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
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
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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.security.AccessController;
30 import java.security.PrivilegedAction;
31 import java.time.Duration;
32 import java.util.ArrayList;
33 import java.util.List;
34 import java.util.NoSuchElementException;
35 import java.util.Objects;
36 import java.util.function.Function;
37 import java.util.function.Predicate;
38 import java.util.function.Supplier;
39 import java.util.stream.Stream;
40 import jdk.internal.javac.PreviewFeature;
41 import jdk.internal.misc.InnocuousThread;
42 import jdk.internal.misc.ThreadFlock;
43
44 /**
45 * An API for <em>structured concurrency</em>. {@code StructuredTaskScope} supports cases
46 * where a main task splits into several concurrent subtasks, and where the subtasks must
47 * complete before the main task continues. A {@code StructuredTaskScope} can be used to
48 * ensure that the lifetime of a concurrent operation is confined by a <em>syntax block</em>,
49 * just like that of a sequential operation in structured programming.
50 *
51 * <p> {@code StructuredTaskScope} defines the static method {@link #open(Joiner) open}
52 * to open a new {@code StructuredTaskScope} and the {@link #close() close} method to close
53 * it. The API is designed to be used with the {@code try-with-resources} statement where
54 * the {@code StructuredTaskScope} is opened as a resource and then closed automatically.
55 * The code in the block uses the {@link #fork(Callable) fork} method to fork subtasks.
56 * After forking, it uses the {@link #join() join} method to wait for all subtasks to
57 * finish (or some other outcome) as a single operation. Forking a subtask starts a new
58 * {@link Thread} to run the subtask. The thread executing the main task does not continue
59 * beyond the {@code close} method until all threads started to execute subtasks have finished.
60 * To ensure correct usage, the {@code fork}, {@code join} and {@code close} methods may
61 * only be invoked by the <em>owner thread</em> (the thread that opened the {@code
62 * StructuredTaskScope}), and the {@code close} method throws an exception after closing
63 * if the owner did not invoke the {@code join} method.
64 *
65 * <p> A {@code StructuredTaskScope} is opened with a {@link Joiner} that handles subtask
66 * completion and produces the outcome (the result or an exception) for the {@link #join()
67 * join} method. The {@code Joiner} interface defines static methods to create a
68 * {@code Joiner} for common cases.
69 *
70 * <p> A {@code Joiner} may <a id="CancelExecution"><em>cancel execution</em></a>
71 * (sometimes called "short-circuiting") when some condition is reached that does not
72 * require the result of subtasks that are still executing. Cancelling execution prevents
73 * new threads from being started to execute further subtasks, {@linkplain Thread#interrupt()
74 * interrupts} the threads executing subtasks that have not completed, and causes the
75 * {@code join} method to wakeup with a result (or exception). The {@link #close() close}
76 * method always waits for threads executing subtasks to finish, even if execution is
77 * cancelled, so it cannot continue beyond the {@code close} method until the interrupted
78 * threads finish. Subtasks should be coded so that they finish as soon as possible when
79 * interrupted. Subtasks that block on methods that are not interruptible may delay the
80 * closing of a task scope.
81 *
82 * <p> Consider the example of a main task that splits into two subtasks to concurrently
83 * fetch resources from two URL locations "left" and "right". Both subtasks may complete
84 * successfully, one subtask may succeed and the other may fail, or both subtasks may
85 * fail. The main task in this example is interested in the successful result from both
86 * subtasks. It uses {@link Joiner#awaitAllSuccessfulOrThrow()
87 * Joiner.awaitAllSuccessfulOrThrow()} to create a {@code Joiner} that waits for both
88 * subtasks to complete successfully or for either subtask to fail.
89 * {@snippet lang=java :
90 * try (var scope = StructuredTaskScope.open(Joiner.awaitAllSuccessfulOrThrow())) {
91 *
92 * // @link substring="fork" target="#fork(Callable)" :
93 * Subtask<String> subtask1 = scope.fork(() -> query(left));
94 * Subtask<Integer> subtask2 = scope.fork(() -> query(right));
95 *
96 * // throws if either subtask fails
97 * scope.join(); // @link substring="join" target="#join()"
98 *
99 * // both subtasks completed successfully
100 * return new MyResult(subtask1.get(), subtask2.get()); // @link substring="get" target="Subtask#get()"
101 *
102 * }
103 * }
104 *
105 * <p> In this example, the main task forks the two subtasks. The {@code fork} method
106 * returns a {@link Subtask Subtask} that is a handle to the forked subtask. The main task
107 * waits in the {@code join} method for both subtasks to complete successfully or for either
108 * subtask to fail. If both subtasks complete successfully then the {@code join} method
109 * completes and the main task uses the {@link Subtask#get() Subtask.get()} method to get
110 * the result of each subtask. If either subtask fails then the {@code Joiner} causes the
111 * other subtask to be cancelled (this will interrupt the thread executing the subtask)
112 * and the {@code join} method throws {@link ExecutionException} with the exception from
113 * the failed subtask as the {@linkplain Throwable#getCause() cause}.
114 *
115 * <p> Now consider another example that also splits into two subtasks to concurrently
116 * fetch resources. In this example, the code in the main task is only interested in the
117 * result from the first subtask to complete successfully. The example uses {@link
118 * Joiner#anySuccessfulResultOrThrow() Joiner.anySuccessfulResultOrThrow()} to
119 * create a {@code Joiner} that makes available the result of the first subtask to
120 * complete successfully. The type parameter in the example is "{@code String}" so that
121 * only subtasks that return a {@code String} can be forked.
122 * {@snippet lang=java :
123 * // @link substring="open" target="#open(Policy)" :
124 * try (var scope = StructuredTaskScope.open(Joiner.<String>anySuccessfulResultOrThrow())) {
125 *
126 * scope.fork(() -> query(left)); // @link substring="fork" target="#fork(Callable)"
127 * scope.fork(() -> query(right));
128 *
129 * // throws if both subtasks fail
130 * String firstResult = scope.join();
131 *
132 * // @link substring="close" target="#close()" :
133 * } // close
134 * }
135 *
136 * <p> In the example, the main task forks the two subtasks, then waits in the {@code
137 * join} method for either subtask to complete successfully or for both subtasks to fail.
138 * If one of the subtasks completes successfully then the {@code Joiner} causes the other
139 * subtask to be cancelled (this will interrupt the thread executing the subtask), and
140 * the {@code join} method returns the result from the first subtask. Cancelling the other
141 * subtask avoids the main task waiting for a result that it doesn't care about. If both
142 * subtasks fail then the {@code join} method throws {@link ExecutionException} with the
143 * exception from one of the subtasks as the {@linkplain Throwable#getCause() cause}.
144 *
145 * <p> Whether code uses the {@code Subtask} returned from {@code fork} will depend on
146 * the {@code Joiner} and usage. Some {@code Joiner} implementations are suited to subtasks
147 * that return results of the same type and where the {@code join} method returns a result
148 * for the main task to use. Code that forks subtasks that return results of different
149 * types, and uses a {@code Joiner} such as {@code Joiner.awaitAllSuccessfulOrThrow()} that
150 * does not return a result, will use {@link Subtask#get() Subtask.get()} after joining.
151 *
152 * <h2>Exception handling</h2>
153 *
154 * <p> A {@code StructuredTaskScope} is opened with a {@link Joiner Joiner} that
155 * handles subtask completion and produces the outcome for the {@link #join() join} method.
156 * In some cases, the outcome will be a result, in other cases it will be an exception.
157 * If the outcome is an exception then the {@code join} method throws {@link
158 * ExecutionException} with the exception as the {@linkplain Throwable#getCause()
159 * cause}. For many {@code Joiner} implementations, the exception will be an exception
160 * thrown by a subtask that failed. In the case of {@link Joiner#allSuccessfulOrThrow()
161 * allSuccessfulOrThrow} and {@link Joiner#awaitAllSuccessfulOrThrow() awaitAllSuccessfulOrThrow}
162 * for example, the exception is from the first subtask to fail.
