<div dir="ltr"><div>Hi Viktor,</div><div><br></div><div>You are right, the documentation already outlines this behavior, and reading the try-with-resources specification makes the rest clear. It is just an unfortunate example in JEP 444. </div><div><br></div><div>If the main thread is only sent a single interrupt, then by the time ExecutorService.close is called, the main thread will no longer be interrupted, since that flag is cleared when the InterruptedException is thrown.</div><div><br></div><div>This means that if you send the main thread a single interrupt, the code ends up blocking for just as long as it would have otherwise, the only effect the interrupt has is to cause the request to fail, but that failure happens "late" only after the two fetches have completed normally. That's not the kind of behavior I think you'd want from this kind of code, you likely would want those two fetches to be cancelled, so it was surprising to me.</div><div><br></div><div>The only way to get this code to actually stop early is to have an external source send repeated interrupts to the main thread, such that the main thread will be interrupted twice: Once to leave future.get, and once to cause ExecutorService.close to interrupt the tasks.</div><div><br></div><div>But I think it's just a slightly unclear example, and it's possible to work around by creating a wrapping ExecutorService that invokes shutdownNow on close.</div><div><br></div><div>Thanks for your answer, and you are right, we likely want to use SC instead of this executor.</div></div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">Den man. 1. dec. 2025 kl. 22.19 skrev Viktor Klang <<a href="mailto:viktor.klang@oracle.com">viktor.klang@oracle.com</a>>:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>
<div>
<p>Hi Stig,<br>
</p>
<p><b>Executors.newVirtualThreadPerTaskExecutor()</b>'s Javadoc
states:<br>
<br>
<i>"Creates an Executor that starts a new virtual Thread for each
task. The number of threads created by the Executor is
unbounded.<br>
<b>This method is equivalent to invoking
newThreadPerTaskExecutor(ThreadFactory) with a thread factory
that creates virtual threads.</b>"</i><br>
<br>
-
<a href="https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/Executors.html#newVirtualThreadPerTaskExecutor()" target="_blank">https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/Executors.html#newVirtualThreadPerTaskExecutor()</a><br>
</p>
<p>And if we look at how `ExecutorService::close()` is specified:<br>
<br>
<i>"<b>Initiates an orderly shutdown in which previously submitted
tasks are executed</b>, but no new tasks will be accepted. <b>This
method waits until all tasks have completed execution and the
executor has terminated.</b>"</i></p>
<p>-
<a href="https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/ExecutorService.html#close()" target="_blank">https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/concurrent/ExecutorService.html#close()</a><br>
<br>
>This led me to believe that I could write code like that, and
get something that reacts to interrupts promptly. But
unfortunately, the close method on this executor does not
interrupt the two submitted fetches. Instead, if the main thread
in this code is interrupted, the thread will block until the two
fetches complete on their own time, and then it will fail the
request, which seems a little silly. <br>
</p>
<p>No, the specification for ExecutorService::close() further
states:<br>
<br>
<i>"<b>If interrupted while waiting, this method stops all
executing tasks as if by invoking shutdownNow().</b> It then
continues to wait until all actively executing tasks have
completed. Tasks that were awaiting execution are not executed.
The interrupt status will be re-asserted before this method
returns."<br>
</i><br>
</p>
<div>Given the above: It looks like you want
to use Structured Concurrency instead of
Executors.newVirtualThreadPerTaskExecutor()</div>
<div><br>
</div>
<div>On 2025-12-01 21:16, Stig Rohde Døssing
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">
<div>Hi,</div>
<div><br>
</div>
<div>I stumbled on a minor landmine with
newVirtualThreadPerTaskExecutor, and figured I'd share in case
something can be done to communicate this behavior a little
better, because it caught me by surprise.</div>
<div><br>
</div>
<div>JEP 444 suggests code like the following<br>
<br>
try (var executor =
Executors.newVirtualThreadPerTaskExecutor()) {<br>
var future1 = executor.submit(() ->
fetchURL(url1));<br>
var future2 = executor.submit(() ->
fetchURL(url2));<br>
response.send(future1.get() + future2.get());<br>
} catch (ExecutionException | InterruptedException e) {<br>
response.fail(e);<br>
}<br>
<br>
</div>
<div>This led me to believe that I could write code like that,
and get something that reacts to interrupts promptly. But
unfortunately, the close method on this executor does not
interrupt the two submitted fetches. Instead, if the main
thread in this code is interrupted, the thread will block
until the two fetches complete on their own time, and then it
will fail the request, which seems a little silly. </div>
<div><br>
</div>
<div>Due to how try-with-resources works, the executor is closed
before the catch block is hit, so in order to "stop early"
when interrupted, you'd need to add a nested try-catch inside
the try-with-resources to either interrupt the two futures or
call shutdownNow on the executor when the main thread is
interrupted.</div>
<div><br>
</div>
<div>I know that the structured concurrency API will be a better
fit for this kind of thing, but given that virtual threads are
likely to spend a lot of their time blocking waiting for
something to occur, it seems a little unfortunate that the
ThreadPerTaskExecutor for virtual threads doesn't do
shutdownNow on close when used in the most straightforward
way.</div>
</div>
</blockquote>
<pre cols="72">--
Cheers,
√
Viktor Klang
Software Architect, Java Platform Group
Oracle</pre>
</div>
</blockquote></div>