OpenJDK 1.23 · java.util.concurrent.ExecutorService · APIdia, the JavaDoc alternative

An Executor that provides methods to manage termination and methods that can produce a Future for tracking progress of one or more asynchronous tasks.

An ExecutorService can be shut down, which will cause it to reject new tasks. Two different methods are provided for shutting down an ExecutorService. The shutdown method will allow previously submitted tasks to execute before terminating, while the shutdownNow method prevents waiting tasks from starting and attempts to stop currently executing tasks. Upon termination, an executor has no tasks actively executing, no tasks awaiting execution, and no new tasks can be submitted. An unused ExecutorService should be shut down to allow reclamation of its resources.

Method submit extends base method Executor#execute(Runnable) by creating and returning a Future that can be used to cancel execution and/or wait for completion. Methods invokeAny and invokeAll perform the most commonly useful forms of bulk execution, executing a collection of tasks and then waiting for at least one, or all, to complete. (Class ExecutorCompletionService can be used to write customized variants of these methods.)

The Executors class provides factory methods for the executor services provided in this package.

Usage Examples

Here is a sketch of a network service in which threads in a thread pool service incoming requests. It uses the preconfigured Executors#newFixedThreadPool factory method:

 class NetworkService implements Runnable {
  private final ServerSocket serverSocket;
  private final ExecutorService pool;

  public NetworkService(int port, int poolSize)
      throws IOException {
    serverSocket = new ServerSocket(port);
    pool = Executors.newFixedThreadPool(poolSize);
  }

  public void run() { // run the service
    try {
      for (;;) {
        pool.execute(new Handler(serverSocket.accept()));
      }
    } catch (IOException ex) {
      pool.shutdown();
    }
  }
}

class Handler implements Runnable {
  private final Socket socket;
  Handler(Socket socket) { this.socket = socket; }
  public void run() {
    // read and service request on socket
  }
}

An ExecutorService may also be established and closed (shutdown, blocking until terminated) as follows; illustrating with a different Executors factory method:

 try (ExecutorService e =  Executors.newWorkStealingPool()) {
  // submit or execute many tasks with e ...
}

Further customization is also possible. For example, the following method shuts down an ExecutorService in two phases, first by calling shutdown to reject incoming tasks, and then calling shutdownNow, if necessary, to cancel any lingering tasks:

 void shutdownAndAwaitTermination(ExecutorService pool) {
  pool.shutdown(); // Disable new tasks from being submitted
  try {
    // Wait a while for existing tasks to terminate
    if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
      pool.shutdownNow(); // Cancel currently executing tasks
      // Wait a while for tasks to respond to being cancelled
      if (!pool.awaitTermination(60, TimeUnit.SECONDS))
          System.err.println("Pool did not terminate");
    }
  } catch (InterruptedException ex) {
    // (Re-)Cancel if current thread also interrupted
    pool.shutdownNow();
    // Preserve interrupt status
    Thread.currentThread().interrupt();
  }
}

Memory consistency effects: Actions in a thread prior to the submission of a Runnable or Callable task to an ExecutorService happen-before any actions taken by that task, which in turn happen-before the result is retrieved via Future.get().

