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https://issues.apache.org/jira/browse/AVRO-1640?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16132166#comment-16132166
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Sean Busbey commented on AVRO-1640:
-----------------------------------
this would be much better as a DISCUSS thread on dev@, could someone please
bring it up there?
> Does SpecificData.schemaCache actually do anything?
> ---------------------------------------------------
>
> Key: AVRO-1640
> URL: https://issues.apache.org/jira/browse/AVRO-1640
> Project: Avro
> Issue Type: Bug
> Components: java
> Affects Versions: 1.7.7
> Reporter: Silas Davis
> Priority: Minor
> Labels: should_be_dev_discussion
>
> I've been looking at the source code for SpecificData. On line 182 a
> schemaCache is defined as a WeakHashMap.
> Having looked into how WeakHashMap works - it stores a WeakReference for the
> keys of the map but not the values, I can't see how the schemaCache actually
> provides a cache. I think it just stores all Schema objects it has ever seen,
> and never expires any. This is because it stores keys of type
> java.lang.reflect.Type key, typlically a Class<?>, which typically never fall
> out of reference. The only way the Class (a static singleton object) will be
> garbage collected is if the ClassLoader that holds a reference to it is
> garbage collected. This mostly doesn't happen, so the schemaCache is really
> just behaving as a HashMap.
> It seems like WeakHashMap is commonly mistakenly used as a cache:
> -
> http://stackoverflow.com/questions/1802809/javas-weakhashmap-and-caching-why-is-it-referencing-the-keys-not-the-values
> -
> http://www.codeinstructions.com/2008/09/weakhashmap-is-not-cache-understanding.html
> Perhaps I am missing something, like Avro's use of ClassLoaders meaning that
> Class objects do get collected, apologies if so.
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