Access principal name from servlet filter

[Etienne Dysli-Metref]

I'm porting a servlet filter that was used with IdPv2 to v3 and it uses
edu.internet2.middleware.shibboleth.idp.util.HttpServletHelper and
edu.internet2.middleware.shibboleth.idp.authn.LoginContext to read the
current principal name, like so:

LoginContext loginContext = HttpServletHelper.getLoginContext(
  HttpServletHelper.getStorageService(servletContext),
  servletContext,
  (HttpServletRequest) request);
String principalName = loginContext.getPrincipalName();

and then writes it into a cookie.
What is the equivalent functionality in v3?

Cheers,
   Etienne

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