1 /*
   2  * Copyright (c) 2018, 2025, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.lang;
  26 



  27 import java.util.Locale;
  28 import java.util.Objects;
  29 import java.util.concurrent.CountDownLatch;
  30 import java.util.concurrent.Executor;
  31 import java.util.concurrent.Executors;
  32 import java.util.concurrent.ForkJoinPool;
  33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
  34 import java.util.concurrent.ForkJoinTask;
  35 import java.util.concurrent.Future;
  36 import java.util.concurrent.RejectedExecutionException;
  37 import java.util.concurrent.ScheduledExecutorService;
  38 import java.util.concurrent.ScheduledThreadPoolExecutor;
  39 import java.util.concurrent.TimeUnit;
  40 import jdk.internal.event.VirtualThreadEndEvent;

  41 import jdk.internal.event.VirtualThreadStartEvent;
  42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;

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



















  67 
  68     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  69     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  70     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  71     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  72     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  73 
  74     // scheduler and continuation
  75     private final Executor scheduler;
  76     private final Continuation cont;
  77     private final Runnable runContinuation;
  78 
  79     // virtual thread state, accessed by VM
  80     private volatile int state;
  81 
  82     /*
  83      * Virtual thread state transitions:
  84      *
  85      *      NEW -> STARTED         // Thread.start, schedule to run
  86      *  STARTED -> TERMINATED      // failed to start
  87      *  STARTED -> RUNNING         // first run
  88      *  RUNNING -> TERMINATED      // done
  89      *
  90      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
  91      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
  92      *  PARKING -> PINNED          // cont.yield failed, parked indefinitely on carrier
  93      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
  94      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
  95      * UNPARKED -> RUNNING         // continue execution after park
  96      *
  97      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
  98      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
  99      * TIMED_PARKING -> TIMED_PINNED    // cont.yield failed, timed-parked on carrier
 100      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 101      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 102      *
 103      *   RUNNING -> BLOCKING       // blocking on monitor enter
 104      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 105      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 106      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 107      *
 108      *   RUNNING -> WAITING        // transitional state during wait on monitor
 109      *   WAITING -> WAIT           // waiting on monitor
 110      *      WAIT -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 111      *      WAIT -> UNBLOCKED      // timed-out/interrupted
 112      *
 113      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 114      * TIMED_WAITING -> TIMED_WAIT      // timed-waiting on monitor
 115      *    TIMED_WAIT -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 116      *    TIMED_WAIT -> UNBLOCKED       // timed-out/interrupted
 117      *
 118      *  RUNNING -> YIELDING        // Thread.yield
 119      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 120      * YIELDING -> RUNNING         // cont.yield failed
 121      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 122      */
 123     private static final int NEW      = 0;
 124     private static final int STARTED  = 1;
 125     private static final int RUNNING  = 2;     // runnable-mounted
 126 
 127     // untimed and timed parking
 128     private static final int PARKING       = 3;
 129     private static final int PARKED        = 4;     // unmounted
 130     private static final int PINNED        = 5;     // mounted
 131     private static final int TIMED_PARKING = 6;
 132     private static final int TIMED_PARKED  = 7;     // unmounted
 133     private static final int TIMED_PINNED  = 8;     // mounted
 134     private static final int UNPARKED      = 9;     // unmounted but runnable
 135 
 136     // Thread.yield
 137     private static final int YIELDING = 10;
 138     private static final int YIELDED  = 11;         // unmounted but runnable
 139 
 140     // monitor enter
 141     private static final int BLOCKING  = 12;
 142     private static final int BLOCKED   = 13;        // unmounted
 143     private static final int UNBLOCKED = 14;        // unmounted but runnable
 144 
 145     // monitor wait/timed-wait
 146     private static final int WAITING       = 15;
 147     private static final int WAIT          = 16;    // waiting in Object.wait
 148     private static final int TIMED_WAITING = 17;
 149     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 150 
 151     private static final int TERMINATED = 99;  // final state
 152 
 153     // can be suspended from scheduling when unmounted
 154     private static final int SUSPENDED = 1 << 8;
 155 
 156     // parking permit made available by LockSupport.unpark
 157     private volatile boolean parkPermit;
 158 
 159     // blocking permit made available by unblocker thread when another thread exits monitor
 160     private volatile boolean blockPermit;
 161 
 162     // true when on the list of virtual threads waiting to be unblocked
 163     private volatile boolean onWaitingList;
 164 
 165     // next virtual thread on the list of virtual threads waiting to be unblocked
 166     private volatile VirtualThread next;
 167 
 168     // notified by Object.notify/notifyAll while waiting in Object.wait
 169     private volatile boolean notified;
 170 
 171     // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
 172     private volatile boolean interruptibleWait;
 173 
 174     // timed-wait support
 175     private byte timedWaitSeqNo;
 176 
 177     // timeout for timed-park and timed-wait, only accessed on current/carrier thread
 178     private long timeout;
 179 
 180     // timer task for timed-park and timed-wait, only accessed on current/carrier thread
 181     private Future<?> timeoutTask;
 182 
 183     // carrier thread when mounted, accessed by VM
 184     private volatile Thread carrierThread;
 185 
 186     // termination object when joining, created lazily if needed
 187     private volatile CountDownLatch termination;
 188 
 189     /**
 190      * Returns the default scheduler.
 191      */
 192     static Executor defaultScheduler() {
 193         return DEFAULT_SCHEDULER;
 194     }
 195 







