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.util.Locale;
  28 import java.util.Objects;
  29 import java.util.concurrent.Executor;
  30 import java.util.concurrent.Executors;
  31 import java.util.concurrent.ForkJoinPool;
  32 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
  33 import java.util.concurrent.ForkJoinTask;
  34 import java.util.concurrent.Future;

  35 import java.util.concurrent.RejectedExecutionException;
  36 import java.util.concurrent.ScheduledExecutorService;

  37 import java.util.concurrent.ScheduledThreadPoolExecutor;


  38 import java.util.concurrent.TimeUnit;
  39 import jdk.internal.event.VirtualThreadEndEvent;
  40 import jdk.internal.event.VirtualThreadStartEvent;
  41 import jdk.internal.event.VirtualThreadSubmitFailedEvent;

  42 import jdk.internal.misc.CarrierThread;
  43 import jdk.internal.misc.InnocuousThread;
  44 import jdk.internal.misc.Unsafe;
  45 import jdk.internal.vm.Continuation;
  46 import jdk.internal.vm.ContinuationScope;
  47 import jdk.internal.vm.StackableScope;
  48 import jdk.internal.vm.ThreadContainer;
  49 import jdk.internal.vm.ThreadContainers;
  50 import jdk.internal.vm.annotation.ChangesCurrentThread;
  51 import jdk.internal.vm.annotation.Hidden;
  52 import jdk.internal.vm.annotation.IntrinsicCandidate;
  53 import jdk.internal.vm.annotation.JvmtiHideEvents;
  54 import jdk.internal.vm.annotation.JvmtiMountTransition;
  55 import jdk.internal.vm.annotation.ReservedStackAccess;
  56 import sun.nio.ch.Interruptible;
  57 import static java.util.concurrent.TimeUnit.*;
  58 
  59 /**
  60  * A thread that is scheduled by the Java virtual machine rather than the operating system.
  61  */
  62 final class VirtualThread extends BaseVirtualThread {
  63     private static final Unsafe U = Unsafe.getUnsafe();
  64     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  65     private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();






















  66 
  67     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  68     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  69     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  70     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  71 
  72     // scheduler and continuation
  73     private final Executor scheduler;
  74     private final Continuation cont;
  75     private final Runnable runContinuation;
  76 
  77     // virtual thread state, accessed by VM
  78     private volatile int state;
  79 
  80     /*
  81      * Virtual thread state transitions:
  82      *
  83      *      NEW -> STARTED         // Thread.start, schedule to run
  84      *  STARTED -> TERMINATED      // failed to start
  85      *  STARTED -> RUNNING         // first run
  86      *  RUNNING -> TERMINATED      // done
  87      *
  88      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
  89      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
  90      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
  91      * UNPARKED -> RUNNING         // continue execution after park
  92      *
  93      *  PARKING -> RUNNING         // cont.yield failed, need to park on carrier
  94      *  RUNNING -> PINNED          // park on carrier
  95      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
  96      *
  97      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
  98      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
  99      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 100      *
 101      * TIMED_PARKING -> RUNNING         // cont.yield failed, need to park on carrier
 102      *       RUNNING -> TIMED_PINNED    // park on carrier
 103      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 104      *
 105      *   RUNNING -> BLOCKING       // blocking on monitor enter
 106      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 107      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 108      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 109      *
 110      *   RUNNING -> WAITING        // transitional state during wait on monitor
 111      *   WAITING -> WAIT           // waiting on monitor
 112      *      WAIT -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 113      *      WAIT -> UNBLOCKED      // interrupted
 114      *
 115      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 116      * TIMED_WAITING -> TIMED_WAIT      // timed-waiting on monitor
 117      *    TIMED_WAIT -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 118      *    TIMED_WAIT -> UNBLOCKED       // timed-out/interrupted
 119      *
 120      *  RUNNING -> YIELDING        // Thread.yield
 121      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 122      * YIELDING -> RUNNING         // cont.yield failed
 123      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 124      */
 125     private static final int NEW      = 0;
 126     private static final int STARTED  = 1;
 127     private static final int RUNNING  = 2;     // runnable-mounted
 128 
 129     // untimed and timed parking
 130     private static final int PARKING       = 3;
 131     private static final int PARKED        = 4;     // unmounted
 132     private static final int PINNED        = 5;     // mounted
 133     private static final int TIMED_PARKING = 6;
 134     private static final int TIMED_PARKED  = 7;     // unmounted
 135     private static final int TIMED_PINNED  = 8;     // mounted
 136     private static final int UNPARKED      = 9;     // unmounted but runnable
 137 
 138     // Thread.yield
 139     private static final int YIELDING = 10;
 140     private static final int YIELDED  = 11;         // unmounted but runnable
 141 
 142     // monitor enter
 143     private static final int BLOCKING  = 12;
 144     private static final int BLOCKED   = 13;        // unmounted
 145     private static final int UNBLOCKED = 14;        // unmounted but runnable
 146 
 147     // monitor wait/timed-wait
 148     private static final int WAITING       = 15;
 149     private static final int WAIT          = 16;    // waiting in Object.wait
 150     private static final int TIMED_WAITING = 17;
 151     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 152 
 153     private static final int TERMINATED = 99;  // final state
 154 
 155     // parking permit made available by LockSupport.unpark
 156     private volatile boolean parkPermit;
 157 
 158     // blocking permit made available by unblocker thread when another thread exits monitor
 159     private volatile boolean blockPermit;
 160 
 161     // true when on the list of virtual threads waiting to be unblocked
 162     private volatile boolean onWaitingList;
 163 
 164     // next virtual thread on the list of virtual threads waiting to be unblocked
 165     private volatile VirtualThread next;
 166 
 167     // notified by Object.notify/notifyAll while waiting in Object.wait
 168     private volatile boolean notified;
 169 
 170     // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
 171     private volatile boolean interruptibleWait;
 172 
 173     // timed-wait support
 174     private byte timedWaitSeqNo;
 175 
 176     // timeout for timed-park and timed-wait, only accessed on current/carrier thread
 177     private long timeout;
 178 
 179     // timer task for timed-park and timed-wait, only accessed on current/carrier thread
 180     private Future<?> timeoutTask;
 181 
 182     // carrier thread when mounted, accessed by VM
 183     private volatile Thread carrierThread;
 184 
 185     // true to notifyAll after this virtual thread terminates
 186     private volatile boolean notifyAllAfterTerminate;
 187 
 188     /**
 189      * Returns the default scheduler.








