本文描述http client使用socks代理过程中需要注意的几个方面：1，socks5支持用户密码授权；2，支持https；3，支持让代理服务器解析DNS；
使用代理创建Socket
从原理上来看，不管用什么http客户端(httpclient，okhttp)，最终都要转换到java.net.Socket的创建上去，看到代码：
package java.net;
public Socket(Proxy proxy) { ... }这是JDK中对网络请求使用Socks代理的入口方法。(http代理是在http协议层之上的，不在此文讨论范围之内)。
HttpClient要实现socks代理，就需要塞进去一个Proxy对象，也就是定制两个类：org.apache.http.conn.ssl.SSLConnectionSocketFactory 和org.apache.http.conn.socket.PlainConnectionSocketFactory，分别对应https和http。
代码如下：
private class SocksSSLConnectionSocketFactory extends SSLConnectionSocketFactory { public SocksSSLConnectionSocketFactory(SSLContext sslContext, HostnameVerifier hostnameVerifier) { super(sslContext, hostnameVerifier); } @Override public Socket createSocket(HttpContext context) throws IOException { ProxyConfig proxyConfig = (ProxyConfig) context.getAttribute(ProxyConfigKey); if (proxyConfig != null) {//需要代理 return new Socket(proxyConfig.getProxy()); } else { return super.createSocket(context); } } @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); } }和
private class SocksSSLConnectionSocketFactory extends SSLConnectionSocketFactory { public SocksSSLConnectionSocketFactory(SSLContext sslContext, HostnameVerifier hostnameVerifier) { super(sslContext, hostnameVerifier); } @Override public Socket createSocket(HttpContext context) throws IOException { ProxyConfig proxyConfig = (ProxyConfig) context.getAttribute(ProxyConfigKey); if (proxyConfig != null) { return new Socket(proxyConfig.getProxy()); } else { return super.createSocket(context); } } @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 formedas follows:
+----+-----+-------+------+----------+----------+
|VER | CMD | RSV | ATYP | DST.ADDR | DST.PORT |
+----+-----+-------+------+----------+----------+
| 1 | 1 | X'00' | 1 | Variable | 2 | +----+-----+-------+------+----------+----------+ Where: o VER protocol version: X'05' o CMD o CONNECT X'01' o BIND X'02' o UDP ASSOCIATE X'03' o RSV RESERVED o ATYP address type of following address o IP V4 address: X'01' o DOMAINNAME: X'03' o IP V6 address: X'04'代码按上面方法写，协议握手发送的是ATYP=X'03'，即采用域名的地址类型。否则，HttpClient会尝试在客户端解析，然后发送ATYP=X'01'进行协商。当然，大多数时候HttpClient在解析域名的时候就挂了。
https中需要注意的问题
在使用httpclient访问https网站的时候，经常会遇到javax.net.ssl包中的异常，例如：
Caused by: javax.net.ssl.SSLException: Received fatal alert: internal_error at sun.security.ssl.Alerts.getSSLException(Unknown Source) ~[na:1.7.0_80] at sun.security.ssl.Alerts.getSSLException(Unknown Source) ~[na:1.7.0_80]一般需要做几个设置：
创建不校验证书链的SSLContext
SSLContext sslContext =null;
try {
sslContext = new SSLContextBuilder().loadTrustMaterial(null, new TrustStrategy() { @Override public boolean isTrusted(X509Certificate[] chain, String authType) throws CertificateException { return true; } }).build(); } catch (Exception e) { throw new com.aliyun.oss.ClientException(e.getMessage()); } ... new SocksSSLConnectionSocketFactory(sslContext, NoopHostnameVerifier.INSTANCE)创建不校验域名的HostnameVerifier
public class NoopHostnameVerifier implements javax.net.ssl.HostnameVerifier { public static final NoopHostnameVerifier INSTANCE = new NoopHostnameVerifier(); @Override public boolean verify(final String s, final SSLSession sslSession) { return true; } }如何使用用户密码授权？
java SDK中给Socks代理授权有点特殊，不是按socket来的，而是在系统层面做的全局配置。比如，可以通过下面代码设置一个全局的Authenticator：
Authenticator.setDefault(new MyAuthenticator("userName", "Password")); ... class MyAuthenticator extends java.net.Authenticator { private String user ; private String password ; public MyAuthenticator(String user, String password) { this.user = user; this.password = password; } protected PasswordAuthentication getPasswordAuthentication() { return new PasswordAuthentication(user, password.toCharArray()); } }这种方法很简单，不过有些不方便的地方，如果你的产品中需要连接不同的Proxy服务器，而他们的用户名密码是不一样的，那么这个方法就不适用了。
基于ThreadLocal的Authenticator
public class ThreadLocalProxyAuthenticator extends Authenticator{ private ThreadLocal<PasswordAuthentication> credentials = null; private static class SingletonHolder { private static final ThreadLocalProxyAuthenticator instance = new ThreadLocalProxyAuthenticator(); } public static final ThreadLocalProxyAuthenticator getInstance() { return SingletonHolder.instance; } public void setCredentials(String user, String password) { credentials.set(new PasswordAuthentication(user, password.toCharArray())); } public static void clearCredentials() { ThreadLocalProxyAuthenticator authenticator = ThreadLocalProxyAuthenticator.getInstance(); Authenticator.setDefault(authenticator); authenticator.credentials.set(null); } public PasswordAuthentication getPasswordAuthentication() { return credentials.get(); } } 这个类意味着，授权信息只会保存到当前调用者的线程中，其他线程的调用者无法访问，在创建Socket的线程中设置密钥和清理密钥，就可以做到授权按照Socket连接进行隔离。Java TheadLocal相关知识本文不赘述。
按连接隔离的授权
class ProxyHttpClient extends CloseableHttpClient{ private CloseableHttpClient httpClient; public ProxyHttpClient(CloseableHttpClient httpClient){ this.httpClient=httpClient; } protected CloseableHttpResponse doExecute(HttpHost target, HttpRequest request, HttpContext context) throws IOException, ClientProtocolException { ProxyConfig proxyConfig = //这里获取当前连接的代理配置信息 boolean clearCredentials = false; if (proxyConfig != null) { if (context == null) { context = HttpClientContext.create(); } context.setAttribute(ProxyConfigKey, proxyConfig); if (proxyConfig.getAuthentication() != null) { ThreadLocalProxyAuthenticator.setCredentials(proxyConfig.getAuthentication());//设置授权信息 clearCredentials = true; } } try { return httpClient.execute(target, request, context); } finally { if (clearCredentials) {//清理授权信息 ThreadLocalProxyAuthenticator.clearCredentials(); } } } }另外，线程是可以复用的，因为每次调用完毕后，都清理了授权信息。
这里有个一POJO类ProxyConfig，保存的是socks代理的IP端口和用户密码信息。
public class ProxyConfig {
private Proxy proxy; private PasswordAuthentication authentication; }
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