Export.cpp

extern "C" HelloCxxModule* createHelloCxxModule() {
  return new HelloCxxModule();
}

HelloCxxPackage.java

public final class HelloCxxPackage implements ReactPackage {
  @Override
  public List<NativeModule> createNativeModules(ReactApplicationContext reactContext) {
    return Arrays.<NativeModule>asList(
        // I have librnpackage-hellocxx.so the exported createHelloCxxModule() above.
        CxxModuleWrapper.makeDso("rnpackage-hellocxx", "createHelloCxxModule")
    );
  }
  @Override
  public List<ViewManager> createViewManagers(ReactApplicationContext reactContext) {
    return Collections.emptyList();
  }
}