Java tutorial
/* * 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.Collection; import java.util.List; /** * 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 {@code ExecutorService} can be shut down, which will cause * it to reject new tasks. Two different methods are provided for * shutting down an {@code 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 {@code ExecutorService} should be shut down to allow * reclamation of its resources. * * <p>Method {@code submit} extends base method {@link * Executor#execute(Runnable)} by creating and returning a {@link Future} * that can be used to cancel execution and/or wait for completion. * Methods {@code invokeAny} and {@code 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.) * * <p>The {@link Executors} class provides factory methods for the * executor services provided in this package. * * <h2>Usage Examples</h2> * * 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> {@code * 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 {@code ExecutorService} in two phases, * first by calling {@code shutdown} to reject incoming tasks, and then * calling {@code shutdownNow}, if necessary, to cancel any lingering tasks: * * <pre> {@code * 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. * * <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}{@code ("modifyThread")}, * or the security manager's {@code checkAccess} 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. * * <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}{@code ("modifyThread")}, * or the security manager's {@code checkAccess} method * denies access. */ List<Runnable> shutdownNow(); /** * Returns {@code true} if this executor has been shut down. * * @return {@code true} if this executor has been shut down */ boolean isShutdown(); /** * Returns {@code true} if all tasks have completed following shut down. * Note that {@code isTerminated} is never {@code true} unless * either {@code shutdown} or {@code shutdownNow} was called first. * * @return {@code true} 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. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return {@code true} if this executor terminated and * {@code false} 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 {@code get} 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 * {@code result = exec.submit(aCallable).get();} * * <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 {@code get} 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 {@code get} method will * return {@code null} 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. * {@link Future#isDone} is {@code true} 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 {@code null} * @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. * {@link Future#isDone} is {@code true} 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 {@code null} * @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. * * @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 {@code null} * @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. * * @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 {@code null} * @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; }