7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.util.Locale;
28 import java.util.Objects;
29 import java.util.concurrent.CountDownLatch;
30 import java.util.concurrent.Executor;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.Future;
36 import java.util.concurrent.RejectedExecutionException;
37 import java.util.concurrent.ScheduledExecutorService;
38 import java.util.concurrent.ScheduledThreadPoolExecutor;
39 import java.util.concurrent.TimeUnit;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66 private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
67
68 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
69 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
70 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
71 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
72 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
73
74 // scheduler and continuation
75 private final Executor scheduler;
76 private final Continuation cont;
77 private final Runnable runContinuation;
78
79 // virtual thread state, accessed by VM
80 private volatile int state;
81
82 /*
83 * Virtual thread state transitions:
84 *
85 * NEW -> STARTED // Thread.start, schedule to run
86 * STARTED -> TERMINATED // failed to start
87 * STARTED -> RUNNING // first run
88 * RUNNING -> TERMINATED // done
89 *
90 * RUNNING -> PARKING // Thread parking with LockSupport.park
91 * PARKING -> PARKED // cont.yield successful, parked indefinitely
92 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
93 * PARKED -> UNPARKED // unparked, may be scheduled to continue
94 * PINNED -> RUNNING // unparked, continue execution on same carrier
95 * UNPARKED -> RUNNING // continue execution after park
151 private static final int TERMINATED = 99; // final state
152
153 // can be suspended from scheduling when unmounted
154 private static final int SUSPENDED = 1 << 8;
155
156 // parking permit made available by LockSupport.unpark
157 private volatile boolean parkPermit;
158
159 // blocking permit made available by unblocker thread when another thread exits monitor
160 private volatile boolean blockPermit;
161
162 // true when on the list of virtual threads waiting to be unblocked
163 private volatile boolean onWaitingList;
164
165 // next virtual thread on the list of virtual threads waiting to be unblocked
166 private volatile VirtualThread next;
167
168 // notified by Object.notify/notifyAll while waiting in Object.wait
169 private volatile boolean notified;
170
171 // timed-wait support
172 private byte timedWaitSeqNo;
173
174 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
175 private long timeout;
176
177 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
178 private Future<?> timeoutTask;
179
180 // carrier thread when mounted, accessed by VM
181 private volatile Thread carrierThread;
182
183 // termination object when joining, created lazily if needed
184 private volatile CountDownLatch termination;
185
186 /**
187 * Returns the default scheduler.
188 */
189 static Executor defaultScheduler() {
190 return DEFAULT_SCHEDULER;
191 }
192
193 /**
194 * Returns the continuation scope used for virtual threads.
195 */
196 static ContinuationScope continuationScope() {
197 return VTHREAD_SCOPE;
198 }
199
200 /**
201 * Creates a new {@code VirtualThread} to run the given task with the given
202 * scheduler. If the given scheduler is {@code null} and the current thread
203 * is a platform thread then the newly created virtual thread will use the
204 * default scheduler. If given scheduler is {@code null} and the current
205 * thread is a virtual thread then the current thread's scheduler is used.
206 *
207 * @param scheduler the scheduler or null
208 * @param name thread name
209 * @param characteristics characteristics
210 * @param task the task to execute
211 */
212 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
213 super(name, characteristics, /*bound*/ false);
214 Objects.requireNonNull(task);
215
216 // choose scheduler if not specified
217 if (scheduler == null) {
218 Thread parent = Thread.currentThread();
219 if (parent instanceof VirtualThread vparent) {
220 scheduler = vparent.scheduler;
221 } else {
222 scheduler = DEFAULT_SCHEDULER;
223 }
224 }
225
226 this.scheduler = scheduler;
227 this.cont = new VThreadContinuation(this, task);
228 this.runContinuation = this::runContinuation;
229 }
230
231 /**
232 * The continuation that a virtual thread executes.
233 */
234 private static class VThreadContinuation extends Continuation {
235 VThreadContinuation(VirtualThread vthread, Runnable task) {
236 super(VTHREAD_SCOPE, wrap(vthread, task));
237 }
238 @Override
239 protected void onPinned(Continuation.Pinned reason) {
240 }
241 private static Runnable wrap(VirtualThread vthread, Runnable task) {
242 return new Runnable() {
243 @Hidden
299 afterYield();
300 }
301 }
302 }
303
304 /**
305 * Cancel timeout task when continuing after timed-park or timed-wait.
306 * The timeout task may be executing, or may have already completed.
307 */
308 private void cancelTimeoutTask() {
309 if (timeoutTask != null) {
310 timeoutTask.cancel(false);
311 timeoutTask = null;
312 }
313 }
314
315 /**
316 * Submits the runContinuation task to the scheduler. For the default scheduler,
317 * and calling it on a worker thread, the task will be pushed to the local queue,
318 * otherwise it will be pushed to an external submission queue.
