java多线程-阻塞队列BlockingQueue

大纲

1. BlockingQueue接口
2. ArrayBlockingQueue

一、BlockingQueue接口

public interface BlockingQueue<E> extends Queue<E> {
    //add、offer向队列插值，返回插入是否成功
    boolean add(E e);
    boolean offer(E e);
    //向队列插值，队列满则阻塞至队列非满
    void put(E e) throws InterruptedException;
    //offer方法加上超时，超时返回false
    boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException;
    //从队列拿值，队列空则阻塞至队列非空
    E take() throws InterruptedException;
    //向队列拿值，返回是否成功，超时返回false
    E poll(long timeout, TimeUnit unit) throws InterruptedException;
    //队列剩余空间
    int remainingCapacity();
    //删除
    boolean remove(Object o);
    //是否包含
    boolean contains(Object o);
    //移除队列中的值到集合中
    int drainTo(Collection<? super E> c);
    //maxElements移除最大个数
    int drainTo(Collection<? super E> c, int maxElements);
}

二、ArrayBlockingQueue

主要成员变量

    /** 队列 */
    final Object[] items;

    /** 拿值的索引，用于 take, poll, peek, remove 方法*/
    int takeIndex;

    /** 存值的索引, 用于 next put, offer, or, add 方法*/
    int putIndex;

    /** 队列中元素数量 */
    int count;

    /** 锁，两个condition都是由这个锁创建 */
    final ReentrantLock lock;

    /** 可以拿值的Condition(不空可拿) */
    private final Condition notEmpty;

    /** 可以存值的Condition(不满可存) */
    private final Condition notFull;

构造函数

    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    public ArrayBlockingQueue(int capacity, boolean fair) {
        //初始化锁与condition，默认非公平所
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);
        //将集合中元素拷贝至队列
        final ReentrantLock lock = this.lock;
        lock.lock(); // Lock only for visibility, not mutual exclusion
        try {
            int i = 0;
            try {
                for (E e : c) {
                    checkNotNull(e);
                    items[i++] = e;
                }
            } catch (ArrayIndexOutOfBoundsException ex) {
                throw new IllegalArgumentException();
            }
            //队列数量等于集合数量
            count = i;
            //存索引赋值：队列满赋0，不满赋集合数量
            putIndex = (i == capacity) ? 0 : i;
        } finally {
            lock.unlock();
        }
    }

主要方法

取：

　　//非阻塞
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }

    //阻塞
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0)
                //队列大小为0拿值线程阻塞
                notEmpty.await();
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    //出队
    private E dequeue() {
        final Object[] items = this.items;
        E x = (E) items[takeIndex];
        items[takeIndex] = null;
        //对应putindex
        if (++takeIndex == items.length)
            takeIndex = 0;
        count--;
        if (itrs != null)
            itrs.elementDequeued();
        //唤醒存值线程
        notFull.signal();
        return x;
    }

存：

//入队
    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();
    }
    //非阻塞
    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();
        }
    }
    //阻塞
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            //队列满则阻塞
            while (count == items.length)
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

阻塞队列阻塞的存取值方法通过2个condition的await和signal方法完成：当队列空时notEmpty.await取值阻塞，当队列满时notFull.await存值阻塞；入队操作完成时调用notEmpty.signal唤醒取值线程，出队才做完成时唤醒存值线程。