JavaEE：Cookie和Session

Cookie是客户端技术，程序把每个用户的数据以cookie的形式写给用户各自的浏览器。当用户使用浏览器再去访问服务器中的web资源时，就会带着各自的数据去。
这样web资源处理的就是用户各自的数据了。
Session
Session是服务器端技术，利用这个技术，服务器在运行时可以为每一个用户的浏览器创建一个其独享的session对象，由于session为用户浏览器独享，所以用户在访问服务器
的web资源时，可以把各自的数据放在各自的session中，当用户再去访问服务器中的其他web资源时，其他web资源再从用户各自的session中取出数据为用户服务。

什么是会话？用户打开浏览器，访问站点，连续进行多次操作，关闭浏览器，整个过程称为会话。
管理HTTP协议会话状态：Cookie和Session
Cookie：将用户相关数据，保存客户端，用户每次访问服务器自动携带cookie数据。
Session：将用户相关数据保存服务器端，为每个客户端生成一个独立Session数据对象，通过对象唯一编号，区分哪个浏览器对应哪个Session

Cookie

Cookie快速入门案例：（上次访问时间）
eg：

package cn.lsl.cookie;

import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
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 LastVisitServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        Cookie[] cookies = request.getCookies();
        response.setContentType("text/html;charset=utf-8");
        if(cookies == null){
            //当前时间毫秒等价new Date().getTime()
            long now = System.currentTimeMillis();
            //向客户端写出cookie需要用到cookie
            Cookie cookie = new Cookie("last", now+"");
            response.addCookie(cookie);
            response.getWriter().println("欢迎第一次访问本网站");
        }else{
            for (Cookie cookie : cookies) {
                if(cookie.getName().equals("last")){
                    long lasttime = Long.parseLong(cookie.getValue());
                    //显示给用户
                    Date date = new Date(lasttime);
                    
                    //格式化日期
                    DateFormat dateFormat = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
                    response.getWriter().println("上次访问时间：" + dateFormat.format(date));
                }
            }
            long now = System.currentTimeMillis();
            Cookie cookie = new Cookie("last", now+"");
            response.addCookie(cookie);
        }
    }

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

}

分析：
1、通过服务器向客户端写cookie
Cookie cookie = new Cookie(name,value);
response.addCookie(cookie);
* 在HTTP协议响应头信息中 Set-Cookie: last=1339556457609

2、当客户端存在cookie之后，以后每次请求自动携带 HTTP协议请求头信息 Cookie: last=1339556456859
服务器端获得需要cookie数据
Cookie[] cookies = request.getCookies(); ---- 获得客户端所有cookie
if(cookies==null){}  判断cookie是否存在

遍历cookie获得需要信息
for (Cookie cookie : cookies) {
    // 获得每个cookie
    if (cookie.getName().equals("last")) {
        // 找到了需要cookie                 
    }
}

CookieAPI详解
1.将cookie写回客户端 response.addCookie(cookie);
2.读取请求中cookie信息 request.getCookies
3.Cookie对象创建 new Cookie(name,value)
*cookie的name不允许改变

什么是会话cookie，什么是持久cookie？
cookie信息默认情况保存在浏览器内存中 ---- 会话cookie
会话cookie会在关闭浏览器时清除

持久Cookie，cookie数据保存客户端硬盘上
通过setMaxAge设置Cookie为持久Cookie
* 在JavaAPI 中所有与时间相关参数，int 类型 单位秒， long类型 单位毫秒
eg：

package cn.lsl.cookie;

import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
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 LastVisitServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setHeader("Cache-Control", "no-cache");
        response.setDateHeader("Expires", -1);
        response.setHeader("Pragma", "no-cache");
        
