1 /*
2 * Copyright (c) 2021, 2024, 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
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8 * particular file as subject to the "Classpath" exception as provided
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10 *
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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).
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24 */
25 package java.util.concurrent;
26
27 import java.lang.invoke.MethodHandles;
28 import java.lang.invoke.VarHandle;
29 import java.time.Duration;
30 import java.time.Instant;
31 import java.util.Objects;
32 import java.util.Optional;
33 import java.util.concurrent.locks.ReentrantLock;
34 import java.util.function.Function;
35 import java.util.function.Supplier;
36 import jdk.internal.javac.PreviewFeature;
37 import jdk.internal.misc.ThreadFlock;
38 import jdk.internal.invoke.MhUtil;
39
40 /**
41 * A basic API for <em>structured concurrency</em>. {@code StructuredTaskScope} supports
42 * cases where a task splits into several concurrent subtasks, and where the subtasks must
43 * complete before the main task continues. A {@code StructuredTaskScope} can be used to
44 * ensure that the lifetime of a concurrent operation is confined by a <em>syntax block</em>,
45 * just like that of a sequential operation in structured programming.
46 *
47 * <h2>Basic operation</h2>
48 *
49 * A {@code StructuredTaskScope} is created with one of its public constructors. It defines
50 * the {@link #fork(Callable) fork} method to start a thread to execute a subtask, the {@link
51 * #join() join} method to wait for all subtasks to finish, and the {@link #close() close}
52 * method to close the task scope. The API is intended to be used with the {@code
53 * try-with-resources} statement. The intention is that code in the try <em>block</em>
54 * uses the {@code fork} method to fork threads to execute the subtasks, wait for the
55 * subtasks to finish with the {@code join} method, and then <em>process the results</em>.
56 * A call to the {@code fork} method returns a {@link Subtask Subtask} to representing
57 * the <em>forked subtask</em>. Once {@code join} is called, the {@code Subtask} can be
58 * used to get the result completed successfully, or the exception if the subtask failed.
59 * {@snippet lang=java :
60 * Callable<String> task1 = ...
61 * Callable<Integer> task2 = ...
62 *
63 * try (var scope = new StructuredTaskScope<Object>()) {
64 *
65 * Subtask<String> subtask1 = scope.fork(task1); // @highlight substring="fork"
66 * Subtask<Integer> subtask2 = scope.fork(task2); // @highlight substring="fork"
67 *
68 * scope.join(); // @highlight substring="join"
69 *
70 * ... process results/exceptions ...
71 *
72 * } // close // @highlight substring="close"
73 * }
74 * <p> The following example forks a collection of homogeneous subtasks, waits for all of
75 * them to complete with the {@code join} method, and uses the {@link Subtask.State
76 * Subtask.State} to partition the subtasks into a set of the subtasks that completed
77 * successfully and another for the subtasks that failed.
78 * {@snippet lang=java :
79 * List<Callable<String>> callables = ...
80 *
81 * try (var scope = new StructuredTaskScope<String>()) {
82 *
83 * List<Subtask<String>> subtasks = callables.stream().map(scope::fork).toList();
84 *
85 * scope.join();
86 *
87 * Map<Boolean, Set<Subtask<String>>> map = subtasks.stream()
88 * .collect(Collectors.partitioningBy(h -> h.state() == Subtask.State.SUCCESS,
89 * Collectors.toSet()));
90 *
91 * } // close
92 * }
93 *
94 * <p> To ensure correct usage, the {@code join} and {@code close} methods may only be
95 * invoked by the <em>owner</em> (the thread that opened/created the task scope), and the
96 * {@code close} method throws an exception after closing if the owner did not invoke the
97 * {@code join} method after forking.
98 *
99 * <p> {@code StructuredTaskScope} defines the {@link #shutdown() shutdown} method to shut
100 * down a task scope without closing it. The {@code shutdown()} method <em>cancels</em> all
101 * unfinished subtasks by {@linkplain Thread#interrupt() interrupting} the threads. It
102 * prevents new threads from starting in the task scope. If the owner is waiting in the
103 * {@code join} method then it will wakeup.
104 *
105 * <p> Shutdown is used for <em>short-circuiting</em> and allow subclasses to implement
106 * <em>policy</em> that does not require all subtasks to finish.
107 *
108 * <h2>Subclasses with policies for common cases</h2>
109 *
110 * Two subclasses of {@code StructuredTaskScope} are defined to implement policy for
111 * common cases:
112 * <ol>
113 * <li> {@link ShutdownOnSuccess ShutdownOnSuccess} captures the result of the first
114 * subtask to complete successfully. Once captured, it shuts down the task scope to
115 * interrupt unfinished threads and wakeup the owner. This class is intended for cases
116 * where the result of any subtask will do ("invoke any") and where there is no need to
117 * wait for results of other unfinished subtasks. It defines methods to get the first
118 * result or throw an exception if all subtasks fail.
119 * <li> {@link ShutdownOnFailure ShutdownOnFailure} captures the exception of the first
120 * subtask to fail. Once captured, it shuts down the task scope to interrupt unfinished
121 * threads and wakeup the owner. This class is intended for cases where the results of all
122 * subtasks are required ("invoke all"); if any subtask fails then the results of other
123 * unfinished subtasks are no longer needed. If defines methods to throw an exception if
124 * any of the subtasks fail.
125 * </ol>
126 *
127 * <p> The following are two examples that use the two classes. In both cases, a pair of
128 * subtasks are forked to fetch resources from two URL locations "left" and "right". The
129 * first example creates a ShutdownOnSuccess object to capture the result of the first
130 * subtask to complete successfully, cancelling the other by way of shutting down the task
131 * scope. The main task waits in {@code join} until either subtask completes with a result
132 * or both subtasks fail. It invokes {@link ShutdownOnSuccess#result(Function)
133 * result(Function)} method to get the captured result. If both subtasks fail then this
134 * method throws a {@code WebApplicationException} with the exception from one of the
135 * subtasks as the cause.
136 * {@snippet lang=java :
137 * try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
138 *
139 * scope.fork(() -> fetch(left));
140 * scope.fork(() -> fetch(right));
141 *
142 * scope.join();
143 *
144 * // @link regex="result(?=\()" target="ShutdownOnSuccess#result" :
145 * String result = scope.result(e -> new WebApplicationException(e));
146 *
147 * ...
