ZK Developer's Reference/UI Composing/Richlet

A richlet is a small Java program that composes a user interface in Java for serving user's request.

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

If the resource is a richlet, the 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 necessary components programmatically in response to the request.

The choice between ZUML pages and richlets depends on your preference. However, the performance shall not be a concern since the parsing of ZUML is optimized.

Implement a Richlet

It is straightforward to implement a richlet. First, implement the Richlet interface and then mapping URL to the richlet.

Implement a Richlet as a Java class

A richlet must implement the Richlet interface. However, you generally don't have to implement it from scratch. Rather, you could extends from GenericRichlet. With GenericRichlet, the only thing you have to do is to implement Richlet.service(Page). It is called when an associated URL is required. For example,

 package org.zkoss.zkdemo;

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

 public class TestRichlet extends GenericRichlet {
     //Richlet//
     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();
         l.setParent(w);

         final Button b = new Button("Change");
         b.addEventListener(Events.ON_CLICK,
             new EventListener() {
                 int count;
                 public void onEvent(Event evt) {
                     l.setValue("" + ++count);
                 }
             });
         b.setParent(w);

         w.setPage(page);
     }
 }

To have better control, you can implement the init and destroy methods to initialize and to destroy the resources required by the richlet when it is loaded. Like a servlet, a richlet is loaded once and serves all requests for the URL with which it is associated. In other words, it must be thread-safe. Sharing components in static members are generally not a good idea.

One Richlet per URL

Like servlets, a single richlet is created and shared for all users. In other words, the richlet (at least the service method) must be thread-safe. On the other hand, components are not shareable. Each desktop has an independent set of components. Therefore, it is generally not a good idea to store components as a data member of a richlet.

There are many ways to solve this issue. A typical one is to use another class for holding the components for each desktop, as illustrated below.

import org.zkoss.zk.ui.Page;
import org.zkoss.zul.Window;


 class MyApp { //one per desktop
     Window _main;
     MyApp(Page page) {
         _main = new Window();
         _main.setPage(page);
     }
 }

 
import org.zkoss.zk.ui.GenericRichlet;
import org.zkoss.zk.ui.Page;

class MyRichlet extends GenericRichlet {
	public void service(Page page) {
		new MyApp(page); //create and forget
    }
}

Configure web.xml and zk.xml

After implementing the richlet, you can define the richlet in zk.xml with the following statement:

<richlet>
    <richlet-name>Test</richlet-name>
    <richlet-class>org.zkoss.zkdemo.TestRichlet</richlet-class>
</richlet>

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

<richlet-mapping>
    <richlet-name>Test</richlet-name>
    <url-pattern>/test</url-pattern>
</richlet-mapping>
<richlet-mapping>
    <richlet-name>Test</richlet-name>
    <url-pattern>/some/more/*</url-pattern>
</richlet-mapping>

By default, richlets are disabled. To enable them, add the following declaration to web.xml. Once enabled, you can add as many as richlets as you want without modifying web.xml any more.

<servlet-mapping>
    <servlet-name>zkLoader</servlet-name>
    <url-pattern>/zk/*</url-pattern>
</servlet-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 /*, then 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.

Version History

Last Update : 2010/11/8

Version	Date	Content