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



 266         }

 267         this.scheduler = scheduler;
 268         this.cont = new VThreadContinuation(this, task);
 269 
 270         if (scheduler == BUILTIN_SCHEDULER) {
 271             this.runContinuation = new BuiltinSchedulerTask(this);
 272         } else {
 273             this.runContinuation = new CustomSchedulerTask(this, att);
 274         }
 275     }
 276 
 277     /**
 278      * The task to execute when using the built-in scheduler.
 279      */
 280     static final class BuiltinSchedulerTask implements VirtualThreadTask {
 281         private final VirtualThread vthread;
 282         BuiltinSchedulerTask(VirtualThread vthread) {
 283             this.vthread = vthread;
 284         }
 285         @Override
 286         public Object attach(Object att) {
 287             throw new UnsupportedOperationException();
 288         }
 289         @Override
 290         public Object attachment() {
 291             throw new UnsupportedOperationException();
 292         }
 293         @Override
 294         public Thread thread() {
 295             return vthread;
 296         }
 297         @Override
 298         public void run() {
 299             vthread.runContinuation();;
 300         }
 301     }
 302 
 303     /**
 304      * The task to execute when using a custom scheduler.
 305      */
 306     static final class CustomSchedulerTask implements VirtualThreadTask {
 307         private static final VarHandle ATT =
 308                 MhUtil.findVarHandle(MethodHandles.lookup(), "att", Object.class);
 309         private final VirtualThread vthread;
 310         private volatile Object att;
 311         CustomSchedulerTask(VirtualThread vthread, Object att) {
 312             this.vthread = vthread;
 313             if (att != null) {
 314                 this.att = att;
 315             }
 316         }
 317         @Override
 318         public Object attach(Object att) {
 319             return ATT.getAndSet(this, att);
 320         }
 321         @Override
 322         public Object attachment() {
 323             return att;
 324         }
 325         @Override
 326         public Thread thread() {
 327             return vthread;
 328         }
 329         @Override
 330         public void run() {
 331             vthread.runContinuation();;
 332         }
 333     }
 334 
 335     /**
 336      * The continuation that a virtual thread executes.
 337      */
 338     private static class VThreadContinuation extends Continuation {
 339         VThreadContinuation(VirtualThread vthread, Runnable task) {
 340             super(VTHREAD_SCOPE, wrap(vthread, task));
 341         }
 342         @Override
 343         protected void onPinned(Continuation.Pinned reason) {
 344         }
 345         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 346             return new Runnable() {
 347                 @Hidden
 348                 @JvmtiHideEvents
 349                 public void run() {
 350                     vthread.notifyJvmtiStart(); // notify JVMTI
 351                     try {
 352                         vthread.run(task);
 353                     } finally {
 354                         vthread.notifyJvmtiEnd(); // notify JVMTI
 355                     }
 356                 }
 357             };
 358         }
 359     }
 360 
 361     /**
 362      * Runs or continues execution on the current thread. The virtual thread is mounted
 363      * on the current thread before the task runs or continues. It unmounts when the
 364      * task completes or yields.
 365      */
 366     @ChangesCurrentThread // allow mount/unmount to be inlined
 367     private void runContinuation() {
 368         // the carrier must be a platform thread
 369         if (Thread.currentThread().isVirtual()) {
 370             throw new WrongThreadException();
 371         }
 372 
 373         // set state to RUNNING
 374         int initialState = state();
 375         if (initialState == STARTED || initialState == UNPARKED
 376                 || initialState == UNBLOCKED || initialState == YIELDED) {
 377             // newly started or continue after parking/blocking/Thread.yield
 378             if (!compareAndSetState(initialState, RUNNING)) {
 379                 return;
 380             }
 381             // consume permit when continuing after parking or blocking. If continue
 382             // after a timed-park or timed-wait then the timeout task is cancelled.
 383             if (initialState == UNPARKED) {
 384                 cancelTimeoutTask();
 385                 setParkPermit(false);
 386             } else if (initialState == UNBLOCKED) {
 387                 cancelTimeoutTask();
 388                 blockPermit = false;
 389             }
 390         } else {
 391             // not runnable
 392             return;
 393         }
 394 
 395         mount();
 396         try {
 397             cont.run();
 398         } finally {
 399             unmount();
 400             if (cont.isDone()) {
 401                 afterDone();
 402             } else {
 403                 afterYield();
 404             }
 405         }
 406     }
 407 
 408     /**
 409      * Cancel timeout task when continuing after timed-park or timed-wait.
 410      * The timeout task may be executing, or may have already completed.
 411      */
 412     private void cancelTimeoutTask() {
 413         if (timeoutTask != null) {
 414             timeoutTask.cancel(false);
 415             timeoutTask = null;
 416         }
 417     }
 418 
 419     /**
 420      * Submits the runContinuation task to the scheduler. For the built-in scheduler,
 421      * the task will be pushed to the local queue if possible, otherwise it will be
 422      * pushed to an external submission queue.

 423      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 424      * @throws RejectedExecutionException
 425      */
 426     private void submitRunContinuation(boolean retryOnOOME) {
 427         boolean done = false;
 428         while (!done) {
 429             try {
 430                 // Pin the continuation to prevent the virtual thread from unmounting
 431                 // when submitting a task. For the default scheduler this ensures that
 432                 // the carrier doesn't change when pushing a task. For other schedulers
 433                 // it avoids deadlock that could arise due to carriers and virtual
 434                 // threads contending for a lock.
