RFR: 8378268: Thread.join can wait on Thread, allows joinNanos to be removed

[Alan Bateman]

On Sun, 22 Feb 2026 14:07:49 GMT, Viktor Klang <vklang at openjdk.org> wrote:

>> Thread.join was historically implemented with a loop of Object.wait conditioned on isAlive. We couldn't for this virtual threads because it would pin a virtual thread invoking join to it carrier.  Now that Object.wait preempts, it means the special implementation of join for virtual threads can be removed. Where possible, the notifyAll is elided as it adds to the cost of termination.
>> 
>> The implNote of the join methods is restored (this is implNote text, not spec, so no CSR required).
>
> src/java.base/share/classes/java/lang/Thread.java line 1909:
> 
>> 1907:         // ensure there is a notifyAll to wake up waiters when this thread terminates
>> 1908:         if (this instanceof VirtualThread vthread) {
>> 1909:             vthread.beforeJoin();
> 
> If deemed valuable, we might be able to do something more sophisticated as the proposed solution will trigger notifyAll even if all joiners bailed out due to timeout.

I think it's best to keep it as simple as possible. It's not wrong to notifyAll if there are no waiters so okay if a joiner bails. My concern with doing anything more sophisticated is that it would add counters or other book keeping. It's enough to just avoid the notifyAll when the thread terminates before anyone calls join.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29839#discussion_r2845582926