RFR: 8314480: Memory ordering spec updates in java.lang.ref [v31]

[Brent Christian]

On 5/10/24 10:54 AM, Hans Boehm wrote:
> I'm not convinced this helps.
> 
> The isAlive() spec says:
> 
> "A thread is alive if it has been started and has not yet terminated."
> 
> Clearly an object is reachable if it can be accessed by a thread that will be started in the 
> future. Is that part of a "potential continuing computation from any live thread"?

I would think; one "computation" a live thread can perform is to start another thread, which in
turn might access the object.

> I think the JLS wording is a bit sloppy about what "live thread" means here. And that sloppiness 
> is probably better than pointing at a precise definition that I'm not sure really applies.
> 
> "in any potential continuing computation from any live thread" really seems to mean something 
> like "in any continuing computation in which no finalizers are run"?

Once an object becomes finalizer-reachable, it can only be reached via a running finalizer. (JLS
12.6.1: "A finalizer-reachable object can be reached from some finalizable object through some chain
of references, but not from any live thread.")

So maybe finalizer threads should not be considered "live" threads.

> Even if the object is later accessed from an existing "live" thread, it should not be considered
>  reachable if that happens only after a finalizer later makes it reachable again.

Agreed - if an object can (*and only can*) be accessed again after a finalizer resurrects it, that
doesn't count as "reachable". In fact, at that point, the object must have transitioned from 
reachable to finalizer-reachable.

If an object gets resurrected by a finalizer, it does become reachable again and can again be 
accessed by the program. (And of course if the object's own finalizer ran, it won't run again if the 
object again stops being reachable.)

> So I don't see why the thread from which the access happens matters at all.

I think it would only matter for accesses from a finalizer thread.

-Brent