LinkedBlockingQueue源码分析

1. 变量和构造方法

private final int capacity; // 容量（最大节点个数），默认Integer.MAX_VALUE
private final AtomicInteger count = new AtomicInteger(); // 当前节点个数
transient Node<E> head; // 队列头结点
private transient Node<E> last; // 队列尾节点
private final ReentrantLock takeLock = new ReentrantLock(); // 头结点锁（取）
private final Condition notEmpty = takeLock.newCondition(); // 等待队列非空
private final ReentrantLock putLock = new ReentrantLock(); // 尾结点锁（置）
private final Condition notFull = putLock.newCondition(); // 等待队列非满

public LinkedBlockingQueue() {
    this(Integer.MAX_VALUE);
}

public LinkedBlockingQueue(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
    last = head = new Node<E>(null); // 头节点不存放元素
}

public LinkedBlockingQueue(Collection<? extends E> c) {
    this(Integer.MAX_VALUE);
    final ReentrantLock putLock = this.putLock;
    putLock.lock(); // 尾结点加锁
    try {
        int n = 0;
        for (E e : c) {
            if (e == null)
                throw new NullPointerException();
            if (n == capacity) // 队列满
                throw new IllegalStateException("Queue full");
            enqueue(new Node<E>(e));
            ++n;
        }
        count.set(n);
    } finally {
        putLock.unlock(); // 释放锁
    }
}

2. 生产（put、offer）

static class Node<E> { // 节点
    E item;
    Node<E> next;
    Node(E x) { item = x; }
}

private void enqueue(Node<E> node) {
    last = last.next = node;
}

private void signalNotEmpty() { // 在获取takeLock状态下，唤醒等待get的线程
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        notEmpty.signal();
    } finally {
        takeLock.unlock();
    }
}

public void put(E e) throws InterruptedException {
    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(); // 加锁
    try {
        while (count.get() == capacity) { // 队列满
            notFull.await();
        }
        enqueue(node);
        c = count.getAndIncrement(); // 必须是原子操作
        if (c + 1 < capacity) // put后，队列非满
            notFull.signal(); // 唤醒后续等待put的线程（如果存在）
    } finally {
        putLock.unlock(); // 释放锁
    }
    if (c == 0) // put前，队列空
        signalNotEmpty(); // 唤醒首个等待get的节点（如果存在）
}

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(); // 加锁
    try {
        while (count.get() == capacity) { // 队列满
            if (nanos <= 0) // 超时（awaitNanos可能已被signal，但在SyncQueue中排队等锁时超时，见ReentrantLock）
                return false;
            nanos = notFull.awaitNanos(nanos); // 等待（在nanos时间内）
        }
        enqueue(new Node<E>(e));
        c = count.getAndIncrement();
        if (c + 1 < capacity) // offer后，队列非满
            notFull.signal(); // 唤醒后续等待offer的线程（如果存在）
    } finally {
        putLock.unlock(); // 释放锁
    }
    if (c == 0) // offer前，队列为空
        signalNotEmpty(); // 唤醒首个等待get的线程（如果存在）
    return true;
}

3. 消费（take、poll）

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;
}

private void signalNotFull() { // 在获取putLock锁状态下，唤醒等待put的线程
    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;
    takeLock.lockInterruptibly(); // 加锁
    try {
        while (count.get() == 0) { // 队列空
            notEmpty.await(); // 等待
        }
        x = dequeue();
        c = count.getAndDecrement();
        if (c > 1) // get后，队列非空
            notEmpty.signal(); // 唤醒后续等待get线程（如果存在）
    } finally {
        takeLock.unlock(); // 释放锁
    }
    if (c == capacity) // get前，队列已满
        signalNotFull(); // 唤醒首个等待put的线程（如果存在）
    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(); // 加锁
    try {
        while (count.get() == 0) { // 队列空
            if (nanos <= 0) // 超时
                return null;
            nanos = notEmpty.awaitNanos(nanos); // 等待poll
        }
        x = dequeue();
        c = count.getAndDecrement();
        if (c > 1) // poll后，队列非空
            notEmpty.signal(); // 唤醒后续等待poll的线程（如果存在）
    } finally {
        takeLock.unlock(); // 释放锁
    }
    if (c == capacity) // poll前，队列已满
        signalNotFull(); // 唤醒首个等待put的线程（如果存在）
    return x;
}

4. 加全锁（remove）

void fullyLock() { // 加全锁（remove、contains、toArray、clear、Itr）
    putLock.lock();
    takeLock.lock();
}

void fullyUnlock() { // 释放全锁
    takeLock.unlock();
    putLock.unlock();
}

void unlink(Node<E> p, Node<E> trail) {
    p.item = null;
    trail.next = p.next;
    if (last == p)
        last = trail;
    if (count.getAndDecrement() == capacity) // 移除节点前，队列已满
        notFull.signal(); // 唤醒等待put/offer的线程
}

public boolean remove(Object o) { // 删除节点
    if (o == null) return false;
    fullyLock(); // 加全锁
    try {
        for (Node<E> trail = head, p = trail.next; p != null; trail = p, p = p.next) {
            if (o.equals(p.item)) {
                unlink(p, trail);
                return true;
            }
        }
        return false;
    } finally {
        fullyUnlock(); // 释放全锁
    }
}