DNS Cache Sample

dns.policy

grant {
    permission java.net.SocketPermission "*", "resolve";
};

DnsDemo.java

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.Calendar;
import java.util.Optional;

public class DnsDemo {
    public static void main(String[] args) throws Exception {
        while (true) {
            System.out.printf("%1$tH:%1$tM:%1$tS%n", Calendar.getInstance());
            printAddressInfo(addressOf("www.google.com"));
            printAddressInfo(addressOf("www.this-does-not-exist.io"));
            Thread.sleep(5000);
        }
    }

    private static Optional<InetAddress> addressOf(String host) {
        try {
            return Optional.of(InetAddress.getByName(host));
        } catch (UnknownHostException ex) {
            return Optional.empty();
        }
    }

    private static void printAddressInfo(Optional<InetAddress> address) {
        address.ifPresent((a) -> System.out.printf("%s => %s%n", a.getHostName(), a.getHostAddress()));
    }
}

Invocation

javac DnsDemo.java
java -cp . DnsDemo -Djava.security.manager -Djava.security.policy==src/main/resources/dns.policy