1 /*
2 * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.util.Locale;
28 import java.util.Objects;
29 import java.util.concurrent.CountDownLatch;
30 import java.util.concurrent.Executor;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.Future;
36 import java.util.concurrent.RejectedExecutionException;
37 import java.util.concurrent.ScheduledExecutorService;
38 import java.util.concurrent.ScheduledThreadPoolExecutor;
39 import java.util.concurrent.TimeUnit;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66 private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
67
68 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
69 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
70 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
71 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
72 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
73
74 // scheduler and continuation
75 private final Executor scheduler;
76 private final Continuation cont;
77 private final Runnable runContinuation;
78
79 // virtual thread state, accessed by VM
80 private volatile int state;
81
82 /*
83 * Virtual thread state transitions:
84 *
85 * NEW -> STARTED // Thread.start, schedule to run
86 * STARTED -> TERMINATED // failed to start
87 * STARTED -> RUNNING // first run
88 * RUNNING -> TERMINATED // done
89 *
90 * RUNNING -> PARKING // Thread parking with LockSupport.park
91 * PARKING -> PARKED // cont.yield successful, parked indefinitely
92 * PARKED -> UNPARKED // unparked, may be scheduled to continue
93 * UNPARKED -> RUNNING // continue execution after park
94 *
95 * PARKING -> RUNNING // cont.yield failed, need to park on carrier
96 * RUNNING -> PINNED // park on carrier
97 * PINNED -> RUNNING // unparked, continue execution on same carrier
98 *
99 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
100 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
101 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
102 *
103 * TIMED_PARKING -> RUNNING // cont.yield failed, need to park on carrier
104 * RUNNING -> TIMED_PINNED // park on carrier
105 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
106 *
107 * RUNNING -> BLOCKING // blocking on monitor enter
108 * BLOCKING -> BLOCKED // blocked on monitor enter
109 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
110 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
111 *
112 * RUNNING -> WAITING // transitional state during wait on monitor
113 * WAITING -> WAIT // waiting on monitor
114 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
115 * WAIT -> UNBLOCKED // interrupted
116 *
117 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
118 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
119 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
120 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
121 *
122 * RUNNING -> YIELDING // Thread.yield
123 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
124 * YIELDING -> RUNNING // cont.yield failed
125 * YIELDED -> RUNNING // continue execution after Thread.yield
126 */
127 private static final int NEW = 0;
128 private static final int STARTED = 1;
129 private static final int RUNNING = 2; // runnable-mounted
130
131 // untimed and timed parking
132 private static final int PARKING = 3;
133 private static final int PARKED = 4; // unmounted
134 private static final int PINNED = 5; // mounted
135 private static final int TIMED_PARKING = 6;
136 private static final int TIMED_PARKED = 7; // unmounted
137 private static final int TIMED_PINNED = 8; // mounted
138 private static final int UNPARKED = 9; // unmounted but runnable
139
140 // Thread.yield
141 private static final int YIELDING = 10;
142 private static final int YIELDED = 11; // unmounted but runnable
143
144 // monitor enter
145 private static final int BLOCKING = 12;
146 private static final int BLOCKED = 13; // unmounted
147 private static final int UNBLOCKED = 14; // unmounted but runnable
148
149 // monitor wait/timed-wait
150 private static final int WAITING = 15;
151 private static final int WAIT = 16; // waiting in Object.wait
152 private static final int TIMED_WAITING = 17;
153 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
154
155 private static final int TERMINATED = 99; // final state
156
157 // 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 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
176 private volatile boolean interruptibleWait;
177
178 // timed-wait support
179 private byte timedWaitSeqNo;
180
181 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
182 private long timeout;
183
184 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
185 private Future<?> timeoutTask;
186
187 // carrier thread when mounted, accessed by VM
188 private volatile Thread carrierThread;
189
190 // termination object when joining, created lazily if needed
191 private volatile CountDownLatch termination;
192
193 /**
194 * Returns the default scheduler.
195 */
196 static Executor defaultScheduler() {
197 return DEFAULT_SCHEDULER;
198 }
199
200 /**
201 * Returns the continuation scope used for virtual threads.
202 */
203 static ContinuationScope continuationScope() {
204 return VTHREAD_SCOPE;
205 }
206
207 /**
208 * Creates a new {@code VirtualThread} to run the given task with the given
209 * scheduler. If the given scheduler is {@code null} and the current thread
210 * is a platform thread then the newly created virtual thread will use the
211 * default scheduler. If given scheduler is {@code null} and the current
212 * thread is a virtual thread then the current thread's scheduler is used.
213 *
214 * @param scheduler the scheduler or null
215 * @param name thread name
216 * @param characteristics characteristics
217 * @param task the task to execute
218 */
219 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
220 super(name, characteristics, /*bound*/ false);
221 Objects.requireNonNull(task);
222
223 // choose scheduler if not specified
224 if (scheduler == null) {
225 Thread parent = Thread.currentThread();
226 if (parent instanceof VirtualThread vparent) {
227 scheduler = vparent.scheduler;
228 } else {
229 scheduler = DEFAULT_SCHEDULER;
230 }
231 }
232
233 this.scheduler = scheduler;
234 this.cont = new VThreadContinuation(this, task);
235 this.runContinuation = this::runContinuation;
236 }
237
238 /**
239 * The continuation that a virtual thread executes.
