java.util.concurrent
interface ExecutorService

All Superinterfaces:

Executor

All Known Subinterfaces:

ScheduledExecutorService

All Known Implementing Classes:

AbstractExecutorService, ThreadPoolExecutor, ScheduledThreadPoolExecutor

Most common way to construct:

ExecutorService pool = Executors.newFixedThreadPool(3);

Based on 84 examples

public interface ExecutorService
extends Executor

An {@link Executor} that provides methods to manage termination and methods that can produce a {@link 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 {@link #shutdown} method will allow previously submitted tasks to execute before terminating, while the {@link #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 {@link Executor#execute} by creating and returning a {@link 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 {@link ExecutorCompletionService} can be used to write customized variants of these methods.)

The {@link Executors} class provides factory methods for the executor services provided in this package.

Usage Examples

 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
   }
 }
 

 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 {@code Runnable} or {@code Callable} task to an {@code ExecutorService} happen-before any actions taken by that task, which in turn happen-before the result is retrieved via {@code Future.get()}.

awaitTermination

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 - 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

invokeAll

public List invokeAll(Collection tasks)
               throws InterruptedException

Executes the given tasks, returning a list of Futures holding their status and results when all complete. {@link 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.

invokeAll

public List invokeAll(Collection 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. {@link 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

invokeAny

public Object invokeAny(Collection 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:

tasks - the collection of tasks

Returns:

the result returned by one of the tasks

Throws:

InterruptedException - if interrupted while waiting

ExecutionException - if no task successfully completes

invokeAny

public Object invokeAny(Collection 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:

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

ExecutionException - if no task successfully completes

TimeoutException - if the given timeout elapses before any task successfully completes

isShutdown

public boolean isShutdown()

Returns true if this executor has been shut down.

Returns:

true if this executor has been shut down

isTerminated

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:

true if all tasks have completed following shut down

shutdown

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.

shutdownNow

public List 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 {@link Thread#interrupt}, so any task that fails to respond to interrupts may never terminate.

Returns:

list of tasks that never commenced execution

submit

public Future submit(Callable 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 {@link Executors} class includes a set of methods that can convert some other common closure-like objects, for example, {@link java.security.PrivilegedAction} to {@link Callable} form so they can be submitted.

Parameters:

task - the task to submit

Returns:

a Future representing pending completion of the task

submit

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 - the task to submit

Returns:

a Future representing pending completion of the task

submit

public Future submit(Runnable task,
                     Object 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