在学习雷神的android视频解码器这篇文章的时候，又不小心踩了许多坑。这不是因为雷神的博客写的不够详细，而是随着技术更新，有很多在当初看来不用关心的细节，在今天一个不小心就会出错。

问题一：No implementation found for decode

No implementation found for int com.xiaguangcheng.cmakedemo.MainActivity.decode(java.lang.String, java.lang.String) (tried Java_com_xiaguangcheng_cmakedemo_MainActivity_decode and Java_com_xiaguangcheng_cmakedemo_MainActivity_decode__Ljava_lang_String_2Ljava_lang_String_2)

这个问题是说找不到我们在cpp文件中写的实现decode的本地方法，但实际上我们已经写了。可能是因为Cmake管理的问题，我们需要在方法之前加上extern "C"

extern "C"
JNIEXPORT jint JNICALL
Java_com_fengfutong_cmakedemo_MainActivity_decode
        (JNIEnv *env, jobject , jstring input_jstr, jstring output_jstr)

问题二：Couldn't open input stream.

我们根据提示在cpp中找到这段代码

if (avformat_open_input(&pFormatCtx, input_str, NULL, NULL) != 0) {
        LOGE("Couldn't open input stream.\n,%s,%d",input_str,avformat_open_input(&pFormatCtx, input_str, NULL, NULL));
        return -1;
}

这个问题是因为我用的项目的目标版本超过了23.

targetSdkVersion 25

因为超过了23之后，有一些危险权限是需要手动申请的。尽管你在AndroidManifest.xml中已经配置了读写sdk的权限

 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

但是报错的时候，并没有任何提示关于permission denied。

这个时候我们只要把targetSdkVersion降低到23以下就可以了。当然也可以稍微麻烦一点点，手动申请一下权限。

下面贴上最新的native-lib.cpp和MainActivity.java代码

#include <jni.h>
#include <string>

extern "C" {
#include "libavcodec/avcodec.h"
#include "libavformat/avformat.h"
#include "libswscale/swscale.h"
#include "libavutil/imgutils.h"

#ifdef ANDROID

#include <android/log.h>

#define LOGE(format, ...)  __android_log_print(ANDROID_LOG_ERROR, "(>_<)", format, ##__VA_ARGS__)
#define LOGI(format, ...)  __android_log_print(ANDROID_LOG_INFO,  "(^_^)", format, ##__VA_ARGS__)
#else
#define LOGE(format, ...)  printf("(>_<) " format "\n", ##__VA_ARGS__)
#define LOGI(format, ...)  printf("(^_^) " format "\n", ##__VA_ARGS__)
#endif
};



extern "C"
JNIEXPORT jstring JNICALL
Java_com_fengfutong_cmakedemo_MainActivity_stringFromJNI(JNIEnv *env,
                                                         jobject /* this */) {
    char info[10000] = {0};
    sprintf(info, "%s\n", avcodec_configuration());
    return env->NewStringUTF(info);
}

//Output FFmpeg's av_log()
void custom_log(void *ptr, int level, const char *fmt, va_list vl) {
    FILE *fp = fopen("/storage/emulated/0/av_log.txt", "a+");
    if (fp) {
        vfprintf(fp, fmt, vl);
        fflush(fp);
        fclose(fp);
    }
}

extern "C"
JNIEXPORT jint JNICALL
Java_com_fengfutong_cmakedemo_MainActivity_decode
        (JNIEnv *env, jobject , jstring input_jstr, jstring output_jstr) {
    AVFormatContext *pFormatCtx;
    int i, videoindex;
    AVCodecContext *pCodecCtx;
    AVCodec *pCodec;
    AVFrame *pFrame, *pFrameYUV;
    uint8_t *out_buffer;
    AVPacket *packet;
    int y_size;
    int ret, got_picture;
    struct SwsContext *img_convert_ctx;
    FILE *fp_yuv;
    int frame_cnt;
    clock_t time_start, time_finish;
    double time_duration = 0.0;

    char input_str[500] = {0};
    char output_str[500] = {0};
    char info[1000] = {0};
    sprintf(input_str, "%s", env->GetStringUTFChars(input_jstr, NULL));
    sprintf(output_str, "%s", env->GetStringUTFChars(output_jstr, NULL));

    //FFmpeg av_log() callback
    av_log_set_callback(custom_log);

    av_register_all();
    avformat_network_init();
    pFormatCtx = avformat_alloc_context();

    if (avformat_open_input(&pFormatCtx, input_str, NULL, NULL) != 0) {
        LOGE("Couldn't open input stream.\n,%s,%d",input_str,avformat_open_input(&pFormatCtx, input_str, NULL, NULL));
        return -1;
    }
    if (avformat_find_stream_info(pFormatCtx, NULL) < 0) {
        LOGE("Couldn't find stream information.\n");
        return -1;
    }
    videoindex = -1;
    for (i = 0; i < pFormatCtx->nb_streams; i++)
        if (pFormatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
            videoindex = i;
            break;
        }
    if (videoindex == -1) {
        LOGE("Couldn't find a video stream.\n");
        return -1;
    }
    pCodecCtx = pFormatCtx->streams[videoindex]->codec;
    pCodec = avcodec_find_decoder(pCodecCtx->codec_id);
    if (pCodec == NULL) {
        LOGE("Couldn't find Codec.\n");
        return -1;
    }
    if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {
        LOGE("Couldn't open codec.\n");
        return -1;
    }

    pFrame = av_frame_alloc();
    pFrameYUV = av_frame_alloc();
    out_buffer = (unsigned char *) av_malloc(
            av_image_get_buffer_size(AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1));
    av_image_fill_arrays(pFrameYUV->data, pFrameYUV->linesize, out_buffer,
                         AV_PIX_FMT_YUV420P, pCodecCtx->width, pCodecCtx->height, 1);


