• 并发包中ScheduledThreadPoolExecutor


    ScheduledThreadPoolExecutor继承了ThreadPoolExecutor,是一个定时任务调度执行的线程池

    一、变量与构造函数

        /**
         * shutdown时是否取消定时任务
         */
        private volatile boolean continueExistingPeriodicTasksAfterShutdown;
    
        /**
         * shutdown时是否取消定时任务
         */
        private volatile boolean executeExistingDelayedTasksAfterShutdown = true;
    
        /**
         * ScheduledFutureTask.cancel 任务取消是否删除队列中的任务
         */
        private volatile boolean removeOnCancel = false;
    
        /**
         * Sequence number to break scheduling ties, and in turn to
         * guarantee FIFO order among tied entries.
         */
        private static final AtomicLong sequencer = new AtomicLong();
    
        //构造方法默认:
        //maximumPoolSize == Integer.MAX_VALUR,
        //KeepAliveTime == 0,
        //workQueue == new DelayedWorkQueue(),
        public ScheduledThreadPoolExecutor(int corePoolSize) {
            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
                  new DelayedWorkQueue());
        }
    
        /**
         * Creates a new {@code ScheduledThreadPoolExecutor} with the
         */
        public ScheduledThreadPoolExecutor(int corePoolSize,
                                           ThreadFactory threadFactory) {
            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
                  new DelayedWorkQueue(), threadFactory);
        }
    
        /**
         * Creates a new ScheduledThreadPoolExecutor with the given
         */
        public ScheduledThreadPoolExecutor(int corePoolSize,
                                           RejectedExecutionHandler handler) {
            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
                  new DelayedWorkQueue(), handler);
        }
    
        /**
         * Creates a new ScheduledThreadPoolExecutor with the given
         */
        public ScheduledThreadPoolExecutor(int corePoolSize,
                                           ThreadFactory threadFactory,
                                           RejectedExecutionHandler handler) {
            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
                  new DelayedWorkQueue(), threadFactory, handler);
        }

    ScheduledThreadPoolExecutor采用自定义的内部类DelayedWorkQueue的实例作为工作队列,DelayedWorkQueue类似于DelayQueue

    二、重要实现

    ScheduledThreadPoolExecutor自定义了两个内部类DelayedWorkQueue、ScheduledFutureTask,现在看看ScheduledFutureTask构造

    1.ScheduledFutureTask类

        private class ScheduledFutureTask<V>
                extends FutureTask<V> implements RunnableScheduledFuture<V> {//继承FutureTask
    
            /** 自增序列号,任务执行时间相同时,序列号小的先执行(FIFO),由上面的AtomicLong原子变量sequence保证线程安全*/
            private final long sequenceNumber;
    
            /** 任务执行的时间yyyyMMddHHmmss,单位纳秒 */
            private long time;
    
            /**
             * 任务周期,单位是纳秒
             * period == 0 时,当前任务为一次性任务,执行完后退出
             * period > 0 时,当前任务为fixed-rate任务,是固定频率的定时任务,具体一点:定时7点执行第一次任务,之后每小时执行一次任务:7/8/9...
             * period < 0 时,当前任务为fixed-delay任务,是固定延时的定时任务,具体一点:定时7点执行第一次任务,第一次任务执行完成的时间+1小时...即可能是7/8.1/9.2...
             */
            private final long period;
    
            /** The actual task to be re-enqueued by reExecutePeriodic */
            RunnableScheduledFuture<V> outerTask = this;
    
            /**
             * Index into delay queue, to support faster cancellation.
             */
            int heapIndex;
    
            /**
             * Creates a one-shot action with given nanoTime-based trigger time.
             */
            ScheduledFutureTask(Runnable r, V result, long ns) {
                super(r, result);
                this.time = ns;
                this.period = 0;
                this.sequenceNumber = sequencer.getAndIncrement();
            }
    
            /**
             * Creates a periodic action with given nano time and period.
             */
            ScheduledFutureTask(Runnable r, V result, long ns, long period) {
                super(r, result);
                this.time = ns;
                this.period = period;
                this.sequenceNumber = sequencer.getAndIncrement();
            }
    