319 * @param scheduler the scheduler
320 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
321 * @throws RejectedExecutionException
322 */
323 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
324 boolean done = false;
325 while (!done) {
326 try {
327 // Pin the continuation to prevent the virtual thread from unmounting
328 // when submitting a task. For the default scheduler this ensures that
329 // the carrier doesn't change when pushing a task. For other schedulers
330 // it avoids deadlock that could arise due to carriers and virtual
331 // threads contending for a lock.
332 if (currentThread().isVirtual()) {
333 Continuation.pin();
334 try {
335 scheduler.execute(runContinuation);
336 } finally {
337 Continuation.unpin();
338 }
339 } else {
340 scheduler.execute(runContinuation);
341 }
342 done = true;
343 } catch (RejectedExecutionException ree) {
344 submitFailed(ree);
345 throw ree;
346 } catch (OutOfMemoryError e) {
347 if (retryOnOOME) {
348 U.park(false, 100_000_000); // 100ms
349 } else {
350 throw e;
351 }
352 }
353 }
354 }
355
356 /**
357 * Submits the runContinuation task to the given scheduler as an external submit.
358 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
359 * @throws RejectedExecutionException
360 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
361 */
362 private void externalSubmitRunContinuation(ForkJoinPool pool) {
363 assert Thread.currentThread() instanceof CarrierThread;
364 try {
365 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
366 } catch (RejectedExecutionException ree) {
367 submitFailed(ree);
368 throw ree;
369 } catch (OutOfMemoryError e) {
370 submitRunContinuation(pool, true);
371 }
372 }
373
374 /**
375 * Submits the runContinuation task to the scheduler. For the default scheduler,
376 * and calling it on a worker thread, the task will be pushed to the local queue,
377 * otherwise it will be pushed to an external submission queue.
378 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
379 * @throws RejectedExecutionException
380 */
381 private void submitRunContinuation() {
382 submitRunContinuation(scheduler, true);
383 }
384
385 /**
386 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
387 * queue is empty. If not empty, or invoked by another thread, then this method works
388 * like submitRunContinuation and just submits the task to the scheduler.
389 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
390 * @throws RejectedExecutionException
391 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
392 */
393 private void lazySubmitRunContinuation() {
394 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
395 ForkJoinPool pool = ct.getPool();
396 try {
397 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
398 } catch (RejectedExecutionException ree) {
399 submitFailed(ree);
400 throw ree;
401 } catch (OutOfMemoryError e) {
402 submitRunContinuation();
403 }
404 } else {
405 submitRunContinuation();
406 }
407 }
408
409 /**
410 * Submits the runContinuation task to the scheduler. For the default scheduler, and
411 * calling it a virtual thread that uses the default scheduler, the task will be
412 * pushed to an external submission queue. This method may throw OutOfMemoryError.
413 * @throws RejectedExecutionException
414 * @throws OutOfMemoryError
415 */
416 private void externalSubmitRunContinuationOrThrow() {
417 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
418 try {
419 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
420 } catch (RejectedExecutionException ree) {
421 submitFailed(ree);
422 throw ree;
423 }
424 } else {
425 submitRunContinuation(scheduler, false);
426 }
427 }
428
429 /**
430 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
431 */
432 private void submitFailed(RejectedExecutionException ree) {
433 var event = new VirtualThreadSubmitFailedEvent();
434 if (event.isEnabled()) {
435 event.javaThreadId = threadId();
436 event.exceptionMessage = ree.getMessage();
437 event.commit();
438 }
439 }
440
441 /**
442 * Runs a task in the context of this virtual thread.
443 */
444 private void run(Runnable task) {
445 assert Thread.currentThread() == this && state == RUNNING;
560 long timeout = this.timeout;
561 assert timeout > 0;
562 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
563 setState(newState = TIMED_PARKED);
564 }
565
566 // may have been unparked while parking
567 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
568 // lazy submit if local queue is empty
569 lazySubmitRunContinuation();
570 }
571 return;
572 }
573
574 // Thread.yield
575 if (s == YIELDING) {
576 setState(YIELDED);
577
578 // external submit if there are no tasks in the local task queue
579 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
580 externalSubmitRunContinuation(ct.getPool());
581 } else {
582 submitRunContinuation();
583 }
584 return;
585 }
586
587 // blocking on monitorenter
588 if (s == BLOCKING) {
589 setState(BLOCKED);
590
591 // may have been unblocked while blocking
592 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
593 // lazy submit if local queue is empty
594 lazySubmitRunContinuation();
595 }
596 return;
597 }
598
599 // Object.wait
600 if (s == WAITING || s == TIMED_WAITING) {
601 int newState;
602 if (s == WAITING) {
603 setState(newState = WAIT);
604 } else {
605 // For timed-wait, a timeout task is scheduled to execute. The timeout
606 // task will change the thread state to UNBLOCKED and submit the thread
607 // to the scheduler. A sequence number is used to ensure that the timeout
608 // task only unblocks the thread for this timed-wait. We synchronize with
609 // the timeout task to coordinate access to the sequence number and to
610 // ensure the timeout task doesn't execute until the thread has got to
611 // the TIMED_WAIT state.
