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.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinTask;
34 import java.util.concurrent.Future;
35 import java.util.concurrent.RejectedExecutionException;
36 import java.util.concurrent.ScheduledExecutorService;
37 import java.util.concurrent.ScheduledThreadPoolExecutor;
38 import java.util.concurrent.TimeUnit;
39 import jdk.internal.event.VirtualThreadEndEvent;
40 import jdk.internal.event.VirtualThreadStartEvent;
41 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
42 import jdk.internal.misc.CarrierThread;
43 import jdk.internal.misc.InnocuousThread;
44 import jdk.internal.misc.Unsafe;
45 import jdk.internal.vm.Continuation;
46 import jdk.internal.vm.ContinuationScope;
47 import jdk.internal.vm.StackableScope;
48 import jdk.internal.vm.ThreadContainer;
49 import jdk.internal.vm.ThreadContainers;
50 import jdk.internal.vm.annotation.ChangesCurrentThread;
51 import jdk.internal.vm.annotation.Hidden;
52 import jdk.internal.vm.annotation.IntrinsicCandidate;
53 import jdk.internal.vm.annotation.JvmtiHideEvents;
54 import jdk.internal.vm.annotation.JvmtiMountTransition;
55 import jdk.internal.vm.annotation.ReservedStackAccess;
56 import sun.nio.ch.Interruptible;
57 import static java.util.concurrent.TimeUnit.*;
58
59 /**
60 * A thread that is scheduled by the Java virtual machine rather than the operating system.
61 */
62 final class VirtualThread extends BaseVirtualThread {
63 private static final Unsafe U = Unsafe.getUnsafe();
64 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
65
66 private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
67 private static final boolean IS_CUSTOM_DEFAULT_SCHEDULER;
68 static {
69 // experimental
70 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
71 if (propValue != null) {
72 DEFAULT_SCHEDULER = createCustomDefaultScheduler(propValue);
73 IS_CUSTOM_DEFAULT_SCHEDULER = true;
74 } else {
75 DEFAULT_SCHEDULER = createDefaultForkJoinPoolScheduler();
76 IS_CUSTOM_DEFAULT_SCHEDULER = false;
77 }
78 }
79
80 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
81 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
82 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
83 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
84 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
85
86 // scheduler and continuation
87 private final VirtualThreadScheduler scheduler;
88 private final Continuation cont;
89 private final Runnable runContinuation;
90
91 // virtual thread state, accessed by VM
92 private volatile int state;
93
94 /*
95 * Virtual thread state transitions:
96 *
97 * NEW -> STARTED // Thread.start, schedule to run
98 * STARTED -> TERMINATED // failed to start
99 * STARTED -> RUNNING // first run
100 * RUNNING -> TERMINATED // done
101 *
102 * RUNNING -> PARKING // Thread parking with LockSupport.park
103 * PARKING -> PARKED // cont.yield successful, parked indefinitely
104 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
105 * PARKED -> UNPARKED // unparked, may be scheduled to continue
106 * PINNED -> RUNNING // unparked, continue execution on same carrier
107 * UNPARKED -> RUNNING // continue execution after park
108 *
109 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
110 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
111 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
112 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
113 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
114 *
115 * RUNNING -> BLOCKING // blocking on monitor enter
116 * BLOCKING -> BLOCKED // blocked on monitor enter
117 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
118 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
119 *
120 * RUNNING -> WAITING // transitional state during wait on monitor
121 * WAITING -> WAIT // waiting on monitor
122 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
123 * WAIT -> UNBLOCKED // timed-out/interrupted
124 *
125 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
126 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
127 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
128 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
129 *
130 * RUNNING -> YIELDING // Thread.yield
131 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
132 * YIELDING -> RUNNING // cont.yield failed
133 * YIELDED -> RUNNING // continue execution after Thread.yield
134 */
135 private static final int NEW = 0;
136 private static final int STARTED = 1;
137 private static final int RUNNING = 2; // runnable-mounted
138
139 // untimed and timed parking
140 private static final int PARKING = 3;
141 private static final int PARKED = 4; // unmounted
142 private static final int PINNED = 5; // mounted
143 private static final int TIMED_PARKING = 6;
144 private static final int TIMED_PARKED = 7; // unmounted
145 private static final int TIMED_PINNED = 8; // mounted
146 private static final int UNPARKED = 9; // unmounted but runnable
147
148 // Thread.yield
149 private static final int YIELDING = 10;
150 private static final int YIELDED = 11; // unmounted but runnable
151
152 // monitor enter
153 private static final int BLOCKING = 12;
154 private static final int BLOCKED = 13; // unmounted
155 private static final int UNBLOCKED = 14; // unmounted but runnable
156
157 // monitor wait/timed-wait
158 private static final int WAITING = 15;
159 private static final int WAIT = 16; // waiting in Object.wait
160 private static final int TIMED_WAITING = 17;
161 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
162
163 private static final int TERMINATED = 99; // final state
164
165 // can be suspended from scheduling when unmounted
166 private static final int SUSPENDED = 1 << 8;
167
168 // parking permit made available by LockSupport.unpark
169 private volatile boolean parkPermit;
170
171 // blocking permit made available by unblocker thread when another thread exits monitor
172 private volatile boolean blockPermit;
173
174 // true when on the list of virtual threads waiting to be unblocked
175 private volatile boolean onWaitingList;
176
177 // next virtual thread on the list of virtual threads waiting to be unblocked
178 private volatile VirtualThread next;
179
180 // notified by Object.notify/notifyAll while waiting in Object.wait
181 private volatile boolean notified;
182
183 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
184 private volatile boolean interruptableWait;
185
186 // timed-wait support
187 private byte timedWaitSeqNo;
188
189 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
190 private long timeout;
191
192 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
193 private Future<?> timeoutTask;
194
195 // carrier thread when mounted, accessed by VM
196 private volatile Thread carrierThread;
197
198 // termination object when joining, created lazily if needed
199 private volatile CountDownLatch termination;
200
201 /**
202 * Returns the default scheduler.
