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.lang.reflect.Constructor;
28 import java.util.Locale;
29 import java.util.Objects;
30 import java.util.concurrent.CountDownLatch;
31 import java.util.concurrent.Executor;
32 import java.util.concurrent.Executors;
33 import java.util.concurrent.ForkJoinPool;
34 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
35 import java.util.concurrent.ForkJoinTask;
36 import java.util.concurrent.Future;
37 import java.util.concurrent.RejectedExecutionException;
38 import java.util.concurrent.ScheduledExecutorService;
39 import java.util.concurrent.ScheduledThreadPoolExecutor;
40 import java.util.concurrent.TimeUnit;
41 import jdk.internal.event.VirtualThreadEndEvent;
42 import jdk.internal.event.VirtualThreadStartEvent;
43 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
44 import jdk.internal.misc.CarrierThread;
45 import jdk.internal.misc.InnocuousThread;
46 import jdk.internal.misc.Unsafe;
47 import jdk.internal.vm.Continuation;
48 import jdk.internal.vm.ContinuationScope;
49 import jdk.internal.vm.StackableScope;
50 import jdk.internal.vm.ThreadContainer;
51 import jdk.internal.vm.ThreadContainers;
52 import jdk.internal.vm.annotation.ChangesCurrentThread;
53 import jdk.internal.vm.annotation.Hidden;
54 import jdk.internal.vm.annotation.IntrinsicCandidate;
55 import jdk.internal.vm.annotation.JvmtiHideEvents;
56 import jdk.internal.vm.annotation.JvmtiMountTransition;
57 import jdk.internal.vm.annotation.ReservedStackAccess;
58 import sun.nio.ch.Interruptible;
59 import static java.util.concurrent.TimeUnit.*;
60
61 /**
62 * A thread that is scheduled by the Java virtual machine rather than the operating system.
63 */
64 final class VirtualThread extends BaseVirtualThread {
65 private static final Unsafe U = Unsafe.getUnsafe();
66 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
67 private static final Executor DEFAULT_SCHEDULER = createDefaultScheduler();
68
69 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
70 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
71 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
72 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
73 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
74
75 // scheduler and continuation
76 private final Executor scheduler;
77 private final Continuation cont;
78 private final Runnable runContinuation;
79
80 // virtual thread state, accessed by VM
81 private volatile int state;
82
83 /*
84 * Virtual thread state transitions:
85 *
86 * NEW -> STARTED // Thread.start, schedule to run
87 * STARTED -> TERMINATED // failed to start
88 * STARTED -> RUNNING // first run
89 * RUNNING -> TERMINATED // done
90 *
91 * RUNNING -> PARKING // Thread parking with LockSupport.park
92 * PARKING -> PARKED // cont.yield successful, parked indefinitely
93 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
94 * PARKED -> UNPARKED // unparked, may be scheduled to continue
95 * PINNED -> RUNNING // unparked, continue execution on same carrier
96 * UNPARKED -> RUNNING // continue execution after park
97 *
98 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
99 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
100 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
101 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
102 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
103 *
104 * RUNNING -> BLOCKING // blocking on monitor enter
105 * BLOCKING -> BLOCKED // blocked on monitor enter
106 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
107 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
108 *
109 * RUNNING -> WAITING // transitional state during wait on monitor
110 * WAITING -> WAIT // waiting on monitor
111 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
112 * WAIT -> UNBLOCKED // timed-out/interrupted
113 *
114 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
115 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
116 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
117 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
118 *
119 * RUNNING -> YIELDING // Thread.yield
120 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
121 * YIELDING -> RUNNING // cont.yield failed
122 * YIELDED -> RUNNING // continue execution after Thread.yield
123 */
124 private static final int NEW = 0;
125 private static final int STARTED = 1;
126 private static final int RUNNING = 2; // runnable-mounted
127
128 // untimed and timed parking
129 private static final int PARKING = 3;
130 private static final int PARKED = 4; // unmounted
131 private static final int PINNED = 5; // mounted
132 private static final int TIMED_PARKING = 6;
133 private static final int TIMED_PARKED = 7; // unmounted
134 private static final int TIMED_PINNED = 8; // mounted
135 private static final int UNPARKED = 9; // unmounted but runnable
136
137 // Thread.yield
138 private static final int YIELDING = 10;
139 private static final int YIELDED = 11; // unmounted but runnable
140
141 // monitor enter
142 private static final int BLOCKING = 12;
143 private static final int BLOCKED = 13; // unmounted
144 private static final int UNBLOCKED = 14; // unmounted but runnable
145
146 // monitor wait/timed-wait
147 private static final int WAITING = 15;
148 private static final int WAIT = 16; // waiting in Object.wait
149 private static final int TIMED_WAITING = 17;
150 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
151
152 private static final int TERMINATED = 99; // final state
153
154 // can be suspended from scheduling when unmounted
155 private static final int SUSPENDED = 1 << 8;
156
157 // parking permit made available by LockSupport.unpark
158 private volatile boolean parkPermit;
159
160 // blocking permit made available by unblocker thread when another thread exits monitor
161 private volatile boolean blockPermit;
162
163 // true when on the list of virtual threads waiting to be unblocked
164 private volatile boolean onWaitingList;
165
166 // next virtual thread on the list of virtual threads waiting to be unblocked
167 private volatile VirtualThread next;
168
169 // notified by Object.notify/notifyAll while waiting in Object.wait
170 private volatile boolean notified;
171
172 // true when virtual thread is executing Java level Object.wait, false on VM internal Object.wait
173 private volatile boolean interruptableWait;
174
175 // timed-wait support
176 private byte timedWaitSeqNo;
177
178 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
179 private long timeout;
180
181 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
182 private Future<?> timeoutTask;
183
184 // carrier thread when mounted, accessed by VM
185 private volatile Thread carrierThread;
186
187 // termination object when joining, created lazily if needed
188 private volatile CountDownLatch termination;
189
190 /**
191 * Returns the default scheduler.
192 */
193 static Executor defaultScheduler() {
194 return DEFAULT_SCHEDULER;
195 }
196
197 /**
198 * Returns the continuation scope used for virtual threads.
199 */
200 static ContinuationScope continuationScope() {
201 return VTHREAD_SCOPE;
202 }
203
204 /**
205 * Creates a new {@code VirtualThread} to run the given task with the given
206 * scheduler. If the given scheduler is {@code null} and the current thread
207 * is a platform thread then the newly created virtual thread will use the
208 * default scheduler. If given scheduler is {@code null} and the current
209 * thread is a virtual thread then the current thread's scheduler is used.
210 *
211 * @param scheduler the scheduler or null
212 * @param name thread name
213 * @param characteristics characteristics
214 * @param task the task to execute
215 */
216 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
217 super(name, characteristics, /*bound*/ false);
218 Objects.requireNonNull(task);
219
220 // choose scheduler if not specified
221 if (scheduler == null) {
222 Thread parent = Thread.currentThread();
223 if (parent instanceof VirtualThread vparent) {
224 scheduler = vparent.scheduler;
225 } else {
226 scheduler = DEFAULT_SCHEDULER;
227 }
228 }
229
230 this.scheduler = scheduler;
231 this.cont = new VThreadContinuation(this, task);
232 this.runContinuation = this::runContinuation;
233 }
234
235 /**
236 * The continuation that a virtual thread executes.
237 */
238 private static class VThreadContinuation extends Continuation {
239 VThreadContinuation(VirtualThread vthread, Runnable task) {
240 super(VTHREAD_SCOPE, wrap(vthread, task));
241 }
242 @Override
243 protected void onPinned(Continuation.Pinned reason) {
244 }
245 private static Runnable wrap(VirtualThread vthread, Runnable task) {
246 return new Runnable() {
247 @Hidden
248 @JvmtiHideEvents
249 public void run() {
250 vthread.notifyJvmtiStart(); // notify JVMTI
251 try {
252 vthread.run(task);
253 } finally {
254 vthread.notifyJvmtiEnd(); // notify JVMTI
255 }
256 }
257 };
258 }
259 }
260
261 /**
262 * Runs or continues execution on the current thread. The virtual thread is mounted
263 * on the current thread before the task runs or continues. It unmounts when the
264 * task completes or yields.
265 */
266 @ChangesCurrentThread // allow mount/unmount to be inlined
267 private void runContinuation() {
268 // the carrier must be a platform thread
269 if (Thread.currentThread().isVirtual()) {
270 throw new WrongThreadException();
271 }
272
273 // set state to RUNNING
274 int initialState = state();
275 if (initialState == STARTED || initialState == UNPARKED
276 || initialState == UNBLOCKED || initialState == YIELDED) {
277 // newly started or continue after parking/blocking/Thread.yield
278 if (!compareAndSetState(initialState, RUNNING)) {
279 return;
280 }
281 // consume permit when continuing after parking or blocking. If continue
282 // after a timed-park or timed-wait then the timeout task is cancelled.
283 if (initialState == UNPARKED) {
284 cancelTimeoutTask();
285 setParkPermit(false);
286 } else if (initialState == UNBLOCKED) {
287 cancelTimeoutTask();
288 blockPermit = false;
289 }
290 } else {
291 // not runnable
292 return;
293 }
294
295 mount();
296 try {
297 cont.run();
298 } finally {
299 unmount();
300 if (cont.isDone()) {
301 afterDone();
302 } else {
303 afterYield();
304 }
305 }
306 }
307
308 /**
309 * Cancel timeout task when continuing after timed-park or timed-wait.
310 * The timeout task may be executing, or may have already completed.
311 */
312 private void cancelTimeoutTask() {
313 if (timeoutTask != null) {
314 timeoutTask.cancel(false);
315 timeoutTask = null;
316 }
317 }
318
319 /**
320 * Submits the runContinuation task to the scheduler. For the default scheduler,
321 * and calling it on a worker thread, the task will be pushed to the local queue,
322 * otherwise it will be pushed to an external submission queue.
323 * @param scheduler the scheduler
324 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
325 * @throws RejectedExecutionException
326 */
327 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
328 boolean done = false;
329 while (!done) {
330 try {
331 // Pin the continuation to prevent the virtual thread from unmounting
332 // when submitting a task. For the default scheduler this ensures that
333 // the carrier doesn't change when pushing a task. For other schedulers
334 // it avoids deadlock that could arise due to carriers and virtual
335 // threads contending for a lock.
336 if (currentThread().isVirtual()) {
337 Continuation.pin();
338 try {
339 scheduler.execute(runContinuation);
340 } finally {
341 Continuation.unpin();
342 }
343 } else {
344 scheduler.execute(runContinuation);
345 }
346 done = true;
347 } catch (RejectedExecutionException ree) {
348 submitFailed(ree);
349 throw ree;
350 } catch (OutOfMemoryError e) {
351 if (retryOnOOME) {
352 U.park(false, 100_000_000); // 100ms
353 } else {
354 throw e;
355 }
356 }
357 }
358 }
359
360 /**
361 * Submits the runContinuation task to the given scheduler as an external submit.
362 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
363 * @throws RejectedExecutionException
364 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
365 */
366 private void externalSubmitRunContinuation(ForkJoinPool pool) {
367 assert Thread.currentThread() instanceof CarrierThread;
368 try {
369 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
370 } catch (RejectedExecutionException ree) {
371 submitFailed(ree);
372 throw ree;
373 } catch (OutOfMemoryError e) {
374 submitRunContinuation(pool, true);
375 }
376 }
377
378 /**
379 * Submits the runContinuation task to the scheduler. For the default scheduler,
380 * and calling it on a worker thread, the task will be pushed to the local queue,
381 * otherwise it will be pushed to an external submission queue.