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








 209      *
 210      * @param scheduler the scheduler or null
 211      * @param name thread name
 212      * @param characteristics characteristics
 213      * @param task the task to execute
 214      */
 215     VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {



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




























































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

 397         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 398             ForkJoinPool pool = ct.getPool();
 399             try {
 400                 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
 401             } catch (RejectedExecutionException ree) {
 402                 submitFailed(ree);
 403                 throw ree;
 404             } catch (OutOfMemoryError e) {
 405                 submitRunContinuation();
 406             }
 407         } else {
 408             submitRunContinuation();
 409         }
 410     }
 411 
 412     /**
 413      * Submits the runContinuation task to the scheduler. For the default scheduler, and
 414      * calling it a virtual thread that uses the default scheduler, the task will be
 415      * pushed to an external submission queue. This method may throw OutOfMemoryError.

 416      * @throws RejectedExecutionException
 417      * @throws OutOfMemoryError
 418      */
 419     private void externalSubmitRunContinuationOrThrow() {
 420         if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {

 421             try {
 422                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 423             } catch (RejectedExecutionException ree) {
 424                 submitFailed(ree);
 425                 throw ree;


 426             }
 427         } else {
 428             submitRunContinuation(scheduler, false);




































 429         }
 430     }
 431 
 432     /**
 433      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 434      */
 435     private void submitFailed(RejectedExecutionException ree) {
 436         var event = new VirtualThreadSubmitFailedEvent();
 437         if (event.isEnabled()) {
 438             event.javaThreadId = threadId();
 439             event.exceptionMessage = ree.getMessage();
 440             event.commit();
 441         }
 442     }
 443 
 444     /**
 445      * Runs a task in the context of this virtual thread.
 446      */
 447     private void run(Runnable task) {
 448         assert Thread.currentThread() == this && state == RUNNING;
 449 
 450         // emit JFR event if enabled
 451         if (VirtualThreadStartEvent.isTurnedOn()) {
 452             var event = new VirtualThreadStartEvent();
 453             event.javaThreadId = threadId();
 454             event.commit();
 455         }
 456 
 457         Object bindings = Thread.scopedValueBindings();
 458         try {
 459             runWith(bindings, task);
 460         } catch (Throwable exc) {
 461             dispatchUncaughtException(exc);
 462         } finally {
 463             // pop any remaining scopes from the stack, this may block
 464             StackableScope.popAll();
 465 
 466             // emit JFR event if enabled
 467             if (VirtualThreadEndEvent.isTurnedOn()) {
 468                 var event = new VirtualThreadEndEvent();
 469                 event.javaThreadId = threadId();
 470                 event.commit();
 471             }
 472         }
 473     }
 474 
 475     /**
 476      * Mounts this virtual thread onto the current platform thread. On
 477      * return, the current thread is the virtual thread.
 478      */
 479     @ChangesCurrentThread
 480     @ReservedStackAccess
 481     private void mount() {
 482         // notify JVMTI before mount
 483         notifyJvmtiMount(/*hide*/true);
 484 
 485         // sets the carrier thread
 486         Thread carrier = Thread.currentCarrierThread();
 487         setCarrierThread(carrier);
 488 
 489         // sync up carrier thread interrupted status if needed
 490         if (interrupted) {
 491             carrier.setInterrupt();
 492         } else if (carrier.isInterrupted()) {
 493             synchronized (interruptLock) {
 494                 // need to recheck interrupted status
 495                 if (!interrupted) {
 496                     carrier.clearInterrupt();
 497                 }
 498             }
 499         }
 500 
 501         // set Thread.currentThread() to return this virtual thread
 502         carrier.setCurrentThread(this);
 503     }
 504 
 505     /**
 506      * Unmounts this virtual thread from the carrier. On return, the
 507      * current thread is the current platform thread.
 508      */
 509     @ChangesCurrentThread
 510     @ReservedStackAccess
 511     private void unmount() {
 512         assert !Thread.holdsLock(interruptLock);
 513 
 514         // set Thread.currentThread() to return the platform thread
 515         Thread carrier = this.carrierThread;
 516         carrier.setCurrentThread(carrier);
 517 
 518         // break connection to carrier thread, synchronized with interrupt
 519         synchronized (interruptLock) {
 520             setCarrierThread(null);
 521         }
 522         carrier.clearInterrupt();
 523 
 524         // notify JVMTI after unmount
 525         notifyJvmtiUnmount(/*hide*/false);
 526     }
 527 
 528     /**
 529      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 530      * the continuation continues.
 531      */
 532     @Hidden
 533     private boolean yieldContinuation() {
 534         notifyJvmtiUnmount(/*hide*/true);
 535         try {
 536             return Continuation.yield(VTHREAD_SCOPE);
 537         } finally {
 538             notifyJvmtiMount(/*hide*/false);
 539         }
 540     }
 541 
 542     /**
 543      * Invoked in the context of the carrier thread after the Continuation yields when
 544      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 545      */
 546     private void afterYield() {
 547         assert carrierThread == null;
 548 
 549         // re-adjust parallelism if the virtual thread yielded when compensating
 550         if (currentThread() instanceof CarrierThread ct) {
 551             ct.endBlocking();
 552         }
 553 
 554         int s = state();
 555 
 556         // LockSupport.park/parkNanos
 557         if (s == PARKING || s == TIMED_PARKING) {
 558             int newState;
 559             if (s == PARKING) {
 560                 setState(newState = PARKED);
 561             } else {
 562                 // schedule unpark
 563                 long timeout = this.timeout;
