public void setActiveSessionsCache(Cache<Serializable, Session> cache) {
        this.activeSessions = cache;
    }
Sets the cache instance to use for storing active sessions.  If one is not set (it remains {@code null}),
it will be {@link CacheManager#getCache(String) acquired} from the {@link #setCacheManager configured}
{@code CacheManager} using the {@link #getActiveSessionsCacheName() activeSessionsCacheName}.

@param cache the cache instance to use for storing active sessions or {@code null} if the cache is to be
             acquired from the {@link #setCacheManager configured} {@code CacheManager}.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java