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Understanding ZK cache breaking by URL

ZK applications rely on a number of resource files loaded by the client. These resources are mainly JS and CSS packages, but can also include images or other binaries necessary on client side. To improve performances, default ZK applications will use the following caching strategy. Resources are served to the client via a URL containing a hash value of the ZK version. If the ZK version doesn't change, this hash value will stay identical between server restarts, sessions, and page loading. This allow clients to cache ZK resources, since the URL doesn't change between page load, and thus reduce page initialization times on client side.

When the ZK version changes, the hash value contained in the resources URLs is changes, and thus breaks caching on client side. This is important, as ZK internal JavaScript files should only be used in their intended ZK version.

In effect, this means that internal ZK files will be cached unless the ZK version is changed. After version changes, the resources are downloaded and the new version is cached, replacing the outdated resources.

Loading resources through the ZK cache enabled URLs

Resources declared from the classpath, or from ZK ClassWebResources folder will be delivered to the clients through the same URLs patterns as the internal ZK resources. This can apply to resources declared globally in lang-addon, or locally in a page with a script or style tag.

Forcing a cache clear on non-ZK resources

Since clients may cache a specific version of resources declared this way, they will not update them unless forced to do so by a clear-cache, a forced refresh, or a ZK version change. A simple way to break the cache when a new version of your resources must be downloaded is to use the built-in hashed value in the resource URLs. As stated before, the value will change if the ZK version change. However, it will also change if the webapp build id changes.

This can be done by creating a new WebApp class extending the default SimpleWebApp and overriding the default getBuild() method. The build returned can be generated automatically on webapp init to force the clients to update on every server restart. It could also be defined manually as a string, or retrieved from a database to only force a restart when the developer updates the version id.


Generating a random number on startup

public class ForceResourcesUpdateWebapp extends SimpleWebApp {

	private String randomizer;
	public void init(Object context, Configuration config) {
		super.init(context, config);
		randomizer = Long.toHexString(System.currentTimeMillis());
	public String getBuild() {
		return super.getBuild()+randomizer;

Manually setting a version id

public class ForceResourcesUpdateWebapp extends SimpleWebApp {
	public String getBuild() {
		return super.getBuild()+"1.1.0";

Once the webapp class has been created, it must be declared in zk.xml such as:


Code samples

The following samples are available in github

Version History

Last Update : 2018/9/18

Version Date Content

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