1 /*
2 * Copyright (c) 2018, 2024, 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.lang.reflect.Constructor;
28 import java.util.Arrays;
29 import java.util.Locale;
30 import java.util.Objects;
31 import java.util.concurrent.CountDownLatch;
32 import java.util.concurrent.Executor;
33 import java.util.concurrent.Executors;
34 import java.util.concurrent.ForkJoinPool;
35 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
36 import java.util.concurrent.ForkJoinTask;
37 import java.util.concurrent.Future;
38 import java.util.concurrent.RejectedExecutionException;
39 import java.util.concurrent.ScheduledExecutorService;
40 import java.util.concurrent.ScheduledThreadPoolExecutor;
41 import java.util.concurrent.TimeUnit;
42 import java.util.stream.Stream;
43 import jdk.internal.event.VirtualThreadEndEvent;
44 import jdk.internal.event.VirtualThreadStartEvent;
45 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
46 import jdk.internal.misc.CarrierThread;
47 import jdk.internal.misc.InnocuousThread;
48 import jdk.internal.misc.Unsafe;
49 import jdk.internal.vm.Continuation;
50 import jdk.internal.vm.ContinuationScope;
51 import jdk.internal.vm.StackableScope;
52 import jdk.internal.vm.ThreadContainer;
53 import jdk.internal.vm.ThreadContainers;
54 import jdk.internal.vm.annotation.ChangesCurrentThread;
55 import jdk.internal.vm.annotation.Hidden;
56 import jdk.internal.vm.annotation.IntrinsicCandidate;
57 import jdk.internal.vm.annotation.JvmtiHideEvents;
58 import jdk.internal.vm.annotation.JvmtiMountTransition;
59 import jdk.internal.vm.annotation.ReservedStackAccess;
60 import sun.nio.ch.Interruptible;
61 import static java.util.concurrent.TimeUnit.*;
62
63 /**
64 * A thread that is scheduled by the Java virtual machine rather than the operating system.
65 */
66 final class VirtualThread extends BaseVirtualThread {
67 private static final Unsafe U = Unsafe.getUnsafe();
68 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
69 private static final Executor DEFAULT_SCHEDULER = createDefaultScheduler();
70 private static final ScheduledExecutorService[] DELAYED_TASK_SCHEDULERS = createDelayedTaskSchedulers();
71
72 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
73 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
74 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
75 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
76 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
77
78 // scheduler and continuation
79 private final Executor scheduler;
80 private final Continuation cont;
81 private final Runnable runContinuation;
82
83 // virtual thread state, accessed by VM
84 private volatile int state;
85
86 /*
87 * Virtual thread state transitions:
88 *
89 * NEW -> STARTED // Thread.start, schedule to run
90 * STARTED -> TERMINATED // failed to start
91 * STARTED -> RUNNING // first run
92 * RUNNING -> TERMINATED // done
93 *
94 * RUNNING -> PARKING // Thread parking with LockSupport.park
95 * PARKING -> PARKED // cont.yield successful, parked indefinitely
96 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
97 * PARKED -> UNPARKED // unparked, may be scheduled to continue
98 * PINNED -> RUNNING // unparked, continue execution on same carrier
99 * UNPARKED -> RUNNING // continue execution after park
100 *
101 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
102 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
103 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
104 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
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 // timed-out/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 // can be suspended from scheduling when unmounted
158 private static final int SUSPENDED = 1 << 8;
159
160 // parking permit made available by LockSupport.unpark
161 private volatile boolean parkPermit;
162
163 // blocking permit made available by unblocker thread when another thread exits monitor
164 private volatile boolean blockPermit;
165
166 // true when on the list of virtual threads waiting to be unblocked
167 private volatile boolean onWaitingList;
168
169 // next virtual thread on the list of virtual threads waiting to be unblocked
170 private volatile VirtualThread next;
171
172 // notified by Object.notify/notifyAll while waiting in Object.wait
173 private volatile boolean notified;
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 a stream of the delayed task schedulers used to support timed operations.
199 */
200 static Stream<ScheduledExecutorService> delayedTaskSchedulers() {
201 return Arrays.stream(DELAYED_TASK_SCHEDULERS);
202 }
203
204 /**
205 * Returns the continuation scope used for virtual threads.
206 */
207 static ContinuationScope continuationScope() {
208 return VTHREAD_SCOPE;
209 }
210
211 /**
212 * Creates a new {@code VirtualThread} to run the given task with the given
213 * scheduler. If the given scheduler is {@code null} and the current thread
214 * is a platform thread then the newly created virtual thread will use the
215 * default scheduler. If given scheduler is {@code null} and the current
216 * thread is a virtual thread then the current thread's scheduler is used.
217 *
218 * @param scheduler the scheduler or null
219 * @param name thread name
220 * @param characteristics characteristics
221 * @param task the task to execute
222 */
223 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
224 super(name, characteristics, /*bound*/ false);
225 Objects.requireNonNull(task);
226
227 // choose scheduler if not specified
228 if (scheduler == null) {
229 Thread parent = Thread.currentThread();
230 if (parent instanceof VirtualThread vparent) {
231 scheduler = vparent.scheduler;
232 } else {
233 scheduler = DEFAULT_SCHEDULER;
234 }
235 }
236
237 this.scheduler = scheduler;
238 this.cont = new VThreadContinuation(this, task);
239 this.runContinuation = this::runContinuation;
240 }
241
242 /**
243 * The continuation that a virtual thread executes.
