最近在研究tomcat的session管理,遇到以下问题请大家帮忙看看

public boolean isValid()
  {
    if (this.expiring) { //如果已经过期返回true
      return true;
    }

    if (!(this.isValid)) { //如果没有过期 则返回false
      return false;
    }

    if ((ACTIVITY_CHECK) && (this.accessCount.get() > 0)) {
      return true;
    }

    if (this.maxInactiveInterval >= 0) { //判断当前访问时间-上次访问时间 如果超过maxInactiveInterval 过期session
      long timeNow = System.currentTimeMillis();
      int timeIdle = (int)((timeNow - this.thisAccessedTime) / 1000L);
      if (timeIdle >= this.maxInactiveInterval) {
        expire(true); //过期方法
      }
    }

    return this.isValid;
  }

  public void expire(boolean notify)
  {
    if (this.expiring) {
      return;
    }
    synchronized (this)
    {
      if (this.manager == null) {
        return;
      }
      this.expiring = true; //设置expiring=true

      Context context = (Context)this.manager.getContainer();
      Object[] listeners = context.getApplicationLifecycleListeners();
      if ((notify) && (listeners != null)) {
        HttpSessionEvent event = new HttpSessionEvent(getSession());

        for (int i = 0; i < listeners.length; ++i) {
          int j = listeners.length - 1 - i;
          if (!(listeners[j] instanceof HttpSessionListener))
            continue;
          HttpSessionListener listener = (HttpSessionListener)listeners[j];
          try
          {
            fireContainerEvent(context, "beforeSessionDestroyed", listener);

            listener.sessionDestroyed(event);
            fireContainerEvent(context, "afterSessionDestroyed", listener);
          }
          catch (Throwable t)
          {
            try {
              fireContainerEvent(context, "afterSessionDestroyed", listener);
            }
            catch (Exception e)
            {
            }

            this.manager.getContainer().getLogger().error(sm.getString("standardSession.sessionEvent"), t);
          }
        }
      }

      if (ACTIVITY_CHECK) {
        this.accessCount.set(0);
      }
      setValid(false); //设置已经过期

      long timeNow = System.currentTimeMillis();
      int timeAlive = (int)((timeNow - this.creationTime) / 1000L);
      synchronized (this.manager) {
        if (timeAlive > this.manager.getSessionMaxAliveTime()) {
          this.manager.setSessionMaxAliveTime(timeAlive);
        }
        int numExpired = this.manager.getExpiredSessions();
        ++numExpired;
        this.manager.setExpiredSessions(numExpired);
        int average = this.manager.getSessionAverageAliveTime();
        average = (average * (numExpired - 1) + timeAlive) / numExpired;
        this.manager.setSessionAverageAliveTime(average);
      }

      this.manager.remove(this);

      if (notify) {
        fireSessionEvent("destroySession", null);
      }

      this.expiring = false; //expiring改为false

      String[] keys = keys();
      for (int i = 0; i < keys.length; ++i)
        removeAttributeInternal(keys[i], notify);
    }
  }