@[Android JNINativeMethod介绍]

Android JNI 数据结构JNINativeMethod说明

Andoird 中使用了一种不同传统Java JNI的方式来定义其native的函数。
其中很重要的区别是Andorid使用了一种Java 和 C 函数的映射表数组，并在其中描述了函数的参数和返回值。这个数组的类型是JNINativeMethod，定义如下：

typedef struct {
  const char* name;
  const char* signature;
  void* fnPtr;
} JNINativeMethod;

• 第一个变量name是Java中函数的名字。

• 第二个变量signature，用字符串是描述了函数的参数和返回值，例如

"()V"
"(II)V"
"(Ljava/lang/String;Ljava/lang/String;)V"

实际上这些字符是与函数的参数类型一一对应的。

“()” 中的字符表示参数，后面的则代表返回值。例如"()V" 就表示void Func();

“(II)V” 表示 void Func(int, int);

普通类型

字符   Java类型        C类型

V      void            void
Z      jboolean        boolean
I      jint            int
J      jlong           long
D      jdouble         double
F      jfloat          float
B      jbyte           byte
C      jchar           char
S      jshort          short

数组类型

以"["开始，用两个字符表示

[I     jintArray       int[]
[F     jfloatArray     float[]
[B     jbyteArray      byte[]
[C     jcharArray      char[]
[S     jshortArray     short[]
[D     jdoubleArray    double[]
[J     jlongArray      long[]
[Z     jbooleanArray   boolean[]

上面的都是基本类型。
如果Java函数的参数是class，则以"L"开头，以";“结尾
中间是用”/" 隔开的包及类名。
而其对应的C函数名的参数则为jobject.
一个例外是String类，其对应的类为jstring

如：

Ljava/lang/String; String jstring
Ljava/net/Socket; Socket jobject
如果JAVA函数位于一个嵌入类，则用$作为类名间的分隔符。
例如 
"(Ljava/lang/String;Landroid/os/FileUtils$FileStatus;)Z"

ijkplayer中的定义为样例参考

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 },
};

• 第三个变量fnPtr是函数指针，指向C函数。