This gist is fork from https://gist.github.com/870666 by @kyon_mm (This gist is used at http://d.hatena.ne.jp/fumokmm/20110315/1300210292)

keyValueXml.groovy

class KeyValue {
    def key
    def value
    KeyValue(key, value){
        this.key = key
        this.value = value
    }
}

def convert(def builder, List list, boolean first = true) {
  if (first) {
    builder.langs(type: 'current') {
      convert(builder, list, false)
    }
  } else {
    list.each { kv ->
      if (kv.value instanceof List)
        builder."${kv.key}" {
          convert(builder, kv.value, false)
        }
      else
        builder."${kv.key}"(kv.value)
    }
  }
}

def sw = new StringWriter()
def xml = new groovy.xml.MarkupBuilder(sw)
xml.doubleQuotes = true // 属性はダブルクォートだよね！
def list = [
  new KeyValue("key1","value1"),
  new KeyValue("key2","value2"),
  new KeyValue("key3", [
    new KeyValue("key3-1","value3-1"),
    new KeyValue("key3-2","value3-2")
  ])
]

def expect = '''\
<langs type="current">
  <key1>value1</key1>
  <key2>value2</key2>
  <key3>
    <key3-1>value3-1</key3-1>
    <key3-2>value3-2</key3-2>
  </key3>
</langs>'''
convert(xml, list)
assert sw.toString() == expect