Android通过OpenSL ES播放音频套路详解

*****我的视频课程：FFmpeg打造Android万能音频播放器*****

先定个小目标

        通过本篇博客，我们会很容易的使用Android底层库OpenSL ES来播放音频文件（包括assets、uri和pcm）。

一、首先OpenSL ES是什么？

        OpenSL ES (Open Sound Library for Embedded Systems)是无授权费、跨平台、针对嵌入式系统精心优化的硬件音频加速API。它为嵌入式移动多媒体设备上的本地应用程序开发者提供标准化, 高性能,低响应时间的音频功能实现方法，并实现软/硬件音频性能的直接跨平台部署，降低执行难度，促进高级音频市场的发展。——摘自百度百科。

        简单来说OpenSL ES是一个嵌入式跨平台免费的音频处理库。

二、OpenSL ES库在哪里？

        Android的OpenSL ES库是在NDK的platforms文件夹对应android平台先相应cpu类型里面，如：

三、OpenSL ES开发流程和重要接口

        通过分析android-ndk中native-audio的代码，发现了用OpenSL ES来开发应用也是有套路(规律)可寻的：
3.1、OpenSL ES的开发流程主要有如下6个步骤：
        1、创建接口对象
        2、设置混音器
        3、创建播放器（录音器）
        4、设置缓冲队列和回调函数
        5、设置播放状态
        6、启动回调函数
        其中4和6是播放PCM等数据格式的音频是需要用到的。

3.2、OpenSL ES中最重要的接口类SLObjectItf
        通过SLObjectItf接口类我们可以创建所需要的各种类型的类接口，比如：
        创建引擎接口对象：SLObjectItf engineObject
        创建混音器接口对象：SLObjectItf outputMixObject
        创建播放器接口对象：SLObjectItf playerObject
        以上等等都是通过SLObjectItf来创建的，知道了这个规则后，看源代码就会轻松很多了
3.3、利用SLObjectItf来创建具体的接口对象实例
        OpenSL ES中也有具体的接口类，比如（引擎：SLEngineItf，播放器：SLPlayItf，声音控制器：SLVolumeItf等等）。
3.4、创建引擎并实现它
        OpenSL ES中开始的第一步都是声明SLObjectItf接口类型的引擎接口对象engineObject，然后用方法slCreateEngine创建一个引擎接口对象；创建好引擎接口对象后，需要用SLObjectItf的Realize方法来实现engineObject；最后用SLObjectItf的GetInterface方法来初始化SLEngnineItf对象实例。如：

SLObjectItf engineObject = NULL;//用SLObjectItf声明引擎接口对象
SLEngineItf engineEngine = NULL;//声明具体的引擎对象实例

void createEngine()
{
SLresult result;//返回结果
result = slCreateEngine(&engineObject, 0, NULL, 0, NULL, NULL);//第一步创建引擎
result = (*engineObject)->Realize(engineObject, SL_BOOLEAN_FALSE);//实现（Realize）engineObject接口对象
result = (*engineObject)->GetInterface(engineObject, SL_IID_ENGINE, &engineEngine);//通过engineObject的GetInterface方法初始化engineEngine
}

3.4、利用引擎对象创建其他接口对象
        其他接口对象（SLObjectItf
outputMixObject，SLObjectItf playerObject）等都是用引擎接口对象创建的（具体的接口对象需要的参数这里就说了，可参照ndk例子里面的），如：

//混音器
SLObjectItf outputMixObject = NULL;//用SLObjectItf创建混音器接口对象
SLEnvironmentalReverbItf outputMixEnvironmentalReverb = NULL;////创建具体的混音器对象实例

result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);//利用引擎接口对象创建混音器接口对象
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);//实现（Realize）混音器接口对象
result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);//利用混音器接口对象初始化具体混音器实例