612 long timeout = this.timeout;
613 assert timeout > 0;
614 synchronized (timedWaitLock()) {
615 byte seqNo = ++timedWaitSeqNo;
616 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
617 setState(newState = TIMED_WAIT);
618 }
619 }
620
621 // may have been notified while in transition to wait state
622 if (notified && compareAndSetState(newState, BLOCKED)) {
623 // may have even been unblocked already
624 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
625 submitRunContinuation();
626 }
627 return;
628 }
629
630 // may have been interrupted while in transition to wait state
631 if (interrupted && compareAndSetState(newState, UNBLOCKED)) {
632 submitRunContinuation();
633 return;
634 }
635 return;
636 }
637
638 assert false;
639 }
640
641 /**
642 * Invoked after the continuation completes.
643 */
644 private void afterDone() {
645 afterDone(true);
646 }
647
648 /**
649 * Invoked after the continuation completes (or start failed). Sets the thread
650 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
651 *
1147 throw new InternalError();
1148 }
1149 }
1150
1151 @Override
1152 boolean alive() {
1153 int s = state;
1154 return (s != NEW && s != TERMINATED);
1155 }
1156
1157 @Override
1158 boolean isTerminated() {
1159 return (state == TERMINATED);
1160 }
1161
1162 @Override
1163 StackTraceElement[] asyncGetStackTrace() {
1164 StackTraceElement[] stackTrace;
1165 do {
1166 stackTrace = (carrierThread != null)
1167 ? super.asyncGetStackTrace() // mounted
1168 : tryGetStackTrace(); // unmounted
1169 if (stackTrace == null) {
1170 Thread.yield();
1171 }
1172 } while (stackTrace == null);
1173 return stackTrace;
1174 }
1175
1176 /**
1177 * Returns the stack trace for this virtual thread if it is unmounted.
1178 * Returns null if the thread is mounted or in transition.
1179 */
1180 private StackTraceElement[] tryGetStackTrace() {
1181 int initialState = state() & ~SUSPENDED;
1182 switch (initialState) {
1183 case NEW, STARTED, TERMINATED -> {
1184 return new StackTraceElement[0]; // unmounted, empty stack
1185 }
1186 case RUNNING, PINNED, TIMED_PINNED -> {
1187 return null; // mounted
1188 }
1189 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1190 // unmounted, not runnable
1191 }
1192 case UNPARKED, UNBLOCKED, YIELDED -> {
1193 // unmounted, runnable
1194 }
1195 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1196 return null; // in transition
1197 }
1198 default -> throw new InternalError("" + initialState);
1199 }
1200
1201 // thread is unmounted, prevent it from continuing
1202 int suspendedState = initialState | SUSPENDED;
1203 if (!compareAndSetState(initialState, suspendedState)) {
1204 return null;
1205 }
1206
1207 // get stack trace and restore state
1208 StackTraceElement[] stack;
1209 try {
1210 stack = cont.getStackTrace();
1211 } finally {
1212 assert state == suspendedState;
1213 setState(initialState);
1214 }
1215 boolean resubmit = switch (initialState) {
1216 case UNPARKED, UNBLOCKED, YIELDED -> {
1217 // resubmit as task may have run while suspended
1218 yield true;
1219 }
1220 case PARKED, TIMED_PARKED -> {
1221 // resubmit if unparked while suspended
1222 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1223 }
1224 case BLOCKED -> {
1225 // resubmit if unblocked while suspended
1226 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1227 }
1228 case WAIT, TIMED_WAIT -> {
1229 // resubmit if notified or interrupted while waiting (Object.wait)
1230 // waitTimeoutExpired will retry if the timed expired when suspended
1231 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1232 }
1233 default -> throw new InternalError();
1234 };
1235 if (resubmit) {
1236 submitRunContinuation();
1237 }
1238 return stack;
1239 }
1240
1241 @Override
1242 public String toString() {
1243 StringBuilder sb = new StringBuilder("VirtualThread[#");
1244 sb.append(threadId());
1245 String name = getName();
1246 if (!name.isEmpty()) {
1247 sb.append(",");
1248 sb.append(name);
1249 }
1250 sb.append("]/");
1251
1252 // add the carrier state and thread name when mounted
1253 boolean mounted;
1254 if (Thread.currentThread() == this) {
1255 mounted = appendCarrierInfo(sb);
1256 } else {
1257 disableSuspendAndPreempt();
1258 try {
1398 @IntrinsicCandidate
1399 @JvmtiMountTransition
1400 private native void notifyJvmtiMount(boolean hide);
1401
1402 @IntrinsicCandidate
1403 @JvmtiMountTransition
1404 private native void notifyJvmtiUnmount(boolean hide);
1405
1406 @IntrinsicCandidate
1407 private static native void notifyJvmtiDisableSuspend(boolean enter);
1408
1409 private static native void registerNatives();
1410 static {
1411 registerNatives();
1412
1413 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1414 var group = Thread.virtualThreadGroup();
1415 }
1416
1417 /**
1418 * Creates the default ForkJoinPool scheduler.
