1 /*
2 * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.util.Locale;
28 import java.util.Objects;
29 import java.util.concurrent.CountDownLatch;
30 import java.util.concurrent.Executor;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.Future;
36 import java.util.concurrent.RejectedExecutionException;
37 import java.util.concurrent.ScheduledExecutorService;
38 import java.util.concurrent.ScheduledThreadPoolExecutor;
39 import java.util.concurrent.TimeUnit;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66 private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
67
68 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
69 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
70 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
71 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
72 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
73
74 // scheduler and continuation
75 private final Executor scheduler;
76 private final Continuation cont;
77 private final Runnable runContinuation;
78
79 // virtual thread state, accessed by VM
80 private volatile int state;
81
82 /*
83 * Virtual thread state transitions:
84 *
85 * NEW -> STARTED // Thread.start, schedule to run
86 * STARTED -> TERMINATED // failed to start
87 * STARTED -> RUNNING // first run
88 * RUNNING -> TERMINATED // done
89 *
90 * RUNNING -> PARKING // Thread parking with LockSupport.park
91 * PARKING -> PARKED // cont.yield successful, parked indefinitely
92 * PARKED -> UNPARKED // unparked, may be scheduled to continue
93 * UNPARKED -> RUNNING // continue execution after park
94 *
95 * PARKING -> RUNNING // cont.yield failed, need to park on carrier
96 * RUNNING -> PINNED // park on carrier
97 * PINNED -> RUNNING // unparked, continue execution on same carrier
98 *
99 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
100 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
101 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
102 *
103 * TIMED_PARKING -> RUNNING // cont.yield failed, need to park on carrier
104 * RUNNING -> TIMED_PINNED // park on carrier
105 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
106 *
107 * RUNNING -> BLOCKING // blocking on monitor enter
108 * BLOCKING -> BLOCKED // blocked on monitor enter
109 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
110 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
111 *
112 * RUNNING -> WAITING // transitional state during wait on monitor
113 * WAITING -> WAIT // waiting on monitor
114 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
115 * WAIT -> UNBLOCKED // interrupted
116 *
117 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
118 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
119 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
120 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
121 *
122 * RUNNING -> YIELDING // Thread.yield
123 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
124 * YIELDING -> RUNNING // cont.yield failed
125 * YIELDED -> RUNNING // continue execution after Thread.yield
126 */
127 private static final int NEW = 0;
128 private static final int STARTED = 1;
129 private static final int RUNNING = 2; // runnable-mounted
130
131 // untimed and timed parking
132 private static final int PARKING = 3;
133 private static final int PARKED = 4; // unmounted
134 private static final int PINNED = 5; // mounted
135 private static final int TIMED_PARKING = 6;
136 private static final int TIMED_PARKED = 7; // unmounted
137 private static final int TIMED_PINNED = 8; // mounted
138 private static final int UNPARKED = 9; // unmounted but runnable
139
140 // Thread.yield
141 private static final int YIELDING = 10;
142 private static final int YIELDED = 11; // unmounted but runnable
143
144 // monitor enter
145 private static final int BLOCKING = 12;
146 private static final int BLOCKED = 13; // unmounted
147 private static final int UNBLOCKED = 14; // unmounted but runnable
148
149 // monitor wait/timed-wait
150 private static final int WAITING = 15;
151 private static final int WAIT = 16; // waiting in Object.wait
152 private static final int TIMED_WAITING = 17;
153 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
154
155 private static final int TERMINATED = 99; // final state
156
157 // parking permit made available by LockSupport.unpark
158 private volatile boolean parkPermit;
159
160 // blocking permit made available by unblocker thread when another thread exits monitor
161 private volatile boolean blockPermit;
162
163 // true when on the list of virtual threads waiting to be unblocked
164 private volatile boolean onWaitingList;
165
166 // next virtual thread on the list of virtual threads waiting to be unblocked
167 private volatile VirtualThread next;
168
169 // notified by Object.notify/notifyAll while waiting in Object.wait
170 private volatile boolean notified;
171
172 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
173 private volatile boolean interruptibleWait;
174
175 // timed-wait support
176 private byte timedWaitSeqNo;
177
178 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
179 private long timeout;
180
181 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
182 private Future<?> timeoutTask;
183
184 // carrier thread when mounted, accessed by VM
185 private volatile Thread carrierThread;
186
187 // termination object when joining, created lazily if needed
188 private volatile CountDownLatch termination;
189
190 /**
191 * Returns the default scheduler.
192 */
193 static Executor defaultScheduler() {
194 return DEFAULT_SCHEDULER;
195 }
196
197 /**
198 * Returns the continuation scope used for virtual threads.
199 */
200 static ContinuationScope continuationScope() {
201 return VTHREAD_SCOPE;
202 }
203
204 /**
205 * Creates a new {@code VirtualThread} to run the given task with the given
206 * scheduler. If the given scheduler is {@code null} and the current thread
207 * is a platform thread then the newly created virtual thread will use the
208 * default scheduler. If given scheduler is {@code null} and the current
209 * thread is a virtual thread then the current thread's scheduler is used.
