# Interface ExecutorService

All Superinterfaces:
  `AutoCloseable`, `Executor`

All Known Subinterfaces:
  `ScheduledExecutorService`

All Known Implementing Classes:
  `AbstractExecutorService`, `ForkJoinPool`, `ScheduledThreadPoolExecutor`, `ThreadPoolExecutor`

public interface ExecutorService extends Executor, AutoCloseable
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
       }
     }

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()`.

Since:
  1.5

## Method Summary

Modifier and Type
Method
Description
`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.
`default void`
`close()`
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
`<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.
`<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.
`<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.
`<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.
`boolean`
`isShutdown()`
Returns `true` if this executor has been shut down.
`boolean`
`isTerminated()`
Returns `true` if all tasks have completed following shut down.
`void`
`shutdown()`
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
`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.
`Future<?>`
`submit(Runnable task)`
Submits a Runnable task for execution and returns a Future representing that task.
`<T> Future<T>`
`submit(Runnable task, T result)`
Submits a Runnable task for execution and returns a Future representing that task.
`<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.

### Methods declared in interface java.util.concurrent.Executor

`execute`

## Method Details

### shutdown

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.

### shutdownNow

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

### isShutdown

boolean isShutdown()
Returns `true` if this executor has been shut down.

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

### isTerminated

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

### awaitTermination

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

### submit

<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, `PrivilegedAction` to `Callable` form so they can be submitted.

Type Parameters:
  `T` - the type of the task's result
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

### submit

<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.

Type Parameters:
  `T` - the type of the result
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

### submit

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

### invokeAll

<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.

Type Parameters:
  `T` - the type of the values returned from the tasks
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

### invokeAll

<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.

Type Parameters:
  `T` - the type of the values returned from the tasks
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

### invokeAny

<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.

Type Parameters:
  `T` - the type of the values returned from the tasks
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

### invokeAny

<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.

Type Parameters:
  `T` - the type of the values returned from the tasks
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

### close

default void 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.

Specified by:
  `close` in interface `AutoCloseable`
Implementation Requirements:
  The default implementation invokes `shutdown()` and waits for tasks to complete execution with `awaitTermination`.
Since:
  19