74.【JavaWeb -02】（一）

1.会话定义:
会话打开一个浏览器，点击了很多超链接，访问多个web资源,关闭浏览器这个过程
可以称之为会话。
2.有状态会话:
服务端Session先给小朋友A发了一个邀请函Cookie，这就证明小朋友A可以来参加
Session聚会,并且有留存登入聚会的信息，曾经来过，就叫做有状态会话。

1.cookie:
(1).客户端技术:(响应，请求)
2.session:
(1).服务器技术: 利用这个技术可以保存会话的用户信息，我们可以把信息或则数据
放在session里面
3.常见的应用:
网站登入之后，下次就不用登入了，第二次直接登入
4.一个网站cookie是否存在上限?
(1).一个cookie只能保存一个信息
(2).一个web站点可以给浏览器发送多个cookie,最多存放20个cookie;
(3).300个cookie浏览器上限
(4).Cookie大小有限制4kb
5.删除cookie:
(1).不设置有效期，关闭浏览器，自动失效
(2).设置有效期为0

1.请求获得cookie：
Cookie[] cookies = req.getCookies();
2.获得cookie的键key
cookie.getName()
3.获得cookie的值value
cookie.getValue();
4.将字符串参数解析为带符号的十进制 long 
parseLong(String s) 
5.创建一个cookie key 和 value
Cookie cookie = new Cookie("lastLoginTime", System.currentTimeMillis()+""); 
6.cookie的生存期
cookie.setMaxAge(24*60*60);
7.响应给客户端cookie
resp.addCookie(cookie);

package com.Jsxs.Cookie;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;
//保存用户上一次访问的时间
public class Demo1 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
//        服务器告诉你来的时间，把这个时间封装成为一个信件，你带来信件，我就知道是你来了
        resp.setCharacterEncoding("utf-16");
        req.setCharacterEncoding("utf-16");
        PrintWriter out = resp.getWriter();
//        cookie,是服务端从客户端获取，客户端需要请求服务端先得到cookie
        Cookie[] cookies = req.getCookies();     //这里返回一个数组，说明存在多个。
//        判断cookie数组是否存在
        if (cookies != null){
//            如果存在了怎么办?
            out.write("您上一次访问的时间是:");
            for (int i = 0; i < cookies.length; i++) {
                Cookie cookie = cookies[i];
                if (cookie.getName().equals("lastLoginTime")){
//                    获取cookie中的值
                    long l = Long.parseLong(cookie.getValue());
                    Date date = new Date(l);
                    out.write(date.toLocaleString());
                }
            }
        }else {
            out.write("您第一次访问本次网站，我们将会对您的信息进行详细的保存");
        }
//        服务器相应一个cookie
        Cookie cookie = new Cookie("lastLoginTime", System.currentTimeMillis()+"");
        cookie.setMaxAge(24*60*60);
        resp.addCookie(cookie);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

再创建一个DEMO2,并且创建一个cookie的key，一定要和demo1的key相等，进行替换的操作。
• 1

package com.Jsxs.Cookie;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
public class Demo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Cookie cookie = new Cookie("lastLoginTime", System.currentTimeMillis()+"");
        cookie.setMaxAge(0);
        resp.addCookie(cookie);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

1.编码:
String string=URLEncoder.encode("字符串","utf-8")
2.解码:
String string=URLDecoder.decode("字符串","UTF-8")
3.JSP文件上写:
<%@ page contentType="text/html;charset=UTF-8" language="java" %>

package com.Jsxs.Cookie;
import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;
import java.net.URLDecoder;
import java.net.URLEncoder;
public class Demo3 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setCharacterEncoding("utf-16");
        req.setCharacterEncoding("utf-16");
        PrintWriter out = resp.getWriter();
        Cookie[] cookies = req.getCookies();
        if(cookies!=null){
            out.write("您带了小饼干，我很欢迎");
            for (int i = 0; i < cookies.length; i++) {
                Cookie cookie = cookies[i];
                if(cookie.getName().equals("name")){
                    System.out.println(URLDecoder.decode(cookie.getValue(),"UTF-8"));
                }
            }
        }else {
            out.write("第一次登入还没创建到cookie");
        }
        Cookie cookie = new Cookie("name", URLEncoder.encode("吉士先生","UTF-8"));
        resp.addCookie(cookie);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}