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Preflighted requests

Unlike simple requests (discussed above), "preflighted" requests first send an HTTP request by the OPTIONS method to the resource on the other domain, in order to determine whether the actual request is safe to send.  Cross-site requests are preflighted like this since they may have implications to user data.  In particular, a request is preflighted if:

• It uses methods other than GET, HEAD or POST.  Also, if POST is used to send request data with a Content-Type other than application/x-www-form-urlencoded, multipart/form-data, or text/plain, e.g. if the POST request sends an XML payload to the server using application/xml or text/xml, then the request is preflighted.
• It sets custom headers in the request (e.g. the request uses a header such as X-PINGOTHER)

https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS

http://91.213.30.153/url?sa=t&rct=j&q=Preflight+Request+server&source=web&cd=1&ved=0CBsQFjAA&url=%68%74%74%70%3a%2f%2f%77%77%77%2e%68%74%6d%6c%35%72%6f%63%6b%73%2e%63%6f%6d%2f%65%6e%2f%74%75%74%6f%72%69%61%6c%73%2f%63%6f%72%73%2f&ei=LysyVczBGKbC7gbQyoGIDw&usg=AFQjCNEqGW6hrCKA6tJWxPT0-r66XZGgeQ&bvm=bv.91071109,d.bGg&cad=rjt

https://spring.io/understanding/CORS

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原文：http://www.cnblogs.com/aloha/p/4437789.html