Возможно, вы можете использовать проверку JS.
JSP:
<s:checkbox id="other" name="other" />
<s:textfield id="otherDetails" name="otherDetails"></s:textfield>
<sx:submit
id="button_submit"
name="button_submit"
onclick="return checkOther();" />
JS:
function checkOther() {
if(document.getElementById('other').checked == true) {
if(document.getElementyById('otherDetails').value.length == 0) {
//you should trim the value
alert("Insert other details");
return false;
}
}
return true;
}