1 /*
   2  * Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.lang;
  26 
  27 import java.lang.reflect.Constructor;
  28 import java.util.Arrays;
  29 import java.util.Locale;
  30 import java.util.Objects;
  31 import java.util.concurrent.CountDownLatch;
  32 import java.util.concurrent.Executor;
  33 import java.util.concurrent.Executors;
  34 import java.util.concurrent.ForkJoinPool;
  35 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
  36 import java.util.concurrent.ForkJoinTask;
  37 import java.util.concurrent.Future;
  38 import java.util.concurrent.RejectedExecutionException;
  39 import java.util.concurrent.ScheduledExecutorService;
  40 import java.util.concurrent.ScheduledThreadPoolExecutor;
  41 import java.util.concurrent.TimeUnit;
  42 import java.util.stream.Stream;
  43 import jdk.internal.event.VirtualThreadEndEvent;
  44 import jdk.internal.event.VirtualThreadStartEvent;
  45 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  46 import jdk.internal.misc.CarrierThread;
  47 import jdk.internal.misc.InnocuousThread;
  48 import jdk.internal.misc.Unsafe;
  49 import jdk.internal.vm.Continuation;
  50 import jdk.internal.vm.ContinuationScope;
  51 import jdk.internal.vm.StackableScope;
  52 import jdk.internal.vm.ThreadContainer;
  53 import jdk.internal.vm.ThreadContainers;
  54 import jdk.internal.vm.annotation.ChangesCurrentThread;
  55 import jdk.internal.vm.annotation.Hidden;
  56 import jdk.internal.vm.annotation.IntrinsicCandidate;
  57 import jdk.internal.vm.annotation.JvmtiHideEvents;
  58 import jdk.internal.vm.annotation.JvmtiMountTransition;
  59 import jdk.internal.vm.annotation.ReservedStackAccess;
  60 import sun.nio.ch.Interruptible;
  61 import static java.util.concurrent.TimeUnit.*;
  62 
  63 /**
  64  * A thread that is scheduled by the Java virtual machine rather than the operating system.
  65  */
  66 final class VirtualThread extends BaseVirtualThread {
  67     private static final Unsafe U = Unsafe.getUnsafe();
  68     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  69     private static final Executor DEFAULT_SCHEDULER = createDefaultScheduler();
  70     private static final ScheduledExecutorService[] DELAYED_TASK_SCHEDULERS = createDelayedTaskSchedulers();
  71 
  72     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  73     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  74     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  75     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  76     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  77 
  78     // scheduler and continuation
  79     private final Executor scheduler;
  80     private final Continuation cont;
  81     private final Runnable runContinuation;
  82 
  83     // virtual thread state, accessed by VM
  84     private volatile int state;
  85 
  86     /*
  87      * Virtual thread state transitions:
  88      *
  89      *      NEW -> STARTED         // Thread.start, schedule to run
  90      *  STARTED -> TERMINATED      // failed to start
  91      *  STARTED -> RUNNING         // first run
  92      *  RUNNING -> TERMINATED      // done
  93      *
  94      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
  95      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
  96      *  PARKING -> PINNED          // cont.yield failed, parked indefinitely on carrier
  97      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
  98      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
  99      * UNPARKED -> RUNNING         // continue execution after park
 100      *
 101      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
 102      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
 103      * TIMED_PARKING -> TIMED_PINNED    // cont.yield failed, timed-parked on carrier
 104      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 105      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 106      *
 107      *   RUNNING -> BLOCKING       // blocking on monitor enter
 108      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 109      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 110      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 111      *
 112      *   RUNNING -> WAITING        // transitional state during wait on monitor
 113      *   WAITING -> WAITED         // waiting on monitor
 114      *    WAITED -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 115      *    WAITED -> UNBLOCKED      // timed-out/interrupted
 116      *
 117      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 118      * TIMED_WAITING -> TIMED_WAITED    // timed-waiting on monitor
 119      *  TIMED_WAITED -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 120      *  TIMED_WAITED -> UNBLOCKED       // timed-out/interrupted
 121      *
 122      *  RUNNING -> YIELDING        // Thread.yield
 123      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 124      * YIELDING -> RUNNING         // cont.yield failed
 125      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 126      */
 127     private static final int NEW      = 0;
 128     private static final int STARTED  = 1;
 129     private static final int RUNNING  = 2;     // runnable-mounted
 130 
 131     // untimed and timed parking
 132     private static final int PARKING       = 3;
 133     private static final int PARKED        = 4;     // unmounted
 134     private static final int PINNED        = 5;     // mounted
 135     private static final int TIMED_PARKING = 6;
 136     private static final int TIMED_PARKED  = 7;     // unmounted
 137     private static final int TIMED_PINNED  = 8;     // mounted
 138     private static final int UNPARKED      = 9;     // unmounted but runnable
 139 
 140     // Thread.yield
 141     private static final int YIELDING = 10;
 142     private static final int YIELDED  = 11;         // unmounted but runnable
 143 
 144     // monitor enter
 145     private static final int BLOCKING  = 12;
 146     private static final int BLOCKED   = 13;        // unmounted
 147     private static final int UNBLOCKED = 14;        // unmounted but runnable
 148 
 149     // monitor wait/timed-wait
 150     private static final int WAITING       = 15;
 151     private static final int WAIT          = 16;    // waiting in Object.wait
 152     private static final int TIMED_WAITING = 17;
 153     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 154 
 155     private static final int TERMINATED = 99;  // final state
 156 
 157     // can be suspended from scheduling when unmounted
 158     private static final int SUSPENDED = 1 << 8;
 159 
 160     // parking permit made available by LockSupport.unpark
 161     private volatile boolean parkPermit;
 162 
 163     // blocking permit made available by unblocker thread when another thread exits monitor
 164     private volatile boolean blockPermit;
 165 
 166     // true when on the list of virtual threads waiting to be unblocked
 167     private volatile boolean onWaitingList;
 168 
 169     // next virtual thread on the list of virtual threads waiting to be unblocked
 170     private volatile VirtualThread next;
 171 
 172     // notified by Object.notify/notifyAll while waiting in Object.wait
 173     private volatile boolean notified;
 174 
 175     // timed-wait support
 176     private byte timedWaitSeqNo;
 177 
 178     // timeout for timed-park and timed-wait, only accessed on current/carrier thread
 179     private long timeout;
 180 
 181     // timer task for timed-park and timed-wait, only accessed on current/carrier thread
 182     private Future<?> timeoutTask;
 183 
 184     // carrier thread when mounted, accessed by VM
 185     private volatile Thread carrierThread;
 186 
 187     // termination object when joining, created lazily if needed
 188     private volatile CountDownLatch termination;
 189 
 190     /**
 191      * Returns the default scheduler.
