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