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<a href="http://www.ruanyifeng.com/blog/2016/04/cors.html" _src="http://www.ruanyifeng.com/blog/2016/04/cors.html">http://www.ruanyifeng.com/blog/2016/04/cors.html</a> CORS是一个W3C标准，全称是&quot;跨域资源共享&quot;（Cross-origin resource sharing）。它允许浏览器向跨源服务器，发出XMLHttpRequest请求，从而克服了AJAX只能同源使用的限制。本文详细介绍CORS的内部机制。 <h2 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 2.88em; line-height: 1.35em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">一、简介</h2><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">CORS需要浏览器和服务器同时支持。目前，所有浏览器都支持该功能，IE浏览器不能低于IE10。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">整个CORS通信过程，都是浏览器自动完成，不需要用户参与。对于开发者来说，CORS通信与同源的AJAX通信没有差别，代码完全一样。浏览器一旦发现AJAX请求跨源，就会自动添加一些附加的头信息，有时还会多出一次附加的请求，但用户不会有感觉。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">因此，实现CORS通信的关键是服务器。只要服务器实现了CORS接口，就可以跨源通信。<h2 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 2.88em; line-height: 1.35em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">二、两种请求</h2><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">浏览器将CORS请求分成两类：简单请求（simple request）和非简单请求（not-so-simple request）。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">只要同时满足以下两大条件，就属于简单请求。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;">（1) 请求方法是以下三种方法之一：<ul style="list-style-type: none;" class=" list-paddingleft-2"><li>HEAD</li><li>GET</li><li>POST</li></ul>（2）HTTP的头信息不超出以下几种字段：<ul style="list-style-type: none;" class=" list-paddingleft-2"><li>Accept</li><li>Accept-Language</li><li>Content-Language</li><li>Last-Event-ID</li><li>Content-Type：只限于三个值<code style="margin: 0px; padding: 0px; list-style-type: none; border: none;">application/x-www-form-urlencoded</code>、<code style="margin: 0px; padding: 0px; list-style-type: none; border: none;">multipart/form-data</code>、<code style="margin: 0px; padding: 0px; list-style-type: none; border: none;">text/plain</code></li></ul></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">凡是不同时满足上面两个条件，就属于非简单请求。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">浏览器对这两种请求的处理，是不一样的。<h2 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 2.88em; line-height: 1.35em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">三、简单请求</h2><h3 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 1.87em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">3.1 基本流程</h3><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">对于简单请求，浏览器直接发出CORS请求。具体来说，就是在头信息之中，增加一个<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">下面是一个例子，浏览器发现这次跨源AJAX请求是简单请求，就自动在头信息之中，添加一个<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">GET&nbsp;/cors&nbsp;HTTP/1.1Origin:&nbsp;http://api.bob.comHost:&nbsp;api.alice.comAccept-Language:&nbsp;en-USConnection:&nbsp;keep-aliveUser-Agent:&nbsp;Mozilla/5.0...</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面的头信息中，<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段用来说明，本次请求来自哪个源（协议 + 域名 + 端口）。服务器根据这个值，决定是否同意这次请求。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">如果<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>指定的源，不在许可范围内，服务器会返回一个正常的HTTP回应。浏览器发现，这个回应的头信息没有包含<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Origin</code>字段（详见下文），就知道出错了，从而抛出一个错误，被<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">XMLHttpRequest</code>的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">onerror</code>回调函数捕获。注意，这种错误无法通过状态码识别，因为HTTP回应的状态码有可能是200。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">如果<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>指定的域名在许可范围内，服务器返回的响应，会多出几个头信息字段。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">Access-Control-Allow-Origin:&nbsp;http://api.bob.comAccess-Control-Allow-Credentials:&nbsp;trueAccess-Control-Expose-Headers:&nbsp;FooBarContent-Type:&nbsp;text/html;&nbsp;charset=utf-8</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面的头信息之中，有三个与CORS请求相关的字段，都以<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-</code>开头。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（1）Access-Control-Allow-Origin<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段是必须的。它的值要么是请求时<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段的值，要么是一个<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">*</code>，表示接受任意域名的请求。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（2）Access-Control-Allow-Credentials<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段可选。它的值是一个布尔值，表示是否允许发送Cookie。默认情况下，Cookie不包括在CORS请求之中。设为<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">true</code>，即表示服务器明确许可，Cookie可以包含在请求中，一起发给服务器。这个值也只能设为<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">true</code>，如果服务器不要浏览器发送Cookie，删除该字段即可。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（3）Access-Control-Expose-Headers<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段可选。CORS请求时，<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">XMLHttpRequest</code>对象的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">getResponseHeader()</code>方法只能拿到6个基本字段：<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Cache-Control</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Content-Language</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Content-Type</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Expires</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Last-Modified</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Pragma</code>。如果想拿到其他字段，就必须在<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Expose-Headers</code>里面指定。