从JNI(Java Native Interface)访问C常量(标题)

Question

如何从Java端(通过JNI)访问C头文件中定义的常量？

我有一个带有头文件c_header.h的C库:

// I'll try to get these values in java
#define PBYTES 64
#define SBYTES 128
#define SGBYTES 128

然后我有这个java代码libbind_jni.java:

package libbind;

public class libbind_jni {
static{
    try {
        System.load("libbind_jni.so");
    } catch (UnsatisfiedLinkError e) {
        System.err.println("Native code library failed to load.\n" + e);
        System.exit(1);
    }
}

public static native String[] pubm(String seed);

然后在libbind_jni.java上运行javah我生成JNI头文件jni_header.h:

#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     libbind_libbind_jni
 * Method:    pubm
 * Signature: (Ljava/lang/String;)[Ljava/lang/String;
 */
JNIEXPORT jobjectArray JNICALL Java_lib_f_1jni_pubm
  (JNIEnv *, jclass, jstring);

然后我为JNI库jni_source.c写了一个小C代码:

#include "jni_header.h"
#include "c_header.h"
#include <stdlib.h>

JNIEXPORT jobjectArray JNICALL Java_libbind_libbind_1jni_pubm
  (JNIEnv *env, jclass cls, jstring jniSeed){
    unsigned char vk[PBYTES];
    unsigned char sk[SBYTES];
    <whatever...>
}

- 这里编译java库的库 - libbind.so

而JAVA源码j_source.java:

import libbind.libbind_jni;

public class test {

    /* trying to access constants from library (libbind.so) */
    System.out.println(libbind_jni.PBYTES);
    System.out.println(libbind_jni.SBYTES);
    System.out.println(libbind_jni.SGBYTES);

}
public static void main(String[] args) {
    test();
}


$ javac test.java

test.java:5: cannot find symbol
symbol  : variable PBYTES
location: class libbind.libbind_jni
    System.out.println(libbind_jni.PBYTES);

所以问题是:如何从Java访问C头文件中的这些定义/常量？

谢谢.

Answer 1

简单的答案是你不能。您必须在 Java 中定义相同的值。

public static final int PBYTES = 64;
public static final int SBYTES = 128;
public static final int SGBYTES = 128;

或者您可以定义一些返回这些值的本机方法。但无法从 Java 访问此类宏。

public static native int PBYTES();
public static native int SBYTES();
public static native int SGBYTES();