struts2 <s:select>标签

Servlet.service() for servlet jsp threw exception
tag 'select', field 'list', name 'user.department.departmentcode': The requested list key '#request.list_department' could not be resolved as a collection/array/map/enumeration/iterator type. Example: people or people.{name} - [unknown location]
biz层
public List findDepartment() {
String hql = "from Department ";
List list = this.getDao().find(hql);
return list;
}
action 层
public String findDepartment(){
List list = this.biz.findDepartment();
ServletActionContext.getRequest().setAttribute("list_department", list);
return "";
}

private Department department;

public Department getDepartment() {
return department;
}

public void setDepartment(Department department) {
this.department = department;
}
jsp
<s:action name="user!findDepartment"></s:action>
<s:select theme="simple" name="user.department.departmentcode" headerKey="-1" headerValue="全部"
list="#request.list_department" listKey="department.departmentcode" listValue="department.departmentname">
</s:select>