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.LinkedTransferQueue;
  38 import java.util.concurrent.RejectedExecutionException;
  39 import java.util.concurrent.ScheduledExecutorService;
  40 import java.util.concurrent.ScheduledFuture;
  41 import java.util.concurrent.ScheduledThreadPoolExecutor;
  42 import java.util.concurrent.ThreadFactory;
  43 import java.util.concurrent.ThreadPoolExecutor;
  44 import java.util.concurrent.TimeUnit;
  45 import jdk.internal.event.VirtualThreadEndEvent;
  46 import jdk.internal.event.VirtualThreadParkEvent;
  47 import jdk.internal.event.VirtualThreadStartEvent;
  48 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  49 import jdk.internal.invoke.MhUtil;
  50 import jdk.internal.misc.CarrierThread;
  51 import jdk.internal.misc.InnocuousThread;
  52 import jdk.internal.misc.Unsafe;
  53 import jdk.internal.vm.Continuation;
  54 import jdk.internal.vm.ContinuationScope;
  55 import jdk.internal.vm.StackableScope;
  56 import jdk.internal.vm.ThreadContainer;
  57 import jdk.internal.vm.ThreadContainers;
  58 import jdk.internal.vm.annotation.ChangesCurrentThread;
  59 import jdk.internal.vm.annotation.Hidden;
  60 import jdk.internal.vm.annotation.IntrinsicCandidate;
  61 import jdk.internal.vm.annotation.JvmtiHideEvents;
  62 import jdk.internal.vm.annotation.JvmtiMountTransition;
  63 import jdk.internal.vm.annotation.ReservedStackAccess;
  64 import sun.nio.ch.Interruptible;
  65 import static java.util.concurrent.TimeUnit.*;
  66 
  67 /**
  68  * A thread that is scheduled by the Java virtual machine rather than the operating system.
  69  */
  70 final class VirtualThread extends BaseVirtualThread {
  71     private static final Unsafe U = Unsafe.getUnsafe();
  72     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  73 
  74     private static final VirtualThreadScheduler BUILTIN_SCHEDULER;
  75     private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
  76     private static final VirtualThreadScheduler EXTERNAL_VIEW;
  77     static {
  78         // experimental
  79         String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
  80         if (propValue != null) {
  81             VirtualThreadScheduler builtinScheduler = createBuiltinScheduler(true);
  82             VirtualThreadScheduler externalView = createExternalView(builtinScheduler);
  83             VirtualThreadScheduler defaultScheduler = loadCustomScheduler(externalView, propValue);
  84             BUILTIN_SCHEDULER = builtinScheduler;
  85             DEFAULT_SCHEDULER = defaultScheduler;
  86             EXTERNAL_VIEW = externalView;
  87         } else {
  88             var builtinScheduler = createBuiltinScheduler(false);
  89             BUILTIN_SCHEDULER = builtinScheduler;
  90             DEFAULT_SCHEDULER = builtinScheduler;
  91             EXTERNAL_VIEW = createExternalView(builtinScheduler);
  92         }
  93     }
  94 
  95     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  96     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  97     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  98     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  99     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
 100 
 101     // scheduler and continuation
 102     private final VirtualThreadScheduler scheduler;
 103     private final Continuation cont;
 104     private final VirtualThreadTask runContinuation;
 105 
 106     // virtual thread state, accessed by VM
 107     private volatile int state;
 108 
 109     /*
 110      * Virtual thread state transitions:
 111      *
 112      *      NEW -> STARTED         // Thread.start, schedule to run
 113      *  STARTED -> TERMINATED      // failed to start
 114      *  STARTED -> RUNNING         // first run
 115      *  RUNNING -> TERMINATED      // done
 116      *
 117      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
 118      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
 119      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
 120      * UNPARKED -> RUNNING         // continue execution after park
 121      *
 122      *  PARKING -> RUNNING         // cont.yield failed, need to park on carrier
 123      *  RUNNING -> PINNED          // park on carrier
 124      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
 125      *
 126      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
 127      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
 128      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 129      *
 130      * TIMED_PARKING -> RUNNING         // cont.yield failed, need to park on carrier
 131      *       RUNNING -> TIMED_PINNED    // park on carrier
 132      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 133      *
 134      *   RUNNING -> BLOCKING       // blocking on monitor enter
 135      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 136      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 137      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 138      *
 139      *   RUNNING -> WAITING        // transitional state during wait on monitor
 140      *   WAITING -> WAIT           // waiting on monitor
 141      *      WAIT -> BLOCKED        // notified, waiting to be unblocked by monitor owner
 142      *      WAIT -> UNBLOCKED      // interrupted
 143      *
 144      *       RUNNING -> TIMED_WAITING   // transition state during timed-waiting on monitor
 145      * TIMED_WAITING -> TIMED_WAIT      // timed-waiting on monitor
 146      *    TIMED_WAIT -> BLOCKED         // notified, waiting to be unblocked by monitor owner
 147      *    TIMED_WAIT -> UNBLOCKED       // timed-out/interrupted
 148      *
 149      *  RUNNING -> YIELDING        // Thread.yield
 150      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 151      * YIELDING -> RUNNING         // cont.yield failed
 152      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 153      */
 154     private static final int NEW      = 0;
 155     private static final int STARTED  = 1;
 156     private static final int RUNNING  = 2;     // runnable-mounted
 157 
 158     // untimed and timed parking
 159     private static final int PARKING       = 3;
 160     private static final int PARKED        = 4;     // unmounted
 161     private static final int PINNED        = 5;     // mounted
 162     private static final int TIMED_PARKING = 6;
 163     private static final int TIMED_PARKED  = 7;     // unmounted
 164     private static final int TIMED_PINNED  = 8;     // mounted
 165     private static final int UNPARKED      = 9;     // unmounted but runnable
 166 
 167     // Thread.yield
 168     private static final int YIELDING = 10;
 169     private static final int YIELDED  = 11;         // unmounted but runnable
 170 
 171     // monitor enter
 172     private static final int BLOCKING  = 12;
 173     private static final int BLOCKED   = 13;        // unmounted
 174     private static final int UNBLOCKED = 14;        // unmounted but runnable
 175 
 176     // monitor wait/timed-wait
 177     private static final int WAITING       = 15;
 178     private static final int WAIT          = 16;    // waiting in Object.wait
 179     private static final int TIMED_WAITING = 17;
 180     private static final int TIMED_WAIT    = 18;    // waiting in timed-Object.wait
 181 
 182     private static final int TERMINATED = 99;  // final state
 183 



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



 280         }

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













 444      * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
 445      * @throws RejectedExecutionException
 446      */
 447     private void submitRunContinuation(boolean retryOnOOME) {
 448         boolean done = false;
 449         while (!done) {
 450             try {
 451                 // Pin the continuation to prevent the virtual thread from unmounting
 452                 // when submitting a task. For the default scheduler this ensures that
 453                 // the carrier doesn't change when pushing a task. For other schedulers
 454                 // it avoids deadlock that could arise due to carriers and virtual
 455                 // threads contending for a lock.