            /**
             * Creates a one-shot action with given nanoTime-based trigger time.
             */
            ScheduledFutureTask(Callable<V> callable, long ns) {
                super(callable);
                this.time = ns;
                this.period = 0;
                this.sequenceNumber = sequencer.getAndIncrement();
            }
    
            public long getDelay(TimeUnit unit) {
                return unit.convert(time - now(), NANOSECONDS);
            }
    
            //延迟队列比较器
            //同一任务返回0
            //不同任务先比较下一次执行的时间,若相同则比较任务初始化时的序列号sequenceNumber
            public int compareTo(Delayed other) {
                if (other == this) // compare zero if same object
                    return 0;
                if (other instanceof ScheduledFutureTask) {
                    ScheduledFutureTask<?> x = (ScheduledFutureTask<?>)other;
                    long diff = time - x.time;
                    if (diff < 0)
                        return -1;
                    else if (diff > 0)
                        return 1;
                    else if (sequenceNumber < x.sequenceNumber)
                        return -1;
                    else
                        return 1;
                }
                long diff = getDelay(NANOSECONDS) - other.getDelay(NANOSECONDS);
                return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
            }
    
            /**
             * 是周期重复执行任务
             */
            public boolean isPeriodic() {
                return period != 0;
            }
    
            /**
             * 设置周期任务的下一次执行的时间
             */
            private void setNextRunTime() {
                long p = period;
                if (p > 0)
                    time += p;
                else
                    time = triggerTime(-p);
            }
    
            public boolean cancel(boolean mayInterruptIfRunning) {
                boolean cancelled = super.cancel(mayInterruptIfRunning);
                if (cancelled && removeOnCancel && heapIndex >= 0)
                    remove(this);
                return cancelled;
            }
    
            /**
    * 定时处理方法,ThreadPoolExecutor.execute(command)中command.run()== ScheduledFutureTask.super.run(),给它再包装了一次run方法 * 一次性任务直接执行
    * 定时任务①执行②设置下一次执行时间③入DelayedWorkerQueue
    */ public void run() { boolean periodic = isPeriodic(); if (!canRunInCurrentRunState(periodic)) cancel(false); else if (!periodic) ScheduledFutureTask.super.run(); else if (ScheduledFutureTask.super.runAndReset()) { setNextRunTime(); reExecutePeriodic(outerTask); } }

    2.setNextRunTime():返回任务的下一次执行时间

        private void setNextRunTime() {
            long p = period;
            if (p > 0)
                time += p;//固定频率定时任务  time = firstTime + n*p
            else
                time = triggerTime(-p);//固定延时定时任务  time = firstTime;第二次任务执行时间是firstTask执行完后+延时(now + delay)
        }
    
        long triggerTime(long delay) {
            return now() +
                ((delay < (Long.MAX_VALUE >> 1)) ? delay : overflowFree(delay));
        }
    
        private long overflowFree(long delay) {
            Delayed head = (Delayed) super.getQueue().peek();
            if (head != null) {
                long headDelay = head.getDelay(NANOSECONDS);
                if (headDelay < 0 && (delay - headDelay < 0))
                    delay = Long.MAX_VALUE + headDelay;
            }
            return delay;
        }

    3.ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit):创建一次性任务(period == 0)

        public ScheduledFuture<?> schedule(Runnable command,
                                           long delay,
                                           TimeUnit unit) {
            if (command == null || unit == null)
                throw new NullPointerException();
            //创建一次性定时任务,period == 0,指定下次执行的时间time
            RunnableScheduledFuture<?> t = decorateTask(command,
                new ScheduledFutureTask<Void>(command, null,
                                              triggerTime(delay, unit)));
            //任务入延迟队列DelayedWorkerQueue
            delayedExecute(t);
            return t;
        }
    
        private void delayedExecute(RunnableScheduledFuture<?> task) {
            if (isShutdown())
                //线程池状态检验
                reject(task);
            else {
                //任务入延迟工作队列
                super.getQueue().add(task);
                if (isShutdown() &&
                    !canRunInCurrentRunState(task.isPeriodic()) &&
                    remove(task))
                    //线程池正在关闭,
                    //
                    //延迟工作队列删除任务,并尝试终止线程池
                    task.cancel(false);
                else
                    //线程池RUNNING状态,工作线程数<corePoolSize,创建工作线程add(null,boolean)
                    ensurePrestart();
            }
        }

