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From Renè Glanzer <>
Subject Re: [collections] LRUMap Problem ConcurrentModificationException
Date Mon, 15 Jun 2009 12:31:51 GMT
I'm calling the remove() method on the iterator. I know when i call
the remove on the map itself it will cause the problem with my
currently running iterator, but i'm aware of that.

Here is the code block which should delete old entries:

store: is the LRUMap
birth: is a long which keeps the creation time when this is set to 0
the item should be deleted

public void removeAllExpiredItems()
       Object o=this.get(key);
       if(o != null)
         Item iEntry=(Item);
           it.remove(); //Here the exception occurs
     this.setLastCleanDate(new Date()); //only to know when the
deleter run the last time

Thanks for any help :-)

2009/6/15 James Carman <>:
> Are you calling remove() on the iterator or on the map itself?
> On Mon, Jun 15, 2009 at 6:37 AM, Renè Glanzer<> wrote:
>> Hello,
>> is there still no help for me?
>> Is somebody able to explain me, why i get this
>> "java.util.ConcurrentModificationException"
>> on iterating and calling remove() on the LRUMap?
>> Please
>> René
>> 2009/6/10 Renè Glanzer <>:
>>> 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
>>> class.
>>> 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
>>>> 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
>>>> 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,
>>>> 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
>>>> 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 <>
>>>>> ... Additionally to the maximum number of the LRUMap i also implemented
>>>>> 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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