The Last-Modified entity header field value is often used as a cache validator.

Last-Modified 实体头部字段值通常用作一个缓存验证器。

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There is another header called the Content-Type header which is used to communicate the media type of the actual entity body.

还有另一种标头称为Content - Type标头，用来表达实际实体正文的媒体类型。

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The Expires entity header field gives the date and time after which the response is considered stale.

Expires 头部字段提供一个日期和时间，响应在该日期和时间后被认为失效。

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