Session技术

•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HttpSession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它web资源时，其它web资源再从用户各自的session中取出数据为用户服务。

数据存储在服务端

服务端会为每一个客户端浏览器创建一个独享的session

Session也是一个域对象。域的范围是一个会话

session的实现原理（一个session为一个用户浏览器服务）：

注意：是大写的JSESSIONID

浏览器在访问servlet的时候一定会携带者一个cookie过去（JSESSIONID）
getSession()：会根据cookie的id到服务器去寻找这个id所对应的session对象，如果找不到，就会创建一个新的session，并给这个session分配一个唯一的id，并把他写入一个响应头（setCookie）返回给客户端（实际还是发送了一个cookie到客户端）

默认存储到客户端的浏览器中，当浏览器再访问另一个servlet的时候，就会携带着那个cookie，到达服务器，此时已经有了session，直接去找对应的session，从中拿取数据进行处理

当另一个浏览器访问时，又会创建一个新的session，并分配一个唯一的id，剩余步骤如上。

lSession和Cookie的主要区别在于：

Cookie是把用户的数据写给用户的浏览器。

Session技术把用户的数据写到用户独占的session中。

Session对象由服务器创建，开发人员可以调用request对象的getSession方法得到session对象。

常用方法：

创建session

1)request.getSession() ;  

此方法做了2件事： 1. 先看浏览器是否携带了一个名字叫JSESSIONiD的Cookie,如果没有，则直接创建一个新的session

2. 如果有,则根据Cookie的值去服务端的内存中寻找id的值是此值的session.并且返回，找不到，创建新的返回

2) request.getSession(booleanflag) ;

如果flag为true,则相当于情况1）

如果flag为 false,则只查不创建。

延长session的生命

服务端发送的JSESSIONID默认情况下存储在客户端的缓存中，所以关闭浏览器，session的生命就结束了，要想多个关闭后还能用，那么必须创建一个新的

Cookie,名字也是JSESSIONID,设置存活时间，此时cookie存储在客户端的硬盘上，下次打开新的浏览器访问，浏览器会携带此Cookie,也就达到了关闭浏览器

还能使用此Session的效果了。

Session1:

package com.heima.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;//演示创建sessionpublic class ServletSession1 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {/** *   getsession()方法做了两件事： *   1. 看客户端是否携带了JSESSIONIDCookie *       2. 如果没有携带，创建一个新的session对象，并分配一个唯一的id,发送到客户端，存储在客户端的缓存中 *      如果携带了，将根据cookie的值(id)到服务端的内存中寻找session，如果找到了则返回此session为客户端服务， *    如果找不到，则创建新的session对象，并并分配一个唯一的id,发送到客户端，存储在客户端的缓存中 */HttpSession session = request.getSession() ;String id = session.getId() ;System.out.println("111:" + id);session.setAttribute("name", "张无忌") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

Session2:

package com.heima.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;//演示从session中取出数据public class ServletSession2 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {//获取session对象HttpSession session = request.getSession() ;System.out.println("222:" + session.getId());//从session对象中获取数据String name = (String) session.getAttribute("name") ;System.out.println(name);}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

应用

1.加入购物车案例

bean：Book:

package com.heima.car.bean;public class Book {private String id ;private String bookName ;private String author ;private float price ;private String description ;private int count ;public Book(String id, String bookName, String author, float price,String description) {this.id = id;this.bookName = bookName;this.author = author;this.price = price;this.description = description;}public String getId() {return id;}public void setId(String id) {this.id = id;}public String getBookName() {return bookName;}public void setBookName(String bookName) {this.bookName = bookName;}public String getAuthor() {return author;}public void setAuthor(String author) {this.author = author;}public float getPrice() {return price;}public void setPrice(float price) {this.price = price;}public String getDescription() {return description;}public void setDescription(String description) {this.description = description;}public int getCount() {return count;}public void setCount(int count) {this.count = count;}@Overridepublic String toString() {return "Book [id=" + id + ", bookName=" + bookName + ", author="+ author + ", price=" + price + ", description=" + description+ "]";}@Overridepublic boolean equals(Object obj) {Book b = (Book)obj ;return id.equals(b.id);//判断id是否一样，id一样就是买过这本书了 不一样就是没买过}}

