java - 为什么我的 DelayQueue 没有延迟并以错误的顺序打印？

Question

我有以下据称非常简单的 DelayQueue 演示.

class DelayedThing implements Delayed {

    private final long waitUntil;
    private final String name;

    public DelayedThing(String name, long wait) {
        this.name = name;
        this.waitUntil = System.currentTimeMillis() + wait;
        System.out.println("DelayedThing(" + name + " wait=" + wait + " until-" + waitUntil);
    }

    @Override
    public long getDelay(TimeUnit unit) {
        System.out.println(name + " getDelay = " + unit.convert(waitUntil - System.currentTimeMillis(), TimeUnit.MILLISECONDS));
        return unit.convert(waitUntil - System.currentTimeMillis(), TimeUnit.MILLISECONDS);
    }

    @Override
    public int compareTo(Delayed o) {
        long diff = this.getDelay(TimeUnit.MILLISECONDS) - o.getDelay(TimeUnit.MILLISECONDS);
        System.out.println(name + ".compareTo(" + o + ") = " + diff);
        return Long.signum(diff);
    }

    @Override
    public String toString() {
        return name;
    }
}

public void test() throws InterruptedException {
    BlockingQueue<Delayed> queue = new DelayQueue<>();
    queue.add(new DelayedThing("one second", 1000));
    queue.add(new DelayedThing("two seconds", 2000));
    queue.add(new DelayedThing("half second", 500));
    for (Delayed d : queue) {
        System.out.println(d);
    }
}

但是它打印

half second
two seconds
one second

这显然是错误的。

Answer 1

这个错误很微妙。我假设 DelayQueue 的 iterator 将对每个元素执行一系列 take 调用。 错了!

参见 iterator() Java文档:

Returns an iterator over all the elements (both expired and unexpired) in this queue.

这是出乎意料的。

一个正确的解决方案如下:

    while (queue.size() > 0) {
        System.out.println(queue.take());
    }

请注意，如果您尝试流式传输队列，也会发生此问题:

    queue.stream().forEach((d) -> {
        System.out.println(d);
    });

由于流式传输将发生在 DelayQueue 提供的iterator 上，这也会产生意想不到的结果。