1 /*
2 * Copyright (c) 2018, 2025, 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 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
93 * PARKED -> UNPARKED // unparked, may be scheduled to continue
94 * PINNED -> RUNNING // unparked, continue execution on same carrier
95 * UNPARKED -> RUNNING // continue execution after park
96 *
97 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
98 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
99 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
100 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
101 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
102 *
103 * RUNNING -> BLOCKING // blocking on monitor enter
104 * BLOCKING -> BLOCKED // blocked on monitor enter
105 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
106 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
107 *
108 * RUNNING -> WAITING // transitional state during wait on monitor
109 * WAITING -> WAIT // waiting on monitor
110 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
111 * WAIT -> UNBLOCKED // timed-out/interrupted
112 *
113 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
114 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
115 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
116 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
117 *
118 * RUNNING -> YIELDING // Thread.yield
119 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
120 * YIELDING -> RUNNING // cont.yield failed
121 * YIELDED -> RUNNING // continue execution after Thread.yield
122 */
123 private static final int NEW = 0;
124 private static final int STARTED = 1;
125 private static final int RUNNING = 2; // runnable-mounted
126
127 // untimed and timed parking
128 private static final int PARKING = 3;
129 private static final int PARKED = 4; // unmounted
130 private static final int PINNED = 5; // mounted
131 private static final int TIMED_PARKING = 6;
132 private static final int TIMED_PARKED = 7; // unmounted
133 private static final int TIMED_PINNED = 8; // mounted
134 private static final int UNPARKED = 9; // unmounted but runnable
135
136 // Thread.yield
137 private static final int YIELDING = 10;
138 private static final int YIELDED = 11; // unmounted but runnable
139
140 // monitor enter
141 private static final int BLOCKING = 12;
142 private static final int BLOCKED = 13; // unmounted
143 private static final int UNBLOCKED = 14; // unmounted but runnable
144
145 // monitor wait/timed-wait
146 private static final int WAITING = 15;
147 private static final int WAIT = 16; // waiting in Object.wait
148 private static final int TIMED_WAITING = 17;
149 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
150
151 private static final int TERMINATED = 99; // final state
152
153 // can be suspended from scheduling when unmounted
154 private static final int SUSPENDED = 1 << 8;
155
156 // parking permit made available by LockSupport.unpark
157 private volatile boolean parkPermit;
158
159 // blocking permit made available by unblocker thread when another thread exits monitor
160 private volatile boolean blockPermit;
161
162 // true when on the list of virtual threads waiting to be unblocked
163 private volatile boolean onWaitingList;
164
165 // next virtual thread on the list of virtual threads waiting to be unblocked
166 private volatile VirtualThread next;
167
168 // notified by Object.notify/notifyAll while waiting in Object.wait
169 private volatile boolean notified;
170
171 // timed-wait support
172 private byte timedWaitSeqNo;
173
174 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
175 private long timeout;
176
177 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
178 private Future<?> timeoutTask;
179
180 // carrier thread when mounted, accessed by VM
181 private volatile Thread carrierThread;
182
183 // termination object when joining, created lazily if needed
184 private volatile CountDownLatch termination;
185
186 /**
187 * Returns the default scheduler.
188 */
189 static Executor defaultScheduler() {
190 return DEFAULT_SCHEDULER;
191 }
192
193 /**
194 * Returns the continuation scope used for virtual threads.
195 */
196 static ContinuationScope continuationScope() {
197 return VTHREAD_SCOPE;
198 }
199
200 /**
201 * Creates a new {@code VirtualThread} to run the given task with the given
202 * scheduler. If the given scheduler is {@code null} and the current thread
203 * is a platform thread then the newly created virtual thread will use the
204 * default scheduler. If given scheduler is {@code null} and the current
205 * thread is a virtual thread then the current thread's scheduler is used.
206 *
207 * @param scheduler the scheduler or null
208 * @param name thread name
209 * @param characteristics characteristics
210 * @param task the task to execute
211 */
212 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
213 super(name, characteristics, /*bound*/ false);
214 Objects.requireNonNull(task);
215
216 // choose scheduler if not specified
217 if (scheduler == null) {
218 Thread parent = Thread.currentThread();
219 if (parent instanceof VirtualThread vparent) {
220 scheduler = vparent.scheduler;
221 } else {
222 scheduler = DEFAULT_SCHEDULER;
223 }
224 }
225
226 this.scheduler = scheduler;
227 this.cont = new VThreadContinuation(this, task);
228 this.runContinuation = this::runContinuation;
229 }
230
231 /**
232 * The continuation that a virtual thread executes.
233 */
234 private static class VThreadContinuation extends Continuation {
235 VThreadContinuation(VirtualThread vthread, Runnable task) {
236 super(VTHREAD_SCOPE, wrap(vthread, task));
237 }
238 @Override
239 protected void onPinned(Continuation.Pinned reason) {
240 }
241 private static Runnable wrap(VirtualThread vthread, Runnable task) {
242 return new Runnable() {
243 @Hidden
244 @JvmtiHideEvents
245 public void run() {
246 vthread.notifyJvmtiStart(); // notify JVMTI
247 try {
248 vthread.run(task);
249 } finally {
250 vthread.notifyJvmtiEnd(); // notify JVMTI
251 }
252 }
253 };
254 }
255 }
256
257 /**
258 * Runs or continues execution on the current thread. The virtual thread is mounted
259 * on the current thread before the task runs or continues. It unmounts when the
260 * task completes or yields.
