Lock和Condition在JDK中ArrayBlockingQueue的应用

ArrayBlockingQueue的实现思路简单描述，ArrayBlockingQueue的底对于互斥访问使用的一个锁。细节参考源码take和put方法：

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.*;
import java.util.*;
 
public class ArrayBlockingQueue<E> extends AbstractQueue<E> implements BlockingQueue<E>, java.io.Serializable {
 
    /** The queued items */
    final Object[] items;//证明了java泛型是语法级别的泛型。
 
    /** items index for next take, poll, peek or remove */
    int takeIndex;
 
    /** items index for next put, offer, or add */
    int putIndex;
 
    // 可以使用一个int变量计数原因是：ArrayBlockingQueue的实现只有一把锁，对count访问时候都是在锁的保护机制下实现互斥的。
    /** Number of elements in the queue */
    int count;
 
    /*
     * Concurrency control uses the classic two-condition algorithm found in any
     * textbook.
     */
    // 使用一把锁
    /** Main lock guarding all access */
    final ReentrantLock lock;
 
    // 两个Condition对象
    /** Condition for waiting takes */
    private final Condition notEmpty;
    /** Condition for waiting puts */
    private final Condition notFull;
 
    /**
     * Circularly decrement i.
     */
    final int dec(int i) {
        return ((i == 0) ? items.length : i) - 1;
    }
 
    @SuppressWarnings("unchecked")
    static <E> E cast(Object item) {
        return (E) item;
    }
 
    /**
     * Returns item at index i.
     */
    final E itemAt(int i) {
        return this.<E>cast(items[i]);
    }
 
    /**
     * Throws NullPointerException if argument is null.
     *
     * @param v
     *            the element
     */
    private static void checkNotNull(Object v) {
        if (v == null)
            throw new NullPointerException();
    }
 
    /**
     * Deletes item at position i. Utility for remove and iterator.remove. Call
     * only when holding lock.
     */
    void removeAt(int i) {
        final Object[] items = this.items;
        // if removing front item, just advance
        if (i == takeIndex) {
            items[takeIndex] = null;
            takeIndex = inc(takeIndex);
        } else {
            // slide over all others up through putIndex.
            for (;;) {
                int nexti = inc(i);
                if (nexti != putIndex) {
                    items[i] = items[nexti];
                    i = nexti;
                } else {
                    items[i] = null;
                    putIndex = i;
                    break;
                }
            }
        }
        --count;
        notFull.signal();
    }
 
    /**
     * Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity and
     * default access policy.
     *
     * @param capacity
     *            the capacity of this queue
     * @throws IllegalArgumentException
     *             if {@code capacity < 1}
     */
    public ArrayBlockingQueue(int capacity) {
        this(capacity, false);
    }
 
    /**
     * Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity and
     * the specified access policy.
     *
     * @param capacity
     *            the capacity of this queue
     * @param fair
     *            if {@code true} then queue accesses for threads blocked on
     *            insertion or removal, are processed in FIFO order; if
     *            {@code false} the access order is unspecified.
     * @throws IllegalArgumentException
     *             if {@code capacity < 1}
     */
    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();
    }
 
    /**
     * Creates an {@code ArrayBlockingQueue} with the given (fixed) capacity,
     * the specified access policy and initially containing the elements of the
     * given collection, added in traversal order of the collection's iterator.
     *
     * @param capacity
     *            the capacity of this queue
     * @param fair
     *            if {@code true} then queue accesses for threads blocked on
     *            insertion or removal, are processed in FIFO order; if
     *            {@code false} the access order is unspecified.
     * @param c
     *            the collection of elements to initially contain
     * @throws IllegalArgumentException
     *             if {@code capacity} is less than {@code c.size()}, or less
     *             than 1.
     * @throws NullPointerException
     *             if the specified collection or any of its elements are null
     */
    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();
        }
    }
 
