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：


代码如下:

<%@ 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：


代码如下:

<!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。