240 */
241 private static class VThreadContinuation extends Continuation {
242 VThreadContinuation(VirtualThread vthread, Runnable task) {
243 super(VTHREAD_SCOPE, wrap(vthread, task));
244 }
245 @Override
246 protected void onPinned(Continuation.Pinned reason) {
247 }
248 private static Runnable wrap(VirtualThread vthread, Runnable task) {
249 return new Runnable() {
250 @Hidden
251 @JvmtiHideEvents
252 public void run() {
253 vthread.endFirstTransition();
254 try {
255 vthread.run(task);
256 } finally {
257 vthread.startFinalTransition();
258 }
259 }
260 };
261 }
262 }
263
264 /**
265 * Runs or continues execution on the current thread. The virtual thread is mounted
266 * on the current thread before the task runs or continues. It unmounts when the
267 * task completes or yields.
268 */
269 @ChangesCurrentThread // allow mount/unmount to be inlined
270 private void runContinuation() {
271 // the carrier must be a platform thread
272 if (Thread.currentThread().isVirtual()) {
273 throw new WrongThreadException();
274 }
275
276 // set state to RUNNING
277 int initialState = state();
278 if (initialState == STARTED || initialState == UNPARKED
279 || initialState == UNBLOCKED || initialState == YIELDED) {
280 // newly started or continue after parking/blocking/Thread.yield
281 if (!compareAndSetState(initialState, RUNNING)) {
282 return;
283 }
284 // consume permit when continuing after parking or blocking. If continue
285 // after a timed-park or timed-wait then the timeout task is cancelled.
286 if (initialState == UNPARKED) {
287 cancelTimeoutTask();
288 setParkPermit(false);
289 } else if (initialState == UNBLOCKED) {
290 cancelTimeoutTask();
291 blockPermit = false;
292 }
293 } else {
294 // not runnable
295 return;
296 }
297
298 mount();
299 try {
300 cont.run();
301 } finally {
302 unmount();
303 if (cont.isDone()) {
304 afterDone();
305 } else {
306 afterYield();
307 }
308 }
309 }
310
311 /**
312 * Cancel timeout task when continuing after timed-park or timed-wait.
313 * The timeout task may be executing, or may have already completed.
314 */
315 private void cancelTimeoutTask() {
316 if (timeoutTask != null) {
317 timeoutTask.cancel(false);
318 timeoutTask = null;
319 }
320 }
321
322 /**
323 * Submits the given task to the given executor. If the scheduler is a
324 * ForkJoinPool then the task is first adapted to a ForkJoinTask.
325 */
326 private void submit(Executor executor, Runnable task) {
327 if (executor instanceof ForkJoinPool pool) {
328 pool.submit(ForkJoinTask.adapt(task));
329 } else {
330 executor.execute(task);
331 }
332 }
333
334 /**
335 * Submits the runContinuation task to the scheduler. For the default scheduler,
336 * and calling it on a worker thread, the task will be pushed to the local queue,
337 * otherwise it will be pushed to an external submission queue.
338 * @param scheduler the scheduler
339 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
340 * @throws RejectedExecutionException
341 */
342 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
343 boolean done = false;
344 while (!done) {
345 try {
346 // Pin the continuation to prevent the virtual thread from unmounting
347 // when submitting a task. For the default scheduler this ensures that
348 // the carrier doesn't change when pushing a task. For other schedulers
349 // it avoids deadlock that could arise due to carriers and virtual
350 // threads contending for a lock.
351 if (currentThread().isVirtual()) {
352 Continuation.pin();
353 try {
354 submit(scheduler, runContinuation);
355 } finally {
356 Continuation.unpin();
357 }
358 } else {
359 submit(scheduler, runContinuation);
360 }
361 done = true;
362 } catch (RejectedExecutionException ree) {
363 submitFailed(ree);
364 throw ree;
365 } catch (OutOfMemoryError e) {
366 if (retryOnOOME) {
367 U.park(false, 100_000_000); // 100ms
368 } else {
369 throw e;
370 }
371 }
372 }
373 }
374
375 /**
376 * Submits the runContinuation task to the given scheduler as an external submit.
377 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
378 * @throws RejectedExecutionException
379 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
380 */
381 private void externalSubmitRunContinuation(ForkJoinPool pool) {
382 assert Thread.currentThread() instanceof CarrierThread;
383 try {
384 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
385 } catch (RejectedExecutionException ree) {
386 submitFailed(ree);
387 throw ree;
388 } catch (OutOfMemoryError e) {
389 submitRunContinuation(pool, true);
390 }
391 }
392
393 /**
394 * Submits the runContinuation task to the scheduler. For the default scheduler,
395 * and calling it on a worker thread, the task will be pushed to the local queue,
396 * otherwise it will be pushed to an external submission queue.
397 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
398 * @throws RejectedExecutionException
399 */
400 private void submitRunContinuation() {
401 submitRunContinuation(scheduler, true);
402 }
403
404 /**
405 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
406 * queue is empty. If not empty, or invoked by another thread, then this method works
407 * like submitRunContinuation and just submits the task to the scheduler.
408 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
409 * @throws RejectedExecutionException
410 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
411 */
412 private void lazySubmitRunContinuation() {
413 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
414 ForkJoinPool pool = ct.getPool();
415 try {
416 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
417 } catch (RejectedExecutionException ree) {
418 submitFailed(ree);
419 throw ree;
420 } catch (OutOfMemoryError e) {
421 submitRunContinuation();
422 }
423 } else {
424 submitRunContinuation();
425 }
426 }
427
428 /**
429 * Submits the runContinuation task to the scheduler. For the default scheduler, and
430 * calling it a virtual thread that uses the default scheduler, the task will be
431 * pushed to an external submission queue. This method may throw OutOfMemoryError.
