def atom = {
    	params.max = 10
     	params.sort = 'datePosted'
     	params.order = 'desc'
       	def tmpList = Item.list( params )	
		
		def feedHeader = """<feed xmlns="http://www.w3.org/2005/Atom">
		  <title type="text">aboutGroovy.com</title>
		  <link rel="alternate" type="text/html" href="http://aboutGroovy.com"/>
		  <link rel="self" type="application/atom+xml" href="http://aboutGroovy.com/item/atom"/>
		  <updated>2006-12-17T18:30:02Z</updated>
		  <author><name>Scott Davis</name></author>
		  <id>tag:aboutgroovy.com,2006-12-18:thisIsUnique</id>
		  <generator uri="http://aboutGroovy.com" version="0.0.1">Hand-rolled Grails code</generator>
		"""
		
		def feedFooter = "</feed>"

		StringBuffer feed = new StringBuffer()
		def df = new java.text.SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss'-07:00'")
		tmpList.each{item ->
			Writer sw = new StringWriter()
			def x = new groovy.xml.MarkupBuilder(sw)
			x.entry(xmlns:"http://www.w3.org/2005/Atom"){
				author{name("Scott Davis")}
				published(df.format(item.datePosted))
				updated(df.format(item.datePosted))
				link(href:"http://aboutGroovy.com/item/show/${item.id}", 
					rel:"alternate", title:item.title, type:"text/html")
				id("tag:aboutgroovy.com,2006:/item/show/${item.id}")
				title(type:"text", item.title)
				content(type:"xhtml"){
					div(xmlns:"http://www.w3.org/1999/xhtml"){
						p("Category: ${item.type}")
						p{a(href:item.url, "Original Source")}
						p(item.shortDescription)
						p(item.description)
					}
				}	
			}
			feed.append(sw.toString() + "\n")
		}
	
		response.setContentType("application/atom+xml")				
		render "${feedHeader}${feed}${feedFooter}"
	}

http://aboutgroovy.com/item/atom

Here's a quick block of code that adds a valid Atom feed to you Grails application. Validate here: http://feedvalidator.org/check?url=http://aboutgroovy.com/item/atom

Atom feed from Grails

groovy

Groovy