210 *
211 * @param scheduler the scheduler or null
212 * @param name thread name
213 * @param characteristics characteristics
214 * @param task the task to execute
215 */
216 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
217 super(name, characteristics, /*bound*/ false);
218 Objects.requireNonNull(task);
219
220 // choose scheduler if not specified
221 if (scheduler == null) {
222 Thread parent = Thread.currentThread();
223 if (parent instanceof VirtualThread vparent) {
224 scheduler = vparent.scheduler;
225 } else {
226 scheduler = DEFAULT_SCHEDULER;
227 }
228 }
229
230 this.scheduler = scheduler;
231 this.cont = new VThreadContinuation(this, task);
232 this.runContinuation = this::runContinuation;
233 }
234
235 /**
236 * The continuation that a virtual thread executes.
237 */
238 private static class VThreadContinuation extends Continuation {
239 VThreadContinuation(VirtualThread vthread, Runnable task) {
240 super(VTHREAD_SCOPE, wrap(vthread, task));
241 }
242 @Override
243 protected void onPinned(Continuation.Pinned reason) {
244 }
245 private static Runnable wrap(VirtualThread vthread, Runnable task) {
246 return new Runnable() {
247 @Hidden
248 @JvmtiHideEvents
249 public void run() {
250 vthread.endFirstTransition();
251 try {
252 vthread.run(task);
253 } finally {
254 vthread.startFinalTransition();
255 }
256 }
257 };
258 }
259 }
260
261 /**
262 * Runs or continues execution on the current thread. The virtual thread is mounted
263 * on the current thread before the task runs or continues. It unmounts when the
264 * task completes or yields.
265 */
266 @ChangesCurrentThread // allow mount/unmount to be inlined
267 private void runContinuation() {
268 // the carrier must be a platform thread
269 if (Thread.currentThread().isVirtual()) {
270 throw new WrongThreadException();
271 }
272
273 // set state to RUNNING
274 int initialState = state();
275 if (initialState == STARTED || initialState == UNPARKED
276 || initialState == UNBLOCKED || initialState == YIELDED) {
277 // newly started or continue after parking/blocking/Thread.yield
278 if (!compareAndSetState(initialState, RUNNING)) {
279 return;
280 }
281 // consume permit when continuing after parking or blocking. If continue
282 // after a timed-park or timed-wait then the timeout task is cancelled.
283 if (initialState == UNPARKED) {
284 cancelTimeoutTask();
285 setParkPermit(false);
286 } else if (initialState == UNBLOCKED) {
287 cancelTimeoutTask();
288 blockPermit = false;
289 }
290 } else {
291 // not runnable
292 return;
293 }
294
295 mount();
296 try {
297 cont.run();
298 } finally {
299 unmount();
300 if (cont.isDone()) {
301 afterDone();
302 } else {
303 afterYield();
304 }
305 }
306 }
307
308 /**
309 * Cancel timeout task when continuing after timed-park or timed-wait.
310 * The timeout task may be executing, or may have already completed.
311 */
312 private void cancelTimeoutTask() {
313 if (timeoutTask != null) {
314 timeoutTask.cancel(false);
315 timeoutTask = null;
316 }
317 }
318
319 /**
320 * Submits the given task to the given executor. If the scheduler is a
321 * ForkJoinPool then the task is first adapted to a ForkJoinTask.
322 */
323 private void submit(Executor executor, Runnable task) {
324 if (executor instanceof ForkJoinPool pool) {
325 pool.submit(ForkJoinTask.adapt(task));
326 } else {
327 executor.execute(task);
328 }
329 }
330
331 /**
332 * Submits the runContinuation task to the scheduler. For the default scheduler,
333 * and calling it on a worker thread, the task will be pushed to the local queue,
334 * otherwise it will be pushed to an external submission queue.
335 * @param scheduler the scheduler
336 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
337 * @throws RejectedExecutionException
338 */
339 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
340 boolean done = false;
341 while (!done) {
342 try {
343 // Pin the continuation to prevent the virtual thread from unmounting
344 // when submitting a task. For the default scheduler this ensures that
345 // the carrier doesn't change when pushing a task. For other schedulers
346 // it avoids deadlock that could arise due to carriers and virtual
347 // threads contending for a lock.
348 if (currentThread().isVirtual()) {
349 Continuation.pin();
350 try {
351 submit(scheduler, runContinuation);
352 } finally {
353 Continuation.unpin();
354 }
355 } else {
356 submit(scheduler, runContinuation);
357 }
358 done = true;
359 } catch (RejectedExecutionException ree) {
360 submitFailed(ree);
361 throw ree;
362 } catch (OutOfMemoryError e) {
363 if (retryOnOOME) {
364 U.park(false, 100_000_000); // 100ms
365 } else {
366 throw e;
367 }
368 }
369 }
370 }
371
372 /**
373 * Submits the runContinuation task to the given scheduler as an external submit.
374 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
375 * @throws RejectedExecutionException
376 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
377 */
378 private void externalSubmitRunContinuation(ForkJoinPool pool) {
379 assert Thread.currentThread() instanceof CarrierThread;
380 try {
381 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
382 } catch (RejectedExecutionException ree) {
383 submitFailed(ree);
384 throw ree;
385 } catch (OutOfMemoryError e) {
386 submitRunContinuation(pool, true);
387 }
388 }
389
390 /**
391 * Submits the runContinuation task to the scheduler. For the default scheduler,
392 * and calling it on a worker thread, the task will be pushed to the local queue,
393 * otherwise it will be pushed to an external submission queue.
