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

  33 import java.util.concurrent.Executors;
  34 import java.util.concurrent.ForkJoinPool;

  35 import java.util.concurrent.ForkJoinTask;
  36 import java.util.concurrent.Future;
  37 import java.util.concurrent.RejectedExecutionException;
  38 import java.util.concurrent.ScheduledExecutorService;
  39 import java.util.concurrent.ScheduledThreadPoolExecutor;
  40 import java.util.concurrent.TimeUnit;
  41 import jdk.internal.event.VirtualThreadEndEvent;
  42 import jdk.internal.event.VirtualThreadParkEvent;
  43 import jdk.internal.event.VirtualThreadStartEvent;
  44 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  45 import jdk.internal.invoke.MhUtil;
  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 
  70     private static final ForkJoinPool BUILTIN_SCHEDULER;
  71     private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
  72     static {
  73         // experimental
  74         String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
  75         if (propValue != null) {
  76             var builtinScheduler = createBuiltinDefaultScheduler(true);
  77             VirtualThreadScheduler defaultScheduler = builtinScheduler.externalView();
  78             for (String cn: propValue.split(",")) {
  79                 defaultScheduler = loadCustomScheduler(defaultScheduler, cn);
  80             }
  81             BUILTIN_SCHEDULER = builtinScheduler;
  82             DEFAULT_SCHEDULER = defaultScheduler;
  83         } else {
  84             var builtinScheduler = createBuiltinDefaultScheduler(false);
  85             BUILTIN_SCHEDULER = builtinScheduler;
  86             DEFAULT_SCHEDULER = builtinScheduler;
  87         }
  88     }
  89 
  90     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  91     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  92     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  93     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  94     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  95 
  96     // scheduler and continuation
  97     private final VirtualThreadScheduler scheduler;
  98     private final Continuation cont;
  99     private final VirtualThreadTask runContinuation;
 100 
 101     // virtual thread state, accessed by VM
 102     private volatile int state;
 103 
 104     /*
 105      * Virtual thread state transitions:
 106      *
 107      *      NEW -> STARTED         // Thread.start, schedule to run
 108      *  STARTED -> TERMINATED      // failed to start
 109      *  STARTED -> RUNNING         // first run
 110      *  RUNNING -> TERMINATED      // done
 111      *
 112      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
 113      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
 114      *  PARKING -> PINNED          // cont.yield failed, parked indefinitely on carrier
 115      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
 116      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
 117      * UNPARKED -> RUNNING         // continue execution after park
 118      *
 119      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
 120      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
 121      * TIMED_PARKING -> TIMED_PINNED    // cont.yield failed, timed-parked on carrier
 122      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 123      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 124      *
 125      *   RUNNING -> BLOCKING       // blocking on monitor enter
 126      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 127      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 128      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 129      *
 130      *   RUNNING -> WAITING        // transitional state during wait on monitor
 131      *   WAITING -> WAIT           // waiting on monitor
 132      *      WAIT -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 133      *      WAIT -> UNBLOCKED      // timed-out/interrupted
 134      *
 135      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 136      * TIMED_WAITING -> TIMED_WAIT      // timed-waiting on monitor
 137      *    TIMED_WAIT -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 138      *    TIMED_WAIT -> UNBLOCKED       // timed-out/interrupted
 139      *
 140      *  RUNNING -> YIELDING        // Thread.yield
 141      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 142      * YIELDING -> RUNNING         // cont.yield failed
 143      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 144      */
 145     private static final int NEW      = 0;
 146     private static final int STARTED  = 1;
 147     private static final int RUNNING  = 2;     // runnable-mounted
 148 
 149     // untimed and timed parking
 150     private static final int PARKING       = 3;
 151     private static final int PARKED        = 4;     // unmounted
 152     private static final int PINNED        = 5;     // mounted
 153     private static final int TIMED_PARKING = 6;
 154     private static final int TIMED_PARKED  = 7;     // unmounted
 155     private static final int TIMED_PINNED  = 8;     // mounted
 156     private static final int UNPARKED      = 9;     // unmounted but runnable
 157 
 158     // Thread.yield
 159     private static final int YIELDING = 10;
 160     private static final int YIELDED  = 11;         // unmounted but runnable
 161 
 162     // monitor enter
 163     private static final int BLOCKING  = 12;
 164     private static final int BLOCKED   = 13;        // unmounted
 165     private static final int UNBLOCKED = 14;        // unmounted but runnable
 166 
 167     // monitor wait/timed-wait
 168     private static final int WAITING       = 15;
 169     private static final int WAIT          = 16;    // waiting in Object.wait
 170     private static final int TIMED_WAITING = 17;
 171     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 172 
 173     private static final int TERMINATED = 99;  // final state
 174 
 175     // can be suspended from scheduling when unmounted
 176     private static final int SUSPENDED = 1 << 8;
 177 
 178     // parking permit made available by LockSupport.unpark
 179     private volatile boolean parkPermit;
 180 
 181     // blocking permit made available by unblocker thread when another thread exits monitor
 182     private volatile boolean blockPermit;
 183 
 184     // true when on the list of virtual threads waiting to be unblocked
 185     private volatile boolean onWaitingList;
 186 
 187     // next virtual thread on the list of virtual threads waiting to be unblocked
 188     private volatile VirtualThread next;
 189 
 190     // notified by Object.notify/notifyAll while waiting in Object.wait
 191     private volatile boolean notified;
 192 
 193     // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
 194     private volatile boolean interruptibleWait;
 195 
 196     // timed-wait support
 197     private byte timedWaitSeqNo;
 198 
 199     // timeout for timed-park and timed-wait, only accessed on current/carrier thread
 200     private long timeout;
 201 
 202     // timer task for timed-park and timed-wait, only accessed on current/carrier thread
 203     private Future<?> timeoutTask;
 204 
 205     // carrier thread when mounted, accessed by VM
 206     private volatile Thread carrierThread;
 207 
 208     // termination object when joining, created lazily if needed
 209     private volatile CountDownLatch termination;
 210 
 211     /**
 212      * Returns the default scheduler.
 213      */
 214     static VirtualThreadScheduler defaultScheduler() {
 215         return DEFAULT_SCHEDULER;
 216     }
 217 
 218     /**
 219      * Returns true if using a custom default scheduler.
 220      */
 221     static boolean isCustomDefaultScheduler() {
 222         return DEFAULT_SCHEDULER != BUILTIN_SCHEDULER;
 223     }
 224 
 225     /**
 226      * Returns the continuation scope used for virtual threads.
 227      */
 228     static ContinuationScope continuationScope() {
 229         return VTHREAD_SCOPE;
 230     }
 231 
 232     /**
 233      * Return the scheduler for this thread.
