ExecutorService

  • The ExecutorService interface in Java manages asynchronous task execution using thread pools and provides methods for task submission, termination, and monitoring.

  • shutdown() allows existing tasks to complete before shutting down, while shutdownNow() attempts to stop all tasks immediately.

  • submit() methods are used for submitting tasks to the ExecutorService for execution, returning a Future object representing the task's result.

  • Future objects enable tracking task progress, retrieving results, and canceling tasks.

  • Specialized implementations like ScheduledExecutorService enable scheduling tasks for future execution or periodic repetition.
public interface ExecutorService implements Executor
Known Indirect Subclasses

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(int) 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
   }
 }
 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 ie) {
     // (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().

Public Method Summary

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<? extends Callable<T>> tasks)
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
abstract <T> List<Future<T>>
invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit 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.
abstract <T> T
invokeAny(Collection<? extends 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 <T> T
invokeAny(Collection<? extends 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 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 <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.
abstract <T> Future<T>
submit(Runnable task, T result)
Submits a Runnable task for execution and returns a Future representing that task.
abstract Future<?>
submit(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.

Inherited Method Summary

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<? extends Callable<T>> tasks)

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
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 List<Future<T>> invokeAll (Collection<? extends Callable<T>> tasks, long timeout, TimeUnit 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. 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
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 these 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 T invokeAny (Collection<? extends 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 element task subject to execution is null
IllegalArgumentException if tasks is empty
ExecutionException if no task successfully completes
RejectedExecutionException if tasks cannot be scheduled for execution

public abstract T invokeAny (Collection<? extends 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, or unit, or any element task subject to execution is 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 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.

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

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.

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 of tasks that never commenced execution

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. 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, 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 the task cannot be scheduled for execution
NullPointerException if the task is null

public abstract 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
task the task to submit
result the result to return
Returns
  • a Future representing pending completion of the task
Throws
RejectedExecutionException if the task cannot be scheduled for execution
NullPointerException if the task is null

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