 564                 assert timeout > 0;
 565                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 566                 setState(newState = TIMED_PARKED);
 567             }
 568 
 569             // may have been unparked while parking
 570             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 571                 // lazy submit if local queue is empty
 572                 lazySubmitRunContinuation();
 573             }
 574             return;
 575         }
 576 
 577         // Thread.yield
 578         if (s == YIELDING) {
 579             setState(YIELDED);
 580 
 581             // external submit if there are no tasks in the local task queue
 582             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 583                 externalSubmitRunContinuation(ct.getPool());
 584             } else {
 585                 submitRunContinuation();
 586             }
 587             return;
 588         }
 589 
 590         // blocking on monitorenter
 591         if (s == BLOCKING) {
 592             setState(BLOCKED);
 593 
 594             // may have been unblocked while blocking
 595             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 596                 // lazy submit if local queue is empty
 597                 lazySubmitRunContinuation();
 598             }
 599             return;
 600         }
 601 
 602         // Object.wait
 603         if (s == WAITING || s == TIMED_WAITING) {
 604             int newState;
 605             boolean interruptible = interruptibleWait;
 606             if (s == WAITING) {
 607                 setState(newState = WAIT);
 608             } else {
 609                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 610                 // task will change the thread state to UNBLOCKED and submit the thread
 611                 // to the scheduler. A sequence number is used to ensure that the timeout
 612                 // task only unblocks the thread for this timed-wait. We synchronize with
 613                 // the timeout task to coordinate access to the sequence number and to
 614                 // ensure the timeout task doesn't execute until the thread has got to
 615                 // the TIMED_WAIT state.
 616                 long timeout = this.timeout;
 617                 assert timeout > 0;
 618                 synchronized (timedWaitLock()) {
 619                     byte seqNo = ++timedWaitSeqNo;
 620                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 621                     setState(newState = TIMED_WAIT);
 622                 }
 623             }
 624 
 625             // may have been notified while in transition to wait state
 626             if (notified && compareAndSetState(newState, BLOCKED)) {
 627                 // may have even been unblocked already
 628                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 629                     submitRunContinuation();
 630                 }
 631                 return;
 632             }
 633 
 634             // may have been interrupted while in transition to wait state
 635             if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 636                 submitRunContinuation();
 637                 return;
 638             }
 639             return;
 640         }
 641 
 642         assert false;
 643     }
 644 
 645     /**
 646      * Invoked after the continuation completes.
 647      */
 648     private void afterDone() {
 649         afterDone(true);
 650     }
 651 
 652     /**
 653      * Invoked after the continuation completes (or start failed). Sets the thread
 654      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 655      *
 656      * @param notifyContainer true if its container should be notified
 657      */
 658     private void afterDone(boolean notifyContainer) {
 659         assert carrierThread == null;
 660         setState(TERMINATED);
 661 
 662         // notify anyone waiting for this virtual thread to terminate
 663         CountDownLatch termination = this.termination;
 664         if (termination != null) {
 665             assert termination.getCount() == 1;
 666             termination.countDown();
 667         }
 668 
 669         // notify container
 670         if (notifyContainer) {
 671             threadContainer().remove(this);
 672         }
 673 
 674         // clear references to thread locals
 675         clearReferences();
 676     }
 677 
 678     /**
 679      * Schedules this {@code VirtualThread} to execute.
 680      *
 681      * @throws IllegalStateException if the container is shutdown or closed
 682      * @throws IllegalThreadStateException if the thread has already been started
 683      * @throws RejectedExecutionException if the scheduler cannot accept a task
 684      */
 685     @Override
 686     void start(ThreadContainer container) {
 687         if (!compareAndSetState(NEW, STARTED)) {
 688             throw new IllegalThreadStateException("Already started");
 689         }
 690 
 691         // bind thread to container
 692         assert threadContainer() == null;
 693         setThreadContainer(container);
 694 
 695         // start thread
 696         boolean addedToContainer = false;
 697         boolean started = false;
 698         try {
 699             container.add(this);  // may throw
 700             addedToContainer = true;
 701 
 702             // scoped values may be inherited
 703             inheritScopedValueBindings(container);
 704 
 705             // submit task to run thread, using externalSubmit if possible
 706             externalSubmitRunContinuationOrThrow();
 707             started = true;
 708         } finally {
 709             if (!started) {
 710                 afterDone(addedToContainer);
 711             }
 712         }
 713     }
 714 
 715     @Override
 716     public void start() {
 717         start(ThreadContainers.root());
 718     }
 719 
 720     @Override
 721     public void run() {
 722         // do nothing
 723     }
 724 
 725     /**
 726      * Parks until unparked or interrupted. If already unparked then the parking
 727      * permit is consumed and this method completes immediately (meaning it doesn't
 728      * yield). It also completes immediately if the interrupted status is set.
 729      */
 730     @Override
 731     void park() {
 732         assert Thread.currentThread() == this;
 733 
 734         // complete immediately if parking permit available or interrupted
 735         if (getAndSetParkPermit(false) || interrupted)
 736             return;
 737 
 738         // park the thread
 739         boolean yielded = false;

