走进MediaPlayer，轻松操控Android10.0的音频音量

不知不觉就2020年了，不知不觉Android就到了10.0了，不知不觉又TM好久没更了，不知不觉新冠病毒还是遥遥无期，考虑了下后续还是基于10.0来继续分析吧。
MediaPlayer的java层API的使用以及说明网上还是很多的，如果深究其原理呢，会发现最终都调了native XXX方法，其实这就调到了jni

static{   
        System.loadLibrary("media_jni");native_init();}

这里调到了media_jin,在frameworks/base/media/jni/android_media_MediaPlayer.cpp 中

staticconst JNINativeMethod gMethods[]={{"nativeSetDataSource","(Landroid/os/IBinder;Ljava/lang/String;[Ljava/lang/String;""[Ljava/lang/String;)V",(void*)android_media_MediaPlayer_setDataSourceAndHeaders
    },{"_setDataSource","(Ljava/io/FileDescriptor;JJ)V",(void*)android_media_MediaPlayer_setDataSourceFD},{"_setDataSource","(Landroid/media/MediaDataSource;)V",(void*)android_media_MediaPlayer_setDataSourceCallback },{"_setVideoSurface","(Landroid/view/Surface;)V",(void*)android_media_MediaPlayer_setVideoSurface},{"_prepare","()V",(void*)android_media_MediaPlayer_prepare},{"prepareAsync","()V",(void*)android_media_MediaPlayer_prepareAsync},{"_start","()V",(void*)android_media_MediaPlayer_start},{"_stop","()V",(void*)android_media_MediaPlayer_stop},{"getVideoWidth","()I",(void*)android_media_MediaPlayer_getVideoWidth},{"getVideoHeight","()I",(void*)android_media_MediaPlayer_getVideoHeight},{"native_getMetrics","()Landroid/os/PersistableBundle;",(void*)android_media_MediaPlayer_native_getMetrics},{"setPlaybackParams","(Landroid/media/PlaybackParams;)V",(void*)android_media_MediaPlayer_setPlaybackParams},{"getPlaybackParams","()Landroid/media/PlaybackParams;",(void*)android_media_MediaPlayer_getPlaybackParams},{"setSyncParams","(Landroid/media/SyncParams;)V",(void*)android_media_MediaPlayer_setSyncParams},{"getSyncParams","()Landroid/media/SyncParams;",(void*)android_media_MediaPlayer_getSyncParams},{"_seekTo","(JI)V",(void*)android_media_MediaPlayer_seekTo},{"_notifyAt","(J)V",(void*)android_media_MediaPlayer_notifyAt},{"_pause","()V",(void*)android_media_MediaPlayer_pause},{"isPlaying","()Z",(void*)android_media_MediaPlayer_isPlaying},{"getCurrentPosition","()I",(void*)android_media_MediaPlayer_getCurrentPosition},{"getDuration","()I",(void*)android_media_MediaPlayer_getDuration},{"_release","()V",(void*)android_media_MediaPlayer_release},{"_reset","()V",(void*)android_media_MediaPlayer_reset},{"_setAudioStreamType","(I)V",(void*)android_media_MediaPlayer_setAudioStreamType},{"_getAudioStreamType","()I",(void*)android_media_MediaPlayer_getAudioStreamType},{"setParameter","(ILandroid/os/Parcel;)Z",(void*)android_media_MediaPlayer_setParameter},{"setLooping","(Z)V",(void*)android_media_MediaPlayer_setLooping},{"isLooping","()Z",(void*)android_media_MediaPlayer_isLooping},{"_setVolume","(FF)V",(void*)android_media_MediaPlayer_setVolume},{"native_invoke","(Landroid/os/Parcel;Landroid/os/Parcel;)I",(void*)android_media_MediaPlayer_invoke},{"native_setMetadataFilter","(Landroid/os/Parcel;)I",(void*)android_media_MediaPlayer_setMetadataFilter},{"native_getMetadata","(ZZLandroid/os/Parcel;)Z",(void*)android_media_MediaPlayer_getMetadata},{"native_init","()V",(void*)android_media_MediaPlayer_native_init},{"native_setup","(Ljava/lang/Object;)V",(void*)android_media_MediaPlayer_native_setup},{"native_finalize","()V",(void*)android_media_MediaPlayer_native_finalize},{"getAudioSessionId","()I",(void*)android_media_MediaPlayer_get_audio_session_id},{"setAudioSessionId","(I)V",(void*)android_media_MediaPlayer_set_audio_session_id},{"_setAuxEffectSendLevel","(F)V",(void*)android_media_MediaPlayer_setAuxEffectSendLevel},{"attachAuxEffect","(I)V",(void*)android_media_MediaPlayer_attachAuxEffect},{"native_pullBatteryData","(Landroid/os/Parcel;)I",(void*)android_media_MediaPlayer_pullBatteryData},{"native_setRetransmitEndpoint","(Ljava/lang/String;I)I",(void*)android_media_MediaPlayer_setRetransmitEndpoint},{"setNextMediaPlayer","(Landroid/media/MediaPlayer;)V",(void*)android_media_MediaPlayer_setNextMediaPlayer},{"native_applyVolumeShaper","(Landroid/media/VolumeShaper$Configuration;Landroid/media/VolumeShaper$Operation;)I",(void*)android_media_MediaPlayer_applyVolumeShaper},{"native_getVolumeShaperState","(I)Landroid/media/VolumeShaper$State;",(void*)android_media_MediaPlayer_getVolumeShaperState},// Modular DRM{"_prepareDrm","([B[B)V",(void*)android_media_MediaPlayer_prepareDrm },{"_releaseDrm","()V",(void*)android_media_MediaPlayer_releaseDrm },// AudioRouting{"native_setOutputDevice","(I)Z",(void*)android_media_MediaPlayer_setOutputDevice},{"native_getRoutedDeviceId","()I",(void*)android_media_MediaPlayer_getRoutedDeviceId},{"native_enableDeviceCallback","(Z)V",(void*)android_media_MediaPlayer_enableDeviceCallback},};

