cookie使用-实现登录时间的记录,和浏览历史的显示

淩亂°似流年 2022-05-20 09:54 192阅读 0赞

## cookie的作用 ##

每次学习一个技术的时候首先要搞明白它的作用是么?首先要明确一点,http的连接是无状态的,它不会保存你之前发送的所有请求,不会知道两次访问服务器的人是不是同一个.cookie的本质是一段文体数据,储存在客户端,可能是硬盘可能是内存

## 实现每次登录时间的记录 ##

`两个步骤:`

*  拿到cookie,读出存在cookie中的值
 *  写cookie,将每次的新访问时间写进cookie里面

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 test1(request, response);
 }
 
 private void test1(HttpServletRequest request, HttpServletResponse response) throws IOException {
 
 // 拿到了cookie了,写在页面上
 // 解决编码问题
 response.setCharacterEncoding("UTF-8");
 response.setHeader("content-type", "text/html;charset=UTF-8");
 PrintWriter writer = response.getWriter();
 
 //读值
 Cookie[] cookies = request.getCookies();
 for (int i = 0; cookies != null && i < cookies.length; i++) {
 Cookie cookie = cookies[i];
 String name = cookie.getName();
 if (name.equals("lastAccessTime")) {
 String lastTime = cookie.getValue();
 writer.write("您上次访问的时间是" + URLDecoder.decode(lastTime, "UTF-8"));
 }
 }
 
 // 获取本地最新时间,设置cookie
 Date now = new Date();
 SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm");
 String currentTime = format.format(now);
 Cookie cookie = new Cookie("lastAccessTime", URLEncoder.encode(currentTime, "UTF-8"));
 response.addCookie(cookie);

`注意点:`

*  编码问题
 *  cookie为空的判断，如果对方是第一次访问，则没有这个值,也就不用显示上次访问时间了
 *  日期格式化一下，这样符合咱中国人的阅读顺序
 *  cookie中的值不能为空值，如果有的话，就用URLEncoder进行编码,再解码,不然会报错`An invalid character [32] was present in the Cookie value`,意思是存在了空值

## 实现浏览历史 ##

`思路:`

*  cookie设置浏览记录id\_id\_id的形式,避免占用多个cookie
 *  两个页面  
    
    
     *  一个页面作为主页,显示有哪些书,显示历史记录
     *  一个页面作为详情页面,同时设置cookie.

代码:

`主页面:`  
DB用来存入数据,容器是Map,直接静态代码块赋值即可  
Book是抽象化的书对象

package cookie;
 
 import javax.servlet.ServletException;
 import javax.servlet.annotation.WebServlet;
 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.Arrays;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 
 @WebServlet(name = "ServletCookieDemo2")
 public class ServletCookieDemo2 extends HttpServlet { 
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 
 }
 
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 
 // 显示所有的书及其链接
 response.setCharacterEncoding("UTF-8");
 response.setHeader("content-type", "text/html;charset=UTF-8");
 PrintWriter writer = response.getWriter();
 
 writer.write("一共有如下书: ");
 Map<String, Book> map = DB.getAll();
 for (Map.Entry<String, Book> entry : map.entrySet()) {
 Book book = entry.getValue();
 writer.write("<a href='/jsp/ServletCookieDemo3?id=" + book.getId() + "' target='_blank'>" + book.getName() + "</a> ");
 }
 writer.write(" ");
 
 writer.write("浏览历史为: ");
 //　写上访问历史
 Cookie[] cookies = request.getCookies();
 for (int i = 0; cookies != null && i < cookies.length; i++) {
 Cookie cookie = cookies[i];
 if (cookie.getName().equals("bookHistory")) {
 String value = cookie.getValue();
 String[] ids = value.split("_");
 for (String id : ids) {
 writer.write(DB.getAll().get(id).getName()+" ");
 }
 }
 }
 }
 }
 
 class DB {
 private static Map<String, Book> map = new LinkedHashMap<>();
 
 static {
 map.put("1", new Book("1", "java", "a", "12"));
 map.put("2", new Book("2", "php", "a", "12"));
 map.put("3", new Book("3", "python", "a", "12"));
 map.put("4", new Book("4", "js", "a", "12"));
 map.put("5", new Book("5", "html", "a", "12"));
 }
 
