3.Cookie

1. 从请求中拿到cookie信息

2. 服务器响应给客户端cookie

   Cookie[] cookies = req.getCookies();//获取cookie
   cookie.getName();//获取cookie中的key
   cookie.getValue();//获取cookie中的value
   Cookie cookie = new Cookie("lastLoginTime", "" + System.currentTimeMillis());//新建一个cookie
   cookie.setMaxAge(24*60*60);//设置cookie的有效期
   resp.addCookie(cookie);//响应给客户端一个cookie
   

   cookie：一般会保存在本地的用户目录下/AppData下

一个网站cookie是否存在上限？

• 一个cookie只能保存一个信息
• 一个web站点可以给浏览器发送多个cookie，每个web站点最多存放20个cookie（ 不同的浏览器会有所不同）
• cookie大小有限制4kb
• 浏览器上限是300个cookie

删除cookie

• 不设置有效期，关闭浏览器，自动失效
• 设置有效期时间为0

演示代码

public class CookieDemo01 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //解决中文乱码
        req.setCharacterEncoding("utf-8");
        resp.setCharacterEncoding("utf-8");

        PrintWriter writer = resp.getWriter();

        //cookie，服务器从客户端中获取
        Cookie[] cookies = req.getCookies();//这里返回的是数组，说明Cookie可能存在多个
        if (cookies == null) {
            writer.print("这是您第一次访问本站！");
        } else {
            writer.print("您上一次访问本站的时间是：" );
            for (int i = 0; i < cookies.length; i++) {
                Cookie cookie = cookies[i];
                if ("lastLoginTime".equals(cookie.getName())) {
                    long lastLoginTime = Long.parseLong(cookie.getValue());
                    Date date = new Date(lastLoginTime);
                    SimpleDateFormat sd = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
                    writer.print(sd.format(date));
                }
            }
        }
        //添加 or 更新
        Cookie cookie = new Cookie("lastLoginTime", "" + System.currentTimeMillis());
        //设置cookie为1天
        cookie.setMaxAge(24*60*60);
        resp.addCookie(cookie);
        writer.flush();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

演示二

public class CookieDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //解决中文乱码
        req.setCharacterEncoding("UTF-8");
        resp.setCharacterEncoding("UTF-8");

        PrintWriter writer = resp.getWriter();

        //cookie，服务器从客户端中获取
        Cookie[] cookies = req.getCookies();
        if (cookies == null) {
            writer.print("这是您第一次访问本站！");
        } else {
            for (int i = 0; i < cookies.length; i++) {
                Cookie cookie = cookies[i];
                if ("name".equals(cookie.getName())) {
                    //防止网络传输中文乱码问题
                    writer.print(URLDecoder.decode(cookie.getValue(), "UTF-8"));
                }
            }
        }
        //添加 or 更新
        Cookie cookie = new Cookie("name", URLEncoder.encode("狂神", "UTF-8"));
        resp.addCookie(cookie);
        writer.flush();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}