JNI之------JNINativeMethod

文章出处：http://blog.youkuaiyun.com/shift_wwx

前言：之前的一篇(Android基础总结之九：JNI详解)是结合实例的简单总结，这边总结一下JNINativeMethod。

1、JNINativeMethod 结构体的官方定义

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1. typedef struct {    
2.     const char* name;    
3.     const char* signature;    
4.     void* fnPtr;    
5. } 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：

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1. static JNINativeMethod methods[] = {    
2.   { "printJNI", "()I",        (void*)printForTest},    
3.       
4. };   

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);

2、第二个参数之基本类型

3、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。

4、举例

5、结合Media中的看看

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1. static JNINativeMethod gMethods[] = {  
2.     {  
3.         "_setDataSource",  
4.         "(Ljava/lang/String;[Ljava/lang/String;[Ljava/lang/String;)V",  
5.         (void *)android_media_MediaPlayer_setDataSourceAndHeaders  
6.     },  
7.   
8.     {"_setDataSource",       "(Ljava/io/FileDescriptor;JJ)V",    (void *)android_media_MediaPlayer_setDataSourceFD},  
9.     {"_setVideoSurface",    "(Landroid/view/Surface;)V",        (void *)android_media_MediaPlayer_setVideoSurface},  
10.     {"prepare",             "()V",                              (void *)android_media_MediaPlayer_prepare},  
11.     {"prepareAsync",        "()V",                              (void *)android_media_MediaPlayer_prepareAsync},  
12.     {"_start",              "()V",                              (void *)android_media_MediaPlayer_start},  
13.     {"_stop",               "()V",                              (void *)android_media_MediaPlayer_stop},  
14.     {"getVideoWidth",       "()I",                              (void *)android_media_MediaPlayer_getVideoWidth},  
15.     {"getVideoHeight",      "()I",                              (void *)android_media_MediaPlayer_getVideoHeight},  
16.     {"seekTo",              "(I)V",                             (void *)android_media_MediaPlayer_seekTo},  
17.     {"_pause",              "()V",                              (void *)android_media_MediaPlayer_pause},  
18.     {"isPlaying",           "()Z",                              (void *)android_media_MediaPlayer_isPlaying},  
19.     {"getCurrentPosition",  "()I",                              (void *)android_media_MediaPlayer_getCurrentPosition},  
20.     {"getDuration",         "()I",                              (void *)android_media_MediaPlayer_getDuration},  
21.     {"_release",            "()V",                              (void *)android_media_MediaPlayer_release},  
22.     {"_reset",              "()V",                              (void *)android_media_MediaPlayer_reset},  
23.     {"setAudioStreamType",  "(I)V",                             (void *)android_media_MediaPlayer_setAudioStreamType},  
24.     {"setLooping",          "(Z)V",                             (void *)android_media_MediaPlayer_setLooping},  
25.     {"isLooping",           "()Z",                              (void *)android_media_MediaPlayer_isLooping},  
26.     {"setVolume",           "(FF)V",                            (void *)android_media_MediaPlayer_setVolume},  
27.     {"native_invoke",       "(Landroid/os/Parcel;Landroid/os/Parcel;)I",(void *)android_media_MediaPlayer_invoke},  
28.     {"native_setMetadataFilter", "(Landroid/os/Parcel;)I",      (void *)android_media_MediaPlayer_setMetadataFilter},  
29.     {"native_getMetadata", "(ZZLandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_getMetadata},  
30.     {"native_init",         "()V",                              (void *)android_media_MediaPlayer_native_init},  
31.     {"native_setup",        "(Ljava/lang/Object;)V",            (void *)android_media_MediaPlayer_native_setup},  
32.     {"native_finalize",     "()V",                              (void *)android_media_MediaPlayer_native_finalize},  
33.     {"getAudioSessionId",   "()I",                              (void *)android_media_MediaPlayer_get_audio_session_id},  
34.     {"setAudioSessionId",   "(I)V",                             (void *)android_media_MediaPlayer_set_audio_session_id},  
35.     {"setAuxEffectSendLevel", "(F)V",                           (void *)android_media_MediaPlayer_setAuxEffectSendLevel},  
36.     {"attachAuxEffect",     "(I)V",                             (void *)android_media_MediaPlayer_attachAuxEffect},  
37.     {"native_pullBatteryData", "(Landroid/os/Parcel;)I",        (void *)android_media_MediaPlayer_pullBatteryData},  
38.     {"setParameter",        "(ILandroid/os/Parcel;)Z",          (void *)android_media_MediaPlayer_setParameter},  
39.     {"getParameter",        "(ILandroid/os/Parcel;)V",          (void *)android_media_MediaPlayer_getParameter},  
40.     {"native_setRetransmitEndpoint", "(Ljava/lang/String;I)I",  (void *)android_media_MediaPlayer_setRetransmitEndpoint},  
41.     {"setNextMediaPlayer",  "(Landroid/media/MediaPlayer;)V",   (void *)android_media_MediaPlayer_setNextMediaPlayer},  
42.     {"updateProxyConfig", "(Landroid/net/ProxyProperties;)V", (void *)android_media_MediaPlayer_updateProxyConfig},  
43. };  

如，

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1. {"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，

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1. {"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