382 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
383 * @throws RejectedExecutionException
384 */
385 private void submitRunContinuation() {
386 submitRunContinuation(scheduler, true);
387 }
388
389 /**
390 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
391 * queue is empty. If not empty, or invoked by another thread, then this method works
392 * like submitRunContinuation and just submits the task to the scheduler.
393 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
394 * @throws RejectedExecutionException
395 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
396 */
397 private void lazySubmitRunContinuation() {
398 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
399 ForkJoinPool pool = ct.getPool();
400 try {
401 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
402 } catch (RejectedExecutionException ree) {
403 submitFailed(ree);
404 throw ree;
405 } catch (OutOfMemoryError e) {
406 submitRunContinuation();
407 }
408 } else {
409 submitRunContinuation();
410 }
411 }
412
413 /**
414 * Submits the runContinuation task to the scheduler. For the default scheduler, and
415 * calling it a virtual thread that uses the default scheduler, the task will be
416 * pushed to an external submission queue. This method may throw OutOfMemoryError.
417 * @throws RejectedExecutionException
418 * @throws OutOfMemoryError
419 */
420 private void externalSubmitRunContinuationOrThrow() {
421 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
422 try {
423 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
424 } catch (RejectedExecutionException ree) {
425 submitFailed(ree);
426 throw ree;
427 }
428 } else {
429 submitRunContinuation(scheduler, false);
430 }
431 }
432
433 /**
434 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
435 */
436 private void submitFailed(RejectedExecutionException ree) {
437 var event = new VirtualThreadSubmitFailedEvent();
438 if (event.isEnabled()) {
439 event.javaThreadId = threadId();
440 event.exceptionMessage = ree.getMessage();
441 event.commit();
442 }
443 }
444
445 /**
446 * Runs a task in the context of this virtual thread.
447 */
448 private void run(Runnable task) {
449 assert Thread.currentThread() == this && state == RUNNING;
450
451 // emit JFR event if enabled
452 if (VirtualThreadStartEvent.isTurnedOn()) {
453 var event = new VirtualThreadStartEvent();
454 event.javaThreadId = threadId();
455 event.commit();
456 }
457
458 Object bindings = Thread.scopedValueBindings();
459 try {
460 runWith(bindings, task);
461 } catch (Throwable exc) {
462 dispatchUncaughtException(exc);
463 } finally {
464 // pop any remaining scopes from the stack, this may block
465 StackableScope.popAll();
466
467 // emit JFR event if enabled
468 if (VirtualThreadEndEvent.isTurnedOn()) {
469 var event = new VirtualThreadEndEvent();
470 event.javaThreadId = threadId();
471 event.commit();
472 }
473 }
474 }
475
476 /**
477 * Mounts this virtual thread onto the current platform thread. On
478 * return, the current thread is the virtual thread.
479 */
480 @ChangesCurrentThread
481 @ReservedStackAccess
482 private void mount() {
483 // notify JVMTI before mount
484 notifyJvmtiMount(/*hide*/true);
485
486 // sets the carrier thread
487 Thread carrier = Thread.currentCarrierThread();
488 setCarrierThread(carrier);
489
490 // sync up carrier thread interrupt status if needed
491 if (interrupted) {
492 carrier.setInterrupt();
493 } else if (carrier.isInterrupted()) {
494 synchronized (interruptLock) {
495 // need to recheck interrupt status
496 if (!interrupted) {
497 carrier.clearInterrupt();
498 }
499 }
500 }
501
502 // set Thread.currentThread() to return this virtual thread
503 carrier.setCurrentThread(this);
504 }
505
506 /**
507 * Unmounts this virtual thread from the carrier. On return, the
508 * current thread is the current platform thread.
509 */
510 @ChangesCurrentThread
511 @ReservedStackAccess
512 private void unmount() {
513 assert !Thread.holdsLock(interruptLock);
514
515 // set Thread.currentThread() to return the platform thread
516 Thread carrier = this.carrierThread;
517 carrier.setCurrentThread(carrier);
518
519 // break connection to carrier thread, synchronized with interrupt
520 synchronized (interruptLock) {
521 setCarrierThread(null);
522 }
523 carrier.clearInterrupt();
524
525 // notify JVMTI after unmount
526 notifyJvmtiUnmount(/*hide*/false);
527 }
528
529 /**
530 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
531 * the continuation continues.
532 */
533 @Hidden
534 private boolean yieldContinuation() {
535 notifyJvmtiUnmount(/*hide*/true);
536 try {
537 return Continuation.yield(VTHREAD_SCOPE);
538 } finally {
539 notifyJvmtiMount(/*hide*/false);
540 }
541 }
542
543 /**
544 * Invoked in the context of the carrier thread after the Continuation yields when
545 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
546 */
547 private void afterYield() {
548 assert carrierThread == null;
549
550 // re-adjust parallelism if the virtual thread yielded when compensating
551 if (currentThread() instanceof CarrierThread ct) {
552 ct.endBlocking();
553 }
554
555 int s = state();
556
557 // LockSupport.park/parkNanos
558 if (s == PARKING || s == TIMED_PARKING) {
559 int newState;
560 if (s == PARKING) {
561 setState(newState = PARKED);
562 } else {
563 // schedule unpark
564 long timeout = this.timeout;
565 assert timeout > 0;
566 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
567 setState(newState = TIMED_PARKED);
568 }
569
570 // may have been unparked while parking
571 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
572 // lazy submit if local queue is empty
573 lazySubmitRunContinuation();
574 }
575 return;
576 }
577
578 // Thread.yield
579 if (s == YIELDING) {
580 setState(YIELDED);
581
582 // external submit if there are no tasks in the local task queue
583 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
584 externalSubmitRunContinuation(ct.getPool());
585 } else {
586 submitRunContinuation();
587 }
588 return;
589 }
590
591 // blocking on monitorenter
592 if (s == BLOCKING) {
593 setState(BLOCKED);
594
595 // may have been unblocked while blocking
596 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
597 // lazy submit if local queue is empty
598 lazySubmitRunContinuation();
599 }
600 return;
601 }
602
603 // Object.wait
604 if (s == WAITING || s == TIMED_WAITING) {
605 int newState;
606 boolean interruptable = interruptableWait;
607 if (s == WAITING) {
608 setState(newState = WAIT);
609 } else {
610 // For timed-wait, a timeout task is scheduled to execute. The timeout
611 // task will change the thread state to UNBLOCKED and submit the thread
612 // to the scheduler. A sequence number is used to ensure that the timeout
613 // task only unblocks the thread for this timed-wait. We synchronize with
614 // the timeout task to coordinate access to the sequence number and to
615 // ensure the timeout task doesn't execute until the thread has got to
616 // the TIMED_WAIT state.
