音频的播放Android提供了像MediaPlayer，SoundPool，AudioTrack（需自己解码音频）等。这些都只是单纯的播放一个声音，支持的音频文件格式也存在有限。比如我们想开发一款像QQ音乐这样的音乐播放器，一款好的音乐器并不是简单的播放歌曲，里面会包含有很多设置，能够对音效的操作编辑。FFmpeg作为音视频操作的库，解码出音频文件PCM数据，PCM作为音频的原始数据我们可以对其进行编辑等等，FFmpeg可对音频添加滤镜。
不管是视频图像解码还是音频解码FFmpeg都有标准的步骤。
看看FFmpeg音频解码的过程：
音频解码流程.png
下面是ffmpeg解码音频流程的代码
//
// Created by Administrator on 2019/8/21.
//
#include "YBFFmpeg.h"
#include "android_log.h"
#include "PlayerContan.h"
#include
YBFFmpeg::YBFFmpeg(PlayerJNICall *playerJNICall, const char *url) {
pPlayerJNICall = playerJNICall;
char *copyUrl = (char *) malloc(strlen(url) + 1);
memcpy(copyUrl, url, strlen(url) + 1);
this->url = url;
}
YBFFmpeg::~YBFFmpeg() {
relese();
}
void *thread_play(void *arg) {
YBFFmpeg *ybfFmpeg = (YBFFmpeg *) arg;
int res;
int audioStramIndex = -1;
int index = 0;
//注册所有组件
av_register_all();
//打开文件
res = avformat_open_input(&(ybfFmpeg->pFormatContext), ybfFmpeg->url, NULL, NULL);
if (res != 0) {
LOGE("%d,%s", res, av_err2str(res));
ybfFmpeg->callPlayerJniError(res, av_err2str(res));
return (void*)res;
}
//查找流信息
res = avformat_find_stream_info(ybfFmpeg->pFormatContext, NULL);
if (res < 0) {
LOGE("%d,%s", res, av_err2str(res));
ybfFmpeg->callPlayerJniError(res, av_err2str(res));
return (void*)res;
}
//查找对应流index 找的是AVMEDIA_TYPE_AUDIO（音频流）
audioStramIndex = av_find_best_stream(ybfFmpeg->pFormatContext, AVMEDIA_TYPE_AUDIO,
-1, -1, NULL,
0);
if (audioStramIndex < 0) {
LOGE("%s", "no find audio stream");
ybfFmpeg->callPlayerJniError(PLAYER_FIND_AUDIO_STREAM_ERRO, "no find audio stream");
return (void*)-1;
}
AVCodecParameters *pCodecParameters = ybfFmpeg->pFormatContext->streams[audioStramIndex]->codecpar;
//查找解码器
AVCodec *pCodec = avcodec_find_decoder(
pCodecParameters->codec_id);
if (pCodec == NULL) {
LOGE("%s", "no find Codec fail");
ybfFmpeg->callPlayerJniError(PLAYER_FIND_CODEC_ERRO, "PLAYER_FIND_CODEC_ERRO");
return (void*)-1;
}
//开辟pCodecContext
ybfFmpeg->pCodecContext = avcodec_alloc_context3(pCodec);
if (ybfFmpeg->pCodecContext == NULL) {
LOGE("%s", "avcodec alloc context fail");
ybfFmpeg->callPlayerJniError(PLAYER_ALLOC_CODECCONTEXT_ERRO,
"PLAYER_ALLOC_CODECCONTEXT_ERRO");
return (void*)-1;
}
res = avcodec_parameters_to_context(ybfFmpeg->pCodecContext, pCodecParameters);
if (res < 0) {
LOGE("%d,%s", res, av_err2str(res));
ybfFmpeg->callPlayerJniError(res, av_err2str(res));
return (void*)res;
}
//打开解码器
res = avcodec_open2(ybfFmpeg->pCodecContext, pCodec, NULL);
if (res < 0) {
LOGE("%d,%s", res, av_err2str(res));
ybfFmpeg->callPlayerJniError(res, av_err2str(res));
return (void*)res;
}
int64_t out_ch_layout = AV_CH_LAYOUT_STEREO;
enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;
int out_sample_rate = 44100;
int64_t in_ch_layout = pCodecParameters->channel_layout;
enum AVSampleFormat in_sample_fmt = ybfFmpeg->pCodecContext->sample_fmt;
int in_sample_rate = ybfFmpeg->pCodecContext->sample_rate;
ybfFmpeg->pSwrContext = swr_alloc_set_opts(NULL, out_ch_layout, out_sample_fmt,
