Android Multimedia框架总结（二十七）MediaCodec回顾

Android App 通过 MediaCodec Java API 获得的编解码器，实际上是由 StageFright 媒体框架提供。android.media.MediaCodec 调用 libmedia_jni.so 中 JNI native 函数，这些 JNI 函数再去调用 libstagefright.so 库获得 StageFright 框架中的编解码器。StageFright再调用OMX组件进行解码。这是之前梳理的流程。这次再读主要是搞明白他们的调用过程。

先看Java层代码：

mediaCodec = MediaCodec.createByCodecName(codecName);//创建Codec
MediaFormat mediaFormat = MediaFormat.createVideoFormat(MINE_TYPE, width, height);mediaFormat.setInteger(MediaFormat.KEY_BIT_RATE, bit);
mediaFormat.setInteger(MediaFormat.KEY_FRAME_RATE, fps);
mediaFormat.setInteger(MediaFormat.KEY_I_FRAME_INTERVAL, 1); // 关键帧间隔时间// 单位s
mediaFormat.setInteger(MediaFormat.KEY_COLOR_FORMAT,                                    MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420SemiPlanar);// mediaFormat.setInteger(MediaFormat.KEY_PROFILE,// CodecProfileLevel.AVCProfileBaseline);// mediaFormat.setInteger(MediaFormat.KEY_LEVEL,// CodecProfileLevel.AVCLevel52);mediaCodec.configure(mediaFormat, null, null,MediaCodec.CONFIGURE_FLAG_ENCODE);mediaCodec.start();

Java层：
createByName->New MediaCode()->setup(native声明方法)
JNI层：
1、映射（android_media_MediaCodec.cpp 中）

    { "native_setup", "(Ljava/lang/String;ZZ)V",(void *)android_media_MediaCodec_native_setup },

2、setup->android_media_MediaCodec_native_setup

static void android_media_MediaCodec_native_setup(JNIEnv *env, jobject thiz,jstring name, jboolean nameIsType, jboolean encoder) {if (name == NULL) {//这里可以学习，jni层如何抛出异常的。使用jniThrowExceptionjniThrowException(env, "java/lang/NullPointerException", NULL);return;}const char *tmp = env->GetStringUTFChars(name, NULL);if (tmp == NULL) {return;}sp<JMediaCodec> codec = new JMediaCodec(env, thiz, tmp, nameIsType, encoder);const status_t err = codec->initCheck();if (err == NAME_NOT_FOUND) {// fail and do not try again.jniThrowException(env, "java/lang/IllegalArgumentException",String8::format("Failed to initialize %s, error %#x", tmp, err));env->ReleaseStringUTFChars(name, tmp);return;} if (err == NO_MEMORY) {throwCodecException(env, err, ACTION_CODE_TRANSIENT,String8::format("Failed to initialize %s, error %#x", tmp, err));env->ReleaseStringUTFChars(name, tmp);return;} else if (err != OK) {// believed possible to try againjniThrowException(env, "java/io/IOException",String8::format("Failed to find matching codec %s, error %#x", tmp, err));env->ReleaseStringUTFChars(name, tmp);return;}env->ReleaseStringUTFChars(name, tmp);codec->registerSelf();setMediaCodec(env,thiz, codec);
}

3、重点是 sp codec = new JMediaCodec(env, thiz, tmp, nameIsType, encoder);
sp是智能指针中强指针（Strong Pointer），智能指针主要灵活管理内存相关。还有wp（Weak Pointer），弱引用指针对象，在、system\core\include\utils\StrongPointer.h 下，直接可以拷贝这套google的东西，运用到项目也是可以的。