617 long timeout = this.timeout;
618 assert timeout > 0;
619 synchronized (timedWaitLock()) {
620 byte seqNo = ++timedWaitSeqNo;
621 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
622 setState(newState = TIMED_WAIT);
623 }
624 }
625
626 // may have been notified while in transition to wait state
627 if (notified && compareAndSetState(newState, BLOCKED)) {
628 // may have even been unblocked already
629 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
630 submitRunContinuation();
631 }
632 return;
633 }
634
635 // may have been interrupted while in transition to wait state
636 if (interruptable && interrupted && compareAndSetState(newState, UNBLOCKED)) {
637 submitRunContinuation();
638 return;
639 }
640 return;
641 }
642
643 assert false;
644 }
645
646 /**
647 * Invoked after the continuation completes.
648 */
649 private void afterDone() {
650 afterDone(true);
651 }
652
653 /**
654 * Invoked after the continuation completes (or start failed). Sets the thread
655 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
656 *
657 * @param notifyContainer true if its container should be notified
658 */
659 private void afterDone(boolean notifyContainer) {
660 assert carrierThread == null;
661 setState(TERMINATED);
662
663 // notify anyone waiting for this virtual thread to terminate
664 CountDownLatch termination = this.termination;
665 if (termination != null) {
666 assert termination.getCount() == 1;
667 termination.countDown();
668 }
669
670 // notify container
671 if (notifyContainer) {
672 threadContainer().remove(this);
673 }
674
675 // clear references to thread locals
676 clearReferences();
677 }
678
679 /**
680 * Schedules this {@code VirtualThread} to execute.
681 *
682 * @throws IllegalStateException if the container is shutdown or closed
683 * @throws IllegalThreadStateException if the thread has already been started
684 * @throws RejectedExecutionException if the scheduler cannot accept a task
685 */
686 @Override
687 void start(ThreadContainer container) {
688 if (!compareAndSetState(NEW, STARTED)) {
689 throw new IllegalThreadStateException("Already started");
690 }
691
692 // bind thread to container
693 assert threadContainer() == null;
694 setThreadContainer(container);
695
696 // start thread
697 boolean addedToContainer = false;
698 boolean started = false;
699 try {
700 container.add(this); // may throw
701 addedToContainer = true;
702
703 // scoped values may be inherited
704 inheritScopedValueBindings(container);
705
706 // submit task to run thread, using externalSubmit if possible
707 externalSubmitRunContinuationOrThrow();
708 started = true;
709 } finally {
710 if (!started) {
711 afterDone(addedToContainer);
712 }
713 }
714 }
715
716 @Override
717 public void start() {
718 start(ThreadContainers.root());
719 }
720
721 @Override
722 public void run() {
723 // do nothing
724 }
725
726 /**
727 * Parks until unparked or interrupted. If already unparked then the parking
728 * permit is consumed and this method completes immediately (meaning it doesn't
729 * yield). It also completes immediately if the interrupt status is set.
730 */
731 @Override
732 void park() {
733 assert Thread.currentThread() == this;
734
735 // complete immediately if parking permit available or interrupted
736 if (getAndSetParkPermit(false) || interrupted)
737 return;
738
739 // park the thread
740 boolean yielded = false;
741 setState(PARKING);
742 try {
743 yielded = yieldContinuation();
744 } catch (OutOfMemoryError e) {
745 // park on carrier
746 } finally {
747 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
748 if (!yielded) {
749 assert state() == PARKING;
750 setState(RUNNING);
751 }
752 }
753
754 // park on the carrier thread when pinned
755 if (!yielded) {
756 parkOnCarrierThread(false, 0);
757 }
758 }
759
760 /**
761 * Parks up to the given waiting time or until unparked or interrupted.
762 * If already unparked then the parking permit is consumed and this method
763 * completes immediately (meaning it doesn't yield). It also completes immediately
764 * if the interrupt status is set or the waiting time is {@code <= 0}.
765 *
766 * @param nanos the maximum number of nanoseconds to wait.
767 */
768 @Override
769 void parkNanos(long nanos) {
770 assert Thread.currentThread() == this;
771
772 // complete immediately if parking permit available or interrupted
773 if (getAndSetParkPermit(false) || interrupted)
774 return;
775
776 // park the thread for the waiting time
777 if (nanos > 0) {
778 long startTime = System.nanoTime();
779
780 // park the thread, afterYield will schedule the thread to unpark
781 boolean yielded = false;
782 timeout = nanos;
783 setState(TIMED_PARKING);
784 try {
785 yielded = yieldContinuation();
786 } catch (OutOfMemoryError e) {
787 // park on carrier
788 } finally {
789 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
790 if (!yielded) {
791 assert state() == TIMED_PARKING;
792 setState(RUNNING);
793 }
794 }
795
796 // park on carrier thread for remaining time when pinned (or OOME)
797 if (!yielded) {
798 long remainingNanos = nanos - (System.nanoTime() - startTime);
799 parkOnCarrierThread(true, remainingNanos);
800 }
801 }
802 }
803
804 /**
805 * Parks the current carrier thread up to the given waiting time or until
806 * unparked or interrupted. If the virtual thread is interrupted then the
807 * interrupt status will be propagated to the carrier thread.
