客戶要在android手機上做個能視訊剪下的app，由於視訊源只是MP4，所以就想到了用MediaExtractor和MediaMuxer來實現功能，直接上程式碼。

public class VideoDecoder {
    private final static String TAG = "VideoDecoder";
    private MediaCodec mediaDecoder;
    private MediaExtractor mediaExtractor;
    private MediaFormat mediaFormat;
    private MediaMuxer mediaMuxer;
    private
 
 String mime = null;

    public boolean decodeVideo(String url, long clipPoint, long clipDuration) {
        int videoTrackIndex = -1;
        int audioTrackIndex = -1;
        int videoMaxInputSize = 0;
        int audioMaxInputSize = 0;
        int sourceVTrack = 0;
        int sourceATrack = 0;
        long
 
 videoDuration, audioDuration;
        //建立分離器
        mediaExtractor = new MediaExtractor();
        try {
            //設定檔案路徑
            mediaExtractor.setDataSource(url);
            //建立合成器
            mediaMuxer = new MediaMuxer(url.substring(0, url.lastIndexOf(".")) + "_output.mp4", OutputFormat.MUXER_OUTPUT_MPEG_4);
        } catch
 
 (Exception e) {
            Log.e(TAG, "error path" + e.getMessage());
        }
        //獲取每個軌道的資訊
        for (int i = 0; i < mediaExtractor.getTrackCount(); i++) {
            try {
                mediaFormat = mediaExtractor.getTrackFormat(i);
                mime = mediaFormat.getString(MediaFormat.KEY_MIME);
                if (mime.startsWith("video/")) {
                    sourceVTrack = i;
                    int width = mediaFormat.getInteger(MediaFormat.KEY_WIDTH);
                    int height = mediaFormat.getInteger(MediaFormat.KEY_HEIGHT);
                    videoMaxInputSize = mediaFormat.getInteger(MediaFormat.KEY_MAX_INPUT_SIZE);
                    videoDuration = mediaFormat.getLong(MediaFormat.KEY_DURATION);
                    //檢測剪輯點和剪輯時長是否正確 
                    if (clipPoint >= videoDuration) {
                        Log.e(TAG, "clip point is error!");
                        return false;
                    }
                    if ((clipDuration != 0) && ((clipDuration + clipPoint) >= videoDuration)) {
                        Log.e(TAG, "clip duration is error!");
                        return false;
                    }
                    Log.d(TAG, "width and height is " + width + " " + height
                                    + ";maxInputSize is " + videoMaxInputSize
                                    + ";duration is " + videoDuration
                    );
                    //向合成器新增視訊軌
                    videoTrackIndex = mediaMuxer.addTrack(mediaFormat);
                }
                else if (mime.startsWith("audio/")) {
                    sourceATrack = i;
                    int sampleRate = mediaFormat.getInteger(MediaFormat.KEY_SAMPLE_RATE);
                    int channelCount = mediaFormat.getInteger(MediaFormat.KEY_CHANNEL_COUNT);
                    audioMaxInputSize = mediaFormat.getInteger(MediaFormat.KEY_MAX_INPUT_SIZE);
                    audioDuration = mediaFormat.getLong(MediaFormat.KEY_DURATION);
                    Log.d(TAG, "sampleRate is " + sampleRate
                                    + ";channelCount is " + channelCount
                                    + ";audioMaxInputSize is " + audioMaxInputSize
                                    + ";audioDuration is " + audioDuration
                    );
                    //新增音軌
                    audioTrackIndex = mediaMuxer.addTrack(mediaFormat);
                }
                Log.d(TAG, "file mime is " + mime);
            } catch (Exception e) {
                Log.e(TAG, " read error " + e.getMessage());
            }
        }
        //分配緩衝
        ByteBuffer inputBuffer = ByteBuffer.allocate(videoMaxInputSize);
        //根據官方文件的解釋MediaMuxer的start一定要在addTrack之後
        mediaMuxer.start();
        //視訊處理部分
        mediaExtractor.selectTrack(sourceVTrack);
        MediaCodec.BufferInfo videoInfo = new MediaCodec.BufferInfo();
        videoInfo.presentationTimeUs = 0;
        long videoSampleTime;
        //獲取源視訊相鄰幀之間的時間間隔。(1)
        {
            mediaExtractor.readSampleData(inputBuffer, 0);
            //skip first I frame
            if (mediaExtractor.getSampleFlags() == MediaExtractor.SAMPLE_FLAG_SYNC)
                mediaExtractor.advance();
            mediaExtractor.readSampleData(inputBuffer, 0);