 192      */
 193     static Executor defaultScheduler() {
 194         return DEFAULT_SCHEDULER;
 195     }
 196 
 197     /**
 198      * Returns a stream of the delayed task schedulers used to support timed operations.
 199      */
 200     static Stream<ScheduledExecutorService> delayedTaskSchedulers() {
 201         return Arrays.stream(DELAYED_TASK_SCHEDULERS);
 202     }
 203 
 204     /**
 205      * Returns the continuation scope used for virtual threads.
 206      */
 207     static ContinuationScope continuationScope() {
 208         return VTHREAD_SCOPE;
 209     }
 210 
 211     /**
 212      * Creates a new {@code VirtualThread} to run the given task with the given
 213      * scheduler. If the given scheduler is {@code null} and the current thread
 214      * is a platform thread then the newly created virtual thread will use the
 215      * default scheduler. If given scheduler is {@code null} and the current
 216      * thread is a virtual thread then the current thread's scheduler is used.
 217      *
 218      * @param scheduler the scheduler or null
 219      * @param name thread name
 220      * @param characteristics characteristics
 221      * @param task the task to execute
 222      */
 223     VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
 224         super(name, characteristics, /*bound*/ false);
 225         Objects.requireNonNull(task);
 226 
 227         // choose scheduler if not specified
 228         if (scheduler == null) {
 229             Thread parent = Thread.currentThread();
 230             if (parent instanceof VirtualThread vparent) {
 231                 scheduler = vparent.scheduler;
 232             } else {
 233                 scheduler = DEFAULT_SCHEDULER;
 234             }
 235         }
 236 
 237         this.scheduler = scheduler;
 238         this.cont = new VThreadContinuation(this, task);
 239         this.runContinuation = this::runContinuation;
 240     }
 241 
 242     /**
 243      * The continuation that a virtual thread executes.
 244      */
 245     private static class VThreadContinuation extends Continuation {
 246         VThreadContinuation(VirtualThread vthread, Runnable task) {
 247             super(VTHREAD_SCOPE, wrap(vthread, task));
 248         }
 249         @Override
 250         protected void onPinned(Continuation.Pinned reason) {
 251         }
 252         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 253             return new Runnable() {
 254                 @Hidden
 255                 @JvmtiHideEvents
 256                 public void run() {
 257                     vthread.notifyJvmtiStart(); // notify JVMTI
 258                     try {
 259                         vthread.run(task);
 260                     } finally {
 261                         vthread.notifyJvmtiEnd(); // notify JVMTI
 262                     }
 263                 }
 264             };
 265         }
 266     }
 267 
 268     /**
 269      * Runs or continues execution on the current thread. The virtual thread is mounted
 270      * on the current thread before the task runs or continues. It unmounts when the
 271      * task completes or yields.
 272      */
 273     @ChangesCurrentThread // allow mount/unmount to be inlined
 274     private void runContinuation() {
 275         // the carrier must be a platform thread
 276         if (Thread.currentThread().isVirtual()) {
 277             throw new WrongThreadException();
 278         }
 279 
 280         // set state to RUNNING
 281         int initialState = state();
 282         if (initialState == STARTED || initialState == UNPARKED
 283                 || initialState == UNBLOCKED || initialState == YIELDED) {
 284             // newly started or continue after parking/blocking/Thread.yield
 285             if (!compareAndSetState(initialState, RUNNING)) {
 286                 return;
 287             }
 288             // consume permit when continuing after parking or blocking. If continue
 289             // after a timed-park or timed-wait then the timeout task is cancelled.
 290             if (initialState == UNPARKED) {
 291                 cancelTimeoutTask();
 292                 setParkPermit(false);
 293             } else if (initialState == UNBLOCKED) {
 294                 cancelTimeoutTask();
 295                 blockPermit = false;
 296             }
 297         } else {
 298             // not runnable
 299             return;
 300         }
 301 
 302         mount();
 303         try {
 304             cont.run();
 305         } finally {
 306             unmount();
 307             if (cont.isDone()) {
 308                 afterDone();
 309             } else {
 310                 afterYield();
 311             }
 312         }
 313     }
 314 
 315     /**
 316      * Cancel timeout task when continuing after timed-park or timed-wait.
 317      * The timeout task may be executing, or may have already completed.
 318      */
 319     private void cancelTimeoutTask() {
 320         if (timeoutTask != null) {
 321             timeoutTask.cancel(false);
 322             timeoutTask = null;
 323         }
 324     }
 325 
 326     /**
 327      * Submits the runContinuation task to the scheduler. For the default scheduler,
 328      * and calling it on a worker thread, the task will be pushed to the local queue,
 329      * otherwise it will be pushed to an external submission queue.
 330      * @param scheduler the scheduler
 331      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 332      * @throws RejectedExecutionException
 333      */
 334     private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
 335         boolean done = false;
 336         while (!done) {
 337             try {
 338                 // Pin the continuation to prevent the virtual thread from unmounting
 339                 // when submitting a task. For the default scheduler this ensures that
 340                 // the carrier doesn't change when pushing a task. For other schedulers
 341                 // it avoids deadlock that could arise due to carriers and virtual
 342                 // threads contending for a lock.
 343                 if (currentThread().isVirtual()) {
 344                     Continuation.pin();
 345                     try {
 346                         scheduler.execute(runContinuation);
 347                     } finally {
 348                         Continuation.unpin();
 349                     }
 350                 } else {
 351                     scheduler.execute(runContinuation);
 352                 }
 353                 done = true;
 354             } catch (RejectedExecutionException ree) {
 355                 submitFailed(ree);
 356                 throw ree;
 357             } catch (OutOfMemoryError e) {
 358                 if (retryOnOOME) {
 359                     U.park(false, 100_000_000); // 100ms
 360                 } else {
 361                     throw e;
 362                 }
 363             }
 364         }
 365     }
 366 
 367     /**
 368      * Submits the runContinuation task to the given scheduler as an external submit.
