HttpSession cannot be resolved to a type in jsf logout

问题

HttpSession cannot be resolved to a type in jsf logout

public class LogoutBean 
{

   public String Logout() 
    {
//      FacesContext context = FacesContext.getCurrentInstance();
//      ExternalContext ec = context.getExternalContext();
// 
//      final HttpServletRequest request = (HttpServletRequest)ec.getRequest();
//       request.getSession( false ).invalidate();


        ((HttpSession) FacesContext.getCurrentInstance().getExternalContext().getSession(false)).invalidate();

         return "login";
    }
}

来源：https://stackoverflow.com/questions/11929508/httpsession-cannot-be-resolved-to-a-type-in-jsf-logout