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how to invoke JSF Validator on keyup event?
<h:inputText id="foo" value="#{bean.foo}">
    <f:ajax event="keyup" execute="@this" render="fooMessage" />
    <f:validator validatorId="fooValidator" />
</h:inputText>
<h:message id="fooMessage" for="foo" />

@FacesValidator("fooValidator")
public class FooValidator implements Validator {

    @Override
    public void validate(FacesContext context, UIComponent component, Object value) throws ValidatorException {
        // ...

        if (invalid) {
            throw new ValidatorException(new FacesMessage("Fail!"));
        }
    }

}