 369      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 370      * @throws RejectedExecutionException
 371      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 372      */
 373     private void externalSubmitRunContinuation(ForkJoinPool pool) {
 374         assert Thread.currentThread() instanceof CarrierThread;
 375         try {
 376             pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
 377         } catch (RejectedExecutionException ree) {
 378             submitFailed(ree);
 379             throw ree;
 380         } catch (OutOfMemoryError e) {
 381             submitRunContinuation(pool, true);
 382         }
 383     }
 384 
 385     /**
 386      * Submits the runContinuation task to the scheduler. For the default scheduler,
 387      * and calling it on a worker thread, the task will be pushed to the local queue,
 388      * otherwise it will be pushed to an external submission queue.
 389      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 390      * @throws RejectedExecutionException
 391      */
 392     private void submitRunContinuation() {
 393         submitRunContinuation(scheduler, true);
 394     }
 395 
 396     /**
 397      * Lazy submit the runContinuation task if invoked on a carrier thread and its local
 398      * queue is empty. If not empty, or invoked by another thread, then this method works
 399      * like submitRunContinuation and just submits the task to the scheduler.
 400      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 401      * @throws RejectedExecutionException
 402      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 403      */
 404     private void lazySubmitRunContinuation() {
 405         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 406             ForkJoinPool pool = ct.getPool();
 407             try {
 408                 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
 409             } catch (RejectedExecutionException ree) {
 410                 submitFailed(ree);
 411                 throw ree;
 412             } catch (OutOfMemoryError e) {
 413                 submitRunContinuation();
 414             }
 415         } else {
 416             submitRunContinuation();
 417         }
 418     }
 419 
 420     /**
 421      * Submits the runContinuation task to the scheduler. For the default scheduler, and
 422      * calling it a virtual thread that uses the default scheduler, the task will be
 423      * pushed to an external submission queue. This method may throw OutOfMemoryError.
 424      * @throws RejectedExecutionException
 425      * @throws OutOfMemoryError
 426      */
 427     private void externalSubmitRunContinuationOrThrow() {
 428         if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
 429             try {
 430                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 431             } catch (RejectedExecutionException ree) {
 432                 submitFailed(ree);
 433                 throw ree;
 434             }
 435         } else {
 436             submitRunContinuation(scheduler, false);
 437         }
 438     }
 439 
 440     /**
 441      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 442      */
 443     private void submitFailed(RejectedExecutionException ree) {
 444         var event = new VirtualThreadSubmitFailedEvent();
 445         if (event.isEnabled()) {
 446             event.javaThreadId = threadId();
 447             event.exceptionMessage = ree.getMessage();
 448             event.commit();
 449         }
 450     }
 451 
 452     /**
 453      * Runs a task in the context of this virtual thread.
 454      */
 455     private void run(Runnable task) {
 456         assert Thread.currentThread() == this && state == RUNNING;
 457 
 458         // emit JFR event if enabled
 459         if (VirtualThreadStartEvent.isTurnedOn()) {
 460             var event = new VirtualThreadStartEvent();
 461             event.javaThreadId = threadId();
 462             event.commit();
 463         }
 464 
 465         Object bindings = Thread.scopedValueBindings();
 466         try {
 467             runWith(bindings, task);
 468         } catch (Throwable exc) {
 469             dispatchUncaughtException(exc);
 470         } finally {
 471             // pop any remaining scopes from the stack, this may block
 472             StackableScope.popAll();
 473 
 474             // emit JFR event if enabled
 475             if (VirtualThreadEndEvent.isTurnedOn()) {
 476                 var event = new VirtualThreadEndEvent();
 477                 event.javaThreadId = threadId();
 478                 event.commit();
 479             }
 480         }
 481     }
 482 
 483     /**
 484      * Mounts this virtual thread onto the current platform thread. On
 485      * return, the current thread is the virtual thread.
 486      */
 487     @ChangesCurrentThread
 488     @ReservedStackAccess
 489     private void mount() {
 490         // notify JVMTI before mount
 491         notifyJvmtiMount(/*hide*/true);
 492 
 493         // sets the carrier thread
 494         Thread carrier = Thread.currentCarrierThread();
 495         setCarrierThread(carrier);
 496 
 497         // sync up carrier thread interrupt status if needed
 498         if (interrupted) {
 499             carrier.setInterrupt();
 500         } else if (carrier.isInterrupted()) {
 501             synchronized (interruptLock) {
 502                 // need to recheck interrupt status
 503                 if (!interrupted) {
 504                     carrier.clearInterrupt();
 505                 }
 506             }
 507         }
 508 
 509         // set Thread.currentThread() to return this virtual thread
 510         carrier.setCurrentThread(this);
 511     }
 512 
 513     /**
 514      * Unmounts this virtual thread from the carrier. On return, the
 515      * current thread is the current platform thread.
 516      */
 517     @ChangesCurrentThread
 518     @ReservedStackAccess
 519     private void unmount() {
 520         assert !Thread.holdsLock(interruptLock);
 521 
 522         // set Thread.currentThread() to return the platform thread
 523         Thread carrier = this.carrierThread;
 524         carrier.setCurrentThread(carrier);
 525 
 526         // break connection to carrier thread, synchronized with interrupt
 527         synchronized (interruptLock) {
 528             setCarrierThread(null);
 529         }
 530         carrier.clearInterrupt();
 531 
 532         // notify JVMTI after unmount
 533         notifyJvmtiUnmount(/*hide*/false);
 534     }
 535 
 536     /**
 537      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 538      * the continuation continues.
