public Session readSession(Serializable sessionId) throws UnknownSessionException {
        Session s = getCachedSession(sessionId);
        if (s == null) {
            s = super.readSession(sessionId);
        }
        return s;
    }
Attempts to acquire the Session from the cache first using the session ID as the cache key.  If no session
is found, {@code super.readSession(sessionId)} is called to perform the actual retrieval.

@param sessionId the id of the session to retrieve from the EIS.
@return the session identified by {@code sessionId} in the EIS.
@throws UnknownSessionException if the id specified does not correspond to any session in the cache or EIS.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java