163 *
164 * <p> Many of the details for how exceptions are handled will depend on usage. In some
165 * cases, the {@code join} method will be called in a {@code try-catch} block to catch
166 * {@code ExecutionException} and handle the cause. The exception handling may use
167 * {@code instanceof} with pattern matching to handle specific causes. In some cases it
168 * may not be useful to catch {@code ExecutionException} but instead leave it to propagate
169 * to the configured {@linkplain Thread.UncaughtExceptionHandler uncaught exception handler}
170 * for logging purposes.
171 *
172 * <p> For cases where a specific exception triggers the use of a default result then it
173 * may be more appropriate to handle this in the subtask itself rather than the subtask
174 * failing and code in the main task handling the exception.
175 *
176 * <h2>Configuration</h2>
177 *
178 * A {@code StructuredTaskScope} is opened with {@linkplain Config configuration} that
179 * consists of a {@link ThreadFactory} to create threads, an optional name for monitoring
180 * and management purposes, and an optional timeout.
181 *
182 * <p> The 1-arg {@link #open(Joiner) open} method creates a {@code StructuredTaskScope}
183 * with the <a id="DefaultConfiguration"> <em>default configuration</em></a>. The default
184 * configuration has a {@code ThreadFactory} that creates unnamed
185 * <a href="{@docRoot}/java.base/java/lang/Thread.html#virtual-threads">virtual threads</a>,
186 * is unnamed for monitoring and management purposes, and has no timeout.
187 *
188 * <p> The 2-arg {@link #open(Joiner, Function) open} method can be used to create a
189 * {@code StructuredTaskScope} that uses a different {@code ThreadFactory}, has a name for
190 * the purposes of monitoring and management, or has a timeout that cancels execution if
191 * the timeout expires before or while waiting for subtasks to complete. The {@code open}
192 * method is called with a {@linkplain Function function} that is applied to the default
193 * configuration and returns a {@link Config Config} for the {@code StructuredTaskScope}
194 * under construction.
195 *
196 * <p> The following example opens a new {@code StructuredTaskScope} with a {@code
197 * ThreadFactory} that creates virtual threads {@linkplain Thread#setName(String) named}
198 * "duke-0", "duke-1" ...
199 * {@snippet lang = java:
200 * // @link substring="name" target="Thread.Builder#name(String, long)" :
201 * ThreadFactory factory = Thread.ofVirtual().name("duke-", 0).factory();
202 *
203 * // @link substring="withThreadFactory" target="Config#withThreadFactory(ThreadFactory)" :
204 * try (var scope = StructuredTaskScope.open(joiner, cf -> cf.withThreadFactory(factory))) {
205 *
206 * scope.fork( .. ); // runs in a virtual thread with name "duke-0"
207 * scope.fork( .. ); // runs in a virtual thread with name "duke-1"
208 *
209 * scope.join();
210 *
211 * }
212 *}
213 *
214 * <p> A second example sets a timeout, represented by a {@link Duration}. The timeout
215 * starts when the new task scope is opened. If the timeout expires before the {@code join}
216 * method has completed then <a href="#CancelExecution">execution is cancelled</a>. This
217 * interrupts the threads executing the two subtasks and causes the {@link #join() join}
218 * method to throw {@link ExecutionException} with {@link TimeoutException} as the cause.
219 * {@snippet lang=java :
220 * Duration timeout = Duration.ofSeconds(10);
221 *
222 * // @link substring="allSuccessfulOrThrow" target="Joiner#allSuccessfulOrThrow()" :
223 * try (var scope = StructuredTaskScope.open(Joiner.<String>allSuccessfulOrThrow(),
224 * // @link substring="withTimeout" target="Config#withTimeout(Duration)" :
225 * cf -> cf.withTimeout(timeout))) {
226 *
227 * scope.fork(() -> query(left));
228 * scope.fork(() -> query(right));
229 *
230 * List<String> result = scope.join()
231 * .map(Subtask::get)
232 * .toList();
233 *
234 * }
235 * }
236 *
237 * <h2>Inheritance of scoped value bindings</h2>
238 *
239 * {@link ScopedValue} supports the execution of a method with a {@code ScopedValue} bound
240 * to a value for the bounded period of execution of the method by the <em>current thread</em>.
241 * It allows a value to be safely and efficiently shared to methods without using method
242 * parameters.
243 *
244 * <p> When used in conjunction with a {@code StructuredTaskScope}, a {@code ScopedValue}
245 * can also safely and efficiently share a value to methods executed by subtasks forked
246 * in the task scope. When a {@code ScopedValue} object is bound to a value in the thread
247 * executing the main task then that binding is inherited by the threads created to
248 * execute the subtasks. The thread executing the main task does not continue beyond the
249 * {@link #close() close} method until all threads executing the subtasks have finished.
250 * This ensures that the {@code ScopedValue} is not reverted to being {@linkplain
251 * ScopedValue#isBound() unbound} (or its previous value) while subtasks are executing.
252 * In addition to providing a safe and efficient means to inherit a value into subtasks,
253 * the inheritance allows sequential code using {@code ScopedValue} be refactored to use
254 * structured concurrency.
255 *
256 * <p> To ensure correctness, opening a new {@code StructuredTaskScope} captures the
257 * current thread's scoped value bindings. These are the scoped values bindings that are
258 * inherited by the threads created to execute subtasks in the task scope. Forking a
259 * subtask checks that the bindings in effect at the time that the subtask is forked
260 * match the bindings when the {@code StructuredTaskScope} was created. This check ensures
261 * that a subtask does not inherit a binding that is reverted in the main task before the
262 * subtask has completed.
263 *
264 * <p> A {@code ScopedValue} that is shared across threads requires that the value be an
265 * immutable object or for all access to the value to be appropriately synchronized.
266 *
267 * <p> The following example demonstrates the inheritance of scoped value bindings. The
268 * scoped value USERNAME is bound to the value "duke" for the bounded period of a lambda
269 * expression by the thread executing it. The code in the block opens a {@code
270 * StructuredTaskScope} and forks two subtasks, it then waits in the {@code join} method
271 * and aggregates the results from both subtasks. If code executed by the threads
272 * running subtask1 and subtask2 uses {@link ScopedValue#get()}, to get the value of
273 * USERNAME, then value "duke" will be returned.
274 * {@snippet lang=java :
275 * // @link substring="newInstance" target="ScopedValue#newInstance()" :
276 * private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();
277 *
278 * // @link substring="callWhere" target="ScopedValue#callWhere" :
279 * Result result = ScopedValue.callWhere(USERNAME, "duke", () -> {
280 *
281 * try (var scope = StructuredTaskScope.open(Joiner.awaitAllSuccessfulOrThrow())) {
282 *
283 * Subtask<String> subtask1 = scope.fork( .. ); // inherits binding
284 * Subtask<Integer> subtask2 = scope.fork( .. ); // inherits binding
285 *
286 * scope.join();
287 * return new MyResult(subtask1.get(), subtask2.get());
288 * }
289 *
290 * });
291 * }
292 *
293 * <p> A scoped value inherited into a subtask may be
294 * <a href="{@docRoot}/java.base/java/lang/ScopedValues.html#rebind">rebound</a> to a new
295 * value in the subtask for the bounded execution of some method executed in the subtask.
296 * When the method completes, the value of the {@code ScopedValue} reverts to its previous
297 * value, the value inherited from the thread executing the main task.
298 *
299 * <p> A subtask may execute code that itself opens a new {@code StructuredTaskScope}.
300 * A main task executing in thread T1 opens a {@code StructuredTaskScope} and forks a
301 * subtask that runs in thread T2. The scoped value bindings captured when T1 opens the
302 * task scope are inherited into T2. The subtask (in thread T2) executes code that opens a
303 * new {@code StructuredTaskScope} and forks a subtask that runs in thread T3. The scoped
304 * value bindings captured when T2 opens the task scope are inherited into T3. These
305 * include (or may be the same) as the bindings that were inherited from T1. In effect,
306 * scoped values are inherited into a tree of subtasks, not just one level of subtask.
307 *
308 * <h2>Memory consistency effects</h2>
309 *
310 * <p> Actions in the owner thread of a {@code StructuredTaskScope} prior to
311 * {@linkplain #fork forking} of a subtask
312 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility">
313 * <i>happen-before</i></a> any actions taken by that subtask, which in turn
314 * <i>happen-before</i> the subtask result is {@linkplain Subtask#get() retrieved}.