148 * }
149 * }
150 * The second example creates a ShutdownOnFailure object to capture the exception of the
151 * first subtask to fail, cancelling the other by way of shutting down the task scope. The
152 * main task waits in {@link #joinUntil(Instant)} until both subtasks complete with a
153 * result, either fails, or a deadline is reached. It invokes {@link
154 * ShutdownOnFailure#throwIfFailed(Function) throwIfFailed(Function)} to throw an exception
155 * if either subtask fails. This method is a no-op if both subtasks complete successfully.
156 * The example uses {@link Supplier#get()} to get the result of each subtask. Using
157 * {@code Supplier} instead of {@code Subtask} is preferred for common cases where the
158 * object returned by fork is only used to get the result of a subtask that completed
159 * successfully.
160 * {@snippet lang=java :
161 * Instant deadline = ...
162 *
163 * try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
164 *
165 * Supplier<String> supplier1 = scope.fork(() -> query(left));
166 * Supplier<String> supplier2 = scope.fork(() -> query(right));
167 *
168 * scope.joinUntil(deadline);
169 *
170 * // @link substring="throwIfFailed" target="ShutdownOnFailure#throwIfFailed" :
171 * scope.throwIfFailed(e -> new WebApplicationException(e));
172 *
173 * // both subtasks completed successfully
174 * String result = Stream.of(supplier1, supplier2)
175 * .map(Supplier::get)
176 * .collect(Collectors.joining(", ", "{ ", " }"));
177 *
178 * ...
179 * }
180 * }
181 *
182 * <h2>Extending StructuredTaskScope</h2>
183 *
184 * {@code StructuredTaskScope} can be extended, and the {@link #handleComplete(Subtask)
185 * handleComplete} method overridden, to implement policies other than those implemented
186 * by {@code ShutdownOnSuccess} and {@code ShutdownOnFailure}. A subclass may, for example,
187 * collect the results of subtasks that complete successfully and ignore subtasks that
188 * fail. It may collect exceptions when subtasks fail. It may invoke the {@link #shutdown()
189 * shutdown} method to shut down and cause {@link #join() join} to wakeup when some
190 * condition arises.
191 *
192 * <p> A subclass will typically define methods to make available results, state, or other
193 * outcome to code that executes after the {@code join} method. A subclass that collects
194 * results and ignores subtasks that fail may define a method that returns the results.
195 * A subclass that implements a policy to shut down when a subtask fails may define a
196 * method to get the exception of the first subtask to fail.
197 *
198 * <p> The following is an example of a simple {@code StructuredTaskScope} implementation
199 * that collects homogenous subtasks that complete successfully. It defines the method
200 * "{@code completedSuccessfully()}" that the main task can invoke after it joins.
201 * {@snippet lang=java :
202 * class CollectingScope<T> extends StructuredTaskScope<T> {
203 * private final Queue<Subtask<? extends T>> subtasks = new LinkedTransferQueue<>();
204 *
205 * @Override
206 * protected void handleComplete(Subtask<? extends T> subtask) {
207 * if (subtask.state() == Subtask.State.SUCCESS) {
208 * subtasks.add(subtask);
209 * }
210 * }
211 *
212 * @Override
213 * public CollectingScope<T> join() throws InterruptedException {
214 * super.join();
215 * return this;
216 * }
217 *
218 * public Stream<Subtask<? extends T>> completedSuccessfully() {
219 * // @link substring="ensureOwnerAndJoined" target="ensureOwnerAndJoined" :
220 * super.ensureOwnerAndJoined();
221 * return subtasks.stream();
222 * }
223 * }
224 * }
225 * <p> The implementations of the {@code completedSuccessfully()} method in the example
226 * invokes {@link #ensureOwnerAndJoined()} to ensure that the method can only be invoked
227 * by the owner thread and only after it has joined.
228 *
229 * <h2><a id="TreeStructure">Tree structure</a></h2>
230 *
231 * Task scopes form a tree where parent-child relations are established implicitly when
232 * opening a new task scope:
233 * <ul>
234 * <li> A parent-child relation is established when a thread started in a task scope
235 * opens its own task scope. A thread started in task scope "A" that opens task scope
236 * "B" establishes a parent-child relation where task scope "A" is the parent of task
237 * scope "B".
238 * <li> A parent-child relation is established with nesting. If a thread opens task
239 * scope "B", then opens task scope "C" (before it closes "B"), then the enclosing task
240 * scope "B" is the parent of the nested task scope "C".
241 * </ul>
242 *
243 * The <i>descendants</i> of a task scope are the child task scopes that it is a parent
244 * of, plus the descendants of the child task scopes, recursively.
245 *
246 * <p> The tree structure supports:
247 * <ul>
248 * <li> Inheritance of {@linkplain ScopedValue scoped values} across threads.
249 * <li> Confinement checks. The phrase "threads contained in the task scope" in method
250 * descriptions means threads started in the task scope or descendant scopes.
251 * </ul>
252 *
253 * <p> The following example demonstrates the inheritance of a scoped value. A scoped
254 * value {@code USERNAME} is bound to the value "{@code duke}". A {@code StructuredTaskScope}
255 * is created and its {@code fork} method invoked to start a thread to execute {@code
256 * childTask}. The thread inherits the scoped value <em>bindings</em> captured when
257 * creating the task scope. The code in {@code childTask} uses the value of the scoped
258 * value and so reads the value "{@code duke}".
259 * {@snippet lang=java :
260 * private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();
261 *
262 * // @link substring="run" target="ScopedValue.Carrier#run(Runnable)" :
263 * ScopedValue.where(USERNAME, "duke").run(() -> {
264 * try (var scope = new StructuredTaskScope<String>()) {
265 *
266 * scope.fork(() -> childTask()); // @highlight substring="fork"
267 * ...
268 * }
269 * });
270 *
271 * ...
272 *
273 * String childTask() {
274 * // @link substring="get" target="ScopedValue#get()" :
275 * String name = USERNAME.get(); // "duke"
276 * ...
277 * }
278 * }
279 *
280 * <p> {@code StructuredTaskScope} does not define APIs that exposes the tree structure
281 * at this time.
