ArrayBlockingQueue

基于数组的有界BlockingQueue。队列的头节点是最早入队的节点

ArrayBlockingQueue核心源码解读

public class ArrayBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    // 队列中元素数组
    final Object[] items;
    // 从队列中获取或取走元素的索引位置
    int takeIndex;
    // 从队列中插入元素的索引位置
    int putIndex;
    // 队列中的元素个数
    int count;
    // 队列中的所有操作的锁
    final ReentrantLock lock;
    // 从队列中取走元素的条件
    private final Condition notEmpty;
    // 往队列中插入元素的条件
    private final Condition notFull;
    // i--
    final int dec(int i) {
        return ((i == 0) ? items.length : i) - 1;
    }
    // 随机访问元素
    @SuppressWarnings("unchecked")
    final E itemAt(int i) {
        return (E) items[i];
    }

    // 入队操作，只能在持有锁的情况下可以调用此方法
    private void enqueue(E x) {
        final Object[] items = this.items;
        // 将元素放入队列
        items[putIndex] = x;
        // putIndex++，如果putIndex等于队列的容量，则重置putIndex = 0的位置
        if (++putIndex == items.length)
            putIndex = 0;
        // 队列中元素个数 + 1
        count++;
        // 唤醒有出队操作的线程，可以取出元素
        notEmpty.signal();
    }
    // 出队操作，只能在持有锁的情况下可以调用此方法
    private E dequeue() {
        final Object[] items = this.items;
        @SuppressWarnings("unchecked")
        E x = (E) items[takeIndex];
        // 将队列中的元素删除
        items[takeIndex] = null;
        // takeIndex++，如果takeIndex等于队列的容量，则重置takeIndex = 0的位置
        if (++takeIndex == items.length)
            takeIndex = 0;
        // 队列中元素个数 - 1
        count--;
        if (itrs != null)
            itrs.elementDequeued();
        // 唤醒有入队操作的线程，可以插入元素了。
        notFull.signal();
        return x;
    }

    // 删除指定位置的元素，注意只能在持有锁的操作可以调用此方法
    // 此方法效率低，通常不建议使用，因为它需要移动元素
    void removeAt(final int removeIndex) {
        final Object[] items = this.items;
        // 如果要删除的元素在队列的头节点，那么和take、poll或remove方法逻辑一样
        if (removeIndex == takeIndex) {
            // 删除元素
            items[takeIndex] = null;
            // 如果takeIndex等队列容量，则将takeIndex重置0
            if (++takeIndex == items.length)
                takeIndex = 0;
            // 队列中的元素个数 - 1
            count--;
            if (itrs != null)
                itrs.elementDequeued();
        } else {
            // 下次插入队列的索引位置
            final int putIndex = this.putIndex;
            // 从删除元素的索引位置遍历直到putIndex位置
            for (int i = removeIndex;;) {
                int next = i + 1;
                // 如果到队列的未尾，将索引位置置0
                if (next == items.length)
                    next = 0;
                // 如果不等于putIndex，将元素移动到前一个位置
                if (next != putIndex) {
                    items[i] = items[next];
                    i = next;
                } else { // 如果等于putIndex，移动元素完毕
                    // 将i位置的元素置null
                    items[i] = null;
                    // putIndex指向i
                    this.putIndex = i;
                    break;
                }
            }
            // 队列中的元素个数 - 1
            count--;
            if (itrs != null)
                itrs.removedAt(removeIndex);
        }
        notFull.signal();
    }
    // 构造指定容量
    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }

    // 构造指定容量，可以设置采用公平或非公平锁的策略
    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        // 创建Object数组
        this.items = new Object[capacity];
        // 创建ReentrantLock对象
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }
    
    public ArrayBlockingQueue(int capacity, boolean fair,
                              Collection<? extends E> c) {
        this(capacity, fair);

        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();
            }
            count = i;
            putIndex = (i == capacity) ? 0 : i;
        } finally {
            lock.unlock();
        }
    }

    // 插入元素，如果队列已满，抛异常
    public boolean add(E e) {
        return super.add(e);
    }

    // 插入元素，如果队列已满，则返回false
    public boolean offer(E e) {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lock(); // 获取锁，此时会阻塞
        try {
            // 队列已满，直接返回false
            if (count == items.length)
                return false;
            else { // 入队操作
                enqueue(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }

    // 插入元素，如果队列已满，则进入阻塞状态。支持中断
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 如果队列已满
            while (count == items.length)
                notFull.await(); // 线程阻塞
            // 入队
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

