The method could be improve to also dump all threads - including virtual thtreads - to a file.

The HotspotDiagnotiscMBean supports that - for instance:

String file = Path.of("threads.json").toAbsolutePath().toString();
ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class)
        .dumpThreads(file, HotSpotDiagnosticMXBean.ThreadDumpFormat.JSON);

Some investigation might be needed to see if we should create several such files, as printThreads can be called multiple time by a test.

Here is an exanple where a test may cause threads to be printed:

"MainThread" prio=5 Id=26 RUNNABLE
at java.management@26-internal/sun.management.ThreadImpl.dumpThreads0(Native Method)
at java.management@26-internal/sun.management.ThreadImpl.dumpAllThreads(ThreadImpl.java:505)
at java.management@26-internal/sun.management.ThreadImpl.dumpAllThreads(ThreadImpl.java:493)
at app//ReferenceTracker.printThreads(ReferenceTracker.java:221)
at app//ReferenceTracker.check(ReferenceTracker.java:283)
at app//ReferenceTracker.check(ReferenceTracker.java:111)
at app//AbstractThrowingSubscribers.testThrowing(AbstractThrowingSubscribers.java:543)
at app//AbstractThrowingSubscribers.testThrowing(AbstractThrowingSubscribers.java:475)
at app//ThrowingSubscribersAsLimiting.test(ThrowingSubscribersAsLimiting.java:63)
at app//ThrowingSubscribersAsLimitingAsync.test(ThrowingSubscribersAsLimitingAsync.java:48)