private AppenderRefComponentBuilder createAppenderRef(final String key, final Properties properties) { final String ref = (String) properties.remove("ref"); if (Strings.isEmpty(ref)) { throw new ConfigurationException("No ref attribute provided for AppenderRef " + key); } final AppenderRefComponentBuilder appenderRefBuilder = builder.newAppenderRef(ref); final String level = Strings.trimToNull((String) properties.remove("level")); if (!Strings.isEmpty(level)) { appenderRefBuilder.addAttribute("level", level); } return addFiltersToComponent(appenderRefBuilder, properties); }