实现论坛管理，1、在mysql中创建论坛表，包含留言时间、留言IP地址、留言内容，2、在Servlet程序中实现发表留言、留言显示、删除留言

1. 在MySQL中创建论坛表

CREATE TABLE forum (
    id INT(11) NOT NULL AUTO_INCREMENT,
    message_time DATETIME NOT NULL,
    message_ip VARCHAR(50) NOT NULL,
    message_content VARCHAR(500) NOT NULL,
    PRIMARY KEY (id)
);

2. 在Servlet程序中实现发表留言、留言显示、删除留言

// 发表留言
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String content = request.getParameter("content");
    String ip = request.getRemoteAddr();
    LocalDateTime now = LocalDateTime.now();
    DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
    String formattedDateTime = now.format(formatter);

    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/forum", "root", "password");
        PreparedStatement ps = con.prepareStatement("INSERT INTO forum (message_time, message_ip, message_content) VALUES (?, ?, ?)");
        ps.setString(1, formattedDateTime);
        ps.setString(2, ip);
        ps.setString(3, content);
        ps.executeUpdate();
        con.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

    response.sendRedirect(request.getContextPath() + "/index.jsp");
}

// 留言显示
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    List<Message> messages = new ArrayList<Message>();

    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/forum", "root", "password");
        PreparedStatement ps = con.prepareStatement("SELECT * FROM forum ORDER BY message_time DESC");
        ResultSet rs = ps.executeQuery();

        while (rs.next()) {
            int id = rs.getInt("id");
            LocalDateTime time = LocalDateTime.parse(rs.getString("message_time"), DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"));
            String ip = rs.getString("message_ip");
            String content = rs.getString("message_content");

            Message message = new Message(id, time, ip, content);
            messages.add(message);
        }

        con.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

    request.setAttribute("messages", messages);
    request.getRequestDispatcher("/index.jsp").forward(request, response);
}

// 删除留言
protected void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    int id = Integer.parseInt(request.getParameter("id"));

    try {
        Class.forName("com.mysql.jdbc.Driver");
        Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/forum", "root", "password");
        PreparedStatement ps = con.prepareStatement("DELETE FROM forum WHERE id = ?");
        ps.setInt(1, id);
        ps.executeUpdate();
        con.close();
    } catch (Exception e) {
        e.printStackTrace();
    }

    response.setStatus(HttpServletResponse.SC_NO_CONTENT);
}

其中，`Message`类表示一条留言，包含`id`、`time`、`ip`和`content`四个字段。在留言显示时，将查询到的所有留言存储在一个`List<Message>`中，并将其传递给`index.jsp`页面进行展示。在删除留言时，根据留言的`id`进行删除操作，并返回`HttpServletResponse.SC_NO_CONTENT`状态码表示删除成功。