315 *
316 * <h2>General exceptions</h2>
317 *
318 * <p> Unless otherwise specified, passing a {@code null} argument to a method in this
319 * class will cause a {@link NullPointerException} to be thrown.
320 *
321 * @param <T> the result type of tasks executed in the task scope
322 * @param <R> the type of the result returned by the join method
323 *
324 * @jls 17.4.5 Happens-before Order
325 * @since 21
326 */
327 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
328 public class StructuredTaskScope<T, R> implements AutoCloseable {
329 private static final VarHandle CANCELLED;
330 static {
331 try {
332 MethodHandles.Lookup l = MethodHandles.lookup();
333 CANCELLED = l.findVarHandle(StructuredTaskScope.class,"cancelled", boolean.class);
334 } catch (Exception e) {
335 throw new ExceptionInInitializerError(e);
336 }
337 }
338
339 private final Joiner<? super T, ? extends R> joiner;
340 private final ThreadFactory threadFactory;
341 private final ThreadFlock flock;
342
343 // fields that are only accessed by owner thread
344 private boolean needToJoin; // set by fork to indicate that owner must join
345 private boolean joined; // set to true when owner joins
346 private boolean closed;
347 private Future<?> timerTask;
348
349 // set or read by any thread
350 private volatile boolean cancelled;
351
352 // set by the timer thread, read by the owner thread
353 private volatile boolean timeoutExpired;
354
355 /**
356 * Throws IllegalStateException if the task scope is closed.
357 */
358 private void ensureOpen() {
359 assert Thread.currentThread() == flock.owner();
360 if (closed) {
361 throw new IllegalStateException("Task scope is closed");
362 }
363 }
364
365 /**
366 * Throws WrongThreadException if the current thread is not the owner thread.
367 */
368 private void ensureOwner() {
369 if (Thread.currentThread() != flock.owner()) {
370 throw new WrongThreadException("Current thread not owner");
371 }
372 }
373
374 /**
375 * Throws IllegalStateException if invoked by the owner thread and the owner thread
376 * has not joined.
377 */
378 private void ensureJoinedIfOwner() {
379 if (Thread.currentThread() == flock.owner() && !joined) {
380 String msg = needToJoin ? "Owner did not join" : "join did not complete";
381 throw new IllegalStateException(msg);
382 }
383 }
384
385 /**
386 * Interrupts all threads in this task scope, except the current thread.
387 */
388 private void implInterruptAll() {
389 flock.threads()
390 .filter(t -> t != Thread.currentThread())
391 .forEach(t -> {
392 try {
393 t.interrupt();
394 } catch (Throwable ignore) { }
395 });
396 }
397
398 @SuppressWarnings("removal")
399 private void interruptAll() {
400 if (System.getSecurityManager() == null) {
401 implInterruptAll();
402 } else {
403 PrivilegedAction<Void> pa = () -> {
404 implInterruptAll();
405 return null;
406 };
407 AccessController.doPrivileged(pa);
408 }
409 }
410
411 /**
412 * Cancel exception if not already cancelled.
413 */
414 private void cancelExecution() {
415 if (!cancelled && CANCELLED.compareAndSet(this, false, true)) {
416 // prevent new threads from starting
417 flock.shutdown();
418
419 // interrupt all unfinished threads
420 interruptAll();
421
422 // wakeup join
423 flock.wakeup();
424 }
425 }
426
427 /**
428 * Schedules a task to cancel execution on timeout.
429 */
430 private void scheduleTimeout(Duration timeout) {
431 assert Thread.currentThread() == flock.owner() && timerTask == null;
432 timerTask = TimerSupport.schedule(timeout, () -> {
433 if (!cancelled) {
434 timeoutExpired = true;
435 cancelExecution();
436 }
437 });
438 }
439
440 /**
441 * Cancels the timer task if set.
442 */
443 private void cancelTimeout() {
444 assert Thread.currentThread() == flock.owner();
445 if (timerTask != null) {
446 timerTask.cancel(false);
447 }
448 }
449
450 /**
451 * Invoked by the thread for a subtask when the subtask completes before execution
452 * was cancelled.
453 */
454 private void onComplete(SubtaskImpl<? extends T> subtask) {
455 assert subtask.state() != Subtask.State.UNAVAILABLE;
456 if (joiner.onComplete(subtask)) {
457 cancelExecution();
458 }
459 }
460
461 /**
462 * Initialize a new StructuredTaskScope.
463 */
464 @SuppressWarnings("this-escape")
465 private StructuredTaskScope(Joiner<? super T, ? extends R> joiner,
466 ThreadFactory threadFactory,
467 String name) {
468 this.joiner = joiner;
469 this.threadFactory = threadFactory;
470
471 if (name == null)
472 name = Objects.toIdentityString(this);
473 this.flock = ThreadFlock.open(name);
474 }
475
476 /**
477 * Represents a subtask forked with {@link #fork(Callable)} or {@link #fork(Runnable)}.
478 *
479 * <p> Code that forks subtasks can use the {@link #get() get()} method after {@linkplain
480 * #join() joining} to obtain the result of a subtask that completed successfully. It
481 * can use the {@link #exception()} method to obtain the exception thrown by a subtask
482 * that failed.
483 *
484 * @param <T> the result type
485 * @since 21
486 */
487 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
488 public sealed interface Subtask<T> extends Supplier<T> permits SubtaskImpl {
489 /**
490 * Represents the state of a subtask.
491 * @see Subtask#state()
492 * @since 21
493 */
494 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
495 enum State {
496 /**
497 * The subtask result or exception is not available. This state indicates that
498 * the subtask was forked but has not completed, it completed after execution
499 * was cancelled, or it was forked after execution was cancelled (in which
500 * case a thread was not created to execute the subtask).
501 */
502 UNAVAILABLE,
503 /**
504 * The subtask completed successfully. The {@link Subtask#get() Subtask.get()}
505 * method can be used to get the result. This is a terminal state.
506 */
507 SUCCESS,
508 /**
509 * The subtask failed with an exception. The {@link Subtask#exception()
510 * Subtask.exception()} method can be used to get the exception. This is a
511 * terminal state.
512 */
513 FAILED,
514 }
515
516 /**
517 * {@return the subtask state}
518 */
519 State state();
520
521 /**
522 * Returns the result of this subtask if it completed successfully. If
523 * {@linkplain #fork(Callable) forked} to execute a value-returning task then the
524 * result from the {@link Callable#call() call} method is returned. If
525 * {@linkplain #fork(Runnable) forked} to execute a task that does not return a
526 * result then {@code null} is returned.
527 *
528 * <p> Code executing in the scope owner thread can use this method to get the
529 * result of a successful subtask only after it has {@linkplain #join() joined}.
530 *
531 * <p> Code executing in the {@code Joiner} {@link Joiner#onComplete(Subtask)
532 * onComplete} method should test that the {@linkplain #state() subtask state} is
533 * {@link State#SUCCESS SUCCESS} before using this method to get the result.
534 *
535 * @return the possibly-null result
536 * @throws IllegalStateException if the subtask has not completed, did not complete
537 * successfully, or the current thread is the task scope owner and it has not joined
538 * @see State#SUCCESS
539 */
540 T get();
541
542 /**
543 * {@return the exception thrown by this subtask if it failed} If
544 * {@linkplain #fork(Callable) forked} to execute a value-returning task then
545 * the exception thrown by the {@link Callable#call() call} method is returned.
546 * If {@linkplain #fork(Runnable) forked} to execute a task that does not return
547 * a result then the exception thrown by the {@link Runnable#run() run} method is
548 * returned.
549 *
550 * <p> Code executing in the scope owner thread can use this method to get the
551 * exception thrown by a failed subtask only after it has {@linkplain #join() joined}.
552 *
553 * <p> Code executing in a {@code Joiner} {@link Joiner#onComplete(Subtask)
554 * onComplete} method should test that the {@linkplain #state() subtask state} is
555 * {@link State#FAILED FAILED} before using this method to get the exception.
