Validate a multiple html:select with validator framework

[Heidy Gutiérrez Guzmán]

I have a multiple  html:select  and i need  to validate it with validator
framework.

The attribute in the ActionForm is an array of strings.
private String[] subtemas= new String[]{}

The validation that i'm using is required.This validation works with others
form fields

This is the code of html:select in jsp page:

<html:select  property="subtemas"  multiple="true" >
       <html:options collection="collectionSubtemas" property="id"
labelProperty="nombre" />
</htmlelect>

Thanks