JNI-Java访问C库-白红宇

JNI-Java访问C库

阅读量：4125 次

发布时间：2019-05-25

本文共 4499 字，大约阅读时间需要 14 分钟。

JNI字段描述符

用"["表示数组，比如"int []"表示为"[I"

对于类，要用全称"L包.子包.类名;"(前面有"L"，后面有";")，比如"Ljava/lang/String;"

除String类外，其他的类都用Object表示，即"Ljava/lang/Object;"

native.c

/*编译：gcc -I/usr/lib/jvm/java-1.7.0-openjdk-amd64/include/ -fPIC -shared -o libnative.so native.c*/#include 
       
        #include 
        
         #include 
         
           //usr/lib/jvm/java-1.7.0-openjdk-amd64/include/jni.h#if 0typedef struct {	char *name;			/* java里调用的函数名 */	char *signature;	/* JNI字段描述符，用来表示Jave里调用的函数的参数和返回值类型 */	void *fnPtr;		/* C语言实现的本地函数 */}JNINativeMethod;#endif/*注意：C函数比Java里的声明多2个参数: (JNIEnv *env, jclass cls)*/void c_hello(JNIEnv *env, jobject cls){	printf("hello world!\n");}/*	形参：int	返回值：int*/jint c_myprintf(JNIEnv *env, jobject cls, jint m){	printf("hello world! %d\n",m);	return 100;}/*	形参：String	返回值：String*/jstring JNICALL c_display_string(JNIEnv *env, jobject cls, jstring str){	//printf("this is c : %s\n",str);	//return "return from C";    const jbyte *cstr;    cstr = (*env)->GetStringUTFChars(env, str, NULL);    if (cstr == NULL) {        return NULL; /* OutOfMemoryError already thrown */    }    printf("get string from java:%s\n", cstr);	(*env)->ReleaseStringUTFChars(env, str, cstr);	return (*env)->NewStringUTF(env, "return from c");}/*	形参：传入数组	返回值：int*/jint JNICALL c_display_data(JNIEnv *env, jobject cls, jintArray arr){	jint *carr;    jint i, sum = 0;    carr = (*env)->GetIntArrayElements(env, arr, NULL);    if (carr == NULL) {        return 0; /* exception occurred */    }    for (i = 0; i < (*env)->GetArrayLength(env,arr); i++) {        sum += carr[i];    }    (*env)->ReleaseIntArrayElements(env, arr, carr, 0);    return sum;}/* * Class:     JNIDemo * Method:    display_array * Signature: ([I)[I * * 形参：传入数组 * 返回值：输出数组 */jintArray JNICALL c_display_array(JNIEnv *env, jobject cls, jintArray arr){	jint *carr;	jint *oarr;	jintArray rarr;    jint i, n = 0;    carr = (*env)->GetIntArrayElements(env, arr, NULL);    if (carr == NULL) {        return 0; /* exception occurred */    }	n = (*env)->GetArrayLength(env,arr);	oarr = malloc(sizeof(jint)*n);	if(oarr == NULL) {		(*env)->ReleaseIntArrayElements(env, arr, carr, 0);		return 0;			}	for(i = 0; i < n; i++) {		oarr[i] = carr[n-1-i];	}	/*Creates an jint array with thegiven length.*/	rarr = (*env)->NewIntArray(env,n);	if(rarr == NULL) {		(*env)->ReleaseIntArrayElements(env, arr, carr, 0);		return 0;	}	/*Copies the contents of primitive arrays to or from a preallocated C buffer.		void (JNICALL *SetIntArrayRegion)		  (JNIEnv *env, jintArray array, jsize start, jsize len, const jint *buf);	*/	(*env)->SetIntArrayRegion(env,rarr,0,n,oarr);    (*env)->ReleaseIntArrayElements(env, arr, carr, 0);	free(oarr);    return rarr;}/* 定义一个映射数组JNINativeMethod[]可以注册多个本地函数*/static const JNINativeMethod methods[] = {	{"hello","()V",(void *)c_hello},	{"myprintf","(I)I",(void *)c_myprintf},	{"display_string","(Ljava/lang/String;)Ljava/lang/String;",(void *)c_display_string},	{"display_data","([I)I",(void *)c_display_data},	{"display_array","([I)[I",(void *)c_display_array},};/* 只要java 加载库System.loadLibrary      就会执行该函数          */JNIEXPORT jint JNICALLJNI_OnLoad(JavaVM *jvm, void *reserved){    JNIEnv *env;    jclass cls;	/* 获得运行环境 */    if ((*jvm)->GetEnv(jvm, (void **)&env, JNI_VERSION_1_4)) {        return JNI_ERR; /* JNI version not supported */    }    cls = (*env)->FindClass(env, "JNIDemo");    if (cls == NULL) {        return JNI_ERR;    }	/* 建立联系 java<--->c*/	if((*env)->RegisterNatives(env,cls,methods,5) < 0)		return JNI_ERR;	    return JNI_VERSION_1_4;}

JNIDemo.java

/*编译:		javac JNIDemo.java		export LD_LIBRARY_PATH=.		java JNIDemo自动生成字段描述符：		javac JNIDemo.java		javah -jni JNIDemo*/public class JNIDemo {	/* 静态代码块，加载库只执行一次 */		static {			/* 1. load 加载libnative.so */			System.loadLibrary("native");/* libnative.so */		}	//public native static void hello();//声明	public native void hello();//声明	public native int myprintf(int m);	public native String display_string(String str);	public native int display_data(int a[]);	public native int[] display_array(int a[]);	public static void main(String args[]){		JNIDemo d = new JNIDemo();		int a[] = {1, 2, 3, 4, 5};		int b[] = null;		int i;		/* 2. java <--> c 建立映射关系 */		/* 3.调用 */		d.hello();		System.out.println(d.myprintf(123));		System.out.println(d.display_string("this is java jni demo"));		System.out.println(d.display_data(a));		b = d.display_array(a);		for(i = 0; i < b.length; i++)			System.out.println(b[i]);	}}

调试结果：