JavaFX FXML, controllers, CSS and Scene Builder

Registration.fxml

<?xml version="1.0" encoding="UTF-8"?>

<?import javafx.geometry.Insets?>
<?import javafx.scene.control.Button?>
<?import javafx.scene.control.ComboBox?>
<?import javafx.scene.control.Label?>
<?import javafx.scene.control.TextField?>
<?import javafx.scene.layout.GridPane?>
<?import javafx.scene.layout.VBox?>

<!-- A course registration form: two fields, a button and a message, wired to RegistrationController. -->
<VBox xmlns="http://javafx.com/javafx" xmlns:fx="http://javafx.com/fxml"
      fx:controller="RegistrationController" spacing="12" styleClass="form">
  <padding><Insets top="16" right="16" bottom="16" left="16"/></padding>
  <Label text="Course registration" styleClass="title"/>
  <GridPane hgap="8" vgap="8">
    <Label text="_Name" mnemonicParsing="true" labelFor="$name" GridPane.rowIndex="0" GridPane.columnIndex="0"/>
    <TextField fx:id="name" promptText="As on your ID card" GridPane.rowIndex="0" GridPane.columnIndex="1"/>
    <Label text="_Branch" mnemonicParsing="true" labelFor="$branch" GridPane.rowIndex="1" GridPane.columnIndex="0"/>
    <ComboBox fx:id="branch" GridPane.rowIndex="1" GridPane.columnIndex="1"/>
  </GridPane>
  <Button fx:id="register" text="Register" defaultButton="true" onAction="#register"/>
  <Label fx:id="message"/>
</VBox>

RegistrationController.java

import javafx.fxml.FXML;
import javafx.scene.control.Button;
import javafx.scene.control.ComboBox;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;

/** The controller of Registration.fxml: FXMLLoader creates it and fills the @FXML fields from the fx:id attributes. */
public class RegistrationController {
    @FXML private TextField name;
    @FXML private ComboBox<String> branch;
    @FXML private Button register;
    @FXML private Label message;

    /** Whether the constructor and initialize() say when they run (LoadCost.java turns it off). */
    static boolean printSteps = true;

    public RegistrationController() {
        if (printSteps) {
            System.out.println("  constructor: name field is " + name); // nothing is injected yet
        }
    }

    /** Called once, after every @FXML field has been injected. */
    @FXML
    private void initialize() {
        if (printSteps) {
            System.out.println("  initialize(): name field is a " + name.getClass().getSimpleName());
        }
        branch.getItems().setAll("Computer Engineering", "Information Technology", "Electrical Engineering");
        branch.getSelectionModel().selectFirst();
        register.disableProperty().bind(name.textProperty().isEmpty());
    }

    /** The handler that onAction="#register" names. */
    @FXML
    private void register() {
        message.setText("Welcome " + name.getText().strip() + ", " + branch.getValue());
    }
}

ShowForm.java

import java.nio.file.Path;
import javafx.application.Platform;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextField;

/** Loads Registration.fxml, prints what the loader built, then types a name and presses Register. */
public class ShowForm {
    public static void main(String[] args) throws Exception {
        FxRun.start();
        FxRun.onFx(() -> {
            try {
                System.out.println("loading Registration.fxml");
                FXMLLoader loader = new FXMLLoader(Path.of("Registration.fxml").toUri().toURL());
                Parent root = loader.load();
                System.out.println("root: " + root.getClass().getSimpleName() + " with " + root.getChildrenUnmodifiable().size() + " children; controller: " + loader.getController().getClass().getSimpleName());
                Button register = (Button) loader.getNamespace().get("register"); // every fx:id is also in the namespace
                System.out.println("Register disabled while the name is empty: " + register.isDisabled());
                ((TextField) loader.getNamespace().get("name")).setText("Kabir Mehta");
                register.fire();
                System.out.println("after typing and pressing Register: " + ((Label) loader.getNamespace().get("message")).getText());
            } catch (java.io.IOException e) {
                throw new java.io.UncheckedIOException(e);
            }
        });
        Platform.exit();
    }
}

FxRun.java

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import javafx.application.Platform;

/** Starts JavaFX without an Application subclass and runs code on the JavaFX Application Thread. */
final class FxRun {
    private FxRun() {
    }

    /** Starts the JavaFX runtime and waits until its thread is running. */
    static void start() throws InterruptedException {
        CountDownLatch started = new CountDownLatch(1);
        Platform.startup(started::countDown);
        started.await();
    }

    /** Computes a value on the JavaFX Application Thread and returns it. */
    static <T> T call(java.util.function.Supplier<T> task) throws InterruptedException {
        Object[] result = new Object[1];
        onFx(() -> result[0] = task.get());
        @SuppressWarnings("unchecked")
        T value = (T) result[0];
        return value;
    }

    /** Runs the task on the JavaFX Application Thread and waits for it, at most 10 seconds. */
    static void onFx(Runnable task) throws InterruptedException {
        CountDownLatch done = new CountDownLatch(1);
        Platform.runLater(() -> {
            try {
                task.run();
            } finally {
                done.countDown();
            }
        });
        if (!done.await(10, TimeUnit.SECONDS)) {
            throw new IllegalStateException("the JavaFX Application Thread did not answer");
        }
    }
}

fx.args

--module-path javafx-sdk-27/lib
--add-modules javafx.controls,javafx.fxml
--enable-native-access=javafx.graphics
-Dglass.platform=headless
-Dprism.order=sw