394 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
395 * @throws RejectedExecutionException
396 */
397 private void submitRunContinuation() {
398 submitRunContinuation(scheduler, true);
399 }
400
401 /**
402 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
403 * queue is empty. If not empty, or invoked by another thread, then this method works
404 * like submitRunContinuation and just submits the task to the scheduler.
405 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
406 * @throws RejectedExecutionException
407 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
408 */
409 private void lazySubmitRunContinuation() {
410 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
411 ForkJoinPool pool = ct.getPool();
412 try {
413 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
414 } catch (RejectedExecutionException ree) {
415 submitFailed(ree);
416 throw ree;
417 } catch (OutOfMemoryError e) {
418 submitRunContinuation();
419 }
420 } else {
421 submitRunContinuation();
422 }
423 }
424
425 /**
426 * Submits the runContinuation task to the scheduler. For the default scheduler, and
427 * calling it a virtual thread that uses the default scheduler, the task will be
428 * pushed to an external submission queue. This method may throw OutOfMemoryError.
429 * @throws RejectedExecutionException
430 * @throws OutOfMemoryError
431 */
432 private void externalSubmitRunContinuationOrThrow() {
433 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
434 try {
435 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
436 } catch (RejectedExecutionException ree) {
437 submitFailed(ree);
438 throw ree;
439 }
440 } else {
441 submitRunContinuation(scheduler, false);
442 }
443 }
444
445 /**
446 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
447 */
448 private void submitFailed(RejectedExecutionException ree) {
449 var event = new VirtualThreadSubmitFailedEvent();
450 if (event.isEnabled()) {
451 event.javaThreadId = threadId();
452 event.exceptionMessage = ree.getMessage();
453 event.commit();
454 }
455 }
456
457 /**
458 * Runs a task in the context of this virtual thread.
459 */
460 private void run(Runnable task) {
461 assert Thread.currentThread() == this && state == RUNNING;
462
463 // emit JFR event if enabled
464 if (VirtualThreadStartEvent.isTurnedOn()) {
465 var event = new VirtualThreadStartEvent();
466 event.javaThreadId = threadId();
467 event.commit();
468 }
469
470 Object bindings = Thread.scopedValueBindings();
471 try {
472 runWith(bindings, task);
473 } catch (Throwable exc) {
474 dispatchUncaughtException(exc);
475 } finally {
476 // pop any remaining scopes from the stack, this may block
477 StackableScope.popAll();
478
479 // emit JFR event if enabled
480 if (VirtualThreadEndEvent.isTurnedOn()) {
481 var event = new VirtualThreadEndEvent();
482 event.javaThreadId = threadId();
483 event.commit();
484 }
485 }
486 }
487
488 /**
489 * Mounts this virtual thread onto the current platform thread. On
490 * return, the current thread is the virtual thread.
491 */
492 @ChangesCurrentThread
493 @ReservedStackAccess
494 private void mount() {
495 startTransition(/*mount*/true);
496 // We assume following volatile accesses provide equivalent
497 // of acquire ordering, otherwise we need U.loadFence() here.
498
499 // sets the carrier thread
500 Thread carrier = Thread.currentCarrierThread();
501 setCarrierThread(carrier);
502
503 // sync up carrier thread interrupted status if needed
504 if (interrupted) {
505 carrier.setInterrupt();
506 } else if (carrier.isInterrupted()) {
507 synchronized (interruptLock) {
508 // need to recheck interrupted status
509 if (!interrupted) {
510 carrier.clearInterrupt();
511 }
512 }
513 }
514
515 // set Thread.currentThread() to return this virtual thread
516 carrier.setCurrentThread(this);
517 }
518
519 /**
520 * Unmounts this virtual thread from the carrier. On return, the
521 * current thread is the current platform thread.
522 */
523 @ChangesCurrentThread
524 @ReservedStackAccess
525 private void unmount() {
526 assert !Thread.holdsLock(interruptLock);
527
528 // set Thread.currentThread() to return the platform thread
529 Thread carrier = this.carrierThread;
530 carrier.setCurrentThread(carrier);
531
532 // break connection to carrier thread, synchronized with interrupt
533 synchronized (interruptLock) {
534 setCarrierThread(null);
535 }
536 carrier.clearInterrupt();
537
538 // We assume previous volatile accesses provide equivalent
539 // of release ordering, otherwise we need U.storeFence() here.
540 endTransition(/*mount*/false);
541 }
542
543 /**
544 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
545 * the continuation continues.
