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/OpenJDK 21

Class StructuredTaskScope<T>

java.lang.Object
java.util.concurrent.StructuredTaskScope<T>
Type Parameters:
T - the result type of tasks executed in the task scope
All Implemented Interfaces:
AutoCloseable
Direct Known Subclasses:
StructuredTaskScope.ShutdownOnFailurePREVIEW, StructuredTaskScope.ShutdownOnSuccessPREVIEW 
public class StructuredTaskScope<T> extends Object implements AutoCloseable
StructuredTaskScope is a preview API of the Java platform.
Programs can only use StructuredTaskScope when preview features are enabled.
Preview features may be removed in a future release, or upgraded to permanent features of the Java platform.
A basic API for structured concurrency. StructuredTaskScope supports cases where a task splits into several concurrent subtasks, and where the subtasks must complete before the main task continues. A StructuredTaskScope can be used to ensure that the lifetime of a concurrent operation is confined by a syntax block, just like that of a sequential operation in structured programming.

Basic operation

A StructuredTaskScope is created with one of its public constructors. It defines the fork method to start a thread to execute a subtask, the join method to wait for all subtasks to finish, and the close method to close the task scope. The API is intended to be used with the try-with-resources statement. The intention is that code in the try block uses the fork method to fork threads to execute the subtasks, wait for the subtasks to finish with the join method, and then process the results. A call to the fork method returns a SubtaskPREVIEW to representing the forked subtask. Once join is called, the Subtask can be used to get the result completed successfully, or the exception if the subtask failed. 
    Callable<String> task1 = ...
    Callable<Integer> task2 = ...

    try (var scope = new StructuredTaskScope<Object>()) {

        Subtask<String> subtask1 = scope.fork(task1);
        Subtask<Integer> subtask2 = scope.fork(task2);

        scope.join();

        ... process results/exceptions ...

    } // close

The following example forks a collection of homogeneous subtasks, waits for all of them to complete with the join method, and uses the Subtask.StatePREVIEW to partition the subtasks into a set of the subtasks that completed successfully and another for the subtasks that failed. 

    List<Callable<String>> callables = ...

    try (var scope = new StructuredTaskScope<String>()) {

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

        scope.join();

        Map<Boolean, Set<Subtask<String>>> map = subtasks.stream()
                .collect(Collectors.partitioningBy(h -> h.state() == Subtask.State.SUCCESS,
                                                   Collectors.toSet()));

    } // close

To ensure correct usage, the join and close methods may only be invoked by the owner (the thread that opened/created the task scope), and the close method throws an exception after closing if the owner did not invoke the join method after forking.

StructuredTaskScope defines the shutdown method to shut down a task scope without closing it. The shutdown() method cancels all unfinished subtasks by interrupting the threads. It prevents new threads from starting in the task scope. If the owner is waiting in the join method then it will wakeup.

Shutdown is used for short-circuiting and allow subclasses to implement policy that does not require all subtasks to finish.

Subclasses with policies for common cases

Two subclasses of StructuredTaskScope are defined to implement policy for common cases:
  1. ShutdownOnSuccessPREVIEW captures the result of the first subtask to complete successfully. Once captured, it shuts down the task scope to interrupt unfinished threads and wakeup the owner. This class is intended for cases where the result of any subtask will do ("invoke any") and where there is no need to wait for results of other unfinished subtasks. It defines methods to get the first result or throw an exception if all subtasks fail.
  2. ShutdownOnFailurePREVIEW captures the exception of the first subtask to fail. Once captured, it shuts down the task scope to interrupt unfinished threads and wakeup the owner. This class is intended for cases where the results of all subtasks are required ("invoke all"); if any subtask fails then the results of other unfinished subtasks are no longer needed. If defines methods to throw an exception if any of the subtasks fail.

The following are two examples that use the two classes. In both cases, a pair of subtasks are forked to fetch resources from two URL locations "left" and "right". The first example creates a ShutdownOnSuccess object to capture the result of the first subtask to complete successfully, cancelling the other by way of shutting down the task scope. The main task waits in join until either subtask completes with a result or both subtasks fail. It invokes result(Function)PREVIEW method to get the captured result. If both subtasks fail then this method throws a WebApplicationException with the exception from one of the subtasks as the cause. 

    try (var scope = new StructuredTaskScope.ShutdownOnSuccess<String>()) {

        scope.fork(() -> fetch(left));
        scope.fork(() -> fetch(right));

        scope.join();

        String result = scope.result(e -> new WebApplicationException(e));

