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From Mark Thomas <>
Subject Re: [pool] equal instances
Date Thu, 09 Jun 2011 15:59:32 GMT
On 09/06/2011 14:50, sebb wrote:
> On 9 June 2011 14:41, Mark Thomas <> wrote:
>> 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
>> unacceptable.
> Note that System.identityHashCode() is not necessarily unique:

I think I might be able to work around that by asking the factory for
another object if it issues one with an identityHashCode we have already

> Could of course use == to disambiguate such objects.

That would have some nasty performance implications and may also needs
some syncs in a few places.


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