JNI 之 JNINativeMethod——安卓

转自：JNI之------JNINativeMethod_私房菜的博客-CSDN博客

JNINativeMethod 结构体的官方定义 

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Java中函数的参数和返回值

第三个变量fnPtr是函数指针，指向native函数。前面都要接 (void *)

第一个变量与第三个变量是对应的，一个是java层方法名，对应着第三个参数的native方法名字

示例：

static JNINativeMethod methods[] = {  
  { "printJNI", "()I",        (void*)printForTest},  
    
}; 

第二个参数介绍

主要是第二个参数比较复杂：

括号里面表示参数的类型，括号后面表示返回值。

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

"(II)V" 表示 void Fun(int a, int b);

第二个参数之基本类型

第二个参数之对象类型和数组类型

对象类型：以"L"开头，以";"结尾，中间用"/"隔开。如表第一行。

数组类型：以"["开头，如表第二行(n维数组的话，则前面就n个"["，如"[[[D"就表示double[ ][ ][ ])。

对象数组类型：就是上述两者结合，如表第三行。
 

举例

 结合Media中的看看

static JNINativeMethod gMethods[] = {
    {
        "_setDataSource",
        "(Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/String;)V",
        (void *)android_media_MediaPlayer_setDataSourceAndHeaders
    },
 
    {"_setDataSource",       "(Ljava/io/FileDescriptor;JJ)V",    (void *)android_media_MediaPlayer_setDataSourceFD},
    {"_setVideoSurface",    "(Landroid/view/Surface;)V",        (void *)android_media_MediaPlayer_setVideoSurface},
    {"prepare",             "()V",                              (void *)android_media_MediaPlayer_prepare},
    {"prepareAsync",        "()V",                              (void *)android_media_MediaPlayer_prepareAsync},
    {"_start",              "()V",                              (void *)android_media_MediaPlayer_start},
    {"_stop",               "()V",                              (void *)android_media_MediaPlayer_stop},
    {"getVideoWidth",       "()I",                              (void *)android_media_MediaPlayer_getVideoWidth},
    {"getVideoHeight",      "()I",                              (void *)android_media_MediaPlayer_getVideoHeight},
    {"seekTo",              "(I)V",                             (void *)android_media_MediaPlayer_seekTo},
    {"_pause",              "()V",                              (void *)android_media_MediaPlayer_pause},
    {"isPlaying",           "()Z",                              (void *)android_media_MediaPlayer_isPlaying},
    {"getCurrentPosition",  "()I",                              (void *)android_media_MediaPlayer_getCurrentPosition},
    {"getDuration",         "()I",                              (void *)android_media_MediaPlayer_getDuration},
    {"_release",            "()V",                              (void *)android_media_MediaPlayer_release},
    {"_reset",              "()V",                              (void *)android_media_MediaPlayer_reset},
    {"setAudioStreamType",  "(I)V",                             (void *)android_media_MediaPlayer_setAudioStreamType},
    {"setLooping",          "(Z)V",                             (void *)android_media_MediaPlayer_setLooping},
    {"isLooping",           "()Z",                              (void *)android_media_MediaPlayer_isLooping},
    {"setVolume",           "(FF)V",                            (void *)android_media_MediaPlayer_setVolume},
    {"native_invoke",       "(Landroid/os/Parcel;Landroid/os/Parcel;)I",(void *)android_media_MediaPlayer_invoke},
    {"native_setMetadataFilter", "(Landroid/os/Parcel;)I",      (void *)android_media_MediaPlayer_setMetadataFilter},
    {"native_getMetadata", "(ZZLandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_getMetadata},
    {"native_init",         "()V",                              (void *)android_media_MediaPlayer_native_init},
    {"native_setup",        "(Ljava/lang/Object;)V",            (void *)android_media_MediaPlayer_native_setup},
    {"native_finalize",     "()V",                              (void *)android_media_MediaPlayer_native_finalize},
    {"getAudioSessionId",   "()I",                              (void *)android_media_MediaPlayer_get_audio_session_id},
    {"setAudioSessionId",   "(I)V",                             (void *)android_media_MediaPlayer_set_audio_session_id},
    {"setAuxEffectSendLevel", "(F)V",                           (void *)android_media_MediaPlayer_setAuxEffectSendLevel},
    {"attachAuxEffect",     "(I)V",                             (void *)android_media_MediaPlayer_attachAuxEffect},
    {"native_pullBatteryData", "(Landroid/os/Parcel;)I",        (void *)android_media_MediaPlayer_pullBatteryData},
    {"setParameter",        "(ILandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_setParameter},
    {"getParameter",        "(ILandroid/os/Parcel;)V",          (void *)android_media_MediaPlayer_getParameter},
    {"native_setRetransmitEndpoint", "(Ljava/lang/String;I)I",  (void *)android_media_MediaPlayer_setRetransmitEndpoint},
    {"setNextMediaPlayer",  "(Landroid/media/MediaPlayer;)V",   (void *)android_media_MediaPlayer_setNextMediaPlayer},
    {"updateProxyConfig", "(Landroid/net/ProxyProperties;)V", (void *)android_media_MediaPlayer_updateProxyConfig},
};

如，

{"getParameter",        "(ILandroid/os/Parcel;)V", 
 (void *)android_media_MediaPlayer_getParameter},

就是代表java层函数void getParameter(int a , Parcel b)，返回值是void，形参是init型和Parcel型
再如，

{"native_setRetransmitEndpoint", "(Ljava/lang/String;I)I", 
 (void *)android_media_MediaPlayer_setRetransmitEndpoint},

就代表java层函数int native_setRetransmitEndpoint(String a, int b)，返回值是int，形参是String型和int型