上面的例子指定，<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">getResponseHeader(&#39;FooBar&#39;)</code>可以返回<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">FooBar</code>字段的值。<h3 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 1.87em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">3.2 withCredentials 属性</h3><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面说到，CORS请求默认不发送Cookie和HTTP认证信息。如果要把Cookie发到服务器，一方面要服务器同意，指定<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Credentials</code>字段。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">Access-Control-Allow-Credentials:&nbsp;true</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">另一方面，开发者必须在AJAX请求中打开<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">withCredentials</code>属性。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-javascript" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">var&nbsp;xhr&nbsp;=&nbsp;new&nbsp;XMLHttpRequest();xhr.withCredentials&nbsp;=&nbsp;true;</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">否则，即使服务器同意发送Cookie，浏览器也不会发送。或者，服务器要求设置Cookie，浏览器也不会处理。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">但是，如果省略<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">withCredentials</code>设置，有的浏览器还是会一起发送Cookie。这时，可以显式关闭<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">withCredentials</code>。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-javascript" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">xhr.withCredentials&nbsp;=&nbsp;false;</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">需要注意的是，如果要发送Cookie，<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Origin</code>就不能设为星号，必须指定明确的、与请求网页一致的域名。同时，Cookie依然遵循同源政策，只有用服务器域名设置的Cookie才会上传，其他域名的Cookie并不会上传，且（跨源）原网页代码中的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">document.cookie</code>也无法读取服务器域名下的Cookie。<h2 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 2.88em; line-height: 1.35em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">四、非简单请求</h2><h3 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 1.87em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">4.1 预检请求</h3><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">非简单请求是那种对服务器有特殊要求的请求，比如请求方法是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">PUT</code>或<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">DELETE</code>，或者<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Content-Type</code>字段的类型是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">application/json</code>。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">非简单请求的CORS请求，会在正式通信之前，增加一次HTTP查询请求，称为&quot;预检&quot;请求（preflight）。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">浏览器先询问服务器，当前网页所在的域名是否在服务器的许可名单之中，以及可以使用哪些HTTP动词和头信息字段。只有得到肯定答复，浏览器才会发出正式的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">XMLHttpRequest</code>请求，否则就报错。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">下面是一段浏览器的JavaScript脚本。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-javascript" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">var&nbsp;url&nbsp;=&nbsp;&#39;http://api.alice.com/cors&#39;;var&nbsp;xhr&nbsp;=&nbsp;new&nbsp;XMLHttpRequest();xhr.open(&#39;PUT&#39;,&nbsp;url,&nbsp;true);xhr.setRequestHeader(&#39;X-Custom-Header&#39;,&nbsp;&#39;value&#39;);xhr.send();</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面代码中，HTTP请求的方法是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">PUT</code>，并且发送一个自定义头信息<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">X-Custom-Header</code>。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">浏览器发现，这是一个非简单请求，就自动发出一个&quot;预检&quot;请求，要求服务器确认可以这样请求。下面是这个&quot;预检&quot;请求的HTTP头信息。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">OPTIONS&nbsp;/cors&nbsp;HTTP/1.1Origin:&nbsp;http://api.bob.comAccess-Control-Request-Method:&nbsp;PUTAccess-Control-Request-Headers:&nbsp;X-Custom-HeaderHost:&nbsp;api.alice.comAccept-Language:&nbsp;en-USConnection:&nbsp;keep-aliveUser-Agent:&nbsp;Mozilla/5.0...</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">&quot;预检&quot;请求用的请求方法是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">OPTIONS</code>，表示这个请求是用来询问的。头信息里面，关键字段是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>，表示请求来自哪个源。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">除了<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段，&quot;预检&quot;请求的头信息包括两个特殊字段。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（1）Access-Control-Request-Method<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段是必须的，用来列出浏览器的CORS请求会用到哪些HTTP方法，上例是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">PUT</code>。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（2）Access-Control-Request-Headers<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段是一个逗号分隔的字符串，指定浏览器CORS请求会额外发送的头信息字段，上例是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">X-Custom-Header</code>。<h3 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 1.87em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">4.2 预检请求的回应</h3><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">服务器收到&quot;预检&quot;请求以后，检查了<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>、<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Request-Method</code>和<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Request-Headers</code>字段以后，确认允许跨源请求，就可以做出回应。