使用 servlets 打开 jsp 使用 id 编辑页面

Question

我对使用 Servlet 和 JSP 还很陌生，我正在尝试获取一个列出所有用户的 JSP 页面，以重定向到编辑表单 jsp，具体取决于在用户 ID 上，列出所有用户的 JSP 页面使用迭代器显示来自我的 JDBC table 的数据，但我似乎无法弄清楚如何单独为编辑 link 以便它可以加载带有该用户数据的编辑表单，我们将不胜感激。

这里是 jsp 页面的当前代码，用于列出所有学生

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1" import="java.util.*" import="model.Student"%>

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, intial-scale=1 shink-to-fit=yes">

    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" 
    integrity="sha384-..." crossorigin="anonymous">
</head>
<body>
    <nav class="navbar navbar-dark bg-primary pd-8">
         <a class="navbar-brand">&nbsp;&nbsp;&nbsp;&nbsp;...University</a>
    </nav>
    <div class="container-fluid">
    <div class="container">

            <div class="form container-fluid p-4">
               <a href="<%=request.getContextPath()%>/new" class="btn btn-success" >Add
                    Student</a>
            </div>
            <br>
            
            <!--Assigning ArrayList object containing student data to the local object -->
            <% ArrayList<Student> studentList = (ArrayList) request.getAttribute("listStudents"); %> 
            <table class="table table-bordered">
            
                <thead>
                    <tr>
                        <th>ID</th>
                        <th>Name</th>
                        <th>Email</th>
                        <th>Actions</th>
                    </tr>
                </thead>
                <tbody>
                    <!-- data from table displayed using iterator-->
                    <%
                     if(request.getAttribute("listStudents") != null)  {
                            Iterator<Student> iterator = studentList.iterator();
                            while(iterator.hasNext()) {
                                Student studentDetails = iterator.next();
                    %>
                        <tr><td><%=studentDetails.getId()%></td>
                            <td><%=studentDetails.getName()%></td>
                            <td><%=studentDetails.getEmail()%></td>
                            <td><a href="<%=request.getContextPath()%>/edit?id=<%=studentDetails.getId()%>">Update</a> <!-- id assigned to edit link-->
                                &nbsp;&nbsp;&nbsp;&nbsp; <a href="<%=request.getContextPath()%>/delete?id=<%=studentDetails.getId()%>">Delete</a></td>  <!-- id assigned to delete link-->
                        </tr>
                    <% 
                            }
                     }
                    %>
                </tbody>
                
            </table>
            </div>
        </div>
        
    </body>
</html>

然后应该将 id 值分配给 servlet 中的一个值，该值用于 select 来自 table 的特定学生，当调用编辑时

这是我的部分 servlet 代码：

import java.io.IOException;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.List;
import jakarta.servlet.RequestDispatcher;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import dao.StudentDao;
import model.Student;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

public class StudentServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    private StudentDao studentDao;

    public StudentServlet() {
        this.studentDao = new StudentDao();
    }

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

    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String sPath = request.getServletPath();
        //switch statement to call appropriate method
        switch (sPath) {
            case "/new":
                try {
                    showNewForm(request, response);
                } catch (ServletException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/insert":
                try {
                    insertStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                } 
                break;
            case "/delete":
                try {
                    deleteStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/update":
                try {
                    updateStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/edit":
                try {
                    editStudent(request, response);
                } catch (ServletException | IOException e) {
                    e.printStackTrace();
                }
                break;
            default:
                try {
                    listAllStudents(request, response); //home page = .../week04/StudentServlet
                } catch (ServletException | IOException | SQLException e) {
                    e.printStackTrace();
                } 
                break; 
            } 
    }

    private void editStudent(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException {
        int id = Integer.parseInt(request.getParameter("id"));
        Student currentStudent = studentDao.selectStudent(id);
        System.out.println(currentStudent);
        RequestDispatcher dispatch = request.getRequestDispatcher("student-form.jsp");
        request.setAttribute("student", currentStudent); 
        dispatch.forward(request, response);
    }

    private void listAllStudents(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException, SQLException {
        List<Student> allStudents = studentDao.selectAllStudents();
        request.setAttribute("listStudents", allStudents);
        RequestDispatcher dispatch = request.getRequestDispatcher("student-list.jsp");
        dispatch.forward(request, response);
    }
}

