From Documentation


The controller is a Java program that is used to glue UI (view) and Data (model) together.

For a simple UI, there is no need of controller. For example, the data of a Listbox could be abstracted by implementing ListModel.

For typical database access, the glue logic (i.e., control) can be handled by a generic feature called Data Binding. In other words, the read and write operations can be handled automatically by a generic Data Binding, and you don't need to write the glue logic at all.

In this section we will discuss how to implement a custom controller (aka., a composer).

Custom Controller

A custom controller is also know as a composer (in ZK). To implement a composer, you could extend from GenericForwardComposer, or implement Composer from scratch. Then, specify it in the element it wants to handle in a ZUML document.

To implement the logic to glue UI and data, a composer usually do:

  • Post-process components after ZK Loader renders a ZUML document. It can be done by overriding Composer.doAfterCompose(Component).
  • Handle events and manipulate components if necessary.

In additions, a composer can be used to involve the lifecycle of ZK Loader for doing:

  • Exception handling
  • Component Instantiation monitoring and filtering

A composer be configured as system-level, such that it will be called when ZK Loader has processed a ZUML document.


Implementing Composer is straightfoward: just override Composer.doAfterCompose(Component) and do whatever you want.

However, it is suggested to extend from GenericForwardComposer since the default implementation of GenericForwardComposer.doAfterCompose(Component) wires variables and event listener automatically.

For example,

package foo;
import org.zkoss.zk.ui.Component;
import org.zkoss.zk.ui.util.GenericForwardComposer;
import org.zkoss.zul.*;

public class MyComposer extends GenericForwardComposer {
    Textbox input;
    Label output;

    void onClick$submit() {
    void onClick$reset() {

where input will be wired to a fellow named input, and onClick$submit will be registered as an event listener for an event named onClick and to a fellow named submit.

To associate a composer to a component, just specify the apply attribute to the element you want to control. For example,

<grid apply="foo.MyComposer">
            <textbox id="input"/>
            <button label="Submit" id="submit"/>
            <button label="Reset" id="reset"/>

If you have to post-process the components after ZK Loader initializes them, you could override GenericForwardComposer.doAfterCompose(Component). It is important to call back super.doAfterCompose(comp). Otherwise, the wiring won't work.

public void doAfterCompose(Component comp) {
   super.doAfterCompose(comp); //wire variables and event listners
   //do whatever you want (you could access wired variables here)

where comp is the component that the composer is applied to. In this example, it is the grid. As the name indicates, doAfterCompose is called after the grid and all its descendants are instantiated.

The apply Attribute

If you could specify multiple composers, just separate them with comma. They will be called from left to right.

In additions to the class name, you could specify an instance too. For example, suppose you have an instance called fooComposer, then

<grid apply="${fooComposer}">

If a class name is specified, each time the component is instantiated, an instance of the class is instantiated too. Thus, you don't have to worry about the concurrency issue. However, if you specify an instance, it will be used directly. Thus, you have to either create an instance for each request, or make it thread-safe.

Composer with More Control

A composer could also handle the exceptions, if any, involve the life cycle of rendering, and monitor and even control how a child component is instantiated. It can be done by implementing the corresponding interfaces.

Exception and Lifecycle Handling with ComposerExt

If you want a composer to handle the exception and/or involve the life cycle of rendering, you could also implement ComposerExt. Since GenericForwardComposer already implements this interface[1], you only need to override the method you care if you extends from it.

For example, we could handle the exception by overriding ComposerExt.doCatch(Throwable) and/or ComposerExt.doFinally().

public class MyComposer extends GenericForwardComposer implements ComposerExt {
    public boolean doCatch(Throwable ex) {
        return _ignorable(ex); //return true if ex could be ingored

For involving the life cycle, you could override ComposerExt.doBeforeCompose(Page, Component, ComponentInfo) and/or ComposerExt.doBeforeComposeChildren(Component).

  1. In ZK 5, ComposerExt is implemented. However, it is not in ZK 6 (for better performance), so it is better to implement it if you need it.

Fine-grained Full Control with FullComposer

In addition to controlling the give component, a composer can monitor the instantiation and exceptions for each child and descendant component. It can be done by implementing ComposerExt. GenericForwardComposer does not implement this interface by default. Thus, you have to implement it explicitly.

There is no method need to implement in this interface. It is like a decorative interface to indicate that it requires the fine-grained full control. In other words, all methods declared in Composer and ComposerExt will be invoked one-by-one against each child and descendant component.

For example, suppose we have a composer implementing both Composer and FullComposer, and it is assigned as followed

<panel apply="foo.MyComposer">

then, Composer.doAfterCompose(Component) will be called for datebox, textbox, div and then panel (in the order of child-first-parent-last). If FullComposer is not implemented, only panel will be called.

System-level Composer

You could register a system-level composer, such that it will be notified for every ZUML document being rendered.

It could be done by specifying the composer you implemented in WEB-INF/zk.xml[1]:


Each time a ZK page, including ZK pages and richlets, is created, ZK will instantiate one instance for each registered system-level composer and the invoke the doAfterCompose method with each root component. The system-level composer is usually used to post-process ZK pages, such as adding a trademark. If you want to process only certain pages, you can check the request path by calling Desktop.getRequestPath() (the desktop instance can be found thru the give component).

If the system-level composer also implements ComposerExt, it can be used to handle more situations, such as exceptions, like any other composer can do.

If the system-level composer also implements FullComposer, it will be invoked when each component is created. It provides the finest grain of control but a wrong implementation might degrade the performance.

Notice that since a new instance of the composer is created for each page, there is no concurrency issues.

  1. For more information, please refer to ZK Configuration Reference


A system-level composer can implement ComposerExt to handle exceptions for a richlet, such as doCatch and doFinally. However, doBeforeCompose and doBeforeComposeChildren won't be called.

FullComposer is not applicable to richlets. In other words, system-level composers are called only for the root components.

Version History

Last Update : 2010/11/12

Version Date Content

Last Update : 2010/11/12

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