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">HTTP/1.1&nbsp;200&nbsp;OKDate:&nbsp;Mon,&nbsp;01&nbsp;Dec&nbsp;2008&nbsp;01:15:39&nbsp;GMTServer:&nbsp;Apache/2.0.61&nbsp;(Unix)Access-Control-Allow-Origin:&nbsp;http://api.bob.comAccess-Control-Allow-Methods:&nbsp;GET,&nbsp;POST,&nbsp;PUTAccess-Control-Allow-Headers:&nbsp;X-Custom-HeaderContent-Type:&nbsp;text/html;&nbsp;charset=utf-8Content-Encoding:&nbsp;gzipContent-Length:&nbsp;0Keep-Alive:&nbsp;timeout=2,&nbsp;max=100Connection:&nbsp;Keep-AliveContent-Type:&nbsp;text/plain</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面的HTTP回应中，关键的是<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Origin</code>字段，表示<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">http://api.bob.com</code>可以请求数据。该字段也可以设为星号，表示同意任意跨源请求。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">Access-Control-Allow-Origin:&nbsp;*</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">如果浏览器否定了&quot;预检&quot;请求，会返回一个正常的HTTP回应，但是没有任何CORS相关的头信息字段。这时，浏览器就会认定，服务器不同意预检请求，因此触发一个错误，被<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">XMLHttpRequest</code>对象的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">onerror</code>回调函数捕获。控制台会打印出如下的报错信息。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-bash" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">XMLHttpRequest&nbsp;cannot&nbsp;load&nbsp;http://api.alice.com.Origin&nbsp;http://api.bob.com&nbsp;is&nbsp;not&nbsp;allowed&nbsp;by&nbsp;Access-Control-Allow-Origin.</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">服务器回应的其他CORS相关字段如下。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">Access-Control-Allow-Methods:&nbsp;GET,&nbsp;POST,&nbsp;PUTAccess-Control-Allow-Headers:&nbsp;X-Custom-HeaderAccess-Control-Allow-Credentials:&nbsp;trueAccess-Control-Max-Age:&nbsp;1728000</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（1）Access-Control-Allow-Methods<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段必需，它的值是逗号分隔的一个字符串，表明服务器支持的所有跨域请求的方法。注意，返回的是所有支持的方法，而不单是浏览器请求的那个方法。这是为了避免多次&quot;预检&quot;请求。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（2）Access-Control-Allow-Headers<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">如果浏览器请求包括<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Request-Headers</code>字段，则<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Headers</code>字段是必需的。它也是一个逗号分隔的字符串，表明服务器支持的所有头信息字段，不限于浏览器在&quot;预检&quot;中请求的字段。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（3）Access-Control-Allow-Credentials<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段与简单请求时的含义相同。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">（4）Access-Control-Max-Age<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">该字段可选，用来指定本次预检请求的有效期，单位为秒。上面结果中，有效期是20天（1728000秒），即允许缓存该条回应1728000秒（即20天），在此期间，不用发出另一条预检请求。<h3 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 1.87em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">4.3 浏览器的正常请求和回应</h3><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">一旦服务器通过了&quot;预检&quot;请求，以后每次浏览器正常的CORS请求，就都跟简单请求一样，会有一个<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>头信息字段。服务器的回应，也都会有一个<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Origin</code>头信息字段。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">下面是&quot;预检&quot;请求之后，浏览器的正常CORS请求。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">PUT&nbsp;/cors&nbsp;HTTP/1.1Origin:&nbsp;http://api.bob.comHost:&nbsp;api.alice.comX-Custom-Header:&nbsp;valueAccept-Language:&nbsp;en-USConnection:&nbsp;keep-aliveUser-Agent:&nbsp;Mozilla/5.0...</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面头信息的<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Origin</code>字段是浏览器自动添加的。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">下面是服务器正常的回应。<blockquote style="margin: 2em; padding: 1em; list-style-type: none; border: 0.3em solid rgb(224, 223, 204); color: rgb(17, 17, 17); background-color: rgb(245, 242, 240); border-radius: 1em; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; text-shadow: white 0px 1px; overflow: auto; font-size: 12px; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px;"><pre class=" language-http" style="margin-top: 0px; margin-bottom: 0px; padding: 0em 0em 0.5em; list-style-type: none; border: none; color: black; text-shadow: white 0px 1px; font-family: Consolas, Monaco, &quot;Andale Mono&quot;, monospace; direction: ltr; word-spacing: normal; font-size: 1.2em; tab-size: 4; hyphens: none; overflow: auto; background-color: rgb(245, 242, 240);">Access-Control-Allow-Origin:&nbsp;http://api.bob.comContent-Type:&nbsp;text/html;&nbsp;charset=utf-8</pre></blockquote><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">上面头信息中，<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">Access-Control-Allow-Origin</code>字段是每次回应都必定包含的。<h2 style="margin: 1.5em 0px 0.2em; padding: 0px 0px 0.2em; list-style-type: none; font-weight: 500; border-top: none; border-right: none; border-bottom: 1px solid rgb(211, 211, 211); border-left: none; border-image: initial; font-size: 2.88em; line-height: 1.35em; letter-spacing: -0.03em; font-family: Georgia, serif; white-space: normal; word-spacing: 2.4px; ">五、与JSONP的比较</h2><p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">CORS与JSONP的使用目的相同，但是比JSONP更强大。<p style="margin-top: 1em; margin-bottom: 0px; margin-left: 0.8em; padding: 0px; list-style-type: none; border: none; color: rgb(17, 17, 17); font-size: 1.6em; line-height: 28.8px; font-family: Georgia, serif; letter-spacing: -0.12px; white-space: normal; word-spacing: 2.4px; ">JSONP只支持<code style="margin: auto 3px; padding: 0px 5px; list-style-type: none; border: none; display: inline-block; font-size: 15.6px; background-color: pink; border-radius: 5px;">GET</code>请求，CORS支持所有类型的HTTP请求。JSONP的优势在于支持老式浏览器，以及可以向不支持CORS的网站请求数据。