808 * @param timed true for a timed park, false for untimed
809 * @param nanos the waiting time in nanoseconds
810 */
811 private void parkOnCarrierThread(boolean timed, long nanos) {
812 assert state() == RUNNING;
813
814 setState(timed ? TIMED_PINNED : PINNED);
815 try {
816 if (!parkPermit) {
817 if (!timed) {
818 U.park(false, 0);
819 } else if (nanos > 0) {
820 U.park(false, nanos);
821 }
822 }
823 } finally {
824 setState(RUNNING);
825 }
826
827 // consume parking permit
828 setParkPermit(false);
829
830 // JFR jdk.VirtualThreadPinned event
831 postPinnedEvent("LockSupport.park");
832 }
833
834 /**
835 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
836 * Recording the event in the VM avoids having JFR event recorded in Java
837 * with the same name, but different ID, to events recorded by the VM.
838 */
839 @Hidden
840 private static native void postPinnedEvent(String op);
841
842 /**
843 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
844 * then its task is scheduled to continue, otherwise its next call to {@code park} or
845 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
846 * @throws RejectedExecutionException if the scheduler cannot accept a task
847 */
848 @Override
849 void unpark() {
850 if (!getAndSetParkPermit(true) && currentThread() != this) {
851 int s = state();
852
853 // unparked while parked
854 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
855 submitRunContinuation();
856 return;
857 }
858
859 // unparked while parked when pinned
860 if (s == PINNED || s == TIMED_PINNED) {
861 // unpark carrier thread when pinned
862 disableSuspendAndPreempt();
863 try {
864 synchronized (carrierThreadAccessLock()) {
865 Thread carrier = carrierThread;
866 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
867 U.unpark(carrier);
868 }
869 }
870 } finally {
871 enableSuspendAndPreempt();
872 }
873 return;
874 }
875 }
876 }
877
878 /**
879 * Invoked by unblocker thread to unblock this virtual thread.
880 */
881 private void unblock() {
882 assert !Thread.currentThread().isVirtual();
883 blockPermit = true;
884 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
885 submitRunContinuation();
886 }
887 }
888
889 /**
890 * Invoked by FJP worker thread or STPE thread when park timeout expires.
891 */
892 private void parkTimeoutExpired() {
893 assert !VirtualThread.currentThread().isVirtual();
894 if (!getAndSetParkPermit(true)
895 && (state() == TIMED_PARKED)
896 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
897 lazySubmitRunContinuation();
898 }
899 }
900
901 /**
902 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
903 * If the virtual thread is in timed-wait then this method will unblock the thread
904 * and submit its task so that it continues and attempts to reenter the monitor.
905 * This method does nothing if the thread has been woken by notify or interrupt.
906 */
907 private void waitTimeoutExpired(byte seqNo) {
908 assert !Thread.currentThread().isVirtual();
909 for (;;) {
910 boolean unblocked = false;
911 synchronized (timedWaitLock()) {
912 if (seqNo != timedWaitSeqNo) {
913 // this timeout task is for a past timed-wait
914 return;
915 }
916 int s = state();
917 if (s == TIMED_WAIT) {
918 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
919 } else if (s != (TIMED_WAIT | SUSPENDED)) {
920 // notified or interrupted, no longer waiting
921 return;
922 }
923 }
924 if (unblocked) {
925 lazySubmitRunContinuation();
926 return;
927 }
928 // need to retry when thread is suspended in time-wait
929 Thread.yield();
930 }
931 }
932
933 /**
934 * Attempts to yield the current virtual thread (Thread.yield).
935 */
936 void tryYield() {
937 assert Thread.currentThread() == this;
938 setState(YIELDING);
939 boolean yielded = false;
940 try {
941 yielded = yieldContinuation(); // may throw
942 } finally {
943 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
944 if (!yielded) {
945 assert state() == YIELDING;
946 setState(RUNNING);
947 }
948 }
949 }
950
951 /**
952 * Sleep the current thread for the given sleep time (in nanoseconds). If
953 * nanos is 0 then the thread will attempt to yield.
954 *
955 * @implNote This implementation parks the thread for the given sleeping time
956 * and will therefore be observed in PARKED state during the sleep. Parking
957 * will consume the parking permit so this method makes available the parking
958 * permit after the sleep. This may be observed as a spurious, but benign,
959 * wakeup when the thread subsequently attempts to park.
960 *
961 * @param nanos the maximum number of nanoseconds to sleep
962 * @throws InterruptedException if interrupted while sleeping
963 */
964 void sleepNanos(long nanos) throws InterruptedException {
965 assert Thread.currentThread() == this && nanos >= 0;
966 if (getAndClearInterrupt())
967 throw new InterruptedException();
968 if (nanos == 0) {
969 tryYield();
970 } else {
971 // park for the sleep time
972 try {
973 long remainingNanos = nanos;
974 long startNanos = System.nanoTime();
975 while (remainingNanos > 0) {
976 parkNanos(remainingNanos);
977 if (getAndClearInterrupt()) {
978 throw new InterruptedException();
979 }
980 remainingNanos = nanos - (System.nanoTime() - startNanos);
981 }
982 } finally {
983 // may have been unparked while sleeping
984 setParkPermit(true);
985 }
986 }
987 }
988
989 /**
990 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
991 * A timeout of {@code 0} means to wait forever.
