When the DNS lookup occurs in this code? By getByName() or getHostName() or getHostAddress()?

问题

In which line the DNS lookup occurs?

import java.net.InetAddress;
import java.net.UnknownHostException;
public class Addresses {
     public static void main(String[] args) {

        try {
           InetAddress address= InetAddress.getByName("www.sun.com"); //line  7
           System.out.println(address.getHostName()+ "-"+address.getHostAddress());//line 8
        } catch (UnknownHostException e) {
           e.printStackTrace();
        }
     }
}

---

As I can understand it should be at line 7. But in a set of tutorials I found that it is said to be at line 8. But there was no any explanation for it. If it is at line 8, but not at line 7, can somebody please explain it.

回答1:

From the JAVA source both InetAddress.getByName("www.sun.com"); and address.getHostName() do the lookup.

InetAddress.getByName("www.sun.com"); method will do lookup here

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 ((addresses = checkLookupTable(host)) == null) {
        try {
            // This is the first thread which looks up the addresses
            // this host or the cache entry for this host has been
            // expired so this thread should do the lookup.
            for (NameService nameService : nameServices) {
                try {
                    /*
                     * Do not put the call to lookup() inside the
                     * constructor.  if you do you will still be
                     * allocating space when the lookup fails.
                     */

                    addresses = nameService.lookupAllHostAddr(host);
                    success = true;
                    break;
                } catch (UnknownHostException uhe) {
                    if (host.equalsIgnoreCase("localhost")) {
                        InetAddress[] local = new InetAddress[] { impl.loopbackAddress() };
                        addresses = local;
                        success = true;
                        break;
                    }
                    else {
                        addresses = unknown_array;
                        success = false;
                        ex = uhe;
                    }
                }
            }

            // More to do?
            if (reqAddr != null && addresses.length > 1 && !addresses[0].equals(reqAddr)) {
                // Find it?
                int i = 1;
                for (; i < addresses.length; i++) {
                    if (addresses[i].equals(reqAddr)) {
                        break;
                    }
                }
                // Rotate
                if (i < addresses.length) {
                    InetAddress tmp, tmp2 = reqAddr;
                    for (int j = 0; j < i; j++) {
                        tmp = addresses[j];
                        addresses[j] = tmp2;
                        tmp2 = tmp;
                    }
                    addresses[i] = tmp2;
                }
            }
            // Cache the address.
            cacheAddresses(host, addresses, success);

            if (!success && ex != null)
                throw ex;

        } finally {
            // Delete host from the lookupTable and notify
            // all threads waiting on the lookupTable monitor.
            updateLookupTable(host);
        }
    }

    return addresses;
}

address.getHostName() method do lookup here

 private static String getHostFromNameService(InetAddress addr, boolean check) {
    String host = null;
    for (NameService nameService : nameServices) {
        try {
            // first lookup the hostname
            host = nameService.getHostByAddr(addr.getAddress());

            /* check to see if calling code is allowed to know
             * the hostname for this IP address, ie, connect to the host
             */
            if (check) {
                SecurityManager sec = System.getSecurityManager();
                if (sec != null) {
                    sec.checkConnect(host, -1);
                }
            }

            /* now get all the IP addresses for this hostname,
             * and make sure one of them matches the original IP
             * address. We do this to try and prevent spoofing.
             */

            InetAddress[] arr = InetAddress.getAllByName0(host, check);
            boolean ok = false;

            if(arr != null) {
                for(int i = 0; !ok && i < arr.length; i++) {
                    ok = addr.equals(arr[i]);
                }
            }

            //XXX: if it looks a spoof just return the address?
            if (!ok) {
                host = addr.getHostAddress();
                return host;
            }

            break;

        } catch (SecurityException e) {
            host = addr.getHostAddress();
            break;
        } catch (UnknownHostException e) {
            host = addr.getHostAddress();
            // let next provider resolve the hostname
        }
    }

    return host;
}

来源：https://stackoverflow.com/questions/22978067/when-the-dns-lookup-occurs-in-this-code-by-getbyname-or-gethostname-or-geth