StructuredTaskScope (Java SE 25 & JDK 25)
Description: declaration: module: java.base, package: java.util.concurrent, interface: StructuredTaskScope
Keywords: java.util.concurrent.StructuredTaskScope interface

# Interface StructuredTaskScope<T,R>

Type Parameters:
  `T` - the result type of subtasks executed in the scope
  `R` - the result type of the scope

All Superinterfaces:
  `AutoCloseable`

public sealed interface StructuredTaskScope<T,R> extends 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.
An API for structured concurrency. `StructuredTaskScope` supports cases where execution of a task (a unit of work) splits into several concurrent subtasks, and where the subtasks must complete before the task continues. A `StructuredTaskScope` can be used to ensure that the lifetime of a concurrent operation is confined by a syntax block, similar to that of a sequential operation in structured programming.

`StructuredTaskScope` defines the static method `open` to open a new `StructuredTaskScope` and the `close` method to close it. The API is designed to be used with the `try`-with-resources statement where the `StructuredTaskScope` is opened as a resource and then closed automatically. The code inside the block uses the `fork` method to fork subtasks. After forking, it uses the `join` method to wait for all subtasks to finish (or some other outcome) as a single operation. Forking a subtask starts a new `Thread` to run the subtask. The thread executing the task does not continue beyond the `close` method until all threads started to execute subtasks have finished. To ensure correct usage, the `fork`, `join` and `close` methods may only be invoked by the owner thread (the thread that opened the `StructuredTaskScope`), the `fork` method may not be called after `join`, the `join` method may only be invoked once, and the `close` method throws an exception after closing if the owner did not invoke the `join` method after forking subtasks.

As a first example, consider a task that splits into two subtasks to concurrently fetch resources from two URL locations "left" and "right". Both subtasks may complete successfully, one subtask may succeed and the other may fail, or both subtasks may fail. The task in this example is interested in the successful result from both subtasks. It waits in the `join` method for both subtasks to complete successfully or for either subtask to fail.

       try (var scope = StructuredTaskScope.open()) {

           Subtask<String> subtask1 = scope.fork(() -> query(left));
           Subtask<Integer> subtask2 = scope.fork(() -> query(right));

           // throws if either subtask fails
           scope.join();

           // both subtasks completed successfully
           return new MyResult(subtask1.get(), subtask2.get());

       } // close

If both subtasks complete successfully then the `join` method completes normally and the task uses the `Subtask.get()`PREVIEW method to get the result of each subtask. If one of the subtasks fails then the other subtask is cancelled (this will interrupt the thread executing the other subtask) and the `join` method throws `StructuredTaskScope.FailedException`PREVIEW with the exception from the failed subtask as the cause.

To allow for cancellation, subtasks must be coded so that they finish as soon as possible when interrupted. Subtasks that do not respond to interrupt, e.g. block on methods that are not interruptible, may delay the closing of a scope indefinitely. The `close` method always waits for threads executing subtasks to finish, even if the scope is cancelled, so execution cannot continue beyond the `close` method until the interrupted threads finish.

In the example, the subtasks produce results of different types (`String` and `Integer`). In other cases the subtasks may all produce results of the same type. If the example had used `StructuredTaskScope.<String>open()` then it could only be used to fork subtasks that return a `String` result.

## Joiners

In the example above, the task fails if any subtask fails. If all subtasks succeed then the `join` method completes normally. Other policy and outcome is supported by creating a `StructuredTaskScope` with a `StructuredTaskScope.Joiner`PREVIEW that implements the desired policy. A `Joiner` handles subtask completion and produces the outcome for the `join` method. In the example above, `join` returns `null`. Depending on the `Joiner`, `join` may return a result, a stream of elements, or some other object. The `Joiner` interface defines factory methods to create `Joiner`s for some common cases.

A `Joiner` may cancel the scope (sometimes called "short-circuiting") when some condition is reached that does not require the result of subtasks that are still executing. Cancelling the scope prevents new threads from being started to execute further subtasks, interrupts the threads executing subtasks that have not completed, and causes the `join` method to wakeup with the outcome (result or exception). In the above example, the outcome is that `join` completes with a result of `null` when all subtasks succeed. The scope is cancelled if any of the subtasks fail and `join` throws `FailedException` with the exception from the failed subtask as the cause. Other `Joiner` implementations may cancel the scope for other reasons.

