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。   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

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。