Modern applications often need the ability to learn information about hosts out on the network. One key class in this process for Java developers is the java.net.InetAddress . This class allows you to figure out various information about hosts, as well as discovering host information by different means.
InetAddress is a deceptively simple class to use, in that it provides a simple API for working with some very complex concepts. For instance, it provides a standard interface for discovering IPv4 IP addresses as well as IPv6 IP addresses. In addition, it distinguishes between multicast and unicast address types transparently. Finally, there are facilities built-in for determining if a host is reachable.
Here are some useful tidbits to understand:
- If the internet address is IPv6, the returned object from the static methods of
InetAddresswill be anInet6Addressobject. Likewise, if the address is IPv4, the returned object from the static methods will be anInet4Addressobject. - The IP Address lookup can be by
byte[], in which case highest-order byte format is used - so for the ip address127.0.0.1, you would have thebyte[]{127,0,0,1}. - Host name resolution is goverened by caching that can be controlled by some Java system properties - from the Javadoc:
networkaddress.cache.ttl (default: -1)
Indicates the caching policy for successful name lookups from the name service. The value is specified as as integer to indicate the number of seconds to cache the successful lookup.
A value of -1 indicates "cache forever".networkaddress.cache.negative.ttl (default: 10)
Indicates the caching policy for un-successful name lookups from the name service. The value is specified as as integer to indicate the number of seconds to cache the failure for un-successful lookups.
A value of 0 indicates "never cache". A value of -1 indicates "cache forever".
Here is a little example class that shows some of the common techniques for using InetAddress to discover various information:
package org.javalobby.tnt.net; import java.net.InetAddress; public class InetAddressTest { public static void main(String[] args) throws Exception { // Get by host name InetAddress javalobby = InetAddress.getByName("javalobby.org"); // Get by IP as host name InetAddress byIpAsName = InetAddress.getByName("64.69.35.190"); // Get by IP as highest-order byte array InetAddress byIp = InetAddress.getByAddress(new byte[] { 64, 69, 35, (byte)190}); // Get Local address InetAddress local = InetAddress.getLocalHost(); // Get Local Address by Loopback IP InetAddress localByIp = InetAddress.getByName("127.0.0.1"); printAddressInfo("By-Name (Javalobby.org)", javalobby); printAddressInfo("By-Name (Using IP as Host)", byIpAsName); printAddressInfo("By-IP: (64.69.35.190)", byIp); printAddressInfo("Special Local Host", local); printAddressInfo("Local Host By IP", localByIp); } private static void printAddressInfo(String name, InetAddress... hosts) throws Exception { System.out.println("===== Printing Info for: '" + name + "' ====="); for(InetAddress host : hosts) { System.out.println("Host Name: " + host.getHostName()); System.out.println("Canonical Host Name: " + host.getCanonicalHostName()); System.out.println("Host Address: " + host.getHostAddress()); System.out.println("Calculated Host Address: " + getIpAsString(host)); System.out.print("Is Any Local: " + host.isAnyLocalAddress()); System.out.print(" - Is Link Local: " + host.isLinkLocalAddress()); System.out.print(" - Is Loopback: " + host.isLoopbackAddress()); System.out.print(" - Is Multicast: " + host.isMulticastAddress()); System.out.println(" - Is Site Local: " + host.isSiteLocalAddress()); System.out.println("Is Reachable in 2 seconds: " + host.isReachable(2000)); } } private static String getIpAsString(InetAddress address) { byte[] ipAddress = address.getAddress(); StringBuffer str = new StringBuffer(); for(int i=0; i<ipAddress.length; i++) { if(i > 0) str.append('.'); str.append(ipAddress[i] & 0xFF); } return str.toString(); } }
Here is an example output:
===== Printing Info for: 'By-Name (Javalobby.org)' ===== Host Name: javalobby.org Canonical Host Name: www.javalobby.org Host Address: 64.69.35.190 Calculated Host Address: 64.69.35.190 Is Any Local: false - Is Link Local: false - Is Loopback: false - Is Multicast: false - Is Site Local: false Is Reachable in 2 seconds: true ===== Printing Info for: 'By-Name (Using IP as Host)' ===== Host Name: www.javalobby.org Canonical Host Name: www.javalobby.org Host Address: 64.69.35.190 Calculated Host Address: 64.69.35.190 Is Any Local: false - Is Link Local: false - Is Loopback: false - Is Multicast: false - Is Site Local: false Is Reachable in 2 seconds: true ===== Printing Info for: 'By-IP: (64.69.35.190)' ===== Host Name: www.javalobby.org Canonical Host Name: www.javalobby.org Host Address: 64.69.35.190 Calculated Host Address: 64.69.35.190 Is Any Local: false - Is Link Local: false - Is Loopback: false - Is Multicast: false - Is Site Local: false Is Reachable in 2 seconds: true ===== Printing Info for: 'Special Local Host' ===== Host Name: COFFEE-BYTES-2 Canonical Host Name: 192.168.1.101 Host Address: 192.168.1.101 Calculated Host Address: 192.168.1.101 Is Any Local: false - Is Link Local: false - Is Loopback: false - Is Multicast: false - Is Site Local: true Is Reachable in 2 seconds: true ===== Printing Info for: 'Local Host By IP' ===== Host Name: localhost Canonical Host Name: localhost Host Address: 127.0.0.1 Calculated Host Address: 127.0.0.1 Is Any Local: false - Is Link Local: false - Is Loopback: true - Is Multicast: false - Is Site Local: false Is Reachable in 2 seconds: true Until next time, R.J. Lorimer Contributing Editor - rj -at- javalobby.org Author - http://www.coffee-bytes.com Software Consultant - http://www.crosslogic.com