如何在struts2的 action中得到<s:optiontransferselect/>标签中的内容

Question

jsp中的关键内容如下:
<s:form action="TestTagLib">
        <s:optiontransferselect
    label="optiontransferselect Test"
    labelposition="top"
    list="{'thomas1','ada','lothar'}"
    name="leftList"
    addToLeftLabel="向左移动"
    addToRightLabel="向右移动"
    addAllToLeftLabel="left"
    addAllToRightLabel="right"
    doubleName="rightList"
  doubleList="{'Expert One-on-One J2EE Design and Development', 'JavaScript: TDefinGuide'}" 
          />
      <s:submit  value="login"/> 
  </s:form>然后提交到TestTagLibAction中,在action中设置了setter(),getter()方法,关键代码如下:private List leftList;
private List rightList;public List getLeftList() {
return leftList;
}public void setLeftList(List leftList) {
this.leftList = leftList;
}public List getRightList() {
return rightList;
}public void setRightList(List rightList) {
this.rightList = rightList;
}为什么在execute()方法中得不到 rightList的值? 其中rightList对应前面jsp页面中 <s:optiontransferselect/>的doublename="rightList"标签,代码如下:                List list1=getRightList();
for(int i=0;i <list1.size();i++){
System.out.println(list1.get(i).toString());
}报下面的错:
exceptionjavax.servlet.ServletException
org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:518)
org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:421)
root causejava.lang.NullPointerException
struts2action.TestTagLibAction.execute(TestTagLibAction.java:55)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)