struts2中从后台读取数据到<s:select>

public class Books implements java.io.Serializable {

// Fields

private String bookId;
private String bookType;
private String bookName;
private String bookAuthor;
private String bookPress;
private Float bookPrice;
private Integer bookNumber;
private Set readers = new HashSet();
//省略相应的set和get方法
}

impl中

//查询所有的图书类型
public List findBookType()
{
Session session = HibernateSessionFactory.getSession();
Transaction tx = session.beginTransaction();
String hql="from Books group by bookType";
Query query = session.createQuery(hql);
List list = query.list();
return list;
}
Action中

public String findBookType()
{
String result="error";
List list = null;

BooksDao booksDao =new BooksImpl();
list = booksDao.findBookType();
if(list!=null)
{
ActionContext ctx = ActionContext.getContext();
Map map = ctx.getSession();
map.put("bookType", list);
//result="error";
System.out.println("--->"+list.size());
}
return result;
}
jsp中

<s:select label="类别" name="books.bookType" list="#session.bookType"
listKey="id" listValue="bookType" headerKey="-1" headerValue="请选择书的类别"
emptyOption="false" />