html+sevrlet+mysql+dao实现注册登录

首先，需要在数据库中创建一个用户表，包含id、username、password等字段。

然后，可以使用以下步骤实现注册登录功能：

1. 注册功能

在注册页面中，用户填写用户名和密码，点击提交按钮后，将数据通过POST方式传递给servlet。在servlet中，根据提交的数据创建一个新用户并插入数据库中。

JSP页面：

<form action="register" method="post">
    <input type="text" name="username" placeholder="用户名"><br>
    <input type="password" name="password" placeholder="密码"><br>
    <button type="submit">注册</button>
</form>

Servlet代码：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String username = request.getParameter("username");
    String password = request.getParameter("password");
    User user = new User(username, password);
    UserDao userDao = new UserDao();
    boolean result = userDao.addUser(user);
    if (result) {
        response.sendRedirect("login.jsp");
    } else {
        response.sendRedirect("register.jsp");
    }
}

UserDao代码：

public boolean addUser(User user) {
    Connection conn = null;
    PreparedStatement ps = null;
    try {
        conn = DBUtil.getConnection();
        String sql = "INSERT INTO user (username, password) VALUES (?, ?)";
        ps = conn.prepareStatement(sql);
        ps.setString(1, user.getUsername());
        ps.setString(2, user.getPassword());
        int result = ps.executeUpdate();
        return result > 0;
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DBUtil.close(conn, ps, null);
    }
    return false;
}

2. 登录功能

在登录页面中，用户填写用户名和密码，点击提交按钮后，将数据通过POST方式传递给servlet。在servlet中，根据提交的数据查询数据库中是否存在该用户，如果存在且密码正确，则跳转到主页；否则返回登录页面。

JSP页面：

<form action="login" method="post">
    <input type="text" name="username" placeholder="用户名"><br>
    <input type="password" name="password" placeholder="密码"><br>
    <button type="submit">登录</button>
</form>

Servlet代码：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    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)) {
        HttpSession session = request.getSession();
        session.setAttribute("user", user);
        response.sendRedirect("index.jsp");
    } else {
        response.sendRedirect("login.jsp");
    }
}

UserDao代码：

public User getUserByUsername(String username) {
    Connection conn = null;
    PreparedStatement ps = null;
    ResultSet rs = null;
    try {
        conn = DBUtil.getConnection();
        String sql = "SELECT * FROM user WHERE username=?";
        ps = conn.prepareStatement(sql);
        ps.setString(1, username);
        rs = ps.executeQuery();
        if (rs.next()) {
            int id = rs.getInt("id");
            String password = rs.getString("password");
            return new User(id, username, password);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    } finally {
        DBUtil.close(conn, ps, rs);
    }
    return null;
}

这里使用了一个DBUtil工具类来获取数据库连接和关闭资源，具体实现可以参考以下代码：

public class DBUtil {
    private static final String URL = "jdbc:mysql://localhost:3306/test?useSSL=false&characterEncoding=utf8";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "root";
    
    public static Connection getConnection() throws SQLException {
        return DriverManager.getConnection(URL, USERNAME, PASSWORD);
    }

    public static void close(Connection conn, Statement stmt, ResultSet rs) {
        try {
            if (rs != null) rs.close();
            if (stmt != null) stmt.close();
            if (conn != null) conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

以上就是使用html+sevrlet+mysql+dao实现注册登录的步骤，希望对你有所帮助。