261 */
262 @ChangesCurrentThread // allow mount/unmount to be inlined
263 private void runContinuation() {
264 // the carrier must be a platform thread
265 if (Thread.currentThread().isVirtual()) {
266 throw new WrongThreadException();
267 }
268
269 // set state to RUNNING
270 int initialState = state();
271 if (initialState == STARTED || initialState == UNPARKED
272 || initialState == UNBLOCKED || initialState == YIELDED) {
273 // newly started or continue after parking/blocking/Thread.yield
274 if (!compareAndSetState(initialState, RUNNING)) {
275 return;
276 }
277 // consume permit when continuing after parking or blocking. If continue
278 // after a timed-park or timed-wait then the timeout task is cancelled.
279 if (initialState == UNPARKED) {
280 cancelTimeoutTask();
281 setParkPermit(false);
282 } else if (initialState == UNBLOCKED) {
283 cancelTimeoutTask();
284 blockPermit = false;
285 }
286 } else {
287 // not runnable
288 return;
289 }
290
291 mount();
292 try {
293 cont.run();
294 } finally {
295 unmount();
296 if (cont.isDone()) {
297 afterDone();
298 } else {
299 afterYield();
300 }
301 }
302 }
303
304 /**
305 * Cancel timeout task when continuing after timed-park or timed-wait.
306 * The timeout task may be executing, or may have already completed.
307 */
308 private void cancelTimeoutTask() {
309 if (timeoutTask != null) {
310 timeoutTask.cancel(false);
311 timeoutTask = null;
312 }
313 }
314
315 /**
316 * Submits the runContinuation task to the scheduler. For the default scheduler,
317 * and calling it on a worker thread, the task will be pushed to the local queue,
318 * otherwise it will be pushed to an external submission queue.
319 * @param scheduler the scheduler
320 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
321 * @throws RejectedExecutionException
322 */
323 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
324 boolean done = false;
325 while (!done) {
326 try {
327 // Pin the continuation to prevent the virtual thread from unmounting
328 // when submitting a task. For the default scheduler this ensures that
329 // the carrier doesn't change when pushing a task. For other schedulers
330 // it avoids deadlock that could arise due to carriers and virtual
331 // threads contending for a lock.
332 if (currentThread().isVirtual()) {
333 Continuation.pin();
334 try {
335 scheduler.execute(runContinuation);
336 } finally {
337 Continuation.unpin();
338 }
339 } else {
340 scheduler.execute(runContinuation);
341 }
342 done = true;
343 } catch (RejectedExecutionException ree) {
344 submitFailed(ree);
345 throw ree;
346 } catch (OutOfMemoryError e) {
347 if (retryOnOOME) {
348 U.park(false, 100_000_000); // 100ms
349 } else {
350 throw e;
351 }
352 }
353 }
354 }
355
356 /**
357 * Submits the runContinuation task to the given scheduler as an external submit.
358 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
359 * @throws RejectedExecutionException
360 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
361 */
362 private void externalSubmitRunContinuation(ForkJoinPool pool) {
363 assert Thread.currentThread() instanceof CarrierThread;
364 try {
365 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
366 } catch (RejectedExecutionException ree) {
367 submitFailed(ree);
368 throw ree;
369 } catch (OutOfMemoryError e) {
370 submitRunContinuation(pool, true);
371 }
372 }
373
374 /**
375 * Submits the runContinuation task to the scheduler. For the default scheduler,
376 * and calling it on a worker thread, the task will be pushed to the local queue,
377 * otherwise it will be pushed to an external submission queue.
378 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
379 * @throws RejectedExecutionException
380 */
381 private void submitRunContinuation() {
382 submitRunContinuation(scheduler, true);
383 }
384
385 /**
386 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
387 * queue is empty. If not empty, or invoked by another thread, then this method works
388 * like submitRunContinuation and just submits the task to the scheduler.
389 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
390 * @throws RejectedExecutionException
391 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
392 */
393 private void lazySubmitRunContinuation() {
394 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
395 ForkJoinPool pool = ct.getPool();
396 try {
397 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
398 } catch (RejectedExecutionException ree) {
399 submitFailed(ree);
400 throw ree;
401 } catch (OutOfMemoryError e) {
402 submitRunContinuation();
403 }
404 } else {
405 submitRunContinuation();
406 }
407 }
408
409 /**
410 * Submits the runContinuation task to the scheduler. For the default scheduler, and
411 * calling it a virtual thread that uses the default scheduler, the task will be
412 * pushed to an external submission queue. This method may throw OutOfMemoryError.
413 * @throws RejectedExecutionException
414 * @throws OutOfMemoryError
415 */
416 private void externalSubmitRunContinuationOrThrow() {
417 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
418 try {
419 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
420 } catch (RejectedExecutionException ree) {
421 submitFailed(ree);
422 throw ree;
423 }
424 } else {
425 submitRunContinuation(scheduler, false);
426 }
427 }
428
429 /**
430 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
431 */
432 private void submitFailed(RejectedExecutionException ree) {
433 var event = new VirtualThreadSubmitFailedEvent();
434 if (event.isEnabled()) {
435 event.javaThreadId = threadId();
436 event.exceptionMessage = ree.getMessage();
437 event.commit();
438 }
439 }
440
441 /**
442 * Runs a task in the context of this virtual thread.
