wicket-user-guide/src/main/asciidoc/resources/resources_10.adoc

In Wicket the best way to add dynamic functionalities to our application (such as csv export, a pdf generated on the fly, etc...) is implementing a custom resource. In this paragraph as example of custom resource we will build a basic RSS feeds generator which can be used to publish feeds on our site (project CustomResourceMounting). Instead of generating a RSS feed by hand we will use Rome framework and its utility classes. 

As hinted above in <<_static_vs_dynamic_resources,paragraph 16.1>>, class _AbstractResource_ can be used as base class to implement new resources. This class defines abstract method _newResourceResponse_ which is invoked when the resource is requested. The following is the code of our RSS feeds generator:

[source,java]
----
public class RSSProducerResource extends AbstractResource {

  @Override
  protected ResourceResponse newResourceResponse(Attributes attributes) {
    ResourceResponse resourceResponse = new ResourceResponse();
    resourceResponse.setContentType("text/xml");
    resourceResponse.setTextEncoding("utf-8");
    
    resourceResponse.setWriteCallback(new WriteCallback()
    {
      @Override
      public void writeData(Attributes attributes) throws IOException
      {
        OutputStream outputStream = attributes.getResponse().getOutputStream();
        Writer writer = new OutputStreamWriter(outputStream);
        SyndFeedOutput output = new SyndFeedOutput();
            try {
          output.output(getFeed(), writer);
        } catch (FeedException e) {
          throw new WicketRuntimeException("Problems writing feed to response...");
        }
      }      
    });
    
    return resourceResponse;
  }
  // method getFeed()...
}
----

Method _newResourceResponse_ returns an instance of _ResourceResponse_ representing the response generated by the custom resource. Since RSS feeds are based on XML, in the code above we have set the type of the response to text/xml and the text encoding to utf-8.

To specify the content that will be returned by our resource we must also provide an implementation of inner class _WriteCallback_ which is responsible for writing content data to response's output stream. In our project we used class SyndFeedOutput from Rome framework to write our feed to response. Method _getFeed()_ is just an utility method that generates a sample RSS feed (which is an instance of interface _com.sun.syndication.feed.synd.SyndFeed_).

Now that we have our custom resource in place, we can use it in the home page of the project. The easiest way to make a resource available to users is to expose it with link component _ResourceLink_: 

[source,java]
----
add(new ResourceLink("rssLink", new RSSProducerResource()));
----

In the next paragraphs we will see how to register a resource at application-level and how to mount it to an arbitrary URL.