How InetAddress.getLocalHost() works?

Question

I'm trying to get how InetAddress.getLocalHost() works. Javadoc says that it retrieves the name of the host from the system and then resolves it into an InetAddress . What exactly does "resolve into InetAddess" mean? Does it simply ask DNS to resolve a hostname?

Answer 1

From InetAddress.java source:

 private static InetAddress[] getAddressesFromNameService(String host, InetAddress reqAddr)
        throws UnknownHostException
    {
        InetAddress[] addresses = null;
        boolean success = false;
        UnknownHostException ex = null;

        // Check whether the host is in the lookupTable.
        // 1) If the host isn't in the lookupTable when
        //    checkLookupTable() is called, checkLookupTable()
        //    would add the host in the lookupTable and
        //    return null. So we will do the lookup.
        // 2) If the host is in the lookupTable when
        //    checkLookupTable() is called, the current thread
        //    would be blocked until the host is removed
        //    from the lookupTable. Then this thread
        //    should try to look up the addressCache.
        //     i) if it found the addresses in the
        //        addressCache, checkLookupTable()  would
        //        return the addresses.
        //     ii) if it didn't find the addresses in the
        //         addressCache for any reason,
        //         it should add the host in the
        //         lookupTable and return null so the
        //         following code would do a lookup itself.

  ...

if (host.equalsIgnoreCase("localhost")) {
  InetAddress[] local = new InetAddress[] { impl.loopbackAddress() }; // {0x7f,0x00,0x00,0x01}
  addresses = local;
  success = true;
  break;
}

To recap:

• both InetAddress.getAllByName() and InetAddress.getLocalHost() resolve the address by calling getAddressesFromNameService()
• The JVM maintains its own cache of hostname -> IP address mapping.
• If the address is not in the cache ( lookupTable or addressCache ) it will call the operating system's DNS (exact behavior may vary per JVM implementation).
• For localhost sepcificly - there is a specific case inside getAddressesFromNameService()