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。3、实现留言查询（可选）。

1. 在MySQL中创建论坛表：

CREATE TABLE forum (
  id INT(11) NOT NULL AUTO_INCREMENT,
  time DATETIME NOT NULL,
  ip VARCHAR(50) NOT NULL,
  content TEXT NOT NULL,
  PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

2. 在Servlet程序中实现发表留言、留言显示、删除留言：

public class ForumServlet extends HttpServlet {
    
    private Connection conn;
    
    public void init() throws ServletException {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");
            String url = "jdbc:mysql://localhost:3306/forum";
            String username = "root";
            String password = "root";
            conn = DriverManager.getConnection(url, username, password);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }
    
    public void destroy() {
        try {
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
    
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String action = request.getParameter("action");
        if ("add".equals(action)) {
            addMessage(request, response);
        } else if ("delete".equals(action)) {
            deleteMessage(request, response);
        } else {
            showMessages(request, response);
        }
    }
    
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        doGet(request, response);
    }
    
    private void addMessage(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String content = request.getParameter("content");
        if (content != null && !content.isEmpty()) {
            try {
                PreparedStatement ps = conn.prepareStatement("INSERT INTO forum (time, ip, content) VALUES (NOW(), ?, ?)");
                ps.setString(1, request.getRemoteAddr());
                ps.setString(2, content);
                ps.executeUpdate();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        response.sendRedirect(request.getContextPath() + "/ForumServlet");
    }
    
    private void showMessages(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        try {
            PreparedStatement ps = conn.prepareStatement("SELECT * FROM forum ORDER BY time DESC");
            ResultSet rs = ps.executeQuery();
            List<Map<String, Object>> messages = new ArrayList<>();
            while (rs.next()) {
                Map<String, Object> message = new HashMap<>();
                message.put("id", rs.getInt("id"));
                message.put("time", rs.getTimestamp("time"));
                message.put("ip", rs.getString("ip"));
                message.put("content", rs.getString("content"));
                messages.add(message);
            }
            request.setAttribute("messages", messages);
            request.getRequestDispatcher("/forum.jsp").forward(request, response);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
    
    private void deleteMessage(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        String idStr = request.getParameter("id");
        if (idStr != null && !idStr.isEmpty()) {
            try {
                int id = Integer.parseInt(idStr);
                PreparedStatement ps = conn.prepareStatement("DELETE FROM forum WHERE id = ?");
                ps.setInt(1, id);
                ps.executeUpdate();
            } catch (NumberFormatException | SQLException e) {
                e.printStackTrace();
            }
        }
        response.sendRedirect(request.getContextPath() + "/ForumServlet");
    }
    
}

3. 实现留言查询：

private void showMessages(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    String keyword = request.getParameter("keyword");
    try {
        PreparedStatement ps;
        if (keyword != null && !keyword.isEmpty()) {
            ps = conn.prepareStatement("SELECT * FROM forum WHERE content LIKE ? ORDER BY time DESC");
            ps.setString(1, "%" + keyword + "%");
        } else {
            ps = conn.prepareStatement("SELECT * FROM forum ORDER BY time DESC");
        }
        ResultSet rs = ps.executeQuery();
        List<Map<String, Object>> messages = new ArrayList<>();
        while (rs.next()) {
            Map<String, Object> message = new HashMap<>();
            message.put("id", rs.getInt("id"));
            message.put("time", rs.getTimestamp("time"));
            message.put("ip", rs.getString("ip"));
            message.put("content", rs.getString("content"));
            messages.add(message);
        }
        request.setAttribute("messages", messages);
        request.getRequestDispatcher("/forum.jsp").forward(request, response);
    } catch (SQLException e) {
        e.printStackTrace();
    }
}