MediaScanner分析 - MediaScanner.cpp

MediaScanner.cpp分析

文件路径 frameworks/base/media/libmedia/MediaScanner.cpp

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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;
}

很简单，对路径参数做了些处理，调用doProcessDirectory

@extensions可能包含多个扩展名，扩展名之间用comma分隔开

client.setLocale(locale()) 目的未知

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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;
}

参数@extensions可能包含多个extensions，用","分割开，这个函数检查给定路径@path的扩展名是否包含在@extensions中

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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 name
105 char* fileSpot = path + strlen(path);
106 struct dirent* entry;
107
108 // ignore directories that contain a ".nomedia" file
109 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 path
119 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, but
140 // 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" directory
165 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 errors
178 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读取出来的dirent 有可能没法获取目录项类型，需要使用stat系统调用来获取，这样既可以在某些情况下提升效率，又能解决兼容性问题。

152 文件是普通文件或者目录

156 ～158 看来MediaScanner并不想处理路径太长的文件，长路径的文件和子目录都不做处理了

162 ~ 181 对于子目录，首先忽略掉.开始的目录，这也符合Linux/Unix的惯例，忽略掉/mnt/sdcard/extsd /mnt/sdcard/udisk udisk, 看来MediaScanner也不处理U盘和sd卡。最后递归处理子目录

182 ~ 189 如果是文件，并且文件的扩展名是否在扫描的扩展名集合@extensions中，那么调用client.scanFile来处理这个文件，注意这里会忽略掉文件尺寸为0的文件。

Ok, 到这里MediaScanner.cpp结束，现在我大概对MediaScanner.cpp有了一个印象，这个MediaScanner.cpp估计也是一个Service，负责处理Media文件扫描，具体到每一个文件的处理过程交给一个MediaScannerClient来处理。

MediaScanner的含义就是对某一个路径进行扫描，找到那些符合扩展名集合的文件进行处理，至于怎么处理，这个要分析java层的frameworks/base/media/java/android/media/MediaScanner.java

MediaScanner.cpp的几个规则：

1. 不处理.开始的目录，但是处理.开始的文件， 哈哈

2. 不处理u盘，SD卡

3. 不处理0尺寸文件

4. 当然也不处理扩展名不匹配的文件

MediaScannerClient.cpp分析

MediaScannerClient.cpp存在的意义是由于MediaScanner.java无法处理locale字串，至于为什么不能放在MediaScanner.java中做，而是很复杂的又实现了一个MediaScannerClient.cpp，还没想明白

MediaScannerClient.cpp为MediaScanner.java提供了如下几个方法;

setLocale, beginFile, addStringTag, convertValues, endFile

这几个方法围绕着MediaScanner.java获取的metadata(用mNames, mValues表示)，由于包含着本地化的字符串，需要把native encoding 转换为UTF-8