我的代码有问题。我正在制作一个经销商网站。我的属性得到一个空值。我的程序在JSP上运行我的ArrayList的foreach循环，我在每辆车上也有购买它们的按钮。当用户选择购买该商品时，假设取下他们选择的汽车并将其转发到购买页面。但出于某种奇怪的原因，当我到达购买页面时，汽车价值为零。任何帮助都将不胜感激。
Servlet：
package com.servlets;
import java.io.IOException;
import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import com.dealership.Car;
import com.dealership.Inventory;
/**
* Servlet implementation class PurchaseServlet
*/
@WebServlet("/PurchaseServlet")
public class PurchaseServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
/**
* @see HttpServlet#HttpServlet()
*/
public PurchaseServlet() {
super();
// TODO Auto-generated constructor stub
}
/**
* @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
HttpSession session = request.getSession(true);
Car purchaseCar = (Car)session.getAttribute("purchaseCar");
Inventory inventory = new Inventory();
if(purchaseCar == null) {
purchaseCar = new Car();
}
if (request.getParameter("purchase") != null) {
inventory.test(purchaseCar);
}
session.setAttribute("purchaseCar", purchaseCar);
RequestDispatcher rs = request.getRequestDispatcher("purchase.jsp");
rs.forward(request, response);
}
}
JSP:
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<jsp:include page="/HomepageServlet" />
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<!doctype html>
<html lang="en">
<head>
<!-- Required meta tags -->
<meta charset="utf-8">
<meta name="viewport"
content="width=device-width, initial-scale=1, shrink-to-fit=no">
<!-- Bootstrap CSS -->
<link rel="stylesheet"
href="https://cdn.jsdelivr.net/npm/bootstrap@4.5.3/dist/css/bootstrap.min.css"
integrity="sha384-TX8t27EcRE3e/ihU7zmQxVncDAy5uIKz4rEkgIXeMed4M0jlfIDPvg6uqKI2xXr2"
crossorigin="anonymous">
<!-- Local CSS -->
<link rel="stylesheet" href="styles.css" />
<!-- Link to font -->
<link rel="preconnect" href="https://fonts.gstatic.com">
<link
href="https://fonts.googleapis.com/css2?family=Cinzel:wght@600&display=swap"
rel="stylesheet">
<title> Tropical Auto</title>
</head>
<body>
<div>
<nav class="navbar navbar-expand-lg navbar-light bg-light">
<a class="navbar-brand" href="#">Nick's Tropical Autocenter</a> <a
class="navbar-brand" href="#"> <img src="mango.svg" width="30"
height="30" alt="" loading="lazy">
</a>
<button class="navbar-toggler" type="button" data-toggle="collapse"
data-target="#navbarNav" aria-controls="navbarNav"
aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarNav">
<ul class="navbar-nav">
<li class="nav-item active"><a class="nav-link"
href="index.jsp"> INVENTORY <span class="sr-only">(current)</span></a>
</li>
<li class="nav-item"><a class="nav-link" href="usedcars.jsp">USED
INVENTORY</a></li>
<li class="nav-item"><a class="nav-link" href="specials.jsp">SPECIALS</a>
</li>
<li class="nav-item"><a class="nav-link" href="#">ABOUT US</a>
</li>
</ul>
<form class="d-flex" action="SearchCarsServlet" method="post">
<input class="form-control me-2" type="search" name="searchInput"
placeholder="Search" aria-label="Search">
<button class="btn btn-outline-success" type="submit">Search</button>
</form>
</div>
</nav>
</div>
<div class=album py-5bg-light">
<div class="container">
<div class="row row-cols-1 row-cols-sm-2 row-cols-md-3 g-3">
<c:forEach var="car" items="${inventory.carList}">
<c:if
test="${car.used == false && car.residence < 120 && car.purchased == false}">
<div class="col">
<div class="card" style="width: 18rem;">
<img src=${car.image } class="card-img-top" alt="...">
<div class="card-body">
<p><span class="mandatory">${car.make}${car.model}</span></p>
<p class="card-text">${car.description}
</div>
<div class="d-flex justify-content-center align-items-center">
<form action="PurchaseServlet" method="POST">
<c:set var="purchaseCar" value="${car}" scope="application" />
<button type="submit" name="purchase" class="btn btn-success">Purchase</button>
</form>
<div class="dropdown">
<a class="btn btn-secondary dropdown-toggle" href="#"
role="button" id="dropdownMenuLink" data-toggle="dropdown"
aria-haspopup="true" aria-expanded="false"> Details </a>
<div class="dropdown-menu details-button"
aria-labelledby="dropdownMenuLink">
<a class="dropdown-item" href="#">Make: ${car.make}</a> <a
class="dropdown-item" href="#">Model: ${car.model}</a> <a
class="dropdown-item" href="#">Year: ${car.year}</a> <a
class="dropdown-item" href="#">Mileage: ${car.mileage}</a> <a
class="dropdown-item" href="#">Price: $${car.price}</a>
</div>
</div>
</div>
</div>
</div>
</c:if>
</c:forEach>
</div>
</div>
</div>
<!-- Optional JavaScript; choose one of the two! -->
<!-- Option 1: jQuery and Bootstrap Bundle (includes Popper) -->
<script src="https://code.jquery.com/jquery-3.5.1.slim.min.js"
integrity="sha384-DfXdz2htPH0lsSSs5nCTpuj/zy4C+OGpamoFVy38MVBnE+IbbVYUew+OrCXaRkfj"
crossorigin="anonymous"></script>
<script
src="https://cdn.jsdelivr.net/npm/bootstrap@4.5.3/dist/js/bootstrap.bundle.min.js"
integrity="sha384-ho+j7jyWK8fNQe+A12Hb8AhRq26LrZ/JpcUGGOn+Y7RsweNrtN/tE3MoK7ZeZDyx"
crossorigin="anonymous"></script>
</body>
</html>
有很多代码需要理解，但我认为问题就在这里：
<foreach>
<form>
<c:set var="purchaseCar" value="${car}" scope="application" />
<submit...
</form>
</foreach>
这里的会话范围就足够了。但真正的问题是，这是对所有记录执行的，并且每个记录都覆盖了前一个记录。因此，purchaseCar变量在每次迭代中都会被修改。
通常在表单中，您希望有<input>标记，它将所需的信息和请求一起发送到servlet。通常是汽车的ID：(