1 /*
2 * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.util.Locale;
28 import java.util.Objects;
29 import java.util.concurrent.CountDownLatch;
30 import java.util.concurrent.Executor;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.Future;
36 import java.util.concurrent.RejectedExecutionException;
37 import java.util.concurrent.ScheduledExecutorService;
38 import java.util.concurrent.ScheduledThreadPoolExecutor;
39 import java.util.concurrent.TimeUnit;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66 private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
67
68 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
69 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
70 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
71 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
72 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
73
74 // scheduler and continuation
75 private final Executor scheduler;
76 private final Continuation cont;
77 private final Runnable runContinuation;
78
79 // virtual thread state, accessed by VM
80 private volatile int state;
81
82 /*
83 * Virtual thread state transitions:
84 *
85 * NEW -> STARTED // Thread.start, schedule to run
86 * STARTED -> TERMINATED // failed to start
87 * STARTED -> RUNNING // first run
88 * RUNNING -> TERMINATED // done
89 *
90 * RUNNING -> PARKING // Thread parking with LockSupport.park
91 * PARKING -> PARKED // cont.yield successful, parked indefinitely
92 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
93 * PARKED -> UNPARKED // unparked, may be scheduled to continue
94 * PINNED -> RUNNING // unparked, continue execution on same carrier
95 * UNPARKED -> RUNNING // continue execution after park
96 *
97 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
98 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
99 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
100 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
101 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
102 *
103 * RUNNING -> BLOCKING // blocking on monitor enter
104 * BLOCKING -> BLOCKED // blocked on monitor enter
105 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
106 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
107 *
108 * RUNNING -> WAITING // transitional state during wait on monitor
109 * WAITING -> WAIT // waiting on monitor
110 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
111 * WAIT -> UNBLOCKED // timed-out/interrupted
112 *
113 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
114 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
115 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
116 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
117 *
118 * RUNNING -> YIELDING // Thread.yield
119 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
120 * YIELDING -> RUNNING // cont.yield failed
121 * YIELDED -> RUNNING // continue execution after Thread.yield
122 */
123 private static final int NEW = 0;
124 private static final int STARTED = 1;
125 private static final int RUNNING = 2; // runnable-mounted
126
127 // untimed and timed parking
128 private static final int PARKING = 3;
129 private static final int PARKED = 4; // unmounted
130 private static final int PINNED = 5; // mounted
131 private static final int TIMED_PARKING = 6;
132 private static final int TIMED_PARKED = 7; // unmounted
133 private static final int TIMED_PINNED = 8; // mounted
134 private static final int UNPARKED = 9; // unmounted but runnable
135
136 // Thread.yield
137 private static final int YIELDING = 10;
138 private static final int YIELDED = 11; // unmounted but runnable
139
140 // monitor enter
141 private static final int BLOCKING = 12;
142 private static final int BLOCKED = 13; // unmounted
143 private static final int UNBLOCKED = 14; // unmounted but runnable
144
145 // monitor wait/timed-wait
146 private static final int WAITING = 15;
147 private static final int WAIT = 16; // waiting in Object.wait
148 private static final int TIMED_WAITING = 17;
149 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
150
151 private static final int TERMINATED = 99; // final state
152
153 // can be suspended from scheduling when unmounted
154 private static final int SUSPENDED = 1 << 8;
155
156 // parking permit made available by LockSupport.unpark
157 private volatile boolean parkPermit;
158
159 // blocking permit made available by unblocker thread when another thread exits monitor
160 private volatile boolean blockPermit;
161
162 // true when on the list of virtual threads waiting to be unblocked
163 private volatile boolean onWaitingList;
164
165 // next virtual thread on the list of virtual threads waiting to be unblocked
166 private volatile VirtualThread next;
167
168 // notified by Object.notify/notifyAll while waiting in Object.wait
169 private volatile boolean notified;
170
171 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
172 private volatile boolean interruptibleWait;
173
174 // timed-wait support
175 private byte timedWaitSeqNo;
176
177 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
178 private long timeout;
179
180 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
181 private Future<?> timeoutTask;
182
183 // carrier thread when mounted, accessed by VM
184 private volatile Thread carrierThread;
185
186 // termination object when joining, created lazily if needed
187 private volatile CountDownLatch termination;
188
189 /**
190 * Returns the default scheduler.
191 */
192 static Executor defaultScheduler() {
193 return DEFAULT_SCHEDULER;
194 }
195
196 /**
197 * Returns the continuation scope used for virtual threads.
198 */
199 static ContinuationScope continuationScope() {
200 return VTHREAD_SCOPE;
201 }
202
203 /**
204 * Creates a new {@code VirtualThread} to run the given task with the given
205 * scheduler. If the given scheduler is {@code null} and the current thread
206 * is a platform thread then the newly created virtual thread will use the
207 * default scheduler. If given scheduler is {@code null} and the current
208 * thread is a virtual thread then the current thread's scheduler is used.
209 *
210 * @param scheduler the scheduler or null
211 * @param name thread name
212 * @param characteristics characteristics
213 * @param task the task to execute
214 */
215 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
216 super(name, characteristics, /*bound*/ false);
217 Objects.requireNonNull(task);
218
219 // choose scheduler if not specified
220 if (scheduler == null) {
221 Thread parent = Thread.currentThread();
222 if (parent instanceof VirtualThread vparent) {
223 scheduler = vparent.scheduler;
224 } else {
225 scheduler = DEFAULT_SCHEDULER;
226 }
227 }
228
229 this.scheduler = scheduler;
230 this.cont = new VThreadContinuation(this, task);
231 this.runContinuation = this::runContinuation;
232 }
233
234 /**
235 * The continuation that a virtual thread executes.