1419 */
1420 private static ForkJoinPool createDefaultScheduler() {
1421 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1422 int parallelism, maxPoolSize, minRunnable;
1423 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1424 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1425 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1426 if (parallelismValue != null) {
1427 parallelism = Integer.parseInt(parallelismValue);
1428 } else {
1429 parallelism = Runtime.getRuntime().availableProcessors();
1430 }
1431 if (maxPoolSizeValue != null) {
1432 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1433 parallelism = Integer.min(parallelism, maxPoolSize);
1434 } else {
1435 maxPoolSize = Integer.max(parallelism, 256);
1436 }
1437 if (minRunnableValue != null) {
1438 minRunnable = Integer.parseInt(minRunnableValue);
1439 } else {
1440 minRunnable = Integer.max(parallelism / 2, 1);
1441 }
1442 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1443 boolean asyncMode = true; // FIFO
1444 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1445 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1446 }
1447
1448 /**
1449 * Schedule a runnable task to run after a delay.
1450 */
1451 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1452 if (scheduler instanceof ForkJoinPool pool) {
1453 return pool.schedule(command, delay, unit);
1454 } else {
1455 return DelayedTaskSchedulers.schedule(command, delay, unit);
1456 }
1457 }
1458
1459 /**
1460 * Supports scheduling a runnable task to run after a delay. It uses a number
1461 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1462 * work queue used. This class is used when using a custom scheduler.
1463 */
1464 private static class DelayedTaskSchedulers {
1465 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1515 assert changed;
1516 vthread.unblock();
1517
1518 vthread = nextThread;
1519 }
1520 }
1521 }
1522
1523 /**
1524 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1525 * if necessary until a list of one or more threads becomes available.
1526 */
1527 private static native VirtualThread takeVirtualThreadListToUnblock();
1528
1529 static {
1530 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1531 VirtualThread::unblockVirtualThreads);
1532 unblocker.setDaemon(true);
1533 unblocker.start();
1534 }
1535 }
|
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.reflect.Constructor;
28 import java.util.Locale;
29 import java.util.Objects;
30 import java.util.concurrent.CountDownLatch;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinTask;
34 import java.util.concurrent.Future;
35 import java.util.concurrent.RejectedExecutionException;
36 import java.util.concurrent.ScheduledExecutorService;
37 import java.util.concurrent.ScheduledThreadPoolExecutor;
38 import java.util.concurrent.TimeUnit;
39 import java.util.function.Supplier;
40 import jdk.internal.event.VirtualThreadEndEvent;
41 import jdk.internal.event.VirtualThreadStartEvent;
42 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
43 import jdk.internal.misc.CarrierThread;
44 import jdk.internal.misc.InnocuousThread;
45 import jdk.internal.misc.Unsafe;
46 import jdk.internal.vm.Continuation;
47 import jdk.internal.vm.ContinuationScope;
48 import jdk.internal.vm.StackableScope;
49 import jdk.internal.vm.ThreadContainer;
50 import jdk.internal.vm.ThreadContainers;
51 import jdk.internal.vm.annotation.ChangesCurrentThread;
52 import jdk.internal.vm.annotation.Hidden;
53 import jdk.internal.vm.annotation.IntrinsicCandidate;
54 import jdk.internal.vm.annotation.JvmtiHideEvents;
55 import jdk.internal.vm.annotation.JvmtiMountTransition;
56 import jdk.internal.vm.annotation.ReservedStackAccess;
57 import sun.nio.ch.Interruptible;
58 import static java.util.concurrent.TimeUnit.*;
59
60 /**
61 * A thread that is scheduled by the Java virtual machine rather than the operating system.