244 */
245 private static class VThreadContinuation extends Continuation {
246 VThreadContinuation(VirtualThread vthread, Runnable task) {
247 super(VTHREAD_SCOPE, wrap(vthread, task));
248 }
249 @Override
250 protected void onPinned(Continuation.Pinned reason) {
251 }
252 private static Runnable wrap(VirtualThread vthread, Runnable task) {
253 return new Runnable() {
254 @Hidden
255 @JvmtiHideEvents
256 public void run() {
257 vthread.notifyJvmtiStart(); // notify JVMTI
258 try {
259 vthread.run(task);
260 } finally {
261 vthread.notifyJvmtiEnd(); // notify JVMTI
262 }
263 }
264 };
265 }
266 }
267
268 /**
269 * Runs or continues execution on the current thread. The virtual thread is mounted
270 * on the current thread before the task runs or continues. It unmounts when the
271 * task completes or yields.
272 */
273 @ChangesCurrentThread // allow mount/unmount to be inlined
274 private void runContinuation() {
275 // the carrier must be a platform thread
276 if (Thread.currentThread().isVirtual()) {
277 throw new WrongThreadException();
278 }
279
280 // set state to RUNNING
281 int initialState = state();
282 if (initialState == STARTED || initialState == UNPARKED
283 || initialState == UNBLOCKED || initialState == YIELDED) {
284 // newly started or continue after parking/blocking/Thread.yield
285 if (!compareAndSetState(initialState, RUNNING)) {
286 return;
287 }
288 // consume permit when continuing after parking or blocking. If continue
289 // after a timed-park or timed-wait then the timeout task is cancelled.
290 if (initialState == UNPARKED) {
291 cancelTimeoutTask();
292 setParkPermit(false);
293 } else if (initialState == UNBLOCKED) {
294 cancelTimeoutTask();
295 blockPermit = false;
296 }
297 } else {
298 // not runnable
299 return;
300 }
301
302 mount();
303 try {
304 cont.run();
305 } finally {
306 unmount();
307 if (cont.isDone()) {
308 afterDone();
309 } else {
310 afterYield();
311 }
312 }
313 }
314
315 /**
316 * Cancel timeout task when continuing after timed-park or timed-wait.
317 * The timeout task may be executing, or may have already completed.
318 */
319 private void cancelTimeoutTask() {
320 if (timeoutTask != null) {
321 timeoutTask.cancel(false);
322 timeoutTask = null;
323 }
324 }
325
326 /**
327 * Submits the runContinuation task to the scheduler. For the default scheduler,
328 * and calling it on a worker thread, the task will be pushed to the local queue,
329 * otherwise it will be pushed to an external submission queue.
330 * @param scheduler the scheduler
331 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
332 * @throws RejectedExecutionException
333 */
334 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
335 boolean done = false;
336 while (!done) {
337 try {
338 // Pin the continuation to prevent the virtual thread from unmounting
339 // when submitting a task. For the default scheduler this ensures that
340 // the carrier doesn't change when pushing a task. For other schedulers
341 // it avoids deadlock that could arise due to carriers and virtual
342 // threads contending for a lock.
343 if (currentThread().isVirtual()) {
344 Continuation.pin();
345 try {
346 scheduler.execute(runContinuation);
347 } finally {
348 Continuation.unpin();
349 }
350 } else {
351 scheduler.execute(runContinuation);
352 }
353 done = true;
354 } catch (RejectedExecutionException ree) {
355 submitFailed(ree);
356 throw ree;
357 } catch (OutOfMemoryError e) {
358 if (retryOnOOME) {
359 U.park(false, 100_000_000); // 100ms
360 } else {
361 throw e;
362 }
363 }
364 }
365 }
366
367 /**
368 * Submits the runContinuation task to the given scheduler as an external submit.
369 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
370 * @throws RejectedExecutionException
371 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
372 */
373 private void externalSubmitRunContinuation(ForkJoinPool pool) {
374 assert Thread.currentThread() instanceof CarrierThread;
375 try {
376 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
377 } catch (RejectedExecutionException ree) {
378 submitFailed(ree);
379 throw ree;
380 } catch (OutOfMemoryError e) {
381 submitRunContinuation(pool, true);
382 }
383 }
384
385 /**
386 * Submits the runContinuation task to the scheduler. For the default scheduler,
387 * and calling it on a worker thread, the task will be pushed to the local queue,
388 * otherwise it will be pushed to an external submission queue.
389 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
390 * @throws RejectedExecutionException
391 */
392 private void submitRunContinuation() {
393 submitRunContinuation(scheduler, true);
394 }
395
396 /**
397 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
398 * queue is empty. If not empty, or invoked by another thread, then this method works
399 * like submitRunContinuation and just submits the task to the scheduler.
400 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
401 * @throws RejectedExecutionException
402 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
403 */
404 private void lazySubmitRunContinuation() {
405 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
406 ForkJoinPool pool = ct.getPool();
407 try {
408 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
409 } catch (RejectedExecutionException ree) {
410 submitFailed(ree);
411 throw ree;
412 } catch (OutOfMemoryError e) {
413 submitRunContinuation();
414 }
415 } else {
416 submitRunContinuation();
417 }
418 }
419
420 /**
421 * Submits the runContinuation task to the scheduler. For the default scheduler, and
422 * calling it a virtual thread that uses the default scheduler, the task will be
423 * pushed to an external submission queue. This method may throw OutOfMemoryError.
