Jdk1.6 JUC源码解析(13)-LinkedBlockingQueue

功能简介：

• LinkedBlockingQueue是一种基于单向链表实现的有界的(可选的，不指定默认int最大值)阻塞队列。队列中的元素遵循先入先出 (FIFO)的规则。新元素插入到队列的尾部，从队列头部取出元素。(在并发程序中，基于链表实现的队列和基于数组实现的队列相比，往往具有更高的吞吐 量，但性能稍差一些)

源码分析：

• 首先看下LinkedBlockingQueue内部的数据结构：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;

    /**
     * Linked list node class
     */
    static class Node<E> {
        /** The item, volatile to ensure barrier separating write and read */
        volatile E item;
        Node<E> next;
        Node(E x) { item = x; }
    }
    /** The capacity bound, or Integer.MAX_VALUE if none */
    private final int capacity;
    /** 这里的count为原子量，避免了一些使用count的地方需要加两把锁。 */
    private final AtomicInteger count = new AtomicInteger(0);
    /** Head of linked list */
    private transient Node<E> head;
    /** Tail of linked list */
    private transient Node<E> last;
    /** Lock held by take, poll, etc */
    private final ReentrantLock takeLock = new ReentrantLock();
    /** Wait queue for waiting takes */
    private final Condition notEmpty = takeLock.newCondition();
    /** Lock held by put, offer, etc */
    private final ReentrantLock putLock = new ReentrantLock();
    /** Wait queue for waiting puts */
    private final Condition notFull = putLock.newCondition();

    /**
     * Creates a <tt>LinkedBlockingQueue</tt> with a capacity of
     * {@link Integer#MAX_VALUE}.
     */
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }
    /**
     * Creates a <tt>LinkedBlockingQueue</tt> with the given (fixed) capacity.
     *
     * @param capacity the capacity of this queue
     * @throws IllegalArgumentException if <tt>capacity</tt> is not greater
     *         than zero
     */
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node<E>(null);
    }

    public LinkedBlockingQueue(Collection<? extends E> c) {
        this(Integer.MAX_VALUE);
        for (E e : c)
            add(e);
    }

　　首先可见，内部为单向链表；其次，内部为两把锁：存锁和取锁，并分别关联一个条件(是一种双锁队列)。

• 还是从put和take入手，先看下put方法：

    public void put(E e) throws InterruptedException {
        if (e == null) throw new NullPointerException();
        // Note: convention in all put/take/etc is to preset
        // local var holding count  negative to indicate failure unless set.
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            /*
             * Note that count is used in wait guard even though it is
             * not protected by lock. This works because count can
             * only decrease at this point (all other puts are shut
             * out by lock), and we (or some other waiting put) are
             * signalled if it ever changes from
             * capacity. Similarly for all other uses of count in
             * other wait guards.
             */
            try {
                while (count.get() == capacity)
                    notFull.await();
            } catch (InterruptedException ie) {
                notFull.signal(); // propagate to a non-interrupted thread
                throw ie;
            }
            insert(e);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                /*
                 * 注意这里的处理：和单锁队列不同，count为原子量，不需要锁保护。
                 * put过程中可能有其他线程执行多次get,所以这里需要判断一下当前
                 * 如果还有剩余容量，那么继续唤醒notFull条件上等待的线程。
                 */
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0) //如果count又0变为1，说明在队列是空的情况下插入了1个元素，唤醒notNull条件上等待的线程。
            signalNotEmpty();
    }
    /**
     * Creates a node and links it at end of queue.
     * @param x the item
     */
    private void insert(E x) {
        last = last.next = new Node<E>(x);
    }
    /**
     * Signals a waiting take. Called only from put/offer (which do not
     * otherwise ordinarily lock takeLock.)
     */
    private void signalNotEmpty() {
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }

代码很容易看懂，再看下take方法实现：

    public E take() throws InterruptedException {
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            try {
                while (count.get() == 0)
                    notEmpty.await();
            } catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to a non-interrupted thread
                throw ie;
            }
            x = extract();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }
    /**
     * Removes a node from head of queue,
     * @return the node
     */
    private E extract() {
        Node<E> first = head.next;
        head = first;
        E x = first.item;
        first.item = null;
        return x;
    }
    /**
     * Signals a waiting put. Called only from take/poll.
     */
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

和put对等的逻辑，也很容易看懂。

• 上面看到，主要方法里并没有同时用两把锁，但有些方法里会同时使用两把锁，比如remove方法等：

    public boolean remove(Object o) {
        if (o == null) return false;
        boolean removed = false;
        fullyLock();
        try {
            Node<E> trail = head;
            Node<E> p = head.next;
            while (p != null) {
                if (o.equals(p.item)) {
                    removed = true;
                    break;
                }
                trail = p;
                p = p.next;
            }
            if (removed) {
                p.item = null;
                trail.next = p.next;
                if (last == p)
                    last = trail;
                if (count.getAndDecrement() == capacity)
                    notFull.signalAll();
            }
        } finally {
            fullyUnlock();
        }
        return removed;
    }
    /**
     * Lock to prevent both puts and takes.
     */
    private void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }
    /**
     * Unlock to allow both puts and takes.
     */
    private void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }