java - ScheduledExecutorService 和取消行为

Question

我正试图为此找到一个满意的答案:

什么可能导致 ScheduledFuture#cancel(false) 返回 false？本质上，我正在像这样创建一个新的调度程序:

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
ScheduledFuture<?> schedulerStatus = scheduler.scheduleWithFixedDelay(this, 0L, 1L, TimeUnit.SECONDS);

在 run 方法中我有如下内容:

public void run() {
    // Do some logic
    schedulerStatus.cancel(false);
}

我看过 JavaDoc但它并没有真正说出任何有用的东西，

"or could not be cancelled for some other reason"

因此，如果能帮助理解这里到底发生了什么，那就太好了!

谢谢

更新

进一步扩展我上面的例子:

private final ScheduledExecutorService scheduler;
private volatile ScheduledFuture<?> schedulerStatus;

@Inject
public IndexChangeManagerImpl(IndexChangeMonitor monitor, IndexChangeObservable observable) {
    this.monitor = monitor;
    scheduler = Executors.newScheduledThreadPool(1);
}

@Override
public  void init() {
    if (schedulerStatus == null || schedulerStatus.isDone()) {
        // Kick of the scheduler to attempt to initiate the listener every second.
        schedulerStatus = scheduler.scheduleWithFixedDelay(this, 0L, 1L, TimeUnit.SECONDS);
    }
}

@Override
public void run() {
    try {
        // Before starting the monitor, ensure it is not currently running.
        monitor.shutdown();

        // Initiate the monitor.
        monitor.start();

        // Listener has been established, no more attempts necessary.
        schedulerStatus.cancel(false);

        lastLog = 0;
    } catch (ChangeMonitorException sfE) {
        long now = System.currentTimeMillis();

        if (lastLog == 0 || now - lastLog > 60000) {
            // Log every 60 seconds.
            DEBUG.error("Error attempting to initiate index change monitor.", sfE);
            lastLog = now;
        }
    }
}

希望这段代码能证明取消返回 false 的两个原因实际上是不可能的(注意这都是单线程的)。由于在单个计划线程内调用取消，显然任务既未完成也无法取消。

需要注意的是，cancel 返回 false 是间歇性的，发生在系统负载不足的情况下。

还有什么想法吗？

Answer 1

如果它已经完成执行或已经被取消，它看起来像返回 false。

来自 Java 默认 Executor 实现所使用的默认 Future 实现的源代码:

boolean innerCancel(boolean mayInterruptIfRunning) {
    for (;;) {
        int s = getState();
        if (ranOrCancelled(s))
            return false;
        if (compareAndSetState(s, CANCELLED))
            break;
    }
    if (mayInterruptIfRunning) {
        Thread r = runner;
        if (r != null)
            r.interrupt();
    }
    releaseShared(0);
    done();
    return true;
}