Now consider another example that splits into two subtasks. In this example, each subtask produces a `String` result and the task is only interested in the result from the first subtask to complete successfully. The example uses `Joiner.anySuccessfulResultOrThrow()`PREVIEW to create a `Joiner` that makes available the result of the first subtask to complete successfully. The type parameter in the example is "`String`" so that only subtasks that return a `String` can be forked.

       try (var scope = StructuredTaskScope.open(Joiner.<String>anySuccessfulResultOrThrow())) {

           scope.fork(callable1);
           scope.fork(callable2);

           // throws if both subtasks fail
           String firstResult = scope.join();

       }

In the example, the task forks the two subtasks, then waits in the `join` method for either subtask to complete successfully or for both subtasks to fail. If one of the subtasks completes successfully then the `Joiner` causes the other subtask to be cancelled (this will interrupt the thread executing the subtask), and the `join` method returns the result from the successful subtask. Cancelling the other subtask avoids the task waiting for a result that it doesn't care about. If both subtasks fail then the `join` method throws `FailedException` with the exception from one of the subtasks as the cause.

Whether code uses the `Subtask` returned from `fork` will depend on the `Joiner` and usage. Some `Joiner` implementations are suited to subtasks that return results of the same type and where the `join` method returns a result for the task to use. Code that forks subtasks that return results of different types, and uses a `Joiner` such as `Joiner.awaitAllSuccessfulOrThrow()` that does not return a result, will use `Subtask.get()`PREVIEW after joining.

## Exception handling

A `StructuredTaskScope` is opened with a `Joiner`PREVIEW that handles subtask completion and produces the outcome for the `join` method. In some cases, the outcome will be a result, in other cases it will be an exception. If the outcome is an exception then the `join` method throws `StructuredTaskScope.FailedException`PREVIEW with the exception as the cause. For many `Joiner` implementations, the exception will be an exception thrown by a subtask that failed. In the case of `allSuccessfulOrThrow`PREVIEW and `awaitAllSuccessfulOrThrow`PREVIEW for example, the exception is from the first subtask to fail.

Many of the details for how exceptions are handled will depend on usage. In some cases it may be useful to add a `catch` block to the `try`-with-resources statement to catch `FailedException`. The exception handling may use `instanceof` with pattern matching to handle specific causes.

       try (var scope = StructuredTaskScope.open()) {

           ..

       } catch (StructuredTaskScope.FailedException e) {

           Throwable cause = e.getCause();
           switch (cause) {
               case IOException ioe -> ..
               default -> ..
           }

       }

In other cases it may not be useful to catch `FailedException` but instead leave it to propagate to the configured uncaught exception handler for logging purposes.

For cases where a specific exception triggers the use of a default result then it may be more appropriate to handle this in the subtask itself rather than the subtask failing and the scope owner handling the exception.

## Configuration

A `StructuredTaskScope` is opened with configuration that consists of a `ThreadFactory` to create threads, an optional name for monitoring and management purposes, and an optional timeout.

The `open()` and `open(Joiner)` methods create a `StructuredTaskScope` with the default configuration. The default configuration has a `ThreadFactory` that creates unnamed virtual threads, is unnamed for monitoring and management purposes, and has no timeout.

The 2-arg `open` method can be used to create a `StructuredTaskScope` that uses a different `ThreadFactory`, has a name for the purposes of monitoring and management, or has a timeout that cancels the scope if the timeout expires before or while waiting for subtasks to complete. The `open` method is called with a function that is applied to the default configuration and returns a `Configuration`PREVIEW for the `StructuredTaskScope` under construction.

The following example opens a new `StructuredTaskScope` with a `ThreadFactory` that creates virtual threads named "duke-0", "duke-1" ...