443 */
444 private void run(Runnable task) {
445 assert Thread.currentThread() == this && state == RUNNING;
446
447 // emit JFR event if enabled
448 if (VirtualThreadStartEvent.isTurnedOn()) {
449 var event = new VirtualThreadStartEvent();
450 event.javaThreadId = threadId();
451 event.commit();
452 }
453
454 Object bindings = Thread.scopedValueBindings();
455 try {
456 runWith(bindings, task);
457 } catch (Throwable exc) {
458 dispatchUncaughtException(exc);
459 } finally {
460 // pop any remaining scopes from the stack, this may block
461 StackableScope.popAll();
462
463 // emit JFR event if enabled
464 if (VirtualThreadEndEvent.isTurnedOn()) {
465 var event = new VirtualThreadEndEvent();
466 event.javaThreadId = threadId();
467 event.commit();
468 }
469 }
470 }
471
472 /**
473 * Mounts this virtual thread onto the current platform thread. On
474 * return, the current thread is the virtual thread.
475 */
476 @ChangesCurrentThread
477 @ReservedStackAccess
478 private void mount() {
479 // notify JVMTI before mount
480 notifyJvmtiMount(/*hide*/true);
481
482 // sets the carrier thread
483 Thread carrier = Thread.currentCarrierThread();
484 setCarrierThread(carrier);
485
486 // sync up carrier thread interrupt status if needed
487 if (interrupted) {
488 carrier.setInterrupt();
489 } else if (carrier.isInterrupted()) {
490 synchronized (interruptLock) {
491 // need to recheck interrupt status
492 if (!interrupted) {
493 carrier.clearInterrupt();
494 }
495 }
496 }
497
498 // set Thread.currentThread() to return this virtual thread
499 carrier.setCurrentThread(this);
500 }
501
502 /**
503 * Unmounts this virtual thread from the carrier. On return, the
504 * current thread is the current platform thread.
505 */
506 @ChangesCurrentThread
507 @ReservedStackAccess
508 private void unmount() {
509 assert !Thread.holdsLock(interruptLock);
510
511 // set Thread.currentThread() to return the platform thread
512 Thread carrier = this.carrierThread;
513 carrier.setCurrentThread(carrier);
514
515 // break connection to carrier thread, synchronized with interrupt
516 synchronized (interruptLock) {
517 setCarrierThread(null);
518 }
519 carrier.clearInterrupt();
520
521 // notify JVMTI after unmount
522 notifyJvmtiUnmount(/*hide*/false);
523 }
524
525 /**
526 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
527 * the continuation continues.
528 */
529 @Hidden
530 private boolean yieldContinuation() {
531 notifyJvmtiUnmount(/*hide*/true);
532 try {
533 return Continuation.yield(VTHREAD_SCOPE);
534 } finally {
535 notifyJvmtiMount(/*hide*/false);
536 }
537 }
538
539 /**
540 * Invoked in the context of the carrier thread after the Continuation yields when
541 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
542 */
543 private void afterYield() {
544 assert carrierThread == null;
545
546 // re-adjust parallelism if the virtual thread yielded when compensating
547 if (currentThread() instanceof CarrierThread ct) {
548 ct.endBlocking();
549 }
550
551 int s = state();
552
553 // LockSupport.park/parkNanos
554 if (s == PARKING || s == TIMED_PARKING) {
555 int newState;
556 if (s == PARKING) {
557 setState(newState = PARKED);
558 } else {
559 // schedule unpark
560 long timeout = this.timeout;
561 assert timeout > 0;
562 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
563 setState(newState = TIMED_PARKED);
564 }
565
566 // may have been unparked while parking
567 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
568 // lazy submit if local queue is empty
569 lazySubmitRunContinuation();
570 }
571 return;
572 }
573
574 // Thread.yield
575 if (s == YIELDING) {
576 setState(YIELDED);
577
578 // external submit if there are no tasks in the local task queue
579 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
580 externalSubmitRunContinuation(ct.getPool());
581 } else {
582 submitRunContinuation();
583 }
584 return;
585 }
586
587 // blocking on monitorenter
588 if (s == BLOCKING) {
589 setState(BLOCKED);
590
591 // may have been unblocked while blocking
592 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
593 // lazy submit if local queue is empty
594 lazySubmitRunContinuation();
595 }
596 return;
597 }
598
599 // Object.wait
600 if (s == WAITING || s == TIMED_WAITING) {
601 int newState;
602 if (s == WAITING) {
603 setState(newState = WAIT);
604 } else {
605 // For timed-wait, a timeout task is scheduled to execute. The timeout
606 // task will change the thread state to UNBLOCKED and submit the thread
607 // to the scheduler. A sequence number is used to ensure that the timeout
608 // task only unblocks the thread for this timed-wait. We synchronize with
609 // the timeout task to coordinate access to the sequence number and to
610 // ensure the timeout task doesn't execute until the thread has got to
611 // the TIMED_WAIT state.
