Audio笔记之AudioTrack::write

public int write(byte[] audioData, int offsetInBytes, int sizeInBytes) {

        if (mState == STATE_UNINITIALIZED) {
            return ERROR_INVALID_OPERATION;
        }

        if ( (audioData == null) || (offsetInBytes < 0 ) || (sizeInBytes < 0)
                || (offsetInBytes + sizeInBytes < 0)    // detect integer overflow
                || (offsetInBytes + sizeInBytes > audioData.length)) {
            return ERROR_BAD_VALUE;
        }

        int ret = native_write_byte(audioData, offsetInBytes, sizeInBytes, mAudioFormat);

        if ((mDataLoadMode == MODE_STATIC)
                && (mState == STATE_NO_STATIC_DATA)
                && (ret > 0)) {
            // benign race with respect to other APIs that read mState
            mState = STATE_INITIALIZED;
        }

        return ret;
}

static jint android_media_AudioTrack_native_write_byte(JNIEnv *env,  jobject thiz,
                                                  jbyteArray javaAudioData,
                                                  jint offsetInBytes, jint sizeInBytes,
                                                  jint javaAudioFormat) {
    //ALOGV("android_media_AudioTrack_native_write_byte(offset=%d, sizeInBytes=%d) called",
    //    offsetInBytes, sizeInBytes);
    sp<AudioTrack> lpTrack = getAudioTrack(env, thiz);
    if (lpTrack == NULL) {
        jniThrowException(env, "java/lang/IllegalStateException",
            "Unable to retrieve AudioTrack pointer for write()");
        return 0;
    }

    // get the pointer for the audio data from the java array
    // NOTE: We may use GetPrimitiveArrayCritical() when the JNI implementation changes in such
    // a way that it becomes much more efficient. When doing so, we will have to prevent the
    // AudioSystem callback to be called while in critical section (in case of media server
    // process crash for instance)
    jbyte* cAudioData = NULL;
    if (javaAudioData) {
        cAudioData = (jbyte *)env->GetByteArrayElements(javaAudioData, NULL);
        if (cAudioData == NULL) {
            ALOGE("Error retrieving source of audio data to play, can't play");
            return 0; // out of memory or no data to load
        }
    } else {
        ALOGE("NULL java array of audio data to play, can't play");
        return 0;
    }

    jint written = writeToTrack(lpTrack, javaAudioFormat, cAudioData, offsetInBytes, sizeInBytes);

    env->ReleaseByteArrayElements(javaAudioData, cAudioData, 0);

    //ALOGV("write wrote %d (tried %d) bytes in the native AudioTrack with offset %d",
    //     (int)written, (int)(sizeInBytes), (int)offsetInBytes);
    return written;
}

jint writeToTrack(const sp<AudioTrack>& track, jint audioFormat, jbyte* data,
                  jint offsetInBytes, jint sizeInBytes) {
    // give the data to the native AudioTrack object (the data starts at the offset)
    ssize_t written = 0;
    // regular write() or copy the data to the AudioTrack's shared memory?
    if (track->sharedBuffer() == 0) {
        written = track->write(data + offsetInBytes, sizeInBytes);
        // for compatibility with earlier behavior of write(), return 0 in this case
        if (written == (ssize_t) WOULD_BLOCK) {
            written = 0;
        }
    } else {
        if (audioFormat == ENCODING_PCM_16BIT) {
            // writing to shared memory, check for capacity
            if ((size_t)sizeInBytes > track->sharedBuffer()->size()) {
                sizeInBytes = track->sharedBuffer()->size();
            }
            memcpy(track->sharedBuffer()->pointer(), data + offsetInBytes, sizeInBytes);
            written = sizeInBytes;
        } else if (audioFormat == ENCODING_PCM_8BIT) {
            // data contains 8bit data we need to expand to 16bit before copying
            // to the shared memory
            // writing to shared memory, check for capacity,
            // note that input data will occupy 2X the input space due to 8 to 16bit conversion
            if (((size_t)sizeInBytes)*2 > track->sharedBuffer()->size()) {
                sizeInBytes = track->sharedBuffer()->size() / 2;
            }
            int count = sizeInBytes;
            int16_t *dst = (int16_t *)track->sharedBuffer()->pointer();
            const int8_t *src = (const int8_t *)(data + offsetInBytes);
            while (count--) {
                *dst++ = (int16_t)(*src++^0x80) << 8;
            }
            // even though we wrote 2*sizeInBytes, we only report sizeInBytes as written to hide
            // the 8bit mixer restriction from the