        ThreadFactory factory = Thread.ofVirtual().name("duke-", 0).factory();

        try (var scope = StructuredTaskScope.open(joiner, cf -> cf.withThreadFactory(factory))) {

            scope.fork( .. );   // runs in a virtual thread with name "duke-0"
            scope.fork( .. );   // runs in a virtual thread with name "duke-1"

            scope.join();

         }

A second example sets a timeout, represented by a `Duration`. The timeout starts when the new scope is opened. If the timeout expires before the `join` method has completed then the scope is cancelled. This interrupts the threads executing the two subtasks and causes the `join` method to throw `StructuredTaskScope.TimeoutException`PREVIEW.

       Duration timeout = Duration.ofSeconds(10);

       try (var scope = StructuredTaskScope.open(Joiner.<String>allSuccessfulOrThrow(),
                                                 cf -> cf.withTimeout(timeout))) {

           scope.fork(callable1);
           scope.fork(callable2);

           List<String> result = scope.join()
                                      .map(Subtask::get)
                                      .toList();

      }

## Inheritance of scoped value bindings

`ScopedValue` supports the execution of a method with a `ScopedValue` bound to a value for the bounded period of execution of the method by the current thread. It allows a value to be safely and efficiently shared to methods without using method parameters.

When used in conjunction with a `StructuredTaskScope`, a `ScopedValue` can also safely and efficiently share a value to methods executed by subtasks forked in the scope. When a `ScopedValue` object is bound to a value in the thread executing the task then that binding is inherited by the threads created to execute the subtasks. The thread executing the task does not continue beyond the `close` method until all threads executing the subtasks have finished. This ensures that the `ScopedValue` is not reverted to being unbound (or its previous value) while subtasks are executing. In addition to providing a safe and efficient means to inherit a value into subtasks, the inheritance allows sequential code using `ScopedValue` be refactored to use structured concurrency.

To ensure correctness, opening a new `StructuredTaskScope` captures the current thread's scoped value bindings. These are the scoped values bindings that are inherited by the threads created to execute subtasks in the scope. Forking a subtask checks that the bindings in effect at the time that the subtask is forked match the bindings when the `StructuredTaskScope` was created. This check ensures that a subtask does not inherit a binding that is reverted in the task before the subtask has completed.

A `ScopedValue` that is shared across threads requires that the value be an immutable object or for all access to the value to be appropriately synchronized.

The following example demonstrates the inheritance of scoped value bindings. The scoped value USERNAME is bound to the value "duke" for the bounded period of a lambda expression by the thread executing it. The code in the block opens a `StructuredTaskScope` and forks two subtasks, it then waits in the `join` method and aggregates the results from both subtasks. If code executed by the threads running subtask1 and subtask2 uses `ScopedValue.get()`, to get the value of USERNAME, then value "duke" will be returned.

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

        MyResult result = ScopedValue.where(USERNAME, "duke").call(() -> {

            try (var scope = StructuredTaskScope.open()) {

                Subtask<String> subtask1 = scope.fork( .. );    // inherits binding
                Subtask<Integer> subtask2 = scope.fork( .. );   // inherits binding

                scope.join();
                return new MyResult(subtask1.get(), subtask2.get());
            }

        });

A scoped value inherited into a subtask may be rebound to a new value in the subtask for the bounded execution of some method executed in the subtask. When the method completes, the value of the `ScopedValue` reverts to its previous value, the value inherited from the thread executing the task.

A subtask may execute code that itself opens a new `StructuredTaskScope`. A task executing in thread T1 opens a `StructuredTaskScope` and forks a subtask that runs in thread T2. The scoped value bindings captured when T1 opens the scope are inherited into T2. The subtask (in thread T2) executes code that opens a new `StructuredTaskScope` and forks a subtask that runs in thread T3. The scoped value bindings captured when T2 opens the scope are inherited into T3. These include (or may be the same) as the bindings that were inherited from T1. In effect, scoped values are inherited into a tree of subtasks, not just one level of subtask.

## Memory consistency effects

Actions in the owner thread of a `StructuredTaskScope` prior to forking of a subtask happen-before any actions taken by that subtask, which in turn happen-before the subtask result is retrievedPREVIEW.

