Java中常用阻塞队列的问题小结

Java常用阻塞队列

ArrayBlockingQueue

内部由一个固定长度的数组来实现阻塞队列

/** The queued items */
final Object[] items;

/** items index for next take, poll, peek or remove */
int takeIndex;

/** items index for next put, offer, or add */
int putIndex;

public ArrayBlockingQueue(int capacity, boolean fair) {
    if (capacity <= 0)
        throw new IllegalArgumentException();
    /** 定长数组 */
    this.items = new Object[capacity]; 
    lock = new ReentrantLock(fair);
    notEmpty = lock.newCondition();
    notFull =  lock.newCondition();
}

提供了两个入队操作方法，offer()和put()
offer方法不会阻塞，但有返回值，如果队列满了，那么直接返回false，否则插入数据并返回true。

/**
 * Inserts the specified element at the tail of this queue if it is
 * possible to do so immediately without exceeding the queue's capacity,
 * returning {@code true} upon success and {@code false} if this queue
 * is full.  This method is generally preferable to method {@link #add},
 * which can fail to insert an element only by throwing an exception.
 *
 * @throws NullPointerException if the specified element is null
 */
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();
    }
}

put()会在队列满了的时候会阻塞生产者线程，知道有消费者线程消费后将其唤醒。

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();
    }
}

private E dequeue() {
    // assert lock.getHoldCount() == 1;
    // assert items[takeIndex] != null;
    final Object[] items = this.items;
    @SuppressWarnings("unchecked")
    E x = (E) items[takeIndex];
    items[takeIndex] = null;
    if (++takeIndex == items.length)
        takeIndex = 0;
    count--;
    if (itrs != null)
        itrs.elementDequeued();
    notFull.signal(); // 出队后唤醒生产者线程
    return x;
}

LinkedBlockingQueue

基于链表的阻塞队列，同ArrayListBlockingQueue类似，其内部也维持着一个数据缓冲队列（该队列由一个链表构成），当生产者往队列中放入一个数据时，队列会从生产者手中获取数据，并缓存在队列内部，而生产者立即返回；只有当队列缓冲区达到最大值缓存容量时，才会阻塞生产者队列，直到消费者从队列中消费掉一份数据，生产者线程会被唤醒，反之对于消费者这端的处理也基于同样的原理。

需要注意的是，如果构造一个LinkedBlockingQueue对象，而没有指定其容量大小，LinkedBlockingQueue会默认一个类似无限大小的容量（Integer.MAX_VALUE），这样的话，如果生产者的速度一旦大于消费者的速度，也许还没有等到队列满阻塞产生，系统内存就有可能已被消耗殆尽了。

/**
 * Creates a {@code LinkedBlockingQueue} with a capacity of
 * {@link Integer#MAX_VALUE}.
 */
public LinkedBlockingQueue() {
    this(Integer.MAX_VALUE);
}

/**
 * Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
 *
 * @param capacity the capacity of this queue
 * @throws IllegalArgumentException if {@code capacity} is not greater
 *         than zero
 */
public LinkedBlockingQueue(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
    last = head = new Node(null);
}

使用 BlockingQueue 实现生产者消费者问题

public class ProducerConsumer {

    private static BlockingQueue queue = new ArrayBlockingQueue<>(5);
    private static class Producer extends Thread {
        @Override
        public void run() {
            try {
                queue.put("product");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.print("produce..");
        }
    }
    private static class Consumer extends Thread {
                String product = queue.take();
            System.out.print("consume..");
}
public static void main(String[] args) {
    for (int i = 0; i < 2; i++) {
        Producer producer = new Producer();
        producer.start();
    for (int i = 0; i < 5; i++) {
        Consumer consumer = new Consumer();
        consumer.start();
    for (int i = 0; i < 3; i++) {
output：
produce..produce..consume..consume..produce..consume..produce..consume..produce..consume..

到此这篇关于Java常用阻塞队列的文章就介绍到这了,更多相关Java阻塞队列内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家！