//JSF page

<h:panelGroup>Session ID</h:panelGroup>
<h:panelGroup>
    <h:inputText id="sessionid" value="#{DatabaseController.formMap['sessionid']}" >
        <f:validateLength minimum="0" maximum="15"/>
        <f:validator binding="#{ValidatorController.validateMessage}" >
            <f:attribute name="type" value="sessionid" />
        </f:validator>
        <f:ajax event="blur" render="sessionid" />                                          
    </h:inputText>
    <h:message id="sessionidMessage" for="sessionid" />
</h:panelGroup>

// validator

import java.io.Serializable;
import javax.enterprise.context.SessionScoped;
import javax.faces.application.FacesMessage;
import javax.faces.component.UIComponent;
import javax.faces.context.FacesContext;
import javax.faces.validator.Validator;
import javax.faces.validator.ValidatorException;
import javax.inject.Named;

@Named("ValidatorController")
@SessionScoped
public class FormValidator implements Validator, Serializable {

    public FormValidator() {
    }

    @Override
    public void validate(FacesContext context, UIComponent component, Object value) throws ValidatorException {
        throw new UnsupportedOperationException("Not supported yet.");
    }

    public void validateMessage(FacesContext context, UIComponent component, Object value) throws ValidatorException {

        if (context.equals("test")) {
            throw new ValidatorException(new FacesMessage(FacesMessage.SEVERITY_ERROR,
                    "Session ID is already in use, please choose another.", null));
        }
    }
}