Uwe Schindler created LUCENE-5887:
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Summary: 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
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 VirtualIdentityMap, the remaining one is
the ByteBuffer stuff in MMapDirectory. Unfortunately I have still no idea how
to remove that one... :(
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