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| 1 | +# DelayQueue |
| 2 | +>DelayQueue（**基于PriorityQueue来实现的**）是一个存放Delayed 元素的无界阻塞队列，只有在延迟期满时才能从中提取元素。该队列的头部是延迟期满后保存时间最长的 Delayed 元素。如果延迟都还没有期满，则队列没有头部，并且poll将返回null。当一个元素的 getDelay(TimeUnit.NANOSECONDS) 方法返回一个小于或等于零的值时，则出现期满，poll就以移除这个元素了。此队列不允许使用 null 元素。 |
| 3 | +>DelayQueue中存储的是Delayed接口的实现 |
| 4 | +
|
| 5 | +## Delayed |
| 6 | +```Java |
| 7 | +public interface Delayed extends Comparable<Delayed> { |
| 8 | + |
| 9 | + /** |
| 10 | + * Returns the remaining delay associated with this object, in the |
| 11 | + * given time unit. |
| 12 | + * |
| 13 | + * @param unit the time unit |
| 14 | + * @return the remaining delay; zero or negative values indicate |
| 15 | + * that the delay has already elapsed |
| 16 | + */ |
| 17 | + long getDelay(TimeUnit unit); |
| 18 | +} |
| 19 | +``` |
| 20 | +我们要实现getDelay()方法，返回还需要等待的时间。 |
| 21 | +同时Delayed接口继承了Comparable方法，所以要实现compareTo方法（用于在优先级队列中排序）。 |
| 22 | + |
| 23 | +### 一个Delayed接口的实现类 |
| 24 | +```Java |
| 25 | +public class DelayMsg<V> implements Delayed{ |
| 26 | + private Long expire; //用于存储什么时候当前对象可以生效。 |
| 27 | + private Long insertTime; //当前对象插入队列的时间，用于优先级排列，基于延迟的基础上的FIFO |
| 28 | + private V v; |
| 29 | + public V getV() { |
| 30 | + return v; |
| 31 | + } |
| 32 | + public Long getInsertTime() { |
| 33 | + return insertTime; |
| 34 | + } |
| 35 | + public DelayMsg(Long expire){ |
| 36 | + this.expire = expire; |
| 37 | + } |
| 38 | + @Override |
| 39 | + public int compareTo(Delayed o) { |
| 40 | + Long delta = insertTime - ((DelayMsg)o).getInsertTime(); |
| 41 | + if(delta > 0) return 1; |
| 42 | + else if(delta == 0) return 0; |
| 43 | + else |
| 44 | + return -1; |
| 45 | + } |
| 46 | + |
| 47 | + /* (non-Javadoc) |
| 48 | + * @see java.util.concurrent.Delayed#getDelay(java.util.concurrent.TimeUnit) |
| 49 | + * 用于定义延时策略,根据需求定义。 |
| 50 | + */ |
| 51 | + @Override |
| 52 | + public long getDelay(TimeUnit unit) { |
| 53 | + return unit.convert(this.expire - System.currentTimeMillis(), TimeUnit.MILLISECONDS);//判断是否当前时间已经超过了生效时间 |
| 54 | + } |
| 55 | +} |
| 56 | +``` |
| 57 | + |
| 58 | +## DelayQueue的API |
| 59 | +### 添加元素 |
| 60 | +* 因为是无界队列，所以不存在插入元素时的阻塞。 |
| 61 | +* put() 通过offer()方法实现。 |
| 62 | +```Java |
| 63 | + public void put(E e) { |
| 64 | + offer(e); |
| 65 | + } |
| 66 | +``` |
| 67 | + |
| 68 | +* add() 通过offer()方法实现。 |
| 69 | +```Java |
| 70 | + public boolean add(E e) { |
| 71 | + return offer(e); |
| 72 | + } |
| 73 | +``` |
| 74 | + |
| 75 | +* offer() 事实上offer方法调用了priorityQueue的插入元素方法 |
| 76 | +```Java |
| 77 | + public boolean offer(E e) { |
| 78 | + final ReentrantLock lock = this.lock; |
| 79 | + lock.lock(); |
| 80 | + try { |
| 81 | + q.offer(e); |
| 82 | + if (q.peek() == e) { //说明当前队列中只有一个元素，解除阻塞。 |