612 long timeout = this.timeout;
613 assert timeout > 0;
614 synchronized (timedWaitLock()) {
615 byte seqNo = ++timedWaitSeqNo;
616 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
617 setState(newState = TIMED_WAIT);
618 }
619 }
620
621 // may have been notified while in transition to wait state
622 if (notified && compareAndSetState(newState, BLOCKED)) {
623 // may have even been unblocked already
624 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
625 submitRunContinuation();
626 }
627 return;
628 }
629
630 // may have been interrupted while in transition to wait state
631 if (interrupted && compareAndSetState(newState, UNBLOCKED)) {
632 submitRunContinuation();
633 return;
634 }
635 return;
636 }
637
638 assert false;
639 }
640
641 /**
642 * Invoked after the continuation completes.
643 */
644 private void afterDone() {
645 afterDone(true);
646 }
647
648 /**
649 * Invoked after the continuation completes (or start failed). Sets the thread
650 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
651 *
652 * @param notifyContainer true if its container should be notified
653 */
654 private void afterDone(boolean notifyContainer) {
655 assert carrierThread == null;
656 setState(TERMINATED);
657
658 // notify anyone waiting for this virtual thread to terminate
659 CountDownLatch termination = this.termination;
660 if (termination != null) {
661 assert termination.getCount() == 1;
662 termination.countDown();
663 }
664
665 // notify container
666 if (notifyContainer) {
667 threadContainer().onExit(this);
668 }
669
670 // clear references to thread locals
671 clearReferences();
672 }
673
674 /**
675 * Schedules this {@code VirtualThread} to execute.
676 *
677 * @throws IllegalStateException if the container is shutdown or closed
678 * @throws IllegalThreadStateException if the thread has already been started
679 * @throws RejectedExecutionException if the scheduler cannot accept a task
680 */
681 @Override
682 void start(ThreadContainer container) {
683 if (!compareAndSetState(NEW, STARTED)) {
684 throw new IllegalThreadStateException("Already started");
685 }
686
687 // bind thread to container
688 assert threadContainer() == null;
689 setThreadContainer(container);
690
691 // start thread
692 boolean addedToContainer = false;
693 boolean started = false;
694 try {
695 container.onStart(this); // may throw
696 addedToContainer = true;
697
698 // scoped values may be inherited
699 inheritScopedValueBindings(container);
700
701 // submit task to run thread, using externalSubmit if possible
702 externalSubmitRunContinuationOrThrow();
703 started = true;
704 } finally {
705 if (!started) {
706 afterDone(addedToContainer);
707 }
708 }
709 }
710
711 @Override
712 public void start() {
713 start(ThreadContainers.root());
714 }
715
716 @Override
717 public void run() {
718 // do nothing
719 }
720
721 /**
722 * Parks until unparked or interrupted. If already unparked then the parking
723 * permit is consumed and this method completes immediately (meaning it doesn't
724 * yield). It also completes immediately if the interrupt status is set.
725 */
726 @Override
727 void park() {
728 assert Thread.currentThread() == this;
729
730 // complete immediately if parking permit available or interrupted
731 if (getAndSetParkPermit(false) || interrupted)
732 return;
733
734 // park the thread
735 boolean yielded = false;
736 setState(PARKING);
737 try {
738 yielded = yieldContinuation();
739 } catch (OutOfMemoryError e) {
740 // park on carrier
741 } finally {
742 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
743 if (!yielded) {
744 assert state() == PARKING;
745 setState(RUNNING);
746 }
747 }
748
749 // park on the carrier thread when pinned
750 if (!yielded) {
751 parkOnCarrierThread(false, 0);
752 }
753 }
754
755 /**
756 * Parks up to the given waiting time or until unparked or interrupted.
757 * If already unparked then the parking permit is consumed and this method
758 * completes immediately (meaning it doesn't yield). It also completes immediately
759 * if the interrupt status is set or the waiting time is {@code <= 0}.
760 *
761 * @param nanos the maximum number of nanoseconds to wait.
762 */
763 @Override
764 void parkNanos(long nanos) {
765 assert Thread.currentThread() == this;
766
767 // complete immediately if parking permit available or interrupted
768 if (getAndSetParkPermit(false) || interrupted)
769 return;
770
771 // park the thread for the waiting time
772 if (nanos > 0) {
773 long startTime = System.nanoTime();
774
775 // park the thread, afterYield will schedule the thread to unpark
776 boolean yielded = false;
777 timeout = nanos;
778 setState(TIMED_PARKING);
779 try {
780 yielded = yieldContinuation();
781 } catch (OutOfMemoryError e) {
782 // park on carrier
783 } finally {
784 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
785 if (!yielded) {
786 assert state() == TIMED_PARKING;
787 setState(RUNNING);
788 }
789 }
790
791 // park on carrier thread for remaining time when pinned (or OOME)
792 if (!yielded) {
793 long remainingNanos = nanos - (System.nanoTime() - startTime);
794 parkOnCarrierThread(true, remainingNanos);
795 }
796 }
797 }
798
799 /**
800 * Parks the current carrier thread up to the given waiting time or until
801 * unparked or interrupted. If the virtual thread is interrupted then the
802 * interrupt status will be propagated to the carrier thread.
