Java Web项目JSP显示数据库学生总数为null问题求助

我希望JSP页面显示数据库中的学生总数，但数据库内已有3条学生数据，页面却显示null。

以下是我编写的代码：

我的DAO类 #

import com.fyp.connection.MySqlConnection;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DashboardLecturerDAO {

    public int getTotalStudentCount() {
        int totalCount = 0;
        Connection connection = null;
        PreparedStatement preparedStatement = null;
        ResultSet resultSet = null;
        try {
            
            connection = new MySqlConnection().getConnection();
            
            String query = "SELECT COUNT(*) AS total FROM student";
            
            preparedStatement = connection.prepareStatement(query);
            
            resultSet = preparedStatement.executeQuery();
            
            if (resultSet.next()) {
                totalCount = resultSet.getInt("total");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            
            try {
                if (resultSet != null) resultSet.close();
                if (preparedStatement != null) preparedStatement.close();
                if (connection != null) connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return totalCount;
    }

}

我的Servlet类 #

import com.fyp.model.Dao.lecturer.DashboardLecturerDAO;
import java.io.IOException;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

@WebServlet(name = "Dashboard_LecturerServlet", urlPatterns = {"/Dashboard_LecturerServlet"})
public class Dashboard_LecturerServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        
        DashboardLecturerDAO dashboardLecturerDAO = new DashboardLecturerDAO();
        
        int totalCount = dashboardLecturerDAO.getTotalStudentCount();
        
        request.setAttribute("totalCount", totalCount);
        
        request.getRequestDispatcher("/Dashboard-Lecturer.jsp").forward(request, response);
    }
    
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        
    }
    
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
    
    @Override
    public String getServletInfo() {
        return "Short description";
    }

}

我的数据库连接类 #

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class MySqlConnection {
private String jdbcURL = "jdbc:mysql://localhost:3306/project";
private String jdbcUsername = "root";
private String jdbcPassword = "";

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

}

我的JSP代码 #

<p><%= request.getAttribute("totalCount") %></p>
  <p>Total Students</p>

排查与解决方法 #

1. 确保请求通过Servlet访问 #

直接访问JSP页面时，request中不会有totalCount属性，自然显示null。必须访问http://你的域名/项目名/Dashboard_LecturerServlet，让Servlet处理请求后转发到JSP。

2. 检查数据库连接有效性 #

在DashboardLecturerDAO的getTotalStudentCount方法中添加打印语句，确认数据库连接是否成功：

connection = new MySqlConnection().getConnection();
System.out.println("数据库连接状态：" + (connection != null ? "成功" : "失败"));

如果连接失败，检查：

• MySQL服务是否启动
• jdbcURL中的数据库名project是否存在
• 用户名root和密码是否正确

3. 验证SQL查询结果 #

在获取总数后添加打印，确认查询是否正确：

if (resultSet.next()) {
    totalCount = resultSet.getInt("total");
    System.out.println("查询到的学生总数：" + totalCount);
}

如果打印结果不是3，检查：

• student表名是否拼写正确（MySQL在区分大小写的系统中表名大小写敏感）
• 表中是否确实有3条有效数据（排除被逻辑删除的记录）

4. JSP代码优化（可选） #

用EL表达式替代脚本代码，更简洁且避免空指针显示问题：

<p>${totalCount}</p>
<p>Total Students</p>

内容的提问来源于stack exchange，提问作者Faris Zack