203 */
204 static VirtualThreadScheduler defaultScheduler() {
205 return DEFAULT_SCHEDULER;
206 }
207
208 /**
209 * Returns true if using a custom default scheduler.
210 */
211 static boolean isCustomDefaultScheduler() {
212 return IS_CUSTOM_DEFAULT_SCHEDULER;
213 }
214
215 /**
216 * Returns the continuation scope used for virtual threads.
217 */
218 static ContinuationScope continuationScope() {
219 return VTHREAD_SCOPE;
220 }
221
222 /**
223 * Return the scheduler for this thread.
224 * @param revealBuiltin true to reveal the built-in default scheduler, false to hide
225 */
226 VirtualThreadScheduler scheduler(boolean revealBuiltin) {
227 if (scheduler instanceof BuiltinDefaultScheduler builtin && !revealBuiltin) {
228 return builtin.externalView();
229 } else {
230 return scheduler;
231 }
232 }
233
234 /**
235 * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
236 *
237 * @param scheduler the scheduler or null for default scheduler
238 * @param name thread name
239 * @param characteristics characteristics
240 * @param task the task to execute
241 */
242 VirtualThread(VirtualThreadScheduler scheduler,
243 String name,
244 int characteristics,
245 Runnable task) {
246 super(name, characteristics, /*bound*/ false);
247 Objects.requireNonNull(task);
248
249 // use default scheduler if not provided
250 if (scheduler == null) {
251 scheduler = DEFAULT_SCHEDULER;
252 }
253
254 this.scheduler = scheduler;
255 this.cont = new VThreadContinuation(this, task);
256 this.runContinuation = this::runContinuation;
257 }
258
259 /**
260 * The continuation that a virtual thread executes.
261 */
262 private static class VThreadContinuation extends Continuation {
263 VThreadContinuation(VirtualThread vthread, Runnable task) {
264 super(VTHREAD_SCOPE, wrap(vthread, task));
265 }
266 @Override
267 protected void onPinned(Continuation.Pinned reason) {
268 }
269 private static Runnable wrap(VirtualThread vthread, Runnable task) {
270 return new Runnable() {
271 @Hidden
272 @JvmtiHideEvents
273 public void run() {
274 vthread.notifyJvmtiStart(); // notify JVMTI
275 try {
276 vthread.run(task);
277 } finally {
278 vthread.notifyJvmtiEnd(); // notify JVMTI
279 }
280 }
281 };
282 }
283 }
284
285 /**
286 * Runs or continues execution on the current thread. The virtual thread is mounted
287 * on the current thread before the task runs or continues. It unmounts when the
288 * task completes or yields.
289 */
290 @ChangesCurrentThread // allow mount/unmount to be inlined
291 private void runContinuation() {
292 // the carrier must be a platform thread
293 if (Thread.currentThread().isVirtual()) {
294 throw new WrongThreadException();
295 }
296
297 // set state to RUNNING
298 int initialState = state();
299 if (initialState == STARTED || initialState == UNPARKED
300 || initialState == UNBLOCKED || initialState == YIELDED) {
301 // newly started or continue after parking/blocking/Thread.yield
302 if (!compareAndSetState(initialState, RUNNING)) {
303 return;
304 }
305 // consume permit when continuing after parking or blocking. If continue
306 // after a timed-park or timed-wait then the timeout task is cancelled.
307 if (initialState == UNPARKED) {
308 cancelTimeoutTask();
309 setParkPermit(false);
310 } else if (initialState == UNBLOCKED) {
311 cancelTimeoutTask();
312 blockPermit = false;
313 }
314 } else {
315 // not runnable
316 return;
317 }
318
319 mount();
320 try {
321 cont.run();
322 } finally {
323 unmount();
324 if (cont.isDone()) {
325 afterDone();
326 } else {
327 afterYield();
328 }
329 }
330 }
331
332 /**
333 * Cancel timeout task when continuing after timed-park or timed-wait.
334 * The timeout task may be executing, or may have already completed.
335 */
336 private void cancelTimeoutTask() {
337 if (timeoutTask != null) {
338 timeoutTask.cancel(false);
339 timeoutTask = null;
340 }
341 }
342
343 /**
344 * Submits the runContinuation task to the scheduler. For the default scheduler,
345 * and calling it on a worker thread, the task will be pushed to the local queue,
346 * otherwise it will be pushed to an external submission queue.
347 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
348 * @throws RejectedExecutionException
349 */
350 private void submitRunContinuation(boolean retryOnOOME) {
351 boolean done = false;
352 while (!done) {
353 try {
354 // Pin the continuation to prevent the virtual thread from unmounting
355 // when submitting a task. For the default scheduler this ensures that
356 // the carrier doesn't change when pushing a task. For other schedulers
357 // it avoids deadlock that could arise due to carriers and virtual
358 // threads contending for a lock.
359 if (currentThread().isVirtual()) {
360 Continuation.pin();
361 try {
362 scheduler.execute(this, runContinuation);
363 } finally {
364 Continuation.unpin();
365 }
366 } else {
367 scheduler.execute(this, runContinuation);
368 }
369 done = true;
370 } catch (RejectedExecutionException ree) {
371 submitFailed(ree);
372 throw ree;
373 } catch (OutOfMemoryError e) {
374 if (retryOnOOME) {
375 U.park(false, 100_000_000); // 100ms
376 } else {
377 throw e;
378 }
379 }
380 }
381 }
382
383 /**
384 * Submits the runContinuation task to the scheduler. For the default scheduler,
385 * and calling it on a worker thread, the task will be pushed to the local queue,
386 * otherwise it will be pushed to an external submission queue.
387 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
388 * @throws RejectedExecutionException
389 */
390 private void submitRunContinuation() {
391 submitRunContinuation(true);
392 }
393
394 /**
395 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
396 * queue is empty. If not empty, or invoked by another thread, then this method works
397 * like submitRunContinuation and just submits the task to the scheduler.
