LinkedBlockingQueue代码阅读

public class LinkedBlockingQueue extends AbstractQueue
        implements BlockingQueue, java.io.Serializable {
    private static final long serialVersionUID = -6903933977591709194L;
    
    //链表节点类
    static class Node {
        E item;

        //下个节点指针
        Node next;

        Node(E x) { item = x; }
    }

    /** 容量 不设置为Integer.MAX_VALUE */
    private final int capacity;

    /** 当前元素数量 */
    private final AtomicInteger count = new AtomicInteger();

    //头节点
    transient Node head;

    //尾节点
    private transient Node last;

    //出队锁
    private final ReentrantLock takeLock = new ReentrantLock();

    //出队信号量，当队列为空时，出队线程阻塞
    private final Condition notEmpty = takeLock.newCondition();

    //入队锁
    private final ReentrantLock putLock = new ReentrantLock();

    //入队信号量，当容量满时，入队线程阻塞
    private final Condition notFull = putLock.newCondition();

    /**
     * 唤醒等待取数据. Called only from put/offer
     */
    private void signalNotEmpty() {
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
    }

    /**
     * 唤醒等待插入数据. Called only from take/poll.
     */
    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

    //插入队列尾部
    private void enqueue(Node node) {
        // assert putLock.isHeldByCurrentThread();
        // assert last.next == null;
        last = last.next = node;
    }

    //从头部出队列
    private E dequeue() {
        // assert takeLock.isHeldByCurrentThread();
        // assert head.item == null;
        Node h = head;
        Node first = h.next;
        h.next = h; // help GC
        head = first;
        E x = first.item;
        first.item = null;
        return x;
    }

    //全部上锁
    void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

    //全部解锁
    void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }
    
    public LinkedBlockingQueue() {
        this(Integer.MAX_VALUE);
    }
   
    public LinkedBlockingQueue(int capacity) {
        if (capacity <= 0) throw new IllegalArgumentException();
        this.capacity = capacity;
        last = head = new Node(null);
    }

    public LinkedBlockingQueue(Collection c) {
        this(Integer.MAX_VALUE);
        final ReentrantLock putLock = this.putLock;
        putLock.lock(); // Never contended, but necessary for visibility
        try {
            int n = 0;
            for (E e : c) {
                if (e == null)
                    throw new NullPointerException();
                if (n == capacity)
                    throw new IllegalStateException("Queue full");
                enqueue(new Node(e));
                ++n;
            }
            count.set(n);
        } finally {
            putLock.unlock();
        }
    }
    
    public int size() {
        return count.get();
    }

    //剩余容量
    public int remainingCapacity() {
        return capacity - count.get();
    }

    /**
     * 插入元素到队列尾部, 当容量满时一直阻塞
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    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;
        Node node = new Node(e);
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            //如果容量已满，则阻塞等待
            while (count.get() == capacity) {
                notFull.await();
            }
            enqueue(node); //入队
            c = count.getAndIncrement(); //已用空间加1
            if (c + 1 < capacity)
                notFull.signal(); //如果有剩余空间，唤醒1个取数据线程
        } finally {
            putLock.unlock();
        }
        if (c == 0) //如果队列中只有一个元素，唤醒一个取出线程
            signalNotEmpty();
    }

    /**
     * 插入元素到队列尾部, 当容量满时阻塞一段时间
     *
     * @return {@code true} if successful, or {@code false} if
     *         the specified waiting time elapses before space is available
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException {
        if (e == null) throw new NullPointerException();
        long nanos = unit.toNanos(timeout);
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        putLock.lockInterruptibly();
        try {
            while (count.get() == capacity) { //如果容量已满
                if (nanos <= 0)
                    return false;
                nanos = notFull.awaitNanos(nanos); //阻塞等待一定时间
            }
            enqueue(new Node(e)); //入队
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0) //如果队列中只有一个元素，唤醒一个取出线程
            signalNotEmpty();
        return true;
    }

    /**
     * 插入元素到队列尾部
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null) throw new NullPointerException();
        final AtomicInteger count = this.count;
        if (count.get() == capacity)
            return false;
        int c = -1;
        Node node = new Node(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            if (count.get() < capacity) {  //有剩余空间
                enqueue(node); //入队
                c = count.getAndIncrement(); 
                if (c + 1 < capacity)  //有剩余空间
                    notFull.signal();  //唤醒1个插入线程
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty(); //唤醒取数据线程
        return c >= 0;
    }

    public E take() throws InterruptedException {
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) { //如果队列数据没有
                notEmpty.await(); //等待
            }
            x = dequeue(); //出队列
            c = count.getAndDecrement();//总数量减1
            if (c > 1) //如果队列不为空
                notEmpty.signal(); //唤醒1个取数据线程
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull(); //尝试唤醒插入线程线程
        return x;
    }

    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        E x = null;
        int c = -1;
        long nanos = unit.toNanos(timeout);
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lockInterruptibly();
        try {
            while (count.get() == 0) {
                if (nanos <= 0)
                    return null;
                nanos = notEmpty.awaitNanos(nanos);
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        return x;
    }

    public E poll() {
        final AtomicInteger count = this.count;
        if (count.get() == 0)
            return null;
        E x = null;
        int c = -1;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            if (count.get() > 0) {
                x = dequeue();
                c = count.getAndDecrement();
                if (c > 1)
                    notEmpty.signal();
            }
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull(); 
        return x;
    }

    /**
     * 获取首节点元素
     * @return
     */
    public E peek() {
        if (count.get() == 0)
            return null;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            Node first = head.next;
            if (first == null)
                return null;
            else
                return first.item;
        } finally {
            takeLock.unlock();
        }
    }   
   
}