 539      */
 540     @Hidden
 541     private boolean yieldContinuation() {
 542         notifyJvmtiUnmount(/*hide*/true);
 543         try {
 544             return Continuation.yield(VTHREAD_SCOPE);
 545         } finally {
 546             notifyJvmtiMount(/*hide*/false);
 547         }
 548     }
 549 
 550     /**
 551      * Invoked in the context of the carrier thread after the Continuation yields when
 552      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 553      */
 554     private void afterYield() {
 555         assert carrierThread == null;
 556 
 557         // re-adjust parallelism if the virtual thread yielded when compensating
 558         if (currentThread() instanceof CarrierThread ct) {
 559             ct.endBlocking();
 560         }
 561 
 562         int s = state();
 563 
 564         // LockSupport.park/parkNanos
 565         if (s == PARKING || s == TIMED_PARKING) {
 566             int newState;
 567             if (s == PARKING) {
 568                 setState(newState = PARKED);
 569             } else {
 570                 // schedule unpark
 571                 assert timeout > 0;
 572                 timeoutTask = schedule(this::unpark, timeout, NANOSECONDS);
 573                 setState(newState = TIMED_PARKED);
 574             }
 575 
 576             // may have been unparked while parking
 577             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 578                 // lazy submit if local queue is empty
 579                 lazySubmitRunContinuation();
 580             }
 581             return;
 582         }
 583 
 584         // Thread.yield
 585         if (s == YIELDING) {
 586             setState(YIELDED);
 587 
 588             // external submit if there are no tasks in the local task queue
 589             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 590                 externalSubmitRunContinuation(ct.getPool());
 591             } else {
 592                 submitRunContinuation();
 593             }
 594             return;
 595         }
 596 
 597         // blocking on monitorenter
 598         if (s == BLOCKING) {
 599             setState(BLOCKED);
 600 
 601             // may have been unblocked while blocking
 602             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 603                 // lazy submit if local queue is empty
 604                 lazySubmitRunContinuation();
 605             }
 606             return;
 607         }
 608 
 609         // Object.wait
 610         if (s == WAITING || s == TIMED_WAITING) {
 611             int newState;
 612             if (s == WAITING) {
 613                 setState(newState = WAIT);
 614             } else {
 615                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 616                 // task will change the thread state to UNBLOCKED and submit the thread
 617                 // to the scheduler. A sequence number is used to ensure that the timeout
 618                 // task only unblocks the thread for this timed-wait. We synchronize with
 619                 // the timeout task to coordinate access to the sequence number and to
 620                 // ensure the timeout task doesn't execute until the thread has got to
 621                 // the TIMED_WAIT state.
 622                 assert timeout > 0;
 623                 synchronized (timedWaitLock()) {
 624                     byte seqNo = ++timedWaitSeqNo;
 625                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 626                     setState(newState = TIMED_WAIT);
 627                 }
 628             }
 629 
 630             // may have been notified while in transition to wait state
 631             if (notified && compareAndSetState(newState, BLOCKED)) {
 632                 // may have even been unblocked already
 633                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 634                     submitRunContinuation();
 635                 }
 636                 return;
 637             }
 638 
 639             // may have been interrupted while in transition to wait state
 640             if (interrupted && compareAndSetState(newState, UNBLOCKED)) {
 641                 submitRunContinuation();
 642                 return;
 643             }
 644             return;
 645         }
 646 
 647         assert false;
 648     }
 649 
 650     /**
 651      * Invoked after the continuation completes.
 652      */
 653     private void afterDone() {
 654         afterDone(true);
 655     }
 656 
 657     /**
 658      * Invoked after the continuation completes (or start failed). Sets the thread
 659      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 660      *
 661      * @param notifyContainer true if its container should be notified
 662      */
 663     private void afterDone(boolean notifyContainer) {
 664         assert carrierThread == null;
 665         setState(TERMINATED);
 666 
 667         // notify anyone waiting for this virtual thread to terminate
 668         CountDownLatch termination = this.termination;
 669         if (termination != null) {
 670             assert termination.getCount() == 1;
 671             termination.countDown();
 672         }
 673 
 674         // notify container
 675         if (notifyContainer) {
 676             threadContainer().onExit(this);
 677         }
 678 
 679         // clear references to thread locals
 680         clearReferences();
 681     }
 682 
 683     /**
 684      * Schedules this {@code VirtualThread} to execute.
 685      *
 686      * @throws IllegalStateException if the container is shutdown or closed
 687      * @throws IllegalThreadStateException if the thread has already been started
 688      * @throws RejectedExecutionException if the scheduler cannot accept a task
 689      */
 690     @Override
 691     void start(ThreadContainer container) {
 692         if (!compareAndSetState(NEW, STARTED)) {
 693             throw new IllegalThreadStateException("Already started");
 694         }
 695 
 696         // bind thread to container
 697         assert threadContainer() == null;
 698         setThreadContainer(container);
 699 
 700         // start thread
 701         boolean addedToContainer = false;
 702         boolean started = false;
 703         try {
 704             container.onStart(this);  // may throw
 705             addedToContainer = true;
 706 
 707             // scoped values may be inherited
 708             inheritScopedValueBindings(container);
 709 
 710             // submit task to run thread, using externalSubmit if possible
 711             externalSubmitRunContinuationOrThrow();
 712             started = true;
 713         } finally {
 714             if (!started) {
 715                 afterDone(addedToContainer);
 716             }
 717         }
 718     }
 719 
 720     @Override
 721     public void start() {
 722         start(ThreadContainers.root());
 723     }
 724 
 725     @Override
 726     public void run() {
 727         // do nothing
 728     }
 729 
 730     /**
 731      * Parks until unparked or interrupted. If already unparked then the parking
 732      * permit is consumed and this method completes immediately (meaning it doesn't
 733      * yield). It also completes immediately if the interrupt status is set.
 734      */
 735     @Override
 736     void park() {
 737         assert Thread.currentThread() == this;
 738 
 739         // complete immediately if parking permit available or interrupted
 740         if (getAndSetParkPermit(false) || interrupted)
 741             return;
 742 
 743         // park the thread
 744         boolean yielded = false;
 745         setState(PARKING);
 746         try {
 747             yielded = yieldContinuation();
 748         } catch (OutOfMemoryError e) {
 749             // park on carrier
 750         } finally {
 751             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 752             if (!yielded) {
 753                 assert state() == PARKING;
 754                 setState(RUNNING);
 755             }
 756         }
 757 
 758         // park on the carrier thread when pinned
 759         if (!yielded) {
 760             parkOnCarrierThread(false, 0);
 761         }
 762     }
 763 
 764     /**
 765      * Parks up to the given waiting time or until unparked or interrupted.