    4.ScheduledFuture<?>  scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit):创建固定延时的定时任务(period < 0)

        public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                         long initialDelay,
                                                         long delay,
                                                         TimeUnit unit) {
            if (command == null || unit == null)
                throw new NullPointerException();
            if (delay <= 0)
                throw new IllegalArgumentException();
            //创建固定延时的定时任务period <0,并设置下次运行的时间
            ScheduledFutureTask<Void> sft =
                new ScheduledFutureTask<Void>(command,
                                              null,
                                              triggerTime(initialDelay, unit),
                                              unit.toNanos(-delay));
            //设置outerTask
            RunnableScheduledFuture<Void> t = decorateTask(command, sft);
            sft.outerTask = t;
            //任务执行(入延迟工作队列)
            delayedExecute(t);
            return t;
        }

    5.ScheduledFuture<?>  scheduleWithFixedRate(Runnable command, long initialDelay, long delay, TimeUnit unit):创建固定频率的定时任务

        public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                      long initialDelay,
                                                      long period,
                                                      TimeUnit unit) {
            if (command == null || unit == null)
                throw new NullPointerException();
            if (period <= 0)
                throw new IllegalArgumentException();
            //创建固定频率的定时任务period > 0,并设置下次运行时间
            ScheduledFutureTask<Void> sft =
                new ScheduledFutureTask<Void>(command,
                                              null,
                                              triggerTime(initialDelay, unit),
                                              unit.toNanos(period));
            //设置outerTask
            RunnableScheduledFuture<Void> t = decorateTask(command, sft);
            sft.outerTask = t;
            //执行定时任务(入延迟工作队列)
            delayedExecute(t);
            return t;
        }

     三.总结

    1.是定时任务线程池,继承ThreadPoolExcutor实现,自定义了延迟队列DelayedWorkerQueue,对command在封装成ScheduledFutureTask

    2.用过ScheduledFutureTask的period参数来区分为3种任务

    ①一次性任务(period == 0)

    ②固定频率定时任务(fixed-rate : period > 0)7/8/9 上一次任务开始时间+period

    ③固定延时定时任务(fixed-delay : period < 0)7/8.1/9.2 上一次任务结束时间+period

    四.实例

    public class SchedulePoolTest {
    
        private static final ScheduledExecutorService pool = Executors.newScheduledThreadPool(2);
        private static final Lock lock = new ReentrantLock();
    
        public static String format(long time){
            Date date = new Date(time);
            return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(date);
        }
    
        public static void main(String[] args){
            Runnable task = new Runnable() {
                @Override
                public void run() {
                    lock.lock();
                    try {
                        System.out.println(format(System.currentTimeMillis()));
                    }finally {
                        lock.unlock();
                    }
                }
            };
            System.out.println(format(System.currentTimeMillis()));
            //pool.schedule(task,5,TimeUnit.SECONDS);
            //pool.scheduleAtFixedRate(task,5,1,TimeUnit.SECONDS);
            Runnable task1 = new Runnable() {
                @Override
                public void run() {
                    lock.lock();
                    try {
                        System.out.println(format(System.currentTimeMillis()));
                        Thread.sleep(2000);
                    }catch (InterruptedException e){
                        e.printStackTrace();
                    }finally {
                        lock.unlock();
                    }
                }
            };
            pool.scheduleWithFixedDelay(task1,5,1,TimeUnit.SECONDS);
            try {
                Thread.sleep(20000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            pool.shutdown();
    
        }
    
    }

     参考自《java并发编程之美》

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  • 原文地址:https://www.cnblogs.com/wqff-biubiu/p/12258103.html
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