6 FFmpeg4Android：音频解码

音频解码，就是将视频文件中的音频部分抽离出来，生成PCM文件，并使用Android控件AudioTrack进行播放。
新建，MedioPlayer项目，其目录结构如下：

目录结构

java端代码，MainActivity.java：

package com.lzp.decoder;

import java.io.File;

import com.lzp.decoder.view.VideoView;

import android.app.Activity;
import android.os.Bundle;
import android.os.Environment;
import android.view.Surface;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Spinner;

public class MainActivity extends Activity {

    private Player player;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        player = new Player();
    }

    public void playAudio(View btn) {
        String input = new File(Environment.getExternalStorageDirectory(), "sintel.mp4").getAbsolutePath();
        String output = new File(Environment.getExternalStorageDirectory(), "output.pcm").getAbsolutePath();
        player.sound(input, output);
    }
}

java端代码，Player.java：

package com.lzp.decoder;

import android.media.AudioFormat;
import android.media.AudioManager;
import android.media.AudioTrack;
import android.util.Log;
import android.view.Surface;

/**
 * 视频播放的控制器
 */
public class Player {

    // 解码视频
    public native void render(String input, Surface surface);
    // 解码音频
    public native void sound(String input, String output);

    /**
     * 创建一个AudioTrac对象，用于播放
     *
     * @param nb_channels
     * @return
     */
    public AudioTrack createAudioTrack(int sampleRateInHz, int nb_channels) {
        // 固定格式的音频码流
        int audioFormat = AudioFormat.ENCODING_PCM_16BIT;
        Log.i("jason", "nb_channels:" + nb_channels);
        // 声道布局
        int channelConfig;
        if (nb_channels == 1) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_MONO;
        } else if (nb_channels == 2) {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        } else {
            channelConfig = android.media.AudioFormat.CHANNEL_OUT_STEREO;
        }

        int bufferSizeInBytes = AudioTrack.getMinBufferSize(sampleRateInHz, channelConfig, audioFormat);

        AudioTrack audioTrack = new AudioTrack(
                AudioManager.STREAM_MUSIC,
                sampleRateInHz, channelConfig,
                audioFormat,
                bufferSizeInBytes, AudioTrack.MODE_STREAM);
        // 播放
        // audioTrack.play();
        // 写入PCM
        // audioTrack.write(audioData, offsetInBytes, sizeInBytes);
        return audioTrack;
    }

    static {
        System.loadLibrary("avutil-54");
        System.loadLibrary("swresample-1");
        System.loadLibrary("avcodec-56");
        System.loadLibrary("avformat-56");
        System.loadLibrary("swscale-3");
        System.loadLibrary("postproc-53");
        System.loadLibrary("avfilter-5");
        System.loadLibrary("avdevice-56");
        System.loadLibrary("myffmpeg");
    }
}

c端解码代码，ffmpeg_audio_player.c：

#include "com_lzp_decoder_Player.h"

#include <stdlib.h>
#include <unistd.h>
#include <android/log.h>

//封装格式
#include "libavformat/avformat.h"
//解码
#include "libavcodec/avcodec.h"
//缩放
#include "libswscale/swscale.h"
//重采样
#include "libswresample/swresample.h"

#define LOGI(FORMAT, ...) __android_log_print(ANDROID_LOG_INFO, "ffmpeg",FORMAT, ##__VA_ARGS__);
#define LOGE(FORMAT, ...) __android_log_print(ANDROID_LOG_ERROR, "ffmpeg",FORMAT, ##__VA_ARGS__);

#define MAX_AUDIO_FRME_SIZE 48000 * 4


JNIEXPORT void JNICALL Java_com_lzp_decoder_Player_sound
(JNIEnv *env, jobject jthiz, jstring input_jstr, jstring output_jstr){
    const char* input_cstr = (*env)->GetStringUTFChars(env, input_jstr, NULL);
    const char* output_cstr = (*env)->GetStringUTFChars(env, output_jstr, NULL);
    LOGI("%s", "sound");
    // 注册组件
    av_register_all();
    AVFormatContext *pFormatCtx = avformat_alloc_context();
    //打开音频文件
    if (avformat_open_input(&pFormatCtx, input_cstr, NULL, NULL) != 0){
        LOGI("无法打开音频文件：%s\n", input_cstr);
        return;
    }
    // 获取输入文件信息
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0){
        LOGI("%s", "无法获取输入文件信息");
        return;
    }
    // 获取音频流索引位置
    int i = 0, audio_stream_idx = -1;
    for (; i < pFormatCtx->nb_streams; i++){
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO){
            audio_stream_idx = i;
            break;
        }
    }