Method Summary

Modifier and Type	Method and Description
public boolean	Returns: `true` if this executor terminated and `false` if the timeout elapsed before termination awaitTermination(long the maximum time to wait timeout, TimeUnit the time unit of the timeout argument unit) Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
public default void	close() Implements java.lang.AutoCloseable.close. Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
public < the type of the values returned from the tasks T> List<Future<T>>	Returns: a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed invokeAll(Collection<? extends Callable<T>> the collection of tasks tasks) Executes the given tasks, returning a list of Futures holding their status and results when all complete.
public < the type of the values returned from the tasks T> List<Future<T>>	Returns: a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list. If the operation did not time out, each task will have completed. If it did time out, some of these tasks will not have completed. invokeAll(Collection<? extends Callable<T>> the collection of tasks tasks, long the maximum time to wait timeout, TimeUnit the time unit of the timeout argument unit) Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first.
public < the type of the values returned from the tasks T> T	Returns: the result returned by one of the tasks invokeAny(Collection<? extends Callable<T>> the collection of tasks tasks) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do.
public < the type of the values returned from the tasks T> T	Returns: the result returned by one of the tasks invokeAny(Collection<? extends Callable<T>> the collection of tasks tasks, long the maximum time to wait timeout, TimeUnit the time unit of the timeout argument unit) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do before the given timeout elapses.
public boolean	Returns: `true` if this executor has been shut down isShutdown() Returns `true` if this executor has been shut down.
public boolean	Returns: `true` if all tasks have completed following shut down isTerminated() Returns `true` if all tasks have completed following shut down.
public void	shutdown() Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
public List<Runnable>	Returns: list of tasks that never commenced execution shutdownNow() Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.
public < the type of the task's result T> Future<T>	Returns: a Future representing pending completion of the task submit(Callable<T> the task to submit task) Submits a value-returning task for execution and returns a Future representing the pending results of the task.
public < the type of the result T> Future<T>	Returns: a Future representing pending completion of the task submit(Runnable the task to submit task, T the result to return result) Submits a Runnable task for execution and returns a Future representing that task.
public Future<?>	Returns: a Future representing pending completion of the task submit(Runnable the task to submit task) Submits a Runnable task for execution and returns a Future representing that task.

Inherited from java.util.concurrent.Executor:: execute

Method Detail

awaitTermination back to summary

awaitTermination	back to summary
public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first. Parameters timeout:long the maximum time to wait unit:TimeUnit the time unit of the timeout argument Returns:boolean `true` if this executor terminated and `false` if the timeout elapsed before termination Exceptions InterruptedException: if interrupted while waiting

public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException

Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.

Parameters

timeout:long: the maximum time to wait
unit:TimeUnit: the time unit of the timeout argument

Returns:boolean

true if this executor terminated and false if the timeout elapsed before termination

Exceptions

InterruptedException:: if interrupted while waiting

close back to summary

close	back to summary
public default void close() Implements java.lang.AutoCloseable.close. Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. This method waits until all tasks have completed execution and the executor has terminated. If interrupted while waiting, this method stops all executing tasks as if by invoking `shutdownNow()`. It then continues to wait until all actively executing tasks have completed. Tasks that were awaiting execution are not executed. The interrupt status will be re-asserted before this method returns. If already terminated, invoking this method has no effect. Implementation Specification The default implementation invokes `shutdown()` and waits for tasks to complete execution with `awaitTermination`. Annotations @Override Exceptions SecurityException: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold `java.lang.RuntimePermission("modifyThread")`, or the security manager's `checkAccess` method denies access. Since 19

public default void close()

Implements java.lang.AutoCloseable.close.

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. This method waits until all tasks have completed execution and the executor has terminated.

If interrupted while waiting, this method stops all executing tasks as if by invoking shutdownNow(). It then continues to wait until all actively executing tasks have completed. Tasks that were awaiting execution are not executed. The interrupt status will be re-asserted before this method returns.

If already terminated, invoking this method has no effect.

Implementation Specification

The default implementation invokes shutdown() and waits for tasks to complete execution with awaitTermination.

Annotations

@Override

Exceptions

SecurityException:: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold java.lang.RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

Since

invokeAll back to summary

invokeAll	back to summary
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException Executes the given tasks, returning a list of Futures holding their status and results when all complete. `Future#isDone` is `true` for each element of the returned list. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress. Parameters <T> the type of the values returned from the tasks tasks:Collection<? extends Callable<T>> the collection of tasks Returns:List<Future<T>> a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed Exceptions InterruptedException: if interrupted while waiting, in which case unfinished tasks are cancelled NullPointerException: if tasks or any of its elements are `null` RejectedExecutionException: if any task cannot be scheduled for execution

public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) throws InterruptedException

Executes the given tasks, returning a list of Futures holding their status and results when all complete. Future#isDone is true for each element of the returned list. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.

