java - InetAddress.getLocalHost() 如何工作？

Question

我正在尝试了解 InetAddress.getLocalHost() 的工作原理。 Javadoc表示它从系统中检索主机名，然后将其解析为 InetAddress。 “解析为 InetAdess”到底是什么意思？它只是要求 DNS 解析主机名吗？

Answer 1

来自 InetAddress.java来源:

 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;
}

回顾一下:

• 两者InetAddress.getAllByName()和InetAddress.getLocalHost()通过调用 getAddressesFromNameService() 解析地址
• JVM 维护自己的主机名缓存 -> IP 地址映射。
• 如果地址不在缓存中( lookupTable 或 addressCache )，它将调用操作系统的 DNS(具体行为可能因 JVM 实现而异)。
• 对于本地主机来说 - getAddressesFromNameService() 中有一个特定的情况