 234      * @param revealBuiltin true to reveal the built-in scheduler, false to hide
 235      */
 236     VirtualThreadScheduler scheduler(boolean revealBuiltin) {
 237         if (scheduler instanceof BuiltinDefaultScheduler builtin && !revealBuiltin) {
 238             return builtin.externalView();
 239         } else {
 240             return scheduler;
 241         }
 242     }
 243 
 244     /**
 245      * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
 246      *
 247      * @param scheduler the scheduler or null for default scheduler
 248      * @param name thread name
 249      * @param characteristics characteristics
 250      * @param task the task to execute
 251      */
 252     VirtualThread(VirtualThreadScheduler scheduler,
 253                   String name,
 254                   int characteristics,
 255                   Runnable task) {
 256         super(name, characteristics, /*bound*/ false);
 257         Objects.requireNonNull(task);
 258 
 259         // use default scheduler if not provided
 260         if (scheduler == null) {
 261             scheduler = DEFAULT_SCHEDULER;





 262         }
 263 
 264         this.scheduler = scheduler;
 265         this.cont = new VThreadContinuation(this, task);
 266 
 267         if (scheduler == BUILTIN_SCHEDULER) {
 268             this.runContinuation = new BuiltinSchedulerTask(this);
 269         } else {
 270             this.runContinuation = new CustomSchedulerTask(this);
 271         }
 272     }
 273 
 274     /**
 275      * The task to execute when using the built-in scheduler.
 276      */
 277     static final class BuiltinSchedulerTask implements VirtualThreadTask {
 278         private final VirtualThread vthread;
 279         BuiltinSchedulerTask(VirtualThread vthread) {
 280             this.vthread = vthread;
 281         }
 282         @Override
 283         public Object attach(Object att) {
 284             throw new UnsupportedOperationException();
 285         }
 286         @Override
 287         public Object attachment() {
 288             throw new UnsupportedOperationException();
 289         }
 290         @Override
 291         public Thread thread() {
 292             return vthread;
 293         }
 294         @Override
 295         public void run() {
 296             vthread.runContinuation();;
 297         }
 298     }
 299 
 300     /**
 301      * The task to execute when using a custom scheduler.
 302      */
 303     static final class CustomSchedulerTask implements VirtualThreadTask {
 304         private static final VarHandle ATT =
 305                 MhUtil.findVarHandle(MethodHandles.lookup(), "att", Object.class);
 306         private final VirtualThread vthread;
 307         private volatile Object att;
 308         CustomSchedulerTask(VirtualThread vthread) {
 309             this.vthread = vthread;
 310         }
 311         @Override
 312         public Object attach(Object att) {
 313             return ATT.getAndSet(this, att);
 314         }
 315         @Override
 316         public Object attachment() {
 317             return att;
 318         }
 319         @Override
 320         public Thread thread() {
 321             return vthread;
 322         }
 323         @Override
 324         public void run() {
 325             vthread.runContinuation();;
 326         }
 327     }
 328 
 329     /**
 330      * The continuation that a virtual thread executes.
 331      */
 332     private static class VThreadContinuation extends Continuation {
 333         VThreadContinuation(VirtualThread vthread, Runnable task) {
 334             super(VTHREAD_SCOPE, wrap(vthread, task));
 335         }
 336         @Override
 337         protected void onPinned(Continuation.Pinned reason) {
 338         }
 339         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 340             return new Runnable() {
 341                 @Hidden
 342                 @JvmtiHideEvents
 343                 public void run() {
 344                     vthread.notifyJvmtiStart(); // notify JVMTI
 345                     try {
 346                         vthread.run(task);
 347                     } finally {
 348                         vthread.notifyJvmtiEnd(); // notify JVMTI
 349                     }
 350                 }
 351             };
 352         }
 353     }
 354 
 355     /**
 356      * Runs or continues execution on the current thread. The virtual thread is mounted
 357      * on the current thread before the task runs or continues. It unmounts when the
 358      * task completes or yields.
 359      */
 360     @ChangesCurrentThread // allow mount/unmount to be inlined
 361     private void runContinuation() {
 362         // the carrier must be a platform thread
 363         if (Thread.currentThread().isVirtual()) {
 364             throw new WrongThreadException();
 365         }
 366 
 367         // set state to RUNNING
 368         int initialState = state();
 369         if (initialState == STARTED || initialState == UNPARKED
 370                 || initialState == UNBLOCKED || initialState == YIELDED) {
 371             // newly started or continue after parking/blocking/Thread.yield
 372             if (!compareAndSetState(initialState, RUNNING)) {
 373                 return;
 374             }
 375             // consume permit when continuing after parking or blocking. If continue
 376             // after a timed-park or timed-wait then the timeout task is cancelled.
 377             if (initialState == UNPARKED) {
 378                 cancelTimeoutTask();
 379                 setParkPermit(false);
 380             } else if (initialState == UNBLOCKED) {
 381                 cancelTimeoutTask();
 382                 blockPermit = false;
 383             }
 384         } else {
 385             // not runnable
 386             return;
 387         }
 388 
 389         mount();
 390         try {
 391             cont.run();
 392         } finally {
 393             unmount();
 394             if (cont.isDone()) {
 395                 afterDone();
 396             } else {
 397                 afterYield();
 398             }
 399         }
 400     }
 401 
 402     /**
 403      * Cancel timeout task when continuing after timed-park or timed-wait.
 404      * The timeout task may be executing, or may have already completed.
 405      */
 406     private void cancelTimeoutTask() {
 407         if (timeoutTask != null) {
 408             timeoutTask.cancel(false);
 409             timeoutTask = null;
 410         }
 411     }
 412 
 413     /**
 414      * Submits the runContinuation task to the scheduler. For the built-in scheduler,
 415      * the task will be pushed to the local queue if possible, otherwise it will be
 416      * pushed to an external submission queue.

 417      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 418      * @throws RejectedExecutionException
 419      */
 420     private void submitRunContinuation(boolean retryOnOOME) {
 421         boolean done = false;
 422         while (!done) {
 423             try {
 424                 // Pin the continuation to prevent the virtual thread from unmounting
 425                 // when submitting a task. For the default scheduler this ensures that
 426                 // the carrier doesn't change when pushing a task. For other schedulers
 427                 // it avoids deadlock that could arise due to carriers and virtual
 428                 // threads contending for a lock.
 429                 if (currentThread().isVirtual()) {
 430                     Continuation.pin();
 431                     try {
 432                         scheduler.onContinue(runContinuation);
 433                     } finally {
 434                         Continuation.unpin();
 435                     }
 436                 } else {
 437                     scheduler.onContinue(runContinuation);
 438                 }
 439                 done = true;
 440             } catch (RejectedExecutionException ree) {
 441                 submitFailed(ree);
 442                 throw ree;
 443             } catch (OutOfMemoryError e) {
 444                 if (retryOnOOME) {
 445                     U.park(false, 100_000_000); // 100ms
 446                 } else {
 447                     throw e;
 448                 }
 449             }
 450         }
 451     }
 452 


















 453     /**
 454      * Submits the runContinuation task to the scheduler. For the default scheduler,
 455      * and calling it on a worker thread, the task will be pushed to the local queue,
 456      * otherwise it will be pushed to an external submission queue.
