Android JNI 使用的数据结构JNINativeMethod详解

最近在做关于Android语音识别的东东。研究Recognizer源码的时候碰到个关于JNINativeMethod的结构体，不解，Google之在网上找到了片文章，见文后。结构体代码如下：

/*
 * Table of methods associated with a single class.
 */
static JNINativeMethod gMethods[] = {
    /* name, signature, funcPtr */
    // PMem
    {"PMemInit",                           "()V",                     (void*)Java_android_speech_srec_Recognizer_PMemInit},
    {"PMemShutdown",                       "()V",                     (void*)Java_android_speech_srec_Recognizer_PMemShutdown},
    // SR_Session
    {"SR_SessionCreate",                   "(Ljava/lang/String;)V",   (void*)Java_android_speech_srec_Recognizer_SR_1SessionCreate},
    {"SR_SessionDestroy",                  "()V",                     (void*)Java_android_speech_srec_Recognizer_SR_1SessionDestroy},
    // SR_Recognizer
    {"SR_RecognizerStart",                 "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerStart},
    {"SR_RecognizerStop",                  "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerStop},
    {"SR_RecognizerCreate",                "()I",                     (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerCreate},
    {"SR_RecognizerDestroy",               "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerDestroy},
    {"SR_RecognizerSetup",                 "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerSetup},
    {"SR_RecognizerUnsetup",               "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerUnsetup},
    {"SR_RecognizerIsSetup",               "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSetup},
    {"SR_RecognizerGetParameter",          "(ILjava/lang/String;)Ljava/lang/String;", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerGetParameter},
    {"SR_RecognizerGetSize_tParameter",    "(ILjava/lang/String;)I",  (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerGetSize_1tParameter},
    {"SR_RecognizerGetBoolParameter",      "(ILjava/lang/String;)Z",  (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerGetBoolParameter},
    {"SR_RecognizerSetParameter",          "(ILjava/lang/String;Ljava/lang/String;)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerSetParameter},
    {"SR_RecognizerSetSize_tParameter",    "(ILjava/lang/String;I)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerSetSize_1tParameter},
    {"SR_RecognizerSetBoolParameter",      "(ILjava/lang/String;Z)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerSetBoolParameter},
    {"SR_RecognizerSetupRule",             "(IILjava/lang/String;)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerSetupRule},
    {"SR_RecognizerHasSetupRules",         "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerHasSetupRules},
    {"SR_RecognizerActivateRule",          "(IILjava/lang/String;I)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerActivateRule},
    {"SR_RecognizerDeactivateRule",        "(IILjava/lang/String;)V", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerDeactivateRule},
    {"SR_RecognizerDeactivateAllRules",    "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerDeactivateAllRules},
    {"SR_RecognizerIsActiveRule",          "(IILjava/lang/String;)Z", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsActiveRule},
    {"SR_RecognizerCheckGrammarConsistency", "(II)Z",                 (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerCheckGrammarConsistency},
    {"SR_RecognizerPutAudio",              "(I[BIIZ)I",               (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerPutAudio},
    {"SR_RecognizerAdvance",               "(I)I",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerAdvance},
    {"SR_RecognizerIsSignalClipping",      "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalClipping},
    {"SR_RecognizerIsSignalDCOffset",      "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalDCOffset},
    {"SR_RecognizerIsSignalNoisy",         "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalNoisy},
    {"SR_RecognizerIsSignalTooQuiet",      "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalTooQuiet},
    {"SR_RecognizerIsSignalTooFewSamples", "(I)Z",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalTooFewSamples},
    {"SR_RecognizerIsSignalTooManySamples", "(I)Z",                   (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerIsSignalTooManySamples},
    // SR_AcousticState
    {"SR_AcousticStateReset",               "(I)V",                   (void*)Java_android_speech_srec_Recognizer_SR_1AcousticStateReset},
    {"SR_AcousticStateSet",                 "(ILjava/lang/String;)V", (void*)Java_android_speech_srec_Recognizer_SR_1AcousticStateSet},
    {"SR_AcousticStateGet",                 "(I)Ljava/lang/String;",  (void*)Java_android_speech_srec_Recognizer_SR_1AcousticStateGet},
    // SR_Grammar
    {"SR_GrammarCompile",                  "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1GrammarCompile},
    {"SR_GrammarAddWordToSlot",            "(ILjava/lang/String;Ljava/lang/String;Ljava/lang/String;ILjava/lang/String;)V", (void*)Java_android_speech_srec_Recognizer_SR_1GrammarAddWordToSlot},
    {"SR_GrammarResetAllSlots",            "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1GrammarResetAllSlots},
    {"SR_GrammarSetupVocabulary",          "(II)V",                   (void*)Java_android_speech_srec_Recognizer_SR_1GrammarSetupVocabulary},
    {"SR_GrammarSetupRecognizer",          "(II)V",                   (void*)Java_android_speech_srec_Recognizer_SR_1GrammarSetupRecognizer},
    {"SR_GrammarUnsetupRecognizer",        "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1GrammarUnsetupRecognizer},
    {"SR_GrammarCreate",                   "()I",                     (void*)Java_android_speech_srec_Recognizer_SR_1GrammarCreate},
    {"SR_GrammarDestroy",                  "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1GrammarDestroy},
    {"SR_GrammarLoad",                     "(Ljava/lang/String;)I",   (void*)Java_android_speech_srec_Recognizer_SR_1GrammarLoad},
    {"SR_GrammarSave",                     "(ILjava/lang/String;)V",  (void*)Java_android_speech_srec_Recognizer_SR_1GrammarSave},
    {"SR_GrammarAllowOnly",                "(ILjava/lang/String;)V",  (void*)Java_android_speech_srec_Recognizer_SR_1GrammarAllowOnly},
    {"SR_GrammarAllowAll",                 "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1GrammarAllowAll},
    // SR_Vocabulary
    {"SR_VocabularyLoad",                  "()I",                     (void*)Java_android_speech_srec_Recognizer_SR_1VocabularyLoad},
    {"SR_VocabularyDestroy",               "(I)V",                    (void*)Java_android_speech_srec_Recognizer_SR_1VocabularyDestroy},
    {"SR_VocabularyGetPronunciation",      "(ILjava/lang/String;)Ljava/lang/String;",  (void*)Java_android_speech_srec_Recognizer_SR_1VocabularyGetPronunciation},
    // SR_RecognizerResult
    {"SR_RecognizerResultGetWaveform",     "(I)[B",                   (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerResultGetWaveform},
    {"SR_RecognizerResultGetSize",         "(I)I",                    (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerResultGetSize},
    {"SR_RecognizerResultGetKeyCount",     "(II)I",                   (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerResultGetKeyCount},
    {"SR_RecognizerResultGetKeyList",      "(II)[Ljava/lang/String;", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerResultGetKeyList},
    {"SR_RecognizerResultGetValue",        "(IILjava/lang/String;)Ljava/lang/String;", (void*)Java_android_speech_srec_Recognizer_SR_1RecognizerResultGetValue},
};

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

第三个变量fnPtr是函数指针，指向C函数。

其中比较难以理解的是第二个参数，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);

具体的每一个字符的对应关系如下

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

数组则以"["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

Ljava/lang/String; String jstring

Ljava/net/Socket; Socket jobject

如果J***A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"

文章出处：http://hi.baidu.com/leicejbgnndjrze/item/b87a4895e06a6c1c924f41f4

另供参考：http://blog.csdn.net/bigapple88/article/details/6756204