public Collection<Session> getActiveSessions() {
        Cache<Serializable, Session> cache = getActiveSessionsCacheLazy();
        if (cache != null) {
            return cache.values();
        } else {
            return Collections.emptySet();
        }
    }
Returns all active sessions in the system.
<p/>
<p>This implementation merely returns the sessions found in the activeSessions cache.  Subclass implementations
may wish to override this method to retrieve them in a different way, perhaps by an RDBMS query or by other
means.

@return the sessions found in the activeSessions cache.
core/src/main/java/org/apache/shiro/session/mgt/eis/CachingSessionDAO.java