shiro session计算timeout

/**
     * Determines if this session is expired.
     *
     * @return true if the specified session has expired, false otherwise.
     */
    protected boolean isTimedOut() {

        if (isExpired()) {
            return true;
        }

        long timeout = getTimeout();

        if (timeout >= 0l) {

            Date lastAccessTime = getLastAccessTime();

            if (lastAccessTime == null) {
                String msg = "session.lastAccessTime for session with id [" +
                        getId() + "] is null.  This value must be set at " +
                        "least once, preferably at least upon instantiation.  Please check the " +
                        getClass().getName() + " implementation and ensure " +
                        "this value will be set (perhaps in the constructor?)";
                throw new IllegalStateException(msg);
            }

            // Calculate at what time a session would have been last accessed
            // for it to be expired at this point.  In other words, subtract
            // from the current time the amount of time that a session can
            // be inactive before expiring.  If the session was last accessed
            // before this time, it is expired.
            long expireTimeMillis = System.currentTimeMillis() - timeout;
            Date expireTime = new Date(expireTimeMillis);
            return lastAccessTime.before(expireTime);
        } else {
            if (log.isTraceEnabled()) {
                log.trace("No timeout for session with id [" + getId() +
                        "].  Session is not considered expired.");
            }
        }

        return false;
    }