@GetMapping("/hello")
public Map<String,String> hello(...) { ... }

Or is that example just the default GET template you get with a Spring
Boot installation, i.e. that code snippet is not relevant or fitted to
make sense for the particular task of running XSLT (2 or 3)?

In general for your last two posts with the hint to email you for a copy
of the guide, can't you just put them up on some web presence like
Github pages or similar you might have?