Parameters

<T>: the type of the values returned from the tasks
tasks:Collection<? extends Callable<T>>: the collection of tasks

Returns:List<Future<T>>

a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed

Exceptions

InterruptedException:: if interrupted while waiting, in which case unfinished tasks are cancelled
NullPointerException:: if tasks or any of its elements are null
RejectedExecutionException:: if any task cannot be scheduled for execution

invokeAll back to summary

invokeAll	back to summary
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first. `Future#isDone` is `true` for each element of the returned list. Upon return, tasks that have not completed are cancelled. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress. Parameters <T> the type of the values returned from the tasks tasks:Collection<? extends Callable<T>> the collection of tasks timeout:long the maximum time to wait unit:TimeUnit the time unit of the timeout argument Returns:List<Future<T>> a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list. If the operation did not time out, each task will have completed. If it did time out, some of these tasks will not have completed. Exceptions InterruptedException: if interrupted while waiting, in which case unfinished tasks are cancelled NullPointerException: if tasks, any of its elements, or unit are `null` RejectedExecutionException: if any task cannot be scheduled for execution

public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException

Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first. Future#isDone is true for each element of the returned list. Upon return, tasks that have not completed are cancelled. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.

Parameters

<T>: the type of the values returned from the tasks
tasks:Collection<? extends Callable<T>>: the collection of tasks
timeout:long: the maximum time to wait
unit:TimeUnit: the time unit of the timeout argument

Returns:List<Future<T>>

a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list. If the operation did not time out, each task will have completed. If it did time out, some of these tasks will not have completed.

Exceptions

InterruptedException:: if interrupted while waiting, in which case unfinished tasks are cancelled
NullPointerException:: if tasks, any of its elements, or unit are null
RejectedExecutionException:: if any task cannot be scheduled for execution

invokeAny back to summary

invokeAny	back to summary
public <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress. Parameters <T> the type of the values returned from the tasks tasks:Collection<? extends Callable<T>> the collection of tasks Returns:T the result returned by one of the tasks Exceptions InterruptedException: if interrupted while waiting ExecutionException: if no task successfully completes NullPointerException: if tasks or any element task subject to execution is `null` IllegalArgumentException: if tasks is empty RejectedExecutionException: if tasks cannot be scheduled for execution

public <T> T invokeAny(Collection<? extends Callable<T>> tasks) throws InterruptedException, ExecutionException

Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress.

Parameters

<T>: the type of the values returned from the tasks
tasks:Collection<? extends Callable<T>>: the collection of tasks

Returns:T

the result returned by one of the tasks

Exceptions

InterruptedException:: if interrupted while waiting
ExecutionException:: if no task successfully completes
NullPointerException:: if tasks or any element task subject to execution is null
IllegalArgumentException:: if tasks is empty
RejectedExecutionException:: if tasks cannot be scheduled for execution

invokeAny back to summary

invokeAny	back to summary
public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do before the given timeout elapses. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress. Parameters <T> the type of the values returned from the tasks tasks:Collection<? extends Callable<T>> the collection of tasks timeout:long the maximum time to wait unit:TimeUnit the time unit of the timeout argument Returns:T the result returned by one of the tasks Exceptions InterruptedException: if interrupted while waiting ExecutionException: if no task successfully completes TimeoutException: if the given timeout elapses before any task successfully completes NullPointerException: if tasks, or unit, or any element task subject to execution is `null` RejectedExecutionException: if tasks cannot be scheduled for execution

public <T> T invokeAny(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException

Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do before the given timeout elapses. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress.

Parameters

<T>: the type of the values returned from the tasks
tasks:Collection<? extends Callable<T>>: the collection of tasks
timeout:long: the maximum time to wait
unit:TimeUnit: the time unit of the timeout argument

Returns:T

the result returned by one of the tasks

Exceptions

InterruptedException:: if interrupted while waiting
ExecutionException:: if no task successfully completes
TimeoutException:: if the given timeout elapses before any task successfully completes
NullPointerException:: if tasks, or unit, or any element task subject to execution is null
RejectedExecutionException:: if tasks cannot be scheduled for execution

isShutdown back to summary

isShutdown	back to summary
public boolean isShutdown() Returns `true` if this executor has been shut down. Returns:boolean `true` if this executor has been shut down

public boolean isShutdown()

Returns true if this executor has been shut down.