BookUtils:

package com.heima.car.utils;import java.util.HashMap;import java.util.Map;import com.heima.car.bean.Book;public class BookUtils {private static Map<String,Book> map = new HashMap<String,Book>() ;static{map.put("1", new Book("1","葵花宝典","安倍晋三",100,"欲练神功，必须先练好基本功")) ;map.put("2", new Book("2","辟邪剑谱","陈冠希",80,"绝世好书")) ;map.put("3", new Book("3","西游记","吴承恩",50,"一群小猴子的故事")) ;map.put("4", new Book("4","水浒传","施耐庵",90,"三个女人和105个男人的故事")) ;map.put("5", new Book("5","西厢记","阿娇",70,"好好看啊。。。。。。")) ;map.put("6", new Book("6","神雕侠侣","金庸",100,"感天动地的旷世绝恋")) ;map.put("7", new Book("7","红楼梦","葫芦娃",60,"男人的梦想。。。。。。。")) ;}//获取所有的书public  static Map<String,Book> getAllBook(){return map ;}//根据书的id获取谋一本书public static Book getBookById(String id){return map.get(id) ;}}

ShowAllBookServlet:

package com.heima.car.servlet;import java.io.IOException;import java.io.PrintWriter;import java.util.Map;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 com.heima.car.bean.Book;import com.heima.car.utils.BookUtils;/** * 功能有两个： * 1. 显示所有的书的信息 * 2. 提供一个购物车的连接 * @author Administrator * */public class ShowAllBookServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;out.write("本站有以下好书：<br>") ;//1.显示所有的书//获取所有的书的集合Map<String ,Book> map = BookUtils.getAllBook() ;//遍历集合for (Map.Entry<String, Book> entry : map.entrySet()) {//拿到每一本的idString id = entry.getKey() ;//拿到每一本书Book book = entry.getValue() ;//输出书的名字out.write(book.getBookName() + "  <a href = '"+ request.getContextPath()+"/servlet/ShowBookDetailServlet?id=" + id +"'>显示详细信息</a><br>") ;}out.write("<br><br><br><br>") ;//提供查看购物车的连接out.write("<a href = '"+ request.getContextPath()+"/servlet/CarServlet'>查看购物车</a>") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

ShowBookDetailServlet:

package com.heima.car.servlet;import java.io.IOException;import java.io.PrintWriter;import java.util.Arrays;import java.util.LinkedList;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 com.heima.car.bean.Book;import com.heima.car.utils.BookUtils;/** * 此类完成两件事情： *   1. 显示书的详细信息 *   2. 提供购物的连接，返回继续浏览的连接 * @author Administrator * */public class ShowBookDetailServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;//1.显示书的详细信息//拿到页面传递的idString id = request.getParameter("id") ;//根据id获取书Book book = BookUtils.getBookById(id) ;//显示信息out.write(book + "  <a href = '" + request.getContextPath() +"/servlet/ShowAllBookServlet'>返回主页继续浏览</a><a href = '" + request.getContextPath()+ "/servlet/BuyServlet?id=" + id + "'>放入购物车</a><br>") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

