16. Resource management with Wicket

One of the biggest challenge for a web framework is to offer an efficient and consistent mechanism to handle internal resources such as CSS/JavaScript files, picture files, pdf and so on. Resources can be static (like an icon used across the site) or dynamic (they can be generated on the fly) and they can be made available to users as a download or as a simple URL.

In paragraph 6.6 we have already seen how to add CSS and JavaScript contents to the header section of the page. In the first half of this chapter we will learn a more sophisticated technique that allows us to manage static resources directly from code and “pack” them with our custom components.

Then, in the second part of the chapter we will see how to implement custom resources to enrich our web application with more complex and dynamic functionalities.

16.1. Static vs dynamic resources

In Wicket a resource is an entity that can interact with the current request and response and It must implement interface org.apache.wicket.request.resource.IResource. This interface defines just method respond(IResource.Attributes attributes) where the nested class IResource. Attributes provides access to request, response and page parameters objects.

Resources can be static or dynamic. Static resources don’t entail any computational effort to be generated and they generally correspond to a resource on the filesystem. On the contrary dynamic resources are generated on the fly when they are requested, following a specific logic coded inside them.

An example of dynamic resource is the built-in class CaptchaImageResource in package org.apache.wicket.extensions.markup.html.captcha which generates a captcha image each time is rendered.

As we will see in paragraph 16.10, developers can build custom resources extending base class org.apache.wicket.request.resource.AbstractResource.

16.2. Resource references

Most of the times in Wicket we won’t directly instantiate a resource but rather we will use a reference to it. Resource references are represented by abstract class org.apache.wicket.request.resource.ResourceReference which returns a concrete resource with factory method getResource(). In this way we can lazy-initialize resources loading them only the first time they are requested.

16.3. Package resources

With HTML we use to include static resources in our pages using tags like