通过Java语言生成MySQL数据库的增删改查操作，主要分为以下五个类：DBUtil类、Mess类、MessDAO类、ShowAllData类以及Main类。的设计思路

1. DBUtil类：用于数据库连接和关闭。

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

public class DBUtil {
    private static final String URL = "jdbc:mysql://localhost:3306/test?useSSL=false&serverTimezone=UTC";
    private static final String USER = "root";
    private static final String PASSWORD = "123456";

    public static Connection getConnection() {
        Connection conn = null;
        try {
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection(URL, USER, PASSWORD);
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }

    public static void closeConnection(Connection conn) {
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

2. Mess类：用于定义实体类，对应数据库中的表。

public class Mess {
    private int id;
    private String name;
    private String message;

    public Mess() {}

    public Mess(String name, String message) {
        this.name = name;
        this.message = message;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

3. MessDAO类：用于对数据库中的表进行增删改查操作。

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 MessDAO {
    private Connection conn;

    public MessDAO(Connection conn) {
        this.conn = conn;
    }

    public boolean addMess(Mess mess) {
        String sql = "insert into mess(name, message) values(?, ?)";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, mess.getName());
            pstmt.setString(2, mess.getMessage());
            pstmt.execute();
            pstmt.close();
            return true;
        } catch (SQLException e) {
            e.printStackTrace();
            return false;
        }
    }

    public boolean deleteMess(int id) {
        String sql = "delete from mess where id = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            pstmt.executeUpdate();
            pstmt.close();
            return true;
        } catch (SQLException e) {
            e.printStackTrace();
            return false;
        }
    }

    public boolean updateMess(Mess mess) {
        String sql = "update mess set name = ?, message = ? where id = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, mess.getName());
            pstmt.setString(2, mess.getMessage());
            pstmt.setInt(3, mess.getId());
            pstmt.executeUpdate();
            pstmt.close();
            return true;
        } catch (SQLException e) {
            e.printStackTrace();
            return false;
        }
    }

    public List<Mess> getAllMess() {
        List<Mess> list = new ArrayList<>();
        String sql = "select * from mess";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            ResultSet rs = pstmt.executeQuery();
            while (rs.next()) {
                Mess mess = new Mess();
                mess.setId(rs.getInt("id"));
                mess.setName(rs.getString("name"));
                mess.setMessage(rs.getString("message"));
                list.add(mess);
            }
            rs.close();
            pstmt.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return list;
    }
}

4. ShowAllData类：用于显示数据库中mess表中的所有数据。

import java.sql.Connection;
import java.util.List;

public class ShowAllData {
    public static void main(String[] args) {
        Connection conn = DBUtil.getConnection();
        MessDAO messDAO = new MessDAO(conn);
        List<Mess> list = messDAO.getAllMess();
        for (Mess mess : list) {
            System.out.println("ID：" + mess.getId() + "，姓名：" + mess.getName() + "，留言：" + mess.getMessage());
        }
        DBUtil.closeConnection(conn);
    }
}

5. Main类：用于测试增删改查操作。

import java.sql.Connection;

public class Main {
    public static void main(String[] args) {
        Connection conn = DBUtil.getConnection();
        MessDAO messDAO = new MessDAO(conn);

        // 添加
        Mess mess1 = new Mess("张三", "你好！");
        messDAO.addMess(mess1);
        Mess mess2 = new Mess("李四", "你好啊！");
        messDAO.addMess(mess2);

        // 修改
        Mess mess3 = new Mess();
        mess3.setId(1);
        mess3.setName("张三");
        mess3.setMessage("你好吗？");
        messDAO.updateMess(mess3);

        // 删除
        messDAO.deleteMess(2);

        DBUtil.closeConnection(conn);
    }
}