398 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
399 * @throws RejectedExecutionException
400 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
401 */
402 private void lazySubmitRunContinuation() {
403 if (scheduler == DEFAULT_SCHEDULER
404 && currentCarrierThread() instanceof CarrierThread ct
405 && ct.getQueuedTaskCount() == 0) {
406 try {
407 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
408 } catch (RejectedExecutionException ree) {
409 submitFailed(ree);
410 throw ree;
411 } catch (OutOfMemoryError e) {
412 submitRunContinuation();
413 }
414 } else {
415 submitRunContinuation();
416 }
417 }
418
419 /**
420 * Submits the runContinuation task to the scheduler. For the default scheduler, and
421 * calling it a virtual thread that uses the default scheduler, the task will be
422 * pushed to an external submission queue.
423 * @throws RejectedExecutionException
424 */
425 private void externalSubmitRunContinuation() {
426 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
427 try {
428 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
429 } catch (RejectedExecutionException ree) {
430 submitFailed(ree);
431 throw ree;
432 } catch (OutOfMemoryError e) {
433 submitRunContinuation();
434 }
435 } else {
436 submitRunContinuation();
437 }
438 }
439
440 /**
441 * Submits the runContinuation task to the scheduler. For the default scheduler, and
442 * calling it a virtual thread that uses the default scheduler, the task will be
443 * pushed to an external submission queue. This method may throw OutOfMemoryError.
444 * @throws RejectedExecutionException
445 * @throws OutOfMemoryError
446 */
447 private void externalSubmitRunContinuationOrThrow() {
448 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
449 try {
450 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
451 } catch (RejectedExecutionException ree) {
452 submitFailed(ree);
453 throw ree;
454 }
455 } else {
456 submitRunContinuation(false);
457 }
458 }
459
460 /**
461 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
462 */
463 private void submitFailed(RejectedExecutionException ree) {
464 var event = new VirtualThreadSubmitFailedEvent();
465 if (event.isEnabled()) {
466 event.javaThreadId = threadId();
467 event.exceptionMessage = ree.getMessage();
468 event.commit();
469 }
470 }
471
472 /**
473 * Runs a task in the context of this virtual thread.
474 */
475 private void run(Runnable task) {
476 assert Thread.currentThread() == this && state == RUNNING;
477
478 // emit JFR event if enabled
479 if (VirtualThreadStartEvent.isTurnedOn()) {
480 var event = new VirtualThreadStartEvent();
481 event.javaThreadId = threadId();
482 event.commit();
483 }
484
485 Object bindings = Thread.scopedValueBindings();
486 try {
487 runWith(bindings, task);
488 } catch (Throwable exc) {
489 dispatchUncaughtException(exc);
490 } finally {
491 // pop any remaining scopes from the stack, this may block
492 StackableScope.popAll();
493
494 // emit JFR event if enabled
495 if (VirtualThreadEndEvent.isTurnedOn()) {
496 var event = new VirtualThreadEndEvent();
497 event.javaThreadId = threadId();
498 event.commit();
499 }
500 }
501 }
502
503 /**
504 * Mounts this virtual thread onto the current platform thread. On
505 * return, the current thread is the virtual thread.
506 */
507 @ChangesCurrentThread
508 @ReservedStackAccess
509 private void mount() {
510 // notify JVMTI before mount
511 notifyJvmtiMount(/*hide*/true);
512
513 // sets the carrier thread
514 Thread carrier = Thread.currentCarrierThread();
515 setCarrierThread(carrier);
516
517 // sync up carrier thread interrupt status if needed
518 if (interrupted) {
519 carrier.setInterrupt();
520 } else if (carrier.isInterrupted()) {
521 synchronized (interruptLock) {
522 // need to recheck interrupt status
523 if (!interrupted) {
524 carrier.clearInterrupt();
525 }
526 }
527 }
528
529 // set Thread.currentThread() to return this virtual thread
530 carrier.setCurrentThread(this);
531 }
532
533 /**
534 * Unmounts this virtual thread from the carrier. On return, the
535 * current thread is the current platform thread.
536 */
537 @ChangesCurrentThread
538 @ReservedStackAccess
539 private void unmount() {
540 assert !Thread.holdsLock(interruptLock);
541
542 // set Thread.currentThread() to return the platform thread
543 Thread carrier = this.carrierThread;
544 carrier.setCurrentThread(carrier);
545
546 // break connection to carrier thread, synchronized with interrupt
547 synchronized (interruptLock) {
548 setCarrierThread(null);
549 }
550 carrier.clearInterrupt();
551
552 // notify JVMTI after unmount
553 notifyJvmtiUnmount(/*hide*/false);
554 }
555
556 /**
557 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
558 * the continuation continues.
559 */
560 @Hidden
561 private boolean yieldContinuation() {
562 notifyJvmtiUnmount(/*hide*/true);
563 try {
564 return Continuation.yield(VTHREAD_SCOPE);
565 } finally {
566 notifyJvmtiMount(/*hide*/false);
567 }
568 }
569
570 /**
571 * Invoked in the context of the carrier thread after the Continuation yields when
572 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
573 */
574 private void afterYield() {
575 assert carrierThread == null;
576
577 // re-adjust parallelism if the virtual thread yielded when compensating
578 if (currentThread() instanceof CarrierThread ct) {
579 ct.endBlocking();
580 }
581
582 int s = state();
583
584 // LockSupport.park/parkNanos
585 if (s == PARKING || s == TIMED_PARKING) {
586 int newState;
587 if (s == PARKING) {
588 setState(newState = PARKED);
589 } else {
590 // schedule unpark
591 long timeout = this.timeout;
592 assert timeout > 0;
593 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
594 setState(newState = TIMED_PARKED);
595 }
596
597 // may have been unparked while parking
598 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
599 // lazy submit if local queue is empty
600 lazySubmitRunContinuation();
601 }
602 return;
603 }
604
605 // Thread.yield
606 if (s == YIELDING) {
607 setState(YIELDED);
608
609 // external submit if there are no tasks in the local task queue
610 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
611 externalSubmitRunContinuation();
612 } else {
613 submitRunContinuation();
614 }
615 return;
616 }
617
618 // blocking on monitorenter
619 if (s == BLOCKING) {
620 setState(BLOCKED);
621
622 // may have been unblocked while blocking
623 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
624 // lazy submit if local queue is empty
625 lazySubmitRunContinuation();
626 }
627 return;
628 }
629
630 // Object.wait
631 if (s == WAITING || s == TIMED_WAITING) {
632 int newState;
633 boolean interruptable = interruptableWait;
634 if (s == WAITING) {
635 setState(newState = WAIT);
636 } else {
637 // For timed-wait, a timeout task is scheduled to execute. The timeout
638 // task will change the thread state to UNBLOCKED and submit the thread
639 // to the scheduler. A sequence number is used to ensure that the timeout
640 // task only unblocks the thread for this timed-wait. We synchronize with
641 // the timeout task to coordinate access to the sequence number and to
642 // ensure the timeout task doesn't execute until the thread has got to
643 // the TIMED_WAIT state.
