frameworks/av/media/extractors下每个目录都会生成对应的so，eg: libmp4extractor 、 libaacextractor 、 libmpeg2extractor 等等
这些so最终都在frameworks/av/apex/Android.bp中被编译成”com.android.media”（apex）
apex_defaults {
name: "com.android.media-defaults",
updatable: true,
java_libs: ["updatable-media"],
multilib: {
first: {
// Extractor process runs only with the primary ABI.
native_shared_libs: [
// Extractor plugins
"libaacextractor",
"libamrextractor",
"libflacextractor",
"libmidiextractor",
"libmkvextractor",
"libmp3extractor",
"libmp4extractor",
"libmpeg2extractor",
"liboggextractor",
"libwavextractor",
],
},
},
prebuilts: [
"mediaextractor.policy",
"code_coverage.policy",
"crash_dump.policy",
],
key: "com.android.media.key",
certificate: ":com.android.media.certificate",
// Use a custom AndroidManifest.xml used for API targeting.
androidManifest: ":com.android.media-androidManifest",
// IMPORTANT: For the APEX to be installed on Android 10 (API 29),
// min_sdk_version should be 29. This enables the build system to make
// sure the package compatible to Android 10 in two ways:
// - build the APEX package compatible to Android 10
// so that the package can be installed.
// - build artifacts (lib/javalib/bin) against Android 10 SDK
// so that the artifacts can run.
min_sdk_version: "29",
}
apex {
name: "com.android.media",
manifest: "manifest.json",
defaults: ["com.android.media-defaults"],
}
本地so存放在: apex/com.android.media/lib64/extractors 目录下
在 MediaExtractorService 的实例化时会去加载 extractors
MediaExtractorService::MediaExtractorService() {
MediaExtractorFactory::LoadExtractors();
}
void MediaExtractorFactory::LoadExtractors() {
Mutex::Autolock autoLock(gPluginMutex);
if (gPluginsRegistered) {
return;
}
//默认为 false
gIgnoreVersion = property_get_bool("debug.extractor.ignore_version", false);
std::shared_ptr<std::list<sp<ExtractorPlugin>>> newList(new std::list<sp<ExtractorPlugin>>());
android_namespace_t *mediaNs = android_get_exported_namespace("com_android_media");
if (mediaNs != NULL) {
const android_dlextinfo dlextinfo = {
.flags = ANDROID_DLEXT_USE_NAMESPACE,
.library_namespace = mediaNs,
};
RegisterExtractors("/apex/com.android.media/lib64/extractors", &dlextinfo, *newList);
} else {
ALOGE("couldn't find media namespace.");
}
RegisterExtractors("/system/lib64/extractors", NULL, *newList);
//qcom 的 libmmparserextractor.so 会放在这里
RegisterExtractors("/system_ext/lib64/extractors", NULL, *newList);
newList->sort(compareFunc);
gPlugins = newList;
for (auto it = gPlugins->begin(); it != gPlugins->end(); ++it) {
if ((*it)->def.def_version == EXTRACTORDEF_VERSION_NDK_V2) {
for (size_t i = 0;; i++) {
const char* ext = (*it)->def.u.v3.supported_types[i];
if (ext == nullptr) {
break;
}
gSupportedExtensions.push_back(std::string(ext));
}
}
}
gPluginsRegistered = true;
}
在我的手机上这里会加载 /apex/com.android.media/lib64/extractors 和 /system_ext/lib64/extractors 下面的so , 后者是 qcom 的 mmparserextractor , 在来看一下加载的方式：
void MediaExtractorFactory::RegisterExtractors(
const char *libDirPath, const android_dlextinfo* dlextinfo,
std::list<sp<ExtractorPlugin>> &pluginList) {
ALOGV("search for plugins at %s", libDirPath);
// 打开so所在的目录
DIR *libDir = opendir(libDirPath);
if (libDir) {
struct dirent* libEntry;
while ((libEntry = readdir(libDir))) {
if (libEntry->d_name[0] == '.') {
continue;
}
String8 libPath = String8(libDirPath) + "/" + libEntry->d_name;
if (!libPath.contains("extractor.so")) {
continue;
}
//打开 lib
void *libHandle = android_dlopen_ext(
libPath.string(),
RTLD_NOW | RTLD_LOCAL, dlextinfo);
//获取 GETEXTRACTORDEF 函数句柄
GetExtractorDef getDef =
(GetExtractorDef) dlsym(libHandle, "GETEXTRACTORDEF");
ALOGV("registering sniffer for %s", libPath.string());
//将新的ExtractorPlugin 放在 pluginList 尾部， 其中会检查同一个 extractor的不同版本
RegisterExtractor(
new ExtractorPlugin(getDef(), libHandle, libPath), pluginList);
}
closedir(libDir);
} else {
ALOGE("couldn't opendir(%s)", libDirPath);
}
}
这里从 lib 中拿到 GETEXTRACTORDEF 函数最后获取到的是 ExtractorDef 结构体
struct ExtractorDef {
// version number of this structure
const uint32_t def_version;
// A unique identifier for this extractor.
