【jsp】案例：显示商品列表 & 问题：List内添加元素，为什么值都变成一样的了

代码：

1.Product：

 package domain;

 public class Product {

 /*    `pid` varchar(50) NOT NULL,
       `pname` varchar(150) DEFAULT NULL,
       `market_price` double DEFAULT NULL,
       `shop_price` double DEFAULT NULL,
       `pimage` varchar(200) DEFAULT NULL,
       `pdate` date DEFAULT NULL,
       `is_hot` int(11) DEFAULT NULL,
       `pdesc` varchar(255) DEFAULT NULL,
       `pflag` int(11) DEFAULT NULL,
       `cid` varchar(50) DEFAULT NULL,
       PRIMARY KEY (`pid`)*/

     private String pid;
     private String pname;
     private double market_price;
     private double shop_price;
     private String pimage;
     private String pdate;
     private int is_hot;
     private String pdesc;
     private int pflag;
     private String cid;

     public Product(){
         pid="";
         pname="";
         pimage="";
         pdate="";
         pdesc="";
         cid="";
     }

     public String getPid() {
         return pid;
     }
     public void setPid(String pid) {
         this.pid = pid;
     }
     public String getPname() {
         return pname;
     }
     public void setPname(String pname) {
         this.pname = pname;
     }
     public double getMarket_price() {
         return market_price;
     }
     public void setMarket_price(double market_price) {
         this.market_price = market_price;
     }
     public double getShop_price() {
         return shop_price;
     }
     public void setShop_price(double shop_price) {
         this.shop_price = shop_price;
     }
     public String getPimage() {
         return pimage;
     }
     public void setPimage(String pimage) {
         this.pimage = pimage;
     }
     public String getPdate() {
         return pdate;
     }
     public void setPdate(String pdate) {
         this.pdate = pdate;
     }
     public int getIs_hot() {
         return is_hot;
     }
     public void setIs_hot(int is_hot) {
         this.is_hot = is_hot;
     }
     public String getPdesc() {
         return pdesc;
     }
     public void setPdesc(String pdesc) {
         this.pdesc = pdesc;
     }
     public int getPflag() {
         return pflag;
     }
     public void setPflag(int pflag) {
         this.pflag = pflag;
     }
     public String getCid() {
         return cid;
     }
     public void setCid(String cid) {
         this.cid = cid;
     }

 }

2.ProductListD

 package dao;

 import java.sql.*;
 import java.util.ArrayList;
 import java.util.List;

 import domain.Product;

 public class ProductListDao {

     public List<Product> getAllProduct(){
         List<Product> productList=new ArrayList<Product>();//List是接口，不能直接new List
 //        Product product=new Product();//不能只定义一个变量，页面上只显示最后一个商品

         try {
             Class.forName("com.mysql.jdbc.Driver");
         } catch (ClassNotFoundException e) {
             e.printStackTrace();
         }
         try {
             Connection con=DriverManager.getConnection("jdbc:mysql://localhost:3306/web17","root","root");
             PreparedStatement ps=con.prepareStatement("select * from product");
             ResultSet rs=ps.executeQuery();
 //            int i=0;
             while(rs.next()) {
                 Product product=new Product();

                 product.setPid(rs.getString(1));
                 product.setPname(rs.getString(2));
                 product.setMarket_price(rs.getInt(3));
                 product.setShop_price(rs.getInt(4));
                 product.setPimage(rs.getString(5));
                 product.setPdate(rs.getString(6));
                 product.setIs_hot(rs.getInt(7));
                 product.setPdate(rs.getString(8));
                 product.setPflag(rs.getInt(9));
                 product.setCid(rs.getString(10));

 //                System.out.println(product.getPname());
                 productList.add(product);
 //                System.out.println(productList.get(i).getPname());
 //                i++;
             }

             rs.close();
             ps.close();
             con.close();
         } catch (SQLException e) {
             e.printStackTrace();
         }

         return productList;
     }

 }

3.ProductListServlet

 package servlet;

 import java.io.IOException;
 import java.util.List;

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

 import dao.ProductListDao;
 import domain.Product;

 public class ProductListServlet extends HttpServlet {

     protected void doGet(HttpServletRequest request, HttpServletResponse response)
             throws ServletException, IOException {

