Tratamento de ViewExpired JSF. Baseado em: http://stackoverflow.com/questions/8144195/check-if-session-exists-jsf

CustomExceptionHandler.java

package br.com.caelum.notasfiscais.exception;

import java.util.Iterator;
import java.util.Map;

import javax.faces.FacesException;
import javax.faces.application.NavigationHandler;
import javax.faces.application.ViewExpiredException;
import javax.faces.context.ExceptionHandler;
import javax.faces.context.ExceptionHandlerWrapper;
import javax.faces.context.FacesContext;
import javax.faces.event.ExceptionQueuedEvent;
import javax.faces.event.ExceptionQueuedEventContext;

public class CustomExceptionHandler extends ExceptionHandlerWrapper {

	private ExceptionHandler wrapped;
	 
    public CustomExceptionHandler(ExceptionHandler wrapped) {
        this.wrapped = wrapped;
    }
 
    @Override
    public ExceptionHandler getWrapped() {
        return this.wrapped;
    }
 
    @Override
    public void handle() throws FacesException {
    	//pega o contexto do JSF
    	FacesContext facesContext = FacesContext.getCurrentInstance();

    	//percore as listas de exceptions esperando tratamento
        for (Iterator<ExceptionQueuedEvent> iter = getUnhandledExceptionQueuedEvents().iterator(); iter.hasNext();) {
        	//obtém a próxima exception da lista
            Throwable exception = iter.next().getContext().getException();

            //se for ViewExpired...
            if (exception instanceof ViewExpiredException) {
            	//redireciona para login
                facesContext.getApplication().getNavigationHandler().handleNavigation(facesContext, null, "login?faces-redirec=true");
                facesContext.renderResponse();
                //remove esta exception da lista
                iter.remove();
            }
        }
        
        //delega para o tratamento default
        getWrapped().handle();
    }
}

CustomExceptionHandlerFactory.java

package br.com.caelum.notasfiscais.exception;

import javax.faces.context.ExceptionHandler;
import javax.faces.context.ExceptionHandlerFactory;

public class CustomExceptionHandlerFactory extends ExceptionHandlerFactory{
	
	private ExceptionHandlerFactory parent;
	 
    public CustomExceptionHandlerFactory(ExceptionHandlerFactory parent) {
        this.parent = parent;
    }
 
    @Override
    public ExceptionHandler getExceptionHandler() {
        ExceptionHandler result = parent.getExceptionHandler();
        result = new CustomExceptionHandler(result);
 
        return result;
    }
}

faces-config.xml

<?xml version="1.0" encoding="UTF-8"?>
<faces-config xmlns="http://xmlns.jcp.org/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-facesconfig_2_2.xsd"
	version="2.2">

	<lifecycle>
		<phase-listener>br.com.caelum.notasfiscais.listener.CicloDeVidaListener</phase-listener>
		<phase-listener>br.com.caelum.notasfiscais.listener.Autorizador</phase-listener>
	</lifecycle>

    <!--Registrando o tratador de exceptiions -->
	<factory>
		<exception-handler-factory>br.com.caelum.notasfiscais.exception.CustomExceptionHandlerFactory</exception-handler-factory>
	</factory>

</faces-config>