992 *
993 * @throws InterruptedException if interrupted while waiting
994 * @return true if the thread has terminated
995 */
996 boolean joinNanos(long nanos) throws InterruptedException {
997 if (state() == TERMINATED)
998 return true;
999
1000 // ensure termination object exists, then re-check state
1001 CountDownLatch termination = getTermination();
1002 if (state() == TERMINATED)
1003 return true;
1004
1005 // wait for virtual thread to terminate
1006 if (nanos == 0) {
1007 termination.await();
1008 } else {
1009 boolean terminated = termination.await(nanos, NANOSECONDS);
1010 if (!terminated) {
1011 // waiting time elapsed
1012 return false;
1013 }
1014 }
1015 assert state() == TERMINATED;
1016 return true;
1017 }
1018
1019 @Override
1020 void blockedOn(Interruptible b) {
1021 disableSuspendAndPreempt();
1022 try {
1023 super.blockedOn(b);
1024 } finally {
1025 enableSuspendAndPreempt();
1026 }
1027 }
1028
1029 @Override
1030 public void interrupt() {
1031 if (Thread.currentThread() != this) {
1032 // if current thread is a virtual thread then prevent it from being
1033 // suspended or unmounted when entering or holding interruptLock
1034 Interruptible blocker;
1035 disableSuspendAndPreempt();
1036 try {
1037 synchronized (interruptLock) {
1038 interrupted = true;
1039 blocker = nioBlocker();
1040 if (blocker != null) {
1041 blocker.interrupt(this);
1042 }
1043
1044 // interrupt carrier thread if mounted
1045 Thread carrier = carrierThread;
1046 if (carrier != null) carrier.setInterrupt();
1047 }
1048 } finally {
1049 enableSuspendAndPreempt();
1050 }
1051
1052 // notify blocker after releasing interruptLock
1053 if (blocker != null) {
1054 blocker.postInterrupt();
1055 }
1056
1057 // make available parking permit, unpark thread if parked
1058 unpark();
1059
1060 // if thread is waiting in Object.wait then schedule to try to reenter
1061 int s = state();
1062 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1063 submitRunContinuation();
1064 }
1065
1066 } else {
1067 interrupted = true;
1068 carrierThread.setInterrupt();
1069 setParkPermit(true);
1070 }
1071 }
1072
1073 @Override
1074 public boolean isInterrupted() {
1075 return interrupted;
1076 }
1077
1078 @Override
1079 boolean getAndClearInterrupt() {
1080 assert Thread.currentThread() == this;
1081 boolean oldValue = interrupted;
1082 if (oldValue) {
1083 disableSuspendAndPreempt();
1084 try {
1085 synchronized (interruptLock) {
1086 interrupted = false;
1087 carrierThread.clearInterrupt();
1088 }
1089 } finally {
1090 enableSuspendAndPreempt();
1091 }
1092 }
1093 return oldValue;
1094 }
1095
1096 @Override
1097 Thread.State threadState() {
1098 int s = state();
1099 switch (s & ~SUSPENDED) {
1100 case NEW:
1101 return Thread.State.NEW;
1102 case STARTED:
1103 // return NEW if thread container not yet set
1104 if (threadContainer() == null) {
1105 return Thread.State.NEW;
1106 } else {
1107 return Thread.State.RUNNABLE;
1108 }
1109 case UNPARKED:
1110 case UNBLOCKED:
1111 case YIELDED:
1112 // runnable, not mounted
1113 return Thread.State.RUNNABLE;
1114 case RUNNING:
1115 // if mounted then return state of carrier thread
1116 if (Thread.currentThread() != this) {
1117 disableSuspendAndPreempt();
1118 try {
1119 synchronized (carrierThreadAccessLock()) {
1120 Thread carrierThread = this.carrierThread;
1121 if (carrierThread != null) {
1122 return carrierThread.threadState();
1123 }
1124 }
1125 } finally {
1126 enableSuspendAndPreempt();
1127 }
1128 }
1129 // runnable, mounted
1130 return Thread.State.RUNNABLE;
1131 case PARKING:
1132 case TIMED_PARKING:
1133 case WAITING:
1134 case TIMED_WAITING:
1135 case YIELDING:
1136 // runnable, in transition
1137 return Thread.State.RUNNABLE;
1138 case PARKED:
1139 case PINNED:
1140 case WAIT:
1141 return Thread.State.WAITING;
1142 case TIMED_PARKED:
1143 case TIMED_PINNED:
1144 case TIMED_WAIT:
1145 return Thread.State.TIMED_WAITING;
1146 case BLOCKING:
1147 case BLOCKED:
1148 return Thread.State.BLOCKED;
1149 case TERMINATED:
1150 return Thread.State.TERMINATED;
1151 default:
1152 throw new InternalError();
1153 }
1154 }
1155
1156 @Override
1157 boolean alive() {
1158 int s = state;
1159 return (s != NEW && s != TERMINATED);
1160 }
1161
1162 @Override
1163 boolean isTerminated() {
1164 return (state == TERMINATED);
1165 }
1166
1167 @Override
1168 StackTraceElement[] asyncGetStackTrace() {
1169 StackTraceElement[] stackTrace;
1170 do {
1171 stackTrace = (carrierThread != null)
1172 ? super.asyncGetStackTrace() // mounted
1173 : tryGetStackTrace(); // unmounted
1174 if (stackTrace == null) {
1175 Thread.yield();
1176 }
1177 } while (stackTrace == null);
1178 return stackTrace;
1179 }
1180
1181 /**
1182 * Returns the stack trace for this virtual thread if it is unmounted.
1183 * Returns null if the thread is mounted or in transition.
