理解ScheduledExecutorService中scheduleAtFixedRate和scheduleWithFixedDelay的区别

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）