如何从Struts 2中的选定对象中获取对象属性

我有一个City对象的列表，其中包含name和id字段.我使用Struts2，我有一个带select标记的jsp页面.

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
}

City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，我已经初始化了source字段，但无法获取sourceId.

In this way I'm getting source field initialized, but I can't fetch sourceId.

我试图将source字段类型更改为City，但是却得到了FieldError

I tried to change source field type to City, but I got FieldError

源"字段的无效字段值.

Invalid field value for field "source".

我应该如何正确获取ID?

How should I properly fetch the id?