1 /*
   2  * Copyright (c) 2018, 2026, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.lang;
  26 
  27 import java.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.ScheduledFuture;
  40 import java.util.concurrent.ScheduledThreadPoolExecutor;
  41 import java.util.concurrent.TimeUnit;
  42 import jdk.internal.event.VirtualThreadEndEvent;
  43 import jdk.internal.event.VirtualThreadParkEvent;
  44 import jdk.internal.event.VirtualThreadStartEvent;
  45 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  46 import jdk.internal.invoke.MhUtil;
  47 import jdk.internal.misc.CarrierThread;
  48 import jdk.internal.misc.InnocuousThread;
  49 import jdk.internal.misc.Unsafe;
  50 import jdk.internal.vm.Continuation;
  51 import jdk.internal.vm.ContinuationScope;
  52 import jdk.internal.vm.StackableScope;
  53 import jdk.internal.vm.ThreadContainer;
  54 import jdk.internal.vm.ThreadContainers;
  55 import jdk.internal.vm.annotation.ChangesCurrentThread;
  56 import jdk.internal.vm.annotation.Hidden;
  57 import jdk.internal.vm.annotation.IntrinsicCandidate;
  58 import jdk.internal.vm.annotation.JvmtiHideEvents;
  59 import jdk.internal.vm.annotation.JvmtiMountTransition;
  60 import jdk.internal.vm.annotation.ReservedStackAccess;
  61 import sun.nio.ch.Interruptible;
  62 import static java.util.concurrent.TimeUnit.*;
  63 
  64 /**
  65  * A thread that is scheduled by the Java virtual machine rather than the operating system.
  66  */
  67 final class VirtualThread extends BaseVirtualThread {
  68     private static final Unsafe U = Unsafe.getUnsafe();
  69     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  70 
  71     private static final BuiltinScheduler BUILTIN_SCHEDULER;
  72     private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
  73     private static final VirtualThreadScheduler EXTERNAL_VIEW;
  74     static {
  75         // experimental
  76         String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
  77         if (propValue != null) {
  78             BuiltinScheduler builtinScheduler = createBuiltinScheduler(true);
  79             VirtualThreadScheduler externalView = builtinScheduler.createExternalView();
  80             VirtualThreadScheduler defaultScheduler = loadCustomScheduler(externalView, propValue);
  81             BUILTIN_SCHEDULER = builtinScheduler;
  82             DEFAULT_SCHEDULER = defaultScheduler;
  83             EXTERNAL_VIEW = externalView;
  84         } else {
  85             var builtinScheduler = createBuiltinScheduler(false);
  86             BUILTIN_SCHEDULER = builtinScheduler;
  87             DEFAULT_SCHEDULER = builtinScheduler;
  88             EXTERNAL_VIEW = builtinScheduler.createExternalView();
  89         }
  90     }
  91 
  92     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  93     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  94     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  95     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  96     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  97 
  98     // scheduler and continuation
  99     private final VirtualThreadScheduler scheduler;
 100     private final Continuation cont;
 101     private final VirtualThreadTask runContinuation;
 102 
 103     // virtual thread state, accessed by VM
 104     private volatile int state;
 105 
 106     /*
 107      * Virtual thread state transitions:
 108      *
 109      *      NEW -> STARTED         // Thread.start, schedule to run
 110      *  STARTED -> TERMINATED      // failed to start
 111      *  STARTED -> RUNNING         // first run
 112      *  RUNNING -> TERMINATED      // done
 113      *
 114      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
 115      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
 116      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
 117      * UNPARKED -> RUNNING         // continue execution after park
 118      *
 119      *  PARKING -> RUNNING         // cont.yield failed, need to park on carrier
 120      *  RUNNING -> PINNED          // park on carrier
 121      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
 122      *
 123      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
 124      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
 125      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 126      *
 127      * TIMED_PARKING -> RUNNING         // cont.yield failed, need to park on carrier
 128      *       RUNNING -> TIMED_PINNED    // park on carrier
 129      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 130      *
 131      *   RUNNING -> BLOCKING       // blocking on monitor enter
 132      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 133      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 134      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 135      *
 136      *   RUNNING -> WAITING        // transitional state during wait on monitor
 137      *   WAITING -> WAIT           // waiting on monitor
 138      *      WAIT -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 139      *      WAIT -> UNBLOCKED      // interrupted
 140      *
 141      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 142      * TIMED_WAITING -> TIMED_WAIT      // timed-waiting on monitor
 143      *    TIMED_WAIT -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 144      *    TIMED_WAIT -> UNBLOCKED       // timed-out/interrupted
 145      *
 146      *  RUNNING -> YIELDING        // Thread.yield
 147      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 148      * YIELDING -> RUNNING         // cont.yield failed
 149      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 150      */
 151     private static final int NEW      = 0;
 152     private static final int STARTED  = 1;
 153     private static final int RUNNING  = 2;     // runnable-mounted
 154 
 155     // untimed and timed parking
 156     private static final int PARKING       = 3;
 157     private static final int PARKED        = 4;     // unmounted
 158     private static final int PINNED        = 5;     // mounted
 159     private static final int TIMED_PARKING = 6;
 160     private static final int TIMED_PARKED  = 7;     // unmounted
 161     private static final int TIMED_PINNED  = 8;     // mounted
 162     private static final int UNPARKED      = 9;     // unmounted but runnable
 163 
 164     // Thread.yield
 165     private static final int YIELDING = 10;
 166     private static final int YIELDED  = 11;         // unmounted but runnable
 167 
 168     // monitor enter
 169     private static final int BLOCKING  = 12;
 170     private static final int BLOCKED   = 13;        // unmounted
 171     private static final int UNBLOCKED = 14;        // unmounted but runnable
 172 
 173     // monitor wait/timed-wait
 174     private static final int WAITING       = 15;
 175     private static final int WAIT          = 16;    // waiting in Object.wait
 176     private static final int TIMED_WAITING = 17;
 177     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 178 
 179     private static final int TERMINATED = 99;  // final state
 180 



 181     // parking permit made available by LockSupport.unpark
 182     private volatile boolean parkPermit;
 183 
 184     // blocking permit made available by unblocker thread when another thread exits monitor
 185     private volatile boolean blockPermit;
 186 
 187     // true when on the list of virtual threads waiting to be unblocked
 188     private volatile boolean onWaitingList;
 189 
 190     // next virtual thread on the list of virtual threads waiting to be unblocked
 191     private volatile VirtualThread next;
 192 
 193     // notified by Object.notify/notifyAll while waiting in Object.wait
 194     private volatile boolean notified;
 195 
 196     // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
 197     private volatile boolean interruptibleWait;
 198 
 199     // timed-wait support
 200     private byte timedWaitSeqNo;
 201 
 202     // timeout for timed-park and timed-wait, only accessed on current/carrier thread
 203     private long timeout;
 204 
 205     // timer task for timed-park and timed-wait, only accessed on current/carrier thread
 206     private Future<?> timeoutTask;
 207 
 208     // carrier thread when mounted, accessed by VM
 209     private volatile Thread carrierThread;
 210 
 211     // termination object when joining, created lazily if needed
 212     private volatile CountDownLatch termination;
 213 
 214     /**
 215      * Return the built-in scheduler.