282 *
283 * <p> Unless otherwise specified, passing a {@code null} argument to a constructor
284 * or method in this class will cause a {@link NullPointerException} to be thrown.
285 *
286 * <h2>Memory consistency effects</h2>
287 *
288 * <p> Actions in the owner thread of, or a thread contained in, the task scope prior to
289 * {@linkplain #fork forking} of a subtask
290 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility">
291 * <i>happen-before</i></a> any actions taken by that subtask, which in turn <i>happen-before</i>
292 * the subtask result is {@linkplain Subtask#get() retrieved} or <i>happen-before</i> any
293 * actions taken in a thread after {@linkplain #join() joining} of the task scope.
294 *
295 * @jls 17.4.5 Happens-before Order
296 *
297 * @param <T> the result type of tasks executed in the task scope
298 * @since 21
299 */
300 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
301 public class StructuredTaskScope<T> implements AutoCloseable {
302 private final ThreadFactory factory;
303 private final ThreadFlock flock;
304 private final ReentrantLock shutdownLock = new ReentrantLock();
305
306 // states: OPEN -> SHUTDOWN -> CLOSED
307 private static final int OPEN = 0; // initial state
308 private static final int SHUTDOWN = 1;
309 private static final int CLOSED = 2;
310
311 // state: set to SHUTDOWN by any thread, set to CLOSED by owner, read by any thread
312 private volatile int state;
313
314 // Counters to support checking that the task scope owner joins before processing
315 // results and attempts join before closing the task scope. These counters are
316 // accessed only by the owner thread.
317 private int forkRound; // incremented when the first subtask is forked after join
318 private int lastJoinAttempted; // set to the current fork round when join is attempted
319 private int lastJoinCompleted; // set to the current fork round when join completes
320
321 /**
322 * Represents a subtask forked with {@link #fork(Callable)}.
323 * @param <T> the result type
324 * @since 21
325 */
326 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
327 public sealed interface Subtask<T> extends Supplier<T> permits SubtaskImpl {
328 /**
329 * {@return the value returning task provided to the {@code fork} method}
330 *
331 * @apiNote Task objects with unique identity may be used for correlation by
332 * implementations of {@link #handleComplete(Subtask) handleComplete}.
333 */
334 Callable<? extends T> task();
335
336 /**
337 * Represents the state of a subtask.
338 * @see Subtask#state()
339 * @since 21
340 */
341 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
342 enum State {
343 /**
344 * The subtask result or exception is not available. This state indicates that
345 * the subtask was forked but has not completed, it completed after the task
346 * scope was {@linkplain #shutdown() shut down}, or it was forked after the
347 * task scope was shut down.
348 */
349 UNAVAILABLE,
350 /**
351 * The subtask completed successfully with a result. The {@link Subtask#get()
352 * Subtask.get()} method can be used to obtain the result. This is a terminal
353 * state.
354 */
355 SUCCESS,
356 /**
357 * The subtask failed with an exception. The {@link Subtask#exception()
358 * Subtask.exception()} method can be used to obtain the exception. This is a
359 * terminal state.
360 */
361 FAILED,
362 }
363
364 /**
365 * {@return the state of the subtask}
366 */
367 State state();
368
369 /**
370 * Returns the result of the subtask.
371 *
372 * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
373 * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
374 * joinUntil}) before it can obtain the result of the subtask.
375 *
376 * @return the possibly-null result
377 * @throws IllegalStateException if the subtask has not completed, did not complete
378 * successfully, or the current thread is the task scope owner and did not join
379 * after forking
380 * @see State#SUCCESS
381 */
382 T get();
383
384 /**
385 * {@return the exception thrown by the subtask}
386 *
387 * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
388 * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
389 * joinUntil}) before it can obtain the exception thrown by the subtask.
390 *
391 * @throws IllegalStateException if the subtask has not completed, completed with
392 * a result, or the current thread is the task scope owner and did not join after
393 * forking
394 * @see State#FAILED
395 */
396 Throwable exception();
397 }
398
399 /**
400 * Creates a structured task scope with the given name and thread factory. The task
401 * scope is optionally named for the purposes of monitoring and management. The thread
402 * factory is used to {@link ThreadFactory#newThread(Runnable) create} threads when
403 * subtasks are {@linkplain #fork(Callable) forked}. The task scope is owned by the
404 * current thread.
405 *
406 * <p> Construction captures the current thread's {@linkplain ScopedValue scoped value}
407 * bindings for inheritance by threads started in the task scope. The
408 * <a href="#TreeStructure">Tree Structure</a> section in the class description details
409 * how parent-child relations are established implicitly for the purpose of inheritance
410 * of scoped value bindings.
411 *
412 * @param name the name of the task scope, can be null
413 * @param factory the thread factory
414 */
415 @SuppressWarnings("this-escape")
416 public StructuredTaskScope(String name, ThreadFactory factory) {
417 this.factory = Objects.requireNonNull(factory, "'factory' is null");
418 if (name == null)
419 name = Objects.toIdentityString(this);
420 this.flock = ThreadFlock.open(name);
421 }
422
423 /**
424 * Creates an unnamed structured task scope that creates virtual threads. The task
425 * scope is owned by the current thread.
426 *
427 * @implSpec This constructor is equivalent to invoking the 2-arg constructor with a
428 * name of {@code null} and a thread factory that creates virtual threads.
429 */
430 public StructuredTaskScope() {
431 this(null, Thread.ofVirtual().factory());
432 }
433
434 private IllegalStateException newIllegalStateExceptionScopeClosed() {
435 return new IllegalStateException("Task scope is closed");
436 }
437
438 private IllegalStateException newIllegalStateExceptionNoJoin() {
439 return new IllegalStateException("Owner did not join after forking subtasks");
440 }
441
442 /**
443 * Throws IllegalStateException if the scope is closed, returning the state if not
444 * closed.
445 */
446 private int ensureOpen() {
447 int s = state;
448 if (s == CLOSED)
449 throw newIllegalStateExceptionScopeClosed();
450 return s;
451 }
452
453 /**
454 * Throws WrongThreadException if the current thread is not the owner.
455 */
456 private void ensureOwner() {
457 if (Thread.currentThread() != flock.owner())
458 throw new WrongThreadException("Current thread not owner");
459 }
460
461 /**
462 * Throws WrongThreadException if the current thread is not the owner
463 * or a thread contained in the tree.
