1 /*
   2  * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.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.time.Instant;


  33 import java.util.Objects;
  34 import java.util.Optional;
  35 import java.util.concurrent.locks.ReentrantLock;
  36 import java.util.function.Function;

  37 import java.util.function.Supplier;

  38 import jdk.internal.javac.PreviewFeature;

  39 import jdk.internal.misc.ThreadFlock;
  40 
  41 /**
  42  * A basic API for <em>structured concurrency</em>. {@code StructuredTaskScope} supports
  43  * cases where a task splits into several concurrent subtasks, and where the subtasks must
  44  * complete before the main task continues. A {@code StructuredTaskScope} can be used to
  45  * ensure that the lifetime of a concurrent operation is confined by a <em>syntax block</em>,
  46  * just like that of a sequential operation in structured programming.
  47  *
  48  * <h2>Basic operation</h2>












  49  *
  50  * A {@code StructuredTaskScope} is created with one of its public constructors. It defines
  51  * the {@link #fork(Callable) fork} method to start a thread to execute a subtask, the {@link
  52  * #join() join} method to wait for all subtasks to finish, and the {@link #close() close}
  53  * method to close the task scope. The API is intended to be used with the {@code
  54  * try-with-resources} statement. The intention is that code in the try <em>block</em>
  55  * uses the {@code fork} method to fork threads to execute the subtasks, wait for the
  56  * subtasks to finish with the {@code join} method, and then <em>process the results</em>.
  57  * A call to the {@code fork} method returns a {@link Subtask Subtask} to representing
  58  * the <em>forked subtask</em>. Once {@code join} is called, the {@code Subtask} can be
  59  * used to get the result completed successfully, or the exception if the subtask failed.
  60  * {@snippet lang=java :
  61  *     Callable<String> task1 = ...
  62  *     Callable<Integer> task2 = ...
  63  *
  64  *     try (var scope = new StructuredTaskScope<Object>()) {










  65  *
  66  *         Subtask<String> subtask1 = scope.fork(task1);   // @highlight substring="fork"
  67  *         Subtask<Integer> subtask2 = scope.fork(task2);  // @highlight substring="fork"










  68  *
  69  *         scope.join();                                   // @highlight substring="join"

  70  *
  71  *         ... process results/exceptions ...

  72  *
  73  *     } // close                                          // @highlight substring="close"

  74  * }
  75  * <p> The following example forks a collection of homogeneous subtasks, waits for all of
  76  * them to complete with the {@code join} method, and uses the {@link Subtask.State
  77  * Subtask.State} to partition the subtasks into a set of the subtasks that completed
  78  * successfully and another for the subtasks that failed.
  79  * {@snippet lang=java :
  80  *     List<Callable<String>> callables = ...
  81  *
  82  *     try (var scope = new StructuredTaskScope<String>()) {







  83  *
  84  *         List<Subtask<String>> subtasks = callables.stream().map(scope::fork).toList();







  85  *
  86  *         scope.join();


  87  *
  88  *         Map<Boolean, Set<Subtask<String>>> map = subtasks.stream()
  89  *                 .collect(Collectors.partitioningBy(h -> h.state() == Subtask.State.SUCCESS,
  90  *                                                    Collectors.toSet()));
  91  *
  92  *     } // close
  93  * }
  94  *
  95  * <p> To ensure correct usage, the {@code join} and {@code close} methods may only be
  96  * invoked by the <em>owner</em> (the thread that opened/created the task scope), and the
  97  * {@code close} method throws an exception after closing if the owner did not invoke the
  98  * {@code join} method after forking.
  99  *
 100  * <p> {@code StructuredTaskScope} defines the {@link #shutdown() shutdown} method to shut
 101  * down a task scope without closing it. The {@code shutdown()} method <em>cancels</em> all
 102  * unfinished subtasks by {@linkplain Thread#interrupt() interrupting} the threads. It
 103  * prevents new threads from starting in the task scope. If the owner is waiting in the
 104  * {@code join} method then it will wakeup.




 105  *
 106  * <p> Shutdown is used for <em>short-circuiting</em> and allow subclasses to implement
 107  * <em>policy</em> that does not require all subtasks to finish.




 108  *
 109  * <h2>Subclasses with policies for common cases</h2>
 110  *
 111  * Two subclasses of {@code StructuredTaskScope} are defined to implement policy for
 112  * common cases:
 113  * <ol>
 114  *   <li> {@link ShutdownOnSuccess ShutdownOnSuccess} captures the result of the first
 115  *   subtask to complete successfully. Once captured, it shuts down the task scope to
 116  *   interrupt unfinished threads and wakeup the owner. This class is intended for cases
 117  *   where the result of any subtask will do ("invoke any") and where there is no need to
 118  *   wait for results of other unfinished subtasks. It defines methods to get the first
 119  *   result or throw an exception if all subtasks fail.
 120  *   <li> {@link ShutdownOnFailure ShutdownOnFailure} captures the exception of the first
 121  *   subtask to fail. Once captured, it shuts down the task scope to interrupt unfinished
 122  *   threads and wakeup the owner. This class is intended for cases where the results of all
 123  *   subtasks are required ("invoke all"); if any subtask fails then the results of other
 124  *   unfinished subtasks are no longer needed. If defines methods to throw an exception if
 125  *   any of the subtasks fail.
 126  * </ol>
 127  *
 128  * <p> The following are two examples that use the two classes. In both cases, a pair of
 129  * subtasks are forked to fetch resources from two URL locations "left" and "right". The
 130  * first example creates a ShutdownOnSuccess object to capture the result of the first
 131  * subtask to complete successfully, cancelling the other by way of shutting down the task
 132  * scope. The main task waits in {@code join} until either subtask completes with a result
 133  * or both subtasks fail. It invokes {@link ShutdownOnSuccess#result(Function)
 134  * result(Function)} method to get the captured result. If both subtasks fail then this
 135  * method throws a {@code WebApplicationException} with the exception from one of the
 136  * subtasks as the cause.
 137  * {@snippet lang=java :
 138  *     try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {
 139  *
 140  *         scope.fork(() -> fetch(left));
 141  *         scope.fork(() -> fetch(right));

 142  *
 143  *         scope.join();
 144  *
 145  *         // @link regex="result(?=\()" target="ShutdownOnSuccess#result" :
 146  *         String result = scope.result(e -> new WebApplicationException(e));

 147  *
 148  *         ...
 149  *     }
 150  * }
 151  * The second example creates a ShutdownOnFailure object to capture the exception of the
 152  * first subtask to fail, cancelling the other by way of shutting down the task scope. The
 153  * main task waits in {@link #joinUntil(Instant)} until both subtasks complete with a
 154  * result, either fails, or a deadline is reached. It invokes {@link
 155  * ShutdownOnFailure#throwIfFailed(Function) throwIfFailed(Function)} to throw an exception
 156  * if either subtask fails. This method is a no-op if both subtasks complete successfully.
 157  * The example uses {@link Supplier#get()} to get the result of each subtask. Using
 158  * {@code Supplier} instead of {@code Subtask} is preferred for common cases where the
 159  * object returned by fork is only used to get the result of a subtask that completed
 160  * successfully.
 161  * {@snippet lang=java :
 162  *    Instant deadline = ...
 163  *
 164  *    try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {






 165  *
 166  *         Supplier<String> supplier1 = scope.fork(() -> query(left));
 167  *         Supplier<String> supplier2 = scope.fork(() -> query(right));




 168  *
 169  *         scope.joinUntil(deadline);

 170  *
 171  *         // @link substring="throwIfFailed" target="ShutdownOnFailure#throwIfFailed" :
 172  *         scope.throwIfFailed(e -> new WebApplicationException(e));
 173  *
 174  *         // both subtasks completed successfully
 175  *         String result = Stream.of(supplier1, supplier2)
 176  *                 .map(Supplier::get)
 177  *                 .collect(Collectors.joining(", ", "{ ", " }"));
 178  *
 179  *         ...
 180  *     }
 181  * }
 182  *
 183  * <h2>Extending StructuredTaskScope</h2>
 184  *
 185  * {@code StructuredTaskScope} can be extended, and the {@link #handleComplete(Subtask)
 186  * handleComplete} method overridden, to implement policies other than those implemented
 187  * by {@code ShutdownOnSuccess} and {@code ShutdownOnFailure}. A subclass may, for example,
 188  * collect the results of subtasks that complete successfully and ignore subtasks that
 189  * fail. It may collect exceptions when subtasks fail. It may invoke the {@link #shutdown()
 190  * shutdown} method to shut down and cause {@link #join() join} to wakeup when some
 191  * condition arises.
 192  *
 193  * <p> A subclass will typically define methods to make available results, state, or other
 194  * outcome to code that executes after the {@code join} method. A subclass that collects
 195  * results and ignores subtasks that fail may define a method that returns the results.
 196  * A subclass that implements a policy to shut down when a subtask fails may define a
 197  * method to get the exception of the first subtask to fail.
 198  *
 199  * <p> The following is an example of a simple {@code StructuredTaskScope} implementation
 200  * that collects homogenous subtasks that complete successfully. It defines the method
 201  * "{@code completedSuccessfully()}" that the main task can invoke after it joins.


 202  * {@snippet lang=java :
 203  *     class CollectingScope<T> extends StructuredTaskScope<T> {
 204  *         private final Queue<Subtask<? extends T>> subtasks = new LinkedTransferQueue<>();
 205  *
 206  *         @Override
 207  *         protected void handleComplete(Subtask<? extends T> subtask) {
 208  *             if (subtask.state() == Subtask.State.SUCCESS) {
 209  *                 subtasks.add(subtask);
 210  *             }
 211  *         }
 212  *
 213  *         @Override
 214  *         public CollectingScope<T> join() throws InterruptedException {
 215  *             super.join();
 216  *             return this;
 217  *         }
 218  *
 219  *         public Stream<Subtask<? extends T>> completedSuccessfully() {
 220  *             // @link substring="ensureOwnerAndJoined" target="ensureOwnerAndJoined" :
 221  *             super.ensureOwnerAndJoined();
 222  *             return subtasks.stream();
 223  *         }
 224  *     }
 225  * }
 226  * <p> The implementations of the {@code completedSuccessfully()} method in the example
 227  * invokes {@link #ensureOwnerAndJoined()} to ensure that the method can only be invoked
 228  * by the owner thread and only after it has joined.
 229  *
 230  * <h2><a id="TreeStructure">Tree structure</a></h2>





 231  *
 232  * Task scopes form a tree where parent-child relations are established implicitly when
 233  * opening a new task scope:
 234  * <ul>
 235  *   <li> A parent-child relation is established when a thread started in a task scope
 236  *   opens its own task scope. A thread started in task scope "A" that opens task scope
 237  *   "B" establishes a parent-child relation where task scope "A" is the parent of task
 238  *   scope "B".
 239  *   <li> A parent-child relation is established with nesting. If a thread opens task
 240  *   scope "B", then opens task scope "C" (before it closes "B"), then the enclosing task
 241  *   scope "B" is the parent of the nested task scope "C".
 242  * </ul>
 243  *
 244  * The <i>descendants</i> of a task scope are the child task scopes that it is a parent
 245  * of, plus the descendants of the child task scopes, recursively.





 246  *
 247  * <p> The tree structure supports:
 248  * <ul>
 249  *   <li> Inheritance of {@linkplain ScopedValue scoped values} across threads.
 250  *   <li> Confinement checks. The phrase "threads contained in the task scope" in method
 251  *   descriptions means threads started in the task scope or descendant scopes.
 252  * </ul>
 253  *
 254  * <p> The following example demonstrates the inheritance of a scoped value. A scoped
 255  * value {@code USERNAME} is bound to the value "{@code duke}". A {@code StructuredTaskScope}
 256  * is created and its {@code fork} method invoked to start a thread to execute {@code
 257  * childTask}. The thread inherits the scoped value <em>bindings</em> captured when
 258  * creating the task scope. The code in {@code childTask} uses the value of the scoped
 259  * value and so reads the value "{@code duke}".

 260  * {@snippet lang=java :

 261  *     private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();
 262  *
 263  *     // @link substring="runWhere" target="ScopedValue#runWhere(ScopedValue, Object, Runnable)" :
 264  *     ScopedValue.runWhere(USERNAME, "duke", () -> {
 265  *         try (var scope = new StructuredTaskScope<String>()) {
 266  *
 267  *             scope.fork(() -> childTask());           // @highlight substring="fork"
 268  *             ...
 269  *          }
 270  *     });
 271  *
 272  *     ...

 273  *
 274  *     String childTask() {
 275  *         // @link substring="get" target="ScopedValue#get()" :
 276  *         String name = USERNAME.get();   // "duke"
 277  *         ...
 278  *     }
 279  * }
 280  *
 281  * <p> {@code StructuredTaskScope} does not define APIs that exposes the tree structure
 282  * at this time.



 283  *
 284  * <p> Unless otherwise specified, passing a {@code null} argument to a constructor
 285  * or method in this class will cause a {@link NullPointerException} to be thrown.






 286  *
 287  * <h2>Memory consistency effects</h2>
 288  *
 289  * <p> Actions in the owner thread of, or a thread contained in, the task scope prior to
 290  * {@linkplain #fork forking} of a subtask
 291  * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility">
 292  * <i>happen-before</i></a> any actions taken by that subtask, which in turn <i>happen-before</i>
 293  * the subtask result is {@linkplain Subtask#get() retrieved} or <i>happen-before</i> any
 294  * actions taken in a thread after {@linkplain #join() joining} of the task scope.
 295  *
 296  * @jls 17.4.5 Happens-before Order



 297  *
 298  * @param <T> the result type of tasks executed in the task scope



 299  * @since 21
 300  */
 301 @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 302 public class StructuredTaskScope<T> implements AutoCloseable {
 303     private final ThreadFactory factory;











 304     private final ThreadFlock flock;
 305     private final ReentrantLock shutdownLock = new ReentrantLock();
 306 
 307     // states: OPEN -> SHUTDOWN -> CLOSED
 308     private static final int OPEN     = 0;   // initial state
 309     private static final int SHUTDOWN = 1;
 310     private static final int CLOSED   = 2;

















































































































 311 
 312     // state: set to SHUTDOWN by any thread, set to CLOSED by owner, read by any thread
 313     private volatile int state;







 314 
 315     // Counters to support checking that the task scope owner joins before processing
 316     // results and attempts join before closing the task scope. These counters are
 317     // accessed only by the owner thread.
 318     private int forkRound;         // incremented when the first subtask is forked after join
 319     private int lastJoinAttempted; // set to the current fork round when join is attempted
 320     private int lastJoinCompleted; // set to the current fork round when join completes
 321 
 322     /**
 323      * Represents a subtask forked with {@link #fork(Callable)}.
 324      * @param <T> the result type
 325      * @since 21
 326      */
 327     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 328     public sealed interface Subtask<T> extends Supplier<T> permits SubtaskImpl {
 329         /**
 330          * {@return the value returning task provided to the {@code fork} method}
 331          *
 332          * @apiNote Task objects with unique identity may be used for correlation by
 333          * implementations of {@link #handleComplete(Subtask) handleComplete}.
 334          */
 335         Callable<? extends T> task();
 336 
 337         /**
 338          * Represents the state of a subtask.
 339          * @see Subtask#state()
 340          * @since 21
 341          */
 342         @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 343         enum State {
 344             /**
 345              * The subtask result or exception is not available. This state indicates that
 346              * the subtask was forked but has not completed, it completed after the task
 347              * scope was {@linkplain #shutdown() shut down}, or it was forked after the
 348              * task scope was shut down.
 349              */
 350             UNAVAILABLE,
 351             /**
 352              * The subtask completed successfully with a result. The {@link Subtask#get()
 353              * Subtask.get()} method can be used to obtain the result. This is a terminal
 354              * state.
 355              */
 356             SUCCESS,
 357             /**
 358              * The subtask failed with an exception. The {@link Subtask#exception()
 359              * Subtask.exception()} method can be used to obtain the exception. This is a
 360              * terminal state.
 361              */
 362             FAILED,
 363         }
 364 
 365         /**
 366          * {@return the state of the subtask}
 367          */
 368         State state();
 369 
 370         /**
 371          * Returns the result of the subtask.




 372          *
 373          * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
 374          * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
 375          * joinUntil}) before it can obtain the result of the subtask.



 376          *
 377          * @return the possibly-null result
 378          * @throws IllegalStateException if the subtask has not completed, did not complete
 379          * successfully, or the current thread is the task scope owner and did not join
 380          * after forking
 381          * @see State#SUCCESS
 382          */
 383         T get();
 384 
 385         /**
 386          * {@return the exception thrown by the subtask}








 387          *
 388          * <p> To ensure correct usage, if the scope owner {@linkplain #fork(Callable) forks}
 389          * a subtask, then it must join (with {@link #join() join} or {@link #joinUntil(Instant)
 390          * joinUntil}) before it can obtain the exception thrown by the subtask.
 391          *
 392          * @throws IllegalStateException if the subtask has not completed, completed with
 393          * a result, or the current thread is the task scope owner and did not join after
 394          * forking
 395          * @see State#FAILED
 396          */
 397         Throwable exception();
 398     }
 399 
 400     /**
 401      * Creates a structured task scope with the given name and thread factory. The task
 402      * scope is optionally named for the purposes of monitoring and management. The thread
 403      * factory is used to {@link ThreadFactory#newThread(Runnable) create} threads when
 404      * subtasks are {@linkplain #fork(Callable) forked}. The task scope is owned by the
 405      * current thread.
 406      *
 407      * <p> Construction captures the current thread's {@linkplain ScopedValue scoped value}
 408      * bindings for inheritance by threads started in the task scope. The
 409      * <a href="#TreeStructure">Tree Structure</a> section in the class description details
 410      * how parent-child relations are established implicitly for the purpose of inheritance
 411      * of scoped value bindings.










 412      *
 413      * @param name the name of the task scope, can be null
 414      * @param factory the thread factory
 415      */
 416     @SuppressWarnings("this-escape")
 417     public StructuredTaskScope(String name, ThreadFactory factory) {
 418         this.factory = Objects.requireNonNull(factory, "'factory' is null");
 419         if (name == null)
 420             name = Objects.toIdentityString(this);
 421         this.flock = ThreadFlock.open(name);
 422     }
 423 
 424     /**
 425      * Creates an unnamed structured task scope that creates virtual threads. The task
 426      * scope is owned by the current thread.












 427      *
 428      * @implSpec This constructor is equivalent to invoking the 2-arg constructor with a
 429      * name of {@code null} and a thread factory that creates virtual threads.









 430      */
 431     public StructuredTaskScope() {
 432         this(null, Thread.ofVirtual().factory());
 433     }
 434 
 435     private IllegalStateException newIllegalStateExceptionScopeClosed() {
 436         return new IllegalStateException("Task scope is closed");
 437     }




















 438 
 439     private IllegalStateException newIllegalStateExceptionNoJoin() {
 440         return new IllegalStateException("Owner did not join after forking subtasks");
 441     }



















 442 
 443     /**
 444      * Throws IllegalStateException if the scope is closed, returning the state if not
 445      * closed.
 446      */
 447     private int ensureOpen() {
 448         int s = state;
 449         if (s == CLOSED)
 450             throw newIllegalStateExceptionScopeClosed();
 451         return s;
 452     }










 453 
 454     /**
 455      * Throws WrongThreadException if the current thread is not the owner.
 456      */
 457     private void ensureOwner() {
 458         if (Thread.currentThread() != flock.owner())
 459             throw new WrongThreadException("Current thread not owner");
 460     }














 461 
 462     /**
 463      * Throws WrongThreadException if the current thread is not the owner
 464      * or a thread contained in the tree.
 465      */
 466     private void ensureOwnerOrContainsThread() {
 467         Thread currentThread = Thread.currentThread();
 468         if (currentThread != flock.owner() && !flock.containsThread(currentThread))
 469             throw new WrongThreadException("Current thread not owner or thread in the tree");
 470     }










 471 
 472     /**
 473      * Throws IllegalStateException if the current thread is the owner, and the owner did
 474      * not join after forking a subtask in the given fork round.
 475      */
 476     private void ensureJoinedIfOwner(int round) {
 477         if (Thread.currentThread() == flock.owner() && (round > lastJoinCompleted)) {
 478             throw newIllegalStateExceptionNoJoin();










































































 479         }
 480     }
 481 
 482     /**
 483      * Ensures that the current thread is the owner of this task scope and that it joined
 484      * (with {@link #join()} or {@link #joinUntil(Instant)}) after {@linkplain #fork(Callable)
 485      * forking} subtasks.
 486      *
 487      * @apiNote This method can be used by subclasses that define methods to make available
 488      * results, state, or other outcome to code intended to execute after the join method.

