JAXB を使用してマップを「value」XML 構造にマーシャリングする方法

<map>
  <entry>
    <key> KEY </key>
    <value> VALUE </value>
  </entry>
  <entry>
    <key> KEY2 </key>
    <value> VALUE2 </value>
  </entry>
  ...
</map>

<map>
  <key> VALUE </key>
  <key2> VALUE2 </key2>
 ...
</map>

public enum KeyType {
    KEY, KEY2;
}

@XmlJavaTypeAdapter(MapAdapter.class)
Map<KeyType, String> mapProperty;

class MapElements {
    @XmlAttribute public String key;
    @XmlAttribute public String value;
}

class MapAdapter extends XmlAdapter<MapElements[], Map<String, String>> {
    public MapElements[] marshal(Map<String, String> arg0) {
        MapElements[] mapElements = new MapElements[arg0.size()];
        int i = 0;
        for (var entry : arg0.entrySet())
            mapElements[i++] = new MapElements(entry.getKey(), entry.getValue());
        return mapElements;
    }
    public Map<String, String> unmarshal(MapElements[] arg0) {
        Map<String, String> r = new TreeMap<>();
        for (MapElements mapelement : arg0)
            r.put(mapelement.key, mapelement.value);
        return r;
    }
}