code: public static void loadExternalSpringConfig(RuntimeSpringConfiguration config, final GrailsApplication application) {
          if (springGroovyResourcesBeanBuilder == null) {
              try {
                  Class<?> groovySpringResourcesClass = null;
                  try {
                      groovySpringResourcesClass = ClassUtils.forName(SPRING_RESOURCES_CLASS,
                          application.getClassLoader());
                  }
                  catch (ClassNotFoundException e) {
                      // ignore
                  }
                  if (groovySpringResourcesClass != null) {
                      reloadSpringResourcesConfig(config, application, groovySpringResourcesClass);
                  }
              }
              catch (Exception ex) {
                  LOG.error("[RuntimeConfiguration] Unable to load beans from resources.groovy", ex);
              }
          }
          else {
              if (!springGroovyResourcesBeanBuilder.getSpringConfig().equals(config)) {
                  springGroovyResourcesBeanBuilder.registerBeans(config);
              }
          }
      }
docs: Loads any external Spring configuration into the given RuntimeSpringConfiguration object.
  @param config The config instance