https://openjdk.java.net/ 
/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;
import java.util.List;
import java.util.Collection;
import java.security.PrivilegedAction;
import java.security.PrivilegedExceptionAction;


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 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
  }
}


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(). 
Author:Doug Lea
Since:1.5
/**
 * 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.
 *
 * <p> An <tt>ExecutorService</tt> can be shut down, which will cause
 * it to reject new tasks.  Two different methods are provided for
 * shutting down an <tt>ExecutorService</tt>. 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 <tt>ExecutorService</tt> should be shut down to allow
 * reclamation of its resources.
 *
 * <p> Method <tt>submit</tt> 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 <tt>invokeAny</tt> and <tt>invokeAll</tt> 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.)
 *
 * <p>The {@link Executors} class provides factory methods for the
 * executor services provided in this package.
 *
 * <h3>Usage Examples</h3>
 *
 * Here is a sketch of a network service in which threads in a thread
 * pool service incoming requests. It uses the preconfigured {@link
 * Executors#newFixedThreadPool} factory method:
 *
 * <pre>
 * 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
 *   }
 * }
 * </pre>
 *
 * The following method shuts down an <tt>ExecutorService</tt> in two phases,
 * first by calling <tt>shutdown</tt> to reject incoming tasks, and then
 * calling <tt>shutdownNow</tt>, if necessary, to cancel any lingering tasks:
 *
 * <pre>
 * 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();
 *   }
 * }
 * </pre>
 *
 * <p>Memory consistency effects: Actions in a thread prior to the
 * submission of a {@code Runnable} or {@code Callable} task to an
 * {@code ExecutorService}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * any actions taken by that task, which in turn <i>happen-before</i> the
 * result is retrieved via {@code Future.get()}.
 *
 * @since 1.5
 * @author Doug Lea
 */
public interface ExecutorService extends Executor {

    
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. 
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.
/**
     * 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.
     *
     * <p>This method does not wait for previously submitted tasks to
     * complete execution.  Use {@link #awaitTermination awaitTermination}
     * to do that.
     *
     * @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 {@link
     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
     *         or the security manager's <tt>checkAccess</tt> method
     *         denies access.
     */
    void shutdown();

    
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. 
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.
Returns:list of tasks that never commenced execution
/**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution.
     *
     * <p>This method does not wait for actively executing tasks to
     * terminate.  Use {@link #awaitTermination awaitTermination} to
     * do that.
     *
     * <p>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.
     *
     * @return 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 {@link
     *         java.lang.RuntimePermission}<tt>("modifyThread")</tt>,
     *         or the security manager's <tt>checkAccess</tt> method
     *         denies access.
     */
    List<Runnable> shutdownNow();

    
Returns true if this executor has been shut down.

Returns:true if this executor has been shut down
/**
     * Returns <tt>true</tt> if this executor has been shut down.
     *
     * @return <tt>true</tt> if this executor has been shut down
     */
    boolean isShutdown();

    
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
/**
     * Returns <tt>true</tt> if all tasks have completed following shut down.
     * Note that <tt>isTerminated</tt> is never <tt>true</tt> unless
     * either <tt>shutdown</tt> or <tt>shutdownNow</tt> was called first.
     *
     * @return <tt>true</tt> if all tasks have completed following shut down
     */
    boolean isTerminated();

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

Params:
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Throws:
Returns:true if this executor terminated and
        false if the timeout elapsed before termination
/**
     * Blocks until all tasks have completed execution after a shutdown
     * request, or the timeout occurs, or the current thread is
     * interrupted, whichever happens first.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return <tt>true</tt> if this executor terminated and
     *         <tt>false</tt> if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException;


    
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. 
Params:
  • task – the task to submit
Type parameters:
  • <T> – the type of the task's result
Throws:
Returns:a Future representing pending completion of the task
/**
     * Submits a value-returning task for execution and returns a
     * Future representing the pending results of the task. The
     * Future's <tt>get</tt> method will return the task's result upon
     * successful completion.
     *
     * <p>
     * If you would like to immediately block waiting
     * for a task, you can use constructions of the form
     * <tt>result = exec.submit(aCallable).get();</tt>
     *
     * <p> 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.
     *
     * @param task the task to submit
     * @param <T> the type of the task's result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <T> Future<T> submit(Callable<T> task);

    
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.

Params:
  • task – the task to submit
  • result – the result to return
Type parameters:
  • <T> – the type of the result
Throws:
Returns:a Future representing pending completion of the task
/**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's <tt>get</tt> method will
     * return the given result upon successful completion.
     *
     * @param task the task to submit
     * @param result the result to return
     * @param <T> the type of the result
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    <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 null upon successful completion.

Params:
  • task – the task to submit
Throws:
Returns:a Future representing pending completion of the task
/**
     * Submits a Runnable task for execution and returns a Future
     * representing that task. The Future's <tt>get</tt> method will
     * return <tt>null</tt> upon <em>successful</em> completion.
     *
     * @param task the task to submit
     * @return a Future representing pending completion of the task
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     * @throws NullPointerException if the task is null
     */
    Future<?> submit(Runnable task);

    
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.

Params:
  • tasks – the collection of tasks
Type parameters:
  • <T> – the type of the values returned from the tasks
Throws:
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.
/**
     * Executes the given tasks, returning a list of Futures holding
     * their status and results when all complete.
     * {@link Future#isDone} is <tt>true</tt> for each
     * element of the returned list.
     * Note that a <em>completed</em> 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.
     *
     * @param tasks the collection of tasks
     * @param <T> the type of the values returned from the tasks
     * @return 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.
     * @throws NullPointerException if tasks or any of its elements are <tt>null</tt>
     * @throws RejectedExecutionException if any task cannot be
     *         scheduled for execution
     */

    <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 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.

Params:
  • tasks – the collection of tasks
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Type parameters:
  • <T> – the type of the values returned from the tasks
Throws:
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.
/**
     * 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 <tt>true</tt> for each
     * element of the returned list.
     * Upon return, tasks that have not completed are cancelled.
     * Note that a <em>completed</em> 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.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @param <T> the type of the values returned from the tasks
     * @return 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
     * @throws NullPointerException if tasks, any of its elements, or
     *         unit are <tt>null</tt>
     * @throws RejectedExecutionException if any task cannot be scheduled
     *         for execution
     */
    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                  long timeout, TimeUnit unit)
        throws InterruptedException;

    
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.

Params:
  • tasks – the collection of tasks
Type parameters:
  • <T> – the type of the values returned from the tasks
Throws:
Returns:the result returned by one of the 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.
     *
     * @param tasks the collection of tasks
     * @param <T> the type of the values returned from the tasks
     * @return the result returned by one of the tasks
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks or any element task
     *         subject to execution is <tt>null</tt>
     * @throws IllegalArgumentException if tasks is empty
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <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 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.

Params:
  • tasks – the collection of tasks
  • timeout – the maximum time to wait
  • unit – the time unit of the timeout argument
Type parameters:
  • <T> – the type of the values returned from the tasks
Throws:
Returns:the result returned by one of the tasks.
/**
     * 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.
     *
     * @param tasks the collection of tasks
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @param <T> the type of the values returned from the tasks
     * @return the result returned by one of the tasks.
     * @throws InterruptedException if interrupted while waiting
     * @throws NullPointerException if tasks, or unit, or any element
     *         task subject to execution is <tt>null</tt>
     * @throws TimeoutException if the given timeout elapses before
     *         any task successfully completes
     * @throws ExecutionException if no task successfully completes
     * @throws RejectedExecutionException if tasks cannot be scheduled
     *         for execution
     */
    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}