JQuery Ajax通过Handler访问外部XML数据的代码

JQuery的使用非常简单，我们只需要从其官方网站上下载一个脚本文件并引用到页面上即可，然后你就可以在你的脚本代码中任意使用JQuery提供的对象和功能了。
　　在JQuery中使用Ajax方法异步获取服务器资源非常简单，读者可以参考其官方网站上提供的例子http://api.jquery.com/category/ajax/。当然，作为客户端脚本，JQuery也会遇到跨域访问资源的问题，什么是跨域访问呢？简单来说就是脚本所要访问的资源属于网站外部的资源，脚本所在的位置和资源所在的位置不在同一区域。默认情况下，浏览器是不允许直接进行资源的跨域访问的，除非客户端浏览器有设置，否则访问会失败。在这种情况下，我们一般都会采用在服务器端使用handler来解决，就是说在脚本和资源之间建立一个桥梁，让脚本访问本站点内的handler，通过handler去访问外部资源。这个是非常普遍的做法，而且操作起来也非常简单，因为会经常使用到，所以在此记录一下，方便日后使用！
　　首先需要在网站中创建一个handler，在Visual Studio中新建一个Generic Handler文件，拷贝下面的代码：

<%@ WebHandler Language="C#" Class="WebApplication1.Stock" %> 
namespace WebApplication1 
{ 
using System; 
using System.IO; 
using System.Net; 
using System.Text; 
using System.Web; 
using System.Collections.Generic; 
using System.Linq; 
/// <summary> 
/// Asynchronous HTTP handler for rendering external xml source. 
/// </summary> 
public class Stock : System.Web.IHttpAsyncHandler 
{ 
private static readonly SafeList safeList = new SafeList(); 
private HttpContext context; 
private WebRequest request; 
/// <summary> 
/// Gets a value indicating whether the HTTP handler is reusable. 
/// </summary> 
public bool IsReusable 
{ 
get { return false; } 
} 
/// <summary> 
/// Verify that the external RSS feed is hosted by a server on the safe list 
/// before making an asynchronous HTTP request for it. 
/// </summary> 
public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData) 
{ 
var u = context.Request.QueryString["u"]; 
var uri = new Uri(u); 
if (safeList.IsSafe(uri.DnsSafeHost)) 
{ 
this.context = context; 
this.request = HttpWebRequest.Create(uri); 
return this.request.BeginGetResponse(cb, extraData); 
} 
else 
{ 
throw new HttpException(204, "No content"); 
} 
} 
/// <summary> 
/// Render the response from the asynchronous HTTP request for the RSS feed 
/// using the response's Expires and Last-Modified headers when caching. 
/// </summary> 
public void EndProcessRequest(IAsyncResult result) 
{ 
string expiresHeader; 
string lastModifiedHeader; 
string rss; 
using (var response = this.request.EndGetResponse(result)) 
{ 
expiresHeader = response.Headers["Expires"]; 
lastModifiedHeader = response.Headers["Last-Modified"]; 
using (var stream = response.GetResponseStream()) 
using (var reader = new StreamReader(stream, true)) 
{ 
rss = reader.ReadToEnd(); 
} 
} 
var output = this.context.Response; 
output.ContentEncoding = Encoding.UTF8; 
output.ContentType = "text/xml;"; // "application/rss+xml; charset=utf-8"; 
output.Write(rss); 
var cache = output.Cache; 
cache.VaryByParams["u"] = true; 
DateTime expires; 
var hasExpires = DateTime.TryParse(expiresHeader, out expires); 
DateTime lastModified; 
var hasLastModified = DateTime.TryParse(lastModifiedHeader, out lastModified); 
cache.SetCacheability(HttpCacheability.Public); 
cache.SetOmitVaryStar(true); 
cache.SetSlidingExpiration(false); 
cache.SetValidUntilExpires(true); 
DateTime expireBy = DateTime.Now.AddHours(1); 
if (hasExpires && expires.CompareTo(expireBy) <= 0) 
{ 
cache.SetExpires(expires); 
} 
else 
{ 
cache.SetExpires(expireBy); 
} 
if (hasLastModified) 
{ 
cache.SetLastModified(lastModified); 
} 
} 
/// <summary> 
/// Do not process requests synchronously. 
/// </summary> 
public void ProcessRequest(HttpContext context) 
{ 
throw new InvalidOperationException(); 
} 
} 
/// <summary> 
/// Methods for matching hostnames to a list of safe hosts. 
/// </summary> 
public class SafeList 
{ 
/// <summary> 
/// Hard-coded list of safe hosts. 
/// </summary> 
private static readonly IEnumerable<string> hostnames = new string[] 
{ 
"cnblogs.com", 
"msn.com", 
"163.com", 
"csdn.com" 
}; 
/// <summary> 
/// Prefix each safe hostname with a period. 
/// </summary> 
private static readonly IEnumerable<string> dottedHostnames = 
from hostname in hostnames 
select string.Concat(".", hostname); 
/// <summary> 
/// Tests if the <paramref name="hostname" /> matches exactly or ends with a 
/// hostname from the safe host list. 
/// </summary> 
/// <param name="hostname">Hostname to test</param> 
/// <returns>True if the hostname matches</returns> 
public bool IsSafe(string hostname) 
{ 
return MatchesHostname(hostname) || MatchesDottedHostname(hostname); 
} 
/// <summary> 
/// Tests if the <paramref name="hostname" /> ends with a hostname from the 
/// safe host list. 
/// </summary> 
/// <param name="hostname">Hostname to test</param> 
/// <returns>True if the hostname matches</returns> 
private static bool MatchesDottedHostname(string hostname) 
{ 
return dottedHostnames.Any(host => hostname.EndsWith(host, StringComparison.InvariantCultureIgnoreCase)); 
} 
/// <summary> 
/// Tests if the <paramref name="hostname" /> matches exactly with a hostname 
/// from the safe host list. 
/// </summary> 
/// <param name="hostname">Hostname to test</param> 
/// <returns>True if the hostname matches</returns> 
private static bool MatchesHostname(string hostname) 
{ 
return hostnames.Contains(hostname, StringComparer.InvariantCultureIgnoreCase); 
} 
} 
}