    packet = (AVPacket *) av_malloc(sizeof(AVPacket));

    img_convert_ctx = sws_getContext(pCodecCtx->width, pCodecCtx->height, pCodecCtx->pix_fmt,
                                     pCodecCtx->width, pCodecCtx->height, AV_PIX_FMT_YUV420P,
                                     SWS_BICUBIC, NULL, NULL, NULL);


    sprintf(info, "[Input     ]%s\n", input_str);
    sprintf(info, "%s[Output    ]%s\n", info, output_str);
    sprintf(info, "%s[Format    ]%s\n", info, pFormatCtx->iformat->name);
    sprintf(info, "%s[Codec     ]%s\n", info, pCodecCtx->codec->name);
    sprintf(info, "%s[Resolution]%dx%d\n", info, pCodecCtx->width, pCodecCtx->height);


    fp_yuv = fopen(output_str, "wb+");
    if (fp_yuv == NULL) {
        printf("Cannot open output file.\n");
        return -1;
    }

    frame_cnt = 0;
    time_start = clock();

    while (av_read_frame(pFormatCtx, packet) >= 0) {
        if (packet->stream_index == videoindex) {
            ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
            if (ret < 0) {
                LOGE("Decode Error.\n");
                return -1;
            }
            if (got_picture) {
                sws_scale(img_convert_ctx, (const uint8_t *const *) pFrame->data, pFrame->linesize,
                          0, pCodecCtx->height,
                          pFrameYUV->data, pFrameYUV->linesize);

                y_size = pCodecCtx->width * pCodecCtx->height;
                fwrite(pFrameYUV->data[0], 1, y_size, fp_yuv);    //Y
                fwrite(pFrameYUV->data[1], 1, y_size / 4, fp_yuv);  //U
                fwrite(pFrameYUV->data[2], 1, y_size / 4, fp_yuv);  //V
                //Output info
                char pictype_str[10] = {0};
                switch (pFrame->pict_type) {
                    case AV_PICTURE_TYPE_I:
                        sprintf(pictype_str, "I");
                        break;
                    case AV_PICTURE_TYPE_P:
                        sprintf(pictype_str, "P");
                        break;
                    case AV_PICTURE_TYPE_B:
                        sprintf(pictype_str, "B");
                        break;
                    default:
                        sprintf(pictype_str, "Other");
                        break;
                }
                LOGI("Frame Index: %5d. Type:%s", frame_cnt, pictype_str);
                frame_cnt++;
            }
        }
        av_free_packet(packet);
    }
    //flush decoder
    //FIX: Flush Frames remained in Codec
    while (1) {
        ret = avcodec_decode_video2(pCodecCtx, pFrame, &got_picture, packet);
        if (ret < 0)
            break;
        if (!got_picture)
            break;
        sws_scale(img_convert_ctx, (const uint8_t *const *) pFrame->data, pFrame->linesize, 0,
                  pCodecCtx->height,
                  pFrameYUV->data, pFrameYUV->linesize);
        int y_size = pCodecCtx->width * pCodecCtx->height;
        fwrite(pFrameYUV->data[0], 1, y_size, fp_yuv);    //Y
        fwrite(pFrameYUV->data[1], 1, y_size / 4, fp_yuv);  //U
        fwrite(pFrameYUV->data[2], 1, y_size / 4, fp_yuv);  //V
        //Output info
        char pictype_str[10] = {0};
        switch (pFrame->pict_type) {
            case AV_PICTURE_TYPE_I:
                sprintf(pictype_str, "I");
                break;
            case AV_PICTURE_TYPE_P:
                sprintf(pictype_str, "P");
                break;
            case AV_PICTURE_TYPE_B:
                sprintf(pictype_str, "B");
                break;
            default:
                sprintf(pictype_str, "Other");
                break;
        }
        LOGI("Frame Index: %5d. Type:%s", frame_cnt, pictype_str);
        frame_cnt++;
    }
    time_finish = clock();
    time_duration = (double) (time_finish - time_start);

    sprintf(info, "%s[Time      ]%fms\n", info, time_duration);
    sprintf(info, "%s[Count     ]%d\n", info, frame_cnt);

    sws_freeContext(img_convert_ctx);

    fclose(fp_yuv);

    av_frame_free(&pFrameYUV);
    av_frame_free(&pFrame);
    avcodec_close(pCodecCtx);
    avformat_close_input(&pFormatCtx);

    return 0;
}

MainActivity:

public class MainActivity extends Activity {

    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("avutil-55");
        System.loadLibrary("avcodec-57");
        System.loadLibrary("avformat-57");
        System.loadLibrary("avdevice-57");
        System.loadLibrary("swresample-2");
        System.loadLibrary("swscale-4");
//        System.loadLibrary("postproc-53");
        System.loadLibrary("avfilter-6");
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        // Example of a call to a native method
//        TextView tv = (TextView) findViewById(R.id.sample_text);
//        tv.setText(stringFromJNI());
        Button button= (Button) findViewById(R.id.button);
        final EditText editText= (EditText) findViewById(R.id.editText);
        final EditText editText2= (EditText) findViewById(R.id.editText2);
        editText.setText("sintel.mp4");
        editText2.setText("1234test.yuv");
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String s = editText.getText().toString();
                String s1 = editText2.getText().toString();
                String absolutePath = Environment.getExternalStorageDirectory().getAbsolutePath();
                String inputPath=absolutePath+"/"+s;
                String outPath=absolutePath+"/"+s1;
                Log.e("main",inputPath+"---->"+outPath);
                decode(inputPath,outPath);
            }
        });

    }

    public native int decode(String inputPath, String outPath) ;

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
}