MediaRecorder音视频框架

MediaRecorder音视频框架 附几篇学习博客链接： Camera视频加水印
JNI使用的数据结构JNINativeMethod详解
MPEG4Writer学习博客 Camera结构分析
Android高通平台Camera录制--MPEG4Writer.cpp 简单跟读
阅代码工具：Source Insight 一、API简要说明
1.1 它Android平台上的一套多媒体API，负责音视频编码。里面编码格式集成丰富，视频录制、录音都可以使用它。作为纯APP开发的话，了解一些音视频参数设置及设置顺序就可以使用了。如果是基于平台去做定制的话，就需要对整个框架流程需要一定的熟悉，假如要扩展一个新的接口，或者更改某个接口功能。那么接下来一起来跟读吧。

二、框架图 MediaRecorder音视频框架_第1张图片

三、代码跟读流程分析 MediaRecorder.java—>android_media_MediaRecorder.cpp—>mediarecorder.cpp—> IMediaRecorder.cpp—>MediaRecorderClient.cpp—>StagefrightRecorder.cpp—>MPEG4Writer.cpp
3.1 MediaRecorder.java--SDK上封装的接口类源码路径：frameworks/base/media/java/android/media/MediaRecorder.java 3.1.1 采用setCamera这个接口来进行跟读，注意Camera参数这个比较特殊。 System.loadLibrary("media_jni");调用JNI编译出来的libmedia_jni.so库

public class MediaRecorder{    static {        System.loadLibrary("media_jni");        native_init();    }    ...    public native void setCamera(Camera c);    ...}

3.2 android_media_MediaRecorder.cpp--JNI接口类。库名：libmedia_jni.so 源码路径：frameworks/base/media/jni/android_media_MediaRecorder.cpp
3.2.1 MediaRecorder.java里面setCamera的JNI接口

static void android_media_MediaRecorder_setCamera(JNIEnv* env, jobject thiz, jobject camera){    // we should not pass a null camera to get_native_camera() call.    if (camera == NULL) {        jniThrowNullPointerException(env, "camera object is a NULL pointer");        return;    }    sp c = get_native_camera(env, camera, NULL);    if (c == NULL) {        // get_native_camera will throw an exception in this case        return;    }    sp mr = getMediaRecorder(env, thiz);    process_media_recorder_call(env, mr->setCamera(c->remote(), c->getRecordingProxy()),            "java/lang/RuntimeException", "setCamera failed.");}

3.2.2 通过getMediaRecorder获取对象实例，调用下一层setCamera接口。const修饰类类型变量指针，表示其不可重新赋值，一次会话只能实例化一次内存地址。

static sp getMediaRecorder(JNIEnv* env, jobject thiz){    Mutex::Autolock l(sLock);    MediaRecorder* const p = (MediaRecorder*)env->GetLongField(thiz, fields.context);    return sp(p);}

3.2.3 JNI里面添加函数需要注册

static JNINativeMethod gMethods[] = {    {"setCamera",            "(Landroid/hardware/Camera;)V",    (void *)android_media_MediaRecorder_setCamera},    ...};1、第一个字符串表示java层函数名2、()里面java层函数参数类型(L+包名;)，括号外面是返回值类型V表示void3、JNI函数指针

// 注册位置// This function only registers the native methods, and is called from// JNI_OnLoad in android_media_MediaPlayer.cppint register_android_media_MediaRecorder(JNIEnv *env){    return AndroidRuntime::registerNativeMethods(env,                "android/media/MediaRecorder", gMethods, NELEM(gMethods));}

3.3 mediarecorder.cpp--就是上一层getMediaRecorder实例化的对象
源码路径：frameworks/av/media/libmedia/mediarecorder.cpp 3.3.1 上面JNI中的 android_media_MediaRecorder_setCamera函数 mr->setCamera(c->remote(), c->getRecordingProxy()

status_t MediaRecorder::setCamera(const sp& camera, const sp& proxy){    ALOGV("setCamera(%p,%p)", camera.get(), proxy.get());    if (mMediaRecorder == NULL) {        ALOGE("media recorder is not initialized yet");        return INVALID_OPERATION;    }    if (!(mCurrentState & MEDIA_RECORDER_IDLE)) {        ALOGE("setCamera called in an invalid state(%d)", mCurrentState);        return INVALID_OPERATION;    }    status_t ret = mMediaRecorder->setCamera(camera, proxy);    if (OK != ret) {        ALOGV("setCamera failed: %d", ret);        mCurrentState = MEDIA_RECORDER_ERROR;        return ret;    }    return ret;}

3.3.2 这部分不是JNI语法，已经是标准C++语法。所以必须要在头文件种声明函数原型。结合头部文件可以看出，cpp函数里面调用到了IMediaRecorder对象中的setCamera 3.3.3 源码路径： frameworks/av/include/media/mediarecorder.h

class MediaRecorder : public BnMediaRecorderClient,                      public virtual IMediaDeathNotifier{public:...    status_t    setCamera(const sp& camera, const sp& proxy);...private:...    sp          mMediaRecorder;...};

