code: private static String formatPrincipal(final String username) {
          if  (StringUtils.isNotBlank(LDAP_AUTH_USERNAME_FMT)) {
              return String.format(LDAP_AUTH_USERNAME_FMT, username);
          }
          return username;
      }
docs: Formats the principal in username@domain format or in a custom format if is specified in the config file.
  If LDAP_AUTH_USERNAME_FMT is configured to a non-empty value, the substring %s in this value will be replaced with the entered username.
  The recommended format of this value depends on your LDAP server(Active Directory, OpenLDAP, etc.).
  Examples:
  alpine.ldap.auth.username.format=%s
  alpine.ldap.auth.username.format=%s@company.com
  @param username the username
  @return a formatted user principal
  @since 1.4.0