464 */
465 private void ensureOwnerOrContainsThread() {
466 Thread currentThread = Thread.currentThread();
467 if (currentThread != flock.owner() && !flock.containsThread(currentThread))
468 throw new WrongThreadException("Current thread not owner or thread in the tree");
469 }
470
471 /**
472 * Throws IllegalStateException if the current thread is the owner, and the owner did
473 * not join after forking a subtask in the given fork round.
474 */
475 private void ensureJoinedIfOwner(int round) {
476 if (Thread.currentThread() == flock.owner() && (round > lastJoinCompleted)) {
477 throw newIllegalStateExceptionNoJoin();
478 }
479 }
480
481 /**
482 * Ensures that the current thread is the owner of this task scope and that it joined
483 * (with {@link #join()} or {@link #joinUntil(Instant)}) after {@linkplain #fork(Callable)
484 * forking} subtasks.
485 *
486 * @apiNote This method can be used by subclasses that define methods to make available
487 * results, state, or other outcome to code intended to execute after the join method.
488 *
489 * @throws WrongThreadException if the current thread is not the task scope owner
490 * @throws IllegalStateException if the task scope is open and task scope owner did
491 * not join after forking
492 */
493 protected final void ensureOwnerAndJoined() {
494 ensureOwner();
495 if (forkRound > lastJoinCompleted) {
496 throw newIllegalStateExceptionNoJoin();
497 }
498 }
499
500 /**
501 * Invoked by a subtask when it completes successfully or fails in this task scope.
502 * This method is not invoked if a subtask completes after the task scope is
503 * {@linkplain #shutdown() shut down}.
504 *
505 * @implSpec The default implementation throws {@code NullPointerException} if the
506 * subtask is {@code null}. It throws {@link IllegalArgumentException} if the subtask
507 * has not completed.
508 *
509 * @apiNote The {@code handleComplete} method should be thread safe. It may be
510 * invoked by several threads concurrently.
511 *
512 * @param subtask the subtask
513 *
514 * @throws IllegalArgumentException if called with a subtask that has not completed
515 */
516 protected void handleComplete(Subtask<? extends T> subtask) {
517 if (subtask.state() == Subtask.State.UNAVAILABLE)
518 throw new IllegalArgumentException();
519 }
520
521 /**
522 * Starts a new thread in this task scope to execute a value-returning task, thus
523 * creating a <em>subtask</em> of this task scope.
524 *
525 * <p> The value-returning task is provided to this method as a {@link Callable}, the
526 * thread executes the task's {@link Callable#call() call} method. The thread is
527 * created with the task scope's {@link ThreadFactory}. It inherits the current thread's
528 * {@linkplain ScopedValue scoped value} bindings. The bindings must match the bindings
529 * captured when the task scope was created.
530 *
531 * <p> This method returns a {@link Subtask Subtask} to represent the <em>forked
532 * subtask</em>. The {@code Subtask} object can be used to obtain the result when
533 * the subtask completes successfully, or the exception when the subtask fails. To
534 * ensure correct usage, the {@link Subtask#get() get()} and {@link Subtask#exception()
535 * exception()} methods may only be called by the task scope owner after it has waited
536 * for all threads to finish with the {@link #join() join} or {@link #joinUntil(Instant)}
537 * methods. When the subtask completes, the thread invokes the {@link
538 * #handleComplete(Subtask) handleComplete} method to consume the completed subtask.
539 * If the task scope is {@linkplain #shutdown() shut down} before the subtask completes
540 * then the {@code handleComplete} method will not be invoked.
541 *
542 * <p> If this task scope is {@linkplain #shutdown() shutdown} (or in the process of
543 * shutting down) then the subtask will not run and the {@code handleComplete} method
544 * will not be invoked.
545 *
546 * <p> This method may only be invoked by the task scope owner or threads contained
547 * in the task scope.
548 *
549 * @implSpec This method may be overridden for customization purposes, wrapping tasks
550 * for example. If overridden, the subclass must invoke {@code super.fork} to start a
551 * new thread in this task scope.
552 *
553 * @param task the value-returning task for the thread to execute
554 * @param <U> the result type
555 * @return the subtask
556 * @throws IllegalStateException if this task scope is closed
557 * @throws WrongThreadException if the current thread is not the task scope owner or a
558 * thread contained in the task scope
559 * @throws StructureViolationException if the current scoped value bindings are not
560 * the same as when the task scope was created
561 * @throws RejectedExecutionException if the thread factory rejected creating a
562 * thread to run the subtask
563 */
564 public <U extends T> Subtask<U> fork(Callable<? extends U> task) {
565 Objects.requireNonNull(task, "'task' is null");
566 int s = ensureOpen(); // throws ISE if closed
567
568 // when forked by the owner, the subtask is forked in the current or next round
569 int round = -1;
570 if (Thread.currentThread() == flock.owner()) {
571 round = forkRound;
572 if (forkRound == lastJoinCompleted) {
573 // new round if first fork after join
574 round++;
575 }
576 }
577
578 SubtaskImpl<U> subtask = new SubtaskImpl<>(this, task, round);
579 if (s < SHUTDOWN) {
580 // create thread to run task
581 Thread thread = factory.newThread(subtask);
582 if (thread == null) {
583 throw new RejectedExecutionException("Rejected by thread factory");
584 }
585
586 // attempt to start the thread
587 try {
588 flock.start(thread);
589 } catch (IllegalStateException e) {
590 // shutdown by another thread, or underlying flock is shutdown due
591 // to unstructured use
592 }
593 }
594
595 // force owner to join if this is the first fork in the round
596 if (Thread.currentThread() == flock.owner() && round > forkRound) {
597 forkRound = round;
598 }
599
600 // return forked subtask or a subtask that did not run
601 return subtask;
602 }
603
604 /**
605 * Wait for all threads to finish or the task scope to shut down.