 216      * @param trusted true if caller is trusted, false if not trusted
 217      */
 218     static VirtualThreadScheduler builtinScheduler(boolean trusted) {
 219         return trusted ? BUILTIN_SCHEDULER : EXTERNAL_VIEW;
 220     }
 221 
 222     /**
 223      * Returns the default scheduler, usually the same as the built-in scheduler.
 224      */
 225     static VirtualThreadScheduler defaultScheduler() {
 226         return DEFAULT_SCHEDULER;
 227     }
 228 
 229     /**
 230      * Returns the continuation scope used for virtual threads.
 231      */
 232     static ContinuationScope continuationScope() {
 233         return VTHREAD_SCOPE;
 234     }
 235 
 236     /**
 237      * Return the scheduler for this thread.
 238      * @param trusted true if caller is trusted, false if not trusted
 239      */
 240     VirtualThreadScheduler scheduler(boolean trusted) {
 241         if (scheduler == BUILTIN_SCHEDULER && !trusted) {
 242             return EXTERNAL_VIEW;
 243         } else {
 244             return scheduler;
 245         }
 246     }
 247 
 248     /**
 249      * Returns the task to start/continue this virtual thread.
 250      */
 251     VirtualThreadTask virtualThreadTask() {
 252         return runContinuation;
 253     }
 254 
 255     /**
 256      * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
 257      *
 258      * @param scheduler the scheduler or null for default scheduler
 259      * @param preferredCarrier the preferred carrier or null
 260      * @param name thread name
 261      * @param characteristics characteristics
 262      * @param task the task to execute
 263      */
 264     VirtualThread(VirtualThreadScheduler scheduler,
 265                   Thread preferredCarrier,
 266                   String name,
 267                   int characteristics,
 268                   Runnable task) {
 269         super(name, characteristics, /*bound*/ false);
 270         Objects.requireNonNull(task);
 271 
 272         // use default scheduler if not provided
 273         if (scheduler == null) {
 274             scheduler = DEFAULT_SCHEDULER;
 275         } else if (scheduler == EXTERNAL_VIEW) {
 276             throw new UnsupportedOperationException();



 277         }

 278         this.scheduler = scheduler;
 279         this.cont = new VThreadContinuation(this, task);
 280 
 281         if (scheduler == BUILTIN_SCHEDULER) {
 282             this.runContinuation = new BuiltinSchedulerTask(this);
 283         } else {
 284             this.runContinuation = new CustomSchedulerTask(this, preferredCarrier);
 285         }
 286     }
 287 
 288     /**
 289      * The task to execute when using the built-in scheduler.
 290      */
 291     static final class BuiltinSchedulerTask implements VirtualThreadTask {
 292         private final VirtualThread vthread;
 293         BuiltinSchedulerTask(VirtualThread vthread) {
 294             this.vthread = vthread;
 295         }
 296         @Override
 297         public Thread thread() {
 298             return vthread;
 299         }
 300         @Override
 301         public void run() {
 302             vthread.runContinuation();;
 303         }
 304         @Override
 305         public Thread preferredCarrier() {
 306             throw new UnsupportedOperationException();
 307         }
 308         @Override
 309         public Object attach(Object att) {
 310             throw new UnsupportedOperationException();
 311         }
 312         @Override
 313         public Object attachment() {
 314             throw new UnsupportedOperationException();
 315         }
 316     }
 317 
 318     /**
 319      * The task to execute when using a custom scheduler.
 320      */
 321     static final class CustomSchedulerTask implements VirtualThreadTask {
 322         private static final VarHandle ATT =
 323                 MhUtil.findVarHandle(MethodHandles.lookup(), "att", Object.class);
 324         private final VirtualThread vthread;
 325         private final Thread preferredCarrier;
 326         private volatile Object att;
 327         CustomSchedulerTask(VirtualThread vthread, Thread preferredCarrier) {
 328             this.vthread = vthread;
 329             this.preferredCarrier = preferredCarrier;
 330         }
 331         @Override
 332         public Thread thread() {
 333             return vthread;
 334         }
 335         @Override
 336         public void run() {
 337             vthread.runContinuation();;
 338         }
 339         @Override
 340         public Thread preferredCarrier() {
 341             return preferredCarrier;
 342         }
 343         @Override
 344         public Object attach(Object att) {
 345             return ATT.getAndSet(this, att);
 346         }
 347         @Override
 348         public Object attachment() {
 349             return att;
 350         }
 351     }
 352 
 353     /**
 354      * The continuation that a virtual thread executes.
 355      */
 356     private static class VThreadContinuation extends Continuation {
 357         VThreadContinuation(VirtualThread vthread, Runnable task) {
 358             super(VTHREAD_SCOPE, wrap(vthread, task));
 359         }
 360         @Override
 361         protected void onPinned(Continuation.Pinned reason) {
 362         }
 363         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 364             return new Runnable() {
 365                 @Hidden
 366                 @JvmtiHideEvents
 367                 public void run() {
 368                     vthread.endFirstTransition();
 369                     try {
 370                         vthread.run(task);
 371                     } finally {
 372                         vthread.startFinalTransition();
 373                     }
 374                 }
 375             };
 376         }
 377     }
 378 
 379     /**
 380      * Runs or continues execution on the current thread. The virtual thread is mounted
 381      * on the current thread before the task runs or continues. It unmounts when the
 382      * task completes or yields.
 383      */
 384     @ChangesCurrentThread // allow mount/unmount to be inlined
 385     private void runContinuation() {
 386         // the carrier must be a platform thread
 387         if (Thread.currentThread().isVirtual()) {
 388             throw new WrongThreadException();
 389         }
 390 
 391         // set state to RUNNING
 392         int initialState = state();
 393         if (initialState == STARTED || initialState == UNPARKED
 394                 || initialState == UNBLOCKED || initialState == YIELDED) {
 395             // newly started or continue after parking/blocking/Thread.yield
 396             if (!compareAndSetState(initialState, RUNNING)) {
 397                 return;
 398             }
 399             // consume permit when continuing after parking or blocking. If continue
 400             // after a timed-park or timed-wait then the timeout task is cancelled.