        Cookie[] cookies = request.getCookies();
        response.setContentType("text/html;charset=utf-8");
        if(cookies == null){
            //当前时间毫秒等价new Date().getTime()
            long now = System.currentTimeMillis();
            //向客户端写出cookie需要用到cookie
            Cookie cookie = new Cookie("last", now+"");
            cookie.setPath("/Cookie");
            //设置cookie有效时间
            cookie.setMaxAge(60*60*24*2);
            response.addCookie(cookie);
            response.getWriter().println("欢迎第一次访问本网站");
        }else{
            for (Cookie cookie : cookies) {
                if(cookie.getName().equals("last")){
                    long lasttime = Long.parseLong(cookie.getValue());
                    //显示给用户
                    Date date = new Date(lasttime);
                    
                    //格式化日期
                    DateFormat dateFormat = new SimpleDateFormat("yyyy年MM月dd日 HH时mm分ss秒");
                    response.getWriter().println("上次访问时间：" + dateFormat.format(date));
                }
            }
            long now = System.currentTimeMillis();
            Cookie cookie = new Cookie("last", now+"");
            cookie.setPath("/Cookie");
            //设置cookie有效时间
            cookie.setMaxAge(60*60*24*2);
            response.addCookie(cookie);
        }
    }

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

}

访问cookie有效路径path
默认情况下，生成cookie时，产生默认有效访问路径 （默认生成cookie程序路径）
http://localhost/Cookie/lastvisit  --- 生成cookie  --- path 默认值： /Cookie
http://localhost/Cookie/servlet/path ---- 生成cookie ---- path 默认值：/Cookie/servlet

第二次访问程序携带cookie信息，如果访问路径与path不一致，不会携带cookie 信息
company cookie : path --- /Cookie/serlvet
last cookie : path --- /Cookie

访问 ：http://localhost/Cookie/servlet/path ---- 同时满足/Cookie/serlvet、/Cookie 携带 company 和 last两个cookie信息
访问 ：http://localhost/Cookie/lastvisit  ---- 满足 /Cookie 不满足/Cookie/serlvet 携带 last 一个cookie 信息
* 以后程序开发，尽量设置path ---- setPath方法

第一方cookie 和 第三方cookie
通过setDomain 设置cookie 有效域名
访问google时，生成cookie过程中 cookie.setDomain(".baidu.com") ---- 生成百度cookie ------ 第三方cookie
* 第三方cookie 属于不安全  ----- 一般浏览器不接受第三方cookie

访问google时，生成cookie，cookie.setDomain(.google.con) ---- 生成google的cookie ------ 第一方cookie


删除Cookie
案例：删除上次访问时间
原理：设置cookie MaxAge为 0

*删除cookie必须设置path与cookie的path一致

eg:

package cn.lsl.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 DeleteCookieServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        Cookie cookie = new Cookie("last", "");
        cookie.setMaxAge(0);
        //path一致
        cookie.setPath("/Cookie");
        response.addCookie(cookie);
    }

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

}

历史浏览记录案例

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@page import="cn.lsl.cookie.CookieUtils"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>product.jsp</title>
  </head>
  
  <body>
    <h1>商品列表</h1>
    <a href="/Cookie/producthistory?id=1">冰箱</a><br/>
    <a href="/Cookie/producthistory?id=2">洗衣机</a><br/>
    <a href="/Cookie/producthistory?id=3">笔记本电脑</a><br/>
    <a href="/Cookie/producthistory?id=4">手机</a><br/>
    <a href="/Cookie/producthistory?id=5">空调</a><br/>
    <a href="/Cookie/producthistory?id=6">热水器</a><br/>
    <a href="/Cookie/producthistory?id=7">微波炉</a><br/>
    <a href="/Cookie/producthistory?id=8">电风扇</a><br/>
    
    <h1>商品浏览记录</h1><h3><a href="/Cookie/cleanhistory">清空浏览记录</a></h3>
    
    <%
        Cookie cookie = CookieUtils.findCookie(request.getCookies(),"history");
        if(cookie == null){
            out.println("无浏览记录！");
        }else{
            String ids = cookie.getValue();
            String[] idArray = ids.split(",");
            String[] arr = { "冰箱", "洗衣机", "笔记本电脑", "手机", "空调", "热水器", "微波炉", "电风扇" };
            for(String id : idArray){
                out.println(arr[Integer.parseInt(id)-1]+"<br/>");
            }
        }
     %>
  </body>
</html>

package cn.lsl.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 ProductHistoryServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        String[] arr = { "冰箱", "洗衣机", "笔记本电脑", "手机", "空调", "热水器", "微波炉", "电风扇" };
        
