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Meet ajax

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##1.1 What is ajax:

Ajax stands for "Asynchronous Javascript And XML" (Asynchronous JavaScript and XML), which refers to a method for creating interactive web applications. Web development technology. Ajax = Asynchronous JavaScript and XML (a subset of Standard Universal Markup Language). Ajax can use web pages to achieve asynchronous updates by exchanging a small amount of data with the server in the background.
This means that parts of the webpage can be updated without reloading the entire webpage (no refresh technology). Traditional web pages (not using Ajax) must reload the entire web page if the content needs to be updated.

1.2Ajax application scenarios:

1.2.1 Check whether the user name has been registered:

Many sites The registration page has a friendly prompt to automatically detect whether the user name exists. The entire page of this function is not refreshed, but data can still be exchanged asynchronously with the server to query whether the user name entered by the user exists in the database.

1.2.2 Province and city level drop-down box linkage:

Many sites have the operation of entering the user address. After completing When entering an address, the province where the user is located is a drop-down box. When different provinces are selected, different city selections will appear. This is the most common province-city linkage effect.

1.2.3 Content auto-completion:

Both Baidu, which focuses on search, and Taobao, which searches for products within the site, have search functions. , when you enter a query keyword in the i search box, the entire page is not refreshed, but relevant query notes will be displayed based on the keywords. This process is asynchronous.

Baidu’s search completion function:

Taobao’s search completion function:

1.3 The difference between synchronous and asynchronous methods :

Send a request in synchronous mode: to send a request, you need to wait for the response to return before you can send the next request. If There is no response to this request and the next request cannot be sent. The client will always be waiting.
Send a request asynchronously: Send a request without waiting for the response to be returned. You can send the next request at any time, that is, there is no need to wait.

1.4 Ajax Principle Analysis:

The AJAX engine will send an asynchronous request without refreshing the browser address bar :

Use JavaScript to obtain the browser's built-in AJAX engine (XMLHttpRequest object)
Use js to determine the request path and request parameters
The AJAX engine object is based on the request path and request parameters Make a request to send

The server receives the request from the Ajax engine and processes it:

Server Obtain request parameter data
The server processes the request business (calling the business layer code)
The server responds with data to the Ajax engine

The Ajax engine obtains the data responded by the server and updates the data to the specific location of the browser page by executing the JavaScript callback function :

By setting the callback to the Ajax engine The function obtains the data of the server response
Use JavaScript to display the response data at the specified location, thereby partially modifying the data of the page to achieve the purpose of partial refresh.

2.1js native Ajax:

js native Ajax development steps:

CreateAjaxEngine Object

Bind listening for the Ajax engine object (the listening server has responded with data to the engine)

Bind submission address

Send request

Listen to process the response data

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css"></style>
    <script type="text/javascript" src="js/jquery-3.3.1.min.js"></script>
    <script type="text/javascript">
        //同步请求点击事件
        function sendRequest() {
            //js刷地址栏请求服务器端
            location.href = "Ajax1Servlet?name=admin&password=123abc";
        }
        
        //异步请求点击事件
        function sendAsynRequest() {
            //1.创建ajax引擎对象
            var xmlHttp = new XMLHttpRequest();
            //2.设置回调函数，目的是处理服务器完全返回的数据
            xmlHttp.onreadystatechange = function () {
                /**
                 * 这个回调函数什么调用呢？是ajax引擎对象与服务器通信状态码改变的时候调用
                 * ajax引擎对象与服务器通信状态码xmlHttp.readystate，范围0~4
                 * 0：请求未初始化
                 * 1：服务器连接已建立
                 * 2：请求已接收
                 * 3：请求处理中
                 * 4：请求已完成，且响应已就绪
                 * 这个回调函数一共被调用4次，但只有状态码4的时候才代表服务器响应完成数据完成。
                 * ajax引擎通信转态码为4和http通信转态码为200
                 */
                if(xmlHttp.readyState==4 && xmlHttp.status==200){
                    //获取响应数据
                    var content = xmlHttp.responseText;
                    alert(content);
                }
            }
            //3.设置请求路径和请求参数
            /**
             * xmlHttp.open(method,url)
             * method,请求方法，get或post请求
             * url：请求路径
             */
            xmlHttp.open("get","Ajax1Servlet?name=admin&psw=abc123");
            //4.发送请求
            xmlHttp.send();
        }
    </script>
</head>
<body>
<input type="button" value="发送同步请求" onclick="sendRequest();"/>
<input type="button" value="发送异步请求" onclick="sendAsynRequest();"/>
</body>
</html>

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package com.sunny.web;

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;

@WebServlet(name = "Ajax1Servlet", urlPatterns = "/Ajax1Servlet")
public class Ajax1Servlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //获取请求参数
        String name = request.getParameter("name");
        String password = request.getParameter("password");
        //打印出来
        System.out.println("name="+name);
        System.out.println("password="+password);
        //输出数据前端
        response.getWriter().write("hello js ajax");
    }
}

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2.2 Ajax engine connection status readyState value 0~4 change process：

Stores the status of XMLHttpRequest. Changes from 0 to 4.
0: The request is not initialized
1: The server connection has been established
2: The request has been received
3: The request is being processed
4: The request has been completed and the response is ready

The status value 4 here can only indicate that the server's response has been received and the server has completed processing the ajax request, but it does not mean that the server's response has been correctly obtained. Response, needs to match the http status code 200. Two conditions can indicate that the server response is correctly obtained. Only when these two conditions are met, xmlhttp.responseText can obtain the correct response data.

xmlhttp.onreadystatechange = function(){
			if(xmlhttp.readyState == 4){
				if(xmlhttp.status == 200){
					alert("响应数据" + xmlhttp.responseText);
				}
			}
		};

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