protected Cache<Serializable, Session> createActiveSessionsCache() {
        Cache<Serializable, Session> cache = null;
        CacheManager mgr = getCacheManager();
        if (mgr != null) {
            String name = getActiveSessionsCacheName();
            cache = mgr.getCache(name);
        }
        return cache;
    }
Creates a cache instance used to store active sessions.  Creation is done by first
{@link #getCacheManager() acquiring} the {@code CacheManager}.  If the cache manager is not null, the
cache returned is that resulting from the following call:
<pre>       String name = {@link #getActiveSessionsCacheName() getActiveSessionsCacheName()};
cacheManager.getCache(name);</pre>

@return a cache instance used to store active sessions, or {@code null} if the {@code CacheManager} has
not been set.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java