DNS查找在哪一行?
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();
}
}
}
我可以理解它应该在第7行。但在一组教程中,我发现据说它在第8行。但是没有任何解释。如果是第8行,而不是第7行,有人可以解释一下。
答案 0 :(得分:1)
来自JAVA源的InetAddress.getByName(" www.sun.com");和address.getHostName()进行查找。
<强> InetAddress.getByName(&#34; www.sun.com&#34);方法会在这里查找
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;
}