Module jdk.incubator.concurrent
Package jdk.incubator.concurrent

Class StructuredTaskScope<T>

java.lang.Object
jdk.incubator.concurrent.StructuredTaskScope<T>
Type Parameters:
T - the result type of tasks executed in the scope
All Implemented Interfaces:
AutoCloseable
Direct Known Subclasses:
StructuredTaskScope.ShutdownOnFailure, StructuredTaskScope.ShutdownOnSuccess
public class StructuredTaskScope<T> extends Object implements AutoCloseable
A basic API for structured concurrency. StructuredTaskScope supports cases where a task splits into several concurrent subtasks, to be executed in their own threads, 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 usage

A StructuredTaskScope is created with one of its public constructors. It defines the fork method to start a thread to execute a task, the join method to wait for all threads to finish, and the close method to close the task scope. The API is intended to be used with the try-with-resources construct. The intention is that code in the block uses the fork method to fork threads to execute the subtasks, wait for the threads to finish with the join method, and then process the results. Processing of results may include handling or re-throwing of exceptions. 
    try (var scope = new StructuredTaskScope<Object>()) {

        Future<Integer> future1 = scope.fork(task1);
        Future<String> future2 = scope.fork(task2);

        scope.join();

        ... process results/exceptions ...

    } // 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. Shutdown is useful for cases where a subtask completes with a result (or exception) and the results of other unfinished subtasks are no longer needed. If a subtask invokes shutdown while the owner is waiting in the join method then it will cause join to wakeup, all unfinished threads to be interrupted and prevents new threads from starting in the task scope.

Subclasses with policies for common cases

Two subclasses of StructuredTaskScope are defined to implement policy for common cases:
  1. ShutdownOnSuccess captures the first result and 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 tasks. It defines methods to get the first result or throw an exception if all subtasks fail.
  2. ShutdownOnFailure captures the first exception and shuts down the task scope. 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 normally, 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) 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) to throw an exception when either subtask fails. This method is a no-op if no subtasks fail. The main task uses Future's resultNow() method to retrieve the results. 
   Instant deadline = ...

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

        Future<String> future1 = scope.fork(() -> query(left));
        Future<String> future2 = scope.fork(() -> query(right));

        scope.joinUntil(deadline);

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

        // both subtasks completed successfully
        String result = Stream.of(future1, future2)
                .map(Future::resultNow)
                .collect(Collectors.joining(", ", "{ ", " }"));

        ...
    }

Extending StructuredTaskScope

StructuredTaskScope can be extended, and the handleComplete overridden, to implement policies other than those implemented by ShutdownOnSuccess and ShutdownOnFailure. The method may be overridden to, for example, collect the results of subtasks that complete with a result 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 a collection of results. A subclass that implements a policy to shut down when a subtask fails may define a method to retrieve the exception of the first subtask to fail.

The following is an example of a StructuredTaskScope implementation that collects the results of subtasks that complete successfully. It defines the method results() to be used by the main task to retrieve the results. 

   class MyScope<T> extends StructuredTaskScope<T> {
       private final Queue<T> results = new ConcurrentLinkedQueue<>();

       MyScope() {
           super(null, Thread.ofVirtual().factory());
       }

       @Override
       protected void handleComplete(Future<T> future) {
           if (future.state() == Future.State.SUCCESS) {
               T result = future.resultNow();
               results.add(result);
           }
       }

       // Returns a stream of results from the subtasks that completed successfully
       public Stream<T> results() {
           return results.stream();
       }
   }

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 values 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.where(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 Callable task happen-before any actions taken by that task, which in turn happen-before the task result is retrieved via its Future, 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:
19

  • 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 tasks are forked. The task scope is owned by the current thread.

      This method captures the current thread's scoped value bindings for inheritance by threads created 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.

      This constructor is equivalent to invoking the 2-arg constructor with a name of null and a thread factory that creates virtual threads.

      Throws:
      UnsupportedOperationException - if preview features are not enabled

  • Method Details

    • handleComplete

      protected void handleComplete(Future<T> future)
      Invoked when a task completes before the scope is shut down.

      The handleComplete method should be thread safe. It may be invoked by several threads concurrently.

      Implementation Requirements:
      The default implementation does nothing.
      Parameters:
      future - the completed task

    • fork

      public <U extends T> Future<U> fork(Callable<? extends U> task)
      Starts a new thread to run the given task.

      The new thread is created with the task scope's ThreadFactory. It inherits the current thread's scoped value bindings. The bindings must match the bindings captured when the task scope was created.

      If the task completes before the task scope is shutdown then the handleComplete method is invoked to consume the completed task. The handleComplete method is run when the task completes with a result or exception. If the Future's cancel method is used to cancel a task before the task scope is shut down, then the handleComplete method is run by the thread that invokes cancel. If the task scope shuts down at or around the same time that the task completes or is cancelled then the handleComplete method may or may not be invoked.

      If this task scope is shutdown (or in the process of shutting down) then fork returns a Future representing a cancelled task that was not run.

      This method may only be invoked by the task scope owner or threads contained in the task scope. The cancel method of the returned Future object is also restricted to the task scope owner or threads contained in the task scope. The cancel method throws WrongThreadException if invoked from another thread. All other methods on the returned Future object, such as get, are not restricted.

      Type Parameters:
      U - the result type
      Parameters:
      task - the task to run
      Returns:
      a future
      Throws:
      IllegalStateException - if this task scope is closed
      WrongThreadException - if the current thread is not the owner or a thread contained in the task scope
      StructureViolationException - 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 task

    • join

      public StructuredTaskScope<T> join() throws InterruptedException
      Wait for all threads to finish or the task scope to shut down. This method waits until all threads started in the task scope finish execution (of both task and handleComplete method), the shutdown method is invoked to shut down the task scope, or the current thread is interrupted.

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

      Returns:
      this task scope
      Throws:
      IllegalStateException - if this task scope is closed
      WrongThreadException - if the current thread is not the owner
      InterruptedException - if interrupted while waiting

    • joinUntil

      public StructuredTaskScope<T> joinUntil(Instant deadline) throws InterruptedException, TimeoutException
      Wait for all threads to finish or the task scope to shut down, up to the given deadline. This method waits until all threads started in the task scope finish execution (of both task and handleComplete method), the shutdown method is invoked to shut down the task scope, the current thread is interrupted, or the deadline is reached.

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

      Parameters:
      deadline - the deadline
      Returns:
      this task scope
      Throws:
      IllegalStateException - if this task scope is closed
      WrongThreadException - if the current thread is not the owner
      InterruptedException - if interrupted while waiting
      TimeoutException - if the deadline is reached while waiting

    • shutdown

      public void shutdown()
      Shut down the 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.

      More specifically, this method:

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

      When this method completes then the Future objects for all tasks will be done, normally or abnormally. There may still 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.

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

      Throws:
      IllegalStateException - if this task scope is closed
      WrongThreadException - if the current thread is not the owner or a thread contained in the task scope

    • 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 then 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 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 StructureViolationException. Similarly, if this method is called to close a task scope while executing with scoped value 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 owner has to wait for threads forked in these task scopes to finish.

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