 401             if (initialState == UNPARKED) {
 402                 cancelTimeoutTask();
 403                 setParkPermit(false);
 404             } else if (initialState == UNBLOCKED) {
 405                 cancelTimeoutTask();
 406                 blockPermit = false;
 407             }
 408         } else {
 409             // not runnable
 410             return;
 411         }
 412 
 413         mount();
 414         try {
 415             cont.run();
 416         } finally {
 417             unmount();
 418             if (cont.isDone()) {
 419                 afterDone();
 420             } else {
 421                 afterYield();
 422             }
 423         }
 424     }
 425 
 426     /**
 427      * Cancel timeout task when continuing after timed-park or timed-wait.
 428      * The timeout task may be executing, or may have already completed.
 429      */
 430     private void cancelTimeoutTask() {
 431         if (timeoutTask != null) {
 432             timeoutTask.cancel(false);
 433             timeoutTask = null;
 434         }
 435     }
 436 
 437     /**
 438      * Submits the runContinuation task to the scheduler. For the built-in scheduler,
 439      * the task will be pushed to the local queue if possible, otherwise it will be
 440      * pushed to an external submission queue.













 441      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 442      * @throws RejectedExecutionException
 443      */
 444     private void submitRunContinuation(boolean retryOnOOME) {
 445         boolean done = false;
 446         while (!done) {
 447             try {
 448                 // Pin the continuation to prevent the virtual thread from unmounting
 449                 // when submitting a task. For the default scheduler this ensures that
 450                 // the carrier doesn't change when pushing a task. For other schedulers
 451                 // it avoids deadlock that could arise due to carriers and virtual
 452                 // threads contending for a lock.
 453                 if (currentThread().isVirtual()) {
 454                     Continuation.pin();
 455                     try {
 456                         scheduler.onContinue(runContinuation);
 457                     } finally {
 458                         Continuation.unpin();
 459                     }
 460                 } else {
 461                     scheduler.onContinue(runContinuation);
 462                 }
 463                 done = true;
 464             } catch (RejectedExecutionException ree) {
 465                 submitFailed(ree);
 466                 throw ree;
 467             } catch (OutOfMemoryError e) {
 468                 if (retryOnOOME) {
 469                     U.park(false, 100_000_000); // 100ms
 470                 } else {
 471                     throw e;
 472                 }
 473             }
 474         }
 475     }
 476 


















 477     /**
 478      * Submits the runContinuation task to the scheduler. For the default scheduler,
 479      * and calling it on a worker thread, the task will be pushed to the local queue,
 480      * otherwise it will be pushed to an external submission queue.
 481      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 482      * @throws RejectedExecutionException
 483      */
 484     private void submitRunContinuation() {
 485         submitRunContinuation(true);
 486     }
 487 
 488     /**
 489      * Invoked from a carrier thread to lazy submit the runContinuation task to the
 490      * carrier's local queue if the queue is empty. If not empty, or invoked by a thread
 491      * for a custom scheduler, then it just submits the task to the scheduler.
 492      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 493      * @throws RejectedExecutionException
 494      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 495      */
 496     private void lazySubmitRunContinuation() {
 497         assert !currentThread().isVirtual();
 498         if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {

 499             try {
 500                 ct.getPool().lazySubmit(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 a carrier thread to externally submit the runContinuation task to the
 514      * scheduler. If invoked by a thread for a custom scheduler, then it just submits the
 515      * task to the scheduler.
 516      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 517      * @throws RejectedExecutionException
 518      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 519      */
 520     private void externalSubmitRunContinuation() {
 521         assert !currentThread().isVirtual();
 522         if (currentThread() instanceof CarrierThread ct) {
 523             try {
 524                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 525             } catch (RejectedExecutionException ree) {
 526                 submitFailed(ree);
 527                 throw ree;
 528             } catch (OutOfMemoryError e) {
 529                 submitRunContinuation();
 530             }
 531         } else {
 532             submitRunContinuation();
 533         }
 534     }
 535 
 536     /**
 537      * Invoked from Thread.start to externally submit the runContinuation task to the
 538      * scheduler. If this virtual thread is scheduled by the built-in scheduler,
 539      * and this method is called from a virtual thread scheduled by the built-in
 540      * scheduler, then it uses externalSubmit to ensure that the task is pushed to an
 541      * external submission queue rather than the local queue.
 542      * @throws RejectedExecutionException
 543      * @throws OutOfMemoryError
 544      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 545      */
 546     private void externalSubmitRunContinuationOrThrow() {
 547         try {
 548             if (currentThread().isVirtual()) {
 549                 // Pin the continuation to prevent the virtual thread from unmounting
 550                 // when submitting a task. This avoids deadlock that could arise due to
 551                 // carriers and virtual threads contending for a lock.
 552                 Continuation.pin();
 553                 try {
 554                     if (scheduler == BUILTIN_SCHEDULER
 555                             && currentCarrierThread() instanceof CarrierThread ct) {
 556                         ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 557                     } else {
 558                         scheduler.onStart(runContinuation);
 559                     }
 560                 } finally {
 561                     Continuation.unpin();
 562                 }
 563             } else {
 564                 scheduler.onStart(runContinuation);
 565             }
 566         } catch (RejectedExecutionException ree) {
 567             submitFailed(ree);
 568             throw ree;
 569         }
 570     }
 571 
 572     /**
 573      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 574      */
 575     private void submitFailed(RejectedExecutionException ree) {
 576         var event = new VirtualThreadSubmitFailedEvent();
 577         if (event.isEnabled()) {
 578             event.javaThreadId = threadId();
 579             event.exceptionMessage = ree.getMessage();
 580             event.commit();
 581         }
 582     }
 583 
 584     /**
 585      * Runs a task in the context of this virtual thread.
 586      */
 587     private void run(Runnable task) {
 588         assert Thread.currentThread() == this && state == RUNNING;
 589 
 590         // emit JFR event if enabled
 591         if (VirtualThreadStartEvent.isTurnedOn()) {
 592             var event = new VirtualThreadStartEvent();
 593             event.javaThreadId = threadId();
 594             event.commit();
 595         }
 596 
 597         Object bindings = Thread.scopedValueBindings();
 598         try {
 599             runWith(bindings, task);
 600         } catch (Throwable exc) {
 601             dispatchUncaughtException(exc);
 602         } finally {
 603             // pop any remaining scopes from the stack, this may block
 604             StackableScope.popAll();
 605 
 606             // emit JFR event if enabled
 607             if (VirtualThreadEndEvent.isTurnedOn()) {
 608                 var event = new VirtualThreadEndEvent();
 609                 event.javaThreadId = threadId();
 610                 event.commit();
 611             }
 612         }
 613     }
 614 
 615     /**
 616      * Mounts this virtual thread onto the current platform thread. On
 617      * return, the current thread is the virtual thread.
 618      */
 619     @ChangesCurrentThread
 620     @ReservedStackAccess
 621     private void mount() {
 622         startTransition(/*mount*/true);
 623         // We assume following volatile accesses provide equivalent
 624         // of acquire ordering, otherwise we need U.loadFence() here.
