在会话到期前调用方法

我已经实现了 sessionListener 但一旦我达到 public void sessionDestroyed(HttpSessionEvent event) FacesConfig.getCurrentInstance() )在会话实际过期之前。

我该怎么做?有什么想法吗?这是我的会话侦听器:

public class MySessionListener implements HttpSessionListener {

    private static final Logger log = LoggerFactory.getLogger(MySessionListener.class);

    public MySessionListener() {

    }

    public void sessionCreated(HttpSessionEvent event) {
        log.debug("Current Session created : "
              + event.getSession().getId()+ " at "+ new Date());
    }

    public void sessionDestroyed(HttpSessionEvent event) {
        // get the destroying session...

        HttpSession session = event.getSession();
        prepareLogoutInfoAndLogoutActiveUser(session);

        log.debug("Current Session destroyed :"
              + session.getId()+ " Logging out user...");

        /*
         * nobody can reach user data after this point because 
         * session is invalidated already.
         * So, get the user data from session and save its 
         * logout information before losing it.
         * User's redirection to the timeout page will be 
         * handled by the SessionTimeoutFilter.
         */

         // Only if needed
    }

    /**
     * Clean your logout operations.
     */
    public void prepareLogoutInfoAndLogoutActiveUser(HttpSession httpSession) {
        UserBean user = FacesContext.getCurrentInstance().getApplication().evaluateExpressionGet(FacesContext.getCurrentInstance(), "#{user}", UserBean.class);
        LockBean lock = FacesContext.getCurrentInstance().getApplication().evaluateExpressionGet(FacesContext.getCurrentInstance(), "#{lock}", LockBean.class);
        lock.unlock(user.getUsername());
        log.info("Unlocked examination for user: "+user.getUsername());
    }
}

NullPointerException 在 FacesContext.getCurrentInstance().getApplication() getCurrentInstance 为空或 getApplication 返回空值