         ProductListDao productListDao=new ProductListDao();
         List<Product> productList=productListDao.getAllProduct();
 //        System.out.println(productList.isEmpty());

         request.setAttribute("productList", productList);
         request.getRequestDispatcher("/product_list.jsp").forward(request, response);

     }

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

 }

4.product_list.jsp

 <%@ page language="java" contentType="text/html; charset=UTF-8"
     pageEncoding="UTF-8"%>
 <%@ page import="java.util.*"%>
 <%@ page import="domain.*"%>
 <!DOCTYPE html>
 <html>
 <head>
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>会员登录</title>
 <link rel="stylesheet" href="css/bootstrap.min.css" type="text/css" />
 <script src="js/jquery-1.11.3.min.js" type="text/javascript"></script>
 <script src="js/bootstrap.min.js" type="text/javascript"></script>
 <!-- 引入自定义css文件 style.css -->
 <link rel="stylesheet" href="css/style.css" type="text/css" />

 <style>
 body {
     margin-top: 20px;
     margin: 0 auto;
     width: 100%;
 }

 .carousel-inner .item img {
     width: 100%;
     height: 300px;
 }
 </style>
 </head>

 <body>

     <!-- 引入header.jsp -->
     <jsp:include page="/header.jsp"></jsp:include>

     <div class="row" style="width: 1210px; margin: 0 auto;">
         <div class="col-md-12">
             <ol class="breadcrumb">
                 <li><a href="#">首页</a></li>
             </ol>
         </div>

         <%
             List<Product> productList = (List<Product>) request.getAttribute("productList");

             if (productList != null) {
                 for (Product product : productList) {

                     out.write("<div class='col-md-2'>");
                     out.write("<a href='product_info.htm'> <img src='"+product.getPimage()+"' width='170' height='170' style='display: inline-block;'>");
                     out.write("</a>");
                     out.write("<p>");
                     out.write("<a href='product_info.html' style='color: green'>"+product.getPname()+"</a>");
                     out.write("</p>");
                     out.write("<p>");
                     out.write("<font color='#FF0000'>商城价：&yen;"+product.getShop_price()+"</font>");
                     out.write("</p>");
                     out.write("</div>");

                 }
             }
         %>

     </div>

     <!--分页 -->
     <div style="width: 380px; margin: 0 auto; margin-top: 50px;">
         <ul class="pagination" style="text-align: center; margin-top: 10px;">
             <li class="disabled"><a href="#" aria-label="Previous"><span
                     aria-hidden="true">&laquo;</span></a></li>
             <li class="active"><a href="#">1</a></li>
             <li><a href="#">2</a></li>
             <li><a href="#">3</a></li>
             <li><a href="#">4</a></li>
             <li><a href="#">5</a></li>
             <li><a href="#">6</a></li>
             <li><a href="#">7</a></li>
             <li><a href="#">8</a></li>
             <li><a href="#">9</a></li>
             <li><a href="#" aria-label="Next"> <span aria-hidden="true">&raquo;</span>
             </a></li>
         </ul>
     </div>
     <!-- 分页结束 -->

     <!--商品浏览记录-->
     <div
         style="width: 1210px; margin: 0 auto; padding: 0 9px; border: 1px solid #ddd; border-top: 2px solid #999; height: 246px;">

         <h4 style="width: 50%; float: left; font: 14px/30px 微软雅黑">浏览记录</h4>
         <div style="width: 50%; float: right; text-align: right;">
             <a href="">more</a>
         </div>
         <div style="clear: both;"></div>

         <div style="overflow: hidden;">

             <ul style="list-style: none;">
                 <li
                     style="width: 150px; height: 216; float: left; margin: 0 8px 0 0; padding: 0 18px 15px; text-align: center;"><img
                     src="products/1/cs10001.jpg" width="130px" height="130px" /></li>
             </ul>

         </div>
     </div>

     <!-- 引入footer.jsp -->
     <jsp:include page="/footer.jsp"></jsp:include>

 </body>

 </html>

结果：

http://localhost:8080/WEB17_2/ProductListServlet

注意：

加入List内的元素只保存了地址，而不是复制元素。将元素加入List后，修改该元素值，会影响到List内的元素值。所以每次放入一个新元素都需要new一个新地址。否则结果只有最后一个元素值，前面的元素值都被覆盖了。