62 */
63 final class VirtualThread extends BaseVirtualThread {
64 private static final Unsafe U = Unsafe.getUnsafe();
65 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
66
67 private static final VirtualThreadScheduler DEFAULT_SCHEDULER;
68 private static final boolean IS_CUSTOM_DEFAULT_SCHEDULER;
69 static {
70 // experimental
71 String propValue = System.getProperty("jdk.virtualThreadScheduler.implClass");
72 if (propValue != null) {
73 DEFAULT_SCHEDULER = createCustomDefaultScheduler(propValue);
74 IS_CUSTOM_DEFAULT_SCHEDULER = true;
75 } else {
76 DEFAULT_SCHEDULER = createDefaultForkJoinPoolScheduler();
77 IS_CUSTOM_DEFAULT_SCHEDULER = false;
78 }
79 }
80
81 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
82 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
83 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
84 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
85 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
86
87 // scheduler and continuation
88 private final VirtualThreadScheduler scheduler;
89 private final Continuation cont;
90 private final Runnable runContinuation;
91
92 // virtual thread state, accessed by VM
93 private volatile int state;
94
95 /*
96 * Virtual thread state transitions:
97 *
98 * NEW -> STARTED // Thread.start, schedule to run
99 * STARTED -> TERMINATED // failed to start
100 * STARTED -> RUNNING // first run
101 * RUNNING -> TERMINATED // done
102 *
103 * RUNNING -> PARKING // Thread parking with LockSupport.park
104 * PARKING -> PARKED // cont.yield successful, parked indefinitely
105 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
106 * PARKED -> UNPARKED // unparked, may be scheduled to continue
107 * PINNED -> RUNNING // unparked, continue execution on same carrier
108 * UNPARKED -> RUNNING // continue execution after park
164 private static final int TERMINATED = 99; // final state
165
166 // can be suspended from scheduling when unmounted
167 private static final int SUSPENDED = 1 << 8;
168
169 // parking permit made available by LockSupport.unpark
170 private volatile boolean parkPermit;
171
172 // blocking permit made available by unblocker thread when another thread exits monitor
173 private volatile boolean blockPermit;
174
175 // true when on the list of virtual threads waiting to be unblocked
176 private volatile boolean onWaitingList;
177
178 // next virtual thread on the list of virtual threads waiting to be unblocked
179 private volatile VirtualThread next;
180
181 // notified by Object.notify/notifyAll while waiting in Object.wait
182 private volatile boolean notified;
183
184 // true when waiting in Object.wait, false for VM internal uninterruptible Object.wait
185 private volatile boolean interruptableWait;
186
187 // timed-wait support
188 private byte timedWaitSeqNo;
189
190 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
191 private long timeout;
192
193 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
194 private Future<?> timeoutTask;
195
196 // carrier thread when mounted, accessed by VM
197 private volatile Thread carrierThread;
198
199 // termination object when joining, created lazily if needed
200 private volatile CountDownLatch termination;
201
202 /**
203 * Returns the default scheduler.
204 */
205 static VirtualThreadScheduler defaultScheduler() {
206 return DEFAULT_SCHEDULER;
207 }
208
209 /**
210 * Returns true if using a custom default scheduler.
211 */
212 static boolean isCustomDefaultScheduler() {
213 return IS_CUSTOM_DEFAULT_SCHEDULER;
214 }
215
216 /**
217 * Returns the continuation scope used for virtual threads.
218 */
219 static ContinuationScope continuationScope() {
220 return VTHREAD_SCOPE;
221 }
222
223 /**
224 * Return the scheduler for this thread.
225 * @param revealBuiltin true to reveal the built-in default scheduler, false to hide
226 */
227 VirtualThreadScheduler scheduler(boolean revealBuiltin) {
228 if (scheduler instanceof BuiltinDefaultScheduler builtin && !revealBuiltin) {
229 return builtin.externalView();
230 } else {
231 return scheduler;
232 }
233 }
234
235 /**
236 * Creates a new {@code VirtualThread} to run the given task with the given scheduler.
237 *
238 * @param scheduler the scheduler or null for default scheduler
239 * @param name thread name
240 * @param characteristics characteristics
241 * @param task the task to execute
242 */
243 VirtualThread(VirtualThreadScheduler scheduler,
244 String name,
245 int characteristics,
246 Runnable task) {
247 super(name, characteristics, /*bound*/ false);
248 Objects.requireNonNull(task);
249
250 // use default scheduler if not provided
251 if (scheduler == null) {
252 scheduler = DEFAULT_SCHEDULER;
253 }
254
255 this.scheduler = scheduler;
256 this.cont = new VThreadContinuation(this, task);
257 this.runContinuation = this::runContinuation;
258 }
259
260 /**
261 * The continuation that a virtual thread executes.
262 */
263 private static class VThreadContinuation extends Continuation {
264 VThreadContinuation(VirtualThread vthread, Runnable task) {
265 super(VTHREAD_SCOPE, wrap(vthread, task));
266 }
267 @Override
268 protected void onPinned(Continuation.Pinned reason) {
269 }
270 private static Runnable wrap(VirtualThread vthread, Runnable task) {
271 return new Runnable() {
272 @Hidden
328 afterYield();
329 }
330 }
331 }
332
333 /**
334 * Cancel timeout task when continuing after timed-park or timed-wait.
335 * The timeout task may be executing, or may have already completed.
336 */
337 private void cancelTimeoutTask() {
338 if (timeoutTask != null) {
339 timeoutTask.cancel(false);
340 timeoutTask = null;
341 }
342 }
343
344 /**
345 * Submits the runContinuation task to the scheduler. For the default scheduler,
346 * and calling it on a worker thread, the task will be pushed to the local queue,
347 * otherwise it will be pushed to an external submission queue.
348 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
349 * @throws RejectedExecutionException
350 */
351 private void submitRunContinuation(boolean retryOnOOME) {
352 boolean done = false;
353 while (!done) {
354 try {
355 // Pin the continuation to prevent the virtual thread from unmounting
356 // when submitting a task. For the default scheduler this ensures that
357 // the carrier doesn't change when pushing a task. For other schedulers
358 // it avoids deadlock that could arise due to carriers and virtual
359 // threads contending for a lock.