 456                 if (currentThread().isVirtual()) {
 457                     Continuation.pin();
 458                     try {
 459                         scheduler.onContinue(runContinuation);
 460                     } finally {
 461                         Continuation.unpin();
 462                     }
 463                 } else {
 464                     scheduler.onContinue(runContinuation);
 465                 }
 466                 done = true;
 467             } catch (RejectedExecutionException ree) {
 468                 submitFailed(ree);
 469                 throw ree;
 470             } catch (OutOfMemoryError e) {
 471                 if (retryOnOOME) {
 472                     U.park(false, 100_000_000); // 100ms
 473                 } else {
 474                     throw e;
 475                 }
 476             }
 477         }
 478     }
 479 


















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

 502             try {
 503                 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
 504             } catch (RejectedExecutionException ree) {
 505                 submitFailed(ree);
 506                 throw ree;
 507             } catch (OutOfMemoryError e) {
 508                 submitRunContinuation();
 509             }
 510         } else {
 511             submitRunContinuation();
 512         }
 513     }
 514 
 515     /**
 516      * Invoked from a carrier thread to externally submit the runContinuation task to the
 517      * scheduler. If invoked by a thread for a custom scheduler, then it just submits the
 518      * task to the scheduler.
 519      * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
 520      * @throws RejectedExecutionException
 521      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 522      */
 523     private void externalSubmitRunContinuation() {
 524         assert !currentThread().isVirtual();
 525         if (currentThread() instanceof CarrierThread ct) {
 526             try {
 527                 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 528             } catch (RejectedExecutionException ree) {
 529                 submitFailed(ree);
 530                 throw ree;
 531             } catch (OutOfMemoryError e) {
 532                 submitRunContinuation();
 533             }
 534         } else {
 535             submitRunContinuation();
 536         }
 537     }
 538 
 539     /**
 540      * Invoked from Thread.start to externally submit the runContinuation task to the
 541      * scheduler. If this virtual thread is scheduled by the built-in scheduler,
 542      * and this method is called from a virtual thread scheduled by the built-in
 543      * scheduler, then it uses externalSubmit to ensure that the task is pushed to an
 544      * external submission queue rather than the local queue.
 545      * @throws RejectedExecutionException
 546      * @throws OutOfMemoryError
 547      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 548      */
 549     private void externalSubmitRunContinuationOrThrow() {
 550         try {
 551             if (currentThread().isVirtual()) {
 552                 // Pin the continuation to prevent the virtual thread from unmounting
 553                 // when submitting a task. This avoids deadlock that could arise due to
 554                 // carriers and virtual threads contending for a lock.
 555                 Continuation.pin();
 556                 try {
 557                     if (scheduler == BUILTIN_SCHEDULER
 558                             && currentCarrierThread() instanceof CarrierThread ct) {
 559                         ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
 560                     } else {
 561                         scheduler.onStart(runContinuation);
 562                     }
 563                 } finally {
 564                     Continuation.unpin();
 565                 }
 566             } else {
 567                 scheduler.onStart(runContinuation);
 568             }
 569         } catch (RejectedExecutionException ree) {
 570             submitFailed(ree);
 571             throw ree;
 572         }
 573     }
 574 
 575     /**
 576      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 577      */
 578     private void submitFailed(RejectedExecutionException ree) {
 579         var event = new VirtualThreadSubmitFailedEvent();
 580         if (event.isEnabled()) {
 581             event.javaThreadId = threadId();
 582             event.exceptionMessage = ree.getMessage();
 583             event.commit();
 584         }
 585     }
 586 
 587     /**
 588      * Runs a task in the context of this virtual thread.
 589      */
 590     private void run(Runnable task) {
 591         assert Thread.currentThread() == this && state == RUNNING;
 592 
 593         // emit JFR event if enabled
 594         if (VirtualThreadStartEvent.isTurnedOn()) {
 595             var event = new VirtualThreadStartEvent();
 596             event.javaThreadId = threadId();
 597             event.commit();
 598         }
 599 
 600         Object bindings = Thread.scopedValueBindings();
 601         try {
 602             runWith(bindings, task);
 603         } catch (Throwable exc) {
 604             dispatchUncaughtException(exc);
 605         } finally {
 606             // pop any remaining scopes from the stack, this may block
 607             StackableScope.popAll();
 608 
 609             // emit JFR event if enabled
 610             if (VirtualThreadEndEvent.isTurnedOn()) {
 611                 var event = new VirtualThreadEndEvent();
 612                 event.javaThreadId = threadId();
 613                 event.commit();
 614             }
 615         }
 616     }
 617 
 618     /**
 619      * Mounts this virtual thread onto the current platform thread. On
 620      * return, the current thread is the virtual thread.
 621      */
 622     @ChangesCurrentThread
 623     @ReservedStackAccess
 624     private void mount() {
 625         startTransition(/*mount*/true);
 626         // We assume following volatile accesses provide equivalent
 627         // of acquire ordering, otherwise we need U.loadFence() here.
 628 
 629         // sets the carrier thread
 630         Thread carrier = Thread.currentCarrierThread();
 631         setCarrierThread(carrier);
 632 
 633         // sync up carrier thread interrupted status if needed
 634         if (interrupted) {
 635             carrier.setInterrupt();
 636         } else if (carrier.isInterrupted()) {
 637             synchronized (interruptLock) {
 638                 // need to recheck interrupted status
 639                 if (!interrupted) {
 640                     carrier.clearInterrupt();
 641                 }
 642             }
 643         }
 644 
 645         // set Thread.currentThread() to return this virtual thread
 646         carrier.setCurrentThread(this);
 647     }
 648 
 649     /**
 650      * Unmounts this virtual thread from the carrier. On return, the
 651      * current thread is the current platform thread.
 652      */
 653     @ChangesCurrentThread
 654     @ReservedStackAccess
 655     private void unmount() {
 656         assert !Thread.holdsLock(interruptLock);
 657 
 658         // set Thread.currentThread() to return the platform thread
 659         Thread carrier = this.carrierThread;
 660         carrier.setCurrentThread(carrier);
 661 
 662         // break connection to carrier thread, synchronized with interrupt
 663         synchronized (interruptLock) {
 664             setCarrierThread(null);
 665         }
 666         carrier.clearInterrupt();
 667 
 668         // We assume previous volatile accesses provide equivalent
 669         // of release ordering, otherwise we need U.storeFence() here.
 670         endTransition(/*mount*/false);
 671     }
 672 
 673     /**
 674      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 675      * the continuation continues.
 676      */
 677     @Hidden
 678     private boolean yieldContinuation() {
 679         startTransition(/*mount*/false);
 680         try {
 681             return Continuation.yield(VTHREAD_SCOPE);
 682         } finally {
 683             endTransition(/*mount*/true);
 684         }
 685     }
 686 
 687     /**
 688      * Invoked in the context of the carrier thread after the Continuation yields when
 689      * parking, blocking on monitor enter, Object.wait, or Thread.yield.
 690      */
 691     private void afterYield() {
 692         assert carrierThread == null;
 693 
 694         // re-adjust parallelism if the virtual thread yielded when compensating
 695         if (currentThread() instanceof CarrierThread ct) {
 696             ct.endBlocking();
 697         }
 698 
 699         int s = state();
 700 
 701         // LockSupport.park/parkNanos
 702         if (s == PARKING || s == TIMED_PARKING) {
 703             int newState;
 704             if (s == PARKING) {
 705                 setState(newState = PARKED);
 706             } else {
 707                 // schedule unpark
 708                 long timeout = this.timeout;
 709                 assert timeout > 0;
 710                 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
 711                 setState(newState = TIMED_PARKED);
 712             }
 713 
 714             // may have been unparked while parking
 715             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 716                 // lazy submit if local queue is empty
 717                 lazySubmitRunContinuation();
 718             }
 719             return;
 720         }
 721 
 722         // Thread.yield
 723         if (s == YIELDING) {
 724             setState(YIELDED);
 725 
 726             // external submit if there are no tasks in the local task queue
 727             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 728                 externalSubmitRunContinuation();
 729             } else {
 730                 submitRunContinuation();
 731             }
 732             return;
 733         }
 734 
 735         // blocking on monitorenter
 736         if (s == BLOCKING) {
 737             setState(BLOCKED);
 738 
 739             // may have been unblocked while blocking
 740             if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 741                 // lazy submit if local queue is empty
 742                 lazySubmitRunContinuation();
 743             }
 744             return;
 745         }
 746 
 747         // Object.wait
 748         if (s == WAITING || s == TIMED_WAITING) {
 749             int newState;
 750             boolean blocked;
 751             boolean interruptible = interruptibleWait;
 752             if (s == WAITING) {
 753                 setState(newState = WAIT);
 754                 // may have been notified while in transition
 755                 blocked = notified && compareAndSetState(WAIT, BLOCKED);
 756             } else {
 757                 // For timed-wait, a timeout task is scheduled to execute. The timeout
 758                 // task will change the thread state to UNBLOCKED and submit the thread
 759                 // to the scheduler. A sequence number is used to ensure that the timeout
 760                 // task only unblocks the thread for this timed-wait. We synchronize with
 761                 // the timeout task to coordinate access to the sequence number and to
 762                 // ensure the timeout task doesn't execute until the thread has got to
 763                 // the TIMED_WAIT state.
 764                 long timeout = this.timeout;
 765                 assert timeout > 0;
 766                 synchronized (timedWaitLock()) {
 767                     byte seqNo = ++timedWaitSeqNo;
 768                     timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
 769                     setState(newState = TIMED_WAIT);
 770                     // May have been notified while in transition. This must be done while
 771                     // holding the monitor to avoid changing the state of a new timed wait call.
 772                     blocked = notified && compareAndSetState(TIMED_WAIT, BLOCKED);
 773                 }
 774             }
 775 
 776             if (blocked) {
 777                 // may have been unblocked already
 778                 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
 779                     lazySubmitRunContinuation();
 780                 }
 781             } else {
 782                 // may have been interrupted while in transition to wait state
 783                 if (interruptible && interrupted && compareAndSetState(newState, UNBLOCKED)) {
 784                     lazySubmitRunContinuation();
 785                 }
 786             }
 787             return;
 788         }
 789 
 790         assert false;
 791     }
 792 
 793     /**
 794      * Invoked after the continuation completes.
 795      */
 796     private void afterDone() {
 797         afterDone(true);
 798     }
 799 
 800     /**
 801      * Invoked after the continuation completes (or start failed). Sets the thread
 802      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 803      *
 804      * @param notifyContainer true if its container should be notified
 805      */
 806     private void afterDone(boolean notifyContainer) {
 807         assert carrierThread == null;
 808         setState(TERMINATED);
 809 
 810         // notify anyone waiting for this virtual thread to terminate
 811         CountDownLatch termination = this.termination;
 812         if (termination != null) {
 813             assert termination.getCount() == 1;
 814             termination.countDown();
 815         }
 816 
 817         // notify container
 818         if (notifyContainer) {
 819             threadContainer().remove(this);
 820         }
 821 
 822         // clear references to thread locals
 823         clearReferences();
 824     }
 825 
 826     /**
 827      * Schedules this {@code VirtualThread} to execute.