Returns:boolean: true if this executor has been shut down

isTerminated back to summary

isTerminated	back to summary
public boolean isTerminated() Returns `true` if all tasks have completed following shut down. Note that `isTerminated` is never `true` unless either `shutdown` or `shutdownNow` was called first. Returns:boolean `true` if all tasks have completed following shut down

public boolean isTerminated()

Returns true if all tasks have completed following shut down. Note that isTerminated is never true unless either shutdown or shutdownNow was called first.

Returns:boolean: true if all tasks have completed following shut down

shutdown back to summary

shutdown	back to summary
public void shutdown() Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down. This method does not wait for previously submitted tasks to complete execution. Use `awaitTermination` to do that. Exceptions SecurityException: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold `java.lang.RuntimePermission("modifyThread")`, or the security manager's `checkAccess` method denies access.

public void shutdown()

Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down.

This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

Exceptions

SecurityException:: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold java.lang.RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

shutdownNow back to summary

shutdownNow	back to summary
public List<Runnable> shutdownNow() Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. This method does not wait for actively executing tasks to terminate. Use `awaitTermination` to do that. There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via `Thread#interrupt`, so any task that fails to respond to interrupts may never terminate. Returns:List<Runnable> list of tasks that never commenced execution Exceptions SecurityException: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold `java.lang.RuntimePermission("modifyThread")`, or the security manager's `checkAccess` method denies access.

public List<Runnable> shutdownNow()

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread#interrupt, so any task that fails to respond to interrupts may never terminate.

Returns:List<Runnable>

list of tasks that never commenced execution

Exceptions

SecurityException:: if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold java.lang.RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

submit back to summary

submit	back to summary
public <T> Future<T> submit(Callable<T> task) Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's `get` method will return the task's result upon successful completion. If you would like to immediately block waiting for a task, you can use constructions of the form `result = exec.submit(aCallable).get();` Note The `Executors` class includes a set of methods that can convert some other common closure-like objects, for example, `java.security.PrivilegedAction` to `Callable` form so they can be submitted. Parameters <T> the type of the task's result task:Callable<T> the task to submit Returns:Future<T> a Future representing pending completion of the task Exceptions RejectedExecutionException: if the task cannot be scheduled for execution NullPointerException: if the task is null

public <T> Future<T> submit(Callable<T> task)

Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's get method will return the task's result upon successful completion.

If you would like to immediately block waiting for a task, you can use constructions of the form result = exec.submit(aCallable).get();

Note

The Executors class includes a set of methods that can convert some other common closure-like objects, for example, java.security.PrivilegedAction to Callable form so they can be submitted.

Parameters

<T>: the type of the task's result
task:Callable<T>: the task to submit

Returns:Future<T>

a Future representing pending completion of the task

Exceptions

RejectedExecutionException:: if the task cannot be scheduled for execution
NullPointerException:: if the task is null

submit back to summary

submit	back to summary
public <T> Future<T> submit(Runnable task, T result) Submits a Runnable task for execution and returns a Future representing that task. The Future's `get` method will return the given result upon successful completion. Parameters <T> the type of the result task:Runnable the task to submit result:T the result to return Returns:Future<T> a Future representing pending completion of the task Exceptions RejectedExecutionException: if the task cannot be scheduled for execution NullPointerException: if the task is null

public <T> Future<T> submit(Runnable task, T result)

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return the given result upon successful completion.

Parameters

<T>: the type of the result
task:Runnable: the task to submit
result:T: the result to return

Returns:Future<T>

a Future representing pending completion of the task

Exceptions

RejectedExecutionException:: if the task cannot be scheduled for execution
NullPointerException:: if the task is null

submit back to summary

submit	back to summary
public Future<?> submit(Runnable task) Submits a Runnable task for execution and returns a Future representing that task. The Future's `get` method will return `null` upon successful completion. Parameters task:Runnable the task to submit Returns:Future<?> a Future representing pending completion of the task Exceptions RejectedExecutionException: if the task cannot be scheduled for execution NullPointerException: if the task is null

public Future<?> submit(Runnable task)

Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.

Parameters

task:Runnable: the task to submit

Returns:Future<?>

a Future representing pending completion of the task

Exceptions

RejectedExecutionException:: if the task cannot be scheduled for execution
NullPointerException:: if the task is null

public Interface ExecutorService

Usage Examples

Method Summary

Method Detail