803 * @param timed true for a timed park, false for untimed
804 * @param nanos the waiting time in nanoseconds
805 */
806 private void parkOnCarrierThread(boolean timed, long nanos) {
807 assert state() == RUNNING;
808
809 setState(timed ? TIMED_PINNED : PINNED);
810 try {
811 if (!parkPermit) {
812 if (!timed) {
813 U.park(false, 0);
814 } else if (nanos > 0) {
815 U.park(false, nanos);
816 }
817 }
818 } finally {
819 setState(RUNNING);
820 }
821
822 // consume parking permit
823 setParkPermit(false);
824
825 // JFR jdk.VirtualThreadPinned event
826 postPinnedEvent("LockSupport.park");
827 }
828
829 /**
830 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
831 * Recording the event in the VM avoids having JFR event recorded in Java
832 * with the same name, but different ID, to events recorded by the VM.
833 */
834 @Hidden
835 private static native void postPinnedEvent(String op);
836
837 /**
838 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
839 * then its task is scheduled to continue, otherwise its next call to {@code park} or
840 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
841 * @throws RejectedExecutionException if the scheduler cannot accept a task
842 */
843 @Override
844 void unpark() {
845 if (!getAndSetParkPermit(true) && currentThread() != this) {
846 int s = state();
847
848 // unparked while parked
849 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
850 submitRunContinuation();
851 return;
852 }
853
854 // unparked while parked when pinned
855 if (s == PINNED || s == TIMED_PINNED) {
856 // unpark carrier thread when pinned
857 disableSuspendAndPreempt();
858 try {
859 synchronized (carrierThreadAccessLock()) {
860 Thread carrier = carrierThread;
861 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
862 U.unpark(carrier);
863 }
864 }
865 } finally {
866 enableSuspendAndPreempt();
867 }
868 return;
869 }
870 }
871 }
872
873 /**
874 * Invoked by unblocker thread to unblock this virtual thread.
875 */
876 private void unblock() {
877 assert !Thread.currentThread().isVirtual();
878 blockPermit = true;
879 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
880 submitRunContinuation();
881 }
882 }
883
884 /**
885 * Invoked by FJP worker thread or STPE thread when park timeout expires.
886 */
887 private void parkTimeoutExpired() {
888 assert !VirtualThread.currentThread().isVirtual();
889 if (!getAndSetParkPermit(true)
890 && (state() == TIMED_PARKED)
891 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
892 lazySubmitRunContinuation();
893 }
894 }
895
896 /**
897 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
898 * If the virtual thread is in timed-wait then this method will unblock the thread
899 * and submit its task so that it continues and attempts to reenter the monitor.
900 * This method does nothing if the thread has been woken by notify or interrupt.
901 */
902 private void waitTimeoutExpired(byte seqNo) {
903 assert !Thread.currentThread().isVirtual();
904 for (;;) {
905 boolean unblocked = false;
906 synchronized (timedWaitLock()) {
907 if (seqNo != timedWaitSeqNo) {
908 // this timeout task is for a past timed-wait
909 return;
910 }
911 int s = state();
912 if (s == TIMED_WAIT) {
913 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
914 } else if (s != (TIMED_WAIT | SUSPENDED)) {
915 // notified or interrupted, no longer waiting
916 return;
917 }
918 }
919 if (unblocked) {
920 lazySubmitRunContinuation();
921 return;
922 }
923 // need to retry when thread is suspended in time-wait
924 Thread.yield();
925 }
926 }
927
928 /**
929 * Attempts to yield the current virtual thread (Thread.yield).
930 */
931 void tryYield() {
932 assert Thread.currentThread() == this;
933 setState(YIELDING);
934 boolean yielded = false;
935 try {
936 yielded = yieldContinuation(); // may throw
937 } finally {
938 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
939 if (!yielded) {
940 assert state() == YIELDING;
941 setState(RUNNING);
942 }
943 }
944 }
945
946 /**
947 * Sleep the current thread for the given sleep time (in nanoseconds). If
948 * nanos is 0 then the thread will attempt to yield.
949 *
950 * @implNote This implementation parks the thread for the given sleeping time
951 * and will therefore be observed in PARKED state during the sleep. Parking
952 * will consume the parking permit so this method makes available the parking
953 * permit after the sleep. This may be observed as a spurious, but benign,
954 * wakeup when the thread subsequently attempts to park.
955 *
956 * @param nanos the maximum number of nanoseconds to sleep
957 * @throws InterruptedException if interrupted while sleeping
958 */
959 void sleepNanos(long nanos) throws InterruptedException {
960 assert Thread.currentThread() == this && nanos >= 0;
961 if (getAndClearInterrupt())
962 throw new InterruptedException();
963 if (nanos == 0) {
964 tryYield();
965 } else {
966 // park for the sleep time
967 try {
968 long remainingNanos = nanos;
969 long startNanos = System.nanoTime();
970 while (remainingNanos > 0) {
971 parkNanos(remainingNanos);
972 if (getAndClearInterrupt()) {
973 throw new InterruptedException();
974 }
975 remainingNanos = nanos - (System.nanoTime() - startNanos);
976 }
977 } finally {
978 // may have been unparked while sleeping
979 setParkPermit(true);
980 }
981 }
982 }
983
984 /**
985 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
986 * A timeout of {@code 0} means to wait forever.
