java并发：阻塞队列之DelayQueue
延时队列
DelayQueue是一个支持延时获取元素的使用优先级队列实现的无界的阻塞队列。
在创建元素时可以指定多久才能从队列中获取当前元素，只有在延迟期满时才能从队列中提取元素。
类图如下：
DelayQueue的定义以及构造函数如下：
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>(); /** * Thread designated to wait for the element at the head of * the queue. This variant of the Leader-Follower pattern * (http://www.cs.wustl.edu/~schmidt/POSA/POSA2/) serves to * minimize unnecessary timed waiting. When a thread becomes * the leader, it waits only for the next delay to elapse, but * other threads await indefinitely. The leader thread must * signal some other thread before returning from take() or * poll(...), unless some other thread becomes leader in the * interim. Whenever the head of the queue is replaced with * an element with an earlier expiration time, the leader * field is invalidated by being reset to null, and some * waiting thread, but not necessarily the current leader, is * signalled. So waiting threads must be prepared to acquire * and lose leadership while waiting. */ private Thread leader; /** * Condition signalled when a newer element becomes available * at the head of the queue or a new thread may need to * become leader. */ private final Condition available = lock.newCondition(); /** * Creates a new {@code DelayQueue} that is initially empty. */ public DelayQueue() {} /** * Creates a {@code DelayQueue} initially containing the elements of the * given collection of {@link Delayed} instances. * * @param c the collection of elements to initially contain * @throws NullPointerException if the specified collection or any * of its elements are null */ public DelayQueue(Collection<? extends E> c) { this.addAll(c); }
解读：
DelayQueue 内部使用 PriorityQueue 存放数据，使用 ReentrantLock 实现线程同步。
重点解释一下变量 leader：
其使用基于 Leader-Follower模式的变体，用于减少不必要的线程等待。
当一个线程调用队列的 take 方法变为 leader 线程后，它会调用 available. awaitNanos(delay) 等待 delay 时间；其他线程（follwer线程) 则调用 available. await()进行无限等待。
leader 线程延迟时间过期后会退出 take 方法，并通过调用 available.signal()方法唤醒一个 follwer线程，被唤醒的 follwer线程被选举为新的 leader线程。
Note：
队列中的元素必须实现Delayed接口和Comparable接口，也就是说DelayQueue里面的元素必须有public void compareTo(To)和long getDelay(TimeUnit unit)方法存在。
原因：由于每个元素都有一个过期时间，所以要实现获取当前元素还剩下多少时间就过期了的接口；由于DelayQueue底层使用优先级队列来实现，所以要实现元素之间相互比较的接口。
Delayed接口的定义如下：
public interface Delayed extends Comparable<Delayed> { /** * Returns the remaining delay associated with this object, in the * given time unit. * * @param unit the time unit * @return the remaining delay; zero or negative values indicate * that the delay has already elapsed */ long getDelay(TimeUnit unit); }
解读：
此接口继承自Comparable接口。
Comparable接口的定义如下：
public interface Comparable<T> { public int compareTo(T o); }
添加元素
offer方法的代码如下：
/** * Inserts the specified element into this delay queue. * * @param e the element to add * @return {@code true} * @throws NullPointerException if the specified element is null */ 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(); } }
解读：
该方法在获取独占锁之后调用优先级队列的offer方法实现入队
/** * Inserts the specified element into this priority queue. * * @return {@code true} (as specified by {@link Queue#offer}) * @throws ClassCastException if the specified element cannot be * compared with elements currently in this priority queue * according to the priority queue's ordering * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { if (e == null) throw new NullPointerException(); modCount++; int i = size; if (i >= queue.length) grow(i + 1); siftUp(i, e); size = i + 1; return true; }