 190      */
 191     static Executor defaultScheduler() {
 192         return DEFAULT_SCHEDULER;
 193     }
 194 
 195     /**
 196      * Returns the continuation scope used for virtual threads.
 197      */
 198     static ContinuationScope continuationScope() {
 199         return VTHREAD_SCOPE;
 200     }
 201 
 202     /**
 203      * Creates a new {@code VirtualThread} to run the given task with the given
 204      * scheduler. If the given scheduler is {@code null} and the current thread
 205      * is a platform thread then the newly created virtual thread will use the
 206      * default scheduler. If given scheduler is {@code null} and the current
 207      * thread is a virtual thread then the current thread's scheduler is used.



 208      *
 209      * @param scheduler the scheduler or null

 210      * @param name thread name
 211      * @param characteristics characteristics
 212      * @param task the task to execute
 213      */
 214     VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {




 215         super(name, characteristics, /*bound*/ false);
 216         Objects.requireNonNull(task);
 217 
 218         // choose scheduler if not specified
 219         if (scheduler == null) {
 220             Thread parent = Thread.currentThread();
 221             if (parent instanceof VirtualThread vparent) {
 222                 scheduler = vparent.scheduler;
 223             } else {
 224                 scheduler = DEFAULT_SCHEDULER;
 225             }
 226         }
 227 
 228         this.scheduler = scheduler;
 229         this.cont = new VThreadContinuation(this, task);
 230         this.runContinuation = this::runContinuation;





























































 231     }
 232 
 233     /**
 234      * The continuation that a virtual thread executes.
 235      */
 236     private static class VThreadContinuation extends Continuation {
 237         VThreadContinuation(VirtualThread vthread, Runnable task) {
 238             super(VTHREAD_SCOPE, wrap(vthread, task));
 239         }
 240         @Override
 241         protected void onPinned(Continuation.Pinned reason) {
 242         }
 243         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 244             return new Runnable() {
 245                 @Hidden
 246                 @JvmtiHideEvents
 247                 public void run() {
 248                     vthread.endFirstTransition();
 249                     try {
 250                         vthread.run(task);
 251                     } finally {
 252                         vthread.startFinalTransition();
 253                     }
 254                 }
 255             };
 256         }
 257     }
 258 
 259     /**
 260      * Runs or continues execution on the current thread. The virtual thread is mounted
 261      * on the current thread before the task runs or continues. It unmounts when the
 262      * task completes or yields.
 263      */
 264     @ChangesCurrentThread // allow mount/unmount to be inlined
 265     private void runContinuation() {
 266         // the carrier must be a platform thread
 267         if (Thread.currentThread().isVirtual()) {
 268             throw new WrongThreadException();
 269         }
 270 
 271         // set state to RUNNING
 272         int initialState = state();
 273         if (initialState == STARTED || initialState == UNPARKED
 274                 || initialState == UNBLOCKED || initialState == YIELDED) {
 275             // newly started or continue after parking/blocking/Thread.yield
 276             if (!compareAndSetState(initialState, RUNNING)) {
 277                 return;
 278             }
 279             // consume permit when continuing after parking or blocking. If continue
 280             // after a timed-park or timed-wait then the timeout task is cancelled.
 281             if (initialState == UNPARKED) {
 282                 cancelTimeoutTask();
 283                 setParkPermit(false);
 284             } else if (initialState == UNBLOCKED) {
 285                 cancelTimeoutTask();
 286                 blockPermit = false;
 287             }
 288         } else {
 289             // not runnable
 290             return;
 291         }
 292 
 293         mount();
 294         try {
 295             cont.run();
 296         } finally {
 297             unmount();
 298             if (cont.isDone()) {
 299                 afterDone();
 300             } else {
 301                 afterYield();
 302             }
 303         }
 304     }
 305 
 306     /**
 307      * Cancel timeout task when continuing after timed-park or timed-wait.
 308      * The timeout task may be executing, or may have already completed.
 309      */
 310     private void cancelTimeoutTask() {
 311         if (timeoutTask != null) {
 312             timeoutTask.cancel(false);
 313             timeoutTask = null;
 314         }
 315     }
 316 
 317     /**
 318      * Submits the given task to the given executor. If the scheduler is a
 319      * ForkJoinPool then the task is first adapted to a ForkJoinTask.
 320      */
 321     private void submit(Executor executor, Runnable task) {
 322         if (executor instanceof ForkJoinPool pool) {
 323             pool.submit(ForkJoinTask.adapt(task));
 324         } else {
 325             executor.execute(task);
 326         }
 327     }
 328 
 329     /**
 330      * Submits the runContinuation task to the scheduler. For the default scheduler,
 331      * and calling it on a worker thread, the task will be pushed to the local queue,
 332      * otherwise it will be pushed to an external submission queue.
 333      * @param scheduler the scheduler
 334      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 335      * @throws RejectedExecutionException
 336      */
 337     private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
 338         boolean done = false;
 339         while (!done) {
 340             try {
 341                 // Pin the continuation to prevent the virtual thread from unmounting
 342                 // when submitting a task. For the default scheduler this ensures that
 343                 // the carrier doesn't change when pushing a task. For other schedulers
 344                 // it avoids deadlock that could arise due to carriers and virtual
 345                 // threads contending for a lock.
 346                 if (currentThread().isVirtual()) {
 347                     Continuation.pin();
 348                     try {
 349                         submit(scheduler, runContinuation);
 350                     } finally {
 351                         Continuation.unpin();
 352                     }
 353                 } else {
 354                     submit(scheduler, runContinuation);
 355                 }
 356                 done = true;
 357             } catch (RejectedExecutionException ree) {
 358                 submitFailed(ree);
 359                 throw ree;
 360             } catch (OutOfMemoryError e) {
 361                 if (retryOnOOME) {
 362                     U.park(false, 100_000_000); // 100ms
 363                 } else {
 364                     throw e;
 365                 }
 366             }
 367         }
 368     }
 369 
 370     /**
 371      * Submits the runContinuation task to the given scheduler as an external submit.
 372      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 373      * @throws RejectedExecutionException
 374      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 375      */
 376     private void externalSubmitRunContinuation(ForkJoinPool pool) {
 377         assert Thread.currentThread() instanceof CarrierThread;
 378         try {
 379             pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
 380         } catch (RejectedExecutionException ree) {
 381             submitFailed(ree);
 382             throw ree;
 383         } catch (OutOfMemoryError e) {
 384             submitRunContinuation(pool, true);
 385         }
 386     }
 387 
 388     /**
 389      * Submits the runContinuation task to the scheduler. For the default scheduler,
 390      * and calling it on a worker thread, the task will be pushed to the local queue,
 391      * otherwise it will be pushed to an external submission queue.
 392      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 393      * @throws RejectedExecutionException
 394      */
 395     private void submitRunContinuation() {
 396         submitRunContinuation(scheduler, true);
 397     }
 398 
 399     /**
 400      * Lazy submit the runContinuation task if invoked on a carrier thread and its local
 401      * queue is empty. If not empty, or invoked by another thread, then this method works
 402      * like submitRunContinuation and just submits the task to the scheduler.
 403      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 404      * @throws RejectedExecutionException
 405      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 406      */
 407     private void lazySubmitRunContinuation() {