// See below for a convenience macro to create this from a string.
media_uuid_t extractor_uuid;
// Version number of this extractor. When two extractors with the same
// uuid are encountered, the one with the largest version number will
// be used.
const uint32_t extractor_version;
// a human readable name
const char *extractor_name;
union {
struct {
SnifferFunc sniff;
} v2;
struct {
SnifferFunc sniff;
// a NULL terminated list of container mime types and/or file extensions
// that this extractor supports
const char **supported_types;
} v3;
} u;
};
其中包含这个 extractor 的 版本/名称/sniff 方法等， 然后再来看 ExtractorPlugin ，这是对 ExtractorDef 的一层封装，其中也会保存 lib 的句柄和 路径。
struct ExtractorPlugin : public RefBase {
ExtractorDef def;
void *libHandle;
String8 libPath;
String8 uuidString;
ExtractorPlugin(ExtractorDef definition, void *handle, String8 &path)
: def(definition), libHandle(handle), libPath(path) {
for (size_t i = 0; i < sizeof ExtractorDef::extractor_uuid; i++) {
uuidString.appendFormat("%02x", def.extractor_uuid.b[i]);
}
}
~ExtractorPlugin() {
if (libHandle != nullptr) {
ALOGV("closing handle for %s %d", libPath.c_str(), def.extractor_version);
dlclose(libHandle);
}
}
};
以 MP4 Extractor 为例子
frameworks/av/media/extractors/mp4/MPEG4Extractor.cpp
extern "C" {
// This is the only symbol that needs to be exported
__attribute__ ((visibility ("default")))
ExtractorDef GETEXTRACTORDEF() {
return {
EXTRACTORDEF_VERSION,
UUID("27575c67-4417-4c54-8d3d-8e626985a164"),
2, // version
"MP4 Extractor",
{ .v3 = {Sniff, extensions} },
};
}
} // extern "C"
static const char *extensions[] = {
"3g2",
"3ga",
"3gp",
"3gpp",
"3gpp2",
"m4a",
"m4r",
"m4v",
"mov",
"mp4",
"qt",
NULL
};
static CreatorFunc Sniff(
CDataSource *source, float *confidence, void **,
FreeMetaFunc *) {
DataSourceHelper helper(source);
if (BetterSniffMPEG4(&helper, confidence)) {
return CreateExtractor;
}
if (LegacySniffMPEG4(&helper, confidence)) {
ALOGW("Identified supported mpeg4 through LegacySniffMPEG4.");
return CreateExtractor;
}
return NULL;
}
这里涉及到相关的封装方法 ：
MP4文件、MOV文件和3GP文件，这三种媒体文件格式采用了相同的封装格式，其基本的组成单元是box。“ftyp”就是整个文件的第一个box，通过判断该box来确定文件的类型， 具体的 ftyp 类型可以参考
直接来看 LegacySniffMPEG4 会比较简单， 匹配固定的 ftyp 类型，如果匹配成功则返回 true ， 并将分数设置为0.4
static bool LegacySniffMPEG4(DataSourceHelper *source, float *confidence) {
uint8_t header[8];
ssize_t n = source->readAt(4, header, sizeof(header));
if (n < (ssize_t)sizeof(header)) {
return false;
}
if (!memcmp(header, "ftyp3gp", 7) || !memcmp(header, "ftypmp42", 8)
|| !memcmp(header, "ftyp3gr6", 8) || !memcmp(header, "ftyp3gs6", 8)
|| !memcmp(header, "ftyp3ge6", 8) || !memcmp(header, "ftyp3gg6", 8)
|| !memcmp(header, "ftypisom", 8) || !memcmp(header, "ftypM4V ", 8)
|| !memcmp(header, "ftypM4A ", 8) || !memcmp(header, "ftypf4v ", 8)
|| !memcmp(header, "ftypkddi", 8) || !memcmp(header, "ftypM4VP", 8)
|| !memcmp(header, "ftypmif1", 8) || !memcmp(header, "ftypheic", 8)
|| !memcmp(header, "ftypmsf1", 8) || !memcmp(header, "ftyphevc", 8)) {
*confidence = 0.4;
return true;
}
return false;
}
另外看一下 Sniff 函数的返回值 ：
static CMediaExtractor* CreateExtractor(CDataSource *source, void *) {
return wrap(new MPEG4Extractor(new DataSourceHelper(source)));
}
可以看到这里返回的是一个 MPEG4Extractor 的实例。
