记得刚学Android开发的时候在2011年，那个时候除了hello world，第一个接触的组件就是这个，这也是目前Android手机运用到最多的组件，有人说没怎么使用过，目前的各类播放器，短视频，都属于多媒体播放，作为Android的多媒体开始鼻祖当属MediaPlayer了。

回顾下官方的API接口

以官方贴图为切入点，API地址

• 被重载的方法必须改变参数列表；
• 被重载的方法可以改变返回类型；
• 被重载的方法可以改变访问修饰符；
• 被重载的方法可以声明新的或更广的检查异常；
• 方法能够在同一个类中或者在一个子类中被重载
  那么如果返回类型不一样算是重载吗？答案是：不是的

setDataSorece

下面常用方法中，列出不同重载后的方法。这时播放器接收到媒体资源后准备进入初始状态。这里需要注意的是，这个顺序不能改变，setDataSourece一定是播放器处于空闲态的时候调用的，否则就会出错。具体我们用代码来分析：

 public void setDataSource(FileDescriptor fd, long offset, long length)
            throws IOException, IllegalArgumentException, IllegalStateException {
        try (ParcelFileDescriptor modernFd = FileUtils.convertToModernFd(fd)) {
            if (modernFd == null) {
                _setDataSource(fd, offset, length);
            } else {
                _setDataSource(modernFd.getFileDescriptor(), offset, length);
            }
        } catch (IOException e) {
            Log.w(TAG, "Ignoring IO error while setting data source", e);
        }
    }

非Idle调用会抛出IllegalStateException异常，剩下两个是在重载的时候抛出的。至于上面的异常，官方给出注释了

Sets the data source (MediaDataSource) to use.
Params:
dataSource – the MediaDataSource for the media you want to play
Throws:
IllegalStateException – if it is called in an invalid state
IllegalArgumentException – if dataSource is not a valid MediaDataSource

看下底层的代码是如何实现的，路径：framework/av/media/mediaplayer.cpp
看起重载后的其中一个方法：

status_t MediaPlayer::setDataSource(const sp<IDataSource> &source)
{
    ALOGV("setDataSource(IDataSource)");
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

这里关注几个点就可以
需要获取mediaplayer的服务也就是getMediaPlayerService，这里就要来到经常提起的binder机制了，其实就是一个典型的CS模型

• Server端
  BnMediaPlayerService 为MediaPlayerService在server端的具体实现；
• Client端
  BpMediaPlayerService为MediaPlayerService在client端的具体代理；
  通过BpMediaPlayerService，可以享用MediaPlayerService所提供的服务
  通过BpMediaPlayerService可以获取到IMediaRecorder、IMediaMetadataRetriever、IMediaPlayer、IOMX在client端的代理类BpXXX（IMediaRecorder、IMediaMetadataRetriever、IMediaPlayer、IOMX，这些皆是Bp和Bn的共同接口，和MediaPlayerService一样，都有Bp和Bn构成）
  引用链接

status_t MediaPlayer::setDataSource(
        const sp<IMediaHTTPService> &httpService,
        const char *url, const KeyedVector<String8, String8> *headers)
{
    ALOGV("setDataSource(%s)", url);
    status_t err = BAD_VALUE;
    if (url != NULL) {
        const sp<IMediaPlayerService> service(getMediaPlayerService());
        if (service != 0) {
            sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
            if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
                (NO_ERROR != player->setDataSource(httpService, url, headers))) {
                player.clear();
            }
            err = attachNewPlayer(player);
        }
    }
    return err;
}

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length)
{
    ALOGV("setDataSource(%d, %" PRId64 ", %" PRId64 ")", fd, offset, length);
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(fd, offset, length))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

status_t MediaPlayer::setDataSource(const sp<IDataSource> &source)
{
    ALOGV("setDataSource(IDataSource)");
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId, mOpPackageName));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

prepareAsync

status_t MediaPlayer::prepareAsync()
{
    ALOGV("prepareAsync");
    //防止重复调用，它的作用域到函数结束
    Mutex::Autolock _l(mLock);
    return prepareAsync_l();
}
// must call with lock held
status_t MediaPlayer::prepareAsync_l()
{
    if ( (mPlayer != 0) && ( mCurrentState & (MEDIA_PLAYER_INITIALIZED | MEDIA_PLAYER_STOPPED) ) ) {
        if (mAudioAttributesParcel != NULL) {
            mPlayer->setParameter(KEY_PARAMETER_AUDIO_ATTRIBUTES, *mAudioAttributesParcel);
        } else {
            mPlayer->setAudioStreamType(mStreamType);
        }
        mCurrentState = MEDIA_PLAYER_PREPARING;
        return mPlayer->prepareAsync();
    }
    ALOGE("prepareAsync called in state %d, mPlayer(%p)", mCurrentState, mPlayer.get());
    return INVALID_OPERATION;
}

• prepare
• start
• pause
• stop
• reset
• seekTo
• release
• setDisplay
• setSurface
• setVolume
• getTrackInfo
• selectTrack