Android-NDK:native-codec

最新推荐文章于 2025-09-29 22:20:09 发布

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该博客介绍了如何在Android中使用C++编写原生库（NDK）进行媒体解码，并与Java层进行交互。通过CMakeLists.txt配置构建过程，然后在Java中声明 native 方法，例如 `createStreamingMediaPlayer` 和 `setSurface`，在C++中实现这些方法以处理文件读取、解码和设置渲染Surface。整个流程涉及到了 ANativeWindow、AMediaCodec 和 AMediaExtractor 等原生API的使用。

CMakeLists.txt

cmake_minimum_required(VERSION 3.4.1)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -UNDEBUG")

add_library(native-codec-jni SHARED
            looper.cpp
            native-codec-jni.cpp)

# Include libraries needed for native-codec-jni lib
target_link_libraries(native-codec-jni
                      android
                      log
                      mediandk
                      OpenMAXAL)

NativeCodec.java

/** Native methods, implemented in jni folder */
public static native void createEngine();
public static native boolean createStreamingMediaPlayer(AssetManager assetMgr, String filename);
public static native void setPlayingStreamingMediaPlayer(boolean isPlaying);
public static native void shutdown();
public static native void setSurface(Surface surface);
public static native void rewindStreamingMediaPlayer();

native-codec-jni.cpp

#include <assert.h>
#include <jni.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <limits.h>

#include "looper.h"
#include "media/NdkMediaCodec.h"
#include "media/NdkMediaExtractor.h"

setSurface

// set the surface
void Java_com_example_nativecodec_NativeCodec_setSurface(JNIEnv *env, jclass clazz, jobject surface)
{
    // obtain a native window from a Java surface
    if (data.window) {
        ANativeWindow_release(data.window);
        data.window = NULL;
    }
    data.window = ANativeWindow_fromSurface(env, surface);
    LOGV("@@@ setsurface %p", data.window);
}

从一个java的surface获取window

createStreamingMediaPlayer

jboolean Java_com_example_nativecodec_NativeCodec_createStreamingMediaPlayer(JNIEnv* env,
        jclass clazz, jobject assetMgr, jstring filename)
{
    LOGV("@@@ create");

    // convert Java string to UTF-8
    const char *utf8 = env->GetStringUTFChars(filename, NULL);获取c文件字符串
    LOGV("opening %s", utf8);

    off_t outStart, outLen;
    int fd = AAsset_openFileDescriptor(AAssetManager_open(AAssetManager_fromJava(env, assetMgr), utf8, 0),
                                       &outStart, &outLen);//获取文件fd

    env->ReleaseStringUTFChars(filename, utf8);
    if (fd < 0) {
        LOGE("failed to open file: %s %d (%s)", utf8, fd, strerror(errno));
        return JNI_FALSE;
    }

    data.fd = fd;

    workerdata *d = &data;

    AMediaExtractor *ex = AMediaExtractor_new();//获取媒体的解析器
    media_status_t err = AMediaExtractor_setDataSourceFd(ex, d->fd,
                                                         static_cast<off64_t>(outStart),
                                                         static_cast<off64_t>(outLen));
    close(d->fd);
    if (err != AMEDIA_OK) {
        LOGV("setDataSource error: %d", err);
        return JNI_FALSE;
    }

    int numtracks = AMediaExtractor_getTrackCount(ex);//获取媒体轨，音频和视频

    AMediaCodec *codec = NULL;

    LOGV("input has %d tracks", numtracks);
    for (int i = 0; i < numtracks; i++) {
        AMediaFormat *format = AMediaExtractor_getTrackFormat(ex, i);//获取相应音频和视频轨的媒体信息
        const char *s = AMediaFormat_toString(format);把相应的媒体信息转换为字符串
        LOGV("track %d format: %s", i, s);
        const char *mime;
        if (!AMediaFormat_getString(format, AMEDIAFORMAT_KEY_MIME, &mime)) {//获取媒体类型，如video/avc等
            LOGV("no mime type");
            return JNI_FALSE;
        } else if (!strncmp(mime, "video/", 6)) {
            // Omitting most error handling for clarity.
            // Production code should check for errors.
            AMediaExtractor_selectTrack(ex, i);
            codec = AMediaCodec_createDecoderByType(mime);//根据媒体类型创建相应的codec
            AMediaCodec_configure(codec, format, d->window, NULL, 0);//codec配置
            d->ex = ex;
            d->codec = codec;
            d->renderstart = -1;
            d->sawInputEOS = false;
            d->sawOutputEOS = false;
            d->isPlaying = false;
            d->renderonce = true;
            AMediaCodec_start(codec);//开始codec
        }
        AMediaFormat_delete(format);删除相应的类型
    }

