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[since 5.0.4][ZK EE] [since 6.0.0][ZK CE]

It is common that the states of some components are not required to maintain on the server. A typical example is that an application might use some components, such as hbox, for layout and won't access it again after rendered. To minimize the memory footprint, ZK supports a special property called stubonly (Component.setStubonly(String)). Once specified with true, its states won't be maintained on the server (and all states are maintained at the client). For example,

<hbox stubonly="true">
  • Notice this feature is available since ZK 5.0.4 EE, and available in CE since ZK 6.0.0.

Values of Stubonly: true, false and inherit

The default value of the stubonly property is inherit. It means the value is the same as its parent's, if any, or false, if no parent at all. Thus, if a component's stubonly is specified with true, all its descendants are stub-only too, unless false is specified explicitly. For example, in the following snippet, only textbox is not stub-only, while hbox, splitter, listbox, listitem and labels are all stub-only.

<hbox stubonly="true">
  a stub-only label
  <textbox stubonly="false"/>
    <listitem label="also stubonly"/>

Limitation of Stub Components

When a component is stub only, it will be replaced with a special component called a stub component (StubComponent) after rendered. In addition, the adjacent stub components might be merged to minimize the memory further. Thus, the application should not access the component again on the server, if it is specified as stub only.


While a stub component cannot be invalidated directly, it is safe to invalidate its parent. ZK will rerender all non-stub components and retain the states of stub components at the client. For example, in the following snippet, it is safe to click the invalidate button. From an end user's point of view, there is no difference whether stubonly is specified or not.

  <button label="self.parent.invalidate()"/>
  <vbox stubonly="true">
  stubonly <textbox/>

It is a special case that paging can not apply stubonly at the same time. For example,

<listbox mold="paging" pageSize="1" >
	<listitem >
		<listcell stubonly="true"/>
		<listcell />

Although paging will invalidate listbox and its children, stubonly needs the referred widget in client side which is detached during paging and throws mounting error.

Detach and Reuse

The detailed information of a stub component is stored at the client. It is removed when the component is removed. Thus, you can't reuse a component if the component has some stub components and it is detached. For example, the following code won't work:

public class MyListener implements EventListener {
   private static Window win;
   public void onEvent(Event evt) throws Exception{
      if (win ==null) {	   
         win = new Window();
         Label testLabel = new Label("My Label");

In the above example[1], win is reused but it also has a stub component (a label). When the window is closed, all the information at the client are removed (since Window's onClose() method detaches the window). Thus, if the event was executed again, the client can restore the detailed information back.

  1. Please refer to ZK-1094 for a real case.

Event Handling

ZK will preserve all registered event listeners and handlers, when converting a stub-only component to a stub component. In other words, the listener will be called if the corresponding event is fired. However, since the original component no longer exists, the event is fired in the most generic format: an instance of Event, rather than a derived class.

For example, in the following snippet, "org.zkoss.zk.ui.event.StubEvent:onStub" will be generated to System.out.

<textbox stubonly="true" onChange='System.out.println(event.getClass().getName()+":"+event.getName())'/>

In addition, the target (Event.getTarget()) is the stub component rather than the original one (text).

Client-side Programming

The client-side widget of a component is the same no matter if it is stub only. Thus, the application can have the full control by registering the client side event listener, such as

<textbox stubonly="true" w:onChange="doSomething(this.value)" xmlns:w="client"/>

In other words, the stub-only components behave the same at the client.

Refer to Client Side Programming and ZK Client-side Reference: General Control for more information.

Version History

Last Update : 2015/9/3

Version Date Content
6.0.2 June, 2012 Bug: stubonly doesn't work, and change the event handle from origin event to StubEvent.

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