Java: Spring AI

import java.net.InetAddress;
import java.util.Arrays;
import java.util.Set;
import java.util.stream.Collectors;

public final class SecurdGuard {
    private static final Set<String> BLOCK_ADDRS = Arrays.stream(
        System.getenv().getOrDefault("SECURD_BLOCK_ADDRS", "").split(","))
        .filter(s -> !s.isBlank()).collect(Collectors.toSet());

    public static final class Held extends RuntimeException {
        public Held(String msg) { super(msg); }
    }

    public static void check(String host, String agentRole) {
        try {
            for (InetAddress a : InetAddress.getAllByName(host)) {
                if (BLOCK_ADDRS.contains(a.getHostAddress())) {
                    throw new Held(host + " is held or blocked for " + agentRole);
                }
            }
        } catch (java.net.UnknownHostException e) {
            throw new Held(host + " did not resolve inside " + agentRole);
        }
    }
}

@Bean
@Description("Fetch an approved URL")
public Function<String, String> fetchUrl(RestClient http) {
    return url -> {
        String host = URI.create(url).getHost();
        try {
            SecurdGuard.check(host, "billing-agent");
        } catch (SecurdGuard.Held held) {
            return "BLOCKED: " + held.getMessage();
        }
        return http.get().uri(url).retrieve().body(String.class);
    };
}