 766      * If already unparked then the parking permit is consumed and this method
 767      * completes immediately (meaning it doesn't yield). It also completes immediately
 768      * if the interrupt status is set or the waiting time is {@code <= 0}.
 769      *
 770      * @param nanos the maximum number of nanoseconds to wait.
 771      */
 772     @Override
 773     void parkNanos(long nanos) {
 774         assert Thread.currentThread() == this;
 775 
 776         // complete immediately if parking permit available or interrupted
 777         if (getAndSetParkPermit(false) || interrupted)
 778             return;
 779 
 780         // park the thread for the waiting time
 781         if (nanos > 0) {
 782             long startTime = System.nanoTime();
 783 
 784             // park the thread, afterYield will schedule the thread to unpark
 785             boolean yielded = false;
 786             timeout = nanos;
 787             setState(TIMED_PARKING);
 788             try {
 789                 yielded = yieldContinuation();
 790             } catch (OutOfMemoryError e) {
 791                 // park on carrier
 792             } finally {
 793                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 794                 if (!yielded) {
 795                     assert state() == TIMED_PARKING;
 796                     setState(RUNNING);
 797                 }
 798             }
 799 
 800             // park on carrier thread for remaining time when pinned (or OOME)
 801             if (!yielded) {
 802                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 803                 parkOnCarrierThread(true, remainingNanos);
 804             }
 805         }
 806     }
 807 
 808     /**
 809      * Parks the current carrier thread up to the given waiting time or until
 810      * unparked or interrupted. If the virtual thread is interrupted then the
 811      * interrupt status will be propagated to the carrier thread.
 812      * @param timed true for a timed park, false for untimed
 813      * @param nanos the waiting time in nanoseconds
 814      */
 815     private void parkOnCarrierThread(boolean timed, long nanos) {
 816         assert state() == RUNNING;
 817 
 818         setState(timed ? TIMED_PINNED : PINNED);
 819         try {
 820             if (!parkPermit) {
 821                 if (!timed) {
 822                     U.park(false, 0);
 823                 } else if (nanos > 0) {
 824                     U.park(false, nanos);
 825                 }
 826             }
 827         } finally {
 828             setState(RUNNING);
 829         }
 830 
 831         // consume parking permit
 832         setParkPermit(false);
 833 
 834         // JFR jdk.VirtualThreadPinned event
 835         postPinnedEvent("LockSupport.park");
 836     }
 837 
 838     /**
 839      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 840      * Recording the event in the VM avoids having JFR event recorded in Java
 841      * with the same name, but different ID, to events recorded by the VM.
 842      */
 843     @Hidden
 844     private static native void postPinnedEvent(String op);
 845 
 846     /**
 847      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 848      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 849      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 850      * @throws RejectedExecutionException if the scheduler cannot accept a task
 851      */
 852     @Override
 853     void unpark() {
 854         if (!getAndSetParkPermit(true) && currentThread() != this) {
 855             int s = state();
 856 
 857             // unparked while parked
 858             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
 859                 submitRunContinuation();
 860                 return;
 861             }
 862 
 863             // unparked while parked when pinned
 864             if (s == PINNED || s == TIMED_PINNED) {
 865                 // unpark carrier thread when pinned
 866                 disableSuspendAndPreempt();
 867                 try {
 868                     synchronized (carrierThreadAccessLock()) {
 869                         Thread carrier = carrierThread;
 870                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 871                             U.unpark(carrier);
 872                         }
 873                     }
 874                 } finally {
 875                     enableSuspendAndPreempt();
 876                 }
 877                 return;
 878             }
 879         }
 880     }
 881 
 882     /**
 883      * Invoked by unblocker thread to unblock this virtual thread.
 884      */
 885     private void unblock() {
 886         assert !Thread.currentThread().isVirtual();
 887         blockPermit = true;
 888         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
 889             submitRunContinuation();
 890         }
 891     }
 892 
 893     /**
 894      * Invoked by timer thread when wait timeout for virtual thread has expired.
 895      * If the virtual thread is in timed-wait then this method will unblock the thread
 896      * and submit its task so that it continues and attempts to reenter the monitor.
 897      * This method does nothing if the thread has been woken by notify or interrupt.
 898      */
 899     private void waitTimeoutExpired(byte seqNo) {
 900         assert !Thread.currentThread().isVirtual();
 901         for (;;) {
 902             boolean unblocked = false;
 903             synchronized (timedWaitLock()) {
 904                 if (seqNo != timedWaitSeqNo) {
 905                     // this timeout task is for a past timed-wait
 906                     return;
 907                 }
 908                 int s = state();
 909                 if (s == TIMED_WAIT) {
 910                     unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
 911                 } else if (s != (TIMED_WAIT | SUSPENDED)) {
 912                     // notified or interrupted, no longer waiting
 913                     return;
 914                 }
 915             }
 916             if (unblocked) {
 917                 submitRunContinuation();
 918                 return;
 919             }
 920             // need to retry when thread is suspended in time-wait
 921             Thread.yield();
 922         }
 923     }
 924 
 925     /**
 926      * Attempts to yield the current virtual thread (Thread.yield).
 927      */
 928     void tryYield() {
 929         assert Thread.currentThread() == this;
 930         setState(YIELDING);
 931         boolean yielded = false;
 932         try {
 933             yielded = yieldContinuation();  // may throw
 934         } finally {
 935             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 936             if (!yielded) {
 937                 assert state() == YIELDING;
 938                 setState(RUNNING);
 939             }
 940         }
 941     }
 942 
 943     /**
 944      * Sleep the current thread for the given sleep time (in nanoseconds). If
 945      * nanos is 0 then the thread will attempt to yield.
 946      *
 947      * @implNote This implementation parks the thread for the given sleeping time
 948      * and will therefore be observed in PARKED state during the sleep. Parking
 949      * will consume the parking permit so this method makes available the parking
 950      * permit after the sleep. This may be observed as a spurious, but benign,
 951      * wakeup when the thread subsequently attempts to park.