4、看下构造函数，

JMediaCodec::JMediaCodec(JNIEnv *env, jobject thiz,const char *name, bool nameIsType, bool encoder): mClass(NULL),mObject(NULL) {jclass clazz = env->GetObjectClass(thiz);//获取classCHECK(clazz != NULL);mClass = (jclass)env->NewGlobalRef(clazz);//全局引用mObject = env->NewWeakGlobalRef(thiz);//弱全局引用cacheJavaObjects(env);mLooper = new ALooper;mLooper->setName("MediaCodec_looper");mLooper->start(false,      // runOnCallingThreadtrue,       // canCallJavaPRIORITY_FOREGROUND);if (nameIsType) {mCodec = MediaCodec::CreateByType(mLooper, name, encoder, &mInitStatus);} else {mCodec = MediaCodec::CreateByComponentName(mLooper, name, &mInitStatus);}CHECK((mCodec != NULL) != (mInitStatus != OK));
}

mCodec = MediaCodec::CreateByType(mLooper, name, encoder, &mInitStatus)，//这里的MedCodec就是stagefright中MediaCodec，位于 \frameworks\av\media\libstagefright\MediaCodec.cpp，隶属于libstagefright.so

5、进入MediaCodec.cpp中

// static
sp<MediaCodec> MediaCodec::CreateByType(const sp<ALooper> &looper, const char *mime, bool encoder, status_t *err, pid_t pid) {sp<MediaCodec> codec = new MediaCodec(looper, pid);const status_t ret = codec->init(mime, true /* nameIsType */, encoder);if (err != NULL) {*err = ret;}return ret == OK ? codec : NULL; // NULL deallocates codec.
}

6、构造

MediaCodec::MediaCodec(const sp<ALooper> &looper, pid_t pid): mState(UNINITIALIZED),mReleasedByResourceManager(false),mLooper(looper),mCodec(NULL),mReplyID(0),mFlags(0),mStickyError(OK),mSoftRenderer(NULL),mResourceManagerClient(new ResourceManagerClient(this)),mResourceManagerService(new ResourceManagerServiceProxy(pid)),mBatteryStatNotified(false),mIsVideo(false),mVideoWidth(0),mVideoHeight(0),mRotationDegrees(0),mDequeueInputTimeoutGeneration(0),mDequeueInputReplyID(0),mDequeueOutputTimeoutGeneration(0),mDequeueOutputReplyID(0),mHaveInputSurface(false),mHavePendingInputBuffers(false) {
}

这里有点意思，直接把MediaCodec.h中头文件中变量进行初始化，注意：（冒号），看MediaCodec.h中，第一句就是struct MediaCodec : public AHandler，也就是说MediaCodec是一个结构体，这个结构体继承AHandler（也是一个结构体），AHandler继承utils/RefBase.h，为什么大量使用结构体，而不是class？

1、结构体在堆栈中创建，是值类型，速度回比较快
2、也考虑到和C兼容，C也有结构体。
3、通常用来处理作为基类型对待的小对象
4、c++ 里面结构体是可以继承的（纠正网上很多说结构体，不能继承的说法，这里源码也可以得到印证，结构体不仅能继承其他，也能被其他继承）

struct 与class本质上应该是相同的，只是默认的访问权限不同（struct默认是public，class默认是private ），网上也有人说结构体不能定义虚函数，这种说法也不对，如下代码也是MediaCodec.h中的，不仅可以定义虚函数，还可以定义友元。

protected:virtual ~MediaCodec();virtual void onMessageReceived(const sp<AMessage> &msg);private:// used by ResourceManagerClientstatus_t reclaim(bool force = false);friend struct ResourceManagerClient;

最后疑惑点

在阅读时，还发现有NdkMediaCodec及NdkMediaCodec.cpp这些个class, 和上面几个class的区别是什么？有什么关系？为什么要这么设计？
frameworks\av\include\ndk\NdkMediaCodec.h