 625 
 626         // sets the carrier thread
 627         Thread carrier = Thread.currentCarrierThread();
 628         setCarrierThread(carrier);
 629 
 630         // sync up carrier thread interrupted status if needed
 631         if (interrupted) {
 632             carrier.setInterrupt();
 633         } else if (carrier.isInterrupted()) {
 634             synchronized (interruptLock) {
 635                 // need to recheck interrupted status
 636                 if (!interrupted) {
 637                     carrier.clearInterrupt();
 638                 }
 639             }
 640         }
 641 
 642         // set Thread.currentThread() to return this virtual thread
 643         carrier.setCurrentThread(this);
 644     }
 645 
 646     /**
 647      * Unmounts this virtual thread from the carrier. On return, the
 648      * current thread is the current platform thread.
 649      */
 650     @ChangesCurrentThread
 651     @ReservedStackAccess
 652     private void unmount() {
 653         assert !Thread.holdsLock(interruptLock);
 654 
 655         // set Thread.currentThread() to return the platform thread
 656         Thread carrier = this.carrierThread;
 657         carrier.setCurrentThread(carrier);
 658 
 659         // break connection to carrier thread, synchronized with interrupt
 660         synchronized (interruptLock) {
 661             setCarrierThread(null);
 662         }
 663         carrier.clearInterrupt();
 664 
 665         // We assume previous volatile accesses provide equivalent
 666         // of release ordering, otherwise we need U.storeFence() here.
 667         endTransition(/*mount*/false);
 668     }
 669 
 670     /**
 671      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 672      * the continuation continues.
 673      */
 674     @Hidden
 675     private boolean yieldContinuation() {
 676         startTransition(/*mount*/false);
 677         try {
 678             return Continuation.yield(VTHREAD_SCOPE);
 679         } finally {
 680             endTransition(/*mount*/true);
 681         }
 682     }
 683 
 684     /**
 685      * Invoked in the context of the carrier thread after the Continuation yields when
 686      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 687      */
 688     private void afterYield() {
 689         assert carrierThread == null;
 690 
 691         // re-adjust parallelism if the virtual thread yielded when compensating
 692         if (currentThread() instanceof CarrierThread ct) {
 693             ct.endBlocking();
 694         }
 695 
 696         int s = state();
 697 
 698         // LockSupport.park/parkNanos
 699         if (s == PARKING || s == TIMED_PARKING) {
 700             int newState;
 701             if (s == PARKING) {
 702                 setState(newState = PARKED);
 703             } else {
 704                 // schedule unpark
 705                 long timeout = this.timeout;
 706                 assert timeout > 0;
 707                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 708                 setState(newState = TIMED_PARKED);
 709             }
 710 
 711             // may have been unparked while parking
 712             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 713                 // lazy submit if local queue is empty
 714                 lazySubmitRunContinuation();
 715             }
 716             return;
 717         }
 718 
 719         // Thread.yield
 720         if (s == YIELDING) {
 721             setState(YIELDED);
 722 
 723             // external submit if there are no tasks in the local task queue
 724             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 725                 externalSubmitRunContinuation();
 726             } else {
 727                 submitRunContinuation();
 728             }
 729             return;
 730         }
 731 
 732         // blocking on monitorenter
 733         if (s == BLOCKING) {
 734             setState(BLOCKED);
 735 
 736             // may have been unblocked while blocking
 737             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 738                 // lazy submit if local queue is empty
 739                 lazySubmitRunContinuation();
 740             }
 741             return;
 742         }
 743 
 744         // Object.wait
 745         if (s == WAITING || s == TIMED_WAITING) {
 746             int newState;
 747             boolean blocked;
 748             boolean interruptible = interruptibleWait;
 749             if (s == WAITING) {
 750                 setState(newState = WAIT);
 751                 // may have been notified while in transition
 752                 blocked = notified && compareAndSetState(WAIT, BLOCKED);
 753             } else {
 754                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 755                 // task will change the thread state to UNBLOCKED and submit the thread
 756                 // to the scheduler. A sequence number is used to ensure that the timeout
 757                 // task only unblocks the thread for this timed-wait. We synchronize with
 758                 // the timeout task to coordinate access to the sequence number and to
 759                 // ensure the timeout task doesn't execute until the thread has got to
 760                 // the TIMED_WAIT state.
 761                 long timeout = this.timeout;
 762                 assert timeout > 0;
 763                 synchronized (timedWaitLock()) {
 764                     byte seqNo = ++timedWaitSeqNo;
 765                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 766                     setState(newState = TIMED_WAIT);
 767                     // May have been notified while in transition. This must be done while
 768                     // holding the monitor to avoid changing the state of a new timed wait call.
 769                     blocked = notified && compareAndSetState(TIMED_WAIT, BLOCKED);
 770                 }
 771             }
 772 
 773             if (blocked) {
 774                 // may have been unblocked already
 775                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 776                     lazySubmitRunContinuation();
 777                 }
 778             } else {
 779                 // may have been interrupted while in transition to wait state
 780                 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 781                     lazySubmitRunContinuation();
 782                 }
 783             }
 784             return;
 785         }
 786 
 787         assert false;
 788     }
 789 
 790     /**
 791      * Invoked after the continuation completes.
 792      */
 793     private void afterDone() {
 794         afterDone(true);
 795     }
 796 
 797     /**
 798      * Invoked after the continuation completes (or start failed). Sets the thread
 799      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 800      *
 801      * @param notifyContainer true if its container should be notified
 802      */
 803     private void afterDone(boolean notifyContainer) {
 804         assert carrierThread == null;
 805         setState(TERMINATED);
 806 
 807         // notify anyone waiting for this virtual thread to terminate
 808         CountDownLatch termination = this.termination;
 809         if (termination != null) {
 810             assert termination.getCount() == 1;
 811             termination.countDown();
 812         }
 813 
 814         // notify container
 815         if (notifyContainer) {
 816             threadContainer().remove(this);
 817         }
 818 
 819         // clear references to thread locals
 820         clearReferences();
 821     }
 822 
 823     /**
 824      * Schedules this {@code VirtualThread} to execute.
