Android(安卓)MediaExtractor setDataSource

0. setDataSource时序图

1. MediaExtractor(java)

file: frameworks/base/media/java/android/mediaf/MediaExtractor.java
目前Android 5.0仅支持本地视频

    public final void setDataSource(FileDescriptor fd) throws IOException {        setDataSource(fd, 0, 0x7ffffffffffffffL);    }

这会调用JNI setDataSource

2. android_media_MediaExtractor(native)

file: frameworks/base/media/jni/android_media_MediaExtractor.cpp
native的setDataSource是调用NuMediaExtractor的setDataSource

status_t JMediaExtractor::setDataSource(int fd, off64_t offset, off64_t size) {    return mImpl->setDataSource(fd, offset, size);}

3. NuMediaExtractor

file：frameworks/av/media/libstagefright/NuMediaExtractor.cpp
NuMediaExtractor先创建FileSource
然后根据FileSource创建相应的Extractor

status_t NuMediaExtractor::setDataSource(int fd, off64_t offset, off64_t size) {    ...    sp<FileSource> fileSource = new FileSource(dup(fd), offset, size);    mImpl = MediaExtractor::Create(fileSource);    ...    mDataSource = fileSource;    updateDurationAndBitrate();    return OK;}

4. New FileSource

FileSource继承于DataSource。
FileSource初始化函数执行操作有：打开文件，获取文件FD，Seek文件

FileSource::FileSource(const char *filename)    : mFd(-1),      mOffset(0),      mLength(-1),      mDecryptHandle(NULL),      mDrmManagerClient(NULL),      mDrmBufOffset(0),      mDrmBufSize(0),      mDrmBuf(NULL){    mFd = open(filename, O_LARGEFILE | O_RDONLY);    if (mFd >= 0) {        mLength = lseek64(mFd, 0, SEEK_END);    } else {        ALOGE("Failed to open file '%s'. (%s)", filename, strerror(errno));    }}

DataSource初始化函数执行操作有：new Sniffer

    DataSource() : mSniffer(new Sniffer()) {}

Sniffer构造过程：注册各个Extractor的Sniffer函数
registerDefaultSniffers

void Sniffer::registerDefaultSniffers() {    Mutex::Autolock autoLock(mSnifferMutex);    registerSniffer_l(SniffMPEG4);    registerSniffer_l(SniffMatroska);    registerSniffer_l(SniffOgg);    registerSniffer_l(SniffWAV);    registerSniffer_l(SniffFLAC);    registerSniffer_l(SniffAMR);    registerSniffer_l(SniffMPEG2TS);    registerSniffer_l(SniffMP3);    registerSniffer_l(SniffAAC);    registerSniffer_l(SniffMPEG2PS);    registerSniffer_l(SniffWVM);    registerSniffer_l(ExtendedExtractor::Sniff);    char value[PROPERTY_VALUE_MAX];    if (property_get("drm.service.enabled", value, NULL)            && (!strcmp(value, "1") || !strcasecmp(value, "true"))) {        registerSniffer_l(SniffDRM);    }}

5. MediaExtractor::Create

file:frameworks/av/media/libstagefright/MediaExtractor.cpp
MediaExtractor（C++）是MediaExtractor体系中核心，是各个extractor（eg.MPEG4Extractor）的父类，管理者.
Create它根据文件的MIME信息创建对应的Extractor，具体逻辑如下：
（1）sniff ：获取文件meta信息
（2）根据meta中的MIME创建对应的Extractor
（3）ExtendedUtils判断创建默认Extractor还是扩展Extractor

