服务器客户模式的文件下载，从客户端把文件地址传给服务器，服务器也文件传回给客户端。这个主要是实现的在一台机子上的演示。可以根据需要修改-Download the file server client mode, the file address　the client to the server, the server file back to the client. This is mainly achieved demonstration on a loom. You can modify it as needed