Use links on stateless pages

A plain Link makes its page stateful: its URL points to a component of a page instance that Wicket keeps in the page store. On a page that has to stay stateless, such as a landing page or a login page that must not create a session, use a StatelessLink for clicks that run code, and a BookmarkablePageLink for plain navigation. Why a page is stateful or stateless is explained in Stateful and stateless pages.

Replace Link with StatelessLink

StatelessLink has the same onClick() as Link:

add(new StatelessLink<Void>("logout") {
    @Override
    public void onClick() {
        getSession().invalidate();
        setResponsePage(getApplication().getHomePage());
    }
});

The page must be bookmarkable, with a constructor without arguments or one that takes PageParameters, and all its other components must be stateless too. One stateful component anywhere on the page makes the whole page stateful.

Keep the state of a click in page parameters

The URL of a StatelessLink names the page class, the page parameters and the link. When the user clicks, Wicket creates a new instance of the page from those parameters, calls onClick() on the link in that new instance, and renders the result. Nothing from the instance the user saw survives. Instance fields start from their initial values on every click:

public class CounterPage extends WebPage {
    private int count;

    public CounterPage(PageParameters parameters) {
        super(parameters);
        add(new StatelessLink<Void>("increment") {
            @Override
            public void onClick() {
                count++;   // always goes from 0 to 1
            }
        });
    }
}

Put the state that a click needs in the page parameters instead, and send the user to a page with new parameters:

CounterPage.java

public class CounterPage extends WebPage {

    public CounterPage(PageParameters parameters) {
        super(parameters);
        int count = parameters.get("count").toInt(0);

        add(new Label("count", count));
        add(new StatelessLink<Void>("increment") {
            @Override
            public void onClick() {
                setResponsePage(CounterPage.class,
                    new PageParameters().add("count", count + 1));
            }
        });
    }
}

CounterPage.html

<p>Clicked <span wicket:id="count">0</span> times.</p>
<a wicket:id="increment">Click</a>

The link URL carries the current page parameters, so the new instance that handles the click sees the same count as the page the user clicked on.

When a click only navigates, a BookmarkablePageLink with the new parameters does the same with one request instead of a click request and a redirect:

add(new BookmarkablePageLink<Void>("increment", CounterPage.class,
    new PageParameters().add("count", count + 1)));

Check that the page stays stateless

A page that you expect to be stateless can turn stateful when someone adds a component later. Keep a page stateless shows how to find the component that makes a page stateful and how to test for it. For forms and Ajax on stateless pages, see Build a stateless form and Use Ajax on stateless pages.