424 * @throws RejectedExecutionException
425 * @throws OutOfMemoryError
426 */
427 private void externalSubmitRunContinuationOrThrow() {
428 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
429 try {
430 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
431 } catch (RejectedExecutionException ree) {
432 submitFailed(ree);
433 throw ree;
434 }
435 } else {
436 submitRunContinuation(scheduler, false);
437 }
438 }
439
440 /**
441 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
442 */
443 private void submitFailed(RejectedExecutionException ree) {
444 var event = new VirtualThreadSubmitFailedEvent();
445 if (event.isEnabled()) {
446 event.javaThreadId = threadId();
447 event.exceptionMessage = ree.getMessage();
448 event.commit();
449 }
450 }
451
452 /**
453 * Runs a task in the context of this virtual thread.
454 */
455 private void run(Runnable task) {
456 assert Thread.currentThread() == this && state == RUNNING;
457
458 // emit JFR event if enabled
459 if (VirtualThreadStartEvent.isTurnedOn()) {
460 var event = new VirtualThreadStartEvent();
461 event.javaThreadId = threadId();
462 event.commit();
463 }
464
465 Object bindings = Thread.scopedValueBindings();
466 try {
467 runWith(bindings, task);
468 } catch (Throwable exc) {
469 dispatchUncaughtException(exc);
470 } finally {
471 // pop any remaining scopes from the stack, this may block
472 StackableScope.popAll();
473
474 // emit JFR event if enabled
475 if (VirtualThreadEndEvent.isTurnedOn()) {
476 var event = new VirtualThreadEndEvent();
477 event.javaThreadId = threadId();
478 event.commit();
479 }
480 }
481 }
482
483 /**
484 * Mounts this virtual thread onto the current platform thread. On
485 * return, the current thread is the virtual thread.
486 */
487 @ChangesCurrentThread
488 @ReservedStackAccess
489 private void mount() {
490 // notify JVMTI before mount
491 notifyJvmtiMount(/*hide*/true);
492
493 // sets the carrier thread
494 Thread carrier = Thread.currentCarrierThread();
495 setCarrierThread(carrier);
496
497 // sync up carrier thread interrupt status if needed
498 if (interrupted) {
499 carrier.setInterrupt();
500 } else if (carrier.isInterrupted()) {
501 synchronized (interruptLock) {
502 // need to recheck interrupt status
503 if (!interrupted) {
504 carrier.clearInterrupt();
505 }
506 }
507 }
508
509 // set Thread.currentThread() to return this virtual thread
510 carrier.setCurrentThread(this);
511 }
512
513 /**
514 * Unmounts this virtual thread from the carrier. On return, the
515 * current thread is the current platform thread.
516 */
517 @ChangesCurrentThread
518 @ReservedStackAccess
519 private void unmount() {
520 assert !Thread.holdsLock(interruptLock);
521
522 // set Thread.currentThread() to return the platform thread
523 Thread carrier = this.carrierThread;
524 carrier.setCurrentThread(carrier);
525
526 // break connection to carrier thread, synchronized with interrupt
527 synchronized (interruptLock) {
528 setCarrierThread(null);
529 }
530 carrier.clearInterrupt();
531
532 // notify JVMTI after unmount
533 notifyJvmtiUnmount(/*hide*/false);
534 }
535
536 /**
537 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
538 * the continuation continues.
539 */
540 @Hidden
541 private boolean yieldContinuation() {
542 notifyJvmtiUnmount(/*hide*/true);
543 try {
544 return Continuation.yield(VTHREAD_SCOPE);
545 } finally {
546 notifyJvmtiMount(/*hide*/false);
547 }
548 }
549
550 /**
551 * Invoked in the context of the carrier thread after the Continuation yields when
552 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
553 */
554 private void afterYield() {
555 assert carrierThread == null;
556
557 // re-adjust parallelism if the virtual thread yielded when compensating
558 if (currentThread() instanceof CarrierThread ct) {
559 ct.endBlocking();
560 }
561
562 int s = state();
563
564 // LockSupport.park/parkNanos
565 if (s == PARKING || s == TIMED_PARKING) {
566 int newState;
567 if (s == PARKING) {
568 setState(newState = PARKED);
569 } else {
570 // schedule unpark
571 assert timeout > 0;
572 timeoutTask = schedule(this::unpark, timeout, NANOSECONDS);
573 setState(newState = TIMED_PARKED);
574 }
575
576 // may have been unparked while parking
577 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
578 // lazy submit if local queue is empty
579 lazySubmitRunContinuation();
580 }
581 return;
582 }
583
584 // Thread.yield
585 if (s == YIELDING) {
586 setState(YIELDED);
587
588 // external submit if there are no tasks in the local task queue
589 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
590 externalSubmitRunContinuation(ct.getPool());
591 } else {
592 submitRunContinuation();
593 }
594 return;
595 }
596
597 // blocking on monitorenter
598 if (s == BLOCKING) {
599 setState(BLOCKED);
600
601 // may have been unblocked while blocking
602 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
603 // lazy submit if local queue is empty
604 lazySubmitRunContinuation();
605 }
606 return;
607 }
608
609 // Object.wait
610 if (s == WAITING || s == TIMED_WAITING) {
611 int newState;
612 if (s == WAITING) {
613 setState(newState = WAIT);
614 } else {
615 // For timed-wait, a timeout task is scheduled to execute. The timeout
616 // task will change the thread state to UNBLOCKED and submit the thread
617 // to the scheduler. A sequence number is used to ensure that the timeout
618 // task only unblocks the thread for this timed-wait. We synchronize with
619 // the timeout task to coordinate access to the sequence number and to
620 // ensure the timeout task doesn't execute until the thread has got to
621 // the TIMED_WAIT state.
