ScheduledExecutorService.java

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 * 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.
 *
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/*
 * 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;

/**
 * An {@link ExecutorService} that can schedule commands to run after a given
 * delay, or to execute periodically.
 *
 * <p>The {@code schedule} methods create tasks with various delays
 * and return a task object that can be used to cancel or check
 * execution. The {@code scheduleAtFixedRate} and
 * {@code scheduleWithFixedDelay} methods create and execute tasks
 * that run periodically until cancelled.
 *
 * <p>Commands submitted using the {@link Executor#execute(Runnable)}
 * and {@link ExecutorService} {@code submit} methods are scheduled
 * with a requested delay of zero. Zero and negative delays (but not
 * periods) are also allowed in {@code schedule} methods, and are
 * treated as requests for immediate execution.
 *
 * <p>All {@code schedule} methods accept <em>relative</em> delays and
 * periods as arguments, not absolute times or dates. It is a simple
 * matter to transform an absolute time represented as a {@link
 * java.util.Date} to the required form. For example, to schedule at
 * a certain future {@code date}, you can use: {@code schedule(task,
 * date.getTime() - System.currentTimeMillis(),
 * TimeUnit.MILLISECONDS)}. Beware however that expiration of a
 * relative delay need not coincide with the current {@code Date} at
 * which the task is enabled due to network time synchronization
 * protocols, clock drift, or other factors.
 *
 * <p>The {@link Executors} class provides convenient factory methods for
 * the ScheduledExecutorService implementations provided in this package.
 *
 * <h3>Usage Example</h3>
 *
 * Here is a class with a method that sets up a ScheduledExecutorService
 * to beep every ten seconds for an hour:
 *
 * <pre> {@code
 * import static java.util.concurrent.TimeUnit.*;
 * class BeeperControl {
 *   private final ScheduledExecutorService scheduler =
 *     Executors.newScheduledThreadPool(1);
 *
 *   public void beepForAnHour() {
 *     Runnable beeper = () -> System.out.println("beep");
 *     ScheduledFuture<?> beeperHandle =
 *       scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
 *     Runnable canceller = () -> beeperHandle.cancel(false);
 *     scheduler.schedule(canceller, 1, HOURS);
 *   }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 1.5
 */
// 【定时任务执行框架】
public interface ScheduledExecutorService extends ExecutorService {
    /**
     * Submits a one-shot task that becomes enabled after the given delay.
     *
     * @param command the task to execute
     * @param delay   the time from now to delay execution
     * @param unit    the time unit of the delay parameter
     *
     * @return a ScheduledFuture representing pending completion of
     * the task and whose {@code get()} method will return
     * {@code null} upon completion
     *
     * @throws RejectedExecutionException if the task cannot be
     *                                    scheduled for execution
     * @throws NullPointerException       if command or unit is null
     */
    // 执行一次性的定时任务(Runnable)
    ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit);
    
    /**
     * Submits a value-returning one-shot task that becomes enabled
     * after the given delay.
     *
     * @param callable the function to execute
     * @param delay    the time from now to delay execution
     * @param unit     the time unit of the delay parameter
     * @param <V>      the type of the callable's result
     *
     * @return a ScheduledFuture that can be used to extract result or cancel
     *
     * @throws RejectedExecutionException if the task cannot be
     *                                    scheduled for execution
     * @throws NullPointerException       if callable or unit is null
     */
    // 执行一次性的定时任务(Callable)
    <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit);
    
    /**
     * Submits a periodic action that becomes enabled first after the
     * given initial delay, and subsequently with the given period;
     * that is, executions will commence after
     * {@code initialDelay}, then {@code initialDelay + period}, then
     * {@code initialDelay + 2 * period}, and so on.
     *
     * <p>The sequence of task executions continues indefinitely until
     * one of the following exceptional completions occur:
     * <ul>
     * <li>The task is {@linkplain Future#cancel explicitly cancelled}
     * via the returned future.
     * <li>The executor terminates, also resulting in task cancellation.
     * <li>An execution of the task throws an exception.  In this case
     * calling {@link Future#get() get} on the returned future will throw
     * {@link ExecutionException}, holding the exception as its cause.
     * </ul>
     * Subsequent executions are suppressed.  Subsequent calls to
     * {@link Future#isDone isDone()} on the returned future will
     * return {@code true}.
     *
     * <p>If any execution of this task takes longer than its period, then
     * subsequent executions may start late, but will not concurrently
     * execute.
     *
     * @param command      the task to execute
     * @param initialDelay the time to delay first execution
     * @param period       the period between successive executions
     * @param unit         the time unit of the initialDelay and period parameters
     *
     * @return a ScheduledFuture representing pending completion of
     * the series of repeated tasks.  The future's {@link
     * Future#get() get()} method will never return normally,
     * and will throw an exception upon task cancellation or
     * abnormal termination of a task execution.
     *
     * @throws RejectedExecutionException if the task cannot be
     *                                    scheduled for execution
     * @throws NullPointerException       if command or unit is null
     * @throws IllegalArgumentException   if period less than or equal to zero
     */
    // 执行重复性的定时任务：固定周期，从任务初次被触发开始，以后每隔period时间就被触发一次
    ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit);
    
    /**
     * Submits a periodic action that becomes enabled first after the
     * given initial delay, and subsequently with the given delay
     * between the termination of one execution and the commencement of
     * the next.
     *
     * <p>The sequence of task executions continues indefinitely until
     * one of the following exceptional completions occur:
     * <ul>
     * <li>The task is {@linkplain Future#cancel explicitly cancelled}
     * via the returned future.
     * <li>The executor terminates, also resulting in task cancellation.
     * <li>An execution of the task throws an exception.  In this case
     * calling {@link Future#get() get} on the returned future will throw
     * {@link ExecutionException}, holding the exception as its cause.
     * </ul>
     * Subsequent executions are suppressed.  Subsequent calls to
     * {@link Future#isDone isDone()} on the returned future will
     * return {@code true}.
     *
     * @param command      the task to execute
     * @param initialDelay the time to delay first execution
     * @param delay        the delay between the termination of one
     *                     execution and the commencement of the next
     * @param unit         the time unit of the initialDelay and delay parameters
     *
     * @return a ScheduledFuture representing pending completion of
     * the series of repeated tasks.  The future's {@link
     * Future#get() get()} method will never return normally,
     * and will throw an exception upon task cancellation or
     * abnormal termination of a task execution.
     *
     * @throws RejectedExecutionException if the task cannot be
     *                                    scheduled for execution
     * @throws NullPointerException       if command or unit is null
     * @throws IllegalArgumentException   if delay less than or equal to zero
     */
    // 执行重复性的定时任务：固定延时，任务下次的开始时间=任务上次结束时间+delay
    ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit);
}