Passing a List of Strings from Java to C

Question

I have need to pass List of Strings from Java to C through JNI. My Java program pass a List argument and C program accepts a list.

Below is the code which I tried.

JNIEXPORT jobject JNICALL Java_jni_CallJNIfunction(JNIEnv *env,  
                                                         jobjectArray jParameters){

    list<const char*> cParameters;

    jsize stringCount = env->GetArrayLength(jParameters);

    for (int i=0; i<stringCount; i++) {
       jstring arrElement = (jstring) (env->GetObjectArrayElement(jParameters, i));
       const char* nativeElement = env->GetStringUTFChars( arrElement, NULL);

       cParameters.push_back(nativeElement);
       env->ReleaseStringUTFChars(arrElement, nativeElement);
    }

    CallCfunction(cParameters);

}

But my JVM crashes at GetStringUTFChars() line. What is the wrong with this program?

When you say that you pass a List of arguments, do you really mean an array? The C function seems to be written to accept an array, and if Java passes a List object, you can expect it to crash at some point. — Joni
– Joni, Commented Oct 16, 2012 at 7:32

Aubin · Accepted Answer · 2012-10-19 18:06:31Z

2

You do:

const char* nativeElement = env->GetStringUTFChars( arrElement, NULL);
cParameters.push_back( nativeElement );
env->ReleaseStringUTFChars(arrElement, nativeElement);

You release the strings you store into a list, so your list contains a lot of bad pointers!

You must copy the string into a long time allocated space, you have the choice between std::string, char*+malloc, or use-it-and-forget-it approach.

Explanation for the third solution :

for( int i = 0; i < stringCount; ++i )
{
   jstring arrElement = (jstring) (env->GetObjectArrayElement(jParameters, i));
   const char* nativeElement = env->GetStringUTFChars( arrElement, NULL);
   CallCfunction( nativeElement ); // modified to process an item not a list<
   env->ReleaseStringUTFChars(arrElement, nativeElement);
}

answered Oct 19, 2012 at 18:06

Aubin

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Comments

GooseSerbus · Accepted Answer · 2012-10-19 18:02:18Z

The following code will take a Set<String> and convert it into a std::vector<std::string>, but I wouldn't do this, you'd be better off converting the set to an array using the toArray method on the Set, you can then use your original code.

JNIEXPORT jobject JNICALL Java_jni_CallJNIfunction(
  JNIEnv *env,
  jclass,
  jobject setObj) {

    jmethodID iteratorID = env->GetMethodID(env->FindClass("java/util/Set"), "iterator", "()Ljava/util/Iterator;");
    jclass iterator = env->FindClass("java/util/Iterator");
    jmethodID hasNextID = env->GetMethodID(iterator, "hasNext", "()Z");
    jmethodID nextID = env->GetMethodID(iterator, "next", "()Ljava/lang/Object;");

    std::vector<std::string> strSet;

    jobject iteratorObj = env->CallObjectMethod(setObj, iteratorID);
    while (env->CallBooleanMethod(iteratorObj, hasNextID) == JNI_TRUE) {
      jstring current = (jstring)env->CallObjectMethod(iteratorObj, nextID);
      const char* str = env->GetStringUTFChars(current, NULL);

      strSet.push_back(str);

      env->ReleaseStringUTFChars(current, str);
    }
}

But unless you've got a massive Set which copying to an array would be too slow or take too much memory, then I would convert to an array.

Jan Bergström · Accepted Answer · 2016-02-18 02:59:54Z

0

The first answer in plain simple C-function is:

char **GetStringsfromJniStringArray(JNIEnv *env, jobjectArray stringArray) {
    size_t stringCount = (size_t)(*env)->GetArrayLength(env, stringArray);
    char **Strings=calloc(stringCount, sizeof(char*));
    int i = 0;
    for(i = 0; i < (int)stringCount; ++i )
    {
        jstring jniString = (jstring) (*env)->GetObjectArrayElement(env, stringArray, i);
        const char *TempString = (*env)->GetStringUTFChars(env, jniString, NULL);
        Strings[i] = calloc(strlen(TempString)+1, sizeof(char));
        strcpy(Strings[i], TempString);
        (*env)->ReleaseStringUTFChars(env, jniString, TempString);
    }
    return Strings;
}

answered Feb 18, 2016 at 2:59

Jan Bergström

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