622 assert timeout > 0;
623 synchronized (timedWaitLock()) {
624 byte seqNo = ++timedWaitSeqNo;
625 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
626 setState(newState = TIMED_WAIT);
627 }
628 }
629
630 // may have been notified while in transition to wait state
631 if (notified && compareAndSetState(newState, BLOCKED)) {
632 // may have even been unblocked already
633 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
634 submitRunContinuation();
635 }
636 return;
637 }
638
639 // may have been interrupted while in transition to wait state
640 if (interrupted && compareAndSetState(newState, UNBLOCKED)) {
641 submitRunContinuation();
642 return;
643 }
644 return;
645 }
646
647 assert false;
648 }
649
650 /**
651 * Invoked after the continuation completes.
652 */
653 private void afterDone() {
654 afterDone(true);
655 }
656
657 /**
658 * Invoked after the continuation completes (or start failed). Sets the thread
659 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
660 *
661 * @param notifyContainer true if its container should be notified
662 */
663 private void afterDone(boolean notifyContainer) {
664 assert carrierThread == null;
665 setState(TERMINATED);
666
667 // notify anyone waiting for this virtual thread to terminate
668 CountDownLatch termination = this.termination;
669 if (termination != null) {
670 assert termination.getCount() == 1;
671 termination.countDown();
672 }
673
674 // notify container
675 if (notifyContainer) {
676 threadContainer().onExit(this);
677 }
678
679 // clear references to thread locals
680 clearReferences();
681 }
682
683 /**
684 * Schedules this {@code VirtualThread} to execute.
685 *
686 * @throws IllegalStateException if the container is shutdown or closed
687 * @throws IllegalThreadStateException if the thread has already been started
688 * @throws RejectedExecutionException if the scheduler cannot accept a task
689 */
690 @Override
691 void start(ThreadContainer container) {
692 if (!compareAndSetState(NEW, STARTED)) {
693 throw new IllegalThreadStateException("Already started");
694 }
695
696 // bind thread to container
697 assert threadContainer() == null;
698 setThreadContainer(container);
699
700 // start thread
701 boolean addedToContainer = false;
702 boolean started = false;
703 try {
704 container.onStart(this); // may throw
705 addedToContainer = true;
706
707 // scoped values may be inherited
708 inheritScopedValueBindings(container);
709
710 // submit task to run thread, using externalSubmit if possible
711 externalSubmitRunContinuationOrThrow();
712 started = true;
713 } finally {
714 if (!started) {
715 afterDone(addedToContainer);
716 }
717 }
718 }
719
720 @Override
721 public void start() {
722 start(ThreadContainers.root());
723 }
724
725 @Override
726 public void run() {
727 // do nothing
728 }
729
730 /**
731 * Parks until unparked or interrupted. If already unparked then the parking
732 * permit is consumed and this method completes immediately (meaning it doesn't
733 * yield). It also completes immediately if the interrupt status is set.
734 */
735 @Override
736 void park() {
737 assert Thread.currentThread() == this;
738
739 // complete immediately if parking permit available or interrupted
740 if (getAndSetParkPermit(false) || interrupted)
741 return;
742
743 // park the thread
744 boolean yielded = false;
745 setState(PARKING);
746 try {
747 yielded = yieldContinuation();
748 } catch (OutOfMemoryError e) {
749 // park on carrier
750 } finally {
751 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
752 if (!yielded) {
753 assert state() == PARKING;
754 setState(RUNNING);
755 }
756 }
757
758 // park on the carrier thread when pinned
759 if (!yielded) {
760 parkOnCarrierThread(false, 0);
761 }
762 }
763
764 /**
765 * Parks up to the given waiting time or until unparked or interrupted.
766 * If already unparked then the parking permit is consumed and this method
767 * completes immediately (meaning it doesn't yield). It also completes immediately
768 * if the interrupt status is set or the waiting time is {@code <= 0}.
769 *
770 * @param nanos the maximum number of nanoseconds to wait.
771 */
772 @Override
773 void parkNanos(long nanos) {
774 assert Thread.currentThread() == this;
775
776 // complete immediately if parking permit available or interrupted
777 if (getAndSetParkPermit(false) || interrupted)
778 return;
779
780 // park the thread for the waiting time
781 if (nanos > 0) {
782 long startTime = System.nanoTime();
783
784 // park the thread, afterYield will schedule the thread to unpark
785 boolean yielded = false;
786 timeout = nanos;
787 setState(TIMED_PARKING);
788 try {
789 yielded = yieldContinuation();
790 } catch (OutOfMemoryError e) {
791 // park on carrier
792 } finally {
793 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
794 if (!yielded) {
795 assert state() == TIMED_PARKING;
796 setState(RUNNING);
797 }
798 }
799
800 // park on carrier thread for remaining time when pinned (or OOME)
801 if (!yielded) {
802 long remainingNanos = nanos - (System.nanoTime() - startTime);
803 parkOnCarrierThread(true, remainingNanos);
804 }
805 }
806 }
807
808 /**
809 * Parks the current carrier thread up to the given waiting time or until
810 * unparked or interrupted. If the virtual thread is interrupted then the
811 * interrupt status will be propagated to the carrier thread.
