首先上层java调用
XXXPlayer
AudioManager audiomanage = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
audiomanager就是我们定义的控制系统声音的对象，（如果context报错，可将其改成XXXPlayer.this）
audiomanager.SetStreamVolume(AA,BB,CC),是我们可以直接使用的AudioManager的成员函数，3个参数表示的意思：AA：有内置的常量,可以在AudioManager里面查到相关的定义，我们在此用AudioManager.STREAM_MUSIC， BB：自己设置音量的值，CC：也是一些标示量，我在此设置为0；
1.AudioManager.java
public void setStreamVolume(int streamType, int index, int flags);上层接口
1)调用IAudioService service = getService(); 当程序开启时会获得service,调用此来获得
2.执行ServiceManager.java
public static IBinder getService(String name)获取audio服务
3.AudioService.java
public void setStreamVolume(int streamType, int index, int flags)//服务接口
1) private void setStreamVolumeInt(int streamType, int index, boolean force, boolean lastAudible)//服务函数
2)调用以下函数
sendMsg(mAudioHandler, MSG_SET_SYSTEM_VOLUME, streamType, SENDMSG_NOOP, 0, 0,streamState, 0)
//Post message to set system volume (it in turn will post a message
// to persist)
3)AudioHandler::setSystemVolume(VolumeStreamState streamState);//sendmsg(...)后执行函数
4)调用AudioHandler::setStreamVolumeIndex(int stream, int index)
5)AudioSystem.setStreamVolumeIndex(stream,index);//audioSystem接口
static int android_media_AudioSystem_setStreamVolumeIndex(JNIEnv *env, jobject thiz, jint stream, jint index)
1)调用AudioSystem::setStreamVolumeIndex
6.status_t AudioSystem::setStreamVolumeIndex(stream_type stream, int index)（处理到这时，也可以直接走AudioFlinger路线，不经过策略）
1)获得服务 const sp<IAudioPolicyService>& aps = AudioSystem::get_audio_policy_service();
2)调用aps->setStreamVolumeIndex(stream, index)
7.status_t AudioPolicyService::setStreamVolumeIndex(AudioSystem::stream_type stream, int index)
1)调用mpPolicyManager->setStreamVolumeIndex(stream, index)
status_t AudioPolicyManager::setStreamVolumeIndex(AudioSystem::stream_type stream, int index)
1)记录音量index: mStreams[stream].mIndexCur = index
2)compute and apply stream volume on all outputs:
checkAndSetVolume(stream, index, mOutputs.keyAt(i), mOutputs.valueAt(i)->device())
8.status_t AudioPolicyManager::checkAndSetVolume(int stream, int index, audio_io_handle_t output, uint32_t device, int delayMs, bool force)
1)计算音量：float volume = computeVolume(stream, index, output, device);
2)调用：mpClientInterface->setStreamVolume((AudioSystem::stream_type)stream, volume, output, delayMs);
9.status_t AudioPolicyService::setStreamVolume(AudioSystem::stream_type stream, float volume, audio_io_handle_t output, int delayMs)
调用mAudioCommandThread->volumeCommand((int)stream, volume, (int)output, delayMs);
10.status_t AudioPolicyService::AudioCommandThread::volumeCommand(int stream, float volume, int output, int delayMs)
调用insertCommand_l(command, delayMs);