644 long timeout = this.timeout;
645 assert timeout > 0;
646 synchronized (timedWaitLock()) {
647 byte seqNo = ++timedWaitSeqNo;
648 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
649 setState(newState = TIMED_WAIT);
650 }
651 }
652
653 // may have been notified while in transition to wait state
654 if (notified && compareAndSetState(newState, BLOCKED)) {
655 // may have even been unblocked already
656 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
657 submitRunContinuation();
658 }
659 return;
660 }
661
662 // may have been interrupted while in transition to wait state
663 if (interruptable && interrupted && compareAndSetState(newState, UNBLOCKED)) {
664 submitRunContinuation();
665 return;
666 }
667 return;
668 }
669
670 assert false;
671 }
672
673 /**
674 * Invoked after the continuation completes.
675 */
676 private void afterDone() {
677 afterDone(true);
678 }
679
680 /**
681 * Invoked after the continuation completes (or start failed). Sets the thread
682 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
683 *
684 * @param notifyContainer true if its container should be notified
685 */
686 private void afterDone(boolean notifyContainer) {
687 assert carrierThread == null;
688 setState(TERMINATED);
689
690 // notify anyone waiting for this virtual thread to terminate
691 CountDownLatch termination = this.termination;
692 if (termination != null) {
693 assert termination.getCount() == 1;
694 termination.countDown();
695 }
696
697 // notify container
698 if (notifyContainer) {
699 threadContainer().remove(this);
700 }
701
702 // clear references to thread locals
703 clearReferences();
704 }
705
706 /**
707 * Schedules this {@code VirtualThread} to execute.
708 *
709 * @throws IllegalStateException if the container is shutdown or closed
710 * @throws IllegalThreadStateException if the thread has already been started
711 * @throws RejectedExecutionException if the scheduler cannot accept a task
712 */
713 @Override
714 void start(ThreadContainer container) {
715 if (!compareAndSetState(NEW, STARTED)) {
716 throw new IllegalThreadStateException("Already started");
717 }
718
719 // bind thread to container
720 assert threadContainer() == null;
721 setThreadContainer(container);
722
723 // start thread
724 boolean addedToContainer = false;
725 boolean started = false;
726 try {
727 container.add(this); // may throw
728 addedToContainer = true;
729
730 // scoped values may be inherited
731 inheritScopedValueBindings(container);
732
733 // submit task to run thread, using externalSubmit if possible
734 externalSubmitRunContinuationOrThrow();
735 started = true;
736 } finally {
737 if (!started) {
738 afterDone(addedToContainer);
739 }
740 }
741 }
742
743 @Override
744 public void start() {
745 start(ThreadContainers.root());
746 }
747
748 @Override
749 public void run() {
750 // do nothing
751 }
752
753 /**
754 * Parks until unparked or interrupted. If already unparked then the parking
755 * permit is consumed and this method completes immediately (meaning it doesn't
756 * yield). It also completes immediately if the interrupt status is set.
757 */
758 @Override
759 void park() {
760 assert Thread.currentThread() == this;
761
762 // complete immediately if parking permit available or interrupted
763 if (getAndSetParkPermit(false) || interrupted)
764 return;
765
766 // park the thread
767 boolean yielded = false;
768 setState(PARKING);
769 try {
770 yielded = yieldContinuation();
771 } catch (OutOfMemoryError e) {
772 // park on carrier
773 } finally {
774 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
775 if (!yielded) {
776 assert state() == PARKING;
777 setState(RUNNING);
778 }
779 }
780
781 // park on the carrier thread when pinned
782 if (!yielded) {
783 parkOnCarrierThread(false, 0);
784 }
785 }
786
787 /**
788 * Parks up to the given waiting time or until unparked or interrupted.
789 * If already unparked then the parking permit is consumed and this method
790 * completes immediately (meaning it doesn't yield). It also completes immediately
791 * if the interrupt status is set or the waiting time is {@code <= 0}.
792 *
793 * @param nanos the maximum number of nanoseconds to wait.
794 */
795 @Override
796 void parkNanos(long nanos) {
797 assert Thread.currentThread() == this;
798
799 // complete immediately if parking permit available or interrupted
800 if (getAndSetParkPermit(false) || interrupted)
801 return;
802
803 // park the thread for the waiting time
804 if (nanos > 0) {
805 long startTime = System.nanoTime();
806
807 // park the thread, afterYield will schedule the thread to unpark
808 boolean yielded = false;
809 timeout = nanos;
810 setState(TIMED_PARKING);
811 try {
812 yielded = yieldContinuation();
813 } catch (OutOfMemoryError e) {
814 // park on carrier
815 } finally {
816 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
817 if (!yielded) {
818 assert state() == TIMED_PARKING;
819 setState(RUNNING);
820 }
821 }
822
823 // park on carrier thread for remaining time when pinned (or OOME)
824 if (!yielded) {
825 long remainingNanos = nanos - (System.nanoTime() - startTime);
826 parkOnCarrierThread(true, remainingNanos);
827 }
828 }
829 }
830
831 /**
832 * Parks the current carrier thread up to the given waiting time or until
833 * unparked or interrupted. If the virtual thread is interrupted then the
834 * interrupt status will be propagated to the carrier thread.