812 * @param timed true for a timed park, false for untimed
813 * @param nanos the waiting time in nanoseconds
814 */
815 private void parkOnCarrierThread(boolean timed, long nanos) {
816 assert state() == RUNNING;
817
818 setState(timed ? TIMED_PINNED : PINNED);
819 try {
820 if (!parkPermit) {
821 if (!timed) {
822 U.park(false, 0);
823 } else if (nanos > 0) {
824 U.park(false, nanos);
825 }
826 }
827 } finally {
828 setState(RUNNING);
829 }
830
831 // consume parking permit
832 setParkPermit(false);
833
834 // JFR jdk.VirtualThreadPinned event
835 postPinnedEvent("LockSupport.park");
836 }
837
838 /**
839 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
840 * Recording the event in the VM avoids having JFR event recorded in Java
841 * with the same name, but different ID, to events recorded by the VM.
842 */
843 @Hidden
844 private static native void postPinnedEvent(String op);
845
846 /**
847 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
848 * then its task is scheduled to continue, otherwise its next call to {@code park} or
849 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
850 * @throws RejectedExecutionException if the scheduler cannot accept a task
851 */
852 @Override
853 void unpark() {
854 if (!getAndSetParkPermit(true) && currentThread() != this) {
855 int s = state();
856
857 // unparked while parked
858 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
859 submitRunContinuation();
860 return;
861 }
862
863 // unparked while parked when pinned
864 if (s == PINNED || s == TIMED_PINNED) {
865 // unpark carrier thread when pinned
866 disableSuspendAndPreempt();
867 try {
868 synchronized (carrierThreadAccessLock()) {
869 Thread carrier = carrierThread;
870 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
871 U.unpark(carrier);
872 }
873 }
874 } finally {
875 enableSuspendAndPreempt();
876 }
877 return;
878 }
879 }
880 }
881
882 /**
883 * Invoked by unblocker thread to unblock this virtual thread.
884 */
885 private void unblock() {
886 assert !Thread.currentThread().isVirtual();
887 blockPermit = true;
888 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
889 submitRunContinuation();
890 }
891 }
892
893 /**
894 * Invoked by timer thread when wait timeout for virtual thread has expired.
895 * If the virtual thread is in timed-wait then this method will unblock the thread
896 * and submit its task so that it continues and attempts to reenter the monitor.
897 * This method does nothing if the thread has been woken by notify or interrupt.
898 */
899 private void waitTimeoutExpired(byte seqNo) {
900 assert !Thread.currentThread().isVirtual();
901 for (;;) {
902 boolean unblocked = false;
903 synchronized (timedWaitLock()) {
904 if (seqNo != timedWaitSeqNo) {
905 // this timeout task is for a past timed-wait
906 return;
907 }
908 int s = state();
909 if (s == TIMED_WAIT) {
910 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
911 } else if (s != (TIMED_WAIT | SUSPENDED)) {
912 // notified or interrupted, no longer waiting
913 return;
914 }
915 }
916 if (unblocked) {
917 submitRunContinuation();
918 return;
919 }
920 // need to retry when thread is suspended in time-wait
921 Thread.yield();
922 }
923 }
924
925 /**
926 * Attempts to yield the current virtual thread (Thread.yield).
927 */
928 void tryYield() {
929 assert Thread.currentThread() == this;
930 setState(YIELDING);
931 boolean yielded = false;
932 try {
933 yielded = yieldContinuation(); // may throw
934 } finally {
935 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
936 if (!yielded) {
937 assert state() == YIELDING;
938 setState(RUNNING);
939 }
940 }
941 }
942
943 /**
944 * Sleep the current thread for the given sleep time (in nanoseconds). If
945 * nanos is 0 then the thread will attempt to yield.
946 *
947 * @implNote This implementation parks the thread for the given sleeping time
948 * and will therefore be observed in PARKED state during the sleep. Parking
949 * will consume the parking permit so this method makes available the parking
950 * permit after the sleep. This may be observed as a spurious, but benign,
951 * wakeup when the thread subsequently attempts to park.
952 *
953 * @param nanos the maximum number of nanoseconds to sleep
954 * @throws InterruptedException if interrupted while sleeping
955 */
956 void sleepNanos(long nanos) throws InterruptedException {
957 assert Thread.currentThread() == this && nanos >= 0;
958 if (getAndClearInterrupt())
959 throw new InterruptedException();
960 if (nanos == 0) {
961 tryYield();
962 } else {
963 // park for the sleep time
964 try {
965 long remainingNanos = nanos;
966 long startNanos = System.nanoTime();
967 while (remainingNanos > 0) {
968 parkNanos(remainingNanos);
969 if (getAndClearInterrupt()) {
970 throw new InterruptedException();
971 }
972 remainingNanos = nanos - (System.nanoTime() - startNanos);
973 }
974 } finally {
975 // may have been unparked while sleeping
976 setParkPermit(true);
977 }
978 }
979 }
980
981 /**
982 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
983 * A timeout of {@code 0} means to wait forever.
