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MediaPlayer本地播放流程解析（一）
应用场景：
MediaPlayer mediaPlayer = new MediaPlayer();
mediaPlayer.setOnCompletionListener(new OnCompletionListener() {
@Override
public void onCompletion(MediaPlayer mp) {
mediaPlayer.release();
mediaPlayer = null;
}
});
mediaPlayer.setDataSource(“abc.mp3”);
mediaPlayer.prepare();
mediaPlayer.start();
一、setDataSource
在MediaPlayer.java 中public void setDataSource(FileDescriptor fd, long offset, long length)
throws IOException, IllegalArgumentException, IllegalStateException {
disableProxyListener();
setDataSource(fd, offset, length);
}
setDataSource最终调用了native函数：_setDataSource(fd,offset, length);
我们直接跳到JNI层来看它的具体实现
根据JNI相关的知识，在android_media_MediaPlayer.cpp中找到了其实现代码：static void
android_media_MediaPlayer_setDataSourceFD(JNIEnv *env, jobject thiz, jobject fileDescriptor, jlong offset, jlong length)
{
sp<MediaPlayer> mp = getMediaPlayer(env, thiz);
if (mp == NULL ) {
jniThrowException(env, "java/lang/IllegalStateException", NULL);
return;
}
if (fileDescriptor == NULL) {
jniThrowException(env, "java/lang/IllegalArgumentException", NULL);
return;
}
int fd = jniGetFDFromFileDescriptor(env, fileDescriptor);
ALOGV("setDataSourceFD: fd %d", fd);
process_media_player_call( env, thiz, mp->setDataSource(fd, offset, length), "java/io/IOException", "setDataSourceFD failed." );
}
mp为MediaPlayer类型的对象，在JNI层创建，在MediaPlayer.cpp中，一起来看setDataSource的实现。status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
status_t err = UNKNOWN_ERROR;
const sp<IMediaPlayerService>& service(getMediaPlayerService());
if (service != 0) {
sp<IMediaPlayer> player(service->create(this, mAudioSessionId));
if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
(NO_ERROR != player->setDataSource(fd, offset, length))) {
player.clear();
}
err = attachNewPlayer(player);
}
return err;
}
getMediaPlayerService()为一个典型的Binder机制向ServiceManager获取服务的方法，Binder这方面的知识可以参考http://blog.csdn.net/super_dc/article/details/37738123和http://blog.csdn.net/super_dc/article/details/37764947
service->create(this, mAudioSessionId)，先看create方法在IMediaPlayerService.cpp中的实现：virtual sp<IMediaPlayer> create(
const sp<IMediaPlayerClient>& client, int audioSessionId) {
Parcel data, reply;
data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());
data.writeStrongBinder(client->asBinder());
data.writeInt32(audioSessionId);
remote()->transact(CREATE, data, &reply);
return interface_cast<IMediaPlayer>(reply.readStrongBinder());
}
这里只是Binder客户端的实现，其最终实现会在MediaPlayerService.cpp中由服务端MediaPlayerService来实现。sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client,
int audioSessionId)
{
pid_t pid = IPCThreadState::self()->getCallingPid();
int32_t connId = android_atomic_inc(&mNextConnId);
sp<Client> c = new Client(
this, pid, connId, client, audioSessionId,
IPCThreadState::self()->getCallingUid());
ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid,
IPCThreadState::self()->getCallingUid());
/* add by Gary. start {{----------------------------------- */
c->setScreen(mScreen);
/* add by Gary. end -----------------------------------}} */
c->setSubGate(mGlobalSubGate); // 2012-03-12, add the global interfaces to control the subtitle gate
wp<Client> w = c;
{
Mutex::Autolock lock(mLock);
mClients.add(w);
}
return c;
}
综合上面两点，sp<IMediaPlayer>player(service->create(this, mAudioSessionId));中player实际上是一个Client类型对象的proxy。其具体实现都在Client中实现。
player->setDataSource(fd, offset, length)就可以直接到MediaPlayerService.cpp中的Client类中来看其具体实现了。status_t MediaPlayerService::Client::setDataSource(int fd, int64_t offset, int64_t length)
{
struct stat sb;
int ret = fstat(fd, &sb);
if (ret != 0) {
ALOGE("fstat(%d) failed: %d, %s", fd, ret, strerror(errno));
return UNKNOWN_ERROR;
}
if (offset >= sb.st_size) {
ALOGE("offset error");
::close(fd);
return UNKNOWN_ERROR;
}
if (offset + length > sb.st_size) {
length = sb.st_size - offset;
ALOGV("calculated length = %lld", length);
}
// 关键点1
player_type playerType = MediaPlayerFactory::getPlayerType(this,