 828      *
 829      * @throws IllegalStateException if the container is shutdown or closed
 830      * @throws IllegalThreadStateException if the thread has already been started
 831      * @throws RejectedExecutionException if the scheduler cannot accept a task
 832      */
 833     @Override
 834     void start(ThreadContainer container) {
 835         if (!compareAndSetState(NEW, STARTED)) {
 836             throw new IllegalThreadStateException("Already started");
 837         }
 838 
 839         // bind thread to container
 840         assert threadContainer() == null;
 841         setThreadContainer(container);
 842 
 843         // start thread
 844         boolean addedToContainer = false;
 845         boolean started = false;
 846         try {
 847             container.add(this);  // may throw
 848             addedToContainer = true;
 849 
 850             // scoped values may be inherited
 851             inheritScopedValueBindings(container);
 852 
 853             // submit task to run thread, using externalSubmit if possible
 854             externalSubmitRunContinuationOrThrow();
 855             started = true;
 856         } finally {
 857             if (!started) {
 858                 afterDone(addedToContainer);
 859             }
 860         }
 861     }
 862 
 863     @Override
 864     public void start() {
 865         start(ThreadContainers.root());
 866     }
 867 
 868     @Override
 869     public void run() {
 870         // do nothing
 871     }
 872 
 873     /**
 874      * Parks until unparked or interrupted. If already unparked then the parking
 875      * permit is consumed and this method completes immediately (meaning it doesn't
 876      * yield). It also completes immediately if the interrupted status is set.
 877      */
 878     @Override
 879     void park() {
 880         assert Thread.currentThread() == this;
 881 
 882         // complete immediately if parking permit available or interrupted
 883         if (getAndSetParkPermit(false) || interrupted)
 884             return;
 885 
 886         // park the thread
 887         boolean yielded = false;
 888         long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 889         setState(PARKING);
 890         try {
 891             yielded = yieldContinuation();
 892         } catch (OutOfMemoryError e) {
 893             // park on carrier
 894         } finally {
 895             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 896             if (yielded) {
 897                 VirtualThreadParkEvent.offer(eventStartTime, Long.MIN_VALUE);
 898             } else {
 899                 assert state() == PARKING;
 900                 setState(RUNNING);
 901             }
 902         }
 903 
 904         // park on the carrier thread when pinned
 905         if (!yielded) {
 906             parkOnCarrierThread(false, 0);
 907         }
 908     }
 909 
 910     /**
 911      * Parks up to the given waiting time or until unparked or interrupted.
 912      * If already unparked then the parking permit is consumed and this method
 913      * completes immediately (meaning it doesn't yield). It also completes immediately
 914      * if the interrupted status is set or the waiting time is {@code <= 0}.
 915      *
 916      * @param nanos the maximum number of nanoseconds to wait.
 917      */
 918     @Override
 919     void parkNanos(long nanos) {
 920         assert Thread.currentThread() == this;
 921 
 922         // complete immediately if parking permit available or interrupted
 923         if (getAndSetParkPermit(false) || interrupted)
 924             return;
 925 
 926         // park the thread for the waiting time
 927         if (nanos > 0) {
 928             long startTime = System.nanoTime();
 929 
 930             // park the thread, afterYield will schedule the thread to unpark
 931             boolean yielded = false;
 932             long eventStartTime = VirtualThreadParkEvent.eventStartTime();
 933             timeout = nanos;
 934             setState(TIMED_PARKING);
 935             try {
 936                 yielded = yieldContinuation();
 937             } catch (OutOfMemoryError e) {
 938                 // park on carrier
 939             } finally {
 940                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 941                 if (yielded) {
 942                     VirtualThreadParkEvent.offer(eventStartTime, nanos);
 943                 } else {
 944                     assert state() == TIMED_PARKING;
 945                     setState(RUNNING);
 946                 }
 947             }
 948 
 949             // park on carrier thread for remaining time when pinned (or OOME)
 950             if (!yielded) {
 951                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 952                 parkOnCarrierThread(true, remainingNanos);
 953             }
 954         }
 955     }
 956 
 957     /**
 958      * Parks the current carrier thread up to the given waiting time or until
 959      * unparked or interrupted. If the virtual thread is interrupted then the
 960      * interrupted status will be propagated to the carrier thread.
 961      * @param timed true for a timed park, false for untimed
 962      * @param nanos the waiting time in nanoseconds
 963      */
 964     private void parkOnCarrierThread(boolean timed, long nanos) {
 965         assert state() == RUNNING;
 966 
 967         setState(timed ? TIMED_PINNED : PINNED);
 968         try {
 969             if (!parkPermit) {
 970                 if (!timed) {
 971                     U.park(false, 0);
 972                 } else if (nanos > 0) {
 973                     U.park(false, nanos);
 974                 }
 975             }
 976         } finally {
 977             setState(RUNNING);
 978         }
 979 
 980         // consume parking permit
 981         setParkPermit(false);
 982 
 983         // JFR jdk.VirtualThreadPinned event
 984         postPinnedEvent("LockSupport.park");
 985     }
 986 
 987     /**
 988      * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
 989      * Recording the event in the VM avoids having JFR event recorded in Java
 990      * with the same name, but different ID, to events recorded by the VM.