 457      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 458      * @throws RejectedExecutionException
 459      */
 460     private void submitRunContinuation() {
 461         submitRunContinuation(true);
 462     }
 463 
 464     /**
 465      * Invoked from a carrier thread to lazy submit the runContinuation task to the
 466      * carrier's local queue if the queue is empty. If not empty, or invoked by a thread
 467      * for a custom scheduler, then it just submits the task to the scheduler.
 468      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 469      * @throws RejectedExecutionException
 470      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 471      */
 472     private void lazySubmitRunContinuation() {
 473         assert !currentThread().isVirtual();
 474         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {

 475             try {
 476                 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
 477             } catch (RejectedExecutionException ree) {
 478                 submitFailed(ree);
 479                 throw ree;
 480             } catch (OutOfMemoryError e) {
 481                 submitRunContinuation();
 482             }
 483         } else {
 484             submitRunContinuation();
 485         }
 486     }
 487 
 488     /**
 489      * Invoked from a carrier thread to externally submit the runContinuation task to the
 490      * scheduler. If invoked by a thread for a custom scheduler, then it just submits the
 491      * task to the scheduler.
 492      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 493      * @throws RejectedExecutionException
 494      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 495      */
 496     private void externalSubmitRunContinuation() {
 497         assert !currentThread().isVirtual();
 498         if (currentThread() instanceof CarrierThread ct) {
 499             try {
 500                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 501             } catch (RejectedExecutionException ree) {
 502                 submitFailed(ree);
 503                 throw ree;
 504             } catch (OutOfMemoryError e) {
 505                 submitRunContinuation();
 506             }
 507         } else {
 508             submitRunContinuation();
 509         }
 510     }
 511 
 512     /**
 513      * Invoked from Thread.start to externally submit the runContinuation task to the
 514      * scheduler. If this virtual thread is scheduled by the built-in scheduler,
 515      * and this method is called from a virtual thread scheduled by the built-in
 516      * scheduler, then it uses externalSubmit to ensure that the task is pushed to an
 517      * external submission queue rather than the local queue.
 518      * @throws RejectedExecutionException
 519      * @throws OutOfMemoryError
 520      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 521      */
 522     private void externalSubmitRunContinuationOrThrow() {
 523         try {
 524             if (currentThread().isVirtual()) {
 525                 // Pin the continuation to prevent the virtual thread from unmounting
 526                 // when submitting a task. This avoids deadlock that could arise due to
 527                 // carriers and virtual threads contending for a lock.
 528                 Continuation.pin();
 529                 try {
 530                     if (scheduler == BUILTIN_SCHEDULER
 531                             && currentCarrierThread() instanceof CarrierThread ct) {
 532                         ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 533                     } else {
 534                         scheduler.onStart(runContinuation);
 535                     }
 536                 } finally {
 537                     Continuation.unpin();
 538                 }
 539             } else {
 540                 scheduler.onStart(runContinuation);
 541             }
 542         } catch (RejectedExecutionException ree) {
 543             submitFailed(ree);
 544             throw ree;
 545         }
 546     }
 547 
 548     /**
 549      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 550      */
 551     private void submitFailed(RejectedExecutionException ree) {
 552         var event = new VirtualThreadSubmitFailedEvent();
 553         if (event.isEnabled()) {
 554             event.javaThreadId = threadId();
 555             event.exceptionMessage = ree.getMessage();
 556             event.commit();
 557         }
 558     }
 559 
 560     /**
 561      * Runs a task in the context of this virtual thread.
 562      */
 563     private void run(Runnable task) {
 564         assert Thread.currentThread() == this && state == RUNNING;
 565 
 566         // emit JFR event if enabled
 567         if (VirtualThreadStartEvent.isTurnedOn()) {
 568             var event = new VirtualThreadStartEvent();
 569             event.javaThreadId = threadId();
 570             event.commit();
 571         }
 572 
 573         Object bindings = Thread.scopedValueBindings();
 574         try {
 575             runWith(bindings, task);
 576         } catch (Throwable exc) {
 577             dispatchUncaughtException(exc);
 578         } finally {
 579             // pop any remaining scopes from the stack, this may block
 580             StackableScope.popAll();
 581 
 582             // emit JFR event if enabled
 583             if (VirtualThreadEndEvent.isTurnedOn()) {
 584                 var event = new VirtualThreadEndEvent();
 585                 event.javaThreadId = threadId();
 586                 event.commit();
 587             }
 588         }
 589     }
 590 
 591     /**
 592      * Mounts this virtual thread onto the current platform thread. On
 593      * return, the current thread is the virtual thread.
 594      */
 595     @ChangesCurrentThread
 596     @ReservedStackAccess
 597     private void mount() {
 598         // notify JVMTI before mount
 599         notifyJvmtiMount(/*hide*/true);
 600 
 601         // sets the carrier thread
 602         Thread carrier = Thread.currentCarrierThread();
 603         setCarrierThread(carrier);
 604 
 605         // sync up carrier thread interrupted status if needed
 606         if (interrupted) {
 607             carrier.setInterrupt();
 608         } else if (carrier.isInterrupted()) {
 609             synchronized (interruptLock) {
 610                 // need to recheck interrupted status
 611                 if (!interrupted) {
 612                     carrier.clearInterrupt();
 613                 }
 614             }
 615         }
 616 
 617         // set Thread.currentThread() to return this virtual thread
 618         carrier.setCurrentThread(this);
 619     }
 620 
 621     /**
 622      * Unmounts this virtual thread from the carrier. On return, the
 623      * current thread is the current platform thread.
 624      */
 625     @ChangesCurrentThread
 626     @ReservedStackAccess
 627     private void unmount() {
 628         assert !Thread.holdsLock(interruptLock);
 629 
 630         // set Thread.currentThread() to return the platform thread
 631         Thread carrier = this.carrierThread;
 632         carrier.setCurrentThread(carrier);
 633 
 634         // break connection to carrier thread, synchronized with interrupt
 635         synchronized (interruptLock) {
 636             setCarrierThread(null);
 637         }
 638         carrier.clearInterrupt();
 639 
 640         // notify JVMTI after unmount
 641         notifyJvmtiUnmount(/*hide*/false);
 642     }
 643 
 644     /**
 645      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 646      * the continuation continues.