236 */
237 private static class VThreadContinuation extends Continuation {
238 VThreadContinuation(VirtualThread vthread, Runnable task) {
239 super(VTHREAD_SCOPE, wrap(vthread, task));
240 }
241 @Override
242 protected void onPinned(Continuation.Pinned reason) {
243 }
244 private static Runnable wrap(VirtualThread vthread, Runnable task) {
245 return new Runnable() {
246 @Hidden
247 @JvmtiHideEvents
248 public void run() {
249 vthread.notifyJvmtiStart(); // notify JVMTI
250 try {
251 vthread.run(task);
252 } finally {
253 vthread.notifyJvmtiEnd(); // notify JVMTI
254 }
255 }
256 };
257 }
258 }
259
260 /**
261 * Runs or continues execution on the current thread. The virtual thread is mounted
262 * on the current thread before the task runs or continues. It unmounts when the
263 * task completes or yields.
264 */
265 @ChangesCurrentThread // allow mount/unmount to be inlined
266 private void runContinuation() {
267 // the carrier must be a platform thread
268 if (Thread.currentThread().isVirtual()) {
269 throw new WrongThreadException();
270 }
271
272 // set state to RUNNING
273 int initialState = state();
274 if (initialState == STARTED || initialState == UNPARKED
275 || initialState == UNBLOCKED || initialState == YIELDED) {
276 // newly started or continue after parking/blocking/Thread.yield
277 if (!compareAndSetState(initialState, RUNNING)) {
278 return;
279 }
280 // consume permit when continuing after parking or blocking. If continue
281 // after a timed-park or timed-wait then the timeout task is cancelled.
282 if (initialState == UNPARKED) {
283 cancelTimeoutTask();
284 setParkPermit(false);
285 } else if (initialState == UNBLOCKED) {
286 cancelTimeoutTask();
287 blockPermit = false;
288 }
289 } else {
290 // not runnable
291 return;
292 }
293
294 mount();
295 try {
296 cont.run();
297 } finally {
298 unmount();
299 if (cont.isDone()) {
300 afterDone();
301 } else {
302 afterYield();
303 }
304 }
305 }
306
307 /**
308 * Cancel timeout task when continuing after timed-park or timed-wait.
309 * The timeout task may be executing, or may have already completed.
310 */
311 private void cancelTimeoutTask() {
312 if (timeoutTask != null) {
313 timeoutTask.cancel(false);
314 timeoutTask = null;
315 }
316 }
317
318 /**
319 * Submits the runContinuation task to the scheduler. For the default scheduler,
320 * and calling it on a worker thread, the task will be pushed to the local queue,
321 * otherwise it will be pushed to an external submission queue.
322 * @param scheduler the scheduler
323 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
324 * @throws RejectedExecutionException
325 */
326 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
327 boolean done = false;
328 while (!done) {
329 try {
330 // Pin the continuation to prevent the virtual thread from unmounting
331 // when submitting a task. For the default scheduler this ensures that
332 // the carrier doesn't change when pushing a task. For other schedulers
333 // it avoids deadlock that could arise due to carriers and virtual
334 // threads contending for a lock.
335 if (currentThread().isVirtual()) {
336 Continuation.pin();
337 try {
338 scheduler.execute(runContinuation);
339 } finally {
340 Continuation.unpin();
341 }
342 } else {
343 scheduler.execute(runContinuation);
344 }
345 done = true;
346 } catch (RejectedExecutionException ree) {
347 submitFailed(ree);
348 throw ree;
349 } catch (OutOfMemoryError e) {
350 if (retryOnOOME) {
351 U.park(false, 100_000_000); // 100ms
352 } else {
353 throw e;
354 }
355 }
356 }
357 }
358
359 /**
360 * Submits the runContinuation task to the given scheduler as an external submit.
361 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
362 * @throws RejectedExecutionException
363 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
364 */
365 private void externalSubmitRunContinuation(ForkJoinPool pool) {
366 assert Thread.currentThread() instanceof CarrierThread;
367 try {
368 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
369 } catch (RejectedExecutionException ree) {
370 submitFailed(ree);
371 throw ree;
372 } catch (OutOfMemoryError e) {
373 submitRunContinuation(pool, true);
374 }
375 }
376
377 /**
378 * Submits the runContinuation task to the scheduler. For the default scheduler,
379 * and calling it on a worker thread, the task will be pushed to the local queue,
380 * otherwise it will be pushed to an external submission queue.
381 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
382 * @throws RejectedExecutionException
383 */
384 private void submitRunContinuation() {
385 submitRunContinuation(scheduler, true);
386 }
387
388 /**
389 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
390 * queue is empty. If not empty, or invoked by another thread, then this method works
391 * like submitRunContinuation and just submits the task to the scheduler.
392 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
393 * @throws RejectedExecutionException
394 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
395 */
396 private void lazySubmitRunContinuation() {
397 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
398 ForkJoinPool pool = ct.getPool();
399 try {
400 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
401 } catch (RejectedExecutionException ree) {
402 submitFailed(ree);
403 throw ree;
404 } catch (OutOfMemoryError e) {
405 submitRunContinuation();
406 }
407 } else {
408 submitRunContinuation();
409 }
410 }
411
412 /**
413 * Submits the runContinuation task to the scheduler. For the default scheduler, and
414 * calling it a virtual thread that uses the default scheduler, the task will be
415 * pushed to an external submission queue. This method may throw OutOfMemoryError.