 408         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 409             ForkJoinPool pool = ct.getPool();
 410             try {
 411                 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
 412             } catch (RejectedExecutionException ree) {
 413                 submitFailed(ree);
 414                 throw ree;
 415             } catch (OutOfMemoryError e) {
 416                 submitRunContinuation();
 417             }
 418         } else {
 419             submitRunContinuation();
 420         }
 421     }
 422 
 423     /**
 424      * Submits the runContinuation task to the scheduler. For the default scheduler, and
 425      * calling it a virtual thread that uses the default scheduler, the task will be
 426      * pushed to an external submission queue. This method may throw OutOfMemoryError.

 427      * @throws RejectedExecutionException
 428      * @throws OutOfMemoryError
 429      */
 430     private void externalSubmitRunContinuationOrThrow() {
 431         if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {

 432             try {
 433                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 434             } catch (RejectedExecutionException ree) {
 435                 submitFailed(ree);
 436                 throw ree;


 437             }
 438         } else {
 439             submitRunContinuation(scheduler, false);




































 440         }
 441     }
 442 
 443     /**
 444      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 445      */
 446     private void submitFailed(RejectedExecutionException ree) {
 447         var event = new VirtualThreadSubmitFailedEvent();
 448         if (event.isEnabled()) {
 449             event.javaThreadId = threadId();
 450             event.exceptionMessage = ree.getMessage();
 451             event.commit();
 452         }
 453     }
 454 
 455     /**
 456      * Runs a task in the context of this virtual thread.
 457      */
 458     private void run(Runnable task) {
 459         assert Thread.currentThread() == this && state == RUNNING;
 460 
 461         // emit JFR event if enabled
 462         if (VirtualThreadStartEvent.isTurnedOn()) {
 463             var event = new VirtualThreadStartEvent();
 464             event.javaThreadId = threadId();
 465             event.commit();
 466         }
 467 
 468         Object bindings = Thread.scopedValueBindings();
 469         try {
 470             runWith(bindings, task);
 471         } catch (Throwable exc) {
 472             dispatchUncaughtException(exc);
 473         } finally {
 474             // pop any remaining scopes from the stack, this may block
 475             StackableScope.popAll();
 476 
 477             // emit JFR event if enabled
 478             if (VirtualThreadEndEvent.isTurnedOn()) {
 479                 var event = new VirtualThreadEndEvent();
 480                 event.javaThreadId = threadId();
 481                 event.commit();
 482             }
 483         }
 484     }
 485 
 486     /**
 487      * Mounts this virtual thread onto the current platform thread. On
 488      * return, the current thread is the virtual thread.
 489      */
 490     @ChangesCurrentThread
 491     @ReservedStackAccess
 492     private void mount() {
 493         startTransition(/*mount*/true);
 494         // We assume following volatile accesses provide equivalent
 495         // of acquire ordering, otherwise we need U.loadFence() here.
 496 
 497         // sets the carrier thread
 498         Thread carrier = Thread.currentCarrierThread();
 499         setCarrierThread(carrier);
 500 
 501         // sync up carrier thread interrupted status if needed
 502         if (interrupted) {
 503             carrier.setInterrupt();
 504         } else if (carrier.isInterrupted()) {
 505             synchronized (interruptLock) {
 506                 // need to recheck interrupted status
 507                 if (!interrupted) {
 508                     carrier.clearInterrupt();
 509                 }
 510             }
 511         }
 512 
 513         // set Thread.currentThread() to return this virtual thread
 514         carrier.setCurrentThread(this);
 515     }
 516 
 517     /**
 518      * Unmounts this virtual thread from the carrier. On return, the
 519      * current thread is the current platform thread.
 520      */
 521     @ChangesCurrentThread
 522     @ReservedStackAccess
 523     private void unmount() {
 524         assert !Thread.holdsLock(interruptLock);
 525 
 526         // set Thread.currentThread() to return the platform thread
 527         Thread carrier = this.carrierThread;
 528         carrier.setCurrentThread(carrier);
 529 
 530         // break connection to carrier thread, synchronized with interrupt
 531         synchronized (interruptLock) {
 532             setCarrierThread(null);
 533         }
 534         carrier.clearInterrupt();
 535 
 536         // We assume previous volatile accesses provide equivalent
 537         // of release ordering, otherwise we need U.storeFence() here.
 538         endTransition(/*mount*/false);
 539     }
 540 
 541     /**
 542      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 543      * the continuation continues.
 544      */
 545     @Hidden
 546     private boolean yieldContinuation() {
 547         startTransition(/*mount*/false);
 548         try {
 549             return Continuation.yield(VTHREAD_SCOPE);
 550         } finally {
 551             endTransition(/*mount*/true);
 552         }
 553     }
 554 
 555     /**
 556      * Invoked in the context of the carrier thread after the Continuation yields when
 557      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 558      */
 559     private void afterYield() {
 560         assert carrierThread == null;
 561 
 562         // re-adjust parallelism if the virtual thread yielded when compensating
 563         if (currentThread() instanceof CarrierThread ct) {
 564             ct.endBlocking();
 565         }
 566 
 567         int s = state();
 568 
 569         // LockSupport.park/parkNanos
 570         if (s == PARKING || s == TIMED_PARKING) {
 571             int newState;
 572             if (s == PARKING) {
 573                 setState(newState = PARKED);
 574             } else {
 575                 // schedule unpark
 576                 long timeout = this.timeout;
 577                 assert timeout > 0;
 578                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 579                 setState(newState = TIMED_PARKED);
 580             }
 581 
 582             // may have been unparked while parking
 583             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 584                 // lazy submit if local queue is empty
 585                 lazySubmitRunContinuation();
 586             }
 587             return;
 588         }
 589 
 590         // Thread.yield