BuyServlet:

package com.heima.car.servlet;import java.io.IOException;import java.io.PrintWriter;import java.util.ArrayList;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;import com.heima.car.bean.Book;import com.heima.car.utils.BookUtils;//此servlet是将购买的书放入购物车public class BuyServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;//获取页面传递的idString id = request.getParameter("id") ;//根据id找到书对象Book book = BookUtils.getBookById(id) ;//将书存入session对象中HttpSession session  = request.getSession() ;//将书放入session中的一个集合中List<Book> list = (List<Book>) session.getAttribute("carlist") ;if(list == null){//说明第一次购买书籍，此时session中是不存在list集合，需要手动创建list = new ArrayList<Book>() ;//将购买的书籍放入book.setCount(1) ;list.add(book) ;//将list存入sessionsession.setAttribute("carlist", list) ;}else{//说明已经购买过书籍了//判断购物车中是否已经购买过此书int index = list.indexOf(book) ;  //这里的这个方法跟重写equals方法有什么联系???????????????????????????//indexOf(Object o)  找出指定元素第一次出现在集合中 的索引值if(index == -1){//说明没有买过此书book.setCount(1) ;list.add(book) ;}else{//说明买过了Book b = list.get(index) ;b.setCount(b.getCount() + 1) ;}//提醒用户，此书已经放入购物车中out.write(book.getBookName() + " 已经放入购物车中,4秒后转向主页，低级<a href = '"+request.getContextPath()+"/servlet/ShowAllBookServlet'>此处</a>直接转向主页") ;response.setHeader("Refresh", "4;url="+ request.getContextPath() + "/servlet/ShowAllBookServlet") ;}}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

CarServlet:

package com.heima.car.servlet;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;import com.heima.car.bean.Book;//显示购物车中的内容public class CarServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ; HttpSession session = request.getSession() ;//如果用户直接在地址栏中输入地址访问此servlet,那此时session是一个新创建的对象 if(session.isNew()){ //直接返回主页面 response.sendRedirect(request.getContextPath() + "/servlet/ShowAllBookServlet") ; return ; } //先从session拿到购物车List<Book> list = (List<Book>)session.getAttribute("carlist") ; //如果用户直接从主页面上点击查看购物车过来,此时还没有购买任何书籍呢，所以提示用户会主页if(list == null){//用户首次访问购物车out.write("你还没有购买任何书籍呢，看个鸟啊,2秒后转向主页") ;response.setHeader("Refresh", "2;url="+ request.getContextPath() + "/servlet/ShowAllBookServlet") ;return ;} //说明购买了书籍out.write("你购买的书籍如下：<br> ") ;out.write("书名\t数量\t总价格<br>") ;for (int i = 0; i < list.size(); i++) {Book b = list.get(i) ;out.write(b.getBookName() + "\t" + b.getCount() + "\t" + b.getPrice() * b.getCount() + "<br>") ;}out.write("<br><br><a href = '" +request.getContextPath()+ "/servlet/ShowAllBookServlet'>返回主页面</a>") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

2.验证码登录

LoginServlet:

package com.heima.servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import javax.servlet.http.HttpSession;//验证用户是否合法public class LoginServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;//拿取页面传递的信息String username = request.getParameter("username") ;String pass = request.getParameter("pass") ;String fcode = request.getParameter("code") ;System.out.println(fcode);//先验证验证码if("".equals(fcode)){out.write("验证码不能为空,2秒后转向登陆页面") ;response.setHeader("Refresh", "2;url=" + request.getContextPath() + "/login.html") ;return ;}else{//从session中拿到存储的验证码HttpSession  session  = request.getSession() ;String scode = (String) session.getAttribute("scode") ;if(!scode.equalsIgnoreCase(fcode)){out.write("验证码填写错误,2秒后转向登陆页面") ;response.setHeader("Refresh", "2;url=" + request.getContextPath() + "/login.html") ;return ;}if("张三".equals(username) && "111".equals(pass)){//需要将用户名存入session  session.setAttribute("username", username) ;request.getRequestDispatcher("MainServlet").forward(request, response) ;}else{out.write("用户名或者密码错误,2秒后转向登陆页面") ;response.setHeader("Refresh", "2;url=" + request.getContextPath() + "/login.html") ;}}}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