556 *
557 * @throws IllegalStateException if the subtask has not completed, completed with
558 * a result, or the current thread is the task scope owner and it has not joined
559 * @see State#FAILED
560 */
561 Throwable exception();
562 }
563
564 /**
565 * An object used with a {@link StructuredTaskScope} to handle subtask completion
566 * and produce the result for a main task waiting in the {@link #join() join} method
567 * for subtasks to complete.
568 *
569 * <p> Joiner defines static methods to create {@code Joiner} objects for common cases:
570 * <ul>
571 * <li> {@link #allSuccessfulOrThrow() allSuccessfulOrThrow()} creates a {@code Joiner}
572 * that yields a stream of the completed subtasks for {@code join} to return when
573 * all subtasks complete successfully. It cancels execution and causes {@code join}
574 * to throw if any subtask fails.
575 * <li> {@link #anySuccessfulResultOrThrow() anySuccessfulResultOrThrow()} creates a
576 * {@code Joiner} that yields the result of the first subtask to succeed. It cancels
577 * execution and causes {@code join} to throw if all subtasks fail.
578 * <li> {@link #awaitAllSuccessfulOrThrow() awaitAllSuccessfulOrThrow()} creates a
579 * {@code Joiner} that waits for all successful subtasks. It cancels execution and
580 * causes {@code join} to throw if any subtask fails.
581 * <li> {@link #awaitAll() awaitAll()} creates a {@code Joiner} that waits for all
582 * subtasks. If does not cancel execution.
583 * </ul>
584 *
585 * <p> In addition to the methods to create {@code Joiner} objects for common cases,
586 * the {@link #all(Predicate) all(Predicate)} method is defined to create a {@code
587 * Joiner} that yields a stream of all subtasks. It is created with a {@link
588 * Predicate Predicate} that determines if execution should continue or be cancelled.
589 * This {@code Joiner} can be built upon to create custom policies that cancel
590 * execution based on some condition.
591 *
592 * <p> More advanced policies can be developed by implementing the {@code Joiner}
593 * interface. The {@link #onFork(Subtask)} method is invoked when subtasks are forked.
594 * The {@link #onComplete(Subtask)} method is invoked when subtasks complete with a
595 * result or exception. These methods return a {@code boolean} to indicate if execution
596 * should be cancelled. These methods can be used to collect subtasks, results, or
597 * exceptions, and control when to cancel execution. The {@link #result()} method
598 * must be implemented to produce the result (or exception) for the {@code join}
599 * method.
600 *
601 * <p> Unless otherwise specified, passing a {@code null} argument to a method
602 * in this class will cause a {@link NullPointerException} to be thrown.
603 *
604 * @implSpec Implementations of this interface must be thread safe. The {@link
605 * #onComplete(Subtask)} method defined by this interface may be invoked by several
606 * threads concurrently.
607 *
608 * @apiNote It is very important that a new {@code Joiner} object is created for each
609 * {@code StructuredTaskScope}. {@code Joiner} objects should never be shared with
610 * different task scopes or re-used after a task is closed.
611 *
612 * <p> Designing a {@code Joiner} should take into account the code at the use-site
613 * where the results from the {@link StructuredTaskScope#join() join} method are
614 * processed. It should be clear what the {@code Joiner} does vs. the application
615 * code at the use-site. In general, the {@code Joiner} implementation is not the
616 * place to code "business logic". A {@code Joiner} should be designed to be as
617 * general purpose as possible.
618 *
619 * @param <T> the result type of tasks executed in the task scope
620 * @param <R> the type of results returned by the join method
621 * @since 24
622 * @see #open(Joiner)
623 */
624 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
625 @FunctionalInterface
626 public interface Joiner<T, R> {
627
628 /**
629 * Invoked by {@link #fork(Callable) fork(Callable)} and {@link #fork(Runnable)
630 * fork(Runnable)} when forking a subtask. The method is invoked from the task
631 * owner thread. The method is invoked before a thread is created to run the
632 * subtask.
633 *
634 * @implSpec The default implementation throws {@code NullPointerException} if the
635 * subtask is {@code null}. It throws {@code IllegalArgumentException} if the
636 * subtask is not in the {@link Subtask.State#UNAVAILABLE UNAVAILABLE} state, it
637 * otherwise returns {@code false}.
638 *
639 * @apiNote This method is invoked by the {@code fork} methods. It should not be
640 * invoked directly.
641 *
642 * @param subtask the subtask
643 * @return {@code true} to cancel execution
644 */
645 default boolean onFork(Subtask<? extends T> subtask) {
646 if (subtask.state() != Subtask.State.UNAVAILABLE) {
647 throw new IllegalArgumentException();
648 }
649 return false;
650 }
651
652 /**
653 * Invoked by the thread started to execute a subtask after the subtask completes
654 * successfully or fails with an exception. This method is not invoked if a
655 * subtask completes after execution has been cancelled.
656 *
657 * @implSpec The default implementation throws {@code NullPointerException} if the
658 * subtask is {@code null}. It throws {@code IllegalArgumentException} if the
659 * subtask is not in the {@link Subtask.State#SUCCESS SUCCESS} or {@link
660 * Subtask.State#FAILED FAILED} state, it otherwise returns {@code false}.
661 *
662 * @apiNote This method is invoked by subtasks when they complete. It should not
663 * be invoked directly.
664 *
665 * @param subtask the subtask
666 * @return {@code true} to cancel execution
667 */
668 default boolean onComplete(Subtask<? extends T> subtask) {
669 if (subtask.state() == Subtask.State.UNAVAILABLE) {
670 throw new IllegalArgumentException();
671 }
672 return false;
673 }
674
675 /**
676 * Invoked by {@link #join()} to produce the result (or exception) after waiting
677 * for all subtasks to complete or execution to be cancelled. The result from this
678 * method is returned by the {@code join} method. If this method throws, then
679 * {@code join} throws {@link ExecutionException} with the exception thrown by
680 * this method as the cause.
681 *
682 * <p> In normal usage, this method will be called at most once to produce the
683 * result (or exception). If the {@code join} method is called more than once
684 * then this method may be called more than once to produce the result. An
685 * implementation should return an equal result (or throw the same exception) on
686 * second or subsequent calls to produce the outcome.
687 *
688 * @apiNote This method is invoked by the {@code join} method. It should not be
689 * invoked directly.
690 *
691 * @return the result
692 * @throws Throwable the exception
693 */
694 R result() throws Throwable;
695
696 /**
697 * {@return a new Joiner object that yields a stream of all subtasks when all
698 * subtasks complete successfully, or throws if any subtask fails}
699 * If any subtask fails then execution is cancelled.
700 *
701 * <p> If all subtasks complete successfully, the joiner's {@link Joiner#result()}
702 * method returns a stream of all subtasks in the order that they were forked.
703 * If any subtask failed then the {@code result} method throws the exception from
704 * the first subtask to fail.
705 *
706 * @apiNote This joiner is intended for cases where the results for all subtasks
707 * are required ("invoke all"); if any subtask fails then the results of other
708 * unfinished subtasks are no longer needed. A typical usage will be when the
709 * subtasks return results of the same type, the returned stream of forked
710 * subtasks can be used to get the results.
711 *
712 * @param <T> the result type of subtasks
713 */
714 static <T> Joiner<T, Stream<Subtask<T>>> allSuccessfulOrThrow() {
715 return new AllSuccessful<>();
716 }
717
718 /**
719 * {@return a new Joiner object that yields the result of a subtask that completed
720 * successfully, or throws if all subtasks fail} If any subtask completes
721 * successfully then execution is cancelled.
722 *
723 * <p> The joiner's {@link Joiner#result()} method returns the result of a subtask
724 * that completed successfully. If all subtasks fail then the {@code result} method
725 * throws the exception from one of the failed subtasks. The {@code result} method
726 * throws {@code NoSuchElementException} if no subtasks were forked.
727 *
728 * @apiNote This joiner is intended for cases where the result of any subtask will
729 * do ("invoke any") and where the results of other unfinished subtasks are no
730 * longer needed.