987 *
988 * @throws InterruptedException if interrupted while waiting
989 * @return true if the thread has terminated
990 */
991 boolean joinNanos(long nanos) throws InterruptedException {
992 if (state() == TERMINATED)
993 return true;
994
995 // ensure termination object exists, then re-check state
996 CountDownLatch termination = getTermination();
997 if (state() == TERMINATED)
998 return true;
999
1000 // wait for virtual thread to terminate
1001 if (nanos == 0) {
1002 termination.await();
1003 } else {
1004 boolean terminated = termination.await(nanos, NANOSECONDS);
1005 if (!terminated) {
1006 // waiting time elapsed
1007 return false;
1008 }
1009 }
1010 assert state() == TERMINATED;
1011 return true;
1012 }
1013
1014 @Override
1015 void blockedOn(Interruptible b) {
1016 disableSuspendAndPreempt();
1017 try {
1018 super.blockedOn(b);
1019 } finally {
1020 enableSuspendAndPreempt();
1021 }
1022 }
1023
1024 @Override
1025 public void interrupt() {
1026 if (Thread.currentThread() != this) {
1027 // if current thread is a virtual thread then prevent it from being
1028 // suspended or unmounted when entering or holding interruptLock
1029 Interruptible blocker;
1030 disableSuspendAndPreempt();
1031 try {
1032 synchronized (interruptLock) {
1033 interrupted = true;
1034 blocker = nioBlocker();
1035 if (blocker != null) {
1036 blocker.interrupt(this);
1037 }
1038
1039 // interrupt carrier thread if mounted
1040 Thread carrier = carrierThread;
1041 if (carrier != null) carrier.setInterrupt();
1042 }
1043 } finally {
1044 enableSuspendAndPreempt();
1045 }
1046
1047 // notify blocker after releasing interruptLock
1048 if (blocker != null) {
1049 blocker.postInterrupt();
1050 }
1051
1052 // make available parking permit, unpark thread if parked
1053 unpark();
1054
1055 // if thread is waiting in Object.wait then schedule to try to reenter
1056 int s = state();
1057 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1058 submitRunContinuation();
1059 }
1060
1061 } else {
1062 interrupted = true;
1063 carrierThread.setInterrupt();
1064 setParkPermit(true);
1065 }
1066 }
1067
1068 @Override
1069 public boolean isInterrupted() {
1070 return interrupted;
1071 }
1072
1073 @Override
1074 boolean getAndClearInterrupt() {
1075 assert Thread.currentThread() == this;
1076 boolean oldValue = interrupted;
1077 if (oldValue) {
1078 disableSuspendAndPreempt();
1079 try {
1080 synchronized (interruptLock) {
1081 interrupted = false;
1082 carrierThread.clearInterrupt();
1083 }
1084 } finally {
1085 enableSuspendAndPreempt();
1086 }
1087 }
1088 return oldValue;
1089 }
1090
1091 @Override
1092 Thread.State threadState() {
1093 int s = state();
1094 switch (s & ~SUSPENDED) {
1095 case NEW:
1096 return Thread.State.NEW;
1097 case STARTED:
1098 // return NEW if thread container not yet set
1099 if (threadContainer() == null) {
1100 return Thread.State.NEW;
1101 } else {
1102 return Thread.State.RUNNABLE;
1103 }
1104 case UNPARKED:
1105 case UNBLOCKED:
1106 case YIELDED:
1107 // runnable, not mounted
1108 return Thread.State.RUNNABLE;
1109 case RUNNING:
1110 // if mounted then return state of carrier thread
1111 if (Thread.currentThread() != this) {
1112 disableSuspendAndPreempt();
1113 try {
1114 synchronized (carrierThreadAccessLock()) {
1115 Thread carrierThread = this.carrierThread;
1116 if (carrierThread != null) {
1117 return carrierThread.threadState();
1118 }
1119 }
1120 } finally {
1121 enableSuspendAndPreempt();
1122 }
1123 }
1124 // runnable, mounted
1125 return Thread.State.RUNNABLE;
1126 case PARKING:
1127 case TIMED_PARKING:
1128 case WAITING:
1129 case TIMED_WAITING:
1130 case YIELDING:
1131 // runnable, in transition
1132 return Thread.State.RUNNABLE;
1133 case PARKED:
1134 case PINNED:
1135 case WAIT:
1136 return Thread.State.WAITING;
1137 case TIMED_PARKED:
1138 case TIMED_PINNED:
1139 case TIMED_WAIT:
1140 return Thread.State.TIMED_WAITING;
1141 case BLOCKING:
1142 case BLOCKED:
1143 return Thread.State.BLOCKED;
1144 case TERMINATED:
1145 return Thread.State.TERMINATED;
1146 default:
1147 throw new InternalError();
1148 }
1149 }
1150
1151 @Override
1152 boolean alive() {
1153 int s = state;
1154 return (s != NEW && s != TERMINATED);
1155 }
1156
1157 @Override
1158 boolean isTerminated() {
1159 return (state == TERMINATED);
1160 }
1161
1162 @Override
1163 StackTraceElement[] asyncGetStackTrace() {
1164 StackTraceElement[] stackTrace;
1165 do {
1166 stackTrace = (carrierThread != null)
1167 ? super.asyncGetStackTrace() // mounted
1168 : tryGetStackTrace(); // unmounted
1169 if (stackTrace == null) {
1170 Thread.yield();
1171 }
1172 } while (stackTrace == null);
1173 return stackTrace;
1174 }
1175
1176 /**
1177 * Returns the stack trace for this virtual thread if it is unmounted.