    /**
     * 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 throwing an {@code IllegalStateException}
     * if this queue is full.
     *
     * @param e
     *            the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws IllegalStateException
     *             if this queue is full
     * @throws NullPointerException
     *             if the specified element is null
     */
    public boolean add(E e) {
        return super.add(e);
    }
 
    /**
     * 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 {
                insert(e);
                return true;
            }
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Inserts the specified element at the tail of this queue, waiting up to
     * the specified wait time for space to become available if the queue is
     * full.
     *
     * @throws InterruptedException
     *             {@inheritDoc}
     * @throws NullPointerException
     *             {@inheritDoc}
     */
    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 {
            while (count == items.length) {
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            insert(e);
            return true;
        } finally {
            lock.unlock();
        }
    }
 
    public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : extract();
        } finally {
            lock.unlock();
        }
    }
 
    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            return extract();
        } finally {
            lock.unlock();
        }
    }
 
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (count == 0) ? null : itemAt(takeIndex);
        } finally {
            lock.unlock();
        }
    }
 
    // this doc comment is overridden to remove the reference to collections
    // greater in size than Integer.MAX_VALUE
    /**
     * Returns the number of elements in this queue.
     *
     * @return the number of elements in this queue
     */
    public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return count;
        } finally {
            lock.unlock();
        }
    }
 
    // this doc comment is a modified copy of the inherited doc comment,
    // without the reference to unlimited queues.
    /**
     * Returns the number of additional elements that this queue can ideally (in
     * the absence of memory or resource constraints) accept without blocking.
     * This is always equal to the initial capacity of this queue less the
     * current {@code size} of this queue.
     *
     * <p>
     * Note that you <em>cannot</em> always tell if an attempt to insert an
     * element will succeed by inspecting {@code remainingCapacity} because it
     * may be the case that another thread is about to insert or remove an
     * element.
     */
    public int remainingCapacity() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return items.length - count;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Removes a single instance of the specified element from this queue, if it
     * is present. More formally, removes an element {@code e} such that
     * {@code o.equals(e)}, if this queue contains one or more such elements.
     * Returns {@code true} if this queue contained the specified element (or
     * equivalently, if this queue changed as a result of the call).
     *
     * <p>
     * Removal of interior elements in circular array based queues is an
     * intrinsically slow and disruptive operation, so should be undertaken only
     * in exceptional circumstances, ideally only when the queue is known not to
     * be accessible by other threads.
     *
     * @param o
     *            element to be removed from this queue, if present
     * @return {@code true} if this queue changed as a result of the call
     */
    public boolean remove(Object o) {
        if (o == null)
            return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--) {
                if (o.equals(items[i])) {
                    removeAt(i);
                    return true;
                }
            }
            return false;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Returns {@code true} if this queue contains the specified element. More
     * formally, returns {@code true} if and only if this queue contains at
     * least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o
     *            object to be checked for containment in this queue
     * @return {@code true} if this queue contains the specified element
     */
    public boolean contains(Object o) {
        if (o == null)
            return false;
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
                if (o.equals(items[i]))
                    return true;
            return false;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Returns an array containing all of the elements in this queue, in proper
     * sequence.
     *
     * <p>
     * The returned array will be "safe" in that no references to it are
     * maintained by this queue. (In other words, this method must allocate a
     * new array). The caller is thus free to modify the returned array.
     *
     * <p>
     * This method acts as bridge between array-based and collection-based APIs.
     *
     * @return an array containing all of the elements in this queue
     */
    public Object[] toArray() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final int count = this.count;
            Object[] a = new Object[count];
            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
                a[k] = items[i];
            return a;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Returns an array containing all of the elements in this queue, in proper
     * sequence; the runtime type of the returned array is that of the specified
     * array. If the queue fits in the specified array, it is returned therein.
     * Otherwise, a new array is allocated with the runtime type of the
     * specified array and the size of this queue.
     *
     * <p>
     * If this queue fits in the specified array with room to spare (i.e., the
     * array has more elements than this queue), the element in the array
     * immediately following the end of the queue is set to {@code null}.
     *
     * <p>
     * Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs. Further, this method allows
     * precise control over the runtime type of the output array, and may, under
     * certain circumstances, be used to save allocation costs.
     *
     * <p>
     * Suppose {@code x} is a queue known to contain only strings. The following
     * code can be used to dump the queue into a newly allocated array of
     * {@code String}:
     *
     * <pre>
     * String[] y = x.toArray(new String[0]);
     * </pre>
     *
     * Note that {@code toArray(new Object[0])} is identical in function to
     * {@code toArray()}.
     *
     * @param a
     *            the array into which the elements of the queue are to be
     *            stored, if it is big enough; otherwise, a new array of the
     *            same runtime type is allocated for this purpose
     * @return an array containing all of the elements in this queue
     * @throws ArrayStoreException
     *             if the runtime type of the specified array is not a supertype
     *             of the runtime type of every element in this queue
     * @throws NullPointerException
     *             if the specified array is null
     */
    @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);
            for (int i = takeIndex, k = 0; k < count; i = inc(i), k++)
                a[k] = (T) items[i];
            if (len > count)
                a[count] = null;
            return a;
        } finally {
            lock.unlock();
        }
    }
 
