docs: public static String getCallerPrincipal() {
          String caller = null;
          SubjectManagerService sms = smServiceRef.getService();
          if (sms != null) {
              Subject subject = sms.getCallerSubject();
              if (subject != null) {
  
                  WSCredential wsCred = getWSCredential(subject);
  
                  if (wsCred != null && !wsCred.isUnauthenticated()) {
                      try {
                          caller = wsCred.getSecurityName();
                      } catch (Exception e) {
                          if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled())
                              Tr.debug(tc, "Internal error: " + e);
                      }
                  }
              }
          }
          return caller;
      }