1 我们知道HTTP协议是无状态协议，也就是说每个请求都是独立的！无法记录前一次请求的状态。但HTTP协议中可以使用Cookie来完成会话跟踪！

在JavaWeb中，使用session来完成会话跟踪，session底层依赖Cookie技术。

2 Cookie

Cookie是由服务器创建，然后通过响应发送给客户端的一个键值对。客户端会保存Cookie，并会标注出Cookie的来源（哪个服务器的Cookie）。当客户端向服务器发出请求时会把所有这个服务器Cookie包含在请求中发送给服务器，这样服务器就可以识别客户端了！

3 cookie例子1

客户端访问AServlet，AServlet在响应中添加Cookie，浏览器会自动保存Cookie。然后客户端访问BServlet，这时浏览器会自动在请求中带上Cookie，BServlet获取请求中的Cookie打印出来。

package com.cug.cookie;

import java.io.IOException;
import java.util.UUID;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class AServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		resp.setContentType("text/html;charset=utf-8");
		
		String id = UUID.randomUUID().toString();
		Cookie ck1 = new Cookie("id",id);
		resp.addCookie(ck1);
		resp.getWriter().print("cookie 已经设置好！");
	}
}

package com.cug.cookie;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class BServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		resp.setContentType("text/html;charset=utf-8");
		
		Cookie[] cookies = req.getCookies();
		for(Cookie c: cookies){
			if(c.getName().equals("id")){
				resp.getWriter().print(c.getValue());
			}
		}
	}
}

package com.cug.cookie;

import java.io.IOException;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		resp.setContentType("text/html;charset=utf-8");
		Cookie ck = new Cookie("lasttime",(new Date()).toString());
		resp.addCookie(ck);
		
		String s = "您是首次访问本站吧！";
		Cookie[] cookies = req.getCookies();
		if(cookies != null){
			for(Cookie c: cookies){
				if(c.getName().equals("lasttime")){
					s = "您上次访问的时间为："+c.getValue();
				}
			}
		}
		resp.getWriter().print(s);
	}
}

package com.cug.cookie;

import java.io.IOException;
import java.net.URLDecoder;
import java.net.URLEncoder;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DServlet extends HttpServlet{
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		resp.setContentType("text/html;charset=utf-8");
		String name = URLEncoder.encode("朱","utf-8");
		String value = http://www.mamicode.com/URLEncoder.encode("zhu","utf-8");>

Java web----Cookie