835 * @param timed true for a timed park, false for untimed
836 * @param nanos the waiting time in nanoseconds
837 */
838 private void parkOnCarrierThread(boolean timed, long nanos) {
839 assert state() == RUNNING;
840
841 setState(timed ? TIMED_PINNED : PINNED);
842 try {
843 if (!parkPermit) {
844 if (!timed) {
845 U.park(false, 0);
846 } else if (nanos > 0) {
847 U.park(false, nanos);
848 }
849 }
850 } finally {
851 setState(RUNNING);
852 }
853
854 // consume parking permit
855 setParkPermit(false);
856
857 // JFR jdk.VirtualThreadPinned event
858 postPinnedEvent("LockSupport.park");
859 }
860
861 /**
862 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
863 * Recording the event in the VM avoids having JFR event recorded in Java
864 * with the same name, but different ID, to events recorded by the VM.
865 */
866 @Hidden
867 private static native void postPinnedEvent(String op);
868
869 /**
870 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
871 * then its task is scheduled to continue, otherwise its next call to {@code park} or
872 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
873 * @throws RejectedExecutionException if the scheduler cannot accept a task
874 */
875 @Override
876 void unpark() {
877 if (!getAndSetParkPermit(true) && currentThread() != this) {
878 int s = state();
879
880 // unparked while parked
881 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
882 submitRunContinuation();
883 return;
884 }
885
886 // unparked while parked when pinned
887 if (s == PINNED || s == TIMED_PINNED) {
888 // unpark carrier thread when pinned
889 disableSuspendAndPreempt();
890 try {
891 synchronized (carrierThreadAccessLock()) {
892 Thread carrier = carrierThread;
893 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
894 U.unpark(carrier);
895 }
896 }
897 } finally {
898 enableSuspendAndPreempt();
899 }
900 return;
901 }
902 }
903 }
904
905 /**
906 * Invoked by unblocker thread to unblock this virtual thread.
907 */
908 private void unblock() {
909 assert !Thread.currentThread().isVirtual();
910 blockPermit = true;
911 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
912 submitRunContinuation();
913 }
914 }
915
916 /**
917 * Invoked by FJP worker thread or STPE thread when park timeout expires.
918 */
919 private void parkTimeoutExpired() {
920 assert !VirtualThread.currentThread().isVirtual();
921 if (!getAndSetParkPermit(true)
922 && (state() == TIMED_PARKED)
923 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
924 lazySubmitRunContinuation();
925 }
926 }
927
928 /**
929 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
930 * If the virtual thread is in timed-wait then this method will unblock the thread
931 * and submit its task so that it continues and attempts to reenter the monitor.
932 * This method does nothing if the thread has been woken by notify or interrupt.
933 */
934 private void waitTimeoutExpired(byte seqNo) {
935 assert !Thread.currentThread().isVirtual();
936 for (;;) {
937 boolean unblocked = false;
938 synchronized (timedWaitLock()) {
939 if (seqNo != timedWaitSeqNo) {
940 // this timeout task is for a past timed-wait
941 return;
942 }
943 int s = state();
944 if (s == TIMED_WAIT) {
945 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
946 } else if (s != (TIMED_WAIT | SUSPENDED)) {
947 // notified or interrupted, no longer waiting
948 return;
949 }
950 }
951 if (unblocked) {
952 lazySubmitRunContinuation();
953 return;
954 }
955 // need to retry when thread is suspended in time-wait
956 Thread.yield();
957 }
958 }
959
960 /**
961 * Attempts to yield the current virtual thread (Thread.yield).
962 */
963 void tryYield() {
964 assert Thread.currentThread() == this;
965 setState(YIELDING);
966 boolean yielded = false;
967 try {
968 yielded = yieldContinuation(); // may throw
969 } finally {
970 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
971 if (!yielded) {
972 assert state() == YIELDING;
973 setState(RUNNING);
974 }
975 }
976 }
977
978 /**
979 * Sleep the current thread for the given sleep time (in nanoseconds). If
980 * nanos is 0 then the thread will attempt to yield.
981 *
982 * @implNote This implementation parks the thread for the given sleeping time
983 * and will therefore be observed in PARKED state during the sleep. Parking
984 * will consume the parking permit so this method makes available the parking
985 * permit after the sleep. This may be observed as a spurious, but benign,
986 * wakeup when the thread subsequently attempts to park.
987 *
988 * @param nanos the maximum number of nanoseconds to sleep
989 * @throws InterruptedException if interrupted while sleeping
990 */
991 void sleepNanos(long nanos) throws InterruptedException {
992 assert Thread.currentThread() == this && nanos >= 0;
993 if (getAndClearInterrupt())
994 throw new InterruptedException();
995 if (nanos == 0) {
996 tryYield();
997 } else {
998 // park for the sleep time
999 try {
1000 long remainingNanos = nanos;
1001 long startNanos = System.nanoTime();
1002 while (remainingNanos > 0) {
1003 parkNanos(remainingNanos);
1004 if (getAndClearInterrupt()) {
1005 throw new InterruptedException();
1006 }
1007 remainingNanos = nanos - (System.nanoTime() - startNanos);
1008 }
1009 } finally {
1010 // may have been unparked while sleeping
1011 setParkPermit(true);
1012 }
1013 }
1014 }
1015
1016 /**
1017 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1018 * A timeout of {@code 0} means to wait forever.