 825      *
 826      * @throws IllegalStateException if the container is shutdown or closed
 827      * @throws IllegalThreadStateException if the thread has already been started
 828      * @throws RejectedExecutionException if the scheduler cannot accept a task
 829      */
 830     @Override
 831     void start(ThreadContainer container) {
 832         if (!compareAndSetState(NEW, STARTED)) {
 833             throw new IllegalThreadStateException("Already started");
 834         }
 835 
 836         // bind thread to container
 837         assert threadContainer() == null;
 838         setThreadContainer(container);
 839 
 840         // start thread
 841         boolean addedToContainer = false;
 842         boolean started = false;
 843         try {
 844             container.add(this);  // may throw
 845             addedToContainer = true;
 846 
 847             // scoped values may be inherited
 848             inheritScopedValueBindings(container);
 849 
 850             // submit task to run thread, using externalSubmit if possible
 851             externalSubmitRunContinuationOrThrow();
 852             started = true;
 853         } finally {
 854             if (!started) {
 855                 afterDone(addedToContainer);
 856             }
 857         }
 858     }
 859 
 860     @Override
 861     public void start() {
 862         start(ThreadContainers.root());
 863     }
 864 
 865     @Override
 866     public void run() {
 867         // do nothing
 868     }
 869 
 870     /**
 871      * Parks until unparked or interrupted. If already unparked then the parking
 872      * permit is consumed and this method completes immediately (meaning it doesn't
 873      * yield). It also completes immediately if the interrupted status is set.
 874      */
 875     @Override
 876     void park() {
 877         assert Thread.currentThread() == this;
 878 
 879         // complete immediately if parking permit available or interrupted
 880         if (getAndSetParkPermit(false) || interrupted)
 881             return;
 882 
 883         // park the thread
 884         boolean yielded = false;
 885         long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 886         setState(PARKING);
 887         try {
 888             yielded = yieldContinuation();
 889         } catch (OutOfMemoryError e) {
 890             // park on carrier
 891         } finally {
 892             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 893             if (yielded) {
 894                 VirtualThreadParkEvent.offer(eventStartTime, Long.MIN_VALUE);
 895             } else {
 896                 assert state() == PARKING;
 897                 setState(RUNNING);
 898             }
 899         }
 900 
 901         // park on the carrier thread when pinned
 902         if (!yielded) {
 903             parkOnCarrierThread(false, 0);
 904         }
 905     }
 906 
 907     /**
 908      * Parks up to the given waiting time or until unparked or interrupted.
 909      * If already unparked then the parking permit is consumed and this method
 910      * completes immediately (meaning it doesn't yield). It also completes immediately
 911      * if the interrupted status is set or the waiting time is {@code <= 0}.
 912      *
 913      * @param nanos the maximum number of nanoseconds to wait.
 914      */
 915     @Override
 916     void parkNanos(long nanos) {
 917         assert Thread.currentThread() == this;
 918 
 919         // complete immediately if parking permit available or interrupted
 920         if (getAndSetParkPermit(false) || interrupted)
 921             return;
 922 
 923         // park the thread for the waiting time
 924         if (nanos > 0) {
 925             long startTime = System.nanoTime();
 926 
 927             // park the thread, afterYield will schedule the thread to unpark
 928             boolean yielded = false;
 929             long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 930             timeout = nanos;
 931             setState(TIMED_PARKING);
 932             try {
 933                 yielded = yieldContinuation();
 934             } catch (OutOfMemoryError e) {
 935                 // park on carrier
 936             } finally {
 937                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 938                 if (yielded) {
 939                     VirtualThreadParkEvent.offer(eventStartTime, nanos);
 940                 } else {
 941                     assert state() == TIMED_PARKING;
 942                     setState(RUNNING);
 943                 }
 944             }
 945 
 946             // park on carrier thread for remaining time when pinned (or OOME)
 947             if (!yielded) {
 948                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 949                 parkOnCarrierThread(true, remainingNanos);
 950             }
 951         }
 952     }
 953 
 954     /**
 955      * Parks the current carrier thread up to the given waiting time or until
 956      * unparked or interrupted. If the virtual thread is interrupted then the
 957      * interrupted status will be propagated to the carrier thread.
 958      * @param timed true for a timed park, false for untimed
 959      * @param nanos the waiting time in nanoseconds
 960      */
 961     private void parkOnCarrierThread(boolean timed, long nanos) {
 962         assert state() == RUNNING;
 963 
 964         setState(timed ? TIMED_PINNED : PINNED);
 965         try {
 966             if (!parkPermit) {
 967                 if (!timed) {
 968                     U.park(false, 0);
 969                 } else if (nanos > 0) {
 970                     U.park(false, nanos);
 971                 }
 972             }
 973         } finally {
 974             setState(RUNNING);
 975         }
 976 
 977         // consume parking permit
 978         setParkPermit(false);
 979 
 980         // JFR jdk.VirtualThreadPinned event
 981         postPinnedEvent("LockSupport.park");
 982     }
 983 
 984     /**
 985      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 986      * Recording the event in the VM avoids having JFR event recorded in Java
 987      * with the same name, but different ID, to events recorded by the VM.
 988      */
 989     @Hidden
 990     private static native void postPinnedEvent(String op);
 991 
 992     /**
 993      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 994      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 995      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 996      * @param lazySubmit to use lazySubmit if possible
 997      * @throws RejectedExecutionException if the scheduler cannot accept a task
 998      */
 999     private void unpark(boolean lazySubmit) {
1000         if (!getAndSetParkPermit(true) && currentThread() != this) {
1001             int s = state();
1002 
1003             // unparked while parked
1004             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
1005                 if (lazySubmit) {
1006                     lazySubmitRunContinuation();
1007                 } else {
1008                     submitRunContinuation();
1009                 }
1010                 return;
1011             }
1012 
1013             // unparked while parked when pinned
1014             if (s == PINNED || s == TIMED_PINNED) {
1015                 // unpark carrier thread when pinned
1016                 disableSuspendAndPreempt();
1017                 try {
1018                     synchronized (carrierThreadAccessLock()) {
1019                         Thread carrier = carrierThread;
1020                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
1021                             U.unpark(carrier);
1022                         }
1023                     }
1024                 } finally {
1025                     enableSuspendAndPreempt();
1026                 }
1027                 return;
1028             }
1029         }
1030     }
1031 
1032     @Override
1033     void unpark() {
1034         unpark(false);
1035     }
1036 
1037     /**
1038      * Invoked by unblocker thread to unblock this virtual thread.
1039      */
1040     private void unblock() {
1041         assert !Thread.currentThread().isVirtual();
1042         blockPermit = true;
1043         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
1044             submitRunContinuation();
1045         }
1046     }
1047 
1048     /**
1049      * Invoked by FJP worker thread or STPE thread when park timeout expires.
1050      */
1051     private void parkTimeoutExpired() {
1052         assert !VirtualThread.currentThread().isVirtual();
1053         unpark(true);
1054     }
1055 
1056     /**
1057      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
1058      * If the virtual thread is in timed-wait then this method will unblock the thread
1059      * and submit its task so that it continues and attempts to reenter the monitor.
1060      * This method does nothing if the thread has been woken by notify or interrupt.