/** This file defines an NDK API.* Do not remove methods.* Do not change method signatures.* Do not change the value of constants.* Do not change the size of any of the classes defined in here.* Do not reference types that are not part of the NDK.* Do not #include files that aren't part of the NDK.*/#ifndef _NDK_MEDIA_CODEC_H
#define _NDK_MEDIA_CODEC_H#include <android/native_window.h>#include "NdkMediaCrypto.h"
#include "NdkMediaError.h"
#include "NdkMediaFormat.h"#ifdef __cplusplus
extern "C" {
#endifstruct AMediaCodec;
typedef struct AMediaCodec AMediaCodec;struct AMediaCodecBufferInfo {int32_t offset;int32_t size;int64_t presentationTimeUs;uint32_t flags;
};
typedef struct AMediaCodecBufferInfo AMediaCodecBufferInfo;
typedef struct AMediaCodecCryptoInfo AMediaCodecCryptoInfo;enum {AMEDIACODEC_BUFFER_FLAG_END_OF_STREAM = 4,AMEDIACODEC_CONFIGURE_FLAG_ENCODE = 1,AMEDIACODEC_INFO_OUTPUT_BUFFERS_CHANGED = -3,AMEDIACODEC_INFO_OUTPUT_FORMAT_CHANGED = -2,AMEDIACODEC_INFO_TRY_AGAIN_LATER = -1
};/*** Create codec by name. Use this if you know the exact codec you want to use.* When configuring, you will need to specify whether to use the codec as an* encoder or decoder.*/
AMediaCodec* AMediaCodec_createCodecByName(const char *name);/*** Create codec by mime type. Most applications will use this, specifying a* mime type obtained from media extractor.*/
AMediaCodec* AMediaCodec_createDecoderByType(const char *mime_type);/*** Create encoder by name.*/
AMediaCodec* AMediaCodec_createEncoderByType(const char *mime_type);/*** delete the codec and free its resources*/
media_status_t AMediaCodec_delete(AMediaCodec*);/*** Configure the codec. For decoding you would typically get the format from an extractor.*/
media_status_t AMediaCodec_configure(AMediaCodec*,const AMediaFormat* format,ANativeWindow* surface,AMediaCrypto *crypto,uint32_t flags);/*** Start the codec. A codec must be configured before it can be started, and must be started* before buffers can be sent to it.*/
media_status_t AMediaCodec_start(AMediaCodec*);/*** Stop the codec.*/
media_status_t AMediaCodec_stop(AMediaCodec*);/** Flush the codec's input and output. All indices previously returned from calls to* AMediaCodec_dequeueInputBuffer and AMediaCodec_dequeueOutputBuffer become invalid.*/
media_status_t AMediaCodec_flush(AMediaCodec*);/*** Get an input buffer. The specified buffer index must have been previously obtained from* dequeueInputBuffer, and not yet queued.*/
uint8_t* AMediaCodec_getInputBuffer(AMediaCodec*, size_t idx, size_t *out_size);/*** Get an output buffer. The specified buffer index must have been previously obtained from* dequeueOutputBuffer, and not yet queued.*/
uint8_t* AMediaCodec_getOutputBuffer(AMediaCodec*, size_t idx, size_t *out_size);/*** Get the index of the next available input buffer. An app will typically use this with* getInputBuffer() to get a pointer to the buffer, then copy the data to be encoded or decoded* into the buffer before passing it to the codec.*/
ssize_t AMediaCodec_dequeueInputBuffer(AMediaCodec*, int64_t timeoutUs);/*** Send the specified buffer to the codec for processing.*/
media_status_t AMediaCodec_queueInputBuffer(AMediaCodec*,size_t idx, off_t offset, size_t size, uint64_t time, uint32_t flags);/*** Send the specified buffer to the codec for processing.*/
media_status_t AMediaCodec_queueSecureInputBuffer(AMediaCodec*,size_t idx, off_t offset, AMediaCodecCryptoInfo*, uint64_t time, uint32_t flags);/*** Get the index of the next available buffer of processed data.*/
ssize_t AMediaCodec_dequeueOutputBuffer(AMediaCodec*, AMediaCodecBufferInfo *info,int64_t timeoutUs);
AMediaFormat* AMediaCodec_getOutputFormat(AMediaCodec*);/*** If you are done with a buffer, use this call to return the buffer to* the codec. If you previously specified a surface when configuring this* video decoder you can optionally render the buffer.*/
media_status_t AMediaCodec_releaseOutputBuffer(AMediaCodec*, size_t idx, bool render);/*** If you are done with a buffer, use this call to update its surface timestamp* and return it to the codec to render it on the output surface. If you* have not specified an output surface when configuring this video codec,* this call will simply return the buffer to the codec.** For more details, see the Java documentation for MediaCodec.releaseOutputBuffer.*/
media_status_t AMediaCodec_releaseOutputBufferAtTime(AMediaCodec *mData, size_t idx, int64_t timestampNs);typedef enum {AMEDIACODECRYPTOINFO_MODE_CLEAR = 0,AMEDIACODECRYPTOINFO_MODE_AES_CTR = 1
} cryptoinfo_mode_t;/*** Create an AMediaCodecCryptoInfo from scratch. Use this if you need to use custom* crypto info, rather than one obtained from AMediaExtractor.** AMediaCodecCryptoInfo describes the structure of an (at least* partially) encrypted input sample.* A buffer's data is considered to be partitioned into "subsamples",* each subsample starts with a (potentially empty) run of plain,* unencrypted bytes followed by a (also potentially empty) run of* encrypted bytes.* numBytesOfClearData can be null to indicate that all data is encrypted.* This information encapsulates per-sample metadata as outlined in* ISO/IEC FDIS 23001-7:2011 "Common encryption in ISO base media file format files".*/
AMediaCodecCryptoInfo *AMediaCodecCryptoInfo_new(int numsubsamples,uint8_t key[16],uint8_t iv[16],cryptoinfo_mode_t mode,size_t *clearbytes,size_t *encryptedbytes);/*** delete an AMediaCodecCryptoInfo created previously with AMediaCodecCryptoInfo_new, or* obtained from AMediaExtractor*/
media_status_t AMediaCodecCryptoInfo_delete(AMediaCodecCryptoInfo*);/*** The number of subsamples that make up the buffer's contents.*/
size_t AMediaCodecCryptoInfo_getNumSubSamples(AMediaCodecCryptoInfo*);/*** A 16-byte opaque key*/
media_status_t AMediaCodecCryptoInfo_getKey(AMediaCodecCryptoInfo*, uint8_t *dst);/*** A 16-byte initialization vector*/
media_status_t AMediaCodecCryptoInfo_getIV(AMediaCodecCryptoInfo*, uint8_t *dst);/*** The type of encryption that has been applied,* one of AMEDIACODECRYPTOINFO_MODE_CLEAR or AMEDIACODECRYPTOINFO_MODE_AES_CTR.*/
cryptoinfo_mode_t AMediaCodecCryptoInfo_getMode(AMediaCodecCryptoInfo*);/*** The number of leading unencrypted bytes in each subsample.*/
media_status_t AMediaCodecCryptoInfo_getClearBytes(AMediaCodecCryptoInfo*, size_t *dst);/*** The number of trailing encrypted bytes in each subsample.*/
media_status_t AMediaCodecCryptoInfo_getEncryptedBytes(AMediaCodecCryptoInfo*, size_t *dst);#ifdef __cplusplus
} // extern "C"
#endif#endif //_NDK_MEDIA_CODEC_H