1184 */
1185 private StackTraceElement[] tryGetStackTrace() {
1186 int initialState = state() & ~SUSPENDED;
1187 switch (initialState) {
1188 case NEW, STARTED, TERMINATED -> {
1189 return new StackTraceElement[0]; // unmounted, empty stack
1190 }
1191 case RUNNING, PINNED, TIMED_PINNED -> {
1192 return null; // mounted
1193 }
1194 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1195 // unmounted, not runnable
1196 }
1197 case UNPARKED, UNBLOCKED, YIELDED -> {
1198 // unmounted, runnable
1199 }
1200 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1201 return null; // in transition
1202 }
1203 default -> throw new InternalError("" + initialState);
1204 }
1205
1206 // thread is unmounted, prevent it from continuing
1207 int suspendedState = initialState | SUSPENDED;
1208 if (!compareAndSetState(initialState, suspendedState)) {
1209 return null;
1210 }
1211
1212 // get stack trace and restore state
1213 StackTraceElement[] stack;
1214 try {
1215 stack = cont.getStackTrace();
1216 } finally {
1217 assert state == suspendedState;
1218 setState(initialState);
1219 }
1220 boolean resubmit = switch (initialState) {
1221 case UNPARKED, UNBLOCKED, YIELDED -> {
1222 // resubmit as task may have run while suspended
1223 yield true;
1224 }
1225 case PARKED, TIMED_PARKED -> {
1226 // resubmit if unparked while suspended
1227 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1228 }
1229 case BLOCKED -> {
1230 // resubmit if unblocked while suspended
1231 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1232 }
1233 case WAIT, TIMED_WAIT -> {
1234 // resubmit if notified or interrupted while waiting (Object.wait)
1235 // waitTimeoutExpired will retry if the timed expired when suspended
1236 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1237 }
1238 default -> throw new InternalError();
1239 };
1240 if (resubmit) {
1241 submitRunContinuation();
1242 }
1243 return stack;
1244 }
1245
1246 @Override
1247 public String toString() {
1248 StringBuilder sb = new StringBuilder("VirtualThread[#");
1249 sb.append(threadId());
1250 String name = getName();
1251 if (!name.isEmpty()) {
1252 sb.append(",");
1253 sb.append(name);
1254 }
1255 sb.append("]/");
1256
1257 // add the carrier state and thread name when mounted
1258 boolean mounted;
1259 if (Thread.currentThread() == this) {
1260 mounted = appendCarrierInfo(sb);
1261 } else {
1262 disableSuspendAndPreempt();
1263 try {
1264 synchronized (carrierThreadAccessLock()) {
1265 mounted = appendCarrierInfo(sb);
1266 }
1267 } finally {
1268 enableSuspendAndPreempt();
1269 }
1270 }
1271
1272 // add virtual thread state when not mounted
1273 if (!mounted) {
1274 String stateAsString = threadState().toString();
1275 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1276 }
1277
1278 return sb.toString();
1279 }
1280
1281 /**
1282 * Appends the carrier state and thread name to the string buffer if mounted.
1283 * @return true if mounted, false if not mounted
1284 */
1285 private boolean appendCarrierInfo(StringBuilder sb) {
1286 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1287 Thread carrier = carrierThread;
1288 if (carrier != null) {
1289 String stateAsString = carrier.threadState().toString();
1290 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1291 sb.append('@');
1292 sb.append(carrier.getName());
1293 return true;
1294 } else {
1295 return false;
1296 }
1297 }
1298
1299 @Override
1300 public int hashCode() {
1301 return (int) threadId();
1302 }
1303
1304 @Override
1305 public boolean equals(Object obj) {
1306 return obj == this;
1307 }
1308
1309 /**
1310 * Returns the termination object, creating it if needed.
1311 */
1312 private CountDownLatch getTermination() {
1313 CountDownLatch termination = this.termination;
1314 if (termination == null) {
1315 termination = new CountDownLatch(1);
1316 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1317 termination = this.termination;
1318 }
1319 }
1320 return termination;
1321 }
1322
1323 /**
1324 * Returns the lock object to synchronize on when accessing carrierThread.
1325 * The lock prevents carrierThread from being reset to null during unmount.
1326 */
1327 private Object carrierThreadAccessLock() {
1328 // return interruptLock as unmount has to coordinate with interrupt
1329 return interruptLock;
1330 }
1331
1332 /**
1333 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1334 */
1335 private Object timedWaitLock() {
1336 // use this object for now to avoid the overhead of introducing another lock
1337 return runContinuation;
1338 }
1339
1340 /**
1341 * Disallow the current thread be suspended or preempted.
1342 */
1343 private void disableSuspendAndPreempt() {
1344 notifyJvmtiDisableSuspend(true);
1345 Continuation.pin();
1346 }
1347
1348 /**
1349 * Allow the current thread be suspended or preempted.
