下拉列表报错

jsp的源码：

请选择国家：
<s:select id="guojia" name="guojia" onchange="select_d('guojia')" list="countrys"  theme="simple"

headerKey="0" headerValue="请选择国家" listKey="id" listValue="CName" >

报错内容：

org.apache.jasper.JasperException: tag 'select', field 'list', name 'guojia': The requested list key 'countrys' could not be resolved as a collection/array/map/enumeration/iterator type. Example: people or people.{name} - [unknown location]

上网找了相同的错误，，原来是action层里得到的值List<country> countrys 没有实例化，

当把action里的 List<Country> countrys = new ArrayLIst<>();时，就好了。