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From "Michael McCandless (JIRA)" <>
Subject [jira] Updated: (LUCENE-1383) Workaround ThreadLocal's "leak"
Date Thu, 11 Sep 2008 21:05:44 GMT


Michael McCandless updated LUCENE-1383:

    Attachment: LUCENE-1383.patch

Attached patch.  All tests pass.

The patch adds o.a.l.util.CloseableThreadLocal.  It's a wrapper around ThreadLocal that wraps
the values inside a WeakReference, but then also holds a strong reference to the value (to
ensure GC doesn't reclaim it) until you call the close method.  On calling close, GC is then
free to reclaim all values you had stored, regardless of how long it takes ThreadLocal's implementation
to actually release its references.

There are a couple places in Lucene where I left the current usage of ThreadLocal.

First, uses ThreadLocal to hold reusable token streams.  There is no "close"
called for Analyzer, so unless we are willing to add a finalizer to call CloseableThreadLocal.close()
I think we can leave it.

Second, some of the contrib/benchmark tasks use ThreadLocal to store per-thread DateFormat
which should use tiny memory.

> Workaround ThreadLocal's "leak"
> -------------------------------
>                 Key: LUCENE-1383
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: Index
>    Affects Versions: 1.9, 2.0.0, 2.1, 2.2, 2.3, 2.3.1, 2.3.2
>            Reporter: Michael McCandless
>            Assignee: Michael McCandless
>             Fix For: 2.4
>         Attachments: LUCENE-1383.patch
> Java's ThreadLocal is dangerous to use because it is able to take a
> surprisingly very long time to release references to the values you
> store in it.  Even when a ThreadLocal instance itself is GC'd, hard
> references to the values you had stored in it are easily kept for
> quite some time later.
> While this is not technically a "memory leak", because eventually
> (when the underlying Map that stores the values cleans up its "stale"
> references) the hard reference will be cleared, and GC can proceed,
> its end behavior is not different from a memory leak in that under the
> right situation you can easily tie up far more memory than you'd
> expect, and then hit unexpected OOM error despite allocating an
> extremely large heap to your JVM.
> Lucene users have hit this many times.  Here's the most recent thread:
> And here's another:
> And then there's LUCENE-436 and LUCENE-529 at least.
> A google search for "ThreadLocal leak" yields many compelling hits.
> Sun does this for performance reasons, but I think it's a terrible
> trap and we should work around it with Lucene.

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