• Android学习——MediaScanner源码分析


    神马是MediaScanner呢?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?

    转载请注明出处

    google对MediaScanner写了一大堆的类注释,如下:

       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:

    这时序图好像不是很规范!点击看大图!请见谅。吐舌笑脸  开始看代码把……

    1,scanDirectories。

    初始化数据并调用processDirectory处理扫描。

       1: public void scanDirectories(String[] directories, String volumeName) {
       2:         try {
       3:             long start = System.currentTimeMillis();
       4:             //初始化
       5:             initialize(volumeName);
       6:             //将数据库中的数据缓存到mFileCache
       7:             /*
       8:              * mFileCache.put(key, new FileCacheEntry(uri, rowId, path, lastModified));
       9:              */
      10:             prescan(null);
      11:             long prescan = System.currentTimeMillis();
      12:  
      13:             for (int i = 0; i < directories.length; i++) {
      14:                 //扫描处理
      15:                 processDirectory(directories[i], MediaFile.sFileExtensions, mClient);
      16:             }
      17:             long scan = System.currentTimeMillis();
      18:             //处理后续数据
      19:             postscan(directories);
      20:             long end = System.currentTimeMillis();

    2,processDirectory

    这是一个native方法,所以我们直接转向jni,代码如下:

       1: static void
       2: android_media_MediaScanner_processDirectory(JNIEnv *env, jobject thiz, jstring path, jstring extensions, jobject client)
       3: {   //获取MediaScanner
       4:     MediaScanner *mp = (MediaScanner *)env->GetIntField(thiz, fields.context);
       5:     //参数判断,并抛出异常
       6:     if (path == NULL) {
       7:         jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
       8:         return;
       9:     }
      10:     if (extensions == NULL) {
      11:         jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
      12:         return;
      13:     }
      14:     
      15:     const char *pathStr = env->GetStringUTFChars(path, NULL);
      16:     if (pathStr == NULL) {  // Out of memory
      17:         jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
      18:         return;
      19:     }
      20:     const char *extensionsStr = env->GetStringUTFChars(extensions, NULL);
      21:     if (extensionsStr == NULL) {  // Out of memory
      22:         env->ReleaseStringUTFChars(path, pathStr);
      23:         jniThrowException(env, "java/lang/RuntimeException", "Out of memory");
      24:         return;
      25:     }
      26:     //初始化client实例
      27:     MyMediaScannerClient myClient(env, client);
      28:     //mp调用processDirectory
      29:     mp->processDirectory(pathStr, extensionsStr, myClient, ExceptionCheck, env);
      30:     //gc
      31:     env->ReleaseStringUTFChars(path, pathStr);
      32:     env->ReleaseStringUTFChars(extensions, extensionsStr);
      33: }

    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大笑

    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:  }

    此致,此文结束,累。忍者

    三篇有关MediaScanner的文章,希望对大家有所帮助。

    对C/C++不是很熟悉,如发现分析有误,请告知。

    转载请注明出处

    --------END-------

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  • 原文地址:https://www.cnblogs.com/halzhang/p/1980319.html
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