多线程-BlockingQueue,Array[Linked]BlockingQueue,DelayQueue,PriorityBlockingQueue,SynchronousQueue

▼魔方 西西 提交于 2020-03-22 22:25:15

 

 

阻塞场景 
BlockingQueue阻塞队列,阻塞的情况主要有如下2种: 
1. 当队列满了,进行入队操作阻塞 
2. 当队列空了,进行出队操作阻塞 
阻塞队列主要用在生产者/消费者模式中,下图展示了一个线程生产,一个线程消费的场景: 

BlockingQueue接口 

操作抛异常特殊值阻塞超时
插入 add(o) offer(o) put(o) offer(o,timeout,unit)
删除 remove(o) poll() take() poll(timeout,unit)

1. 抛出异常:如果操作不能马上进行,则抛出异常。 
2. 特殊值:如果操作不能马上进行,将会返回一个特殊的值,一般是true/false。 
3. 阻塞:如果操作不能马上进行,操作会阻塞。 
4. 超时:如果操作不能马上进行,操作会阻塞指定时间,如果指定时间没执行,则返回一个特殊值,一般为true/false。 
不能向BlockingQueue中插入null.否则会报NullPointerException异常。 
BlockingQueue子类 
由以上的图片可以知道BlockingQueue具有如下的几个子类: 
1. ArrayBlockingQueue 
2. DelayQueue 
3. LinkedBlockingQueue 
4. PriorityBlockingQueue 
5. SynchronousQueue

ArrayBlockingQueue 
一个有边界的(容量是有限的)阻塞队列,它的内部实现是一个数组。必须在初始化的时候指定它的容量大小,容量大小一旦指定就不可改变。ArrayBlockingQueue是以先进先出的方式存储数据,最新插入的对象是尾部,最新移除的对象是头部。

public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    /** The queued items */
    final Object[] items;
    public ArrayBlockingQueue(int capacity)
    public ArrayBlockingQueue(int capacity, boolean fair)
    public ArrayBlockingQueue(int capacity, boolean fair, Collection<? extends E> c)
}

LinkedBlockingQueue 
队列大小的配置是可选的,如果我们初始时指定一个大小,它就是有边界的,如果不指定它就是无边界采用默认值Integer.MAX_VALUE的容量,它的内部是一个链表。

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    public LinkedBlockingQueue();
    public LinkedBlockingQueue(int capacity);
    public LinkedBlockingQueue(Collection<? extends E> c);
}

PriorityBlockingQueue 
是一个没有边界的队列,它的排序规则和ProrityQueue一样,需要注意,PriorityBlockingQueue中允许插入null对象。 
所有插入PriorityBlockingQueue队列的对象必须实现Comparable接口,队列优先级的排序规则就是按照我们队这个接口的实现来定义的。 
另外PriorityBlockingQueue可以获得一个Iterator,但是这个迭代器并不保证按照优先级顺序进行迭代。

package org.github.lujiango;

import java.util.Iterator;
import java.util.Random;
import java.util.concurrent.PriorityBlockingQueue;

class PriorityElement implements Comparable<PriorityElement> {
    private int priority;

    public PriorityElement(int priority) {
        this.priority = priority;
    }

    @Override
    public int compareTo(PriorityElement o) {

        return priority >= o.priority ? 1 : -1;
    }

    @Override
    public String toString() {
        return "PriorityElement[priority= " + priority + "]";
    }

}

public class Test13 {

    public static void main(String[] args) throws InterruptedException {
        PriorityBlockingQueue<PriorityElement> queue = new PriorityBlockingQueue<PriorityElement>();
        for (int i = 0; i < 5; i++) {
            Random rand = new Random();
            PriorityElement ele = new PriorityElement(rand.nextInt(10));
            queue.put(ele);
        }
        System.out.println("Iterator----------------");
        Iterator<PriorityElement> it = queue.iterator();
        while (it.hasNext()) {
            System.out.println(it.next());
        }
        System.out.println("PriorityBlockingQueue.tak()-----------");
        while (!queue.isEmpty()) {
            System.out.println(queue.take());
        }
    }
}

SynchronousQueue 
内部仅仅容纳一个元素,当一个线程插入一个元素之后就被阻塞(放入元素的线程立刻被阻塞),除非这个元素被另一个线程消费。

package org.github.lujiango;

import java.util.concurrent.SynchronousQueue;
import java.util.concurrent.TimeUnit;

class MyThread1 implements Runnable {
    private SynchronousQueue<String> queue;

    public MyThread1(SynchronousQueue<String> queue) {
        this.queue = queue;
    }

    @Override
    public void run() {

        try {
            TimeUnit.SECONDS.sleep(3);
            System.out.println("take a from queue...");
            queue.take();
        } catch (InterruptedException e) {
        }
    }
}

public class Test14 {

    public static void main(String[] args) throws InterruptedException {
        SynchronousQueue<String> queue = new SynchronousQueue<String>();
        Thread t = new Thread(new MyThread1(queue));
        t.start();
        System.out.println("put a  into queue...");
        queue.put("a");

    }

}

DelayQueue 
DelayQueue阻塞的是其内部元素,DelayQueue中的元素必须实现Delayed接口。

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);
}

getDelay()返回值就是队列元素被释放前的存活时间,如果返回<=0,就意味着该元素已经到期需要被释放,此时DelayedQueue会通过其take()方法释放此对象,如无可释放(超期元素)元素,则take方法会阻塞。

package org.github.lujiango;

import java.util.concurrent.DelayQueue;
import java.util.concurrent.Delayed;
import java.util.concurrent.TimeUnit;

class DelayedElement implements Delayed {
    private long expired;
    private long delay;
    private String name;

    public DelayedElement(String name, long delay) {
        this.name = name;
        this.delay = delay;
        this.expired = (delay + System.currentTimeMillis());
    }

    @Override
    public int compareTo(Delayed o) {
        DelayedElement cache = (DelayedElement) o;
        return cache.expired > expired ? 1 : -1;
    }

    @Override
    public long getDelay(TimeUnit unit) {
        return (expired - System.currentTimeMillis());
    }

    @Override
    public String toString() {
        return "DelayedElement[delay=" + delay + ",name=" + name + "]";
    }

}

public class Test15 {

    public static void main(String[] args) throws InterruptedException {
        DelayQueue<Delayed> queue = new DelayQueue<Delayed>();
        DelayedElement ele = new DelayedElement("3s", 3000);
        queue.put(ele);
        System.out.println(queue.take());
    }

}

DelayQueue的应用场景很多,比如定时关闭连接,缓存对象,超时处理等各种场景。

 

易学教程内所有资源均来自网络或用户发布的内容,如有违反法律规定的内容欢迎反馈
该文章没有解决你所遇到的问题?点击提问,说说你的问题,让更多的人一起探讨吧!