InetAddress之域名解析
1. getByName
public static InetAddress getByName(String host) throws UnknownHostException { return InetAddress.getAllByName(host)[0]; } public static InetAddress[] getAllByName(String host) throws UnknownHostException { return getAllByName(host, null); } private static InetAddress[] getAllByName(String host, InetAddress reqAddr) throws UnknownHostException { // 1. host == null || host.length() == 0 // loopbackAddress（与系统属性java.net.preferIPv6Addresses相关）：Inet4Address(localhost/127.0.0.1) || Inet6Address(localhost/::1) // 2. host表示一个IP地址 // Inet4Address(null/IPAddressUtil.textToNumericFormatV4(host)) || Inet6Address(null/IPAddressUtil.textToNumericFormatV6(host)) ... ... return getAllByName0(host, reqAddr, true); } private static InetAddress[] getAllByName0 (String host, InetAddress reqAddr, boolean check) throws UnknownHostException { ... ... InetAddress[] addresses = getCachedAddresses(host); // 在缓存中查找记录 if (addresses == null) { addresses = getAddressesFromNameService(host, reqAddr); // 请求域名解析 } if (addresses == unknown_array) // 缓存中存在失败记录 || 域名解析失败 throw new UnknownHostException(host); return addresses.clone(); }
1）InetAddress.Cache
static final class CacheEntry { CacheEntry(InetAddress[] addresses, long expiration) { this.addresses = addresses; this.expiration = expiration; } InetAddress[] addresses; // 地址 long expiration; // 过期时间 } static final class Cache { private LinkedHashMap<String, CacheEntry> cache; private Type type; enum Type {Positive, Negative}; public Cache(Type type) { this.type = type; cache = new LinkedHashMap<String, CacheEntry>(); } private int getPolicy() { // 缓存过期时间 if (type == Type.Positive) { return InetAddressCachePolicy.get(); // -Dsun.net.inetaddr.ttl=-1（启动参数） || networkaddress.cache.ttl=-1（配置文件） } else { return InetAddressCachePolicy.getNegative(); // -Dsun.net.inetaddr.negative.ttl=10（启动参数） || networkaddress.cache.negative.ttl=10（配置文件） } } public Cache put(String host, InetAddress[] addresses) { int policy = getPolicy(); if (policy == InetAddressCachePolicy.NEVER) { // 0：不缓存 return this; } if (policy != InetAddressCachePolicy.FOREVER) { // 非永久缓存：需要清除过期记录 LinkedList<String> expired = new LinkedList<>(); long now = System.currentTimeMillis(); for (String key : cache.keySet()) { CacheEntry entry = cache.get(key); if (entry.expiration >= 0 && entry.expiration < now) { // 当前记录已过期 expired.add(key); } else { // 当前记录未过期 break; // 后续记录的插入时间肯定比当期记录晚 } } for (String key : expired) { // 清除过期记录 cache.remove(key); } } long expiration; if (policy == InetAddressCachePolicy.FOREVER) { // -1：永久缓存 expiration = -1; } else { // 非永久缓存 expiration = System.currentTimeMillis() + (policy * 1000); // 过期时间：policy秒 } CacheEntry entry = new CacheEntry(addresses, expiration); cache.put(host, entry); // 添加记录 return this; } public CacheEntry get(String host) { int policy = getPolicy(); if (policy == InetAddressCachePolicy.NEVER) { // 一定没有缓存记录 return null; } CacheEntry entry = cache.get(host); // 获取记录 if (entry != null && policy != InetAddressCachePolicy.FOREVER) { // 非永久缓存 if (entry.expiration >= 0 && entry.expiration < System.currentTimeMillis()) { // 检查是否过期 cache.remove(host); entry = null; } } return entry; } }
2）getCachedAddresses
private static boolean addressCacheInit = false; // 缓存未初始化 static InetAddress[] unknown_array; // 域名解析失败时使用的地址：任意地址{0.0.0.0, 0.0.0.0} || {::, ::} private static Cache addressCache = new Cache(Cache.Type.Positive); // 正面缓存 private static Cache negativeCache = new Cache(Cache.Type.Negative); // 负面缓存 private static final HashMap<String, Void> lookupTable = new HashMap<>(); private static InetAddress[] getCachedAddresses(String hostname) { hostname = hostname.toLowerCase(); synchronized (addressCache) { cacheInitIfNeeded(); CacheEntry entry = addressCache.get(hostname); // 在正面缓存中查找记录 if (entry == null) { entry = negativeCache.get(hostname); // 在负面缓存中查找记录 } if (entry != null) { return entry.addresses; // return 缓存中记录的地址 } } return null; // 在缓存中未找到记录 } private static void cacheInitIfNeeded() { assert Thread.holdsLock(addressCache); if (addressCacheInit) { return; } unknown_array = new InetAddress[1]; // 大小为1 unknown_array[0] = impl.anyLocalAddress(); // Inet4Address(0.0.0.0/0.0.0.0) || Inet6Address(::/::) addressCache.put(impl.anyLocalAddress().getHostName(), unknown_array); addressCacheInit = true; }
