ScheduledExecutorService任務的運行時間晚於預期

Question

我定期運行任務並為間隔提供靈活性，下一個超時在每個任務結束時計算，從Instant.now（）轉換為毫秒，並使用ScheduledExecutorService#schedule 。

這段代碼通常工作正常（左邊的藍色曲線），但其他日子則不太好。

在我看來，事情有時會在啟動時變壞（機器每晚重啟），雖然程序應該並且確實糾正自己，但ScheduledExecutorService#schedule不能恢復，並且計划任務一直運行得很晚 。 似乎完整的JVM重啟是唯一的解決方案。

我最初的想法是，這是一個錯誤，根據機器啟動的時間，事情可能會出錯。 但是以下日志輸出表明該問題與我對ScheduledExecutorService#schedule使用有關：

// Log time in GMT+2, other times are in GMT
// The following lines are written following system startup (all times are correct)
08 juin 00:08:49.993 [main] WARN  com.pgscada.webdyn.Webdyn - Scheduling next webdyn service time. Currently 2018-06-07T22:08:49.993Z, last connection null
08 juin 00:08:50.586 [main] INFO  com.pgscada.webdyn.Webdyn - The next data sample at 2018-06-07T22:10:00Z and the next FTP connection at 2018-06-07T22:30:00Z
08 juin 00:08:50.586 [main] WARN  com.pgscada.webdyn.Webdyn - Completed webdyn schedule in 9ms, next execution at 2018-06-07T22:10:00Z (in 69414 ms) will run as data-sample
// So we are expecting the next execution to occur at 00:10:00 (or in 69.4 seconds)
// Except that it runs at 00:11:21
08 juin 00:11:21.206 [pool-1-thread-4] INFO  com.pgscada.webdyn.Webdyn - Executing Webdyn service, isDataSample=true, isFtpConnection=false, nextTimeout=2018-06-07T22:10:00Z, lastFtpConnection=null
// But thats OK because it should correct itself
08 juin 00:13:04.151 [pool-1-thread-4] WARN  com.pgscada.webdyn.Webdyn - Scheduling next webdyn service time. Currently 2018-06-07T22:10:00Z, last connection null
08 juin 00:13:04.167 [pool-1-thread-4] INFO  com.pgscada.webdyn.Webdyn - The next data sample at 2018-06-07T22:20:00Z and the next FTP connection at 2018-06-07T22:30:00Z
08 juin 00:13:04.167 [pool-1-thread-4] WARN  com.pgscada.webdyn.Webdyn - Completed webdyn schedule in 0ms, next execution at 2018-06-07T22:20:00Z (in 415833 ms) will run as data-sample
// So now we are expecting the next execution to occur at 00:20:00 (or in 415.8 seconds)
// But it runs at 00:28:06
08 juin 00:28:06.145 [pool-1-thread-4] INFO  com.pgscada.webdyn.Webdyn - Executing Webdyn service, isDataSample=true, isFtpConnection=false, nextTimeout=2018-06-07T22:20:00Z, lastFtpConnection=null

以下是調度功能的實際生產代碼。

ScheduledExecutorService EXECUTORS = Executors.newScheduledThreadPool(10);


private void scheduleNextTimeout(Instant currentTime, Instant lastFtpConnection) {

    try {

        log.info("Scheduling next webdyn service time. Currently {}, last connection {}", currentTime, lastFtpConnection);

        // Parse config files first
        getConfigIni().parse();

        long time = System.nanoTime();
        final Instant earliestPossibleTimeout = Instant.now().plusSeconds(5); 

        Instant nextDataSample = nextTimeout(currentTime);

        if (nextDataSample.isBefore(earliestPossibleTimeout)) {
            final Instant oldTime = nextDataSample;
            nextDataSample = nextTimeout(earliestPossibleTimeout);
            log.warn("Next data sample was calculated to a time in the past '{}', resetting to a future time: {}", oldTime, nextDataSample);
        }

        Instant nextFtp = nextFtpConnection(currentTime, lastFtpConnection);

        if (nextFtp.isBefore(earliestPossibleTimeout)) {
            final Instant oldTime = nextFtp;
            nextFtp = nextFtpConnection(earliestPossibleTimeout, lastFtpConnection);
            log.warn("Next FTP connection was calculated to a time in the past '{}', resetting to a future time: {}", oldTime, nextFtp);
        }

        final boolean isFtpConnection = !nextDataSample.isBefore(nextFtp);
        final boolean isDataSample = !isFtpConnection || nextDataSample.equals(nextFtp);
        log.info("The next data sample at {} and the next FTP connection at {}", nextDataSample, nextFtp);

        final Instant nextTimeout = nextDataSample.isBefore(nextFtp) ? nextDataSample : nextFtp;
        final long millis = Duration.between(Instant.now(), nextTimeout).toMillis();
        EXECUTORS.schedule(() -> {
            log.info("Executing Webdyn service, isDataSample={}, isFtpConnection={}, nextTimeout={}, lastFtpConnection={}",
                    isDataSample, isFtpConnection, nextTimeout, lastFtpConnection);

            long tme = System.nanoTime();
            try {
                connect(isDataSample, isFtpConnection, nextTimeout, lastFtpConnection);
                log.warn("Completed webdyn service in {}s", (System.nanoTime() - tme) / 1000000);
            } catch (final Throwable ex) {
                log.error("Failed webdyn service after {}ms : {}", (System.nanoTime() - tme) / 1000000, ex.getMessage(), ex);
            } finally {
                scheduleNextTimeout(nextTimeout, isFtpConnection ? nextTimeout : lastFtpConnection);
            }
        }, millis, TimeUnit.MILLISECONDS);

        log.warn("Completed webdyn schedule in {}ms, next execution at {} (in {} ms) will run as {}",
                (System.nanoTime() - time) / 1000000, nextTimeout, millis, isFtpConnection ? "ftp-connection" : "data-sample");

    } catch (final Throwable ex) {
        log.error("Fatal error in webdyn schedule : {}", ex.getMessage(), ex);
    }
}

Answer 1

正如我在評論中所提到的那樣，這里的問題是有一個共享的，可變的，非線程安全的資源（EXECUTORS atribute）被多個線程改變了。 它在啟動時由主線程更改，並且從池中使用任何線程來執行任務。

需要注意的是， 即使您一次只有一個線程訪問共享資源（僅僅因為一次只運行一個任務），您仍然需要考慮一致性 。 這是因為沒有同步，Java內存模型不保證一個線程所做的更改對其他線程一直可見，無論它們運行多久。

因此解決方案是使方法scheduleNextTimeout同步 ，從而保證更改不會保持在本地執行線程並寫入主存儲器。

您還可以在部件周圍創建一個同步塊（在“this”上同步），這樣可以訪問共享資源，但由於系統似乎不是重型的，其余的代碼似乎不需要很長一段時間，沒有必要......

在我第一次遇到這類問題的時候，我曾經從這篇很好的短篇文章中了解到這一點:) https://www.cs.umd.edu/~pugh/java/memoryModel/jsr- 133-的faq.html＃jsr133

我很高興我可以幫忙。