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NNTP-Posting-Date: Tue, 15 May 2012 04:14:01 -0500
Date: Tue, 15 May 2012 10:14:01 +0100
From: bugbear <bugbear@trim_papermule.co.uk_trim>
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Subject: multithreaded cache?
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Xref: csiph.com comp.lang.java.programmer:14544

I'm using various Apache Commons Maps as a
multithread cache, protected using
ReentrantReadWriteLock, so that getting() uses a read lock,
and putting() uses a write lock.

But I've got an issue; in the
case of a cache miss (protected by a read lock),
the required value is acquired using the "underlying function"
that the cache is over; this value is then put() into
the cache (protected by a write lock)

This is all perfectly thread safe, and gives
correct results.

However, if the underlying function is slow
and/or resource hungry (consider cacheing
a ray traced image!) many threads can
end up calling the real function (second
and subsequent threads to the first get a miss
during the first threads call to the underlying function).

"obviously..." what I want is for only
the thread that FIRST has a cache miss
calls the underlying function, whilst other
threads (for the same key) wait.

This seems "obvious" enough that I'm guessing
there's a standard solution.

Googling led me to several "heavy" libraries;

This appears more a locking/cacheing issue
than a Java issue, although my implementation
is in Java.

Can anyone (please) point me at a
canonical solution/implementation?

  BugBear