 489      *
 490      * @throws WrongThreadException if the current thread is not the task scope owner
 491      * @throws IllegalStateException if the task scope is open and task scope owner did
 492      * not join after forking














 493      */
 494     protected final void ensureOwnerAndJoined() {
 495         ensureOwner();
 496         if (forkRound > lastJoinCompleted) {
 497             throw newIllegalStateExceptionNoJoin();
 498         }

































 499     }
 500 
 501     /**
 502      * Invoked by a subtask when it completes successfully or fails in this task scope.
 503      * This method is not invoked if a subtask completes after the task scope is
 504      * {@linkplain #shutdown() shut down}.





 505      *
 506      * @implSpec The default implementation throws {@code NullPointerException} if the
 507      * subtask is {@code null}. It throws {@link IllegalArgumentException} if the subtask
 508      * has not completed.
 509      *
 510      * @apiNote The {@code handleComplete} method should be thread safe. It may be
 511      * invoked by several threads concurrently.


 512      *
 513      * @param subtask the subtask


 514      *
 515      * @throws IllegalArgumentException if called with a subtask that has not completed








 516      */
 517     protected void handleComplete(Subtask<? extends T> subtask) {
 518         if (subtask.state() == Subtask.State.UNAVAILABLE)
 519             throw new IllegalArgumentException();



















 520     }
 521 
 522     /**
 523      * Starts a new thread in this task scope to execute a value-returning task, thus
 524      * creating a <em>subtask</em> of this task scope.



 525      *
 526      * <p> The value-returning task is provided to this method as a {@link Callable}, the
 527      * thread executes the task's {@link Callable#call() call} method. The thread is
 528      * created with the task scope's {@link ThreadFactory}. It inherits the current thread's
 529      * {@linkplain ScopedValue scoped value} bindings. The bindings must match the bindings
 530      * captured when the task scope was created.
 531      *
 532      * <p> This method returns a {@link Subtask Subtask} to represent the <em>forked
 533      * subtask</em>. The {@code Subtask} object can be used to obtain the result when
 534      * the subtask completes successfully, or the exception when the subtask fails. To
 535      * ensure correct usage, the {@link Subtask#get() get()} and {@link Subtask#exception()
 536      * exception()} methods may only be called by the task scope owner after it has waited
 537      * for all threads to finish with the {@link #join() join} or {@link #joinUntil(Instant)}
 538      * methods. When the subtask completes, the thread invokes the {@link
 539      * #handleComplete(Subtask) handleComplete} method to consume the completed subtask.
 540      * If the task scope is {@linkplain #shutdown() shut down} before the subtask completes
 541      * then the {@code handleComplete} method will not be invoked.





 542      *
 543      * <p> If this task scope is {@linkplain #shutdown() shutdown} (or in the process of
 544      * shutting down) then the subtask will not run and the {@code handleComplete} method
 545      * will not be invoked.












 546      *
 547      * <p> This method may only be invoked by the task scope owner or threads contained
 548      * in the task scope.







 549      *
 550      * @implSpec This method may be overridden for customization purposes, wrapping tasks
 551      * for example. If overridden, the subclass must invoke {@code super.fork} to start a
 552      * new thread in this task scope.
 553      *
 554      * @param task the value-returning task for the thread to execute
 555      * @param <U> the result type
 556      * @return the subtask
 557      * @throws IllegalStateException if this task scope is closed
 558      * @throws WrongThreadException if the current thread is not the task scope owner or a
 559      * thread contained in the task scope
 560      * @throws StructureViolationException if the current scoped value bindings are not
 561      * the same as when the task scope was created
 562      * @throws RejectedExecutionException if the thread factory rejected creating a
 563      * thread to run the subtask
 564      */
 565     public <U extends T> Subtask<U> fork(Callable<? extends U> task) {
 566         Objects.requireNonNull(task, "'task' is null");
 567         int s = ensureOpen();   // throws ISE if closed
 568 
 569         // when forked by the owner, the subtask is forked in the current or next round
 570         int round = -1;
 571         if (Thread.currentThread() == flock.owner()) {
 572             round = forkRound;
 573             if (forkRound == lastJoinCompleted) {
 574                 // new round if first fork after join
 575                 round++;
 576             }

 577         }
 578 
 579         SubtaskImpl<U> subtask = new SubtaskImpl<>(this, task, round);
 580         if (s < SHUTDOWN) {
 581             // create thread to run task
 582             Thread thread = factory.newThread(subtask);
 583             if (thread == null) {
 584                 throw new RejectedExecutionException("Rejected by thread factory");
 585             }
 586 
 587             // attempt to start the thread
 588             try {
 589                 flock.start(thread);
 590             } catch (IllegalStateException e) {
 591                 // shutdown by another thread, or underlying flock is shutdown due
 592                 // to unstructured use
 593             }
 594         }
 595 
 596         // force owner to join if this is the first fork in the round
 597         if (Thread.currentThread() == flock.owner() && round > forkRound) {
 598             forkRound = round;
 599         }
 600 
 601         // return forked subtask or a subtask that did not run
 602         return subtask;
 603     }
 604 
 605     /**
 606      * Wait for all threads to finish or the task scope to shut down.
 607      */
 608     private void implJoin(Duration timeout)
 609         throws InterruptedException, TimeoutException
 610     {
 611         ensureOwner();
 612         lastJoinAttempted = forkRound;
 613         int s = ensureOpen();  // throws ISE if closed
 614         if (s == OPEN) {
 615             // wait for all threads, wakeup, interrupt, or timeout
 616             if (timeout != null) {
 617                 flock.awaitAll(timeout);
 618             } else {
 619                 flock.awaitAll();
 620             }
 621         }
 622         lastJoinCompleted = forkRound;
 623     }
 624 
 625     /**
 626      * Wait for all subtasks started in this task scope to finish or the task scope to
 627      * shut down.
 628      *
 629      * <p> This method waits for all subtasks by waiting for all threads {@linkplain
 630      * #fork(Callable) started} in this task scope to finish execution. It stops waiting
 631      * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, or
 632      * the current thread is {@linkplain Thread#interrupt() interrupted}.
 633      *
 634      * <p> This method may only be invoked by the task scope owner.
 635      *
 636      * @implSpec This method may be overridden for customization purposes or to return a
 637      * more specific return type. If overridden, the subclass must invoke {@code
 638      * super.join} to ensure that the method waits for threads in this task scope to
 639      * finish.
 640      *
 641      * @return this task scope
 642      * @throws IllegalStateException if this task scope is closed


 643      * @throws WrongThreadException if the current thread is not the task scope owner
 644      * @throws InterruptedException if interrupted while waiting




 645      */
 646     public StructuredTaskScope<T> join() throws InterruptedException {
 647         try {
 648             implJoin(null);
 649         } catch (TimeoutException e) {
 650             throw new InternalError();
 651         }
 652         return this;
 653     }
 654 
 655     /**
 656      * Wait for all subtasks started in this task scope to finish or the task scope to
 657      * shut down, up to the given deadline.





 658      *
 659      * <p> This method waits for all subtasks by waiting for all threads {@linkplain
 660      * #fork(Callable) started} in this task scope to finish execution. It stops waiting
 661      * when all threads finish, the task scope is {@linkplain #shutdown() shut down}, the
 662      * deadline is reached, or the current thread is {@linkplain Thread#interrupt()
 663      * interrupted}.

 664      *
 665      * <p> This method may only be invoked by the task scope owner.
 666      *
 667      * @implSpec This method may be overridden for customization purposes or to return a
 668      * more specific return type. If overridden, the subclass must invoke {@code
 669      * super.joinUntil} to ensure that the method waits for threads in this task scope to
 670      * finish.
 671      *
 672      * @param deadline the deadline
 673      * @return this task scope
 674      * @throws IllegalStateException if this task scope is closed
 675      * @throws WrongThreadException if the current thread is not the task scope owner


 676      * @throws InterruptedException if interrupted while waiting
 677      * @throws TimeoutException if the deadline is reached while waiting
 678      */
 679     public StructuredTaskScope<T> joinUntil(Instant deadline)
 680         throws InterruptedException, TimeoutException
 681     {
 682         Duration timeout = Duration.between(Instant.now(), deadline);
 683         implJoin(timeout);
 684         return this;
 685     }
 686 
 687     /**
 688      * Interrupt all unfinished threads.
 689      */
 690     private void implInterruptAll() {
 691         flock.threads()
 692             .filter(t -> t != Thread.currentThread())
 693             .forEach(t -> {
 694                 try {
 695                     t.interrupt();
 696                 } catch (Throwable ignore) { }
 697             });
 698     }
 699 
 700     @SuppressWarnings("removal")
 701     private void interruptAll() {
 702         if (System.getSecurityManager() == null) {
 703             implInterruptAll();
 704         } else {
 705             PrivilegedAction<Void> pa = () -> {
 706                 implInterruptAll();
 707                 return null;
 708             };
 709             AccessController.doPrivileged(pa);
 710         }
 711     }
 712 
 713     /**
 714      * Shutdown the task scope if not already shutdown. Return true if this method
 715      * shutdowns the task scope, false if already shutdown.
 716      */
 717     private boolean implShutdown() {
 718         shutdownLock.lock();
 719         try {
 720             if (state < SHUTDOWN) {
 721                 // prevent new threads from starting
 722                 flock.shutdown();
 723 
 724                 // set status before interrupting tasks
 725                 state = SHUTDOWN;


 726 
 727                 // interrupt all unfinished threads
 728                 interruptAll();


 729 
 730                 return true;
 731             } else {
 732                 // already shutdown
 733                 return false;
 734             }
 735         } finally {
 736             shutdownLock.unlock();
 737         }
 738     }
 739 
 740     /**
 741      * Shut down this task scope without closing it. Shutting down a task scope prevents
 742      * new threads from starting, interrupts all unfinished threads, and causes the
 743      * {@link #join() join} method to wakeup. Shutdown is useful for cases where the
 744      * results of unfinished subtasks are no longer needed. It will typically be called
 745      * by the {@link #handleComplete(Subtask)} implementation of a subclass that
 746      * implements a policy to discard unfinished tasks once some outcome is reached.
 747      *
 748      * <p> More specifically, this method:
 749      * <ul>
 750      * <li> {@linkplain Thread#interrupt() Interrupts} all unfinished threads in the
 751      * task scope (except the current thread).
 752      * <li> Wakes up the task scope owner if it is waiting in {@link #join()} or {@link
 753      * #joinUntil(Instant)}. If the task scope owner is not waiting then its next call to
 754      * {@code join} or {@code joinUntil} will return immediately.
 755      * </ul>
 756      *
 757      * <p> The {@linkplain Subtask.State state} of unfinished subtasks that complete at
 758      * around the time that the task scope is shutdown is not defined. A subtask that
 759      * completes successfully with a result, or fails with an exception, at around
 760      * the time that the task scope is shutdown may or may not <i>transition</i> to a
 761      * terminal state.
 762      *
 763      * <p> This method may only be invoked by the task scope owner or threads contained
 764      * in the task scope.
 765      *
 766      * @implSpec This method may be overridden for customization purposes. If overridden,
 767      * the subclass must invoke {@code super.shutdown} to ensure that the method shuts
 768      * down the task scope.


 769      *
 770      * @apiNote
 771      * There may be threads that have not finished because they are executing code that
 772      * did not respond (or respond promptly) to thread interrupt. This method does not wait
 773      * for these threads. When the owner invokes the {@link #close() close} method
 774      * to close the task scope then it will wait for the remaining threads to finish.
 775      *
 776      * @throws IllegalStateException if this task scope is closed
 777      * @throws WrongThreadException if the current thread is not the task scope owner or
 778      * a thread contained in the task scope
 779      * @see #isShutdown()
 780      */
 781     public void shutdown() {
 782         ensureOwnerOrContainsThread();
 783         int s = ensureOpen();  // throws ISE if closed
 784         if (s < SHUTDOWN && implShutdown())
 785             flock.wakeup();
 786     }
 787 
 788     /**
 789      * {@return true if this task scope is shutdown, otherwise false}
 790      * @see #shutdown()
 791      */
 792     public final boolean isShutdown() {
 793         return state >= SHUTDOWN;
 794     }
 795 
 796     /**
 797      * Closes this task scope.
 798      *
 799      * <p> This method first shuts down the task scope (as if by invoking the {@link
 800      * #shutdown() shutdown} method). It then waits for the threads executing any
 801      * unfinished tasks to finish. If interrupted, this method will continue to wait for
 802      * the threads to finish before completing with the interrupt status set.

