2，预检请求
预检请求首先需要向另外一个域名的资源发送一个 HTTP OPTIONS 请求头，其目的就是为了判断实际发送的请求是否是安全的。下面的2种情况需要进行预检：
a，不是上面的简单请求，比如使用Content-Type 为 application/xml 或 text/xml 的 POST 请求
b，在请求中设置自定义头，比如 X-JSON、X-MENGXIANHUI 等
注意：在 iis 里进行测试，必须在“应用程序扩展”里面配置 .aspx 扩展的动作允许 OPTIONS。
下面我们举一个预检的请求：
<!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>孟宪会之AJAX跨域请求测试</title>
</head>
<body>
<input type='button' value='开始测试' nclick='crossDomainRequest()' />
<div id="content"></div>
<script. type="text/javascript">
//<![CDATA[
var xhr = new XMLHttpRequest();
var url = 'http://dotnet.aspx.cc/PreflightedRequests.aspx';
function crossDomainRequest() {
document.getElementById("content").innerHTML = "开始进行请求……";
if (xhr) {
var xml = "<root>测试</root>";
xhr.open('POST', url, true);
xhr.setRequestHeader("POWERED-BY-MENGXIANHUI", "Approve");
xhr.setRequestHeader("Content-Type", "application/xml");
xhr.onreadystatechange = handler;
xhr.send(xml);
} else {
document.getElementById("content").innerHTML = "不能创建 XMLHttpRequest。";
}
}
function handler(evtXHR) {
if (xhr.readyState == 4) {
if (xhr.status == 200) {
var response = xhr.responseText;
document.getElementById("content").innerHTML = "结果：" + response;
} else {
document.getElementById("content").innerHTML = "不能进行跨越访问。";
}
}
else {
document.getElementById("content").innerHTML += "<br/>执行状态 readyState：" + xhr.readyState;
}
}
//]]>
</script>
</body>
</html>
上面的例子我们发送 xml 格式的数据，并且，发送一个非标准的HTTP头 POWERED-BY-MENGXIANHUI 来说明服务器端该如何设置响应头的。
在服务器端，PreflightedRequests.aspx 的内容如下：
<%@ Page Language="C#" %>
<script. runat="server">
protected void Page_Load(object sender, EventArgs e)
{
if (Request.HttpMethod.Equals("GET"))
{
Response.Write("这个页面是用来测试跨域 POST 请求的，直接浏览意义不大。");
}
else if (Request.HttpMethod.Equals("OPTIONS"))
{
//通知客户端允许预检请求。并设置缓存时间
Response.ClearContent();
Response.AddHeader("Access-Control-Allow-Origin", "http://www.meng_xian_hui.com:801");
Response.AddHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS");
Response.AddHeader("Access-Control-Allow-Headers", "POWERED-BY-MENGXIANHUI");
Response.AddHeader("Access-Control-Max-Age", "30");
//此过程无需返回数据
Response.End();
}
else if (Request.HttpMethod.Equals("POST"))
{
if (Request.Headers["Origin"].Equals("http://www.meng_xian_hui.com:801"))
{
System.Xml.XmlDocument doc = new System.Xml.XmlDocument();
doc.Load(Request.InputStream);
Response.AddHeader("Access-Control-Allow-Origin", "http://www.meng_xian_hui.com:801");
Response.Write("您提交的数据是：<br/><br/>" + Server.HtmlEncode(doc.OuterXml));
}
else
{
Response.Write("不允许你的网站请求。");
}
}
}
</script>
点击“开始测试”按钮，将会执行下面的一系列请求。
OPTIONS /PreflightedRequests.aspx HTTP/1.1
Host: dotnet.aspx.cc
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; zh-CN; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: GB2312,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
Origin: http://www.meng_xian_hui.com:801
Access-Control-Request-Method: POST
Access-Control-Request-Headers: powered-by-mengxianhui
HTTP/1.x 200 OK
Date: Sun, 10 Jan 2010 14:00:34 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801
Access-Control-Allow-Methods: POST, GET, OPTIONS
Access-Control-Allow-Headers: POWERED-BY-MENGXIANHUI
Access-Control-Max-Age: 30
Set-Cookie: ASP.NET_SessionId=5npqri55dl1k1zvij1tlw3re; path=/; HttpOnly
Cache-Control: private
Content-Length: 0
POST /PreflightedRequests.aspx HTTP/1.1
Host: dotnet.aspx.cc
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; zh-CN; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: GB2312,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
POWERED-BY-MENGXIANHUI: Approve
Content-Type: application/xml; charset=UTF-8
Referer: http://www.meng_xian_hui.com:801/CrossDomainAjax/PreflightedRequests.html
Content-Length: 19
Origin: http://www.meng_xian_hui.com:801
Pragma: no-cache
Cache-Control: no-cache
<root>测试</root>
HTTP/1.x 200 OK
Date: Sun, 10 Jan 2010 14:00:34 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801
Set-Cookie: ASP.NET_SessionId=byvose45zmtbqy45d2a1jf2i; path=/; HttpOnly
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 65
以上的代码反映了预检请求的执行过程：首先发送 OPTIONS 请求头，用来向服务器咨询服务器的更多信息，以便为后续的真实请求做准备。比如是否支持 POST 方法等。值得注意的是：
浏览器还发送 Access-Control-Request-Method: POST 和 Access-Control-Request-Headers: powered-by-mengxianhui 请求头。
注意：以上过程是第一次请求的时候的过程，如果在 30 秒内重复点击按钮，你可以看不到 OPTIONS 这一过程。则执行过程是这样的：
POST /PreflightedRequests.aspx HTTP/1.1
Host: dotnet.aspx.cc
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.2; zh-CN; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: zh-cn,zh;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: GB2312,utf-8;q=0.7,*;q=0.7
Keep-Alive: 300
Connection: keep-alive
POWERED-BY-MENGXIANHUI: Approve
Content-Type: application/xml; charset=UTF-8
Referer: http://www.meng_xian_hui.com:801/CrossDomainAjax/PreflightedRequests.html
Content-Length: 19
Origin: http://www.meng_xian_hui.com:801
Pragma: no-cache
Cache-Control: no-cache
<root>测试</root>
HTTP/1.x 200 OK
Date: Sun, 10 Jan 2010 14:06:32 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801
Set-Cookie: ASP.NET_SessionId=qs1c4urxywdbdx55u04pvual; path=/; HttpOnly
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 65
为什么会这样？细心的童鞋可能注意到了，在服务器端有一行代码 Response.AddHeader("Access-Control-Max-Age", "30"); 它是用来设置预检的有效时间的，单位是秒。这一点要特别注意。