java - ScheduledExecutorService#scheduleAtFixedRate 不起作用

Question

当我定期执行一项耗时的任务时，结果没有达到我的预期。

public static void main(String[] args) {
    ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(10);

    scheduler.scheduleAtFixedRate(() -> {
        System.out.println("start: " + System.currentTimeMillis());
        try {
            TimeUnit.SECONDS.sleep(3);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("end  : " + System.currentTimeMillis());
    }, 0, 1, TimeUnit.SECONDS);
}
/*
    start: 1539688734439
    end  : 1539688737442
    start: 1539688737443
    end  : 1539688740447
    start: 1539688740447
    end  : 1539688743452
    start: 1539688743452
    end  : 1539688746456
    start: 1539688746456
    end  : 1539688749461
    ...
 */

如上所示，两次开始之间间隔 3 秒，而不是 1 秒。

Answer 1

这正是它的设计方式，来自 doc :

If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.