606 */
607 private void implJoin(Duration timeout)
608 throws InterruptedException, TimeoutException
609 {
610 ensureOwner();
611 lastJoinAttempted = forkRound;
612 int s = ensureOpen(); // throws ISE if closed
613 if (s == OPEN) {
614 // wait for all threads, wakeup, interrupt, or timeout
615 if (timeout != null) {
616 flock.awaitAll(timeout);
617 } else {
618 flock.awaitAll();
619 }
620 }
621 lastJoinCompleted = forkRound;
622 }
623
624 /**
625 * Wait for all subtasks started in this task scope to finish or the task scope to
626 * shut down.
627 *
628 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
629 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
630 * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, or
631 * the current thread is {@linkplain Thread#interrupt() interrupted}.
632 *
633 * <p> This method may only be invoked by the task scope owner.
634 *
635 * @implSpec This method may be overridden for customization purposes or to return a
636 * more specific return type. If overridden, the subclass must invoke {@code
637 * super.join} to ensure that the method waits for threads in this task scope to
638 * finish.
639 *
640 * @return this task scope
641 * @throws IllegalStateException if this task scope is closed
642 * @throws WrongThreadException if the current thread is not the task scope owner
643 * @throws InterruptedException if interrupted while waiting
644 */
645 public StructuredTaskScope<T> join() throws InterruptedException {
646 try {
647 implJoin(null);
648 } catch (TimeoutException e) {
649 throw new InternalError();
650 }
651 return this;
652 }
653
654 /**
655 * Wait for all subtasks started in this task scope to finish or the task scope to
656 * shut down, up to the given deadline.
657 *
658 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
659 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
660 * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, the
661 * deadline is reached, or the current thread is {@linkplain Thread#interrupt()
662 * interrupted}.
663 *
664 * <p> This method may only be invoked by the task scope owner.
665 *
666 * @implSpec This method may be overridden for customization purposes or to return a
667 * more specific return type. If overridden, the subclass must invoke {@code
668 * super.joinUntil} to ensure that the method waits for threads in this task scope to
669 * finish.
670 *
671 * @param deadline the deadline
672 * @return this task scope
673 * @throws IllegalStateException if this task scope is closed
674 * @throws WrongThreadException if the current thread is not the task scope owner
675 * @throws InterruptedException if interrupted while waiting
676 * @throws TimeoutException if the deadline is reached while waiting
677 */
678 public StructuredTaskScope<T> joinUntil(Instant deadline)
679 throws InterruptedException, TimeoutException
680 {
681 Duration timeout = Duration.between(Instant.now(), deadline);
682 implJoin(timeout);
683 return this;
684 }
685
686 /**
687 * Interrupt all unfinished threads.
688 */
689 private void interruptAll() {
690 flock.threads()
691 .filter(t -> t != Thread.currentThread())
692 .forEach(t -> {
693 try {
694 t.interrupt();
695 } catch (Throwable ignore) { }
696 });
697 }
698
699 /**
700 * Shutdown the task scope if not already shutdown. Return true if this method
701 * shutdowns the task scope, false if already shutdown.
702 */
703 private boolean implShutdown() {
704 shutdownLock.lock();
705 try {
706 if (state < SHUTDOWN) {
707 // prevent new threads from starting
708 flock.shutdown();
709
710 // set status before interrupting tasks
711 state = SHUTDOWN;
712
713 // interrupt all unfinished threads
714 interruptAll();
715
716 return true;
717 } else {
718 // already shutdown
719 return false;
720 }
721 } finally {
722 shutdownLock.unlock();
723 }
724 }
725
726 /**
727 * Shut down this task scope without closing it. Shutting down a task scope prevents
728 * new threads from starting, interrupts all unfinished threads, and causes the
729 * {@link #join() join} method to wakeup. Shutdown is useful for cases where the
730 * results of unfinished subtasks are no longer needed. It will typically be called
731 * by the {@link #handleComplete(Subtask)} implementation of a subclass that
732 * implements a policy to discard unfinished tasks once some outcome is reached.
733 *
734 * <p> More specifically, this method:
735 * <ul>
736 * <li> {@linkplain Thread#interrupt() Interrupts} all unfinished threads in the
737 * task scope (except the current thread).
738 * <li> Wakes up the task scope owner if it is waiting in {@link #join()} or {@link
739 * #joinUntil(Instant)}. If the task scope owner is not waiting then its next call to
740 * {@code join} or {@code joinUntil} will return immediately.
741 * </ul>
742 *
743 * <p> The {@linkplain Subtask.State state} of unfinished subtasks that complete at
744 * around the time that the task scope is shutdown is not defined. A subtask that
745 * completes successfully with a result, or fails with an exception, at around
746 * the time that the task scope is shutdown may or may not <i>transition</i> to a
747 * terminal state.
748 *
749 * <p> This method may only be invoked by the task scope owner or threads contained
750 * in the task scope.
751 *
752 * @implSpec This method may be overridden for customization purposes. If overridden,
753 * the subclass must invoke {@code super.shutdown} to ensure that the method shuts
754 * down the task scope.
755 *
756 * @apiNote
757 * There may be threads that have not finished because they are executing code that
758 * did not respond (or respond promptly) to thread interrupt. This method does not wait
759 * for these threads. When the owner invokes the {@link #close() close} method
760 * to close the task scope then it will wait for the remaining threads to finish.
761 *
762 * @throws IllegalStateException if this task scope is closed
763 * @throws WrongThreadException if the current thread is not the task scope owner or
764 * a thread contained in the task scope
765 * @see #isShutdown()
766 */
767 public void shutdown() {
768 ensureOwnerOrContainsThread();
769 int s = ensureOpen(); // throws ISE if closed
770 if (s < SHUTDOWN && implShutdown())
771 flock.wakeup();
772 }
773
774 /**
775 * {@return true if this task scope is shutdown, otherwise false}
776 * @see #shutdown()
777 */
778 public final boolean isShutdown() {
779 return state >= SHUTDOWN;
780 }
781
782 /**
783 * Closes this task scope.
784 *
785 * <p> This method first shuts down the task scope (as if by invoking the {@link
786 * #shutdown() shutdown} method). It then waits for the threads executing any
787 * unfinished tasks to finish. If interrupted, this method will continue to wait for
788 * the threads to finish before completing with the interrupt status set.
789 *
790 * <p> This method may only be invoked by the task scope owner. If the task scope
791 * is already closed then the task scope owner invoking this method has no effect.
