@Override
        public Socket connectSocket(int connectTimeout, Socket socket, HttpHost host, InetSocketAddress remoteAddress,
                                    InetSocketAddress localAddress, HttpContext context) throws IOException {
            ProxyConfig proxyConfig = (ProxyConfig) context.getAttribute(ProxyConfigKey);
            if (proxyConfig != null) {//make proxy server to resolve host in http url
                remoteAddress = InetSocketAddress
                        .createUnresolved(host.getHostName(), host.getPort());
            }
            return super.connectSocket(connectTimeout, socket, host, remoteAddress, localAddress, context);
        }
    }

然后在创建httpclient对象时，给HttpClientConnectionManager设置socketFactoryRegistry
            Registry<ConnectionSocketFactory> socketFactoryRegistry = RegistryBuilder.<ConnectionSocketFactory>create()
                .register(Protocol.HTTP.toString(), new SocksConnectionSocketFactory())
                .register(Protocol.HTTPS.toString(), new SocksSSLConnectionSocketFactory(sslContext, NoopHostnameVerifier.INSTANCE))
                .build();

        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager(socketFactoryRegistry);
   

让代理服务器解析域名

场景：运行httpClient的进程所在主机可能并不能上公网，大部分时候，也无法进行DNS解析，这时通常会出现域名无法解析的IO异常，下面介绍怎么避免在客户端解析域名。

上面有一行代码非常关键：
remoteAddress = InetSocketAddress
                        .createUnresolved(host.getHostName(), host.getPort());

变量host是你发起http请求的目标主机和端口信息，这里创建了一个未解析(Unresolved)的SocketAddress，在socks协议握手阶段，InetSocketAddress信息会原封不动的发送到代理服务器，由代理服务器解析出具体的IP地址。
Socks的协议描述中有个片段：
  The SOCKS request is formed as follows:

        +----+-----+-------+------+----------+----------+
        |VER | CMD |  RSV  | ATYP | DST.ADDR | DST.PORT |
        +----+-----+-------+------+----------+----------+
        | 1  |  1  | X'00' |  1  | Variable |    2    |
        +----+-----+-------+------+----------+----------+

    Where: