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 java.util.function.Supplier;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66
67 private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
68 private static final boolean IS_CUSTOM_DEFAULT_SCHEDULER;
69 static {
70 // experimental
71 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
72 if (propValue != null) {
73 DEFAULT_SCHEDULER = createCustomDefaultScheduler(propValue);
74 IS_CUSTOM_DEFAULT_SCHEDULER = true;
75 } else {
76 DEFAULT_SCHEDULER = createDefaultForkJoinPoolScheduler();
77 IS_CUSTOM_DEFAULT_SCHEDULER = false;
78 }
79 }
80
81 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
82 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
83 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
84 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
85 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
86
87 // scheduler and continuation
88 private final VirtualThreadScheduler scheduler;
89 private final Continuation cont;
90 private final Runnable runContinuation;
91
92 // virtual thread state, accessed by VM
93 private volatile int state;
94
95 /*
96 * Virtual thread state transitions:
97 *
98 * NEW -> STARTED // Thread.start, schedule to run
99 * STARTED -> TERMINATED // failed to start
100 * STARTED -> RUNNING // first run
101 * RUNNING -> TERMINATED // done
102 *
103 * RUNNING -> PARKING // Thread parking with LockSupport.park
104 * PARKING -> PARKED // cont.yield successful, parked indefinitely
105 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
106 * PARKED -> UNPARKED // unparked, may be scheduled to continue
107 * PINNED -> RUNNING // unparked, continue execution on same carrier
108 * UNPARKED -> RUNNING // continue execution after park
109 *
110 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
111 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
112 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
113 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
114 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
115 *
116 * RUNNING -> BLOCKING // blocking on monitor enter
117 * BLOCKING -> BLOCKED // blocked on monitor enter
118 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
119 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
120 *
121 * RUNNING -> WAITING // transitional state during wait on monitor
122 * WAITING -> WAIT // waiting on monitor
123 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
124 * WAIT -> UNBLOCKED // timed-out/interrupted
125 *
126 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
127 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
128 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
129 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
130 *
131 * RUNNING -> YIELDING // Thread.yield
132 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
133 * YIELDING -> RUNNING // cont.yield failed
134 * YIELDED -> RUNNING // continue execution after Thread.yield
135 */
136 private static final int NEW = 0;
137 private static final int STARTED = 1;
138 private static final int RUNNING = 2; // runnable-mounted
139
140 // untimed and timed parking
141 private static final int PARKING = 3;
142 private static final int PARKED = 4; // unmounted
143 private static final int PINNED = 5; // mounted
144 private static final int TIMED_PARKING = 6;
145 private static final int TIMED_PARKED = 7; // unmounted
146 private static final int TIMED_PINNED = 8; // mounted
147 private static final int UNPARKED = 9; // unmounted but runnable
148
149 // Thread.yield
150 private static final int YIELDING = 10;
151 private static final int YIELDED = 11; // unmounted but runnable
152
153 // monitor enter
154 private static final int BLOCKING = 12;
155 private static final int BLOCKED = 13; // unmounted
156 private static final int UNBLOCKED = 14; // unmounted but runnable
157
158 // monitor wait/timed-wait
159 private static final int WAITING = 15;
160 private static final int WAIT = 16; // waiting in Object.wait
161 private static final int TIMED_WAITING = 17;
162 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
163
164 private static final int TERMINATED = 99; // final state
165
166 // can be suspended from scheduling when unmounted
167 private static final int SUSPENDED = 1 << 8;
168
169 // parking permit made available by LockSupport.unpark
170 private volatile boolean parkPermit;
171
172 // blocking permit made available by unblocker thread when another thread exits monitor
173 private volatile boolean blockPermit;
174
175 // true when on the list of virtual threads waiting to be unblocked
176 private volatile boolean onWaitingList;
177
178 // next virtual thread on the list of virtual threads waiting to be unblocked
179 private volatile VirtualThread next;
180
181 // notified by Object.notify/notifyAll while waiting in Object.wait
182 private volatile boolean notified;
183
184 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
185 private volatile boolean interruptableWait;
186
187 // timed-wait support
188 private byte timedWaitSeqNo;
189
190 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
191 private long timeout;
192
193 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
194 private Future<?> timeoutTask;
195
196 // carrier thread when mounted, accessed by VM
197 private volatile Thread carrierThread;
198
199 // termination object when joining, created lazily if needed
200 private volatile CountDownLatch termination;
201
202 /**
203 * Returns the default scheduler.
204 */
205 static VirtualThreadScheduler defaultScheduler() {
206 return DEFAULT_SCHEDULER;
207 }
208
209 /**
210 * Returns true if using a custom default scheduler.
211 */
212 static boolean isCustomDefaultScheduler() {
213 return IS_CUSTOM_DEFAULT_SCHEDULER;
214 }
215
216 /**
217 * Returns the continuation scope used for virtual threads.
218 */
219 static ContinuationScope continuationScope() {
220 return VTHREAD_SCOPE;
221 }
222
223 /**
224 * Return the scheduler for this thread.
225 * @param revealBuiltin true to reveal the built-in default scheduler, false to hide
226 */
227 VirtualThreadScheduler scheduler(boolean revealBuiltin) {
228 if (scheduler instanceof BuiltinDefaultScheduler builtin && !revealBuiltin) {
229 return builtin.externalView();
230 } else {
231 return scheduler;
232 }
233 }
234
235 /**
236 * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
237 *
238 * @param scheduler the scheduler or null for default scheduler
239 * @param name thread name
240 * @param characteristics characteristics
241 * @param task the task to execute
242 */
243 VirtualThread(VirtualThreadScheduler scheduler,
244 String name,
245 int characteristics,
246 Runnable task) {
247 super(name, characteristics, /*bound*/ false);
248 Objects.requireNonNull(task);
249
250 // use default scheduler if not provided
251 if (scheduler == null) {
252 scheduler = DEFAULT_SCHEDULER;
253 }
254
255 this.scheduler = scheduler;
256 this.cont = new VThreadContinuation(this, task);
257 this.runContinuation = this::runContinuation;
258 }
259
260 /**
261 * The continuation that a virtual thread executes.