在AMediaCodec中，有一个createAMediaCodec，如下：

static AMediaCodec * createAMediaCodec(const char *name, bool name_is_type, bool encoder) {AMediaCodec *mData = new AMediaCodec();mData->mLooper = new ALooper;mData->mLooper->setName("NDK MediaCodec_looper");status_t ret = mData->mLooper->start(false,      // runOnCallingThreadtrue,       // canCallJava XXXPRIORITY_FOREGROUND);if (name_is_type) {mData->mCodec = android::MediaCodec::CreateByType(mData->mLooper, name, encoder);} else {mData->mCodec = android::MediaCodec::CreateByComponentName(mData->mLooper, name);}if (mData->mCodec == NULL) {  // failed to create codecAMediaCodec_delete(mData);return NULL;}mData->mHandler = new CodecHandler(mData);mData->mLooper->registerHandler(mData->mHandler);mData->mGeneration = 1;mData->mRequestedActivityNotification = false;mData->mCallback = NULL;return mData;
}

其中也有一个android::MediaCodec::CreateByType(mData->mLooper, name, encoder);调用的是MediaCodec的CreateByType函数。这个类作用暂时不明确，看Google标识是勿动，勿改，勿乱搞，这是个NDK API。

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Android Multimedia框架总结（二十七）MediaCodec回顾

最后疑惑点

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