/**
  * The set of currently active Sessions for this Manager, keyed by
  * session identifier.
  */
 protected Map<String, Session> sessions = new ConcurrentHashMap<>();

/**
  * This method will be invoked by the context/container on a periodic
  * basis and allows the manager to implement
  * a method that executes periodic tasks, such as expiring sessions etc.
  */
 public void backgroundProcess();

/**
 * {@inheritDoc}
 * <p>
 * Direct call to {@link #processExpires()}
 */
@Override
public void backgroundProcess() {
 count = (count + 1) % processExpiresFrequency;
 if (count == 0) //如果达到检查频率则开始检查
  processExpires();
}

/**
 * Invalidate all sessions that have expired.
 */
public void processExpires() {

 long timeNow = System.currentTimeMillis();
 Session sessions[] = findSessions(); //获取所有session对象
 int expireHere = 0 ; //过期session的数量，不要被这个变量名骗了

 if(log.isDebugEnabled())
  log.debug("Start expire sessions " + getName() + " at " + timeNow + " sessioncount " + sessions.length);
 for (int i = 0; i < sessions.length; i++) {
  if (sessions[i]!=null && !sessions[i].isValid()) {
   expireHere++;
  }
 }
 long timeEnd = System.currentTimeMillis();
 if(log.isDebugEnabled()) //打印记录
   log.debug("End expire sessions " + getName() + " processingTime " + (timeEnd - timeNow) + " expired sessions: " + expireHere);
 processingTime += ( timeEnd - timeNow );

}

/**
 * Return the <code>isValid</code> flag for this session.
 */
@Override
public boolean isValid() {

 if (!this.isValid) {
  return false;
 }

 if (this.expiring) {
  return true;
 }

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

 //关键所在
 //如果有设置最大空闲时间
 //就获取此Session的空闲时间进行判断
 //如果已超时，则执行expire操作
 if (maxInactiveInterval > 0) { 
  int timeIdle = (int) (getIdleTimeInternal() / 1000L);
  if (timeIdle >= maxInactiveInterval) {
   expire(true);
  }
 }

 return this.isValid;
}