1 /*
2 * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.lang.invoke.MethodHandles;
28 import java.lang.invoke.VarHandle;
29 import java.lang.reflect.Constructor;
30 import java.util.Locale;
31 import java.util.Objects;
32 import java.util.concurrent.Executors;
33 import java.util.concurrent.ForkJoinPool;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.Future;
36 import java.util.concurrent.LinkedTransferQueue;
37 import java.util.concurrent.RejectedExecutionException;
38 import java.util.concurrent.ScheduledExecutorService;
39 import java.util.concurrent.ScheduledFuture;
40 import java.util.concurrent.ScheduledThreadPoolExecutor;
41 import java.util.concurrent.ThreadFactory;
42 import java.util.concurrent.ThreadPoolExecutor;
43 import java.util.concurrent.TimeUnit;
44 import jdk.internal.event.VirtualThreadEndEvent;
45 import jdk.internal.event.VirtualThreadStartEvent;
46 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
47 import jdk.internal.invoke.MhUtil;
48 import jdk.internal.misc.CarrierThread;
49 import jdk.internal.misc.InnocuousThread;
50 import jdk.internal.misc.Unsafe;
51 import jdk.internal.vm.Continuation;
52 import jdk.internal.vm.ContinuationScope;
53 import jdk.internal.vm.StackableScope;
54 import jdk.internal.vm.ThreadContainer;
55 import jdk.internal.vm.ThreadContainers;
56 import jdk.internal.vm.annotation.ChangesCurrentThread;
57 import jdk.internal.vm.annotation.Hidden;
58 import jdk.internal.vm.annotation.IntrinsicCandidate;
59 import jdk.internal.vm.annotation.JvmtiHideEvents;
60 import jdk.internal.vm.annotation.JvmtiMountTransition;
61 import jdk.internal.vm.annotation.ReservedStackAccess;
62 import sun.nio.ch.Interruptible;
63 import static java.util.concurrent.TimeUnit.*;
64
65 /**
66 * A thread that is scheduled by the Java virtual machine rather than the operating system.
67 */
68 final class VirtualThread extends BaseVirtualThread {
69 private static final Unsafe U = Unsafe.getUnsafe();
70 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
71
72 private static final VirtualThreadScheduler BUILTIN_SCHEDULER;
73 private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
74 private static final VirtualThreadScheduler EXTERNAL_VIEW;
75 private static final boolean USE_STPE;
76 static {
77 // experimental
78 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
79 if (propValue != null) {
80 VirtualThreadScheduler builtinScheduler = createBuiltinScheduler(true);
81 VirtualThreadScheduler externalView = createExternalView(builtinScheduler);
82 VirtualThreadScheduler defaultScheduler = loadCustomScheduler(externalView, propValue);
83 BUILTIN_SCHEDULER = builtinScheduler;
84 DEFAULT_SCHEDULER = defaultScheduler;
85 EXTERNAL_VIEW = externalView;
86 } else {
87 var builtinScheduler = createBuiltinScheduler(false);
88 BUILTIN_SCHEDULER = builtinScheduler;
89 DEFAULT_SCHEDULER = builtinScheduler;
90 EXTERNAL_VIEW = createExternalView(builtinScheduler);
91 }
92 propValue = System.getProperty("jdk.virtualThreadScheduler.useSTPE");
93 if (propValue != null) {
94 USE_STPE = Boolean.parseBoolean(propValue);
95 } else {
96 USE_STPE = Runtime.getRuntime().availableProcessors() < 4;
97 }
98 }
99
100 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
101 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
102 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
103 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
104
105 // scheduler and continuation
106 private final VirtualThreadScheduler scheduler;
107 private final Continuation cont;
108 private final VThreadTask runContinuation;
109
110 // virtual thread state, accessed by VM
111 private volatile int state;
112
113 /*
114 * Virtual thread state transitions:
115 *
116 * NEW -> STARTED // Thread.start, schedule to run
117 * STARTED -> TERMINATED // failed to start
118 * STARTED -> RUNNING // first run
119 * RUNNING -> TERMINATED // done
120 *
121 * RUNNING -> PARKING // Thread parking with LockSupport.park
122 * PARKING -> PARKED // cont.yield successful, parked indefinitely
123 * PARKED -> UNPARKED // unparked, may be scheduled to continue
124 * UNPARKED -> RUNNING // continue execution after park
125 *
126 * PARKING -> RUNNING // cont.yield failed, need to park on carrier
127 * RUNNING -> PINNED // park on carrier
128 * PINNED -> RUNNING // unparked, continue execution on same carrier
129 *
130 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
131 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
132 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
133 *
134 * TIMED_PARKING -> RUNNING // cont.yield failed, need to park on carrier
135 * RUNNING -> TIMED_PINNED // park on carrier
136 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
137 *
138 * RUNNING -> BLOCKING // blocking on monitor enter
139 * BLOCKING -> BLOCKED // blocked on monitor enter
140 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
141 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
142 *
143 * RUNNING -> WAITING // transitional state during wait on monitor
144 * WAITING -> WAIT // waiting on monitor
145 * WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
146 * WAIT -> UNBLOCKED // interrupted
147 *
148 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
149 * TIMED_WAITING -> TIMED_WAIT // timed-waiting on monitor
150 * TIMED_WAIT -> BLOCKED // notified, waiting to be unblocked by monitor owner
151 * TIMED_WAIT -> UNBLOCKED // timed-out/interrupted
152 *
153 * RUNNING -> YIELDING // Thread.yield
154 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
155 * YIELDING -> RUNNING // cont.yield failed
156 * YIELDED -> RUNNING // continue execution after Thread.yield
157 */
158 private static final int NEW = 0;
159 private static final int STARTED = 1;
160 private static final int RUNNING = 2; // runnable-mounted
161
162 // untimed and timed parking
163 private static final int PARKING = 3;
164 private static final int PARKED = 4; // unmounted
165 private static final int PINNED = 5; // mounted
166 private static final int TIMED_PARKING = 6;
167 private static final int TIMED_PARKED = 7; // unmounted
168 private static final int TIMED_PINNED = 8; // mounted
169 private static final int UNPARKED = 9; // unmounted but runnable
170
171 // Thread.yield
172 private static final int YIELDING = 10;
173 private static final int YIELDED = 11; // unmounted but runnable
174
175 // monitor enter
176 private static final int BLOCKING = 12;
177 private static final int BLOCKED = 13; // unmounted
178 private static final int UNBLOCKED = 14; // unmounted but runnable
179
180 // monitor wait/timed-wait
181 private static final int WAITING = 15;
182 private static final int WAIT = 16; // waiting in Object.wait
183 private static final int TIMED_WAITING = 17;
184 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
185
186 private static final int TERMINATED = 99; // final state
187
188 // parking permit made available by LockSupport.unpark
189 private volatile boolean parkPermit;
190
191 // blocking permit made available by unblocker thread when another thread exits monitor
192 private volatile boolean blockPermit;
193
194 // true when on the list of virtual threads waiting to be unblocked
195 private volatile boolean onWaitingList;
196
197 // next virtual thread on the list of virtual threads waiting to be unblocked
198 private volatile VirtualThread next;
199
200 // notified by Object.notify/notifyAll while waiting in Object.wait
201 private volatile boolean notified;
202
203 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
204 private volatile boolean interruptibleWait;
205
206 // timed-wait support
207 private byte timedWaitSeqNo;
208
209 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
210 private long timeout;
211
212 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
213 private Future<?> timeoutTask;
214
215 // carrier thread when mounted, accessed by VM
216 private volatile Thread carrierThread;
217
218 // true to notifyAll after this virtual thread terminates
219 private volatile boolean notifyAllAfterTerminate;
220
221 /**
222 * Return the built-in scheduler.
223 * @param trusted true if caller is trusted, false if not trusted
224 */
225 static VirtualThreadScheduler builtinScheduler(boolean trusted) {
226 return trusted ? BUILTIN_SCHEDULER : EXTERNAL_VIEW;
227 }
228
229 /**
230 * Returns the default scheduler, usually the same as the built-in scheduler.
231 */
232 static VirtualThreadScheduler defaultScheduler() {
233 return DEFAULT_SCHEDULER;
234 }
235
236 /**
237 * Returns the continuation scope used for virtual threads.
238 */
239 static ContinuationScope continuationScope() {
240 return VTHREAD_SCOPE;
241 }
242
243 /**
244 * Returns the task to start/continue this virtual thread.
245 */
246 VirtualThreadTask virtualThreadTask() {
247 return runContinuation;
248 }
249
250 /**
251 * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
252 *
253 * @param scheduler the scheduler or null for default scheduler
254 * @param preferredCarrier the preferred carrier or null
255 * @param name thread name
256 * @param characteristics characteristics
257 * @param task the task to execute
258 */
259 VirtualThread(VirtualThreadScheduler scheduler,
260 Thread preferredCarrier,
261 String name,
262 int characteristics,
263 Runnable task) {
264 super(name, characteristics, /*bound*/ false);
265 Objects.requireNonNull(task);
266
267 // use default scheduler if not provided
268 if (scheduler == null) {
269 scheduler = DEFAULT_SCHEDULER;
270 } else if (scheduler == EXTERNAL_VIEW) {
271 throw new UnsupportedOperationException();
272 }
273 this.scheduler = scheduler;
274 this.cont = new VThreadContinuation(this, task);
275
276 if (scheduler == BUILTIN_SCHEDULER) {
277 this.runContinuation = new VThreadTask(this);
278 } else {
279 this.runContinuation = new CustomVThreadTask(this, preferredCarrier);
280 }
281 }
282
283 /**
284 * The task to start/continue a virtual thread.
285 */
286 static non-sealed class VThreadTask implements VirtualThreadTask {
287 private final VirtualThread vthread;
288 VThreadTask(VirtualThread vthread) {
289 this.vthread = vthread;
290 }
291 @Override
292 public final Thread thread() {
293 return vthread;
294 }
295 @Override
296 public final void run() {
297 vthread.runContinuation();;
298 }
299 @Override
300 public Thread preferredCarrier() {
301 throw new UnsupportedOperationException();
302 }
303 @Override
304 public Object attach(Object att) {
305 throw new UnsupportedOperationException();
306 }
307 @Override
308 public Object attachment() {
309 throw new UnsupportedOperationException();
310 }
311 }
312
313 /**
314 * The task to start/continue a virtual thread when using a custom scheduler.
315 */
316 static final class CustomVThreadTask extends VThreadTask {
317 private static final VarHandle ATT =
318 MhUtil.findVarHandle(MethodHandles.lookup(), "att", Object.class);
319 private final Thread preferredCarrier;
320 private volatile Object att;
321 CustomVThreadTask(VirtualThread vthread, Thread preferredCarrier) {
322 super(vthread);
323 this.preferredCarrier = preferredCarrier;
324 }
325 @Override
326 public Thread preferredCarrier() {
327 return preferredCarrier;
328 }
329 @Override
330 public Object attach(Object att) {
331 return ATT.getAndSet(this, att);
332 }
333 @Override
334 public Object attachment() {
335 return att;
336 }
337 }
338
339 /**
340 * The continuation that a virtual thread executes.
341 */
342 private static class VThreadContinuation extends Continuation {
343 VThreadContinuation(VirtualThread vthread, Runnable task) {
344 super(VTHREAD_SCOPE, wrap(vthread, task));
345 }
346 @Override
347 protected void onPinned(Continuation.Pinned reason) {
348 }
349 private static Runnable wrap(VirtualThread vthread, Runnable task) {
350 return new Runnable() {
351 @Hidden
352 @JvmtiHideEvents
353 public void run() {
354 vthread.endFirstTransition();
355 try {
356 vthread.run(task);
357 } finally {
358 vthread.startFinalTransition();
359 }
360 }
361 };
362 }
363 }
364
365 /**
366 * Runs or continues execution on the current thread. The virtual thread is mounted
367 * on the current thread before the task runs or continues. It unmounts when the
368 * task completes or yields.
369 */
370 @ChangesCurrentThread // allow mount/unmount to be inlined
371 private void runContinuation() {
372 // the carrier must be a platform thread
373 if (Thread.currentThread().isVirtual()) {
374 throw new WrongThreadException();
375 }
376
377 // set state to RUNNING
378 int initialState = state();
379 if (initialState == STARTED || initialState == UNPARKED
380 || initialState == UNBLOCKED || initialState == YIELDED) {
381 // newly started or continue after parking/blocking/Thread.yield
382 if (!compareAndSetState(initialState, RUNNING)) {
383 return;
384 }
385 // consume permit when continuing after parking or blocking. If continue
386 // after a timed-park or timed-wait then the timeout task is cancelled.
387 if (initialState == UNPARKED) {
388 cancelTimeoutTask();
389 setParkPermit(false);
390 } else if (initialState == UNBLOCKED) {
391 cancelTimeoutTask();
392 blockPermit = false;
393 }
394 } else {
395 // not runnable
396 return;
397 }
398
399 mount();
400 try {
401 cont.run();
402 } finally {
403 unmount();
404 if (cont.isDone()) {
405 afterDone();
406 } else {
407 afterYield();
408 }
409 }
410 }
411
412 /**
413 * Cancel timeout task when continuing after timed-park or timed-wait.
414 * The timeout task may be executing, or may have already completed.
415 */
416 private void cancelTimeoutTask() {
417 if (timeoutTask != null) {
418 timeoutTask.cancel(false);
419 timeoutTask = null;
420 }
421 }
422
423 /**
424 * Submits the runContinuation task to the scheduler. For the built-in scheduler,
425 * the task will be pushed to the local queue if possible, otherwise it will be
426 * pushed to an external submission queue.
427 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
428 * @throws RejectedExecutionException
429 */
430 private void submitRunContinuation(boolean retryOnOOME) {
431 boolean done = false;
432 while (!done) {
433 try {
434 // Pin the continuation to prevent the virtual thread from unmounting
435 // when submitting a task. For the default scheduler this ensures that
436 // the carrier doesn't change when pushing a task. For other schedulers
437 // it avoids deadlock that could arise due to carriers and virtual
438 // threads contending for a lock.
439 if (currentThread().isVirtual()) {
440 Continuation.pin();
441 try {
442 scheduler.onContinue(runContinuation);
443 } finally {
444 Continuation.unpin();
445 }
446 } else {
447 scheduler.onContinue(runContinuation);
448 }
449 done = true;
450 } catch (RejectedExecutionException ree) {
451 submitFailed(ree);
452 throw ree;
453 } catch (OutOfMemoryError e) {
454 if (retryOnOOME) {
455 U.park(false, 100_000_000); // 100ms
456 } else {
457 throw e;
458 }
459 }
460 }
461 }
462
463 /**
464 * Submits the runContinuation task to the scheduler. For the default scheduler,
465 * and calling it on a worker thread, the task will be pushed to the local queue,
466 * otherwise it will be pushed to an external submission queue.
467 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
468 * @throws RejectedExecutionException
469 */
470 private void submitRunContinuation() {
471 submitRunContinuation(true);
472 }
473
474 /**
475 * Invoked from a carrier thread to lazy submit the runContinuation task to the
476 * carrier's local queue if the queue is empty. If not empty, or invoked by a thread
477 * for a custom scheduler, then it just submits the task to the scheduler.
478 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
479 * @throws RejectedExecutionException
480 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
481 */
482 private void lazySubmitRunContinuation() {
483 assert !currentThread().isVirtual();
484 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
485 try {
486 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
487 } catch (RejectedExecutionException ree) {
488 submitFailed(ree);
489 throw ree;
490 } catch (OutOfMemoryError e) {
491 submitRunContinuation();
492 }
493 } else {
494 submitRunContinuation();
495 }
496 }
497
498 /**
499 * Invoked from a carrier thread to externally submit the runContinuation task to the
500 * scheduler. If invoked by a thread for a custom scheduler, then it just submits the
501 * task to the scheduler.
502 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
503 * @throws RejectedExecutionException
504 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
505 */
506 private void externalSubmitRunContinuation() {
507 assert !currentThread().isVirtual();
508 if (currentThread() instanceof CarrierThread ct) {
509 try {
510 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
511 } catch (RejectedExecutionException ree) {
512 submitFailed(ree);
513 throw ree;
514 } catch (OutOfMemoryError e) {
515 submitRunContinuation();
516 }
517 } else {
518 submitRunContinuation();
519 }
520 }
521
522 /**
523 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
524 */
525 private void submitFailed(RejectedExecutionException ree) {
526 var event = new VirtualThreadSubmitFailedEvent();
527 if (event.isEnabled()) {
528 event.javaThreadId = threadId();
529 event.exceptionMessage = ree.getMessage();
530 event.commit();
531 }
532 }
533
534 /**
535 * Runs a task in the context of this virtual thread.
536 */
537 private void run(Runnable task) {
538 assert Thread.currentThread() == this && state == RUNNING;
539
540 // emit JFR event if enabled
541 if (VirtualThreadStartEvent.isTurnedOn()) {
542 var event = new VirtualThreadStartEvent();
543 event.javaThreadId = threadId();
544 event.commit();
545 }
546
547 Object bindings = Thread.scopedValueBindings();
548 try {
549 runWith(bindings, task);
550 } catch (Throwable exc) {
551 dispatchUncaughtException(exc);
552 } finally {
553 // pop any remaining scopes from the stack, this may block
554 StackableScope.popAll();
555
556 // emit JFR event if enabled
557 if (VirtualThreadEndEvent.isTurnedOn()) {
558 var event = new VirtualThreadEndEvent();
559 event.javaThreadId = threadId();
560 event.commit();
561 }
562 }
563 }
564
565 /**
566 * Mounts this virtual thread onto the current platform thread. On
567 * return, the current thread is the virtual thread.
568 */
569 @ChangesCurrentThread
570 @ReservedStackAccess
571 private void mount() {
572 startTransition(/*mount*/true);
573 // We assume following volatile accesses provide equivalent
574 // of acquire ordering, otherwise we need U.loadFence() here.
575
576 // sets the carrier thread
577 Thread carrier = Thread.currentCarrierThread();
578 setCarrierThread(carrier);
579
580 // sync up carrier thread interrupted status if needed
581 if (interrupted) {
582 carrier.setInterrupt();
583 } else if (carrier.isInterrupted()) {
584 synchronized (interruptLock) {
585 // need to recheck interrupted status
586 if (!interrupted) {
587 carrier.clearInterrupt();
588 }
589 }
590 }
591
592 // set Thread.currentThread() to return this virtual thread
593 carrier.setCurrentThread(this);
594 }
595
596 /**
597 * Unmounts this virtual thread from the carrier. On return, the
598 * current thread is the current platform thread.
599 */
600 @ChangesCurrentThread
601 @ReservedStackAccess
602 private void unmount() {
603 assert !Thread.holdsLock(interruptLock);
604
605 // set Thread.currentThread() to return the platform thread
606 Thread carrier = this.carrierThread;
607 carrier.setCurrentThread(carrier);
608
609 // break connection to carrier thread, synchronized with interrupt
610 synchronized (interruptLock) {
611 setCarrierThread(null);
612 }
613 carrier.clearInterrupt();
614
615 // We assume previous volatile accesses provide equivalent
616 // of release ordering, otherwise we need U.storeFence() here.
617 endTransition(/*mount*/false);
618 }
619
620 /**
621 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
622 * the continuation continues.
623 */
624 @Hidden
625 private boolean yieldContinuation() {
626 startTransition(/*mount*/false);
627 try {
628 return Continuation.yield(VTHREAD_SCOPE);
629 } finally {
630 endTransition(/*mount*/true);
631 }
632 }
633
634 /**
635 * Invoked in the context of the carrier thread after the Continuation yields when
636 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
637 */
638 private void afterYield() {
639 assert carrierThread == null;
640
641 // re-adjust parallelism if the virtual thread yielded when compensating
642 if (currentThread() instanceof CarrierThread ct) {
643 ct.endBlocking();
644 }
645
646 int s = state();
647
648 // LockSupport.park/parkNanos
649 if (s == PARKING || s == TIMED_PARKING) {
650 int newState;
651 if (s == PARKING) {
652 setState(newState = PARKED);
653 } else {
654 // schedule unpark
655 long timeout = this.timeout;
656 assert timeout > 0;
657 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
658 setState(newState = TIMED_PARKED);
659 }
660
661 // may have been unparked while parking
662 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
663 // lazy submit if local queue is empty
664 lazySubmitRunContinuation();
665 }
666 return;
667 }
668
669 // Thread.yield
670 if (s == YIELDING) {
671 setState(YIELDED);
672
673 // external submit if there are no tasks in the local task queue
674 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
675 externalSubmitRunContinuation();
676 } else {
677 submitRunContinuation();
678 }
679 return;
680 }
681
682 // blocking on monitorenter
683 if (s == BLOCKING) {
684 setState(BLOCKED);
685
686 // may have been unblocked while blocking
687 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
688 // lazy submit if local queue is empty
689 lazySubmitRunContinuation();
690 }
691 return;
692 }
693
694 // Object.wait
695 if (s == WAITING || s == TIMED_WAITING) {
696 int newState;
697 boolean blocked;
698 boolean interruptible = interruptibleWait;
699 if (s == WAITING) {
700 setState(newState = WAIT);
701 // may have been notified while in transition
702 blocked = notified && compareAndSetState(WAIT, BLOCKED);
703 } else {
704 // For timed-wait, a timeout task is scheduled to execute. The timeout
705 // task will change the thread state to UNBLOCKED and submit the thread
706 // to the scheduler. A sequence number is used to ensure that the timeout
707 // task only unblocks the thread for this timed-wait. We synchronize with
708 // the timeout task to coordinate access to the sequence number and to
709 // ensure the timeout task doesn't execute until the thread has got to
710 // the TIMED_WAIT state.
711 long timeout = this.timeout;
712 assert timeout > 0;
713 synchronized (timedWaitLock()) {
714 byte seqNo = ++timedWaitSeqNo;
715 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
716 setState(newState = TIMED_WAIT);
717 // May have been notified while in transition. This must be done while
718 // holding the monitor to avoid changing the state of a new timed wait call.
719 blocked = notified && compareAndSetState(TIMED_WAIT, BLOCKED);
720 }
721 }
722
723 if (blocked) {
724 // may have been unblocked already
725 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
726 lazySubmitRunContinuation();
727 }
728 } else {
729 // may have been interrupted while in transition to wait state
730 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
731 lazySubmitRunContinuation();
732 }
733 }
734 return;
735 }
736
737 assert false;
738 }
739
740 /**
741 * Invoked after the continuation completes.
742 */
743 private void afterDone() {
744 afterDone(true);
745 }
746
747 /**
748 * Invoked after the continuation completes (or start failed). Sets the thread
749 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
750 *
751 * @param notifyContainer true if its container should be notified
752 */
753 private void afterDone(boolean notifyContainer) {
754 assert carrierThread == null;
755 setState(TERMINATED);
756
757 // notifyAll to wakeup any threads waiting for this thread to terminate
758 if (notifyAllAfterTerminate) {
759 synchronized (this) {
760 notifyAll();
761 }
762 }
763
764 // notify container
765 if (notifyContainer) {
766 threadContainer().remove(this);
767 }
768
769 // clear references to thread locals
770 clearReferences();
771 }
772
773 /**
774 * Schedules this {@code VirtualThread} to execute.
775 *
776 * @throws IllegalStateException if the container is shutdown or closed
777 * @throws IllegalThreadStateException if the thread has already been started
778 * @throws RejectedExecutionException if the scheduler cannot accept a task
779 */
780 private void start(ThreadContainer container, boolean lazy) {
781 if (!compareAndSetState(NEW, STARTED)) {
782 throw new IllegalThreadStateException("Already started");
783 }
784
785 // bind thread to container
786 assert threadContainer() == null;
787 setThreadContainer(container);
788
789 // start thread
790 boolean addedToContainer = false;
791 boolean started = false;
792 try {
793 container.add(this); // may throw
794 addedToContainer = true;
795
796 // scoped values may be inherited
797 inheritScopedValueBindings(container);
798
799 // submit task to schedule
800 try {
801 if (currentThread().isVirtual()) {
802 Continuation.pin();
803 try {
804 if (scheduler == BUILTIN_SCHEDULER
805 && currentCarrierThread() instanceof CarrierThread ct) {
806 ForkJoinPool pool = ct.getPool();
807 ForkJoinTask<?> task = ForkJoinTask.adapt(runContinuation);
808 if (lazy) {
809 pool.lazySubmit(task);
810 } else {
811 pool.externalSubmit(task);
812 }
813 } else {
814 scheduler.onStart(runContinuation);
815 }
816 } finally {
817 Continuation.unpin();
818 }
819 } else {
820 scheduler.onStart(runContinuation);
821 }
822 } catch (RejectedExecutionException ree) {
823 submitFailed(ree);
824 throw ree;
825 }
826
827 started = true;
828 } finally {
829 if (!started) {
830 afterDone(addedToContainer);
831 }
832 }
833 }
834
835 @Override
836 void start(ThreadContainer container) {
837 start(container, false);
838 }
839
840 @Override
841 public void start() {
842 start(ThreadContainers.root(), false);
843 }
844
845 /**
846 * Schedules this thread to begin execution without guarantee that it will execute.
847 */
848 void lazyStart() {
849 start(ThreadContainers.root(), true);
850 }
851
852 @Override
853 public void run() {
854 // do nothing
855 }
856
857 /**
858 * Invoked by Thread.join before a thread waits for this virtual thread to terminate.
859 */
860 void beforeJoin() {
861 notifyAllAfterTerminate = true;
862 }
863
864 /**
865 * Parks until unparked or interrupted. If already unparked then the parking
866 * permit is consumed and this method completes immediately (meaning it doesn't
867 * yield). It also completes immediately if the interrupted status is set.
868 */
869 @Override
870 void park() {
871 assert Thread.currentThread() == this;
872
873 // complete immediately if parking permit available or interrupted
874 if (getAndSetParkPermit(false) || interrupted)
875 return;
876
877 // park the thread
878 boolean yielded = false;
879 setState(PARKING);
880 try {
881 yielded = yieldContinuation();
882 } catch (OutOfMemoryError e) {
883 // park on carrier
884 } finally {
885 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
886 if (!yielded) {
887 assert state() == PARKING;
888 setState(RUNNING);
889 }
890 }
891
892 // park on the carrier thread when pinned
893 if (!yielded) {
894 parkOnCarrierThread(false, 0);
895 }
896 }
897
898 /**
899 * Parks up to the given waiting time or until unparked or interrupted.
900 * If already unparked then the parking permit is consumed and this method
901 * completes immediately (meaning it doesn't yield). It also completes immediately
902 * if the interrupted status is set or the waiting time is {@code <= 0}.
903 *
904 * @param nanos the maximum number of nanoseconds to wait.
905 */
906 @Override
907 void parkNanos(long nanos) {
908 assert Thread.currentThread() == this;
909
910 // complete immediately if parking permit available or interrupted
911 if (getAndSetParkPermit(false) || interrupted)
912 return;
913
914 // park the thread for the waiting time
915 if (nanos > 0) {
916 long startTime = System.nanoTime();
917
918 // park the thread, afterYield will schedule the thread to unpark
919 boolean yielded = false;
920 timeout = nanos;
921 setState(TIMED_PARKING);
922 try {
923 yielded = yieldContinuation();
924 } catch (OutOfMemoryError e) {
925 // park on carrier
926 } finally {
927 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
928 if (!yielded) {
929 assert state() == TIMED_PARKING;
930 setState(RUNNING);
931 }
932 }
933
934 // park on carrier thread for remaining time when pinned (or OOME)
935 if (!yielded) {
936 long remainingNanos = nanos - (System.nanoTime() - startTime);
937 parkOnCarrierThread(true, remainingNanos);
938 }
939 }
940 }
941
942 /**
943 * Parks the current carrier thread up to the given waiting time or until
944 * unparked or interrupted. If the virtual thread is interrupted then the
945 * interrupted status will be propagated to the carrier thread.
946 * @param timed true for a timed park, false for untimed
947 * @param nanos the waiting time in nanoseconds
948 */
949 private void parkOnCarrierThread(boolean timed, long nanos) {
950 assert state() == RUNNING;
951
952 setState(timed ? TIMED_PINNED : PINNED);
953 try {
954 if (!parkPermit) {
955 if (!timed) {
956 U.park(false, 0);
957 } else if (nanos > 0) {
958 U.park(false, nanos);
959 }
960 }
961 } finally {
962 setState(RUNNING);
963 }
964
965 // consume parking permit
966 setParkPermit(false);
967
968 // JFR jdk.VirtualThreadPinned event
969 postPinnedEvent("LockSupport.park");
970 }
971
972 /**
973 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
974 * Recording the event in the VM avoids having JFR event recorded in Java
975 * with the same name, but different ID, to events recorded by the VM.
976 */
977 @Hidden
978 private static native void postPinnedEvent(String op);
979
980 /**
981 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
982 * then its task is scheduled to continue, otherwise its next call to {@code park} or
983 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
984 * @param lazySubmit to use lazySubmit if possible
985 * @throws RejectedExecutionException if the scheduler cannot accept a task
986 */
987 private void unpark(boolean lazySubmit) {
988 if (!getAndSetParkPermit(true) && currentThread() != this) {
989 int s = state();
990
991 // unparked while parked
992 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
993
994 if (lazySubmit && currentThread().isVirtual()) {
995 Continuation.pin();
996 try {
997 if (scheduler == BUILTIN_SCHEDULER
998 && currentCarrierThread() instanceof CarrierThread ct) {
999 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
1000 return;
1001 }
1002 } catch (RejectedExecutionException ree) {
1003 submitFailed(ree);
1004 throw ree;
1005 } catch (OutOfMemoryError e) {
1006 // fall-though
1007 } finally {
1008 Continuation.unpin();
1009 }
1010 }
1011
1012 submitRunContinuation();
1013 return;
1014 }
1015
1016 // unparked while parked when pinned
1017 if (s == PINNED || s == TIMED_PINNED) {
1018 // unpark carrier thread when pinned
1019 disableSuspendAndPreempt();
1020 try {
1021 synchronized (carrierThreadAccessLock()) {
1022 Thread carrier = carrierThread;
1023 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
1024 U.unpark(carrier);
1025 }
1026 }
1027 } finally {
1028 enableSuspendAndPreempt();
1029 }
1030 return;
1031 }
1032 }
1033 }
1034
1035 @Override
1036 void unpark() {
1037 unpark(false);
1038 }
1039
1040 @Override
1041 void lazyUnpark() {
1042 unpark(true);
1043 }
1044
1045 /**
1046 * Invoked by unblocker thread to unblock this virtual thread.
1047 */
1048 private void unblock() {
1049 assert !Thread.currentThread().isVirtual();
1050 blockPermit = true;
1051 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
1052 submitRunContinuation();
1053 }
1054 }
1055
1056 /**
1057 * Invoked by FJP worker thread or STPE thread when park timeout expires.
1058 */
1059 private void parkTimeoutExpired() {
1060 assert !VirtualThread.currentThread().isVirtual();
1061 unpark(true);
1062 }
1063
1064 /**
1065 * Invoked by FJP worker thread or STPE thread when wait timeout expires.
1066 * If the virtual thread is in timed-wait then this method will unblock the thread
1067 * and submit its task so that it continues and attempts to reenter the monitor.
1068 * This method does nothing if the thread has been woken by notify or interrupt.
1069 */
1070 private void waitTimeoutExpired(byte seqNo) {
1071 assert !Thread.currentThread().isVirtual();
1072
1073 synchronized (timedWaitLock()) {
1074 if (seqNo != timedWaitSeqNo) {
1075 // this timeout task is for a past timed-wait
1076 return;
1077 }
1078 if (!compareAndSetState(TIMED_WAIT, UNBLOCKED)) {
1079 // already notified (or interrupted)
1080 return;
1081 }
1082 }
1083
1084 lazySubmitRunContinuation();
1085 }
1086
1087 /**
1088 * Attempts to yield the current virtual thread (Thread.yield).
1089 */
1090 void tryYield() {
1091 assert Thread.currentThread() == this;
1092 setState(YIELDING);
1093 boolean yielded = false;
1094 try {
1095 yielded = yieldContinuation(); // may throw
1096 } finally {
1097 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
1098 if (!yielded) {
1099 assert state() == YIELDING;
1100 setState(RUNNING);
1101 }
1102 }
1103 }
1104
1105 /**
1106 * Sleep the current thread for the given sleep time (in nanoseconds). If
1107 * nanos is 0 then the thread will attempt to yield.
1108 *
1109 * @implNote This implementation parks the thread for the given sleeping time
1110 * and will therefore be observed in PARKED state during the sleep. Parking
1111 * will consume the parking permit so this method makes available the parking
1112 * permit after the sleep. This may be observed as a spurious, but benign,
1113 * wakeup when the thread subsequently attempts to park.
1114 *
1115 * @param nanos the maximum number of nanoseconds to sleep
1116 * @throws InterruptedException if interrupted while sleeping
1117 */
1118 void sleepNanos(long nanos) throws InterruptedException {
1119 assert Thread.currentThread() == this && nanos >= 0;
1120 if (getAndClearInterrupt())
1121 throw new InterruptedException();
1122 if (nanos == 0) {
1123 tryYield();
1124 } else {
1125 // park for the sleep time
1126 try {
1127 long remainingNanos = nanos;
1128 long startNanos = System.nanoTime();
1129 while (remainingNanos > 0) {
1130 parkNanos(remainingNanos);
1131 if (getAndClearInterrupt()) {
1132 throw new InterruptedException();
1133 }
1134 remainingNanos = nanos - (System.nanoTime() - startNanos);
1135 }
1136 } finally {
1137 // may have been unparked while sleeping
1138 setParkPermit(true);
1139 }
1140 }
1141 }
1142
1143 @Override
1144 void blockedOn(Interruptible b) {
1145 disableSuspendAndPreempt();
1146 try {
1147 super.blockedOn(b);
1148 } finally {
1149 enableSuspendAndPreempt();
1150 }
1151 }
1152
1153 @Override
1154 public void interrupt() {
1155 if (Thread.currentThread() != this) {
1156 // if current thread is a virtual thread then prevent it from being
1157 // suspended or unmounted when entering or holding interruptLock
1158 Interruptible blocker;
1159 disableSuspendAndPreempt();
1160 try {
1161 synchronized (interruptLock) {
1162 interrupted = true;
1163 blocker = nioBlocker();
1164 if (blocker != null) {
1165 blocker.interrupt(this);
1166 }
1167
1168 // interrupt carrier thread if mounted
1169 Thread carrier = carrierThread;
1170 if (carrier != null) carrier.setInterrupt();
1171 }
1172 } finally {
1173 enableSuspendAndPreempt();
1174 }
1175
1176 // notify blocker after releasing interruptLock
1177 if (blocker != null) {
1178 blocker.postInterrupt();
1179 }
1180
1181 // make available parking permit, unpark thread if parked
1182 unpark();
1183
1184 // if thread is waiting in Object.wait then schedule to try to reenter
1185 int s = state();
1186 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1187 submitRunContinuation();
1188 }
1189
1190 } else {
1191 interrupted = true;
1192 carrierThread.setInterrupt();
1193 setParkPermit(true);
1194 }
1195 }
1196
1197 @Override
1198 public boolean isInterrupted() {
1199 return interrupted;
1200 }
1201
1202 @Override
1203 boolean getAndClearInterrupt() {
1204 assert Thread.currentThread() == this;
1205 boolean oldValue = interrupted;
1206 if (oldValue) {
1207 disableSuspendAndPreempt();
1208 try {
1209 synchronized (interruptLock) {
1210 interrupted = false;
1211 carrierThread.clearInterrupt();
1212 }
1213 } finally {
1214 enableSuspendAndPreempt();
1215 }
1216 }
1217 return oldValue;
1218 }
1219
1220 @Override
1221 Thread.State threadState() {
1222 switch (state()) {
1223 case NEW:
1224 return Thread.State.NEW;
1225 case STARTED:
1226 // return NEW if thread container not yet set
1227 if (threadContainer() == null) {
1228 return Thread.State.NEW;
1229 } else {
1230 return Thread.State.RUNNABLE;
1231 }
1232 case UNPARKED:
1233 case UNBLOCKED:
1234 case YIELDED:
1235 // runnable, not mounted
1236 return Thread.State.RUNNABLE;
1237 case RUNNING:
1238 // if mounted then return state of carrier thread
1239 if (Thread.currentThread() != this) {
1240 disableSuspendAndPreempt();
1241 try {
1242 synchronized (carrierThreadAccessLock()) {
1243 Thread carrierThread = this.carrierThread;
1244 if (carrierThread != null) {
1245 return carrierThread.threadState();
1246 }
1247 }
1248 } finally {
1249 enableSuspendAndPreempt();
1250 }
1251 }
1252 // runnable, mounted
1253 return Thread.State.RUNNABLE;
1254 case PARKING:
1255 case TIMED_PARKING:
1256 case WAITING:
1257 case TIMED_WAITING:
1258 case YIELDING:
1259 // runnable, in transition
1260 return Thread.State.RUNNABLE;
1261 case PARKED:
1262 case PINNED:
1263 case WAIT:
1264 return Thread.State.WAITING;
1265 case TIMED_PARKED:
1266 case TIMED_PINNED:
1267 case TIMED_WAIT:
1268 return Thread.State.TIMED_WAITING;
1269 case BLOCKING:
1270 case BLOCKED:
1271 return Thread.State.BLOCKED;
1272 case TERMINATED:
1273 return Thread.State.TERMINATED;
1274 default:
1275 throw new InternalError();
1276 }
1277 }
1278
1279 @Override
1280 boolean alive() {
1281 int s = state;
1282 return (s != NEW && s != TERMINATED);
1283 }
1284
1285 @Override
1286 boolean isTerminated() {
1287 return (state == TERMINATED);
1288 }
1289
1290 @Override
1291 public String toString() {
1292 StringBuilder sb = new StringBuilder("VirtualThread[#");
1293 sb.append(threadId());
1294 String name = getName();
1295 if (!name.isEmpty()) {
1296 sb.append(",");
1297 sb.append(name);
1298 }
1299 sb.append("]/");
1300
1301 // add the carrier state and thread name when mounted
1302 boolean mounted;
1303 if (Thread.currentThread() == this) {
1304 mounted = appendCarrierInfo(sb);
1305 } else {
1306 disableSuspendAndPreempt();
1307 try {
1308 synchronized (carrierThreadAccessLock()) {
1309 mounted = appendCarrierInfo(sb);
1310 }
1311 } finally {
1312 enableSuspendAndPreempt();
1313 }
1314 }
1315
1316 // add virtual thread state when not mounted
1317 if (!mounted) {
1318 String stateAsString = threadState().toString();
1319 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1320 }
1321
1322 return sb.toString();
1323 }
1324
1325 /**
1326 * Appends the carrier state and thread name to the string buffer if mounted.
1327 * @return true if mounted, false if not mounted
1328 */
1329 private boolean appendCarrierInfo(StringBuilder sb) {
1330 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1331 Thread carrier = carrierThread;
1332 if (carrier != null) {
1333 String stateAsString = carrier.threadState().toString();
1334 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1335 sb.append('@');
1336 sb.append(carrier.getName());
1337 return true;
1338 } else {
1339 return false;
1340 }
1341 }
1342
1343 @Override
1344 public int hashCode() {
1345 return (int) threadId();
1346 }
1347
1348 @Override
1349 public boolean equals(Object obj) {
1350 return obj == this;
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 // The following four methods notify the VM when a "transition" starts and ends.
1424 // A "mount transition" embodies the steps to transfer control from a platform
1425 // thread to a virtual thread, changing the thread identity, and starting or
1426 // resuming the virtual thread's continuation on the carrier.
1427 // An "unmount transition" embodies the steps to transfer control from a virtual
1428 // thread to its carrier, suspending the virtual thread's continuation, and
1429 // restoring the thread identity to the platform thread.
1430 // The notifications to the VM are necessary in order to coordinate with functions
1431 // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1432 // a transition may block if transitions are disabled. Ending a transition may
1433 // notify a thread that is waiting to disable transitions. The notifications are
1434 // also used to post JVMTI events for virtual thread start and end.
1435
1436 @IntrinsicCandidate
1437 @JvmtiMountTransition
1438 private native void endFirstTransition();
1439
1440 @IntrinsicCandidate
1441 @JvmtiMountTransition
1442 private native void startFinalTransition();
1443
1444 @IntrinsicCandidate
1445 @JvmtiMountTransition
1446 private native void startTransition(boolean mount);
1447
1448 @IntrinsicCandidate
1449 @JvmtiMountTransition
1450 private native void endTransition(boolean mount);
1451
1452 @IntrinsicCandidate
1453 private static native void notifyJvmtiDisableSuspend(boolean enter);
1454
1455 private static native void registerNatives();
1456 static {
1457 registerNatives();
1458
1459 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1460 var group = Thread.virtualThreadGroup();
1461 }
1462
1463 /**
1464 * Loads a VirtualThreadScheduler with the given class name. The class must be public
1465 * in an exported package, with public one-arg or no-arg constructor, and be visible
1466 * to the system class loader.
1467 * @param delegate the scheduler that the custom scheduler may delegate to
1468 * @param cn the class name of the custom scheduler
1469 */
1470 private static VirtualThreadScheduler loadCustomScheduler(VirtualThreadScheduler delegate, String cn) {
1471 VirtualThreadScheduler scheduler;
1472 try {
1473 Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1474 // 1-arg constructor
1475 try {
1476 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1477 return (VirtualThreadScheduler) ctor.newInstance(delegate);
1478 } catch (NoSuchMethodException e) {
1479 // 0-arg constructor
1480 Constructor<?> ctor = clazz.getConstructor();
1481 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1482 }
1483 } catch (Exception ex) {
1484 throw new Error(ex);
1485 }
1486 System.err.println("WARNING: Using custom default scheduler, this is an experimental feature!");
1487 return scheduler;
1488 }
1489
1490 /**
1491 * Creates the built-in ForkJoinPool scheduler.
1492 * @param wrapped true if wrapped by a custom default scheduler
1493 */
1494 private static VirtualThreadScheduler createBuiltinScheduler(boolean wrapped) {
1495 int parallelism, maxPoolSize, minRunnable;
1496 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1497 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1498 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1499 if (parallelismValue != null) {
1500 parallelism = Integer.parseInt(parallelismValue);
1501 } else {
1502 parallelism = Runtime.getRuntime().availableProcessors();
1503 }
1504 if (maxPoolSizeValue != null) {
1505 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1506 parallelism = Integer.min(parallelism, maxPoolSize);
1507 } else {
1508 maxPoolSize = Integer.max(parallelism, 256);
1509 }
1510 if (minRunnableValue != null) {
1511 minRunnable = Integer.parseInt(minRunnableValue);
1512 } else {
1513 minRunnable = Integer.max(parallelism / 2, 1);
1514 }
1515 if (Boolean.getBoolean("jdk.virtualThreadScheduler.useTPE")) {
1516 return new BuiltinThreadPoolExecutorScheduler(parallelism);
1517 } else {
1518 return new BuiltinForkJoinPoolScheduler(parallelism, maxPoolSize, minRunnable, wrapped);
1519 }
1520 }
1521
1522 /**
1523 * The built-in ForkJoinPool scheduler.