 991      */
 992     @Hidden
 993     private static native void postPinnedEvent(String op);
 994 
 995     /**
 996      * Re-enables this virtual thread for scheduling. If this virtual thread is parked
 997      * then its task is scheduled to continue, otherwise its next call to {@code park} or
 998      * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
 999      * @param lazySubmit to use lazySubmit if possible
1000      * @throws RejectedExecutionException if the scheduler cannot accept a task
1001      */
1002     private void unpark(boolean lazySubmit) {
1003         if (!getAndSetParkPermit(true) && currentThread() != this) {
1004             int s = state();
1005 
1006             // unparked while parked
1007             if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
1008                 if (lazySubmit) {
1009                     lazySubmitRunContinuation();
1010                 } else {
1011                     submitRunContinuation();
1012                 }
1013                 return;
1014             }
1015 
1016             // unparked while parked when pinned
1017             if (s == PINNED || s == TIMED_PINNED) {
1018                 // unpark carrier thread when pinned
1019                 disableSuspendAndPreempt();
1020                 try {
1021                     synchronized (carrierThreadAccessLock()) {
1022                         Thread carrier = carrierThread;
1023                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
1024                             U.unpark(carrier);
1025                         }
1026                     }
1027                 } finally {
1028                     enableSuspendAndPreempt();
1029                 }
1030                 return;
1031             }
1032         }
1033     }
1034 
1035     @Override
1036     void unpark() {
1037         unpark(false);
1038     }
1039 
1040     /**
1041      * Invoked by unblocker thread to unblock this virtual thread.
1042      */
1043     private void unblock() {
1044         assert !Thread.currentThread().isVirtual();
1045         blockPermit = true;
1046         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
1047             submitRunContinuation();
1048         }
1049     }
1050 
1051     /**
1052      * Invoked by FJP worker thread or STPE thread when park timeout expires.
1053      */
1054     private void parkTimeoutExpired() {
1055         assert !VirtualThread.currentThread().isVirtual();
1056         unpark(true);
1057     }
1058 
1059     /**
1060      * Invoked by FJP worker thread or STPE thread when wait timeout expires.
1061      * If the virtual thread is in timed-wait then this method will unblock the thread
1062      * and submit its task so that it continues and attempts to reenter the monitor.
1063      * This method does nothing if the thread has been woken by notify or interrupt.
1064      */
1065     private void waitTimeoutExpired(byte seqNo) {
1066         assert !Thread.currentThread().isVirtual();
1067 
1068         synchronized (timedWaitLock()) {
1069             if (seqNo != timedWaitSeqNo) {
1070                 // this timeout task is for a past timed-wait
1071                 return;









1072             }
1073             if (!compareAndSetState(TIMED_WAIT, UNBLOCKED)) {
1074                 // already unblocked
1075                 return;
1076             }


1077         }
1078 
1079         lazySubmitRunContinuation();
1080     }
1081 
1082     /**
1083      * Attempts to yield the current virtual thread (Thread.yield).
1084      */
1085     void tryYield() {
1086         assert Thread.currentThread() == this;
1087         setState(YIELDING);
1088         boolean yielded = false;
1089         try {
1090             yielded = yieldContinuation();  // may throw
1091         } finally {
1092             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
1093             if (!yielded) {
1094                 assert state() == YIELDING;
1095                 setState(RUNNING);
1096             }
1097         }
1098     }
1099 
1100     /**
1101      * Sleep the current thread for the given sleep time (in nanoseconds). If
1102      * nanos is 0 then the thread will attempt to yield.
1103      *
1104      * @implNote This implementation parks the thread for the given sleeping time
1105      * and will therefore be observed in PARKED state during the sleep. Parking
1106      * will consume the parking permit so this method makes available the parking
1107      * permit after the sleep. This may be observed as a spurious, but benign,
1108      * wakeup when the thread subsequently attempts to park.
1109      *
1110      * @param nanos the maximum number of nanoseconds to sleep
1111      * @throws InterruptedException if interrupted while sleeping
1112      */
1113     void sleepNanos(long nanos) throws InterruptedException {
1114         assert Thread.currentThread() == this && nanos >= 0;
1115         if (getAndClearInterrupt())
1116             throw new InterruptedException();
1117         if (nanos == 0) {
1118             tryYield();
1119         } else {
1120             // park for the sleep time
1121             try {
1122                 long remainingNanos = nanos;
1123                 long startNanos = System.nanoTime();
1124                 while (remainingNanos > 0) {
1125                     parkNanos(remainingNanos);
1126                     if (getAndClearInterrupt()) {
1127                         throw new InterruptedException();
1128                     }
1129                     remainingNanos = nanos - (System.nanoTime() - startNanos);
1130                 }
1131             } finally {
1132                 // may have been unparked while sleeping
1133                 setParkPermit(true);
1134             }
1135         }
1136     }
1137 
1138     /**
1139      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
1140      * A timeout of {@code 0} means to wait forever.
