How to convert a jcharArray to a char[] in c

问题

I have a jcharArray that is passed into a C program through Java, and I need to know how to use the array in the C program. How do I convert my jcharArray bits into something I can use (char bits[])?

I tried writing this code using JNI

JNIEXPORT jint JNICALL Java_ex_NistStatisticalTestSuite_frequency
  (JNIEnv *env, jclass cls, jcharArray bits, jint jn)
{

    printf("running frequency test");

    int     i;
    double  f, s_obs, p_value, sum, sqrt2 = 1.41421356237309504880;
    int n=jn;
    char deletethis=(char)bits[0];
    sum = 0.0;
    for ( i=0; i<n; i++ )
        sum += 2*1-1;
    s_obs = fabs(sum)/sqrt(n);
    f = s_obs/sqrt2;
    p_value = erfc(f);

    return (jint)p_value;

}

but it fails to compile, saying:

frequency.c:19:2: error: invalid use of undefined type ‘struct _jobject’
  char deletethis=(char)bits[0];
  ^~~~
frequency.c:19:28: error: dereferencing pointer to incomplete type ‘struct _jobject’
  char deletethis=(char)bits[0];

回答1:

You must use jni functions, there are at least two methods:

Copy a region:

jchar buf[10]; 
(*env)->GetCharArrayRegion(env, bits, 0, 10, buf);

lock a memory region in JVM, then access it and finally release:

jchar *carr; 
carr = (*env)->GetCharArrayElements(env, bits, NULL); 
if (carr == NULL) {
    return 0; /* exception occurred */ 
} 
//for (int i=0; i<10; i++) {
//    do something with carr[i]; 
//} 
(*env)->ReleaseCharArrayElements(env, bits, carr, 0);

Here I assume your array is 10 elements in length. To find out number of elements in the array use GetArrayLength jni function.

来源：https://stackoverflow.com/questions/38882650/how-to-convert-a-jchararray-to-a-char-in-c

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