 591         if (s == YIELDING) {
 592             setState(YIELDED);
 593 
 594             // external submit if there are no tasks in the local task queue
 595             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 596                 externalSubmitRunContinuation(ct.getPool());
 597             } else {
 598                 submitRunContinuation();
 599             }
 600             return;
 601         }
 602 
 603         // blocking on monitorenter
 604         if (s == BLOCKING) {
 605             setState(BLOCKED);
 606 
 607             // may have been unblocked while blocking
 608             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 609                 // lazy submit if local queue is empty
 610                 lazySubmitRunContinuation();
 611             }
 612             return;
 613         }
 614 
 615         // Object.wait
 616         if (s == WAITING || s == TIMED_WAITING) {
 617             int newState;
 618             boolean blocked;
 619             boolean interruptible = interruptibleWait;
 620             if (s == WAITING) {
 621                 setState(newState = WAIT);
 622                 // may have been notified while in transition
 623                 blocked = notified && compareAndSetState(WAIT, BLOCKED);
 624             } else {
 625                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 626                 // task will change the thread state to UNBLOCKED and submit the thread
 627                 // to the scheduler. A sequence number is used to ensure that the timeout
 628                 // task only unblocks the thread for this timed-wait. We synchronize with
 629                 // the timeout task to coordinate access to the sequence number and to
 630                 // ensure the timeout task doesn't execute until the thread has got to
 631                 // the TIMED_WAIT state.
 632                 long timeout = this.timeout;
 633                 assert timeout > 0;
 634                 synchronized (timedWaitLock()) {
 635                     byte seqNo = ++timedWaitSeqNo;
 636                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 637                     setState(newState = TIMED_WAIT);
 638                     // May have been notified while in transition. This must be done while
 639                     // holding the monitor to avoid changing the state of a new timed wait call.
 640                     blocked = notified && compareAndSetState(TIMED_WAIT, BLOCKED);
 641                 }
 642             }
 643 
 644             if (blocked) {
 645                 // may have been unblocked already
 646                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 647                     lazySubmitRunContinuation();
 648                 }
 649             } else {
 650                 // may have been interrupted while in transition to wait state
 651                 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 652                     lazySubmitRunContinuation();
 653                 }
 654             }
 655             return;
 656         }
 657 
 658         assert false;
 659     }
 660 
 661     /**
 662      * Invoked after the continuation completes.
 663      */
 664     private void afterDone() {
 665         afterDone(true);
 666     }
 667 
 668     /**
 669      * Invoked after the continuation completes (or start failed). Sets the thread
 670      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 671      *
 672      * @param notifyContainer true if its container should be notified
 673      */
 674     private void afterDone(boolean notifyContainer) {
 675         assert carrierThread == null;
 676         setState(TERMINATED);
 677 
 678         // notifyAll to wakeup any threads waiting for this thread to terminate
 679         if (notifyAllAfterTerminate) {
 680             synchronized (this) {
 681                 notifyAll();
 682             }
 683         }
 684 
 685         // notify container
 686         if (notifyContainer) {
 687             threadContainer().remove(this);
 688         }
 689 
 690         // clear references to thread locals
 691         clearReferences();
 692     }
 693 
 694     /**
 695      * Schedules this {@code VirtualThread} to execute.
 696      *
 697      * @throws IllegalStateException if the container is shutdown or closed
 698      * @throws IllegalThreadStateException if the thread has already been started
 699      * @throws RejectedExecutionException if the scheduler cannot accept a task
 700      */
 701     @Override
 702     void start(ThreadContainer container) {
 703         if (!compareAndSetState(NEW, STARTED)) {
 704             throw new IllegalThreadStateException("Already started");
 705         }
 706 
 707         // bind thread to container
 708         assert threadContainer() == null;
 709         setThreadContainer(container);
 710 
 711         // start thread
 712         boolean addedToContainer = false;
 713         boolean started = false;
 714         try {
 715             container.add(this);  // may throw
 716             addedToContainer = true;
 717 
 718             // scoped values may be inherited
 719             inheritScopedValueBindings(container);
 720 
 721             // submit task to run thread, using externalSubmit if possible
 722             externalSubmitRunContinuationOrThrow();
 723             started = true;
 724         } finally {
 725             if (!started) {
 726                 afterDone(addedToContainer);
 727             }
 728         }
 729     }
 730 
 731     @Override
 732     public void start() {
 733         start(ThreadContainers.root());
 734     }
 735 
 736     @Override
 737     public void run() {
 738         // do nothing
 739     }
 740 
 741     /**
 742      * Invoked by Thread.join before a thread waits for this virtual thread to terminate.
 743      */
 744     void beforeJoin() {
 745         notifyAllAfterTerminate = true;
 746     }
 747 
 748     /**
 749      * Parks until unparked or interrupted. If already unparked then the parking
 750      * permit is consumed and this method completes immediately (meaning it doesn't
 751      * yield). It also completes immediately if the interrupted status is set.
 752      */
 753     @Override
 754     void park() {
 755         assert Thread.currentThread() == this;
 756 
 757         // complete immediately if parking permit available or interrupted
 758         if (getAndSetParkPermit(false) || interrupted)
 759             return;
 760 
 761         // park the thread
 762         boolean yielded = false;
 763         setState(PARKING);
 764         try {
 765             yielded = yieldContinuation();
 766         } catch (OutOfMemoryError e) {
 767             // park on carrier
 768         } finally {
 769             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 770             if (!yielded) {
 771                 assert state() == PARKING;