3.4 IMediaRecorder.cpp--这部分采用了IBinder通信源码路径：frameworks/av/media/libmedia/IMediaRecorder.cpp 3.4.1 首先得在枚举量中添加一个符号常量

enum {    RELEASE = IBinder::FIRST_CALL_TRANSACTION,...    SET_CAMERA,...};

3.4.2 实现函数部分

class BpMediaRecorder: public BpInterface{public:...    status_t setCamera(const sp& camera, const sp& proxy)    {        ALOGV("setCamera(%p,%p)", camera.get(), proxy.get());        Parcel data, reply;        data.writeInterfaceToken(IMediaRecorder::getInterfaceDescriptor());        data.writeStrongBinder(camera->asBinder());        data.writeStrongBinder(proxy->asBinder());        remote()->transact(SET_CAMERA, data, &reply);        return reply.readInt32();    }...};

3.4.3 transact调用

status_t BnMediaRecorder::onTransact(                                     uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){    switch (code) {        case SET_CAMERA: {            ALOGV("SET_CAMERA");            CHECK_INTERFACE(IMediaRecorder, data, reply);            sp camera = interface_cast(data.readStrongBinder());            sp proxy =                interface_cast(data.readStrongBinder());            reply->writeInt32(setCamera(camera, proxy));            return NO_ERROR;        } break;        default:            return BBinder::onTransact(code, data, reply, flags);    }}

3.4.4 函数原型源码路径：frameworks/av/include/media/IMediaRecorder.h

class IMediaRecorder: public IInterface{public:    ...    virtual status_t setCamera(const sp& camera,                               const sp& proxy) = 0;    ...};

3.5 MediaRecorderClient.cpp 源码路径：frameworks/av/media/libmediaplayerservice/MediaRecorderClient.cpp 3.5.1 这里的mRecorder是MediaRecorderBase类变量指针，而MediaRecorderBase只是一个.h头文件。
它是一个基类，定义了函数原型，由StagefrightRecorder去继承实现，从这里看出接下来调到了StagefrightRecorder里面的setCamera具体实现。

status_t MediaRecorderClient::setCamera(const sp& camera,                                        const sp& proxy){    ALOGV("setCamera");    Mutex::Autolock lock(mLock);    if (mRecorder == NULL) {        ALOGE("recorder is not initialized");        return NO_INIT;    }    return mRecorder->setCamera(camera, proxy);}

3.5.2 MediaRecorderClient.cpp函数原型源码路径： frameworks/av/media/libmediaplayerservice/MediaRecorderClient.h

class MediaRecorderClient : public BnMediaRecorder{public:    virtual     status_t   setCamera(const sp& camera,                                    const sp& proxy);    ...    MediaRecorderBase      *mRecorder;};

3.6 StagefrightRecorder.cpp--看到了这里也就马上到尽头了，MPEG4Writer封装类就是在这里实例化的。
源码路径：frameworks/av/media/libmediaplayerservice/StagefrightRecorder.cpp 3.6.1 mCamera这个全局首先会在setupCameraSource配置， setupCameraSource会在setupMediaSource进行调用，这里不co代码出来了。

status_t StagefrightRecorder::setCamera(const sp &camera,                                        const sp &proxy) {    ALOGV("setCamera");    if (camera == 0) {        ALOGE("camera is NULL");        return BAD_VALUE;    }    if (proxy == 0) {        ALOGE("camera proxy is NULL");        return BAD_VALUE;    }    mCamera = camera;    mCameraProxy = proxy;    return OK;}

3.6.2 实例化文件封装类，准备进入封装流程

status_t StagefrightRecorder::setupMPEG4orWEBMRecording() {    ...    writer = new MPEG4Writer(mOutputFd);    ...    // 实例化对象之后调用setupMediaSource把参数设置下去}

3.6.3 函数原型：源码路径：frameworks/av/media/libmediaplayerservice/StagefrightRecorder.h

struct StagefrightRecorder : public MediaRecorderBase {    ...    virtual status_t setCamera(const sp& camera, const sp& proxy);    ...}

3.6.4 跟读到这差不多了，根据流程可以添加自己想要添加的接口了。从这里就可以调用到MPEG4Writer.cpp 例如：

status_t StagefrightRecorder::test() {    if (mWriter == NULL) {        ALOGE("File writer is not avaialble");    }    mWriter->test();    return OK;}

3.7 MPEG4Writer.cpp--媒体文件封装类，这个cpp需要单独拿出来讲，所以这里不做过多介绍。阅读此封装类之前需要熟悉多媒体文件封装流程。
源码路径：frameworks/av/media/libstagefright/MPEG4Writer.cpp 源码路径：frameworks/av/include/media/stagefright/MPEG4Writer.h

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