 435                 if (currentThread().isVirtual()) {
 436                     Continuation.pin();
 437                     try {
 438                         scheduler.onContinue(runContinuation);
 439                     } finally {
 440                         Continuation.unpin();
 441                     }
 442                 } else {
 443                     scheduler.onContinue(runContinuation);
 444                 }
 445                 done = true;
 446             } catch (RejectedExecutionException ree) {
 447                 submitFailed(ree);
 448                 throw ree;
 449             } catch (OutOfMemoryError e) {
 450                 if (retryOnOOME) {
 451                     U.park(false, 100_000_000); // 100ms
 452                 } else {
 453                     throw e;
 454                 }
 455             }
 456         }
 457     }
 458 


















 459     /**
 460      * Submits the runContinuation task to the scheduler. For the default scheduler,
 461      * and calling it on a worker thread, the task will be pushed to the local queue,
 462      * otherwise it will be pushed to an external submission queue.
 463      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 464      * @throws RejectedExecutionException
 465      */
 466     private void submitRunContinuation() {
 467         submitRunContinuation(true);
 468     }
 469 
 470     /**
 471      * Invoked from a carrier thread to lazy submit the runContinuation task to the
 472      * carrier's local queue if the queue is empty. If not empty, or invoked by a thread
 473      * for a custom scheduler, then it just submits the task to the scheduler.
 474      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 475      * @throws RejectedExecutionException
 476      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 477      */
 478     private void lazySubmitRunContinuation() {
 479         assert !currentThread().isVirtual();
 480         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {

 481             try {
 482                 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
 483             } catch (RejectedExecutionException ree) {
 484                 submitFailed(ree);
 485                 throw ree;
 486             } catch (OutOfMemoryError e) {
 487                 submitRunContinuation();
 488             }
 489         } else {
 490             submitRunContinuation();
 491         }
 492     }
 493 
 494     /**
 495      * Invoked from a carrier thread to externally submit the runContinuation task to the
 496      * scheduler. If invoked by a thread for a custom scheduler, then it just submits the
 497      * task to the scheduler.
 498      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 499      * @throws RejectedExecutionException
 500      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 501      */
 502     private void externalSubmitRunContinuation() {
 503         assert !currentThread().isVirtual();
 504         if (currentThread() instanceof CarrierThread ct) {
 505             try {
 506                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 507             } catch (RejectedExecutionException ree) {
 508                 submitFailed(ree);
 509                 throw ree;
 510             } catch (OutOfMemoryError e) {
 511                 submitRunContinuation();
 512             }
 513         } else {
 514             submitRunContinuation();
 515         }
 516     }
 517 
 518     /**
 519      * Invoked from Thread.start to externally submit the runContinuation task to the
 520      * scheduler. If this virtual thread is scheduled by the built-in scheduler,
 521      * and this method is called from a virtual thread scheduled by the built-in
 522      * scheduler, then it uses externalSubmit to ensure that the task is pushed to an
 523      * external submission queue rather than the local queue.
 524      * @throws RejectedExecutionException
 525      * @throws OutOfMemoryError
 526      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 527      */
 528     private void externalSubmitRunContinuationOrThrow() {
 529         try {
 530             if (currentThread().isVirtual()) {
 531                 // Pin the continuation to prevent the virtual thread from unmounting
 532                 // when submitting a task. This avoids deadlock that could arise due to
 533                 // carriers and virtual threads contending for a lock.
 534                 Continuation.pin();
 535                 try {
 536                     if (scheduler == BUILTIN_SCHEDULER
 537                             && currentCarrierThread() instanceof CarrierThread ct) {
 538                         ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 539                     } else {
 540                         scheduler.onStart(runContinuation);
 541                     }
 542                 } finally {
 543                     Continuation.unpin();
 544                 }
 545             } else {
 546                 scheduler.onStart(runContinuation);
 547             }
 548         } catch (RejectedExecutionException ree) {
 549             submitFailed(ree);
 550             throw ree;
 551         }
 552     }
 553 
 554     /**
 555      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 556      */
 557     private void submitFailed(RejectedExecutionException ree) {
 558         var event = new VirtualThreadSubmitFailedEvent();
 559         if (event.isEnabled()) {
 560             event.javaThreadId = threadId();
 561             event.exceptionMessage = ree.getMessage();
 562             event.commit();
 563         }
 564     }
 565 
 566     /**
 567      * Runs a task in the context of this virtual thread.
 568      */
 569     private void run(Runnable task) {
 570         assert Thread.currentThread() == this && state == RUNNING;
 571 
 572         // emit JFR event if enabled
 573         if (VirtualThreadStartEvent.isTurnedOn()) {
 574             var event = new VirtualThreadStartEvent();
 575             event.javaThreadId = threadId();
 576             event.commit();
 577         }
 578 
 579         Object bindings = Thread.scopedValueBindings();
 580         try {
 581             runWith(bindings, task);
 582         } catch (Throwable exc) {
 583             dispatchUncaughtException(exc);
 584         } finally {
 585             // pop any remaining scopes from the stack, this may block
 586             StackableScope.popAll();
 587 
 588             // emit JFR event if enabled
 589             if (VirtualThreadEndEvent.isTurnedOn()) {
 590                 var event = new VirtualThreadEndEvent();
 591                 event.javaThreadId = threadId();
 592                 event.commit();
 593             }
 594         }
 595     }
 596 
 597     /**
 598      * Mounts this virtual thread onto the current platform thread. On
 599      * return, the current thread is the virtual thread.
 600      */
 601     @ChangesCurrentThread
 602     @ReservedStackAccess
 603     private void mount() {
 604         // notify JVMTI before mount
 605         notifyJvmtiMount(/*hide*/true);
 606 
 607         // sets the carrier thread
 608         Thread carrier = Thread.currentCarrierThread();
 609         setCarrierThread(carrier);
 610 
 611         // sync up carrier thread interrupted status if needed
 612         if (interrupted) {
 613             carrier.setInterrupt();
 614         } else if (carrier.isInterrupted()) {
 615             synchronized (interruptLock) {
 616                 // need to recheck interrupted status
 617                 if (!interrupted) {
 618                     carrier.clearInterrupt();
 619                 }
 620             }
 621         }
 622 
 623         // set Thread.currentThread() to return this virtual thread
 624         carrier.setCurrentThread(this);
 625     }
 626 
 627     /**
 628      * Unmounts this virtual thread from the carrier. On return, the
 629      * current thread is the current platform thread.
