MediaCodec是Android（api>=16）提供的一个多媒体硬解编码库，能实现音视频的编解码。

工作原理：其内部有2个队列，一个是输入队列，一个是输出队列。输入队列负责存储编
解码前的原始数据存储，并输送给MediaCodec处理；输出队列负责存储编解码后
的新数据，可以直接处理或保存到文件中。

AAC 的头部信息介绍 ：https://blog.csdn.net/jay100500/article/details/52955232

 //mediacodec
    private MediaFormat encoderFormat = null;
    private MediaCodec encoder = null;
    private FileOutputStream outputStream = null;
    private MediaCodec.BufferInfo info = null;
    private int perpcmsize = 0;
    private byte[] outByteBuffer = null;
    private int aacsamplerate = 4;
    private double recordTime = 0;
    private int audioSamplerate = 0;

    private void initMediacodec(int samperate, File outfile)
    {
        try {
            aacsamplerate = getADTSsamplerate(samperate);
            //立体声
            encoderFormat = MediaFormat.createAudioFormat(MediaFormat.MIMETYPE_AUDIO_AAC, samperate, 2);
            //96kbps fm音质
            encoderFormat.setInteger(MediaFormat.KEY_BIT_RATE, 96000);
            encoderFormat.setInteger(MediaFormat.KEY_AAC_PROFILE, MediaCodecInfo.CodecProfileLevel.AACObjectLC);
            encoderFormat.setInteger(MediaFormat.KEY_MAX_INPUT_SIZE, 4096);
            encoder = MediaCodec.createEncoderByType(MediaFormat.MIMETYPE_AUDIO_AAC);
            info = new MediaCodec.BufferInfo();
            if(encoder == null)
            {
                MyLog.d("craete encoder wrong");
                return;
            }
            recordTime = 0;
            encoder.configure(encoderFormat, null, null, MediaCodec.CONFIGURE_FLAG_ENCODE);
            outputStream = new FileOutputStream(outfile);
            encoder.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void encodecPcmToAAc(int size, byte[] buffer)
    {
        if(buffer != null && encoder != null)
        {
            //录音时间  size/ 采样率*声道数 * bits/8
            recordTime += size * 1.0 / (audioSamplerate * 2 * (16 / 8));
            MyLog.d("recordTime = " + recordTime);
            //回掉
            if(wlOnRecordTimeListener != null)
            {
                wlOnRecordTimeListener.onRecordTime((int) recordTime);
            }

            int inputBufferindex = encoder.dequeueInputBuffer(0);
            if(inputBufferindex >= 0)
            {
                ByteBuffer byteBuffer = encoder.getInputBuffers()[inputBufferindex];
                byteBuffer.clear();
                byteBuffer.put(buffer);
                encoder.queueInputBuffer(inputBufferindex, 0, size, 0, 0);
            }

            int index = encoder.dequeueOutputBuffer(info, 0);
            while(index >= 0)
            {
                try {
                    perpcmsize = info.size + 7;
                    outByteBuffer = new byte[perpcmsize];

                    ByteBuffer byteBuffer = encoder.getOutputBuffers()[index];
                    byteBuffer.position(info.offset);
                    byteBuffer.limit(info.offset + info.size);

                    addADtsHeader(outByteBuffer, perpcmsize, aacsamplerate);

                    byteBuffer.get(outByteBuffer, 7, info.size);
                    byteBuffer.position(info.offset);
                    outputStream.write(outByteBuffer, 0, perpcmsize);

                    encoder.releaseOutputBuffer(index, false);
                    index = encoder.dequeueOutputBuffer(info, 0);
                    outByteBuffer = null;
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    private void addADtsHeader(byte[] packet, int packetLen, int samplerate)
    {
        int profile = 2; // AAC LC
        int freqIdx = samplerate; // samplerate
        int chanCfg = 2; // CPE

        packet[0] = (byte) 0xFF; // 0xFFF(12bit) 这里只取了8位，所以还差4位放到下一个里面
        packet[1] = (byte) 0xF9; // 第一个t位放F
        packet[2] = (byte) (((profile - 1) << 6) + (freqIdx << 2) + (chanCfg >> 2));
        packet[3] = (byte) (((chanCfg & 3) << 6) + (packetLen >> 11));
        packet[4] = (byte) ((packetLen & 0x7FF) >> 3);
        packet[5] = (byte) (((packetLen & 7) << 5) + 0x1F);
        packet[6] = (byte) 0xFC;
    }

    private int getADTSsamplerate(int samplerate)
    {
        int rate = 4;
        switch (samplerate)
        {
            case 96000:
                rate = 0;
                break;
            case 88200:
                rate = 1;
                break;
            case 64000:
                rate = 2;
                break;
            case 48000:
                rate = 3;
                break;
            case 44100:
                rate = 4;
                break;
            case 32000:
                rate = 5;
                break;
            case 24000:
                rate = 6;
                break;
            case 22050:
                rate = 7;
                break;
            case 16000:
                rate = 8;
                break;
            case 12000:
                rate = 9;
                break;
            case 11025:
                rate = 10;
                break;
            case 8000:
                rate = 11;
                break;
            case 7350:
                rate = 12;
                break;
        }
        return rate;
    }

    private void releaseMedicacodec()
    {
        if(encoder == null)
        {
            return;
        }
        try {
            recordTime = 0;
            outputStream.close();
            outputStream = null;
            encoder.stop();
            encoder.release();
            encoder = null;
            encoderFormat = null;
            info = null;
            initmediacodec = false;

            MyLog.d("录制完成...");
        } catch (IOException e) {
            e.printStackTrace();
        }
        finally {
            if(outputStream != null)
            {
                try {
                    outputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                outputStream = null;
            }
        }
    }