fd,
offset,
length);
// 关键点2
sp<MediaPlayerBase> p = setDataSource_pre(playerType);
if (p == NULL) {
return NO_INIT;
}
// now set data source
// 关键点3
setDataSource_post(p, p->setDataSource(fd, offset, length));
return mStatus;
}
这里有3个关键点，我们分别破解之，先看getPlayerTypeplayer_type MediaPlayerFactory::getPlayerType(const sp<IMediaPlayer>& client,
int fd,
int64_t offset,
int64_t length) {
GET_PLAYER_TYPE_IMPL(client, fd, offset, length);
}
#define GET_PLAYER_TYPE_IMPL(a...) Mutex::Autolock lock_(&sLock); player_type ret = STAGEFRIGHT_PLAYER; float bestScore = 0.0; for (size_t i = 0; i < sFactoryMap.size(); ++i) { IFactory* v = sFactoryMap.valueAt(i); float thisScore; CHECK(v != NULL); thisScore = v->scoreFactory(a, bestScore); if (thisScore > bestScore) { ret = sFactoryMap.keyAt(i); bestScore = thisScore; } } if (0.0 == bestScore) { ret = getDefaultPlayerType(); } return ret;
MediaPlayerFactory作为一个工厂类，各种mediaplayer向它注册，并各自实现scoreFactory和createPlayer用来判断当前多媒体文件是否适合用此mediaplayer来播放和创建mediaplayer。在哪儿注册mediaplayer呢？在MediaPlayerService的构造函数中，也就是说当向系统注册MediaPlayerService服务时，就已经注册了一些mediaplayer了。
播放mp3文件时，会创建STAGEFRIGHT_PLAYER，这也是默认的播放器。下面就以STAGEFRIGHT_PLAYER来继续下面的流程。
到目前为止，我们知道playerType返回了STAGEFRIGHT_PLAYER，接着来看关键点2.sp<MediaPlayerBase> MediaPlayerService::Client::setDataSource_pre(
player_type playerType)
{
ALOGV("player type = %d", playerType);
// create the right type of player
sp<MediaPlayerBase> p = createPlayer(playerType);
if (p == NULL) {
return p;
}
if (!p->hardwareOutput()) {
mAudioOutput = new AudioOutput(mAudioSessionId);
static_cast<MediaPlayerInterface*>(p.get())->setAudioSink(mAudioOutput);
}
return p;
}
根据playerType创建播放器,实际上就是创建StagefrightPlayer
再看关键点3，p->setDataSource(fd,offset, length)实际上就是调用了StagefrightPlayer的setDataSource。看代码：StagefrightPlayer::StagefrightPlayer()
: mPlayer(new AwesomePlayer) {
ALOGV("StagefrightPlayer");
mPlayer->setListener(this);
}
status_t StagefrightPlayer::setDataSource(int fd, int64_t offset, int64_t length) {
ALOGV("setDataSource(%d, %lld, %lld)", fd, offset, length);
return mPlayer->setDataSource(dup(fd), offset, length);
}
由代码可知，StagefrightPlayer只是AwesomePlayer的代理类，具体实现还在AwesomePlayer里面。status_t AwesomePlayer::setDataSource(
int fd, int64_t offset, int64_t length) {
Mutex::Autolock autoLock(mLock);
reset_l();
sp<DataSource> dataSource = new FileSource(fd, offset, length);
status_t err = dataSource->initCheck();
if (err != OK) {
return err;
}
mFileSource = dataSource;
{
Mutex::Autolock autoLock(mStatsLock);
mStats.mFd = fd;
mStats.mURI = String8();
}
return setDataSource_l(dataSource);
}
FileSource类实现了数据读取，播放器调用dataSource->readAt来获取数据，另外，其基类DataSource提供了一些分离器如下。RegisterDefaultSniffers将在AwesomePlayer的构造函数中被调用。// static
void DataSource::RegisterDefaultSniffers() {
RegisterSniffer(SniffMPEG4);
RegisterSniffer(SniffMatroska);
RegisterSniffer(SniffOgg);
RegisterSniffer(SniffWAV);
RegisterSniffer(SniffFLAC);
RegisterSniffer(SniffAMR);
RegisterSniffer(SniffMPEG2TS);
RegisterSniffer(SniffMP3);
RegisterSniffer(SniffAAC);
RegisterSniffer(SniffMPEG2PS);
RegisterSniffer(SniffWVM);
char value[PROPERTY_VALUE_MAX];
if (property_get("drm.service.enabled", value, NULL)
&& (!strcmp(value, "1") || !strcasecmp(value, "true"))) {
RegisterSniffer(SniffDRM);
}
}
接着往下看setDataSource_l(dataSource)status_t AwesomePlayer::setDataSource_l(
const sp<DataSource> &dataSource) {
// 对于不同的文件格式会创建不同的MediaExtractor，MP3文件会创建MP3Extractor
// 文件格式靠source->sniff(&tmp, &confidence, &meta)来区分，这个函数会遍历之前通过RegisterSniffer注册的分离器，得到最合适的文件格式
sp<MediaExtractor> extractor = MediaExtractor::Create(dataSource);
if (extractor == NULL) {
return UNKNOWN_ERROR;
}
if (extractor->getDrmFlag()) {
checkDrmStatus(dataSource);
}
return setDataSource_l(extractor);
}
status_t AwesomePlayer::setDataSource_l(const sp<MediaExtractor> &extractor) {
// Attempt to approximate overall stream bitrate by summing all
// tracks' individual bitrates, if not all of them advertise bitrate,
// we have to fail.