416 * @throws RejectedExecutionException
417 * @throws OutOfMemoryError
418 */
419 private void externalSubmitRunContinuationOrThrow() {
420 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
421 try {
422 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
423 } catch (RejectedExecutionException ree) {
424 submitFailed(ree);
425 throw ree;
426 }
427 } else {
428 submitRunContinuation(scheduler, false);
429 }
430 }
431
432 /**
433 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
434 */
435 private void submitFailed(RejectedExecutionException ree) {
436 var event = new VirtualThreadSubmitFailedEvent();
437 if (event.isEnabled()) {
438 event.javaThreadId = threadId();
439 event.exceptionMessage = ree.getMessage();
440 event.commit();
441 }
442 }
443
444 /**
445 * Runs a task in the context of this virtual thread.
446 */
447 private void run(Runnable task) {
448 assert Thread.currentThread() == this && state == RUNNING;
449
450 // emit JFR event if enabled
451 if (VirtualThreadStartEvent.isTurnedOn()) {
452 var event = new VirtualThreadStartEvent();
453 event.javaThreadId = threadId();
454 event.commit();
455 }
456
457 Object bindings = Thread.scopedValueBindings();
458 try {
459 runWith(bindings, task);
460 } catch (Throwable exc) {
461 dispatchUncaughtException(exc);
462 } finally {
463 // pop any remaining scopes from the stack, this may block
464 StackableScope.popAll();
465
466 // emit JFR event if enabled
467 if (VirtualThreadEndEvent.isTurnedOn()) {
468 var event = new VirtualThreadEndEvent();
469 event.javaThreadId = threadId();
470 event.commit();
471 }
472 }
473 }
474
475 /**
476 * Mounts this virtual thread onto the current platform thread. On
477 * return, the current thread is the virtual thread.
478 */
479 @ChangesCurrentThread
480 @ReservedStackAccess
481 private void mount() {
482 // notify JVMTI before mount
483 notifyJvmtiMount(/*hide*/true);
484
485 // sets the carrier thread
486 Thread carrier = Thread.currentCarrierThread();
487 setCarrierThread(carrier);
488
489 // sync up carrier thread interrupted status if needed
490 if (interrupted) {
491 carrier.setInterrupt();
492 } else if (carrier.isInterrupted()) {
493 synchronized (interruptLock) {
494 // need to recheck interrupted status
495 if (!interrupted) {
496 carrier.clearInterrupt();
497 }
498 }
499 }
500
501 // set Thread.currentThread() to return this virtual thread
502 carrier.setCurrentThread(this);
503 }
504
505 /**
506 * Unmounts this virtual thread from the carrier. On return, the
507 * current thread is the current platform thread.
508 */
509 @ChangesCurrentThread
510 @ReservedStackAccess
511 private void unmount() {
512 assert !Thread.holdsLock(interruptLock);
513
514 // set Thread.currentThread() to return the platform thread
515 Thread carrier = this.carrierThread;
516 carrier.setCurrentThread(carrier);
517
518 // break connection to carrier thread, synchronized with interrupt
519 synchronized (interruptLock) {
520 setCarrierThread(null);
521 }
522 carrier.clearInterrupt();
523
524 // notify JVMTI after unmount
525 notifyJvmtiUnmount(/*hide*/false);
526 }
527
528 /**
529 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
530 * the continuation continues.
531 */
532 @Hidden
533 private boolean yieldContinuation() {
534 notifyJvmtiUnmount(/*hide*/true);
535 try {
536 return Continuation.yield(VTHREAD_SCOPE);
537 } finally {
538 notifyJvmtiMount(/*hide*/false);
539 }
540 }
541
542 /**
543 * Invoked in the context of the carrier thread after the Continuation yields when
544 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
545 */
546 private void afterYield() {
547 assert carrierThread == null;
548
549 // re-adjust parallelism if the virtual thread yielded when compensating
550 if (currentThread() instanceof CarrierThread ct) {
551 ct.endBlocking();
552 }
553
554 int s = state();
555
556 // LockSupport.park/parkNanos
557 if (s == PARKING || s == TIMED_PARKING) {
558 int newState;
559 if (s == PARKING) {
560 setState(newState = PARKED);
561 } else {
562 // schedule unpark
563 long timeout = this.timeout;
564 assert timeout > 0;
565 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
566 setState(newState = TIMED_PARKED);
567 }
568
569 // may have been unparked while parking
570 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
571 // lazy submit if local queue is empty
572 lazySubmitRunContinuation();
573 }
574 return;
575 }
576
577 // Thread.yield
578 if (s == YIELDING) {
579 setState(YIELDED);
580
581 // external submit if there are no tasks in the local task queue
582 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
583 externalSubmitRunContinuation(ct.getPool());
584 } else {
585 submitRunContinuation();
586 }
587 return;
588 }
589
590 // blocking on monitorenter
591 if (s == BLOCKING) {
592 setState(BLOCKED);
593
594 // may have been unblocked while blocking
595 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
596 // lazy submit if local queue is empty
597 lazySubmitRunContinuation();
598 }
599 return;
600 }
601
602 // Object.wait
603 if (s == WAITING || s == TIMED_WAITING) {
604 int newState;
605 boolean interruptible = interruptibleWait;
606 if (s == WAITING) {
607 setState(newState = WAIT);
608 } else {
609 // For timed-wait, a timeout task is scheduled to execute. The timeout
610 // task will change the thread state to UNBLOCKED and submit the thread
611 // to the scheduler. A sequence number is used to ensure that the timeout
612 // task only unblocks the thread for this timed-wait. We synchronize with
613 // the timeout task to coordinate access to the sequence number and to
614 // ensure the timeout task doesn't execute until the thread has got to
615 // the TIMED_WAIT state.