1178 * Returns null if the thread is mounted or in transition.
1179 */
1180 private StackTraceElement[] tryGetStackTrace() {
1181 int initialState = state() & ~SUSPENDED;
1182 switch (initialState) {
1183 case NEW, STARTED, TERMINATED -> {
1184 return new StackTraceElement[0]; // unmounted, empty stack
1185 }
1186 case RUNNING, PINNED, TIMED_PINNED -> {
1187 return null; // mounted
1188 }
1189 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1190 // unmounted, not runnable
1191 }
1192 case UNPARKED, UNBLOCKED, YIELDED -> {
1193 // unmounted, runnable
1194 }
1195 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1196 return null; // in transition
1197 }
1198 default -> throw new InternalError("" + initialState);
1199 }
1200
1201 // thread is unmounted, prevent it from continuing
1202 int suspendedState = initialState | SUSPENDED;
1203 if (!compareAndSetState(initialState, suspendedState)) {
1204 return null;
1205 }
1206
1207 // get stack trace and restore state
1208 StackTraceElement[] stack;
1209 try {
1210 stack = cont.getStackTrace();
1211 } finally {
1212 assert state == suspendedState;
1213 setState(initialState);
1214 }
1215 boolean resubmit = switch (initialState) {
1216 case UNPARKED, UNBLOCKED, YIELDED -> {
1217 // resubmit as task may have run while suspended
1218 yield true;
1219 }
1220 case PARKED, TIMED_PARKED -> {
1221 // resubmit if unparked while suspended
1222 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1223 }
1224 case BLOCKED -> {
1225 // resubmit if unblocked while suspended
1226 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1227 }
1228 case WAIT, TIMED_WAIT -> {
1229 // resubmit if notified or interrupted while waiting (Object.wait)
1230 // waitTimeoutExpired will retry if the timed expired when suspended
1231 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1232 }
1233 default -> throw new InternalError();
1234 };
1235 if (resubmit) {
1236 submitRunContinuation();
1237 }
1238 return stack;
1239 }
1240
1241 @Override
1242 public String toString() {
1243 StringBuilder sb = new StringBuilder("VirtualThread[#");
1244 sb.append(threadId());
1245 String name = getName();
1246 if (!name.isEmpty()) {
1247 sb.append(",");
1248 sb.append(name);
1249 }
1250 sb.append("]/");
1251
1252 // add the carrier state and thread name when mounted
1253 boolean mounted;
1254 if (Thread.currentThread() == this) {
1255 mounted = appendCarrierInfo(sb);
1256 } else {
1257 disableSuspendAndPreempt();
1258 try {
1259 synchronized (carrierThreadAccessLock()) {
1260 mounted = appendCarrierInfo(sb);
1261 }
1262 } finally {
1263 enableSuspendAndPreempt();
1264 }
1265 }
1266
1267 // add virtual thread state when not mounted
1268 if (!mounted) {
1269 String stateAsString = threadState().toString();
1270 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1271 }
1272
1273 return sb.toString();
1274 }
1275
1276 /**
1277 * Appends the carrier state and thread name to the string buffer if mounted.
1278 * @return true if mounted, false if not mounted
1279 */
1280 private boolean appendCarrierInfo(StringBuilder sb) {
1281 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1282 Thread carrier = carrierThread;
1283 if (carrier != null) {
1284 String stateAsString = carrier.threadState().toString();
1285 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1286 sb.append('@');
1287 sb.append(carrier.getName());
1288 return true;
1289 } else {
1290 return false;
1291 }
1292 }
1293
1294 @Override
1295 public int hashCode() {
1296 return (int) threadId();
1297 }
1298
1299 @Override
1300 public boolean equals(Object obj) {
1301 return obj == this;
1302 }
1303
1304 /**
1305 * Returns the termination object, creating it if needed.
1306 */
1307 private CountDownLatch getTermination() {
1308 CountDownLatch termination = this.termination;
1309 if (termination == null) {
1310 termination = new CountDownLatch(1);
1311 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1312 termination = this.termination;
1313 }
1314 }
1315 return termination;
1316 }
1317
1318 /**
1319 * Returns the lock object to synchronize on when accessing carrierThread.
1320 * The lock prevents carrierThread from being reset to null during unmount.
1321 */
1322 private Object carrierThreadAccessLock() {
1323 // return interruptLock as unmount has to coordinate with interrupt
1324 return interruptLock;
1325 }
1326
1327 /**
1328 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1329 */
1330 private Object timedWaitLock() {
1331 // use this object for now to avoid the overhead of introducing another lock
1332 return runContinuation;
1333 }
1334
1335 /**
1336 * Disallow the current thread be suspended or preempted.
1337 */
1338 private void disableSuspendAndPreempt() {
1339 notifyJvmtiDisableSuspend(true);
1340 Continuation.pin();
1341 }
1342
1343 /**
1344 * Allow the current thread be suspended or preempted.
