JSP中继续出现列表为空的错误？

Question

我想将数据库中的数据值放入

Employee

类中，然后在JSP文件中显示所有值。但是，继续出现错误，提示列表为空。

员工.java

package employee;


import java.util.Date;

public class Employee {
    private int id;
    private String firstName;
    private String lastName;
    private String email;
    private Date hireDate;

    public Employee() {
    }

   
    public Employee(int id, String firstName, String lastName, String email, Date hireDate) {
        this.id = id;
        this.firstName = firstName;
        this.lastName = lastName;
        this.email = email;
        this.hireDate = hireDate;
    }


    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public Date getHireDate() {
        return hireDate;
    }

    public void setHireDate(Date hireDate) {
        this.hireDate = hireDate;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "id=" + id +
                ", firstName='" + firstName + '\'' +
                ", lastName='" + lastName + '\'' +
                ", email='" + email + '\'' +
                ", hireDate=" + hireDate +
                '}';
    }
}

EmployeeServlet.java

package employee;

import jakarta.servlet.RequestDispatcher;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;

@WebServlet(urlPatterns="/employees", loadOnStartup = 1)
public class EmployeeServlet extends HttpServlet {
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<Employee> employees = new ArrayList<>();

        String url = "jdbc:mariadb://127.0.0.1:3306/test03";
        String user = "root";
        String password = "1234";

        try {
            Class.forName("org.mariadb.jdbc.Driver");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            throw new ServletException("JDBC Driver not found.");
        }


        try (Connection conn = DriverManager.getConnection(url, user, password);
             Statement stmt = conn.createStatement();
             ResultSet rs = stmt.executeQuery("SELECT id, first_name, last_name, email, hire_date FROM employees")) {

            while (rs.next()) {
                Employee employee = new Employee();
                employee.setId(rs.getInt("id"));
                employee.setFirstName(rs.getString("first_name"));
                employee.setLastName(rs.getString("last_name"));
                employee.setEmail(rs.getString("email"));
                employee.setHireDate(rs.getDate("hire_date"));
                employees.add(employee);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }

        request.setAttribute("employees", employees);
        RequestDispatcher dispatcher = request.getRequestDispatcher("/employees.jsp");
        dispatcher.forward(request, response);
    }
}

employees.jsp

<%@ page import="java.util.List" %>
<%@ page import="employee.Employee" %>
<%@ page import="java.util.ArrayList" %>


<%
    List<Employee> employees = (List<Employee>) request.getAttribute("employees");
    if (employees == null || employees.isEmpty()) {
        System.out.println("Employee list is empty or null.");
    }
%>
<table border="1">
    <tr>
        <th>ID</th>
        <th>First Name</th>
        <th>Last Name</th>
        <th>Email</th>
        <th>Hire Date</th>
    </tr>
    <%
        if (employees != null) {
            for (Employee employee : employees) {
    %>
    <tr>
        <td><%= employee.getId() %></td>
        <td><%= employee.getFirstName() %></td>
        <td><%= employee.getLastName() %></td>
        <td><%= employee.getEmail() %></td>
        <td><%= employee.getHireDate() %></td>
    </tr>
    <%
        }
    } else {
    %>
    <tr>
        <td colspan="5">No data found.</td>
    </tr>
    <%
        }
    %>
</table>

数据明显存在，并且似乎已正确连接，但当我运行它时，它是空的。

Answer 1

您没有在 servlet 中设置 request 属性，因为您使用了请求 url

http://localhost:8080/ex01_war/employees.jsp

代码

request.getAttribute("employees")

返回

null

。

当 JSP 从 servlet 转发到 JSP 时，它会保留其属性。

但是你应该使用

http://localhost:8080/ex01_war/employees

。

如果您在浏览器中手动输入此 url，则会执行 Http GET 请求，该请求由

doGet()

方法处理。然后填写员工列表并设置请求属性

request.setAttribute("employees", employees);

，然后转发到 JSP。

当 JSP 渲染时，它将获取先前在 servlet 中设置的 request 属性。因此，它不会是

null

并且列表将在

for

循环中迭代。