    // 获取解码器
    AVCodecContext *codecCtx = pFormatCtx->streams[audio_stream_idx]->codec;
    AVCodec *codec = avcodec_find_decoder(codecCtx->codec_id);
    if (codec == NULL){
        LOGI("%s", "无法获取解码器");
        return;
    }
    // 打开解码器
    if (avcodec_open2(codecCtx, codec, NULL) < 0){
        LOGI("%s", "无法打开解码器");
        return;
    }
    // 压缩数据
    AVPacket *packet = (AVPacket *)av_malloc(sizeof(AVPacket));
    // 解压缩数据
    AVFrame *frame = av_frame_alloc();
    // frame->16bit 44100 PCM 统一音频采样格式与采样率
    SwrContext *swrCtx = swr_alloc();

    // 重采样设置参数-------------start
    // 输入的采样格式
    enum AVSampleFormat in_sample_fmt = codecCtx->sample_fmt;
    // 输出采样格式16bit PCM
    enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
    // 输入采样率
    int in_sample_rate = codecCtx->sample_rate;
    // 输出采样率
    int out_sample_rate = in_sample_rate;
    // 获取输入的声道布局
    // 根据声道个数获取默认的声道布局（2个声道，默认立体声stereo）
    // av_get_default_channel_layout(codecCtx->channels);
    uint64_t in_ch_layout = codecCtx->channel_layout;
    // 输出的声道布局（立体声）
    uint64_t out_ch_layout = AV_CH_LAYOUT_STEREO;

    swr_alloc_set_opts(swrCtx,
        out_ch_layout, out_sample_fmt, out_sample_rate,
        in_ch_layout, in_sample_fmt, in_sample_rate,
        0, NULL);
    swr_init(swrCtx);

    // 输出的声道个数
    int out_channel_nb = av_get_channel_layout_nb_channels(out_ch_layout);

    // 重采样设置参数-------------end

    // JNI begin------------------
    // JasonPlayer
    jclass player_class = (*env)->GetObjectClass(env, jthiz);

    // AudioTrack对象
    jmethodID create_audio_track_mid = (*env)->GetMethodID(env, player_class, "createAudioTrack", "(II)Landroid/media/AudioTrack;");
    jobject audio_track = (*env)->CallObjectMethod(env, jthiz, create_audio_track_mid, out_sample_rate, out_channel_nb);

    // 调用AudioTrack.play方法
    jclass audio_track_class = (*env)->GetObjectClass(env, audio_track);
    jmethodID audio_track_play_mid = (*env)->GetMethodID(env, audio_track_class, "play", "()V");
    (*env)->CallVoidMethod(env, audio_track, audio_track_play_mid);

    // AudioTrack.write
    jmethodID audio_track_write_mid = (*env)->GetMethodID(env, audio_track_class, "write", "([BII)I");

    // JNI end------------------
    FILE *fp_pcm = fopen(output_cstr, "wb");

    // 16bit 44100 PCM 数据
    uint8_t *out_buffer = (uint8_t *)av_malloc(MAX_AUDIO_FRME_SIZE);

    int got_frame = 0, index = 0, ret;
    // 不断读取压缩数据
    while (av_read_frame(pFormatCtx, packet) >= 0){
        // 解码音频类型的Packet
        if (packet->stream_index == audio_stream_idx){
            // 解码
            ret = avcodec_decode_audio4(codecCtx, frame, &got_frame, packet);

            if (ret < 0){
                LOGI("%s", "解码完成");
            }
            // 解码一帧成功
            if (got_frame > 0){
                LOGI("解码：%d", index++);
                swr_convert(swrCtx, &out_buffer, MAX_AUDIO_FRME_SIZE, (const uint8_t **)frame->data, frame->nb_samples);
                // 获取sample的size
                int out_buffer_size = av_samples_get_buffer_size(NULL, out_channel_nb,
                    frame->nb_samples, out_sample_fmt, 1);
                fwrite(out_buffer, 1, out_buffer_size, fp_pcm);

                // out_buffer缓冲区数据，转成byte数组
                jbyteArray audio_sample_array = (*env)->NewByteArray(env, out_buffer_size);
                jbyte* sample_bytep = (*env)->GetByteArrayElements(env, audio_sample_array, NULL);
                // out_buffer的数据复制到sampe_bytep
                memcpy(sample_bytep, out_buffer, out_buffer_size);
                // 同步
                (*env)->ReleaseByteArrayElements(env, audio_sample_array, sample_bytep, 0);

                // AudioTrack.write PCM数据
                (*env)->CallIntMethod(env, audio_track, audio_track_write_mid,
                    audio_sample_array, 0, out_buffer_size);
                // 释放局部引用
                (*env)->DeleteLocalRef(env, audio_sample_array);
                usleep(1000 * 16);
            }
        }
        av_free_packet(packet);
    }

    av_frame_free(&frame);
    av_free(out_buffer);

    swr_free(&swrCtx);
    avcodec_close(codecCtx);
    avformat_close_input(&pFormatCtx);

    (*env)->ReleaseStringUTFChars(env, input_jstr, input_cstr);
    (*env)->ReleaseStringUTFChars(env, output_jstr, output_cstr);
}

运行结果日志：

运行结果

源码+视频文件下载：http://download.csdn.net/download/itismelzp/10229694