360 if (currentThread().isVirtual()) {
361 Continuation.pin();
362 try {
363 scheduler.execute(this, runContinuation);
364 } finally {
365 Continuation.unpin();
366 }
367 } else {
368 scheduler.execute(this, runContinuation);
369 }
370 done = true;
371 } catch (RejectedExecutionException ree) {
372 submitFailed(ree);
373 throw ree;
374 } catch (OutOfMemoryError e) {
375 if (retryOnOOME) {
376 U.park(false, 100_000_000); // 100ms
377 } else {
378 throw e;
379 }
380 }
381 }
382 }
383
384 /**
385 * Submits the runContinuation task to the scheduler. For the default scheduler,
386 * and calling it on a worker thread, the task will be pushed to the local queue,
387 * otherwise it will be pushed to an external submission queue.
388 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
389 * @throws RejectedExecutionException
390 */
391 private void submitRunContinuation() {
392 submitRunContinuation(true);
393 }
394
395 /**
396 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
397 * queue is empty. If not empty, or invoked by another thread, then this method works
398 * like submitRunContinuation and just submits the task to the scheduler.
399 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
400 * @throws RejectedExecutionException
401 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
402 */
403 private void lazySubmitRunContinuation() {
404 if (scheduler == DEFAULT_SCHEDULER
405 && currentCarrierThread() instanceof CarrierThread ct
406 && ct.getQueuedTaskCount() == 0) {
407 try {
408 ct.getPool().lazySubmit(ForkJoinTask.adapt(runContinuation));
409 } catch (RejectedExecutionException ree) {
410 submitFailed(ree);
411 throw ree;
412 } catch (OutOfMemoryError e) {
413 submitRunContinuation();
414 }
415 } else {
416 submitRunContinuation();
417 }
418 }
419
420 /**
421 * Submits the runContinuation task to the scheduler. For the default scheduler, and
422 * calling it a virtual thread that uses the default scheduler, the task will be
423 * pushed to an external submission queue.
424 * @throws RejectedExecutionException
425 */
426 private void externalSubmitRunContinuation() {
427 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
428 try {
429 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
430 } catch (RejectedExecutionException ree) {
431 submitFailed(ree);
432 throw ree;
433 } catch (OutOfMemoryError e) {
434 submitRunContinuation();
435 }
436 } else {
437 submitRunContinuation();
438 }
439 }
440
441 /**
442 * Submits the runContinuation task to the scheduler. For the default scheduler, and
443 * calling it a virtual thread that uses the default scheduler, the task will be
444 * pushed to an external submission queue. This method may throw OutOfMemoryError.
445 * @throws RejectedExecutionException
446 * @throws OutOfMemoryError
447 */
448 private void externalSubmitRunContinuationOrThrow() {
449 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
450 try {
451 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
452 } catch (RejectedExecutionException ree) {
453 submitFailed(ree);
454 throw ree;
455 }
456 } else {
457 submitRunContinuation(false);
458 }
459 }
460
461 /**
462 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
463 */
464 private void submitFailed(RejectedExecutionException ree) {
465 var event = new VirtualThreadSubmitFailedEvent();
466 if (event.isEnabled()) {
467 event.javaThreadId = threadId();
468 event.exceptionMessage = ree.getMessage();
469 event.commit();
470 }
471 }
472
473 /**
474 * Runs a task in the context of this virtual thread.
475 */
476 private void run(Runnable task) {
477 assert Thread.currentThread() == this && state == RUNNING;
592 long timeout = this.timeout;
593 assert timeout > 0;
594 timeoutTask = schedule(this::parkTimeoutExpired, timeout, NANOSECONDS);
595 setState(newState = TIMED_PARKED);
596 }
597
598 // may have been unparked while parking
599 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
600 // lazy submit if local queue is empty
601 lazySubmitRunContinuation();
602 }
603 return;
604 }
605
606 // Thread.yield
607 if (s == YIELDING) {
608 setState(YIELDED);
609
610 // external submit if there are no tasks in the local task queue
611 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
612 externalSubmitRunContinuation();
613 } else {
614 submitRunContinuation();
615 }
616 return;
617 }
618
619 // blocking on monitorenter
620 if (s == BLOCKING) {
621 setState(BLOCKED);
622
623 // may have been unblocked while blocking
624 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
625 // lazy submit if local queue is empty
626 lazySubmitRunContinuation();
627 }
628 return;
629 }
630
631 // Object.wait
632 if (s == WAITING || s == TIMED_WAITING) {
633 int newState;
634 boolean interruptable = interruptableWait;
635 if (s == WAITING) {
636 setState(newState = WAIT);
637 } else {
638 // For timed-wait, a timeout task is scheduled to execute. The timeout
639 // task will change the thread state to UNBLOCKED and submit the thread
640 // to the scheduler. A sequence number is used to ensure that the timeout
641 // task only unblocks the thread for this timed-wait. We synchronize with
642 // the timeout task to coordinate access to the sequence number and to
643 // ensure the timeout task doesn't execute until the thread has got to
644 // the TIMED_WAIT state.
