10.3使用Servlet写一个表白墙网站

一.创建项目,引入依赖和前端页面.

1.注意,静态页面放在webapp目录下,不是WEB-INF,

2.存档约定:前端使用post(/messageWall/message)发送http请求,使用json格式把数据传输到后端.服务器返回HTTP/1.1 200 ok.

3.读档约定:使用get(/messageWall/message),服务器返回HTTP/1.1 200 ok,和json数组.

4.使用maven管理数据库资源包.

5.在mysql客户端创建数据库和表.

二.代码

1.pom.xml:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>hello_servlet2023-9-18</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <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>2.14.2</version>
        </dependency>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.49</version>
        </dependency>

    </dependencies>
    <packaging>war</packaging>
    <build>
        <finalName>messageWall</finalName>
    </build>
</project>

2.web.xml:

<!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>

3.messageWall.html:

<!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>
    <style>
        /* * 通配符选择器, 是选中页面所有元素 */
        * {
            /* 消除浏览器的默认样式. */
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        .container {
            width: 600px;
            margin: 20px auto;
        }

        h1 {
            text-align: center;
        }

        p {
            text-align: center;
            color: #666;
            margin: 20px 0;
        }

        .row {
            /* 开启弹性布局 */
            display: flex;
            height: 40px;
            /* 水平方向居中 */
            justify-content: center;
            /* 垂直方向居中 */
            align-items: center;
        }

        .row span {
            width: 80px;
        }

        .row input {
            width: 200px;
            height: 30px;
        }

        .row button {
            width: 280px;
            height: 30px;
            color: white;
            background-color: orange;
            /* 去掉边框 */
            border: none;
            border-radius: 5px;
        }

        /* 点击的时候有个反馈 */
        .row button:active {
            background-color: grey;
        }
    </style>
</head>
<body>
<div class="container">
    <h1>表白墙</h1>
    <p>输入内容后点击提交, 信息会显示到下方表格中</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 class="row">
        xxx 对 xx 说 xxxx
    </div> -->
</div>

<script src="https://cdn.staticfile.org/jquery/1.10.2/jquery.min.js"></script>

<script>
        // 实现提交操作. 点击提交按钮, 就能够把用户输入的内容提交到页面上显示.
        // 点击的时候, 获取到三个输入框中的文本内容
        // 创建一个新的 div.row 把内容构造到这个 div 中即可.
        let containerDiv = document.querySelector('.container');
        let inputs = document.querySelectorAll('input');
        let button = document.querySelector('#submit');
        button.onclick = function() {
            // 1. 获取到三个输入框的内容
            let from = inputs[0].value;
            let to = inputs[1].value;
            let msg = inputs[2].value;
            if (from == '' || to == '' || msg == '') {
                return;
            }
            // 2. 构造新 div
            let rowDiv = document.createElement('div');
            rowDiv.className = 'row message';
            rowDiv.innerHTML = from + ' 对 ' + to + ' 说: ' + msg;
            containerDiv.appendChild(rowDiv);
            // 3. 清空之前的输入框内容
            for (let input of inputs) {
                input.value = '';
            }

            // 4. 把用户输入的数据, 构造出 HTTP 请求, 发送给服务器.
            let body = {
                from: from,
                to: to,
                message: msg
            };
            $.ajax({
                type: 'post',
                // url: '/messageWall/message',
                url: 'message',
                contentType: 'application/json; charset=utf8',
                data: JSON.stringify(body),
                success: function(body) {
                    // 预期 body 中返回 ok
                    console.log(body);
                }
            });
        }

        // 在页面加载的时候, 发起一个 GET 请求给服务器, 从服务器拿到提交过的数据.
        $.ajax({
            type: 'get',
            url: 'message',
            success: function(body) {
                // 但是由于 jquery 见到响应中的 application/json , 就会自动的把响应
                // 转换成 js 对象数组. body 其实是 js 对象数组, 而不是 json 字符串了.
                // 就可以直接按照数组的方式来操作 body, 每个元素都是 js 对象