731 *
732 * @param <T> the result type of subtasks
733 */
734 static <T> Joiner<T, T> anySuccessfulResultOrThrow() {
735 return new AnySuccessful<>();
736 }
737
738 /**
739 * {@return a new Joiner object that waits for all successful subtasks. It
740 * <a href="StructuredTaskScope.html#CancelExecution">cancels execution</a> if
741 * any subtask fails}
742 *
743 * <p> The joiner's {@link Joiner#result() result} method returns {@code null}
744 * if all subtasks complete successfully, or throws the exception from the first
745 * subtask to fail.
746 *
747 * @apiNote This joiner is intended for cases where the results for all subtasks
748 * are required ("invoke all"), and where the code {@linkplain #fork(Callable)
749 * forking} subtasks keeps a reference to the {@linkplain Subtask Subtask} objects.
750 * A typical usage will be when subtasks return results of different types.
751 *
752 * @param <T> the result type of subtasks
753 */
754 static <T> Joiner<T, Void> awaitAllSuccessfulOrThrow() {
755 return new AwaitSuccessful<>();
756 }
757
758 /**
759 * {@return a new Joiner object that waits for all subtasks}
760 *
761 * <p> The joiner's {@link Joiner#result() result} method returns {@code null}.
762 *
763 * @apiNote This joiner is intended for cases where subtasks make use of
764 * <em>side-effects</em> rather than return results or fail with exceptions.
765 * The {@link #fork(Runnable) fork(Runnable)} method can be used to fork subtasks
766 * that do not return a result.
767 *
768 * @param <T> the result type of subtasks
769 */
770 static <T> Joiner<T, Void> awaitAll() {
771 // ensure that new Joiner object is returned
772 return new Joiner<T, Void>() {
773 @Override
774 public Void result() {
775 return null;
776 }
777 };
778 }
779
780 /**
781 * {@return a new Joiner object that yields a stream of all subtasks, cancelling
782 * execution when evaluating a completed subtask with the given predicate returns
783 * {@code true}}
784 *
785 * <p> The joiner's {@link Joiner#onComplete(Subtask)} method invokes the
786 * predicate's {@link Predicate#test(Object) test} method with the subtask that
787 * completed successfully or failed with an exception. If the {@code test} method
788 * returns {@code true} then <a href="StructuredTaskScope.html#CancelExecution">
789 * execution is cancelled</a>. The {@code test} method must be thread safe as it
790 * may be invoked concurrently from several threads.
791 *
792 * <p> The joiner's {@link #result()} method returns the stream of all subtasks,
793 * in fork order. The stream may contain subtasks that have completed
794 * (in {@link Subtask.State#SUCCESS SUCCESS} or {@link Subtask.State#FAILED FAILED}
795 * state) or subtasks in the {@link Subtask.State#UNAVAILABLE UNAVAILABLE} state
796 * if execution was cancelled before all subtasks were forked or completed.
797 *
798 * <p> The following example uses this method to create a {@code Joiner} that
799 * <a href="StructuredTaskScope.html#CancelExecution">cancels execution</a> when
800 * two or more subtasks fail.
801 * {@snippet lang=java :
802 * class CancelAfterTwoFailures<T> implements Predicate<Subtask<? extends T>> {
803 * private final AtomicInteger failedCount = new AtomicInteger();
804 * @Override
805 * public boolean test(Subtask<? extends T> subtask) {
806 * return subtask.state() == Subtask.State.FAILED
807 * && failedCount.incrementAndGet() >= 2;
808 * }
809 * }
810 *
811 * var joinPolicy = Joiner.all(new CancelAfterTwoFailures<String>());
812 * }
813 *
814 * @param isDone the predicate to evaluate completed subtasks
815 * @param <T> the result type of subtasks
816 */
817 static <T> Joiner<T, Stream<Subtask<T>>> all(Predicate<Subtask<? extends T>> isDone) {
818 return new AllSubtasks<>(isDone);
819 }
820 }
821
822 /**
823 * Represents the configuration for a {@code StructuredTaskScope}.
824 *
825 * <p> The configuration for a {@code StructuredTaskScope} consists of a {@link
826 * ThreadFactory} to create threads, an optional name for the purposes of monitoring
827 * and management, and an optional timeout.
828 *
829 * <p> Creating a {@code StructuredTaskScope} with its 1-arg {@link #open(Joiner) open}
830 * method uses the <a href="StructuredTaskScope.html#DefaultConfiguration">default
831 * configuration</a>. The default configuration consists of a thread factory that
832 * creates unnamed <a href="{@docRoot}/java.base/java/lang/Thread.html#virtual-threads">
833 * virtual threads</a>, no name for monitoring and management purposes, and no timeout.
834 *
835 * <p> Creating a {@code StructuredTaskScope} with its 2-arg {@link #open(Joiner, Function)
836 * open} method allows a different configuration to be used. The function specified
837 * to the {@code open} method is applied to the default configuration and returns the
838 * configuration for the {@code StructuredTaskScope} under construction. The function
839 * can use the {@code with-} prefixed methods defined here to specify the components
840 * of the configuration to use.
841 *
842 * <p> Unless otherwise specified, passing a {@code null} argument to a method
843 * in this class will cause a {@link NullPointerException} to be thrown.
844 *
845 * @since 24
846 */
847 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
848 public sealed interface Config permits ConfigImpl {
849 /**
850 * {@return a new {@code Config} object with the given thread factory}
851 * The other components are the same as this object. The thread factory is used by
852 * a task scope to create threads when {@linkplain #fork(Callable) forking} subtasks.
853 * @param threadFactory the thread factory
854 *
855 * @apiNote The thread factory will typically create
856 * <a href="{@docRoot}/java.base/java/lang/Thread.html#virtual-threads">virtual threads</a>,
857 * maybe with names for monitoring purposes, an {@linkplain Thread.UncaughtExceptionHandler
858 * uncaught exception handler}, or other properties configured.
859 *
860 * @see #fork(Callable)
861 */
862 Config withThreadFactory(ThreadFactory threadFactory);
863
864 /**
865 * {@return a new {@code Config} object with the given name}
866 * The other components are the same as this object. A task scope is optionally
867 * named for the purposes of monitoring and management.
868 * @param name the name
869 * @see StructuredTaskScope#toString()
870 */
871 Config withName(String name);
872
873 /**
874 * {@return a new {@code Config} object with the given timeout}
875 * The other components are the same as this object.
876 * @param timeout the timeout
877 *
878 * @apiNote Applications using deadlines, expressed as an {@link java.time.Instant},
879 * can use {@link Duration#between Duration.between(Instant.now(), deadline)} to
880 * compute the timeout for this method.
881 *
882 * @see #join()
883 */
884 Config withTimeout(Duration timeout);
885 }
886
887 /**
888 * Opens a new structured task scope to use the given {@code Joiner} object and with
889 * configuration that is the result of applying the given function to the
890 * <a href="#DefaultConfiguration">default configuration</a>.
891 *
892 * <p> The {@code configFunction} is called with the default configuration and returns
893 * the configuration for the new structured task scope. The function may, for example,
894 * set the {@linkplain Config#withThreadFactory(ThreadFactory) ThreadFactory} or set
895 * a {@linkplain Config#withTimeout(Duration) timeout}.
896 *
897 * <p> If a {@linkplain Config#withThreadFactory(ThreadFactory) ThreadFactory} is set
898 * then the {@code ThreadFactory}'s {@link ThreadFactory#newThread(Runnable) newThread}
899 * method will be used to create threads when forking subtasks in this task scope.
900 *
901 * <p> If a {@linkplain Config#withTimeout(Duration) timeout} is set then it starts
902 * when the task scope is opened. If the timeout expires before the task scope has
903 * {@linkplain #join() joined} then execution is cancelled and the {@code join} method
904 * throws {@link ExecutionException} with {@link TimeoutException} as the cause.
905 *
906 * <p> The new task scope is owned by the current thread. Only code executing in this
907 * thread can {@linkplain #fork(Callable) fork}, {@linkplain #join() join}, or
908 * {@linkplain #close close} the task scope.
909 *
910 * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
911 * value} bindings for inheritance by threads started in the task scope.