//播放器
SLObjectItf playerObject = NULL;//用SLObjectItf创建播放器接口对象
SLPlayItf playerPlay = NULL;//创建具体的播放器对象实例

result = (*engineEngine)->CreateAudioPlayer(engineEngine, &playerObject, &audioSrc, &audioSnk, 3, ids, req);//利用引擎接口对象创建播放器接口对象
result = (*playerObject)->Realize(playerObject, SL_BOOLEAN_FALSE);//实现（Realize）播放器接口对象
result = (*playerObject)->GetInterface(playerObject, SL_IID_PLAY, &playerPlay);//初始化具体的播放器对象实例

3.6、最后就是使用创建好的具体对象实例来实现具体的功能。

四、具体列子

首先导入OpenSL ES和其他必须的库

target_link_libraries( # Specifies the target library.
openslaudioywl5320
OpenSLES
android
# Links the target library to the log library
# included in the NDK.
${log-lib} )

4.1、播放assets文件
第一步：创建引擎
第二步：创建混音器
第三步：创建播放器
第四步：设置播放状态

JNIEXPORT void JNICALL
Java_com_ywl5320_openslaudio_MainActivity_playAudioByOpenSL_1assets(JNIEnv *env, jobject instance, jobject assetManager, jstring filename) {

release();
const char *utf8 = (*env)->GetStringUTFChars(env, filename, NULL);

// use asset manager to open asset by filename
AAssetManager* mgr = AAssetManager_fromJava(env, assetManager);
AAsset* asset = AAssetManager_open(mgr, utf8, AASSET_MODE_UNKNOWN);
(*env)->ReleaseStringUTFChars(env, filename, utf8);

// open asset as file descriptor
off_t start, length;
int fd = AAsset_openFileDescriptor(asset, &start, &length);
AAsset_close(asset);

SLresult result;

//第一步，创建引擎
createEngine();

//第二步，创建混音器
const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
(void)result;
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
(void)result;
result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
if (SL_RESULT_SUCCESS == result) {
result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(outputMixEnvironmentalReverb, &reverbSettings);
(void)result;
}
//第三步，设置播放器参数和创建播放器
// 1、 配置 audio source
SLDataLocator_AndroidFD loc_fd = {SL_DATALOCATOR_ANDROIDFD, fd, start, length};
SLDataFormat_MIME format_mime = {SL_DATAFORMAT_MIME, NULL, SL_CONTAINERTYPE_UNSPECIFIED};
SLDataSource audioSrc = {&loc_fd, &format_mime};

// 2、 配置 audio sink
SLDataLocator_OutputMix loc_outmix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};
SLDataSink audioSnk = {&loc_outmix, NULL};

// 创建播放器
const SLInterfaceID ids[3] = {SL_IID_SEEK, SL_IID_MUTESOLO, SL_IID_VOLUME};
const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
result = (*engineEngine)->CreateAudioPlayer(engineEngine, &fdPlayerObject, &audioSrc, &audioSnk, 3, ids, req);
(void)result;

// 实现播放器
result = (*fdPlayerObject)->Realize(fdPlayerObject, SL_BOOLEAN_FALSE);
(void)result;

// 得到播放器接口
result = (*fdPlayerObject)->GetInterface(fdPlayerObject, SL_IID_PLAY, &fdPlayerPlay);
(void)result;

// 得到声音控制接口
result = (*fdPlayerObject)->GetInterface(fdPlayerObject, SL_IID_VOLUME, &fdPlayerVolume);
(void)result;

//第四步，设置播放状态
if (NULL != fdPlayerPlay) {

result = (*fdPlayerPlay)->SetPlayState(fdPlayerPlay, SL_PLAYSTATE_PLAYING);
(void)result;
}

//设置播放音量 （100 * -50：静音 ）
(*fdPlayerVolume)->SetVolumeLevel(fdPlayerVolume, 20 * -50);

}

4.2、播放Uri连接

第一步：创建引擎
第二步：创建混音器
第三步：创建播放器
第四步：设置播放状态

JNIEXPORT void JNICALL
Java_com_ywl5320_openslaudio_MainActivity_playAudioByOpenSL_1uri(JNIEnv *env, jobject instance, jstring uri) {
SLresult result;
release();
// convert Java string to UTF-8
const char *utf8 = (*env)->GetStringUTFChars(env, uri, NULL);

