手把手带你理解java线程池之工作队列workQueue

手把手带你理解java线程池之工作队列workQueue

目录ArrayBlockingQueueSynchronousQueueLinkedBlockingDequeLinkedBlockingQueueDelayDequeLinkedTransferQueuePriorityBlockingQueue

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。

图示：

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;

/**

* xArlY测试SynchronousQueue

*/

public class SynchronousQueueTest {

private static final SynchronousQueue synchronousQueue = new SynchronousQueue<>();

private static final ExecutorService service = Executors.newCachedThreadPool();

public static void main(String[] args) {

/**

* Provider

*/

service.submit(() -&ghttp://t; {

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 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 node = new Node(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 如图所示，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

示例

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 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());

}

}

}

result

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;

}

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