 803      *
 804      * <p> This method may only be invoked by the task scope owner. If the task scope
 805      * is already closed then the task scope owner invoking this method has no effect.
 806      *
 807      * <p> A {@code StructuredTaskScope} is intended to be used in a <em>structured
 808      * manner</em>. If this method is called to close a task scope before nested task
 809      * scopes are closed then it closes the underlying construct of each nested task scope
 810      * (in the reverse order that they were created in), closes this task scope, and then
 811      * throws {@link StructureViolationException}.
 812      * Similarly, if this method is called to close a task scope while executing with
 813      * {@linkplain ScopedValue scoped value} bindings, and the task scope was created
 814      * before the scoped values were bound, then {@code StructureViolationException} is
 815      * thrown after closing the task scope.
 816      * If a thread terminates without first closing task scopes that it owns then
 817      * termination will cause the underlying construct of each of its open tasks scopes to
 818      * be closed. Closing is performed in the reverse order that the task scopes were
 819      * created in. Thread termination may therefore be delayed when the task scope owner
 820      * has to wait for threads forked in these task scopes to finish.
 821      *
 822      * @implSpec This method may be overridden for customization purposes. If overridden,
 823      * the subclass must invoke {@code super.close} to close the task scope.
 824      *
 825      * @throws IllegalStateException thrown after closing the task scope if the task scope
 826      * owner did not attempt to join after forking
 827      * @throws WrongThreadException if the current thread is not the task scope owner
 828      * @throws StructureViolationException if a structure violation was detected
 829      */
 830     @Override
 831     public void close() {
 832         ensureOwner();
 833         int s = state;
 834         if (s == CLOSED)
 835             return;







 836 

 837         try {
 838             if (s < SHUTDOWN)
 839                 implShutdown();
 840             flock.close();
 841         } finally {
 842             state = CLOSED;
 843         }
 844 
 845         // throw ISE if the owner didn't attempt to join after forking
 846         if (forkRound > lastJoinAttempted) {
 847             lastJoinCompleted = forkRound;
 848             throw newIllegalStateExceptionNoJoin();
 849         }
 850     }
 851 




 852     @Override
 853     public String toString() {
 854         String name = flock.name();
 855         return switch (state) {
 856             case OPEN     -> name;
 857             case SHUTDOWN -> name + "/shutdown";
 858             case CLOSED   -> name + "/closed";
 859             default -> throw new InternalError();
 860         };
 861     }
 862 
 863     /**
 864      * Subtask implementation, runs the task specified to the fork method.
 865      */
 866     private static final class SubtaskImpl<T> implements Subtask<T>, Runnable {
 867         private static final AltResult RESULT_NULL = new AltResult(Subtask.State.SUCCESS);
 868 
 869         private record AltResult(Subtask.State state, Throwable exception) {
 870             AltResult(Subtask.State state) {
 871                 this(state, null);
 872             }
 873         }
 874 
 875         private final StructuredTaskScope<? super T> scope;
 876         private final Callable<? extends T> task;
 877         private final int round;
 878         private volatile Object result;
 879 
 880         SubtaskImpl(StructuredTaskScope<? super T> scope,
 881                     Callable<? extends T> task,
 882                     int round) {
 883             this.scope = scope;
 884             this.task = task;
 885             this.round = round;
 886         }
 887 
 888         @Override
 889         public void run() {
 890             T result = null;
 891             Throwable ex = null;
 892             try {
 893                 result = task.call();
 894             } catch (Throwable e) {
 895                 ex = e;
 896             }
 897 
 898             // nothing to do if task scope is shutdown
 899             if (scope.isShutdown())
 900                 return;
 901 
 902             // capture result or exception, invoke handleComplete
 903             if (ex == null) {
 904                 this.result = (result != null) ? result : RESULT_NULL;
 905             } else {
 906                 this.result = new AltResult(State.FAILED, ex);
 907             }
 908             scope.handleComplete(this);
 909         }
 910 
 911         @Override
 912         public Callable<? extends T> task() {
 913             return task;
 914         }
 915 
 916         @Override
 917         public Subtask.State state() {
 918             Object result = this.result;
 919             if (result == null) {
 920                 return State.UNAVAILABLE;
 921             } else if (result instanceof AltResult alt) {
 922                 // null or failed
 923                 return alt.state();
 924             } else {
 925                 return State.SUCCESS;
 926             }
 927         }
 928 

 929         @Override
 930         public T get() {
 931             scope.ensureJoinedIfOwner(round);
 932             Object result = this.result;
 933             if (result instanceof AltResult) {
 934                 if (result == RESULT_NULL) return null;
 935             } else if (result != null) {
 936                 @SuppressWarnings("unchecked")
 937                 T r = (T) result;
 938                 return r;
 939             }
 940             throw new IllegalStateException(
 941                     "Result is unavailable or subtask did not complete successfully");
 942         }
 943 
 944         @Override
 945         public Throwable exception() {
 946             scope.ensureJoinedIfOwner(round);
 947             Object result = this.result;
 948             if (result instanceof AltResult alt && alt.state() == State.FAILED) {
 949                 return alt.exception();
 950             }
 951             throw new IllegalStateException(
 952                     "Exception is unavailable or subtask did not complete with exception");
 953         }
 954 
 955         @Override
 956         public String toString() {
 957             String stateAsString = switch (state()) {
 958                 case UNAVAILABLE -> "[Unavailable]";
 959                 case SUCCESS     -> "[Completed successfully]";
 960                 case FAILED      -> {
 961                     Throwable ex = ((AltResult) result).exception();
 962                     yield "[Failed: " + ex + "]";
 963                 }
 964             };
 965             return Objects.toIdentityString(this) + stateAsString;
 966         }
 967     }
 968 
 969     /**
 970      * A {@code StructuredTaskScope} that captures the result of the first subtask to
 971      * complete {@linkplain Subtask.State#SUCCESS successfully}. Once captured, it
 972      * {@linkplain #shutdown() shuts down} the task scope to interrupt unfinished threads
 973      * and wakeup the task scope owner. The policy implemented by this class is intended
 974      * for cases where the result of any subtask will do ("invoke any") and where the
 975      * results of other unfinished subtasks are no longer needed.
 976      *
 977      * <p> Unless otherwise specified, passing a {@code null} argument to a method
 978      * in this class will cause a {@link NullPointerException} to be thrown.
 979      *
 980      * @apiNote This class implements a policy to shut down the task scope when a subtask
 981      * completes successfully. There shouldn't be any need to directly shut down the task
 982      * scope with the {@link #shutdown() shutdown} method.
 983      *
 984      * @param <T> the result type
 985      * @since 21
 986      */
 987     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
 988     public static final class ShutdownOnSuccess<T> extends StructuredTaskScope<T> {
 989         private static final Object RESULT_NULL = new Object();
 990         private static final VarHandle FIRST_RESULT;
 991         private static final VarHandle FIRST_EXCEPTION;
 992         static {
 993             try {
 994                 MethodHandles.Lookup l = MethodHandles.lookup();
 995                 FIRST_RESULT = l.findVarHandle(ShutdownOnSuccess.class, "firstResult", Object.class);
 996                 FIRST_EXCEPTION = l.findVarHandle(ShutdownOnSuccess.class, "firstException", Throwable.class);
 997             } catch (Exception e) {
 998                 throw new ExceptionInInitializerError(e);
 999             }
1000         }
1001         private volatile Object firstResult;

