private void enableSessionValidationIfNecessary() {
        SessionValidationScheduler scheduler = getSessionValidationScheduler();
        if (isSessionValidationSchedulerEnabled() && (scheduler == null || !scheduler.isEnabled())) {
            enableSessionValidation();
        }
    }
Scheduler used to validate sessions on a regular basis.
core/src/main/java/org/apache/shiro/session/mgt/AbstractValidatingSessionManager.java