 647      */
 648     @Hidden
 649     private boolean yieldContinuation() {
 650         notifyJvmtiUnmount(/*hide*/true);
 651         try {
 652             return Continuation.yield(VTHREAD_SCOPE);
 653         } finally {
 654             notifyJvmtiMount(/*hide*/false);
 655         }
 656     }
 657 
 658     /**
 659      * Invoked in the context of the carrier thread after the Continuation yields when
 660      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 661      */
 662     private void afterYield() {
 663         assert carrierThread == null;
 664 
 665         // re-adjust parallelism if the virtual thread yielded when compensating
 666         if (currentThread() instanceof CarrierThread ct) {
 667             ct.endBlocking();
 668         }
 669 
 670         int s = state();
 671 
 672         // LockSupport.park/parkNanos
 673         if (s == PARKING || s == TIMED_PARKING) {
 674             int newState;
 675             if (s == PARKING) {
 676                 setState(newState = PARKED);
 677             } else {
 678                 // schedule unpark
 679                 long timeout = this.timeout;
 680                 assert timeout > 0;
 681                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 682                 setState(newState = TIMED_PARKED);
 683             }
 684 
 685             // may have been unparked while parking
 686             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 687                 // lazy submit if local queue is empty
 688                 lazySubmitRunContinuation();
 689             }
 690             return;
 691         }
 692 
 693         // Thread.yield
 694         if (s == YIELDING) {
 695             setState(YIELDED);
 696 
 697             // external submit if there are no tasks in the local task queue
 698             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 699                 externalSubmitRunContinuation();
 700             } else {
 701                 submitRunContinuation();
 702             }
 703             return;
 704         }
 705 
 706         // blocking on monitorenter
 707         if (s == BLOCKING) {
 708             setState(BLOCKED);
 709 
 710             // may have been unblocked while blocking
 711             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 712                 // lazy submit if local queue is empty
 713                 lazySubmitRunContinuation();
 714             }
 715             return;
 716         }
 717 
 718         // Object.wait
 719         if (s == WAITING || s == TIMED_WAITING) {
 720             int newState;
 721             boolean interruptible = interruptibleWait;
 722             if (s == WAITING) {
 723                 setState(newState = WAIT);
 724             } else {
 725                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 726                 // task will change the thread state to UNBLOCKED and submit the thread
 727                 // to the scheduler. A sequence number is used to ensure that the timeout
 728                 // task only unblocks the thread for this timed-wait. We synchronize with
 729                 // the timeout task to coordinate access to the sequence number and to
 730                 // ensure the timeout task doesn't execute until the thread has got to
 731                 // the TIMED_WAIT state.
 732                 long timeout = this.timeout;
 733                 assert timeout > 0;
 734                 synchronized (timedWaitLock()) {
 735                     byte seqNo = ++timedWaitSeqNo;
 736                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 737                     setState(newState = TIMED_WAIT);
 738                 }
 739             }
 740 
 741             // may have been notified while in transition to wait state
 742             if (notified && compareAndSetState(newState, BLOCKED)) {
 743                 // may have even been unblocked already
 744                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 745                     submitRunContinuation();
 746                 }
 747                 return;
 748             }
 749 
 750             // may have been interrupted while in transition to wait state
 751             if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 752                 submitRunContinuation();
 753                 return;
 754             }
 755             return;
 756         }
 757 
 758         assert false;
 759     }
 760 
 761     /**
 762      * Invoked after the continuation completes.
 763      */
 764     private void afterDone() {
 765         afterDone(true);
 766     }
 767 
 768     /**
 769      * Invoked after the continuation completes (or start failed). Sets the thread
 770      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 771      *
 772      * @param notifyContainer true if its container should be notified
 773      */
 774     private void afterDone(boolean notifyContainer) {
 775         assert carrierThread == null;
 776         setState(TERMINATED);
 777 
 778         // notify anyone waiting for this virtual thread to terminate
 779         CountDownLatch termination = this.termination;
 780         if (termination != null) {
 781             assert termination.getCount() == 1;
 782             termination.countDown();
 783         }
 784 
 785         // notify container
 786         if (notifyContainer) {
 787             threadContainer().remove(this);
 788         }
 789 
 790         // clear references to thread locals
 791         clearReferences();
 792     }
 793 
 794     /**
 795      * Schedules this {@code VirtualThread} to execute.
 796      *
 797      * @throws IllegalStateException if the container is shutdown or closed
 798      * @throws IllegalThreadStateException if the thread has already been started
 799      * @throws RejectedExecutionException if the scheduler cannot accept a task
 800      */
 801     @Override
 802     void start(ThreadContainer container) {
 803         if (!compareAndSetState(NEW, STARTED)) {
 804             throw new IllegalThreadStateException("Already started");
 805         }
 806 
 807         // bind thread to container
 808         assert threadContainer() == null;
 809         setThreadContainer(container);
 810 
 811         // start thread
 812         boolean addedToContainer = false;
 813         boolean started = false;
 814         try {
 815             container.add(this);  // may throw
 816             addedToContainer = true;
 817 
 818             // scoped values may be inherited
 819             inheritScopedValueBindings(container);
 820 
 821             // submit task to run thread, using externalSubmit if possible
 822             externalSubmitRunContinuationOrThrow();
 823             started = true;
 824         } finally {
 825             if (!started) {
 826                 afterDone(addedToContainer);
 827             }
 828         }
 829     }
 830 
 831     @Override
 832     public void start() {
 833         start(ThreadContainers.root());
 834     }
 835 
 836     @Override
 837     public void run() {
 838         // do nothing
 839     }
 840 
 841     /**
 842      * Parks until unparked or interrupted. If already unparked then the parking
 843      * permit is consumed and this method completes immediately (meaning it doesn't
 844      * yield). It also completes immediately if the interrupted status is set.
 845      */
 846     @Override
 847     void park() {
 848         assert Thread.currentThread() == this;
 849 
 850         // complete immediately if parking permit available or interrupted
 851         if (getAndSetParkPermit(false) || interrupted)
 852             return;
 853 
 854         // park the thread
 855         boolean yielded = false;
 856         long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 857         setState(PARKING);
 858         try {
 859             yielded = yieldContinuation();
 860         } catch (OutOfMemoryError e) {
 861             // park on carrier
 862         } finally {
 863             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 864             if (yielded) {
 865                 VirtualThreadParkEvent.offer(eventStartTime, Long.MIN_VALUE);
 866             } else {
 867                 assert state() == PARKING;
 868                 setState(RUNNING);
 869             }
 870         }
 871 
 872         // park on the carrier thread when pinned
 873         if (!yielded) {
 874             parkOnCarrierThread(false, 0);
 875         }
 876     }
 877 
 878     /**
 879      * Parks up to the given waiting time or until unparked or interrupted.
 880      * If already unparked then the parking permit is consumed and this method
 881      * completes immediately (meaning it doesn't yield). It also completes immediately
 882      * if the interrupted status is set or the waiting time is {@code <= 0}.
 883      *
 884      * @param nanos the maximum number of nanoseconds to wait.
 885      */
 886     @Override
 887     void parkNanos(long nanos) {
 888         assert Thread.currentThread() == this;
 889 
 890         // complete immediately if parking permit available or interrupted
 891         if (getAndSetParkPermit(false) || interrupted)
 892             return;
 893 
 894         // park the thread for the waiting time
 895         if (nanos > 0) {
 896             long startTime = System.nanoTime();
 897 
 898             // park the thread, afterYield will schedule the thread to unpark
 899             boolean yielded = false;
 900             long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 901             timeout = nanos;
 902             setState(TIMED_PARKING);
 903             try {
 904                 yielded = yieldContinuation();
 905             } catch (OutOfMemoryError e) {
 906                 // park on carrier
 907             } finally {
 908                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 909                 if (yielded) {
 910                     VirtualThreadParkEvent.offer(eventStartTime, nanos);
 911                 } else {
 912                     assert state() == TIMED_PARKING;
 913                     setState(RUNNING);
 914                 }
 915             }
 916 
 917             // park on carrier thread for remaining time when pinned (or OOME)
 918             if (!yielded) {
 919                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 920                 parkOnCarrierThread(true, remainingNanos);
 921             }
 922         }
 923     }
 924 
 925     /**
 926      * Parks the current carrier thread up to the given waiting time or until
 927      * unparked or interrupted. If the virtual thread is interrupted then the
 928      * interrupted status will be propagated to the carrier thread.
 929      * @param timed true for a timed park, false for untimed
 930      * @param nanos the waiting time in nanoseconds
 931      */
 932     private void parkOnCarrierThread(boolean timed, long nanos) {
 933         assert state() == RUNNING;
 934 
 935         setState(timed ? TIMED_PINNED : PINNED);
 936         try {
 937             if (!parkPermit) {
 938                 if (!timed) {
 939                     U.park(false, 0);
 940                 } else if (nanos > 0) {
 941                     U.park(false, nanos);
 942                 }
 943             }
 944         } finally {
 945             setState(RUNNING);
 946         }
 947 
 948         // consume parking permit
 949         setParkPermit(false);
 950 
 951         // JFR jdk.VirtualThreadPinned event
 952         postPinnedEvent("LockSupport.park");
 953     }
 954 
 955     /**
 956      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 957      * Recording the event in the VM avoids having JFR event recorded in Java
 958      * with the same name, but different ID, to events recorded by the VM.
 959      */
 960     @Hidden
 961     private static native void postPinnedEvent(String op);
 962 
 963     /**
 964      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 965      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 966      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 967      * @throws RejectedExecutionException if the scheduler cannot accept a task
 968      */
 969     @Override
 970     void unpark() {
 971         if (!getAndSetParkPermit(true) && currentThread() != this) {
 972             int s = state();
 973 
 974             // unparked while parked
 975             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
 976                 submitRunContinuation();
 977                 return;
 978             }
 979 
 980             // unparked while parked when pinned
 981             if (s == PINNED || s == TIMED_PINNED) {
 982                 // unpark carrier thread when pinned
 983                 disableSuspendAndPreempt();
 984                 try {
 985                     synchronized (carrierThreadAccessLock()) {
 986                         Thread carrier = carrierThread;
 987                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 988                             U.unpark(carrier);
 989                         }
 990                     }
 991                 } finally {
 992                     enableSuspendAndPreempt();
 993                 }
 994                 return;
 995             }
 996         }
 997     }
 998 
 999     /**
1000      * Invoked by unblocker thread to unblock this virtual thread.
1001      */
1002     private void unblock() {
1003         assert !Thread.currentThread().isVirtual();
1004         blockPermit = true;
1005         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
1006             submitRunContinuation();
1007         }
1008     }
1009 
1010     /**
1011      * Invoked by FJP worker thread or STPE thread when park timeout expires.
1012      */
1013     private void parkTimeoutExpired() {
1014         assert !VirtualThread.currentThread().isVirtual();
1015         if (!getAndSetParkPermit(true)
1016                 && (state() == TIMED_PARKED)
1017                 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
1018             lazySubmitRunContinuation();
1019         }
1020     }
1021 
1022     /**
1023      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
1024      * If the virtual thread is in timed-wait then this method will unblock the thread
1025      * and submit its task so that it continues and attempts to reenter the monitor.
1026      * This method does nothing if the thread has been woken by notify or interrupt.
1027      */
1028     private void waitTimeoutExpired(byte seqNo) {
1029         assert !Thread.currentThread().isVirtual();
1030         for (;;) {
1031             boolean unblocked = false;
1032             synchronized (timedWaitLock()) {
1033                 if (seqNo != timedWaitSeqNo) {
1034                     // this timeout task is for a past timed-wait
1035                     return;
1036                 }
1037                 int s = state();
1038                 if (s == TIMED_WAIT) {
1039                     unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
1040                 } else if (s != (TIMED_WAIT | SUSPENDED)) {
1041                     // notified or interrupted, no longer waiting
1042                     return;
1043                 }
1044             }
1045             if (unblocked) {
1046                 lazySubmitRunContinuation();
1047                 return;
1048             }
1049             // need to retry when thread is suspended in time-wait
1050             Thread.yield();
1051         }
1052     }
1053 
1054     /**
1055      * Attempts to yield the current virtual thread (Thread.yield).
1056      */
1057     void tryYield() {
1058         assert Thread.currentThread() == this;
1059         setState(YIELDING);
1060         boolean yielded = false;
1061         try {
1062             yielded = yieldContinuation();  // may throw
1063         } finally {
1064             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
1065             if (!yielded) {
1066                 assert state() == YIELDING;
1067                 setState(RUNNING);
1068             }
1069         }
1070     }
1071 
1072     /**
1073      * Sleep the current thread for the given sleep time (in nanoseconds). If
1074      * nanos is 0 then the thread will attempt to yield.
1075      *
1076      * @implNote This implementation parks the thread for the given sleeping time
1077      * and will therefore be observed in PARKED state during the sleep. Parking
1078      * will consume the parking permit so this method makes available the parking
1079      * permit after the sleep. This may be observed as a spurious, but benign,
1080      * wakeup when the thread subsequently attempts to park.
1081      *
1082      * @param nanos the maximum number of nanoseconds to sleep
1083      * @throws InterruptedException if interrupted while sleeping
1084      */
1085     void sleepNanos(long nanos) throws InterruptedException {
1086         assert Thread.currentThread() == this && nanos >= 0;
1087         if (getAndClearInterrupt())
1088             throw new InterruptedException();
1089         if (nanos == 0) {
1090             tryYield();
1091         } else {
1092             // park for the sleep time
1093             try {
1094                 long remainingNanos = nanos;
1095                 long startNanos = System.nanoTime();
1096                 while (remainingNanos > 0) {
1097                     parkNanos(remainingNanos);
1098                     if (getAndClearInterrupt()) {
1099                         throw new InterruptedException();
1100                     }
1101                     remainingNanos = nanos - (System.nanoTime() - startNanos);
1102                 }
1103             } finally {
1104                 // may have been unparked while sleeping
1105                 setParkPermit(true);
1106             }
1107         }
1108     }
1109 
1110     /**
1111      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1112      * A timeout of {@code 0} means to wait forever.
