<!DOCTYPE web-app PUBLIC
        "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
        "http://java.sun.com/dtd/web-app_2_3.dtd" >
<web-app>
    <display-name>Archetype Created Web Application</display-name>
</web-app>

2.2 pom.xml

在pom.xml文件下增加以下代码片段，如果代码出现报红出错状态的话，点击右上角Maven 的刷新图标，等待即可。

<dependencies>
        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>3.1.0</version>
            <scope>provided</scope>
        </dependency>

        <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
            <version>2.12.6.1</version>
        </dependency>

        <!-- 引入  mysql  驱动包       -->
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>

    </dependencies>

2.3 ConfessionServelet 类

?用来存在服务器代码。

其中 @WebServlet("/confession")? 是客户端向服务器约定好的请求方式

import com.fasterxml.jackson.databind.ObjectMapper;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

class Confession {
    public String from;
    public String to;
    public String confession;
}

@WebServlet("/confession")
public class ConfessionServlet extends HttpServlet {
    private ObjectMapper objectMapper = new ObjectMapper();
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //处理提交消息请求
        Confession confession = objectMapper.readValue(req.getInputStream(),Confession.class);
        //保存到内存中
        // 通过 ContentType 来告知页面, 返回的数据是 json 格式.
        // 有了这样的声明, 此时 jquery ajax 就会自动的帮我们把字符串转成 js 对象.
        // 如果没有, jquery ajax 就只是当成字符串来处理的
        save(confession);
        resp.setContentType("application/json; charset=utf8");
        resp.getWriter().write("{\"ok\":true}");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取到消息列表，只要把消息列表中的内容整个都返回给客户端即可
        //此处需要使用objectMapper 把 Java 对象，转换成json 格式的字符串
        List<Confession> confessions = load();
        String jsonString = objectMapper.writeValueAsString(confessions);
        System.out.println("jsonString: " + jsonString);
        resp.setContentType("application/json; charset=utf8");
        resp.getWriter().write(jsonString);
    }

    private void save(Confession confession)  {
        //把第一条消息保存到数据库中
        Connection connection = null;
        PreparedStatement statement = null;

        //1.和数据库建立连接
        try {
            connection = DBUtil.getConnection();
            //2.构造 SQL
            String sql = "insert into confession values(?, ?, ?)";
            statement = connection.prepareStatement(sql);
            statement.setString(1,confession.from);
            statement.setString(2,confession.to);
            statement.setString(3,confession.confession);
            //3.执行SQL
            statement.executeUpdate();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            //关闭连接，释放资源
            DBUtil.close(connection,statement,null);
        }
    }

    private List<Confession> load() {
        //从数据库中获取到所有的消息
        List<Confession> confessions = new ArrayList<>();
        Connection connection = null;
        PreparedStatement statement = null;
        ResultSet resultSet = null;
        try {
            connection = DBUtil.getConnection();
            String sql = "select * from confession";
            statement = connection.prepareStatement(sql);
            resultSet = statement.executeQuery();
            while (resultSet.next()) {
                Confession confession = new Confession();
                confession.from = resultSet.getString("from");
                confession.from = resultSet.getString("to");
                confession.from = resultSet.getString("confession");
                confessions.add(confession);
            }
        } catch (SQLException throwables) {
            throwables.printStackTrace();
        } finally {
            DBUtil.close(connection, statement, resultSet);
        }
        return confessions;
    }

}

2.4 DBUtil 类

这个类是用来与Mysql 建立连接，将表白的话存放到 Mysql 中，使用到的是JDBC编程。

使用JDBC基本流程:
1.创建数据源
2.和数据库建立连接
3.构造sq|语句
4.执行sq|语句
5.如果是查询语句,需要遍历结果集.如果是插入/删除/修改,则不需要
6.关闭连接,释放资源?

import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;
import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

public class DBUtil {
    //与数据库建立连接
    private static final String URL = "jdbc:mysql://127.0.0.1:3306/java?characterEncoding=utf8&useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";

    private volatile static DataSource dataSource = null;

    private static DataSource getDataSource() {
        if (dataSource == null) {
            synchronized (DBUtil.class) {
                if (dataSource == null) {
                    dataSource = new MysqlDataSource();
                    ((MysqlDataSource)dataSource).setUrl(URL);
                    ((MysqlDataSource)dataSource).setUser(USERNAME);
                    ((MysqlDataSource)dataSource).setPassword(PASSWORD);
                }
            }
        }
        return dataSource;
    }

    public static Connection getConnection() throws SQLException {
        return getDataSource().getConnection();
    }

    public static void close(Connection connection, PreparedStatement statement, ResultSet resultSet) {
        if (resultSet != null) {
            try {
                resultSet.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (statement != null) {
            try {
                statement.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

