Question

I know I can get the old value through UIInput#getValue().

But in many cases where the field is bound to a bean value I want to get the default value that I don’t need to validate if the input equals the default value.

This is very usefull if a certain field has a unique constraint and you have a editing form.
Validation will always fail since in a check constraint method it will always find its own value thus validating to false.

One way would be to pass that default value as an attribute using <f:attribute> and check inside the validator. But is there an easier built-in way?

Answer 1

The submitted value is just available as value argument in the validate() implementation.

public void validate(FacesContext context, UIComponent component, Object value) throws ValidatorException {
    Object oldValue = ((UIInput) component).getValue();

    if (value != null ? value.equals(oldValue) : oldValue == null) {
        // Value has not changed.
        return;
    }

    // Continue validation here.
}

An alternative is to design the Validator as a ValueChangeListener. It’ll then only be invoked when the value has really changed. It’s somewhat hacky, but it does the job you really need.

<h:inputText ... valueChangeListener="#{uniqueValueValidator}" />

or

<h:inputText ...>
    <f:valueChangeListener binding="#{uniqueValueValidator}" />
</h:inputText>

with

@ManagedBean
public class UniqueValueValidator implements ValueChangeListener {

    @Override
    public void processValueChange(ValueChangeEvent event) throws AbortProcessingException {
        FacesContext context = FacesContext.getCurrentInstance();
        UIInput input = (UIInput) event.getComponent();
        Object oldValue = event.getOldValue();
        Object newValue = event.getNewValue();

        // Validate newValue here against DB or something.
        // ...

        if (invalid) {
            input.setValid(false);
            context.validationFailed();
            context.addMessage(input.getClientId(context),
                new FacesMessage(FacesMessage.SEVERITY_ERROR, "Please enter unique value", null));
        }
    }

}

Note that you cannot throw a ValidatorException there, that’s why need to manually set both the component and faces context to invalid and manually add the message for the component. The context.validationFailed() will force JSF to skip the update model values and invoke action phases.