616 long timeout = this.timeout;
617 assert timeout > 0;
618 synchronized (timedWaitLock()) {
619 byte seqNo = ++timedWaitSeqNo;
620 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
621 setState(newState = TIMED_WAIT);
622 }
623 }
624
625 // may have been notified while in transition to wait state
626 if (notified && compareAndSetState(newState, BLOCKED)) {
627 // may have even been unblocked already
628 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
629 submitRunContinuation();
630 }
631 return;
632 }
633
634 // may have been interrupted while in transition to wait state
635 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
636 submitRunContinuation();
637 return;
638 }
639 return;
640 }
641
642 assert false;
643 }
644
645 /**
646 * Invoked after the continuation completes.
647 */
648 private void afterDone() {
649 afterDone(true);
650 }
651
652 /**
653 * Invoked after the continuation completes (or start failed). Sets the thread
654 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
655 *
656 * @param notifyContainer true if its container should be notified
657 */
658 private void afterDone(boolean notifyContainer) {
659 assert carrierThread == null;
660 setState(TERMINATED);
661
662 // notify anyone waiting for this virtual thread to terminate
663 CountDownLatch termination = this.termination;
664 if (termination != null) {
665 assert termination.getCount() == 1;
666 termination.countDown();
667 }
668
669 // notify container
670 if (notifyContainer) {
671 threadContainer().remove(this);
672 }
673
674 // clear references to thread locals
675 clearReferences();
676 }
677
678 /**
679 * Schedules this {@code VirtualThread} to execute.
680 *
681 * @throws IllegalStateException if the container is shutdown or closed
682 * @throws IllegalThreadStateException if the thread has already been started
683 * @throws RejectedExecutionException if the scheduler cannot accept a task
684 */
685 @Override
686 void start(ThreadContainer container) {
687 if (!compareAndSetState(NEW, STARTED)) {
688 throw new IllegalThreadStateException("Already started");
689 }
690
691 // bind thread to container
692 assert threadContainer() == null;
693 setThreadContainer(container);
694
695 // start thread
696 boolean addedToContainer = false;
697 boolean started = false;
698 try {
699 container.add(this); // may throw
700 addedToContainer = true;
701
702 // scoped values may be inherited
703 inheritScopedValueBindings(container);
704
705 // submit task to run thread, using externalSubmit if possible
706 externalSubmitRunContinuationOrThrow();
707 started = true;
708 } finally {
709 if (!started) {
710 afterDone(addedToContainer);
711 }
712 }
713 }
714
715 @Override
716 public void start() {
717 start(ThreadContainers.root());
718 }
719
720 @Override
721 public void run() {
722 // do nothing
723 }
724
725 /**
726 * Parks until unparked or interrupted. If already unparked then the parking
727 * permit is consumed and this method completes immediately (meaning it doesn't
728 * yield). It also completes immediately if the interrupted status is set.
729 */
730 @Override
731 void park() {
732 assert Thread.currentThread() == this;
733
734 // complete immediately if parking permit available or interrupted
735 if (getAndSetParkPermit(false) || interrupted)
736 return;
737
738 // park the thread
739 boolean yielded = false;
740 setState(PARKING);
741 try {
742 yielded = yieldContinuation();
743 } catch (OutOfMemoryError e) {
744 // park on carrier
745 } finally {
746 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
747 if (!yielded) {
748 assert state() == PARKING;
749 setState(RUNNING);
750 }
751 }
752
753 // park on the carrier thread when pinned
754 if (!yielded) {
755 parkOnCarrierThread(false, 0);
756 }
757 }
758
759 /**
760 * Parks up to the given waiting time or until unparked or interrupted.
761 * If already unparked then the parking permit is consumed and this method
762 * completes immediately (meaning it doesn't yield). It also completes immediately
763 * if the interrupted status is set or the waiting time is {@code <= 0}.
764 *
765 * @param nanos the maximum number of nanoseconds to wait.
766 */
767 @Override
768 void parkNanos(long nanos) {
769 assert Thread.currentThread() == this;
770
771 // complete immediately if parking permit available or interrupted
772 if (getAndSetParkPermit(false) || interrupted)
773 return;
774
775 // park the thread for the waiting time
776 if (nanos > 0) {
777 long startTime = System.nanoTime();
778
779 // park the thread, afterYield will schedule the thread to unpark
780 boolean yielded = false;
781 timeout = nanos;
782 setState(TIMED_PARKING);
783 try {
784 yielded = yieldContinuation();
785 } catch (OutOfMemoryError e) {
786 // park on carrier
787 } finally {
788 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
789 if (!yielded) {
790 assert state() == TIMED_PARKING;
791 setState(RUNNING);
792 }
793 }
794
795 // park on carrier thread for remaining time when pinned (or OOME)
796 if (!yielded) {
797 long remainingNanos = nanos - (System.nanoTime() - startTime);
798 parkOnCarrierThread(true, remainingNanos);
799 }
800 }
801 }
802
803 /**
804 * Parks the current carrier thread up to the given waiting time or until
805 * unparked or interrupted. If the virtual thread is interrupted then the
806 * interrupted status will be propagated to the carrier thread.