792 *
793 * <p> A {@code StructuredTaskScope} is intended to be used in a <em>structured
794 * manner</em>. If this method is called to close a task scope before nested task
795 * scopes are closed then it closes the underlying construct of each nested task scope
796 * (in the reverse order that they were created in), closes this task scope, and then
797 * throws {@link StructureViolationException}.
798 * Similarly, if this method is called to close a task scope while executing with
799 * {@linkplain ScopedValue scoped value} bindings, and the task scope was created
800 * before the scoped values were bound, then {@code StructureViolationException} is
801 * thrown after closing the task scope.
802 * If a thread terminates without first closing task scopes that it owns then
803 * termination will cause the underlying construct of each of its open tasks scopes to
804 * be closed. Closing is performed in the reverse order that the task scopes were
805 * created in. Thread termination may therefore be delayed when the task scope owner
806 * has to wait for threads forked in these task scopes to finish.
807 *
808 * @implSpec This method may be overridden for customization purposes. If overridden,
809 * the subclass must invoke {@code super.close} to close the task scope.
810 *
811 * @throws IllegalStateException thrown after closing the task scope if the task scope
812 * owner did not attempt to join after forking
813 * @throws WrongThreadException if the current thread is not the task scope owner
814 * @throws StructureViolationException if a structure violation was detected
815 */
816 @Override
817 public void close() {
818 ensureOwner();
819 int s = state;
820 if (s == CLOSED)
821 return;
822
823 try {
824 if (s < SHUTDOWN)
825 implShutdown();
826 flock.close();
827 } finally {
828 state = CLOSED;
829 }
830
831 // throw ISE if the owner didn't attempt to join after forking
832 if (forkRound > lastJoinAttempted) {
833 lastJoinCompleted = forkRound;
834 throw newIllegalStateExceptionNoJoin();
835 }
836 }
837
838 @Override
839 public String toString() {
840 String name = flock.name();
841 return switch (state) {
842 case OPEN -> name;
843 case SHUTDOWN -> name + "/shutdown";
844 case CLOSED -> name + "/closed";
845 default -> throw new InternalError();
846 };
847 }
848
849 /**
850 * Subtask implementation, runs the task specified to the fork method.
851 */
852 private static final class SubtaskImpl<T> implements Subtask<T>, Runnable {
853 private static final AltResult RESULT_NULL = new AltResult(Subtask.State.SUCCESS);
854
855 private record AltResult(Subtask.State state, Throwable exception) {
856 AltResult(Subtask.State state) {
857 this(state, null);
858 }
859 }
860
861 private final StructuredTaskScope<? super T> scope;
862 private final Callable<? extends T> task;
863 private final int round;
864 private volatile Object result;
865
866 SubtaskImpl(StructuredTaskScope<? super T> scope,
867 Callable<? extends T> task,
868 int round) {
869 this.scope = scope;
870 this.task = task;
871 this.round = round;
872 }
873
874 @Override
875 public void run() {
876 T result = null;
877 Throwable ex = null;
878 try {
879 result = task.call();
880 } catch (Throwable e) {
881 ex = e;
882 }
883
884 // nothing to do if task scope is shutdown
885 if (scope.isShutdown())
886 return;
887
888 // capture result or exception, invoke handleComplete
889 if (ex == null) {
890 this.result = (result != null) ? result : RESULT_NULL;
891 } else {
892 this.result = new AltResult(State.FAILED, ex);
893 }
894 scope.handleComplete(this);
895 }
896
897 @Override
898 public Callable<? extends T> task() {
899 return task;
900 }
901
902 @Override
903 public Subtask.State state() {
904 Object result = this.result;
905 if (result == null) {
906 return State.UNAVAILABLE;
907 } else if (result instanceof AltResult alt) {
908 // null or failed
909 return alt.state();
910 } else {
911 return State.SUCCESS;
912 }
913 }
914
915 @Override
916 public T get() {
917 scope.ensureJoinedIfOwner(round);
918 Object result = this.result;
919 if (result instanceof AltResult) {
920 if (result == RESULT_NULL) return null;
921 } else if (result != null) {
922 @SuppressWarnings("unchecked")
923 T r = (T) result;
924 return r;
925 }
926 throw new IllegalStateException(
927 "Result is unavailable or subtask did not complete successfully");
928 }
929
930 @Override
931 public Throwable exception() {
932 scope.ensureJoinedIfOwner(round);
933 Object result = this.result;
934 if (result instanceof AltResult alt && alt.state() == State.FAILED) {
935 return alt.exception();
936 }
937 throw new IllegalStateException(
938 "Exception is unavailable or subtask did not complete with exception");
939 }
940
941 @Override
942 public String toString() {
943 String stateAsString = switch (state()) {
944 case UNAVAILABLE -> "[Unavailable]";
945 case SUCCESS -> "[Completed successfully]";
946 case FAILED -> {
947 Throwable ex = ((AltResult) result).exception();
948 yield "[Failed: " + ex + "]";
949 }
950 };
951 return Objects.toIdentityString(this) + stateAsString;
952 }
953 }
954
955 /**
956 * A {@code StructuredTaskScope} that captures the result of the first subtask to
957 * complete {@linkplain Subtask.State#SUCCESS successfully}. Once captured, it
958 * {@linkplain #shutdown() shuts down} the task scope to interrupt unfinished threads
959 * and wakeup the task scope owner. The policy implemented by this class is intended
960 * for cases where the result of any subtask will do ("invoke any") and where the
961 * results of other unfinished subtasks are no longer needed.
962 *
963 * <p> Unless otherwise specified, passing a {@code null} argument to a method
964 * in this class will cause a {@link NullPointerException} to be thrown.
965 *
966 * @apiNote This class implements a policy to shut down the task scope when a subtask
967 * completes successfully. There shouldn't be any need to directly shut down the task
968 * scope with the {@link #shutdown() shutdown} method.
969 *
970 * @param <T> the result type
971 * @since 21
972 */
973 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
974 public static final class ShutdownOnSuccess<T> extends StructuredTaskScope<T> {
975 private static final Object RESULT_NULL = new Object();
976 private static final VarHandle FIRST_RESULT;
977 private static final VarHandle FIRST_EXCEPTION;
978 static {
979 MethodHandles.Lookup l = MethodHandles.lookup();
980 FIRST_RESULT = MhUtil.findVarHandle(l, "firstResult", Object.class);
981 FIRST_EXCEPTION = MhUtil.findVarHandle(l, "firstException", Throwable.class);
982 }
983 private volatile Object firstResult;
984 private volatile Throwable firstException;
985
986 /**
987 * Constructs a new {@code ShutdownOnSuccess} with the given name and thread factory.