 952      *
 953      * @param nanos the maximum number of nanoseconds to sleep
 954      * @throws InterruptedException if interrupted while sleeping
 955      */
 956     void sleepNanos(long nanos) throws InterruptedException {
 957         assert Thread.currentThread() == this && nanos >= 0;
 958         if (getAndClearInterrupt())
 959             throw new InterruptedException();
 960         if (nanos == 0) {
 961             tryYield();
 962         } else {
 963             // park for the sleep time
 964             try {
 965                 long remainingNanos = nanos;
 966                 long startNanos = System.nanoTime();
 967                 while (remainingNanos > 0) {
 968                     parkNanos(remainingNanos);
 969                     if (getAndClearInterrupt()) {
 970                         throw new InterruptedException();
 971                     }
 972                     remainingNanos = nanos - (System.nanoTime() - startNanos);
 973                 }
 974             } finally {
 975                 // may have been unparked while sleeping
 976                 setParkPermit(true);
 977             }
 978         }
 979     }
 980 
 981     /**
 982      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
 983      * A timeout of {@code 0} means to wait forever.
 984      *
 985      * @throws InterruptedException if interrupted while waiting
 986      * @return true if the thread has terminated
 987      */
 988     boolean joinNanos(long nanos) throws InterruptedException {
 989         if (state() == TERMINATED)
 990             return true;
 991 
 992         // ensure termination object exists, then re-check state
 993         CountDownLatch termination = getTermination();
 994         if (state() == TERMINATED)
 995             return true;
 996 
 997         // wait for virtual thread to terminate
 998         if (nanos == 0) {
 999             termination.await();
1000         } else {
1001             boolean terminated = termination.await(nanos, NANOSECONDS);
1002             if (!terminated) {
1003                 // waiting time elapsed
1004                 return false;
1005             }
1006         }
1007         assert state() == TERMINATED;
1008         return true;
1009     }
1010 
1011     @Override
1012     void blockedOn(Interruptible b) {
1013         disableSuspendAndPreempt();
1014         try {
1015             super.blockedOn(b);
1016         } finally {
1017             enableSuspendAndPreempt();
1018         }
1019     }
1020 
1021     @Override
1022     public void interrupt() {
1023         if (Thread.currentThread() != this) {
1024             // if current thread is a virtual thread then prevent it from being
1025             // suspended or unmounted when entering or holding interruptLock
1026             Interruptible blocker;
1027             disableSuspendAndPreempt();
1028             try {
1029                 synchronized (interruptLock) {
1030                     interrupted = true;
1031                     blocker = nioBlocker();
1032                     if (blocker != null) {
1033                         blocker.interrupt(this);
1034                     }
1035 
1036                     // interrupt carrier thread if mounted
1037                     Thread carrier = carrierThread;
1038                     if (carrier != null) carrier.setInterrupt();
1039                 }
1040             } finally {
1041                 enableSuspendAndPreempt();
1042             }
1043 
1044             // notify blocker after releasing interruptLock
1045             if (blocker != null) {
1046                 blocker.postInterrupt();
1047             }
1048 
1049             // make available parking permit, unpark thread if parked
1050             unpark();
1051 
1052             // if thread is waiting in Object.wait then schedule to try to reenter
1053             int s = state();
1054             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1055                 submitRunContinuation();
1056             }
1057 
1058         } else {
1059             interrupted = true;
1060             carrierThread.setInterrupt();
1061             setParkPermit(true);
1062         }
1063     }
1064 
1065     @Override
1066     public boolean isInterrupted() {
1067         return interrupted;
1068     }
1069 
1070     @Override
1071     boolean getAndClearInterrupt() {
1072         assert Thread.currentThread() == this;
1073         boolean oldValue = interrupted;
1074         if (oldValue) {
1075             disableSuspendAndPreempt();
1076             try {
1077                 synchronized (interruptLock) {
1078                     interrupted = false;
1079                     carrierThread.clearInterrupt();
1080                 }
1081             } finally {
1082                 enableSuspendAndPreempt();
1083             }
1084         }
1085         return oldValue;
1086     }
1087 
1088     @Override
1089     Thread.State threadState() {
1090         int s = state();
1091         switch (s & ~SUSPENDED) {
1092             case NEW:
1093                 return Thread.State.NEW;
1094             case STARTED:
1095                 // return NEW if thread container not yet set
1096                 if (threadContainer() == null) {
1097                     return Thread.State.NEW;
1098                 } else {
1099                     return Thread.State.RUNNABLE;
1100                 }
1101             case UNPARKED:
1102             case UNBLOCKED:
1103             case YIELDED:
1104                 // runnable, not mounted
1105                 return Thread.State.RUNNABLE;
1106             case RUNNING:
1107                 // if mounted then return state of carrier thread
1108                 if (Thread.currentThread() != this) {
1109                     disableSuspendAndPreempt();
1110                     try {
1111                         synchronized (carrierThreadAccessLock()) {
1112                             Thread carrierThread = this.carrierThread;
1113                             if (carrierThread != null) {
1114                                 return carrierThread.threadState();
1115                             }
1116                         }
1117                     } finally {
1118                         enableSuspendAndPreempt();
1119                     }
1120                 }
1121                 // runnable, mounted
1122                 return Thread.State.RUNNABLE;
1123             case PARKING:
1124             case TIMED_PARKING:
1125             case WAITING:
1126             case TIMED_WAITING:
1127             case YIELDING:
1128                 // runnable, in transition
1129                 return Thread.State.RUNNABLE;
1130             case PARKED:
1131             case PINNED:
1132             case WAIT:
1133                 return Thread.State.WAITING;
1134             case TIMED_PARKED:
1135             case TIMED_PINNED:
1136             case TIMED_WAIT:
1137                 return Thread.State.TIMED_WAITING;
1138             case BLOCKING:
1139             case BLOCKED:
1140                 return Thread.State.BLOCKED;
1141             case TERMINATED:
1142                 return Thread.State.TERMINATED;
1143             default:
1144                 throw new InternalError();
1145         }
1146     }
1147 
1148     @Override
1149     boolean alive() {
1150         int s = state;
1151         return (s != NEW && s != TERMINATED);
1152     }
1153 
1154     @Override
1155     boolean isTerminated() {
1156         return (state == TERMINATED);
1157     }
1158 
1159     @Override
1160     StackTraceElement[] asyncGetStackTrace() {
1161         StackTraceElement[] stackTrace;
1162         do {
1163             stackTrace = (carrierThread != null)
1164                     ? super.asyncGetStackTrace()  // mounted
1165                     : tryGetStackTrace();         // unmounted
1166             if (stackTrace == null) {
1167                 Thread.yield();
1168             }
1169         } while (stackTrace == null);
1170         return stackTrace;
1171     }
1172 
1173     /**
1174      * Returns the stack trace for this virtual thread if it is unmounted.