//第一步，创建引擎
createEngine();

//第二步，创建混音器
const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
(void)result;
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
(void)result;
result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
if (SL_RESULT_SUCCESS == result) {
result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(
outputMixEnvironmentalReverb, &reverbSettings);
(void)result;
}

//第三步，设置播放器参数和创建播放器
// configure audio source
// (requires the INTERNET permission depending on the uri parameter)
SLDataLocator_URI loc_uri = {SL_DATALOCATOR_URI, (SLchar *) utf8};
SLDataFormat_MIME format_mime = {SL_DATAFORMAT_MIME, NULL, SL_CONTAINERTYPE_UNSPECIFIED};
SLDataSource audioSrc = {&loc_uri, &format_mime};

// configure audio sink
SLDataLocator_OutputMix loc_outmix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};
SLDataSink audioSnk = {&loc_outmix, NULL};

// create audio player
const SLInterfaceID ids[3] = {SL_IID_SEEK, SL_IID_MUTESOLO, SL_IID_VOLUME};
const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};
result = (*engineEngine)->CreateAudioPlayer(engineEngine, &uriPlayerObject, &audioSrc, &audioSnk, 3, ids, req);

(void)result;

// release the Java string and UTF-8
(*env)->ReleaseStringUTFChars(env, uri, utf8);

// realize the player
result = (*uriPlayerObject)->Realize(uriPlayerObject, SL_BOOLEAN_FALSE);
// this will always succeed on Android, but we check result for portability to other platforms
if (SL_RESULT_SUCCESS != result) {
(*uriPlayerObject)->Destroy(uriPlayerObject);
uriPlayerObject = NULL;
return;
}

// get the play interface
result = (*uriPlayerObject)->GetInterface(uriPlayerObject, SL_IID_PLAY, &uriPlayerPlay);
(void)result;

// get the volume interface
result = (*uriPlayerObject)->GetInterface(uriPlayerObject, SL_IID_VOLUME, &uriPlayerVolume);
(void)result;
//第四步，设置播放状态
if (NULL != uriPlayerPlay) {

// set the player's state
result = (*uriPlayerPlay)->SetPlayState(uriPlayerPlay, SL_PLAYSTATE_PLAYING);
(void)result;
}

//设置播放音量 （100 * -50：静音 ）
//    (*uriPlayerVolume)->SetVolumeLevel(uriPlayerVolume, 0 * -50);
}

4.3、播放pcm文件（集成到ffmpeg时，也是播放ffmpeg转换成的pcm格式的数据），这里为了模拟是直接读取的pcm格式的音频文件。

4.3.1、创建播放器和混音器

//第一步，创建引擎
createEngine();

//第二步，创建混音器
const SLInterfaceID mids[1] = {SL_IID_ENVIRONMENTALREVERB};
const SLboolean mreq[1] = {SL_BOOLEAN_FALSE};
result = (*engineEngine)->CreateOutputMix(engineEngine, &outputMixObject, 1, mids, mreq);
(void)result;
result = (*outputMixObject)->Realize(outputMixObject, SL_BOOLEAN_FALSE);
(void)result;
result = (*outputMixObject)->GetInterface(outputMixObject, SL_IID_ENVIRONMENTALREVERB, &outputMixEnvironmentalReverb);
if (SL_RESULT_SUCCESS == result) {
result = (*outputMixEnvironmentalReverb)->SetEnvironmentalReverbProperties(
outputMixEnvironmentalReverb, &reverbSettings);
(void)result;
}
SLDataLocator_OutputMix outputMix = {SL_DATALOCATOR_OUTPUTMIX, outputMixObject};
SLDataSink audioSnk = {&outputMix, NULL};