1113      *
1114      * @throws InterruptedException if interrupted while waiting
1115      * @return true if the thread has terminated
1116      */
1117     boolean joinNanos(long nanos) throws InterruptedException {
1118         if (state() == TERMINATED)
1119             return true;
1120 
1121         // ensure termination object exists, then re-check state
1122         CountDownLatch termination = getTermination();
1123         if (state() == TERMINATED)
1124             return true;
1125 
1126         // wait for virtual thread to terminate
1127         if (nanos == 0) {
1128             termination.await();
1129         } else {
1130             boolean terminated = termination.await(nanos, NANOSECONDS);
1131             if (!terminated) {
1132                 // waiting time elapsed
1133                 return false;
1134             }
1135         }
1136         assert state() == TERMINATED;
1137         return true;
1138     }
1139 
1140     @Override
1141     void blockedOn(Interruptible b) {
1142         disableSuspendAndPreempt();
1143         try {
1144             super.blockedOn(b);
1145         } finally {
1146             enableSuspendAndPreempt();
1147         }
1148     }
1149 
1150     @Override
1151     public void interrupt() {
1152         if (Thread.currentThread() != this) {
1153             // if current thread is a virtual thread then prevent it from being
1154             // suspended or unmounted when entering or holding interruptLock
1155             Interruptible blocker;
1156             disableSuspendAndPreempt();
1157             try {
1158                 synchronized (interruptLock) {
1159                     interrupted = true;
1160                     blocker = nioBlocker();
1161                     if (blocker != null) {
1162                         blocker.interrupt(this);
1163                     }
1164 
1165                     // interrupt carrier thread if mounted
1166                     Thread carrier = carrierThread;
1167                     if (carrier != null) carrier.setInterrupt();
1168                 }
1169             } finally {
1170                 enableSuspendAndPreempt();
1171             }
1172 
1173             // notify blocker after releasing interruptLock
1174             if (blocker != null) {
1175                 blocker.postInterrupt();
1176             }
1177 
1178             // make available parking permit, unpark thread if parked
1179             unpark();
1180 
1181             // if thread is waiting in Object.wait then schedule to try to reenter
1182             int s = state();
1183             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1184                 submitRunContinuation();
1185             }
1186 
1187         } else {
1188             interrupted = true;
1189             carrierThread.setInterrupt();
1190             setParkPermit(true);
1191         }
1192     }
1193 
1194     @Override
1195     public boolean isInterrupted() {
1196         return interrupted;
1197     }
1198 
1199     @Override
1200     boolean getAndClearInterrupt() {
1201         assert Thread.currentThread() == this;
1202         boolean oldValue = interrupted;
1203         if (oldValue) {
1204             disableSuspendAndPreempt();
1205             try {
1206                 synchronized (interruptLock) {
1207                     interrupted = false;
1208                     carrierThread.clearInterrupt();
1209                 }
1210             } finally {
1211                 enableSuspendAndPreempt();
1212             }
1213         }
1214         return oldValue;
1215     }
1216 
1217     @Override
1218     Thread.State threadState() {
1219         int s = state();
1220         switch (s & ~SUSPENDED) {
1221             case NEW:
1222                 return Thread.State.NEW;
1223             case STARTED:
1224                 // return NEW if thread container not yet set
1225                 if (threadContainer() == null) {
1226                     return Thread.State.NEW;
1227                 } else {
1228                     return Thread.State.RUNNABLE;
1229                 }
1230             case UNPARKED:
1231             case UNBLOCKED:
1232             case YIELDED:
1233                 // runnable, not mounted
1234                 return Thread.State.RUNNABLE;
1235             case RUNNING:
1236                 // if mounted then return state of carrier thread
1237                 if (Thread.currentThread() != this) {
1238                     disableSuspendAndPreempt();
1239                     try {
1240                         synchronized (carrierThreadAccessLock()) {
1241                             Thread carrierThread = this.carrierThread;
1242                             if (carrierThread != null) {
1243                                 return carrierThread.threadState();
1244                             }
1245                         }
1246                     } finally {
1247                         enableSuspendAndPreempt();
1248                     }
1249                 }
1250                 // runnable, mounted
1251                 return Thread.State.RUNNABLE;
1252             case PARKING:
1253             case TIMED_PARKING:
1254             case WAITING:
1255             case TIMED_WAITING:
1256             case YIELDING:
1257                 // runnable, in transition
1258                 return Thread.State.RUNNABLE;
1259             case PARKED:
1260             case PINNED:
1261             case WAIT:
1262                 return Thread.State.WAITING;
1263             case TIMED_PARKED:
1264             case TIMED_PINNED:
1265             case TIMED_WAIT:
1266                 return Thread.State.TIMED_WAITING;
1267             case BLOCKING:
1268             case BLOCKED:
1269                 return Thread.State.BLOCKED;
1270             case TERMINATED:
1271                 return Thread.State.TERMINATED;
1272             default:
1273                 throw new InternalError();
1274         }
1275     }
1276 
1277     @Override
1278     boolean alive() {
1279         int s = state;
1280         return (s != NEW && s != TERMINATED);
1281     }
1282 
1283     @Override
1284     boolean isTerminated() {
1285         return (state == TERMINATED);
1286     }
1287 
1288     @Override
1289     StackTraceElement[] asyncGetStackTrace() {
1290         StackTraceElement[] stackTrace;
1291         do {
1292             stackTrace = (carrierThread != null)
1293                     ? super.asyncGetStackTrace()  // mounted
1294                     : tryGetStackTrace();         // unmounted
1295             if (stackTrace == null) {
1296                 Thread.yield();
1297             }
1298         } while (stackTrace == null);
1299         return stackTrace;
1300     }
1301 
1302     /**
1303      * Returns the stack trace for this virtual thread if it is unmounted.
1304      * Returns null if the thread is mounted or in transition.