1061      */
1062     private void waitTimeoutExpired(byte seqNo) {
1063         assert !Thread.currentThread().isVirtual();
1064 
1065         synchronized (timedWaitLock()) {
1066             if (seqNo != timedWaitSeqNo) {
1067                 // this timeout task is for a past timed-wait
1068                 return;









1069             }
1070             if (!compareAndSetState(TIMED_WAIT, UNBLOCKED)) {
1071                 // already unblocked
1072                 return;
1073             }


1074         }
1075 
1076         lazySubmitRunContinuation();
1077     }
1078 
1079     /**
1080      * Attempts to yield the current virtual thread (Thread.yield).
1081      */
1082     void tryYield() {
1083         assert Thread.currentThread() == this;
1084         setState(YIELDING);
1085         boolean yielded = false;
1086         try {
1087             yielded = yieldContinuation();  // may throw
1088         } finally {
1089             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
1090             if (!yielded) {
1091                 assert state() == YIELDING;
1092                 setState(RUNNING);
1093             }
1094         }
1095     }
1096 
1097     /**
1098      * Sleep the current thread for the given sleep time (in nanoseconds). If
1099      * nanos is 0 then the thread will attempt to yield.
1100      *
1101      * @implNote This implementation parks the thread for the given sleeping time
1102      * and will therefore be observed in PARKED state during the sleep. Parking
1103      * will consume the parking permit so this method makes available the parking
1104      * permit after the sleep. This may be observed as a spurious, but benign,
1105      * wakeup when the thread subsequently attempts to park.
1106      *
1107      * @param nanos the maximum number of nanoseconds to sleep
1108      * @throws InterruptedException if interrupted while sleeping
1109      */
1110     void sleepNanos(long nanos) throws InterruptedException {
1111         assert Thread.currentThread() == this && nanos >= 0;
1112         if (getAndClearInterrupt())
1113             throw new InterruptedException();
1114         if (nanos == 0) {
1115             tryYield();
1116         } else {
1117             // park for the sleep time
1118             try {
1119                 long remainingNanos = nanos;
1120                 long startNanos = System.nanoTime();
1121                 while (remainingNanos > 0) {
1122                     parkNanos(remainingNanos);
1123                     if (getAndClearInterrupt()) {
1124                         throw new InterruptedException();
1125                     }
1126                     remainingNanos = nanos - (System.nanoTime() - startNanos);
1127                 }
1128             } finally {
1129                 // may have been unparked while sleeping
1130                 setParkPermit(true);
1131             }
1132         }
1133     }
1134 
1135     /**
1136      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1137      * A timeout of {@code 0} means to wait forever.
1138      *
1139      * @throws InterruptedException if interrupted while waiting
1140      * @return true if the thread has terminated
1141      */
1142     boolean joinNanos(long nanos) throws InterruptedException {
1143         if (state() == TERMINATED)
1144             return true;
1145 
1146         // ensure termination object exists, then re-check state
1147         CountDownLatch termination = getTermination();
1148         if (state() == TERMINATED)
1149             return true;
1150 
1151         // wait for virtual thread to terminate
1152         if (nanos == 0) {
1153             termination.await();
1154         } else {
1155             boolean terminated = termination.await(nanos, NANOSECONDS);
1156             if (!terminated) {
1157                 // waiting time elapsed
1158                 return false;
1159             }
1160         }
1161         assert state() == TERMINATED;
1162         return true;
1163     }
1164 
1165     @Override
1166     void blockedOn(Interruptible b) {
1167         disableSuspendAndPreempt();
1168         try {
1169             super.blockedOn(b);
1170         } finally {
1171             enableSuspendAndPreempt();
1172         }
1173     }
1174 
1175     @Override
1176     public void interrupt() {
1177         if (Thread.currentThread() != this) {
1178             // if current thread is a virtual thread then prevent it from being
1179             // suspended or unmounted when entering or holding interruptLock
1180             Interruptible blocker;
1181             disableSuspendAndPreempt();
1182             try {
1183                 synchronized (interruptLock) {
1184                     interrupted = true;
1185                     blocker = nioBlocker();
1186                     if (blocker != null) {
1187                         blocker.interrupt(this);
1188                     }
1189 
1190                     // interrupt carrier thread if mounted
1191                     Thread carrier = carrierThread;
1192                     if (carrier != null) carrier.setInterrupt();
1193                 }
1194             } finally {
1195                 enableSuspendAndPreempt();
1196             }
1197 
1198             // notify blocker after releasing interruptLock
1199             if (blocker != null) {
1200                 blocker.postInterrupt();
1201             }
1202 
1203             // make available parking permit, unpark thread if parked
1204             unpark();
1205 
1206             // if thread is waiting in Object.wait then schedule to try to reenter
1207             int s = state();
1208             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1209                 submitRunContinuation();
1210             }
1211 
1212         } else {
1213             interrupted = true;
1214             carrierThread.setInterrupt();
1215             setParkPermit(true);
1216         }
1217     }
1218 
1219     @Override
1220     public boolean isInterrupted() {
1221         return interrupted;
1222     }
1223 
1224     @Override
1225     boolean getAndClearInterrupt() {
1226         assert Thread.currentThread() == this;
1227         boolean oldValue = interrupted;
1228         if (oldValue) {
1229             disableSuspendAndPreempt();
1230             try {
1231                 synchronized (interruptLock) {
1232                     interrupted = false;
1233                     carrierThread.clearInterrupt();
1234                 }
1235             } finally {
1236                 enableSuspendAndPreempt();
1237             }
1238         }
1239         return oldValue;
1240     }
1241 
1242     @Override
1243     Thread.State threadState() {
1244         switch (state()) {

1245             case NEW:
1246                 return Thread.State.NEW;
1247             case STARTED:
1248                 // return NEW if thread container not yet set
1249                 if (threadContainer() == null) {
1250                     return Thread.State.NEW;
1251                 } else {
1252                     return Thread.State.RUNNABLE;
1253                 }
1254             case UNPARKED:
1255             case UNBLOCKED:
1256             case YIELDED:
1257                 // runnable, not mounted
1258                 return Thread.State.RUNNABLE;
1259             case RUNNING:
1260                 // if mounted then return state of carrier thread
1261                 if (Thread.currentThread() != this) {
1262                     disableSuspendAndPreempt();
1263                     try {
1264                         synchronized (carrierThreadAccessLock()) {
1265                             Thread carrierThread = this.carrierThread;
1266                             if (carrierThread != null) {
1267                                 return carrierThread.threadState();
1268                             }
1269                         }
1270                     } finally {
1271                         enableSuspendAndPreempt();
1272                     }
1273                 }
1274                 // runnable, mounted
1275                 return Thread.State.RUNNABLE;
1276             case PARKING:
1277             case TIMED_PARKING:
1278             case WAITING:
1279             case TIMED_WAITING:
1280             case YIELDING:
1281                 // runnable, in transition
1282                 return Thread.State.RUNNABLE;
1283             case PARKED:
1284             case PINNED:
1285             case WAIT:
1286                 return Thread.State.WAITING;
1287             case TIMED_PARKED:
1288             case TIMED_PINNED:
1289             case TIMED_WAIT:
1290                 return Thread.State.TIMED_WAITING;
1291             case BLOCKING:
1292             case BLOCKED:
1293                 return Thread.State.BLOCKED;
1294             case TERMINATED:
1295                 return Thread.State.TERMINATED;
1296             default:
1297                 throw new InternalError();
1298         }
1299     }
1300 
1301     @Override
1302     boolean alive() {
1303         int s = state;
1304         return (s != NEW && s != TERMINATED);
1305     }
1306 
1307     @Override
1308     boolean isTerminated() {
1309         return (state == TERMINATED);
1310     }
1311 















































































1312     @Override
1313     public String toString() {
1314         StringBuilder sb = new StringBuilder("VirtualThread[#");
1315         sb.append(threadId());
1316         String name = getName();
1317         if (!name.isEmpty()) {
1318             sb.append(",");
1319             sb.append(name);
1320         }
1321         sb.append("]/");
1322 
1323         // add the carrier state and thread name when mounted
1324         boolean mounted;
1325         if (Thread.currentThread() == this) {
1326             mounted = appendCarrierInfo(sb);
1327         } else {
1328             disableSuspendAndPreempt();
1329             try {
1330                 synchronized (carrierThreadAccessLock()) {
1331                     mounted = appendCarrierInfo(sb);
1332                 }
1333             } finally {
1334                 enableSuspendAndPreempt();
1335             }
1336         }
1337 
1338         // add virtual thread state when not mounted
1339         if (!mounted) {
1340             String stateAsString = threadState().toString();
1341             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1342         }
1343 
1344         return sb.toString();
1345     }
1346 
1347     /**
1348      * Appends the carrier state and thread name to the string buffer if mounted.
1349      * @return true if mounted, false if not mounted
1350      */
1351     private boolean appendCarrierInfo(StringBuilder sb) {
1352         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1353         Thread carrier = carrierThread;
1354         if (carrier != null) {
1355             String stateAsString = carrier.threadState().toString();
1356             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1357             sb.append('@');
1358             sb.append(carrier.getName());
1359             return true;
1360         } else {
1361             return false;
1362         }
1363     }
1364 
1365     @Override
1366     public int hashCode() {
1367         return (int) threadId();
1368     }
1369 
1370     @Override
1371     public boolean equals(Object obj) {
1372         return obj == this;
1373     }
1374 
1375     /**
1376      * Returns the termination object, creating it if needed.
1377      */
1378     private CountDownLatch getTermination() {
1379         CountDownLatch termination = this.termination;
1380         if (termination == null) {
1381             termination = new CountDownLatch(1);
1382             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1383                 termination = this.termination;
1384             }
1385         }
1386         return termination;
1387     }
1388 
1389     /**
1390      * Returns the lock object to synchronize on when accessing carrierThread.
1391      * The lock prevents carrierThread from being reset to null during unmount.
1392      */
1393     private Object carrierThreadAccessLock() {
1394         // return interruptLock as unmount has to coordinate with interrupt
1395         return interruptLock;
1396     }
1397 
1398     /**
1399      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1400      */
1401     private Object timedWaitLock() {
1402         // use this object for now to avoid the overhead of introducing another lock
1403         return runContinuation;
1404     }
1405 
1406     /**
1407      * Disallow the current thread be suspended or preempted.
1408      */
1409     private void disableSuspendAndPreempt() {
1410         notifyJvmtiDisableSuspend(true);
1411         Continuation.pin();
1412     }
1413 
1414     /**
1415      * Allow the current thread be suspended or preempted.
1416      */
1417     private void enableSuspendAndPreempt() {
1418         Continuation.unpin();
1419         notifyJvmtiDisableSuspend(false);
1420     }
1421 
1422     // -- wrappers for get/set of state, parking permit, and carrier thread --
1423 
1424     private int state() {
1425         return state;  // volatile read
1426     }
1427 
1428     private void setState(int newValue) {
1429         state = newValue;  // volatile write
1430     }
1431 
1432     private boolean compareAndSetState(int expectedValue, int newValue) {
1433         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1434     }
1435 
1436     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1437         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1438     }
1439 
1440     private void setParkPermit(boolean newValue) {
1441         if (parkPermit != newValue) {
1442             parkPermit = newValue;
1443         }
1444     }
1445 
1446     private boolean getAndSetParkPermit(boolean newValue) {
1447         if (parkPermit != newValue) {
1448             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1449         } else {
1450             return newValue;
1451         }
1452     }
1453 
1454     private void setCarrierThread(Thread carrier) {
1455         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1456         this.carrierThread = carrier;
1457     }
1458 
1459     // The following four methods notify the VM when a "transition" starts and ends.
1460     // A "mount transition" embodies the steps to transfer control from a platform
1461     // thread to a virtual thread, changing the thread identity, and starting or
1462     // resuming the virtual thread's continuation on the carrier.
1463     // An "unmount transition" embodies the steps to transfer control from a virtual
1464     // thread to its carrier, suspending the virtual thread's continuation, and
1465     // restoring the thread identity to the platform thread.
1466     // The notifications to the VM are necessary in order to coordinate with functions
1467     // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1468     // a transition may block if transitions are disabled. Ending a transition may
1469     // notify a thread that is waiting to disable transitions. The notifications are
1470     // also used to post JVMTI events for virtual thread start and end.
1471 
1472     @IntrinsicCandidate
1473     @JvmtiMountTransition
1474     private native void endFirstTransition();
1475 
1476     @IntrinsicCandidate
1477     @JvmtiMountTransition
1478     private native void startFinalTransition();
1479 
1480     @IntrinsicCandidate
1481     @JvmtiMountTransition
1482     private native void startTransition(boolean mount);
1483 
1484     @IntrinsicCandidate
1485     @JvmtiMountTransition
1486     private native void endTransition(boolean mount);
1487 
1488     @IntrinsicCandidate
1489     private static native void notifyJvmtiDisableSuspend(boolean enter);
1490 
1491     private static native void registerNatives();
1492     static {
1493         registerNatives();
1494 
1495         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1496         var group = Thread.virtualThreadGroup();
1497 
1498         // ensure event class is initialized
1499         try {
1500             MethodHandles.lookup().ensureInitialized(VirtualThreadParkEvent.class);
1501         } catch (IllegalAccessException e) {
1502             throw new ExceptionInInitializerError(e);
1503         }
1504     }
1505 
1506     /**
1507      * Loads a VirtualThreadScheduler with the given class name. The class must be public
1508      * in an exported package, with public one-arg or no-arg constructor, and be visible
1509      * to the system class loader.
