Apereo Cas - Custom Principal Id release

I need to edit the principal id that will be returned to a cas client application.

The principal I need to edit is returned from an external idp which I do not own and its format is like this:

Id: idp autogenerated ID having no meaning for my setup
Attributes:
... Some attributes I do not need ...
**ImportantId**(this is the property I need)

I have read the official documentation so from my understanding the related part of my service json file should be:

...       
"usernameAttributeProvider" : {
        "@class" : "com.mypackage.cas.principal.MyCustomRegisteredServiceUsernameProvider",
        "canonicalizationMode" : "NONE"

In my custom UserNameProvider class, I need to access db in order to get my setup specific username based on the importantId I mentioned above, so I need to do something like this :

public class MyCustomRegisteredServiceUsernameProvider implements RegisteredServiceUsernameAttributeProvider {

    @Autowired
    UsersService usersService;

    @Override
    public String resolveUsername(Principal principal, Service service, RegisteredService registeredService) {
        // String importantId = ...getting it from principal attributes.. 
        return usersService.getUserNameFromImportant(importantId);

    }   
}

The problem is that Autowired annotation unfortunately is not working, thus my UsersService is not initialized resulting in a NPE at runtime.

So, firstly I would like to know why my approach is not working and secondly what is the cas "official" way to achieve this behavior?

Thank you in advance.

Cas Version: 5.3.4 Cas overlay template

Env: W10 x64, Java 8

Deployed on Wildfly 12