 630      */
 631     @ChangesCurrentThread
 632     @ReservedStackAccess
 633     private void unmount() {
 634         assert !Thread.holdsLock(interruptLock);
 635 
 636         // set Thread.currentThread() to return the platform thread
 637         Thread carrier = this.carrierThread;
 638         carrier.setCurrentThread(carrier);
 639 
 640         // break connection to carrier thread, synchronized with interrupt
 641         synchronized (interruptLock) {
 642             setCarrierThread(null);
 643         }
 644         carrier.clearInterrupt();
 645 
 646         // notify JVMTI after unmount
 647         notifyJvmtiUnmount(/*hide*/false);
 648     }
 649 
 650     /**
 651      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 652      * the continuation continues.
 653      */
 654     @Hidden
 655     private boolean yieldContinuation() {
 656         notifyJvmtiUnmount(/*hide*/true);
 657         try {
 658             return Continuation.yield(VTHREAD_SCOPE);
 659         } finally {
 660             notifyJvmtiMount(/*hide*/false);
 661         }
 662     }
 663 
 664     /**
 665      * Invoked in the context of the carrier thread after the Continuation yields when
 666      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 667      */
 668     private void afterYield() {
 669         assert carrierThread == null;
 670 
 671         // re-adjust parallelism if the virtual thread yielded when compensating
 672         if (currentThread() instanceof CarrierThread ct) {
 673             ct.endBlocking();
 674         }
 675 
 676         int s = state();
 677 
 678         // LockSupport.park/parkNanos
 679         if (s == PARKING || s == TIMED_PARKING) {
 680             int newState;
 681             if (s == PARKING) {
 682                 setState(newState = PARKED);
 683             } else {
 684                 // schedule unpark
 685                 long timeout = this.timeout;
 686                 assert timeout > 0;
 687                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 688                 setState(newState = TIMED_PARKED);
 689             }
 690 
 691             // may have been unparked while parking
 692             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 693                 // lazy submit if local queue is empty
 694                 lazySubmitRunContinuation();
 695             }
 696             return;
 697         }
 698 
 699         // Thread.yield
 700         if (s == YIELDING) {
 701             setState(YIELDED);
 702 
 703             // external submit if there are no tasks in the local task queue
 704             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 705                 externalSubmitRunContinuation();
 706             } else {
 707                 submitRunContinuation();
 708             }
 709             return;
 710         }
 711 
 712         // blocking on monitorenter
 713         if (s == BLOCKING) {
 714             setState(BLOCKED);
 715 
 716             // may have been unblocked while blocking
 717             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 718                 // lazy submit if local queue is empty
 719                 lazySubmitRunContinuation();
 720             }
 721             return;
 722         }
 723 
 724         // Object.wait
 725         if (s == WAITING || s == TIMED_WAITING) {
 726             int newState;
 727             boolean interruptible = interruptibleWait;
 728             if (s == WAITING) {
 729                 setState(newState = WAIT);
 730             } else {
 731                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 732                 // task will change the thread state to UNBLOCKED and submit the thread
 733                 // to the scheduler. A sequence number is used to ensure that the timeout
 734                 // task only unblocks the thread for this timed-wait. We synchronize with
 735                 // the timeout task to coordinate access to the sequence number and to
 736                 // ensure the timeout task doesn't execute until the thread has got to
 737                 // the TIMED_WAIT state.
 738                 long timeout = this.timeout;
 739                 assert timeout > 0;
 740                 synchronized (timedWaitLock()) {
 741                     byte seqNo = ++timedWaitSeqNo;
 742                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 743                     setState(newState = TIMED_WAIT);
 744                 }
 745             }
 746 
 747             // may have been notified while in transition to wait state
 748             if (notified && compareAndSetState(newState, BLOCKED)) {
 749                 // may have even been unblocked already
 750                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 751                     submitRunContinuation();
 752                 }
 753                 return;
 754             }
 755 
 756             // may have been interrupted while in transition to wait state
 757             if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 758                 submitRunContinuation();
 759                 return;
 760             }
 761             return;
 762         }
 763 
 764         assert false;
 765     }
 766 
 767     /**
 768      * Invoked after the continuation completes.
 769      */
 770     private void afterDone() {
 771         afterDone(true);
 772     }
 773 
 774     /**
 775      * Invoked after the continuation completes (or start failed). Sets the thread
 776      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 777      *
 778      * @param notifyContainer true if its container should be notified
 779      */
 780     private void afterDone(boolean notifyContainer) {
 781         assert carrierThread == null;
 782         setState(TERMINATED);
 783 
 784         // notify anyone waiting for this virtual thread to terminate
 785         CountDownLatch termination = this.termination;
 786         if (termination != null) {
 787             assert termination.getCount() == 1;
 788             termination.countDown();
 789         }
 790 
 791         // notify container
 792         if (notifyContainer) {
 793             threadContainer().remove(this);
 794         }
 795 
 796         // clear references to thread locals
 797         clearReferences();
 798     }
 799 
 800     /**
 801      * Schedules this {@code VirtualThread} to execute.
