并发容器分析（三）--LinkedBlockingQueue

一、简介

    LinkedBlockingQueue是BlockingQueue的一种使用Link List的实现，它对头和尾（取和添加操作）采用两把不同的锁，相对于ArrayBlockingQueue提高了吞吐量。它也是一种阻塞型的容器，适合于实现“消费者生产者”模式。

二、具体实现

    LinkedBlockingQueue底层的定义如下：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    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; }
    }

    // 支持原子操作
    private final AtomicInteger count = new AtomicInteger(0);

    // 链表的头和尾
    private transient Node<E> head;
    private transient Node<E> last;

    // 针对取和添加操作的两把锁及其上的条件
   private final ReentrantLock takeLock = new ReentrantLock();
    private final Condition notEmpty = takeLock.newCondition();
    private final ReentrantLock putLock = new ReentrantLock();
    private final Condition notFull = putLock.newCondition();

   ...
}

    LinkedBlockingQueue的添加操作：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    private void insert(E x) {
        last = last.next = new Node<E>(x);
    }

    /**
     * signal方法在被调用时，当前线程必须拥有该condition相关的锁!
     * Signal 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();
        }
    }

    public void put(E o) throws InterruptedException {
        if (o == null) throw new NullPointerException();
        int c = -1;
        final ReentrantLock putLock = this.putLock;
        final AtomicInteger count = this.count;
        // 使用putLock
        putLock.lockInterruptibly();
        try {
            try {
            	  // 当容量已满时，等待notFull条件
            while (count.get() == capacity)
                    notFull.await();
            } catch (InterruptedException ie) {
                notFull.signal(); // propagate to a non-interrupted thread
                throw ie;
            }
            insert(o);
            // 取出当前值，并将原数据增加1
            c = count.getAndIncrement();
            // 容量不满，再次激活notFull上等待的put线程
        if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        // 必须先释放putLock再在notEmpty上signal，否则会造成死锁
     if (c == 0)
            signalNotEmpty();
    }

  ...
}

    LinkedBlockingQueue的取操作：

public class LinkedBlockingQueue<E> extends AbstractQueue<E>
        implements BlockingQueue<E>, java.io.Serializable {

    private E extract() {
        Node<E> first = head.next;
        head = first;
        E x = first.item;
        first.item = null;
        return x;
    }

    private void signalNotFull() {
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            notFull.signal();
        } finally {
            putLock.unlock();
        }
    }

    public E take() throws InterruptedException {
        E x;
        int c = -1;
        final AtomicInteger count = this.count;
        final ReentrantLock takeLock = this.takeLock;
        // 使用takeLock
        takeLock.lockInterruptibly();
        try {
            try {
            	  // 若容量为空，等待notEmpty
                while (count.get() == 0)
                    notEmpty.await();
            } catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to a non-interrupted thread
                throw ie;
            }

            x = extract();
            c = count.getAndDecrement();
            // 再次激活notEmpty
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        // take执行之前容量已满，则激活notFull
        if (c == capacity)
            signalNotFull();
        return x;
    }

  ...
}