/*
 * 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;

/**
 * 执行已提交的{@link Runnable}任务的对象。
 * 此接口提供了一种将任务提交与每个任务的运行机制(包括线程使用,调度的详细信息)解耦的方式。
 * 通常使用{@code Executor}来代替显式创建线程。
 * 例如,不用为一组任务中的每一个任务都去调用{@code new Thread(new(RunnableTask())).start()},
 * 你可以使用:
 *
 * <pre>
 * Executor executor = <em>anExecutor</em>;
 * executor.execute(new RunnableTask1());
 * executor.execute(new RunnableTask2());
 * ...
 * </pre>
 *
 * 但是,{@code Executor}接口并不严格要求执行是异步的。
 * 在最简单的情况下,执行器可以在调用者的线程中立即运行提交的任务:
 *
 * <pre> {@code
 * class DirectExecutor implements Executor {
 * public void execute(Runnable r) {
 * r.run();
 * }
 * }}</pre>
 *
 * 更典型的情况是,任务在调用者线程之外的某个线程中执行。
 * 下面的执行器为每个任务创建一个新的线程。
 *
 * <pre> {@code
 * class ThreadPerTaskExecutor implements Executor {
 * public void execute(Runnable r) {
 * new Thread(r).start();
 * }
 * }}</pre>
 *
 * 许多{@code Executor}实现对如何以及何时调度任务施加了某种限制。
 * 下面的执行器将任务的提交串行化到第二个执行器,演示了组合执行器。
 *
 * <pre> {@code
 * class SerialExecutor implements Executor {
 * final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
 * final Executor executor;
 * Runnable active;
 *
 * SerialExecutor(Executor executor) {
 * this.executor = executor;
 * }
 *
 * public synchronized void execute(final Runnable r) {
 * tasks.offer(new Runnable() {
 * public void run() {
 * try {
 * r.run();
 * } finally {
 * scheduleNext();
 * }
 * }
 * });
 * if (active == null) {
 * scheduleNext();
 * }
 * }
 *
 * protected synchronized void scheduleNext() {
 * if ((active = tasks.poll()) != null) {
 * executor.execute(active);
 * }
 * }
 * }}</pre>
 *
 * 这个包中提供的{@code Executor}实现implement了{@link ExecutorService},
 * 这是一个更加扩展的接口。
 * {@link ThreadPoolExecutor}类提供了可扩展的线程池实现。
 * {@link Executors}类提供了更加方便的工厂方法用来生成{@code Executor}。
 *
 * <p>内存一致性影响: Actions in a thread prior to
 * submitting a {@code Runnable} object to an {@code Executor}
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * its execution begins, perhaps in another thread.
 *
 * @since 1.5
 * @author Doug Lea
 */
public interface Executor {

 /**
 * 在将来的某个时候执行给定的命令。
 * 该命令可能在新线程、线程池或调用线程中执行,
 * 具体由{@code Executor}实现决定。
 *
 * @param command the runnable task
 * @throws RejectedExecutionException if this task cannot be
 * accepted for execution
 * @throws NullPointerException if command is null
 */
 void execute(Runnable command);
}