int64_t totalBitRate = 0;
mExtractor = extractor;
for (size_t i = 0; i < extractor->countTracks(); ++i) {
sp<MetaData> meta = extractor->getTrackMetaData(i);
int32_t bitrate;
if (!meta->findInt32(kKeyBitRate, &bitrate)) {
const char *mime;
CHECK(meta->findCString(kKeyMIMEType, &mime));
ALOGV("track of type '%s' does not publish bitrate", mime);
totalBitRate = -1;
break;
}
totalBitRate += bitrate;
}
mBitrate = totalBitRate;
ALOGV("mBitrate = %lld bits/sec", mBitrate);
{
Mutex::Autolock autoLock(mStatsLock);
mStats.mBitrate = mBitrate;
mStats.mTracks.clear();
mStats.mAudioTrackIndex = -1;
mStats.mVideoTrackIndex = -1;
}
bool haveAudio = false;
bool haveVideo = false;
for (size_t i = 0; i < extractor->countTracks(); ++i) {
sp<MetaData> meta = extractor->getTrackMetaData(i);
const char *_mime;
CHECK(meta->findCString(kKeyMIMEType, &_mime));
String8 mime = String8(_mime);
if (!haveVideo && !strncasecmp(mime.string(), "video/", 6)) {
setVideoSource(extractor->getTrack(i));
haveVideo = true;
// Set the presentation/display size
int32_t displayWidth, displayHeight;
bool success = meta->findInt32(kKeyDisplayWidth, &displayWidth);
if (success) {
success = meta->findInt32(kKeyDisplayHeight, &displayHeight);
}
if (success) {
mDisplayWidth = displayWidth;
mDisplayHeight = displayHeight;
}
{
Mutex::Autolock autoLock(mStatsLock);
mStats.mVideoTrackIndex = mStats.mTracks.size();
mStats.mTracks.push();
TrackStat *stat =
&mStats.mTracks.editItemAt(mStats.mVideoTrackIndex);
stat->mMIME = mime.string();
}
} else if (!haveAudio && !strncasecmp(mime.string(), "audio/", 6)) {
setAudioSource(extractor->getTrack(i));
haveAudio = true;
mActiveAudioTrackIndex = i;
{
Mutex::Autolock autoLock(mStatsLock);
mStats.mAudioTrackIndex = mStats.mTracks.size();
mStats.mTracks.push();
TrackStat *stat =
&mStats.mTracks.editItemAt(mStats.mAudioTrackIndex);
stat->mMIME = mime.string();
}
if (!strcasecmp(mime.string(), MEDIA_MIMETYPE_AUDIO_VORBIS)) {
// Only do this for vorbis audio, none of the other audio
// formats even support this ringtone specific hack and
// retrieving the metadata on some extractors may turn out
// to be very expensive.
sp<MetaData> fileMeta = extractor->getMetaData();
int32_t loop;
if (fileMeta != NULL
&& fileMeta->findInt32(kKeyAutoLoop, &loop) && loop != 0) {
modifyFlags(AUTO_LOOPING, SET);
}
}
} else if (!strcasecmp(mime.string(), MEDIA_MIMETYPE_TEXT_3GPP)) {
addTextSource_l(i, extractor->getTrack(i));
}
}
if (!haveAudio && !haveVideo) {
if (mWVMExtractor != NULL) {
return mWVMExtractor->getError();
} else {
return UNKNOWN_ERROR;
}
}
mExtractorFlags = extractor->flags();
return OK;
}
MediaExtractor涉及到媒体文件格式的很多内容，比如track的构成，有几种track等等，后面再做讲解，这里我们播放的是MP3文件，所以countTracks的值为1，sp<MetaData> meta = extractor->getTrackMetaData(i)中meta的kKeyMIMEType值为"audio/"，将会执行到setAudioSource(extractor->getTrack(i))，再看代码：sp<MediaSource> MP3Extractor::getTrack(size_t index) {
if (mInitCheck != OK || index != 0) {
return NULL;
}
// 返回的是一个MP3Source对象
return new MP3Source(
mMeta, mDataSource, mFirstFramePos, mFixedHeader,
mSeeker);
}
void AwesomePlayer::setAudioSource(sp<MediaSource> source) {
CHECK(source != NULL);
mAudioTrack = source;
}
至此，setdatasource就分析完成，下一篇将分析prepare的实现过程。
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