ScheduledThreadPoolExecutor

java提供了方便的定时器功能，代码示例：

public class ScheduledThreadPool_Test {
    static class Command implements Runnable {
        @Override
        public void run() {
            System.out.println("zhang");
        }
    }

    public static void main(String[] args) throws IOException {
        ScheduledExecutorService pool = Executors.newScheduledThreadPool(1);
        pool.scheduleWithFixedDelay(new Command(), 1000, 5000, TimeUnit.MILLISECONDS);
        System.in.read();
    }
}

接下来分析ScheduledThreadPoolExecutor：

// 省略其他代码
public class Executors {
    public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }
}

public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor implements ScheduledExecutorService {

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

    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();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(-delay));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        //把任务添加到队列中，创建工作线程
        delayedExecute(t);
        return t;
    }
}

调用scheduleWithFixedDelay方法，把任务添加到DelayedWorkQueue，并启动工作线程。

private void delayedExecute(RunnableScheduledFuture<?> task) {
    if (isShutdown())
        reject(task);
    else {
        //把任务添加到队列
        super.getQueue().add(task);
        if (isShutdown() &&
            !canRunInCurrentRunState(task.isPeriodic()) &&
            remove(task))
            task.cancel(false);
        else
            ensurePrestart(); // 创建线程
    }
}

从队列中取任务的调用栈：

任务在执行的时候，会新建一个任务，放入队列中，这样就实现了定时任务的功能。

从上面能看出：定时的功能主要是由DelayedWorkQueue和ScheduledFutureTask保证的。

DelayedWorkQueue的底层数据结构是由数组实现的堆（堆是一棵完全二叉树，以小顶堆为例，parent节点值小于左右孩子节点的值）：

// 省略其他代码
static class DelayedWorkQueue extends AbstractQueue<Runnable>
        implements BlockingQueue<Runnable> {
    private static final int INITIAL_CAPACITY = 16;
    private RunnableScheduledFuture[] queue =
        new RunnableScheduledFuture[INITIAL_CAPACITY];
    private final ReentrantLock lock = new ReentrantLock();
    private int size = 0;
    private Thread leader = null;
    private final Condition available = lock.newCondition();

    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);
    }

    private void siftDown(int k, RunnableScheduledFuture key) {
        int half = size >>> 1;
        while (k < half) {
            int child = (k << 1) + 1;
            RunnableScheduledFuture c = queue[child];
            int right = child + 1;
            if (right < size && c.compareTo(queue[right]) > 0)
                c = queue[child = right];
            if (key.compareTo(c) <= 0)
                break;
            queue[k] = c;
            setIndex(c, k);
            k = child;
        }
        queue[k] = key;
        setIndex(key, k);
    }

   public boolean offer(Runnable x) {
        if (x == null)
            throw new NullPointerException();
        RunnableScheduledFuture e = (RunnableScheduledFuture)x;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int i = size;
            if (i >= queue.length)
                grow();
            size = i + 1;
            if (i == 0) {
                queue[0] = e;
                setIndex(e, 0);
            } else {
                siftUp(i, e);
            }
            if (queue[0] == e) {
                leader = null;
                available.signal();
            }
        } finally {
            lock.unlock();
        }
        return true;
    }

    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(TimeUnit.NANOSECONDS);
                    if (delay <= 0)
                        return finishPoll(first);
                    else if (leader != null)
                        available.await();
                    else {
                        Thread thisThread = Thread.currentThread();
                        leader = thisThread;
                        try {
                            available.awaitNanos(delay);
                        } finally {
                            if (leader == thisThread)
                                leader = null;
                        }
                    }
                }
            }
        } finally {
            if (leader == null && queue[0] != null)
                available.signal();
            lock.unlock();
        }
    }
}

ScheduledFutureTask是周期任务：

private class ScheduledFutureTask<V> extends FutureTask<V> implements RunnableScheduledFuture<V> {
    //当2个task的时间相同时，用来比较task优先级
    private final long sequenceNumber;
    //任务执行时间 nanoTime units
    private long time;
    /**
     * Period in nanoseconds for repeating tasks.  A positive
     * value indicates fixed-rate execution.  A negative value
     * indicates fixed-delay execution.  A value of 0 indicates a
     * non-repeating task.
     */
    private final long period;
    /** The actual task to be re-enqueued by reExecutePeriodic */
    RunnableScheduledFuture<V> outerTask = this;
    // DelayedWorkQueue中堆的下标
    int heapIndex;

    ScheduledFutureTask(Runnable r, V result, long ns, long period) {
        super(r, result);
        this.time = ns;
        this.period = period;
        this.sequenceNumber = sequencer.getAndIncrement();
    }

    // 堆在siftUp和siftDown时需要比较大小
    public int compareTo(Delayed other) {
        if (other == this) // compare zero ONLY 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 d = (getDelay(TimeUnit.NANOSECONDS) -
                  other.getDelay(TimeUnit.NANOSECONDS));
        return (d == 0) ? 0 : ((d < 0) ? -1 : 1);
    }

    // 设置周期任务的下一次执行时间
    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;
    }

    /**
     * Overrides FutureTask version so as to reset/requeue if periodic.
     */
    public void run() {
        boolean periodic = isPeriodic();
        if (!canRunInCurrentRunState(periodic))
            cancel(false);
        else if (!periodic)
            ScheduledFutureTask.super.run();
        else if (ScheduledFutureTask.super.runAndReset()) {
            //设置下次任务的时间
            setNextRunTime();
            reExecutePeriodic(outerTask);
        }
    }
}

// ScheduledThreadPoolExecutor
void reExecutePeriodic(RunnableScheduledFuture<?> task) {
    if (canRunInCurrentRunState(true)) {
        super.getQueue().add(task);
        if (!canRunInCurrentRunState(true) && remove(task))
            task.cancel(false);
        else
            ensurePrestart();
    }
}

// ThreadPoolExecutor
void ensurePrestart() {
    int wc = workerCountOf(ctl.get());
    if (wc < corePoolSize)
        addWorker(null, true);
    else if (wc == 0)
        addWorker(null, false);
}