code: public Principal authenticate(String username,
                                    Object credentials,
                                    HttpRequest request)
      {
          KnownUser user;
          synchronized (this)
          {
              user = (KnownUser)super.get(username);
          }
          if (user==null)
              return null;
  
          if (user.authenticate(credentials))
              return user;
          
          return null;
      }
docs: /* ------------------------------------------------------------