接上一篇讨论实现静态资源访问的问题，除了可部分获取资源，还有条件参数，服务器端判断当前资源状况满足这些条件才处理请求。

这些条件有If-Match,If-Modified-Since,If-Range,If-Unmodified-Since,If-None-Match等。最常见的If-None-Match这个header，用来在获取资源时比较本地的etag，如果服务器端不一致才获取。

先获取一个资源的etag:

$ curl -I http://localhost:8080/chain.jpg

HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Accept-Ranges: bytes
ETag: W/"4932-1448300760000"
Last-Modified: Tue, 24 Nov 2015 01:46:00 CST
Content-Type: image/jpeg
Content-Length: 4932
Date: Mon, 23 Nov 2015 18:12:49 GMT

然后在请求header里增加If-None-Match内容：

$ curl -v -H 'If-None-Match: W/"4932-1448300760000"'  http://localhost:8080/chain.jpg

* Hostname was NOT found in DNS cache
*   Trying ::1...
* Connected to localhost (::1) port 8080 (#0)
> GET /_files_/img/chain.jpg HTTP/1.1
> User-Agent: curl/7.37.1
> Host: localhost:8080
> Accept: */*
> If-None-Match: W/"4932-1448300760000"
>
< HTTP/1.1 304 Not Modified
* Server Apache-Coyote/1.1 is not blacklisted
< Server: Apache-Coyote/1.1
< ETag: W/"4932-1448300760000"
< Date: Mon, 23 Nov 2015 18:13:01 GMT
<
* Connection #0 to host localhost left intact

etag相等的话，条件不符合返回304。