 802      *
 803      * @throws IllegalStateException if the container is shutdown or closed
 804      * @throws IllegalThreadStateException if the thread has already been started
 805      * @throws RejectedExecutionException if the scheduler cannot accept a task
 806      */
 807     @Override
 808     void start(ThreadContainer container) {
 809         if (!compareAndSetState(NEW, STARTED)) {
 810             throw new IllegalThreadStateException("Already started");
 811         }
 812 
 813         // bind thread to container
 814         assert threadContainer() == null;
 815         setThreadContainer(container);
 816 
 817         // start thread
 818         boolean addedToContainer = false;
 819         boolean started = false;
 820         try {
 821             container.add(this);  // may throw
 822             addedToContainer = true;
 823 
 824             // scoped values may be inherited
 825             inheritScopedValueBindings(container);
 826 
 827             // submit task to run thread, using externalSubmit if possible
 828             externalSubmitRunContinuationOrThrow();
 829             started = true;
 830         } finally {
 831             if (!started) {
 832                 afterDone(addedToContainer);
 833             }
 834         }
 835     }
 836 
 837     @Override
 838     public void start() {
 839         start(ThreadContainers.root());
 840     }
 841 
 842     @Override
 843     public void run() {
 844         // do nothing
 845     }
 846 
 847     /**
 848      * Parks until unparked or interrupted. If already unparked then the parking
 849      * permit is consumed and this method completes immediately (meaning it doesn't
 850      * yield). It also completes immediately if the interrupted status is set.
 851      */
 852     @Override
 853     void park() {
 854         assert Thread.currentThread() == this;
 855 
 856         // complete immediately if parking permit available or interrupted
 857         if (getAndSetParkPermit(false) || interrupted)
 858             return;
 859 
 860         // park the thread
 861         boolean yielded = false;
 862         long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 863         setState(PARKING);
 864         try {
 865             yielded = yieldContinuation();
 866         } catch (OutOfMemoryError e) {
 867             // park on carrier
 868         } finally {
 869             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 870             if (yielded) {
 871                 VirtualThreadParkEvent.offer(eventStartTime, Long.MIN_VALUE);
 872             } else {
 873                 assert state() == PARKING;
 874                 setState(RUNNING);
 875             }
 876         }
 877 
 878         // park on the carrier thread when pinned
 879         if (!yielded) {
 880             parkOnCarrierThread(false, 0);
 881         }
 882     }
 883 
 884     /**
 885      * Parks up to the given waiting time or until unparked or interrupted.
 886      * If already unparked then the parking permit is consumed and this method
 887      * completes immediately (meaning it doesn't yield). It also completes immediately
 888      * if the interrupted status is set or the waiting time is {@code <= 0}.
 889      *
 890      * @param nanos the maximum number of nanoseconds to wait.
 891      */
 892     @Override
 893     void parkNanos(long nanos) {
 894         assert Thread.currentThread() == this;
 895 
 896         // complete immediately if parking permit available or interrupted
 897         if (getAndSetParkPermit(false) || interrupted)
 898             return;
 899 
 900         // park the thread for the waiting time
 901         if (nanos > 0) {
 902             long startTime = System.nanoTime();
 903 
 904             // park the thread, afterYield will schedule the thread to unpark
 905             boolean yielded = false;
 906             long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 907             timeout = nanos;
 908             setState(TIMED_PARKING);
 909             try {
 910                 yielded = yieldContinuation();
 911             } catch (OutOfMemoryError e) {
 912                 // park on carrier
 913             } finally {
 914                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 915                 if (yielded) {
 916                     VirtualThreadParkEvent.offer(eventStartTime, nanos);
 917                 } else {
 918                     assert state() == TIMED_PARKING;
 919                     setState(RUNNING);
 920                 }
 921             }
 922 
 923             // park on carrier thread for remaining time when pinned (or OOME)
 924             if (!yielded) {
 925                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 926                 parkOnCarrierThread(true, remainingNanos);
 927             }
 928         }
 929     }
 930 
 931     /**
 932      * Parks the current carrier thread up to the given waiting time or until
 933      * unparked or interrupted. If the virtual thread is interrupted then the
 934      * interrupted status will be propagated to the carrier thread.
 935      * @param timed true for a timed park, false for untimed
 936      * @param nanos the waiting time in nanoseconds
 937      */
 938     private void parkOnCarrierThread(boolean timed, long nanos) {
 939         assert state() == RUNNING;
 940 
 941         setState(timed ? TIMED_PINNED : PINNED);
 942         try {
 943             if (!parkPermit) {
 944                 if (!timed) {
 945                     U.park(false, 0);
 946                 } else if (nanos > 0) {
 947                     U.park(false, nanos);
 948                 }
 949             }
 950         } finally {
 951             setState(RUNNING);
 952         }
 953 
 954         // consume parking permit
 955         setParkPermit(false);
 956 
 957         // JFR jdk.VirtualThreadPinned event
 958         postPinnedEvent("LockSupport.park");
 959     }
 960 
 961     /**
 962      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 963      * Recording the event in the VM avoids having JFR event recorded in Java
 964      * with the same name, but different ID, to events recorded by the VM.
 965      */
 966     @Hidden
 967     private static native void postPinnedEvent(String op);
 968 
 969     /**
 970      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 971      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 972      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 973      * @throws RejectedExecutionException if the scheduler cannot accept a task
 974      */
 975     @Override
 976     void unpark() {
 977         if (!getAndSetParkPermit(true) && currentThread() != this) {
 978             int s = state();
 979 
 980             // unparked while parked
 981             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
 982                 submitRunContinuation();
 983                 return;
 984             }
 985 
 986             // unparked while parked when pinned
 987             if (s == PINNED || s == TIMED_PINNED) {
 988                 // unpark carrier thread when pinned
 989                 disableSuspendAndPreempt();
 990                 try {
 991                     synchronized (carrierThreadAccessLock()) {
 992                         Thread carrier = carrierThread;
 993                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 994                             U.unpark(carrier);
 995                         }
 996                     }
 997                 } finally {
 998                     enableSuspendAndPreempt();
 999                 }
1000                 return;
1001             }
1002         }
1003     }
1004 
1005     /**
1006      * Invoked by unblocker thread to unblock this virtual thread.
1007      */
1008     private void unblock() {
1009         assert !Thread.currentThread().isVirtual();
1010         blockPermit = true;
1011         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
1012             submitRunContinuation();
1013         }
1014     }
1015 
1016     /**
1017      * Invoked by FJP worker thread or STPE thread when park timeout expires.
