回顧 mac系統 ijk（Android）編譯，ijk的整個編譯過程大致分為下載、編譯ffmpeg、編譯ijk，編譯ffmpeg我們可以得到libijkffmpeg.so，編譯ijk可以得到libijksdl.so和libijkplayer.so，編譯ijk的時候需要依賴libijkffmpeg.so，這就是在ijk加載這三個so庫的時候，為什么要先加載libijkffmpeg.so的原因。
下面以moudle ijkplayer-armv7a中的源碼學習ijk中的jni實現，ijk c源碼位于ijkplayer-armv7a/src/main/jni/目錄下

如上圖，jni目錄下是c源碼，obj目錄下存放的是編譯之后生成的文件，Android.mk 是提供給NDK編譯的腳本。

一、Android.mk

LOCAL_PATH := $(call my-dir)

MY_APP_JNI_ROOT := $(realpath $(LOCAL_PATH))
MY_APP_PRJ_ROOT := $(realpath $(MY_APP_JNI_ROOT)/..)
MY_APP_ANDROID_ROOT := $(realpath $(MY_APP_PRJ_ROOT)/../../../..)

ifeq ($(TARGET_ARCH_ABI),armeabi-v7a)
MY_APP_FFMPEG_OUTPUT_PATH := $(realpath $(MY_APP_ANDROID_ROOT)/contrib/build/ffmpeg-armv7a/output)
MY_APP_FFMPEG_INCLUDE_PATH := $(realpath $(MY_APP_FFMPEG_OUTPUT_PATH)/include)
endif
ifeq ($(TARGET_ARCH_ABI),armeabi)
MY_APP_FFMPEG_OUTPUT_PATH := $(realpath $(MY_APP_ANDROID_ROOT)/contrib/build/ffmpeg-armv5/output)
MY_APP_FFMPEG_INCLUDE_PATH := $(realpath $(MY_APP_FFMPEG_OUTPUT_PATH)/include)
endif
ifeq ($(TARGET_ARCH_ABI),arm64-v8a)
MY_APP_FFMPEG_OUTPUT_PATH := $(realpath $(MY_APP_ANDROID_ROOT)/contrib/build/ffmpeg-arm64/output)
MY_APP_FFMPEG_INCLUDE_PATH := $(realpath $(MY_APP_FFMPEG_OUTPUT_PATH)/include)
endif
ifeq ($(TARGET_ARCH_ABI),x86)
MY_APP_FFMPEG_OUTPUT_PATH := $(realpath $(MY_APP_ANDROID_ROOT)/contrib/build/ffmpeg-x86/output)
MY_APP_FFMPEG_INCLUDE_PATH := $(realpath $(MY_APP_FFMPEG_OUTPUT_PATH)/include)
endif
ifeq ($(TARGET_ARCH_ABI),x86_64)
MY_APP_FFMPEG_OUTPUT_PATH := $(realpath $(MY_APP_ANDROID_ROOT)/contrib/build/ffmpeg-x86_64/output)
MY_APP_FFMPEG_INCLUDE_PATH := $(realpath $(MY_APP_FFMPEG_OUTPUT_PATH)/include)
endif

include $(call all-subdir-makefiles)

Android.mk通過 include $(call all-subdir-makefiles)調用編譯所有子目錄的mk文件

二、ijkplayer_jni.c

ijkplayer_jni.c在Android Studio中的路徑是
ijkplayer-android/android/ijkplayer/ijkplayer-armv7a/src/main/jni/ijkmedia/ijkplayer/android/ijkplayer_jni.c，這個路徑其實是個軟鏈，其真實存放路徑為 ijkplayer-android/ijkmedia/ijkplayer/android/ijkplayer_jni.c。

ijkplayer_jni.c文件中包含所有的在IjkMediaPlayer中定義的native方法的c實現。

static JNINativeMethod g_methods[] = {
    {
        "_setDataSource",
        "(Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/String;)V",
        (void *) IjkMediaPlayer_setDataSourceAndHeaders
    },
    { "_setDataSourceFd",       "(I)V",     (void *) IjkMediaPlayer_setDataSourceFd },
    { "_setDataSource",         "(Ltv/danmaku/ijk/media/player/misc/IMediaDataSource;)V", (void *)IjkMediaPlayer_setDataSourceCallback },
    { "_setAndroidIOCallback",  "(Ltv/danmaku/ijk/media/player/misc/IAndroidIO;)V", (void *)IjkMediaPlayer_setAndroidIOCallback },