out_sample_rate, in_ch_layout,
in_sample_fmt, in_sample_rate, 0, NULL);
int errocode = swr_init(ybfFmpeg->pSwrContext);
// size 是播放指定的大小，是最终输出的大小
int outChannels = av_get_channel_layout_nb_channels(out_ch_layout);
int dataSize = av_samples_get_buffer_size(NULL, outChannels,
pCodecParameters->frame_size,
out_sample_fmt, 0);
int out_sample_fmt_track;
if (out_sample_fmt == AV_SAMPLE_FMT_U8) {
out_sample_fmt_track = 3;
} else {
out_sample_fmt_track = 2;
}
uint8_t *resampleOutBuffer = (uint8_t *) malloc(dataSize);
JNIEnv* tehread_Env;
ybfFmpeg->pPlayerJNICall->javaVM->AttachCurrentThread(&tehread_Env,NULL);
ybfFmpeg->pPlayerJNICall->initCrateAudioTrack(tehread_Env,outChannels, out_sample_fmt_track);
jbyteArray jPcmByteArray =tehread_Env->NewByteArray(dataSize);
jbyte *jPcmData = tehread_Env->GetByteArrayElements(jPcmByteArray, NULL);
AVPacket *pkt = av_packet_alloc();
AVFrame *pFrame = av_frame_alloc();
//开始解码流
while (av_read_frame(ybfFmpeg->pFormatContext, pkt) == 0) {
if (audioStramIndex == pkt->stream_index) {
//音频流
if (avcodec_send_packet(ybfFmpeg->pCodecContext, pkt) == 0) {
if (avcodec_receive_frame(ybfFmpeg->pCodecContext, pFrame) == 0) {
//解码数据
index++;
LOGE("解码音频%d帧", index);
swr_convert(ybfFmpeg->pSwrContext, &resampleOutBuffer, pFrame->nb_samples,
(const uint8_t **) (pFrame->data),
pFrame->nb_samples);
memcpy(jPcmData, resampleOutBuffer, dataSize);
// 0 把 c 的数组的数据同步到 jbyteArray , 然后释放native数组
tehread_Env->ReleaseByteArrayElements(jPcmByteArray,
jPcmData,
JNI_COMMIT);
ybfFmpeg->pPlayerJNICall->callAudioTrackWrite(tehread_Env,jPcmByteArray, 0, dataSize);
}
}
}
av_packet_unref(pkt);
av_frame_unref(pFrame);
}
av_packet_free(&pkt);
av_frame_free(&pFrame);
tehread_Env->DeleteLocalRef(jPcmByteArray);
ybfFmpeg->pPlayerJNICall->javaVM->DetachCurrentThread()
free(resampleOutBuffer);
}
void YBFFmpeg::play() {
pthread_t play_thread;
pthread_create(&play_thread, NULL, thread_play, this);
pthread_join(play_thread, NULL);
}
void YBFFmpeg::callPlayerJniError(int code, char *msg) {
relese();
callPlayerJniError(code, msg);
}
void YBFFmpeg::relese() {
if (pSwrContext != NULL) {
swr_close(pSwrContext);
swr_free(&pSwrContext);
pSwrContext = NULL;
}
if (pCodecContext != NULL) {
avcodec_free_context(&pCodecContext);
pCodecContext = NULL;
}
if (pFormatContext != NULL) {
avformat_close_input(&pFormatContext);
avformat_free_context(pFormatContext);
pFormatContext = NULL;
}
}
上述代码包含媒体文件的读取（IO），解码模块，音频渲染。简单的展示FFmpeg的解码音频的流程。制作播放器，要考虑东西比较多，比如网络的抖动，解码抖动，秒开等优化。
avformat_open_input()主要是连接网络或者本地资源以及码流头部信息的拉取,
avformat_find_stream_info()媒体信息的探测与分析，做完这步操作一些媒体的基本信息都被填入上下文。看看AVFormatContext 有哪些重要的信息
typedef struct AVFormatContext {
struct AVInputFormat *iformat;
struct AVOutputFormat *oformat;
unsigned int nb_streams;
AVStream **streams;
char filename[1024];
int64_t start_time;
int64_t duration;
int64_t bit_rate;
unsigned int packet_size;
int max_delay;
enum AVCodecID video_codec_id;
AVDictionary *metadata;
AVCodec *video_codec;
......
