java - 这段代码中何时发生 DNS 查找？通过 getByName() 或 getHostName() 或 getHostAddress()？

Question

DNS 查找发生在哪一行？

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();
        }
     }
}

<小时/>

据我了解应该是在第7行。但是在一组教程中我发现据说是在第8行。但是没有任何解释。如果它在第 8 行，但不在第 7 行，有人可以解释一下吗？

Answer 1

来自 JAVA 源 InetAddress.getByName("www.sun.com");和 address.getHostName() 进行查找。

InetAddress.getByName("www.sun.com");方法将在这里进行查找

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() 方法在此处查找

 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;
}