        ...
    }
The second example creates a ShutdownOnFailure object to capture the exception of the first subtask to fail, cancelling the other by way of shutting down the task scope. The main task waits in joinUntil(Instant) until both subtasks complete with a result, either fails, or a deadline is reached. It invokes throwIfFailed(Function)PREVIEW to throw an exception if either subtask fails. This method is a no-op if both subtasks complete successfully. The example uses Supplier.get() to get the result of each subtask. Using Supplier instead of Subtask is preferred for common cases where the object returned by fork is only used to get the result of a subtask that completed successfully. 
   Instant deadline = ...

   try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {

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

        scope.joinUntil(deadline);

        scope.throwIfFailed(e -> new WebApplicationException(e));

        // both subtasks completed successfully
        String result = Stream.of(supplier1, supplier2)
                .map(Supplier::get)
                .collect(Collectors.joining(", ", "{ ", " }"));

        ...
    }

Extending StructuredTaskScope

StructuredTaskScope can be extended, and the handleComplete method overridden, to implement policies other than those implemented by ShutdownOnSuccess and ShutdownOnFailure. A subclass may, for example, collect the results of subtasks that complete successfully and ignore subtasks that fail. It may collect exceptions when subtasks fail. It may invoke the shutdown method to shut down and cause join to wakeup when some condition arises.

A subclass will typically define methods to make available results, state, or other outcome to code that executes after the join method. A subclass that collects results and ignores subtasks that fail may define a method that returns the results. A subclass that implements a policy to shut down when a subtask fails may define a method to get the exception of the first subtask to fail.

The following is an example of a simple StructuredTaskScope implementation that collects homogenous subtasks that complete successfully. It defines the method "completedSuccessfully()" that the main task can invoke after it joins. 

    class CollectingScope<T> extends StructuredTaskScope<T> {
        private final Queue<Subtask<? extends T>> subtasks = new LinkedTransferQueue<>();

        @Override
        protected void handleComplete(Subtask<? extends T> subtask) {
            if (subtask.state() == Subtask.State.SUCCESS) {
                subtasks.add(subtask);
            }
        }

        @Override
        public CollectingScope<T> join() throws InterruptedException {
            super.join();
            return this;
        }

        public Stream<Subtask<? extends T>> completedSuccessfully() {
            super.ensureOwnerAndJoined();
            return subtasks.stream();
        }
    }

The implementations of the completedSuccessfully() method in the example invokes ensureOwnerAndJoined() to ensure that the method can only be invoked by the owner thread and only after it has joined.

Tree structure

Task scopes form a tree where parent-child relations are established implicitly when opening a new task scope:
  • A parent-child relation is established when a thread started in a task scope opens its own task scope. A thread started in task scope "A" that opens task scope "B" establishes a parent-child relation where task scope "A" is the parent of task scope "B".
  • A parent-child relation is established with nesting. If a thread opens task scope "B", then opens task scope "C" (before it closes "B"), then the enclosing task scope "B" is the parent of the nested task scope "C".
The descendants of a task scope are the child task scopes that it is a parent of, plus the descendants of the child task scopes, recursively.

The tree structure supports:

  • Inheritance of scoped valuesPREVIEW across threads.
  • Confinement checks. The phrase "threads contained in the task scope" in method descriptions means threads started in the task scope or descendant scopes.

The following example demonstrates the inheritance of a scoped value. A scoped value USERNAME is bound to the value "duke". A StructuredTaskScope is created and its fork method invoked to start a thread to execute childTask. The thread inherits the scoped value bindings captured when creating the task scope. The code in childTask uses the value of the scoped value and so reads the value "duke". 

    private static final ScopedValue<String> USERNAME = ScopedValue.newInstance();

    ScopedValue.runWhere(USERNAME, "duke", () -> {
        try (var scope = new StructuredTaskScope<String>()) {

            scope.fork(() -> childTask());
            ...
         }
    });

    ...

    String childTask() {
        String name = USERNAME.get();   // "duke"
        ...
    }

StructuredTaskScope does not define APIs that exposes the tree structure at this time.

Unless otherwise specified, passing a null argument to a constructor or method in this class will cause a NullPointerException to be thrown.

Memory consistency effects

Actions in the owner thread of, or a thread contained in, the task scope prior to forking of a subtask happen-before any actions taken by that subtask, which in turn happen-before the subtask result is retrievedPREVIEW or happen-before any actions taken in a thread after joining of the task scope.

See Java Language Specification:
17.4.5 Happens-before Order
Since:
21

Nested Class Summary

Modifier and Type Class Description
static final class  StructuredTaskScope.ShutdownOnFailurePREVIEW
Preview.
A StructuredTaskScope that captures the exception of the first subtask to failPREVIEW.
static final class  StructuredTaskScope.ShutdownOnSuccessPREVIEW<T>
Preview.
A StructuredTaskScope that captures the result of the first subtask to complete successfullyPREVIEW.
static interface  StructuredTaskScope.SubtaskPREVIEW<T>
Preview.
Represents a subtask forked with fork(Callable).