        //获得用户点击查看商品编号
        String id = request.getParameter("id");
        
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().println("用户正在浏览商品：" + arr[Integer.parseInt(id) - 1]);
        response.getWriter().println("<a href='/Cookie/product.jsp'>返回</a>");
        
        Cookie cookie = CookieUtils.findCookie(request.getCookies(), "history");
        if(cookie == null){
            cookie = new Cookie("history", id);
            cookie.setMaxAge(60 * 60);
            cookie.setPath("/Cookie");
            response.addCookie(cookie);
        }else{
            //已存在商品浏览记录，判断当前浏览商品是否已经在记录中
            String ids = cookie.getValue();
            String[] idArray = ids.split(",");
            for(String existId : idArray){
                if(existId.equals(id)){
                    //当前浏览商品已经在浏览器记录中
                    return;
                }
            }
            
            //当前id没有在浏览器记录中
            cookie.setValue(ids + "," + id);
            cookie.setMaxAge(60 * 60);
            cookie.setPath("/Cookie");
            response.addCookie(cookie);
        }
    }

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

}

package cn.lsl.cookie;

import javax.servlet.http.Cookie;

public class CookieUtils {
    /**
     * 在Cookie数组中查找指定name cookie
     */
    public static Cookie findCookie(Cookie[] cookies, String name){
        if(cookies == null){
            return null;
        }else{
            for (Cookie cookie : cookies) {
                if(cookie.getName().equals(name)){
                    return cookie;
                }
            }
            return null;
        }
    }
}

package cn.lsl.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 CleanHistoryServlet extends HttpServlet {

    
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        Cookie cookie = new Cookie("history","");
        cookie.setMaxAge(0);
        cookie.setPath("/Cookie");
        response.addCookie(cookie);
        //跳转到商品列表页面：重定向
        response.sendRedirect("/Cookie/product.jsp");
    }

    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
    }

}

Session

1.Session是服务器端会话管理技术，服务器会为每个浏览器创建一个单独Session对象，保存该浏览器（会话）操作相关信息。
与Cookie区别：cookie是保存在客户端，Session保存在服务器端

2.为什么有Cokie技术？还需要Session？
Cookie存在客户端---- 存在安全问题
Session将数据存在服务器，数据更加安全
*Session将数据保存在服务器端，占用服务器内存资源，Cookie不会占用服务器资源
例如：京东购物车信息保存Cookie，淘宝购物车信息保存Session

3.Session共享

实验：用IE6 向Session保存一个数据，用另一个IE6读取数据？
用第一个IE6保存session数据，当前浏览器可以获得，但是第二个IE6 无法获得第一个浏览器保存Session 数据
IE8以上或火狐浏览器当打开多个浏览器窗口，之间Session共享
原因：IE6的cookie中保存jsessionId默认是会话级别；IE8或者火狐的cookie中保存jsessionId默认是持久cookie

三个问题：
问题一：如何让多个IE6 共享同一个Session  ---- 将session id 持久化
问题二：如果客户端关闭浏览器，是否就删除了Session ？
没有，Session保存在服务器端
问题三：IE6 保存Session，关闭浏览器，再次打开，数据丢失了？
IE6默认jsessionId保存会话Cookie中，关闭浏览器，会话cookie就会被删除，客户端丢失jsessionid 无法找到服务器对应Session对象
* 服务器Session对象还存在

案例一：实现多个浏览器的session共享和关闭浏览器再次打开数据还存在

package cn.lsl.session;

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;
import javax.servlet.http.HttpSession;

public class WriteSessionServlet extends HttpServlet {

    
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //获得Session对象
        HttpSession httpSession = request.getSession();
        //向session保存一个数据
        httpSession.setAttribute("company", "软件公司");
        
