Passing enums as arguments within a jni function

Question

I am trying to pass enum arguments from a Java/JNI function to a C function. For example,
public native void java_func(enum pname);
java_jni_func(JNIEnv * env, jobject panel, enum pname)
c_func(pname);
where
c_func(enum pname);
But I get the error message "illegal use of this type as an expression"
Any ideas on how I go about solving my problem.

Answer 1

Hi DavidBray,
Your task is very simple, you can pass enum value as argument ( do not pay attention to the message above ). You need only to get the name of an enum value in native method. I wrote you a simple example in MS Windows.
Here is the code in Java:
package test;
public class JEnumToJNI {
     static {
          System.loadLibrary("JNIJEnumToJNI");
     public enum JEnum {
         Ok,
         Cancel
     public static void main(String[] args) {
          PrintEnumValue(JEnum.Ok);
          PrintEnumValue(JEnum.Cancel);
     static native void PrintEnumValue(JEnum val);
}The native method extracts the name of enum value and prints it. This is code of JNI module:
#define WIN32_LEAN_AND_MEAN
#include <windows.h>
#include <stdio.h>
#include <string>
#include "test_JEnumToJNI.h"
using namespace std;
// Converts jstring to string object
string jstring2string(JNIEnv *env, jstring js)
    string result;
    long len = env->GetStringUTFLength(js);
    char* p = (char*)malloc(len + 1);
    memset(p, 0, len + 1);
    if(len > 0)
        env->GetStringUTFRegion(js, 0, len, p);
    result = p;
    free(p);
    return result;
BOOL APIENTRY DllMain( HANDLE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
    return TRUE;
JNIEXPORT void JNICALL Java_test_JEnumToJNI_PrintEnumValue(JNIEnv* env, jclass, jobject enumVal)
    // Get enum class
    jclass jclassEnum = env->GetObjectClass(enumVal);
    if(jclassEnum != 0)
         //Get toString() method
        jmethodID toString_ID = env->GetMethodID(jclassEnum, "toString", "()Ljava/lang/String;");
        //Get enumVal name
        jstring jstrEnum = (jstring)env->CallObjectMethod(enumVal, toString_ID);
        fprintf(stdout, "enum value: %s\n", jstring2string(env, jstrEnum).c_str());
        // Delete local references created
        env->DeleteLocalRef(jstrEnum);
        env->DeleteLocalRef(jclassEnum);
    else
        // If jclassEnum == 0 clear Java Exception
        env->ExceptionClear();
}The header generated with javah.exe
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class test_JEnumToJNI */
#ifndef _Included_test_JEnumToJNI
#define _Included_test_JEnumToJNI
#ifdef __cplusplus
extern "C" {
#endif
* Class:     test_JEnumToJNI
* Method:    PrintEnumValue
* Signature: (Ltest/JEnumToJNI/JEnum;)V
JNIEXPORT void JNICALL Java_test_JEnumToJNI_PrintEnumValue
  (JNIEnv *, jclass, jobject);
#ifdef __cplusplus
#endif
#endifYour task is very simple and you need not to substitute enum values with numbers.