Constructor Summary

Constructor Description
StructuredTaskScope()
Creates an unnamed structured task scope that creates virtual threads.
StructuredTaskScope(String name, ThreadFactory factory)
Creates a structured task scope with the given name and thread factory.

Method Summary

Modifier and Type Method Description
void close()
Closes this task scope.
protected final void ensureOwnerAndJoined()
Ensures that the current thread is the owner of this task scope and that it joined (with join() or joinUntil(Instant)) after forking subtasks.
<U extends T>
StructuredTaskScope.SubtaskPREVIEW<U>		 fork(Callable<? extends U> task)
Starts a new thread in this task scope to execute a value-returning task, thus creating a subtask of this task scope.
protected void handleComplete(StructuredTaskScope.SubtaskPREVIEW<? extends T> subtask)
Invoked by a subtask when it completes successfully or fails in this task scope.
final boolean isShutdown()
Returns true if this task scope is shutdown, otherwise false.
StructuredTaskScopePREVIEW<T> join()
Wait for all subtasks started in this task scope to finish or the task scope to shut down.
StructuredTaskScopePREVIEW<T> joinUntil(Instant deadline)
Wait for all subtasks started in this task scope to finish or the task scope to shut down, up to the given deadline.
void shutdown()
Shut down this task scope without closing it.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

StructuredTaskScope

public StructuredTaskScope(String name, ThreadFactory factory)
Creates a structured task scope with the given name and thread factory. The task scope is optionally named for the purposes of monitoring and management. The thread factory is used to create threads when subtasks are forked. The task scope is owned by the current thread.

Construction captures the current thread's scoped valuePREVIEW bindings for inheritance by threads started in the task scope. The Tree Structure section in the class description details how parent-child relations are established implicitly for the purpose of inheritance of scoped value bindings.

Parameters:
name - the name of the task scope, can be null
factory - the thread factory

StructuredTaskScope

public StructuredTaskScope()
Creates an unnamed structured task scope that creates virtual threads. The task scope is owned by the current thread.
Implementation Requirements:
This constructor is equivalent to invoking the 2-arg constructor with a name of null and a thread factory that creates virtual threads.

Method Details

ensureOwnerAndJoined

protected final void ensureOwnerAndJoined()
Ensures that the current thread is the owner of this task scope and that it joined (with join() or joinUntil(Instant)) after forking subtasks.
API Note:
This method can be used by subclasses that define methods to make available results, state, or other outcome to code intended to execute after the join method.
Throws:
WrongThreadException - if the current thread is not the task scope owner
IllegalStateException - if the task scope is open and task scope owner did not join after forking

handleComplete

protected void handleComplete(StructuredTaskScope.SubtaskPREVIEW<? extends T> subtask)
Invoked by a subtask when it completes successfully or fails in this task scope. This method is not invoked if a subtask completes after the task scope is shut down.
API Note:
The handleComplete method should be thread safe. It may be invoked by several threads concurrently.
Implementation Requirements:
The default implementation throws NullPointerException if the subtask is null. It throws IllegalArgumentException if the subtask has not completed.
Parameters:
subtask - the subtask
Throws:
IllegalArgumentException - if called with a subtask that has not completed

fork

public <U extends T> StructuredTaskScope.SubtaskPREVIEW<U> fork(Callable<? extends U> task)
Starts a new thread in this task scope to execute a value-returning task, thus creating a subtask of this task scope.

The value-returning task is provided to this method as a Callable, the thread executes the task's call method. The thread is created with the task scope's ThreadFactory. It inherits the current thread's scoped valuePREVIEW bindings. The bindings must match the bindings captured when the task scope was created.

This method returns a SubtaskPREVIEW to represent the forked subtask. The Subtask object can be used to obtain the result when the subtask completes successfully, or the exception when the subtask fails. To ensure correct usage, the get()PREVIEW and exception()PREVIEW methods may only be called by the task scope owner after it has waited for all threads to finish with the join or joinUntil(Instant) methods. When the subtask completes, the thread invokes the handleComplete method to consume the completed subtask. If the task scope is shut down before the subtask completes then the handleComplete method will not be invoked.

If this task scope is shutdown (or in the process of shutting down) then the subtask will not run and the handleComplete method will not be invoked.

This method may only be invoked by the task scope owner or threads contained in the task scope.

Implementation Requirements:
This method may be overridden for customization purposes, wrapping tasks for example. If overridden, the subclass must invoke super.fork to start a new thread in this task scope.
Type Parameters:
U - the result type
Parameters:
task - the value-returning task for the thread to execute
Returns:
the subtask
Throws:
IllegalStateException - if this task scope is closed
WrongThreadException - if the current thread is not the task scope owner or a thread contained in the task scope
StructureViolationExceptionPREVIEW - if the current scoped value bindings are not the same as when the task scope was created
RejectedExecutionException - if the thread factory rejected creating a thread to run the subtask

join

public StructuredTaskScopePREVIEW<T> join() throws InterruptedException
Wait for all subtasks started in this task scope to finish or the task scope to shut down.

