java - JNI String return value

Question

I have a Java instance method which returns a String and I'm calling this method through JNI in C++. I have written the following code:

const char *DiagLayerContainer_getDESC(JNIEnv *env, jobject diagLayer) {
    jclass diagLayerClass = env->FindClass(PARSER_CLASS);
    jmethodID getDESCDiagLayerMethodID = env->GetMethodID(diagLayerClass, "getDESCDiagLayer", "(Ljava/lang/Object;)Ljava/lang/String;");
    jstring returnString = (jstring) env->CallObjectMethod(diagLayer, getDESCDiagLayerMethodID);
    return env->GetStringUTFChars(returnString, JNI_FALSE);
}

How do I get the string and convert it to a const char *?

My program crashes on the last line with access violation to 0x00000000. returnString is not NULL.

Answer 1

According to GetStringUTFChars, the last parameter is a pointer to jboolean.

Change

return env->GetStringUTFChars(returnString, JNI_FALSE);

to

return env->GetStringUTFChars(returnString, NULL);

Or better yet, return a std::string

std::string DiagLayerContainer_getDESC(...) {
    ...
    const char *js = env->GetStringUTFChars(returnString, NULL);
    std::string cs(js);
    env->ReleaseStringUTFChars(returnString, js);
    return cs;
}

I've built a similar simple example and the code as is, seems fine so far.

Although, there are two possible error sources.

The first one is the method signature. Try "()Ljava/lang/String;" instead of "(Ljava/lang/Object;)Ljava/lang/String;".

The second one is in the java source itself. If the java method returns a null string, CallObjectMethod() will return a NULL jstring and GetStringUTFChars() fails.

Add a

if (returnString == NULL)
    return NULL;

after CallObjectMethod().

So look into the java source and see, whether the method getDESCDiagLayer() might return a null string.