988 * The task scope is optionally named for the purposes of monitoring and management.
989 * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
990 * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
991 * is owned by the current thread.
992 *
993 * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
994 * value} bindings for inheritance by threads started in the task scope. The
995 * {@linkplain StructuredTaskScope##TreeStructure Tree Structure} section
996 * in the class description details how parent-child relations are established
997 * implicitly for the purpose of inheritance of scoped value bindings.
998 *
999 * @param name the name of the task scope, can be null
1000 * @param factory the thread factory
1001 */
1002 public ShutdownOnSuccess(String name, ThreadFactory factory) {
1003 super(name, factory);
1004 }
1005
1006 /**
1007 * Constructs a new unnamed {@code ShutdownOnSuccess} that creates virtual threads.
1008 *
1009 * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1010 * a name of {@code null} and a thread factory that creates virtual threads.
1011 */
1012 public ShutdownOnSuccess() {
1013 this(null, Thread.ofVirtual().factory());
1014 }
1015
1016 @Override
1017 protected void handleComplete(Subtask<? extends T> subtask) {
1018 if (firstResult != null) {
1019 // already captured a result
1020 return;
1021 }
1022
1023 if (subtask.state() == Subtask.State.SUCCESS) {
1024 // task succeeded
1025 T result = subtask.get();
1026 Object r = (result != null) ? result : RESULT_NULL;
1027 if (FIRST_RESULT.compareAndSet(this, null, r)) {
1028 super.shutdown();
1029 }
1030 } else if (firstException == null) {
1031 // capture the exception thrown by the first subtask that failed
1032 FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
1033 }
1034 }
1035
1036 /**
1037 * Wait for a subtask started in this task scope to complete {@linkplain
1038 * Subtask.State#SUCCESS successfully} or all subtasks to complete.
1039 *
1040 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1041 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1042 * when all threads finish, a subtask completes successfully, or the current
1043 * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1044 * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1045 * this task scope.
1046 *
1047 * <p> This method may only be invoked by the task scope owner.
1048 *
1049 * @throws IllegalStateException {@inheritDoc}
1050 * @throws WrongThreadException {@inheritDoc}
1051 */
1052 @Override
1053 public ShutdownOnSuccess<T> join() throws InterruptedException {
1054 super.join();
1055 return this;
1056 }
1057
1058 /**
1059 * Wait for a subtask started in this task scope to complete {@linkplain
1060 * Subtask.State#SUCCESS successfully} or all subtasks to complete, up to the
1061 * given deadline.
1062 *
1063 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1064 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1065 * when all threads finish, a subtask completes successfully, the deadline is
1066 * reached, or the current thread is {@linkplain Thread#interrupt() interrupted}.
1067 * It also stops waiting if the {@link #shutdown() shutdown} method is invoked
1068 * directly to shut down this task scope.
1069 *
1070 * <p> This method may only be invoked by the task scope owner.
1071 *
1072 * @throws IllegalStateException {@inheritDoc}
1073 * @throws WrongThreadException {@inheritDoc}
1074 */
1075 @Override
1076 public ShutdownOnSuccess<T> joinUntil(Instant deadline)
1077 throws InterruptedException, TimeoutException
1078 {
1079 super.joinUntil(deadline);
1080 return this;
1081 }
1082
1083 /**
1084 * {@return the result of the first subtask that completed {@linkplain
1085 * Subtask.State#SUCCESS successfully}}
1086 *
1087 * <p> When no subtask completed successfully, but a subtask {@linkplain
1088 * Subtask.State#FAILED failed} then {@code ExecutionException} is thrown with
1089 * the subtask's exception as the {@linkplain Throwable#getCause() cause}.
1090 *
1091 * @throws ExecutionException if no subtasks completed successfully but at least
1092 * one subtask failed
1093 * @throws IllegalStateException if no subtasks completed or the task scope owner
1094 * did not join after forking
1095 * @throws WrongThreadException if the current thread is not the task scope owner
1096 */
1097 public T result() throws ExecutionException {
1098 return result(ExecutionException::new);
1099 }
1100
1101 /**
1102 * Returns the result of the first subtask that completed {@linkplain
1103 * Subtask.State#SUCCESS successfully}, otherwise throws an exception produced
1104 * by the given exception supplying function.
1105 *
1106 * <p> When no subtask completed successfully, but a subtask {@linkplain
1107 * Subtask.State#FAILED failed}, then the exception supplying function is invoked
1108 * with subtask's exception.
1109 *
1110 * @param esf the exception supplying function
1111 * @param <X> type of the exception to be thrown
1112 * @return the result of the first subtask that completed with a result
1113 *
1114 * @throws X if no subtasks completed successfully but at least one subtask failed
1115 * @throws IllegalStateException if no subtasks completed or the task scope owner
1116 * did not join after forking
1117 * @throws WrongThreadException if the current thread is not the task scope owner
1118 */
1119 public <X extends Throwable> T result(Function<Throwable, ? extends X> esf) throws X {
1120 Objects.requireNonNull(esf);
1121 ensureOwnerAndJoined();
1122
1123 Object result = firstResult;
1124 if (result == RESULT_NULL) {
1125 return null;
1126 } else if (result != null) {
1127 @SuppressWarnings("unchecked")
1128 T r = (T) result;
1129 return r;
1130 }
1131
1132 Throwable exception = firstException;
1133 if (exception != null) {
1134 X ex = esf.apply(exception);
1135 Objects.requireNonNull(ex, "esf returned null");
1136 throw ex;
1137 }
1138
1139 throw new IllegalStateException("No completed subtasks");
1140 }
1141 }
1142
1143 /**
1144 * A {@code StructuredTaskScope} that captures the exception of the first subtask to
1145 * {@linkplain Subtask.State#FAILED fail}. Once captured, it {@linkplain #shutdown()
1146 * shuts down} the task scope to interrupt unfinished threads and wakeup the task
1147 * scope owner. The policy implemented by this class is intended for cases where the
1148 * results for all subtasks are required ("invoke all"); if any subtask fails then the
1149 * results of other unfinished subtasks are no longer needed.