1305      */
1306     private StackTraceElement[] tryGetStackTrace() {
1307         int initialState = state() & ~SUSPENDED;
1308         switch (initialState) {
1309             case NEW, STARTED, TERMINATED -> {
1310                 return new StackTraceElement[0];  // unmounted, empty stack
1311             }
1312             case RUNNING, PINNED, TIMED_PINNED -> {
1313                 return null;   // mounted
1314             }
1315             case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1316                 // unmounted, not runnable
1317             }
1318             case UNPARKED, UNBLOCKED, YIELDED -> {
1319                 // unmounted, runnable
1320             }
1321             case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1322                 return null;  // in transition
1323             }
1324             default -> throw new InternalError("" + initialState);
1325         }
1326 
1327         // thread is unmounted, prevent it from continuing
1328         int suspendedState = initialState | SUSPENDED;
1329         if (!compareAndSetState(initialState, suspendedState)) {
1330             return null;
1331         }
1332 
1333         // get stack trace and restore state
1334         StackTraceElement[] stack;
1335         try {
1336             stack = cont.getStackTrace();
1337         } finally {
1338             assert state == suspendedState;
1339             setState(initialState);
1340         }
1341         boolean resubmit = switch (initialState) {
1342             case UNPARKED, UNBLOCKED, YIELDED -> {
1343                 // resubmit as task may have run while suspended
1344                 yield true;
1345             }
1346             case PARKED, TIMED_PARKED -> {
1347                 // resubmit if unparked while suspended
1348                 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1349             }
1350             case BLOCKED -> {
1351                 // resubmit if unblocked while suspended
1352                 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1353             }
1354             case WAIT, TIMED_WAIT -> {
1355                 // resubmit if notified or interrupted while waiting (Object.wait)
1356                 // waitTimeoutExpired will retry if the timed expired when suspended
1357                 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1358             }
1359             default -> throw new InternalError();
1360         };
1361         if (resubmit) {
1362             submitRunContinuation();
1363         }
1364         return stack;
1365     }
1366 
1367     @Override
1368     public String toString() {
1369         StringBuilder sb = new StringBuilder("VirtualThread[#");
1370         sb.append(threadId());
1371         String name = getName();
1372         if (!name.isEmpty()) {
1373             sb.append(",");
1374             sb.append(name);
1375         }
1376         sb.append("]/");
1377 
1378         // add the carrier state and thread name when mounted
1379         boolean mounted;
1380         if (Thread.currentThread() == this) {
1381             mounted = appendCarrierInfo(sb);
1382         } else {
1383             disableSuspendAndPreempt();
1384             try {
1385                 synchronized (carrierThreadAccessLock()) {
1386                     mounted = appendCarrierInfo(sb);
1387                 }
1388             } finally {
1389                 enableSuspendAndPreempt();
1390             }
1391         }
1392 
1393         // add virtual thread state when not mounted
1394         if (!mounted) {
1395             String stateAsString = threadState().toString();
1396             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1397         }
1398 
1399         return sb.toString();
1400     }
1401 
1402     /**
1403      * Appends the carrier state and thread name to the string buffer if mounted.
1404      * @return true if mounted, false if not mounted
1405      */
1406     private boolean appendCarrierInfo(StringBuilder sb) {
1407         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1408         Thread carrier = carrierThread;
1409         if (carrier != null) {
1410             String stateAsString = carrier.threadState().toString();
1411             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1412             sb.append('@');
1413             sb.append(carrier.getName());
1414             return true;
1415         } else {
1416             return false;
1417         }
1418     }
1419 
1420     @Override
1421     public int hashCode() {
1422         return (int) threadId();
1423     }
1424 
1425     @Override
1426     public boolean equals(Object obj) {
1427         return obj == this;
1428     }
1429 
1430     /**
1431      * Returns the termination object, creating it if needed.
1432      */
1433     private CountDownLatch getTermination() {
1434         CountDownLatch termination = this.termination;
1435         if (termination == null) {
1436             termination = new CountDownLatch(1);
1437             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1438                 termination = this.termination;
1439             }
1440         }
1441         return termination;
1442     }
1443 
1444     /**
1445      * Returns the lock object to synchronize on when accessing carrierThread.
1446      * The lock prevents carrierThread from being reset to null during unmount.
1447      */
1448     private Object carrierThreadAccessLock() {
1449         // return interruptLock as unmount has to coordinate with interrupt
1450         return interruptLock;
1451     }
1452 
1453     /**
1454      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1455      */
1456     private Object timedWaitLock() {
1457         // use this object for now to avoid the overhead of introducing another lock
1458         return runContinuation;
1459     }
1460 
1461     /**
1462      * Disallow the current thread be suspended or preempted.
1463      */
1464     private void disableSuspendAndPreempt() {
1465         notifyJvmtiDisableSuspend(true);
1466         Continuation.pin();
1467     }
1468 
1469     /**
1470      * Allow the current thread be suspended or preempted.
1471      */
1472     private void enableSuspendAndPreempt() {
1473         Continuation.unpin();
1474         notifyJvmtiDisableSuspend(false);
1475     }
1476 
1477     // -- wrappers for get/set of state, parking permit, and carrier thread --
1478 
1479     private int state() {
1480         return state;  // volatile read
1481     }
1482 
1483     private void setState(int newValue) {
1484         state = newValue;  // volatile write
1485     }
1486 
1487     private boolean compareAndSetState(int expectedValue, int newValue) {
1488         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1489     }
1490 
1491     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1492         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1493     }
1494 
1495     private void setParkPermit(boolean newValue) {
1496         if (parkPermit != newValue) {
1497             parkPermit = newValue;
1498         }
1499     }
1500 
1501     private boolean getAndSetParkPermit(boolean newValue) {
1502         if (parkPermit != newValue) {
1503             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1504         } else {
1505             return newValue;
1506         }
1507     }
1508 
1509     private void setCarrierThread(Thread carrier) {
1510         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1511         this.carrierThread = carrier;
1512     }
1513 
1514     // -- JVM TI support --
1515 
1516     @IntrinsicCandidate
1517     @JvmtiMountTransition
1518     private native void notifyJvmtiStart();
1519 
1520     @IntrinsicCandidate
1521     @JvmtiMountTransition
1522     private native void notifyJvmtiEnd();
1523 
1524     @IntrinsicCandidate
1525     @JvmtiMountTransition
1526     private native void notifyJvmtiMount(boolean hide);
1527 
1528     @IntrinsicCandidate
1529     @JvmtiMountTransition
1530     private native void notifyJvmtiUnmount(boolean hide);
1531 
1532     @IntrinsicCandidate
1533     private static native void notifyJvmtiDisableSuspend(boolean enter);
1534 
1535     private static native void registerNatives();
1536     static {
1537         registerNatives();
1538 
1539         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1540         var group = Thread.virtualThreadGroup();
1541 
1542         // ensure event class is initialized
1543         try {
1544             MethodHandles.lookup().ensureInitialized(VirtualThreadParkEvent.class);
1545         } catch (IllegalAccessException e) {
1546             throw new ExceptionInInitializerError(e);
1547         }
1548     }
1549 
1550     /**
1551      * Loads a VirtualThreadScheduler with the given class name. The class must be public
1552      * in an exported package, with public one-arg or no-arg constructor, and be visible
1553      * to the system class loader.
