protected Session getCachedSession(Serializable sessionId, Cache<Serializable, Session> cache) {
        return cache.get(sessionId);
    }
Returns the Session with the specified id from the specified cache.  This method simply calls
{@code cache.get(sessionId)} and can be overridden by subclasses for custom acquisition behavior.

@param sessionId the id of the session to acquire.
@param cache     the cache to acquire the session from
@return the cached session, or {@code null} if the session wasn't in the cache.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java