645 long timeout = this.timeout;
646 assert timeout > 0;
647 synchronized (timedWaitLock()) {
648 byte seqNo = ++timedWaitSeqNo;
649 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
650 setState(newState = TIMED_WAIT);
651 }
652 }
653
654 // may have been notified while in transition to wait state
655 if (notified && compareAndSetState(newState, BLOCKED)) {
656 // may have even been unblocked already
657 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
658 submitRunContinuation();
659 }
660 return;
661 }
662
663 // may have been interrupted while in transition to wait state
664 if (interruptable && interrupted && compareAndSetState(newState, UNBLOCKED)) {
665 submitRunContinuation();
666 return;
667 }
668 return;
669 }
670
671 assert false;
672 }
673
674 /**
675 * Invoked after the continuation completes.
676 */
677 private void afterDone() {
678 afterDone(true);
679 }
680
681 /**
682 * Invoked after the continuation completes (or start failed). Sets the thread
683 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
684 *
1180 throw new InternalError();
1181 }
1182 }
1183
1184 @Override
1185 boolean alive() {
1186 int s = state;
1187 return (s != NEW && s != TERMINATED);
1188 }
1189
1190 @Override
1191 boolean isTerminated() {
1192 return (state == TERMINATED);
1193 }
1194
1195 @Override
1196 StackTraceElement[] asyncGetStackTrace() {
1197 StackTraceElement[] stackTrace;
1198 do {
1199 stackTrace = (carrierThread != null)
1200 ? super.asyncGetStackTrace() // mounted
1201 : supplyIfUnmounted(cont::getStackTrace, // unmounted
1202 () -> new StackTraceElement[0]);
1203 if (stackTrace == null) {
1204 Thread.yield();
1205 }
1206 } while (stackTrace == null);
1207 return stackTrace;
1208 }
1209
1210 /**
1211 * Invokes a supplier to produce a non-null result if this virtual thread is not mounted.
1212 * @param supplier1 invoked if this virtual thread is alive and unmounted
1213 * @param supplier2 invoked if this virtual thread is not alive
1214 * @return the result; {@code null} if this virtual thread is mounted or in transition
1215 */
1216 <T> T supplyIfUnmounted(Supplier<T> supplier1, Supplier<T> supplier2) {
1217 int initialState = state() & ~SUSPENDED;
1218 switch (initialState) {
1219 case NEW, STARTED, TERMINATED -> {
1220 return supplier2.get(); // terminated or not started
1221 }
1222 case RUNNING, PINNED, TIMED_PINNED -> {
1223 return null; // mounted
1224 }
1225 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1226 // unmounted, not runnable
1227 }
1228 case UNPARKED, UNBLOCKED, YIELDED -> {
1229 // unmounted, runnable
1230 }
1231 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1232 return null; // in transition
1233 }
1234 default -> throw new InternalError("" + initialState);
1235 }
1236
1237 // thread is unmounted, prevent it from continuing
1238 int suspendedState = initialState | SUSPENDED;
1239 if (!compareAndSetState(initialState, suspendedState)) {
1240 return null;
1241 }
1242
1243 try {
1244 return supplier1.get();
1245 } finally {
1246 assert state == suspendedState;
1247 setState(initialState);
1248
1249 boolean resubmit = switch (initialState) {
1250 case UNPARKED, UNBLOCKED, YIELDED -> {
1251 // resubmit as task may have run while suspended
1252 yield true;
1253 }
1254 case PARKED, TIMED_PARKED -> {
1255 // resubmit if unparked while suspended
1256 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1257 }
1258 case BLOCKED -> {
1259 // resubmit if unblocked while suspended
1260 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1261 }
1262 case WAIT, TIMED_WAIT -> {
1263 // resubmit if notified or interrupted while waiting (Object.wait)
1264 // waitTimeoutExpired will retry if the timed expired when suspended
1265 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1266 }
1267 default -> throw new InternalError();
1268 };
1269 if (resubmit) {
1270 submitRunContinuation();
1271 }
1272 }
1273
1274 }
1275
1276 @Override
1277 public String toString() {
1278 StringBuilder sb = new StringBuilder("VirtualThread[#");
1279 sb.append(threadId());
1280 String name = getName();
1281 if (!name.isEmpty()) {
1282 sb.append(",");
1283 sb.append(name);
1284 }
1285 sb.append("]/");
1286
1287 // add the carrier state and thread name when mounted
1288 boolean mounted;
1289 if (Thread.currentThread() == this) {
1290 mounted = appendCarrierInfo(sb);
1291 } else {
1292 disableSuspendAndPreempt();
1293 try {
1433 @IntrinsicCandidate
1434 @JvmtiMountTransition
1435 private native void notifyJvmtiMount(boolean hide);
1436
1437 @IntrinsicCandidate
1438 @JvmtiMountTransition
1439 private native void notifyJvmtiUnmount(boolean hide);
1440
1441 @IntrinsicCandidate
1442 private static native void notifyJvmtiDisableSuspend(boolean enter);
1443
1444 private static native void registerNatives();
1445 static {
1446 registerNatives();
1447
1448 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1449 var group = Thread.virtualThreadGroup();
1450 }
1451
1452 /**
1453 * Loads a VirtualThreadScheduler with the given class name to use at the
1454 * default scheduler. The class is public in an exported package, has a public
1455 * one-arg or no-arg constructor, and is visible to the system class loader.
