说明：  简单的代理服务器的java程序 开启程序之后 打开浏览器 设置proxy 需与javac Proxy.java 使用的测试端口号一致
(work as proxy in between client and server， after compile the program, opent the brower and set the proxy, the port number should be the same as the one used in javac Proxy.java)