Stateless pages

Being stateful

By nature a page is stateless, i.e. a new instance is created for each request and discarded at the end. A page becomes stateful as soon as a stateful behavior or component is added in the tree. Stateful pages are stored in a storage for later use, i.e. in following requests the same page instance is reused instead of creating a new instance. Technically, the page may be deserialized so it is not always the same JVM instance but the important thing is that any state/data in the page will be preserved.

A Component declares that it needs to be stateful by returning false in #getStatelessHint() method:

protected boolean getStatelessHint()
{
    return false;
}

and Behavior by overriding:

public boolean getStatelessHint(Component component)
{
    return false;
}

Example

package org.apache.wicket.reference.stateless;

import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.link.Link;

public class StatefulPage extends WebPage
{
	private int counter = 0;

	public StatefulPage()
	{
		add(new Link<Void>("link") {

			@Override
			public void onClick()
			{
				counter++;
			}
		});
	}

	public int getCounter() {
		return counter;
	}
}

Click on an Link will lead to a request that will try to find the Page object that contains this link, then find the link itself in that page and finally execute its #onClick() method. The value of counter field will increment for each click on the link.

If StatelessLink is used instead:

package org.apache.wicket.reference.stateless;

import org.apache.wicket.markup.html.WebPage;
import org.apache.wicket.markup.html.link.StatelessLink;

public class StatelessPage extends WebPage
{
	private int counter = 0;

	public StatelessPage()
	{
		add(new StatelessLink<Void>("link") {

			@Override
			public void onClick()
			{
				counter++;
			}
		});
	}

	public int getCounter() {
		return counter;
	}
}

then the value of counter field will always be 0 because a new instance of StatelessPage is created for each click on the link.

Keeping it stateless

To make sure that a page is stateless and there is no stateful component/behavior added to it by accident StatelessChecker can be used. If there is a stateful component or behavior then this checker will throw an exception with a message explaining where is the problem.

1. Add dependency to wicket-devutils

<dependency>
    <groupId>org.apache.wicket</groupId>
    <artifactId>wicket-devutils</artifactId>
    <version>${wicket.version}<version>
</dependency>

2. Register StatelessChecker

Register StatelessChecker as a listener that is invoked before rendering the page:

MyApplication.java

	@Override
	protected void init()
	{
		super.init();

		getComponentPreOnBeforeRenderListeners().add(new org.apache.wicket.devutils.stateless.StatelessChecker());
	}

3. Annotate the pages

Use StatelessComponent annotation to require the page to be stateless.

@org.apache.wicket.devutils.stateless.StatelessComponent
public class CheckedPage extends WebPage