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https://issues.apache.org/jira/browse/LUCENE-5887?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14098455#comment-14098455
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ASF subversion and git services commented on LUCENE-5887:
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Commit 1618148 from [~thetaphi] in branch 'dev/trunk'
[ https://svn.apache.org/r1618148 ]
LUCENE-5887: Remove WeakIdentityMap caching in AttributeFactory,
AttributeSource, and VirtualMethod in favour of Java 7's ClassValue. Always use
MethodHandles to create AttributeImpl classes.
> Remove horrible WeakIdentityMap caching in AttributeFactory, AttributeSource
> and VirtualMethod
> ----------------------------------------------------------------------------------------------
>
> Key: LUCENE-5887
> URL: https://issues.apache.org/jira/browse/LUCENE-5887
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 5.0, 4.10
>
> Attachments: LUCENE-5887.patch, LUCENE-5887.patch
>
>
> Especially the use case in AttributeFactory is horrible:
> Because of ClassLoader issues we cannot hold strong references (see
> LUCENE-5640 for explanation), we need WeakIdentityMap<Class,
> WeakReference<someVal>>. You could say: let's use a strong value for stuff
> like MethodHandles (used in AttributeFactory), but because those have a
> strong reference to the class, our reference to key would be strong, so
> garbage collector can no longer unload the class. This is why we use the
> WeakReference also on the value.
> The problem is if the value is something like a MethodHandle, which itsself
> has hard reference to (so it gets garbage collected). Then the cache is
> useless.
> In DefaultAttributeFactory I decided, to make methodhandles strong
> references, but then I needed to restrict it to our own classloader,
> otherwise we would have strong references to foreign classloaders.
> Since Java 7 there is java.lang.ClassValue, that fixes the following JVM bug:
> http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6389107
> See also: http://stackoverflow.com/questions/7444420/classvalue-in-java-7
> In fact internally, there is a also a WeakReference/WeakHashMap used, but
> only as fallback - and its only one globally, used by many other JVM
> internals, too. By default it has a very fast path and the call to
> ClassValue.get() is incredibly fast. This should therefore also improve
> AttributeFactory alltogether.
> Next to AttributeFactory, I also improved the Interfaces cache of
> AttributeSource (this one assigns an array of Attribute interfaces to an
> AttributeImpl). The other one is VirtualMethod (assigns its own
> implementationDistance for every seen subclass).
> This also removes almost all uses of WeakIdentityMap, the remaining one is
> the ByteBuffer stuff in MMapDirectory. Unfortunately I have still no idea how
> to remove that one... :(
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