1345 */
1346 private void enableSuspendAndPreempt() {
1347 Continuation.unpin();
1348 notifyJvmtiDisableSuspend(false);
1349 }
1350
1351 // -- wrappers for get/set of state, parking permit, and carrier thread --
1352
1353 private int state() {
1354 return state; // volatile read
1355 }
1356
1357 private void setState(int newValue) {
1358 state = newValue; // volatile write
1359 }
1360
1361 private boolean compareAndSetState(int expectedValue, int newValue) {
1362 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1363 }
1364
1365 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1366 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1367 }
1368
1369 private void setParkPermit(boolean newValue) {
1370 if (parkPermit != newValue) {
1371 parkPermit = newValue;
1372 }
1373 }
1374
1375 private boolean getAndSetParkPermit(boolean newValue) {
1376 if (parkPermit != newValue) {
1377 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1378 } else {
1379 return newValue;
1380 }
1381 }
1382
1383 private void setCarrierThread(Thread carrier) {
1384 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1385 this.carrierThread = carrier;
1386 }
1387
1388 // -- JVM TI support --
1389
1390 @IntrinsicCandidate
1391 @JvmtiMountTransition
1392 private native void notifyJvmtiStart();
1393
1394 @IntrinsicCandidate
1395 @JvmtiMountTransition
1396 private native void notifyJvmtiEnd();
1397
1398 @IntrinsicCandidate
1399 @JvmtiMountTransition
1400 private native void notifyJvmtiMount(boolean hide);
1401
1402 @IntrinsicCandidate
1403 @JvmtiMountTransition
1404 private native void notifyJvmtiUnmount(boolean hide);
1405
1406 @IntrinsicCandidate
1407 private static native void notifyJvmtiDisableSuspend(boolean enter);
1408
1409 private static native void registerNatives();
1410 static {
1411 registerNatives();
1412
1413 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1414 var group = Thread.virtualThreadGroup();
1415 }
1416
1417 /**
1418 * Creates the default ForkJoinPool scheduler.
1419 */
1420 private static ForkJoinPool createDefaultScheduler() {
1421 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1422 int parallelism, maxPoolSize, minRunnable;
1423 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1424 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1425 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1426 if (parallelismValue != null) {
1427 parallelism = Integer.parseInt(parallelismValue);
1428 } else {
1429 parallelism = Runtime.getRuntime().availableProcessors();
1430 }
1431 if (maxPoolSizeValue != null) {
1432 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1433 parallelism = Integer.min(parallelism, maxPoolSize);
1434 } else {
1435 maxPoolSize = Integer.max(parallelism, 256);
1436 }
1437 if (minRunnableValue != null) {
1438 minRunnable = Integer.parseInt(minRunnableValue);
1439 } else {
1440 minRunnable = Integer.max(parallelism / 2, 1);
1441 }
1442 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1443 boolean asyncMode = true; // FIFO
1444 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1445 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1446 }
1447
1448 /**
1449 * Schedule a runnable task to run after a delay.
1450 */
1451 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1452 if (scheduler instanceof ForkJoinPool pool) {
1453 return pool.schedule(command, delay, unit);
1454 } else {
1455 return DelayedTaskSchedulers.schedule(command, delay, unit);
1456 }
1457 }
1458
1459 /**
1460 * Supports scheduling a runnable task to run after a delay. It uses a number
1461 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1462 * work queue used. This class is used when using a custom scheduler.
1463 */
1464 private static class DelayedTaskSchedulers {
1465 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1466
1467 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1468 long tid = Thread.currentThread().threadId();
1469 int index = (int) tid & (INSTANCE.length - 1);
1470 return INSTANCE[index].schedule(command, delay, unit);
1471 }
1472
1473 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1474 String propName = "jdk.virtualThreadScheduler.timerQueues";
1475 String propValue = System.getProperty(propName);
1476 int queueCount;
1477 if (propValue != null) {
1478 queueCount = Integer.parseInt(propValue);
1479 if (queueCount != Integer.highestOneBit(queueCount)) {
1480 throw new RuntimeException("Value of " + propName + " must be power of 2");
1481 }
1482 } else {
1483 int ncpus = Runtime.getRuntime().availableProcessors();
1484 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1485 }
1486 var schedulers = new ScheduledExecutorService[queueCount];
1487 for (int i = 0; i < queueCount; i++) {
1488 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1489 Executors.newScheduledThreadPool(1, task -> {
1490 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1491 t.setDaemon(true);
1492 return t;
1493 });
1494 stpe.setRemoveOnCancelPolicy(true);
1495 schedulers[i] = stpe;
1496 }
1497 return schedulers;
1498 }
1499 }
1500
1501 /**
1502 * Schedule virtual threads that are ready to be scheduled after they blocked on
1503 * monitor enter.
1504 */
1505 private static void unblockVirtualThreads() {
1506 while (true) {
1507 VirtualThread vthread = takeVirtualThreadListToUnblock();
1508 while (vthread != null) {
1509 assert vthread.onWaitingList;
1510 VirtualThread nextThread = vthread.next;
1511
1512 // remove from list and unblock
1513 vthread.next = null;
1514 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1515 assert changed;
1516 vthread.unblock();
1517
1518 vthread = nextThread;
1519 }
1520 }
1521 }
1522
1523 /**
1524 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1525 * if necessary until a list of one or more threads becomes available.
1526 */
1527 private static native VirtualThread takeVirtualThreadListToUnblock();
1528
1529 static {
1530 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1531 VirtualThread::unblockVirtualThreads);
1532 unblocker.setDaemon(true);
1533 unblocker.start();
1534 }
1535 }