1141      *
1142      * @throws InterruptedException if interrupted while waiting
1143      * @return true if the thread has terminated
1144      */
1145     boolean joinNanos(long nanos) throws InterruptedException {
1146         if (state() == TERMINATED)
1147             return true;
1148 
1149         // ensure termination object exists, then re-check state
1150         CountDownLatch termination = getTermination();
1151         if (state() == TERMINATED)
1152             return true;
1153 
1154         // wait for virtual thread to terminate
1155         if (nanos == 0) {
1156             termination.await();
1157         } else {
1158             boolean terminated = termination.await(nanos, NANOSECONDS);
1159             if (!terminated) {
1160                 // waiting time elapsed
1161                 return false;
1162             }
1163         }
1164         assert state() == TERMINATED;
1165         return true;
1166     }
1167 
1168     @Override
1169     void blockedOn(Interruptible b) {
1170         disableSuspendAndPreempt();
1171         try {
1172             super.blockedOn(b);
1173         } finally {
1174             enableSuspendAndPreempt();
1175         }
1176     }
1177 
1178     @Override
1179     public void interrupt() {
1180         if (Thread.currentThread() != this) {
1181             // if current thread is a virtual thread then prevent it from being
1182             // suspended or unmounted when entering or holding interruptLock
1183             Interruptible blocker;
1184             disableSuspendAndPreempt();
1185             try {
1186                 synchronized (interruptLock) {
1187                     interrupted = true;
1188                     blocker = nioBlocker();
1189                     if (blocker != null) {
1190                         blocker.interrupt(this);
1191                     }
1192 
1193                     // interrupt carrier thread if mounted
1194                     Thread carrier = carrierThread;
1195                     if (carrier != null) carrier.setInterrupt();
1196                 }
1197             } finally {
1198                 enableSuspendAndPreempt();
1199             }
1200 
1201             // notify blocker after releasing interruptLock
1202             if (blocker != null) {
1203                 blocker.postInterrupt();
1204             }
1205 
1206             // make available parking permit, unpark thread if parked
1207             unpark();
1208 
1209             // if thread is waiting in Object.wait then schedule to try to reenter
1210             int s = state();
1211             if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1212                 submitRunContinuation();
1213             }
1214 
1215         } else {
1216             interrupted = true;
1217             carrierThread.setInterrupt();
1218             setParkPermit(true);
1219         }
1220     }
1221 
1222     @Override
1223     public boolean isInterrupted() {
1224         return interrupted;
1225     }
1226 
1227     @Override
1228     boolean getAndClearInterrupt() {
1229         assert Thread.currentThread() == this;
1230         boolean oldValue = interrupted;
1231         if (oldValue) {
1232             disableSuspendAndPreempt();
1233             try {
1234                 synchronized (interruptLock) {
1235                     interrupted = false;
1236                     carrierThread.clearInterrupt();
1237                 }
1238             } finally {
1239                 enableSuspendAndPreempt();
1240             }
1241         }
1242         return oldValue;
1243     }
1244 
1245     @Override
1246     Thread.State threadState() {
1247         switch (state()) {

1248             case NEW:
1249                 return Thread.State.NEW;
1250             case STARTED:
1251                 // return NEW if thread container not yet set
1252                 if (threadContainer() == null) {
1253                     return Thread.State.NEW;
1254                 } else {
1255                     return Thread.State.RUNNABLE;
1256                 }
1257             case UNPARKED:
1258             case UNBLOCKED:
1259             case YIELDED:
1260                 // runnable, not mounted
1261                 return Thread.State.RUNNABLE;
1262             case RUNNING:
1263                 // if mounted then return state of carrier thread
1264                 if (Thread.currentThread() != this) {
1265                     disableSuspendAndPreempt();
1266                     try {
1267                         synchronized (carrierThreadAccessLock()) {
1268                             Thread carrierThread = this.carrierThread;
1269                             if (carrierThread != null) {
1270                                 return carrierThread.threadState();
1271                             }
1272                         }
1273                     } finally {
1274                         enableSuspendAndPreempt();
1275                     }
1276                 }
1277                 // runnable, mounted
1278                 return Thread.State.RUNNABLE;
1279             case PARKING:
1280             case TIMED_PARKING:
1281             case WAITING:
1282             case TIMED_WAITING:
1283             case YIELDING:
1284                 // runnable, in transition
1285                 return Thread.State.RUNNABLE;
1286             case PARKED:
1287             case PINNED:
1288             case WAIT:
1289                 return Thread.State.WAITING;
1290             case TIMED_PARKED:
1291             case TIMED_PINNED:
1292             case TIMED_WAIT:
1293                 return Thread.State.TIMED_WAITING;
1294             case BLOCKING:
1295             case BLOCKED:
1296                 return Thread.State.BLOCKED;
1297             case TERMINATED:
1298                 return Thread.State.TERMINATED;
1299             default:
1300                 throw new InternalError();
1301         }
1302     }
1303 
1304     @Override
1305     boolean alive() {
1306         int s = state;
1307         return (s != NEW && s != TERMINATED);
1308     }
1309 
1310     @Override
1311     boolean isTerminated() {
1312         return (state == TERMINATED);
1313     }
1314 















































































1315     @Override
1316     public String toString() {
1317         StringBuilder sb = new StringBuilder("VirtualThread[#");
1318         sb.append(threadId());
1319         String name = getName();
1320         if (!name.isEmpty()) {
1321             sb.append(",");
1322             sb.append(name);
1323         }
1324         sb.append("]/");
1325 
1326         // add the carrier state and thread name when mounted
1327         boolean mounted;
1328         if (Thread.currentThread() == this) {
1329             mounted = appendCarrierInfo(sb);
1330         } else {
1331             disableSuspendAndPreempt();
1332             try {
1333                 synchronized (carrierThreadAccessLock()) {
1334                     mounted = appendCarrierInfo(sb);
1335                 }
1336             } finally {
1337                 enableSuspendAndPreempt();
1338             }
1339         }
1340 
1341         // add virtual thread state when not mounted
1342         if (!mounted) {
1343             String stateAsString = threadState().toString();
1344             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1345         }
1346 
1347         return sb.toString();
1348     }
1349 
1350     /**
1351      * Appends the carrier state and thread name to the string buffer if mounted.
1352      * @return true if mounted, false if not mounted
1353      */
1354     private boolean appendCarrierInfo(StringBuilder sb) {
1355         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1356         Thread carrier = carrierThread;
1357         if (carrier != null) {
1358             String stateAsString = carrier.threadState().toString();
1359             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1360             sb.append('@');
1361             sb.append(carrier.getName());
1362             return true;
1363         } else {
1364             return false;
1365         }
1366     }
1367 
1368     @Override
1369     public int hashCode() {
1370         return (int) threadId();
1371     }
1372 
1373     @Override
1374     public boolean equals(Object obj) {
1375         return obj == this;
1376     }
1377 
1378     /**
1379      * Returns the termination object, creating it if needed.
1380      */
1381     private CountDownLatch getTermination() {
1382         CountDownLatch termination = this.termination;
1383         if (termination == null) {
1384             termination = new CountDownLatch(1);
1385             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1386                 termination = this.termination;
1387             }
1388         }
1389         return termination;
1390     }
1391 
1392     /**
1393      * Returns the lock object to synchronize on when accessing carrierThread.
