oauth2/oauth2-client/src/main/java/org/springframework/security/oauth2/client/web/reactive/function/client/OAuth2AuthorizedClientResolver.java
private Mono<String> clientRegistrationId(Mono<Authentication> authentication) {
		return authentication
				.filter(t -> this.defaultOAuth2AuthorizedClient && t instanceof OAuth2AuthenticationToken)
				.cast(OAuth2AuthenticationToken.class)
				.map(OAuth2AuthenticationToken::getAuthorizedClientRegistrationId);
	}
private Mono<String> clientRegistrationId(Mono<Authentication> authentication) {
		return authentication
				.filter(t -> this.defaultOAuth2AuthorizedClient && t instanceof OAuth2AuthenticationToken)
				.cast(OAuth2AuthenticationToken.class)
				.map(OAuth2AuthenticationToken::getAuthorizedClientRegistrationId);
	}
Attempts to load the client registration id from the current {@link Authentication}
@return