## General exceptions

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

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

- ## Nested Class Summary

  Nested Classes
  Modifier and Type
  Interface
  Description
  `static interface`
  `StructuredTaskScope.ConfigurationPREVIEW`
  Preview.
  Represents the configuration for a `StructuredTaskScope`.
  `static final class`
  `StructuredTaskScope.FailedExceptionPREVIEW`
  Preview.
  Exception thrown by `join()` when the outcome is an exception rather than a result.
  `static interface`
  `StructuredTaskScope.JoinerPREVIEW<T,R>`
  Preview.
  An object used with a `StructuredTaskScope`PREVIEW to handle subtask completion and produce the result for the scope owner waiting in the `join` method for subtasks to complete.
  `static interface`
  `StructuredTaskScope.SubtaskPREVIEW<T>`
  Preview.
  Represents a subtask forked with `fork(Callable)` or `fork(Runnable)`.
  `static final class`
  `StructuredTaskScope.TimeoutExceptionPREVIEW`
  Preview.
  Exception thrown by `join()` if the scope was created with a timeout and the timeout expired before or while waiting in `join`.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `close()`
  Closes this scope.
  `<U extends T>
  StructuredTaskScope.SubtaskPREVIEW<U>`
  `fork(Runnable task)`
  Fork a subtask by starting a new thread in this scope to execute a method that does not return a result.
  `<U extends T>
  StructuredTaskScope.SubtaskPREVIEW<U>`
  `fork(Callable<? extends U> task)`
  Fork a subtask by starting a new thread in this scope to execute a value-returning method.
  `boolean`
  `isCancelled()`
  Returns `true` if this scope is cancelled or in the process of being cancelled, otherwise `false`.
  `R`
  `join()`
  Returns the result, or throws, after waiting for all subtasks to complete or the scope to be cancelled.
  `static <T> StructuredTaskScopePREVIEW<T,Void>`
  `open()`
  Opens a new `StructuredTaskScope` that can be used to fork subtasks that return results of any type.
  `static <T,R> StructuredTaskScopePREVIEW<T,R>`
  `open(StructuredTaskScope.JoinerPREVIEW<? super T, ? extends R> joiner)`
  Opens a new `StructuredTaskScope`to use the given `Joiner` object.
  `static <T,R> StructuredTaskScopePREVIEW<T,R>`
  `open(StructuredTaskScope.JoinerPREVIEW<? super T, ? extends R> joiner, Function<StructuredTaskScope.ConfigurationPREVIEW, StructuredTaskScope.ConfigurationPREVIEW> configFunction)`
  Opens a new `StructuredTaskScope` to use the given `Joiner` object and with configuration that is the result of applying the given function to the default configuration.

- ## Method Details

  - ### open

    static <T,R> StructuredTaskScopePREVIEW<T,R> open(StructuredTaskScope.JoinerPREVIEW<? super T, ? extends R> joiner, Function<StructuredTaskScope.ConfigurationPREVIEW, StructuredTaskScope.ConfigurationPREVIEW> configFunction)
    Opens a new `StructuredTaskScope` to use the given `Joiner` object and with configuration that is the result of applying the given function to the default configuration.

    The `configFunction` is called with the default configuration and returns the configuration for the new scope. The function may, for example, set the ThreadFactoryPREVIEW or set a timeoutPREVIEW. If the function completes with an exception or error then it is propagated by this method. If the function returns `null` then `NullPointerException` is thrown.

    If a `ThreadFactory` is set then its `newThread` method will be called to create threads when forking subtasks in this scope. If a `ThreadFactory` is not set then forking subtasks will create an unnamed virtual thread for each subtask.

    If a timeoutPREVIEW is set then it starts when the scope is opened. If the timeout expires before the scope has joined then the scope is cancelled and the `join` method throws `StructuredTaskScope.TimeoutException`PREVIEW.

    The new scope is owned by the current thread. Only code executing in this thread can fork, join, or close the scope.

    Construction captures the current thread's scoped value bindings for inheritance by threads started in the scope.