546 */
547 @Hidden
548 private boolean yieldContinuation() {
549 startTransition(/*mount*/false);
550 try {
551 return Continuation.yield(VTHREAD_SCOPE);
552 } finally {
553 endTransition(/*mount*/true);
554 }
555 }
556
557 /**
558 * Invoked in the context of the carrier thread after the Continuation yields when
559 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
560 */
561 private void afterYield() {
562 assert carrierThread == null;
563
564 // re-adjust parallelism if the virtual thread yielded when compensating
565 if (currentThread() instanceof CarrierThread ct) {
566 ct.endBlocking();
567 }
568
569 int s = state();
570
571 // LockSupport.park/parkNanos
572 if (s == PARKING || s == TIMED_PARKING) {
573 int newState;
574 if (s == PARKING) {
575 setState(newState = PARKED);
576 } else {
577 // schedule unpark
578 long timeout = this.timeout;
579 assert timeout > 0;
580 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
581 setState(newState = TIMED_PARKED);
582 }
583
584 // may have been unparked while parking
585 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
586 // lazy submit if local queue is empty
587 lazySubmitRunContinuation();
588 }
589 return;
590 }
591
592 // Thread.yield
593 if (s == YIELDING) {
594 setState(YIELDED);
595
596 // external submit if there are no tasks in the local task queue
597 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
598 externalSubmitRunContinuation(ct.getPool());
599 } else {
600 submitRunContinuation();
601 }
602 return;
603 }
604
605 // blocking on monitorenter
606 if (s == BLOCKING) {
607 setState(BLOCKED);
608
609 // may have been unblocked while blocking
610 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
611 // lazy submit if local queue is empty
612 lazySubmitRunContinuation();
613 }
614 return;
615 }
616
617 // Object.wait
618 if (s == WAITING || s == TIMED_WAITING) {
619 int newState;
620 boolean blocked;
621 boolean interruptible = interruptibleWait;
622 if (s == WAITING) {
623 setState(newState = WAIT);
624 // may have been notified while in transition
625 blocked = notified && compareAndSetState(WAIT, BLOCKED);
626 } else {
627 // For timed-wait, a timeout task is scheduled to execute. The timeout
628 // task will change the thread state to UNBLOCKED and submit the thread
629 // to the scheduler. A sequence number is used to ensure that the timeout
630 // task only unblocks the thread for this timed-wait. We synchronize with
631 // the timeout task to coordinate access to the sequence number and to
632 // ensure the timeout task doesn't execute until the thread has got to
633 // the TIMED_WAIT state.
634 long timeout = this.timeout;
635 assert timeout > 0;
636 synchronized (timedWaitLock()) {
637 byte seqNo = ++timedWaitSeqNo;
638 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
639 setState(newState = TIMED_WAIT);
640 // May have been notified while in transition. This must be done while
641 // holding the monitor to avoid changing the state of a new timed wait call.
642 blocked = notified && compareAndSetState(TIMED_WAIT, BLOCKED);
643 }
644 }
645
646 if (blocked) {
647 // may have been unblocked already
648 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
649 lazySubmitRunContinuation();
650 }
651 } else {
652 // may have been interrupted while in transition to wait state
653 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
654 lazySubmitRunContinuation();
655 }
656 }
657 return;
658 }
659
660 assert false;
661 }
662
663 /**
664 * Invoked after the continuation completes.
665 */
666 private void afterDone() {
667 afterDone(true);
668 }
669
670 /**
671 * Invoked after the continuation completes (or start failed). Sets the thread
672 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
673 *
674 * @param notifyContainer true if its container should be notified
675 */
676 private void afterDone(boolean notifyContainer) {
677 assert carrierThread == null;
678 setState(TERMINATED);
679
680 // notify anyone waiting for this virtual thread to terminate
681 CountDownLatch termination = this.termination;
682 if (termination != null) {
683 assert termination.getCount() == 1;
684 termination.countDown();
685 }
686
687 // notify container
688 if (notifyContainer) {
689 threadContainer().remove(this);
690 }
691
692 // clear references to thread locals
693 clearReferences();
694 }
695
696 /**
697 * Schedules this {@code VirtualThread} to execute.
698 *
699 * @throws IllegalStateException if the container is shutdown or closed
700 * @throws IllegalThreadStateException if the thread has already been started
701 * @throws RejectedExecutionException if the scheduler cannot accept a task
702 */
703 @Override
704 void start(ThreadContainer container) {
705 if (!compareAndSetState(NEW, STARTED)) {
706 throw new IllegalThreadStateException("Already started");
707 }
708
709 // bind thread to container
710 assert threadContainer() == null;
711 setThreadContainer(container);
712
713 // start thread
714 boolean addedToContainer = false;
715 boolean started = false;
716 try {
717 container.add(this); // may throw
718 addedToContainer = true;
719
720 // scoped values may be inherited
721 inheritScopedValueBindings(container);
722
723 // submit task to run thread, using externalSubmit if possible
724 externalSubmitRunContinuationOrThrow();
725 started = true;
726 } finally {
727 if (!started) {
728 afterDone(addedToContainer);
729 }
730 }
731 }
732
733 @Override
734 public void start() {
735 start(ThreadContainers.root());
736 }
737
738 @Override
739 public void run() {
740 // do nothing
741 }
742
743 /**
744 * Parks until unparked or interrupted. If already unparked then the parking
745 * permit is consumed and this method completes immediately (meaning it doesn't
746 * yield). It also completes immediately if the interrupted status is set.
747 */
748 @Override
749 void park() {
750 assert Thread.currentThread() == this;
751
752 // complete immediately if parking permit available or interrupted
753 if (getAndSetParkPermit(false) || interrupted)
754 return;
755
756 // park the thread
757 boolean yielded = false;
758 setState(PARKING);
759 try {
760 yielded = yieldContinuation();
761 } catch (OutOfMemoryError e) {
762 // park on carrier
763 } finally {
764 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
765 if (!yielded) {
766 assert state() == PARKING;
767 setState(RUNNING);
768 }
769 }
770
771 // park on the carrier thread when pinned
772 if (!yielded) {
773 parkOnCarrierThread(false, 0);
774 }
775 }
776
777 /**
778 * Parks up to the given waiting time or until unparked or interrupted.