这是我的 DAO，以防我没有正确配置它：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import model.Student;

public class StudentDao {
    private String jdbcURL = "jdbc:mysql://localhost:3308/xyzuniversity";
    private String jdbcUsername = "User";
    private String jdbcPassword = "password";
    
    
    private static final String SELECT_STUDENT_ID = "SELECT name, email FROM student WHERE id =?";

    protected Connection getConnection() {
    
            Connection connection = null;
            try {
                Class.forName("com.mysql.cj.jdbc.Driver");
                connection = DriverManager.getConnection(jdbcURL, jdbcUsername, jdbcPassword);
            }catch (SQLException | ClassNotFoundException e) {
                e.printStackTrace();
            } 
            return connection;
        }

    public Student selectStudent(int id) {
        Student student = null;
        try {
            Connection connection = getConnection();
            PreparedStatement prepStatement = connection.prepareStatement(SELECT_STUDENT_ID);
            prepStatement.setInt(1, id);
            ResultSet rSet = prepStatement.executeQuery();
            while(rSet.next()) {
                String name = rSet.getString("name");
                String email = rSet.getString("email");
                student = new Student(id, name, email);
            }
        } catch (SQLException e) {
            e.printStackTrace();
            }
        return student;
    }

最后是应该显示的 jsp 表单

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, intial-scale=1 shink-to-fit=yes">

    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css"
     integrity="sha384..." crossorigin="anonymous">
</head>
<body>
    <nav class="navbar navbar-dark bg-primary pd-8">
         <a class="navbar-brand">&nbsp;&nbsp;&nbsp;&nbsp;XYZ University</a>
    </nav>
    <div class="container col-md-5 p-4">
        <div class="card">
            <div class="card-body">

                <% if (request.getAttribute("student") != null) { %>
               <!-- form display depends on whether there is data in the table -->
                    <form action="<%=request.getContextPath()%>/update" method="post">
                    <% } else { %>
                    <form action="<%=request.getContextPath()%>/insert" method="post">
                    <% } %>
    
                    <div>
                        <h2>
                        <% if (request.getAttribute("student") != null) { %>
                            Edit Student
                        <% } else { %>
                            Add New Student
                        <% } %>
                        </h2>
                    </div>

                <% if (request.getAttribute("student") != null) { %>
                    <input type="hidden" name="id" value="<%=request.getAttribute("student.id") %>" />
                <% } %>

                <fieldset class="form-group">
                <legend>Name</legend>
                <% if (request.getAttribute("student") != null) { %>
                   <input type="text" value="<%=request.getAttribute("student.name") %>" class="form-control" name="name" required="required">
                <% }  else { %>
                    <input type="text" value="" class="form-control" name="name" required="required">
                <% } %>
                </fieldset>

                <fieldset class="form-group">
                <legend>Email</legend>
                <% if (request.getAttribute("student") != null) { %>
                    <input type="text" value="<%=request.getAttribute("student.email") %>" class="form-control" name="email">
                <% }  else { %>
                    <input type="text" value="" class="form-control" name="email">
                <% } %>     
                </fieldset>
                <button type="submit" class="t-3 btn btn-success">Save</button>
                </form>
            </div>
        </div>
    </div>

</body>
</html>

我假设我通过将编辑 url 设置为 href="<%=request.getContextPath()%>/edit?id=<%=studentDetails.getId 获得了 id 值()%>" 但表单以空值打开，我不知道哪里出错了，也不知道现在如何分配值

真的不知道现在该怎么办，我已经尝试寻找解决方案，但 none 似乎对我的问题有所帮助。如有任何帮助，我们将不胜感激。

Answer 1

问题是您的 request 属性名称是 student

request.setAttribute("student", currentStudent);

但是您的属性查找代码正在寻找 student.id，然后 returns null.

<input type="hidden" name="id" value="<%=request.getAttribute("student.id") %>" />

因此，您需要先获取属性，然后获取其属性以将表单 <input> 填充为

<input type="hidden" name="id" value="<%= ((Student) request.getAttribute("student")).getId() %>" />
<input type="text" name="name" value="<%= ((Student) request.getAttribute("student")).getName() %>" ... />
<input type="text" name="email" value="<%= ((Student) request.getAttribute("student")).getEmail() %>" ... />

request.getAttribute() 方法的 return 类型是 Object，因此，在使用 getter 访问属性之前需要强制转换为 Student。这意味着 JSP 页面也需要一个 import 指令。

<%@ page import="model.Student" %>

但是，我建议使用 JSP EL 表达式语言 语法将这些查找缩短为

<input type="hidden" name="id" value="${student.id}" />
<input type="text" name="name" value="${student.name}" class="form-control" required="required" />
<input type="text" name="email" value="${student.email}" class="form-control" />

这看起来更简洁，更具可读性，无需类型转换并优雅地处理 null 值。您甚至不需要所有这些 null 检查，因为如果 student 属性为 null.

，value 将自动设置为 ""

所以，你所有的 form-groups

<fieldset class="form-group">
    <legend>First Name</legend>
    <% if (request.getAttribute("student") != null) { %>
        <input type="text" value="${student.firstname}" class="form-control" 
               name="firstname" required="required">
    <% }  else { %>
        <input type="text" value="" class="form-control" name="firstname" required="required">
    <% } %>
</fieldset>

可以使用 EL 简化为几行。

<fieldset class="form-group">
    <legend>First Name</legend>
    <input type="text" value="${student.firstname}" class="form-control"
           name="fistname" required="required">
</fieldset>

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