import javafx.application.Application;
import javafx.application.HostServices;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.stage.Modality;
import javafx.stage.Stage;
import javafx.stage.StageStyle;
import netscape.javascript.JSObject;

public class Main extends Application {
  public static void main(String[] args) {
    Application.launch(args);
  }

  @Override
  public void start(Stage stage) {

    Button openURLButton = new Button("Go!");
    openURLButton.setOnAction(e ->{
      HostServices host = getHostServices();
      JSObject js = host.getWebContext();
      if (js == null) {
          Stage s = new Stage(StageStyle.UTILITY);
          s.initModality(Modality.WINDOW_MODAL);
          Label msgLabel = new Label("This is an FX alert!");
          Group root = new Group(msgLabel);
          Scene scene = new Scene(root);
          s.setScene(scene);
          s.setTitle("FX Alert");
          s.show();
      } else {
          js.eval("window.alert('This is a JavaScript alert!')");
      }      
    });

    Scene scene = new Scene(openURLButton);
    stage.setScene(scene);
    stage.setTitle("Knowing the Host");
    stage.show();
  }
}