how to populate Struts 2 UI select tag?

How can I populate Struts 2 UI开发者_StackOverflow社区 select tag?

I keep getting the error:

tag select, field list: The requested list key roles could not be resolved as a collection/array/map/enumeration/iterator type.

Most likely that would be if your list roles is not initialized ( null ) . Or another reason can be if you have not declared an appropriate getter .