为什么Spring Boot调度器中的@Async任务不是每隔1秒运行一次？

Question

我无法理解用以下方法定义的行为 @Async 注解.根据我的理解，该方法应该每1秒执行一次。但我看到的不是这样。

@EnableAsync
@Component
public class AsyncSchedulingDemo {

    private static final SimpleDateFormat dateFormat = new SimpleDateFormat("MM/dd/yyyy HH:mm:ss");

    //every 1 second, INSTANT start of job but waits for the first job to finish hence takes 10 seconds
    //here the next execution will start after 10 seconds because the job takes 10 seconds to finish
    @Scheduled(fixedRate = 1000)
    public void performFixedRateTask_W_LongRunningJob() throws InterruptedException {
        System.out.println("Task performed every 1 second but waits for the first one to finish before the next execution starts " + dateFormat.format(new Date()));
        Thread.sleep(10000);
    }

    //should be executed every 1 second because of @Async
    //here the next execution should start IMMEDIATELY and does NOT wait for the first one to finish
    @Async
    @Scheduled(fixedRate = 1000)
    public void performFixedRateTask_With_Async() throws InterruptedException {
        System.out.println("Async Task performed every 1 second as it does NOT wait for the first one to finish " + dateFormat.format(new Date()));
        Thread.sleep(10000);
    }

}

控制台日志。

Task performed every 1 second but waits for the first one to finish before the next execution starts 05/13/2020 15:54:51
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 15:54:51

Task performed every 1 second but waits for the first one to finish before the next execution starts 05/13/2020 15:55:01
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 15:55:01

Task performed every 1 second but waits for the first one to finish before the next execution starts 05/13/2020 15:55:11
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 15:55:11

===UPDATED====

如果我把第一个方法完全注释掉，然后再次执行应用程序，我看到第二个方法每秒钟都在执行。为什么呢？第一个方法是如何阻止第二个方法的执行的？

控制台日志。

Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 16:45:48
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 16:45:49
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 16:45:50
Async Task performed every 1 second as it does NOT wait for the first one to finish 05/13/2020 16:45:51

Answer 1

我想，这是因为用于执行方法的线程的默认池大小是1秒。@Scheduled 注释的方法是1，根据 春季文档: "如果不提供'pool-size'属性，默认线程池将只有一个线程。"

由于第一个方法不是 @Async挡住了唯一的一条线。