984 *
985 * @throws InterruptedException if interrupted while waiting
986 * @return true if the thread has terminated
987 */
988 boolean joinNanos(long nanos) throws InterruptedException {
989 if (state() == TERMINATED)
990 return true;
991
992 // ensure termination object exists, then re-check state
993 CountDownLatch termination = getTermination();
994 if (state() == TERMINATED)
995 return true;
996
997 // wait for virtual thread to terminate
998 if (nanos == 0) {
999 termination.await();
1000 } else {
1001 boolean terminated = termination.await(nanos, NANOSECONDS);
1002 if (!terminated) {
1003 // waiting time elapsed
1004 return false;
1005 }
1006 }
1007 assert state() == TERMINATED;
1008 return true;
1009 }
1010
1011 @Override
1012 void blockedOn(Interruptible b) {
1013 disableSuspendAndPreempt();
1014 try {
1015 super.blockedOn(b);
1016 } finally {
1017 enableSuspendAndPreempt();
1018 }
1019 }
1020
1021 @Override
1022 public void interrupt() {
1023 if (Thread.currentThread() != this) {
1024 // if current thread is a virtual thread then prevent it from being
1025 // suspended or unmounted when entering or holding interruptLock
1026 Interruptible blocker;
1027 disableSuspendAndPreempt();
1028 try {
1029 synchronized (interruptLock) {
1030 interrupted = true;
1031 blocker = nioBlocker();
1032 if (blocker != null) {
1033 blocker.interrupt(this);
1034 }
1035
1036 // interrupt carrier thread if mounted
1037 Thread carrier = carrierThread;
1038 if (carrier != null) carrier.setInterrupt();
1039 }
1040 } finally {
1041 enableSuspendAndPreempt();
1042 }
1043
1044 // notify blocker after releasing interruptLock
1045 if (blocker != null) {
1046 blocker.postInterrupt();
1047 }
1048
1049 // make available parking permit, unpark thread if parked
1050 unpark();
1051
1052 // if thread is waiting in Object.wait then schedule to try to reenter
1053 int s = state();
1054 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1055 submitRunContinuation();
1056 }
1057
1058 } else {
1059 interrupted = true;
1060 carrierThread.setInterrupt();
1061 setParkPermit(true);
1062 }
1063 }
1064
1065 @Override
1066 public boolean isInterrupted() {
1067 return interrupted;
1068 }
1069
1070 @Override
1071 boolean getAndClearInterrupt() {
1072 assert Thread.currentThread() == this;
1073 boolean oldValue = interrupted;
1074 if (oldValue) {
1075 disableSuspendAndPreempt();
1076 try {
1077 synchronized (interruptLock) {
1078 interrupted = false;
1079 carrierThread.clearInterrupt();
1080 }
1081 } finally {
1082 enableSuspendAndPreempt();
1083 }
1084 }
1085 return oldValue;
1086 }
1087
1088 @Override
1089 Thread.State threadState() {
1090 int s = state();
1091 switch (s & ~SUSPENDED) {
1092 case NEW:
1093 return Thread.State.NEW;
1094 case STARTED:
1095 // return NEW if thread container not yet set
1096 if (threadContainer() == null) {
1097 return Thread.State.NEW;
1098 } else {
1099 return Thread.State.RUNNABLE;
1100 }
1101 case UNPARKED:
1102 case UNBLOCKED:
1103 case YIELDED:
1104 // runnable, not mounted
1105 return Thread.State.RUNNABLE;
1106 case RUNNING:
1107 // if mounted then return state of carrier thread
1108 if (Thread.currentThread() != this) {
1109 disableSuspendAndPreempt();
1110 try {
1111 synchronized (carrierThreadAccessLock()) {
1112 Thread carrierThread = this.carrierThread;
1113 if (carrierThread != null) {
1114 return carrierThread.threadState();
1115 }
1116 }
1117 } finally {
1118 enableSuspendAndPreempt();
1119 }
1120 }
1121 // runnable, mounted
1122 return Thread.State.RUNNABLE;
1123 case PARKING:
1124 case TIMED_PARKING:
1125 case WAITING:
1126 case TIMED_WAITING:
1127 case YIELDING:
1128 // runnable, in transition
1129 return Thread.State.RUNNABLE;
1130 case PARKED:
1131 case PINNED:
1132 case WAIT:
1133 return Thread.State.WAITING;
1134 case TIMED_PARKED:
1135 case TIMED_PINNED:
1136 case TIMED_WAIT:
1137 return Thread.State.TIMED_WAITING;
1138 case BLOCKING:
1139 case BLOCKED:
1140 return Thread.State.BLOCKED;
1141 case TERMINATED:
1142 return Thread.State.TERMINATED;
1143 default:
1144 throw new InternalError();
1145 }
1146 }
1147
1148 @Override
1149 boolean alive() {
1150 int s = state;
1151 return (s != NEW && s != TERMINATED);
1152 }
1153
1154 @Override
1155 boolean isTerminated() {
1156 return (state == TERMINATED);
1157 }
1158
1159 @Override
1160 StackTraceElement[] asyncGetStackTrace() {
1161 StackTraceElement[] stackTrace;
1162 do {
1163 stackTrace = (carrierThread != null)
1164 ? super.asyncGetStackTrace() // mounted
1165 : tryGetStackTrace(); // unmounted
1166 if (stackTrace == null) {
1167 Thread.yield();
1168 }
1169 } while (stackTrace == null);
1170 return stackTrace;
1171 }
1172
1173 /**
1174 * Returns the stack trace for this virtual thread if it is unmounted.
1175 * Returns null if the thread is mounted or in transition.