 772                 setState(RUNNING);
 773             }
 774         }
 775 
 776         // park on the carrier thread when pinned
 777         if (!yielded) {
 778             parkOnCarrierThread(false, 0);
 779         }
 780     }
 781 
 782     /**
 783      * Parks up to the given waiting time or until unparked or interrupted.
 784      * If already unparked then the parking permit is consumed and this method
 785      * completes immediately (meaning it doesn't yield). It also completes immediately
 786      * if the interrupted status is set or the waiting time is {@code <= 0}.
 787      *
 788      * @param nanos the maximum number of nanoseconds to wait.
 789      */
 790     @Override
 791     void parkNanos(long nanos) {
 792         assert Thread.currentThread() == this;
 793 
 794         // complete immediately if parking permit available or interrupted
 795         if (getAndSetParkPermit(false) || interrupted)
 796             return;
 797 
 798         // park the thread for the waiting time
 799         if (nanos > 0) {
 800             long startTime = System.nanoTime();
 801 
 802             // park the thread, afterYield will schedule the thread to unpark
 803             boolean yielded = false;
 804             timeout = nanos;
 805             setState(TIMED_PARKING);
 806             try {
 807                 yielded = yieldContinuation();
 808             } catch (OutOfMemoryError e) {
 809                 // park on carrier
 810             } finally {
 811                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 812                 if (!yielded) {
 813                     assert state() == TIMED_PARKING;
 814                     setState(RUNNING);
 815                 }
 816             }
 817 
 818             // park on carrier thread for remaining time when pinned (or OOME)
 819             if (!yielded) {
 820                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 821                 parkOnCarrierThread(true, remainingNanos);
 822             }
 823         }
 824     }
 825 
 826     /**
 827      * Parks the current carrier thread up to the given waiting time or until
 828      * unparked or interrupted. If the virtual thread is interrupted then the
 829      * interrupted status will be propagated to the carrier thread.
 830      * @param timed true for a timed park, false for untimed
 831      * @param nanos the waiting time in nanoseconds
 832      */
 833     private void parkOnCarrierThread(boolean timed, long nanos) {
 834         assert state() == RUNNING;
 835 
 836         setState(timed ? TIMED_PINNED : PINNED);
 837         try {
 838             if (!parkPermit) {
 839                 if (!timed) {
 840                     U.park(false, 0);
 841                 } else if (nanos > 0) {
 842                     U.park(false, nanos);
 843                 }
 844             }
 845         } finally {
 846             setState(RUNNING);
 847         }
 848 
 849         // consume parking permit
 850         setParkPermit(false);
 851 
 852         // JFR jdk.VirtualThreadPinned event
 853         postPinnedEvent("LockSupport.park");
 854     }
 855 
 856     /**
 857      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 858      * Recording the event in the VM avoids having JFR event recorded in Java
 859      * with the same name, but different ID, to events recorded by the VM.
 860      */
 861     @Hidden
 862     private static native void postPinnedEvent(String op);
 863 
 864     /**
 865      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 866      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 867      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 868      * @param lazySubmit to use lazySubmit if possible
 869      * @throws RejectedExecutionException if the scheduler cannot accept a task
 870      */
 871     private void unpark(boolean lazySubmit) {
 872         if (!getAndSetParkPermit(true) && currentThread() != this) {
 873             int s = state();
 874 
 875             // unparked while parked
 876             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
 877                 if (lazySubmit) {
 878                     lazySubmitRunContinuation();
 879                 } else {
 880                     submitRunContinuation();
 881                 }
 882                 return;
 883             }
 884 
 885             // unparked while parked when pinned
 886             if (s == PINNED || s == TIMED_PINNED) {
 887                 // unpark carrier thread when pinned
 888                 disableSuspendAndPreempt();
 889                 try {
 890                     synchronized (carrierThreadAccessLock()) {
 891                         Thread carrier = carrierThread;
 892                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 893                             U.unpark(carrier);
 894                         }
 895                     }
 896                 } finally {
 897                     enableSuspendAndPreempt();
 898                 }
 899                 return;
 900             }
 901         }
 902     }
 903 
 904     @Override
 905     void unpark() {
 906         unpark(false);
 907     }
 908 
 909     /**
 910      * Invoked by unblocker thread to unblock this virtual thread.
 911      */
 912     private void unblock() {
 913         assert !Thread.currentThread().isVirtual();
 914         blockPermit = true;
 915         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
 916             submitRunContinuation();
 917         }
 918     }
 919 
 920     /**
 921      * Invoked by FJP worker thread or STPE thread when park timeout expires.
 922      */
 923     private void parkTimeoutExpired() {
 924         assert !VirtualThread.currentThread().isVirtual();
 925         unpark(true);
 926     }
 927 
 928     /**
 929      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
 930      * If the virtual thread is in timed-wait then this method will unblock the thread
 931      * and submit its task so that it continues and attempts to reenter the monitor.
 932      * This method does nothing if the thread has been woken by notify or interrupt.
 933      */
 934     private void waitTimeoutExpired(byte seqNo) {
 935         assert !Thread.currentThread().isVirtual();
 936 
 937         synchronized (timedWaitLock()) {
 938             if (seqNo != timedWaitSeqNo) {
 939                 // this timeout task is for a past timed-wait
 940                 return;
 941             }
 942             if (!compareAndSetState(TIMED_WAIT, UNBLOCKED)) {
 943                 // already notified (or interrupted)
 944                 return;
 945             }
 946         }
 947 
 948         lazySubmitRunContinuation();
 949     }
 950 
 951     /**
 952      * Attempts to yield the current virtual thread (Thread.yield).
 953      */
 954     void tryYield() {