1018      */
1019     private void parkTimeoutExpired() {
1020         assert !VirtualThread.currentThread().isVirtual();
1021         if (!getAndSetParkPermit(true)
1022                 && (state() == TIMED_PARKED)
1023                 && compareAndSetState(TIMED_PARKED, UNPARKED)) {
1024             lazySubmitRunContinuation();
1025         }
1026     }
1027 
1028     /**
1029      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
1030      * If the virtual thread is in timed-wait then this method will unblock the thread
1031      * and submit its task so that it continues and attempts to reenter the monitor.
1032      * This method does nothing if the thread has been woken by notify or interrupt.
1033      */
1034     private void waitTimeoutExpired(byte seqNo) {
1035         assert !Thread.currentThread().isVirtual();
1036         for (;;) {
1037             boolean unblocked = false;
1038             synchronized (timedWaitLock()) {
1039                 if (seqNo != timedWaitSeqNo) {
1040                     // this timeout task is for a past timed-wait
1041                     return;
1042                 }
1043                 int s = state();
1044                 if (s == TIMED_WAIT) {
1045                     unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
1046                 } else if (s != (TIMED_WAIT | SUSPENDED)) {
1047                     // notified or interrupted, no longer waiting
1048                     return;
1049                 }
1050             }
1051             if (unblocked) {
1052                 lazySubmitRunContinuation();
1053                 return;
1054             }
1055             // need to retry when thread is suspended in time-wait
1056             Thread.yield();
1057         }
1058     }
1059 
1060     /**
1061      * Attempts to yield the current virtual thread (Thread.yield).
1062      */
1063     void tryYield() {
1064         assert Thread.currentThread() == this;
1065         setState(YIELDING);
1066         boolean yielded = false;
1067         try {
1068             yielded = yieldContinuation();  // may throw
1069         } finally {
1070             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
1071             if (!yielded) {
1072                 assert state() == YIELDING;
1073                 setState(RUNNING);
1074             }
1075         }
1076     }
1077 
1078     /**
1079      * Sleep the current thread for the given sleep time (in nanoseconds). If
1080      * nanos is 0 then the thread will attempt to yield.
1081      *
1082      * @implNote This implementation parks the thread for the given sleeping time
1083      * and will therefore be observed in PARKED state during the sleep. Parking
1084      * will consume the parking permit so this method makes available the parking
1085      * permit after the sleep. This may be observed as a spurious, but benign,
1086      * wakeup when the thread subsequently attempts to park.
1087      *
1088      * @param nanos the maximum number of nanoseconds to sleep
1089      * @throws InterruptedException if interrupted while sleeping
1090      */
1091     void sleepNanos(long nanos) throws InterruptedException {
1092         assert Thread.currentThread() == this && nanos >= 0;
1093         if (getAndClearInterrupt())
1094             throw new InterruptedException();
1095         if (nanos == 0) {
1096             tryYield();
1097         } else {
1098             // park for the sleep time
1099             try {
1100                 long remainingNanos = nanos;
1101                 long startNanos = System.nanoTime();
1102                 while (remainingNanos > 0) {
1103                     parkNanos(remainingNanos);
1104                     if (getAndClearInterrupt()) {
1105                         throw new InterruptedException();
1106                     }
1107                     remainingNanos = nanos - (System.nanoTime() - startNanos);
1108                 }
1109             } finally {
1110                 // may have been unparked while sleeping
1111                 setParkPermit(true);
1112             }
1113         }
1114     }
1115 
1116     /**
1117      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1118      * A timeout of {@code 0} means to wait forever.
1119      *
1120      * @throws InterruptedException if interrupted while waiting
1121      * @return true if the thread has terminated
1122      */
1123     boolean joinNanos(long nanos) throws InterruptedException {
1124         if (state() == TERMINATED)
1125             return true;
1126 
1127         // ensure termination object exists, then re-check state
1128         CountDownLatch termination = getTermination();
1129         if (state() == TERMINATED)
1130             return true;
1131 
1132         // wait for virtual thread to terminate
1133         if (nanos == 0) {
1134             termination.await();
1135         } else {
1136             boolean terminated = termination.await(nanos, NANOSECONDS);
1137             if (!terminated) {
1138                 // waiting time elapsed
1139                 return false;
1140             }
1141         }
1142         assert state() == TERMINATED;
1143         return true;
1144     }
1145 
1146     @Override
1147     void blockedOn(Interruptible b) {
1148         disableSuspendAndPreempt();
1149         try {
1150             super.blockedOn(b);
1151         } finally {
1152             enableSuspendAndPreempt();
1153         }
1154     }
1155 
1156     @Override
1157     public void interrupt() {
1158         if (Thread.currentThread() != this) {
1159             // if current thread is a virtual thread then prevent it from being
1160             // suspended or unmounted when entering or holding interruptLock
1161             Interruptible blocker;
1162             disableSuspendAndPreempt();
1163             try {
1164                 synchronized (interruptLock) {
1165                     interrupted = true;
1166                     blocker = nioBlocker();
1167                     if (blocker != null) {
1168                         blocker.interrupt(this);
1169                     }
1170 
1171                     // interrupt carrier thread if mounted
1172                     Thread carrier = carrierThread;
1173                     if (carrier != null) carrier.setInterrupt();
1174                 }
1175             } finally {
1176                 enableSuspendAndPreempt();
1177             }
1178 
1179             // notify blocker after releasing interruptLock
1180             if (blocker != null) {
1181                 blocker.postInterrupt();
1182             }
1183 
1184             // make available parking permit, unpark thread if parked
1185             unpark();
1186 
1187             // if thread is waiting in Object.wait then schedule to try to reenter
1188             int s = state();
1189             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1190                 submitRunContinuation();
1191             }
1192 
1193         } else {
1194             interrupted = true;
1195             carrierThread.setInterrupt();
1196             setParkPermit(true);
1197         }
1198     }
1199 
1200     @Override
1201     public boolean isInterrupted() {
1202         return interrupted;
1203     }
1204 
1205     @Override
1206     boolean getAndClearInterrupt() {
1207         assert Thread.currentThread() == this;
1208         boolean oldValue = interrupted;
1209         if (oldValue) {
1210             disableSuspendAndPreempt();
1211             try {
