java ScheduledFuture getDelay 返回负值答案

【问题标题】：java ScheduledFuture getDelay return negative valuejava ScheduledFuture getDelay 返回负值
【发布时间】：2019-03-02 11:32:05
【问题描述】：

我正在使用ScheduledExecutorService、Semaphore 和ScheduledFuture 编写一个限速函数，简单来说，当客户端达到限制时，服务器会返回错误429 并显示“msg please try after %d second”。我使用 scheduleFuture.getDelay(TimeUnit.SECONDS) 来获取 %d 的值。对于第一次或第二次尝试，它表现正常，即允许访问单元达到限制并显示之后等待多少秒。然后 getDelay 开始显示负值。这是否意味着 ScheduledExecutorService 无法正常工作？以下是sn-p

public RateLimiter(int permits, long durationInMillis){
    this.semaphore = new Semaphore(permits);
    this.permits = permits;
    this.durationInMillis = durationInMillis;       
    scheduleReplenishment();
}

public boolean allowAccess() {      
    return semaphore.tryAcquire(); 
}

public long nextReplenishmentTime() {
    return scheduledFuture.getDelay(TimeUnit.SECONDS);
}


public void stop() {
    scheduler.shutdownNow();
}   

public void scheduleReplenishment() {
    scheduledFuture = scheduler.schedule(() -> {
        semaphore.release(permits - semaphore.availablePermits());
    }, durationInMillis, TimeUnit.MILLISECONDS);    
}

【问题讨论】：

标签： java semaphore scheduledexecutorservice

【解决方案1】：

如果任务已完成，getDelay(TimeUnit) 将为负数。为了显示它，我在scheduleReplenishment() 中添加了两个参数，并将getReplenishmentTime() 更改为printReplenishmentTime()。

注意1：如果你创建一个Future<>，并用另一个替换一个，你应该关心被删除的......

注意2：如果要测试Future<>和Semaphore，不要立即释放分配的资源。

private final ConcurrentSkipListMap<String, ScheduledFuture<?>> scheduledFutures
                                    = new ConcurrentSkipListMap<>();
private final AtomicInteger counter = new AtomicInteger();

public void printReplenishmentTime() {

    scheduledFutures.forEach((name, f) -> {

        final long delay = f.getDelay(TimeUnit.SECONDS);
        System.out.println(name + " delay " + delay);
    });
}

/**
 * try acquire one permit once from {@code semaphore}, 
 * then wait {@code waitInMillis}, until all permits used.
 * 
 * @param waitInMillis after successfully used one permit, wait
 * @param permits all permits to use, best if permits @gt; 2
 */
public void scheduleReplenishment(final long waitInMillis, final int permits) {

    final String name = "future" + counter.getAndIncrement();
    scheduledFutures.put(name, scheduler.schedule(() -> {

        try {

            for (int permit = permits; 0 < permit;) {

                final boolean ack = semaphore.tryAcquire(1);
                System.out.println(name + " " + (ack ? "acquire" : "not acquire")
                        + " one, but need " + permit);
                if (ack) {

                    permit--;
                }
                if (0 < permit) {

                    try {
                        Thread.sleep(waitInMillis);
                    } catch (final InterruptedException e) {

                        System.out.println(name + " interrupted, exiting...");
                        return;
                    }
                }
            }
            System.out.println(name + " done");

        } finally {

            semaphore.release(permits - permit);
        }

        // BAD CODE: semaphore.availablePermits() for debugging purposes
        // only, maybe 0 release...
        // semaphore.release(permits - semaphore.availablePermits());
    }, durationInMillis, TimeUnit.MILLISECONDS));
}

【讨论】：

【解决方案2】：

scheduler.schedule() 是一次性函数，这就是为什么它显示负的getDelay() 值。

【讨论】：