432 * @throws RejectedExecutionException
433 * @throws OutOfMemoryError
434 */
435 private void externalSubmitRunContinuationOrThrow() {
436 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
437 try {
438 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
439 } catch (RejectedExecutionException ree) {
440 submitFailed(ree);
441 throw ree;
442 }
443 } else {
444 submitRunContinuation(scheduler, false);
445 }
446 }
447
448 /**
449 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
450 */
451 private void submitFailed(RejectedExecutionException ree) {
452 var event = new VirtualThreadSubmitFailedEvent();
453 if (event.isEnabled()) {
454 event.javaThreadId = threadId();
455 event.exceptionMessage = ree.getMessage();
456 event.commit();
457 }
458 }
459
460 /**
461 * Runs a task in the context of this virtual thread.
462 */
463 private void run(Runnable task) {
464 assert Thread.currentThread() == this && state == RUNNING;
465
466 // emit JFR event if enabled
467 if (VirtualThreadStartEvent.isTurnedOn()) {
468 var event = new VirtualThreadStartEvent();
469 event.javaThreadId = threadId();
470 event.commit();
471 }
472
473 Object bindings = Thread.scopedValueBindings();
474 try {
475 runWith(bindings, task);
476 } catch (Throwable exc) {
477 dispatchUncaughtException(exc);
478 } finally {
479 // pop any remaining scopes from the stack, this may block
480 StackableScope.popAll();
481
482 // emit JFR event if enabled
483 if (VirtualThreadEndEvent.isTurnedOn()) {
484 var event = new VirtualThreadEndEvent();
485 event.javaThreadId = threadId();
486 event.commit();
487 }
488 }
489 }
490
491 /**
492 * Mounts this virtual thread onto the current platform thread. On
493 * return, the current thread is the virtual thread.
494 */
495 @ChangesCurrentThread
496 @ReservedStackAccess
497 private void mount() {
498 startTransition(/*is_mount*/true);
499 // We assume following volatile accesses provide equivalent
500 // of acquire ordering, otherwise we need U.loadFence() here.
501
502 // sets the carrier thread
503 Thread carrier = Thread.currentCarrierThread();
504 setCarrierThread(carrier);
505
506 // sync up carrier thread interrupted status if needed
507 if (interrupted) {
508 carrier.setInterrupt();
509 } else if (carrier.isInterrupted()) {
510 synchronized (interruptLock) {
511 // need to recheck interrupted status
512 if (!interrupted) {
513 carrier.clearInterrupt();
514 }
515 }
516 }
517
518 // set Thread.currentThread() to return this virtual thread
519 carrier.setCurrentThread(this);
520 }
521
522 /**
523 * Unmounts this virtual thread from the carrier. On return, the
524 * current thread is the current platform thread.
525 */
526 @ChangesCurrentThread
527 @ReservedStackAccess
528 private void unmount() {
529 assert !Thread.holdsLock(interruptLock);
530
531 // set Thread.currentThread() to return the platform thread
532 Thread carrier = this.carrierThread;
533 carrier.setCurrentThread(carrier);
534
535 // break connection to carrier thread, synchronized with interrupt
536 synchronized (interruptLock) {
537 setCarrierThread(null);
538 }
539 carrier.clearInterrupt();
540
541 // We assume previous volatile accesses provide equivalent
542 // of release ordering, otherwise we need U.storeFence() here.
543 endTransition(/*is_mount*/false);
544 }
545
546 /**
547 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
548 * the continuation continues.
549 */
550 @Hidden
551 private boolean yieldContinuation() {
552 startTransition(/*is_mount*/false);
553 try {
554 return Continuation.yield(VTHREAD_SCOPE);
555 } finally {
556 endTransition(/*is_mount*/true);
557 }
558 }
559
560 /**
561 * Invoked in the context of the carrier thread after the Continuation yields when
562 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
563 */
564 private void afterYield() {
565 assert carrierThread == null;
566
567 // re-adjust parallelism if the virtual thread yielded when compensating
568 if (currentThread() instanceof CarrierThread ct) {
569 ct.endBlocking();
570 }
571
572 int s = state();
573
574 // LockSupport.park/parkNanos
575 if (s == PARKING || s == TIMED_PARKING) {
576 int newState;
577 if (s == PARKING) {
578 setState(newState = PARKED);
579 } else {
580 // schedule unpark
581 long timeout = this.timeout;
582 assert timeout > 0;
583 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
584 setState(newState = TIMED_PARKED);
585 }
586
587 // may have been unparked while parking
588 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
589 // lazy submit if local queue is empty
590 lazySubmitRunContinuation();
591 }
592 return;
593 }
594
595 // Thread.yield
596 if (s == YIELDING) {
597 setState(YIELDED);
598
599 // external submit if there are no tasks in the local task queue
600 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
601 externalSubmitRunContinuation(ct.getPool());
602 } else {
603 submitRunContinuation();
604 }
605 return;
606 }
607
608 // blocking on monitorenter
609 if (s == BLOCKING) {
610 setState(BLOCKED);
611
612 // may have been unblocked while blocking
613 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
614 // lazy submit if local queue is empty
615 lazySubmitRunContinuation();
616 }
617 return;
618 }
619
620 // Object.wait
621 if (s == WAITING || s == TIMED_WAITING) {
622 int newState;
623 boolean interruptible = interruptibleWait;
624 if (s == WAITING) {
625 setState(newState = WAIT);
626 } else {
627 // For timed-wait, a timeout task is scheduled to execute. The timeout
628 // task will change the thread state to UNBLOCKED and submit the thread
629 // to the scheduler. A sequence number is used to ensure that the timeout
630 // task only unblocks the thread for this timed-wait. We synchronize with
631 // the timeout task to coordinate access to the sequence number and to
632 // ensure the timeout task doesn't execute until the thread has got to
633 // the TIMED_WAIT state.
634 long timeout = this.timeout;
635 assert timeout > 0;
636 synchronized (timedWaitLock()) {
637 byte seqNo = ++timedWaitSeqNo;
638 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
639 setState(newState = TIMED_WAIT);
640 }
641 }
642
643 // may have been notified while in transition to wait state
644 if (notified && compareAndSetState(newState, BLOCKED)) {
645 // may have even been unblocked already
646 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
647 submitRunContinuation();
648 }
649 return;
650 }
651
652 // may have been interrupted while in transition to wait state
653 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
654 submitRunContinuation();
655 return;
656 }
657 return;
658 }
659
660 assert false;
661 }
662
663 /**
664 * Invoked after the continuation completes.
665 */
666 private void afterDone() {
667 afterDone(true);
668 }
669
670 /**
671 * Invoked after the continuation completes (or start failed). Sets the thread
672 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
673 *
674 * @param notifyContainer true if its container should be notified
675 */
676 private void afterDone(boolean notifyContainer) {
677 assert carrierThread == null;
678 setState(TERMINATED);
679
680 // notify anyone waiting for this virtual thread to terminate
681 CountDownLatch termination = this.termination;
682 if (termination != null) {
683 assert termination.getCount() == 1;
684 termination.countDown();
685 }
686
687 // notify container
688 if (notifyContainer) {
689 threadContainer().remove(this);
690 }
691
692 // clear references to thread locals
693 clearReferences();
694 }
695
696 /**
697 * Schedules this {@code VirtualThread} to execute.
698 *
699 * @throws IllegalStateException if the container is shutdown or closed
700 * @throws IllegalThreadStateException if the thread has already been started
701 * @throws RejectedExecutionException if the scheduler cannot accept a task
702 */
703 @Override
704 void start(ThreadContainer container) {
705 if (!compareAndSetState(NEW, STARTED)) {
706 throw new IllegalThreadStateException("Already started");
707 }
708
709 // bind thread to container
710 assert threadContainer() == null;
711 setThreadContainer(container);
712
713 // start thread
714 boolean addedToContainer = false;
715 boolean started = false;
716 try {
717 container.add(this); // may throw
718 addedToContainer = true;
719
720 // scoped values may be inherited
721 inheritScopedValueBindings(container);
722
723 // submit task to run thread, using externalSubmit if possible
724 externalSubmitRunContinuationOrThrow();
725 started = true;
726 } finally {
727 if (!started) {
728 afterDone(addedToContainer);
729 }
730 }
731 }
732
733 @Override
734 public void start() {
735 start(ThreadContainers.root());
736 }
737
738 @Override
739 public void run() {
740 // do nothing
741 }
742
743 /**
744 * Parks until unparked or interrupted. If already unparked then the parking
745 * permit is consumed and this method completes immediately (meaning it doesn't
746 * yield). It also completes immediately if the interrupted status is set.
747 */
748 @Override
749 void park() {
750 assert Thread.currentThread() == this;
751
752 // complete immediately if parking permit available or interrupted
753 if (getAndSetParkPermit(false) || interrupted)
754 return;
755
756 // park the thread
757 boolean yielded = false;
758 setState(PARKING);
759 try {
760 yielded = yieldContinuation();
761 } catch (OutOfMemoryError e) {
762 // park on carrier
763 } finally {
764 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
765 if (!yielded) {
766 assert state() == PARKING;
767 setState(RUNNING);
768 }
769 }
770
771 // park on the carrier thread when pinned
772 if (!yielded) {
773 parkOnCarrierThread(false, 0);
774 }
775 }
776
777 /**
778 * Parks up to the given waiting time or until unparked or interrupted.
779 * If already unparked then the parking permit is consumed and this method
780 * completes immediately (meaning it doesn't yield). It also completes immediately
781 * if the interrupted status is set or the waiting time is {@code <= 0}.
782 *
783 * @param nanos the maximum number of nanoseconds to wait.
784 */
785 @Override
786 void parkNanos(long nanos) {
787 assert Thread.currentThread() == this;
788
789 // complete immediately if parking permit available or interrupted
790 if (getAndSetParkPermit(false) || interrupted)
791 return;
792
793 // park the thread for the waiting time
794 if (nanos > 0) {
795 long startTime = System.nanoTime();
796
797 // park the thread, afterYield will schedule the thread to unpark
798 boolean yielded = false;
799 timeout = nanos;
800 setState(TIMED_PARKING);
801 try {
802 yielded = yieldContinuation();
803 } catch (OutOfMemoryError e) {
804 // park on carrier
805 } finally {
806 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
807 if (!yielded) {
808 assert state() == TIMED_PARKING;
809 setState(RUNNING);
810 }
811 }
812
813 // park on carrier thread for remaining time when pinned (or OOME)
814 if (!yielded) {
815 long remainingNanos = nanos - (System.nanoTime() - startTime);
816 parkOnCarrierThread(true, remainingNanos);
817 }
818 }
819 }
820
821 /**
822 * Parks the current carrier thread up to the given waiting time or until
823 * unparked or interrupted. If the virtual thread is interrupted then the
824 * interrupted status will be propagated to the carrier thread.