在上述代码中

2.5 confessWall.html

这个代码是客户端代码

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>表白墙</title>
</head>

<body>
    <style>
        /* 整个页面放背景图的，html, 不可少*/
        
        html,
        body {
            height: 100%;
        }
        
        body {
            background-image: url(wall.jpg);
            background-repeat: no-repeat;
            background-position: center center;
            background-size: cover;
        }
        
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }
        
        .container {
            width: 100%;
        }
        
        h3 {
            text-align: center;
            padding: 30px 0;
            font-size: 24px;
            color: black;
        }
        
        p {
            text-align: center;
            color: rgb(241, 236, 35);
            padding: 10px 0;
        }
        
        .row {
            width: 400px;
            height: 50px;
            margin: 0 auto;
            display: flex;
            justify-content: center;
            align-items: center;
        }
        
        .row span {
            width: 60px;
            font-size: 20px;
            /* color: azure; */
        }
        
        .row input {
            width: 300px;
            height: 40px;
            line-height: 40px;
            font-size: 20px;
            text-indent: 0.5em;
            /* 去掉输入框的轮廓线 */
            outline: none;
        }
        
        .row #submit {
            width: 200px;
            height: 40px;
            font-size: 20px;
            line-height: 40px;
            margin: 0 auto;
            color: white;
            background-color: rgb(241, 238, 35);
            /* 去掉框的轮廓线 */
            border: none;
            border-radius: 10px;
        }
        
        .row #submit:active {
            background-color: gray;
        }
    </style>


    <div class="container">
        <h3>表白墙</h3>
        <p>欢迎来到表白墙，勇敢说出你心灵深处的爱吧！</p>
        <div class="row">
            <span>谁：</span>
            <input type="text">
        </div>
        <div class="row">
            <span>对谁：</span>
            <input type="text">
        </div>
        <div class="row">
            <span>说：</span>
            <input type="text">
        </div>
        <div class="row">
            <button id="submit">提交</button>
        </div>
    </div>

    <script src="https://cdn.bootcdn.net/ajax/libs/jquery/3.6.0/jquery.min.js"></script>
    <script>
        //加入Ajax的代码，此处加入的逻辑有两部分
        //1) 点击按钮提交的时候，Ajax 要构造数据发送给服务器，
        // 2） 页面加载的时候，从服务器获取消息列表，并在界面上显示
        function getMessages() {
            $.ajax({
                type: 'get',
                url: 'confession',
                success: function(body) {
                    //当前body 已经是一个js 对象数组了，Ajax 会根据响应的 content type 来进行解析
                    //如果服务器返回的content-type 已经是application/json 了，Ajax就会把body自动转换成js对象
                    //如果客户端没有自动链接，也可以通过JSON.parse() 这个函数自动转换

                    //依次来取数组转换的每个元素
                    let container = document.querySelector('.container');
                    for (let confession of body) {
                        let div = document.createElement('div');
                        div.innerHTML = from + ' 对 ' + to + ' 说: ' + msg;
                        div.className = 'row';
                        container.appendChild(div);
                    }
                }
            });
        }
        //加上函数调用
        getMessages();

        // 当用户点击 submit, 就会获取到 input 中的内容, 从而把内容构造成一个 div, 插入到页面末尾
        let submitBtn = document.querySelector('#submit');
        submitBtn.onclick = function() {
            // 1.获取到3个input中的内容
            let inputs = document.querySelectorAll('input');
            let from = inputs[0].value;
            let to = inputs[1].value;
            let msg = inputs[2].value;
            if (from == '' || to == '' || msg == '') {
                //用户没填写完，暂时部提交数据
                ruturn;
            }
            // 2. 生成一个新的 div, 内容就是 input 里的内容. 把这个新的 div 加到页面中. 
            let div = document.createElement('div');
            div.innerHTML = from + ' 对 ' + to + ' 说: ' + msg;
            div.className = 'row';
            let container = document.querySelector('.container');
            container.appendChild(div);
            // 3. 清空之前输入框的内容. 
            for (let i = 0; i < inputs.length; i++) {
                inputs[i].value = '';
            }

            //4.把当前获取到的输入框内容，构造成一个HTTP post 请求，通过Ajax 发送给服务器
            let body = {
                from: from,
                to: to,
                confession: msg
            };
            $.ajax({
                type: "post",
                url: "confession",
                contentType: "application/json;charset=utf8",
                data: JSON.stringify(body),
                success: function(body) {
                    alert("表白成功");

                },
                error: function() {
                    alert("表白失败");
                }
            })
        }
    </script>
</body>

</html>

在上述代码中