code: public void init() throws Exception {
          Iterator<UIProvider> providers = ServiceLoader.load(UIProvider.class).iterator();
          uiProvider = providers.hasNext() ? providers.next() : new DefaultUIProvider();
      }
docs: Initialize the UI Provider by looking for an UIProvider inside the launcher
  or fallback to the default UI.
  <p>
  A custom implementation must be embedded inside the launcher jar, and
  /META-INF/services/fxlauncher.UIProvider must point to the new implementation class.
  <p>
  You must do this manually/in your build right around the "embed manifest" step.