4.3.2、设置pcm格式的频率位数等信息并创建播放器

// 第三步，配置PCM格式信息
SLDataLocator_AndroidSimpleBufferQueue android_queue={SL_DATALOCATOR_ANDROIDSIMPLEBUFFERQUEUE,2};
SLDataFormat_PCM pcm={
SL_DATAFORMAT_PCM,//播放pcm格式的数据
2,//2个声道（立体声）
SL_SAMPLINGRATE_44_1,//44100hz的频率
SL_PCMSAMPLEFORMAT_FIXED_16,//位数 16位
SL_PCMSAMPLEFORMAT_FIXED_16,//和位数一致就行
SL_SPEAKER_FRONT_LEFT | SL_SPEAKER_FRONT_RIGHT,//立体声（前左前右）
SL_BYTEORDER_LITTLEENDIAN//结束标志
};
SLDataSource slDataSource = {&android_queue, &pcm};

const SLInterfaceID ids[3] = {SL_IID_BUFFERQUEUE, SL_IID_EFFECTSEND, SL_IID_VOLUME};
const SLboolean req[3] = {SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE, SL_BOOLEAN_TRUE};

result = (*engineEngine)->CreateAudioPlayer(engineEngine, &pcmPlayerObject, &slDataSource, &audioSnk, 3, ids, req);
//初始化播放器
(*pcmPlayerObject)->Realize(pcmPlayerObject, SL_BOOLEAN_FALSE);

//    得到接口后调用  获取Player接口
(*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_PLAY, &pcmPlayerPlay);

4.3.3、设置缓冲队列和回调函数

//    注册回调缓冲区 获取缓冲队列接口
(*pcmPlayerObject)->GetInterface(pcmPlayerObject, SL_IID_BUFFERQUEUE, &pcmBufferQueue);
//缓冲接口回调
(*pcmBufferQueue)->RegisterCallback(pcmBufferQueue, pcmBufferCallBack, NULL);

回调函数：

void * pcmBufferCallBack(SLAndroidBufferQueueItf bf, void * context)
{
//assert(NULL == context);
getPcmData(&buffer);
// for streaming playback, replace this test by logic to find and fill the next buffer
if (NULL != buffer) {
SLresult result;
// enqueue another buffer
result = (*pcmBufferQueue)->Enqueue(pcmBufferQueue, buffer, 44100 * 2 * 2);
// the most likely other result is SL_RESULT_BUFFER_INSUFFICIENT,
// which for this code example would indicate a programming error
}
}

读取pcm格式的文件：

void getPcmData(void **pcm)
{
while(!feof(pcmFile))
{
fread(out_buffer, 44100 * 2 * 2, 1, pcmFile);
if(out_buffer == NULL)
{
LOGI("%s", "read end");
break;
} else{
LOGI("%s", "reading");
}
*pcm = out_buffer;
break;
}
}

4.3.4、设置播放状态并手动开始调用回调函数

//    获取播放状态接口
(*pcmPlayerPlay)->SetPlayState(pcmPlayerPlay, SL_PLAYSTATE_PLAYING);

//    主动调用回调函数开始工作
pcmBufferCallBack(pcmBufferQueue, NULL);

        注意：在回调函数中result = (*pcmBufferQueue)->Enqueue(pcmBufferQueue, buffer, 44100 * 2 * 2);，最后的“44100*2*2”是buffer的大小，因为我这里是指定了没读取一次就从pcm文件中读取了“44100*2*2”个字节，所以可以正常播放，如果是利用ffmpeg来获取pcm数据源，那么实际大小要根据没个AVframe的具体大小来定，这样才能正常播放出声音！！！（44100
* 2 * 2 表示：44100是频率HZ，2是立体声双通道，2是采用的16位采样即2个字节，所以总的字节数就是：44100 * 2 * 2）

        好的，以上就是android利用OpenSL ES来播放音频的过程，不知各位看官是否明白了，文末提供源码和音频文件的下载。好了，开始双十一的枪枪抢吧~

GitHub：Android_OpenSl_Audio