public interface

ExecutorService

implements Executor
java.util.concurrent.ExecutorService
Known Indirect Subclasses

Class Overview

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 stop accepting new tasks. After being shut down, the executor will eventually terminate, at which point no tasks are actively executing, no tasks are awaiting execution, and no new tasks can be submitted.

Method submit extends base method 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 Example

Here is a sketch of a network service in which threads in a thread pool service incoming requests. It uses the preconfigured newFixedThreadPool(int) factory method: 
 class NetworkService {
    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 serve() {
      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
    }
 }

Summary

Public Methods
abstract boolean awaitTermination(long timeout, TimeUnit 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.
abstract <T> List<Future<T>> invokeAll(Collection<Callable<T>> tasks, long timeout, TimeUnit unit)
Executes the given tasks, returning their results when all complete or the timeout expires, whichever happens first.
abstract <T> List<Future<T>> invokeAll(Collection<Callable<T>> tasks)
Executes the given tasks, returning their results when all complete.
abstract <T> T invokeAny(Collection<Callable<T>> tasks, long timeout, TimeUnit 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.
abstract <T> T invokeAny(Collection<Callable<T>> tasks)
Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do.
abstract boolean isShutdown()
Returns true if this executor has been shut down.
abstract boolean isTerminated()
Returns true if all tasks have completed following shut down.
abstract void shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
abstract 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.
abstract Future<?> submit(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.
abstract <T> Future<T> submit(Runnable task, T result)
Submits a Runnable task for execution and returns a Future representing that task that will upon completion return the given result
abstract <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.
[Expand]
Inherited Methods
From interface java.util.concurrent.Executor

Public Methods

public abstract boolean awaitTermination (long timeout, TimeUnit 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.

Parameters
timeout the maximum time to wait
unit the time unit of the timeout argument
Returns
  • true if this executor terminated and false if the timeout elapsed before termination
Throws
InterruptedException if interrupted while waiting

public abstract List<Future<T>> invokeAll (Collection<Callable<T>> tasks, long timeout, TimeUnit unit)

Executes the given tasks, returning their results when all complete or the timeout expires, whichever happens first. 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
tasks the collection of tasks
timeout the maximum time to wait
unit the time unit of the timeout argument
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 thiese tasks will not have completed.
Throws
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 abstract List<Future<T>> invokeAll (Collection<Callable<T>> tasks)

Executes the given tasks, returning their results when all complete. 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
tasks the collection of tasks
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.
Throws
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 abstract T invokeAny (Collection<Callable<T>> tasks, long timeout, TimeUnit 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. 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
tasks the collection of tasks
timeout the maximum time to wait
unit the time unit of the timeout argument
Returns
  • The result returned by one of the tasks.
Throws
InterruptedException if interrupted while waiting
NullPointerException if tasks, any of its elements, or unit are null
TimeoutException if the given timeout elapses before any task successfully completes
ExecutionException if no task successfully completes
RejectedExecutionException if tasks cannot be scheduled for execution

public abstract T invokeAny (Collection<Callable<T>> tasks)

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
tasks the collection of tasks
Returns
  • The result returned by one of the tasks.
Throws
InterruptedException if interrupted while waiting
NullPointerException if tasks or any of its elements are null
IllegalArgumentException if tasks empty
ExecutionException if no task successfully completes
RejectedExecutionException if tasks cannot be scheduled for execution

public abstract boolean isShutdown ()

Returns true if this executor has been shut down.

Returns
  • true if this executor has been shut down

public abstract 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
  • true if all tasks have completed following shut down

public abstract 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.

Throws
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 RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

public abstract 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.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via interrupt(), so if any tasks mask or fail to respond to interrupts, they may never terminate.

Returns
  • list of tasks that never commenced execution
Throws
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 RuntimePermission("modifyThread"), or the security manager's checkAccess method denies access.

public abstract Future<?> submit (Runnable task)

Submits a Runnable task for execution and returns a Future representing that task.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task, and whose get() method will return null upon completion.
Throws
RejectedExecutionException if task cannot be scheduled for execution
NullPointerException if task null

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

Submits a Runnable task for execution and returns a Future representing that task that will upon completion return the given result

Parameters
task the task to submit
result the result to return
Returns
  • a Future representing pending completion of the task, and whose get() method will return the given result upon completion.
Throws
RejectedExecutionException if task cannot be scheduled for execution
NullPointerException if task null

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

Submits a value-returning task for execution and returns a Future representing the pending results of the task.

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, PrivilegedAction to Callable form so they can be submitted.

Parameters
task the task to submit
Returns
  • a Future representing pending completion of the task
Throws
RejectedExecutionException if task cannot be scheduled for execution
NullPointerException if task null