JSF: Validate value provided in both fields or none of them

I have two fields representing data range ("from" and "to"). I need to check if either both fields are filled on none of them, so only moment there should be validation message shown is when one is filled and not second one. How can I do that? I thied this custom validator and add it to both fields (as JSF doesn't validate empty fields) but it always show that they are not valid.

public void validate(FacesContext conte开发者_JS百科xt, UIComponent component, Object value) throws ValidatorException {
  String otherControlId = (String) component.getAttributes().get("otherControlId");
  UIInput otherControlInput = (UIInput) context.getViewRoot().findComponent(otherControlId);
  Object otherControlValue = otherControlInput.getValue();
  if ((value == null && otherControlValue != null) || (value != null && otherControlValue == null)) {
 //show message
  }
}

otherControlId points to second control ID, and I get a control in validator. But otherControlValue is always null.

Components are processed in the order as they appear in the component tree. So if the otherControlInput appears after the currently validated component in the component tree, then it's not processed yet. You would then need to access its (unconverted and unvalidated!) value by UIInput#getSubmittedValue() instead of UIInput#getValue().

Object otherControlValue = otherControlInput.getSubmittedValue();

An alternative is to put the validator on the other component (the one that appears the last in the tree) instead of the current component, then you can get the value by UIInput#getValue().

A completely different alternative is to let required attribute depend on each other:

<h:inputText binding="#{from}" value="#{bean.from}" required="#{not empty to.submittedValue}" />
<h:inputText binding="#{to}" value="#{bean.to}" required="#{not empty from.value}" />