1524 */
1525 private static class BuiltinForkJoinPoolScheduler
1526 extends ForkJoinPool implements VirtualThreadScheduler {
1527
1528 BuiltinForkJoinPoolScheduler(int parallelism, int maxPoolSize, int minRunnable, boolean wrapped) {
1529 ForkJoinWorkerThreadFactory factory = wrapped
1530 ? ForkJoinPool.defaultForkJoinWorkerThreadFactory
1531 : CarrierThread::new;
1532 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1533 boolean asyncMode = true; // FIFO
1534 super(parallelism, factory, handler, asyncMode,
1535 0, maxPoolSize, minRunnable, pool -> true, 30L, SECONDS);
1536 }
1537
1538 @Override
1539 public void onStart(VirtualThreadTask task) {
1540 execute(ForkJoinTask.adapt(task));
1541 }
1542
1543 @Override
1544 public void onContinue(VirtualThreadTask task) {
1545 execute(ForkJoinTask.adapt(task));
1546 }
1547
1548 @Override
1549 public ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit) {
1550 return super.schedule(task, delay, unit);
1551 }
1552 }
1553
1554 /**
1555 * Built-in ThreadPoolExecutor scheduler.
1556 */
1557 private static class BuiltinThreadPoolExecutorScheduler
1558 extends ThreadPoolExecutor implements VirtualThreadScheduler {
1559
1560 BuiltinThreadPoolExecutorScheduler(int maxPoolSize) {
1561 ThreadFactory factory = task -> {
1562 Thread t = InnocuousThread.newThread(task);
1563 t.setDaemon(true);
1564 return t;
1565 };
1566 super(maxPoolSize, maxPoolSize,
1567 0L, SECONDS,
1568 new LinkedTransferQueue<>(),
1569 factory);
1570 }
1571
1572 @Override
1573 public void onStart(VirtualThreadTask task) {
1574 execute(task);
1575 }
1576
1577 @Override
1578 public void onContinue(VirtualThreadTask task) {
1579 execute(task);
1580 }
1581 }
1582
1583 /**
1584 * Wraps the scheduler to avoid leaking a direct reference to built-in scheduler.
1585 */
1586 static VirtualThreadScheduler createExternalView(VirtualThreadScheduler delegate) {
1587 return new VirtualThreadScheduler() {
1588 private void check(VirtualThreadTask task) {
1589 var vthread = (VirtualThread) task.thread();
1590 VirtualThreadScheduler scheduler = vthread.scheduler;
1591 if (scheduler != this && scheduler != DEFAULT_SCHEDULER) {
1592 throw new IllegalArgumentException();
1593 }
1594 }
1595 @Override
1596 public void onStart(VirtualThreadTask task) {
1597 check(task);
1598 delegate.onStart(task);
1599 }
1600 @Override
1601 public void onContinue(VirtualThreadTask task) {
1602 check(task);
1603 delegate.onContinue(task);
1604 }
1605 @Override
1606 public String toString() {
1607 return delegate.toString();
1608 }
1609 };
1610 }
1611
1612 /**
1613 * Schedule a runnable task to run after a delay.
1614 */
1615 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1616 if (USE_STPE) {
1617 return DelayedTaskSchedulers.schedule(command, delay, unit);
1618 } else {
1619 return scheduler.schedule(command, delay, unit);
1620 }
1621 }
1622
1623 /**
1624 * Supports scheduling a runnable task to run after a delay. It uses a number
1625 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1626 * work queue used. This class is used when using a custom scheduler.
1627 */
1628 static class DelayedTaskSchedulers {
1629 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1630
1631 static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1632 long tid = Thread.currentThread().threadId();
1633 int index = (int) tid & (INSTANCE.length - 1);
1634 return INSTANCE[index].schedule(command, delay, unit);
1635 }
1636
1637 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1638 String propName = "jdk.virtualThreadScheduler.timerQueues";
1639 String propValue = System.getProperty(propName);
1640 int queueCount;
1641 if (propValue != null) {
1642 queueCount = Integer.parseInt(propValue);
1643 if (queueCount != Integer.highestOneBit(queueCount)) {
1644 throw new RuntimeException("Value of " + propName + " must be power of 2");
1645 }
1646 } else {
1647 int ncpus = Runtime.getRuntime().availableProcessors();
1648 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1649 }
1650 var schedulers = new ScheduledExecutorService[queueCount];
1651 for (int i = 0; i < queueCount; i++) {
1652 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1653 Executors.newScheduledThreadPool(1, task -> {
1654 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1655 t.setDaemon(true);
1656 return t;
1657 });
1658 stpe.setRemoveOnCancelPolicy(true);
1659 schedulers[i] = stpe;
1660 }
1661 return schedulers;
1662 }
1663 }
1664
1665 /**
1666 * Schedule virtual threads that are ready to be scheduled after they blocked on
1667 * monitor enter.
1668 */
1669 private static void unblockVirtualThreads() {
1670 while (true) {
1671 VirtualThread vthread = takeVirtualThreadListToUnblock();
1672 while (vthread != null) {
1673 assert vthread.onWaitingList;
1674 VirtualThread nextThread = vthread.next;
1675
1676 // remove from list and unblock
1677 vthread.next = null;
1678 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1679 assert changed;
1680 vthread.unblock();
1681
1682 vthread = nextThread;
1683 }
1684 }
1685 }
1686
1687 /**
1688 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1689 * if necessary until a list of one or more threads becomes available.
1690 */
1691 private static native VirtualThread takeVirtualThreadListToUnblock();
1692
1693 static {
1694 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1695 VirtualThread::unblockVirtualThreads);
1696 unblocker.setDaemon(true);
1697 unblocker.start();
1698 }
1699 }