Example of how to enable IPv6 only after testing if it works

Test.java

package test;

import java.lang.reflect.Field;
import java.net.Inet4Address;
import java.net.Inet6Address;
import java.net.InetAddress;
import java.net.Socket;
import java.net.UnknownHostException;

/**
 * This sample code shows how to determine if there is IPv6 connectivity, and
 * alters java properties to enable IPv6 again if it works. This works with
 * "-Djava.net.preferIPv6Address=false, and the goal is to not interfere with
 * applications that currently prefer IPv4 for some reason.
 * 
 * @author hugo
 * 
 */
public class Test {

	/**
	 * Enables IPv6 only if it works. Returns true if IPv6 was enabled. Upon any
	 * sort of error will bail and not change anything.
	 */
	public static boolean enableIPv6OnlyIfItWorks() {
		try {
			// This flag is set to true after an IPv6 connection was
			// established.
			boolean ipv6Works = false;
			// Temporarily undo this setting. Altering preferIPv6Address will
			// suffice
			System.setProperty("java.net.preferIPv4Stack", "false");
			for (InetAddress addr : InetAddress.getAllByName("hugo.barrera.io")) {
				// InetAddress#isReachable would fail in some special scenarios
				// (ie: filtered ICMP)
				if (addr instanceof Inet4Address)
					continue;
				if (addr instanceof Inet6Address) {
					new Socket(addr, 80);
					ipv6Works = true;
				}
			}

			// Set preferIPv6Address according to our results:
			Field f = InetAddress.class.getDeclaredField("preferIPv6Address");
			f.setAccessible(true);
			f.set(null, ipv6Works);
			return true;
		} catch (Exception e) {
			// Catching Exception isn't generally a good idea, but in this
			// scenario we want to bail regardless of what the error was. The
			// purpose is to not add any new possible points of failure.
			System.setProperty("java.net.preferIPv4Stack", "true");
			System.err
					.println("Something went wrong while testing or configuring IPv6.");
			System.err.println("Settings were not altered.");
			e.printStackTrace();

		}
		return false;
	}

	public static void main(String[] args) throws UnknownHostException {
		// The sample code will work even if this property is set to false:
		System.setProperty("java.net.preferIPv6Address", "false");
		enableIPv6OnlyIfItWorks();
		System.out.println(InetAddress.getByName("elysion.barrera.io"));
	}
}