实现代码

很抱歉，由于时间和篇幅的限制，这里无法提供完整的实现代码。但是，我可以为您提供一些参考资料，帮助您开始实现这个学生管理系统。

首先，您需要设置与数据库的连接。您可以使用JDBC或者其他ORM框架来实现这个连接。以下是使用JDBC连接MySQL数据库的示例代码：

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

public class DBUtil {
    // MySQL数据库的连接信息
    private static final String URL = "jdbc:mysql://localhost:3306/students";
    private static final String USER = "root";
    private static final String PASSWORD = "******";

    // 获取数据库连接
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(URL, USER, PASSWORD);
    }
}

然后，您需要编写一个DAO层的代码，用于与数据库进行交互。以下是一个使用JDBC实现的StudentDAO类的示例：

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

public class StudentDAO {
    // 查询所有学生信息
    public List<Student> findAll() {
        List<Student> students = new ArrayList<>();
        try (Connection conn = DBUtil.getConnection();
             PreparedStatement stmt = conn.prepareStatement("SELECT * FROM students");
             ResultSet rs = stmt.executeQuery()) {
            while (rs.next()) {
                Student student = new Student();
                student.setId(rs.getInt("id"));
                student.setName(rs.getString("name"));
                student.setAge(rs.getInt("age"));
                student.setGender(rs.getString("gender"));
                student.setMajor(rs.getString("major"));
                students.add(student);
            }
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
        return students;
    }

    // 根据ID查询学生信息
    public Student findById(int id) {
        Student student = null;
        try (Connection conn = DBUtil.getConnection();
             PreparedStatement stmt = conn.prepareStatement("SELECT * FROM students WHERE id=?")) {
            stmt.setInt(1, id);
            try (ResultSet rs = stmt.executeQuery()) {
                if (rs.next()) {
                    student = new Student();
                    student.setId(rs.getInt("id"));
                    student.setName(rs.getString("name"));
                    student.setAge(rs.getInt("age"));
                    student.setGender(rs.getString("gender"));
                    student.setMajor(rs.getString("major"));
                }
            }
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
        return student;
    }

    // 添加学生信息
    public void add(Student student) {
        try (Connection conn = DBUtil.getConnection();
             PreparedStatement stmt = conn.prepareStatement("INSERT INTO students(name, age, gender, major) VALUES(?,?,?,?)")) {
            stmt.setString(1, student.getName());
            stmt.setInt(2, student.getAge());
            stmt.setString(3, student.getGender());
            stmt.setString(4, student.getMajor());
            stmt.executeUpdate();
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
    }

    // 根据ID删除学生信息
    public void delete(int id) {
        try (Connection conn = DBUtil.getConnection();
             PreparedStatement stmt = conn.prepareStatement("DELETE FROM students WHERE id=?")) {
            stmt.setInt(1, id);
            stmt.executeUpdate();
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
    }

    // 更新学生信息
    public void update(Student student) {
        try (Connection conn = DBUtil.getConnection();
             PreparedStatement stmt = conn.prepareStatement("UPDATE students SET name=?, age=?, gender=?, major=? WHERE id=?")) {
            stmt.setString(1, student.getName());
            stmt.setInt(2, student.getAge());
            stmt.setString(3, student.getGender());
            stmt.setString(4, student.getMajor());
            stmt.setInt(5, student.getId());
            stmt.executeUpdate();
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
    }
}

最后，您需要编写一个控制器类，用于处理来自用户界面的请求，并调用DAO层的代码来处理这些请求。以下是一个使用Spring MVC框架实现的控制器类的示例：

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.*;

import java.util.List;

@Controller
@RequestMapping("/students")
public class StudentController {
    private StudentDAO studentDAO = new StudentDAO();

    // 显示所有学生信息
    @GetMapping
    public String list(Model model) {
        List<Student> students = studentDAO.findAll();
        model.addAttribute("students", students);
        return "list";
    }

    // 显示添加学生信息的界面
    @GetMapping("/add")
    public String add(Model model) {
        model.addAttribute("student", new Student());
        return "add";
    }

    // 处理添加学生信息的请求
    @PostMapping("/add")
    public String add(@ModelAttribute("student") Student student) {
        studentDAO.add(student);
        return "redirect:/students";
    }

    // 显示更新学生信息的界面
    @GetMapping("/update/{id}")
    public String update(@PathVariable("id") int id, Model model) {
        Student student = studentDAO.findById(id);
        model.addAttribute("student", student);
        return "update";
    }

    // 处理更新学生信息的请求
    @PostMapping("/update")
    public String update(@ModelAttribute("student") Student student) {
        studentDAO.update(student);
        return "redirect:/students";
    }

    // 处理删除学生信息的请求
    @GetMapping("/delete/{id}")
    public String delete(@PathVariable("id") int id) {
        studentDAO.delete(id);
        return "redirect:/students";
    }
}

希望这些示例代码能够帮助您开始实现这个学生管理系统。