    // 插入元素，如果在指定的超时时间内没有插入成功，则返回false
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {

        checkNotNull(e);
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 如果队列已满
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos); // 线程阻塞指定的时间
            }
            // 入队
            enqueue(e);
            return true;
        } finally {
            lock.unlock();
        }
    }

    // 从队列中拿走元素，如果队列已空，则返回null
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            // 如果队列已空返回null，否则出队操作
            return (count == 0) ? null : dequeue();
        } finally {
            lock.unlock();
        }
    }

    // 从队列中拿走元素，如果队列已空，则该线程阻塞
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 队列为空
            while (count == 0)
                notEmpty.await(); // 线程阻塞
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

    // 从队列中拿走元素，如果队列已空，则该线程阻塞指定的超时时间，如果在指定时间没有拿到元素，则返回null
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            // 队列为空
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos); // 线程阻塞指定时间
            }
            return dequeue();
        } finally {
            lock.unlock();
        }
    }
    
    // 从队列头部获取元素，如果为空则返回null
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return itemAt(takeIndex); // null when queue is empty
        } finally {
            lock.unlock();
        }
    }

    // 队列的元素个数
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }

    // 队列中剩余的容量大小
    public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }

    public boolean remove(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    if (o.equals(items[i])) {
                        removeAt(i);
                        return true;
                    }
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    public boolean contains(Object o) {
        if (o == null) return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            if (count > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    if (o.equals(items[i]))
                        return true;
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
            }
            return false;
        } finally {
            lock.unlock();
        }
    }

    public Object[] toArray() {
        Object[] a;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final int count = this.count;
            a = new Object[count];
            int n = items.length - takeIndex;
            if (count <= n)
                System.arraycopy(items, takeIndex, a, 0, count);
            else {
                System.arraycopy(items, takeIndex, a, 0, n);
                System.arraycopy(items, 0, a, n, count - n);
            }
        } finally {
            lock.unlock();
        }
        return a;
    }

    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final int count = this.count;
            final int len = a.length;
            if (len < count)
                a = (T[])java.lang.reflect.Array.newInstance(
                    a.getClass().getComponentType(), count);
            int n = items.length - takeIndex;
            if (count <= n)
                System.arraycopy(items, takeIndex, a, 0, count);
            else {
                System.arraycopy(items, takeIndex, a, 0, n);
                System.arraycopy(items, 0, a, n, count - n);
            }
            if (len > count)
                a[count] = null;
        } finally {
            lock.unlock();
        }
        return a;
    }

    public String toString() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int k = count;
            if (k == 0)
                return "[]";

            final Object[] items = this.items;
            StringBuilder sb = new StringBuilder();
            sb.append('[');
            for (int i = takeIndex; ; ) {
                Object e = items[i];
                sb.append(e == this ? "(this Collection)" : e);
                if (--k == 0)
                    return sb.append(']').toString();
                sb.append(',').append(' ');
                if (++i == items.length)
                    i = 0;
            }
        } finally {
            lock.unlock();
        }
    }

    public void clear() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int k = count;
            if (k > 0) {
                final int putIndex = this.putIndex;
                int i = takeIndex;
                do {
                    items[i] = null;
                    if (++i == items.length)
                        i = 0;
                } while (i != putIndex);
                takeIndex = putIndex;
                count = 0;
                if (itrs != null)
                    itrs.queueIsEmpty();
                for (; k > 0 && lock.hasWaiters(notFull); k--)
                    notFull.signal();
            }
        } finally {
            lock.unlock();
        }
    }

    public int drainTo(Collection<? super E> c) {
        return drainTo(c, Integer.MAX_VALUE);
    }

    public int drainTo(Collection<? super E> c, int maxElements) {
        checkNotNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = Math.min(maxElements, count);
            int take = takeIndex;
            int i = 0;
            try {
                while (i < n) {
                    @SuppressWarnings("unchecked")
                    E x = (E) items[take];
                    c.add(x);
                    items[take] = null;
                    if (++take == items.length)
                        take = 0;
                    i++;
                }
                return n;
            } finally {
                // Restore invariants even if c.add() threw
                if (i > 0) {
                    count -= i;
                    takeIndex = take;
                    if (itrs != null) {
                        if (count == 0)
                            itrs.queueIsEmpty();
                        else if (i > take)
                            itrs.takeIndexWrapped();
                    }
                    for (; i > 0 && lock.hasWaiters(notFull); i--)
                        notFull.signal();
                }
            }
        } finally {
            lock.unlock();
        }
    }
}