1350 */
1351 private void enableSuspendAndPreempt() {
1352 Continuation.unpin();
1353 notifyJvmtiDisableSuspend(false);
1354 }
1355
1356 // -- wrappers for get/set of state, parking permit, and carrier thread --
1357
1358 int state() {
1359 return state; // volatile read
1360 }
1361
1362 private void setState(int newValue) {
1363 state = newValue; // volatile write
1364 }
1365
1366 private boolean compareAndSetState(int expectedValue, int newValue) {
1367 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1368 }
1369
1370 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1371 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1372 }
1373
1374 private void setParkPermit(boolean newValue) {
1375 if (parkPermit != newValue) {
1376 parkPermit = newValue;
1377 }
1378 }
1379
1380 private boolean getAndSetParkPermit(boolean newValue) {
1381 if (parkPermit != newValue) {
1382 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1383 } else {
1384 return newValue;
1385 }
1386 }
1387
1388 Thread carrierThread() {
1389 return carrierThread;
1390 }
1391
1392 private void setCarrierThread(Thread carrier) {
1393 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1394 this.carrierThread = carrier;
1395 }
1396
1397 // -- JVM TI support --
1398
1399 @IntrinsicCandidate
1400 @JvmtiMountTransition
1401 private native void notifyJvmtiStart();
1402
1403 @IntrinsicCandidate
1404 @JvmtiMountTransition
1405 private native void notifyJvmtiEnd();
1406
1407 @IntrinsicCandidate
1408 @JvmtiMountTransition
1409 private native void notifyJvmtiMount(boolean hide);
1410
1411 @IntrinsicCandidate
1412 @JvmtiMountTransition
1413 private native void notifyJvmtiUnmount(boolean hide);
1414
1415 @IntrinsicCandidate
1416 private static native void notifyJvmtiDisableSuspend(boolean enter);
1417
1418 private static native void registerNatives();
1419 static {
1420 registerNatives();
1421
1422 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1423 var group = Thread.virtualThreadGroup();
1424 }
1425
1426 /**
1427 * Creates the default scheduler.
1428 * If the system property {@code jdk.virtualThreadScheduler.implClass} is set then
1429 * its value is the name of a class that implements java.util.concurrent.Executor.
1430 * The class is public in an exported package, has a public no-arg constructor,
1431 * and is visible to the system class loader.
1432 * If the system property is not set then the default scheduler will be a
1433 * ForkJoinPool instance.
1434 */
1435 private static Executor createDefaultScheduler() {
1436 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
1437 if (propValue != null) {
1438 try {
1439 Class<?> clazz = Class.forName(propValue, true,
1440 ClassLoader.getSystemClassLoader());
1441 Constructor<?> ctor = clazz.getConstructor();
1442 var scheduler = (Executor) ctor.newInstance();
1443 System.err.println("""
1444 WARNING: Using custom scheduler, this is an experimental feature.""");
1445 return scheduler;
1446 } catch (Exception ex) {
1447 throw new Error(ex);
1448 }
1449 } else {
1450 return createDefaultForkJoinPoolScheduler();
1451 }
1452 }
1453
1454 /**
1455 * Creates the default ForkJoinPool scheduler.
1456 */
1457 private static ForkJoinPool createDefaultForkJoinPoolScheduler() {
1458 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1459 int parallelism, maxPoolSize, minRunnable;
1460 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1461 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1462 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1463 if (parallelismValue != null) {
1464 parallelism = Integer.parseInt(parallelismValue);
1465 } else {
1466 parallelism = Runtime.getRuntime().availableProcessors();
1467 }
1468 if (maxPoolSizeValue != null) {
1469 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1470 parallelism = Integer.min(parallelism, maxPoolSize);
1471 } else {
1472 maxPoolSize = Integer.max(parallelism, 256);
1473 }
1474 if (minRunnableValue != null) {
1475 minRunnable = Integer.parseInt(minRunnableValue);
1476 } else {
1477 minRunnable = Integer.max(parallelism / 2, 1);
1478 }
1479 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1480 boolean asyncMode = true; // FIFO
1481 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1482 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1483 }
1484
1485 /**
1486 * Schedule a runnable task to run after a delay.
1487 */
1488 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1489 if (scheduler instanceof ForkJoinPool pool) {
1490 return pool.schedule(command, delay, unit);
1491 } else {
1492 return DelayedTaskSchedulers.schedule(command, delay, unit);
1493 }
1494 }
1495
1496 /**
1497 * Supports scheduling a runnable task to run after a delay. It uses a number
1498 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1499 * work queue used. This class is used when using a custom scheduler.
1500 */
1501 private static class DelayedTaskSchedulers {
1502 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1503
1504 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1505 long tid = Thread.currentThread().threadId();
1506 int index = (int) tid & (INSTANCE.length - 1);
1507 return INSTANCE[index].schedule(command, delay, unit);
1508 }
1509
1510 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1511 String propName = "jdk.virtualThreadScheduler.timerQueues";
1512 String propValue = System.getProperty(propName);
1513 int queueCount;
1514 if (propValue != null) {
1515 queueCount = Integer.parseInt(propValue);
1516 if (queueCount != Integer.highestOneBit(queueCount)) {
1517 throw new RuntimeException("Value of " + propName + " must be power of 2");
1518 }
1519 } else {
1520 int ncpus = Runtime.getRuntime().availableProcessors();
1521 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1522 }
1523 var schedulers = new ScheduledExecutorService[queueCount];
1524 for (int i = 0; i < queueCount; i++) {
1525 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1526 Executors.newScheduledThreadPool(1, task -> {
1527 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1528 t.setDaemon(true);
1529 return t;
1530 });
1531 stpe.setRemoveOnCancelPolicy(true);
1532 schedulers[i] = stpe;
1533 }
1534 return schedulers;
1535 }
1536 }
1537
1538 /**
1539 * Schedule virtual threads that are ready to be scheduled after they blocked on
1540 * monitor enter.
1541 */
1542 private static void unblockVirtualThreads() {
1543 while (true) {
1544 VirtualThread vthread = takeVirtualThreadListToUnblock();
1545 while (vthread != null) {
1546 assert vthread.onWaitingList;
1547 VirtualThread nextThread = vthread.next;
1548
1549 // remove from list and unblock
1550 vthread.next = null;
1551 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1552 assert changed;
1553 vthread.unblock();
1554
1555 vthread = nextThread;
1556 }
1557 }
1558 }
1559
1560 /**
1561 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1562 * if necessary until a list of one or more threads becomes available.
1563 */
1564 private static native VirtualThread takeVirtualThreadListToUnblock();
1565
1566 static {
1567 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1568 VirtualThread::unblockVirtualThreads);
1569 unblocker.setDaemon(true);
1570 unblocker.start();
1571 }
1572 }