912 *
913 * @param joiner the joiner
914 * @param configFunction a function to produce the configuration
915 * @return a new task scope
916 * @param <T> the result type of tasks executed in the task scope
917 * @param <R> the type of the result returned by the join method
918 * @since 24
919 */
920 public static <T, R> StructuredTaskScope<T, R> open(Joiner<? super T, ? extends R> joiner,
921 Function<Config, Config> configFunction) {
922 Objects.requireNonNull(joiner);
923
924 var config = (ConfigImpl) configFunction.apply(ConfigImpl.defaultConfig());
925 var scope = new StructuredTaskScope<T, R>(joiner, config.threadFactory(), config.name());
926
927 // schedule timeout
928 Duration timeout = config.timeout();
929 if (timeout != null) {
930 boolean done = false;
931 try {
932 scope.scheduleTimeout(timeout);
933 done = true;
934 } finally {
935 if (!done) {
936 scope.close(); // pop if scheduling timeout failed
937 }
938 }
939 }
940
941 return scope;
942 }
943
944 /**
945 * Opens a new structured task scope to use the given {@code Joiner} object. The
946 * task scope is created with the <a href="#DefaultConfiguration">default configuration</a>.
947 * The default configuration has a {@code ThreadFactory} that creates unnamed
948 * <a href="{@docRoot}/java.base/java/lang/Thread.html#virtual-threads">virtual threads</a>,
949 * is unnamed for monitoring and management purposes, and has no timeout.
950 *
951 * @implSpec
952 * This factory method is equivalent to invoking the 2-arg open method with the given
953 * joiner and the {@linkplain Function#identity() identity function}.
954 *
955 * @param joiner the joiner
956 * @return a new task scope
957 * @param <T> the result type of tasks executed in the task scope
958 * @param <R> the type of the result returned by the join method
959 * @since 24
960 */
961 public static <T, R> StructuredTaskScope<T, R> open(Joiner<? super T, ? extends R> joiner) {
962 return open(joiner, Function.identity());
963 }
964
965 /**
966 * Starts a new thread in this task scope to execute a value-returning task, thus
967 * creating a <em>subtask</em>. The value-returning task is provided to this method
968 * as a {@link Callable}, the thread executes the task's {@link Callable#call() call}
969 * method.
970 *
971 * <p> This method first creates a {@link Subtask Subtask} to represent the <em>forked
972 * subtask</em>. It invokes the joiner's {@link Joiner#onFork(Subtask) onFork} method
973 * with the {@code Subtask} object. If the {@code onFork} completes with an exception
974 * or error then it is propagated by the {@code fork} method. If execution is
975 * {@linkplain #isCancelled() cancelled}, or {@code onFork} returns {@code true} to
976 * cancel execution, then this method returns the {@code Subtask} (in the {@link
977 * Subtask.State#UNAVAILABLE UNAVAILABLE} state) without creating a thread to execute
978 * the subtask. If execution is not cancelled then a thread is created with the
979 * {@link ThreadFactory} configured when the task scope was created, and the thread is
980 * started. Forking a subtask inherits the current thread's {@linkplain ScopedValue
981 * scoped value} bindings. The bindings must match the bindings captured when the
982 * task scope was opened. If the subtask completes (successfully or with an exception)
983 * before execution is cancelled, then the thread invokes the joiner's
984 * {@link Joiner#onComplete(Subtask) onComplete} method with subtask in the
985 * {@link Subtask.State#SUCCESS SUCCESS} or {@link Subtask.State#FAILED FAILED} state.
986 *
987 * <p> This method returns the {@link Subtask Subtask} object. In some usages, this
988 * object may be used to get its result. In other cases it may be used for correlation
989 * or just discarded. To ensure correct usage, the {@link Subtask#get() Subtask.get()}
990 * method may only be called by the task scope owner to get the result after it has
991 * waited for subtasks to complete with the {@link #join() join} method and the subtask
992 * completed successfully. Similarly, the {@link Subtask#exception() Subtask.exception()}
993 * method may only be called by the task scope owner after it has joined and the subtask
994 * failed. If execution was cancelled before the subtask was forked, or before it
995 * completes, then neither method can be used to obtain the outcome.
996 *
997 * <p> This method may only be invoked by the task scope owner.
998 *
999 * @param task the value-returning task for the thread to execute
1000 * @param <U> the result type
1001 * @return the subtask
1002 * @throws IllegalStateException if this task scope is closed or the owner has already
1003 * joined
1004 * @throws WrongThreadException if the current thread is not the task scope owner
1005 * @throws StructureViolationException if the current scoped value bindings are not
1006 * the same as when the task scope was created
1007 * @throws RejectedExecutionException if the thread factory rejected creating a
1008 * thread to run the subtask
1009 */
1010 public <U extends T> Subtask<U> fork(Callable<? extends U> task) {
1011 Objects.requireNonNull(task);
1012 ensureOwner();
1013 ensureOpen();
1014 if (joined) {
1015 throw new IllegalStateException("Already joined");
1016 }
1017
1018 var subtask = new SubtaskImpl<U>(this, task);
1019
1020 // notify joiner, even if cancelled
1021 if (joiner.onFork(subtask)) {
1022 cancelExecution();
1023 }
1024
1025 if (!cancelled) {
1026 // create thread to run task
1027 Thread thread = threadFactory.newThread(subtask);
1028 if (thread == null) {
1029 throw new RejectedExecutionException("Rejected by thread factory");
1030 }
1031
1032 // attempt to start the thread
1033 try {
1034 flock.start(thread);
1035 } catch (IllegalStateException e) {
1036 // shutdown by another thread, or underlying flock is shutdown due
1037 // to unstructured use
1038 }
1039 }
1040
1041 needToJoin = true;
1042 return subtask;
1043 }
1044
1045 /**
1046 * Starts a new thread in this task scope to execute a task that does not return a
1047 * result, creating a <em>subtask</em>.
1048 *
1049 * <p> This method works exactly the same as {@link #fork(Callable)} except that
1050 * the task is provided to this method as a {@link Runnable}, the thread executes
1051 * the task's {@link Runnable#run() run} method, and its result is {@code null}.
1052 *
1053 * @param task the task for the thread to execute
1054 * @return the subtask
1055 * @throws IllegalStateException if this task scope is closed or the owner has already
1056 * joined
1057 * @throws WrongThreadException if the current thread is not the task scope owner
1058 * @throws StructureViolationException if the current scoped value bindings are not
1059 * the same as when the task scope was created
1060 * @throws RejectedExecutionException if the thread factory rejected creating a
1061 * thread to run the subtask
1062 * @since 24
1063 */
1064 public Subtask<? extends T> fork(Runnable task) {
1065 Objects.requireNonNull(task);
1066 return fork(() -> { task.run(); return null; });
1067 }
1068
1069 /**
1070 * Waits for all subtasks started in this task scope to complete or execution to be
1071 * cancelled. If a {@linkplain Config#withTimeout(Duration) timeout} has been set
1072 * then execution will be cancelled if the timeout expires before or while waiting.
1073 * Once finished waiting, the {@code Joiner}'s {@link Joiner#result() result}
1074 * method is invoked to get the result or throw an exception. If the {@code result}
1075 * method throws then this method throws {@code ExecutionException} with the
1076 * exception thrown by the {@code result()} method as the cause.
1077 *
1078 * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1079 * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1080 * when all threads finish, the {@code Joiner}'s {@link Joiner#onFork(Subtask)
1081 * onFork} or {@link Joiner#onComplete(Subtask) onComplete} returns {@code true}
1082 * to cancel execution, the timeout (if set) expires, or the current thread is
1083 * {@linkplain Thread#interrupt() interrupted}.
1084 *
1085 * <p> This method may only be invoked by the task scope owner.