 740         setState(PARKING);
 741         try {
 742             yielded = yieldContinuation();
 743         } catch (OutOfMemoryError e) {
 744             // park on carrier
 745         } finally {
 746             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 747             if (!yielded) {


 748                 assert state() == PARKING;
 749                 setState(RUNNING);
 750             }
 751         }
 752 
 753         // park on the carrier thread when pinned
 754         if (!yielded) {
 755             parkOnCarrierThread(false, 0);
 756         }
 757     }
 758 
 759     /**
 760      * Parks up to the given waiting time or until unparked or interrupted.
 761      * If already unparked then the parking permit is consumed and this method
 762      * completes immediately (meaning it doesn't yield). It also completes immediately
 763      * if the interrupted status is set or the waiting time is {@code <= 0}.
 764      *
 765      * @param nanos the maximum number of nanoseconds to wait.
 766      */
 767     @Override
 768     void parkNanos(long nanos) {
 769         assert Thread.currentThread() == this;
 770 
 771         // complete immediately if parking permit available or interrupted
 772         if (getAndSetParkPermit(false) || interrupted)
 773             return;
 774 
 775         // park the thread for the waiting time
 776         if (nanos > 0) {
 777             long startTime = System.nanoTime();
 778 
 779             // park the thread, afterYield will schedule the thread to unpark
 780             boolean yielded = false;

 781             timeout = nanos;
 782             setState(TIMED_PARKING);
 783             try {
 784                 yielded = yieldContinuation();
 785             } catch (OutOfMemoryError e) {
 786                 // park on carrier
 787             } finally {
 788                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 789                 if (!yielded) {


