结果显示如下：

首选是创建一个实体类LeaveAMessage.java作为留言实体对象

package bean;

import lombok.Data;

import java.io.Serializable;
import java.util.Date;

/**
 * 留言实体类
 *
 * @author lck100
 */
@Data
public class LeaveAMessage implements Serializable {
    private int id;// 留言id
    private int userId;// 用户id
    private Date date;// 日期
    private String title;// 留言标题
    private String content;// 留言内容

    /**
     * 空的构造方法
     */
    public LeaveAMessage() {

    }

    /**
     * 带参数的构造方法
     *
     * @param userId  用户id
     * @param date    日期
     * @param title   标题
     * @param content 内容
     */
    public LeaveAMessage(int userId, Date date, String title, String content) {
        this.userId = userId;
        this.date = date;
        this.title = title;
        this.content = content;
    }

    public int getId() {
        return id;
    }

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

    public int getUserId() {
        return userId;
    }

    public void setUserId(int userId) {
        this.userId = userId;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    @Override
    public String toString() {
        return "LeaveAMessage{" +
                "id=" + id +
                ", userId=" + userId +
                ", date=" + date +
                ", title='" + title + '\'' +
                ", content='" + content + '\'' +
                '}';
    }
}

接着是创建数据库表

# 留言表
create table leaveAMessage
(
  id      int primary key not null auto_increment comment 'id编号',
  userId  int             not null comment '用户id编号',
  date    datetime        not null comment '留言发表时间',
  title   varchar(20)     not null comment '留言标题',
  content varchar(500)    not null comment '留言内容'
)

然后是创建操作数据库留言表的方法：

package dao;

import bean.LeaveAMessage;
import utils.JDBCUtils;

import java.sql.*;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;

/**
 * 留言dao
 *
 * @author lck100
 */
public class LeaveAMessageDao {
    /**
     * 查询留言表中的所有留言
     *
     * @return 返回所有留言的结果集合
     */
    public List<LeaveAMessage> checkAll() {
        // 创建一个集合来放置从数据库查询到的所有LeaveAMessage对象
        List<LeaveAMessage> list = new ArrayList<>();
        try {
            // 获取数据库连接对象
            Connection connection = JDBCUtils.getConnection();
            // 查询语句
            String sql = "select id, userid, date, title, content from leaveamessage";
            // 创建数据库执行对象
            PreparedStatement ps = connection.prepareStatement(sql);
            // 获取结果集对象
            ResultSet rs = ps.executeQuery();
            // 遍历结果集
            while (rs.next()) {
                // 实例化LeaveAMessage对象
                LeaveAMessage lam = new LeaveAMessage();
                // 设置留言id
                lam.setId(rs.getInt(1));
                // 设置用户id
                lam.setUserId(rs.getInt(2));
                // 设置留言日期
                lam.setDate(rs.getDate(3));
                // 设置留言标题
                lam.setTitle(rs.getString(4));
                // 设置留言内容
                lam.setContent(rs.getString(5));
                // 将留言对象添加到集合中
                list.add(lam);
            }
            // 返回查询到的结果集合
            return list;
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return null;
    }

    /**
     * 插入留言
     *
     * @param lam 留言对象
     * @return 返回受影响行数
     */
    public int insertLeaveAMessage(LeaveAMessage lam) {
        try {
            // 获取数据库连接对象
            Connection connection = JDBCUtils.getConnection();
            // 数据库语句
            String sql = "insert into leaveamessage(userId, date, title, content) values (?,?,?,?);";
            // 创建数据库执行对象
            PreparedStatement ps = connection.prepareStatement(sql);
            // 设置参数
            ps.setInt(1, lam.getUserId());
            // 需要注意的是，Java的日期格式和数据库的日期格式是不一致的，需要进行转换
            ps.setDate(2, Date.valueOf(new SimpleDateFormat("yyyy-MM-dd").format(lam.getDate())));
            ps.setString(3, lam.getTitle());
            ps.setString(4, lam.getContent());
            // 执行插入语句并返回受影响行
            int i = ps.executeUpdate();
            // 返回受影响行数
            return i;
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return 0;
    }
}

对写的方法进行简单测试，查看是否能够成功从数据库中查询出来数据。

package dao;

import bean.LeaveAMessage;
import org.junit.Test;

import java.util.Date;
import java.util.List;

public class LeaveAMessageTest {
    private LeaveAMessageDao leaveAMessageDao = new LeaveAMessageDao();