1456 */
1457 private static VirtualThreadScheduler createCustomDefaultScheduler(String cn) {
1458 try {
1459 Class<?> clazz = Class.forName(cn, true, ClassLoader.getSystemClassLoader());
1460 VirtualThreadScheduler scheduler;
1461 try {
1462 // 1-arg constructor
1463 Constructor<?> ctor = clazz.getConstructor(VirtualThreadScheduler.class);
1464 var builtin = createDefaultForkJoinPoolScheduler();
1465 scheduler = (VirtualThreadScheduler) ctor.newInstance(builtin.externalView());
1466 } catch (NoSuchMethodException e) {
1467 // 0-arg constructor
1468 Constructor<?> ctor = clazz.getConstructor();
1469 scheduler = (VirtualThreadScheduler) ctor.newInstance();
1470 }
1471 System.err.println("""
1472 WARNING: Using custom default scheduler, this is an experimental feature!""");
1473 return scheduler;
1474 } catch (Exception ex) {
1475 throw new Error(ex);
1476 }
1477 }
1478
1479 /**
1480 * Creates the built-in default ForkJoinPool scheduler.
1481 */
1482 private static BuiltinDefaultScheduler createDefaultForkJoinPoolScheduler() {
1483 int parallelism, maxPoolSize, minRunnable;
1484 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1485 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1486 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1487 if (parallelismValue != null) {
1488 parallelism = Integer.parseInt(parallelismValue);
1489 } else {
1490 parallelism = Runtime.getRuntime().availableProcessors();
1491 }
1492 if (maxPoolSizeValue != null) {
1493 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1494 parallelism = Integer.min(parallelism, maxPoolSize);
1495 } else {
1496 maxPoolSize = Integer.max(parallelism, 256);
1497 }
1498 if (minRunnableValue != null) {
1499 minRunnable = Integer.parseInt(minRunnableValue);
1500 } else {
1501 minRunnable = Integer.max(parallelism / 2, 1);
1502 }
1503 return new BuiltinDefaultScheduler(parallelism, maxPoolSize, minRunnable);
1504 }
1505
1506 /**
1507 * The built-in default ForkJoinPool scheduler.
1508 */
1509 private static class BuiltinDefaultScheduler
1510 extends ForkJoinPool implements VirtualThreadScheduler {
1511
1512 private static final StableValue<VirtualThreadScheduler> VIEW = StableValue.of();
1513
1514 BuiltinDefaultScheduler(int parallelism, int maxPoolSize, int minRunnable) {
1515 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1516 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1517 boolean asyncMode = true; // FIFO
1518 super(parallelism, factory, handler, asyncMode,
1519 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1520 }
1521
1522 @Override
1523 public void execute(Thread vthread, Runnable task) {
1524 execute(ForkJoinTask.adapt(task));
1525 }
1526
1527 /**
1528 * Wraps the scheduler to avoid leaking a direct reference.
1529 */
1530 VirtualThreadScheduler externalView() {
1531 VirtualThreadScheduler builtin = this;
1532 return VIEW.orElseSet(() -> {
1533 return new VirtualThreadScheduler() {
1534 @Override
1535 public void execute(Thread thread, Runnable task) {
1536 Objects.requireNonNull(thread);
1537 if (thread instanceof VirtualThread vthread) {
1538 VirtualThreadScheduler scheduler = vthread.scheduler;
1539 if (scheduler == this || scheduler == DEFAULT_SCHEDULER) {
1540 builtin.execute(thread, task);
1541 } else {
1542 throw new IllegalArgumentException();
1543 }
1544 } else {
1545 throw new UnsupportedOperationException();
1546 }
1547 }
1548 };
1549 });
1550 }
1551 }
1552
1553 /**
1554 * Schedule a runnable task to run after a delay.
1555 */
1556 private Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1557 if (scheduler instanceof ForkJoinPool pool) {
1558 return pool.schedule(command, delay, unit);
1559 } else {
1560 return DelayedTaskSchedulers.schedule(command, delay, unit);
1561 }
1562 }
1563
1564 /**
1565 * Supports scheduling a runnable task to run after a delay. It uses a number
1566 * of ScheduledThreadPoolExecutor instances to reduce contention on the delayed
1567 * work queue used. This class is used when using a custom scheduler.
1568 */
1569 private static class DelayedTaskSchedulers {
1570 private static final ScheduledExecutorService[] INSTANCE = createDelayedTaskSchedulers();
1620 assert changed;
1621 vthread.unblock();
1622
1623 vthread = nextThread;
1624 }
1625 }
1626 }
1627
1628 /**
1629 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1630 * if necessary until a list of one or more threads becomes available.
1631 */
1632 private static native VirtualThread takeVirtualThreadListToUnblock();
1633
1634 static {
1635 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1636 VirtualThread::unblockVirtualThreads);
1637 unblocker.setDaemon(true);
1638 unblocker.start();
1639 }
1640 }
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