 790                     assert state() == TIMED_PARKING;
 791                     setState(RUNNING);
 792                 }
 793             }
 794 
 795             // park on carrier thread for remaining time when pinned (or OOME)
 796             if (!yielded) {
 797                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 798                 parkOnCarrierThread(true, remainingNanos);
 799             }
 800         }
 801     }
 802 
 803     /**
 804      * Parks the current carrier thread up to the given waiting time or until
 805      * unparked or interrupted. If the virtual thread is interrupted then the
 806      * interrupted status will be propagated to the carrier thread.
 807      * @param timed true for a timed park, false for untimed
 808      * @param nanos the waiting time in nanoseconds
 809      */
 810     private void parkOnCarrierThread(boolean timed, long nanos) {
 811         assert state() == RUNNING;
 812 
 813         setState(timed ? TIMED_PINNED : PINNED);
 814         try {
 815             if (!parkPermit) {
 816                 if (!timed) {
 817                     U.park(false, 0);
 818                 } else if (nanos > 0) {
 819                     U.park(false, nanos);
 820                 }
 821             }
 822         } finally {
 823             setState(RUNNING);
 824         }
 825 
 826         // consume parking permit
 827         setParkPermit(false);
 828 
 829         // JFR jdk.VirtualThreadPinned event
 830         postPinnedEvent("LockSupport.park");
 831     }
 832 
 833     /**
 834      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 835      * Recording the event in the VM avoids having JFR event recorded in Java
 836      * with the same name, but different ID, to events recorded by the VM.
 837      */
 838     @Hidden
 839     private static native void postPinnedEvent(String op);
 840 
 841     /**
 842      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 843      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 844      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 845      * @throws RejectedExecutionException if the scheduler cannot accept a task
 846      */
 847     @Override
 848     void unpark() {
 849         if (!getAndSetParkPermit(true) && currentThread() != this) {
 850             int s = state();
 851 
 852             // unparked while parked
 853             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
 854                 submitRunContinuation();
 855                 return;
 856             }
 857 
 858             // unparked while parked when pinned
 859             if (s == PINNED || s == TIMED_PINNED) {
 860                 // unpark carrier thread when pinned
 861                 disableSuspendAndPreempt();
 862                 try {
 863                     synchronized (carrierThreadAccessLock()) {
 864                         Thread carrier = carrierThread;
 865                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 866                             U.unpark(carrier);
 867                         }
 868                     }
 869                 } finally {
 870                     enableSuspendAndPreempt();
 871                 }
 872                 return;
 873             }
 874         }
 875     }
 876 
 877     /**
 878      * Invoked by unblocker thread to unblock this virtual thread.
 879      */
 880     private void unblock() {
 881         assert !Thread.currentThread().isVirtual();
 882         blockPermit = true;
 883         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
 884             submitRunContinuation();
 885         }
 886     }
 887 
 888     /**
 889      * Invoked by FJP worker thread or STPE thread when park timeout expires.
 890      */
 891     private void parkTimeoutExpired() {
 892         assert !VirtualThread.currentThread().isVirtual();
 893         if (!getAndSetParkPermit(true)
 894                 && (state() == TIMED_PARKED)
 895                 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
 896             lazySubmitRunContinuation();
 897         }
 898     }
 899 
 900     /**
 901      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
 902      * If the virtual thread is in timed-wait then this method will unblock the thread
 903      * and submit its task so that it continues and attempts to reenter the monitor.
 904      * This method does nothing if the thread has been woken by notify or interrupt.
 905      */
 906     private void waitTimeoutExpired(byte seqNo) {
 907         assert !Thread.currentThread().isVirtual();
 908         for (;;) {
 909             boolean unblocked = false;
 910             synchronized (timedWaitLock()) {
 911                 if (seqNo != timedWaitSeqNo) {
 912                     // this timeout task is for a past timed-wait
 913                     return;
 914                 }
 915                 int s = state();
 916                 if (s == TIMED_WAIT) {
 917                     unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
 918                 } else if (s != (TIMED_WAIT | SUSPENDED)) {
 919                     // notified or interrupted, no longer waiting
 920                     return;
 921                 }
 922             }
 923             if (unblocked) {
 924                 lazySubmitRunContinuation();
 925                 return;
 926             }
 927             // need to retry when thread is suspended in time-wait
 928             Thread.yield();
 929         }
 930     }
 931 
 932     /**
 933      * Attempts to yield the current virtual thread (Thread.yield).
 934      */
 935     void tryYield() {
 936         assert Thread.currentThread() == this;
 937         setState(YIELDING);
 938         boolean yielded = false;
 939         try {
 940             yielded = yieldContinuation();  // may throw
 941         } finally {
 942             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 943             if (!yielded) {
 944                 assert state() == YIELDING;
 945                 setState(RUNNING);
 946             }
 947         }
 948     }
 949 
 950     /**
 951      * Sleep the current thread for the given sleep time (in nanoseconds). If
 952      * nanos is 0 then the thread will attempt to yield.
 953      *
 954      * @implNote This implementation parks the thread for the given sleeping time
 955      * and will therefore be observed in PARKED state during the sleep. Parking
 956      * will consume the parking permit so this method makes available the parking
 957      * permit after the sleep. This may be observed as a spurious, but benign,
 958      * wakeup when the thread subsequently attempts to park.
 959      *
 960      * @param nanos the maximum number of nanoseconds to sleep
 961      * @throws InterruptedException if interrupted while sleeping
 962      */
 963     void sleepNanos(long nanos) throws InterruptedException {
 964         assert Thread.currentThread() == this && nanos >= 0;
 965         if (getAndClearInterrupt())
 966             throw new InterruptedException();
 967         if (nanos == 0) {
 968             tryYield();
 969         } else {
 970             // park for the sleep time
 971             try {
 972                 long remainingNanos = nanos;
 973                 long startNanos = System.nanoTime();
 974                 while (remainingNanos > 0) {
 975                     parkNanos(remainingNanos);
 976                     if (getAndClearInterrupt()) {
 977                         throw new InterruptedException();
 978                     }
 979                     remainingNanos = nanos - (System.nanoTime() - startNanos);
 980                 }
 981             } finally {
 982                 // may have been unparked while sleeping
 983                 setParkPermit(true);
 984             }
 985         }
 986     }
 987 
 988     /**
 989      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
 990      * A timeout of {@code 0} means to wait forever.
 991      *
 992      * @throws InterruptedException if interrupted while waiting
 993      * @return true if the thread has terminated
 994      */
 995     boolean joinNanos(long nanos) throws InterruptedException {
 996         if (state() == TERMINATED)
 997             return true;
 998 
 999         // ensure termination object exists, then re-check state
1000         CountDownLatch termination = getTermination();
1001         if (state() == TERMINATED)
1002             return true;
1003 
1004         // wait for virtual thread to terminate
1005         if (nanos == 0) {
1006             termination.await();
1007         } else {
1008             boolean terminated = termination.await(nanos, NANOSECONDS);
1009             if (!terminated) {
1010                 // waiting time elapsed
1011                 return false;
1012             }
1013         }
1014         assert state() == TERMINATED;
1015         return true;
1016     }
1017 
1018     @Override
1019     void blockedOn(Interruptible b) {
1020         disableSuspendAndPreempt();
1021         try {
1022             super.blockedOn(b);
1023         } finally {
1024             enableSuspendAndPreempt();
1025         }
1026     }
1027 
1028     @Override
1029     public void interrupt() {
1030         if (Thread.currentThread() != this) {
1031             // if current thread is a virtual thread then prevent it from being
1032             // suspended or unmounted when entering or holding interruptLock
1033             Interruptible blocker;
1034             disableSuspendAndPreempt();
1035             try {
1036                 synchronized (interruptLock) {
1037                     interrupted = true;
1038                     blocker = nioBlocker();
1039                     if (blocker != null) {
1040                         blocker.interrupt(this);
1041                     }
1042 
1043                     // interrupt carrier thread if mounted
1044                     Thread carrier = carrierThread;
1045                     if (carrier != null) carrier.setInterrupt();
1046                 }
1047             } finally {
1048                 enableSuspendAndPreempt();
1049             }
1050 
1051             // notify blocker after releasing interruptLock
1052             if (blocker != null) {
1053                 blocker.postInterrupt();
1054             }
1055 
1056             // make available parking permit, unpark thread if parked
1057             unpark();
1058 
1059             // if thread is waiting in Object.wait then schedule to try to reenter
1060             int s = state();
1061             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1062                 submitRunContinuation();
1063             }
1064 
1065         } else {
1066             interrupted = true;
1067             carrierThread.setInterrupt();
1068             setParkPermit(true);
1069         }
1070     }
1071 
1072     @Override
1073     public boolean isInterrupted() {
1074         return interrupted;
1075     }
1076 
1077     @Override
1078     boolean getAndClearInterrupt() {
1079         assert Thread.currentThread() == this;
1080         boolean oldValue = interrupted;
1081         if (oldValue) {
1082             disableSuspendAndPreempt();
1083             try {
1084                 synchronized (interruptLock) {
1085                     interrupted = false;
1086                     carrierThread.clearInterrupt();
1087                 }
1088             } finally {
1089                 enableSuspendAndPreempt();
1090             }
1091         }
1092         return oldValue;
1093     }
1094 
1095     @Override
1096     Thread.State threadState() {
1097         int s = state();
1098         switch (s & ~SUSPENDED) {
1099             case NEW:
1100                 return Thread.State.NEW;
1101             case STARTED:
1102                 // return NEW if thread container not yet set
1103                 if (threadContainer() == null) {
1104                     return Thread.State.NEW;
1105                 } else {
1106                     return Thread.State.RUNNABLE;
1107                 }
1108             case UNPARKED:
1109             case UNBLOCKED:
1110             case YIELDED:
1111                 // runnable, not mounted
1112                 return Thread.State.RUNNABLE;
1113             case RUNNING:
1114                 // if mounted then return state of carrier thread
1115                 if (Thread.currentThread() != this) {
1116                     disableSuspendAndPreempt();
1117                     try {
1118                         synchronized (carrierThreadAccessLock()) {
1119                             Thread carrierThread = this.carrierThread;
1120                             if (carrierThread != null) {
1121                                 return carrierThread.threadState();
1122                             }
1123                         }
1124                     } finally {
1125                         enableSuspendAndPreempt();
1126                     }
1127                 }
1128                 // runnable, mounted
1129                 return Thread.State.RUNNABLE;
1130             case PARKING:
1131             case TIMED_PARKING:
1132             case WAITING:
1133             case TIMED_WAITING:
1134             case YIELDING:
1135                 // runnable, in transition
1136                 return Thread.State.RUNNABLE;
1137             case PARKED:
1138             case PINNED:
1139             case WAIT:
1140                 return Thread.State.WAITING;
1141             case TIMED_PARKED:
1142             case TIMED_PINNED:
1143             case TIMED_WAIT:
1144                 return Thread.State.TIMED_WAITING;
1145             case BLOCKING:
1146             case BLOCKED:
1147                 return Thread.State.BLOCKED;
1148             case TERMINATED:
1149                 return Thread.State.TERMINATED;
1150             default:
1151                 throw new InternalError();
1152         }
1153     }
1154 
1155     @Override
1156     boolean alive() {
1157         int s = state;
1158         return (s != NEW && s != TERMINATED);
1159     }
1160 
1161     @Override
1162     boolean isTerminated() {
1163         return (state == TERMINATED);
1164     }
1165 
1166     @Override
1167     StackTraceElement[] asyncGetStackTrace() {
1168         StackTraceElement[] stackTrace;
1169         do {
1170             stackTrace = (carrierThread != null)
1171                     ? super.asyncGetStackTrace()  // mounted
1172                     : tryGetStackTrace();         // unmounted
1173             if (stackTrace == null) {
1174                 Thread.yield();
1175             }
1176         } while (stackTrace == null);
1177         return stackTrace;
1178     }
1179 
1180     /**
1181      * Returns the stack trace for this virtual thread if it is unmounted.
1182      * Returns null if the thread is mounted or in transition.
1183      */
1184     private StackTraceElement[] tryGetStackTrace() {
1185         int initialState = state() & ~SUSPENDED;
1186         switch (initialState) {
1187             case NEW, STARTED, TERMINATED -> {
1188                 return new StackTraceElement[0];  // unmounted, empty stack
1189             }
1190             case RUNNING, PINNED, TIMED_PINNED -> {
1191                 return null;   // mounted
1192             }
1193             case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1194                 // unmounted, not runnable
1195             }
1196             case UNPARKED, UNBLOCKED, YIELDED -> {
1197                 // unmounted, runnable