807 * @param timed true for a timed park, false for untimed
808 * @param nanos the waiting time in nanoseconds
809 */
810 private void parkOnCarrierThread(boolean timed, long nanos) {
811 assert state() == RUNNING;
812
813 setState(timed ? TIMED_PINNED : PINNED);
814 try {
815 if (!parkPermit) {
816 if (!timed) {
817 U.park(false, 0);
818 } else if (nanos > 0) {
819 U.park(false, nanos);
820 }
821 }
822 } finally {
823 setState(RUNNING);
824 }
825
826 // consume parking permit
827 setParkPermit(false);
828
829 // JFR jdk.VirtualThreadPinned event
830 postPinnedEvent("LockSupport.park");
831 }
832
833 /**
834 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
835 * Recording the event in the VM avoids having JFR event recorded in Java
836 * with the same name, but different ID, to events recorded by the VM.
837 */
838 @Hidden
839 private static native void postPinnedEvent(String op);
840
841 /**
842 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
843 * then its task is scheduled to continue, otherwise its next call to {@code park} or
844 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
845 * @throws RejectedExecutionException if the scheduler cannot accept a task
846 */
847 @Override
848 void unpark() {
849 if (!getAndSetParkPermit(true) && currentThread() != this) {
850 int s = state();
851
852 // unparked while parked
853 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
854 submitRunContinuation();
855 return;
856 }
857
858 // unparked while parked when pinned
859 if (s == PINNED || s == TIMED_PINNED) {
860 // unpark carrier thread when pinned
861 disableSuspendAndPreempt();
862 try {
863 synchronized (carrierThreadAccessLock()) {
864 Thread carrier = carrierThread;
865 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
866 U.unpark(carrier);
867 }
868 }
869 } finally {
870 enableSuspendAndPreempt();
871 }
872 return;
873 }
874 }
875 }
876
877 /**
878 * Invoked by unblocker thread to unblock this virtual thread.
879 */
880 private void unblock() {
881 assert !Thread.currentThread().isVirtual();
882 blockPermit = true;
883 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
884 submitRunContinuation();
885 }
886 }
887
888 /**
889 * Invoked by FJP worker thread or STPE thread when park timeout expires.
890 */
891 private void parkTimeoutExpired() {
892 assert !VirtualThread.currentThread().isVirtual();
893 if (!getAndSetParkPermit(true)
894 && (state() == TIMED_PARKED)
895 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
896 lazySubmitRunContinuation();
897 }
898 }
899
900 /**
901 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
902 * If the virtual thread is in timed-wait then this method will unblock the thread
903 * and submit its task so that it continues and attempts to reenter the monitor.
904 * This method does nothing if the thread has been woken by notify or interrupt.
905 */
906 private void waitTimeoutExpired(byte seqNo) {
907 assert !Thread.currentThread().isVirtual();
908 for (;;) {
909 boolean unblocked = false;
910 synchronized (timedWaitLock()) {
911 if (seqNo != timedWaitSeqNo) {
912 // this timeout task is for a past timed-wait
913 return;
914 }
915 int s = state();
916 if (s == TIMED_WAIT) {
917 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
918 } else if (s != (TIMED_WAIT | SUSPENDED)) {
919 // notified or interrupted, no longer waiting
920 return;
921 }
922 }
923 if (unblocked) {
924 lazySubmitRunContinuation();
925 return;
926 }
927 // need to retry when thread is suspended in time-wait
928 Thread.yield();
929 }
930 }
931
932 /**
933 * Attempts to yield the current virtual thread (Thread.yield).
934 */
935 void tryYield() {
936 assert Thread.currentThread() == this;
937 setState(YIELDING);
938 boolean yielded = false;
939 try {
940 yielded = yieldContinuation(); // may throw
941 } finally {
942 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
943 if (!yielded) {
944 assert state() == YIELDING;
945 setState(RUNNING);
946 }
947 }
948 }
949
950 /**
951 * Sleep the current thread for the given sleep time (in nanoseconds). If
952 * nanos is 0 then the thread will attempt to yield.
953 *
954 * @implNote This implementation parks the thread for the given sleeping time
955 * and will therefore be observed in PARKED state during the sleep. Parking
956 * will consume the parking permit so this method makes available the parking
957 * permit after the sleep. This may be observed as a spurious, but benign,
958 * wakeup when the thread subsequently attempts to park.
959 *
960 * @param nanos the maximum number of nanoseconds to sleep
961 * @throws InterruptedException if interrupted while sleeping
962 */
963 void sleepNanos(long nanos) throws InterruptedException {
964 assert Thread.currentThread() == this && nanos >= 0;
965 if (getAndClearInterrupt())
966 throw new InterruptedException();
967 if (nanos == 0) {
968 tryYield();
969 } else {
970 // park for the sleep time
971 try {
972 long remainingNanos = nanos;
973 long startNanos = System.nanoTime();
974 while (remainingNanos > 0) {
975 parkNanos(remainingNanos);
976 if (getAndClearInterrupt()) {
977 throw new InterruptedException();
978 }
979 remainingNanos = nanos - (System.nanoTime() - startNanos);
980 }
981 } finally {
982 // may have been unparked while sleeping
983 setParkPermit(true);
984 }
985 }
986 }
987
988 /**
989 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
990 * A timeout of {@code 0} means to wait forever.
991 *
992 * @throws InterruptedException if interrupted while waiting
993 * @return true if the thread has terminated
994 */
995 boolean joinNanos(long nanos) throws InterruptedException {
996 if (state() == TERMINATED)
997 return true;
998
999 // ensure termination object exists, then re-check state
1000 CountDownLatch termination = getTermination();
1001 if (state() == TERMINATED)
1002 return true;
1003
1004 // wait for virtual thread to terminate
1005 if (nanos == 0) {
1006 termination.await();
1007 } else {
1008 boolean terminated = termination.await(nanos, NANOSECONDS);
1009 if (!terminated) {
1010 // waiting time elapsed
1011 return false;
1012 }
1013 }
1014 assert state() == TERMINATED;
1015 return true;
1016 }
1017
1018 @Override
1019 void blockedOn(Interruptible b) {
1020 disableSuspendAndPreempt();
1021 try {
1022 super.blockedOn(b);
1023 } finally {
1024 enableSuspendAndPreempt();
1025 }
1026 }
1027
1028 @Override
1029 public void interrupt() {
1030 if (Thread.currentThread() != this) {
1031 // if current thread is a virtual thread then prevent it from being
1032 // suspended or unmounted when entering or holding interruptLock
1033 Interruptible blocker;
1034 disableSuspendAndPreempt();
1035 try {
1036 synchronized (interruptLock) {
1037 interrupted = true;
1038 blocker = nioBlocker();
1039 if (blocker != null) {
1040 blocker.interrupt(this);
1041 }
1042
1043 // interrupt carrier thread if mounted
1044 Thread carrier = carrierThread;
1045 if (carrier != null) carrier.setInterrupt();
1046 }
1047 } finally {
1048 enableSuspendAndPreempt();
1049 }
1050
1051 // notify blocker after releasing interruptLock
1052 if (blocker != null) {
1053 blocker.postInterrupt();
1054 }
1055
1056 // make available parking permit, unpark thread if parked
1057 unpark();
1058
1059 // if thread is waiting in Object.wait then schedule to try to reenter
1060 int s = state();
1061 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1062 submitRunContinuation();
1063 }
1064
1065 } else {
1066 interrupted = true;
1067 carrierThread.setInterrupt();
1068 setParkPermit(true);
1069 }
1070 }
1071
1072 @Override
1073 public boolean isInterrupted() {
1074 return interrupted;
1075 }
1076
1077 @Override
1078 boolean getAndClearInterrupt() {
1079 assert Thread.currentThread() == this;
1080 boolean oldValue = interrupted;
1081 if (oldValue) {
1082 disableSuspendAndPreempt();
1083 try {
1084 synchronized (interruptLock) {
1085 interrupted = false;
1086 carrierThread.clearInterrupt();
1087 }
1088 } finally {
1089 enableSuspendAndPreempt();
1090 }
1091 }
1092 return oldValue;
1093 }
1094
1095 @Override
1096 Thread.State threadState() {
1097 int s = state();
1098 switch (s & ~SUSPENDED) {
1099 case NEW:
1100 return Thread.State.NEW;
1101 case STARTED:
1102 // return NEW if thread container not yet set
1103 if (threadContainer() == null) {
1104 return Thread.State.NEW;
1105 } else {
1106 return Thread.State.RUNNABLE;
1107 }
1108 case UNPARKED:
1109 case UNBLOCKED:
1110 case YIELDED:
1111 // runnable, not mounted
1112 return Thread.State.RUNNABLE;
1113 case RUNNING:
1114 // if mounted then return state of carrier thread
1115 if (Thread.currentThread() != this) {
1116 disableSuspendAndPreempt();
1117 try {
1118 synchronized (carrierThreadAccessLock()) {
1119 Thread carrierThread = this.carrierThread;
1120 if (carrierThread != null) {
1121 return carrierThread.threadState();
1122 }
1123 }
1124 } finally {
1125 enableSuspendAndPreempt();
1126 }
1127 }
1128 // runnable, mounted
1129 return Thread.State.RUNNABLE;
1130 case PARKING:
1131 case TIMED_PARKING:
1132 case WAITING:
1133 case TIMED_WAITING:
1134 case YIELDING:
1135 // runnable, in transition
1136 return Thread.State.RUNNABLE;
1137 case PARKED:
1138 case PINNED:
1139 case WAIT:
1140 return Thread.State.WAITING;
1141 case TIMED_PARKED:
1142 case TIMED_PINNED:
1143 case TIMED_WAIT:
1144 return Thread.State.TIMED_WAITING;
1145 case BLOCKING:
1146 case BLOCKED:
1147 return Thread.State.BLOCKED;
1148 case TERMINATED:
1149 return Thread.State.TERMINATED;
1150 default:
1151 throw new InternalError();
1152 }
1153 }
1154
1155 @Override
1156 boolean alive() {
1157 int s = state;
1158 return (s != NEW && s != TERMINATED);
1159 }
1160
1161 @Override
1162 boolean isTerminated() {
1163 return (state == TERMINATED);
1164 }
1165
1166 @Override
1167 StackTraceElement[] asyncGetStackTrace() {
1168 StackTraceElement[] stackTrace;
1169 do {
1170 stackTrace = (carrierThread != null)
1171 ? super.asyncGetStackTrace() // mounted
1172 : tryGetStackTrace(); // unmounted
1173 if (stackTrace == null) {
1174 Thread.yield();
1175 }
1176 } while (stackTrace == null);
1177 return stackTrace;
1178 }
1179
1180 /**
1181 * Returns the stack trace for this virtual thread if it is unmounted.
