Grails XML marshalling: change default “<list>” ro

By default Grails renders List in XML with a <list> element tag at its root. Likewise it renders Map with <map>. I would like to control the name of the root element.

If I'm returning an ArrayList of User, then I'd like to see:

<users>
    <user>...</user>
    <user>...</user>
</users>

How can I achieve the above? Here are the Requirements:

Easy to apply this serialization for 50+ domain classes
Abstracted from developers so no explicit coding is required during rendering domain objects (i.e., when render() or respond() is invoked, an ArrayList is still passed in, no explicit casting/converting like as MyNewType)
Able to handle the edge case of an empty list (should return <users/>)

Nice-to-haves:

If this formula can be applied to Map as well, great :)

I have been semi-successful in achieving the goals above, except I don't know how to account for the empty list case. I implemented my own ObjectMarshaller which renders all objects of type List. So long as the list contains one element, I can check the element's type and determine what the plural tag name should be (User => users). But if the list is empty, and since Java generics are by erasure (unless that's different in Groovy?) then I have no way to properly name an empty list other than defaulting to something like <list/>, which is not acceptable.

Some resources that I've been through:

http://www.cacoethes.co.uk/blog/groovyandgrails/dry-json-and-xml-with-grails
http://grails.1312388.n4.nabble.com/Custom-XML-Marshaller-change-the-root-element-name-td4649949.html
http://jwicz.wordpress.com/2011/07/11/grails-custom-xml-marshaller/
http://mrhaki.blogspot.com/2013/11/grails-goodness-register-custom.html
http://manbuildswebsite.com/2010/02/15/rendering-json-in-grails-part-3-customise-your-json-with-object-marshallers/

A way to achieve this is to write a subclass for the CollectionMarshaller class and register it in our Grails application. We can for example register a custom implementation in BootStrap.groovy with the following code:

import org.codehaus.groovy.grails.web.converters.marshaller.xml.CollectionMarshaller
import grails.converters.XML

class BootStrap {

  def init = { servletContext ->
    // Register custom collection marshaller for List with User instances.
    // The root element name is set to users.
    XML.registerObjectMarshaller(new CollectionMarshaller() {
        @Override
        public boolean supports(Object object) {
            object instanceof List<User>
        }

        @Override
        String getElementName(final Object o) {
            'users'
        }
    })
  }
}

To make this work for more domain classes we might get a reference to all domain classes in BootStrap.groovy and loop through them to configure custom CollectionMarshaller instances.

For maps you can extend MapMarshaller

Also described in http://mrhaki.blogspot.com/2014/02/grails-goodness-customize-root-element.html