1002         private volatile Throwable firstException;
1003 
1004         /**
1005          * Constructs a new {@code ShutdownOnSuccess} with the given name and thread factory.
1006          * The task scope is optionally named for the purposes of monitoring and management.
1007          * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
1008          * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
1009          * is owned by the current thread.
1010          *
1011          * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
1012          * value} bindings for inheritance by threads started in the task scope. The
1013          * <a href="#TreeStructure">Tree Structure</a> section in the class description
1014          * details how parent-child relations are established implicitly for the purpose
1015          * of inheritance of scoped value bindings.
1016          *
1017          * @param name the name of the task scope, can be null
1018          * @param factory the thread factory
1019          */
1020         public ShutdownOnSuccess(String name, ThreadFactory factory) {
1021             super(name, factory);
1022         }
1023 
1024         /**
1025          * Constructs a new unnamed {@code ShutdownOnSuccess} that creates virtual threads.
1026          *
1027          * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1028          * a name of {@code null} and a thread factory that creates virtual threads.
1029          */
1030         public ShutdownOnSuccess() {
1031             this(null, Thread.ofVirtual().factory());
1032         }
1033 
1034         @Override
1035         protected void handleComplete(Subtask<? extends T> subtask) {
1036             if (firstResult != null) {
1037                 // already captured a result
1038                 return;


1039             }


1040 
1041             if (subtask.state() == Subtask.State.SUCCESS) {
1042                 // task succeeded
1043                 T result = subtask.get();
1044                 Object r = (result != null) ? result : RESULT_NULL;
1045                 if (FIRST_RESULT.compareAndSet(this, null, r)) {
1046                     super.shutdown();
1047                 }
1048             } else if (firstException == null) {
1049                 // capture the exception thrown by the first subtask that failed
1050                 FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception());




1051             }
1052         }


1053 
1054         /**
1055          * Wait for a subtask started in this task scope to complete {@linkplain
1056          * Subtask.State#SUCCESS successfully} or all subtasks to complete.
1057          *
1058          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1059          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1060          * when all threads finish, a subtask completes successfully, or the current
1061          * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1062          * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1063          * this task scope.
1064          *
1065          * <p> This method may only be invoked by the task scope owner.
1066          *
1067          * @throws IllegalStateException {@inheritDoc}
1068          * @throws WrongThreadException {@inheritDoc}
1069          */
1070         @Override
1071         public ShutdownOnSuccess<T> join() throws InterruptedException {
1072             super.join();
1073             return this;








1074         }
1075 
1076         /**
1077          * Wait for a subtask started in this task scope to complete {@linkplain
1078          * Subtask.State#SUCCESS successfully} or all subtasks to complete, up to the
1079          * given deadline.
1080          *
1081          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1082          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1083          * when all threads finish, a subtask completes successfully, the deadline is
1084          * reached, or the current thread is {@linkplain Thread#interrupt() interrupted}.
1085          * It also stops waiting if the {@link #shutdown() shutdown} method is invoked
1086          * directly to shut down this task scope.
1087          *
1088          * <p> This method may only be invoked by the task scope owner.
1089          *
1090          * @throws IllegalStateException {@inheritDoc}
1091          * @throws WrongThreadException {@inheritDoc}
1092          */
1093         @Override
1094         public ShutdownOnSuccess<T> joinUntil(Instant deadline)
1095             throws InterruptedException, TimeoutException
1096         {
1097             super.joinUntil(deadline);
1098             return this;
1099         }
1100 
1101         /**
1102          * {@return the result of the first subtask that completed {@linkplain
1103          * Subtask.State#SUCCESS successfully}}
1104          *
1105          * <p> When no subtask completed successfully, but a subtask {@linkplain
1106          * Subtask.State#FAILED failed} then {@code ExecutionException} is thrown with
1107          * the subtask's exception as the {@linkplain Throwable#getCause() cause}.
1108          *
1109          * @throws ExecutionException if no subtasks completed successfully but at least
1110          * one subtask failed
1111          * @throws IllegalStateException if no subtasks completed or the task scope owner
1112          * did not join after forking
1113          * @throws WrongThreadException if the current thread is not the task scope owner
1114          */
1115         public T result() throws ExecutionException {
1116             return result(ExecutionException::new);
1117         }
1118 
1119         /**
1120          * Returns the result of the first subtask that completed {@linkplain
1121          * Subtask.State#SUCCESS successfully}, otherwise throws an exception produced
1122          * by the given exception supplying function.
1123          *
1124          * <p> When no subtask completed successfully, but a subtask {@linkplain
1125          * Subtask.State#FAILED failed}, then the exception supplying function is invoked
1126          * with subtask's exception.
1127          *
1128          * @param esf the exception supplying function
1129          * @param <X> type of the exception to be thrown
1130          * @return the result of the first subtask that completed with a result
1131          *
1132          * @throws X if no subtasks completed successfully but at least one subtask failed
1133          * @throws IllegalStateException if no subtasks completed or the task scope owner
1134          * did not join after forking
1135          * @throws WrongThreadException if the current thread is not the task scope owner
1136          */
1137         public <X extends Throwable> T result(Function<Throwable, ? extends X> esf) throws X {
1138             Objects.requireNonNull(esf);
1139             ensureOwnerAndJoined();
1140 
1141             Object result = firstResult;
1142             if (result == RESULT_NULL) {
1143                 return null;
1144             } else if (result != null) {
1145                 @SuppressWarnings("unchecked")
1146                 T r = (T) result;
1147                 return r;
1148             }
1149 
1150             Throwable exception = firstException;
1151             if (exception != null) {
1152                 X ex = esf.apply(exception);
1153                 Objects.requireNonNull(ex, "esf returned null");
1154                 throw ex;
1155             }
1156 
1157             throw new IllegalStateException("No completed subtasks");
1158         }
1159     }
1160 
1161     /**
1162      * A {@code StructuredTaskScope} that captures the exception of the first subtask to
1163      * {@linkplain Subtask.State#FAILED fail}. Once captured, it {@linkplain #shutdown()
1164      * shuts down} the task scope to interrupt unfinished threads and wakeup the task
1165      * scope owner. The policy implemented by this class is intended for cases where the
1166      * results for all subtasks are required ("invoke all"); if any subtask fails then the
1167      * results of other unfinished subtasks are no longer needed.
1168      *
1169      * <p> Unless otherwise specified, passing a {@code null} argument to a method
1170      * in this class will cause a {@link NullPointerException} to be thrown.
1171      *
1172      * @apiNote This class implements a policy to shut down the task scope when a subtask
1173      * fails. There shouldn't be any need to directly shut down the task scope with the
1174      * {@link #shutdown() shutdown} method.
1175      *
1176      * @since 21
1177      */
1178     @PreviewFeature(feature = PreviewFeature.Feature.STRUCTURED_CONCURRENCY)
1179     public static final class ShutdownOnFailure extends StructuredTaskScope<Object> {
1180         private static final VarHandle FIRST_EXCEPTION;
1181         static {
1182             try {
1183                 MethodHandles.Lookup l = MethodHandles.lookup();
1184                 FIRST_EXCEPTION = l.findVarHandle(ShutdownOnFailure.class, "firstException", Throwable.class);
1185             } catch (Exception e) {
1186                 throw new ExceptionInInitializerError(e);
1187             }
1188         }
1189         private volatile Throwable firstException;
1190 
1191         /**
1192          * Constructs a new {@code ShutdownOnFailure} with the given name and thread factory.
1193          * The task scope is optionally named for the purposes of monitoring and management.
1194          * The thread factory is used to {@link ThreadFactory#newThread(Runnable) create}
1195          * threads when subtasks are {@linkplain #fork(Callable) forked}. The task scope
1196          * is owned by the current thread.
1197          *
1198          * <p> Construction captures the current thread's {@linkplain ScopedValue scoped
1199          * value} bindings for inheritance by threads started in the task scope. The
1200          * <a href="#TreeStructure">Tree Structure</a> section in the class description
1201          * details how parent-child relations are established implicitly for the purpose
1202          * of inheritance of scoped value bindings.
1203          *
1204          * @param name the name of the task scope, can be null
1205          * @param factory the thread factory
1206          */
1207         public ShutdownOnFailure(String name, ThreadFactory factory) {
1208             super(name, factory);
1209         }
1210 
1211         /**
1212          * Constructs a new unnamed {@code ShutdownOnFailure} that creates virtual threads.
1213          *
1214          * @implSpec This constructor is equivalent to invoking the 2-arg constructor with
1215          * a name of {@code null} and a thread factory that creates virtual threads.
1216          */
1217         public ShutdownOnFailure() {
1218             this(null, Thread.ofVirtual().factory());