1554      * @param delegate the scheduler that the custom scheduler may delegate to
1555      * @param cn the class name of the custom scheduler
1556      */
1557     private static VirtualThreadScheduler loadCustomScheduler(VirtualThreadScheduler delegate, String cn) {
1558         try {
1559             Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1560             VirtualThreadScheduler scheduler;
1561             try {
1562                 // 1-arg constructor
1563                 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1564                 scheduler = (VirtualThreadScheduler) ctor.newInstance(delegate);
1565             } catch (NoSuchMethodException e) {
1566                 // 0-arg constructor
1567                 Constructor<?> ctor = clazz.getConstructor();
1568                 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1569             }
1570             System.err.println("""
1571                 WARNING: Using custom default scheduler, this is an experimental feature!""");
1572             return scheduler;
1573         } catch (Exception ex) {
1574             throw new Error(ex);
1575         }
1576     }
1577 
1578     /**
1579      * Creates the built-in ForkJoinPool scheduler.
1580      * @param wrapped true if wrapped by a custom default scheduler
1581      */
1582     private static BuiltinDefaultScheduler createBuiltinDefaultScheduler(boolean wrapped) {

1583         int parallelism, maxPoolSize, minRunnable;
1584         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1585         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1586         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1587         if (parallelismValue != null) {
1588             parallelism = Integer.parseInt(parallelismValue);
1589         } else {
1590             parallelism = Runtime.getRuntime().availableProcessors();
1591         }
1592         if (maxPoolSizeValue != null) {
1593             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1594             parallelism = Integer.min(parallelism, maxPoolSize);
1595         } else {
1596             maxPoolSize = Integer.max(parallelism, 256);
1597         }
1598         if (minRunnableValue != null) {
1599             minRunnable = Integer.parseInt(minRunnableValue);
1600         } else {
1601             minRunnable = Integer.max(parallelism / 2, 1);
1602         }
1603         return new BuiltinDefaultScheduler(parallelism, maxPoolSize, minRunnable, wrapped);
1604     }
1605 
1606     /**
1607      * The built-in ForkJoinPool scheduler.
1608      */
1609     private static class BuiltinDefaultScheduler
1610             extends ForkJoinPool implements VirtualThreadScheduler {
1611 
1612         private static final StableValue<VirtualThreadScheduler> VIEW = StableValue.of();
1613 
1614         BuiltinDefaultScheduler(int parallelism, int maxPoolSize, int minRunnable, boolean wrapped) {
1615             ForkJoinWorkerThreadFactory factory = wrapped
1616                     ? ForkJoinPool.defaultForkJoinWorkerThreadFactory
1617                     : CarrierThread::new;
1618             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1619             boolean asyncMode = true; // FIFO
1620             super(parallelism, factory, handler, asyncMode,
1621                     0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1622         }
1623 
1624         private void adaptAndExecute(Runnable task) {
1625             execute(ForkJoinTask.adapt(task));
1626         }
1627 
1628         @Override
1629         public void onStart(VirtualThreadTask task) {
1630             adaptAndExecute(task);
1631         }
1632 
1633         @Override
1634         public void onContinue(VirtualThreadTask task) {
1635             adaptAndExecute(task);
1636         }
1637 
1638         /**
1639          * Wraps the scheduler to avoid leaking a direct reference.
1640          */
1641         VirtualThreadScheduler externalView() {
1642             BuiltinDefaultScheduler builtin = this;
1643             return VIEW.orElseSet(() -> {
1644                 return new VirtualThreadScheduler() {
1645                     private void execute(VirtualThreadTask task) {
1646                         var vthread = (VirtualThread) task.thread();
1647                         VirtualThreadScheduler scheduler = vthread.scheduler;
1648                         if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1649                             builtin.adaptAndExecute(task);
1650                         } else {
1651                             throw new IllegalArgumentException();
1652                         }
1653                     }
1654                     @Override
1655                     public void onStart(VirtualThreadTask task) {
1656                         execute(task);
1657                     }
1658                     @Override
1659                     public void onContinue(VirtualThreadTask task) {
1660                         execute(task);
1661                     }
1662                 };
1663             });
1664         }
1665     }
1666 
1667     /**
1668      * Schedule a runnable task to run after a delay.
1669      */
1670     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1671         if (scheduler instanceof ForkJoinPool pool) {
1672             return pool.schedule(command, delay, unit);
1673         } else {
1674             return DelayedTaskSchedulers.schedule(command, delay, unit);
1675         }
1676     }
1677 
1678     /**
1679      * Supports scheduling a runnable task to run after a delay. It uses a number
1680      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1681      * work queue used. This class is used when using a custom scheduler.
1682      */
1683     private static class DelayedTaskSchedulers {
1684         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1685 
1686         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1687             long tid = Thread.currentThread().threadId();
1688             int index = (int) tid & (INSTANCE.length - 1);
1689             return INSTANCE[index].schedule(command, delay, unit);
1690         }
1691 
1692         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1693             String propName = "jdk.virtualThreadScheduler.timerQueues";
1694             String propValue = System.getProperty(propName);
1695             int queueCount;
1696             if (propValue != null) {
1697                 queueCount = Integer.parseInt(propValue);
1698                 if (queueCount != Integer.highestOneBit(queueCount)) {
1699                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1700                 }
1701             } else {
1702                 int ncpus = Runtime.getRuntime().availableProcessors();
1703                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1704             }
1705             var schedulers = new ScheduledExecutorService[queueCount];
1706             for (int i = 0; i < queueCount; i++) {
1707                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1708                     Executors.newScheduledThreadPool(1, task -> {
1709                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1710                         t.setDaemon(true);
1711                         return t;
1712                     });
1713                 stpe.setRemoveOnCancelPolicy(true);
1714                 schedulers[i] = stpe;
1715             }
1716             return schedulers;
1717         }
1718     }
1719 
1720     /**
1721      * Schedule virtual threads that are ready to be scheduled after they blocked on
1722      * monitor enter.
1723      */
1724     private static void unblockVirtualThreads() {
1725         while (true) {
1726             VirtualThread vthread = takeVirtualThreadListToUnblock();
1727             while (vthread != null) {
1728                 assert vthread.onWaitingList;
1729                 VirtualThread nextThread = vthread.next;
1730 
1731                 // remove from list and unblock
1732                 vthread.next = null;
1733                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1734                 assert changed;
1735                 vthread.unblock();
1736 
1737                 vthread = nextThread;
1738             }
1739         }
1740     }
1741 
1742     /**
1743      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1744      * if necessary until a list of one or more threads becomes available.
1745      */
1746     private static native VirtualThread takeVirtualThreadListToUnblock();
1747 
1748     static {
1749         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1750                 VirtualThread::unblockVirtualThreads);
1751         unblocker.setDaemon(true);
1752         unblocker.start();
1753     }
1754 }
--- EOF ---