关于AJAX异步功能的小实验

为了实验ajax的异步

性，先建一个Web项目，结构大概是这个样子

360截图20180408171612437.jpg

TestServlet.java（主要是提供ajax后台调用的程序）

import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.Date;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class TestServelet
 */
 @WebServlet("/TestServelet")
 public class TestServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;
    
     /**
     * Default constructor. 
     */
    public TestServlet() {
    }    
    
    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        
    //为了体现程序的异步性，先让它睡3s
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        response.setCharacterEncoding("utf-8");  
        //打印出程序运行的时间
        System.out.println("异步程序运行时间："+new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS") .format(new Date()));
    }    
    
    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

testAjax.jsp(前台页面和js，触发异步调用)

<%@ page language="java" contentType="text/html;charset=UTF-8" %>
<html >
<head></head>
<body onload="testajax()">
    Hello Ajax!<br>
</body>
</html>
<SCRIPT LANGUAGE="JavaScript">
    function testajax(){
        fnGetAjaxReturn('http://localhost:8080/TestProject/TestServlet?a='+Math.random());
    }    
    
    function fnGetAjaxReturn(url)
    {   
        var userAgent = navigator.userAgent;        
        var http_request = false;        
        //开始初始化XMLHttpRequest对象
        if(window.XMLHttpRequest) { //Mozilla 浏览器
            http_request = new XMLHttpRequest();            
            if (http_request.overrideMimeType) {//设置MiME类别
                http_request.overrideMimeType("text/xml");
            }
        }        
        //else if (window.ActiveXObject) { // IE浏览器
        else if (window.ActiveXObject||userAgent.indexOf("Trident") > -1){          
           try {
                http_request = new ActiveXObject("Msxml2.XMLHTTP");
            } catch (e) {                try {
                    http_request = new ActiveXObject("Microsoft.XMLHTTP");
                } catch (e) {alert("错了吧");}
            }
        }        if (!http_request) { // 异常，创建对象实例失败
            window.alert("不能创建XMLHttpRequest对象实例.");            
            return false;
        }
        http_request.open("GET", url, true);//true 异步  false 同步 
        http_request.send();
        alert("异步请求之后执行时间："+new Date +'\n毫秒数：'+ new Date().getMilliseconds());
    }</script>

说明：jsp页面加载之后调用一个js，js中会先发出一个异步请求，再执行一个alert弹出操作。

下面开始试验

用IE浏览器，运行URL
http://localhost:8080/TestProject/testAjax.jsp

发现，程序先弹出了alert，过了3s钟，ajax才返回了后台结果，充分证明了ajax的异步性。

运行结果：

360截图20180408172134306.jpg