CH22-ScheduledThreadPoolExecutor

//为相同延时任务提供的顺序编号
private final long sequenceNumber;

//任务可以执行的时间，纳秒级
private long time;

//重复任务的执行周期时间，纳秒级。
private final long period;

//重新入队的任务
RunnableScheduledFuture<V> outerTask = this;

//延迟队列的索引，以支持更快的取消操作
int heapIndex;

public void run() {
    boolean periodic = isPeriodic();//是否为周期任务
    if (!canRunInCurrentRunState(periodic))//当前状态是否可以执行
        cancel(false);
    else if (!periodic)
        //不是周期任务，直接执行
        ScheduledFutureTask.super.run();
    else if (ScheduledFutureTask.super.runAndReset()) {
        setNextRunTime();//设置下一次运行时间
        reExecutePeriodic(outerTask);//重排序一个周期任务
    }
}

public boolean cancel(boolean mayInterruptIfRunning) {
    boolean cancelled = super.cancel(mayInterruptIfRunning);
    if (cancelled && removeOnCancel && heapIndex >= 0)
        remove(this);
    return cancelled;
}

//关闭后继续执行已经存在的周期任务 
private volatile boolean continueExistingPeriodicTasksAfterShutdown;

//关闭后继续执行已经存在的延时任务 
private volatile boolean executeExistingDelayedTasksAfterShutdown = true;

//取消任务后移除 
private volatile boolean removeOnCancel = false;

//为相同延时的任务提供的顺序编号，保证任务之间的FIFO顺序
private static final AtomicLong sequencer = new AtomicLong();

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

public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                       long delay,
                                       TimeUnit unit) {
    if (callable == null || unit == null)
        throw new NullPointerException();
    RunnableScheduledFuture<V> t = decorateTask(callable,
        new ScheduledFutureTask<V>(callable, triggerTime(delay, unit)));//构造ScheduledFutureTask任务
    delayedExecute(t);//任务执行主方法
    return t;
}

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

/**
 * 创建一个周期执行的任务，第一次执行延期时间为initialDelay，
 * 之后每隔period执行一次，不等待第一次执行完成就开始计时
 */
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();
    //构建RunnableScheduledFuture任务类型
    ScheduledFutureTask<Void> sft =
        new ScheduledFutureTask<Void>(command,
                                      null,
                                      triggerTime(initialDelay, unit),//计算任务的延迟时间
                                      unit.toNanos(period));//计算任务的执行周期
    RunnableScheduledFuture<Void> t = decorateTask(command, sft);//执行用户自定义逻辑
    sft.outerTask = t;//赋值给outerTask，准备重新入队等待下一次执行
    delayedExecute(t);//执行任务
    return t;
}

/**
 * 创建一个周期执行的任务，第一次执行延期时间为initialDelay，
 * 在第一次执行完之后延迟delay后开始下一次执行
 */
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();
    //构建RunnableScheduledFuture任务类型
    ScheduledFutureTask<Void> sft =
        new ScheduledFutureTask<Void>(command,
                                      null,
                                      triggerTime(initialDelay, unit),//计算任务的延迟时间
                                      unit.toNanos(-delay));//计算任务的执行周期
    RunnableScheduledFuture<Void> t = decorateTask(command, sft);//执行用户自定义逻辑
    sft.outerTask = t;//赋值给outerTask，准备重新入队等待下一次执行
    delayedExecute(t);//执行任务
    return t;
}

public void shutdown() {
    super.shutdown();
}
//取消并清除由于关闭策略不应该运行的所有任务
@Override void onShutdown() {
    BlockingQueue<Runnable> q = super.getQueue();
    //获取run-after-shutdown参数
    boolean keepDelayed =
        getExecuteExistingDelayedTasksAfterShutdownPolicy();
    boolean keepPeriodic =
        getContinueExistingPeriodicTasksAfterShutdownPolicy();
    if (!keepDelayed && !keepPeriodic) {//池关闭后不保留任务
        //依次取消任务
        for (Object e : q.toArray())
            if (e instanceof RunnableScheduledFuture<?>)
                ((RunnableScheduledFuture<?>) e).cancel(false);
        q.clear();//清除等待队列
    }
    else {//池关闭后保留任务
        // Traverse snapshot to avoid iterator exceptions
        //遍历快照以避免迭代器异常
        for (Object e : q.toArray()) {
            if (e instanceof RunnableScheduledFuture) {
                RunnableScheduledFuture<?> t =
                    (RunnableScheduledFuture<?>)e;
                if ((t.isPeriodic() ? !keepPeriodic : !keepDelayed) ||
                    t.isCancelled()) { // also remove if already cancelled
                    //如果任务已经取消，移除队列中的任务
                    if (q.remove(t))
                        t.cancel(false);
                }
            }
        }
    }
    tryTerminate(); //终止线程池
}