Android JNI 使用的数据结构JNINativeMethod详解

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：

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typedef struct {
<wbr><wbr><wbr>const char* name;<br><wbr><wbr><wbr>const char* signature;<br><wbr><wbr><wbr>void* <wbr><wbr><wbr><wbr><wbr><wbr>fnPtr;<br> } JNINativeMethod;</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

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。

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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"

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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);

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具体的每一个字符的对应关系如下

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字符 <wbr><wbr><wbr><wbr>Java类型 <wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>C类型</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

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

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数组则以"["开始，用两个字符表示

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

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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

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Ljava/lang/String; <wbr><wbr><wbr><wbr><wbr>String <wbr><wbr><wbr><wbr><wbr><wbr><wbr>jstring<br> Ljava/net/Socket; <wbr><wbr><wbr><wbr><wbr>Socket <wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr><wbr>jobject</wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

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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"

下边是我在做串口通信时的代码：

staticconstchar*classPathName="android/serialport/SerialPort";

//注意Ljava/io/FileDescriptor;最后的分号，刚开始做时漏了这个分号，查了两天时间，汗

staticJNINativeMethodmethods[]={

{"open","(Ljava/lang/String;I)Ljava/io/FileDescriptor;",(void*)android_serialport_SerialPort_open},

{"close","()V",(void*)android_serialport_SerialPort_close},

};


/*

*Registerseveralnativemethodsforoneclass.

*/

staticintregisterNativeMethods(JNIEnv*env,constchar*className,
JNINativeMethod*gMethods,intnumMethods)
{
jclassclazz;

clazz=(*env)->FindClass(env,className);
if(clazz==NULL)
returnJNI_FALSE;

if((*env)->RegisterNatives(env,clazz,gMethods,numMethods)<0)
{
LOGE("registernativerserror");
returnJNI_FALSE;
}

returnJNI_TRUE;
}



/*

*Registernativemethodsforallclassesweknowabout.

*

*returnsJNI_TRUEonsuccess.

*/

staticintregisterNatives(JNIEnv*env)

{

if(!registerNativeMethods(env,classPathName,

methods,sizeof(methods)/sizeof(methods[0]))){

returnJNI_FALSE;

}

returnJNI_TRUE;

}

/*

*ThisiscalledbytheVMwhenthesharedlibraryisfirstloaded.

*/

jintJNI_OnLoad(JavaVM*vm,void*reserved)
{
JNIEnv*env=NULL;
jintresult=-1;

LOGI("EnteringJNI_OnLoad\n");

if((*vm)->GetEnv(vm,(void**)&env,JNI_VERSION_1_4)!=JNI_OK)
gotobail;

assert(env!=NULL);

if(!registerNatives(env))
gotobail;

/*success--returnvalidversionnumber*/
result=JNI_VERSION_1_4;

bail:
LOGI("LeavingJNI_OnLoad(result=0x%x)\n",result);
returnresult;
}

static const char *classPathName = "android/serialport/SerialPort";//注意Ljava/io/FileDescriptor;最后的分号，刚开始做时漏了这个分号，查了两天时间，汗static JNINativeMethod methods[] = {  {"open", "(Ljava/lang/String;I)Ljava/io/FileDescriptor;", (void*)android_serialport_SerialPort_open },  {"close", "()V", (void*)android_serialport_SerialPort_close },};/* * Register several native methods for one class. */static int registerNativeMethods(JNIEnv* env, const char* className,    JNINativeMethod* gMethods, int numMethods){    jclass clazz;    clazz = (*env)->FindClass(env, className);    if (clazz == NULL)        return JNI_FALSE;    if ((*env)->RegisterNatives(env, clazz, gMethods, numMethods) < 0)    {LOGE("register nativers error");        return JNI_FALSE;    }    return JNI_TRUE;}/* * Register native methods for all classes we know about. * * returns JNI_TRUE on success. */static int registerNatives(JNIEnv* env){  if (!registerNativeMethods(env, classPathName,                 methods, sizeof(methods) / sizeof(methods[0]))) {    return JNI_FALSE;  }  return JNI_TRUE;}/* * This is called by the VM when the shared library is first loaded. */jint JNI_OnLoad(JavaVM* vm, void* reserved){    JNIEnv* env = NULL;    jint result = -1;    LOGI("Entering JNI_OnLoad\n");    if ((*vm)->GetEnv(vm, (void**) &env, JNI_VERSION_1_4) != JNI_OK)        goto bail;    assert(env != NULL);    if (!registerNatives(env))        goto bail;    /* success -- return valid version number */    result = JNI_VERSION_1_4;bail:    LOGI("Leaving JNI_OnLoad (result=0x%x)\n", result);    return result;}

android_serialport_SeriaPort_open的函数原型为：

static jobject android_serialport_SerialPort_open(JNIEnv *env, jobject thiz, jstring path, jint baudrate)；

static void android_serialport_SerialPort_close(JNIEnv *env, jobject thiz)；

另外还要注意一点，如果是C++,使用的是env, 如果是C,使用的是(*env)，最好参考相应系统中的代码来写。