JDK容器学习之Queue：LinkedBlockingQueue

/** 队列的容量, or Integer.MAX_VALUE if none */
private final int capacity;

/** 队列中实际的个数 */
private final AtomicInteger count = new AtomicInteger();

/**
 * 队列头，但其中没有有效数据，它的下一个才保存实际的数据
 * Head of linked list.
 * Invariant: head.item == null
 */
transient Node<E> head;

/**
 * 队列尾，其内包含有效的数据
 * Invariant: last.next == null
 */
private transient Node<E> last;

/** 出队的锁, etc */
private final ReentrantLock takeLock = new ReentrantLock();

/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();

/** 进队的锁, etc */
private final ReentrantLock putLock = new ReentrantLock();

/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();


static class Node<E> {
  // 存放在队列中的数据; 队列头的item为null
  E item;
  
  // 队列中，该节点的下一个节点，队列尾的next为null
  Node<E> next;

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

public void put(E e) throws InterruptedException {
    // 队列中不能存在null
    if (e == null) throw new NullPointerException();
    
    int c = -1;
    Node<E> node = new Node<E>(e);
    final ReentrantLock putLock = this.putLock;
    final AtomicInteger count = this.count;
    putLock.lockInterruptibly();
    try {
        // 若队列已满，则等待`notFull(出队后，队列未满时).signal()`唤醒
        while (count.get() == capacity) {
            notFull.await();
        }
        enqueue(node); // 进队
        c = count.getAndIncrement(); // 计数+1，并获取队列的实际元素个数
        if (c + 1 < capacity) // 若进队后，队列依然没有满，则释放一个信号 (why?)
            notFull.signal();
    } finally {
        putLock.unlock();
    }
    if (c == 0) // 表示队列从空到有一个数据，唤醒因为队列为空被阻塞的线程
        signalNotEmpty();
}
private void enqueue(Node<E> node) {
    // assert putLock.isHeldByCurrentThread();
    // assert last.next == null;
    last = last.next = node;
}
// 唤醒阻塞的出队线程，注意使用姿势，Condition的使用必须放在对应的锁中间，否则会报错
private void signalNotEmpty() {
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        notEmpty.signal();
    } finally {
        takeLock.unlock();
    }
}

public E take() throws InterruptedException {
    E x;
    int c = -1;
    final AtomicInteger count = this.count;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lockInterruptibly();
    try {
        // 如果队列为空，则阻塞，等待入队之后，被唤醒
        while (count.get() == 0) {     notEmpty.await(); }
        x = dequeue(); // 进队
        c = count.getAndDecrement();
        if (c > 1) // 如果队列依然非空，则唤醒其他因为队列为空被阻塞的线程
            notEmpty.signal();
    } finally { takeLock.unlock();}
    if (c == capacity) 
    // 原来队列为满的，此时出队一个后，正好非满，唤醒因为队列满被阻塞的线程
        signalNotFull();
    return x;
}
// 出队逻辑，实现逻辑是把出队Node节点设置为新的head，释放老的head节点 
private E dequeue() {
    Node<E> h = head;
    Node<E> first = h.next;
    h.next = h; // help GC
    head = first;
    E x = first.item;
    first.item = null;
    return x;
}

// 唤醒阻塞的出队线程，注意使用姿势，Condition的使用必须放在对应的锁中间，否则会报错
private void signalNotFull() {
    final ReentrantLock putLock = this.putLock;
    putLock.lock();
    try {
        notFull.signal();
    } finally {
        putLock.unlock();
    }
}

此时线程C执行入队，入队完成，因为队列由空到非空，会唤醒一个被阻塞的出队线程（假设为A）；

因为出队和入队是可以并发的，现在在线程A执行`c = count.getAndDecrement();`之前，若线程D又入队成功一个，因为此时队列非空，所以不会调用`signalNotEmpty`

现在如果线程A执行出队之后，获取到的c应该为2，如果不执行`notEmpty.signal();`，就会导致线程B一直被阻塞，显然不符合我们的预期

public boolean contains(Object o) {
    if (o == null) return false;
    fullyLock();
    try {
        for (Node<E> p = head.next; p != null; p = p.next)
            if (o.equals(p.item))
                return true;
        return false;
    } finally {
        fullyUnlock();
    }
}

public Object[] toArray() {
    fullyLock();
    try {
        int size = count.get();
        Object[] a = new Object[size];
        int k = 0;
        for (Node<E> p = head.next; p != null; p = p.next)
            a[k++] = p.item;
        return a;
    } finally {
        fullyUnlock();
    }
}

// java.util.concurrent.Executors
public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}