1182 * Returns null if the thread is mounted or in transition.
1183 */
1184 private StackTraceElement[] tryGetStackTrace() {
1185 int initialState = state() & ~SUSPENDED;
1186 switch (initialState) {
1187 case NEW, STARTED, TERMINATED -> {
1188 return new StackTraceElement[0]; // unmounted, empty stack
1189 }
1190 case RUNNING, PINNED, TIMED_PINNED -> {
1191 return null; // mounted
1192 }
1193 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1194 // unmounted, not runnable
1195 }
1196 case UNPARKED, UNBLOCKED, YIELDED -> {
1197 // unmounted, runnable
1198 }
1199 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1200 return null; // in transition
1201 }
1202 default -> throw new InternalError("" + initialState);
1203 }
1204
1205 // thread is unmounted, prevent it from continuing
1206 int suspendedState = initialState | SUSPENDED;
1207 if (!compareAndSetState(initialState, suspendedState)) {
1208 return null;
1209 }
1210
1211 // get stack trace and restore state
1212 StackTraceElement[] stack;
1213 try {
1214 stack = cont.getStackTrace();
1215 } finally {
1216 assert state == suspendedState;
1217 setState(initialState);
1218 }
1219 boolean resubmit = switch (initialState) {
1220 case UNPARKED, UNBLOCKED, YIELDED -> {
1221 // resubmit as task may have run while suspended
1222 yield true;
1223 }
1224 case PARKED, TIMED_PARKED -> {
1225 // resubmit if unparked while suspended
1226 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1227 }
1228 case BLOCKED -> {
1229 // resubmit if unblocked while suspended
1230 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1231 }
1232 case WAIT, TIMED_WAIT -> {
1233 // resubmit if notified or interrupted while waiting (Object.wait)
1234 // waitTimeoutExpired will retry if the timed expired when suspended
1235 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1236 }
1237 default -> throw new InternalError();
1238 };
1239 if (resubmit) {
1240 submitRunContinuation();
1241 }
1242 return stack;
1243 }
1244
1245 @Override
1246 public String toString() {
1247 StringBuilder sb = new StringBuilder("VirtualThread[#");
1248 sb.append(threadId());
1249 String name = getName();
1250 if (!name.isEmpty()) {
1251 sb.append(",");
1252 sb.append(name);
1253 }
1254 sb.append("]/");
1255
1256 // add the carrier state and thread name when mounted
1257 boolean mounted;
1258 if (Thread.currentThread() == this) {
1259 mounted = appendCarrierInfo(sb);
1260 } else {
1261 disableSuspendAndPreempt();
1262 try {
1263 synchronized (carrierThreadAccessLock()) {
1264 mounted = appendCarrierInfo(sb);
1265 }
1266 } finally {
1267 enableSuspendAndPreempt();
1268 }
1269 }
1270
1271 // add virtual thread state when not mounted
1272 if (!mounted) {
1273 String stateAsString = threadState().toString();
1274 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1275 }
1276
1277 return sb.toString();
1278 }
1279
1280 /**
1281 * Appends the carrier state and thread name to the string buffer if mounted.
1282 * @return true if mounted, false if not mounted
1283 */
1284 private boolean appendCarrierInfo(StringBuilder sb) {
1285 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1286 Thread carrier = carrierThread;
1287 if (carrier != null) {
1288 String stateAsString = carrier.threadState().toString();
1289 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1290 sb.append('@');
1291 sb.append(carrier.getName());
1292 return true;
1293 } else {
1294 return false;
1295 }
1296 }
1297
1298 @Override
1299 public int hashCode() {
1300 return (int) threadId();
1301 }
1302
1303 @Override
1304 public boolean equals(Object obj) {
1305 return obj == this;
1306 }
1307
1308 /**
1309 * Returns the termination object, creating it if needed.
1310 */
1311 private CountDownLatch getTermination() {
1312 CountDownLatch termination = this.termination;
1313 if (termination == null) {
1314 termination = new CountDownLatch(1);
1315 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1316 termination = this.termination;
1317 }
1318 }
1319 return termination;
1320 }
1321
1322 /**
1323 * Returns the lock object to synchronize on when accessing carrierThread.
1324 * The lock prevents carrierThread from being reset to null during unmount.
1325 */
1326 private Object carrierThreadAccessLock() {
1327 // return interruptLock as unmount has to coordinate with interrupt
1328 return interruptLock;
1329 }
1330
1331 /**
1332 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1333 */
1334 private Object timedWaitLock() {
1335 // use this object for now to avoid the overhead of introducing another lock
1336 return runContinuation;
1337 }
1338
1339 /**
1340 * Disallow the current thread be suspended or preempted.
1341 */
1342 private void disableSuspendAndPreempt() {
1343 notifyJvmtiDisableSuspend(true);
1344 Continuation.pin();
1345 }
1346
1347 /**
1348 * Allow the current thread be suspended or preempted.