这里我列了所有java调下来的native方法。我们简单分析一个native_init对应到jin里是android_media_Mediaplayer_native_init

staticvoidandroid_media_MediaPlayer_native_init(JNIEnv *env){
    jclass clazz;
    clazz = env->FindClass("android/media/MediaPlayer");if(clazz ==NULL){return;}//指向Java层的mNativeContext的变量
    fields.context = env->GetFieldID(clazz,"mNativeContext","J");if(fields.context ==NULL){return;}//主要处理native向java层回调callabck的
    fields.post_event = env->GetStaticMethodID(clazz,"postEventFromNative","(Ljava/lang/Object;IIILjava/lang/Object;)V");if(fields.post_event ==NULL){return;}
    fields.surface_texture = env->GetFieldID(clazz,"mNativeSurfaceTexture","J");if(fields.surface_texture ==NULL){return;}
    env->DeleteLocalRef(clazz);
    clazz = env->FindClass("android/net/ProxyInfo");if(clazz ==NULL){return;}
    fields.proxyConfigGetHost =
        env->GetMethodID(clazz,"getHost","()Ljava/lang/String;");
    fields.proxyConfigGetPort =
        env->GetMethodID(clazz,"getPort","()I");
    fields.proxyConfigGetExclusionList =
        env->GetMethodID(clazz,"getExclusionListAsString","()Ljava/lang/String;");
    env->DeleteLocalRef(clazz);// Modular DRMFIND_CLASS(clazz,"android/media/MediaDrm$MediaDrmStateException");if(clazz){GET_METHOD_ID(gStateExceptionFields.init, clazz,"<init>","(ILjava/lang/String;)V");
        gStateExceptionFields.classId =static_cast<jclass>(env->NewGlobalRef(clazz));
        env->DeleteLocalRef(clazz);}else{ALOGE("JNI android_media_MediaPlayer_native_init couldn't ""get clazz android/media/MediaDrm$MediaDrmStateException");}
    gPlaybackParamsFields.init(env);
    gSyncParamsFields.init(env);
    gVolumeShaperFields.init(env);}

