Ajax实例

HelloWorld.jsp
<%@ page contentType="text/html;charset=utf-8" language="java" errorPage="" %>
<html>
	<head>
		<meta http-equiv="pragma"content="no-cache"> 
		<meta http-equiv="cache-control"content="no-cache,must-revalidate"> 
		<meta http-equiv="expires"content="0"> 
		

		<title>Ajax</title>
		<script type="text/javascript">
			function ok(){
				var xmlhttp;
				if(window.XMLHttpRequest){
					//非IE
					xmlhttp=new XMLHttpRequest();
				}else if(window.ActiveXObject){
					try{
						xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
					}catch(e){
						xmlhttp= new ActiveXObject("Msxml2.XMLHTTP");
					}
				}
				xmlhttp.onreadystatechange=function(){
					if(4==xmlhttp.readystate){
						if(200==xmlhttp.status){
							//获取返回值
							var msgWelcome=xmlhttp.responseText;
							var msg = document.getElementById("msg");
							msg.innerHTML=msgWelcome;
						}
					}
				}
				//设定请求
				xmlhttp.open("get","http://localhost:8080/Ajax/HelloWorld.do",true);
				//设定http头
				xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
				//发送请求
				xmlhttp.send(null);
			}
		</script>
	</head>
	<body>
		<span id="msg"></span>
		<br>
		<input type="button" onclick="ok();" value="单击此按钮" />
	</body>
</html>


HelloWorld.java
package com.cn.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloWorld extends HttpServlet{

	/**
	 * 
	 */
	private static final long serialVersionUID = 463701915463621765L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {
		response.setContentType("text/plain;charset=utf-8");
		//无缓存
		response.addHeader("pragma", "no-cache"); 
		response.addHeader("cache-control", "no-cache,must-revalidate"); 
		response.addHeader("expires", "0"); 

		PrintWriter out=response.getWriter();
		out.write("当前的系统时间是:"+getCurrentDateAndTime()+"欢迎(HelloWorld)");
	}

	private String getCurrentDateAndTime() {
		// TODO Auto-generated method stub
		String currentDate="";
		//设定时间格式
		SimpleDateFormat format=new SimpleDateFormat("yyyy'-'MM'-'dd HH:mm:ss");
		format.setLenient(false);
		currentDate=format.format(new Date());
		//返回当前日期
		return currentDate;
	}
	
}


很简单的一个实例,大家用的时候要注意的是:
  记得关闭缓存

response.addHeader("pragma", "no-cache"); 
		response.addHeader("cache-control", "no-cache,must-revalidate"); 
		response.addHeader("expires", "0"); 



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