ajax的常规操作（原始方式）

package com.csdn.servlet;
import jakarta.servlet.GenericServlet;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import jakarta.servlet.ServletResponse;
import jakarta.servlet.annotation.WebServlet;
import java.io.IOException;
import java.io.PrintWriter;
//演示Ajax
@WebServlet("/h09")
public class AjaxServlet extends GenericServlet {
    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        String uname = servletRequest.getParameter("uname");
        uname = uname.toUpperCase();
        PrintWriter out = servletResponse.getWriter();
        out.println(uname);
        out.flush();
    }
}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>ajax01</title>
    <script>
        let xmlHttpRequest;
        function sendAjax01() {
            let uname = document.getElementById("uname").value;

            xmlHttpRequest = new XMLHttpRequest();
            xmlHttpRequest.open("Get","h09?uname="+uname,true);//Get：请求方式    h09:请求的目的地   true：异步请求
            //onreadystatechange:当状态发生改变的时候调用sendAjax01CB
            //状态取值有 0-4 ，共 5 个，我们感兴趣的是 4
            //0 表示 xmlHttpRequest未创建
            //1 表示开始发送请求
            //2 表示发送请求完成
            //3 表示开始接收响应
            //4 表示接收响应成功，客户端成功解析服务器返回的响应
            xmlHttpRequest.onreadystatechange = sendAjax01CB;    //CB:call back 回调函数
            //发送请求
            xmlHttpRequest.send();
        }
        function sendAjax01CB() {
            if (xmlHttpRequest.readyState == 4 && xmlHttpRequest.status == 200) {
                //获取响应文本
                let msg = xmlHttpRequest.responseText;
                document.getElementById("uname2").innerText = msg;
            }
        }
    </script>
</head>
<body>
        uname:<input type="text" id="uname"/>
        <input type="button" value="发生异步请求" onclick="sendAjax01()"><br/>
        服务器响应回来的内容：<span id="uname2">&nbsp</span>
</body>
</html>