@Configuration
@ConditionalOnClass(DefaultUrlMappingsHolder.class)
protected static class GspAutoConfiguration.UrlMappingsHolderConfiguration
extends java.lang.Object

The mappings a link generator looks a controller and action up in. A Grails application gives its generator the mappings of its UrlMappings artefact. An application routing with Spring MVC has no such artefact and so no holder, but can still have a generator - the asset pipeline contributes one, which its <asset:...> tags build their URLs with - and the generator requires a holder; the empty one here is what the application would otherwise have to declare itself.