    mlooper = new mylooper();
    mlooper->post(kMsgCodecBuffer, d);//发送msg

    return JNI_TRUE;
}

setPlayingStreamingMediaPlayer

// set the playing state for the streaming media player
void Java_com_example_nativecodec_NativeCodec_setPlayingStreamingMediaPlayer(JNIEnv* env,
        jclass clazz, jboolean isPlaying)
{
    LOGV("@@@ playpause: %d", isPlaying);
    if (mlooper) {
        if (isPlaying) {
            mlooper->post(kMsgResume, &data);//设置播放的状态
        } else {
            mlooper->post(kMsgPause, &data);
        }
    }
}

shutdown

// shut down the native media system
void Java_com_example_nativecodec_NativeCodec_shutdown(JNIEnv* env, jclass clazz)
{
    LOGV("@@@ shutdown");
    if (mlooper) {
        mlooper->post(kMsgDecodeDone, &data, true /* flush */);//decode完毕
        mlooper->quit();
        delete mlooper;
        mlooper = NULL;
    }
    if (data.window) {
        ANativeWindow_release(data.window);
        data.window = NULL;
    }
}

rewindStreamingMediaPlayer

seek动作

// rewind the streaming media player
void Java_com_example_nativecodec_NativeCodec_rewindStreamingMediaPlayer(JNIEnv *env, jclass clazz)
{
    LOGV("@@@ rewind");
    if (mlooper) {
        mlooper->post(kMsgSeek, &data);
    }
}

looper.h

#include <pthread.h>
#include <semaphore.h>

struct loopermessage;

class looper {
    public:
        looper();
        looper& operator=(const looper& ) = delete;
        looper(looper&) = delete;
        virtual ~looper();

        void post(int what, void *data, bool flush = false);
        void quit();

        virtual void handle(int what, void *data);

    private:
        void addmsg(loopermessage *msg, bool flush);
        static void* trampoline(void* p);
        void loop();
        loopermessage *head;
        pthread_t worker;
        sem_t headwriteprotect;
        sem_t headdataavailable;
        bool running;
};

looper.cpp

// for __android_log_print(ANDROID_LOG_INFO, "YourApp", "formatted message");
#include <android/log.h>
#define TAG "NativeCodec-looper"
#define LOGV(...) __android_log_print(ANDROID_LOG_VERBOSE, TAG, __VA_ARGS__)

//定义消息结构体

struct loopermessage;
typedef struct loopermessage loopermessage;

struct loopermessage {
    int what;
    void *obj;
    loopermessage *next;
    bool quit;
};

void* looper::trampoline(void* p) {
    ((looper*)p)->loop();
    return NULL;
}
//创建线程和初始化信号量
looper::looper() {
    sem_init(&headdataavailable, 0, 0);
    sem_init(&headwriteprotect, 0, 1);
    pthread_attr_t attr;
    pthread_attr_init(&attr);

    pthread_create(&worker, &attr, trampoline, this);
    running = true;
}

//把消息放到同步队列里面

void looper::post(int what, void *data, bool flush) {
    loopermessage *msg = new loopermessage();
    msg->what = what;
    msg->obj = data;
    msg->next = NULL;
    msg->quit = false;
    addmsg(msg, flush);
}

void looper::addmsg(loopermessage *msg, bool flush) {
    sem_wait(&headwriteprotect);
    loopermessage *h = head;

    if (flush) {
        while(h) {
            loopermessage *next = h->next;
            delete h;
            h = next;
        }
        h = NULL;
    }
    if (h) {
        while (h->next) {
            h = h->next;
        }
        h->next = msg;
    } else {
        head = msg;
    }
    LOGV("post msg %d", msg->what);
    sem_post(&headwriteprotect);
    sem_post(&headdataavailable);
}

//获取并处理消息，读写要同步

void looper::loop() {
    while(true) {
        // wait for available message
        sem_wait(&headdataavailable);

        // get next available message
        sem_wait(&headwriteprotect);
        loopermessage *msg = head;
        if (msg == NULL) {
            LOGV("no msg");
            sem_post(&headwriteprotect);
            continue;
        }
        head = msg->next;
        sem_post(&headwriteprotect);

        if (msg->quit) {
            LOGV("quitting");
            delete msg;
            return;
        }
        LOGV("processing msg %d", msg->what);
        handle(msg->what, msg->obj);
        delete msg;
    }
}

//退出添加退出的消息

void looper::quit() {
    LOGV("quit");
    loopermessage *msg = new loopermessage();
    msg->what = 0;
    msg->obj = NULL;
    msg->next = NULL;
    msg->quit = true;
    addmsg(msg, false);
    void *retval;
    pthread_join(worker, &retval);
    sem_destroy(&headdataavailable);
    sem_destroy(&headwriteprotect);
    running = false;
}

//处理消息

void looper::handle(int what, void* obj) {
    LOGV("dropping msg %d %p", what, obj);
}