Java阻塞队列

时间:2022-05-23 码出地球人气:0

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<E>(null);
}

使用 BlockingQueue 实现生产者消费者问题

public class ProducerConsumer {

    private static BlockingQueue<String> 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..

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