ArrayBlockingQuque摘要

Posted 2020-10-22 六月风花雪

ArrayBlockingQuque

优势

1. 线程同步，线程安全
2. 对应空或满时，take\put操作将阻塞
3. 内部是一个数组，每个元素不会产生额外的处理对象，如Node

基于什么

• ReentrantLock
• Condition

解析

构造函数

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

• 默认创建一个不公平的锁，有利于吞吐量的提高
• 基于锁创建两个条件，notEmpty\notFull，操作这两个条件的前提必须是已经获取到锁，condition.await()会释放锁进行等待直到被唤醒或中断

非阻塞添加元素 offer(X)

添加元素offer(),offer其实是非阻塞的，添加失败直接放回false

  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();
    }
}

/**
 * Inserts element at current put position
 *  advances, and signals.
 * Call only when holding lock.
 */
private void enqueue(E x) {
    // assert lock.getHoldCount() == 1;
    // assert items[putIndex] == null;
    final Object[] items = this.items;
    items[putIndex] = x;
    if (++putIndex == items.length)
        putIndex = 0;
    count++;
    notEmpty.signal();
}

offer方法锁定代码块，因此enqueue的操作是安全的，在enque结束后，调用

notEmpty.signal()

唤醒notEmpty.await()状态中的线程，让它退出阻塞，尝试获取锁

阻塞添加元素put

 /**
 * Inserts the specified element at the tail of this
 *  queue, waiting
 * for space to become available if the queue is full.
 *
 * @throws InterruptedException {@inheritDoc}
 * @throws NullPointerException {@inheritDoc}
 */
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();
    }
}

可以发现，如果count == items.length = true，会一直调用notFull.await()方法，释放锁，且当前线程阻塞等待，直到条件不成立，并且此处await是没有指定超时时间的，意味着它需要被其他线程唤醒

阻塞取走元素take

 public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        while (count == 0)
            notEmpty.await();
        return dequeue();
    } finally {
        lock.unlock();
    }
}

take方法先获取到锁，锁定代码块，可以看见

 while (count == 0)
      notEmpty.await();

它的作用是如果没有可take的元素就await,await时释放锁，当前线程陷入睡眠，当offer方法被调用，notEpty.signal()被调用后，take方法的线程从沉睡中醒来，尝试重新获取锁，此时，count == 0 = false，程序进行dequeue

 /**
 * Extracts element at current take position
 * advances, and signals.
 * Call only when holding lock.
 */
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();
    return x;
}

在获取到元素后，调用notFull.signal()，唤醒阻塞与notFull.await的线程，告诉它可以往队列里面放数据了