1.//添加轨道
addTrack
2.写数据
writeSampleData
3.释放
release

注意点

buffinfo 必须设置相应的有效值，不然会引发错误
//写入数据大小
bufferInfo.size = readSampleDataSize;
//偏移量
bufferInfo.offset = 0;
//是否为关键帧
bufferInfo.flags = mediaExtractor.getSampleFlags();
//PTS 单位为微妙，必须为递增不然会报错
bufferInfo.presentationTimeUs += videoSampleTime;
mediaMuxer.writeSampleData(trackIndex, buffer, bufferInfo);

这里重点关注下
bufferInfo.presentationTimeUs
mediaExtractor.getSampleTime()
因为mediaExtractor的提取顺序应该是dts的顺序不是pts的顺序，如果视频中存在b帧则getSampleTime不可能递增的，所以bufferInfo.presentationTimeUs=mediaExtractor.getSampleTime()可能会报错，前面说了这个值必须递增。如果不存在b帧，pts==dts，使用没问题。

设置PTS的办法
1.算出帧时间距,github上看见的，排除i帧来计算不知道是为什么，不排除感觉也一样的

long videoSampleTime;
// mediaExtractor.readSampleData(buffer, 0);
// //skip first I frame
// if (mediaExtractor.getSampleFlags() == MediaExtractor.SAMPLE_FLAG_SYNC)
// mediaExtractor.advance();
mediaExtractor.readSampleData(buffer, 0);
long firstVideoPTS = mediaExtractor.getSampleTime();
mediaExtractor.advance();
mediaExtractor.readSampleData(buffer, 0);
long SecondVideoPTS = mediaExtractor.getSampleTime();
videoSampleTime = Math.abs(SecondVideoPTS - firstVideoPTS);

2.通过帧率来计算

//每秒多少帧
int frameRate=mediaFormat=getInteger(MediaFormat.KEY_FRAME_RATE)
//得出平均每一帧间隔多少微妙
videoSampleTime=1000*1000/frameRate

最后提供2个简单的运用示例
有耗时，不要ui线程使用，只是演示

/**

• 提取音/视频
• @param inPath
• @param outPath
• @param isVideo
  */
  @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
  public static void trackExtractor(String inPath, String outPath, boolean isVideo) {
  //提取器
  MediaExtractor mediaExtractor = new MediaExtractor();
  try {
  mediaExtractor.setDataSource(inPath);
  } catch (IOException e) {
  e.printStackTrace();
  }

MediaMuxer mediaMuxer = null;
//复用器
try {
mediaMuxer = new MediaMuxer(outPath, MediaMuxer.OutputFormat.MUXER_OUTPUT_MPEG_4);
} catch (IOException e) {
e.printStackTrace();
}

int trackIndex = 0;
int trackCount = mediaExtractor.getTrackCount();
for (int i = 0; i < trackCount; i++) {
MediaFormat trackFormat = mediaExtractor.getTrackForma
t(i);
String mime = trackFormat.getString(MediaFormat.KEY_MIME);
if (mime.startsWith(“video/”) && isVideo) {
trackIndex = mediaMuxer.addTrack(trackFormat);
mediaExtractor.selectTrack(i);
break;
} else if (mime.startsWith(“audio/”) && !isVideo) {
trackIndex = mediaMuxer.addTrack(trackFormat);
mediaExtractor.selectTrack(i);
break;
}
}
mediaMuxer.start();

ByteBuffer buffer = ByteBuffer.allocate(1024 * 1000);
MediaCodec.BufferInfo bufferInfo = new MediaCodec.BufferInfo();

long videoSampleTime=getSampleTime(mediaExtractor,buffer);
while (true) {
int readSampleDataSize = mediaExtractor.readSampleData(buffer, 0);
if (readSampleDataSize < 0) {
break;
}
bufferInfo.size = readSampleDataSize;
bufferInfo.offset = 0;
bufferInfo.flags = mediaExtractor.getSampleFlags();
bufferInfo.presentationTimeUs += videoSampleTime;
mediaMuxer.writeSampleData(trackIndex, buffer, bufferInfo);
mediaExtractor.advance();
}

mediaExtractor.release();
mediaMuxer.stop();
//内部也会执行stop，所以可以不用执行stop
mediaMuxer.release();

}

private static long getSampleTime(MediaExtractor mediaExtractor, ByteBuffer buffer) {
long videoSampleTime;
// mediaExtractor.readSampleData(buffer, 0);
// //skip first I frame
// if (mediaExtractor.getSampleFlags() == MediaExtractor.SAMPLE_FLAG_SYNC)
// mediaExtractor.advance();
mediaExtractor.readSampleData(buffer, 0);
long firstVideoPTS = mediaExtractor.getSampleTime();
mediaExtractor.advance();
mediaExtractor.readSampleData(buffer, 0);
long SecondVideoPTS = mediaExtractor.getSampleTime();
videoSampleTime = Math.abs(SecondVideoPTS - firstVideoPTS);
Log.d(“MediaMuxerUtil”, "videoSampleTime is " + videoSampleTime);
return videoSampleTime;
}

/**

• 合成音视频
• @param videoPath
• @param audioPath
• @param outPath
  */
  @TargetApi(Build.VERSION_CODES.JELLY_BEAN_MR2)
  public static void trackMuxer(String videoPath, String audioPath, String outPath) {
  MediaExtractor videoExtractor = new MediaExtractor();
  try {
  videoExtractor.setDataSource(videoPath);
  } catch (IOException e) {
  e.printStackTrace();
  }
  MediaFormat videoFormat = null;
  int videoTrackIndex = -1;
  int videoTrackCount = videoExtractor.getTrackCount();
  for (int i = 0; i < videoTrackCount; i++) {
  videoFormat = videoExtractor.getTrackFormat(i);
  String mime = videoFormat.getString(MediaFormat.KEY_MIME);
  if (mime.startsWith(“video/”)) {
  videoTrackIndex = i;
  break;
  }
  }

eoExtractor.getTrackFormat(i);
String mime = videoFormat.getString(MediaFormat.KEY_MIME);
if (mime.startsWith(“video/”)) {
videoTrackIndex = i;
break;
}
}