我给出的例子中是想通过Ajax异步取得msn站点上微软的股票信息，其外部资源地址为http://money.service.msn.com/StockQuotes.aspx?symbols=msft，我们在页面上这样使用JQuery api通过Handler来访问数据：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
<html xmlns="http://www.w3.org/1999/xhtml"> 
<head> 
<title></title> 
<script type="text/javascript" src="jquery-1.3.2.min.js"></script> 
</head> 
<body> 
<div id="con"> 
<span id="loader">loading...</span> 
</div> 
<script type="text/javascript"> 
function getData() { 
$("#loader").ajaxStart(function() { 
$(this).show(); 
}); 
$("#loader").ajaxComplete(function() { 
$(this).hide(); 
}); 
$.ajax({ 
type: "GET", 
url: "Stock.ashx?u=http://money.service.msn.com/StockQuotes.aspx?symbols=msft", 
dataType: "xml", 
success: function(data) { 
var last = ""; 
var change = ""; 
var percentchange = ""; 
var volume = ""; 
var cap = ""; 
var yearhigh = ""; 
var yearlow = ""; 
$(data).find('ticker').each(function() { 
last = $(this).attr('last'); 
change = $(this).attr('change'); 
percentchange = $(this).attr('percentchange'); 
volume = $(this).attr('volume'); 
cap = $(this).attr('marketcap'); 
yearhigh = $(this).attr('yearhigh'); 
yearlow = $(this).attr('yearlow'); 
document.getElementById('con').innerHTML = '<span>name:' + last + '    high:' + volume + '    low:' + cap + '</span>'; 
}) 
} 
}); 
} 
$(window).load(getData); 
</script> 
</body> 
</html>

下面是实现的结果：
name:25.8 high:67,502,221 low:$226,107,039,514
　　Handler的写法基本都大同小异，因此可以写成一个通用的例子，以后如遇到在脚本中需要跨域访问资源时便可以直接使用！代码记录于此，方便查阅。