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From "Silas Davis (JIRA)" <j...@apache.org>
Subject [jira] [Created] (AVRO-1640) Does SpecificData.schemaCache actually do anything?
Date Mon, 23 Feb 2015 17:40:11 GMT
Silas Davis created AVRO-1640:

             Summary: 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

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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