Create the build. The application plugin runs the program on the module path and passes the one JVM flag native access needs. On macOS the UI thread has to be the first thread, so the second flag is added there (ADR-0039).

plugins {
    id 'java'
    id 'application'
}

java {
    toolchain { languageVersion = JavaLanguageVersion.of(25) }
}

repositories {
    mavenCentral()
    maven { url = 'https://central.sonatype.com/repository/maven-snapshots/' }
}

dependencies {
    implementation platform('dev.goldberry:goldberry-bom:2026.1-SNAPSHOT')
    implementation 'dev.goldberry:goldberry'
    runtimeOnly 'dev.goldberry:goldberry-natives::linux-x64'
    runtimeOnly 'dev.goldberry:goldberry-natives::linux-aarch64'
    runtimeOnly 'dev.goldberry:goldberry-natives::macos-aarch64'
    runtimeOnly 'dev.goldberry:goldberry-natives::windows-x64'
}

application {
    mainModule = 'com.example.hello'
    mainClass = 'com.example.hello.Hello'
    applicationDefaultJvmArgs = ['--enable-native-access=dev.goldberry.natives']
    if (System.getProperty('os.name').toLowerCase().contains('mac')) {
        applicationDefaultJvmArgs += '-XstartOnFirstThread'
    }
}

settings.gradle is one line: rootProject.name = 'hello'.

Declare the module. Two requires, and one opens so the toolkit can read the model’s fields and the resources beside it (ADR-0395).

module com.example.hello {
    requires dev.goldberry.core;
    requires dev.goldberry.widgets;

    opens com.example.hello to dev.goldberry.core;
}

Write the model. A class of fields marked @Bind, with the actions nested inside it as a record. Assigning a field is what notifies the window, so there is no setter to call and no event to raise (ADR-0134).

package com.example.hello;

import dev.goldberry.bind.Action;
import dev.goldberry.bind.Actions;
import dev.goldberry.bind.Bind;
import dev.goldberry.bind.Model;

@Model
public final class Counter {

    @Bind("counter.label") private String label = "Pressed 0 times";

    private int presses;

    @Actions
    public record Commands(Counter values) {

        @Action("counter.press")
        public void press() {
            values.presses++;
            values.label = "Pressed " + values.presses + (values.presses == 1 ? " time" : " times");
        }

        @Action("counter.reset")
        public void reset() {
            values.presses = 0;
            values.label = "Pressed 0 times";
        }
    }
}

The record is called Commands and not Actions: a nested type named Actions would shadow the annotation (Building an application).

Write the screen. The document names the value with bind= and the actions with press=. The text after bind= is what a preview shows when nothing is bound.

column id="hello" {
  text class="heading" "Hello, Goldberry"
  text id="count" bind="counter.label" "Pressed 0 times"
  row id="buttons" {
    button class="primary" press="counter.press" "Press me"
    button press="counter.reset" "Reset"
  }
}

Give it a look. The toolkit’s stylesheets draw the controls. Yours lays out the window. Every colour is a token, so the same rule is right on both themes.

#hello   { padding: 24px; gap: 12px; }
#buttons { gap: 8px; }
#count   { color: var(--gb-accent); }

Write the application. The one class that knows a window exists. It names the models, inflates the document, lists the stylesheets, and hands itself to the launcher.

package com.example.hello;

import java.util.ArrayList;
import java.util.List;

import dev.goldberry.Application;
import dev.goldberry.Goldberry;
import dev.goldberry.css.CascadeLayer;
import dev.goldberry.css.Stylesheet;
import dev.goldberry.css.Theme;
import dev.goldberry.kdl.KdlParser;
import dev.goldberry.widget.Widget;
import dev.goldberry.widgets.Controls;
import dev.goldberry.widgets.Widgets;

public final class Hello implements Application {

    private final Counter counter = new Counter();
    private final Counter.Commands commands = new Counter.Commands(counter);
    private final Stylesheet styles = Stylesheet.resource(CascadeLayer.APPLICATION, Hello.class, "hello.css");

    @Override
    public String title() {
        return "Hello";
    }

    @Override
    public List<Object> models() {
        return List.of(counter, commands);
    }

    @Override
    public Widget root() {
        return Widgets.inflater(models().toArray())
                .inflate(KdlParser.resource(Hello.class, "hello.kdl").getFirst());
    }

    @Override
    public List<Stylesheet> stylesheets() {
        var sheets = new ArrayList<>(Controls.stylesheets(Theme.NORD_DARK));
        sheets.add(styles);
        return sheets;
    }

    public static void main(String[] args) {
        Goldberry.launch(new Hello(), args);
    }
}

Run it.

./gradlew run

A window opens with a heading, a line of text and two buttons. Press the first and the line counts. Nothing in the application repainted: assigning label told the bound text and asked the window for a frame (ADR-0128).