On 10/01/2026 15:42, Alan Bateman wrote:
On 10/01/2026 11:04, Andrew Haley wrote:
:
Maybe it's waiting on some kind of semaphore that has become
unreachable. In that case, the thread cannot make any progress. It
makes no difference whether you GC it or not. The native resources it
holds will never be released. Semantically, an unreachable thread is
not a thing: it has no properties.
Therefore, any thread that is unreachable may be GC'd, surely.
I guess I may be missing some way that an unreachable thread is
observable? If so, what is it?
Diagnosing resource leaks, semaphore permit "leaks" etc. is much harder
if threads can be GC'ed before they terminate.
We looked into the implications of ephemeral threads a few years ago.
Aside from being fragile to use, they lead to issues that range from
mildly spooky to truly horrifying. Think of cleaners running when
objects are in inconsistent state. Think of objects with finalizers that
put a "goodbye" element to a queue and cause a thread to come back from
the dead.
OK, I think I get it. The issue here is that a thread is unreachable but
can be resurrected if it's finalized.
So, at some risk of flogging a dead horse, I'm wondering: what if we
could figure out a way in which an unreachable thread could be GC'd, but
only if it was honest-to-goodness unreachable, i.e. it wasn't on a
reference queue and it had no finalizer. We'd be good, right?
--
Andrew Haley (he/him)
Java Platform Lead Engineer
https://keybase.io/andrewhaley
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