1175      * Returns null if the thread is mounted or in transition.
1176      */
1177     private StackTraceElement[] tryGetStackTrace() {
1178         int initialState = state() & ~SUSPENDED;
1179         switch (initialState) {
1180             case NEW, STARTED, TERMINATED -> {
1181                 return new StackTraceElement[0];  // unmounted, empty stack
1182             }
1183             case RUNNING, PINNED, TIMED_PINNED -> {
1184                 return null;   // mounted
1185             }
1186             case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1187                 // unmounted, not runnable
1188             }
1189             case UNPARKED, UNBLOCKED, YIELDED -> {
1190                 // unmounted, runnable
1191             }
1192             case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1193                 return null;  // in transition
1194             }
1195             default -> throw new InternalError("" + initialState);
1196         }
1197 
1198         // thread is unmounted, prevent it from continuing
1199         int suspendedState = initialState | SUSPENDED;
1200         if (!compareAndSetState(initialState, suspendedState)) {
1201             return null;
1202         }
1203 
1204         // get stack trace and restore state
1205         StackTraceElement[] stack;
1206         try {
1207             stack = cont.getStackTrace();
1208         } finally {
1209             assert state == suspendedState;
1210             setState(initialState);
1211         }
1212         boolean resubmit = switch (initialState) {
1213             case UNPARKED, UNBLOCKED, YIELDED -> {
1214                 // resubmit as task may have run while suspended
1215                 yield true;
1216             }
1217             case PARKED, TIMED_PARKED -> {
1218                 // resubmit if unparked while suspended
1219                 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1220             }
1221             case BLOCKED -> {
1222                 // resubmit if unblocked while suspended
1223                 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1224             }
1225             case WAIT, TIMED_WAIT -> {
1226                 // resubmit if notified or interrupted while waiting (Object.wait)
1227                 // waitTimeoutExpired will retry if the timed expired when suspended
1228                 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1229             }
1230             default -> throw new InternalError();
1231         };
1232         if (resubmit) {
1233             submitRunContinuation();
1234         }
1235         return stack;
1236     }
1237 
1238     @Override
1239     public String toString() {
1240         StringBuilder sb = new StringBuilder("VirtualThread[#");
1241         sb.append(threadId());
1242         String name = getName();
1243         if (!name.isEmpty()) {
1244             sb.append(",");
1245             sb.append(name);
1246         }
1247         sb.append("]/");
1248 
1249         // add the carrier state and thread name when mounted
1250         boolean mounted;
1251         if (Thread.currentThread() == this) {
1252             mounted = appendCarrierInfo(sb);
1253         } else {
1254             disableSuspendAndPreempt();
1255             try {
1256                 synchronized (carrierThreadAccessLock()) {
1257                     mounted = appendCarrierInfo(sb);
1258                 }
1259             } finally {
1260                 enableSuspendAndPreempt();
1261             }
1262         }
1263 
1264         // add virtual thread state when not mounted
1265         if (!mounted) {
1266             String stateAsString = threadState().toString();
1267             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1268         }
1269 
1270         return sb.toString();
1271     }
1272 
1273     /**
1274      * Appends the carrier state and thread name to the string buffer if mounted.
1275      * @return true if mounted, false if not mounted
1276      */
1277     private boolean appendCarrierInfo(StringBuilder sb) {
1278         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1279         Thread carrier = carrierThread;
1280         if (carrier != null) {
1281             String stateAsString = carrier.threadState().toString();
1282             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1283             sb.append('@');
1284             sb.append(carrier.getName());
1285             return true;
1286         } else {
1287             return false;
1288         }
1289     }
1290 
1291     @Override
1292     public int hashCode() {
1293         return (int) threadId();
1294     }
1295 
1296     @Override
1297     public boolean equals(Object obj) {
1298         return obj == this;
1299     }
1300 
1301     /**
1302      * Returns the termination object, creating it if needed.
1303      */
1304     private CountDownLatch getTermination() {
1305         CountDownLatch termination = this.termination;
1306         if (termination == null) {
1307             termination = new CountDownLatch(1);
1308             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1309                 termination = this.termination;
1310             }
1311         }
1312         return termination;
1313     }
1314 
1315     /**
1316      * Returns the lock object to synchronize on when accessing carrierThread.
1317      * The lock prevents carrierThread from being reset to null during unmount.
1318      */
1319     private Object carrierThreadAccessLock() {
1320         // return interruptLock as unmount has to coordinate with interrupt
1321         return interruptLock;
1322     }
1323 
1324     /**
1325      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1326      */
1327     private Object timedWaitLock() {
1328         // use this object for now to avoid the overhead of introducing another lock
1329         return runContinuation;
1330     }
1331 
1332     /**
1333      * Disallow the current thread be suspended or preempted.
1334      */
1335     private void disableSuspendAndPreempt() {
1336         notifyJvmtiDisableSuspend(true);
1337         Continuation.pin();
1338     }
1339 
1340     /**
1341      * Allow the current thread be suspended or preempted.