        //将写回给浏览器JSESSIONID持久化
        Cookie cookie = new Cookie("JSESSIONID", httpSession.getId());
        cookie.setMaxAge(60 * 60);
        cookie.setPath("/SessionTest");
        response.addCookie(cookie);
        
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().println("已向session保存一个数据！");
    }

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

}

package cn.lsl.session;

import java.io.IOException;

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

public class ReadSessionServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        HttpSession httpSession = request.getSession();
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().println("读取之前保存session信息：" + httpSession.getAttribute("company"));
    }

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

}

Session购物车案例
1、商品列表、用户点击列表中商品、将商品添加购物车
2、查看购物车中商品，以及商品购买件数

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>product.jsp</title>
  </head>
  
  <body>
        <h1>商品列表</h1>
        冰箱  <a href="/SessionTest/buy?id=1">购买</a> <br/> 
        洗衣机  <a href="/SessionTest/buy?id=2">购买</a> <br/> 
        热水器  <a href="/SessionTest/buy?id=3">购买</a> <br/> 
        微波炉  <a href="/SessionTest/buy?id=4">购买</a> <br/> 
        空调  <a href="/SessionTest/buy?id=5">购买</a> <br/> 
        电饭锅  <a href="/SessionTest/buy?id=6">购买</a> <br/> 
  </body>
</html>

package cn.lsl.session;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

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

public class BuyServlet extends HttpServlet {
    
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //获得购买商品编号
        String id = request.getParameter("id");
        //根据编号得到商品名称
        String[] arr = { "冰箱", "洗衣机", "热水器", "微波炉", "空调", "电饭锅" };
        String productName = arr[Integer.parseInt(id)-1];
        //判断商品名称是否在购物车中
        //先判断购物车是否存在
        Map<String, Integer> cart = (Map<String, Integer>) request.getSession().getAttribute("cart");//可能返回null
        if(cart == null){
            //购物车对象不存在
            cart = new HashMap<String, Integer>();
        }
//        Map  containsKey(String Key),判断key有没有对应的value值；有，则返回true
        if(cart.containsKey(productName)){
            //购物车存在该商品，数量+1
            int number = cart.get(productName);
            cart.put(productName, number+1);
        }else{
            //商品没有在购物车中
            cart.put(productName, 1);
        }
        //将购物车加入session
        request.getSession().setAttribute("cart", cart);
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().println("商品已经被加入购物车！");
        response.getWriter().println("<a href='/SessionTest/cart.jsp'>查看购物车</a>");
    }

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

}

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>cart.jsp</title>
  </head>
  
  <body>
    <h1>查看购物车</h1>
    <h3><a href="/SessionTest/invalidate.jsp">清空购物车</a></h3>
    <!-- 购物车信息保存session中 Map<String, Integer> -->
    <%
        Map<String, Integer> cart = (Map<String,Integer>)request.getSession().getAttribute("cart");
        if(cart == null){
            out.println("无购物信息");
        }else{
            for(String productName : cart.keySet()){
                out.println("<h3>商品名称："+productName+",数量：" + cart.get(productName)+"</h3>");
            }
        }
     %>
  </body>
</html>

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!-- 清空购物车 -->
<%
    //销毁Session
    request.getSession().invalidate();
    response.sendRedirect("/SessionTest/cart.jsp");
 %>

4.禁用Cookie后，Session还能否使用？
可以，可以对url进行重写，原理在url;jsessionid=xxx ---- 在程序中可以调用response.encodeURL进行URL重写
* 如果服务器进行URL重写，所有路径都必须重写

5.Cookie生命周期和Session生命周期
Cookie生命周期
创建：Cookie cookie = new Cookie(name,value); response.add(cookie);
销毁：会话cookie会在浏览器关闭时销毁，持久cookie会在cookie过期(MaxAge)后销毁
Session生命周期
创建：request.getSession()创建session
销毁：三种：1.服务器关闭时销毁  2.session过期时销毁 3.手动调用session.invalidate