1198             }
1199             case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1200                 return null;  // in transition
1201             }
1202             default -> throw new InternalError("" + initialState);
1203         }
1204 
1205         // thread is unmounted, prevent it from continuing
1206         int suspendedState = initialState | SUSPENDED;
1207         if (!compareAndSetState(initialState, suspendedState)) {
1208             return null;
1209         }
1210 
1211         // get stack trace and restore state
1212         StackTraceElement[] stack;
1213         try {
1214             stack = cont.getStackTrace();
1215         } finally {
1216             assert state == suspendedState;
1217             setState(initialState);
1218         }
1219         boolean resubmit = switch (initialState) {
1220             case UNPARKED, UNBLOCKED, YIELDED -> {
1221                 // resubmit as task may have run while suspended
1222                 yield true;
1223             }
1224             case PARKED, TIMED_PARKED -> {
1225                 // resubmit if unparked while suspended
1226                 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1227             }
1228             case BLOCKED -> {
1229                 // resubmit if unblocked while suspended
1230                 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1231             }
1232             case WAIT, TIMED_WAIT -> {
1233                 // resubmit if notified or interrupted while waiting (Object.wait)
1234                 // waitTimeoutExpired will retry if the timed expired when suspended
1235                 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1236             }
1237             default -> throw new InternalError();
1238         };
1239         if (resubmit) {
1240             submitRunContinuation();
1241         }
1242         return stack;
1243     }
1244 
1245     @Override
1246     public String toString() {
1247         StringBuilder sb = new StringBuilder("VirtualThread[#");
1248         sb.append(threadId());
1249         String name = getName();
1250         if (!name.isEmpty()) {
1251             sb.append(",");
1252             sb.append(name);
1253         }
1254         sb.append("]/");
1255 
1256         // add the carrier state and thread name when mounted
1257         boolean mounted;
1258         if (Thread.currentThread() == this) {
1259             mounted = appendCarrierInfo(sb);
1260         } else {
1261             disableSuspendAndPreempt();
1262             try {
1263                 synchronized (carrierThreadAccessLock()) {
1264                     mounted = appendCarrierInfo(sb);
1265                 }
1266             } finally {
1267                 enableSuspendAndPreempt();
1268             }
1269         }
1270 
1271         // add virtual thread state when not mounted
1272         if (!mounted) {
1273             String stateAsString = threadState().toString();
1274             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1275         }
1276 
1277         return sb.toString();
1278     }
1279 
1280     /**
1281      * Appends the carrier state and thread name to the string buffer if mounted.
1282      * @return true if mounted, false if not mounted
1283      */
1284     private boolean appendCarrierInfo(StringBuilder sb) {
1285         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1286         Thread carrier = carrierThread;
1287         if (carrier != null) {
1288             String stateAsString = carrier.threadState().toString();
1289             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1290             sb.append('@');
1291             sb.append(carrier.getName());
1292             return true;
1293         } else {
1294             return false;
1295         }
1296     }
1297 
1298     @Override
1299     public int hashCode() {
1300         return (int) threadId();
1301     }
1302 
1303     @Override
1304     public boolean equals(Object obj) {
1305         return obj == this;
1306     }
1307 
1308     /**
1309      * Returns the termination object, creating it if needed.
1310      */
1311     private CountDownLatch getTermination() {
1312         CountDownLatch termination = this.termination;
1313         if (termination == null) {
1314             termination = new CountDownLatch(1);
1315             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1316                 termination = this.termination;
1317             }
1318         }
1319         return termination;
1320     }
1321 
1322     /**
1323      * Returns the lock object to synchronize on when accessing carrierThread.
1324      * The lock prevents carrierThread from being reset to null during unmount.
1325      */
1326     private Object carrierThreadAccessLock() {
1327         // return interruptLock as unmount has to coordinate with interrupt
1328         return interruptLock;
1329     }
1330 
1331     /**
1332      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1333      */
1334     private Object timedWaitLock() {
1335         // use this object for now to avoid the overhead of introducing another lock
1336         return runContinuation;
1337     }
1338 
1339     /**
1340      * Disallow the current thread be suspended or preempted.
1341      */
1342     private void disableSuspendAndPreempt() {
1343         notifyJvmtiDisableSuspend(true);
1344         Continuation.pin();
1345     }
1346 
1347     /**
1348      * Allow the current thread be suspended or preempted.
1349      */
1350     private void enableSuspendAndPreempt() {
1351         Continuation.unpin();
1352         notifyJvmtiDisableSuspend(false);
1353     }
1354 
1355     // -- wrappers for get/set of state, parking permit, and carrier thread --
1356 
1357     private int state() {
1358         return state;  // volatile read
1359     }
1360 
1361     private void setState(int newValue) {
1362         state = newValue;  // volatile write
1363     }
1364 
1365     private boolean compareAndSetState(int expectedValue, int newValue) {
1366         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1367     }
1368 
1369     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1370         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1371     }
1372 
1373     private void setParkPermit(boolean newValue) {
1374         if (parkPermit != newValue) {
1375             parkPermit = newValue;
1376         }
1377     }
1378 
1379     private boolean getAndSetParkPermit(boolean newValue) {
1380         if (parkPermit != newValue) {
1381             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1382         } else {
1383             return newValue;
1384         }
1385     }
1386 
1387     private void setCarrierThread(Thread carrier) {
1388         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1389         this.carrierThread = carrier;
1390     }
1391 
1392     // -- JVM TI support --
1393 
1394     @IntrinsicCandidate
1395     @JvmtiMountTransition
1396     private native void notifyJvmtiStart();
1397 
1398     @IntrinsicCandidate
1399     @JvmtiMountTransition
1400     private native void notifyJvmtiEnd();
1401 
1402     @IntrinsicCandidate
1403     @JvmtiMountTransition
1404     private native void notifyJvmtiMount(boolean hide);
1405 
1406     @IntrinsicCandidate
1407     @JvmtiMountTransition
1408     private native void notifyJvmtiUnmount(boolean hide);
1409 
1410     @IntrinsicCandidate
1411     private static native void notifyJvmtiDisableSuspend(boolean enter);
1412 
1413     private static native void registerNatives();
1414     static {
1415         registerNatives();
1416 
1417         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1418         var group = Thread.virtualThreadGroup();



