1342      */
1343     private void enableSuspendAndPreempt() {
1344         Continuation.unpin();
1345         notifyJvmtiDisableSuspend(false);
1346     }
1347 
1348     // -- wrappers for get/set of state, parking permit, and carrier thread --
1349 
1350     private int state() {
1351         return state;  // volatile read
1352     }
1353 
1354     private void setState(int newValue) {
1355         state = newValue;  // volatile write
1356     }
1357 
1358     private boolean compareAndSetState(int expectedValue, int newValue) {
1359         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1360     }
1361 
1362     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1363         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1364     }
1365 
1366     private void setParkPermit(boolean newValue) {
1367         if (parkPermit != newValue) {
1368             parkPermit = newValue;
1369         }
1370     }
1371 
1372     private boolean getAndSetParkPermit(boolean newValue) {
1373         if (parkPermit != newValue) {
1374             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1375         } else {
1376             return newValue;
1377         }
1378     }
1379 
1380     private void setCarrierThread(Thread carrier) {
1381         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1382         this.carrierThread = carrier;
1383     }
1384 
1385     // -- JVM TI support --
1386 
1387     @IntrinsicCandidate
1388     @JvmtiMountTransition
1389     private native void notifyJvmtiStart();
1390 
1391     @IntrinsicCandidate
1392     @JvmtiMountTransition
1393     private native void notifyJvmtiEnd();
1394 
1395     @IntrinsicCandidate
1396     @JvmtiMountTransition
1397     private native void notifyJvmtiMount(boolean hide);
1398 
1399     @IntrinsicCandidate
1400     @JvmtiMountTransition
1401     private native void notifyJvmtiUnmount(boolean hide);
1402 
1403     @IntrinsicCandidate
1404     private static native void notifyJvmtiDisableSuspend(boolean enter);
1405 
1406     private static native void registerNatives();
1407     static {
1408         registerNatives();
1409 
1410         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1411         var group = Thread.virtualThreadGroup();
1412     }
1413 
1414     /**
1415      * Creates the default scheduler.
1416      * If the system property {@code jdk.virtualThreadScheduler.implClass} is set then
1417      * its value is the name of a class that implements java.util.concurrent.Executor.
1418      * The class is public in an exported package, has a public no-arg constructor,
1419      * and is visible to the system class loader.
1420      * If the system property is not set then the default scheduler will be a
1421      * ForkJoinPool instance.
1422      */
1423     private static Executor createDefaultScheduler() {
1424         String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
1425         if (propValue != null) {
1426             try {
1427                 Class<?> clazz = Class.forName(propValue, true,
1428                         ClassLoader.getSystemClassLoader());
1429                 Constructor<?> ctor = clazz.getConstructor();
1430                 var scheduler = (Executor) ctor.newInstance();
1431                 System.err.println("""
1432                     WARNING: Using custom scheduler, this is an experimental feature.""");
1433                 return scheduler;
1434             } catch (Exception ex) {
1435                 throw new Error(ex);
1436             }
1437         } else {
1438             return createDefaultForkJoinPoolScheduler();
1439         }
1440     }
1441 
1442     /**
1443      * Creates the default ForkJoinPool scheduler.
1444      */
1445     private static ForkJoinPool createDefaultForkJoinPoolScheduler() {
1446         ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1447         int parallelism, maxPoolSize, minRunnable;
1448         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1449         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1450         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1451         if (parallelismValue != null) {
1452             parallelism = Integer.parseInt(parallelismValue);
1453         } else {
1454             parallelism = Runtime.getRuntime().availableProcessors();
1455         }
1456         if (maxPoolSizeValue != null) {
1457             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1458             parallelism = Integer.min(parallelism, maxPoolSize);
1459         } else {
1460             maxPoolSize = Integer.max(parallelism, 256);
1461         }
1462         if (minRunnableValue != null) {
1463             minRunnable = Integer.parseInt(minRunnableValue);
1464         } else {
1465             minRunnable = Integer.max(parallelism / 2, 1);
1466         }
1467         Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1468         boolean asyncMode = true; // FIFO
1469         return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1470                      0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1471     }
1472 
1473     /**
1474      * Schedule a runnable task to run after a delay.
1475      */
1476     private static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1477         long tid = Thread.currentThread().threadId();
1478         int index = (int) tid & (DELAYED_TASK_SCHEDULERS.length - 1);
1479         return DELAYED_TASK_SCHEDULERS[index].schedule(command, delay, unit);
1480     }
1481 
1482     /**
1483      * Creates the ScheduledThreadPoolExecutors used to execute delayed tasks.
1484      */
1485     private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1486         String propName = "jdk.virtualThreadScheduler.timerQueues";
1487         String propValue = System.getProperty(propName);
1488         int queueCount;
1489         if (propValue != null) {
1490             queueCount = Integer.parseInt(propValue);
1491             if (queueCount != Integer.highestOneBit(queueCount)) {
1492                 throw new RuntimeException("Value of " + propName + " must be power of 2");
1493             }
1494         } else {
1495             int ncpus = Runtime.getRuntime().availableProcessors();
1496             queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1497         }
1498         var schedulers = new ScheduledExecutorService[queueCount];
1499         for (int i = 0; i < queueCount; i++) {
1500             ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1501                 Executors.newScheduledThreadPool(1, task -> {
1502                     Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1503                     t.setDaemon(true);
1504                     return t;
1505                 });
1506             stpe.setRemoveOnCancelPolicy(true);
1507             schedulers[i] = stpe;
1508         }
1509         return schedulers;
1510     }
1511 
1512     /**
1513      * Schedule virtual threads that are ready to be scheduled after they blocked on
1514      * monitor enter.
1515      */
1516     private static void unblockVirtualThreads() {
1517         while (true) {
1518             VirtualThread vthread = takeVirtualThreadListToUnblock();
1519             while (vthread != null) {
1520                 assert vthread.onWaitingList;
1521                 VirtualThread nextThread = vthread.next;
1522 
1523                 // remove from list and unblock
1524                 vthread.next = null;
1525                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1526                 assert changed;
1527                 vthread.unblock();
1528 
1529                 vthread = nextThread;
1530             }
1531         }
1532     }
1533 
1534     /**
1535      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1536      * if necessary until a list of one or more threads becomes available.
1537      */
1538     private static native VirtualThread takeVirtualThreadListToUnblock();
1539 
1540     static {
1541         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1542                 VirtualThread::unblockVirtualThreads);
1543         unblocker.setDaemon(true);
1544         unblocker.start();
1545     }
1546 }