private Cache<Serializable, Session> getActiveSessionsCacheLazy() {
        if (this.activeSessions == null) {
            this.activeSessions = createActiveSessionsCache();
        }
        return activeSessions;
    }
Returns the active sessions cache, but if that cache instance is null, first lazily creates the cache instance
via the {@link #createActiveSessionsCache()} method and then returns the instance.
<p/>
Note that this method will only return a non-null value code if the {@code CacheManager} has been set.  If
not set, there will be no cache.

@return the active sessions cache instance.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java