 955         assert Thread.currentThread() == this;
 956         setState(YIELDING);
 957         boolean yielded = false;
 958         try {
 959             yielded = yieldContinuation();  // may throw
 960         } finally {
 961             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 962             if (!yielded) {
 963                 assert state() == YIELDING;
 964                 setState(RUNNING);
 965             }
 966         }
 967     }
 968 
 969     /**
 970      * Sleep the current thread for the given sleep time (in nanoseconds). If
 971      * nanos is 0 then the thread will attempt to yield.
 972      *
 973      * @implNote This implementation parks the thread for the given sleeping time
 974      * and will therefore be observed in PARKED state during the sleep. Parking
 975      * will consume the parking permit so this method makes available the parking
 976      * permit after the sleep. This may be observed as a spurious, but benign,
 977      * wakeup when the thread subsequently attempts to park.
 978      *
 979      * @param nanos the maximum number of nanoseconds to sleep
 980      * @throws InterruptedException if interrupted while sleeping
 981      */
 982     void sleepNanos(long nanos) throws InterruptedException {
 983         assert Thread.currentThread() == this && nanos >= 0;
 984         if (getAndClearInterrupt())
 985             throw new InterruptedException();
 986         if (nanos == 0) {
 987             tryYield();
 988         } else {
 989             // park for the sleep time
 990             try {
 991                 long remainingNanos = nanos;
 992                 long startNanos = System.nanoTime();
 993                 while (remainingNanos > 0) {
 994                     parkNanos(remainingNanos);
 995                     if (getAndClearInterrupt()) {
 996                         throw new InterruptedException();
 997                     }
 998                     remainingNanos = nanos - (System.nanoTime() - startNanos);
 999                 }
1000             } finally {
1001                 // may have been unparked while sleeping
1002                 setParkPermit(true);
1003             }
1004         }
1005     }
1006 
1007     @Override
1008     void blockedOn(Interruptible b) {
1009         disableSuspendAndPreempt();
1010         try {
1011             super.blockedOn(b);
1012         } finally {
1013             enableSuspendAndPreempt();
1014         }
1015     }
1016 
1017     @Override
1018     public void interrupt() {
1019         if (Thread.currentThread() != this) {
1020             // if current thread is a virtual thread then prevent it from being
1021             // suspended or unmounted when entering or holding interruptLock
1022             Interruptible blocker;
1023             disableSuspendAndPreempt();
1024             try {
1025                 synchronized (interruptLock) {
1026                     interrupted = true;
1027                     blocker = nioBlocker();
1028                     if (blocker != null) {
1029                         blocker.interrupt(this);
1030                     }
1031 
1032                     // interrupt carrier thread if mounted
1033                     Thread carrier = carrierThread;
1034                     if (carrier != null) carrier.setInterrupt();
1035                 }
1036             } finally {
1037                 enableSuspendAndPreempt();
1038             }
1039 
1040             // notify blocker after releasing interruptLock
1041             if (blocker != null) {
1042                 blocker.postInterrupt();
1043             }
1044 
1045             // make available parking permit, unpark thread if parked
1046             unpark();
1047 
1048             // if thread is waiting in Object.wait then schedule to try to reenter
1049             int s = state();
1050             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1051                 submitRunContinuation();
1052             }
1053 
1054         } else {
1055             interrupted = true;
1056             carrierThread.setInterrupt();
1057             setParkPermit(true);
1058         }
1059     }
1060 
1061     @Override
1062     public boolean isInterrupted() {
1063         return interrupted;
1064     }
1065 
1066     @Override
1067     boolean getAndClearInterrupt() {
1068         assert Thread.currentThread() == this;
1069         boolean oldValue = interrupted;
1070         if (oldValue) {
1071             disableSuspendAndPreempt();
1072             try {
1073                 synchronized (interruptLock) {
1074                     interrupted = false;
1075                     carrierThread.clearInterrupt();
1076                 }
1077             } finally {
1078                 enableSuspendAndPreempt();
1079             }
1080         }
1081         return oldValue;
1082     }
1083 
1084     @Override
1085     Thread.State threadState() {
1086         switch (state()) {
1087             case NEW:
1088                 return Thread.State.NEW;
1089             case STARTED:
1090                 // return NEW if thread container not yet set
1091                 if (threadContainer() == null) {
1092                     return Thread.State.NEW;
1093                 } else {
1094                     return Thread.State.RUNNABLE;
1095                 }
1096             case UNPARKED:
1097             case UNBLOCKED:
1098             case YIELDED:
1099                 // runnable, not mounted
1100                 return Thread.State.RUNNABLE;
1101             case RUNNING:
1102                 // if mounted then return state of carrier thread
1103                 if (Thread.currentThread() != this) {
1104                     disableSuspendAndPreempt();
1105                     try {
1106                         synchronized (carrierThreadAccessLock()) {
1107                             Thread carrierThread = this.carrierThread;
1108                             if (carrierThread != null) {
1109                                 return carrierThread.threadState();
1110                             }
1111                         }
1112                     } finally {
1113                         enableSuspendAndPreempt();
1114                     }
1115                 }
1116                 // runnable, mounted
1117                 return Thread.State.RUNNABLE;
1118             case PARKING:
1119             case TIMED_PARKING:
1120             case WAITING:
1121             case TIMED_WAITING:
1122             case YIELDING:
1123                 // runnable, in transition
1124                 return Thread.State.RUNNABLE;
1125             case PARKED:
1126             case PINNED:
1127             case WAIT:
1128                 return Thread.State.WAITING;
1129             case TIMED_PARKED:
1130             case TIMED_PINNED:
1131             case TIMED_WAIT:
1132                 return Thread.State.TIMED_WAITING;
1133             case BLOCKING:
1134             case BLOCKED:
1135                 return Thread.State.BLOCKED;
1136             case TERMINATED:
1137                 return Thread.State.TERMINATED;
1138             default:
1139                 throw new InternalError();
1140         }
1141     }
1142 
1143     @Override
1144     boolean alive() {
1145         int s = state;
1146         return (s != NEW && s != TERMINATED);
1147     }
1148 
1149     @Override
1150     boolean isTerminated() {
1151         return (state == TERMINATED);
1152     }
1153 
1154     @Override
1155     public String toString() {
1156         StringBuilder sb = new StringBuilder("VirtualThread[#");
1157         sb.append(threadId());
1158         String name = getName();
1159         if (!name.isEmpty()) {
1160             sb.append(",");
1161             sb.append(name);
1162         }
1163         sb.append("]/");
1164 
1165         // add the carrier state and thread name when mounted
1166         boolean mounted;
1167         if (Thread.currentThread() == this) {
1168             mounted = appendCarrierInfo(sb);
1169         } else {
1170             disableSuspendAndPreempt();
1171             try {
1172                 synchronized (carrierThreadAccessLock()) {
1173                     mounted = appendCarrierInfo(sb);
1174                 }
1175             } finally {
1176                 enableSuspendAndPreempt();
1177             }
1178         }
1179 
1180         // add virtual thread state when not mounted
1181         if (!mounted) {
1182             String stateAsString = threadState().toString();
1183             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1184         }
1185 
1186         return sb.toString();
1187     }
1188 
1189     /**
1190      * Appends the carrier state and thread name to the string buffer if mounted.
1191      * @return true if mounted, false if not mounted
1192      */
1193     private boolean appendCarrierInfo(StringBuilder sb) {
1194         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1195         Thread carrier = carrierThread;
1196         if (carrier != null) {
1197             String stateAsString = carrier.threadState().toString();
1198             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1199             sb.append('@');
1200             sb.append(carrier.getName());
1201             return true;
1202         } else {
1203             return false;
1204         }
1205     }
1206 
1207     @Override
1208     public int hashCode() {
1209         return (int) threadId();
1210     }
1211 
1212     @Override
1213     public boolean equals(Object obj) {
1214         return obj == this;
1215     }
1216 
1217     /**
1218      * Returns the lock object to synchronize on when accessing carrierThread.
1219      * The lock prevents carrierThread from being reset to null during unmount.
1220      */
1221     private Object carrierThreadAccessLock() {
1222         // return interruptLock as unmount has to coordinate with interrupt
1223         return interruptLock;
1224     }
1225 
1226     /**
1227      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1228      */
1229     private Object timedWaitLock() {
1230         // use this object for now to avoid the overhead of introducing another lock
1231         return runContinuation;
1232     }
1233 
1234     /**
1235      * Disallow the current thread be suspended or preempted.
1236      */
1237     private void disableSuspendAndPreempt() {
1238         notifyJvmtiDisableSuspend(true);
1239         Continuation.pin();
1240     }
1241 
1242     /**
1243      * Allow the current thread be suspended or preempted.
1244      */
1245     private void enableSuspendAndPreempt() {
1246         Continuation.unpin();
1247         notifyJvmtiDisableSuspend(false);
1248     }
1249 
1250     // -- wrappers for get/set of state, parking permit, and carrier thread --
1251 
1252     private int state() {
1253         return state;  // volatile read
1254     }
1255 
1256     private void setState(int newValue) {
1257         state = newValue;  // volatile write
1258     }
1259 
1260     private boolean compareAndSetState(int expectedValue, int newValue) {
1261         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1262     }
1263 
1264     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1265         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1266     }
1267 
1268     private void setParkPermit(boolean newValue) {
1269         if (parkPermit != newValue) {
1270             parkPermit = newValue;
1271         }
1272     }
1273 
1274     private boolean getAndSetParkPermit(boolean newValue) {
1275         if (parkPermit != newValue) {
1276             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1277         } else {
1278             return newValue;
1279         }
1280     }
1281 
1282     private void setCarrierThread(Thread carrier) {
1283         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1284         this.carrierThread = carrier;
1285     }
1286 
1287     // The following four methods notify the VM when a "transition" starts and ends.
1288     // A "mount transition" embodies the steps to transfer control from a platform
1289     // thread to a virtual thread, changing the thread identity, and starting or
1290     // resuming the virtual thread's continuation on the carrier.
1291     // An "unmount transition" embodies the steps to transfer control from a virtual
1292     // thread to its carrier, suspending the virtual thread's continuation, and
1293     // restoring the thread identity to the platform thread.
1294     // The notifications to the VM are necessary in order to coordinate with functions
1295     // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1296     // a transition may block if transitions are disabled. Ending a transition may
1297     // notify a thread that is waiting to disable transitions. The notifications are
1298     // also used to post JVMTI events for virtual thread start and end.
1299 
1300     @IntrinsicCandidate
1301     @JvmtiMountTransition
1302     private native void endFirstTransition();
1303 
1304     @IntrinsicCandidate
1305     @JvmtiMountTransition
1306     private native void startFinalTransition();
1307 
1308     @IntrinsicCandidate
1309     @JvmtiMountTransition
1310     private native void startTransition(boolean mount);
1311 
1312     @IntrinsicCandidate
1313     @JvmtiMountTransition
1314     private native void endTransition(boolean mount);
1315 
1316     @IntrinsicCandidate
1317     private static native void notifyJvmtiDisableSuspend(boolean enter);
1318 
1319     private static native void registerNatives();
1320     static {
1321         registerNatives();
1322 
1323         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1324         var group = Thread.virtualThreadGroup();
1325     }
1326 
1327     /**
1328      * Creates the default ForkJoinPool scheduler.




