1394      * The lock prevents carrierThread from being reset to null during unmount.
1395      */
1396     private Object carrierThreadAccessLock() {
1397         // return interruptLock as unmount has to coordinate with interrupt
1398         return interruptLock;
1399     }
1400 
1401     /**
1402      * Returns a lock object for coordinating timed-wait setup and timeout handling.
1403      */
1404     private Object timedWaitLock() {
1405         // use this object for now to avoid the overhead of introducing another lock
1406         return runContinuation;
1407     }
1408 
1409     /**
1410      * Disallow the current thread be suspended or preempted.
1411      */
1412     private void disableSuspendAndPreempt() {
1413         notifyJvmtiDisableSuspend(true);
1414         Continuation.pin();
1415     }
1416 
1417     /**
1418      * Allow the current thread be suspended or preempted.
1419      */
1420     private void enableSuspendAndPreempt() {
1421         Continuation.unpin();
1422         notifyJvmtiDisableSuspend(false);
1423     }
1424 
1425     // -- wrappers for get/set of state, parking permit, and carrier thread --
1426 
1427     private int state() {
1428         return state;  // volatile read
1429     }
1430 
1431     private void setState(int newValue) {
1432         state = newValue;  // volatile write
1433     }
1434 
1435     private boolean compareAndSetState(int expectedValue, int newValue) {
1436         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1437     }
1438 
1439     private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1440         return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1441     }
1442 
1443     private void setParkPermit(boolean newValue) {
1444         if (parkPermit != newValue) {
1445             parkPermit = newValue;
1446         }
1447     }
1448 
1449     private boolean getAndSetParkPermit(boolean newValue) {
1450         if (parkPermit != newValue) {
1451             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1452         } else {
1453             return newValue;
1454         }
1455     }
1456 
1457     private void setCarrierThread(Thread carrier) {
1458         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1459         this.carrierThread = carrier;
1460     }
1461 
1462     // The following four methods notify the VM when a "transition" starts and ends.
1463     // A "mount transition" embodies the steps to transfer control from a platform
1464     // thread to a virtual thread, changing the thread identity, and starting or
1465     // resuming the virtual thread's continuation on the carrier.
1466     // An "unmount transition" embodies the steps to transfer control from a virtual
1467     // thread to its carrier, suspending the virtual thread's continuation, and
1468     // restoring the thread identity to the platform thread.
1469     // The notifications to the VM are necessary in order to coordinate with functions
1470     // (JVMTI mostly) that disable transitions for one or all virtual threads. Starting
1471     // a transition may block if transitions are disabled. Ending a transition may
1472     // notify a thread that is waiting to disable transitions. The notifications are
1473     // also used to post JVMTI events for virtual thread start and end.
1474 
1475     @IntrinsicCandidate
1476     @JvmtiMountTransition
1477     private native void endFirstTransition();
1478 
1479     @IntrinsicCandidate
1480     @JvmtiMountTransition
1481     private native void startFinalTransition();
1482 
1483     @IntrinsicCandidate
1484     @JvmtiMountTransition
1485     private native void startTransition(boolean mount);
1486 
1487     @IntrinsicCandidate
1488     @JvmtiMountTransition
1489     private native void endTransition(boolean mount);
1490 
1491     @IntrinsicCandidate
1492     private static native void notifyJvmtiDisableSuspend(boolean enter);
1493 
1494     private static native void registerNatives();
1495     static {
1496         registerNatives();
1497 
1498         // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1499         var group = Thread.virtualThreadGroup();
1500 
1501         // ensure event class is initialized
1502         try {
1503             MethodHandles.lookup().ensureInitialized(VirtualThreadParkEvent.class);
1504         } catch (IllegalAccessException e) {
1505             throw new ExceptionInInitializerError(e);
1506         }
1507     }
1508 
1509     /**
1510      * Loads a VirtualThreadScheduler with the given class name. The class must be public
1511      * in an exported package, with public one-arg or no-arg constructor, and be visible
1512      * to the system class loader.
1513      * @param delegate the scheduler that the custom scheduler may delegate to
1514      * @param cn the class name of the custom scheduler
1515      */
1516     private static VirtualThreadScheduler loadCustomScheduler(VirtualThreadScheduler delegate, String cn) {
1517         VirtualThreadScheduler scheduler;
1518         try {
1519             Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1520             // 1-arg constructor
1521             try {
1522                 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1523                 return (VirtualThreadScheduler) ctor.newInstance(delegate);
1524             } catch (NoSuchMethodException e) {
1525                 // 0-arg constructor
1526                 Constructor<?> ctor = clazz.getConstructor();
1527                 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1528             }
1529         } catch (Exception ex) {
1530             throw new Error(ex);
1531         }
1532         System.err.println("WARNING: Using custom default scheduler, this is an experimental feature!");
1533         return scheduler;
1534     }
1535 
1536     /**
1537      * Creates the built-in ForkJoinPool scheduler.
1538      * @param wrapped true if wrapped by a custom default scheduler
1539      */
1540     private static VirtualThreadScheduler createBuiltinScheduler(boolean wrapped) {
1541         int parallelism, maxPoolSize, minRunnable;
1542         String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1543         String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1544         String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1545         if (parallelismValue != null) {
1546             parallelism = Integer.parseInt(parallelismValue);
1547         } else {
1548             parallelism = Runtime.getRuntime().availableProcessors();
1549         }
1550         if (maxPoolSizeValue != null) {
1551             maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1552             parallelism = Integer.min(parallelism, maxPoolSize);
1553         } else {
1554             maxPoolSize = Integer.max(parallelism, 256);
1555         }
1556         if (minRunnableValue != null) {
1557             minRunnable = Integer.parseInt(minRunnableValue);
1558         } else {
1559             minRunnable = Integer.max(parallelism / 2, 1);
1560         }
1561         if (System.getProperty("jdk.virtualThreadScheduler.threadPoolExecutor") != null) {
1562             return new BuiltinThreadPoolExecutorScheduler(parallelism);
1563         } else {
1564             return new BuiltinForkJoinPoolScheduler(parallelism, maxPoolSize, minRunnable, wrapped);
1565         }
1566     }
1567 
1568     /**
1569      * The built-in ForkJoinPool scheduler.
