The Entry type, which is used to store the key-value mapping, extends WeakReference, so when expungeStaleEntries() asks for the next expired weak reference, it gets back an Entry.
用于存储键-值映射的 Entry 类型扩展了 WeakReference，因此当 expungeStaleEntries() 要求下一个失效的弱引用时，它得到一个 Entry。
When a feed reader accesses an Atom feed or entry that has expired, it should alert the user in some way.
当feed阅读器访问一个已经过期的Atom feed或条目时，它会以某种方式向用户发出警告。
If, on the other hand, the content is not in the cache or the cache entry has expired, then the edge server makes a request to the origin server to retrieve the information.
另一方面，如果在请求的内容不在缓存中或缓存已经过期，边缘服务器就会向源服务器发送请求来获得内容。
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