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From Renè Glanzer <>
Subject Re: [collections] LRUMap Problem ConcurrentModificationException
Date Wed, 10 Jun 2009 07:43:52 GMT
Hello Ted,

thanks for the fast response. I understand that simultaneously puts
and gets do not cause the exception cause they are synchronized in my
And my stated code-fragment is the part which wants to delete the
entries from the underlying LRUMap. And my code is wrapped in the
synchronized block. But i think the LRUMap herselfe also iterates the
entries and this maybe happens at the same time when my iterator is
checking and trying to delete elements. My class is not implementing
the LRUMap but has one LRUMap as a variable.
So your solution to override the removeLRU(Entry) methode would not
help me....unfortunatelly :-(

For now i really do not know how to solve the problem in an easy way
without refractoring the entire code?
Any other solutions?
Thanks René

2009/6/9 Ted Dunning <>:
> I apologize for not reading your thread with great care, but I think that
> your problem is pretty clear.
> The issue is not to do with gets and puts overlapping.  The issue is that a
> put or remove happened during the life of your iterator.
> One way to avoid this is to synchronize the entire method that does the
> deletion of old elements.  To speed that up, you might just synchronize the
> scan for elements to delete and collect them into a list.  Then you can
> delete them outside the loop.  Since your scan is probably pretty fast, this
> may be sufficient.  With very high levels of updates and threading, it would
> cause problems.
> Another option is to clone the table.  I believe that some implementations
> of LRUMap in Lucene do copy-on-write semantics, but I don't think that
> collections has these.  Without that, cloning will be as slow or slower than
> your scan so the first option would be better.
> I am curious, however, why you didn't make use of the built-in capabilities
> of the LRUMap to help you with this.  Notably, you should probably just
> over-ride the removeLRU(Entry) method and set the scanUntilRemovable flag.
> I think that this would take the place of your loop and would be much more
> efficient.
> On Tue, Jun 9, 2009 at 9:33 AM, Renè Glanzer <> wrote:
>> ... Additionally to the maximum number of the LRUMap i also implemented a
>> thread which periodicly checks the age of the entries and deletes them
>> when they are too old.
>> For this i generated an Iterator which delivers me each entry and if
>> the creation date is too old i call iterator.remove().
>> But exactly on this line i get an
>> "java.util.ConcurrentModificationException" although i wrapped all
>> access to the map with synchronized blocks. So every put and get
>> methods are synchronized.

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