Invalidate browser cache with custom JSF resource handler

Browser caches static resources like CSS, JavaScript files, images if they don't have proper expire time in the response header. An usual task is to find a solution which allows refetching of resources with each software release. We can modify all resource URLs by appending "&rv=1234" or "?rv=1234" to the end. rv is a current software revision. Such modified URL leads to the refetching of resources. The current revision can be acquired during project build and stored somewhere, e.g. in a text file, in order to make it available in the web application. Maven has e.g. a special plugin for this task called buildnumber-maven-plugin. This plugin allows to get the current revision from a version control system.

We will write a custom JSF resource handler CustomResourceHandler which will accomplish the URL modifying. The resource handler can be registered in faces-config.xml as follows:

<application>
    <resource-handler>packagename.CustomResourceHandler</resource-handler>
</application>

Writing of any resource handler is straightforward. We need to extend javax.faces.application.ResourceHandlerWrapper

public class CustomResourceHandler extends javax.faces.application.ResourceHandlerWrapper
{
    private ResourceHandler wrapped;

    public CustomResourceHandler(ResourceHandler wrapped) {
        this.wrapped = wrapped;
    }

    @Override
    public ResourceHandler getWrapped() {
        return this.wrapped;
    }

    @Override
    public Resource createResource(String resourceName, String libraryName) {
        Resource resource = super.createResource(resourceName, libraryName);

        // here a check of library name could be necessary, etc.
        ...
        return new CustomResource(resource);
    }
}

We need now a class CustomResource. CustomResource should extend javax.faces.application.ResourceWrapper and delegate all calls to this wrapper class except one call getRequestPath(). Method getRequestPath() needs to be overwritten.

public class CustomResource extends javax.faces.application.ResourceWrapper
{
    private javax.faces.application.Resource resource;

    public CustomResource(Resource resource) {
        this.resource = resource;
    }

    @Override
    public Resource getWrapped() {
        return this.resource;
    }

    @Override
    public String getRequestPath() {
        String requestPath = resource.getRequestPath();
                 
        // get current revision
        String revision = ...

        if(requestPath.contains("?"))
            requestPath = requestPath + "&rv=" + revision;
        else
            requestPath = requestPath + "?rv=" + revision;

        return requestPath;
    }

    @Override
    public String getContentType() {
        return getWrapped().getContentType();
    }

    ...
}

That's all. For more control in Mojarra JSF implementation there are two handy config parameters for web.xml.

com.sun.faces.defaultResourceMaxAge set expire time (in milliseconds) into response header and is responsible for browser-caching. Default value in Production ProjectStage is ca. 10 min.
com.sun.faces. resourceUpdateCheckPeriod gives frequency in minutes to check for changes to webapp artifacts that contain resources.