map: { key1 : value1, key2: value2 }

"storage":
  {"entry":[
    {"key":"IRON","value":792},
    {"key":"COPPER","value":2489},
    {"key":"CARDBOARD","value":14536}
    ]
  }
}

@XmlRootElement
public class MapDTO<K, V> {

    @XmlAnyElement
    private List<JAXBElement<V>> elements = new ArrayList<>();

    public MapDTO(){}

    public MapDTO(Map<K,V> map) {
        map.forEach((k, v) ->
                elements.add(
                        new JAXBElement(new QName(k.toString()), v.getClass(), v))
        );
    }
}

"storage":{
  "CIRCUITS":[3379992],
  "CARDBOARD":[84838963545],
  "COPPER_CABLE":[1673987]
}