How do I access return value of a Java method returning java.lang.String from C++ in JNI?

Question

I am trying to pass back a string from a Java method called from C++. I am not able to find out what JNI function should I call to access the method and be returned a jstring value.

My code follows:

C++ part

main() {
    jclass cls;
    jmethodID mid;
    jstring rv;

/** ... omitted code ... */

    cls = env->FindClass("ClassifierWrapper");
    mid = env->GetMethodID(cls, "getString","()Ljava/lang/String");

    rv = env->CallStatic<TYPE>Method(cls, mid, 0);
    const char *strReturn = env->GetStringUTFChars(env, rv, 0);

    env->ReleaseStringUTFChars(rv, strReturn);
}

Java Code

public class ClassifierWrapper {
    public String getString() { return "TEST";}
}

The Method Signature (from "javap -s Class")

public java.lang.String getString();
  Signature: ()Ljava/lang/String;

Answer 1

You should have

cls = env->FindClass("ClassifierWrapper"); 

Then you need to invoke the constructor to get a new object:

jmethodID classifierConstructor = env->GetMethodID(cls,"<init>", "()V"); 
if (classifierConstructor == NULL) {
  return NULL; /* exception thrown */
}

jobject classifierObj = env->NewObject( cls, classifierConstructor);

You are getting static method (even though the method name is wrong). But you need to get the instance method since getString() is not static.

jmethodID getStringMethod = env->GetMethodID(cls, "getString", "()Ljava/lang/String;"); 

Now invoke the method:

rv = env->CallObjectMethod(classifierObj, getStringMethod, 0); 
const char *strReturn = env->GetStringUTFChars(env, rv, 0);