779 * If already unparked then the parking permit is consumed and this method
780 * completes immediately (meaning it doesn't yield). It also completes immediately
781 * if the interrupted status is set or the waiting time is {@code <= 0}.
782 *
783 * @param nanos the maximum number of nanoseconds to wait.
784 */
785 @Override
786 void parkNanos(long nanos) {
787 assert Thread.currentThread() == this;
788
789 // complete immediately if parking permit available or interrupted
790 if (getAndSetParkPermit(false) || interrupted)
791 return;
792
793 // park the thread for the waiting time
794 if (nanos > 0) {
795 long startTime = System.nanoTime();
796
797 // park the thread, afterYield will schedule the thread to unpark
798 boolean yielded = false;
799 timeout = nanos;
800 setState(TIMED_PARKING);
801 try {
802 yielded = yieldContinuation();
803 } catch (OutOfMemoryError e) {
804 // park on carrier
805 } finally {
806 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
807 if (!yielded) {
808 assert state() == TIMED_PARKING;
809 setState(RUNNING);
810 }
811 }
812
813 // park on carrier thread for remaining time when pinned (or OOME)
814 if (!yielded) {
815 long remainingNanos = nanos - (System.nanoTime() - startTime);
816 parkOnCarrierThread(true, remainingNanos);
817 }
818 }
819 }
820
821 /**
822 * Parks the current carrier thread up to the given waiting time or until
823 * unparked or interrupted. If the virtual thread is interrupted then the
824 * interrupted status will be propagated to the carrier thread.
825 * @param timed true for a timed park, false for untimed
826 * @param nanos the waiting time in nanoseconds
827 */
828 private void parkOnCarrierThread(boolean timed, long nanos) {
829 assert state() == RUNNING;
830
831 setState(timed ? TIMED_PINNED : PINNED);
832 try {
833 if (!parkPermit) {
834 if (!timed) {
835 U.park(false, 0);
836 } else if (nanos > 0) {
837 U.park(false, nanos);
838 }
839 }
840 } finally {
841 setState(RUNNING);
842 }
843
844 // consume parking permit
845 setParkPermit(false);
846
847 // JFR jdk.VirtualThreadPinned event
848 postPinnedEvent("LockSupport.park");
849 }
850
851 /**
852 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
853 * Recording the event in the VM avoids having JFR event recorded in Java
854 * with the same name, but different ID, to events recorded by the VM.
855 */
856 @Hidden
857 private static native void postPinnedEvent(String op);
858
859 /**
860 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
861 * then its task is scheduled to continue, otherwise its next call to {@code park} or
862 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
863 * @param lazySubmit to use lazySubmit if possible
864 * @throws RejectedExecutionException if the scheduler cannot accept a task
865 */
866 private void unpark(boolean lazySubmit) {
867 if (!getAndSetParkPermit(true) && currentThread() != this) {
868 int s = state();
869
870 // unparked while parked
871 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
872 if (lazySubmit) {
873 lazySubmitRunContinuation();
874 } else {
875 submitRunContinuation();
876 }
877 return;
878 }
879
880 // unparked while parked when pinned
881 if (s == PINNED || s == TIMED_PINNED) {
882 // unpark carrier thread when pinned
883 disableSuspendAndPreempt();
884 try {
885 synchronized (carrierThreadAccessLock()) {
886 Thread carrier = carrierThread;
887 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
888 U.unpark(carrier);
889 }
890 }
891 } finally {
892 enableSuspendAndPreempt();
893 }
894 return;
895 }
896 }
897 }
898
899 @Override
900 void unpark() {
901 unpark(false);
902 }
903
904 /**
905 * Invoked by unblocker thread to unblock this virtual thread.
906 */
907 private void unblock() {
908 assert !Thread.currentThread().isVirtual();
909 blockPermit = true;
910 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
911 submitRunContinuation();
912 }
913 }
914
915 /**
916 * Invoked by FJP worker thread or STPE thread when park timeout expires.
917 */
918 private void parkTimeoutExpired() {
919 assert !VirtualThread.currentThread().isVirtual();
920 unpark(true);
921 }
922
923 /**
924 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
925 * If the virtual thread is in timed-wait then this method will unblock the thread
926 * and submit its task so that it continues and attempts to reenter the monitor.
927 * This method does nothing if the thread has been woken by notify or interrupt.
928 */
929 private void waitTimeoutExpired(byte seqNo) {
930 assert !Thread.currentThread().isVirtual();
931
932 synchronized (timedWaitLock()) {
933 if (seqNo != timedWaitSeqNo) {
934 // this timeout task is for a past timed-wait
935 return;
936 }
937 if (!compareAndSetState(TIMED_WAIT, UNBLOCKED)) {
938 // already notified (or interrupted)
939 return;
940 }
941 }
942
943 lazySubmitRunContinuation();
944 }
945
946 /**
947 * Attempts to yield the current virtual thread (Thread.yield).
