问JSF，将空值注册到绑定的实例

EN

    <h:selectOneMenu id="city" value="#{cc.attrs.city}"
                     converter="anySelectConverter">
        <f:selectItem itemLabel="#{msg['general.choose']}" itemValue="#{null}"/>
        <f:selectItems itemLabel="#{city.name}" itemValue="#{city}"
                       var="city" value="#{locationComponentController.getAllCities()}" />
        <p:ajax event="valueChange" update="district subdistrict village neighbourhood #{cc.attrs.updateOnSelection}" />
    </h:selectOneMenu>

@FacesConverter("anySelectConverter")
public class AnySelectConverter implements Converter{


     private static Map<Object, String> entities = new ConcurrentHashMap<Object, String>();

        @Override
        public String getAsString(FacesContext context, UIComponent component, Object entity) {

            // TODO : Geçici çözüm sebebi araştırılmalı
            if(entity == null)
                return "";

            synchronized (entities) {
                if (!entities.containsKey(entity)) {
                    String uuid = UUID.randomUUID().toString();
                    entities.put(entity, uuid);
                    return uuid;
                } else {
                    return entities.get(entity);
                }
            }
        }

        @Override
        public Object getAsObject(FacesContext context, UIComponent component, String uuid) {
            for (Entry<Object, String> entry : entities.entrySet()) {
                if (entry.getValue().equals(uuid)) {
                    return entry.getKey();
                }
            }
            return null;
        }

    }