1349 */
1350 private void enableSuspendAndPreempt() {
1351 Continuation.unpin();
1352 notifyJvmtiDisableSuspend(false);
1353 }
1354
1355 // -- wrappers for get/set of state, parking permit, and carrier thread --
1356
1357 private int state() {
1358 return state; // volatile read
1359 }
1360
1361 private void setState(int newValue) {
1362 state = newValue; // volatile write
1363 }
1364
1365 private boolean compareAndSetState(int expectedValue, int newValue) {
1366 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1367 }
1368
1369 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1370 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1371 }
1372
1373 private void setParkPermit(boolean newValue) {
1374 if (parkPermit != newValue) {
1375 parkPermit = newValue;
1376 }
1377 }
1378
1379 private boolean getAndSetParkPermit(boolean newValue) {
1380 if (parkPermit != newValue) {
1381 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1382 } else {
1383 return newValue;
1384 }
1385 }
1386
1387 private void setCarrierThread(Thread carrier) {
1388 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1389 this.carrierThread = carrier;
1390 }
1391
1392 // -- JVM TI support --
1393
1394 @IntrinsicCandidate
1395 @JvmtiMountTransition
1396 private native void notifyJvmtiStart();
1397
1398 @IntrinsicCandidate
1399 @JvmtiMountTransition
1400 private native void notifyJvmtiEnd();
1401
1402 @IntrinsicCandidate
1403 @JvmtiMountTransition
1404 private native void notifyJvmtiMount(boolean hide);
1405
1406 @IntrinsicCandidate
1407 @JvmtiMountTransition
1408 private native void notifyJvmtiUnmount(boolean hide);
1409
1410 @IntrinsicCandidate
1411 private static native void notifyJvmtiDisableSuspend(boolean enter);
1412
1413 private static native void registerNatives();
1414 static {
1415 registerNatives();
1416
1417 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1418 var group = Thread.virtualThreadGroup();
1419 }
1420
1421 /**
1422 * Creates the default ForkJoinPool scheduler.
1423 */
1424 private static ForkJoinPool createDefaultScheduler() {
1425 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1426 int parallelism, maxPoolSize, minRunnable;
1427 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1428 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1429 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1430 if (parallelismValue != null) {
1431 parallelism = Integer.parseInt(parallelismValue);
1432 } else {
1433 parallelism = Runtime.getRuntime().availableProcessors();
1434 }
1435 if (maxPoolSizeValue != null) {
1436 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1437 parallelism = Integer.min(parallelism, maxPoolSize);
1438 } else {
1439 maxPoolSize = Integer.max(parallelism, 256);
1440 }
1441 if (minRunnableValue != null) {
1442 minRunnable = Integer.parseInt(minRunnableValue);
1443 } else {
1444 minRunnable = Integer.max(parallelism / 2, 1);
1445 }
1446 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1447 boolean asyncMode = true; // FIFO
1448 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1449 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1450 }
1451
1452 /**
1453 * Schedule a runnable task to run after a delay.
1454 */
1455 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1456 if (scheduler instanceof ForkJoinPool pool) {
1457 return pool.schedule(command, delay, unit);
1458 } else {
1459 return DelayedTaskSchedulers.schedule(command, delay, unit);
1460 }
1461 }
1462
1463 /**
1464 * Supports scheduling a runnable task to run after a delay. It uses a number
1465 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1466 * work queue used. This class is used when using a custom scheduler.
1467 */
1468 private static class DelayedTaskSchedulers {
1469 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1470
1471 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1472 long tid = Thread.currentThread().threadId();
1473 int index = (int) tid & (INSTANCE.length - 1);
1474 return INSTANCE[index].schedule(command, delay, unit);
1475 }
1476
1477 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1478 String propName = "jdk.virtualThreadScheduler.timerQueues";
1479 String propValue = System.getProperty(propName);
1480 int queueCount;
1481 if (propValue != null) {
1482 queueCount = Integer.parseInt(propValue);
1483 if (queueCount != Integer.highestOneBit(queueCount)) {
1484 throw new RuntimeException("Value of " + propName + " must be power of 2");
1485 }
1486 } else {
1487 int ncpus = Runtime.getRuntime().availableProcessors();
1488 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1489 }
1490 var schedulers = new ScheduledExecutorService[queueCount];
1491 for (int i = 0; i < queueCount; i++) {
1492 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1493 Executors.newScheduledThreadPool(1, task -> {
1494 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1495 t.setDaemon(true);
1496 return t;
1497 });
1498 stpe.setRemoveOnCancelPolicy(true);
1499 schedulers[i] = stpe;
1500 }
1501 return schedulers;
1502 }
1503 }
1504
1505 /**
1506 * Schedule virtual threads that are ready to be scheduled after they blocked on
1507 * monitor enter.
1508 */
1509 private static void unblockVirtualThreads() {
1510 while (true) {
1511 VirtualThread vthread = takeVirtualThreadListToUnblock();
1512 while (vthread != null) {
1513 assert vthread.onWaitingList;
1514 VirtualThread nextThread = vthread.next;
1515
1516 // remove from list and unblock
1517 vthread.next = null;
1518 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1519 assert changed;
1520 vthread.unblock();
1521
1522 vthread = nextThread;
1523 }
1524 }
1525 }
1526
1527 /**
1528 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1529 * if necessary until a list of one or more threads becomes available.
1530 */
1531 private static native VirtualThread takeVirtualThreadListToUnblock();
1532
1533 static {
1534 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1535 VirtualThread::unblockVirtualThreads);
1536 unblocker.setDaemon(true);
1537 unblocker.start();
1538 }
1539 }