MediaScanner分析 – MediaScanner.cpp

MediaScanner.cpp分析

檔案路徑 frameworks/base/media/libmedia/MediaScanner.cpp

status_t MediaScanner::processDirectory(        const char *path, const char *extensions,        MediaScannerClient &client,        ExceptionCheck exceptionCheck, void *exceptionEnv) {    int pathLength = strlen(path);    if (pathLength >= PATH_MAX) {        return UNKNOWN_ERROR;    }    char* pathBuffer = (char *)malloc(PATH_MAX + 1);    if (!pathBuffer) {        return UNKNOWN_ERROR;    }    int pathRemaining = PATH_MAX - pathLength;    strcpy(pathBuffer, path);    if (pathLength > 0 && pathBuffer[pathLength - 1] != '/') {        pathBuffer[pathLength] = '/';        pathBuffer[pathLength + 1] = 0;        --pathRemaining;    }    client.setLocale(locale());    status_t result =        doProcessDirectory(                pathBuffer, pathRemaining, extensions, client,                exceptionCheck, exceptionEnv);    free(pathBuffer);    return result;}

很簡單，對路徑參數做了些處理，調用doProcessDirectory

@extensions可能包含多個副檔名，副檔名之間用comma分隔開

client.setLocale(locale()) 目的未知

static bool fileMatchesExtension(const char* path, const char* extensions) {    char* extension = strrchr(path, '.');    if (!extension) return false;    ++extension;    // skip the dot    if (extension[0] == 0) return false;    while (extensions[0]) {        char* comma = strchr(extensions, ',');        size_t length = (comma ? comma - extensions : strlen(extensions));        if (length == strlen(extension) && strncasecmp(extension, extensions, length) == 0) return true;        extensions += length;        if (extensions[0] == ',') ++extensions;    }    return false;}

參數@extensions可能包含多個extensions，用","分割開，這個函數檢查給定路徑@path的副檔名是否包含在@extensions中

100 status_t MediaScanner::doProcessDirectory(101         char *path, int pathRemaining, const char *extensions,102         MediaScannerClient &client, ExceptionCheck exceptionCheck,103         void *exceptionEnv) {104     // place to copy file or directory name105     char* fileSpot = path + strlen(path);106     struct dirent* entry;107 108     // ignore directories that contain a  ".nomedia" file109     if (pathRemaining >= 8 /* strlen(".nomedia") */ ) {110         strcpy(fileSpot, ".nomedia");111         if (access(path, F_OK) == 0) {112             LOGD("found .nomedia, skipping directory\n");113             fileSpot[0] = 0;114             client.addNoMediaFolder(path);115             return OK;116         }117 118         // restore path119         fileSpot[0] = 0;120     }121 122     DIR* dir = opendir(path);123     if (!dir) {124         LOGD("opendir %s failed, errno: %d", path, errno);125         return UNKNOWN_ERROR;126     }127 128     while ((entry = readdir(dir))) {129         const char* name = entry->d_name;130 131         // ignore "." and ".."132         if (name[0] == '.' && (name[1] == 0 || (name[1] == '.' && name[2] == 0))) {133             continue;134         }135 136         int type = entry->d_type;137         if (type == DT_UNKNOWN) {138             // If the type is unknown, stat() the file instead.139             // This is sometimes necessary when accessing NFS mounted filesystems, but140             // could be needed in other cases well.141             struct stat statbuf;142             if (stat(path, &statbuf) == 0) {143                 if (S_ISREG(statbuf.st_mode)) {144                     type = DT_REG;145                 } else if (S_ISDIR(statbuf.st_mode)) {146                     type = DT_DIR;147                 }148             } else {149                 LOGD("stat() failed for %s: %s", path, strerror(errno) );150             }151         }152         if (type == DT_REG || type == DT_DIR) {153             int nameLength = strlen(name);154             bool isDirectory = (type == DT_DIR);155 156             if (nameLength > pathRemaining || (isDirectory && nameLength + 1 > pathRemaining)) {157                 // path too long!158                 continue;159             }160 161             strcpy(fileSpot, name);162             if (isDirectory) {163                 // ignore directories with a name that starts with '.'164                 // for example, the Mac ".Trashes" directory165                 if (name[0] == '.') continue;166 167                 //When path is "/mnt/sdcard", ignore "/mnt/sdcard/extsd" and "/mnt/sdcard/udisk"168                 if ((!strcmp(path, "/mnt/sdcard/extsd") && !strcmp(name, "extsd"))169                         || (!strcmp(path, "/mnt/sdcard/udisk") && !strcmp(name, "udisk"))) {170                     SLOGI("Ignore %s:%s.", path, name);171                     continue;172                 }173 174                 strcat(fileSpot, "/");175                 int err = doProcessDirectory(path, pathRemaining - nameLength - 1, extensions, client, exceptionCheck, exceptionEnv);176                 if (err) {177                     // pass exceptions up - ignore other errors178                     if (exceptionCheck && exceptionCheck(exceptionEnv)) goto failure;179                     LOGE("Error processing '%s' - skipping\n", path);180                     continue;181                 }182             } else if (fileMatchesExtension(path, extensions)) {183                 struct stat statbuf;184                 stat(path, &statbuf);185                 if (statbuf.st_size > 0) {186                     client.scanFile(path, statbuf.st_mtime, statbuf.st_size);187                 }188                 if (exceptionCheck && exceptionCheck(exceptionEnv)) goto failure;189             }190         }191     }192 193     closedir(dir);194     return OK;195 failure:196     closedir(dir);197     return -1;198 }199 200 }  // namespace android

