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：

java 调用dataworks openapi_ide

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uri
	mAudioUri = 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 media
		mProcessPlaylists = 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

java 调用dataworks openapi_MediaScanner_02

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:  }