设置session过期时间
1、配置web.xml
  <session-config>
      <session-timeout>30</session-timeout>
  </session-config>
2、调用session对象 setMaxInactiveInterval(int interval) 单位是秒
HttpSession session = request.getSession();
session.setMaxInactiveInterval(60*60); 设置session过期时间1小时

用户登陆案例和一次性验证码：
验证码从session获取后，马上删除   ---- 验证码只能使用一次
eg:

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>My JSP 'login.jsp' starting page</title>
    <script type="text/javascript">
        function change(){
            //切换验证码
            document.getElementById("myimg").src="/SessionTest/checkcode?"+new Date().getTime();
        }
    </script>
  </head>
  
  <body>
    <h1>登陆页面</h1>
    <h3 style="color:red;">${requestScope.msg}</h3>
    <form action="/SessionTest/login" method="post">
        用户名<input type="text" name="username" /><br/>
        密码<input type="password" name="password" /><br/>
        请输入验证码<input type="text" name="checkcode" /><img id="myimg" src="/SessionTest/checkcode" style="cursor:pointer;" onclick="change();"><br/>
        <input type="submit" value="登陆" />
    </form>
  </body>
</html>

package cn.lsl.session;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

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

public class LoginServlet extends HttpServlet {
    private Map<String, String> userInfo = new HashMap<String, String>();
    
    @Override
    public void init() throws ServletException {
        //制作用户数据
        userInfo.put("aaa", "111");
        userInfo.put("bbb", "222");
        userInfo.put("ccc", "333");
        userInfo.put("ddd", "444");
    }
    
    
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //中文验证码，需要解决乱码问题
        request.setCharacterEncoding("utf-8");
        //判断验证码输入是否正确
        String checkcode = request.getParameter("checkcode");
        String checkcode_session = (String)request.getSession().getAttribute("checkcode_session");
        //删除session中验证码
        request.getSession().removeAttribute("checkcode_session");
        
        if(checkcode_session == null || !checkcode_session.equals(checkcode)){
            //传递出错错误信息给jsp
            request.setAttribute("msg", "验证码错误");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
            return;
        }
        
        //从请求中获得数据
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        //判断用户信息是否正确
        if(userInfo.containsKey(username)){
            //如果用户名存在
            if(userInfo.get(username).equals(password)){
                //密码正确 -- 登陆成功
                //将用户信息保存session
                request.getSession().setAttribute("username", username);
                //跳转登陆成功，系统主页
                response.sendRedirect("/SessionTest/welcome.jsp");
            }else{
                request.setAttribute("msg", "密码码错误");
                request.getRequestDispatcher("/login.jsp").forward(request, response);
            }
        }else{
            //用户名不存在
            request.setAttribute("msg", "用户名不存在");
            request.getRequestDispatcher("/login.jsp").forward(request, response);
        }
    }

    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        doGet(request,response);
    }

}

package cn.lsl.session;

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CheckcodeServlet extends HttpServlet {

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
//        response.setHeader("Cache-Control", "no-cache");
//        response.setDateHeader("Expires", -1);
//        response.setHeader("Pragma", "no-cache");
        
        int width = 120;
        int height = 30;
        //创建一张内存中缓冲图片
        BufferedImage bufferedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_BGR);
        //背景色
        Graphics graphics = bufferedImage.getGraphics();    //通过graphics对象绘制图片
        graphics.setColor(Color.YELLOW);
        graphics.fillRect(0, 0, width, height);
        
        //边框
        graphics.setColor(Color.BLUE);
        graphics.drawRect(0, 0, width-1, height-1);
        
        //写验证内容
        Graphics2D graphics2d = (Graphics2D)bufferedImage.getGraphics();
        graphics2d.setColor(Color.RED);
        graphics2d.setFont(new Font("宋体",Font.BOLD,18));
        
