在应用程序的安装和下载过程中，进度条的使用已经是非常普遍了。进度条可以用来表识项目的完成进度，可以用百分比或数字表示，可以水平放置。利用Ajax技术创建进度条，功能显得更加强大的快捷。
现在创建一个实例，以演示使用Ajax技术实现进度条。该实例同样可以分为客户端代码和服务端代码。
1，服务端代码
服务端代码主要实现一句客户端的请求信息，返回相应的百分比数字。打开记事本，输入下列代码：
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" errorPage="" %>
<%!
int counter = 1;//注意：多用户将共享此变量，此进度条只适合单用户
%>
<%
String task = request.getParameter("task");
String res = "";
if (task.equals("create")) {
res = "1";
counter = 1;
}
else {
String percent = "";
switch (counter) {
case 1: percent = "10"; break;
case 2: percent = "23"; break;
case 3: percent = "35"; break;
case 4: percent = "51"; break;
case 5: percent = "64"; break;
case 6: percent = "73"; break;
case 7: percent = "89"; break;
case 8: percent = "100"; break;
}
counter++;
res = "<percent>" + percent + "</percent>";
}
// PrintWriter out = response.getWriter();
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
out.println("<response>");
out.println(res);
out.println("</response>");
out.close();
%>
<html>
<head>
<title>JSP+Ajax 进度条</title>
<script type="text/javascript">
var xmlHttp;
var bar_color = 'blue';
var span_id = "yellow";
var clear = " "
function createXMLHttpRequest() {
if (window.ActiveXObject) {
xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
}
else if (window.XMLHttpRequest) {
xmlHttp = new XMLHttpRequest();
}
}
function go() {
createXMLHttpRequest();
checkDiv();
var url = "ProgressBarJsp.jsp?task=create";
xmlHttp.open("GET", url, true);
xmlHttp.onreadystatechange = goCallback;
xmlHttp.send(null);
}
function goCallback() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
setTimeout("pollServer()", 2000);
}
}
}
function pollServer() {
createXMLHttpRequest();
var url = "ProgressBarJsp.jsp?task=poll";
xmlHttp.open("GET", url, true);
xmlHttp.onreadystatechange = pollCallback;
xmlHttp.send(null);
}
function pollCallback() {
if (xmlHttp.readyState == 4) {
if (xmlHttp.status == 200) {
var percent_complete = xmlHttp.responseXML.getElementsByTagName("percent")[0].firstChild.data;
var index = processResult(percent_complete);
for (var i = 1; i <= index; i++) {
var elem = document.getElementById("block" + i);
elem.innerHTML = clear;
elem.style.backgroundColor = bar_color;
var next_cell = i + 1;
if (next_cell > index && next_cell <= 9) {
document.getElementById("block" + next_cell).innerHTML = percent_complete + "%";
}
}
if (index < 9) {
setTimeout("pollServer()", 2000);
} else {
document.getElementById("complete").innerHTML = "网站已完成加载!";
}
}
}
}
function processResult(percent_complete) {
var ind;
if (percent_complete.length == 1) {
ind = 1;
} else if (percent_complete.length == 2) {
ind = percent_complete.substring(0, 1);
} else {
ind = 9;
}
return ind;
}
function checkDiv() {
var progress_bar = document.getElementById("progressBar");
if (progress_bar.style.visibility == "visible") {
clearBar();
document.getElementById("complete").innerHTML = "";
} else {
progress_bar.style.visibility = "visible"
}
}
function clearBar() {
for (var i = 1; i < 10; i++) {
var elem = document.getElementById("block" + i);
elem.innerHTML = clear;
elem.style.backgroundColor = "white";
}
}
</script>
</head>
<body onload="go();">
<h1 align=center>网站正在加载中，请稍候</h1>
<p>
<table align="center">
<tbody>
<tr><td>
<div id="progressBar" style="padding:2px;border:solid yellow 2px;visibility:hidden">
<span id="block1"> </span>
<span id="block2"> </span>
<span id="block3"> </span>
<span id="block4"> </span>
<span id="block5"> </span>
<span id="block6"> </span>
<span id="block7"> </span>
<span id="block8"> </span>
<span id="block9"> </span>
</div>
</td></tr>
<tr><td align="center" id="complete"></td></tr>
</tbody>
</table>
</body>
</html>
将上述代码保存，名称为JspprogressBar.html。在该文件中，JavaScript函数createXMLHttpRequest()主要用来创建XMLHttpRequest对象，go()函数完成想服务器端发送异步请求，该函数在网页加载时被调用，其主要作用就是通知服务器，并在客户端开始运行进度条。GoCallback()函数主要用于处理服务端的响应，并每隔2秒调用pollServer()函数也是用于向服务端发送异步请求，主要请求服务器端响应的百分数字。PollCallback()函数主要用于处理服务器端响应，即依据服务器端的返回的数字，指定进度条的显示状态。这里需要注意的是，goCallback()函数只执行一次，而PollCallback()函数可以执行多次，其余的三个函数都是实现进度条的辅助函数。
3，运行
将上述两个Jsp文件复制到JSP目录下。运行效果图如下：