1019 *
1020 * @throws InterruptedException if interrupted while waiting
1021 * @return true if the thread has terminated
1022 */
1023 boolean joinNanos(long nanos) throws InterruptedException {
1024 if (state() == TERMINATED)
1025 return true;
1026
1027 // ensure termination object exists, then re-check state
1028 CountDownLatch termination = getTermination();
1029 if (state() == TERMINATED)
1030 return true;
1031
1032 // wait for virtual thread to terminate
1033 if (nanos == 0) {
1034 termination.await();
1035 } else {
1036 boolean terminated = termination.await(nanos, NANOSECONDS);
1037 if (!terminated) {
1038 // waiting time elapsed
1039 return false;
1040 }
1041 }
1042 assert state() == TERMINATED;
1043 return true;
1044 }
1045
1046 @Override
1047 void blockedOn(Interruptible b) {
1048 disableSuspendAndPreempt();
1049 try {
1050 super.blockedOn(b);
1051 } finally {
1052 enableSuspendAndPreempt();
1053 }
1054 }
1055
1056 @Override
1057 public void interrupt() {
1058 if (Thread.currentThread() != this) {
1059 // if current thread is a virtual thread then prevent it from being
1060 // suspended or unmounted when entering or holding interruptLock
1061 Interruptible blocker;
1062 disableSuspendAndPreempt();
1063 try {
1064 synchronized (interruptLock) {
1065 interrupted = true;
1066 blocker = nioBlocker();
1067 if (blocker != null) {
1068 blocker.interrupt(this);
1069 }
1070
1071 // interrupt carrier thread if mounted
1072 Thread carrier = carrierThread;
1073 if (carrier != null) carrier.setInterrupt();
1074 }
1075 } finally {
1076 enableSuspendAndPreempt();
1077 }
1078
1079 // notify blocker after releasing interruptLock
1080 if (blocker != null) {
1081 blocker.postInterrupt();
1082 }
1083
1084 // make available parking permit, unpark thread if parked
1085 unpark();
1086
1087 // if thread is waiting in Object.wait then schedule to try to reenter
1088 int s = state();
1089 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1090 submitRunContinuation();
1091 }
1092
1093 } else {
1094 interrupted = true;
1095 carrierThread.setInterrupt();
1096 setParkPermit(true);
1097 }
1098 }
1099
1100 @Override
1101 public boolean isInterrupted() {
1102 return interrupted;
1103 }
1104
1105 @Override
1106 boolean getAndClearInterrupt() {
1107 assert Thread.currentThread() == this;
1108 boolean oldValue = interrupted;
1109 if (oldValue) {
1110 disableSuspendAndPreempt();
1111 try {
1112 synchronized (interruptLock) {
1113 interrupted = false;
1114 carrierThread.clearInterrupt();
1115 }
1116 } finally {
1117 enableSuspendAndPreempt();
1118 }
1119 }
1120 return oldValue;
1121 }
1122
1123 @Override
1124 Thread.State threadState() {
1125 int s = state();
1126 switch (s & ~SUSPENDED) {
1127 case NEW:
1128 return Thread.State.NEW;
1129 case STARTED:
1130 // return NEW if thread container not yet set
1131 if (threadContainer() == null) {
1132 return Thread.State.NEW;
1133 } else {
1134 return Thread.State.RUNNABLE;
1135 }
1136 case UNPARKED:
1137 case UNBLOCKED:
1138 case YIELDED:
1139 // runnable, not mounted
1140 return Thread.State.RUNNABLE;
1141 case RUNNING:
1142 // if mounted then return state of carrier thread
1143 if (Thread.currentThread() != this) {
1144 disableSuspendAndPreempt();
1145 try {
1146 synchronized (carrierThreadAccessLock()) {
1147 Thread carrierThread = this.carrierThread;
1148 if (carrierThread != null) {
1149 return carrierThread.threadState();
1150 }
1151 }
1152 } finally {
1153 enableSuspendAndPreempt();
1154 }
1155 }
1156 // runnable, mounted
1157 return Thread.State.RUNNABLE;
1158 case PARKING:
1159 case TIMED_PARKING:
1160 case WAITING:
1161 case TIMED_WAITING:
1162 case YIELDING:
1163 // runnable, in transition
1164 return Thread.State.RUNNABLE;
1165 case PARKED:
1166 case PINNED:
1167 case WAIT:
1168 return Thread.State.WAITING;
1169 case TIMED_PARKED:
1170 case TIMED_PINNED:
1171 case TIMED_WAIT:
1172 return Thread.State.TIMED_WAITING;
1173 case BLOCKING:
1174 case BLOCKED:
1175 return Thread.State.BLOCKED;
1176 case TERMINATED:
1177 return Thread.State.TERMINATED;
1178 default:
1179 throw new InternalError();
1180 }
1181 }
1182
1183 @Override
1184 boolean alive() {
1185 int s = state;
1186 return (s != NEW && s != TERMINATED);
1187 }
1188
1189 @Override
1190 boolean isTerminated() {
1191 return (state == TERMINATED);
1192 }
1193
1194 @Override
1195 StackTraceElement[] asyncGetStackTrace() {
1196 StackTraceElement[] stackTrace;
1197 do {
1198 stackTrace = (carrierThread != null)
1199 ? super.asyncGetStackTrace() // mounted
1200 : tryGetStackTrace(); // unmounted
1201 if (stackTrace == null) {
1202 Thread.yield();
1203 }
1204 } while (stackTrace == null);
1205 return stackTrace;
1206 }
1207
1208 /**
1209 * Returns the stack trace for this virtual thread if it is unmounted.
1210 * Returns null if the thread is mounted or in transition.