解读：
如果待插入元素 e 为 null，则抛出 NullPointerException 异常。
由于DelayQueue是无界队列，所以方法一直返回 true。
Note：
前述offer中 q.peek()方法返回的并不一定是当前添加的元素。
如果 q.peek()方法返回的是 e，则说明当前元素 e将是最先过期的，于是重置 leader线程为 null，进而激活 avaliable变量对应的条件队列里的一个线程，告诉它队列里面有元素了。
put方法的代码如下：
/** * Inserts the specified element into this delay queue. As the queue is * unbounded this method will never block. * * @param e the element to add * @throws NullPointerException {@inheritDoc} */ public void put(E e) { offer(e); }
解读：
此方法直接调用offer方法来实现。
获取元素
poll方法的代码如下：
/** * Retrieves and removes the head of this queue, or returns {@code null} * if this queue has no elements with an expired delay. * * @return the head of this queue, or {@code null} if this * queue has no elements with an expired delay */ public E poll() { final ReentrantLock lock = this.lock; lock.lock(); try { E first = q.peek(); return (first == null || first.getDelay(NANOSECONDS) > 0) ? null : q.poll(); } finally { lock.unlock(); } }
解读：
如果队列里面没有过期元素，则返回null；否则返回队首元素。
take方法的代码如下：
/** * Retrieves and removes the head of this queue, waiting if necessary * until an element with an expired delay is available on this queue. * * @return the head of this queue * @throws InterruptedException {@inheritDoc} */ public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { for (;;) { E first = q.peek(); if (first == null) available.await(); else { long delay = first.getDelay(NANOSECONDS); if (delay <= 0L) return q.poll(); first = null; // don't retain ref while waiting if (leader != null) available.await(); else { Thread thisThread = Thread.currentThread(); leader = thisThread; try { available.awaitNanos(delay); } finally { if (leader == thisThread) leader = null; } } } } } finally { if (leader == null && q.peek() != null) available.signal(); lock.unlock(); } }
解读：
如果队列里面没有过期元素，则等待。
示例
可以将延时队列DelayQueue运用在以下场景中：
（1）缓存系统的设计：可以用DelayQueue保存元素的有效期，使用一个线程循环查询DelayQueue，一旦能从DelayQueue中获取元素，则表示缓存有效期到了。
（2）定时任务调度：使用DelayQueue保存当天将要执行的任务和执行时间，一旦从DelayQueue中获取到任务就开始执行任务，比如TimerQueue就是使用DelayQueue实现的。
具体示例如下：
（1）Student对象作为DelayQueue的元素，其必须实现Delayed接口的两个方法
package com.test; import java.util.concurrent.Delayed; import java.util.concurrent.TimeUnit; public class Student implements Delayed {//必须实现Delayed接口 private String name; private long submitTime;// 交卷时间 private long workTime;// 考试时间 public Student(String name, long submitTime) { this.name = name; this.workTime = submitTime; this.submitTime = TimeUnit.NANOSECONDS.convert(submitTime, TimeUnit.MILLISECONDS) + System.nanoTime(); System.out.println(this.name + " 交卷，用时" + workTime); } public String getName() { return this.name + " 交卷，用时" + workTime; } //必须实现getDelay方法 public long getDelay(TimeUnit unit) { //返回一个延迟时间 return unit.convert(submitTime - System.nanoTime(), unit.NANOSECONDS); } //必须实现compareTo方法 public int compareTo(Delayed o) { Student that = (Student) o; return submitTime > that.submitTime ? 1 : (submitTime < that.submitTime ? -1 : 0); } }
（2）主线程程序
package com.test;
import java.util.concurrent.DelayQueue;
public class DelayQueueTest { public static void main(String[] args) throws Exception { // 新建一个等待队列 final DelayQueue<Student> bq = new DelayQueue<Student>(); for (int i = 0; i < 5; i++) { Student student = new Student("学生"+i,Math.round((Math.random()*10+i))); bq.put(student); // 将数据存到队列里！ } //获取但不移除此队列的头部；如果此队列为空，则返回 null。 System.out.println(bq.peek().getName()); } }
小结：
