BlockingQueue---DelayQueue

总结

　　一个无界阻塞队列；

　　FIFO；

　　只包含实现了 Delayed 接口的元素，每个元素都有一个延迟时间，在该延迟时间结束之前，该元素不会从队列中可用。一旦元素的延迟到期，它就可以被取出了，并且取出的顺序是按照延迟到期的时间先后进行的。

　　通常用于实现定时任务调度、缓存过期等场景。例如，可以用来管理超时连接或自动删除过期的数据。

特性

无界：可以容纳任意数量的元素。
基于延迟：元素只有在其延迟时间到期后才可被取出。
线程安全：支持多线程同时进行插入和移除操作。
阻塞：取元素的操作（如 take()）如果在没有延迟到期的元素时会被阻塞，直到有元素的延迟到期。

构造函数

　　DelayQueue(): 创建一个新的 DelayQueue 实例。

方法

插入操作:
- put(E e): 将指定的延迟元素插入此队列。由于 DelayQueue 只接受实现了 Delayed 接口的对象，所以这里的 E 必须是 Delayed 类型。
- offer(E e): 尝试将指定的延迟元素插入此队列。与 put 方法类似，但不会抛出异常。
移除操作:
- take(): 检索并移除此队列的头（其延迟已过的元素），如果队列为空，则等待有元素可用。
- poll(): 检索并移除此队列的头（其延迟已过的元素），如果队列为空，则立即返回 null。
- poll(long timeout, TimeUnit unit): 检索并移除此队列的头（其延迟已过的元素），如果队列为空，则最多等待指定的时间，如果超时队列仍然为空则返回 null。
检查操作:
- peek(): 检索但不移除此队列的头（其延迟已过的元素）；如果队列为空，则返回 null。

public class DelayQueue<E extends Delayed> extends AbstractQueue<E> implements BlockingQueue<E> {
        private final transient ReentrantLock lock = new ReentrantLock();
        private final PriorityQueue<E> q = new PriorityQueue<E>();
 
        public DelayQueue() {}
 
        public boolean offer(E e) {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                q.offer(e);
                if (q.peek() == e) {
                    leader = null;
                    available.signal();
                }
                return true;
            } finally {
                lock.unlock();
            }
        }
 
        public E poll() {
            final ReentrantLock lock = this.lock;
            lock.lock();
            try {
                E first = q.peek();
                if (first == null || first.getDelay(NANOSECONDS) > 0)
                    return null;
                else
                    return q.poll();
            } finally {
                lock.unlock();
            }
        }
    }
 
    public class PriorityQueue<E> extends AbstractQueue<E> implements java.io.Serializable {
        private static final int DEFAULT_INITIAL_CAPACITY = 11;
        transient Object[] queue;
        private int size = 0;
        private final Comparator<? super E> comparator;
 
        public PriorityQueue() {
            this(DEFAULT_INITIAL_CAPACITY, null);
        }
 
        public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) {
            // Note: This restriction of at least one is not actually needed,
            // but continues for 1.5 compatibility
            if (initialCapacity < 1)
                throw new IllegalArgumentException();
            this.queue = new Object[initialCapacity];
            this.comparator = comparator;
        }
 
        public boolean offer(E e) {
            if (e == null)
                throw new NullPointerException();
            modCount++;
            int i = size;
            if (i >= queue.length)
                grow(i + 1);
            size = i + 1;
            if (i == 0)
                queue[0] = e;
            else
                siftUp(i, e);
            return true;
        }
 
        public E poll() {
            if (size == 0)
                return null;
            int s = --size;
            modCount++;
            E result = (E) queue[0];
            E x = (E) queue[s];
            queue[s] = null;
            if (s != 0)
                siftDown(0, x);
            return result;
        }
    }

示例

static class DelayedElement implements Delayed {
        private final long delayTime; // 延迟时间
        private final long expire; // 到期时间
        private final String name; // 元素名称
 
        public DelayedElement(long delay, String name) {
            this.delayTime = delay;
            this.name = name;
            this.expire = System.nanoTime() + TimeUnit.NANOSECONDS.convert(delay, TimeUnit.MILLISECONDS);
        }
 
        @Override
        public long getDelay(TimeUnit unit) {
            return unit.convert(expire - System.nanoTime(), TimeUnit.NANOSECONDS);
        }
 
        @Override
        public int compareTo(Delayed other) {
            if (this == other) {
                return 0;
            }
            if (other instanceof DelayedElement) {
                DelayedElement otherDE = (DelayedElement) other;
                return Long.compare(this.expire, otherDE.expire);
            }
            long diff = getDelay(TimeUnit.NANOSECONDS) - other.getDelay(TimeUnit.NANOSECONDS);
            return (diff < 0) ? -1 : (diff > 0) ? 1 : 0;
        }
 
        @Override
        public String toString() {
            return "DelayedElement{" +
                    "name='" + name + '\'' +
                    ", delay=" + delayTime + "ms" +
                    '}';
        }
    }
 
    public static void main(String[] args) throws InterruptedException {
        DelayQueue<DelayedElement> queue = new DelayQueue<>();
 
        // 添加一些延迟元素
        queue.put(new DelayedElement(1000, "One"));
        queue.put(new DelayedElement(500, "Two"));
        queue.put(new DelayedElement(2000, "Three"));
 
        // 消费者线程
        Thread consumer = new Thread(() -> {
            try {
                while (true) {
                    DelayedElement element = queue.take();
                    System.out.println("Consumed: " + element);
                }
            } catch (InterruptedException e) {
                Thread.currentThread().interrupt();
            }
        });
 
        consumer.start();
 
        // 等待一段时间以观察输出
        Thread.sleep(3000);
        consumer.interrupt(); // 停止消费者
 
        // 结果：
        // Consumed: DelayedElement{name='Two', delay=500ms}
        //Consumed: DelayedElement{name='One', delay=1000ms}
        //Consumed: DelayedElement{name='Three', delay=2000ms}
    }