| 83 | + leader = null; |
| 84 | + available.signal(); |
| 85 | + } |
| 86 | + return true; |
| 87 | + } finally { |
| 88 | + lock.unlock(); |
| 89 | + } |
| 90 | + } |
| 91 | +``` |
| 92 | + |
| 93 | +### 读取元素 |
| 94 | +* poll() 返回根节点，如果根节点为空或根节点未完成其延时要求，返回null，非阻塞 |
| 95 | +```Java |
| 96 | + public E poll() { |
| 97 | + final ReentrantLock lock = this.lock; |
| 98 | + lock.lock(); |
| 99 | + try { |
| 100 | + E first = q.peek(); //取出根节点 |
| 101 | + if (first == null || first.getDelay(NANOSECONDS) > 0) |
| 102 | + return null; |
| 103 | + else |
| 104 | + return q.poll();//完成延时的要求，从优先队列中取出根节点并返回。 |
| 105 | + } finally { |
| 106 | + lock.unlock(); |
| 107 | + } |
| 108 | + } |
| 109 | +``` |
| 110 | + |
| 111 | +* take() |
| 112 | +>leader-follower |
| 113 | +>所有线程会有三种身份中的一种：leader和 follower，以及一个干活中的状态：proccesser。它的基本原则就是，永远最多只有一个leader。而所有follower都在等待成为 leader。线程池启动时会自动产生一个Leader负责等待网络IO事件，当有一个事件产生时，Leader线程首先通知一个Follower线程将 其提拔为新的Leader，然后自己就去干活了，去处理这个网络事件，处理完毕后加入Follower线程等待队列，等待下次成为Leader。这种方法 可以增强CPU高速缓存相似性，及消除动态内存分配和线程间的数据交换。 |
| 114 | +
|
| 115 | +```Java |
| 116 | + public E take() throws InterruptedException { |
| 117 | + final ReentrantLock lock = this.lock; |
| 118 | + lock.lockInterruptibly(); |
| 119 | + try { |
| 120 | + for (;;) { |
| 121 | + E first = q.peek(); |
| 122 | + if (first == null) //判断队列中是否有元素 |
| 123 | + available.await(); |
| 124 | + else { |
| 125 | + long delay = first.getDelay(NANOSECONDS);//如果有元素，则判断有没有完成延时要求 |
| 126 | + if (delay <= 0) |
| 127 | + return q.poll(); //完成延时要求，返回元素 |
| 128 | + first = null; // don't retain ref while waiting |
| 129 | + if (leader != null) //如果leader不为空，则继续进入等待 |
| 130 | + available.await(); |
| 131 | + else { //如果leader为空，则将当前处理的线程提升为leader并继续进入等待 |
| 132 | + Thread thisThread = Thread.currentThread(); |
| 133 | + leader = thisThread; |
| 134 | + try { |
| 135 | + available.awaitNanos(delay); |
| 136 | + } finally { |
| 137 | + if (leader == thisThread) |
| 138 | + leader = null; //完成所有操作后，将leader置空 |
| 139 | + } |
| 140 | + } |
| 141 | + } |
| 142 | + } |
| 143 | + } finally { |
| 144 | + if (leader == null && q.peek() != null) |
| 145 | + available.signal(); |
| 146 | + lock.unlock(); |
| 147 | + } |
| 148 | + } |
| 149 | +``` |
| 150 | + |
| 151 | +### Test |
| 152 | +我们创建10个元素放入延迟队列中，设置延时10s，消费者通过while（true）不断读取。 |
| 153 | +* DelayQueueProducer |
| 154 | +```Java |
| 155 | +public class DelayQueueProducer implements Runnable { |
| 156 | + private DelayQueue<DelayMsg<Integer>> q; |
| 157 | + public DelayQueueProducer(DelayQueue<DelayMsg<Integer>> q) { |
| 158 | + super(); |
| 159 | + this.q = q; |
| 160 | + } |
| 161 | + @Override |
| 162 | + public void run() { |
| 163 | + DateFormat df = new SimpleDateFormat("HH:mm:ss"); |
