Struts s:checkbox conversion fails to List<Integer>

Struts <s:checkbox> is not converted correctly to List<Integer> when only one checkbox element is present and checkbox is not checked.

[Given]
1) Mapping s:checkbox to a List<Integer>
2) there is only one checkbox in the list
3) the checkbox is not checked when the form is submitted.

[Result]
Input invalid

The issue is caused by the checkbox tag using the "submit unchecked values" design pattern. As a result, the code base tries to convert the value of the single unchecked element to false (rather than not setting any value of the List<Integer>).

Fix: Allow the developer to choose if the "submit unchecked values" is to be implemented. Add a new parameter to the s:checkbox tag ("requiredValue"). Default of requiredValue=true for backwards compatibility. When set to false, no hidden input values will be displayed in the view. This flexibility might be needed in other scenarios as well.

Pull Request: https://github.com/apache/struts/pull/169