This method waits for all subtasks by waiting for all threads started in this task scope to finish execution. It stops waiting when all threads finish, the task scope is shut down, or the current thread is interrupted.

This method may only be invoked by the task scope owner.

Implementation Requirements:
This method may be overridden for customization purposes or to return a more specific return type. If overridden, the subclass must invoke super.join to ensure that the method waits for threads in this task scope to finish.
Returns:
this task scope
Throws:
IllegalStateException - if this task scope is closed
WrongThreadException - if the current thread is not the task scope owner
InterruptedException - if interrupted while waiting

joinUntil

public StructuredTaskScopePREVIEW<T> joinUntil(Instant deadline) throws InterruptedException, TimeoutException
Wait for all subtasks started in this task scope to finish or the task scope to shut down, up to the given deadline.

This method waits for all subtasks by waiting for all threads started in this task scope to finish execution. It stops waiting when all threads finish, the task scope is shut down, the deadline is reached, or the current thread is interrupted.

This method may only be invoked by the task scope owner.

Implementation Requirements:
This method may be overridden for customization purposes or to return a more specific return type. If overridden, the subclass must invoke super.joinUntil to ensure that the method waits for threads in this task scope to finish.
Parameters:
deadline - the deadline
Returns:
this task scope
Throws:
IllegalStateException - if this task scope is closed
WrongThreadException - if the current thread is not the task scope owner
InterruptedException - if interrupted while waiting
TimeoutException - if the deadline is reached while waiting

shutdown

public void shutdown()
Shut down this task scope without closing it. Shutting down a task scope prevents new threads from starting, interrupts all unfinished threads, and causes the join method to wakeup. Shutdown is useful for cases where the results of unfinished subtasks are no longer needed. It will typically be called by the handleComplete(Subtask) implementation of a subclass that implements a policy to discard unfinished tasks once some outcome is reached.

More specifically, this method:

  • Interrupts all unfinished threads in the task scope (except the current thread).
  • Wakes up the task scope owner if it is waiting in join() or joinUntil(Instant). If the task scope owner is not waiting then its next call to join or joinUntil will return immediately.

This method may only be invoked by the task scope owner or threads contained in the task scope.

API Note:
There may be threads that have not finished because they are executing code that did not respond (or respond promptly) to thread interrupt. This method does not wait for these threads. When the owner invokes the close method to close the task scope then it will wait for the remaining threads to finish.
Implementation Requirements:
This method may be overridden for customization purposes. If overridden, the subclass must invoke super.shutdown to ensure that the method shuts down the task scope.
Throws:
IllegalStateException - if this task scope is closed
WrongThreadException - if the current thread is not the task scope owner or a thread contained in the task scope
See Also:

isShutdown

public final boolean isShutdown()
Returns true if this task scope is shutdown, otherwise false.
Returns:
true if this task scope is shutdown, otherwise false
See Also:

close

public void close()
Closes this task scope.

This method first shuts down the task scope (as if by invoking the shutdown method). It then waits for the threads executing any unfinished tasks to finish. If interrupted, this method will continue to wait for the threads to finish before completing with the interrupt status set.

This method may only be invoked by the task scope owner. If the task scope is already closed then the task scope owner invoking this method has no effect.

A StructuredTaskScope is intended to be used in a structured manner. If this method is called to close a task scope before nested task scopes are closed then it closes the underlying construct of each nested task scope (in the reverse order that they were created in), closes this task scope, and then throws StructureViolationExceptionPREVIEW. Similarly, if this method is called to close a task scope while executing with scoped valuePREVIEW bindings, and the task scope was created before the scoped values were bound, then StructureViolationException is thrown after closing the task scope. If a thread terminates without first closing task scopes that it owns then termination will cause the underlying construct of each of its open tasks scopes to be closed. Closing is performed in the reverse order that the task scopes were created in. Thread termination may therefore be delayed when the task scope owner has to wait for threads forked in these task scopes to finish.

Specified by:
close in interface AutoCloseable
Implementation Requirements:
This method may be overridden for customization purposes. If overridden, the subclass must invoke super.close to close the task scope.
Throws:
IllegalStateException - thrown after closing the task scope if the task scope owner did not attempt to join after forking
WrongThreadException - if the current thread is not the task scope owner
StructureViolationExceptionPREVIEW - if a structure violation was detected

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Documentation extracted from Debian's OpenJDK Development Kit package.
Licensed under the GNU General Public License, version 2, with the Classpath Exception.
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https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/StructuredTaskScope.html