From Documentation


A richlet is a small Java program that composes a user interface in Java for serving the user's request. Before composing UI in Java, we suggest you to know basic concept: UI Composing/Component-based UI first.

When a user requests the content of an URL, ZK Loader checks if the resource of the specified URL is a ZUML page or a richlet. If it is a ZUML page, ZK Loader will create components automatically based on the ZUML page's content as we described in the previous chapters.

If the resource is a richlet, ZK Loader hands over the processing to the richlet. What and how to create components are all handled by the richlet. In other words, it is the developer's job to create all the necessary components programmatically in response to the request.

The choice between the ZUML pages and richlets depends on your preference. However, the performance should not cause any concern since parsing ZUML is optimized.

Implement a Richlet

It is straightforward to implement a richlet. First, you have to implement the Richlet interface before mapping a URL to the richlet.

Implement a Richlet as a Java class

A richlet must implement the Richlet interface. However, you generally do not have to implement it from scratch. Rather, you could extend GenericRichlet, and the only thing you have to do is to override Richlet.service(Page). The method is called when an associated URL is requested. For example,

 package org.zkoss.reference.developer.uicomposing;

import org.zkoss.zk.ui.*;
import org.zkoss.zk.ui.event.*;
import org.zkoss.zul.*;

public class TestRichlet extends GenericRichlet {
	public void service(Page page) {
		page.setTitle("Richlet Test");

		final Window w = new Window("Richlet Test", "normal", false);
		new Label("Hello World!").setParent(w);
		final Label l = new Label();

		final Button b = new Button("Change");
				new EventListener() {
			int count;
			public void onEvent(Event evt) {
				l.setValue("" + ++count);


	public void init(RichletConfig config) {
		//initialize resources

	public void destroy() {
		//destroy resources

In Richlet, you have to compose UI by your own, but some components only support specific child components. We recommend you to read ZK Component Reference before you start to build.

As shown above (line 27), we have to invoke Component.setPage(Page page) explicitly to attach a root component to a page so it will be available at the client.

To have better control, you can even implement the Richlet.init(RichletConfig) and Richlet.destroy() methods to initialize and to destroy any resources required by the richlet when it is loaded.

In addition, you could implement Richlet.getLanguageDefinition() to use a different language as default (for example, implementing a richlet for mobile devices). By default, ZUL (aka., xul/html) is assumed.

Richlet Must Be Thread-Safe

Like a servlet, a single instance of richlet is created and shared with all users for all requests for the mapped URL. A richlet must handle the concurrent requests, and be careful to synchronize access to shared resources. In other words, a richlet (the implementation of the service method) must be thread-safe.

Don't Share Components

When a request (not Ajax request but regular HTTP request) is made by an user, a Desktop and a Page are created first, and then Richlet.service(Page) is invoked to serve the request[1]. In other words, each request is served with an individual desktop and page. Therefore, we cannot share components among different invocation of Richlet.service(Page).

For example, the following code is illegal:

public class MyRichlet extends GenericRichlet {
    private Window main; //Not a good idea to share
    public void service(Page page) {
        if (main == null) {
            main = new Window();
        main.setPage(main); //ERROR! Causes an exception if the same URL is requested twice!

Why? Each deskop should have its own set of component instances[2]. When the URL associated MyRichlet is requested a second time, an exception will be thrown because the main window is already instantiated and associated with the first desktop created from the first request. We cannot assign it to the second desktop.

  1. A normal HTTP request; not an Ajax request. Ajax requests are handled in the same way as ZUML. For more information please refer to the Event Handling section
  2. For more information, please refer to Component-based UI section

Map URL to a Richlet

To map URL to a richlet, there are two steps.

  1. Turn on the support of Richlet (in WEB-INF/web.xml)
  2. Map URL pattern to Richlet (in WEB-INF/zk.xml)

Turn on Richlet

By default, richlets are disabled. To enable them, please add the following declaration to WEB-INF/web.xml. Once enabled, you can add as many richlets as you want without modifying web.xml.

With servlet-mapping:


Since 7.0.0 You can use RichletFilter instead.



where you can replace /zk/* to any pattern you like, such as /do/*. Notice that you cannot map it to an extension (such as *.do) since it will be considered as a ZUML page (rather than a richlet).

Map URL pattern to Richlet

For each richlet you implement, you can define it in WEB-INF/zk.xml with the statement similar to the following:

    <richlet-name>Test</richlet-name><!-- your preferred name -->
    <richlet-class>org.zkoss.zkdemo.TestRichlet</richlet-class><!-- your class name, of course -->

After defining a richlet, you can map it to any number of URLs using the richlet-mapping element as shown below.


Note: With Richlet Filter (since ZK 7.0.0), you should add the prefix of url-pattern of the filter-mapping into the url-pattern of richlet-mapping. For example,


As you can see the highlight above, the /zk is added which is according to the filter-mapping.

Then, you can visit http://localhost:8080/PROJECT_NAME/zk/test to request the richlet.

The URL specified in the url-pattern element must start with /. If the URI ends with /*, it is matched to all request with the same prefix. To retrieve the request's actual URL, you can check the value returned by the getRequestPath method of the current page.

 public void service(Page page) {
     if ("/some/more/hi".equals(page.getRequestPath()) {

Tip: By specifying /* as the url-pattern, you can map all unmatched URLs to your richlet.

Load ZUML in Richlet

Execution provides a collection of methods, such as Execution.createComponents(String, Component, Map), allowing developers to load ZUML documents dynamically. You could load a ZUML document from any source you like, such as database. Please refer to the Load ZUML in Java for details.

Use Spring in Richlet

To use Spring-managed beans in richlets you need the context loader listener that creates spring application context as described in ZK Spring Essentials/Getting Started with ZK Spring/Setting Up ZK Spring. Then you could load Spring beans by using a utility class SpringUtil:

    Object bean = SpringUtil.getBean(beanName);

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Last Update : 2022/07/08

Copyright © Potix Corporation. This article is licensed under GNU Free Documentation License.