Keep a page stateless

Keep a page stateless when it is public, visited by many anonymous users or meant to be bookmarked, such as a home page, a product page or a login page. A stateless page needs no session. Stateful and stateless pages explains the trade-off.

Make the page bookmarkable

Wicket must be able to create the page itself, for every request. Give it a public constructor without arguments, or one that takes only PageParameters, and read everything else from the parameters:

public class ProductPage extends WebPage {
    public ProductPage(PageParameters parameters) {
        long id = parameters.get("id").toLong();
        IModel<Product> product = LoadableDetachableModel.of(() -> productService.find(id));
        add(new Label("name", product.map(Product::getName)));
    }
}

Link to it by class, with BookmarkablePageLink or setResponsePage(ProductPage.class, parameters), so Wicket creates it from the URL. Pass parameters to a page shows how to build those URLs.

Use stateless components

A page is stateless only when every visible, enabled component on it, and every behavior on those components, is stateless. These components are:

Labels, images, containers and other components without a callback are stateless already. A stateful component that is hidden or disabled does not make the page stateful.

Calling getPageReference() on a page makes it stateful, and so does setStatelessHint(false).

Find the component that makes the page stateful

Page.isPageStateless() tells you whether the page is stateless. To find the culprit, visit the components and log those that are not:

@Override
protected void onBeforeRender() {
    super.onBeforeRender();
    if (getApplication().usesDevelopmentConfig() && !isPageStateless()) {
        visitChildren((component, visit) -> {
            if (!component.isStateless()) {
                log.warn("Stateful component: {}", component.getPageRelativePath());
            }
        });
    }
}

With debug logging on for org.apache.wicket.Page, Wicket logs the same thing itself: “Page ‘…’ is not stateless because of component with path ‘…’”.

Fail fast with StatelessChecker

wicket-devutils has an annotation and a checker that throws an exception when a page or component marked as stateless is not. Register the checker in development mode:

WicketApplication.java

@Override
protected void init() {
    super.init();
    if (usesDevelopmentConfig()) {
        getComponentPostOnBeforeRenderListeners().add(new StatelessChecker());
    }
}

and annotate the page:

@StatelessComponent
public class ProductPage extends WebPage {
    // ...
}

When ProductPage renders with a stateful component, StatelessChecker throws a StatelessCheckFailureException that names the component. The annotation has no effect without the checker, and is inherited by subclasses; @StatelessComponent(enabled = false) turns it off for a subclass that is allowed to be stateful.

Test it

Assert the statelessness in a test, so a later change cannot break it unnoticed:

@Test
void productPageIsStateless() {
    tester.startPage(ProductPage.class, new PageParameters().add("id", 42));
    assertTrue(tester.getLastRenderedPage().isPageStateless());
}

Test pages and components shows how to set up WicketTester.