Writing a background checker

Hi! I'm sorry, this must have been asked before but I really can't find an answer!

I need to code a background checker that checks if all of my users that are working need to change shift.
The way I thought this is to start a scheduled thread when the server starts (I'm using servlet) and each time
the thread checks, it updates the UI of each user via 'push'.

I can't seem to grasp though were the 'server starts'...
What I managed to understand is that each user gets a UI for each 'browser session' he creates. Because of this
I suppose the init() method of the UI corresponds to its initialization and not the server's. So I can't use it to start my
threaded checker (since it will call start multiple times -> my threaded checker is a static class).

What is the best way of doing such thing in vaadin?

Thanks!

After taking a deeper look into vaadin, (LOL) should I use this:
public static class Servlet extends VaadinServlet {}

?
and overrite a start method?

I'm sorry, I found it:
https://vaadin.com/book/-/page/application.lifecycle.html

I'll let this here for other people to use the search words that I used.