ERROR [jsp]: Servlet.service() for servlet jsp threw exception
tag 'select', field 'list': The requested list key '' could not be resolved
as a collection/array/map/enumeration/iterator type. Example: people or
people.{name} - [unknown location]
  at org.apache.struts2.components.Component.fieldError(Component.java
:221)
  at org.apache.struts2.components.Component.findValue (Component.java:283)
----------------------------------------------------------------------------

<s:select list="" listKey="city.keys" listValue="city.values"
theme="simple"/>

I don't know what to write to "list" attribute it is required but I want to
use the value in struts.properties file as I said on the first message.