public CacheManager getCacheManager() {
        return cacheManager;
    }
Returns the CacheManager to use for acquiring the {@link #getActiveSessionsCache() activeSessionsCache} if
one is not configured.  That is, the {@code CacheManager} will only be used if the
{@link #getActiveSessionsCache() activeSessionsCache} property is {@code null}.

@return the CacheManager used by the implementation that creates the activeSessions Cache.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java