Ajax simple real-time verification function implementation method

Ajax stands for "Asynchronous Javascript And XML" (Asynchronous JavaScript and XML), which refers to a web development technology for creating interactive web applications. This article mainly introduces the simple real-time verification function of ajax. Friends who need it can refer to it. I hope it can help everyone.

Ajax = Asynchronous JavaScript and XML (a subset of Standard Universal Markup Language).

Ajax is a technology for creating fast, dynamic web pages.

Ajax is a technology that allows you to update parts of a web page without reloading the entire page.

By exchanging a small amount of data with the server in the background, Ajax allows web pages to be updated asynchronously. This means that parts of a web page can be updated without reloading the entire page.

Traditional web pages (without Ajax) must reload the entire web page if content needs to be updated.

This is Baidu's definition of it, which is detailed enough.
One thing worth adding is the understanding of asynchronous. Asynchronous is relative to synchronization. Here they refer to the interaction mode between the server and the browser.

Synchronous, after each request is issued, the user operation is blocked and must require a response to be returned before continuing the operation. Asynchronous means that after sending a request, the user does not need to wait for a response. Everything is implemented by ajax, and the data can be partially updated without refreshing the web page. Improved communication efficiency between both ends.

Let’s have a small demo

Make a demo of a non-refresh verification form, enter the user name in the dialog box, and perform verification in the background, using ajax technology.

Project structure, use maven to build

login.jsp

  <title>login</title>

Welcome to log in:

    用户名：<input>
    <!-- 显示提示信息 -->
    <p></p>
    <!-- 在jsp页面中引入js，绝对路径的方式 -->
    <script></script>

main.js

alert("use ajax!")
//创建XMLHttpRequest对象，在不同浏览器
function createXMLHTTP() {
  if(window.XMLHttpRequest){
    // IE7+, Firefox, Chrome, Opera, Safari 浏览器执行代码
    xmlhttp = new XMLHttpRequest();
  }else {
    // IE6, IE5 浏览器执行代码
    xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
  }
  return xmlhttp;
}
function CallServer() {
  var username = document.getElementById("username").value;
  // 判断为空
  if ((username == null) || (username == "")) return;
  var xmlhttp = createXMLHTTP();
  // 构建请求url
  var url = "/loginServlet"+"?"+"username="+username;
  //状态码改变调用事件
  xmlhttp.onreadystatechange = function () {
    //正常返回，替换msg内容
    if(xmlhttp.readyState == 4 && xmlhttp.status == 200){
      document.getElementById("msg").innerHTML = xmlhttp.responseText;
    }
  }
  //异步提交请求
  xmlhttp.open("GET",url,true);
  //发送请求
  xmlhttp.send();
}

web.xml

nbsp;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>
 <welcome-file-list>
  <welcome-file>index.jsp</welcome-file>
 </welcome-file-list>
 <servlet>
  <servlet-name>loginServlet</servlet-name>
  <servlet-class>com.lbw.servlet.loginServlet</servlet-class>
 </servlet>
 <servlet-mapping>
  <servlet-name>loginServlet</servlet-name>
  <url-pattern>/loginServlet</url-pattern>
 </servlet-mapping>
</web-app>

loginServlet.java

package com.lbw.servlet;
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.io.PrintWriter;
/**
 * 后端使用Servlet处理请求
 */
public class loginServlet extends HttpServlet {
  protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    //设置编码和响应头
    request.setCharacterEncoding("UTF-8");
    response.setContentType("text/xml;charset=UTF-8");
    response.setHeader("Cache-Control", "no-cache");
    //获取参数
    String username = request.getParameter("username");
    String msg = "";
    if("lbw".equals(username)){
      msg = "名称正确";
    }else {
      msg = "名称错误";
    }
    PrintWriter out = response.getWriter();
    out.println(msg);
  }
  protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    doPost(request,response);
  }
}

start testing

Enter localhost:8888/login.jsp, and a pop-up window

will appear. Represents the successful introduction of js into jsp

Enter test data

in the input box Determined by the logic in the Servlet, error message

is returned Determined by the logic in the Servlet, success information is returned

As a result, ajax asynchronous requests have been initially implemented, meeting the requirements of real-time verification

Some small details

1. When using maven to build the project, pay attention to Project Structure -> Facets. Set the paths of web.xml and webapp here. Idea will use

2. When introducing js, pay attention to using relative paths. To map, and use EL expressions, turn on isELIgnored="false"·` to avoid no parsing.

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