[concurrency-interest] Reusing CompletableFuture

Pavel Rappo pavel.rappo at gmail.com
Sun Jul 24 14:11:39 EDT 2016

Hi Martin,

Quite a strong statement. I don't think an isolated fact of mutability
of an entity precludes us from reusing this entity (pool of objects?
reusable buffers?).

You're talking about obtrudeException(Throwable ex) and obtrudeValue(T
value), right? In my example the CF was returned by an API to a user
and this CF was completed prior to be returned.

On Sun, Jul 24, 2016 at 5:19 PM, Martin Buchholz <martinrb at google.com> wrote:
> Don't reuse CompletableFutures.
> Most obviously because they're mutable.
> See also https://bugs.openjdk.java.net/browse/JDK-8161600
> On Sun, Jul 24, 2016 at 5:57 AM, Pavel Rappo <pavel.rappo at gmail.com> wrote:
>> Hi,
>> Is it generally safe to reuse an instance of CompletableFuture? Or am I
>> risking
>> with memory leaks due to dependants build up?
>> Say, I have a method that returns a CF. In some cases it may return a CF
>> straight away indicating that the action has been already done. Should I
>> always
>> create a new completed CF, or can I reuse a pre-cached one?
>> There's a comment in the class itself, but I'm not sure I understand it
>> enough
>> to make a conclusion:
>>      * ...Without precautions, CompletableFutures would be prone to
>>      * garbage accumulation as chains of Completions build up, each
>>      * pointing back to its sources. So we null out fields as soon as
>>      * possible.  The screening checks needed anyway harmlessly ignore
>>      * null arguments that may have been obtained during races with
>>      * threads nulling out fields.  We also try to unlink non-isLive
>>      * (fired or cancelled) Completions from stacks that might
>>      * otherwise never be popped: Method cleanStack always unlinks non
>>      * isLive completions from the head of stack; others may
>>      * occasionally remain if racing with other cancellations or
>>      * removals...
>> Thanks,
>> -Pavel
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