ServletResponse:

package com.heima.servlet;import java.awt.Color;import java.awt.Font;import java.awt.Graphics;import java.awt.image.BufferedImage;import java.io.IOException;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;import com.sun.org.apache.xerces.internal.xni.grammars.Grammar;//输出验证码public class ServletResponse3 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {//创建变量int width = 120 ;int height = 30 ;//创建一个内存图像BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB) ;//创建画笔Graphics g = image.getGraphics() ;//指定边框的颜色g.setColor(Color.RED) ;//画图像的边框g.drawRect(0, 0, width, height) ;//填充一下矩形的背景色//设定画笔的颜色g.setColor(Color.YELLOW) ;//填充矩形的背景g.fillRect(1, 1, width-2, height-2) ;//设置字体大小g.setFont(new Font("幼圆",Font.BOLD + Font.ITALIC,22)) ;//填充内容Random r = new Random() ;//设置画笔的颜色g.setColor(Color.GRAY) ;//画30条干扰线for (int i = 0; i < 30; i++) {g.drawLine(r.nextInt(width), r.nextInt(height), r.nextInt(width), r.nextInt(height)) ;}//设定画笔的颜色g.setColor(Color.RED) ;//随机产生4个汉字String s = "黑马训练营北京传智播客IhavanoMoney" ;//先将汉字转换为unicode编码s = "\u9ED1\u9A6C\u8BAD\u7EC3\u8425\u5317\u4EAC\u4F20\u667A\u64AD\u5BA2IhavanoMoney" ;StringBuffer sb = new StringBuffer() ;for (int i = 0; i < 4; i++) {char c = s.charAt(r.nextInt(s.length())) ;sb.append(c) ;int flag = r.nextBoolean()?1:-1 ; g.drawString(c+"", 20 + 20*i + flag*r.nextInt(5), 23 + flag * r.nextInt(5)) ; }//将验证码存入sessionrequest.getSession().setAttribute("scode", sb.toString()) ;////随机产生4个数字输出到页面//for (int i = 0; i < 4; i++) {//int n  = r.nextInt(10) ;////画到到图片中//g.drawString(n+"", 20 + 20*i, 20) ;//}//告诉客户端不要缓存图像response.setHeader("Expires", -1 + "") ;response.setHeader("Cache-control", "no-cache") ;response.setHeader("Pragma", "no-cache") ;//将图片输出到客户端ImageIO.write(image, "jpg", response.getOutputStream()) ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

MainServlet:

package com.heima.servlet;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class MainServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;String name = (String)request.getSession().getAttribute("username") ;out.write(name  + ",欢迎你") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

3.解决重复提交的问题

解决：

利用第三方变量：

在页面产生的时候同时给一个变量，同时为改变量赋一个值，当请求第一次提交后，把这个值改变：该变量存储在session中，把其删除即可。如果刷新，一个新的请求把刚才的信息又提交了一遍：当提交到服务端的时候，先判断该变量的值，如果该变量的值已经不存在了，就认为重复提交了，如果变量的值还在，则认为是第一次提交。

RegisterServletUI:

package com.heima.token;import java.io.IOException;import java.io.PrintWriter;import java.net.URLEncoder;import java.security.MessageDigest;import java.security.NoSuchAlgorithmException;import java.util.UUID;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;import sun.misc.BASE64Encoder;//此servlet的目的是产生页面，同时提供一个唯一的口令：tokenpublic class RegisterServletUI extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;//产生一个唯一的值String token = "" ;//token = System.nanoTime() + "" ;//采用第三种方式:数据指纹token = UUID.randomUUID().toString() ;try {MessageDigest md = MessageDigest.getInstance("MD5") ;byte[] bs = md.digest(token.getBytes()) ;BASE64Encoder base = new BASE64Encoder() ;token = base.encode(bs) ;System.out.println("session:" + token);} catch (NoSuchAlgorithmException e) {// TODO Auto-generated catch blocke.printStackTrace();}//由于数据指纹中可能存在特殊字符，因此再次进行编码//将口令存入session request.getSession().setAttribute("stoken", token) ;//拿到错误信息显示String error = (String) request.getAttribute("error") ;if(error !=null){out.write(error) ;}//创建页面out.write("<form action = '" + request.getContextPath()+"/servlet/ResiterServlet' method = 'post'>") ;out.write("姓名：<input type = 'text'  name = 'username'> <br>") ;out.write("<input type = 'hidden'  name = 'ftoken' value = '" + token + "'> <br>") ;out.write("<input type = 'submit'  value = '注册'> <br>") ;out.write("</form>") ;}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