1212                 synchronized (interruptLock) {
1213                     interrupted = false;
1214                     carrierThread.clearInterrupt();
1215                 }
1216             } finally {
1217                 enableSuspendAndPreempt();
1218             }
1219         }
1220         return oldValue;
1221     }
1222 
1223     @Override
1224     Thread.State threadState() {
1225         int s = state();
1226         switch (s & ~SUSPENDED) {
1227             case NEW:
1228                 return Thread.State.NEW;
1229             case STARTED:
1230                 // return NEW if thread container not yet set
1231                 if (threadContainer() == null) {
1232                     return Thread.State.NEW;
1233                 } else {
1234                     return Thread.State.RUNNABLE;
1235                 }
1236             case UNPARKED:
1237             case UNBLOCKED:
1238             case YIELDED:
1239                 // runnable, not mounted
1240                 return Thread.State.RUNNABLE;
1241             case RUNNING:
1242                 // if mounted then return state of carrier thread
1243                 if (Thread.currentThread() != this) {
1244                     disableSuspendAndPreempt();
1245                     try {
1246                         synchronized (carrierThreadAccessLock()) {
1247                             Thread carrierThread = this.carrierThread;
1248                             if (carrierThread != null) {
1249                                 return carrierThread.threadState();
1250                             }
1251                         }
1252                     } finally {
1253                         enableSuspendAndPreempt();
1254                     }
1255                 }
1256                 // runnable, mounted
1257                 return Thread.State.RUNNABLE;
1258             case PARKING:
1259             case TIMED_PARKING:
1260             case WAITING:
1261             case TIMED_WAITING:
1262             case YIELDING:
1263                 // runnable, in transition
1264                 return Thread.State.RUNNABLE;
1265             case PARKED:
1266             case PINNED:
1267             case WAIT:
1268                 return Thread.State.WAITING;
1269             case TIMED_PARKED:
1270             case TIMED_PINNED:
1271             case TIMED_WAIT:
1272                 return Thread.State.TIMED_WAITING;
1273             case BLOCKING:
1274             case BLOCKED:
1275                 return Thread.State.BLOCKED;
1276             case TERMINATED:
1277                 return Thread.State.TERMINATED;
1278             default:
1279                 throw new InternalError();
1280         }
1281     }
1282 
1283     @Override
1284     boolean alive() {
1285         int s = state;
1286         return (s != NEW && s != TERMINATED);
1287     }
1288 
1289     @Override
1290     boolean isTerminated() {
1291         return (state == TERMINATED);
1292     }
1293 
1294     @Override
1295     StackTraceElement[] asyncGetStackTrace() {
1296         StackTraceElement[] stackTrace;
1297         do {
1298             stackTrace = (carrierThread != null)
1299                     ? super.asyncGetStackTrace()  // mounted
1300                     : tryGetStackTrace();         // unmounted
1301             if (stackTrace == null) {
1302                 Thread.yield();
1303             }
1304         } while (stackTrace == null);
1305         return stackTrace;
1306     }
1307 
1308     /**
1309      * Returns the stack trace for this virtual thread if it is unmounted.
1310      * Returns null if the thread is mounted or in transition.
1311      */
1312     private StackTraceElement[] tryGetStackTrace() {
1313         int initialState = state() & ~SUSPENDED;
1314         switch (initialState) {
1315             case NEW, STARTED, TERMINATED -> {
1316                 return new StackTraceElement[0];  // unmounted, empty stack
1317             }
1318             case RUNNING, PINNED, TIMED_PINNED -> {
1319                 return null;   // mounted
1320             }
1321             case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1322                 // unmounted, not runnable
1323             }
1324             case UNPARKED, UNBLOCKED, YIELDED -> {
1325                 // unmounted, runnable
1326             }
1327             case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1328                 return null;  // in transition
1329             }
1330             default -> throw new InternalError("" + initialState);
1331         }
1332 
1333         // thread is unmounted, prevent it from continuing
1334         int suspendedState = initialState | SUSPENDED;
1335         if (!compareAndSetState(initialState, suspendedState)) {
1336             return null;
1337         }
1338 
1339         // get stack trace and restore state
1340         StackTraceElement[] stack;
1341         try {
1342             stack = cont.getStackTrace();
1343         } finally {
1344             assert state == suspendedState;
1345             setState(initialState);
1346         }
1347         boolean resubmit = switch (initialState) {
1348             case UNPARKED, UNBLOCKED, YIELDED -> {
1349                 // resubmit as task may have run while suspended
1350                 yield true;
1351             }
1352             case PARKED, TIMED_PARKED -> {
1353                 // resubmit if unparked while suspended
1354                 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1355             }
1356             case BLOCKED -> {
1357                 // resubmit if unblocked while suspended
1358                 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1359             }
1360             case WAIT, TIMED_WAIT -> {
1361                 // resubmit if notified or interrupted while waiting (Object.wait)
1362                 // waitTimeoutExpired will retry if the timed expired when suspended
1363                 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1364             }
1365             default -> throw new InternalError();
1366         };
1367         if (resubmit) {
1368             submitRunContinuation();
1369         }
1370         return stack;
1371     }
1372 
1373     @Override
1374     public String toString() {
1375         StringBuilder sb = new StringBuilder("VirtualThread[#");
1376         sb.append(threadId());
1377         String name = getName();
1378         if (!name.isEmpty()) {
1379             sb.append(",");
1380             sb.append(name);
1381         }
1382         sb.append("]/");
1383 
1384         // add the carrier state and thread name when mounted
1385         boolean mounted;
1386         if (Thread.currentThread() == this) {
1387             mounted = appendCarrierInfo(sb);
1388         } else {
1389             disableSuspendAndPreempt();
1390             try {
1391                 synchronized (carrierThreadAccessLock()) {
1392                     mounted = appendCarrierInfo(sb);
1393                 }
1394             } finally {
1395                 enableSuspendAndPreempt();
1396             }
1397         }
1398 
1399         // add virtual thread state when not mounted
1400         if (!mounted) {
1401             String stateAsString = threadState().toString();
1402             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1403         }
1404 
1405         return sb.toString();
1406     }
1407 
1408     /**
1409      * Appends the carrier state and thread name to the string buffer if mounted.
