[concurrency-interest] ConcurrentReferenceMap enhancement to 166 - Feedback Requested

Bob Lee crazybob at crazybob.org
Thu Apr 17 16:21:40 EDT 2008


On Thu, Apr 17, 2008 at 11:49 AM, Jason T. Greene <jason.greene at redhat.com>
wrote:

> I disagree. I think object lifetime and equality are orthogonal. Just
> because a key can be collected earlier than a normal java reference, does
> not mean that a map containing it should allow semantical duplicates, or not
> resolve an object that is semantically equivalent to a key.
>

You might be right, but after three years of heavy use, we have yet to
encounter a use case which supports your assertion that we want equality
instead of identity comparisons for weak and soft references. Not that it's
not trivial to support both strategies, but I do like to steer users in the
right direction. I've found that users have always needed identity
comparisons, and I've also seen cases where using equality with weak
references would have introduced subtle bugs (Class objects for example).


> You're "interning" use case is a red herring--ReferenceMap is not an
>> efficient way to implement that.
>>
>
> I am referring to an intern pool that does not grow without bounds (could
> be soft or weak), not the exact implementation of String.intern().
>

I knew what you meant. We implement that sort of thing more efficiently and
cleanly with a different data structure, so we shouldn't bend ReferenceMap
too hard toward that use case.


> It's not a huge deal to require one additional thread, however as more and
> more frameworks do this, it creates a problem in a shared environment. Then
> once you need a pool you now have yet another management aspect. So for this
> to be practical IMO it needs to be part of the JVM.
>

Like I said, we only need one thread here for the whole VM and we can even
reuse an existing thread and avoid the overhead of a reference queue.


> The goal of the lazy implementation is to self-scale, since cleanup is
> reusing mutating threads. It also has a nice secondary benefits of not
> requiring changes to the reference system, so the candidate is easily
> backported (the implementation I posted runs on JDK5).


We've been running ReferenceMap on Java 5 for years.

The last thread I saw, he was asking you and Kevin for feedback
> (specifically API feedback), and since you had not yet responded, and others
> where asking for this functionality, we decided to move the discussion here.


I was forwarded an email where you sent out what amounted to a concurrent
version of WeakHashMap, and we provided feedback--we pointed you to
ReferenceMap.

Bob
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