[2.2.1客户端请求](2.2.1%E5%AE%A2%E6%88%B7%E7%AB%AF%E8%AF%B7%E6%B1%82.md) [2.2.2服务器响应](2.2.2%E6%9C%8D%E5%8A%A1%E5%99%A8%E5%93%8D%E5%BA%94.md) [2.2.3Servlet HTTP 状态码](2.2.3ServletHTTP%E7%8A%B6%E6%80%81%E7%A0%81.md) [2.2.4图片验证码类](2.2.4%E5%9B%BE%E7%89%87%E9%AA%8C%E8%AF%81%E7%A0%81%E7%B1%BB.md) [2.2.5注册模拟实现（带验证码）](2.2.5%E5%86%8C%E6%A8%A1%E6%8B%9F%E5%AE%9E%E7%8E%B0%EF%BC%88%E5%B8%A6%E9%AA%8C%E8%AF%81%E7%A0%81%EF%BC%89.md)