Java – JNI string return value

I have a Java instance method that returns a string. I call this method through JNI in C I wrote 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 a string and convert it to const char *?

My program crashed on the last line with an access violation of 0x00000000 Returnstring is not null

Solution

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 an 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 code, which seems good so far

Although, there are two possible sources of error

The first is method signature Try "() ljava / Lang / string;" Instead of "(ljava / Lang / object;) Ljava / lang / String;”.

The second is the Java source code itself If the Java method returns an empty string, callobjectmethod() will return null jstring and getstringutfchars() will fail

Add a

if (returnString == NULL)
    return NULL;

After callobjectmethod()

So check the Java source code to see if the method getdescdiaglayer () can return an empty string