Android10.0Auidio之MediaPlayer（二）

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我是靠谱客的博主称心钢笔，最近开发中收集的这篇文章主要介绍Android10.0Auidio之MediaPlayer（二），觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

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

static const 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

static void
android_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 DRM
FIND_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。

static void
android_media_MediaPlayer_native_setup(JNIEnv *env, jobject thiz, jobject weak_this)
{
ALOGV("native_setup");
// 创建了native层的mediaplayer
sp<MediaPlayer> mp = new MediaPlayer();
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 = new JNIMediaPlayerListener(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