A) FacesContext facesContext = FacesContext.getCurrentInstance();
ExternalContext externalContext=facesContext.getExternalContext();
HttpSession session = (HttpSession) externalContext.getSession(false);
if(session.isNew()) { // java.lang.NullPointerException
B) HttpServletRequest req1 = (HttpServletRequest)FacesContext.getCurrentInstance()
.getExternalContext().getRequest();
HttpSession session1=req1.getSession();
if(session1.isNew()) { // no Exception
为什么案例 A 抛出 NullPointerException 而案例 B 不是。