825 * @param timed true for a timed park, false for untimed
826 * @param nanos the waiting time in nanoseconds
827 */
828 private void parkOnCarrierThread(boolean timed, long nanos) {
829 assert state() == RUNNING;
830
831 setState(timed ? TIMED_PINNED : PINNED);
832 try {
833 if (!parkPermit) {
834 if (!timed) {
835 U.park(false, 0);
836 } else if (nanos > 0) {
837 U.park(false, nanos);
838 }
839 }
840 } finally {
841 setState(RUNNING);
842 }
843
844 // consume parking permit
845 setParkPermit(false);
846
847 // JFR jdk.VirtualThreadPinned event
848 postPinnedEvent("LockSupport.park");
849 }
850
851 /**
852 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
853 * Recording the event in the VM avoids having JFR event recorded in Java
854 * with the same name, but different ID, to events recorded by the VM.
855 */
856 @Hidden
857 private static native void postPinnedEvent(String op);
858
859 /**
860 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
861 * then its task is scheduled to continue, otherwise its next call to {@code park} or
862 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
863 * @param lazySubmit to use lazySubmit if possible
864 * @throws RejectedExecutionException if the scheduler cannot accept a task
865 */
866 private void unpark(boolean lazySubmit) {
867 if (!getAndSetParkPermit(true) && currentThread() != this) {
868 int s = state();
869
870 // unparked while parked
871 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
872 if (lazySubmit) {
873 lazySubmitRunContinuation();
874 } else {
875 submitRunContinuation();
876 }
877 return;
878 }
879
880 // unparked while parked when pinned
881 if (s == PINNED || s == TIMED_PINNED) {
882 // unpark carrier thread when pinned
883 disableSuspendAndPreempt();
884 try {
885 synchronized (carrierThreadAccessLock()) {
886 Thread carrier = carrierThread;
887 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
888 U.unpark(carrier);
889 }
890 }
891 } finally {
892 enableSuspendAndPreempt();
893 }
894 return;
895 }
896 }
897 }
898
899 @Override
900 void unpark() {
901 unpark(false);
902 }
903
904 /**
905 * Invoked by unblocker thread to unblock this virtual thread.
906 */
907 private void unblock() {
908 assert !Thread.currentThread().isVirtual();
909 blockPermit = true;
910 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
911 submitRunContinuation();
912 }
913 }
914
915 /**
916 * Invoked by FJP worker thread or STPE thread when park timeout expires.
917 */
918 private void parkTimeoutExpired() {
919 assert !VirtualThread.currentThread().isVirtual();
920 unpark(true);
921 }
922
923 /**
924 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
925 * If the virtual thread is in timed-wait then this method will unblock the thread
926 * and submit its task so that it continues and attempts to reenter the monitor.
927 * This method does nothing if the thread has been woken by notify or interrupt.
928 */
929 private void waitTimeoutExpired(byte seqNo) {
930 assert !Thread.currentThread().isVirtual();
931 for (;;) {
932 boolean unblocked = false;
933 synchronized (timedWaitLock()) {
934 if (seqNo != timedWaitSeqNo) {
935 // this timeout task is for a past timed-wait
936 return;
937 }
938 int s = state();
939 if (s == TIMED_WAIT) {
940 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
941 } else if (s != (TIMED_WAIT | SUSPENDED)) {
942 // notified or interrupted, no longer waiting
943 return;
944 }
945 }
946 if (unblocked) {
947 lazySubmitRunContinuation();
948 return;
949 }
950 // need to retry when thread is suspended in time-wait
951 Thread.yield();
952 }
953 }
954
955 /**
956 * Attempts to yield the current virtual thread (Thread.yield).
957 */
958 void tryYield() {
959 assert Thread.currentThread() == this;
960 setState(YIELDING);
961 boolean yielded = false;
962 try {
963 yielded = yieldContinuation(); // may throw
964 } finally {
965 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
966 if (!yielded) {
967 assert state() == YIELDING;
968 setState(RUNNING);
969 }
970 }
971 }
972
973 /**
974 * Sleep the current thread for the given sleep time (in nanoseconds). If
975 * nanos is 0 then the thread will attempt to yield.
976 *
977 * @implNote This implementation parks the thread for the given sleeping time
978 * and will therefore be observed in PARKED state during the sleep. Parking
979 * will consume the parking permit so this method makes available the parking
980 * permit after the sleep. This may be observed as a spurious, but benign,
981 * wakeup when the thread subsequently attempts to park.
982 *
983 * @param nanos the maximum number of nanoseconds to sleep
984 * @throws InterruptedException if interrupted while sleeping
985 */
986 void sleepNanos(long nanos) throws InterruptedException {
987 assert Thread.currentThread() == this && nanos >= 0;
988 if (getAndClearInterrupt())
989 throw new InterruptedException();
990 if (nanos == 0) {
991 tryYield();
992 } else {
993 // park for the sleep time
994 try {
995 long remainingNanos = nanos;
996 long startNanos = System.nanoTime();
997 while (remainingNanos > 0) {
998 parkNanos(remainingNanos);
999 if (getAndClearInterrupt()) {
1000 throw new InterruptedException();
1001 }
1002 remainingNanos = nanos - (System.nanoTime() - startNanos);
1003 }
1004 } finally {
1005 // may have been unparked while sleeping
1006 setParkPermit(true);
1007 }
1008 }
1009 }
1010
1011 /**
1012 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1013 * A timeout of {@code 0} means to wait forever.
