org.apache.struts2.interceptor.CheckboxInterceptor is in the
defaultStack. It checks each form parameter submitted to the action and if it finds one with a prefix of
_checkbox it inserts a value for a parameter whose name is derived from the suffix to
_checkbox if it does not exist. The default value inserted is
false but this can be changed by setting the
uncheckedValue parameter on the interceptor.
This means that a checkbox can be accompanied by a hidden input with the same name but a prefix of
_checkbox so that if the checkbox is not checked on the form the action will still receive a value rather than the default HTML action of not providing a value for unchecked checkboxes.