public class ZKInitializer implements WebApplicationInitializer {

	@Override
	public void onStartup(ServletContext servletContext) {
		servletContext.addListener(new HttpSessionListener());
...

Failed to invoke class org.zkoss.zkmax.init.WebAppInit
java.lang.UnsupportedOperationException
	at org.eclipse.jetty.servlet.ServletContextHandler$Context.addServlet(ServletContextHandler.java:1029)
	at org.zkoss.zkmax.ui.comet.CometAsyncServlet.install(CometAsyncServlet.java:56)
	at org.zkoss.zkmax.init.WebAppInit.init(WebAppInit.java:32)
	at org.zkoss.zk.ui.util.Configuration.invokeWebAppInits(Configuration.java:733)
	at org.zkoss.zk.ui.impl.AbstractWebApp.init(AbstractWebApp.java:204)
	at org.zkoss.zk.ui.http.SimpleWebApp.init(SimpleWebApp.java:58)
	at org.zkoss.zk.ui.http.WebManager.<init>(WebManager.java:228)
	at org.zkoss.zk.ui.http.HttpSessionListener23.contextInitialized(HttpSessionListener23.java:140)
	at org.eclipse.jetty.server.handler.ContextHandler.callContextInitialized(ContextHandler.java:800)
	at org.eclipse.jetty.servlet.ServletContextHandler.callContextInitialized(ServletContextHandler.java:446)
	at org.eclipse.jetty.server.handler.ContextHandler.startContext(ContextHandler.java:792)
	at org.eclipse.jetty.servlet.ServletContextHandler.startContext(ServletContextHandler.java:296)
	at org.eclipse.jetty.webapp.WebAppContext.startWebapp(WebAppContext.java:1359)