Ephemerons for Java

[John Rose]

On Jan 9, 2017, at 4:36 PM, Jochen Theodorou <blackdrag at gmx.org> wrote:
> 
> Hi all,
> 
> This is kind of a feature request... What I would like to have is a new Reference variant that allows me to keep an instance alive for as long as the class of the object is alive, without the instance keeping the class alive.
> 
> In the end I would like to be able to store values, but don´t keep the classes involved from being garbage collected. I am especially thinking here about a combination of ClassValue and this Ephemeron (?) that allows me to cache values on classes, without preventing class unloading.
> 
> Is there any chance for this making its way into the JVM? What would prevent this?

This is a good thing to file a JEP on.

I think the main reason we don't have such a thing yet is the extraordinary difficulty of correctly specifying such a thing, where "correct" means "covers enough use cases to be worth it" (your example is just one use case) and also means "has a natural implementation that is not too complex".  And also means "is unambiguous and precise".  And, come to think of it, also means "fits into the Java ecosystem, especially with respect to tradeoffs involving determinism or static typing".  In the case of ephemerons there is some good literature but nobody has translated it into a compelling Java API.  AFAIK.  Anyway, a correct spec. is the bottleneck.  After that it's just budgeting resources and/or awaiting volunteers.

— John