获取一些java层的变量以及方法，这里之分析了fields.context和fields.post_event，一个指向java层的变量一个指向Java层的函数。后面的在使用到时在具体在分析。
还记得在new MediaPlayer创建对象时，调用了native_setup。

staticvoidandroid_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this){ALOGV("native_setup");// 创建了native层的mediaplayer
    sp<MediaPlayer> mp =newMediaPlayer();if(mp ==NULL){jniThrowException(env,"java/lang/RuntimeException","Out of memory");return;}// create new listener and give it to MediaPlayer// 创建了native层的listener并且注册到了mediaplayer中
    sp<JNIMediaPlayerListener> listener =newJNIMediaPlayerListener(env, thiz, weak_this);
    mp->setListener(listener);// Stow our new C++ MediaPlayer in an opaque field in the Java object.setMediaPlayer(env, thiz, mp);}

继续分析这个setMediaPlayer

static sp<MediaPlayer>setMediaPlayer(JNIEnv* env, jobject thiz,const sp<MediaPlayer>& player){
    Mutex::Autolock l(sLock);//fields.context在native_init的时候指向了Java层的mNativeContex
    sp<MediaPlayer> old =(MediaPlayer*)env->GetLongField(thiz, fields.context);if(player.get()){
        player->incStrong((void*)setMediaPlayer);}if(old !=0){
        old->decStrong((void*)setMediaPlayer);}
    env->SetLongField(thiz, fields.context,(jlong)player.get());return old;}

这个方法的作用就时把创建的native Mediaplayer赋值给到java层的MediaPlayer的mNativeContex了
最后再看下JNIMediaPlayerListener，其实就一个回调notify

void JNIMediaPlayerListener::notify(int msg,int ext1,int ext2,const Parcel *obj){
    JNIEnv *env = AndroidRuntime::getJNIEnv();if(obj && obj->dataSize()>0){
        jobject jParcel =createJavaParcelObject(env);if(jParcel !=NULL){
            Parcel* nativeParcel =parcelForJavaObject(env, jParcel);
            nativeParcel->setData(obj->data(), obj->dataSize());//fields.post_event对应java中 postEventFromNative
            env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                    msg, ext1, ext2, jParcel);
            env->DeleteLocalRef(jParcel);}}else{
        env->CallStaticVoidMethod(mClass, fields.post_event, mObject,
                msg, ext1, ext2,NULL);}if(env->ExceptionCheck()){ALOGW("An exception occurred while notifying an event.");LOGW_EX(env);
        env->ExceptionClear();}}

也就是native层的callback最终会通过postEventFromNative返回给java层，好了，我们在看下刚才创建的native的nediaplayer，代码位于/frameworks/av/media/libmedia/mediaplayer.cpp ，

MediaPlayer::MediaPlayer(){ALOGV("constructor");
    mListener =NULL;
    mCookie =NULL;
    mStreamType = AUDIO_STREAM_MUSIC;
    mAudioAttributesParcel =NULL;
    mCurrentPosition =-1;
    mCurrentSeekMode = MediaPlayerSeekMode::SEEK_PREVIOUS_SYNC;
    mSeekPosition =-1;
    mSeekMode = MediaPlayerSeekMode::SEEK_PREVIOUS_SYNC;
    mCurrentState = MEDIA_PLAYER_IDLE;
    mPrepareSync =false;
    mPrepareStatus = NO_ERROR;
    mLoop =false;
    mLeftVolume = mRightVolume =1.0;
    mVideoWidth = mVideoHeight =0;
    mLockThreadId =0;
    mAudioSessionId =(audio_session_t) AudioSystem::newAudioUniqueId(AUDIO_UNIQUE_ID_USE_SESSION);
    AudioSystem::acquireAudioSessionId(mAudioSessionId,-1);
    mSendLevel =0;
    mRetransmitEndpointValid =false;}

一堆参数的初始化，这里不一一说明了，等用到时在具体细说。
在看下刚才的setListener

status_t MediaPlayer::setListener(const sp<MediaPlayerListener>& listener){ALOGV("setListener");
    Mutex::Autolock _l(mLock);
    mListener = listener;return NO_ERROR;}

也是非常简单，这里mListener就赋值了。

总结

从java层创建MediaPlayer开始主要做了几件事：
1.java层调到jni并创建native层的mediaplayer
2.把native层的mediaplayer赋值给java层的mNativeContex
3.在native层注册listener并绑定到java层的postEventFromNative

下篇我们就从MediaPlayer的setDataSource开始，看看java层调用setDataSource后，native层都做了什么