Superclass for almost all {@link Authenticator} implementations that performs the common work around authentication attempts. <p/> This class delegates the actual authentication attempt to subclasses but supports notification for successful and failed logins as well as logouts. Notification is sent to one or more registered {@link AuthenticationListener AuthenticationListener}s to allow for custom processing logic when these conditions occur. <p/> In most cases, the only thing a subclass needs to do (via its {@link #doAuthenticate} implementation) is perform the actual principal/credential verification process for the submitted {@code AuthenticationToken}.