                // 1. 遍历数组, 取出每个 js 对象.
                // 2. 根据这里的 js 对象构造出 页面元素, 显示到页面上
                let container = document.querySelector('.container');
                for (let i = 0; i < body.length; i++) {
                    let message = body[i];
                    // 此处 message 对象, 就形如
                    // {
                    //   from: '黑猫',
                    //   to: '白猫',
                    //   message: '喵'
                    // }
                    // 构造出 html 元素!!
                    // 使用浏览器提供的 api (dom api) 创建 div 标签.
                    let div = document.createElement('div');
                    // 设置一个 CSS 的类. 应用到 CSS 样式了.
                    div.className = 'row';
                    // 给 div 标签里设置内容, 此处显示一行文本.
                    div.innerHTML = message.from + " 对 " + message.to + " 说: " + message.message;
                    container.appendChild(div);
                }
            }
        });

    </script>
</body>
</html>

4.MessageServlet.java:

import com.fasterxml.jackson.databind.ObjectMapper;
import com.mysql.jdbc.jdbc2.optional.MysqlDataSource;

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 javax.sql.DataSource;
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 Message {
    // 这里的变量名要和前端js的变量名完全匹配
    public String from;
    public String to;
    public String message;

    @Override
    public String toString() {
        return "Message{" +
                "from='" + from + '\'' +
                ", to='" + to + '\'' +
                ", message='" + message + '\'' +
                '}';
    }
}

@WebServlet("/message")
public class MessageServlet extends HttpServlet {
    private ObjectMapper objectMapper = new ObjectMapper();
    // private List<Message> list = new ArrayList<>();

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 存档
        // 读取body中的json格式,转化成Java格式,存入链表
        Message message = objectMapper.readValue(req.getInputStream(), Message.class);
        try {
            save(message);
        } catch (SQLException e) {
            e.printStackTrace();
        }
        System.out.println("收到请求" + message);
        resp.setStatus(200);
        resp.getWriter().write("ok");
    }

    // 使用jdbc,往数据库中插入数据
    private void save(Message message) throws SQLException {
        DataSource dataSource = new MysqlDataSource();
        ((MysqlDataSource)dataSource).setUrl("jdbc:mysql://127.0.0.1:3306/messageWall?characterEncoding=utf8&useSSL=false");
        ((MysqlDataSource)dataSource).setUser("root");
        ((MysqlDataSource)dataSource).setPassword("123456");

        Connection connection = dataSource.getConnection();
        String sql = "insert into message values(?, ?, ?)";
        PreparedStatement statement = connection.prepareStatement(sql);
        statement.setString(1, message.from);
        statement.setString(2, message.to);
        statement.setString(3, message.message);
        statement.executeUpdate();
        
        statement.close();
        connection.close();
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        // 读档
        resp.setContentType("application/json; charset=utf8");
        String respJson = null;
        try {
            respJson = objectMapper.writeValueAsString(load());
        } catch (SQLException e) {
            e.printStackTrace();
        }
        resp.getWriter().write(respJson);
    }

    private List<Message> load() throws SQLException {
        DataSource dataSource = new MysqlDataSource();
        ((MysqlDataSource) dataSource).setUrl("jdbc:mysql://127.0.0.1:3306/messageWall?characterEncoding=utf8&useSSL=false");
        ((MysqlDataSource) dataSource).setUser("root");
        ((MysqlDataSource) dataSource).setPassword("123456");

        Connection connection = dataSource.getConnection();
        String sql = "select * from message";
        PreparedStatement statement = connection.prepareStatement(sql);
        ResultSet resultSet = statement.executeQuery();
        List<Message> list = new ArrayList<>();
        while (resultSet.next()) {
            Message message = new Message();
            message.from = resultSet.getString("from");
            message.to = resultSet.getString("to");
            message.message = resultSet.getString("message");
            list.add(message);
        }
        resultSet.close();
        statement.close();
        connection.close();
        return list;
    }
}

三.出错处理

1.首先使用Fiddler抓包,判断是前端问题还是后端问题.

2.如果是前端问题,去浏览器的开发者工具(F12)找错.