(六)web四君子：Request&Response以及Cookie&Session

.

Request：客户端请求
Response:服务器响应
Cookie:客户端会话技术
Session: 服务器端会话技术

1. Request与Response

基本概念
1）Request和Reponse对象是由服务器创建的，我们来使用
2）Request对象是来获取请求消息；Reponse对象是来设置响应消息
Request知识点
Reponse知识点

2.Cookie与Session

会话技术
Cookie与Session都属于会话技术。利用该技术可以做免登陆等功能。

如下图是cookie的原理示例图：

如下图是Session的原理示例图（Session依赖于Cookie，将Session对象的id存入在Cookie）：

CookIe知识点

//2.1设置cookie
@WebServlet("/CookieDemo04")
public class CookieDemo04 extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.创建Cookie对象
        Cookie c1 = new Cookie("msg","setMaxAge");
        //2.设置Cookie的存活时间
        c1.setMaxAge(30);       //将cookie持久化到硬盘，30秒后会自动删除cookie文件
        //3.发送Cookie
        response.addCookie(c1);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doPost(request, response);
    }
}

//2.2获取cookie
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //3.获取Cookie
        Cookie[] cs = request.getCookies();
        if(cs != null){
            for(Cookie c:cs){
                String name = c.getName();
                String value = c.getValue();
                System.out.println(name + "; " + value);
            }
        }
    }

Session知识点
1）Session用于存储一次会话的多次请求的数据，数据存储在服务器端；
2）可以存储任意类型，任意大小的数据；

//3.1Session存储数据
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1.获取session
        HttpSession session = request.getSession();
        //2.存储数据
        session.setAttribute("msg","hello session");
}

//3.2Session获取数据
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //1.获取session
  HttpSession session = request.getSession();
  //2.获取数据
  Object msg = session.getAttribute("msg");
  System.out.println(msg); 
}