Android源代码分析(2) MediaScanner源码分析(下)

Android源代码分析(二) 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

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:  }