Android audio recorder(草稿)

参考：

http://androidcommunity.com/forums/f4/recording-the-audio-from-mic-in-pcm-format-into-a-buffer-using-the-native-libraries-26690/

http://www.devdaily.com/java/jwarehouse/android/media/java/android/media/AudioRecord.java.shtml

http://www.wuzhaopai.com/html/mobile/android/2011101015289.html

http://blog.csdn.net/innost/article/details/6125779

http://www.cnblogs.com/innost/archive/2011/01/15/1936425.html

 最上层，在android java程序里使用了AudioRecord read 函数,

{
...
AudioRecord audioRecord;
short[] buffer = new short[minBufferSize];
int minBufferSize;

int res = audioRecord.read(buffer, 0, minBufferSize);
...
}

首先见识见识这个read的原型：

public int read(short[] audioData, int offsetInShorts, int sizeInShorts) {
        if (mState != STATE_INITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }
        
        if ( (audioData == null) || (offsetInShorts < 0 ) || (sizeInShorts < 0) 
                || (offsetInShorts + sizeInShorts > audioData.length)) {
            return ERROR_BAD_VALUE;
        }

        return native_read_in_short_array(audioData, offsetInShorts, sizeInShorts);//在这里调用native c++ read
    }

native c++ read 又会被重命名，看看他是怎么定义的

static JNINativeMethod gMethods[] = {
    // name,               signature,  funcPtr
    {"native_read_in_byte_array", 
                             "([BII)I", (void *)android_media_AudioRecord_readInByteArray},
    {"native_read_in_short_array",
                             "([SII)I", (void *)android_media_AudioRecord_readInShortArray} //看见没，他们改名换姓了
}；

//----------------------------------------------------------------------------------------
static jint android_media_AudioRecord_readInShortArray(JNIEnv *env,  jobject thiz,
                                                        jshortArray javaAudioData,
                                                        jint offsetInShorts, jint sizeInShorts) {

    return (android_media_AudioRecord_readInByteArray(env, thiz,
                                                        (jbyteArray) javaAudioData,
                                                        offsetInShorts*2, sizeInShorts*2)
            / 2);//都调用了 readInByteArry
}

很好奇吧，不妨先看看readInByteArray他长什么样：

static jint android_media_AudioRecord_readInByteArray(JNIEnv *env,  jobject thiz,
                                                        jbyteArray javaAudioData,
                                                        jint offsetInBytes, jint sizeInBytes) {
    jbyte* recordBuff = NULL;
    AudioRecord *lpRecorder = NULL;

    lpRecorder = 
            (AudioRecord *)env->GetIntField(thiz, javaAudioRecordFields.nativeRecorderInJavaObj);
    if (lpRecorder == NULL) {
        LOGE("Unable to retrieve AudioRecord object, can't record");
        return 0;
    }

    if (!javaAudioData) {
        LOGE("Invalid Java array to store recorded audio, can't record");
        return 0;
    }
 

    recordBuff = (jbyte *)env->GetByteArrayElements(javaAudioData, NULL); //jbyteArray 转成 c++中的BYTE[]

    if (recordBuff == NULL) {
        LOGE("Error retrieving destination for recorded audio data, can't record");
        return 0;
    }

    ssize_t recorderBuffSize = lpRecorder->frameCount()*lpRecorder->frameSize();
    ssize_t readSize = lpRecorder->read(recordBuff + offsetInBytes, 
                                        sizeInBytes > (jint)recorderBuffSize ? 
                                            (jint)recorderBuffSize : sizeInBytes );//这是表哥的核心秘密，他难以忘却read小姐，c++家族里的read小姐
    env->ReleaseByteArrayElements(javaAudioData, recordBuff, 0);

    return (jint) readSize;
}

 ssize_t readSize = lpRecorder->read(recordBuff + offsetInBytes, 
                                        sizeInBytes > (jint)recorderBuffSize ? 
                                            (jint)recorderBuffSize : sizeInBytes );

c++的read要繁文缛节的多，相比java的，请看：

ssize_t AudioRecord::read(void* buffer, size_t userSize)
{
    ssize_t read = 0;
    Buffer audioBuffer;
    int8_t *dst = static_cast<int8_t*>(buffer);

    if (ssize_t(userSize) < 0) {
        // sanity-check. user is most-likely passing an error code.
        LOGE("AudioRecord::read(buffer=%p, size=%u (%d)",
                buffer, userSize, userSize);
        return BAD_VALUE;
    }

    mLock.lock();
    // acquire a strong reference on the IAudioRecord and IMemory so that they cannot be destroyed
    // while we are accessing the cblk
    sp <IAudioRecord> audioRecord = mAudioRecord;
    sp <IMemory> iMem = mCblkMemory;
    mLock.unlock();

    do {

        audioBuffer.frameCount = userSize/frameSize();

        // By using a wait count corresponding to twice the timeout period in
        // obtainBuffer() we give a chance to recover once for a read timeout
        // (if media_server crashed for instance) before returning a length of
        // 0 bytes read to the client
        status_t err = obtainBuffer(&audioBuffer, ((2 * MAX_RUN_TIMEOUT_MS) / WAIT_PERIOD_MS));
        if (err < 0) {
            // out of buffers, return #bytes written
            if (err == status_t(NO_MORE_BUFFERS))
                break;
            if (err == status_t(TIMED_OUT))
                err = 0;
            return ssize_t(err);
        }

        size_t bytesRead = audioBuffer.size;
        memcpy(dst, audioBuffer.i8, bytesRead);

        dst += bytesRead;
        userSize -= bytesRead;
        read += bytesRead;

        releaseBuffer(&audioBuffer);
    } while (userSize);

    return read;
}

关于这个延时是怎么回事：

status_t AudioSystem::getOutputLatency(uint32_t* latency, int streamType)
{
    OutputDescriptor *outputDesc;
    audio_io_handle_t output;

    if (streamType == AUDIO_STREAM_DEFAULT) {
        streamType = AUDIO_STREAM_MUSIC;
    }

    output = getOutput((audio_stream_type_t)streamType);
    if (output == 0) {
        return PERMISSION_DENIED;
    }

    gLock.lock();
    outputDesc = AudioSystem::gOutputs.valueFor(output);
    if (outputDesc == 0) {
        gLock.unlock();
        const sp<IAudioFlinger>& af = AudioSystem::get_audio_flinger();
        if (af == 0) return PERMISSION_DENIED;
        *latency = af->latency(output);
    } else {
        *latency = outputDesc->latency;
        gLock.unlock();
    }