JNLP: How to use javax.jnlp.DownloadService

Question

I'm working on a project where I'm supposed to launch an Eclipse RCP-Project with JNLP. Now, I've just got introduced to the JNLP-World and don't really get everything straight. I saw that, there's a way to structure the download of resources with a DownloadService to manage the java cache.

After searching on the internet i found some examples... But none really explains how and where to use the DownloadService. So these are my questions.

Let say I've got a simple RCP-Plugin:

1. I already exported and signed it the jar.
2. The jar with the jnlp files are already on my tomcat server running locally
3. Where am I supposed to implement my main class running an instance of my DownloadService and how should i register it in my jnlp file?
4. I guess my question is how can i get jnlp only to download the resources, which have changed?

I would be more than thankful if anyone provide me with some explanations or simple examples.

Thanks.