sp<MediaExtractor> MediaExtractor::Create(        const sp<DataSource> &source, const char *mime) {    sp<AMessage> meta;    String8 tmp;    if (mime == NULL) {        float confidence;        if (!source->sniff(&tmp, &confidence, &meta)) {            ALOGV("FAILED to autodetect media content.");            return NULL;        }        mime = tmp.string();        ALOGV("Autodetected media content as '%s' with confidence %.2f",             mime, confidence);    }    bool isDrm = false;    // DRM MIME type syntax is "drm+type+original" where    // type is "es_based" or "container_based" and    // original is the content's cleartext MIME type    if (!strncmp(mime, "drm+", 4)) {        const char *originalMime = strchr(mime+4, '+');        if (originalMime == NULL) {            // second + not found            return NULL;        }        ++originalMime;        if (!strncmp(mime, "drm+es_based+", 13)) {            // DRMExtractor sets container metadata kKeyIsDRM to 1            return new DRMExtractor(source, originalMime);        } else if (!strncmp(mime, "drm+container_based+", 20)) {            mime = originalMime;            isDrm = true;        } else {            return NULL;        }    }    MediaExtractor *ret = NULL;    if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG4)            || !strcasecmp(mime, "audio/mp4")) {        ret = new MPEG4Extractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_MPEG)) {        ret = new MP3Extractor(source, meta);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_NB)            || !strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AMR_WB)) {        ret = new AMRExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_FLAC)) {        ret = new FLACExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_WAV)) {        ret = new WAVExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_OGG)) {        ret = new OggExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MATROSKA)) {        ret = new MatroskaExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG2TS)) {        ret = new MPEG2TSExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_WVM)) {        // Return now. WVExtractor should not have the DrmFlag set in the block below.        return new WVMExtractor(source);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_AUDIO_AAC_ADTS)) {        ret = new AACExtractor(source, meta);    } else if (!strcasecmp(mime, MEDIA_MIMETYPE_CONTAINER_MPEG2PS)) {        ret = new MPEG2PSExtractor(source);    }    if (ret != NULL) {       if (isDrm) {           ret->setDrmFlag(true);       } else {           ret->setDrmFlag(false);       }    }    return ExtendedUtils::MediaExtractor_CreateIfNeeded(ret, source, mime);}

6. ExtendedUtils::MediaExtractor_CreateIfNeeded

MediaExtractor_CreateIfNeeded的作用如其名就是判断是否需要启用ExtentedExtractor。
如果看源代码的话会发现整个过程比较繁琐，下面说下启用ExtentedExtractor的原则，（如果需要深究还是仔细阅读源代码好了），原则如下：
（1）尽量采用ExtentedExtractor
因为DefaultExtractor是google的decoder，采用的是软解；ExtentedExtractor一般是厂商的decoder，通常采用硬解。
（2）如果无法正确获取多媒体信息则采用ExtentedExtractor
（3）如果没有找到DefaultExtractor则采用ExtentedExtractor
（4）对于某些特殊格式采用ExtentedExtractor
比如MPEG4格式，这个一般厂商都支持硬解。