            long firstVideoPTS = mediaExtractor.getSampleTime();
            mediaExtractor.advance();
            mediaExtractor.readSampleData(inputBuffer, 0);
            long SecondVideoPTS = mediaExtractor.getSampleTime();
            videoSampleTime = Math.abs(SecondVideoPTS - firstVideoPTS);
            Log.d(TAG, "videoSampleTime is " + videoSampleTime);
        }
        //選擇起點
        mediaExtractor.seekTo(clipPoint, MediaExtractor.SEEK_TO_PREVIOUS_SYNC);
        while (true) {
            int sampleSize = mediaExtractor.readSampleData(inputBuffer, 0);
            if (sampleSize < 0) {
                //這裡一定要釋放選擇的軌道，不然另一個軌道就無法選中了
                mediaExtractor.unselectTrack(sourceVTrack);
                break;
            }
            int trackIndex = mediaExtractor.getSampleTrackIndex();
            //獲取時間戳
            long presentationTimeUs = mediaExtractor.getSampleTime();
            //獲取幀型別，只能識別是否為I幀
            int sampleFlag = mediaExtractor.getSampleFlags();
            Log.d(TAG, "trackIndex is " + trackIndex
                    + ";presentationTimeUs is " + presentationTimeUs
                    + ";sampleFlag is " + sampleFlag
                    + ";sampleSize is " + sampleSize);
            //剪輯時間到了就跳出
            if ((clipDuration != 0) && (presentationTimeUs > (clipPoint + clipDuration))) {
                mediaExtractor.unselectTrack(sourceVTrack);
                break;
            }
            mediaExtractor.advance();
            videoInfo.offset = 0;
            videoInfo.size = sampleSize;
            videoInfo.flags = sampleFlag;
            mediaMuxer.writeSampleData(videoTrackIndex, inputBuffer, videoInfo);
            videoInfo.presentationTimeUs += videoSampleTime;//presentationTimeUs;
        }
        //音訊部分
        mediaExtractor.selectTrack(sourceATrack);
        MediaCodec.BufferInfo audioInfo = new MediaCodec.BufferInfo();
        audioInfo.presentationTimeUs = 0;
        long audioSampleTime;
        //獲取音訊幀時長
        {
            mediaExtractor.readSampleData(inputBuffer, 0);
            //skip first sample
            if (mediaExtractor.getSampleTime() == 0)
                mediaExtractor.advance();
            mediaExtractor.readSampleData(inputBuffer, 0);
            long firstAudioPTS = mediaExtractor.getSampleTime();
            mediaExtractor.advance();
            mediaExtractor.readSampleData(inputBuffer, 0);
            long SecondAudioPTS = mediaExtractor.getSampleTime();
            audioSampleTime = Math.abs(SecondAudioPTS - firstAudioPTS);
            Log.d(TAG, "AudioSampleTime is " + audioSampleTime);
        }
        mediaExtractor.seekTo(clipPoint, MediaExtractor.SEEK_TO_CLOSEST_SYNC);
        while (true) {
            int sampleSize = mediaExtractor.readSampleData(inputBuffer, 0);
            if (sampleSize < 0) {
                mediaExtractor.unselectTrack(sourceATrack);
                break;
            }
            int trackIndex = mediaExtractor.getSampleTrackIndex();
            long presentationTimeUs = mediaExtractor.getSampleTime();
            Log.d(TAG, "trackIndex is " + trackIndex
                    + ";presentationTimeUs is " + presentationTimeUs);
            if ((clipDuration != 0) && (presentationTimeUs > (clipPoint + clipDuration))) {
                mediaExtractor.unselectTrack(sourceATrack);
                break;
            }
            mediaExtractor.advance();
            audioInfo.offset = 0;
            audioInfo.size = sampleSize;
            mediaMuxer.writeSampleData(audioTrackIndex, inputBuffer, audioInfo);
            audioInfo.presentationTimeUs += audioSampleTime;//presentationTimeUs;
        }
        //全部寫完後釋放MediaMuxer和MediaExtractor
        mediaMuxer.stop();
        mediaMuxer.release();
        mediaExtractor.release();
        mediaExtractor = null;
        return true;
    }
}

這裡要對(1)進行一下解釋，如果視訊中帶了B幀，源視訊時間戳會按照播放順序來打，這樣通過讀每幀獲取到的時間戳就不是一直增長的，MediaMuxer的writeSampleData函式在寫每幀的時間戳時如果時間戳不是增長順序就會報錯，幀率也無法通過MediaFormat.KEY_FRAME_RATE來獲取，也可以通過查詢MP4檔案的stsz關鍵字來獲取總幀數，進而算出幀率，來按幀率給每一幀打時間戳，但是stsz一般都在檔案的後面而且含有音視訊的檔案會有兩個stsz，如果檔案很大，查詢起來就會很慢，所以這裡我用了個偷懶的辦法，用I幀後面兩幀的時間戳絕對差值來當做每幀的時間，然後+=這個時間來為寫入的幀打時間戳，雖然時間戳不是源視訊的順序了，但並不影響解碼。
還有一點就是unselectTrack這個函式，用selectTrack選擇軌道後，如果要切換軌道一定要呼叫unselectTrack函式來釋放軌道，不然讀的還是之前的軌道。