}AVFormatContext
从AVFormatContext 的结构体中可以看出包含的码流数量，码流，文件名，时长，解码器等等，如果想要知道码流的详细数据（比如音频流的采样率，采样点个数，通道等详细信息），通过av_find_best_stream()可以找到我们关注的码流，AVStream结构体中包含了对应码流的详细信息
typedef struct AVStream {
int index; /**< stream index in AVFormatContext */
AVCodecContext *codec;//解码上下文
int64_t start_time; 第一帧数据显示时间
int64_t nb_frames; 多少帧数据
AVCodecParameters *codecpar; //解码参数
}AVStream
AVCodecParameters 解码的一些信息参数
typedef struct AVCodecParameters {
enum AVCodecID codec_id;
int64_t bit_rate;
int width;
int height;
uint64_t channel_layout;
int channels;
int sample_rate;
int block_align;
int frame_size;
}AVCodecParameters
经过层层的解析媒体文件信息几乎都可以取到。有些信息也可能没有比如AVCodecParameters 中的width，height信息码流中可能并不含盖，要等具体解码来覆盖该值。
创建JNI层创建AudioTrack 上篇文章讲到了子线程与主线程调用Java的区别，这里把播放放到了子线程，JNIEnv包括对象都是线程私有的，主要是通过JavaVM来获取JNIEnv,包括创建对象的全局引用
//
// Created by Administrator on 2019/8/21.
//
#include "PlayerJNICall.h"
#include "android_log.h"
PlayerJNICall::PlayerJNICall(JNIEnv *env, JavaVM *javaVM, jobject jPlayerObj) {
this->javaVM = javaVM;
this->JniEnv = env;
this->jPlayerObj = JniEnv->NewGlobalRef(jPlayerObj);
this->instance_clazz = (jclass) (JniEnv->NewGlobalRef(
JniEnv->GetObjectClass(this->jPlayerObj)));
jErroMid = JniEnv->GetMethodID(instance_clazz, "onErro", "(ILjava/lang/String;)V");
}
PlayerJNICall::~PlayerJNICall() {
if (audioTrackInstance != NULL) {
JniEnv->DeleteLocalRef(audioTrackInstance);
audioTrackInstance = NULL;
}
if (jPlayerObj != NULL) {
JniEnv->DeleteGlobalRef(jPlayerObj);
jPlayerObj = NULL;
}
if (instance_clazz != NULL) {
JniEnv->DeleteGlobalRef(instance_clazz);
instance_clazz = NULL;
}
}
/**
* 创建AudioTrack
* @param pEnv
*
* public AudioTrack(int streamType, int sampleRateInHz, int channelConfig, int audioFormat,
int bufferSizeInBytes, int mode)
*/
//对象也是线程私有的
void PlayerJNICall::initCrateAudioTrack(JNIEnv *env, int channelConfig, int audioFormat) {
jmethodID jm_id = env->GetMethodID(instance_clazz, "initAudioTrack",
"(II)Landroid/media/AudioTrack;");
audioTrackInstance = env->CallObjectMethod(jPlayerObj, jm_id, channelConfig,
audioFormat);
jclass audioTrackClazz = env->GetObjectClass(audioTrackInstance);
jmethodID play_mID = env->GetMethodID(audioTrackClazz, "play", "()V");
jWriteMid = env->GetMethodID(audioTrackClazz, "write", "([BII)I");
env->CallVoidMethod(audioTrackInstance, play_mID);
}
void PlayerJNICall::callAudioTrackWrite(JNIEnv *env, jbyteArray audioData, int offsetInBytes,
int sizeInBytes) {
env->CallIntMethod(audioTrackInstance, jWriteMid, audioData, offsetInBytes, sizeInBytes);
}
void PlayerJNICall::callErro(int code, char *msg) {
jstring jMsg = JniEnv->NewStringUTF(msg);
JniEnv->CallVoidMethod(audioTrackInstance, jErroMid, code, jMsg);
JniEnv->ReleaseStringUTFChars(jMsg, msg);
}