1329      */
1330     private static ForkJoinPool createDefaultScheduler() {
1331         ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1332         int parallelism, maxPoolSize, minRunnable;
1333         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1334         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1335         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1336         if (parallelismValue != null) {
1337             parallelism = Integer.parseInt(parallelismValue);
1338         } else {
1339             parallelism = Runtime.getRuntime().availableProcessors();
1340         }
1341         if (maxPoolSizeValue != null) {
1342             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1343             parallelism = Integer.min(parallelism, maxPoolSize);
1344         } else {
1345             maxPoolSize = Integer.max(parallelism, 256);
1346         }
1347         if (minRunnableValue != null) {
1348             minRunnable = Integer.parseInt(minRunnableValue);
1349         } else {
1350             minRunnable = Integer.max(parallelism / 2, 1);
1351         }
1352         Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1353         boolean asyncMode = true; // FIFO
1354         return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1355                      0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);



























































































1356     }
1357 
1358     /**
1359      * Schedule a runnable task to run after a delay.
1360      */
1361     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1362         if (scheduler instanceof ForkJoinPool pool) {
1363             return pool.schedule(command, delay, unit);
1364         } else {
1365             return DelayedTaskSchedulers.schedule(command, delay, unit);


1366         }
1367     }
1368 
1369     /**
1370      * Supports scheduling a runnable task to run after a delay. It uses a number
1371      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1372      * work queue used. This class is used when using a custom scheduler.
1373      */
1374     private static class DelayedTaskSchedulers {
1375         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1376 
1377         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1378             long tid = Thread.currentThread().threadId();
1379             int index = (int) tid & (INSTANCE.length - 1);
1380             return INSTANCE[index].schedule(command, delay, unit);
1381         }
1382 
1383         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1384             String propName = "jdk.virtualThreadScheduler.timerQueues";
1385             String propValue = System.getProperty(propName);
1386             int queueCount;
1387             if (propValue != null) {
1388                 queueCount = Integer.parseInt(propValue);
1389                 if (queueCount != Integer.highestOneBit(queueCount)) {
1390                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1391                 }
1392             } else {
1393                 int ncpus = Runtime.getRuntime().availableProcessors();
1394                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1395             }
1396             var schedulers = new ScheduledExecutorService[queueCount];
1397             for (int i = 0; i < queueCount; i++) {
1398                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1399                     Executors.newScheduledThreadPool(1, task -> {
1400                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1401                         t.setDaemon(true);
1402                         return t;
1403                     });
1404                 stpe.setRemoveOnCancelPolicy(true);
1405                 schedulers[i] = stpe;
1406             }
1407             return schedulers;
1408         }
1409     }
1410 
1411     /**
1412      * Schedule virtual threads that are ready to be scheduled after they blocked on
1413      * monitor enter.
1414      */
1415     private static void unblockVirtualThreads() {
1416         while (true) {
1417             VirtualThread vthread = takeVirtualThreadListToUnblock();
1418             while (vthread != null) {
1419                 assert vthread.onWaitingList;
1420                 VirtualThread nextThread = vthread.next;
1421 
1422                 // remove from list and unblock
1423                 vthread.next = null;
1424                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1425                 assert changed;
1426                 vthread.unblock();
1427 
1428                 vthread = nextThread;
1429             }
1430         }
1431     }
1432 
1433     /**
1434      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1435      * if necessary until a list of one or more threads becomes available.
1436      */
1437     private static native VirtualThread takeVirtualThreadListToUnblock();
1438 
1439     static {
1440         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1441                 VirtualThread::unblockVirtualThreads);
1442         unblocker.setDaemon(true);
1443         unblocker.start();
1444     }
1445 }
--- EOF ---