1014 *
1015 * @throws InterruptedException if interrupted while waiting
1016 * @return true if the thread has terminated
1017 */
1018 boolean joinNanos(long nanos) throws InterruptedException {
1019 if (state() == TERMINATED)
1020 return true;
1021
1022 // ensure termination object exists, then re-check state
1023 CountDownLatch termination = getTermination();
1024 if (state() == TERMINATED)
1025 return true;
1026
1027 // wait for virtual thread to terminate
1028 if (nanos == 0) {
1029 termination.await();
1030 } else {
1031 boolean terminated = termination.await(nanos, NANOSECONDS);
1032 if (!terminated) {
1033 // waiting time elapsed
1034 return false;
1035 }
1036 }
1037 assert state() == TERMINATED;
1038 return true;
1039 }
1040
1041 @Override
1042 void blockedOn(Interruptible b) {
1043 disableSuspendAndPreempt();
1044 try {
1045 super.blockedOn(b);
1046 } finally {
1047 enableSuspendAndPreempt();
1048 }
1049 }
1050
1051 @Override
1052 public void interrupt() {
1053 if (Thread.currentThread() != this) {
1054 // if current thread is a virtual thread then prevent it from being
1055 // suspended or unmounted when entering or holding interruptLock
1056 Interruptible blocker;
1057 disableSuspendAndPreempt();
1058 try {
1059 synchronized (interruptLock) {
1060 interrupted = true;
1061 blocker = nioBlocker();
1062 if (blocker != null) {
1063 blocker.interrupt(this);
1064 }
1065
1066 // interrupt carrier thread if mounted
1067 Thread carrier = carrierThread;
1068 if (carrier != null) carrier.setInterrupt();
1069 }
1070 } finally {
1071 enableSuspendAndPreempt();
1072 }
1073
1074 // notify blocker after releasing interruptLock
1075 if (blocker != null) {
1076 blocker.postInterrupt();
1077 }
1078
1079 // make available parking permit, unpark thread if parked
1080 unpark();
1081
1082 // if thread is waiting in Object.wait then schedule to try to reenter
1083 int s = state();
1084 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1085 submitRunContinuation();
1086 }
1087
1088 } else {
1089 interrupted = true;
1090 carrierThread.setInterrupt();
1091 setParkPermit(true);
1092 }
1093 }
1094
1095 @Override
1096 public boolean isInterrupted() {
1097 return interrupted;
1098 }
1099
1100 @Override
1101 boolean getAndClearInterrupt() {
1102 assert Thread.currentThread() == this;
1103 boolean oldValue = interrupted;
1104 if (oldValue) {
1105 disableSuspendAndPreempt();
1106 try {
1107 synchronized (interruptLock) {
1108 interrupted = false;
1109 carrierThread.clearInterrupt();
1110 }
1111 } finally {
1112 enableSuspendAndPreempt();
1113 }
1114 }
1115 return oldValue;
1116 }
1117
1118 @Override
1119 Thread.State threadState() {
1120 int s = state();
1121 switch (s & ~SUSPENDED) {
1122 case NEW:
1123 return Thread.State.NEW;
1124 case STARTED:
1125 // return NEW if thread container not yet set
1126 if (threadContainer() == null) {
1127 return Thread.State.NEW;
1128 } else {
1129 return Thread.State.RUNNABLE;
1130 }
1131 case UNPARKED:
1132 case UNBLOCKED:
1133 case YIELDED:
1134 // runnable, not mounted
1135 return Thread.State.RUNNABLE;
1136 case RUNNING:
1137 // if mounted then return state of carrier thread
1138 if (Thread.currentThread() != this) {
1139 disableSuspendAndPreempt();
1140 try {
1141 synchronized (carrierThreadAccessLock()) {
1142 Thread carrierThread = this.carrierThread;
1143 if (carrierThread != null) {
1144 return carrierThread.threadState();
1145 }
1146 }
1147 } finally {
1148 enableSuspendAndPreempt();
1149 }
1150 }
1151 // runnable, mounted
1152 return Thread.State.RUNNABLE;
1153 case PARKING:
1154 case TIMED_PARKING:
1155 case WAITING:
1156 case TIMED_WAITING:
1157 case YIELDING:
1158 // runnable, in transition
1159 return Thread.State.RUNNABLE;
1160 case PARKED:
1161 case PINNED:
1162 case WAIT:
1163 return Thread.State.WAITING;
1164 case TIMED_PARKED:
1165 case TIMED_PINNED:
1166 case TIMED_WAIT:
1167 return Thread.State.TIMED_WAITING;
1168 case BLOCKING:
1169 case BLOCKED:
1170 return Thread.State.BLOCKED;
1171 case TERMINATED:
1172 return Thread.State.TERMINATED;
1173 default:
1174 throw new InternalError();
1175 }
1176 }
1177
1178 @Override
1179 boolean alive() {
1180 int s = state;
1181 return (s != NEW && s != TERMINATED);
1182 }
1183
1184 @Override
1185 boolean isTerminated() {
1186 return (state == TERMINATED);
1187 }
1188
1189 @Override
1190 StackTraceElement[] asyncGetStackTrace() {
1191 StackTraceElement[] stackTrace;
1192 do {
1193 stackTrace = (carrierThread != null)
1194 ? super.asyncGetStackTrace() // mounted
1195 : tryGetStackTrace(); // unmounted
1196 if (stackTrace == null) {
1197 Thread.yield();
1198 }
1199 } while (stackTrace == null);
1200 return stackTrace;
1201 }
1202
1203 /**
1204 * Returns the stack trace for this virtual thread if it is unmounted.
1205 * Returns null if the thread is mounted or in transition.
