上篇介绍如何解决跨域请求问题，后来在使用过程中发现，通过XHR发送get/post请求时，都会派发options请求，如下图
后来查资料发现是CORS里的preflight预检机制HTTP access control (CORS) by MDN
在CORS标准中，定义了新的HTTP消息头Access-Control-Allow-Origin，使得服务端可以定义允许通过浏览器请求的域集合。另外，标准定义了当跨域影响用户数据HTTP请求(如用XMLHttpRequest发送get/post)时，浏览器会发送预检请求(OPTIONS请求)给服务端征求支持的请求方法，然后根据服务端响应允许才发送真正的请求
OPTIONS Request Headers
OPTIONS /cvbs/api/offer/querySubsPlanList HTTP/1.1
Host: 10.45.7.243:8080
Connection: keep-alive
Pragma: no-cache
Cache-Control: no-cache
Access-Control-Request-Method: POST
Origin: http://localhost:63342
User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2272.76 Safari/537.36
Access-Control-Request-Headers: accept, content-type
Accept: */*
Referer: http://localhost:63342/WebContent/index.html
Accept-Encoding: gzip, deflate, sdch
Accept-Language: en,zh;q=0.8,en-US;q=0.6,zh-CN;q=0.4
OPTIONS Response Header
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Access-Control-Allow-Origin: http://localhost:63342
Access-Control-Allow-Credentials: true
Access-Control-Max-Age: 1800
Access-Control-Allow-Methods: POST
Access-Control-Allow-Headers: content-type,access-control-request-headers,access-control-request-method,accept,origin,x-requested-with
Content-Length: 0
Date: Wed, 11 Mar 2015 05:16:31 GMT