Question

My issue is fairly straightforward. My program requires immediate notification if a network connection is lost. I’m using Java 5, so I’m unable to use the very handy features of NetworkInterface.

Currently I have two different methods of checking for a network connection:
(try..catches removed)

Method One:

URL url = new URL("http://www.google.com");
HttpURLConnection urlConnect = (HttpURLConnection) url.openConnection();
// trying to retrieve data from the source. If offline, this line will fail:
Object objData = urlConnect.getContent();
return true;

Method 2:

Socket socket = new Socket("www.google.com", 80);
netAccess = socket.isConnected();
socket.close();
return netAccess;

However, both of these methods block until a Timeout is reached before returning false. I need a method that will return immediately, but is compatible with Java 5.

Thanks!

EDIT:
I forgot to mention that my program is reliant on IP addresses, not DNS names. So checking for an error when resolving a host is out…

2nd EDIT:

Figured out a final solution using NetworkInterface:

Enumeration<NetworkInterface> eni = NetworkInterface.getNetworkInterfaces();
        while(eni.hasMoreElements()) {
            Enumeration<InetAddress> eia = eni.nextElement().getInetAddresses();
            while(eia.hasMoreElements()) {
                InetAddress ia = eia.nextElement();
                if (!ia.isAnyLocalAddress() && !ia.isLoopbackAddress() && !ia.isSiteLocalAddress()) {
                    if (!ia.getHostName().equals(ia.getHostAddress()))
                        return true;
                }
            }
        }

Answer 1

From JGuru

Starting with Java 5, there is an isReachable() method in the
InetAddress class. You can specify either a timeout or the
NetworkInterface to use. For more information on the underlying
Internet Control Message Protocol (ICMP) used by ping, see RFC 792
(http://www.faqs.org/rfcs/rfc792.html).

Usage from Java2s

  int timeout = 2000;
  InetAddress[] addresses = InetAddress.getAllByName("www.google.com");
  for (InetAddress address : addresses) {
    if (address.isReachable(timeout))
      System.out.printf("%s is reachable%n", address);
    else
      System.out.printf("%s could not be contacted%n", address);
  }

If you want to avoid blocking use Java NIO (Non-blocking IO) in the java.nio package

String host = ...; 
InetSocketAddress socketAddress = new InetSocketAddress(host, 80); 
channel = SocketChannel.open(); 
channel.configureBlocking(false); 
channel.connect(socketAddress);