1176 */
1177 private StackTraceElement[] tryGetStackTrace() {
1178 int initialState = state() & ~SUSPENDED;
1179 switch (initialState) {
1180 case NEW, STARTED, TERMINATED -> {
1181 return new StackTraceElement[0]; // unmounted, empty stack
1182 }
1183 case RUNNING, PINNED, TIMED_PINNED -> {
1184 return null; // mounted
1185 }
1186 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1187 // unmounted, not runnable
1188 }
1189 case UNPARKED, UNBLOCKED, YIELDED -> {
1190 // unmounted, runnable
1191 }
1192 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1193 return null; // in transition
1194 }
1195 default -> throw new InternalError("" + initialState);
1196 }
1197
1198 // thread is unmounted, prevent it from continuing
1199 int suspendedState = initialState | SUSPENDED;
1200 if (!compareAndSetState(initialState, suspendedState)) {
1201 return null;
1202 }
1203
1204 // get stack trace and restore state
1205 StackTraceElement[] stack;
1206 try {
1207 stack = cont.getStackTrace();
1208 } finally {
1209 assert state == suspendedState;
1210 setState(initialState);
1211 }
1212 boolean resubmit = switch (initialState) {
1213 case UNPARKED, UNBLOCKED, YIELDED -> {
1214 // resubmit as task may have run while suspended
1215 yield true;
1216 }
1217 case PARKED, TIMED_PARKED -> {
1218 // resubmit if unparked while suspended
1219 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1220 }
1221 case BLOCKED -> {
1222 // resubmit if unblocked while suspended
1223 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1224 }
1225 case WAIT, TIMED_WAIT -> {
1226 // resubmit if notified or interrupted while waiting (Object.wait)
1227 // waitTimeoutExpired will retry if the timed expired when suspended
1228 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1229 }
1230 default -> throw new InternalError();
1231 };
1232 if (resubmit) {
1233 submitRunContinuation();
1234 }
1235 return stack;
1236 }
1237
1238 @Override
1239 public String toString() {
1240 StringBuilder sb = new StringBuilder("VirtualThread[#");
1241 sb.append(threadId());
1242 String name = getName();
1243 if (!name.isEmpty()) {
1244 sb.append(",");
1245 sb.append(name);
1246 }
1247 sb.append("]/");
1248
1249 // add the carrier state and thread name when mounted
1250 boolean mounted;
1251 if (Thread.currentThread() == this) {
1252 mounted = appendCarrierInfo(sb);
1253 } else {
1254 disableSuspendAndPreempt();
1255 try {
1256 synchronized (carrierThreadAccessLock()) {
1257 mounted = appendCarrierInfo(sb);
1258 }
1259 } finally {
1260 enableSuspendAndPreempt();
1261 }
1262 }
1263
1264 // add virtual thread state when not mounted
1265 if (!mounted) {
1266 String stateAsString = threadState().toString();
1267 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1268 }
1269
1270 return sb.toString();
1271 }
1272
1273 /**
1274 * Appends the carrier state and thread name to the string buffer if mounted.
1275 * @return true if mounted, false if not mounted
1276 */
1277 private boolean appendCarrierInfo(StringBuilder sb) {
1278 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1279 Thread carrier = carrierThread;
1280 if (carrier != null) {
1281 String stateAsString = carrier.threadState().toString();
1282 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1283 sb.append('@');
1284 sb.append(carrier.getName());
1285 return true;
1286 } else {
1287 return false;
1288 }
1289 }
1290
1291 @Override
1292 public int hashCode() {
1293 return (int) threadId();
1294 }
1295
1296 @Override
1297 public boolean equals(Object obj) {
1298 return obj == this;
1299 }
1300
1301 /**
1302 * Returns the termination object, creating it if needed.
1303 */
1304 private CountDownLatch getTermination() {
1305 CountDownLatch termination = this.termination;
1306 if (termination == null) {
1307 termination = new CountDownLatch(1);
1308 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1309 termination = this.termination;
1310 }
1311 }
1312 return termination;
1313 }
1314
1315 /**
1316 * Returns the lock object to synchronize on when accessing carrierThread.
1317 * The lock prevents carrierThread from being reset to null during unmount.
1318 */
1319 private Object carrierThreadAccessLock() {
1320 // return interruptLock as unmount has to coordinate with interrupt
1321 return interruptLock;
1322 }
1323
1324 /**
1325 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1326 */
1327 private Object timedWaitLock() {
1328 // use this object for now to avoid the overhead of introducing another lock
1329 return runContinuation;
1330 }
1331
1332 /**
1333 * Disallow the current thread be suspended or preempted.
1334 */
1335 private void disableSuspendAndPreempt() {
1336 notifyJvmtiDisableSuspend(true);
1337 Continuation.pin();
1338 }
1339
1340 /**
1341 * Allow the current thread be suspended or preempted.
