AJAX（XMLHttpRequest）进行跨域请求方法详解（二）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
AJAX（XMLHttpRequest）进行跨域请求方法详解（二）
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' onclick='crossDomainRequest()' /> <div id="content"></div> <mce:script type="text/javascript"><!-- 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; } }// --></mce: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#" %><mce: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。"); } } }// --></mce: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/1.1Host: dotnet.aspx.ccUser-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=0.8Accept-Language: zh-cn,zh;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: GB2312,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-aliveOrigin: http://www.meng_xian_hui.com:801Access-Control-Request-Method: POSTAccess-Control-Request-Headers: powered-by-mengxianhuiHTTP/1.x 200 OKDate: Sun, 10 Jan 2010 14:00:34 GMTServer: Microsoft-IIS/6.0X-Powered-By: ASP.NETX-AspNet-Version: 2.0.50727Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801Access-Control-Allow-Methods: POST, GET, OPTIONSAccess-Control-Allow-Headers: POWERED-BY-MENGXIANHUIAccess-Control-Max-Age: 30Set-Cookie: ASP.NET_SessionId=5npqri55dl1k1zvij1tlw3re; path=/; HttpOnlyCache-Control: privateContent-Length: 0POST /PreflightedRequests.aspx HTTP/1.1Host: dotnet.aspx.ccUser-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=0.8Accept-Language: zh-cn,zh;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: GB2312,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-alivePOWERED-BY-MENGXIANHUI: ApproveContent-Type: application/xml; charset=UTF-8Referer: http://www.meng_xian_hui.com:801/CrossDomainAjax/PreflightedRequests.htmlContent-Length: 19Origin: http://www.meng_xian_hui.com:801Pragma: no-cacheCache-Control: no-cache<root>测试</root>HTTP/1.x 200 OKDate: Sun, 10 Jan 2010 14:00:34 GMTServer: Microsoft-IIS/6.0X-Powered-By: ASP.NETX-AspNet-Version: 2.0.50727Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801Set-Cookie: ASP.NET_SessionId=byvose45zmtbqy45d2a1jf2i; path=/; HttpOnlyCache-Control: privateContent-Type: text/html; charset=utf-8Content-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/1.1Host: dotnet.aspx.ccUser-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=0.8Accept-Language: zh-cn,zh;q=0.5Accept-Encoding: gzip,deflateAccept-Charset: GB2312,utf-8;q=0.7,*;q=0.7Keep-Alive: 300Connection: keep-alivePOWERED-BY-MENGXIANHUI: ApproveContent-Type: application/xml; charset=UTF-8Referer: http://www.meng_xian_hui.com:801/CrossDomainAjax/PreflightedRequests.htmlContent-Length: 19Origin: http://www.meng_xian_hui.com:801Pragma: no-cacheCache-Control: no-cache<root>测试</root>HTTP/1.x 200 OKDate: Sun, 10 Jan 2010 14:06:32 GMTServer: Microsoft-IIS/6.0X-Powered-By: ASP.NETX-AspNet-Version: 2.0.50727Access-Control-Allow-Origin: http://www.meng_xian_hui.com:801Set-Cookie: ASP.NET_SessionId=qs1c4urxywdbdx55u04pvual; path=/; HttpOnlyCache-Control: privateContent-Type: text/html; charset=utf-8Content-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。
