Object creation in JNI

Question

I have a class and an enum inside it like so:

public class ppmerrJNI 
{
   public enum ppm_err_e {
        ONE(0),
        TWO(1),
        THREE(2);

         private int code;

         private ppm_err_e(int code) {
           this.code = code;
         }

         public int getValue() {
           return code;
         }

        ppm_err_e getObj(int i) {
            return ppm_err_e.values()[i];
        }
    };
...
}

and I have JNI wrapper function declared like so:

JNIEXPORT jobject JNICALL Java_ppmerrJNI_ppm_1get_1last_1error(JNIEnv *env, jobject thisObj) {
       int someNumber = 5;

       jclass employeeClass = (*env)->FindClass(env,"ppmerrJNI$ppm_err_e");
       jmethodID midConstructor = (*env)->GetMethodID(env, employeeClass, "<init>", "(I)V");
       jobject employeeObject = (*env)->NewObject(env, employeeClass, midConstructor, someNumber);
       return employeeObject ;
}

On the second line (GetMethodId) I get: "Exception in thread "main" java.lang.NoSuchMethodError: ".

Basically, I want to call constructor of enum type "ppm_err_e", which resides inside of a class "ppmerrJNI". I want to return an enum object based on someNumber number and this is the approach I've taken; can settle for any other possible solution too.

I've also tried with:

jmethodID constructor = (*env)->GetMethodID(env, enumClass, "getObj", "(I)LppmerrJNI$ppm_err_e;");

but it always returned null.

Thank you in advance!

Have you verified that those names are the actual names for your class? `"ppmerrJNI$ppm_err_e"`? You don't check if `FindClass` was successful. — PaulMcKenzie, Aug 11 '14 at 16:35
Is it possible that I can't call enum's constructor because enums have private constructor? — user2340939, Aug 12 '14 at 09:40
Yes, that seems to be the case: http://stackoverflow.com/questions/9614282/how-to-create-an-instance-of-enum-using-reflection-in-java — user2340939, Aug 12 '14 at 09:45

score 2 · Accepted Answer · edited Jun 10 '15 at 07:30

You can't instantiate enums. That was the reason why I couldn't call JNI's NewObject() method (enum permits only private constructors, so instantiation is not possible - you need public constructor). I solved it by making a method inside the outer class which takes enums index as an argument and returns corresponding enum instance. The method is called in JNI via CallObjectMethod() instead of NewObject().

score 1 · Answer 2 · answered Aug 11 '14 at 16:44

Try something like the following. You should be accessing static fields, not constructing the enum.

JNIEXPORT jobject JNICALL Java_ppmerrJNI_ppm_1get_1last_1error(JNIEnv *env, jobject thisObj) {
       int someNumber = 5;

       jclass employeeClass   = (*env)->FindClass(env, "ppmerrJNI");
       jfieldID oneField = (*env)->GetStaticFieldID(env, employeeClass , "ONE", "ppmerrJNI$ppm_err_e;");
       jobject STATE_ONE      = (*env)->GetStaticObjectField(env, employeeClass, oneField);

       return employeeObject ;
}

I know how to get enum value by ist name/string, but I need to get it by a number (someNumber)... — user2340939, Aug 12 '14 at 07:29

Object creation in JNI

2 Answers2