ScheduledThreadPoolExecutor源码

普通的线程池执行流程，应该很多人都知道了，可是定时任务线程更为重要，很多框架都用到了，应用场景有很多，比如心跳，拉取信息，redisson锁的续约，同步数据等

public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
}

//shutdown后是否继续执行定时任务
 private volatile boolean continueExistingPeriodicTasksAfterShutdown;

 //shutdown后是否继续执行延迟任务
 private volatile boolean executeExistingDelayedTasksAfterShutdown = true;

//cancle方法收费需要将该任务从队列移除
 private volatile boolean removeOnCancel = false;

 //任务的序列号
 private static final AtomicLong sequencer = new AtomicLong();

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();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(period));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
 }

ScheduledFutureTask

public void run() {
            //判断任务是否周期执行
            boolean periodic = isPeriodic();
            //检查当前状态能否执行任务
            if (!canRunInCurrentRunState(periodic))
                cancel(false);
            //非周期任务直接执行，只执行一次
            else if (!periodic)
                ScheduledFutureTask.super.run();
            //周期任务的执行
            else if (ScheduledFutureTask.super.runAndReset()) {
                //设置下一次执行任务时间
                setNextRunTime();
                //任务的下一次执行
                reExecutePeriodic(outerTask);
            }
}

//没做什么事情，留给后面的扩展点
protected <V> RunnableScheduledFuture<V> decorateTask(
        Runnable runnable, RunnableScheduledFuture<V> task) {
        return task;
}

private void delayedExecute(RunnableScheduledFuture<?> task) {
        //校验线程池状态，如果已经停止，则拒绝
        if (isShutdown())
            reject(task);
        else {
            //加入队列
            super.getQueue().add(task);
            //再次检查，线程池状态发生变化，不能够执行，移出队列，取消任务
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                //开启线程
                ensurePrestart();
        }
    }

DelayedWorkQueue,阻塞获取头结点

public RunnableScheduledFuture<?> take() throws InterruptedException {
            final ReentrantLock lock = this.lock;
            lock.lockInterruptibly();
            try {
                for (;;) {
                    RunnableScheduledFuture<?> first = queue[0];
                    if (first == null)
                        available.await();
                    else {
                        long delay = first.getDelay(NANOSECONDS);
                        if (delay <= 0)
                            return finishPoll(first);
                        first = null; // don't retain ref while waiting
                        if (leader != null)
                            //已经存在leader线程，当前线程await阻塞
                            available.await();
                        else {
                            //当前线程作为leader线程，并制定头结点的延迟时间作为阻塞时间
                            Thread thisThread = Thread.currentThread();
                            leader = thisThread;
                            try {
                                available.awaitNanos(delay);
                            } finally {
                                if (leader == thisThread)
                                    leader = null;
                            }
                        }
                    }
                }
            } finally {
                //leader线程没有阻塞，可以找到头结点，唤醒阻塞线程
                if (leader == null && queue[0] != null)
                    available.signal();
                lock.unlock();
            }
        }

//根据前面设置的时间，跟当前时间比较，计算要等待的时间 
public long getDelay(TimeUnit unit) {
            return unit.convert(time - now(), NANOSECONDS);
        }

 

//进行按时间排序，形成一个小顶堆
private void siftUp(int k, RunnableScheduledFuture<?> key) {
            while (k > 0) {
                int parent = (k - 1) >>> 1;
                RunnableScheduledFuture<?> e = queue[parent];
                if (key.compareTo(e) >= 0)
                    break;
                queue[k] = e;
                setIndex(e, k);
                k = parent;
            }
            queue[k] = key;
            setIndex(key, k);
        }

 

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