3）getAddressesFromNameService
private static InetAddress[] getAddressesFromNameService(String host, InetAddress reqAddr) throws UnknownHostException { ... ... if ((addresses = checkLookupTable(host)) == null) { // 是否正在解析该host ？ 等待 try { for (NameService nameService : nameServices) { try { addresses = nameService.lookupAllHostAddr(host); // 向该域名服务请求域名解析 success = true; break; // 只要有一个域名服务成功解析host则退出循环 } catch (UnknownHostException uhe) { // 1. 若host = localhost，则addresses = [loopbackAdress] & 退出循环 // 2. 否则准备抛异常（addresses = unknown_array & success = false & 记录uhe） & 继续循环 ... ... } } // 若reqAddr不为空 && addresses中存在与reqAddr相等的InetAddress，则将该InetAddress挪到第一个位置 ... ... cacheAddresses(host, addresses, success); // 缓存记录 // 域名解析失败则抛异常uhe ... ... } finally { updateLookupTable(host); // 唤醒所有等待解析该host的线程 } } return addresses; } private static InetAddress[] checkLookupTable(String host) { synchronized (lookupTable) { if (lookupTable.containsKey(host) == false) { // 其它线程是否也在请求解析该host lookupTable.put(host, null); // 告知之后请求解析该host的线程：正在进行域名解析 return null; // return & 进行域名解析 } while (lookupTable.containsKey(host)) { // 域名解析尚未完成 try { lookupTable.wait(); // 等待 } catch (InterruptedException e) { } } } InetAddress[] addresses = getCachedAddresses(host); // 在缓存中查找记录 if (addresses == null) { // 在缓存中未找到记录（之前线程对host进行域名解析失败：addresses == unknown_array） synchronized (lookupTable) { lookupTable.put(host, null); return null; // return & 继续进行域名解析 } } return addresses; // 在缓存中找到记录 } private static void cacheAddresses(String hostname, InetAddress[] addresses, boolean success) { hostname = hostname.toLowerCase(); synchronized (addressCache) { cacheInitIfNeeded(); if (success) { // 域名解析成功 addressCache.put(hostname, addresses); // 在正面缓存中记录 } else { // 域名解析失败 negativeCache.put(hostname, addresses); // 在负面缓存中记录 } } } private static void updateLookupTable(String host) { synchronized (lookupTable) { lookupTable.remove(host); // 告知所有等待解析该host的线程：域名解析完毕 lookupTable.notifyAll(); // 唤醒所有等待解析该host的线程 } }
2. getByAddress
getByName会立刻请求解析域名为IP地址，而getByAddress不会立刻请求解析IP地址为域名，获取域名在调用getHostName时执行。
public static InetAddress getByAddress(byte[] addr) throws UnknownHostException { return getByAddress(null, addr); } public static InetAddress getByAddress(String host, byte[] addr) throws UnknownHostException { // host截子串：[host] -> host ... ... if (addr != null) { if (addr.length == Inet4Address.INADDRSZ) { // 4字节：IPv4地址 return new Inet4Address(host, addr); } else if (addr.length == Inet6Address.INADDRSZ) { // 16字节：IPv6地址 byte[] newAddr = IPAddressUtil.convertFromIPv4MappedAddress(addr); // 转IPv4地址 if (newAddr != null) { // 转换成功 return new Inet4Address(host, newAddr); } else { // 转换失败 return new Inet6Address(host, addr); } } } throw new UnknownHostException("addr is of illegal length"); }
getHostName
public String getHostName() { return getHostName(true); } String getHostName(boolean check) { if (holder().getHostName() == null) { holder().hostName = InetAddress.getHostFromNameService(this, check); // 根据IP地址请求域名 } return holder().getHostName(); } private static String getHostFromNameService(InetAddress addr, boolean check) { String host = null; for (NameService nameService : nameServices) { try { host = nameService.getHostByAddr(addr.getAddress()); // 根据IP地址向该域名服务请求域名 ... ... // prevent spoofing？？？ InetAddress[] arr = InetAddress.getAllByName0(host, check); // 反向由域名获取InetAddress：可能将进行域名解析 boolean ok = false; if(arr != null) { for(int i = 0; !ok && i < arr.length; i++) {// 在arr中查找与addr（IP地址）相等的InetAddress ok = addr.equals(arr[i]); // InetAddress.equals：IP地址相等 } } if (!ok) { // arr == null || 在arr中未找到与addr（IP地址）相等的InetAddress host = addr.getHostAddress(); // IP地址字符串 return host; } break; } catch (SecurityException e) { host = addr.getHostAddress(); // IP地址字符串 break; } catch (UnknownHostException e) { host = addr.getHostAddress(); // IP地址字符串 } } return host; }
