java - ScheduledExecutorService 不执行具有 initialDelay 0 的任务

Question

ScheduledExecutorService 在使用 scheduleWithFixedDelay 或 时，initialDelay 未执行提交的任务 0 时间单位>scheduleAtFixedRate。但是当我调用 schedule 时它正在执行任务在上面

ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
Runnable runnable = () -> System.out.println("I am a runnable");
scheduledExecutorService.schedule(runnable, 0, TimeUnit.SECONDS);
scheduledExecutorService.shutdown();

输出

I am a runnable

但是当我使用 scheduleWithFixedDelay 或 scheduleAtFixedRate 而不是 schedule 时，任务没有被执行。

ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();
Runnable runnable = () -> System.out.println("I am a runnable");
scheduledExecutorService.scheduleWithFixedDelay(runnable, 0, 1, TimeUnit.SECONDS);
// or  scheduledExecutorService.scheduleAtFixedRate(runnable, 0, 1, TimeUnit.SECONDS);
scheduledExecutorService.shutdown();

为什么在这种情况下没有执行任务？我预计此任务将在 initialDelay 设置为 0

时执行

Answer 1

背后的原因可以在ScheduledThreadPoolExecutor中找到，它是Executors.newSingleThreadScheduledExecutor()的实现类

1。任务在调用schedule时执行，

这是由于executeExistingDelayedTasksAfterShutdown is true by default

    /**
     * False if should cancel non-periodic not-yet-expired tasks on shutdown.
     */
    private volatile boolean executeExistingDelayedTasksAfterShutdown = true;

2。 scheduleWithFixedDelay 或 scheduleAtFixedRate

未执行任务

这是由于continueExistingPeriodicTasksAfterShutdown is false by default

    /**
     * False if should cancel/suppress periodic tasks on shutdown.
     */
    private volatile boolean continueExistingPeriodicTasksAfterShutdown;

我们可以通过 ScheduledThreadPoolExecutor#setExecuteExistingDelayedTasksAfterShutdownPolicy 覆盖这些参数和 ScheduledThreadPoolExecutor#setContinueExistingPeriodicTasksAfterShutdownPolicy在安排任务之前。