    public String toString() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int k = count;
            if (k == 0)
                return "[]";
 
            StringBuilder sb = new StringBuilder();
            sb.append('[');
            for (int i = takeIndex;; i = inc(i)) {
                Object e = items[i];
                sb.append(e == this ? "(this Collection)" : e);
                if (--k == 0)
                    return sb.append(']').toString();
                sb.append(',').append(' ');
            }
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Atomically removes all of the elements from this queue. The queue will be
     * empty after this call returns.
     */
    public void clear() {
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            for (int i = takeIndex, k = count; k > 0; i = inc(i), k--)
                items[i] = null;
            count = 0;
            putIndex = 0;
            takeIndex = 0;
            notFull.signalAll();
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * @throws UnsupportedOperationException
     *             {@inheritDoc}
     * @throws ClassCastException
     *             {@inheritDoc}
     * @throws NullPointerException
     *             {@inheritDoc}
     * @throws IllegalArgumentException
     *             {@inheritDoc}
     */
    public int drainTo(Collection<? super E> c) {
        checkNotNull(c);
        if (c == this)
            throw new IllegalArgumentException();
        final Object[] items = this.items;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int i = takeIndex;
            int n = 0;
            int max = count;
            while (n < max) {
                c.add(this.<E>cast(items[i]));
                items[i] = null;
                i = inc(i);
                ++n;
            }
            if (n > 0) {
                count = 0;
                putIndex = 0;
                takeIndex = 0;
                notFull.signalAll();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * @throws UnsupportedOperationException
     *             {@inheritDoc}
     * @throws ClassCastException
     *             {@inheritDoc}
     * @throws NullPointerException
     *             {@inheritDoc}
     * @throws IllegalArgumentException
     *             {@inheritDoc}
     */
    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 i = takeIndex;
            int n = 0;
            int max = (maxElements < count) ? maxElements : count;
            while (n < max) {
                c.add(this.<E>cast(items[i]));
                items[i] = null;
                i = inc(i);
                ++n;
            }
            if (n > 0) {
                count -= n;
                takeIndex = i;
                notFull.signalAll();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }
 
    /**
     * Returns an iterator over the elements in this queue in proper sequence.
     * The elements will be returned in order from first (head) to last (tail).
     *
     * <p>
     * The returned {@code Iterator} is a "weakly consistent" iterator that will
     * never throw {@link java.util.ConcurrentModificationException
     * ConcurrentModificationException}, and guarantees to traverse elements as
     * they existed upon construction of the iterator, and may (but is not
     * guaranteed to) reflect any modifications subsequent to construction.
     *
     * @return an iterator over the elements in this queue in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }
 
    /**
     * 重点分析方法:
     *
     * @return
     * @throws InterruptedException
     */
    public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            //解决伪唤醒
            while (count == 0)
                notEmpty.await();//如果缓冲区为空的话，则阻塞
            return extract();
        } finally {
            //释放锁
            lock.unlock();
        }
    }
 
