I think the token is changing because you are using the file without preserve state between restarts. That means every time your app starts up, the file store is re-initialized. If you want to preserve state, please try something like that:
The IdGenerator is about generating UUIDs for every single type stored by PicketLink IDM. I'm not sure if you really need something different than what PL provides. It is changing every time because the file store is not preserving state, like I previously mentioned.
Maybe, what you can do is extract the token from the response and send it every time during your tests. In this case, even if not preserving state, you will get the same token along with your requests.