1570      */
1571     private static class BuiltinForkJoinPoolScheduler
1572             extends ForkJoinPool implements VirtualThreadScheduler {
1573 
1574         BuiltinForkJoinPoolScheduler(int parallelism, int maxPoolSize, int minRunnable, boolean wrapped) {
1575             ForkJoinWorkerThreadFactory factory = wrapped
1576                     ? ForkJoinPool.defaultForkJoinWorkerThreadFactory
1577                     : CarrierThread::new;
1578             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1579             boolean asyncMode = true; // FIFO
1580             super(parallelism, factory, handler, asyncMode,
1581                     0, maxPoolSize, minRunnable, pool -> true, 30L, SECONDS);
1582         }
1583 
1584         private void adaptAndExecute(Runnable task) {
1585             execute(ForkJoinTask.adapt(task));
1586         }
1587 
1588         @Override
1589         public void onStart(VirtualThreadTask task) {
1590             adaptAndExecute(task);
1591         }
1592 
1593         @Override
1594         public void onContinue(VirtualThreadTask task) {
1595             adaptAndExecute(task);
1596         }
1597 
1598         @Override
1599         public ScheduledFuture<?> schedule(Runnable task, long delay, TimeUnit unit) {
1600             return super.schedule(task, delay, unit);
1601         }
1602     }
1603 
1604     /**
1605      * Built-in ThreadPoolExecutor scheduler.
1606      */
1607     private static class BuiltinThreadPoolExecutorScheduler
1608             extends ThreadPoolExecutor implements VirtualThreadScheduler {
1609 
1610         BuiltinThreadPoolExecutorScheduler(int maxPoolSize) {
1611             ThreadFactory factory = task -> InnocuousThread.newThread(task);
1612             super(maxPoolSize, maxPoolSize,
1613                     0L, SECONDS,
1614                     new LinkedTransferQueue<>(),
1615                     factory);
1616         }
1617 
1618         @Override
1619         public void onStart(VirtualThreadTask task) {
1620             execute(task);
1621         }
1622 
1623         @Override
1624         public void onContinue(VirtualThreadTask task) {
1625             execute(task);
1626         }
1627     }
1628 
1629     /**
1630      * Wraps the scheduler to avoid leaking a direct reference with
1631      * {@link VirtualThreadScheduler#current()}.
1632      */
1633     static VirtualThreadScheduler createExternalView(VirtualThreadScheduler delegate) {
1634         return new VirtualThreadScheduler() {
1635             private void check(VirtualThreadTask task) {
1636                 var vthread = (VirtualThread) task.thread();
1637                 VirtualThreadScheduler scheduler = vthread.scheduler;
1638                 if (scheduler != this && scheduler != DEFAULT_SCHEDULER) {
1639                     throw new IllegalArgumentException();
1640                 }
1641             }
1642             @Override
1643             public void onStart(VirtualThreadTask task) {
1644                 check(task);
1645                 delegate.onStart(task);
1646             }
1647             @Override
1648             public void onContinue(VirtualThreadTask task) {
1649                 check(task);
1650                 delegate.onContinue(task);
1651             }
1652             @Override
1653             public String toString() {
1654                 return delegate.toString();
1655             }
1656         };
1657     }
1658 
1659     /**
1660      * Schedule a runnable task to run after a delay.
1661      */
1662     private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1663         return scheduler.schedule(command, delay, unit);




1664     }
1665 
1666     /**
1667      * Supports scheduling a runnable task to run after a delay. It uses a number
1668      * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1669      * work queue used. This class is used when using a custom scheduler.
1670      */
1671     static class DelayedTaskSchedulers {
1672         private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1673 
1674         static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1675             long tid = Thread.currentThread().threadId();
1676             int index = (int) tid & (INSTANCE.length - 1);
1677             return INSTANCE[index].schedule(command, delay, unit);
1678         }
1679 
1680         private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1681             String propName = "jdk.virtualThreadScheduler.timerQueues";
1682             String propValue = System.getProperty(propName);
1683             int queueCount;
1684             if (propValue != null) {
1685                 queueCount = Integer.parseInt(propValue);
1686                 if (queueCount != Integer.highestOneBit(queueCount)) {
1687                     throw new RuntimeException("Value of " + propName + " must be power of 2");
1688                 }
1689             } else {
1690                 int ncpus = Runtime.getRuntime().availableProcessors();
1691                 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1692             }
1693             var schedulers = new ScheduledExecutorService[queueCount];
1694             for (int i = 0; i < queueCount; i++) {
1695                 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1696                     Executors.newScheduledThreadPool(1, task -> {
1697                         Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1698                         t.setDaemon(true);
1699                         return t;
1700                     });
1701                 stpe.setRemoveOnCancelPolicy(true);
1702                 schedulers[i] = stpe;
1703             }
1704             return schedulers;
1705         }
1706     }
1707 
1708     /**
1709      * Schedule virtual threads that are ready to be scheduled after they blocked on
1710      * monitor enter.
1711      */
1712     private static void unblockVirtualThreads() {
1713         while (true) {
1714             VirtualThread vthread = takeVirtualThreadListToUnblock();
1715             while (vthread != null) {
1716                 assert vthread.onWaitingList;
1717                 VirtualThread nextThread = vthread.next;
1718 
1719                 // remove from list and unblock
1720                 vthread.next = null;
1721                 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1722                 assert changed;
1723                 vthread.unblock();
1724 
1725                 vthread = nextThread;
1726             }
1727         }
1728     }
1729 
1730     /**
1731      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1732      * if necessary until a list of one or more threads becomes available.
1733      */
1734     private static native VirtualThread takeVirtualThreadListToUnblock();
1735 
1736     static {
1737         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1738                 VirtualThread::unblockVirtualThreads);
1739         unblocker.setDaemon(true);
1740         unblocker.start();
1741     }
1742 }
--- EOF ---