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From "Phil Steitz" <>
Subject Re: [Pool] - GenericKeyedObjectPool - one object instance per key?
Date Fri, 20 Jul 2007 07:28:06 GMT
On 7/19/07, Nicholas Hemley <> wrote:
> Hello,
> I am using the commons pool for a project and would like only one object instance to
be stored per key using the GenericKeyedObjectPool - is this actually possible?

No, not directly.
> The javadoc for KeyedObjectPool states that:
> "KeyedObjectPool implementations may choose to store at most  one instance per key value,
or may choose to maintain a pool of instances  for each key (essentially creating a Map of
> However, having looked in the codebase, the GenericKeyedObjectPool implements a linkedlist
of objects, with no option for setting one object per key only...

The problem is not actually with the backing store, but the maxTotal
property not configurable per key.  If this is what you want, or
anyone else thinks this would be useful, please open a Jira ticket
asking for this enhancement.
> Also, I am unable to subclass GenericKeyedObjectPool to alter the behaviour since the
members are all private, so that is not an option.

One workaround would be to subclass and override borrowObject to check
numIdle(key) and numActive(key) before calling super.borrowObject().
If numIdle(key) is 1, borrow the instance. If it is 0 and
numActive(key) is 1, wait or fail, depending on config.  If both are
0, call addObject and then borrowObject.  This is awkward, but could
be made to work.

If what you really want is a KeyedObjectPool that has maxTotal
configurable per key, then open a JIRA (ideally with a patch) and we
can look at implementing that.


> Another option would be to add a custom implementation to my project, however certain
member variables e.g. GenericObjectPool.EVICTION_TIMER are not visible, which procludes this
approach. The only option would be to add a new class to the commons jar, which I am loathe
to do since then it will not be compatible with future releases e.g. v2.
> Many thanks for any feedback.
> Cheerio,
> Nic Hemley
> ---------------------------------
>  Yahoo! Answers - Get better answers from someone who knows. Tryit now.

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