262 */
263 private static class VThreadContinuation extends Continuation {
264 VThreadContinuation(VirtualThread vthread, Runnable task) {
265 super(VTHREAD_SCOPE, wrap(vthread, task));
266 }
267 @Override
268 protected void onPinned(Continuation.Pinned reason) {
269 }
270 private static Runnable wrap(VirtualThread vthread, Runnable task) {
271 return new Runnable() {
272 @Hidden
273 @JvmtiHideEvents
274 public void run() {
275 vthread.notifyJvmtiStart(); // notify JVMTI
276 try {
277 vthread.run(task);
278 } finally {
279 vthread.notifyJvmtiEnd(); // notify JVMTI
280 }
281 }
282 };
283 }
284 }
285
286 /**
287 * Runs or continues execution on the current thread. The virtual thread is mounted
288 * on the current thread before the task runs or continues. It unmounts when the
289 * task completes or yields.
290 */
291 @ChangesCurrentThread // allow mount/unmount to be inlined
292 private void runContinuation() {
293 // the carrier must be a platform thread
294 if (Thread.currentThread().isVirtual()) {
295 throw new WrongThreadException();
296 }
297
298 // set state to RUNNING
299 int initialState = state();
300 if (initialState == STARTED || initialState == UNPARKED
301 || initialState == UNBLOCKED || initialState == YIELDED) {
302 // newly started or continue after parking/blocking/Thread.yield
303 if (!compareAndSetState(initialState, RUNNING)) {
304 return;
305 }
306 // consume permit when continuing after parking or blocking. If continue
307 // after a timed-park or timed-wait then the timeout task is cancelled.
308 if (initialState == UNPARKED) {
309 cancelTimeoutTask();
310 setParkPermit(false);
311 } else if (initialState == UNBLOCKED) {
312 cancelTimeoutTask();
313 blockPermit = false;
314 }
315 } else {
316 // not runnable
317 return;
318 }
319
320 mount();
321 try {
322 cont.run();
323 } finally {
324 unmount();
325 if (cont.isDone()) {
326 afterDone();
327 } else {
328 afterYield();
329 }
330 }
331 }
332
333 /**
334 * Cancel timeout task when continuing after timed-park or timed-wait.
335 * The timeout task may be executing, or may have already completed.
336 */
337 private void cancelTimeoutTask() {
338 if (timeoutTask != null) {
339 timeoutTask.cancel(false);
340 timeoutTask = null;
341 }
342 }
343
344 /**
345 * Submits the runContinuation task to the scheduler. For the default scheduler,
346 * and calling it on a worker thread, the task will be pushed to the local queue,
347 * otherwise it will be pushed to an external submission queue.
348 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
349 * @throws RejectedExecutionException
350 */
351 private void submitRunContinuation(boolean retryOnOOME) {
352 boolean done = false;
353 while (!done) {
354 try {
355 // Pin the continuation to prevent the virtual thread from unmounting
356 // when submitting a task. For the default scheduler this ensures that
357 // the carrier doesn't change when pushing a task. For other schedulers
358 // it avoids deadlock that could arise due to carriers and virtual
359 // threads contending for a lock.
360 if (currentThread().isVirtual()) {
361 Continuation.pin();
362 try {
363 scheduler.execute(this, runContinuation);
364 } finally {
365 Continuation.unpin();
366 }
367 } else {
368 scheduler.execute(this, runContinuation);
369 }
370 done = true;
371 } catch (RejectedExecutionException ree) {
372 submitFailed(ree);
373 throw ree;
374 } catch (OutOfMemoryError e) {
375 if (retryOnOOME) {
376 U.park(false, 100_000_000); // 100ms
377 } else {
378 throw e;
379 }
380 }
381 }
382 }
383
384 /**
385 * Submits the runContinuation task to the scheduler. For the default scheduler,
386 * and calling it on a worker thread, the task will be pushed to the local queue,
387 * otherwise it will be pushed to an external submission queue.
388 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
389 * @throws RejectedExecutionException
390 */
391 private void submitRunContinuation() {
392 submitRunContinuation(true);
393 }
394
395 /**
396 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
397 * queue is empty. If not empty, or invoked by another thread, then this method works
398 * like submitRunContinuation and just submits the task to the scheduler.
399 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
400 * @throws RejectedExecutionException
401 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
402 */
403 private void lazySubmitRunContinuation() {
404 if (scheduler == DEFAULT_SCHEDULER
405 && currentCarrierThread() instanceof CarrierThread ct
406 && ct.getQueuedTaskCount() == 0) {
407 try {
408 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
409 } catch (RejectedExecutionException ree) {
410 submitFailed(ree);
411 throw ree;
412 } catch (OutOfMemoryError e) {
413 submitRunContinuation();
414 }
415 } else {
416 submitRunContinuation();
417 }
418 }
419
420 /**
421 * Submits the runContinuation task to the scheduler. For the default scheduler, and
422 * calling it a virtual thread that uses the default scheduler, the task will be
423 * pushed to an external submission queue.
424 * @throws RejectedExecutionException
425 */
426 private void externalSubmitRunContinuation() {
427 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
428 try {
429 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
430 } catch (RejectedExecutionException ree) {
431 submitFailed(ree);
432 throw ree;
433 } catch (OutOfMemoryError e) {
434 submitRunContinuation();
435 }
436 } else {
437 submitRunContinuation();
438 }
439 }
440
441 /**
442 * Submits the runContinuation task to the scheduler. For the default scheduler, and
443 * calling it a virtual thread that uses the default scheduler, the task will be
444 * pushed to an external submission queue. This method may throw OutOfMemoryError.
445 * @throws RejectedExecutionException
446 * @throws OutOfMemoryError
447 */
448 private void externalSubmitRunContinuationOrThrow() {
449 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
450 try {
451 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
452 } catch (RejectedExecutionException ree) {
453 submitFailed(ree);
454 throw ree;
455 }
456 } else {
457 submitRunContinuation(false);
458 }
459 }
460
461 /**
462 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
463 */
464 private void submitFailed(RejectedExecutionException ree) {
465 var event = new VirtualThreadSubmitFailedEvent();
466 if (event.isEnabled()) {
467 event.javaThreadId = threadId();
468 event.exceptionMessage = ree.getMessage();
469 event.commit();
470 }
471 }
472
473 /**
474 * Runs a task in the context of this virtual thread.
475 */
476 private void run(Runnable task) {
477 assert Thread.currentThread() == this && state == RUNNING;
478
479 // emit JFR event if enabled
480 if (VirtualThreadStartEvent.isTurnedOn()) {
481 var event = new VirtualThreadStartEvent();
482 event.javaThreadId = threadId();
483 event.commit();
484 }
485
486 Object bindings = Thread.scopedValueBindings();
487 try {
488 runWith(bindings, task);
489 } catch (Throwable exc) {
490 dispatchUncaughtException(exc);
491 } finally {
492 // pop any remaining scopes from the stack, this may block
493 StackableScope.popAll();
494
495 // emit JFR event if enabled
496 if (VirtualThreadEndEvent.isTurnedOn()) {
497 var event = new VirtualThreadEndEvent();
498 event.javaThreadId = threadId();
499 event.commit();
500 }
501 }
502 }
503
504 /**
505 * Mounts this virtual thread onto the current platform thread. On
506 * return, the current thread is the virtual thread.
507 */
508 @ChangesCurrentThread
509 @ReservedStackAccess
510 private void mount() {
511 // notify JVMTI before mount
512 notifyJvmtiMount(/*hide*/true);
513
514 // sets the carrier thread
515 Thread carrier = Thread.currentCarrierThread();
516 setCarrierThread(carrier);
517
518 // sync up carrier thread interrupt status if needed
519 if (interrupted) {
520 carrier.setInterrupt();
521 } else if (carrier.isInterrupted()) {
522 synchronized (interruptLock) {
523 // need to recheck interrupt status
524 if (!interrupted) {
525 carrier.clearInterrupt();
526 }
527 }
528 }
529
530 // set Thread.currentThread() to return this virtual thread
531 carrier.setCurrentThread(this);
532 }
533
534 /**
535 * Unmounts this virtual thread from the carrier. On return, the
536 * current thread is the current platform thread.
537 */
538 @ChangesCurrentThread
539 @ReservedStackAccess
540 private void unmount() {
541 assert !Thread.holdsLock(interruptLock);
542
543 // set Thread.currentThread() to return the platform thread
544 Thread carrier = this.carrierThread;
545 carrier.setCurrentThread(carrier);
546
547 // break connection to carrier thread, synchronized with interrupt
548 synchronized (interruptLock) {
549 setCarrierThread(null);
550 }
551 carrier.clearInterrupt();
552
553 // notify JVMTI after unmount
554 notifyJvmtiUnmount(/*hide*/false);
555 }
556
557 /**
558 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
559 * the continuation continues.
560 */
561 @Hidden
562 private boolean yieldContinuation() {
563 notifyJvmtiUnmount(/*hide*/true);
564 try {
565 return Continuation.yield(VTHREAD_SCOPE);
566 } finally {
567 notifyJvmtiMount(/*hide*/false);
568 }
569 }
570
571 /**
572 * Invoked in the context of the carrier thread after the Continuation yields when
573 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
574 */
575 private void afterYield() {
576 assert carrierThread == null;
577
578 // re-adjust parallelism if the virtual thread yielded when compensating
579 if (currentThread() instanceof CarrierThread ct) {
580 ct.endBlocking();
581 }
582
583 int s = state();
584
585 // LockSupport.park/parkNanos
586 if (s == PARKING || s == TIMED_PARKING) {
587 int newState;
588 if (s == PARKING) {
589 setState(newState = PARKED);
590 } else {
591 // schedule unpark
592 long timeout = this.timeout;
593 assert timeout > 0;
594 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
595 setState(newState = TIMED_PARKED);
596 }
597
598 // may have been unparked while parking
599 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
600 // lazy submit if local queue is empty
601 lazySubmitRunContinuation();
602 }
603 return;
604 }
605
606 // Thread.yield
607 if (s == YIELDING) {
608 setState(YIELDED);
609
610 // external submit if there are no tasks in the local task queue
611 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
612 externalSubmitRunContinuation();
613 } else {
614 submitRunContinuation();
615 }
616 return;
617 }
618
619 // blocking on monitorenter
620 if (s == BLOCKING) {
621 setState(BLOCKED);
622
623 // may have been unblocked while blocking
624 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
625 // lazy submit if local queue is empty
626 lazySubmitRunContinuation();
627 }
628 return;
629 }
630
631 // Object.wait
632 if (s == WAITING || s == TIMED_WAITING) {
633 int newState;
634 boolean interruptable = interruptableWait;
635 if (s == WAITING) {
636 setState(newState = WAIT);
637 } else {
638 // For timed-wait, a timeout task is scheduled to execute. The timeout
639 // task will change the thread state to UNBLOCKED and submit the thread
640 // to the scheduler. A sequence number is used to ensure that the timeout
641 // task only unblocks the thread for this timed-wait. We synchronize with
642 // the timeout task to coordinate access to the sequence number and to
643 // ensure the timeout task doesn't execute until the thread has got to
644 // the TIMED_WAIT state.
645 long timeout = this.timeout;
646 assert timeout > 0;
647 synchronized (timedWaitLock()) {
648 byte seqNo = ++timedWaitSeqNo;
649 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
650 setState(newState = TIMED_WAIT);
651 }
652 }
653
654 // may have been notified while in transition to wait state
655 if (notified && compareAndSetState(newState, BLOCKED)) {
656 // may have even been unblocked already
657 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
658 submitRunContinuation();
659 }
660 return;
661 }
662
663 // may have been interrupted while in transition to wait state
664 if (interruptable && interrupted && compareAndSetState(newState, UNBLOCKED)) {
665 submitRunContinuation();
666 return;
667 }
668 return;
669 }
670
671 assert false;
672 }
673
674 /**
675 * Invoked after the continuation completes.
676 */
677 private void afterDone() {
678 afterDone(true);
679 }
680
681 /**
682 * Invoked after the continuation completes (or start failed). Sets the thread
683 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
684 *
685 * @param notifyContainer true if its container should be notified
686 */
687 private void afterDone(boolean notifyContainer) {
688 assert carrierThread == null;
689 setState(TERMINATED);
690
691 // notify anyone waiting for this virtual thread to terminate
692 CountDownLatch termination = this.termination;
693 if (termination != null) {
694 assert termination.getCount() == 1;
695 termination.countDown();
696 }
697
698 // notify container
699 if (notifyContainer) {
700 threadContainer().remove(this);
701 }
702
703 // clear references to thread locals
704 clearReferences();
705 }
706
707 /**
708 * Schedules this {@code VirtualThread} to execute.
709 *
710 * @throws IllegalStateException if the container is shutdown or closed
711 * @throws IllegalThreadStateException if the thread has already been started
712 * @throws RejectedExecutionException if the scheduler cannot accept a task
713 */
714 @Override
715 void start(ThreadContainer container) {
716 if (!compareAndSetState(NEW, STARTED)) {
717 throw new IllegalThreadStateException("Already started");
718 }
719
720 // bind thread to container
721 assert threadContainer() == null;
722 setThreadContainer(container);
723
724 // start thread
725 boolean addedToContainer = false;
726 boolean started = false;
727 try {
728 container.add(this); // may throw
729 addedToContainer = true;
730
731 // scoped values may be inherited
732 inheritScopedValueBindings(container);
733
734 // submit task to run thread, using externalSubmit if possible
735 externalSubmitRunContinuationOrThrow();
736 started = true;
737 } finally {
738 if (!started) {
739 afterDone(addedToContainer);
740 }
741 }
742 }
743
744 @Override
745 public void start() {
746 start(ThreadContainers.root());
747 }
748
749 @Override
750 public void run() {
751 // do nothing
752 }
753
754 /**
755 * Parks until unparked or interrupted. If already unparked then the parking
756 * permit is consumed and this method completes immediately (meaning it doesn't
757 * yield). It also completes immediately if the interrupt status is set.
758 */
759 @Override
760 void park() {
761 assert Thread.currentThread() == this;
762
763 // complete immediately if parking permit available or interrupted
764 if (getAndSetParkPermit(false) || interrupted)
765 return;
766
767 // park the thread
768 boolean yielded = false;
769 setState(PARKING);
770 try {
771 yielded = yieldContinuation();
772 } catch (OutOfMemoryError e) {
773 // park on carrier
774 } finally {
775 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
776 if (!yielded) {
777 assert state() == PARKING;
778 setState(RUNNING);
779 }
780 }
781
782 // park on the carrier thread when pinned
783 if (!yielded) {
784 parkOnCarrierThread(false, 0);
785 }
786 }
787
788 /**
789 * Parks up to the given waiting time or until unparked or interrupted.
790 * If already unparked then the parking permit is consumed and this method
791 * completes immediately (meaning it doesn't yield). It also completes immediately
792 * if the interrupt status is set or the waiting time is {@code <= 0}.
793 *
794 * @param nanos the maximum number of nanoseconds to wait.
795 */
796 @Override
797 void parkNanos(long nanos) {
798 assert Thread.currentThread() == this;
799
800 // complete immediately if parking permit available or interrupted
801 if (getAndSetParkPermit(false) || interrupted)
802 return;
803
804 // park the thread for the waiting time
805 if (nanos > 0) {
806 long startTime = System.nanoTime();
807
808 // park the thread, afterYield will schedule the thread to unpark
809 boolean yielded = false;
810 timeout = nanos;
811 setState(TIMED_PARKING);
812 try {
813 yielded = yieldContinuation();
814 } catch (OutOfMemoryError e) {
815 // park on carrier
816 } finally {
817 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
818 if (!yielded) {
819 assert state() == TIMED_PARKING;
820 setState(RUNNING);
821 }
822 }
823
824 // park on carrier thread for remaining time when pinned (or OOME)
825 if (!yielded) {
826 long remainingNanos = nanos - (System.nanoTime() - startTime);
827 parkOnCarrierThread(true, remainingNanos);
828 }
829 }
830 }
831
832 /**
833 * Parks the current carrier thread up to the given waiting time or until
834 * unparked or interrupted. If the virtual thread is interrupted then the
835 * interrupt status will be propagated to the carrier thread.
836 * @param timed true for a timed park, false for untimed
837 * @param nanos the waiting time in nanoseconds
838 */
839 private void parkOnCarrierThread(boolean timed, long nanos) {
840 assert state() == RUNNING;
841
842 setState(timed ? TIMED_PINNED : PINNED);
843 try {
844 if (!parkPermit) {
845 if (!timed) {
846 U.park(false, 0);
847 } else if (nanos > 0) {
848 U.park(false, nanos);
849 }
850 }
851 } finally {
852 setState(RUNNING);
853 }
854
855 // consume parking permit
856 setParkPermit(false);
857
858 // JFR jdk.VirtualThreadPinned event
859 postPinnedEvent("LockSupport.park");
860 }
861
862 /**
863 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
864 * Recording the event in the VM avoids having JFR event recorded in Java
865 * with the same name, but different ID, to events recorded by the VM.
866 */
867 @Hidden
868 private static native void postPinnedEvent(String op);
869
870 /**
871 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
872 * then its task is scheduled to continue, otherwise its next call to {@code park} or
873 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
874 * @throws RejectedExecutionException if the scheduler cannot accept a task
875 */
876 @Override
877 void unpark() {
878 if (!getAndSetParkPermit(true) && currentThread() != this) {
879 int s = state();
880
881 // unparked while parked
882 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
883 submitRunContinuation();
884 return;
885 }
886
887 // unparked while parked when pinned
888 if (s == PINNED || s == TIMED_PINNED) {
889 // unpark carrier thread when pinned
890 disableSuspendAndPreempt();
891 try {
892 synchronized (carrierThreadAccessLock()) {
893 Thread carrier = carrierThread;
894 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
895 U.unpark(carrier);
896 }
897 }
898 } finally {
899 enableSuspendAndPreempt();
900 }
901 return;
902 }
903 }
904 }
905
906 /**
907 * Invoked by unblocker thread to unblock this virtual thread.
908 */
909 private void unblock() {
910 assert !Thread.currentThread().isVirtual();
911 blockPermit = true;
912 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
913 submitRunContinuation();
914 }
915 }
916
917 /**
918 * Invoked by FJP worker thread or STPE thread when park timeout expires.
919 */
920 private void parkTimeoutExpired() {
921 assert !VirtualThread.currentThread().isVirtual();
922 if (!getAndSetParkPermit(true)
923 && (state() == TIMED_PARKED)
924 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
925 lazySubmitRunContinuation();
926 }
927 }
928
929 /**
930 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
931 * If the virtual thread is in timed-wait then this method will unblock the thread
932 * and submit its task so that it continues and attempts to reenter the monitor.
933 * This method does nothing if the thread has been woken by notify or interrupt.
934 */
935 private void waitTimeoutExpired(byte seqNo) {
936 assert !Thread.currentThread().isVirtual();
937 for (;;) {
938 boolean unblocked = false;
939 synchronized (timedWaitLock()) {
940 if (seqNo != timedWaitSeqNo) {
941 // this timeout task is for a past timed-wait
942 return;
943 }
944 int s = state();
945 if (s == TIMED_WAIT) {
946 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
947 } else if (s != (TIMED_WAIT | SUSPENDED)) {
948 // notified or interrupted, no longer waiting
949 return;
950 }
951 }
952 if (unblocked) {
953 lazySubmitRunContinuation();
954 return;
955 }
956 // need to retry when thread is suspended in time-wait
957 Thread.yield();
958 }
959 }
960
961 /**
962 * Attempts to yield the current virtual thread (Thread.yield).
963 */
964 void tryYield() {
965 assert Thread.currentThread() == this;
966 setState(YIELDING);
967 boolean yielded = false;
968 try {
969 yielded = yieldContinuation(); // may throw
970 } finally {
971 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
972 if (!yielded) {
973 assert state() == YIELDING;
974 setState(RUNNING);
975 }
976 }
977 }
978
979 /**
980 * Sleep the current thread for the given sleep time (in nanoseconds). If
981 * nanos is 0 then the thread will attempt to yield.
982 *
983 * @implNote This implementation parks the thread for the given sleeping time
984 * and will therefore be observed in PARKED state during the sleep. Parking
985 * will consume the parking permit so this method makes available the parking
986 * permit after the sleep. This may be observed as a spurious, but benign,
987 * wakeup when the thread subsequently attempts to park.
988 *
989 * @param nanos the maximum number of nanoseconds to sleep
990 * @throws InterruptedException if interrupted while sleeping
991 */
992 void sleepNanos(long nanos) throws InterruptedException {
993 assert Thread.currentThread() == this && nanos >= 0;
994 if (getAndClearInterrupt())
995 throw new InterruptedException();
996 if (nanos == 0) {
997 tryYield();
998 } else {
999 // park for the sleep time
1000 try {
1001 long remainingNanos = nanos;
1002 long startNanos = System.nanoTime();
1003 while (remainingNanos > 0) {
1004 parkNanos(remainingNanos);
1005 if (getAndClearInterrupt()) {
1006 throw new InterruptedException();
1007 }
1008 remainingNanos = nanos - (System.nanoTime() - startNanos);
1009 }
1010 } finally {
1011 // may have been unparked while sleeping
1012 setParkPermit(true);
1013 }
1014 }
1015 }
1016
1017 /**
1018 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1019 * A timeout of {@code 0} means to wait forever.
1020 *
1021 * @throws InterruptedException if interrupted while waiting
1022 * @return true if the thread has terminated
1023 */
1024 boolean joinNanos(long nanos) throws InterruptedException {
1025 if (state() == TERMINATED)
1026 return true;
1027
1028 // ensure termination object exists, then re-check state
1029 CountDownLatch termination = getTermination();
1030 if (state() == TERMINATED)
1031 return true;
1032
1033 // wait for virtual thread to terminate
1034 if (nanos == 0) {
1035 termination.await();
1036 } else {
1037 boolean terminated = termination.await(nanos, NANOSECONDS);
1038 if (!terminated) {
1039 // waiting time elapsed
1040 return false;
1041 }
1042 }
1043 assert state() == TERMINATED;
1044 return true;
1045 }
1046
1047 @Override
1048 void blockedOn(Interruptible b) {
1049 disableSuspendAndPreempt();
1050 try {
1051 super.blockedOn(b);
1052 } finally {
1053 enableSuspendAndPreempt();
1054 }
1055 }
1056
1057 @Override
1058 public void interrupt() {
1059 if (Thread.currentThread() != this) {
1060 // if current thread is a virtual thread then prevent it from being
1061 // suspended or unmounted when entering or holding interruptLock
1062 Interruptible blocker;
1063 disableSuspendAndPreempt();
1064 try {
1065 synchronized (interruptLock) {
1066 interrupted = true;
1067 blocker = nioBlocker();
1068 if (blocker != null) {
1069 blocker.interrupt(this);
1070 }
1071
1072 // interrupt carrier thread if mounted
1073 Thread carrier = carrierThread;
1074 if (carrier != null) carrier.setInterrupt();
1075 }
1076 } finally {
1077 enableSuspendAndPreempt();
1078 }
1079
1080 // notify blocker after releasing interruptLock
1081 if (blocker != null) {
1082 blocker.postInterrupt();
1083 }
1084
1085 // make available parking permit, unpark thread if parked
1086 unpark();
1087
1088 // if thread is waiting in Object.wait then schedule to try to reenter
1089 int s = state();
1090 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1091 submitRunContinuation();
1092 }
1093
1094 } else {
1095 interrupted = true;
1096 carrierThread.setInterrupt();
1097 setParkPermit(true);
1098 }
1099 }
1100
1101 @Override
1102 public boolean isInterrupted() {
1103 return interrupted;
1104 }
1105
1106 @Override
1107 boolean getAndClearInterrupt() {
1108 assert Thread.currentThread() == this;
1109 boolean oldValue = interrupted;
1110 if (oldValue) {
1111 disableSuspendAndPreempt();
1112 try {
1113 synchronized (interruptLock) {
1114 interrupted = false;
1115 carrierThread.clearInterrupt();
1116 }
1117 } finally {
1118 enableSuspendAndPreempt();
1119 }
1120 }
1121 return oldValue;
1122 }
1123
1124 @Override
1125 Thread.State threadState() {
1126 int s = state();
1127 switch (s & ~SUSPENDED) {
1128 case NEW:
1129 return Thread.State.NEW;
1130 case STARTED:
1131 // return NEW if thread container not yet set
1132 if (threadContainer() == null) {
1133 return Thread.State.NEW;
1134 } else {
1135 return Thread.State.RUNNABLE;
1136 }
1137 case UNPARKED:
1138 case UNBLOCKED:
1139 case YIELDED:
1140 // runnable, not mounted
1141 return Thread.State.RUNNABLE;
1142 case RUNNING:
1143 // if mounted then return state of carrier thread
1144 if (Thread.currentThread() != this) {
1145 disableSuspendAndPreempt();
1146 try {
1147 synchronized (carrierThreadAccessLock()) {
1148 Thread carrierThread = this.carrierThread;
1149 if (carrierThread != null) {
1150 return carrierThread.threadState();
1151 }
1152 }
1153 } finally {
1154 enableSuspendAndPreempt();
1155 }
1156 }
1157 // runnable, mounted
1158 return Thread.State.RUNNABLE;
1159 case PARKING:
1160 case TIMED_PARKING:
1161 case WAITING:
1162 case TIMED_WAITING:
1163 case YIELDING:
1164 // runnable, in transition
1165 return Thread.State.RUNNABLE;
1166 case PARKED:
1167 case PINNED:
1168 case WAIT:
1169 return Thread.State.WAITING;
1170 case TIMED_PARKED:
1171 case TIMED_PINNED:
1172 case TIMED_WAIT:
1173 return Thread.State.TIMED_WAITING;
1174 case BLOCKING:
1175 case BLOCKED:
1176 return Thread.State.BLOCKED;
1177 case TERMINATED:
1178 return Thread.State.TERMINATED;
1179 default:
1180 throw new InternalError();
1181 }
1182 }
1183
1184 @Override
1185 boolean alive() {
1186 int s = state;
1187 return (s != NEW && s != TERMINATED);
1188 }
1189
1190 @Override
1191 boolean isTerminated() {
1192 return (state == TERMINATED);
1193 }
1194
1195 @Override
1196 StackTraceElement[] asyncGetStackTrace() {
1197 StackTraceElement[] stackTrace;
1198 do {
1199 stackTrace = (carrierThread != null)
1200 ? super.asyncGetStackTrace() // mounted
1201 : supplyIfUnmounted(cont::getStackTrace, // unmounted
1202 () -> new StackTraceElement[0]);
1203 if (stackTrace == null) {
1204 Thread.yield();
1205 }
1206 } while (stackTrace == null);
1207 return stackTrace;
1208 }
1209
1210 /**
1211 * Invokes a supplier to produce a non-null result if this virtual thread is not mounted.
1212 * @param supplier1 invoked if this virtual thread is alive and unmounted
1213 * @param supplier2 invoked if this virtual thread is not alive
1214 * @return the result; {@code null} if this virtual thread is mounted or in transition
1215 */
1216 <T> T supplyIfUnmounted(Supplier<T> supplier1, Supplier<T> supplier2) {
1217 int initialState = state() & ~SUSPENDED;
1218 switch (initialState) {
1219 case NEW, STARTED, TERMINATED -> {
1220 return supplier2.get(); // terminated or not started
1221 }
1222 case RUNNING, PINNED, TIMED_PINNED -> {
1223 return null; // mounted
1224 }
1225 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1226 // unmounted, not runnable
1227 }
1228 case UNPARKED, UNBLOCKED, YIELDED -> {
1229 // unmounted, runnable
1230 }
1231 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1232 return null; // in transition
1233 }
1234 default -> throw new InternalError("" + initialState);
1235 }
1236
1237 // thread is unmounted, prevent it from continuing
1238 int suspendedState = initialState | SUSPENDED;
1239 if (!compareAndSetState(initialState, suspendedState)) {
1240 return null;
1241 }
1242
1243 try {
1244 return supplier1.get();
1245 } finally {
1246 assert state == suspendedState;
1247 setState(initialState);
1248
1249 boolean resubmit = switch (initialState) {
1250 case UNPARKED, UNBLOCKED, YIELDED -> {
1251 // resubmit as task may have run while suspended
1252 yield true;
1253 }
1254 case PARKED, TIMED_PARKED -> {
1255 // resubmit if unparked while suspended
1256 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1257 }
1258 case BLOCKED -> {
1259 // resubmit if unblocked while suspended
1260 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1261 }
1262 case WAIT, TIMED_WAIT -> {
1263 // resubmit if notified or interrupted while waiting (Object.wait)
1264 // waitTimeoutExpired will retry if the timed expired when suspended
1265 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1266 }
1267 default -> throw new InternalError();
1268 };
1269 if (resubmit) {
1270 submitRunContinuation();
1271 }
1272 }
1273
1274 }
1275
1276 @Override
1277 public String toString() {
1278 StringBuilder sb = new StringBuilder("VirtualThread[#");
1279 sb.append(threadId());
1280 String name = getName();
1281 if (!name.isEmpty()) {
1282 sb.append(",");
1283 sb.append(name);
1284 }
1285 sb.append("]/");
1286
1287 // add the carrier state and thread name when mounted
1288 boolean mounted;
1289 if (Thread.currentThread() == this) {
1290 mounted = appendCarrierInfo(sb);
1291 } else {
1292 disableSuspendAndPreempt();
1293 try {
1294 synchronized (carrierThreadAccessLock()) {
1295 mounted = appendCarrierInfo(sb);
1296 }
1297 } finally {
1298 enableSuspendAndPreempt();
1299 }
1300 }
1301
1302 // add virtual thread state when not mounted
1303 if (!mounted) {
1304 String stateAsString = threadState().toString();
1305 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1306 }
1307
1308 return sb.toString();
1309 }
1310
1311 /**
1312 * Appends the carrier state and thread name to the string buffer if mounted.
1313 * @return true if mounted, false if not mounted
1314 */
1315 private boolean appendCarrierInfo(StringBuilder sb) {
1316 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1317 Thread carrier = carrierThread;
1318 if (carrier != null) {
1319 String stateAsString = carrier.threadState().toString();
1320 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1321 sb.append('@');
1322 sb.append(carrier.getName());
1323 return true;
1324 } else {
1325 return false;
1326 }
1327 }
1328
1329 @Override
1330 public int hashCode() {
1331 return (int) threadId();
1332 }
1333
1334 @Override
1335 public boolean equals(Object obj) {
1336 return obj == this;
1337 }
1338
1339 /**
1340 * Returns the termination object, creating it if needed.
1341 */
1342 private CountDownLatch getTermination() {
1343 CountDownLatch termination = this.termination;
1344 if (termination == null) {
1345 termination = new CountDownLatch(1);
1346 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1347 termination = this.termination;
1348 }
1349 }
1350 return termination;
1351 }
1352
1353 /**
1354 * Returns the lock object to synchronize on when accessing carrierThread.
1355 * The lock prevents carrierThread from being reset to null during unmount.
1356 */
1357 private Object carrierThreadAccessLock() {
1358 // return interruptLock as unmount has to coordinate with interrupt
1359 return interruptLock;
1360 }
1361
1362 /**
1363 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1364 */
1365 private Object timedWaitLock() {
1366 // use this object for now to avoid the overhead of introducing another lock
1367 return runContinuation;
1368 }
1369
1370 /**
1371 * Disallow the current thread be suspended or preempted.
1372 */
1373 private void disableSuspendAndPreempt() {
1374 notifyJvmtiDisableSuspend(true);
1375 Continuation.pin();
1376 }
1377
1378 /**
1379 * Allow the current thread be suspended or preempted.
1380 */
1381 private void enableSuspendAndPreempt() {
1382 Continuation.unpin();
1383 notifyJvmtiDisableSuspend(false);
1384 }
1385
1386 // -- wrappers for get/set of state, parking permit, and carrier thread --
1387
1388 private int state() {
1389 return state; // volatile read
1390 }
1391
1392 private void setState(int newValue) {
1393 state = newValue; // volatile write
1394 }
1395
1396 private boolean compareAndSetState(int expectedValue, int newValue) {
1397 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1398 }
1399
1400 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1401 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1402 }
1403
1404 private void setParkPermit(boolean newValue) {
1405 if (parkPermit != newValue) {
1406 parkPermit = newValue;
1407 }
1408 }
1409
1410 private boolean getAndSetParkPermit(boolean newValue) {
1411 if (parkPermit != newValue) {
1412 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1413 } else {
1414 return newValue;
1415 }
1416 }
1417
1418 private void setCarrierThread(Thread carrier) {
1419 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1420 this.carrierThread = carrier;
1421 }
1422
1423 // -- JVM TI support --
1424
1425 @IntrinsicCandidate
1426 @JvmtiMountTransition
1427 private native void notifyJvmtiStart();
1428
1429 @IntrinsicCandidate
1430 @JvmtiMountTransition
1431 private native void notifyJvmtiEnd();
1432
1433 @IntrinsicCandidate
1434 @JvmtiMountTransition
1435 private native void notifyJvmtiMount(boolean hide);
1436
1437 @IntrinsicCandidate
1438 @JvmtiMountTransition
1439 private native void notifyJvmtiUnmount(boolean hide);
1440
1441 @IntrinsicCandidate
1442 private static native void notifyJvmtiDisableSuspend(boolean enter);
1443
1444 private static native void registerNatives();
1445 static {
1446 registerNatives();
1447
1448 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1449 var group = Thread.virtualThreadGroup();
1450 }
1451
1452 /**
1453 * Loads a VirtualThreadScheduler with the given class name to use at the
1454 * default scheduler. The class is public in an exported package, has a public
1455 * one-arg or no-arg constructor, and is visible to the system class loader.