1206 */
1207 private StackTraceElement[] tryGetStackTrace() {
1208 int initialState = state() & ~SUSPENDED;
1209 switch (initialState) {
1210 case NEW, STARTED, TERMINATED -> {
1211 return new StackTraceElement[0]; // unmounted, empty stack
1212 }
1213 case RUNNING, PINNED, TIMED_PINNED -> {
1214 return null; // mounted
1215 }
1216 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1217 // unmounted, not runnable
1218 }
1219 case UNPARKED, UNBLOCKED, YIELDED -> {
1220 // unmounted, runnable
1221 }
1222 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1223 return null; // in transition
1224 }
1225 default -> throw new InternalError("" + initialState);
1226 }
1227
1228 // thread is unmounted, prevent it from continuing
1229 int suspendedState = initialState | SUSPENDED;
1230 if (!compareAndSetState(initialState, suspendedState)) {
1231 return null;
1232 }
1233
1234 // get stack trace and restore state
1235 StackTraceElement[] stack;
1236 try {
1237 stack = cont.getStackTrace();
1238 } finally {
1239 assert state == suspendedState;
1240 setState(initialState);
1241 }
1242 boolean resubmit = switch (initialState) {
1243 case UNPARKED, UNBLOCKED, YIELDED -> {
1244 // resubmit as task may have run while suspended
1245 yield true;
1246 }
1247 case PARKED, TIMED_PARKED -> {
1248 // resubmit if unparked while suspended
1249 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1250 }
1251 case BLOCKED -> {
1252 // resubmit if unblocked while suspended
1253 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1254 }
1255 case WAIT, TIMED_WAIT -> {
1256 // resubmit if notified or interrupted while waiting (Object.wait)
1257 // waitTimeoutExpired will retry if the timed expired when suspended
1258 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1259 }
1260 default -> throw new InternalError();
1261 };
1262 if (resubmit) {
1263 submitRunContinuation();
1264 }
1265 return stack;
1266 }
1267
1268 @Override
1269 public String toString() {
1270 StringBuilder sb = new StringBuilder("VirtualThread[#");
1271 sb.append(threadId());
1272 String name = getName();
1273 if (!name.isEmpty()) {
1274 sb.append(",");
1275 sb.append(name);
1276 }
1277 sb.append("]/");
1278
1279 // add the carrier state and thread name when mounted
1280 boolean mounted;
1281 if (Thread.currentThread() == this) {
1282 mounted = appendCarrierInfo(sb);
1283 } else {
1284 disableSuspendAndPreempt();
1285 try {
1286 synchronized (carrierThreadAccessLock()) {
1287 mounted = appendCarrierInfo(sb);
1288 }
1289 } finally {
1290 enableSuspendAndPreempt();
1291 }
1292 }
1293
1294 // add virtual thread state when not mounted
1295 if (!mounted) {
1296 String stateAsString = threadState().toString();
1297 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1298 }
1299
1300 return sb.toString();
1301 }
1302
1303 /**
1304 * Appends the carrier state and thread name to the string buffer if mounted.
1305 * @return true if mounted, false if not mounted
1306 */
1307 private boolean appendCarrierInfo(StringBuilder sb) {
1308 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1309 Thread carrier = carrierThread;
1310 if (carrier != null) {
1311 String stateAsString = carrier.threadState().toString();
1312 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1313 sb.append('@');
1314 sb.append(carrier.getName());
1315 return true;
1316 } else {
1317 return false;
1318 }
1319 }
1320
1321 @Override
1322 public int hashCode() {
1323 return (int) threadId();
1324 }
1325
1326 @Override
1327 public boolean equals(Object obj) {
1328 return obj == this;
1329 }
1330
1331 /**
1332 * Returns the termination object, creating it if needed.
1333 */
1334 private CountDownLatch getTermination() {
1335 CountDownLatch termination = this.termination;
1336 if (termination == null) {
1337 termination = new CountDownLatch(1);
1338 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1339 termination = this.termination;
1340 }
1341 }
1342 return termination;
1343 }
1344
1345 /**
1346 * Returns the lock object to synchronize on when accessing carrierThread.
1347 * The lock prevents carrierThread from being reset to null during unmount.
1348 */
1349 private Object carrierThreadAccessLock() {
1350 // return interruptLock as unmount has to coordinate with interrupt
1351 return interruptLock;
1352 }
1353
1354 /**
1355 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1356 */
1357 private Object timedWaitLock() {
1358 // use this object for now to avoid the overhead of introducing another lock
1359 return runContinuation;
1360 }
1361
1362 /**
1363 * Disallow the current thread be suspended or preempted.
1364 */
1365 private void disableSuspendAndPreempt() {
1366 notifyJvmtiDisableSuspend(true);
1367 Continuation.pin();
1368 }
1369
1370 /**
1371 * Allow the current thread be suspended or preempted.
1372 */
1373 private void enableSuspendAndPreempt() {
1374 Continuation.unpin();
1375 notifyJvmtiDisableSuspend(false);
1376 }
1377
1378 // -- wrappers for get/set of state, parking permit, and carrier thread --
1379
1380 private int state() {
1381 return state; // volatile read
1382 }
1383
1384 private void setState(int newValue) {
1385 state = newValue; // volatile write
1386 }
1387
1388 private boolean compareAndSetState(int expectedValue, int newValue) {
1389 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1390 }
1391
1392 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1393 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1394 }
1395
1396 private void setParkPermit(boolean newValue) {
1397 if (parkPermit != newValue) {
1398 parkPermit = newValue;
1399 }
1400 }
1401
1402 private boolean getAndSetParkPermit(boolean newValue) {
1403 if (parkPermit != newValue) {
1404 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1405 } else {
1406 return newValue;
1407 }
1408 }
1409
1410 private void setCarrierThread(Thread carrier) {
1411 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1412 this.carrierThread = carrier;
1413 }
1414
1415 // The following four methods notify the VM when a "transition" starts and ends.
