[concurrency-interest] Reusing CompletableFuture

Martin Buchholz martinrb at google.com
Sun Jul 24 12:19:43 EDT 2016

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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