304响应， CORS问题: 没有 Access-Control-Allow-Origin 这个头信息时，以前次返回的200请求为准。
示例：可能已被删除
http://7af3zm.com1.z0.glb.clouddn.com/ajax_304_cors.html
附上源码：
html
access: 200、304响应都有 Access-Control-Allow-Origin:*; access-non: 200、304响应都 没 有 Access-Control-Allow-Origin; access-200: 200响应都有 Access-Control-Allow-Origin:*, 304响应 没 有 Access-Control-Allow-Origin;
clojure
(ns base-web.handler (:require [compojure.core :refer :all] [compojure.route :as route] [ring.middleware.defaults :refer [wrap-defaults site-defaults]] [clojure.pprint :as log]))(def my-route (-> site-defaults (assoc-in [:security :anti-forgery] false) (assoc-in [:params] false) ))(defn print-req [req] (println ) (log/pprint my-route) (log/pprint req) (log/pprint (slurp (:body req))) (println "done"))(defn access-200 [req] (let [etag "e-access-200"] (if (not= etag (get-in req [:headers "if-none-match"])) (do (println "200") {:status 200, :headers {"Content-Type" "application/json", "Access-Control-Allow-Origin" "*", "ETag" etag} :body "{\"success\":true, \"frame\": \"ring\"}"}) (do (println "304") {:status 304, :headers {"Content-Type" "application/json", "ETag" etag}}))))(defn access-non [req] (let [etag "e-access-non"] (if (not= etag (get-in req [:headers "if-none-match"])) (do (println "200") {:status 200, :headers {"Content-Type" "application/json", "ETag" etag} :body "{\"success\":true, \"frame\": \"ring\"}"}) (do (println "304") {:status 304, :headers {"Content-Type" "application/json", "ETag" etag}}))))(defn access [req] (let [etag "e-access"] (if (not= etag (get-in req [:headers "if-none-match"])) (do (println "200") {:status 200, :headers {"Content-Type" "application/json", "Access-Control-Allow-Origin" "*", "ETag" etag} :body "{\"success\":true, \"frame\": \"ring\"}"}) (do (println "304") {:status 304, :headers {"Content-Type" "application/json", "ETag" etag}}))))(defroutes app-routes (GET "/access-200" req (access-200 req) ) (GET "/access-non" req (access-non req) ) (GET "/access" req (access req) ) (ANY "*" req (print-req req) "{\"success\":true, \"frame\": \"compojure\"}") (route/not-found "Not Found"))(def app (wrap-defaults app-routes my-route));; lein ring server []