1342 */
1343 private void enableSuspendAndPreempt() {
1344 Continuation.unpin();
1345 notifyJvmtiDisableSuspend(false);
1346 }
1347
1348 // -- wrappers for get/set of state, parking permit, and carrier thread --
1349
1350 private int state() {
1351 return state; // volatile read
1352 }
1353
1354 private void setState(int newValue) {
1355 state = newValue; // volatile write
1356 }
1357
1358 private boolean compareAndSetState(int expectedValue, int newValue) {
1359 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1360 }
1361
1362 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1363 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1364 }
1365
1366 private void setParkPermit(boolean newValue) {
1367 if (parkPermit != newValue) {
1368 parkPermit = newValue;
1369 }
1370 }
1371
1372 private boolean getAndSetParkPermit(boolean newValue) {
1373 if (parkPermit != newValue) {
1374 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1375 } else {
1376 return newValue;
1377 }
1378 }
1379
1380 private void setCarrierThread(Thread carrier) {
1381 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1382 this.carrierThread = carrier;
1383 }
1384
1385 // -- JVM TI support --
1386
1387 @IntrinsicCandidate
1388 @JvmtiMountTransition
1389 private native void notifyJvmtiStart();
1390
1391 @IntrinsicCandidate
1392 @JvmtiMountTransition
1393 private native void notifyJvmtiEnd();
1394
1395 @IntrinsicCandidate
1396 @JvmtiMountTransition
1397 private native void notifyJvmtiMount(boolean hide);
1398
1399 @IntrinsicCandidate
1400 @JvmtiMountTransition
1401 private native void notifyJvmtiUnmount(boolean hide);
1402
1403 @IntrinsicCandidate
1404 private static native void notifyJvmtiDisableSuspend(boolean enter);
1405
1406 private static native void registerNatives();
1407 static {
1408 registerNatives();
1409
1410 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1411 var group = Thread.virtualThreadGroup();
1412 }
1413
1414 /**
1415 * Creates the default scheduler.
1416 * If the system property {@code jdk.virtualThreadScheduler.implClass} is set then
1417 * its value is the name of a class that implements java.util.concurrent.Executor.
1418 * The class is public in an exported package, has a public no-arg constructor,
1419 * and is visible to the system class loader.
1420 * If the system property is not set then the default scheduler will be a
1421 * ForkJoinPool instance.
1422 */
1423 private static Executor createDefaultScheduler() {
1424 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
1425 if (propValue != null) {
1426 try {
1427 Class<?> clazz = Class.forName(propValue, true,
1428 ClassLoader.getSystemClassLoader());
1429 Constructor<?> ctor = clazz.getConstructor();
1430 var scheduler = (Executor) ctor.newInstance();
1431 System.err.println("""
1432 WARNING: Using custom scheduler, this is an experimental feature.""");
1433 return scheduler;
1434 } catch (Exception ex) {
1435 throw new Error(ex);
1436 }
1437 } else {
1438 return createDefaultForkJoinPoolScheduler();
1439 }
1440 }
1441
1442 /**
1443 * Creates the default ForkJoinPool scheduler.
1444 */
1445 private static ForkJoinPool createDefaultForkJoinPoolScheduler() {
1446 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1447 int parallelism, maxPoolSize, minRunnable;
1448 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1449 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1450 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1451 if (parallelismValue != null) {
1452 parallelism = Integer.parseInt(parallelismValue);
1453 } else {
1454 parallelism = Runtime.getRuntime().availableProcessors();
1455 }
1456 if (maxPoolSizeValue != null) {
1457 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1458 parallelism = Integer.min(parallelism, maxPoolSize);
1459 } else {
1460 maxPoolSize = Integer.max(parallelism, 256);
1461 }
1462 if (minRunnableValue != null) {
1463 minRunnable = Integer.parseInt(minRunnableValue);
1464 } else {
1465 minRunnable = Integer.max(parallelism / 2, 1);
1466 }
1467 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1468 boolean asyncMode = true; // FIFO
1469 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1470 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1471 }
1472
1473 /**
1474 * Schedule a runnable task to run after a delay.
1475 */
1476 private static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1477 long tid = Thread.currentThread().threadId();
1478 int index = (int) tid & (DELAYED_TASK_SCHEDULERS.length - 1);
1479 return DELAYED_TASK_SCHEDULERS[index].schedule(command, delay, unit);
1480 }
1481
1482 /**
1483 * Creates the ScheduledThreadPoolExecutors used to execute delayed tasks.
1484 */
1485 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1486 String propName = "jdk.virtualThreadScheduler.timerQueues";
1487 String propValue = System.getProperty(propName);
1488 int queueCount;
1489 if (propValue != null) {
1490 queueCount = Integer.parseInt(propValue);
1491 if (queueCount != Integer.highestOneBit(queueCount)) {
1492 throw new RuntimeException("Value of " + propName + " must be power of 2");
1493 }
1494 } else {
1495 int ncpus = Runtime.getRuntime().availableProcessors();
1496 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1497 }
1498 var schedulers = new ScheduledExecutorService[queueCount];
1499 for (int i = 0; i < queueCount; i++) {
1500 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1501 Executors.newScheduledThreadPool(1, task -> {
1502 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1503 t.setDaemon(true);
1504 return t;
1505 });
1506 stpe.setRemoveOnCancelPolicy(true);
1507 schedulers[i] = stpe;
1508 }
1509 return schedulers;
1510 }
1511
1512 /**
1513 * Schedule virtual threads that are ready to be scheduled after they blocked on
1514 * monitor enter.
1515 */
1516 private static void unblockVirtualThreads() {
1517 while (true) {
1518 VirtualThread vthread = takeVirtualThreadListToUnblock();
1519 while (vthread != null) {
1520 assert vthread.onWaitingList;
1521 VirtualThread nextThread = vthread.next;
1522
1523 // remove from list and unblock
1524 vthread.next = null;
1525 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1526 assert changed;
1527 vthread.unblock();
1528
1529 vthread = nextThread;
1530 }
1531 }
1532 }
1533
1534 /**
1535 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1536 * if necessary until a list of one or more threads becomes available.
1537 */
1538 private static native VirtualThread takeVirtualThreadListToUnblock();
1539
1540 static {
1541 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1542 VirtualThread::unblockVirtualThreads);
1543 unblocker.setDaemon(true);
1544 unblocker.start();
1545 }
1546 }