如何从 Struts 2 中的选定对象中获取对象属性

I have a list of City objects with name and id fields. I use Struts2 and I a have jsp page with a select tag.

<s:select label="Source city" 
          list="cities" 
          name="source"/>

这里是 Action 类

public class CalculationAction extends ActionSupport {

    private List<City> cities;
    private DataAccessPerformer dao = new DataAccessPerformer();
    private String source;
    private int sourceId;

    public CalculationAction() {
        cities = new ArrayList<City>();
        // getting cities from database
        setCities(dao.getAllCities());
    }

    // getters and setters
}

城市类

public class City {

    private int id;
    private String name;

    @Override
    public String toString() {
        return getCityName();
    }

    // getters and setters
}

通过这种方式，我正在初始化 source 字段，但我无法获取 sourceId.

<s:select label="Source city" 
          list="cities" 
          listKey="id"
          listValue="name"
          name="sourceId"/>