948 */
949 void tryYield() {
950 assert Thread.currentThread() == this;
951 setState(YIELDING);
952 boolean yielded = false;
953 try {
954 yielded = yieldContinuation(); // may throw
955 } finally {
956 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
957 if (!yielded) {
958 assert state() == YIELDING;
959 setState(RUNNING);
960 }
961 }
962 }
963
964 /**
965 * Sleep the current thread for the given sleep time (in nanoseconds). If
966 * nanos is 0 then the thread will attempt to yield.
967 *
968 * @implNote This implementation parks the thread for the given sleeping time
969 * and will therefore be observed in PARKED state during the sleep. Parking
970 * will consume the parking permit so this method makes available the parking
971 * permit after the sleep. This may be observed as a spurious, but benign,
972 * wakeup when the thread subsequently attempts to park.
973 *
974 * @param nanos the maximum number of nanoseconds to sleep
975 * @throws InterruptedException if interrupted while sleeping
976 */
977 void sleepNanos(long nanos) throws InterruptedException {
978 assert Thread.currentThread() == this && nanos >= 0;
979 if (getAndClearInterrupt())
980 throw new InterruptedException();
981 if (nanos == 0) {
982 tryYield();
983 } else {
984 // park for the sleep time
985 try {
986 long remainingNanos = nanos;
987 long startNanos = System.nanoTime();
988 while (remainingNanos > 0) {
989 parkNanos(remainingNanos);
990 if (getAndClearInterrupt()) {
991 throw new InterruptedException();
992 }
993 remainingNanos = nanos - (System.nanoTime() - startNanos);
994 }
995 } finally {
996 // may have been unparked while sleeping
997 setParkPermit(true);
998 }
999 }
1000 }
1001
1002 /**
1003 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1004 * A timeout of {@code 0} means to wait forever.
1005 *
1006 * @throws InterruptedException if interrupted while waiting
1007 * @return true if the thread has terminated
1008 */
1009 boolean joinNanos(long nanos) throws InterruptedException {
1010 if (state() == TERMINATED)
1011 return true;
1012
1013 // ensure termination object exists, then re-check state
1014 CountDownLatch termination = getTermination();
1015 if (state() == TERMINATED)
1016 return true;
1017
1018 // wait for virtual thread to terminate
1019 if (nanos == 0) {
1020 termination.await();
1021 } else {
1022 boolean terminated = termination.await(nanos, NANOSECONDS);
1023 if (!terminated) {
1024 // waiting time elapsed
1025 return false;
1026 }
1027 }
1028 assert state() == TERMINATED;
1029 return true;
1030 }
1031
1032 @Override
1033 void blockedOn(Interruptible b) {
1034 disableSuspendAndPreempt();
1035 try {
1036 super.blockedOn(b);
1037 } finally {
1038 enableSuspendAndPreempt();
1039 }
1040 }
1041
1042 @Override
1043 public void interrupt() {
1044 if (Thread.currentThread() != this) {
1045 // if current thread is a virtual thread then prevent it from being
1046 // suspended or unmounted when entering or holding interruptLock
1047 Interruptible blocker;
1048 disableSuspendAndPreempt();
1049 try {
1050 synchronized (interruptLock) {
1051 interrupted = true;
1052 blocker = nioBlocker();
1053 if (blocker != null) {
1054 blocker.interrupt(this);
1055 }
1056
1057 // interrupt carrier thread if mounted
1058 Thread carrier = carrierThread;
1059 if (carrier != null) carrier.setInterrupt();
1060 }
1061 } finally {
1062 enableSuspendAndPreempt();
1063 }
1064
1065 // notify blocker after releasing interruptLock
1066 if (blocker != null) {
1067 blocker.postInterrupt();
1068 }
1069
1070 // make available parking permit, unpark thread if parked
1071 unpark();
1072
1073 // if thread is waiting in Object.wait then schedule to try to reenter
1074 int s = state();
1075 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1076 submitRunContinuation();
1077 }
1078
1079 } else {
1080 interrupted = true;
1081 carrierThread.setInterrupt();
1082 setParkPermit(true);
1083 }
1084 }
1085
1086 @Override
1087 public boolean isInterrupted() {
1088 return interrupted;
1089 }
1090
1091 @Override
1092 boolean getAndClearInterrupt() {
1093 assert Thread.currentThread() == this;
1094 boolean oldValue = interrupted;
1095 if (oldValue) {
1096 disableSuspendAndPreempt();
1097 try {
1098 synchronized (interruptLock) {
1099 interrupted = false;
1100 carrierThread.clearInterrupt();
1101 }
1102 } finally {
1103 enableSuspendAndPreempt();
1104 }
1105 }
1106 return oldValue;
1107 }
1108
1109 @Override
1110 Thread.State threadState() {
1111 switch (state()) {
1112 case NEW:
1113 return Thread.State.NEW;
1114 case STARTED:
1115 // return NEW if thread container not yet set
1116 if (threadContainer() == null) {
1117 return Thread.State.NEW;
1118 } else {
1119 return Thread.State.RUNNABLE;
1120 }
1121 case UNPARKED:
1122 case UNBLOCKED:
1123 case YIELDED:
1124 // runnable, not mounted
1125 return Thread.State.RUNNABLE;
1126 case RUNNING:
1127 // if mounted then return state of carrier thread
1128 if (Thread.currentThread() != this) {
1129 disableSuspendAndPreempt();
1130 try {
1131 synchronized (carrierThreadAccessLock()) {
1132 Thread carrierThread = this.carrierThread;