RegisterServlet:

package com.heima.token;import java.io.IOException;import java.io.PrintWriter;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;//完成注册的功能public class ResiterServlet extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {request.setCharacterEncoding("UTF-8") ;response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;//拿到页面传递的数据String name = request.getParameter("username") ;String ftoken = request.getParameter("ftoken") ;System.out.println("页面： " + ftoken);//判断信息进行注册if("".equals(name)){//返回注册页面\request.setAttribute("error", "用户名必须填写") ;request.getRequestDispatcher("RegisterServletUI").forward(request, response) ;}else{//在注册用户的时候先判断口令，口令一致再进行注册，否则不允许注册//注意，保存成功后要将口令从session中删除//拿到session中存储的口令String stoken = (String) request.getSession().getAttribute("stoken") ;if(ftoken.equals(stoken)){out.write("用户已保存到数据库中") ;//删除口令request.getSession().removeAttribute("stoken") ;}else{out.write("不要重复提交，2秒后转向注册页面") ;response.setHeader("Refresh", "2;url=" + request.getContextPath() + "/servlet/RegisterServletUI") ;}}}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

4.url重写技术

当客户端浏览器禁止了所有的Cookie时，服务器发送的session在客户端的缓存里就不存在了，每次刷新都会被创建新的session。

此时，服务端还要向客户端服务，办法：

1.告诉客户端：不要禁止

  客户端在访问网站的时候，该网站会先去检测：客户端浏览器是不是禁止了，如果禁止了，就会提示消息：不要禁止客户端的Cookie

2.url重写技术：

把session的id附加到网址后面，这样就能找到对应的session为其服务了。

encodeURL()这个方法的原理就是把id附加在了网址的后面
缺点：所有的URL都要重写，麻烦

package com.heima.url;import java.io.IOException;import java.io.PrintWriter;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 ServletSession1 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {response.setContentType("text/html;charset=UTF-8") ;PrintWriter out = response.getWriter() ;HttpSession session  =request.getSession() ;String id = session.getId() ;session.setAttribute("name", "张三") ;//只要将产生的id附加到每一个网址(url)之后,服务端就可以根据id找到对应的session对象为客户端服务String url = request.getContextPath() + "/servlet/ServletSession2" ;url = response.encodeURL(url) ;out.write("<a href = '" + url+"'>访问ServletSession2</a>") ;//System.out.println(session.getId());}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

package com.heima.url;import java.io.IOException;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class ServletSession2 extends HttpServlet {public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {String name = (String)request.getSession().getAttribute("name") ;System.out.println(name);}public void doPost(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {doGet(request, response);}}

5.session的状态

当我们创建了一个session后，此session就在服务器的内存里

当服务器里存在着很多没用的session时，希望可以把这些没用的session清除掉：

利用Tomcat的一种机制：固化或搁置：把session序列化到硬盘上，

固化或搁置：停止服务器时，把有用的session序列化到硬盘上。

再次启动时：把有用的session重新激活，读取到内存里（激活或活化）

当服务器停止的时候，内存中的session将会固化到硬盘上，路径在\work\Catalina\localhost\day11_00_session下，

文件后缀名是ser.当重新启动的时候，此文件对象会自动又被读入内存继续服务。