1416 // A "mount transition" embodies the steps to transfer control from a platform
1417 // thread to a virtual thread, changing the thread identity, and starting or
1418 // resuming the virtual thread's continuation on the carrier.
1419 // An "unmount transition" embodies the steps to transfer control from a virtual
1420 // thread to its carrier, suspending the virtual thread's continuation, and
1421 // restoring the thread identity to the platform thread.
1422 // The notifications to the VM are necessary in order to coordinate with functions
1423 // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1424 // a transition may block if transitions are disabled. Ending a transition may
1425 // notify a thread that is waiting to disable transitions. The notifications are
1426 // also used to post JVMTI events for virtual thread start and end.
1427
1428 @IntrinsicCandidate
1429 @JvmtiMountTransition
1430 private native void endFirstTransition();
1431
1432 @IntrinsicCandidate
1433 @JvmtiMountTransition
1434 private native void startFinalTransition();
1435
1436 @IntrinsicCandidate
1437 @JvmtiMountTransition
1438 private native void startTransition(boolean is_mount);
1439
1440 @IntrinsicCandidate
1441 @JvmtiMountTransition
1442 private native void endTransition(boolean is_mount);
1443
1444 @IntrinsicCandidate
1445 private static native void notifyJvmtiDisableSuspend(boolean enter);
1446
1447 private static native void registerNatives();
1448 static {
1449 registerNatives();
1450
1451 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1452 var group = Thread.virtualThreadGroup();
1453 }
1454
1455 /**
1456 * Creates the default ForkJoinPool scheduler.
1457 */
1458 private static ForkJoinPool createDefaultScheduler() {
1459 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1460 int parallelism, maxPoolSize, minRunnable;
1461 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1462 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1463 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1464 if (parallelismValue != null) {
1465 parallelism = Integer.parseInt(parallelismValue);
1466 } else {
1467 parallelism = Runtime.getRuntime().availableProcessors();
1468 }
1469 if (maxPoolSizeValue != null) {
1470 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1471 parallelism = Integer.min(parallelism, maxPoolSize);
1472 } else {
1473 maxPoolSize = Integer.max(parallelism, 256);
1474 }
1475 if (minRunnableValue != null) {
1476 minRunnable = Integer.parseInt(minRunnableValue);
1477 } else {
1478 minRunnable = Integer.max(parallelism / 2, 1);
1479 }
1480 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1481 boolean asyncMode = true; // FIFO
1482 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1483 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1484 }
1485
1486 /**
1487 * Schedule a runnable task to run after a delay.
1488 */
1489 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1490 if (scheduler instanceof ForkJoinPool pool) {
1491 return pool.schedule(command, delay, unit);
1492 } else {
1493 return DelayedTaskSchedulers.schedule(command, delay, unit);
1494 }
1495 }
1496
1497 /**
1498 * Supports scheduling a runnable task to run after a delay. It uses a number
1499 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1500 * work queue used. This class is used when using a custom scheduler.
1501 */
1502 private static class DelayedTaskSchedulers {
1503 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1504
1505 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1506 long tid = Thread.currentThread().threadId();
1507 int index = (int) tid & (INSTANCE.length - 1);
1508 return INSTANCE[index].schedule(command, delay, unit);
1509 }
1510
1511 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1512 String propName = "jdk.virtualThreadScheduler.timerQueues";
1513 String propValue = System.getProperty(propName);
1514 int queueCount;
1515 if (propValue != null) {
1516 queueCount = Integer.parseInt(propValue);
1517 if (queueCount != Integer.highestOneBit(queueCount)) {
1518 throw new RuntimeException("Value of " + propName + " must be power of 2");
1519 }
1520 } else {
1521 int ncpus = Runtime.getRuntime().availableProcessors();
1522 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1523 }
1524 var schedulers = new ScheduledExecutorService[queueCount];
1525 for (int i = 0; i < queueCount; i++) {
1526 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1527 Executors.newScheduledThreadPool(1, task -> {
1528 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1529 t.setDaemon(true);
1530 return t;
1531 });
1532 stpe.setRemoveOnCancelPolicy(true);
1533 schedulers[i] = stpe;
1534 }
1535 return schedulers;
1536 }
1537 }
1538
1539 /**
1540 * Schedule virtual threads that are ready to be scheduled after they blocked on
1541 * monitor enter.
1542 */
1543 private static void unblockVirtualThreads() {
1544 while (true) {
1545 VirtualThread vthread = takeVirtualThreadListToUnblock();
1546 while (vthread != null) {
1547 assert vthread.onWaitingList;
1548 VirtualThread nextThread = vthread.next;
1549
1550 // remove from list and unblock
1551 vthread.next = null;
1552 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1553 assert changed;
1554 vthread.unblock();
1555
1556 vthread = nextThread;
1557 }
1558 }
1559 }
1560
1561 /**
1562 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1563 * if necessary until a list of one or more threads becomes available.
1564 */
1565 private static native VirtualThread takeVirtualThreadListToUnblock();
1566
1567 static {
1568 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1569 VirtualThread::unblockVirtualThreads);
1570 unblocker.setDaemon(true);
1571 unblocker.start();
1572 }
1573 }