1211 */
1212 private StackTraceElement[] tryGetStackTrace() {
1213 int initialState = state() & ~SUSPENDED;
1214 switch (initialState) {
1215 case NEW, STARTED, TERMINATED -> {
1216 return new StackTraceElement[0]; // unmounted, empty stack
1217 }
1218 case RUNNING, PINNED, TIMED_PINNED -> {
1219 return null; // mounted
1220 }
1221 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1222 // unmounted, not runnable
1223 }
1224 case UNPARKED, UNBLOCKED, YIELDED -> {
1225 // unmounted, runnable
1226 }
1227 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1228 return null; // in transition
1229 }
1230 default -> throw new InternalError("" + initialState);
1231 }
1232
1233 // thread is unmounted, prevent it from continuing
1234 int suspendedState = initialState | SUSPENDED;
1235 if (!compareAndSetState(initialState, suspendedState)) {
1236 return null;
1237 }
1238
1239 // get stack trace and restore state
1240 StackTraceElement[] stack;
1241 try {
1242 stack = cont.getStackTrace();
1243 } finally {
1244 assert state == suspendedState;
1245 setState(initialState);
1246 }
1247 boolean resubmit = switch (initialState) {
1248 case UNPARKED, UNBLOCKED, YIELDED -> {
1249 // resubmit as task may have run while suspended
1250 yield true;
1251 }
1252 case PARKED, TIMED_PARKED -> {
1253 // resubmit if unparked while suspended
1254 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1255 }
1256 case BLOCKED -> {
1257 // resubmit if unblocked while suspended
1258 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1259 }
1260 case WAIT, TIMED_WAIT -> {
1261 // resubmit if notified or interrupted while waiting (Object.wait)
1262 // waitTimeoutExpired will retry if the timed expired when suspended
1263 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1264 }
1265 default -> throw new InternalError();
1266 };
1267 if (resubmit) {
1268 submitRunContinuation();
1269 }
1270 return stack;
1271 }
1272
1273 @Override
1274 public String toString() {
1275 StringBuilder sb = new StringBuilder("VirtualThread[#");
1276 sb.append(threadId());
1277 String name = getName();
1278 if (!name.isEmpty()) {
1279 sb.append(",");
1280 sb.append(name);
1281 }
1282 sb.append("]/");
1283
1284 // add the carrier state and thread name when mounted
1285 boolean mounted;
1286 if (Thread.currentThread() == this) {
1287 mounted = appendCarrierInfo(sb);
1288 } else {
1289 disableSuspendAndPreempt();
1290 try {
1291 synchronized (carrierThreadAccessLock()) {
1292 mounted = appendCarrierInfo(sb);
1293 }
1294 } finally {
1295 enableSuspendAndPreempt();
1296 }
1297 }
1298
1299 // add virtual thread state when not mounted
1300 if (!mounted) {
1301 String stateAsString = threadState().toString();
1302 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1303 }
1304
1305 return sb.toString();
1306 }
1307
1308 /**
1309 * Appends the carrier state and thread name to the string buffer if mounted.
1310 * @return true if mounted, false if not mounted
1311 */
1312 private boolean appendCarrierInfo(StringBuilder sb) {
1313 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1314 Thread carrier = carrierThread;
1315 if (carrier != null) {
1316 String stateAsString = carrier.threadState().toString();
1317 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1318 sb.append('@');
1319 sb.append(carrier.getName());
1320 return true;
1321 } else {
1322 return false;
1323 }
1324 }
1325
1326 @Override
1327 public int hashCode() {
1328 return (int) threadId();
1329 }
1330
1331 @Override
1332 public boolean equals(Object obj) {
1333 return obj == this;
1334 }
1335
1336 /**
1337 * Returns the termination object, creating it if needed.
1338 */
1339 private CountDownLatch getTermination() {
1340 CountDownLatch termination = this.termination;
1341 if (termination == null) {
1342 termination = new CountDownLatch(1);
1343 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1344 termination = this.termination;
1345 }
1346 }
1347 return termination;
1348 }
1349
1350 /**
1351 * Returns the lock object to synchronize on when accessing carrierThread.
1352 * The lock prevents carrierThread from being reset to null during unmount.
1353 */
1354 private Object carrierThreadAccessLock() {
1355 // return interruptLock as unmount has to coordinate with interrupt
1356 return interruptLock;
1357 }
1358
1359 /**
1360 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1361 */
1362 private Object timedWaitLock() {
1363 // use this object for now to avoid the overhead of introducing another lock
1364 return runContinuation;
1365 }
1366
1367 /**
1368 * Disallow the current thread be suspended or preempted.
1369 */
1370 private void disableSuspendAndPreempt() {
1371 notifyJvmtiDisableSuspend(true);
1372 Continuation.pin();
1373 }
1374
1375 /**
1376 * Allow the current thread be suspended or preempted.
1377 */
1378 private void enableSuspendAndPreempt() {
1379 Continuation.unpin();
1380 notifyJvmtiDisableSuspend(false);
1381 }
1382
1383 // -- wrappers for get/set of state, parking permit, and carrier thread --
1384
1385 private int state() {
1386 return state; // volatile read
1387 }
1388
1389 private void setState(int newValue) {
1390 state = newValue; // volatile write
1391 }
1392
1393 private boolean compareAndSetState(int expectedValue, int newValue) {
1394 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1395 }
1396
1397 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1398 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1399 }
1400
1401 private void setParkPermit(boolean newValue) {
1402 if (parkPermit != newValue) {
1403 parkPermit = newValue;
1404 }
1405 }
1406
1407 private boolean getAndSetParkPermit(boolean newValue) {
1408 if (parkPermit != newValue) {
1409 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1410 } else {
1411 return newValue;
1412 }
1413 }
1414
1415 private void setCarrierThread(Thread carrier) {
1416 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1417 this.carrierThread = carrier;
1418 }
1419
1420 // -- JVM TI support --
1421
1422 @IntrinsicCandidate
1423 @JvmtiMountTransition
1424 private native void notifyJvmtiStart();
1425
1426 @IntrinsicCandidate
1427 @JvmtiMountTransition
1428 private native void notifyJvmtiEnd();
1429
1430 @IntrinsicCandidate
1431 @JvmtiMountTransition
1432 private native void notifyJvmtiMount(boolean hide);
1433
1434 @IntrinsicCandidate
1435 @JvmtiMountTransition
1436 private native void notifyJvmtiUnmount(boolean hide);
1437
1438 @IntrinsicCandidate
1439 private static native void notifyJvmtiDisableSuspend(boolean enter);
1440
1441 private static native void registerNatives();
1442 static {
1443 registerNatives();
1444
1445 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1446 var group = Thread.virtualThreadGroup();
1447 }
1448
1449 /**
1450 * Loads a VirtualThreadScheduler with the given class name to use at the
1451 * default scheduler. The class is public in an exported package, has a public
1452 * one-arg or no-arg constructor, and is visible to the system class loader.
