Test.java

package com.test;

public class TestClass {

    private native String stringFromJNI();

    public String getString(){
        return stringFromJNI();
    }

    public TestClass(){
        // Replace with your assembly name
         System.loadLibrary( "test_assembly" );
    }
  
    public static void main(String[] args) {
        TestClass test = new TestClass();
        System.out.println( test.getString() );
    }
}

test_assembly.cpp

class TestClass{
    public:
        static constexpr const char* Name(){ return "com/test/TestClass"; };

        static jni::Local<jni::String> TestMethod( jni::JNIEnv& environment, jni::Class<TestClass>& ){
            return jni::Make<jni::String>( environment, "Hello from JNI !" );
        }
};

JNIEXPORT jni::jint JNICALL JNI_OnLoad( jni::JavaVM* vm, void* ){
    jni::JNIEnv& environment { jni::GetEnv( *vm ) };

    jni::RegisterNatives(
        environment,
        *jni::Class<TestClass>::Find( environment ),
        jni::MakeNativeMethod<decltype(&TestClass::TestMethod), &TestClass::TestMethod>( "stringFromJNI" )
    );

    return jni::Unwrap( jni::jni_version_1_6 );
}