1410      * @return true if mounted, false if not mounted
1411      */
1412     private boolean appendCarrierInfo(StringBuilder sb) {
1413         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1414         Thread carrier = carrierThread;
1415         if (carrier != null) {
1416             String stateAsString = carrier.threadState().toString();
1417             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1418             sb.append('@');
1419             sb.append(carrier.getName());
1420             return true;
1421         } else {
1422             return false;
1423         }
1424     }
1425 
1426     @Override
1427     public int hashCode() {
1428         return (int) threadId();
1429     }
1430 
1431     @Override
1432     public boolean equals(Object obj) {
1433         return obj == this;
1434     }
1435 
1436     /**
1437      * Returns the termination object, creating it if needed.
1438      */
1439     private CountDownLatch getTermination() {
1440         CountDownLatch termination = this.termination;
1441         if (termination == null) {
1442             termination = new CountDownLatch(1);
1443             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1444                 termination = this.termination;
1445             }
1446         }
1447         return termination;
1448     }
1449 
1450     /**
1451      * Returns the lock object to synchronize on when accessing carrierThread.
1452      * The lock prevents carrierThread from being reset to null during unmount.
1453      */
1454     private Object carrierThreadAccessLock() {
1455         // return interruptLock as unmount has to coordinate with interrupt
1456         return interruptLock;
1457     }
1458 
1459     /**
1460      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1461      */
1462     private Object timedWaitLock() {
1463         // use this object for now to avoid the overhead of introducing another lock
1464         return runContinuation;
1465     }
1466 
1467     /**
1468      * Disallow the current thread be suspended or preempted.
1469      */
1470     private void disableSuspendAndPreempt() {
1471         notifyJvmtiDisableSuspend(true);
1472         Continuation.pin();
1473     }
1474 
1475     /**
1476      * Allow the current thread be suspended or preempted.
1477      */
1478     private void enableSuspendAndPreempt() {
1479         Continuation.unpin();
1480         notifyJvmtiDisableSuspend(false);
1481     }
1482 
1483     // -- wrappers for get/set of state, parking permit, and carrier thread --
1484 
1485     private int state() {
1486         return state;  // volatile read
1487     }
1488 
1489     private void setState(int newValue) {
1490         state = newValue;  // volatile write
1491     }
1492 
1493     private boolean compareAndSetState(int expectedValue, int newValue) {
1494         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1495     }
1496 
1497     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1498         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1499     }
1500 
1501     private void setParkPermit(boolean newValue) {
1502         if (parkPermit != newValue) {
1503             parkPermit = newValue;
1504         }
1505     }
1506 
1507     private boolean getAndSetParkPermit(boolean newValue) {
1508         if (parkPermit != newValue) {
1509             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1510         } else {
1511             return newValue;
1512         }
1513     }
1514 
1515     private void setCarrierThread(Thread carrier) {
1516         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1517         this.carrierThread = carrier;
1518     }
1519 
1520     // -- JVM TI support --
1521 
1522     @IntrinsicCandidate
1523     @JvmtiMountTransition
1524     private native void notifyJvmtiStart();
1525 
1526     @IntrinsicCandidate
1527     @JvmtiMountTransition
1528     private native void notifyJvmtiEnd();
1529 
1530     @IntrinsicCandidate
1531     @JvmtiMountTransition
1532     private native void notifyJvmtiMount(boolean hide);
1533 
1534     @IntrinsicCandidate
1535     @JvmtiMountTransition
1536     private native void notifyJvmtiUnmount(boolean hide);
1537 
1538     @IntrinsicCandidate
1539     private static native void notifyJvmtiDisableSuspend(boolean enter);
1540 
1541     private static native void registerNatives();
1542     static {
1543         registerNatives();
1544 
1545         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1546         var group = Thread.virtualThreadGroup();
1547 
1548         // ensure event class is initialized
1549         try {
1550             MethodHandles.lookup().ensureInitialized(VirtualThreadParkEvent.class);
1551         } catch (IllegalAccessException e) {
1552             throw new ExceptionInInitializerError(e);
1553         }
1554     }
1555 
1556     /**
1557      * Loads a VirtualThreadScheduler with the given class name. The class must be public
1558      * in an exported package, with public one-arg or no-arg constructor, and be visible
1559      * to the system class loader.
1560      * @param delegate the scheduler that the custom scheduler may delegate to
1561      * @param cn the class name of the custom scheduler
1562      */
1563     private static VirtualThreadScheduler loadCustomScheduler(VirtualThreadScheduler delegate, String cn) {
1564         VirtualThreadScheduler scheduler;
1565         try {
1566             Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1567             // 1-arg constructor
1568             try {
1569                 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1570                 return (VirtualThreadScheduler) ctor.newInstance(delegate);
1571             } catch (NoSuchMethodException e) {
1572                 // 0-arg constructor
1573                 Constructor<?> ctor = clazz.getConstructor();
1574                 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1575             }
1576         } catch (Exception ex) {
1577             throw new Error(ex);
1578         }
1579         System.err.println("WARNING: Using custom default scheduler, this is an experimental feature!");
1580         return scheduler;
1581     }
1582 
1583     /**
1584      * Creates the built-in ForkJoinPool scheduler.
