Android 音视频开发(5)：使用 MediaExtractor 和 MediaMuxer API 解析和封装 mp4 文件

一个音视频文件是由音频和视频组成的，我们可以通过MediaExtractor、MediaMuxer把音频或视频给单独抽取出来，抽取出来的音频和视频能单独播放； 

一、MediaExtractor API介绍

MediaExtractor的作用是把音频和视频的数据进行分离。

主要API介绍：

• setDataSource(String path)：即可以设置本地文件又可以设置网络文件
• getTrackCount()：得到源文件通道数 
• getTrackFormat(int index)：获取指定（index）的通道格式
• getSampleTime()：返回当前的时间戳 
• readSampleData(ByteBuffer byteBuf, int offset)：把指定通道中的数据按偏移量读取到ByteBuffer中；
• advance()：读取下一帧数据
• release(): 读取结束后释放资源

使用示例：

 MediaExtractor extractor = new MediaExtractor();
 extractor.setDataSource(...);
 int numTracks = extractor.getTrackCount();
 for (int i = 0; i < numTracks; ++i) {
   MediaFormat format = extractor.getTrackFormat(i);
   String mime = format.getString(MediaFormat.KEY_MIME);
   if (weAreInterestedInThisTrack) {
     extractor.selectTrack(i);
   }
 }
 ByteBuffer inputBuffer = ByteBuffer.allocate(...)
 while (extractor.readSampleData(inputBuffer, ...) >= 0) {
   int trackIndex = extractor.getSampleTrackIndex();
   long presentationTimeUs = extractor.getSampleTime();
   ...
   extractor.advance();
 }

 extractor.release();
 extractor = null;

二、MediaMuxer API介绍

MediaMuxer的作用是生成音频或视频文件；还可以把音频与视频混合成一个音视频文件。

相关API介绍：

• MediaMuxer(String path, int format)：path:输出文件的名称  format:输出文件的格式；当前只支持MP4格式；
• addTrack(MediaFormat format)：添加通道；我们更多的是使用MediaCodec.getOutpurForma()或Extractor.getTrackFormat(int index)来获取MediaFormat;也可以自己创建；
• start()：开始合成文件
• writeSampleData(int trackIndex, ByteBuffer byteBuf, MediaCodec.BufferInfo bufferInfo)：把ByteBuffer中的数据写入到在构造器设置的文件中；
• stop()：停止合成文件
• release()：释放资源

使用示例：

MediaMuxer muxer = new MediaMuxer("temp.mp4", OutputFormat.MUXER_OUTPUT_MPEG_4);
 // More often, the MediaFormat will be retrieved from MediaCodec.getOutputFormat()
 // or MediaExtractor.getTrackFormat().
 MediaFormat audioFormat = new MediaFormat(...);
 MediaFormat videoFormat = new MediaFormat(...);
 int audioTrackIndex = muxer.addTrack(audioFormat);
 int videoTrackIndex = muxer.addTrack(videoFormat);
 ByteBuffer inputBuffer = ByteBuffer.allocate(bufferSize);
 boolean finished = false;
 BufferInfo bufferInfo = new BufferInfo();

 muxer.start();
 while(!finished) {
   // getInputBuffer() will fill the inputBuffer with one frame of encoded
   // sample from either MediaCodec or MediaExtractor, set isAudioSample to
   // true when the sample is audio data, set up all the fields of bufferInfo,
   // and return true if there are no more samples.
   finished = getInputBuffer(inputBuffer, isAudioSample, bufferInfo);
   if (!finished) {
     int currentTrackIndex = isAudioSample ? audioTrackIndex : videoTrackIndex;
     muxer.writeSampleData(currentTrackIndex, inputBuffer, bufferInfo);
   }
 };
 muxer.stop();
 muxer.release();

三、使用情境 

3.1 从MP4文件中提取视频并生成新的视频文件

public class MainActivity extends AppCompatActivity {

    private static final String SDCARD_PATH = Environment.getExternalStorageDirectory().getPath();

    private MediaExtractor mMediaExtractor;
    private MediaMuxer mMediaMuxer;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // 获取权限
        int checkWriteExternalPermission = ContextCompat.checkSelfPermission(this, Manifest.permission.WRITE_EXTERNAL_STORAGE);
        int checkReadExternalPermission = ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE);if (checkWriteExternalPermission != PackageManager.PERMISSION_GRANTED ||
                checkReadExternalPermission != PackageManager.PERMISSION_GRANTED) {

            ActivityCompat.requestPermissions(this, new String[]{
                    Manifest.permission.WRITE_EXTERNAL_STORAGE,
                    Manifest.permission.READ_EXTERNAL_STORAGE}, 0);
        }

        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    process();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

    private boolean process() throws IOException {
        mMediaExtractor = new MediaExtractor();
        mMediaExtractor.setDataSource(SDCARD_PATH + "/ss.mp4");

        int mVideoTrackIndex = -1;
        int framerate = 0;
        for (int i = 0; i < mMediaExtractor.getTrackCount(); i++) {
            MediaFormat format = mMediaExtractor.getTrackFormat(i);
            String mime = format.getString(MediaFormat.KEY_MIME);
            if (!mime.startsWith("video/")) {
                continue;
            }
            framerate = format.getInteger(MediaFormat.KEY_FRAME_RATE);
            mMediaExtractor.selectTrack(i);
            mMediaMuxer = new MediaMuxer(SDCARD_PATH + "/ouput.mp4", MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);
            mVideoTrackIndex = mMediaMuxer.addTrack(format);
            mMediaMuxer.start();
        }

        if (mMediaMuxer == null) {
            return false;
        }

        MediaCodec.BufferInfo info = new MediaCodec.BufferInfo();
        info.presentationTimeUs = 0;
        ByteBuffer buffer = ByteBuffer.allocate(500 * 1024);
        int sampleSize = 0;
        while ((sampleSize = mMediaExtractor.readSampleData(buffer, 0)) > 0) {

            info.offset = 0;
            info.size = sampleSize;
            info.flags = MediaCodec.BUFFER_FLAG_SYNC_FRAME;
            info.presentationTimeUs += 1000 * 1000 / framerate;
            mMediaMuxer.writeSampleData(mVideoTrackIndex, buffer, info);
            mMediaExtractor.advance();
        }

        mMediaExtractor.release();

        mMediaMuxer.stop();
        mMediaMuxer.release();

        return true;
    }
}