1086 *
1087 * @return the {@link Joiner#result() result}
1088 * @throws IllegalStateException if this task scope is closed
1089 * @throws WrongThreadException if the current thread is not the task scope owner
1090 * @throws ExecutionException if the joiner's {@code result} method throws, or with
1091 * cause {@link TimeoutException} if a timeout is set and the timeout expires
1092 * @throws InterruptedException if interrupted while waiting
1093 * @since 24
1094 */
1095 public R join() throws ExecutionException, InterruptedException {
1096 ensureOwner();
1097 ensureOpen();
1098
1099 if (!joined) {
1100 // owner has attempted to join
1101 needToJoin = false;
1102
1103 // wait for all threads, execution to be cancelled, or interrupt
1104 flock.awaitAll();
1105
1106 // throw if timeout expired while waiting
1107 if (timeoutExpired) {
1108 throw new ExecutionException(new TimeoutException());
1109 }
1110
1111 // join completed successfully
1112 cancelTimeout();
1113 joined = true;
1114 }
1115
1116 // invoke joiner to get result
1117 try {
1118 return joiner.result();
1119 } catch (Throwable e) {
1120 throw new ExecutionException(e);
1121 }
1122 }
1123
1124 /**
1125 * Cancels execution. This method allows the task scope owner to explicitly
1126 * <a href="#CancelExecution">cancel execution</a>. If not already cancelled, this
1127 * method {@linkplain Thread#interrupt() interrupts} the threads executing subtasks
1128 * that have not completed, and prevents new threads from being started in the task
1129 * scope.
1130 *
1131 * @apiNote This method is intended for cases where a task scope is created with a
1132 * {@link Joiner Joiner} that doesn't cancel execution or where the code in the main
1133 * task needs to cancel execution due to some exception or other condition in the main
1134 * task. The following example accepts network connections indefinitely, forking a
1135 * subtask to handle each connection. If the {@code accept()} method in the example
1136 * throws then the main task cancels execution before joining and closing the scope.
1137 * The {@link #close() close} method waits for the interrupted threads to finish.
1138 *
1139 * {@snippet lang=java :
1140 * // @link substring="awaitAll" target="Joiner#awaitAll()" :
1141 * try (var scope = StructuredTaskScope.open(Joiner.awaitAll())) {
1142 *
1143 * try {
1144 * while (true) {
1145 * Socket peer = listener.accept();
1146 * // @link substring="fork" target="#fork(Runnable)" :
1147 * scope.fork(() -> handle(peer));
1148 * }
1149 * } finally {
1150 * scope.cancel();
1151 * scope.join(); // completes immediately
1152 * }
1153 *
1154 * }
1155 * }
1156 *
1157 * @throws IllegalStateException if this task scope is closed
1158 * @throws WrongThreadException if the current thread is not the task scope owner
1159 * @since 24
1160 */
1161 public void cancel() {
1162 ensureOwner();
1163 ensureOpen();
1164 cancelExecution();
1165 }
1166
1167 /**
1168 * {@return {@code true} if <a href="#CancelExecution">execution is cancelled</a>,
1169 * or in the process of being cancelled, otherwise {@code false}}
1170 *
1171 * <p> Cancelling execution prevents new threads from starting in the task scope and
1172 * {@linkplain Thread#interrupt() interrupts} threads executing unfinished subtasks.
1173 * It may take some time before the interrupted threads finish execution; this
1174 * method may return {@code true} before all threads have been interrupted or before
1175 * all threads have finished.
1176 *
1177 * @apiNote A main task with a lengthy "forking phase" (the code that executes before
1178 * the main task invokes {@link #join() join}) may use this method to avoid doing work
1179 * in cases where execution was cancelled by the completion of a previously forked
1180 * subtask or timeout.
1181 *
1182 * @since 24
1183 */
1184 public boolean isCancelled() {
1185 return cancelled;
1186 }
1187
1188 /**
1189 * Closes this task scope.
1190 *
1191 * <p> This method first <a href="#CancelExecution">cancels execution</a>, if not
1192 * already cancelled. This interrupts the threads executing unfinished subtasks. This
1193 * method then waits for all threads to finish. If interrupted while waiting then it
1194 * will continue to wait until the threads finish, before completing with the interrupt
1195 * status set.
1196 *
1197 * <p> This method may only be invoked by the task scope owner. If the task scope
1198 * is already closed then the task scope owner invoking this method has no effect.
1199 *
1200 * <p> A {@code StructuredTaskScope} is intended to be used in a <em>structured
1201 * manner</em>. If this method is called to close a task scope before nested task
1202 * scopes are closed then it closes the underlying construct of each nested task scope
1203 * (in the reverse order that they were created in), closes this task scope, and then
1204 * throws {@link StructureViolationException}.
1205 * Similarly, if this method is called to close a task scope while executing with
1206 * {@linkplain ScopedValue scoped value} bindings, and the task scope was created
1207 * before the scoped values were bound, then {@code StructureViolationException} is
1208 * thrown after closing the task scope.
1209 * If a thread terminates without first closing task scopes that it owns then
1210 * termination will cause the underlying construct of each of its open tasks scopes to
1211 * be closed. Closing is performed in the reverse order that the task scopes were
1212 * created in. Thread termination may therefore be delayed when the task scope owner
1213 * has to wait for threads forked in these task scopes to finish.
1214 *
1215 * @throws IllegalStateException thrown after closing the task scope if the task scope
1216 * owner did not attempt to join after forking
1217 * @throws WrongThreadException if the current thread is not the task scope owner
1218 * @throws StructureViolationException if a structure violation was detected
1219 */
1220 @Override
1221 public void close() {
1222 ensureOwner();
1223 if (closed) {
1224 return;
1225 }
1226
1227 // cancel execution if not already joined
1228 if (!joined) {
1229 cancelExecution();
1230 cancelTimeout();
1231 }
1232
1233 // wait for stragglers
1234 try {
1235 flock.close();
1236 } finally {
1237 closed = true;
1238 }
1239
1240 // throw ISE if the owner didn't join after forking
1241 if (needToJoin) {
1242 needToJoin = false;
1243 throw new IllegalStateException("Owner did not join");
1244 }
1245 }
1246
1247 /**
1248 * {@inheritDoc} If a {@link Config#withName(String) name} for monitoring and
1249 * monitoring purposes has been set then the string representation includes the name.
1250 */
1251 @Override
1252 public String toString() {
1253 return flock.name();
1254 }
1255
1256 /**
1257 * Subtask implementation, runs the task specified to the fork method.
1258 */
1259 private static final class SubtaskImpl<T> implements Subtask<T>, Runnable {
1260 private static final AltResult RESULT_NULL = new AltResult(Subtask.State.SUCCESS);
1261
1262 private record AltResult(Subtask.State state, Throwable exception) {
1263 AltResult(Subtask.State state) {
1264 this(state, null);
1265 }
1266 }
1267
1268 private final StructuredTaskScope<? super T, ?> scope;
1269 private final Callable<? extends T> task;
1270 private volatile Object result;
1271
1272 SubtaskImpl(StructuredTaskScope<? super T, ?> scope, Callable<? extends T> task) {
1273 this.scope = scope;
1274 this.task = task;
1275 }
1276
1277 @Override
1278 public void run() {
1279 T result = null;
1280 Throwable ex = null;
1281 try {
1282 result = task.call();
1283 } catch (Throwable e) {
1284 ex = e;
1285 }
1286
1287 // nothing to do if task scope is cancelled
1288 if (scope.isCancelled())
1289 return;
1290
1291 // set result/exception and invoke onComplete
1292 if (ex == null) {
1293 this.result = (result != null) ? result : RESULT_NULL;
1294 } else {
1295 this.result = new AltResult(State.FAILED, ex);
1296 }
1297 scope.onComplete(this);
1298 }
1299
1300 @Override
1301 public Subtask.State state() {
1302 Object result = this.result;
1303 if (result == null) {
1304 return State.UNAVAILABLE;
1305 } else if (result instanceof AltResult alt) {
1306 // null or failed
1307 return alt.state();
1308 } else {
1309 return State.SUCCESS;
1310 }
1311 }
1312
1313
1314 @Override
1315 public T get() {
1316 scope.ensureJoinedIfOwner();
1317 Object result = this.result;
1318 if (result instanceof AltResult) {
1319 if (result == RESULT_NULL) return null;
1320 } else if (result != null) {
1321 @SuppressWarnings("unchecked")
1322 T r = (T) result;
1323 return r;
1324 }
1325 throw new IllegalStateException(
1326 "Result is unavailable or subtask did not complete successfully");
1327 }
1328
1329 @Override
1330 public Throwable exception() {
1331 scope.ensureJoinedIfOwner();
1332 Object result = this.result;
1333 if (result instanceof AltResult alt && alt.state() == State.FAILED) {
1334 return alt.exception();
1335 }
1336 throw new IllegalStateException(
1337 "Exception is unavailable or subtask did not complete with exception");
1338 }
1339
1340 @Override
1341 public String toString() {
1342 String stateAsString = switch (state()) {
1343 case UNAVAILABLE -> "[Unavailable]";
1344 case SUCCESS -> "[Completed successfully]";
1345 case FAILED -> {
1346 Throwable ex = ((AltResult) result).exception();
1347 yield "[Failed: " + ex + "]";
1348 }
1349 };
1350 return Objects.toIdentityString(this) + stateAsString;
1351 }
1352 }
1353
1354 /**
1355 * A joiner that returns a stream of all subtasks when all subtasks complete
1356 * successfully. If any subtask fails then execution is cancelled.
