tcp 或 udp 实现文件的上传和下载，只是一个简单的例子-tcp or udp for file upload and download, just a simple example

java SE版的简单聊天室，实现了群聊，私聊，上传下载，查看聊天记录，修改密码等功能。(Java SE version of a simple chat room, has realized the group chat, private chat and upload download, view the chat logs, change passwords, and other functions.)