1510      * @param delegate the scheduler that the custom scheduler may delegate to
1511      * @param cn the class name of the custom scheduler
1512      */
1513     private static VirtualThreadScheduler loadCustomScheduler(VirtualThreadScheduler delegate, String cn) {
1514         VirtualThreadScheduler scheduler;
1515         try {
1516             Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1517             // 1-arg constructor
1518             try {
1519                 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1520                 return (VirtualThreadScheduler) ctor.newInstance(delegate);
1521             } catch (NoSuchMethodException e) {
1522                 // 0-arg constructor
1523                 Constructor<?> ctor = clazz.getConstructor();
1524                 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1525             }
1526         } catch (Exception ex) {
1527             throw new Error(ex);
1528         }
1529         System.err.println("WARNING: Using custom default scheduler, this is an experimental feature!");
1530         return scheduler;
1531     }
1532 
1533     /**
1534      * Creates the built-in ForkJoinPool scheduler.
1535      * @param wrapped true if wrapped by a custom default scheduler
1536      */
1537     private static BuiltinScheduler createBuiltinScheduler(boolean wrapped) {
1538         int parallelism, maxPoolSize, minRunnable;
1539         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1540         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1541         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1542         if (parallelismValue != null) {
1543             parallelism = Integer.parseInt(parallelismValue);
1544         } else {
1545             parallelism = Runtime.getRuntime().availableProcessors();
1546         }
1547         if (maxPoolSizeValue != null) {
1548             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1549             parallelism = Integer.min(parallelism, maxPoolSize);
1550         } else {
1551             maxPoolSize = Integer.max(parallelism, 256);
1552         }
1553         if (minRunnableValue != null) {
1554             minRunnable = Integer.parseInt(minRunnableValue);
1555         } else {
1556             minRunnable = Integer.max(parallelism / 2, 1);
1557         }
1558         return new BuiltinScheduler(parallelism, maxPoolSize, minRunnable, wrapped);
1559     }
1560 
1561     /**
1562      * The built-in ForkJoinPool scheduler.
1563      */
1564     private static class BuiltinScheduler
1565             extends ForkJoinPool implements VirtualThreadScheduler {
1566 
1567         BuiltinScheduler(int parallelism, int maxPoolSize, int minRunnable, boolean wrapped) {
1568             ForkJoinWorkerThreadFactory factory = wrapped
1569                     ? ForkJoinPool.defaultForkJoinWorkerThreadFactory
1570                     : CarrierThread::new;
1571             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1572             boolean asyncMode = true; // FIFO
1573             super(parallelism, factory, handler, asyncMode,
1574                     0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1575         }
1576 
1577         private void adaptAndExecute(Runnable task) {
1578             execute(ForkJoinTask.adapt(task));
1579         }
1580 
1581         @Override
1582         public void onStart(VirtualThreadTask task) {
1583             adaptAndExecute(task);
1584         }
1585 
1586         @Override
1587         public void onContinue(VirtualThreadTask task) {
1588             adaptAndExecute(task);
1589         }
1590 
1591         @Override
1592         public ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit) {
1593             return super.schedule(task, delay, unit);
1594         }
1595 
1596         /**
1597          * Wraps the scheduler to avoid leaking a direct reference with
1598          * {@link VirtualThreadScheduler#current()}.
1599          */
1600         VirtualThreadScheduler createExternalView() {
1601             BuiltinScheduler builtin = this;
1602             return new VirtualThreadScheduler() {
1603                 private void execute(VirtualThreadTask task) {
1604                     var vthread = (VirtualThread) task.thread();
1605                     VirtualThreadScheduler scheduler = vthread.scheduler;
1606                     if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1607                         builtin.adaptAndExecute(task);
1608                     } else {
1609                         throw new IllegalArgumentException();
1610                     }
1611                 }
1612                 @Override
1613                 public void onStart(VirtualThreadTask task) {
1614                     execute(task);
1615                 }
1616                 @Override
1617                 public void onContinue(VirtualThreadTask task) {
1618                     execute(task);
1619                 }
1620                 @Override
1621                 public String toString() {
1622                     return builtin.toString();
1623                 }
1624             };
1625         }
1626     }
1627 
1628     /**
1629      * Schedule a runnable task to run after a delay.
1630      */
1631     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1632         return scheduler.schedule(command, delay, unit);




1633     }
1634 
1635     /**
1636      * Supports scheduling a runnable task to run after a delay. It uses a number
1637      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1638      * work queue used. This class is used when using a custom scheduler.
1639      */
1640     static class DelayedTaskSchedulers {
1641         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1642 
1643         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1644             long tid = Thread.currentThread().threadId();
1645             int index = (int) tid & (INSTANCE.length - 1);
1646             return INSTANCE[index].schedule(command, delay, unit);
1647         }
1648 
1649         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1650             String propName = "jdk.virtualThreadScheduler.timerQueues";
1651             String propValue = System.getProperty(propName);
1652             int queueCount;
1653             if (propValue != null) {
1654                 queueCount = Integer.parseInt(propValue);
1655                 if (queueCount != Integer.highestOneBit(queueCount)) {
1656                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1657                 }
1658             } else {
1659                 int ncpus = Runtime.getRuntime().availableProcessors();
1660                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1661             }
1662             var schedulers = new ScheduledExecutorService[queueCount];
1663             for (int i = 0; i < queueCount; i++) {
1664                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1665                     Executors.newScheduledThreadPool(1, task -> {
1666                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1667                         t.setDaemon(true);
1668                         return t;
1669                     });
1670                 stpe.setRemoveOnCancelPolicy(true);
1671                 schedulers[i] = stpe;
1672             }
1673             return schedulers;
1674         }
1675     }
1676 
1677     /**
1678      * Schedule virtual threads that are ready to be scheduled after they blocked on
1679      * monitor enter.
1680      */
1681     private static void unblockVirtualThreads() {
1682         while (true) {
1683             VirtualThread vthread = takeVirtualThreadListToUnblock();
1684             while (vthread != null) {
1685                 assert vthread.onWaitingList;
1686                 VirtualThread nextThread = vthread.next;
1687 
1688                 // remove from list and unblock
1689                 vthread.next = null;
1690                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1691                 assert changed;
1692                 vthread.unblock();
1693 
1694                 vthread = nextThread;
1695             }
1696         }
1697     }
1698 
1699     /**
1700      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1701      * if necessary until a list of one or more threads becomes available.
1702      */
1703     private static native VirtualThread takeVirtualThreadListToUnblock();
1704 
1705     static {
1706         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1707                 VirtualThread::unblockVirtualThreads);
1708         unblocker.setDaemon(true);
1709         unblocker.start();
1710     }
1711 }
--- EOF ---