    /**
     * Extracts element at current take position, advances, and signals. Call
     * only when holding lock.
     */
    private E extract() {
        final Object[] items = this.items;
        //takeIndexe为消费数据
        E x = this.<E>cast(items[takeIndex]);
        items[takeIndex] = null;
        // 计算下一个消费数据的位置
        takeIndex = inc(takeIndex);
        --count;
        //通知生产者生产数据
        notFull.signal();
        return x;
    }
 
    // Internal helper methods
    /**
     * Circularly increment i.
     */
    final int inc(int i) {
        return (++i == items.length) ? 0 : i;
    }
 
    /**
     * 重点分析方法:
     *
     * 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 {
            //解决伪唤醒
            while (count == items.length)
                notFull.await();//如果容器满了则阻塞此处
            insert(e);
        } finally {
            // 释放锁
            lock.unlock();
        }
    }
 
    /**
     * Inserts element at current put position, advances, and signals. Call only
     * when holding lock.
     */
    private void insert(E x) {
        items[putIndex] = x;
        putIndex = inc(putIndex);
        ++count;
        //唤醒消费者进行消费
        notEmpty.signal();
    }
 
    /**
     * Iterator for ArrayBlockingQueue. To maintain weak consistency with
     * respect to puts and takes, we (1) read ahead one slot, so as to not
     * report hasNext true but then not have an element to return -- however we
     * later recheck this slot to use the most current value; (2) ensure that
     * each array slot is traversed at most once (by tracking "remaining"
     * elements); (3) skip over null slots, which can occur if takes race ahead
     * of iterators. However, for circular array-based queues, we cannot rely on
     * any well established definition of what it means to be weakly consistent
     * with respect to interior removes since these may require slot overwrites
     * in the process of sliding elements to cover gaps. So we settle for
     * resiliency, operating on established apparent nexts, which may miss some
     * elements that have moved between calls to next.
     */
    private class Itr implements Iterator<E> {
        private int remaining; // Number of elements yet to be returned
        private int nextIndex; // Index of element to be returned by next
        private E nextItem; // Element to be returned by next call to next
        private E lastItem; // Element returned by last call to next
        private int lastRet; // Index of last element returned, or -1 if none
 
        Itr() {
            final ReentrantLock lock = ArrayBlockingQueue.this.lock;
            lock.lock();
            try {
                lastRet = -1;
                if ((remaining = count) > 0)
                    nextItem = itemAt(nextIndex = takeIndex);
            } finally {
                lock.unlock();
            }
        }
 
        public boolean hasNext() {
            return remaining > 0;
        }
 
        public E next() {
            final ReentrantLock lock = ArrayBlockingQueue.this.lock;
            lock.lock();
            try {
                if (remaining <= 0)
                    throw new NoSuchElementException();
                lastRet = nextIndex;
                E x = itemAt(nextIndex); // check for fresher value
                if (x == null) {
                    x = nextItem; // we are forced to report old value
                    lastItem = null; // but ensure remove fails
                } else
                    lastItem = x;
                while (--remaining > 0 && // skip over nulls
                        (nextItem = itemAt(nextIndex = inc(nextIndex))) == null)
                    ;
                return x;
            } finally {
                lock.unlock();
            }
        }
 
        public void remove() {
            final ReentrantLock lock = ArrayBlockingQueue.this.lock;
            lock.lock();
            try {
                int i = lastRet;
                if (i == -1)
                    throw new IllegalStateException();
                lastRet = -1;
                E x = lastItem;
                lastItem = null;
                // only remove if item still at index
                if (x != null && x == items[i]) {
                    boolean removingHead = (i == takeIndex);
                    removeAt(i);
                    if (!removingHead)
                        nextIndex = dec(nextIndex);
                }
            } finally {
                lock.unlock();
            }
        }
 
    }
 
}