定时任务和延时任务的理解

• ScheduledExecutorService#scheduleAtFixedRate() 指的是“以固定的频率”执行，period（周期）指的是两次成功执行之间的时间。上一个任务开始的时间计时，一个period后，检测上一个任务是否执行完毕，如果上一个任务执行完毕，则当前任务立即执行，如果上一个任务没有执行完毕，则需要等上一个任务执行完毕后立即执行。
• ScheduledExecutorService#scheduleWithFixedDelay() 指的是“以固定的延时”执行，delay（延时）指的是一次执行终止和下一次执行开始之间的延迟。

示例

scheduleAtFixedRate

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleAtFixedRate(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
        }, 2, 3, TimeUnit.SECONDS);
    }

输出：

startTime: Sun Feb 16 11:17:09 CST 2020
beginTime: Sun Feb 16 11:17:11 CST 2020
endTime: Sun Feb 16 11:17:16 CST 2020
beginTime: Sun Feb 16 11:17:16 CST 2020
endTime: Sun Feb 16 11:17:21 CST 2020
beginTime: Sun Feb 16 11:17:21 CST 2020
endTime: Sun Feb 16 11:17:26 CST 2020
beginTime: Sun Feb 16 11:17:26 CST 2020
endTime: Sun Feb 16 11:17:31 CST 2020
beginTime: Sun Feb 16 11:17:31 CST 2020
endTime: Sun Feb 16 11:17:36 CST 2020
beginTime: Sun Feb 16 11:17:36 CST 2020

当前任务执行时间大于等于间隔时间，任务执行后立即执行下一次任务。相当于连续执行了。

scheduleWithFixedDelay

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleWithFixedDelay(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
        }, 2, 3, TimeUnit.SECONDS);
    }

输出：

startTime: Sun Feb 16 11:27:26 CST 2020
beginTime: Sun Feb 16 11:27:28 CST 2020
endTime: Sun Feb 16 11:27:33 CST 2020
beginTime: Sun Feb 16 11:27:36 CST 2020
endTime: Sun Feb 16 11:27:41 CST 2020
beginTime: Sun Feb 16 11:27:44 CST 2020
endTime: Sun Feb 16 11:27:49 CST 2020
beginTime: Sun Feb 16 11:27:52 CST 2020

每当上次任务执行完毕后，间隔一段时间执行。不管当前任务执行时间大于、等于还是小于间隔时间，执行效果都是一样的。

常见的“坑”

关于定时线程池（定时任务scheduleAtFixedRate和延时任务scheduleWithFixedDelay），好多人认为设置好频率（比如1Min），它会按照这个间隔按部就班的工作。但是，如果其中一次调度任务卡住的话，不仅这次调度失败，而且整个线程池也会停在这次调度上。

示例

如果run方法中抛出异常，而又没有捕获，会导致整个定时任务卡住。建议在整个run方法中使用try...catch...fianlly...处理。

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleAtFixedRate(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
            throw new NullPointerException();
        }, 2, 3, TimeUnit.SECONDS);
    }