136～151 readdir讀取出來的dirent 有可能沒法擷取目錄項類型，需要使用stat系統調用來擷取，這樣既可以在某些情況下提升效率，又能解決相容性問題。

152 檔案是普通檔案或者目錄

156 ～158 看來MediaScanner並不想處理路徑太長的檔案，長路徑的檔案和子目錄都不做處理了

162 ~ 181 對於子目錄，首先忽略掉.開始的目錄，這也符合Linux/Unix的慣例，忽略掉/mnt/sdcard/extsd /mnt/sdcard/udisk udisk, 看來MediaScanner也不處理隨身碟和sd卡。最後遞迴處理子目錄

182 ~ 189 如果是檔案，並且檔案的副檔名是否在掃描的副檔名集合@extensions中，那麼調用client.scanFile來處理這個檔案，注意這裡會忽略掉檔案尺寸為0的檔案。

Ok, 到這裡MediaScanner.cpp結束，現在我大概對MediaScanner.cpp有了一個印象，這個MediaScanner.cpp估計也是一個Service，負責處理Media檔案掃描，具體到每一個檔案的處理過程交給一個MediaScannerClient來處理。

MediaScanner的含義就是對某一個路徑進行掃描，找到那些符合副檔名集合的檔案進行處理，至於怎麼處理，這個要分析java層的frameworks/base/media/java/android/media/MediaScanner.java

MediaScanner.cpp的幾個規則：

1. 不處理.開始的目錄，但是處理.開始的檔案， 哈哈

2. 不處理u盤，SD卡

3. 不處理0尺寸檔案

4. 當然也不處理副檔名不匹配的檔案

MediaScannerClient.cpp分析

MediaScannerClient.cpp存在的意義是由於MediaScanner.java無法處理locale字串，至於為什麼不能放在MediaScanner.java中做，而是很複雜的又實現了一個MediaScannerClient.cpp，還沒想明白

MediaScannerClient.cpp為MediaScanner.java提供了如下幾個方法;

setLocale, beginFile, addStringTag, convertValues, endFile

這幾個方法圍繞著MediaScanner.java擷取的metadata(用mNames, mValues表示)，由於包含著本地化的字串，需要把native encoding 轉換為UTF-8