1150 *
1151 * <p> Unless otherwise specified, passing a {@code null} argument to a method
1152 * in this class will cause a {@link NullPointerException} to be thrown.
1153 *
1154 * @apiNote This class implements a policy to shut down the task scope when a subtask
1155 * fails. There shouldn't be any need to directly shut down the task scope with the
1156 * {@link #shutdown() shutdown} method.
1157 *
1158 * @since 21
1159 */
1160 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
1161 public static final class ShutdownOnFailure extends StructuredTaskScope<Object> {
1162 private static final VarHandle FIRST_EXCEPTION =
1163 MhUtil.findVarHandle(MethodHandles.lookup(), "firstException", Throwable.class);
1164 private volatile Throwable firstException;
1165
1166 /**
1167 * Constructs a new {@code ShutdownOnFailure} with the given name and thread factory.
1168 * The task scope is optionally named for the purposes of monitoring and management.
1169 * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
1170 * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
1171 * is owned by the current thread.
1172 *
1173 * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
1174 * value} bindings for inheritance by threads started in the task scope. The
1175 * {@linkplain StructuredTaskScope##TreeStructure Tree Structure} section in the class description
1176 * details how parent-child relations are established implicitly for the purpose
1177 * of inheritance of scoped value bindings.
1178 *
1179 * @param name the name of the task scope, can be null
1180 * @param factory the thread factory
1181 */
1182 public ShutdownOnFailure(String name, ThreadFactory factory) {
1183 super(name, factory);
1184 }
1185
1186 /**
1187 * Constructs a new unnamed {@code ShutdownOnFailure} that creates virtual threads.
1188 *
1189 * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1190 * a name of {@code null} and a thread factory that creates virtual threads.
1191 */
1192 public ShutdownOnFailure() {
1193 this(null, Thread.ofVirtual().factory());
1194 }
1195
1196 @Override
1197 protected void handleComplete(Subtask<?> subtask) {
1198 if (subtask.state() == Subtask.State.FAILED
1199 && firstException == null
1200 && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception())) {
1201 super.shutdown();
1202 }
1203 }
1204
1205 /**
1206 * Wait for all subtasks started in this task scope to complete or for a subtask
1207 * to {@linkplain Subtask.State#FAILED fail}.
1208 *
1209 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1210 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1211 * when all threads finish, a subtask fails, or the current thread is {@linkplain
1212 * Thread#interrupt() interrupted}. It also stops waiting if the {@link #shutdown()
1213 * shutdown} method is invoked directly to shut down this task scope.
1214 *
1215 * <p> This method may only be invoked by the task scope owner.
1216 *
1217 * @throws IllegalStateException {@inheritDoc}
1218 * @throws WrongThreadException {@inheritDoc}
1219 */
1220 @Override
1221 public ShutdownOnFailure join() throws InterruptedException {
1222 super.join();
1223 return this;
1224 }
1225
1226 /**
1227 * Wait for all subtasks started in this task scope to complete or for a subtask
1228 * to {@linkplain Subtask.State#FAILED fail}, up to the given deadline.
1229 *
1230 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1231 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1232 * when all threads finish, a subtask fails, the deadline is reached, or the current
1233 * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1234 * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1235 * this task scope.
1236 *
1237 * <p> This method may only be invoked by the task scope owner.
1238 *
1239 * @throws IllegalStateException {@inheritDoc}
1240 * @throws WrongThreadException {@inheritDoc}
1241 */
1242 @Override
1243 public ShutdownOnFailure joinUntil(Instant deadline)
1244 throws InterruptedException, TimeoutException
1245 {
1246 super.joinUntil(deadline);
1247 return this;
1248 }
1249
1250 /**
1251 * Returns the exception of the first subtask that {@linkplain Subtask.State#FAILED
1252 * failed}. If no subtasks failed then an empty {@code Optional} is returned.
1253 *
1254 * @return the exception for the first subtask to fail or an empty optional if no
1255 * subtasks failed
1256 *
1257 * @throws WrongThreadException if the current thread is not the task scope owner
1258 * @throws IllegalStateException if the task scope owner did not join after forking
1259 */
1260 public Optional<Throwable> exception() {
1261 ensureOwnerAndJoined();
1262 return Optional.ofNullable(firstException);
1263 }
1264
1265 /**
1266 * Throws if a subtask {@linkplain Subtask.State#FAILED failed}.
1267 * If any subtask failed with an exception then {@code ExecutionException} is
1268 * thrown with the exception of the first subtask to fail as the {@linkplain
1269 * Throwable#getCause() cause}. This method does nothing if no subtasks failed.
1270 *
1271 * @throws ExecutionException if a subtask failed
1272 * @throws WrongThreadException if the current thread is not the task scope owner
1273 * @throws IllegalStateException if the task scope owner did not join after forking
1274 */
1275 public void throwIfFailed() throws ExecutionException {
1276 throwIfFailed(ExecutionException::new);
1277 }
1278
1279 /**
1280 * Throws the exception produced by the given exception supplying function if a
1281 * subtask {@linkplain Subtask.State#FAILED failed}. If any subtask failed with
1282 * an exception then the function is invoked with the exception of the first
1283 * subtask to fail. The exception returned by the function is thrown. This method
1284 * does nothing if no subtasks failed.
1285 *
1286 * @param esf the exception supplying function
1287 * @param <X> type of the exception to be thrown
1288 *
1289 * @throws X produced by the exception supplying function
1290 * @throws WrongThreadException if the current thread is not the task scope owner
1291 * @throws IllegalStateException if the task scope owner did not join after forking
1292 */
1293 public <X extends Throwable>
1294 void throwIfFailed(Function<Throwable, ? extends X> esf) throws X {
1295 ensureOwnerAndJoined();
1296 Objects.requireNonNull(esf);
1297 Throwable exception = firstException;
1298 if (exception != null) {
1299 X ex = esf.apply(exception);
1300 Objects.requireNonNull(ex, "esf returned null");
1301 throw ex;
1302 }
1303 }
1304 }
1305 }