Question

I am using JNI to extend my API in Java, in order to make it accessible through C++ (I am creating wrapper classes). I am experienced in Java but new to C++.

Now, I am creating a wrapper in C++ for a Java function getString() which returns a String. I am using GetStringUTFChars to get the C string from the Java String but I don’t know where to use ReleaseStringUTFChars which releases the C string. Here is my code (aClass and aClassInstance are private date members inside the wrapper class):

const char* getString() {
jmethodID methodID = env->GetMethodID(aClass,
        "methodName",
        "()Ljava/lang/String;");
if (methodID == NULL) {
    cout << "--methodID = NULL";
    exit(0);
}
jstring jstringResult = (jstring) env->CallObjectMethod(aClassInstance, methodID);
const char* result = env->GetStringUTFChars(jstringResult, NULL);
// env->ReleaseStringUTFChars(jstringResult, result); 
return result;
}

If I remove the // and use ReleaseStringUTFChars, result will be released and I will no longer be able to return it, right? Should I copy result to another char array before releasing it?

Thanks, any help is welcome!

K

Answer 1

Mahesh is right

std::string getString()
{
    jmethodID methodID = env->GetMethodID(aClass,
        "methodName",
        "()Ljava/lang/String;");
    if (methodID == NULL)
    {
        cout << "--methodID = NULL";
        exit(0);
    }
    jstring jstringResult = (jstring) env->CallObjectMethod(aClassInstance, methodID);
    const char* result = env->GetStringUTFChars(jstringResult, NULL);
    std::string result2(result);
    env->ReleaseStringUTFChars(jstringResult, result); 
    return result2;
}

std::string takes copies your characters for you and will automatically free when needed. Using raw pointers in C++ is generally frowned upon. Coming from Java you probably have little idea of the problems they can cause.