        //String content = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";
        String content = "u7684u4e00u4e86u662fu6211u4e0du5728u4ebau4eecu6709u6765u4ed6u8fd9u4e0a" +
                "u7740u4e2au5730u5230u5927u91ccu8bf4u5c31u53bbu5b50u5f97u4e5fu548cu90a3u8981u4e0b" +
                "u770bu5929u65f6u8fc7u51fau5c0fu4e48u8d77u4f60u90fdu628au597du8fd8u591au6ca1u4e3a" +
                "u53c8u53efu5bb6u5b66u53eau4ee5u4e3bu4f1au6837u5e74u60f3u751fu540cu8001u4e2du5341" +
                "u4eceu81eau9762u524du5934u9053u5b83u540eu7136u8d70u5f88u50cfu89c1u4e24u7528u5979" +
                "u56fdu52a8u8fdbu6210u56deu4ec0u8fb9u4f5cu5bf9u5f00u800cu5df1u4e9bu73b0u5c71u6c11" +
                "u5019u7ecfu53d1u5de5u5411u4e8bu547du7ed9u957fu6c34u51e0u4e49u4e09u58f0u4e8eu9ad8" +
                "u624bu77e5u7406u773cu5fd7u70b9u5fc3u6218u4e8cu95eeu4f46u8eabu65b9u5b9eu5403u505a" +
                "u53ebu5f53u4f4fu542cu9769u6253u5462u771fu5168u624du56dbu5df2u6240u654cu4e4bu6700" +
                "u5149u4ea7u60c5u8defu5206u603bu6761u767du8bddu4e1cu5e2du6b21u4eb2u5982u88abu82b1" +
                "u53e3u653eu513fu5e38u6c14u4e94u7b2cu4f7fu5199u519bu5427u6587u8fd0u518du679cu600e" +
                "u5b9au8bb8u5febu660eu884cu56e0u522bu98deu5916u6811u7269u6d3bu90e8u95e8u65e0u5f80" +
                "u8239u671bu65b0u5e26u961fu5148u529bu5b8cu5374u7ad9u4ee3u5458u673au66f4u4e5du60a8" +
                "u6bcfu98ceu7ea7u8ddfu7b11u554au5b69u4e07u5c11u76f4u610fu591cu6bd4u9636u8fdeu8f66" +
                "u91cdu4fbfu6597u9a6cu54eau5316u592au6307u53d8u793eu4f3cu58ebu8005u5e72u77f3u6ee1" +
                "u65e5u51b3u767eu539fu62ffu7fa4u7a76u5404u516du672cu601du89e3u7acbu6cb3u6751u516b" +
                "u96beu65e9u8bbau5417u6839u5171u8ba9u76f8u7814u4ecau5176u4e66u5750u63a5u5e94u5173" +
                "u4fe1u89c9u6b65u53cdu5904u8bb0u5c06u5343u627eu4e89u9886u6216u5e08u7ed3u5757u8dd1" +
                "u8c01u8349u8d8au5b57u52a0u811au7d27u7231u7b49u4e60u9635u6015u6708u9752u534au706b" +
                "u6cd5u9898u5efau8d76u4f4du5531u6d77u4e03u5973u4efbu4ef6u611fu51c6u5f20u56e2u5c4b" +
                "u79bbu8272u8138u7247u79d1u5012u775bu5229u4e16u521au4e14u7531u9001u5207u661fu5bfc" +
                "u665au8868u591fu6574u8ba4u54cdu96eau6d41u672au573au8be5u5e76u5e95u6df1u523bu5e73" +
                "u4f1fu5fd9u63d0u786eu8fd1u4eaeu8f7bu8bb2u519cu53e4u9ed1u544au754cu62c9u540du5440" +
                "u571fu6e05u9633u7167u529eu53f2u6539u5386u8f6cu753bu9020u5634u6b64u6cbbu5317u5fc5" +
                "u670du96e8u7a7fu5185u8bc6u9a8cu4f20u4e1au83dcu722cu7761u5174u5f62u91cfu54b1u89c2" +
                "u82e6u4f53u4f17u901au51b2u5408u7834u53cbu5ea6u672fu996du516cu65c1u623fu6781u5357" +
                "u67aau8bfbu6c99u5c81u7ebfu91ceu575au7a7au6536u7b97u81f3u653fu57ceu52b3u843du94b1" +
                "u7279u56f4u5f1fu80dcu6559u70edu5c55u5305u6b4cu7c7bu6e10u5f3au6570u4e61u547cu6027" +
                "u97f3u7b54u54e5u9645u65e7u795eu5ea7u7ae0u5e2eu5566u53d7u7cfbu4ee4u8df3u975eu4f55" +
                "u725bu53d6u5165u5cb8u6562u6389u5ffdu79cdu88c5u9876u6025u6797u505cu606fu53e5u533a" +
                "u8863u822cu62a5u53f6u538bu6162u53d4u80ccu7ec6";
        
