性,先建一个Web项目,结构大概是这个样子

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(':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
: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:

从时间的差异性,发现程序先执行结束(执行了alert),过了3s钟,异步程序才返回结果。

反过来,如果把jsp文件中的

http_request.open("GET", url, true);

true 改为 false 呢?

运行URL,发现程序在苦苦等待Servlet后台请求返回之后,才执行了js中的最后一段代码alert,正是所谓的同步调用。

运行结果如下: