Proper way to get string list in JNI

Question

I have a JNI function in C which is getting passed in a Java FILE class which represents a directory listing. I would like to call the list() function and get the list of strings (files in the directory). What is the best way to do this?

Right now I have

static void* my_function(JNIEnv *env, jobject obj, jobject dir){
    jarray listRet;
    jclass cls = (*env)->GetObjectClass(env, dir);
    jmethodID method = (*env)->GetMethodID(env, cls, "list", "()[Ljava/lang/String");
    listRet = (*env)->CallObjectMethod(env, cls, method);

    jsize stringCount = (*env)->GetArrayLength(env, listRet);
}

However, by adding logging statements, it seems that it never gets past the GetObjectClass call. So, is this call correct? Further, is the GetMethodID call correct? The return type of list() is a (java) String[]. Is there anywhere else that I'm going wrong?

Answer 1

list is not a static method of File. That is, it belongs to an instance of File (dir in your case), not to the File class.

So instead of:

listRet = (*env)->CallObjectMethod(env, cls, method);

you should be using:

listRet = (*env)->CallObjectMethod(env, dir, method);

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Also, you seem to be missing a semicolon in the signature for list. It should be "()[Ljava/lang/String;"