    Type Parameters:
      `T` - the result type of subtasks executed in the scope
      `R` - the result type of the scope
    Parameters:
      `joiner` - the joiner
      `configFunction` - a function to produce the configuration
    Returns:
      a new scope
    Since:
      25

  - ### open

    static <T,R> StructuredTaskScopePREVIEW<T,R> open(StructuredTaskScope.JoinerPREVIEW<? super T, ? extends R> joiner)
    Opens a new `StructuredTaskScope`to use the given `Joiner` object. The scope is created with the default configuration. The default configuration has a `ThreadFactory` that creates unnamed virtual threads, is unnamed for monitoring and management purposes, and has no timeout.
    Implementation Requirements:
      This factory method is equivalent to invoking the 2-arg open method with the given joiner and the identity function.
    Type Parameters:
      `T` - the result type of subtasks executed in the scope
      `R` - the result type of the scope
    Parameters:
      `joiner` - the joiner
    Returns:
      a new scope
    Since:
      25

  - ### open

    static <T> StructuredTaskScopePREVIEW<T,Void> open()
    Opens a new `StructuredTaskScope` that can be used to fork subtasks that return results of any type. The scope's `join()` method waits for all subtasks to succeed or any subtask to fail.

    The `join` method returns `null` if all subtasks complete successfully. It throws `StructuredTaskScope.FailedException`PREVIEW if any subtask fails, with the exception from the first subtask to fail as the cause.

    The scope is created with the default configuration. The default configuration has a `ThreadFactory` that creates unnamed virtual threads, is unnamed for monitoring and management purposes, and has no timeout.

    Implementation Requirements:
      This factory method is equivalent to invoking the 2-arg open method with a joiner created with `awaitAllSuccessfulOrThrow()`PREVIEW and the identity function.
    Type Parameters:
      `T` - the result type of subtasks
    Returns:
      a new scope
    Since:
      25

  - ### fork

    <U extends T> StructuredTaskScope.SubtaskPREVIEW<U> fork(Callable<? extends U> task)
    Fork a subtask by starting a new thread in this scope to execute a value-returning method. The new thread executes the subtask concurrently with the current thread. The parameter to this method is a `Callable`, the new thread executes its `call()` method.

    This method first creates a `Subtask`PREVIEW object to represent the forked subtask. It invokes the joiner's `onFork`PREVIEW method with the subtask in the `UNAVAILABLE`PREVIEW state. If the `onFork` completes with an exception or error then it is propagated by the `fork` method without creating a thread. If the scope is already cancelled, or `onFork` returns `true` to cancel the scope, then this method returns the `Subtask`, in the `UNAVAILABLE`PREVIEW state, without creating a thread to execute the subtask.

    If the scope is not cancelled, and the `onFork` method returns `false`, then a thread is created with the `ThreadFactory` configured when the scope was opened, and the thread is started. Forking a subtask inherits the current thread's scoped value bindings. The bindings must match the bindings captured when the scope was opened. If the subtask completes (successfully or with an exception) before the scope is cancelled, then the thread invokes the joiner's `onComplete`PREVIEW method with the subtask in the `SUCCESS`PREVIEW or `FAILED`PREVIEW state. If the `onComplete` method completes with an exception or error, then the uncaught exception handler is invoked with the exception or error before the thread terminates.

    This method returns the `Subtask`PREVIEW object. In some usages, this object may be used to get its result. In other cases it may be used for correlation or be discarded. To ensure correct usage, the `Subtask.get()`PREVIEW method may only be called by the scope owner to get the result after it has waited for subtasks to complete with the `join` method and the subtask completed successfully. Similarly, the `Subtask.exception()`PREVIEW method may only be called by the scope owner after it has joined and the subtask failed. If the scope was cancelled before the subtask was forked, or before it completes, then neither method can be used to obtain the outcome.

    This method may only be invoked by the scope owner.

