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From Mark Thomas <>
Subject Re: [pool] equal instances
Date Thu, 09 Jun 2011 13:41:48 GMT
On 09/06/2011 10:01, Julien Aymé wrote:
> 2011/6/9 Mark Thomas <>:
>> On 09/06/2011 04:39, Phil Steitz wrote:
>>> Code in trunk now does not work when distinct pooled instances are
>>> equal - i.e., if a factory produces instances A and B and
>>> A.equals(B), this causes problems.   I think this situation should
>>> be allowed - i.e. it is an unacceptable restriction to put on object
>>> factories that distinct the poolable objects they produce be
>>> distinguishable under equals.  This would be a new requirement for
>>> [pool] and I don't think we should require it.  What do others think?
>> As I start to answer this, I can see a very long response developing. I
>> will do my best to keep it short. That may mean I gloss over some aspects.
>> The requirement that objects obtained from the factories meet
>> A.equals(B) == false greatly simplifies the implementation of a number
>> of requirements. Let me explain by using a single requirement although
>> there are a number of other requirements that have very similar
>> consequences.
>> The Requirement:
>> It shall not be possible to return an object to the pool more than once.
>> The pool maintains a list of idle objects. The simplest implementation
>> of the above requirement is to test if any returned object already
>> exists in the pool. This doesn't catch all scenarios but it is a start.
>> If we know that for objects obtained from the factories A.equals(B) ==
>> false then we can use a HashSet to store idle instances and it is very
>> easy to determine if the object being returned exists in the set of idle
>> objects. This makes determining if the object is being returned twice
>> relatively inexpensive. It also makes a reasonable multi-threaded
>> implementation possible.
> </snip>
> And what about using an IdentityHashSet (or IdentityHashMap) to store
> idle objects.
> This would meet the Requirement without having to enforce A.equals(B) == false.

That would be one of the aspects I glossed over. They aren't always
maps/sets and they need to support concurrent access by multiple threads.

A wrapper for pooled objects that uses System.identityHashCode(Object)
may be a possible solution that isn't too complex. It would add a
requirement for the pool to unwrap/wrap objects on borrow/return. I can
look at this if folks think the new restriction on factories is


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