@Override
    protected final Session doGetSession(final SessionKey key) throws InvalidSessionException {
        enableSessionValidationIfNecessary();

        LOGGER.trace("Attempting to retrieve session with key {}", key);

        Session s = retrieveSession(key);
        if (s != null) {
            validate(s, key);
        }
        return s;
    }
If using the underlying default <tt>SessionValidationScheduler</tt> (that is, the
{@link #setSessionValidationScheduler(SessionValidationScheduler) setSessionValidationScheduler} method is
never called) , this method allows one to specify how
frequently session should be validated (to check for orphans).  The default value is
{@link #DEFAULT_SESSION_VALIDATION_INTERVAL}.
<p/>
If you override the default scheduler, it is assumed that overriding instance 'knows' how often to
validate sessions, and this attribute will be ignored.
<p/>
Unless this method is called, the default value is {@link #DEFAULT_SESSION_VALIDATION_INTERVAL}.

@param sessionValidationInterval the time in milliseconds between checking for valid sessions to reap orphans.
core/src/main/java/org/apache/shiro/session/mgt/AbstractValidatingSessionManager.java