Apache Wicket , version 11

HelloWorld.html

<h1 wicket:id="message">Message goes here</h1>

HelloWorld.java

public class HelloWorld extends WebPage {
    public HelloWorld() {
        add(new Label("message", "Hello World!"));
    }
}

AddressPanel.java

public class AddressPanel extends Panel {
    public AddressPanel(String id, IModel<Address> address) {
        super(id, new CompoundPropertyModel<>(address));
        add(new TextField<String>("street"));
        add(new TextField<String>("city"));
    }
}

// one component, used twice on the same form
form.add(new AddressPanel("billing", billingAddress));
form.add(new AddressPanel("shipping", shippingAddress));

CounterPage.java

Label count = new Label("count", () -> clicks);
add(count.setOutputMarkupId(true));

add(new AjaxLink<Void>("increment") {
    @Override
    public void onClick(AjaxRequestTarget target) {
        clicks++;
        target.add(count);
    }
});

MyApplication.java

@Override
protected void init() {
    super.init();
    // CSP is on by default; allow one extra image host
    getCspSettings().blocking()
        .add(CSPDirective.IMG_SRC, "https://images.example.org");
}

HelloWorldTest.java

WicketTester tester = new WicketTester(new MyApplication());
tester.startPage(HelloWorld.class);
tester.assertLabel("message", "Hello World!");