1456 */
1457 private static VirtualThreadScheduler createCustomDefaultScheduler(String cn) {
1458 try {
1459 Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1460 VirtualThreadScheduler scheduler;
1461 try {
1462 // 1-arg constructor
1463 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1464 var builtin = createDefaultForkJoinPoolScheduler();
1465 scheduler = (VirtualThreadScheduler) ctor.newInstance(builtin.externalView());
1466 } catch (NoSuchMethodException e) {
1467 // 0-arg constructor
1468 Constructor<?> ctor = clazz.getConstructor();
1469 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1470 }
1471 System.err.println("""
1472 WARNING: Using custom default scheduler, this is an experimental feature!""");
1473 return scheduler;
1474 } catch (Exception ex) {
1475 throw new Error(ex);
1476 }
1477 }
1478
1479 /**
1480 * Creates the built-in default ForkJoinPool scheduler.
1481 */
1482 private static BuiltinDefaultScheduler createDefaultForkJoinPoolScheduler() {
1483 int parallelism, maxPoolSize, minRunnable;
1484 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1485 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1486 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1487 if (parallelismValue != null) {
1488 parallelism = Integer.parseInt(parallelismValue);
1489 } else {
1490 parallelism = Runtime.getRuntime().availableProcessors();
1491 }
1492 if (maxPoolSizeValue != null) {
1493 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1494 parallelism = Integer.min(parallelism, maxPoolSize);
1495 } else {
1496 maxPoolSize = Integer.max(parallelism, 256);
1497 }
1498 if (minRunnableValue != null) {
1499 minRunnable = Integer.parseInt(minRunnableValue);
1500 } else {
1501 minRunnable = Integer.max(parallelism / 2, 1);
1502 }
1503 return new BuiltinDefaultScheduler(parallelism, maxPoolSize, minRunnable);
1504 }
1505
1506 /**
1507 * The built-in default ForkJoinPool scheduler.
1508 */
1509 private static class BuiltinDefaultScheduler
1510 extends ForkJoinPool implements VirtualThreadScheduler {
1511
1512 private static final StableValue<VirtualThreadScheduler> VIEW = StableValue.of();
1513
1514 BuiltinDefaultScheduler(int parallelism, int maxPoolSize, int minRunnable) {
1515 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1516 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1517 boolean asyncMode = true; // FIFO
1518 super(parallelism, factory, handler, asyncMode,
1519 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1520 }
1521
1522 @Override
1523 public void execute(Thread vthread, Runnable task) {
1524 execute(ForkJoinTask.adapt(task));
1525 }
1526
1527 /**
1528 * Wraps the scheduler to avoid leaking a direct reference.
1529 */
1530 VirtualThreadScheduler externalView() {
1531 VirtualThreadScheduler builtin = this;
1532 return VIEW.orElseSet(() -> {
1533 return new VirtualThreadScheduler() {
1534 @Override
1535 public void execute(Thread thread, Runnable task) {
1536 Objects.requireNonNull(thread);
1537 if (thread instanceof VirtualThread vthread) {
1538 VirtualThreadScheduler scheduler = vthread.scheduler;
1539 if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1540 builtin.execute(thread, task);
1541 } else {
1542 throw new IllegalArgumentException();
1543 }
1544 } else {
1545 throw new UnsupportedOperationException();
1546 }
1547 }
1548 };
1549 });
1550 }
1551 }
1552
1553 /**
1554 * Schedule a runnable task to run after a delay.
1555 */
1556 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1557 if (scheduler instanceof ForkJoinPool pool) {
1558 return pool.schedule(command, delay, unit);
1559 } else {
1560 return DelayedTaskSchedulers.schedule(command, delay, unit);
1561 }
1562 }
1563
1564 /**
1565 * Supports scheduling a runnable task to run after a delay. It uses a number
1566 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1567 * work queue used. This class is used when using a custom scheduler.
1568 */
1569 private static class DelayedTaskSchedulers {
1570 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1571
1572 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1573 long tid = Thread.currentThread().threadId();
1574 int index = (int) tid & (INSTANCE.length - 1);
1575 return INSTANCE[index].schedule(command, delay, unit);
1576 }
1577
1578 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1579 String propName = "jdk.virtualThreadScheduler.timerQueues";
1580 String propValue = System.getProperty(propName);
1581 int queueCount;
1582 if (propValue != null) {
1583 queueCount = Integer.parseInt(propValue);
1584 if (queueCount != Integer.highestOneBit(queueCount)) {
1585 throw new RuntimeException("Value of " + propName + " must be power of 2");
1586 }
1587 } else {
1588 int ncpus = Runtime.getRuntime().availableProcessors();
1589 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1590 }
1591 var schedulers = new ScheduledExecutorService[queueCount];
1592 for (int i = 0; i < queueCount; i++) {
1593 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1594 Executors.newScheduledThreadPool(1, task -> {
1595 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1596 t.setDaemon(true);
1597 return t;
1598 });
1599 stpe.setRemoveOnCancelPolicy(true);
1600 schedulers[i] = stpe;
1601 }
1602 return schedulers;
1603 }
1604 }
1605
1606 /**
1607 * Schedule virtual threads that are ready to be scheduled after they blocked on
1608 * monitor enter.
1609 */
1610 private static void unblockVirtualThreads() {
1611 while (true) {
1612 VirtualThread vthread = takeVirtualThreadListToUnblock();
1613 while (vthread != null) {
1614 assert vthread.onWaitingList;
1615 VirtualThread nextThread = vthread.next;
1616
1617 // remove from list and unblock
1618 vthread.next = null;
1619 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1620 assert changed;
1621 vthread.unblock();
1622
1623 vthread = nextThread;
1624 }
1625 }
1626 }
1627
1628 /**
1629 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1630 * if necessary until a list of one or more threads becomes available.
1631 */
1632 private static native VirtualThread takeVirtualThreadListToUnblock();
1633
1634 static {
1635 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1636 VirtualThread::unblockVirtualThreads);
1637 unblocker.setDaemon(true);
1638 unblocker.start();
1639 }
1640 }