| 164 | + for(int i = 0; i < 10; i++){ //创建10个元素放入延迟队列 |
| 165 | + q.offer(new DelayMsg<Integer>(System.currentTimeMillis() + 10000, System.currentTimeMillis(), i)); |
| 166 | + System.out.println(df.format(new Date(System.currentTimeMillis())) + ": Producer: put " + i + " into queue..."); |
| 167 | + try { |
| 168 | + Thread.sleep(1); |
| 169 | + } catch (InterruptedException e) { |
| 170 | + e.printStackTrace(); |
| 171 | + } |
| 172 | + } |
| 173 | + } |
| 174 | +} |
| 175 | +``` |
| 176 | +* DelayQueueConsumer一直循环读取元素 |
| 177 | +```Java |
| 178 | +public class DelayQueueConsumer implements Runnable { |
| 179 | + private DelayQueue<DelayMsg<Integer>> q; |
| 180 | + public DelayQueueConsumer(DelayQueue<DelayMsg<Integer>> q) { |
| 181 | + super(); |
| 182 | + this.q = q; |
| 183 | + } |
| 184 | + @Override |
| 185 | + public void run() { |
| 186 | + DateFormat df = new SimpleDateFormat("HH:mm:ss"); |
| 187 | + while (true) { |
| 188 | + try { |
| 189 | + System.out.println(df.format(new Date(System.currentTimeMillis())) + ": Consumer: take " + ((DelayMsg<Integer>)q.take()).getV() + " from queue..."); |
| 190 | + Thread.sleep(1); |
| 191 | + } catch (InterruptedException e) { |
| 192 | + e.printStackTrace(); |
| 193 | + } |
| 194 | + } |
| 195 | + } |
| 196 | + public static void main(String[] args) { |
| 197 | + DelayQueue<DelayMsg<Integer>> q = new DelayQueue<>(); |
| 198 | + new Thread(new DelayQueueConsumer(q)).start(); |
| 199 | + new Thread(new DelayQueueProducer(q)).start(); |
| 200 | + System.out.println("Main finish..."); |
| 201 | + } |
| 202 | +} |
| 203 | +``` |
| 204 | +结果 |
| 205 | +Main finish... |
| 206 | +15:54:00: Producer: put 0 into queue... |
| 207 | +15:54:00: Producer: put 1 into queue... |
| 208 | +15:54:00: Producer: put 2 into queue... |
| 209 | +15:54:00: Producer: put 3 into queue... |
| 210 | +15:54:00: Producer: put 4 into queue... |
| 211 | +15:54:00: Producer: put 5 into queue... |
| 212 | +15:54:00: Producer: put 6 into queue... |
| 213 | +15:54:00: Producer: put 7 into queue... |
| 214 | +15:54:00: Producer: put 8 into queue... |
| 215 | +15:54:00: Producer: put 9 into queue... |
| 216 | +15:54:00: Consumer: take 0 from queue... |
| 217 | +15:54:10: Consumer: take 1 from queue... |
| 218 | +15:54:10: Consumer: take 2 from queue... |
| 219 | +15:54:10: Consumer: take 3 from queue... |
| 220 | +15:54:10: Consumer: take 4 from queue... |
| 221 | +15:54:10: Consumer: take 5 from queue... |
| 222 | +15:54:10: Consumer: take 6 from queue... |
| 223 | +15:54:10: Consumer: take 7 from queue... |
| 224 | +15:54:10: Consumer: take 8 from queue... |
| 225 | +15:54:10: Consumer: take 9 from queue... |
| 226 | +>延迟10s才从队列中取出元素，之后因为队列为空开始阻塞。 |
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