Ajax、Json对象及完成添加购物车的功能

Ajax：Asynchronous JavaScript and XML，其实这并不是一项新的技术，不过是融合了几种技术，实现了异步请求，即不需要刷新页面，用户并不会察觉到有请求的发生，但是实际上浏览器引擎发送了请求，只不过用户不再需要等到服务器的响应才去执行其他操作。
关键对象：XMLHttpRequest
关键方法：
open();发送请求
send();发送数据
属性：
readyState
status （服务器返回的状态码）
responseText
responseXML
事件：onreadystatechange

function getXHR() {     var xmlhttp;    if (window.XMLHttpRequest) {// code for IE7+, Firefox, Chrome, Opera, Safari         xmlhttp=new XMLHttpRequest();    } else {// code for IE6, IE5         xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");    }   return xmlhttp;}        //1.得到XMLHttpRequest对象            var xhr = getXHR();            //4.根据状态的改变执行函数            xhr.onreadystatechange = function () {              if(xhr.readyState == 4){                  if(xhr.status == 200){                       if(xhr.responseText == 'true') {                           document.getElementById("msg").innerHTML ='用户名已存在';                       }else if(xhr.responseText == 'false'){                           document.getElementById("msg").innerHTML ='用户名可用';                       }                  }              }            };            //2.open方法发起请求            xhr.open("get","${pageContext.request.contextPath}/servlet/ajaxServlet?name="+name);            //3.send方法            xhr.send(null);

JSONObject对象所依赖的jar包：

package com.itdream.json;import com.itdream.domain.Book;import net.sf.json.JSONArray;import net.sf.json.JSONObject;import net.sf.json.JsonConfig;import org.junit.Test;import java.util.ArrayList;import java.util.List;/** * Created by Dream on 2017/12/7. */public class JsonTest {    @Test  //使用JSONObject封装数据对象类型数据    public void test1(){        Book b = new Book();        b.setId("123");        b.setName("Is your light still shinning?");        b.setPrice(99);        b.setCategory("启发");        b.setPnum(15);        b.setDescription("启发思维");        String s = JSONObject.fromObject(b).toString();        System.out.println(s);    }    //{"category":"启发","description":"启发思维","id":"123","name":"Is your light still shinning?","pnum":15,"price":99}    @Test  //使用JSONArray封装List<Book>对象数据类型    public void test2(){        List<Book> list = new ArrayList<>();        Book b1 = new Book();        b1.setId("123");        b1.setName("Is your light still shinning?");        b1.setPrice(99);        b1.setCategory("启发");        b1.setPnum(15);        b1.setDescription("启发思维");        Book b2 = new Book();        b2.setId("123");        b2.setName("Is your light still shinning?");        b2.setPrice(99);        b2.setCategory("启发");        b2.setPnum(15);        b2.setDescription("启发思维");        Book b3 = new Book();        b3.setId("123");        b3.setName("Is your light still shinning?");        b3.setPrice(99);        b3.setCategory("启发");        b3.setPnum(15);        b3.setDescription("启发思维");        list.add(b1);        list.add(b2);        list.add(b3);        String s = JSONArray.fromObject(list).toString();        System.out.println(s);    }    //[{"category":"启发","description":"启发思维","id":"123","name":"Is your light still shinning?","pnum":15,"price":99},{"category":"启发","description":"启发思维","id":"123","name":"Is your light still shinning?","pnum":15,"price":99},{"category":"启发","description":"启发思维","id":"123","name":"Is your light still shinning?","pnum":15,"price":99}]    @Test  //使用JsonConfig去除不要的字段数据    public void test3(){        List<Book> list = new ArrayList<>();        Book b1 = new Book();        b1.setId("123");        b1.setName("Is your light still shinning?");        b1.setPrice(99);        b1.setCategory("启发");        b1.setPnum(15);        b1.setDescription("启发思维");        Book b2 = new Book();        b2.setId("123");        b2.setName("Is your light still shinning?");        b2.setPrice(99);        b2.setCategory("启发");        b2.setPnum(15);        b2.setDescription("启发思维");        Book b3 = new Book();        b3.setId("123");        b3.setName("Is your light still shinning?");        b3.setPrice(99);        b3.setCategory("启发");        b3.setPnum(15);        b3.setDescription("启发思维");        list.add(b1);        list.add(b2);        list.add(b3);        JsonConfig jc = new JsonConfig();        jc.setExcludes(new String[]{"pnum","id","category","description"});        String s = JSONArray.fromObject(list,jc).toString();        System.out.println(s);    }    //结果：    //[{"name":"Is your light still shinning?","price":99},{"name":"Is your light still shinning?","price":99},{"name":"Is your light still shinning?","price":99}]}

在进行购物的时候，我们将欲购买的商品添加至购物车，购物车应该存放于session对象中
思路：
初始化的数据num=1，便于对第一次加入购物车时的数据进行数量赋值
1.首先获取session对象中的cart数据
2.如果是第一次添加购物车，那么获取的cart对象则为空，这时我们new一个购物车对象
3.判断添加的商品是否存在于购物车中，如果存在，则需要将原先存在于购物车中的商品的数量取出并进行+1操作；如果不存在于购物车中，则将初始化的num赋予给对应商品的数量
4.将商品及其num值添加到cart中
5.将cart设置到Session域中

public class AddCartServlet extends HttpServlet {    public void doGet(HttpServletRequest request,HttpServletResponse response)throws IOException{        response.setContentType("text/html;charset=utf-8");        String id = request.getParameter("id");        BookService bs = new BookService();        Book book = bs.findBookById(id);        Map<Book,String> cart = (Map<Book, String>) request.getSession().getAttribute("cart");        int num = 1; //记录数量        if(cart == null){  //第一次加购物车，购物车还没放session            cart = new HashMap<>();        }        if(cart.containsKey(book)){  //如果这本书已经存在于购物车，那么则需要取出cart中的value值，+1            String n = cart.get(book);            num = Integer.parseInt(n)+1;        }        cart.put(book,num+"");        request.getSession().setAttribute("cart",cart);        response.getWriter().write("<a href='"+request.getContextPath()+"/servlet/pageServlet'>继续购物</a>，<a href='"+request.getContextPath()+"/cart.jsp'>查看购物车</a>");    }    public void doPost(HttpServletRequest request,HttpServletResponse response)throws IOException{        doGet(request,response);    }}