1419     }
1420 
1421     /**
1422      * Creates the default ForkJoinPool scheduler.

1423      */
1424     private static ForkJoinPool createDefaultScheduler() {
1425         ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1426         int parallelism, maxPoolSize, minRunnable;
1427         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1428         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1429         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1430         if (parallelismValue != null) {
1431             parallelism = Integer.parseInt(parallelismValue);
1432         } else {
1433             parallelism = Runtime.getRuntime().availableProcessors();
1434         }
1435         if (maxPoolSizeValue != null) {
1436             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1437             parallelism = Integer.min(parallelism, maxPoolSize);
1438         } else {
1439             maxPoolSize = Integer.max(parallelism, 256);
1440         }
1441         if (minRunnableValue != null) {
1442             minRunnable = Integer.parseInt(minRunnableValue);
1443         } else {
1444             minRunnable = Integer.max(parallelism / 2, 1);
1445         }
1446         Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1447         boolean asyncMode = true; // FIFO
1448         return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1449                      0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);


























































1450     }
1451 
1452     /**
1453      * Schedule a runnable task to run after a delay.
1454      */
1455     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1456         if (scheduler instanceof ForkJoinPool pool) {
1457             return pool.schedule(command, delay, unit);
1458         } else {
1459             return DelayedTaskSchedulers.schedule(command, delay, unit);
1460         }
1461     }
1462 
1463     /**
1464      * Supports scheduling a runnable task to run after a delay. It uses a number
1465      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1466      * work queue used. This class is used when using a custom scheduler.
1467      */
1468     private static class DelayedTaskSchedulers {
1469         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1470 
1471         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1472             long tid = Thread.currentThread().threadId();
1473             int index = (int) tid & (INSTANCE.length - 1);
1474             return INSTANCE[index].schedule(command, delay, unit);
1475         }
1476 
1477         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1478             String propName = "jdk.virtualThreadScheduler.timerQueues";
1479             String propValue = System.getProperty(propName);
1480             int queueCount;
1481             if (propValue != null) {
1482                 queueCount = Integer.parseInt(propValue);
1483                 if (queueCount != Integer.highestOneBit(queueCount)) {
1484                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1485                 }
1486             } else {
1487                 int ncpus = Runtime.getRuntime().availableProcessors();
1488                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1489             }
1490             var schedulers = new ScheduledExecutorService[queueCount];
1491             for (int i = 0; i < queueCount; i++) {
1492                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1493                     Executors.newScheduledThreadPool(1, task -> {
1494                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1495                         t.setDaemon(true);
1496                         return t;
1497                     });
1498                 stpe.setRemoveOnCancelPolicy(true);
1499                 schedulers[i] = stpe;
1500             }
1501             return schedulers;
1502         }
1503     }
1504 
1505     /**
1506      * Schedule virtual threads that are ready to be scheduled after they blocked on
1507      * monitor enter.
1508      */
1509     private static void unblockVirtualThreads() {
1510         while (true) {
1511             VirtualThread vthread = takeVirtualThreadListToUnblock();
1512             while (vthread != null) {
1513                 assert vthread.onWaitingList;
1514                 VirtualThread nextThread = vthread.next;
1515 
1516                 // remove from list and unblock
1517                 vthread.next = null;
1518                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1519                 assert changed;
1520                 vthread.unblock();
1521 
1522                 vthread = nextThread;
1523             }
1524         }
1525     }
1526 
1527     /**
1528      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1529      * if necessary until a list of one or more threads becomes available.
1530      */
1531     private static native VirtualThread takeVirtualThreadListToUnblock();
1532 
1533     static {
1534         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1535                 VirtualThread::unblockVirtualThreads);
1536         unblocker.setDaemon(true);
1537         unblocker.start();
1538     }
1539 }
--- EOF ---