1219         }
1220 
1221         @Override
1222         protected void handleComplete(Subtask<?> subtask) {
1223             if (subtask.state() == Subtask.State.FAILED
1224                     && firstException == null
1225                     && FIRST_EXCEPTION.compareAndSet(this, null, subtask.exception())) {
1226                 super.shutdown();
1227             }
1228         }
1229 
1230         /**
1231          * Wait for all subtasks started in this task scope to complete or for a subtask
1232          * to {@linkplain Subtask.State#FAILED fail}.
1233          *
1234          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1235          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1236          * when all threads finish, a subtask fails, or the current thread is {@linkplain
1237          * Thread#interrupt() interrupted}. It also stops waiting if the {@link #shutdown()
1238          * shutdown} method is invoked directly to shut down this task scope.
1239          *
1240          * <p> This method may only be invoked by the task scope owner.
1241          *
1242          * @throws IllegalStateException {@inheritDoc}
1243          * @throws WrongThreadException {@inheritDoc}
1244          */
1245         @Override
1246         public ShutdownOnFailure join() throws InterruptedException {
1247             super.join();
1248             return this;
1249         }
1250 
1251         /**
1252          * Wait for all subtasks started in this task scope to complete or for a subtask
1253          * to {@linkplain Subtask.State#FAILED fail}, up to the given deadline.
1254          *
1255          * <p> This method waits for all subtasks by waiting for all threads {@linkplain
1256          * #fork(Callable) started} in this task scope to finish execution. It stops waiting
1257          * when all threads finish, a subtask fails, the deadline is reached, or the current
1258          * thread is {@linkplain Thread#interrupt() interrupted}. It also stops waiting
1259          * if the {@link #shutdown() shutdown} method is invoked directly to shut down
1260          * this task scope.
1261          *
1262          * <p> This method may only be invoked by the task scope owner.
1263          *
1264          * @throws IllegalStateException {@inheritDoc}
1265          * @throws WrongThreadException {@inheritDoc}
1266          */
1267         @Override
1268         public ShutdownOnFailure joinUntil(Instant deadline)
1269             throws InterruptedException, TimeoutException
1270         {
1271             super.joinUntil(deadline);
1272             return this;
1273         }

1274 
1275         /**
1276          * Returns the exception of the first subtask that {@linkplain Subtask.State#FAILED
1277          * failed}. If no subtasks failed then an empty {@code Optional} is returned.
1278          *
1279          * @return the exception for the first subtask to fail or an empty optional if no
1280          * subtasks failed
1281          *
1282          * @throws WrongThreadException if the current thread is not the task scope owner
1283          * @throws IllegalStateException if the task scope owner did not join after forking
1284          */
1285         public Optional<Throwable> exception() {
1286             ensureOwnerAndJoined();
1287             return Optional.ofNullable(firstException);
1288         }
1289 
1290         /**
1291          * Throws if a subtask {@linkplain Subtask.State#FAILED failed}.
1292          * If any subtask failed with an exception then {@code ExecutionException} is
1293          * thrown with the exception of the first subtask to fail as the {@linkplain
1294          * Throwable#getCause() cause}. This method does nothing if no subtasks failed.
1295          *
1296          * @throws ExecutionException if a subtask failed
1297          * @throws WrongThreadException if the current thread is not the task scope owner
1298          * @throws IllegalStateException if the task scope owner did not join after forking
1299          */
1300         public void throwIfFailed() throws ExecutionException {
1301             throwIfFailed(ExecutionException::new);
1302         }
1303 
1304         /**
1305          * Throws the exception produced by the given exception supplying function if a
1306          * subtask {@linkplain Subtask.State#FAILED failed}. If any subtask failed with
1307          * an exception then the function is invoked with the exception of the first
1308          * subtask to fail. The exception returned by the function is thrown. This method
1309          * does nothing if no subtasks failed.
1310          *
1311          * @param esf the exception supplying function
1312          * @param <X> type of the exception to be thrown
1313          *
1314          * @throws X produced by the exception supplying function
1315          * @throws WrongThreadException if the current thread is not the task scope owner
1316          * @throws IllegalStateException if the task scope owner did not join after forking
1317          */
1318         public <X extends Throwable>
1319         void throwIfFailed(Function<Throwable, ? extends X> esf) throws X {
1320             ensureOwnerAndJoined();
1321             Objects.requireNonNull(esf);
1322             Throwable exception = firstException;
1323             if (exception != null) {
1324                 X ex = esf.apply(exception);
1325                 Objects.requireNonNull(ex, "esf returned null");
1326                 throw ex;
1327             }






1328         }
1329     }
1330 }
--- EOF ---