任务案例 

@Scheduled(cron = "0/3 * * * * ? ")
public void run() {int sleep = new Random().nextInt(10);log.info("say sleep: {}s", sleep);try {Thread.sleep(sleep * 1000);} catch (InterruptedException e) {e.printStackTrace();}
}

任务设定的是，从 0 秒开始，每 3 秒钟执行一次。任务耗时通过线程 sleep 来模拟，时长则是一个 0-10 的随机数。

打印效果

2019-03-01 14:25:24  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 9s
2019-03-01 14:25:36  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 7s
2019-03-01 14:25:45  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 9s
2019-03-01 14:25:57  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 4s
2019-03-01 14:26:03  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 2s
2019-03-01 14:26:06  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 8s
2019-03-01 14:26:15  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 0s
2019-03-01 14:26:18  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 2s
2019-03-01 14:26:21  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 9s
2019-03-01 14:26:33  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 1s
2019-03-01 14:26:36  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 1s
2019-03-01 14:26:39  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 0s
2019-03-01 14:26:42  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 3s
2019-03-01 14:26:48  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 4s
2019-03-01 14:26:54  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 4s
2019-03-01 14:27:00  INFO 8008 --- [   scheduling-1] c.yclouds.service.demo.TestJob  : say sleep: 3s

结果分析 

• line1和line2对比：24秒执行的任务，耗时9秒。line2并没有在27秒执行打印，说明上次任务未结束时，下次任务不会启动
• line1和line2对比：理论上line1会在33秒时结束，此时刚好下次任务的触发点，结果可以看出，line2并不是在33秒执行的打印，而是下一个周期36秒打印的。也就是说，如果上一个任务结束时，刚好是下次任务的触发点，下次任务不会立马执行，而是会顺延一个周期

• 结论：上次未结束，下次任务不会启动。它会自动顺延到任务结束后的，下一个周期触发