1357 */
1358 private static final class AllSuccessful<T> implements Joiner<T, Stream<Subtask<T>>> {
1359 private static final VarHandle FIRST_EXCEPTION;
1360 static {
1361 try {
1362 MethodHandles.Lookup l = MethodHandles.lookup();
1363 FIRST_EXCEPTION = l.findVarHandle(AllSuccessful.class, "firstException", Throwable.class);
1364 } catch (Exception e) {
1365 throw new ExceptionInInitializerError(e);
1366 }
1367 }
1368 // list of forked subtasks, only accessed by owner thread
1369 private final List<Subtask<T>> subtasks = new ArrayList<>();
1370 private volatile Throwable firstException;
1371
1372 @Override
1373 public boolean onFork(Subtask<? extends T> subtask) {
1374 @SuppressWarnings("unchecked")
1375 var tmp = (Subtask<T>) subtask;
1376 subtasks.add(tmp);
1377 return false;
1378 }
1379
1380 @Override
1381 public boolean onComplete(Subtask<? extends T> subtask) {
1382 return (subtask.state() == Subtask.State.FAILED)
1383 && (firstException == null)
1384 && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
1385 }
1386
1387 @Override
1388 public Stream<Subtask<T>> result() throws Throwable {
1389 Throwable ex = firstException;
1390 if (ex != null) {
1391 throw ex;
1392 } else {
1393 return subtasks.stream();
1394 }
1395 }
1396 }
1397
1398 /**
1399 * A joiner that returns the result of the first subtask to complete successfully.
1400 * If any subtask completes successfully then execution is cancelled.
1401 */
1402 private static final class AnySuccessful<T> implements Joiner<T, T> {
1403 private static final VarHandle FIRST_SUCCESS;
1404 private static final VarHandle FIRST_EXCEPTION;
1405 static {
1406 try {
1407 MethodHandles.Lookup l = MethodHandles.lookup();
1408 FIRST_SUCCESS = l.findVarHandle(AnySuccessful.class, "firstSuccess", Subtask.class);
1409 FIRST_EXCEPTION = l.findVarHandle(AnySuccessful.class, "firstException", Throwable.class);
1410 } catch (Exception e) {
1411 throw new ExceptionInInitializerError(e);
1412 }
1413 }
1414 private volatile Subtask<T> firstSuccess;
1415 private volatile Throwable firstException;
1416
1417 @Override
1418 public boolean onComplete(Subtask<? extends T> subtask) {
1419 if (firstSuccess == null) {
1420 if (subtask.state() == Subtask.State.SUCCESS) {
1421 // capture the first subtask that completes successfully
1422 return FIRST_SUCCESS.compareAndSet(this, null, subtask);
1423 } else if (firstException == null) {
1424 // capture the exception thrown by the first task to fail
1425 FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
1426 }
1427 }
1428 return false;
1429 }
1430
1431 @Override
1432 public T result() throws Throwable {
1433 Subtask<T> firstSuccess = this.firstSuccess;
1434 if (firstSuccess != null) {
1435 return firstSuccess.get();
1436 }
1437 Throwable firstException = this.firstException;
1438 if (firstException != null) {
1439 throw firstException;
1440 } else {
1441 throw new NoSuchElementException("No subtasks completed");
1442 }
1443 }
1444 }
1445
1446 /**
1447 * A joiner that that waits for all successful subtasks. If any subtask fails the
1448 * execution is cancelled.
1449 */
1450 private static final class AwaitSuccessful<T> implements Joiner<T, Void> {
1451 private static final VarHandle FIRST_EXCEPTION;
1452 static {
1453 try {
1454 MethodHandles.Lookup l = MethodHandles.lookup();
1455 FIRST_EXCEPTION = l.findVarHandle(AwaitSuccessful.class, "firstException", Throwable.class);
1456 } catch (Exception e) {
1457 throw new ExceptionInInitializerError(e);
1458 }
1459 }
1460 private volatile Throwable firstException;
1461
1462 @Override
1463 public boolean onComplete(Subtask<? extends T> subtask) {
1464 return (subtask.state() == Subtask.State.FAILED)
1465 && (firstException == null)
1466 && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());
1467 }
1468
1469 @Override
1470 public Void result() throws Throwable {
1471 Throwable ex = firstException;
1472 if (ex != null) {
1473 throw ex;
1474 } else {
1475 return null;
1476 }
1477 }
1478 }
1479
1480 /**
1481 * A joiner that returns a stream of all subtasks.
1482 */
1483 private static class AllSubtasks<T> implements Joiner<T, Stream<Subtask<T>>> {
1484 private final Predicate<Subtask<? extends T>> isDone;
1485 // list of forked subtasks, only accessed by owner thread
1486 private final List<Subtask<T>> subtasks = new ArrayList<>();
1487
1488 AllSubtasks(Predicate<Subtask<? extends T>> isDone) {
1489 this.isDone = Objects.requireNonNull(isDone);
1490 }
1491
1492 @Override
1493 public boolean onFork(Subtask<? extends T> subtask) {
1494 @SuppressWarnings("unchecked")
1495 var tmp = (Subtask<T>) subtask;
1496 subtasks.add(tmp);
1497 return false;
1498 }
1499
1500 @Override
1501 public boolean onComplete(Subtask<? extends T> subtask) {
1502 return isDone.test(subtask);
1503 }
1504
1505 @Override
1506 public Stream<Subtask<T>> result() {
1507 return subtasks.stream();
1508 }
1509 }
1510
1511 /**
1512 * Implementation of Config.
1513 */
1514 private record ConfigImpl(ThreadFactory threadFactory,
1515 String name,
1516 Duration timeout) implements Config {
1517 static Config defaultConfig() {
1518 return new ConfigImpl(Thread.ofVirtual().factory(), null, null);
1519 }
1520
1521 @Override
1522 public Config withThreadFactory(ThreadFactory threadFactory) {
1523 return new ConfigImpl(Objects.requireNonNull(threadFactory), name, timeout);
1524 }
1525
1526 @Override
1527 public Config withName(String name) {
1528 return new ConfigImpl(threadFactory, Objects.requireNonNull(name), timeout);
1529 }
1530
1531 @Override
1532 public Config withTimeout(Duration timeout) {
1533 return new ConfigImpl(threadFactory, name, Objects.requireNonNull(timeout));
1534 }
1535 }
1536
1537 /**
1538 * Used to schedule a task to cancel exception when a timeout expires.
1539 */
1540 private static class TimerSupport {
1541 private static final ScheduledExecutorService DELAYED_TASK_SCHEDULER;
1542 static {
1543 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1544 Executors.newScheduledThreadPool(1, task -> {
1545 Thread t = InnocuousThread.newThread("StructuredTaskScope-Timer", task);
1546 t.setDaemon(true);
1547 return t;
1548 });
1549 stpe.setRemoveOnCancelPolicy(true);
1550 DELAYED_TASK_SCHEDULER = stpe;
1551 }
1552
1553 static Future<?> schedule(Duration timeout, Runnable task) {
1554 long nanos = TimeUnit.NANOSECONDS.convert(timeout);
1555 return DELAYED_TASK_SCHEDULER.schedule(task, nanos, TimeUnit.NANOSECONDS);
1556 }
1557 }
1558 }