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https://issues.apache.org/jira/browse/LANG-334?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12516136
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Jason Madden commented on LANG-334:
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We have also encountered the issue of thread safety in the Enum
class. In our case, it shows up when running in a application server
where there are many threads and other applications are loading
classes at arbitrary times.
I've attached the patch that we are using which seems to correct this
issue for us. Rather than synchronize all access to the cEnumClasses
map (which seems like it would be rather expensive and highly
contended), we adopted a copy-on-write approach that only involves
synchronization when new classes are loaded.
> Enum is not thread-safe
> -----------------------
>
> Key: LANG-334
> URL: https://issues.apache.org/jira/browse/LANG-334
> Project: Commons Lang
> Issue Type: Bug
> Reporter: Michael Sclafani
> Fix For: 2.3.1
>
> Attachments: 334.patch, EnumPlay.java
>
>
> Enum uses no synchronization. Even if you assume that instances are only
> declared statically, the cEnumClasses map is global and can be written to
> when a thread triggers static initialization of B.class while some other
> thread is doing getEnumList(A.class). Unsynchronized access of a map
> undergoing mutation is not thread-safe.
> This isn't theoretical. We're seeing ValuedEnum.getEnum(X.class, 0) return
> null after returning the correct value over 100,000 times, and then return
> the correct value again on the next invocation.
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