 public static Map<String, Book> getAll() {
 return map;
 }
 }
 
 class Book {
 
 private String id;
 private String name;
 private String auther;
 private String price;
 
 public Book(String id, String name, String auther, String price) {
 this.id = id;
 this.name = name;
 this.auther = auther;
 this.price = price;
 }
 
 public String getId() {
 return id;
 }
 
 public void setId(String id) {
 this.id = id;
 }
 
 public String getName() {
 return name;
 }
 
 public void setName(String name) {
 this.name = name;
 }
 
 public String getAuther() {
 return auther;
 }
 
 public void setAuther(String auther) {
 this.auther = auther;
 }
 
 public String getPrice() {
 return price;
 }
 
 public void setPrice(String price) {
 this.price = price;
 }
 
 @Override
 public String toString() {
 return "Book{" +
 "id='" + id + '\'' +
 ", name='" + name + '\'' +
 ", auther='" + auther + '\'' +
 ", price='" + price + '\'' +
 '}';
 }
 }

`详情页面:`

*  将cookie的字符串数组分割然后装进linkedHashMap中,因为LinkedHashMap是有序的,不会乱了访问顺序.
 *  用StringBuilder对添加后的LinkedHashMap进行重组,得到最后的Cookie的字符串
 *  给cookie加上期限

package cookie;
 
 import javax.servlet.ServletException;
 import javax.servlet.annotation.WebServlet;
 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.Arrays;
 import java.util.LinkedList;
 import java.util.List;
 import java.util.Map;
 
 @WebServlet(name = "ServletCookieDemo3")
 public class ServletCookieDemo3 extends HttpServlet { 
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 
 }
 
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 //显示书的详情
 response.setCharacterEncoding("UTF-8");
 response.setHeader("content-type", "text/html;charset=UTF-8");
 PrintWriter writer = response.getWriter();
 
 String id = request.getParameter("id");
 Book book = DB.getAll().get(id);
 writer.write(book.toString());
 
 String bookHistory = makeHistory(request, id);
 Cookie cookie = new Cookie("bookHistory", bookHistory);
 cookie.setMaxAge(1*30*24*3600);
 response.addCookie(cookie);
 
 }
 
 private String makeHistory(HttpServletRequest request, String id) {
 /* 写cookie,新访问的是1 -----以前没有1(不包含1)----超过最大数3个:删除最后一个再头插 | | | | | |--不超过:直接头插 | |--以前有1(包含1):去除,重新添加进头部 old new null 1 3_1 1_3 3_1_2 1_3_2 3_2 1_3_1 */
 String bookHistory = null;
 Cookie[] cookies = request.getCookies();
 for (int i = 0; cookies != null && i < cookies.length; i++) {
 Cookie cookie = cookies[i];
 if (cookie.getName().equals("bookHistory")) {
 bookHistory = cookie.getValue();
 System.out.println(bookHistory);
 }
 }
 
 if (bookHistory == null) {
 return id;
 }
 List<String> list = Arrays.asList(bookHistory.split("_"));
 LinkedList<String> linkedList = new LinkedList<>(list);
 // 包含
 if (linkedList.contains(id)) {
 System.out.println("包含"+id);
 linkedList.remove(id);
 linkedList.addFirst(id);
 } else {
 // 超过最大数3个:删除最后一个再头插
 if (linkedList.size() >= 3) {
 linkedList.removeLast();
 linkedList.addFirst(id);
 } else {
 // 不超过:直接头插
 linkedList.addFirst(id);
 System.out.println("头插成功");
 }
 }
 
 StringBuilder builder = new StringBuilder();
 for (String value : linkedList) {
 builder.append(value+"_");
 }
 
 System.out.println(builder.toString());
 return builder.deleteCharAt(builder.length() - 1).toString();
 }
 }

## 总结要点 ##

关键就在于历史记录如何构成的问题,一共分了几块去写它,保证最新的排在最前面,这个顺序很重要.其它小细节有LinkedHashMap保证有序性,id\_id\_id的历史记录写法,字符串数组到list的转换,cookie的遍历获得指定的那个．

纸上得来终觉浅,绝知此事要躬行．