Build a stateless form

You need this for forms on public pages, such as a login, search or contact form. A normal Form makes its page stateful: Wicket stores the page in the session, and a submit after the session has timed out ends in a “page expired” error. A StatelessForm keeps the page stateless, so a submit always works and a crawler that requests the page does not create sessions.

Replace Form with StatelessForm

LoginPage.java

public class LoginPage extends WebPage {
    private String username;
    private String password;

    public LoginPage(PageParameters parameters) {
        super(parameters);

        StatelessForm<Void> form = new StatelessForm<>("form") {
            @Override
            protected void onSubmit() {
                if (AuthenticatedWebSession.get().signIn(username, password)) {
                    continueToOriginalDestination();
                    setResponsePage(getApplication().getHomePage());
                } else {
                    error("Unknown username or password");
                }
            }
        };
        add(form);

        IModel<String> usernameModel = LambdaModel.of(() -> username, value -> username = value);
        IModel<String> passwordModel = LambdaModel.of(() -> password, value -> password = value);
        form.add(new TextField<>("username", usernameModel).setRequired(true));
        form.add(new PasswordTextField("password", passwordModel));
        add(new FeedbackPanel("feedback"));
    }
}

LoginPage.html

<form wicket:id="form">
    <input type="text" wicket:id="username">
    <input type="password" wicket:id="password">
    <button type="submit">Log in</button>
</form>
<div wicket:id="feedback"></div>

Keep everything on the page stateless

A StatelessForm only helps when the whole page is stateless. The page must be bookmarkable: it needs a constructor without arguments or one that takes PageParameters. Every other component on the page must be stateless too. Labels, fields and buttons are; a normal Link or an Ajax behavior makes the page stateful again. Use a StatelessLink or a BookmarkablePageLink for links. Keep a page stateless shows how to find the component that breaks it.

Understand what the form gets on submit

When the form is submitted, Wicket does not find the page in the session. It creates a new instance of the page from its class and page parameters, and then processes the form on that new instance. For your code this means:

• The models start out the way the constructor sets them up. Anything you changed on the earlier instance, such as a value set by an earlier submit or a choice list filtered at runtime, is gone.
• Only what the browser submits reaches the models. A DropDownChoice can only find the submitted option if the constructor builds the same list of choices again.
• Hidden state belongs in page parameters or in a HiddenField, not in fields of the page.

A failed submit renders the new instance with the errors and the user’s input, as with a normal form.

Pass page parameters along

The form posts back to the page with the page’s current parameters, so a search form on /search?category=books keeps category across the submit. Read the parameters in the constructor, as the page is built anew on each submit:

public SearchPage(PageParameters parameters) {
    super(parameters);
    String category = parameters.get("category").toString("all");
    // build the form for this category
}

To show the result on a bookmarkable URL, redirect with new parameters from onSubmit():

setResponsePage(SearchPage.class, new PageParameters().set("q", query));