您可以尝试从将ServerWebExchange传递给控制器方法开始。
然后获取ServerHttpRequest并调用getSslInfo()。从getSslInfo()文档：

/**
 * Return the SSL session information if the request has been transmitted
 * over a secure protocol including SSL certificates, if available.
 * @return the session information, or {@code null} if none available
 * @since 5.0.2
 */

从SslInfo可以获得X509Certificate[]

@GetMapping
public Mono<Void> get(ServerWebExchange serverWebExchange) {
    ServerHttpRequest request = serverWebExchange.getRequest();

    Optional<X509Certificate[]> peerCertificates =
            Optional.ofNullable(request.getSslInfo())
                    .map(SslInfo::getPeerCertificates);

    if (peerCertificates.isPresent()) {
        ...
    }

    return Mono.empty();
}

如果您希望将此逻辑放在请求到达控制器之前**，则可以实现自己的WebFilter并在其中写入所需的逻辑
WebFilter方法：

Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain);

另外，据我所知，Spring Security有一些验证X509证书的特性。