1133 if (carrierThread != null) {
1134 return carrierThread.threadState();
1135 }
1136 }
1137 } finally {
1138 enableSuspendAndPreempt();
1139 }
1140 }
1141 // runnable, mounted
1142 return Thread.State.RUNNABLE;
1143 case PARKING:
1144 case TIMED_PARKING:
1145 case WAITING:
1146 case TIMED_WAITING:
1147 case YIELDING:
1148 // runnable, in transition
1149 return Thread.State.RUNNABLE;
1150 case PARKED:
1151 case PINNED:
1152 case WAIT:
1153 return Thread.State.WAITING;
1154 case TIMED_PARKED:
1155 case TIMED_PINNED:
1156 case TIMED_WAIT:
1157 return Thread.State.TIMED_WAITING;
1158 case BLOCKING:
1159 case BLOCKED:
1160 return Thread.State.BLOCKED;
1161 case TERMINATED:
1162 return Thread.State.TERMINATED;
1163 default:
1164 throw new InternalError();
1165 }
1166 }
1167
1168 @Override
1169 boolean alive() {
1170 int s = state;
1171 return (s != NEW && s != TERMINATED);
1172 }
1173
1174 @Override
1175 boolean isTerminated() {
1176 return (state == TERMINATED);
1177 }
1178
1179 @Override
1180 public String toString() {
1181 StringBuilder sb = new StringBuilder("VirtualThread[#");
1182 sb.append(threadId());
1183 String name = getName();
1184 if (!name.isEmpty()) {
1185 sb.append(",");
1186 sb.append(name);
1187 }
1188 sb.append("]/");
1189
1190 // add the carrier state and thread name when mounted
1191 boolean mounted;
1192 if (Thread.currentThread() == this) {
1193 mounted = appendCarrierInfo(sb);
1194 } else {
1195 disableSuspendAndPreempt();
1196 try {
1197 synchronized (carrierThreadAccessLock()) {
1198 mounted = appendCarrierInfo(sb);
1199 }
1200 } finally {
1201 enableSuspendAndPreempt();
1202 }
1203 }
1204
1205 // add virtual thread state when not mounted
1206 if (!mounted) {
1207 String stateAsString = threadState().toString();
1208 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1209 }
1210
1211 return sb.toString();
1212 }
1213
1214 /**
1215 * Appends the carrier state and thread name to the string buffer if mounted.
1216 * @return true if mounted, false if not mounted
1217 */
1218 private boolean appendCarrierInfo(StringBuilder sb) {
1219 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1220 Thread carrier = carrierThread;
1221 if (carrier != null) {
1222 String stateAsString = carrier.threadState().toString();
1223 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1224 sb.append('@');
1225 sb.append(carrier.getName());
1226 return true;
1227 } else {
1228 return false;
1229 }
1230 }
1231
1232 @Override
1233 public int hashCode() {
1234 return (int) threadId();
1235 }
1236
1237 @Override
1238 public boolean equals(Object obj) {
1239 return obj == this;
1240 }
1241
1242 /**
1243 * Returns the termination object, creating it if needed.
1244 */
1245 private CountDownLatch getTermination() {
1246 CountDownLatch termination = this.termination;
1247 if (termination == null) {
1248 termination = new CountDownLatch(1);
1249 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1250 termination = this.termination;
1251 }
1252 }
1253 return termination;
1254 }
1255
1256 /**
1257 * Returns the lock object to synchronize on when accessing carrierThread.
1258 * The lock prevents carrierThread from being reset to null during unmount.
1259 */
1260 private Object carrierThreadAccessLock() {
1261 // return interruptLock as unmount has to coordinate with interrupt
1262 return interruptLock;
1263 }
1264
1265 /**
1266 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1267 */
1268 private Object timedWaitLock() {
1269 // use this object for now to avoid the overhead of introducing another lock
1270 return runContinuation;
1271 }
1272
1273 /**
1274 * Disallow the current thread be suspended or preempted.
1275 */
1276 private void disableSuspendAndPreempt() {
1277 notifyJvmtiDisableSuspend(true);
1278 Continuation.pin();
1279 }
1280
1281 /**
1282 * Allow the current thread be suspended or preempted.
1283 */
1284 private void enableSuspendAndPreempt() {
1285 Continuation.unpin();
1286 notifyJvmtiDisableSuspend(false);
1287 }
1288
1289 // -- wrappers for get/set of state, parking permit, and carrier thread --
1290
1291 private int state() {
1292 return state; // volatile read
1293 }
1294
1295 private void setState(int newValue) {
1296 state = newValue; // volatile write
1297 }
1298
1299 private boolean compareAndSetState(int expectedValue, int newValue) {
1300 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1301 }
1302
1303 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1304 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1305 }
1306
1307 private void setParkPermit(boolean newValue) {
1308 if (parkPermit != newValue) {
1309 parkPermit = newValue;
1310 }
1311 }
1312
1313 private boolean getAndSetParkPermit(boolean newValue) {
1314 if (parkPermit != newValue) {
1315 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1316 } else {
1317 return newValue;
1318 }
1319 }
1320
1321 private void setCarrierThread(Thread carrier) {
1322 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1323 this.carrierThread = carrier;
1324 }
1325
1326 // The following four methods notify the VM when a "transition" starts and ends.