1453 */
1454 private static VirtualThreadScheduler createCustomDefaultScheduler(String cn) {
1455 try {
1456 Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1457 VirtualThreadScheduler scheduler;
1458 try {
1459 // 1-arg constructor
1460 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1461 var builtin = createDefaultForkJoinPoolScheduler();
1462 scheduler = (VirtualThreadScheduler) ctor.newInstance(builtin.externalView());
1463 } catch (NoSuchMethodException e) {
1464 // 0-arg constructor
1465 Constructor<?> ctor = clazz.getConstructor();
1466 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1467 }
1468 System.err.println("""
1469 WARNING: Using custom default scheduler, this is an experimental feature!""");
1470 return scheduler;
1471 } catch (Exception ex) {
1472 throw new Error(ex);
1473 }
1474 }
1475
1476 /**
1477 * Creates the built-in default ForkJoinPool scheduler.
1478 */
1479 private static BuiltinDefaultScheduler createDefaultForkJoinPoolScheduler() {
1480 int parallelism, maxPoolSize, minRunnable;
1481 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1482 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1483 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1484 if (parallelismValue != null) {
1485 parallelism = Integer.parseInt(parallelismValue);
1486 } else {
1487 parallelism = Runtime.getRuntime().availableProcessors();
1488 }
1489 if (maxPoolSizeValue != null) {
1490 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1491 parallelism = Integer.min(parallelism, maxPoolSize);
1492 } else {
1493 maxPoolSize = Integer.max(parallelism, 256);
1494 }
1495 if (minRunnableValue != null) {
1496 minRunnable = Integer.parseInt(minRunnableValue);
1497 } else {
1498 minRunnable = Integer.max(parallelism / 2, 1);
1499 }
1500 return new BuiltinDefaultScheduler(parallelism, maxPoolSize, minRunnable);
1501 }
1502
1503 /**
1504 * The built-in default ForkJoinPool scheduler.
1505 */
1506 private static class BuiltinDefaultScheduler
1507 extends ForkJoinPool implements VirtualThreadScheduler {
1508
1509 private static final StableValue<VirtualThreadScheduler> VIEW = StableValue.of();
1510
1511 BuiltinDefaultScheduler(int parallelism, int maxPoolSize, int minRunnable) {
1512 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1513 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1514 boolean asyncMode = true; // FIFO
1515 super(parallelism, factory, handler, asyncMode,
1516 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1517 }
1518
1519 @Override
1520 public void execute(Thread vthread, Runnable task) {
1521 execute(ForkJoinTask.adapt(task));
1522 }
1523
1524 /**
1525 * Wraps the scheduler to avoid leaking a direct reference.
1526 */
1527 VirtualThreadScheduler externalView() {
1528 VirtualThreadScheduler builtin = this;
1529 return VIEW.orElseSet(() -> {
1530 return new VirtualThreadScheduler() {
1531 @Override
1532 public void execute(Thread thread, Runnable task) {
1533 Objects.requireNonNull(thread);
1534 if (thread instanceof VirtualThread vthread) {
1535 VirtualThreadScheduler scheduler = vthread.scheduler;
1536 if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1537 builtin.execute(thread, task);
1538 } else {
1539 throw new IllegalArgumentException();
1540 }
1541 } else {
1542 throw new UnsupportedOperationException();
1543 }
1544 }
1545 };
1546 });
1547 }
1548 }
1549
1550 /**
1551 * Schedule a runnable task to run after a delay.
1552 */
1553 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1554 if (scheduler instanceof ForkJoinPool pool) {
1555 return pool.schedule(command, delay, unit);
1556 } else {
1557 return DelayedTaskSchedulers.schedule(command, delay, unit);
1558 }
1559 }
1560
1561 /**
1562 * Supports scheduling a runnable task to run after a delay. It uses a number
1563 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1564 * work queue used. This class is used when using a custom scheduler.
1565 */
1566 private static class DelayedTaskSchedulers {
1567 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1568
1569 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1570 long tid = Thread.currentThread().threadId();
1571 int index = (int) tid & (INSTANCE.length - 1);
1572 return INSTANCE[index].schedule(command, delay, unit);
1573 }
1574
1575 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1576 String propName = "jdk.virtualThreadScheduler.timerQueues";
1577 String propValue = System.getProperty(propName);
1578 int queueCount;
1579 if (propValue != null) {
1580 queueCount = Integer.parseInt(propValue);
1581 if (queueCount != Integer.highestOneBit(queueCount)) {
1582 throw new RuntimeException("Value of " + propName + " must be power of 2");
1583 }
1584 } else {
1585 int ncpus = Runtime.getRuntime().availableProcessors();
1586 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1587 }
1588 var schedulers = new ScheduledExecutorService[queueCount];
1589 for (int i = 0; i < queueCount; i++) {
1590 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1591 Executors.newScheduledThreadPool(1, task -> {
1592 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1593 t.setDaemon(true);
1594 return t;
1595 });
1596 stpe.setRemoveOnCancelPolicy(true);
1597 schedulers[i] = stpe;
1598 }
1599 return schedulers;
1600 }
1601 }
1602
1603 /**
1604 * Schedule virtual threads that are ready to be scheduled after they blocked on
1605 * monitor enter.
1606 */
1607 private static void unblockVirtualThreads() {
1608 while (true) {
1609 VirtualThread vthread = takeVirtualThreadListToUnblock();
1610 while (vthread != null) {
1611 assert vthread.onWaitingList;
1612 VirtualThread nextThread = vthread.next;
1613
1614 // remove from list and unblock
1615 vthread.next = null;
1616 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1617 assert changed;
1618 vthread.unblock();
1619
1620 vthread = nextThread;
1621 }
1622 }
1623 }
1624
1625 /**
1626 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1627 * if necessary until a list of one or more threads becomes available.
1628 */
1629 private static native VirtualThread takeVirtualThreadListToUnblock();
1630
1631 static {
1632 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1633 VirtualThread::unblockVirtualThreads);
1634 unblocker.setDaemon(true);
1635 unblocker.start();
1636 }
1637 }