Android學習——MediaScanner源碼分析

神馬是MediaScanner呢？在Android的SDK裡面是看不到這個類的，因為被google隱藏了。通過Android的源碼我們可以看到MediaScanner的類注解多了一個@hide的標注。所以對於一般應用開發者，此文意義不是很大，大家可以繞道。

在前兩篇文章中，最後我們都了解了Android的媒體文件的掃描是在MediaScannerService中調用MediaScanner的scanDirectories或者scanSingleFile完成最終的掃描的。那麼MediaScanner是如何工作的呢？

google對MediaScanner寫了一大堆的類注釋，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: * [email protected]}

  17: */

下面為調用時序圖，如下：

這時序圖好像不是很規范！點擊上圖看大圖！請見諒。  開始看代碼把……

1，scanDirectories。

初始化數據並調用processDirectory處理掃描。

   1: public void scanDirectories(String[] directories, String volumeName) {

   2:         try {

   3:             long start = System.currentTimeMillis();

   4:             //初始化

   5:             initialize(volumeName);

   6:             //將數據庫中的數據緩存到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:                 //掃描處理

  15:                 processDirectory(directories[i], MediaFile.sFileExtensions, mClient);

  16:             }

  17:             long scan = System.currentTimeMillis();

  18:             //處理後續數據

  19:             postscan(directories);

  20:             long end = System.currentTimeMillis();

2，processDirectory

這是一個native方法，所以我們直接轉向jni，代碼如下：

   1: static void

   2: android_media_MediaScanner_processDirectory(JNIEnv *env, jobject thiz, jstring path, jstring extensions, jobject client)

   3: {   //獲取MediaScanner

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

   5:     //參數判斷，並拋出異常

   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實例

  27:     MyMediaScannerClient myClient(env, client);

  28:     //mp調用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: {//這方法不知道干嗎的，估計跟線程有關

   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:     //有是一個關鍵點

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

  26:     //釋放內存

  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:                 //文件夾，遞歸調用

  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，擴展名符合

  32:                 struct stat statbuf;

  33:                 stat(path, &statbuf);

  34:                 if (statbuf.st_size > 0) {

  35:                     //調用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:      //有點反射的感覺，調用java裡面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); 讓我們回到Java

在android.media.MediaScanner.MyMediaScannerClient中的scanFile方法是直接調用doScanFile的，來看看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:                         // 調用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:         }

補充：result = endFile(entry, ringtones, notifications, alarms, music, podcasts);就是在這裡將媒體數據信息存放到數據庫的

7，接著是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:     //調用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); 在此方法中根據不同的文件擴展名調用更加底層的解析方法，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:     //根據擴展名調用不同的解析方法

  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:     //調用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中是通過類反射的方法調用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，這個方法主要處理那些在底層解析後的數據返回到java層

   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，此文結束。

轉自：http://www.cnblogs.com/halzhang/archive/2011/03/10/1980319.html