    /**
     * 测试查询所有留言方法
     */
    @Test
    public void testCheckAll() {
        List<LeaveAMessage> list = leaveAMessageDao.checkAll();
        for (LeaveAMessage leaveAMessage : list) {
            System.out.println(leaveAMessage);
        }
    }

    /**
     * 测试插入留言方法
     */
    @Test
    public void testInsertLeaveAMessage(){
        LeaveAMessage leaveAMessage=new LeaveAMessage(1,new Date(),"诗词","人生若只如初见");
        int i = leaveAMessageDao.insertLeaveAMessage(leaveAMessage);
        System.out.println(i);
    }
}

测试成功后，接着往下写：

留言信息显示面板，显示所有的留言信息的messageBoard.jsp

<%@ page import="bean.LeaveAMessage" %>
<%@ page import="java.util.ArrayList" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>留言板信息</title>
</head>
<body>
<form action="leaveAMessage.jsp" method="get" id="app">
    <table border="1">
        <caption>所有留言信息</caption>
        <tr>
            <th>留言人姓名</th>
            <th>留言时间</th>
            <th>留言标题</th>
            <th>留言内容</th>
        </tr>
        <%
            // 获取保存在session中的留言信息
            ArrayList<LeaveAMessage> leaveAMessage = (ArrayList) session.getAttribute("leaveAMessage");
            for (LeaveAMessage aMessage : leaveAMessage) {
        %>
        <tr>
            <td><%=
            session.getAttribute("username")
            %>
            </td>
            <td><%=
            aMessage.getDate()
            %>
            </td>
            <td><%=
            aMessage.getTitle()
            %>
            </td>
            <td><%=
            aMessage.getContent()
            %>
            </td>
        </tr>
        <%
            }
        %>
    </table>
    <input type="submit" value="留言">
</form>
</body>
</html>

以及写留言的页面leaveMessage.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>留言</title>
</head>
<body>
<form action="leaveAMessageServlet" method="post">
    <table>
        <caption>填写留言信息</caption>
        <tr>
            <td>留言标题</td>
            <td><input type="text" name="title"></td>
        </tr>
        <tr>
            <td>留言内容</td>
            <td><textarea name="content" cols="30" rows="10"></textarea></td>
        </tr>
    </table>
    <br><br>
    <input type="submit" value="提交">
    <input type="reset" value="重置">
</form>
</body>
</html>

对留言的处理LeaveAMessageServlet.java

package servlet;

import bean.LeaveAMessage;
import dao.LeaveAMessageDao;
import dao.UserDao;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.Serializable;
import java.util.Date;

/**
 * 留言Servlet
 *
 * @author lck100
 */
public class LeaveAMessageServlet extends HttpServlet implements Serializable {
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
        // 设置请求编码
        request.setCharacterEncoding("UTF-8");
        // 设置响应编码
        response.setContentType("UTF-8");
        // 获取请求的session
        HttpSession session = request.getSession();
        // 获取留言标题
        String title = request.getParameter("title");
        // 获取留言内容
        String content = request.getParameter("content");
        // 实例化LeaveAMessage对象
        LeaveAMessage lam = new LeaveAMessage();
        // 设置用户id，用户id通过使用名字用数据库查询得来
        lam.setUserId(new UserDao().checkUserByName(session.getAttribute("username").toString()).getId());
        // 设置日期，日期为当前日期
        lam.setDate(new Date());
        // 设置留言标题
        lam.setTitle(title);
        // 设置留言内容
        lam.setContent(content);
        // 插入数据到数据库
        int i = new LeaveAMessageDao().insertLeaveAMessage(lam);
        if (i > 0) {
            session.setAttribute("success", "留言成功，点击<a href='messageBoard.jsp'>这里</a>返回主界面");
            request.getRequestDispatcher("success.jsp").forward(request, response);
        } else {
            session.setAttribute("error", "留言失败，点击<a href='leaveAMessage.jsp'>这里</a>返回留言界面");
            request.getRequestDispatcher("error.jsp").forward(request, response);
        }
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }
}

和在web.xml中的配置

    <!--留言Servlet-->
    <servlet>
        <servlet-name>LeaveAMessageServlet</servlet-name>
        <servlet-class>servlet.LeaveAMessageServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>LeaveAMessageServlet</servlet-name>
        <url-pattern>/leaveAMessageServlet</url-pattern>
    </servlet-mapping>