        //内容四个字 -- 随机从content中抽取四个字
        Random random = new Random();
        int x=10;
        int y=20;
        
        StringBuffer stringBuffer = new StringBuffer();
        
        for(int i=0; i<4; i++){
            //为字生成旋转角度 -30 -- 30
            double jiaodu = random.nextInt(60) - 30;
            //将旋转角度换算弧度
            double theta = jiaodu/180*Math.PI;
            System.out.println(theta);
            
            int index = random.nextInt(content.length());
            char letter = content.charAt(index);
            
            stringBuffer.append(letter);
            
            graphics2d.rotate(theta, x, y);
            graphics2d.drawString(letter+"", x, y);
            //将角度还原
            graphics2d.rotate(-theta, x, y);
            x += 30;
        }
        
        //将StringBuffer中四个字转换String
        String checkcode = stringBuffer.toString();
        //将验证码存入session
        request.getSession().setAttribute("checkcode_session", checkcode);
        
        //绘制随机干扰线
        int x1;
        int x2;
        int y1;
        int y2;
        graphics.setColor(Color.LIGHT_GRAY);
        for(int i=0; i<10; i++){
            x1 = random.nextInt(width);
            x2 = random.nextInt(width);
            y1 = random.nextInt(height);
            y2 = random.nextInt(height);
            //根据两点  绘制直线
            graphics.drawLine(x1, y1, x2, y2);
        }
        
        
        //内存中资源释放
        graphics.dispose();
        
        
        //将图片输出到浏览器 ImageIO
        //将内存的图片通过浏览器输出流写成jpg格式图片
        ImageIO.write(bufferedImage, "jpg", response.getOutputStream());
    }

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

}

<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
    <title>welcome.jsp</title>
  </head>
  
  <body>
    <h1>系统主页</h1>
    <%
        //因为session会保存登陆用户信息，如果session中没有该信息，说明用户未登陆
        if(request.getSession().getAttribute("username")==null){
            out.println("你还没有登陆，<a href='/SessionTest/login.jsp'>去登陆</a>");
        }else{
            //已经登陆
            out.println("欢迎你，" + request.getSession().getAttribute("username"));
        }
     %>
  </body>
</html>

Servlet三种数据范围总结：
ServletContext
HttpServletRequest
HttpSession

三种数据范围，每个对象各自维护类似map集合结构，具备相同几个方法
setAttribute 存入一个属性
getAttribute 取出一个属性
removeAttribute 移除一个属性

三种数据范围对象在哪些情况使用？
ServletContext服务器启动时创建，服务器关闭销毁，所有Servlet共享 --- 保存一些全局数据
例如：数据库连接池、工程配置属性、配置文件内容

HttpSession 在request.getSession时创建，在三种情况下销毁 ----- 保存与用户相关数据
例如：系统登陆信息、购物车数据

HttpServletRequest在客户端发起请求时，服务器创建对象，在响应结束时，对象销毁 ---- 保存Servlet向JSP传输数据信息
例如：执行某个操作，Servlet将操作结果传递JSP（Servlet将登陆错误信息传递JSP）、Servlet将数据查询结果传递JSP

以上三种数据范围  ServletContext > HttpSession > HttpServletRequest
*宗旨：优先使用生命周期短的