docs: public static String getUserName() throws Exception {
          Subject subject = getRunAsSubjectInternal();
          if (subject == null) {
              return null;
          }
          Set<Principal> principals = subject.getPrincipals();
          Iterator<Principal> principalsIterator = principals.iterator();
          if (principalsIterator.hasNext()) {
              Principal principal = principalsIterator.next();
              return principal.getName();
          }
          return null;
      }