sp<MediaExtractor> ExtendedUtils::MediaExtractor_CreateIfNeeded(sp<MediaExtractor> defaultExt,                                                            const sp<DataSource> &source,                                                            const char *mime) {    bool bCheckExtendedExtractor = false;    bool videoTrackFound         = false;    bool audioTrackFound         = false;    bool amrwbAudio              = false;    bool hevcVideo               = false;    bool dolbyAudio              = false;    bool mpeg4Container          = false;    bool aacAudioTrack           = false;    int  numOfTrack              = 0;    mpeg4Container = !strncasecmp(mime,                                MEDIA_MIMETYPE_CONTAINER_MPEG4,                                strlen(MEDIA_MIMETYPE_CONTAINER_MPEG4));    if (defaultExt != NULL) {        for (size_t trackItt = 0; trackItt < defaultExt->countTracks(); ++trackItt) {            ++numOfTrack;            sp<MetaData> meta = defaultExt->getTrackMetaData(trackItt);            const char *_mime;            CHECK(meta->findCString(kKeyMIMEType, &_mime));            String8 mime = String8(_mime);            const char * dolbyFormats[ ] = {                MEDIA_MIMETYPE_AUDIO_AC3,                MEDIA_MIMETYPE_AUDIO_EAC3,#ifdef DOLBY_UDC                MEDIA_MIMETYPE_AUDIO_EAC3_JOC,#endif            };            if (!strncasecmp(mime.string(), "audio/", 6)) {                audioTrackFound = true;                amrwbAudio = !strncasecmp(mime.string(),                                          MEDIA_MIMETYPE_AUDIO_AMR_WB,                                          strlen(MEDIA_MIMETYPE_AUDIO_AMR_WB));                aacAudioTrack = !strncasecmp(mime.string(),                                          MEDIA_MIMETYPE_AUDIO_AAC,                                          strlen(MEDIA_MIMETYPE_AUDIO_AAC));                for (size_t i = 0; i < ARRAY_SIZE(dolbyFormats); i++) {                    if (!strncasecmp(mime.string(), dolbyFormats[i], strlen(dolbyFormats[i]))) {                        dolbyAudio = true;                    }                }                if (amrwbAudio || dolbyAudio) {                    break;                }            } else if (!strncasecmp(mime.string(), "video/", 6)) {                videoTrackFound = true;                if(!strncasecmp(mime.string(), "video/hevc", 10)) {                    hevcVideo = true;                }            }        }        if (amrwbAudio || dolbyAudio) {            bCheckExtendedExtractor = true;        } else if (numOfTrack  == 0) {            bCheckExtendedExtractor = true;        } else if (numOfTrack == 1) {            if ((videoTrackFound) ||                (!videoTrackFound && !audioTrackFound) ||                (audioTrackFound && mpeg4Container && aacAudioTrack)) {                bCheckExtendedExtractor = true;            }        } else if (numOfTrack >= 2) {            if (videoTrackFound && audioTrackFound) {                if (amrwbAudio || hevcVideo ) {                    bCheckExtendedExtractor = true;                }            } else {                bCheckExtendedExtractor = true;            }        }    } else {        bCheckExtendedExtractor = true;    }    if (!bCheckExtendedExtractor) {        ALOGD("extended extractor not needed, return default");        return defaultExt;    }    //Create Extended Extractor only if default extractor is not selected    ALOGD("Try creating ExtendedExtractor");    sp<MediaExtractor>  retExtExtractor = ExtendedExtractor::Create(source, mime);    if (retExtExtractor == NULL) {        ALOGD("Couldn't create the extended extractor, return default one");        return defaultExt;    }    if (defaultExt == NULL) {        ALOGD("default extractor is NULL, return extended extractor");        return retExtExtractor;    }    //bCheckExtendedExtractor is true which means default extractor was found    //but we want to give preference to extended extractor based on certain    //conditions.    //needed to prevent a leak in case both extractors are valid    //but we still dont want to use the extended one. we need    //to delete the new one    bool bUseDefaultExtractor = true;    const char * extFormats[ ] = {        MEDIA_MIMETYPE_AUDIO_AMR_WB_PLUS,        MEDIA_MIMETYPE_VIDEO_HEVC,        MEDIA_MIMETYPE_AUDIO_AC3,        MEDIA_MIMETYPE_AUDIO_EAC3,#ifdef DOLBY_UDC        MEDIA_MIMETYPE_AUDIO_EAC3_JOC,#endif        MEDIA_MIMETYPE_AUDIO_AAC,    };    for (size_t trackItt = 0; (trackItt < retExtExtractor->countTracks()); ++trackItt) {        sp<MetaData> meta = retExtExtractor->getTrackMetaData(trackItt);        const char *mime;        bool success = meta->findCString(kKeyMIMEType, &mime);        bool isExtFormat = false;        for (size_t i = 0; i < ARRAY_SIZE(extFormats); i++) {            if (!strncasecmp(mime, extFormats[i], strlen(extFormats[i]))) {                isExtFormat = true;                break;            }        }        if ((success == true) && isExtFormat) {            ALOGD("Discarding default extractor and using the extended one");            bUseDefaultExtractor = false;            break;        }    }    if (bUseDefaultExtractor) {        ALOGD("using default extractor inspite of having a new extractor");        retExtExtractor.clear();        return defaultExt;    } else {        defaultExt.clear();        return retExtExtractor;    }}