+ * <p>
+ * If an originally requested ressource was stored in the {@link HttpSession}
+ * by the {@link SocialAuthenticationEntryPoint}, that URL will be returned,
+ * so that the {@link RequestCache} can restore the request.
+ * Otherwise, <code>null</code> will be returned, to indicate, that the user
+ * should be redirected to the default-post-sign-in-URL (configured in
+ * {@link ProviderSinInController}) after a successfull authentication.