解析方法

  12:     if (extension && strcasecmp(extension, ".mp3") == 0) {

  13:         result = parseMP3(path, client);

  14:     } else if (extension &&

  15:         (strcasecmp(extension, ".mp4") == 0 || strcasecmp(extension, ".m4a") == 0 ||

  16:          strcasecmp(extension, ".3gp") == 0 || strcasecmp(extension, ".3gpp") == 0 ||

  17:          strcasecmp(extension, ".3g2") == 0 || strcasecmp(extension, ".3gpp2") == 0)) {

  18:         result = parseMP4(path, client);

  19:     } else if (extension && strcasecmp(extension, ".ogg") == 0) {

  20:         result = parseOgg(path, client);

  21:     } else if (extension &&

  22:         ( strcasecmp(extension, ".mid") == 0 || strcasecmp(extension, ".smf") == 0

  23:         || strcasecmp(extension, ".imy") == 0)) {

  24:         result = parseMidi(path, client);

  25:     } else if (extension &&

  26:        (strcasecmp(extension, ".wma") == 0 || strcasecmp(extension, ".aac") == 0)) {

  27:         //TODO: parseWMA needs to be renamed to reflect what it is really doing,

  28:         //ie. using OpenCORE frame metadata utility(FMU) to retrieve metadata.

  29:         result = parseWMA(path, client);

  30:     } else {

  31:         result = PVMFFailure;

  32:     }

  33:     //调用client

  34:     client.endFile();

35:

  36:     return result;

  37: }

9，client.endFile()

   1: void MediaScannerClient::endFile()

   2: {

   3:     if (mLocaleEncoding != kEncodingNone) {

   4:         int size = mNames->size();

   5:         uint32_t encoding = kEncodingAll;

6:

   7:         // compute a bit mask containing all possible encodings

   8:         for (int i = 0; i < mNames->size(); i++)

   9:             encoding &= possibleEncodings(mValues->getEntry(i));

10:

  11:         // if the locale encoding matches, then assume we have a native encoding.

  12:         if (encoding & mLocaleEncoding)

  13:             convertValues(mLocaleEncoding);

14:

  15:         // finally, push all name/value pairs to the client

  16:         for (int i = 0; i < mNames->size(); i++) {

  17:             //在handleStringTag中是通过类反射的方法调用java中的handleStringTag

  18:             if (!handleStringTag(mNames->getEntry(i), mValues->getEntry(i)))

  19:                 break;

  20:         }

  21:     }

  22:     // else addStringTag() has done all the work so we have nothing to do

23:

  24:     delete mNames;

  25:     delete mValues;

  26:     mNames = NULL;

  27:     mValues = NULL;

  28: }

10，java中的handleStringTag，这个方法主要处理那些在底层解析后的数据返回到java层

   1: public void handleStringTag(String name, String value) {

   2:      if (name.equalsIgnoreCase("title") || name.startsWith("title;")) {

   3:          // Don't trim() here, to preserve the special \001 character

   4:          // used to force sorting. The media provider will trim() before

   5:          // inserting the title in to the database.

   6:          mTitle = value;

   7:      } else if (name.equalsIgnoreCase("artist") || name.startsWith("artist;")) {

   8:          mArtist = value.trim();

   9:      } else if (name.equalsIgnoreCase("albumartist") || name.startsWith("albumartist;")) {

  10:          mAlbumArtist = value.trim();

  11:      } else if (name.equalsIgnoreCase("album") || name.startsWith("album;")) {

  12:          mAlbum = value.trim();

  13:      } else if (name.equalsIgnoreCase("composer") || name.startsWith("composer;")) {

  14:          mComposer = value.trim();

  15:      } else if (name.equalsIgnoreCase("genre") || name.startsWith("genre;")) {

  16:          // handle numeric genres, which PV sometimes encodes like "(20)"

  17:          if (value.length() > 0) {

  18:              int genreCode = -1;

  19:              char ch = value.charAt(0);

  20:              if (ch == '(') {

  21:                  genreCode = parseSubstring(value, 1, -1);

  22:              } else if (ch >= '0' && ch <= '9') {

  23:                  genreCode = parseSubstring(value, 0, -1);

  24:              }

  25:              if (genreCode >= 0 && genreCode < ID3_GENRES.length) {

  26:                  value = ID3_GENRES[genreCode];

  27:              }

  28:          }

  29:          mGenre = value;

  30:      } else if (name.equalsIgnoreCase("year") || name.startsWith("year;")) {

  31:          mYear = parseSubstring(value, 0, 0);

  32:      } else if (name.equalsIgnoreCase("tracknumber") || name.startsWith("tracknumber;")) {

  33:          // track number might be of the form "2/12"

  34:          // we just read the number before the slash

  35:          int num = parseSubstring(value, 0, 0);

  36:          mTrack = (mTrack / 1000) * 1000 + num;

  37:      } else if (name.equalsIgnoreCase("discnumber") ||

  38:              name.equals("set") || name.startsWith("set;")) {

  39:          // set number might be of the form "1/3"

  40:          // we just read the number before the slash

  41:          int num = parseSubstring(value, 0, 0);

  42:          mTrack = (num * 1000) + (mTrack % 1000);

  43:      } else if (name.equalsIgnoreCase("duration")) {

  44:          mDuration = parseSubstring(value, 0, 0);

  45:      } else if (name.equalsIgnoreCase("writer") || name.startsWith("writer;")) {

  46:          mWriter = value.trim();

  47:      }

  48:  }

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