理解ScheduledExecutorService中scheduleAtFixedRate和scheduleWithFixedDelay的區別

scheduleAtFixedRate

每間隔一段時間執行，分為兩種情況：

1. 當前任務執行時間小於間隔時間，每次到點即執行；

   /**
    * 任務執行時間（8s）小於間隔時間(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
   ...
       
   程序啟動時間是14:36:00，以后每間隔10s執行一次(即14:36:10、14:36:20、14:36:30等)。
   

2. 當前任務執行時間大於等於間隔時間，任務執行后立即執行下一次任務。相當於連續執行了。

   /**
    * 任務執行時間（12s）大於間隔時間(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
       
   程序啟動時間是14:30:13，按理說應該每間隔10s執行一次（即14:30:23、14:30:33等），但由於任務執行時間長於10s，下一次的任務要開始的時候發現上次的任務還沒有完成，因此阻塞等待，一旦發現上次的任務完成，就馬上啟動。表現出來就是任務延時啟動，最終的效果就是連續執行。
   

scheduleWithFixedDelay

每當上次任務執行完畢后，間隔一段時間執行。不管當前任務執行時間大於、等於還是小於間隔時間，執行效果都是一樣的。

/**
 * 任務執行時間（8s）小於間隔時間(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
...

可以看出每個End后，等待了10秒，才啟動下一次Start。

/**
 * 任務執行時間（12s）大於間隔時間(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
...

可以看出每個End后，等待了10秒，才啟動下一次Start。

參考

scheduleAtFixedRate vs scheduleWithFixedDelay

擴展

Spring定時任務@Scheduled注解使用方式淺窺（cron表達式、fixedRate和fixedDelay）