线程池之工作队列
ArrayBlockingQueue
采用数组来实现,并采用可重入锁ReentrantLock来做并发控制，无论是添加还是读取，都先要获得锁才能进行操作 可看出进行读写操作都使用了ReentrantLock，ArrayBlockingQueue需要为其指定容量
public boolean offer(E e) { checkNotNull(e); final ReentrantLock lock = this.lock; lock.lock(); try { if (count == items.length) return false; else { enqueue(e); return true; } } finally { lock.unlock(); } } public void put(E e) throws InterruptedException { checkNotNull(e); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == items.length) notFull.await(); enqueue(e); } finally { lock.unlock(); } }
SynchronousQueue
由于SynchronousQueue源码比较复杂，里面大量的Cas操作，SynchronousQueue没有容器，所以里面是装不了任务的，当一个生产者线程生产一个任务的 时候，如果没有对应的消费者消费，那么该生产者会一直阻塞，知道有消费者消费为止。
如下代码，如果我们将消费者线程注释掉执行，那么生产者哪里将会一直阻塞
package thread.customthreadpool; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.SynchronousQueue; import java.util.concurrent.ThreadPoolExecutor; /** * 测试SynchronousQueue */ public class SynchronousQueueTest { private static final SynchronousQueue<String> synchronousQueue = new SynchronousQueue<>(); private static final ExecutorService service = Executors.newCachedThreadPool(); public static void main(String[] args) { /** * Provider */ service.submit(() -> { try { synchronousQueue.put("liu"); }catch (Exception e){ e.printStackTrace(); } System.out.println("Consumer finished spending"); }); /** * Consumer */ service.submit(() ->{ try { synchronousQueue.take(); }catch (Exception e){ e.printStackTrace(); } System.out.println("take over"); }); } }
LinkedBlockingDeque
LinkedBlockingDeque是一个双向队列，底层使用单链表实现，任何一段都可进行元素的读写操作，在初始化LinkedBlockingDeque的时候， 我们可以指定容量，也可不指定，如果不指定，则容量为Integer.MAX_VALUE，
注：Deque是双端队列，而Queue是单端队列，双端意思是两端都可以进行读写操作，而单端则只能从一端进，一端出（FIFO）
public LinkedBlockingDeque() { this(Integer.MAX_VALUE); }
package thread.customthreadpool; import java.util.concurrent.LinkedBlockingDeque; public class LinkedBlockingDequeTest { private static final LinkedBlockingDeque<Integer> deque = new LinkedBlockingDeque<>(); public static void main(String[] args) throws InterruptedException { deque.put(1); deque.put(2); deque.put(3); deque.put(4); deque.put(5); System.out.println(deque); System.out.println("deque size "+deque.size()); deque.take(); deque.take(); deque.take(); deque.take(); deque.take(); System.out.println(deque); System.out.println("deque size "+deque.size()); } }
LinkedBlockingQueue
底层基于单向连表实现，是一个单向队列，具有先进先出(FIFO)特点，使用了ReentrantLock来做并发控制,读写操作都上锁
private final ReentrantLock putLock = new ReentrantLock(); public void put(E e) throws InterruptedException { if (e == null) throw new NullPointerException(); int c = -1; Node<E> node = new Node<E>(e); final ReentrantLock putLock = this.putLock; final AtomicInteger count = this.count; putLock.lockInterruptibly(); try { while (count.get() == capacity) { notFull.await(); } enqueue(node); c = count.getAndIncrement(); if (c + 1 < capacity) notFull.signal(); } finally { putLock.unlock(); } if (c == 0) signalNotEmpty(); } public E take() throws InterruptedException { E x; int c = -1; final AtomicInteger count = this.count; final ReentrantLock takeLock = this.takeLock; takeLock.lockInterruptibly(); try { while (count.get() == 0) { notEmpty.await(); } x = dequeue(); c = count.getAndDecrement(); if (c > 1) notEmpty.signal(); } finally { takeLock.unlock(); } if (c == capacity) signalNotFull(); return x; }
DelayDeque
DelayDeque是一个无界队列，添加进DelayDeque的元素会经过compareTo方法计算，然后按照时间 进行排序，排在队头的元素是最早到期的，越往后到期时间越长，DelayDeque只能接受Delayed接口类型，队列里的元素并不是按照先进先出的规则，而是按照过期时间
package thread.customthreadpool.delayDeque; import java.util.concurrent.Delayed; import java.util.concurrent.TimeUnit; public class MyDelayed implements Delayed { private final String taskName ; private final long nowTime = System.currentTimeMillis(); private final long expireTime ; public MyDelayed(String taskName,long expireTime) { this.taskName = taskName; this.expireTime = expireTime; } @Override public long getDelay(TimeUnit unit) { return unit.convert((nowTime+expireTime) - System.currentTimeMillis(),TimeUnit.MILLISECONDS); } @Override public int compareTo(Delayed o) { MyDelayed myDelayed = (MyDelayed) o; return (int) (this.getDelay(TimeUnit.MILLISECONDS) - o.getDelay(TimeUnit.MILLISECONDS)); } @Override public String toString() { return "MyDelayed{" + "taskName='" + taskName + '\'' + ", nowTime=" + nowTime + ", expireTime=" + expireTime + '}'; } }
package thread.customthreadpool.delayDeque; import java.util.concurrent.*; public class MyDelayQueue { private static final DelayQueue<MyDelayed> delayQueue = new DelayQueue<>(); private static final ExecutorService service = Executors.newCachedThreadPool(); public static void main(String[] args) throws InterruptedException { service.submit(() -> { delayQueue.put(new MyDelayed("A-Task",5000)); delayQueue.put(new MyDelayed("B-Task",4000)); delayQueue.put(new MyDelayed("C-Task",3000)); delayQueue.put(new MyDelayed("D-Task",2000)); delayQueue.put(new MyDelayed("E-Task",1000)); }); while (true){ System.out.println(delayQueue.take()); } } }
应用场景
1.美团外卖订单：当我们下单后没付款 ，30分钟后将自动取消订单
2.缓存，对于某些任务，需要在特定的时间清理；
and so on
LinkedTransferQueue
当消费线程从队列中取元素时，如果队列为空，那么生成一个为null的节点，消费者线程就一直等待，此时如果生产者线程发现队列中有一个null节点， 它就不入队了，而是将元素填充到这个null节点并唤醒消费者线程，然后消费者线程取走元素。
LinkedTransferQueue是 SynchronousQueue 和 LinkedBlockingQueue 的整合，性能比较高，因为没有锁操作， SynchronousQueue不能存储元素，而LinkedTransferQueue能存储元素，
PriorityBlockingQueue
PriorityBlockingQueue是一个无界的阻塞队列，同时是一个支持优先级的队列，读写操作都是基于ReentrantLock, 内部使用堆算法保证每次出队都是优先级最高的元素
public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); E result; try { while ( (result = dequeue()) == null) notEmpty.await(); } finally { lock.unlock(); } return result; }
