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

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);

The complete working solution is as below:

Java Side

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

Native Side

jclass cls;
jmethodID mid;
jstring rv;


cls = jniEnv->FindClass("ClassifierWrapper"); //plase also consider your package name as package\name\classname

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

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

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


jniEnv->ReleaseStringUTFChars(rv, strReturn);