踩坑系列之scheduleWithFixedDelay 和 scheduleAtFixedRate

定時任務和延時任務的了解

• ScheduledExecutorService#scheduleAtFixedRate() 指的是“以固定的頻率”執行，period（周期）指的是兩次成功執行之間的時間。上一個任務開始的時間計時，一個period後，檢測上一個任務是否執行完畢，如果上一個任務執行完畢，則目前任務立即執行，如果上一個任務沒有執行完畢，則需要等上一個任務執行完畢後立即執行。
• ScheduledExecutorService#scheduleWithFixedDelay() 指的是“以固定的延時”執行，delay（延時）指的是一次執行終止和下一次執行開始之間的延遲。

示例

scheduleAtFixedRate

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleAtFixedRate(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
        }, 2, 3, TimeUnit.SECONDS);
    }           

輸出：

startTime: Sun Feb 16 11:17:09 CST 2020
beginTime: Sun Feb 16 11:17:11 CST 2020
endTime: Sun Feb 16 11:17:16 CST 2020
beginTime: Sun Feb 16 11:17:16 CST 2020
endTime: Sun Feb 16 11:17:21 CST 2020
beginTime: Sun Feb 16 11:17:21 CST 2020
endTime: Sun Feb 16 11:17:26 CST 2020
beginTime: Sun Feb 16 11:17:26 CST 2020
endTime: Sun Feb 16 11:17:31 CST 2020
beginTime: Sun Feb 16 11:17:31 CST 2020
endTime: Sun Feb 16 11:17:36 CST 2020
beginTime: Sun Feb 16 11:17:36 CST 2020           

目前任務執行時間大于等于間隔時間，任務執行後立即執行下一次任務。相當于連續執行了。

scheduleWithFixedDelay

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleWithFixedDelay(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
        }, 2, 3, TimeUnit.SECONDS);
    }           

startTime: Sun Feb 16 11:27:26 CST 2020
beginTime: Sun Feb 16 11:27:28 CST 2020
endTime: Sun Feb 16 11:27:33 CST 2020
beginTime: Sun Feb 16 11:27:36 CST 2020
endTime: Sun Feb 16 11:27:41 CST 2020
beginTime: Sun Feb 16 11:27:44 CST 2020
endTime: Sun Feb 16 11:27:49 CST 2020
beginTime: Sun Feb 16 11:27:52 CST 2020           

每當上次任務執行完畢後，間隔一段時間執行。不管目前任務執行時間大于、等于還是小于間隔時間，執行效果都是一樣的。

常見的“坑”

關于定時線程池（定時任務scheduleAtFixedRate和延時任務scheduleWithFixedDelay），好多人認為設定好頻率（比如1Min），它會按照這個間隔按部就班的工作。但是，如果其中一次排程任務卡住的話，不僅這次排程失敗，而且整個線程池也會停在這次排程上。

如果run方法中抛出異常，而又沒有捕獲，會導緻整個定時任務卡住。建議在整個run方法中使用try...catch...fianlly...處理。

public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat(
            "my-test-pool-%d").build();
        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1, threadFactory);
        Date startTime = new Date();
        System.out.println("startTime: " + startTime.toString());
        scheduledExecutorService.scheduleAtFixedRate(() -> {
            System.out.println("beginTime: " + new Date().toString());
            try {
                Thread.sleep(5 * 1000L);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("endTime: " + new Date().toString());
            throw new NullPointerException();
        }, 2, 3, TimeUnit.SECONDS);
    }