1585      * @param wrapped true if wrapped by a custom default scheduler
1586      */
1587     private static BuiltinScheduler createBuiltinScheduler(boolean wrapped) {

1588         int parallelism, maxPoolSize, minRunnable;
1589         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1590         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1591         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1592         if (parallelismValue != null) {
1593             parallelism = Integer.parseInt(parallelismValue);
1594         } else {
1595             parallelism = Runtime.getRuntime().availableProcessors();
1596         }
1597         if (maxPoolSizeValue != null) {
1598             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1599             parallelism = Integer.min(parallelism, maxPoolSize);
1600         } else {
1601             maxPoolSize = Integer.max(parallelism, 256);
1602         }
1603         if (minRunnableValue != null) {
1604             minRunnable = Integer.parseInt(minRunnableValue);
1605         } else {
1606             minRunnable = Integer.max(parallelism / 2, 1);
1607         }
1608         return new BuiltinScheduler(parallelism, maxPoolSize, minRunnable, wrapped);
1609     }
1610 
1611     /**
1612      * The built-in ForkJoinPool scheduler.
1613      */
1614     private static class BuiltinScheduler
1615             extends ForkJoinPool implements VirtualThreadScheduler {
1616 
1617         BuiltinScheduler(int parallelism, int maxPoolSize, int minRunnable, boolean wrapped) {
1618             ForkJoinWorkerThreadFactory factory = wrapped
1619                     ? ForkJoinPool.defaultForkJoinWorkerThreadFactory
1620                     : CarrierThread::new;
1621             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1622             boolean asyncMode = true; // FIFO
1623             super(parallelism, factory, handler, asyncMode,
1624                     0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1625         }
1626 
1627         private void adaptAndExecute(Runnable task) {
1628             execute(ForkJoinTask.adapt(task));
1629         }
1630 
1631         @Override
1632         public void onStart(VirtualThreadTask task) {
1633             adaptAndExecute(task);
1634         }
1635 
1636         @Override
1637         public void onContinue(VirtualThreadTask task) {
1638             adaptAndExecute(task);
1639         }
1640 
1641         /**
1642          * Wraps the scheduler to avoid leaking a direct reference with
1643          * {@link VirtualThreadScheduler#current()}.
1644          */
1645         VirtualThreadScheduler createExternalView() {
1646             BuiltinScheduler builtin = this;
1647             return new VirtualThreadScheduler() {
1648                 private void execute(VirtualThreadTask task) {
1649                     var vthread = (VirtualThread) task.thread();
1650                     VirtualThreadScheduler scheduler = vthread.scheduler;
1651                     if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1652                         builtin.adaptAndExecute(task);
1653                     } else {
1654                         throw new IllegalArgumentException();
1655                     }
1656                 }
1657                 @Override
1658                 public void onStart(VirtualThreadTask task) {
1659                     execute(task);
1660                 }
1661                 @Override
1662                 public void onContinue(VirtualThreadTask task) {
1663                     execute(task);
1664                 }
1665                 @Override
1666                 public String toString() {
1667                     return builtin.toString();
1668                 }
1669             };
1670         }
1671     }
1672 
1673     /**
1674      * Schedule a runnable task to run after a delay.
1675      */
1676     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1677         if (scheduler == BUILTIN_SCHEDULER) {
1678             return BUILTIN_SCHEDULER.schedule(command, delay, unit);
1679         } else {
1680             return DelayedTaskSchedulers.schedule(command, delay, unit);
1681         }
1682     }
1683 
1684     /**
1685      * Supports scheduling a runnable task to run after a delay. It uses a number
1686      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1687      * work queue used. This class is used when using a custom scheduler.
1688      */
1689     private static class DelayedTaskSchedulers {
1690         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1691 
1692         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1693             long tid = Thread.currentThread().threadId();
1694             int index = (int) tid & (INSTANCE.length - 1);
1695             return INSTANCE[index].schedule(command, delay, unit);
1696         }
1697 
1698         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1699             String propName = "jdk.virtualThreadScheduler.timerQueues";
1700             String propValue = System.getProperty(propName);
1701             int queueCount;
1702             if (propValue != null) {
1703                 queueCount = Integer.parseInt(propValue);
1704                 if (queueCount != Integer.highestOneBit(queueCount)) {
1705                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1706                 }
1707             } else {
1708                 int ncpus = Runtime.getRuntime().availableProcessors();
1709                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1710             }
1711             var schedulers = new ScheduledExecutorService[queueCount];
1712             for (int i = 0; i < queueCount; i++) {
1713                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1714                     Executors.newScheduledThreadPool(1, task -> {
1715                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1716                         t.setDaemon(true);
1717                         return t;
1718                     });
1719                 stpe.setRemoveOnCancelPolicy(true);
1720                 schedulers[i] = stpe;
1721             }
1722             return schedulers;
1723         }
1724     }
1725 
1726     /**
1727      * Schedule virtual threads that are ready to be scheduled after they blocked on
1728      * monitor enter.
1729      */
1730     private static void unblockVirtualThreads() {
1731         while (true) {
1732             VirtualThread vthread = takeVirtualThreadListToUnblock();
1733             while (vthread != null) {
1734                 assert vthread.onWaitingList;
1735                 VirtualThread nextThread = vthread.next;
1736 
1737                 // remove from list and unblock
1738                 vthread.next = null;
1739                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1740                 assert changed;
1741                 vthread.unblock();
1742 
1743                 vthread = nextThread;
1744             }
1745         }
1746     }
1747 
1748     /**
1749      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1750      * if necessary until a list of one or more threads becomes available.
1751      */
1752     private static native VirtualThread takeVirtualThreadListToUnblock();
1753 
1754     static {
1755         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1756                 VirtualThread::unblockVirtualThreads);
1757         unblocker.setDaemon(true);
1758         unblocker.start();
1759     }
1760 }
--- EOF ---