理解ScheduledExecutorService中scheduleAtFixedRate和scheduleWithFixedDelay的区别

时间:2023-02-12 09:29:49

scheduleAtFixedRate

每间隔一段时间执行,分为两种情况:

  1. 当前任务执行时间小于间隔时间,每次到点即执行;

    /**
    * 任务执行时间(8s)小于间隔时间(10s)
    */
    public class ScheduleTest {
    static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1); public static void main(String[] args) {
    scheduler.scheduleAtFixedRate(new Runnable() {
    @Override
    public void run() {
    System.out.println("Start: scheduleAtFixedRate: " + new Date());
    try {
    Thread.sleep(8000);
    } catch (InterruptedException e) {
    e.printStackTrace();
    }
    System.out.println("End : scheduleAtFixedRate: " + new Date());
    }
    }, 0, 10 , SECONDS);
    }
    } output: Start: scheduleAtFixedRate: Sun Apr 28 14:36:00 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:36:08 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:36:10 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:36:18 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:36:20 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:36:28 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:36:30 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:36:38 CST 2019
    ... 程序启动时间是14:36:00,以后每间隔10s执行一次(即14:36:10、14:36:20、14:36:30等)。
  2. 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。

    /**
    * 任务执行时间(12s)大于间隔时间(10s)
    */
    public class ScheduleTest {
    static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1); public static void main(String[] args) {
    scheduler.scheduleAtFixedRate(new Runnable() {
    @Override
    public void run() {
    System.out.println("Start: scheduleAtFixedRate: " + new Date());
    try {
    Thread.sleep(12000);
    } catch (InterruptedException e) {
    e.printStackTrace();
    }
    System.out.println("End : scheduleAtFixedRate: " + new Date());
    }
    }, 0, 10 , SECONDS);
    }
    } output: Start: scheduleAtFixedRate: Sun Apr 28 14:30:13 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:30:25 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:30:25 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:30:37 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:30:37 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:30:49 CST 2019
    Start: scheduleAtFixedRate: Sun Apr 28 14:30:49 CST 2019
    End : scheduleAtFixedRate: Sun Apr 28 14:31:01 CST 2019 程序启动时间是14:30:13,按理说应该每间隔10s执行一次(即14:30:23、14:30:33等),但由于任务执行时间长于10s,下一次的任务要开始的时候发现上次的任务还没有完成,因此阻塞等待,一旦发现上次的任务完成,就马上启动。表现出来就是任务延时启动,最终的效果就是连续执行。

scheduleWithFixedDelay

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。

/**
* 任务执行时间(8s)小于间隔时间(10s)
*/
public class ScheduleTest {
static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1); public static void main(String[] args) {
scheduler.scheduleWithFixedDelay(new Runnable() {
@Override
public void run() {
System.out.println("Start: scheduleWithFixedDelay: " + new Date());
try {
Thread.sleep(12000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("End : scheduleWithFixedDelay: " + new Date());
}
}, 0, 10 , SECONDS);
}
} output: Start: scheduleWithFixedDelay: Sun Apr 28 14:27:59 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:28:07 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:28:17 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:28:25 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:28:35 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:28:43 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:28:53 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:29:01 CST 2019
... 可以看出每个End后,等待了10秒,才启动下一次Start。
/**
* 任务执行时间(12s)大于间隔时间(10s)
*/
public class ScheduleTest {
static ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1); public static void main(String[] args) {
scheduler.scheduleWithFixedDelay(new Runnable() {
@Override
public void run() {
System.out.println("Start: scheduleWithFixedDelay: " + new Date());
try {
Thread.sleep(12000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("End : scheduleWithFixedDelay: " + new Date());
}
}, 0, 10 , SECONDS);
}
} output: Start: scheduleWithFixedDelay: Sun Apr 28 14:26:29 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:26:41 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:26:51 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:27:03 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:27:13 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:27:25 CST 2019
Start: scheduleWithFixedDelay: Sun Apr 28 14:27:35 CST 2019
End : scheduleWithFixedDelay: Sun Apr 28 14:27:47 CST 2019
... 可以看出每个End后,等待了10秒,才启动下一次Start。

参考

scheduleAtFixedRate vs scheduleWithFixedDelay

扩展

Spring定时任务@Scheduled注解使用方式浅窥(cron表达式、fixedRate和fixedDelay)