1327 // A "mount transition" embodies the steps to transfer control from a platform
1328 // thread to a virtual thread, changing the thread identity, and starting or
1329 // resuming the virtual thread's continuation on the carrier.
1330 // An "unmount transition" embodies the steps to transfer control from a virtual
1331 // thread to its carrier, suspending the virtual thread's continuation, and
1332 // restoring the thread identity to the platform thread.
1333 // The notifications to the VM are necessary in order to coordinate with functions
1334 // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1335 // a transition may block if transitions are disabled. Ending a transition may
1336 // notify a thread that is waiting to disable transitions. The notifications are
1337 // also used to post JVMTI events for virtual thread start and end.
1338
1339 @IntrinsicCandidate
1340 @JvmtiMountTransition
1341 private native void endFirstTransition();
1342
1343 @IntrinsicCandidate
1344 @JvmtiMountTransition
1345 private native void startFinalTransition();
1346
1347 @IntrinsicCandidate
1348 @JvmtiMountTransition
1349 private native void startTransition(boolean mount);
1350
1351 @IntrinsicCandidate
1352 @JvmtiMountTransition
1353 private native void endTransition(boolean mount);
1354
1355 @IntrinsicCandidate
1356 private static native void notifyJvmtiDisableSuspend(boolean enter);
1357
1358 private static native void registerNatives();
1359 static {
1360 registerNatives();
1361
1362 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1363 var group = Thread.virtualThreadGroup();
1364 }
1365
1366 /**
1367 * Creates the default ForkJoinPool scheduler.
1368 */
1369 private static ForkJoinPool createDefaultScheduler() {
1370 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1371 int parallelism, maxPoolSize, minRunnable;
1372 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1373 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1374 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1375 if (parallelismValue != null) {
1376 parallelism = Integer.parseInt(parallelismValue);
1377 } else {
1378 parallelism = Runtime.getRuntime().availableProcessors();
1379 }
1380 if (maxPoolSizeValue != null) {
1381 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1382 parallelism = Integer.min(parallelism, maxPoolSize);
1383 } else {
1384 maxPoolSize = Integer.max(parallelism, 256);
1385 }
1386 if (minRunnableValue != null) {
1387 minRunnable = Integer.parseInt(minRunnableValue);
1388 } else {
1389 minRunnable = Integer.max(parallelism / 2, 1);
1390 }
1391 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1392 boolean asyncMode = true; // FIFO
1393 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1394 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1395 }
1396
1397 /**
1398 * Schedule a runnable task to run after a delay.
1399 */
1400 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1401 if (scheduler instanceof ForkJoinPool pool) {
1402 return pool.schedule(command, delay, unit);
1403 } else {
1404 return DelayedTaskSchedulers.schedule(command, delay, unit);
1405 }
1406 }
1407
1408 /**
1409 * Supports scheduling a runnable task to run after a delay. It uses a number
1410 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1411 * work queue used. This class is used when using a custom scheduler.
1412 */
1413 private static class DelayedTaskSchedulers {
1414 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1415
1416 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1417 long tid = Thread.currentThread().threadId();
1418 int index = (int) tid & (INSTANCE.length - 1);
1419 return INSTANCE[index].schedule(command, delay, unit);
1420 }
1421
1422 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1423 String propName = "jdk.virtualThreadScheduler.timerQueues";
1424 String propValue = System.getProperty(propName);
1425 int queueCount;
1426 if (propValue != null) {
1427 queueCount = Integer.parseInt(propValue);
1428 if (queueCount != Integer.highestOneBit(queueCount)) {
1429 throw new RuntimeException("Value of " + propName + " must be power of 2");
1430 }
1431 } else {
1432 int ncpus = Runtime.getRuntime().availableProcessors();
1433 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1434 }
1435 var schedulers = new ScheduledExecutorService[queueCount];
1436 for (int i = 0; i < queueCount; i++) {
1437 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1438 Executors.newScheduledThreadPool(1, task -> {
1439 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1440 t.setDaemon(true);
1441 return t;
1442 });
1443 stpe.setRemoveOnCancelPolicy(true);
1444 schedulers[i] = stpe;
1445 }
1446 return schedulers;
1447 }
1448 }
1449
1450 /**
1451 * Schedule virtual threads that are ready to be scheduled after they blocked on
1452 * monitor enter.
1453 */
1454 private static void unblockVirtualThreads() {
1455 while (true) {
1456 VirtualThread vthread = takeVirtualThreadListToUnblock();
1457 while (vthread != null) {
1458 assert vthread.onWaitingList;
1459 VirtualThread nextThread = vthread.next;
1460
1461 // remove from list and unblock
1462 vthread.next = null;
1463 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1464 assert changed;
1465 vthread.unblock();
1466
1467 vthread = nextThread;
1468 }
1469 }
1470 }
1471
1472 /**
1473 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1474 * if necessary until a list of one or more threads becomes available.
1475 */
1476 private static native VirtualThread takeVirtualThreadListToUnblock();
1477
1478 static {
1479 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1480 VirtualThread::unblockVirtualThreads);
1481 unblocker.setDaemon(true);
1482 unblocker.start();
1483 }
1484 }