    Type Parameters:
      `U` - the result type
    Parameters:
      `task` - the value-returning task for the thread to execute
    Returns:
      the subtask
    Throws:
      `WrongThreadException` - if the current thread is not the scope owner
      `IllegalStateException` - if the owner has already joined or the scope is closed
      `StructureViolationExceptionPREVIEW` - if the current scoped value bindings are not the same as when the scope was created
      `RejectedExecutionException` - if the thread factory rejected creating a thread to run the subtask

  - ### fork

    <U extends T> StructuredTaskScope.SubtaskPREVIEW<U> fork(Runnable task)
    Fork a subtask by starting a new thread in this scope to execute a method that does not return a result.

    This method works exactly the same as `fork(Callable)` except that the parameter to this method is a `Runnable`, the new thread executes its `run` method, and `Subtask.get()`PREVIEW returns `null` if the subtask completes successfully.

    Type Parameters:
      `U` - the result type
    Parameters:
      `task` - the task for the thread to execute
    Returns:
      the subtask
    Throws:
      `WrongThreadException` - if the current thread is not the scope owner
      `IllegalStateException` - if the owner has already joined or the scope is closed
      `StructureViolationExceptionPREVIEW` - if the current scoped value bindings are not the same as when the scope was created
      `RejectedExecutionException` - if the thread factory rejected creating a thread to run the subtask
    Since:
      25

  - ### join

    R join() throws InterruptedException
    Returns the result, or throws, after waiting for all subtasks to complete or the scope to be cancelled.

    This method waits for all subtasks started in this scope to complete or the scope to be cancelled. If a timeoutPREVIEW is configured and the timeout expires before or while waiting, then the scope is cancelled and `TimeoutException`PREVIEW is thrown. Once finished waiting, the `Joiner`'s `result()`PREVIEW method is invoked to get the result or throw an exception. If the `result()` method throws then this method throws `FailedException` with the exception as the cause.

    This method may only be invoked by the scope owner, and only once.

    Returns:
      the result
    Throws:
      `WrongThreadException` - if the current thread is not the scope owner
      `IllegalStateException` - if already joined or this scope is closed
      `StructuredTaskScope.FailedExceptionPREVIEW` - if the outcome is an exception, thrown with the exception from `Joiner.result()`PREVIEW as the cause
      `StructuredTaskScope.TimeoutExceptionPREVIEW` - if a timeout is set and the timeout expires before or while waiting
      `InterruptedException` - if interrupted while waiting
    Since:
      25

  - ### isCancelled

    boolean isCancelled()
    Returns `true` if this scope is cancelled or in the process of being cancelled, otherwise `false`.

    Cancelling the scope prevents new threads from starting in the scope and interrupts threads executing unfinished subtasks. It may take some time before the interrupted threads finish execution; this method may return `true` before all threads have been interrupted or before all threads have finished.

    API Note:
      A task with a lengthy "forking phase" (the code that executes before it invokes `join`) may use this method to avoid doing work in cases where scope is cancelled by the completion of a previously forked subtask or timeout.
    Returns:
      `true` if this scope is cancelled or in the process of being cancelled, otherwise `false`
    Since:
      25

  - ### close

    void close()
    Closes this scope.

    This method first cancels the scope, if not already cancelled. This interrupts the threads executing unfinished subtasks. This method then waits for all threads to finish. If interrupted while waiting then it will continue to wait until the threads finish, before completing with the interrupt status set.

    This method may only be invoked by the scope owner. If the scope is already closed then the 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 scope before nested task scopes are closed then it closes the underlying construct of each nested scope (in the reverse order that they were created in), closes this scope, and then throws `StructureViolationException`PREVIEW. Similarly, if this method is called to close a scope while executing with scoped value bindings, and the scope was created before the scoped values were bound, then `StructureViolationException` is thrown after closing the scope. If a thread terminates without first closing 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 scopes were created in. Thread termination may therefore be delayed when the scope owner has to wait for threads forked in these scopes to finish.

    Specified by:
      `close` in interface `AutoCloseable`
    Throws:
      `IllegalStateException` - thrown after closing the scope if the scope owner did not attempt to join after forking
      `WrongThreadException` - if the current thread is not the scope owner
      `StructureViolationExceptionPREVIEW` - if a structure violation was detected