    { "_setVideoSurface",       "(Landroid/view/Surface;)V", (void *) IjkMediaPlayer_setVideoSurface },
    { "_prepareAsync",          "()V",      (void *) IjkMediaPlayer_prepareAsync },
    { "_start",                 "()V",      (void *) IjkMediaPlayer_start },
    { "_stop",                  "()V",      (void *) IjkMediaPlayer_stop },
    { "seekTo",                 "(J)V",     (void *) IjkMediaPlayer_seekTo },
    { "_pause",                 "()V",      (void *) IjkMediaPlayer_pause },
    { "isPlaying",              "()Z",      (void *) IjkMediaPlayer_isPlaying },
    { "getCurrentPosition",     "()J",      (void *) IjkMediaPlayer_getCurrentPosition },
    { "getDuration",            "()J",      (void *) IjkMediaPlayer_getDuration },
    { "_release",               "()V",      (void *) IjkMediaPlayer_release },
    { "_reset",                 "()V",      (void *) IjkMediaPlayer_reset },
    { "setVolume",              "(FF)V",    (void *) IjkMediaPlayer_setVolume },
    { "getAudioSessionId",      "()I",      (void *) IjkMediaPlayer_getAudioSessionId },
    { "native_init",            "()V",      (void *) IjkMediaPlayer_native_init },
    { "native_setup",           "(Ljava/lang/Object;)V", (void *) IjkMediaPlayer_native_setup },
    { "native_finalize",        "()V",      (void *) IjkMediaPlayer_native_finalize },

    { "_setOption",             "(ILjava/lang/String;Ljava/lang/String;)V", (void *) IjkMediaPlayer_setOption },
    { "_setOption",             "(ILjava/lang/String;J)V",                  (void *) IjkMediaPlayer_setOptionLong },

    { "_getColorFormatName",    "(I)Ljava/lang/String;",    (void *) IjkMediaPlayer_getColorFormatName },
    { "_getVideoCodecInfo",     "()Ljava/lang/String;",     (void *) IjkMediaPlayer_getVideoCodecInfo },
    { "_getAudioCodecInfo",     "()Ljava/lang/String;",     (void *) IjkMediaPlayer_getAudioCodecInfo },
    { "_getMediaMeta",          "()Landroid/os/Bundle;",    (void *) IjkMediaPlayer_getMediaMeta },
    { "_setLoopCount",          "(I)V",                     (void *) IjkMediaPlayer_setLoopCount },
    { "_getLoopCount",          "()I",                      (void *) IjkMediaPlayer_getLoopCount },
    { "_getPropertyFloat",      "(IF)F",                    (void *) ijkMediaPlayer_getPropertyFloat },
    { "_setPropertyFloat",      "(IF)V",                    (void *) ijkMediaPlayer_setPropertyFloat },
    { "_getPropertyLong",       "(IJ)J",                    (void *) ijkMediaPlayer_getPropertyLong },
    { "_setPropertyLong",       "(IJ)V",                    (void *) ijkMediaPlayer_setPropertyLong },
    { "_setStreamSelected",     "(IZ)V",                    (void *) ijkMediaPlayer_setStreamSelected },

    { "native_profileBegin",    "(Ljava/lang/String;)V",    (void *) IjkMediaPlayer_native_profileBegin },
    { "native_profileEnd",      "()V",                      (void *) IjkMediaPlayer_native_profileEnd },

    { "native_setLogLevel",     "(I)V",                     (void *) IjkMediaPlayer_native_setLogLevel },
    { "_setFrameAtTime",        "(Ljava/lang/String;JJII)V", (void *) IjkMediaPlayer_setFrameAtTime },
};

JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *reserved)
{
    JNIEnv* env = NULL;

    g_jvm = vm;
    if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK) {
        return -1;
    }
    assert(env != NULL);

    pthread_mutex_init(&g_clazz.mutex, NULL );

    // FindClass returns LocalReference
    IJK_FIND_JAVA_CLASS(env, g_clazz.clazz, JNI_CLASS_IJKPLAYER);
    (*env)->RegisterNatives(env, g_clazz.clazz, g_methods, NELEM(g_methods) );

    ijkmp_global_init();
    ijkmp_global_set_inject_callback(inject_callback);

    FFmpegApi_global_init(env);

    return JNI_VERSION_1_4;
}

JNIEXPORT void JNI_OnUnload(JavaVM *jvm, void *reserved)
{
    ijkmp_global_uninit();

    pthread_mutex_destroy(&g_clazz.mutex);
}

我們可以從這里入手學習ijk是如何調用ffplay實現播放的。