使用MVC模式链接数据库实现用户注册登录功能，要求有User类，UserDao类，UserController类

好的，我来回答你的问题。

首先，MVC是一种常用的软件架构模式，它将应用程序分为三个核心部分：模型（Model）、视图（View）和控制器（Controller）。

在这个应用程序中，我们需要实现用户注册登录功能，因此需要三个类：User类、UserDao类和UserController类。

User类是一个简单的Java类，它包含了用户的基本信息，如用户名、密码、邮箱等。它的代码如下：

public class User {
    private int id;
    private String username;
    private String password;
    private String email;

    // 构造方法
    public User(int id, String username, String password, String email) {
        this.id = id;
        this.username = username;
        this.password = password;
        this.email = email;
    }

    // getter和setter方法
    public int getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getEmail() {
        return email;
    }

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

UserDao类是一个数据访问对象类，它用于与数据库进行交互，实现对用户信息的增删改查等操作。它的代码如下：

public class UserDao {
    // 连接数据库
    private Connection conn;

    // 构造方法
    public UserDao() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/test";
            String username = "root";
            String password = "password";
            conn = DriverManager.getConnection(url, username, password);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    // 添加用户
    public boolean addUser(User user) {
        boolean flag = false;
        PreparedStatement pstmt = null;
        try {
            String sql = "insert into user(username,password,email) values(?,?,?)";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getUsername());
            pstmt.setString(2, user.getPassword());
            pstmt.setString(3, user.getEmail());
            int count = pstmt.executeUpdate();
            if (count > 0) {
                flag = true;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return flag;
    }

    // 根据用户名查询用户
    public User getUserByUsername(String username) {
        User user = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        try {
            String sql = "select * from user where username=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            rs = pstmt.executeQuery();
            if (rs.next()) {
                int id = rs.getInt("id");
                String password = rs.getString("password");
                String email = rs.getString("email");
                user = new User(id, username, password, email);
            }
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                if (rs != null) {
                    rs.close();
                }
                if (pstmt != null) {
                    pstmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return user;
    }
}

UserController类是一个控制器类，它用于处理用户的请求和响应，实现用户注册登录等功能。它的代码如下：

public class UserController {
    // 用户注册
    public String register(HttpServletRequest request, HttpServletResponse response) {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        String email = request.getParameter("email");
        User user = new User(0, username, password, email);
        UserDao userDao = new UserDao();
        boolean flag = userDao.addUser(user);
        if (flag) {
            return "register_success.jsp";
        } else {
            return "register_failure.jsp";
        }
    }

    // 用户登录
    public String login(HttpServletRequest request, HttpServletResponse response) {
        String username = request.getParameter("username");
        String password = request.getParameter("password");
        UserDao userDao = new UserDao();
        User user = userDao.getUserByUsername(username);
        if (user != null && user.getPassword().equals(password)) {
            return "login_success.jsp";
        } else {
            return "login_failure.jsp";
        }
    }
}

以上就是使用MVC模式链接数据库实现用户注册登录功能的完整示例代码。