LinkedBlockingQueue

LinkedBlockingQueue

LinkedBlockingQueue是基於鏈表的阻塞FIFO隊列，可以指定一個最大的長度限制以防止過度擴展，未指定情況下其大小爲Integer.MAX_VALUE

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);
    }

使用Node來存放元素：

static class Node<E> {
        E item;

        /**
         * One of:
         * - the real successor Node
         * - this Node, meaning the successor is head.next
         * - null, meaning there is no successor (this is the last node)
         */
        Node<E> next;

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

每個Node都有一個指向next的指針，形成一個鏈表。

幾個重要的參數：

/** Current number of elements */
    private final AtomicInteger count = new AtomicInteger();

使用AtomicInteger創建一個volatile的count變量

     * Head of linked list.
     * Invariant: head.item == null
     */
    transient Node<E> head;

    /**
     * Tail of linked list.
     * Invariant: last.next == null
     */
    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();

兩個全局鎖，take一個，put一個。

兩個可以進行線程間通信的鎖notEmpty和notFull，當進行添加或者刪除刪除操作後，需要通知那些因爲鏈表爲空或者滿時不能進行put和remove操作的線程。

鏈表添加和刪除元素就比數組簡單了

private void enqueue(Node<E> node) {   //將新的添加到尾部
        // assert putLock.isHeldByCurrentThread();
        // assert last.next == null;
        last = last.next = node;
    }

private E dequeue() {  //取出隊列首部
        // assert takeLock.isHeldByCurrentThread();
        // assert head.item == null;
        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;
    }

有一些操作需要同時獲得put和take鎖，

/**
     * Locks to prevent both puts and takes.
     */
    void fullyLock() {
        putLock.lock();
        takeLock.lock();
    }

    /**
     * Unlocks to allow both puts and takes.
     */
    void fullyUnlock() {
        takeLock.unlock();
        putLock.unlock();
    }

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;
        Node<E> node = new Node<E>(e);
        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.
             */
            while (count.get() == capacity) {
                notFull.await();
            }
            enqueue(node);
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
    }

put之前要先獲得鎖和count變量的值。噹噹前count達到最大限制時，就等待直到有空餘位置。

ArrayBlockingQueue和LinkedBlockingQueue均不能存儲null

offer方法

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) {
                if (nanos <= 0L)
                    return false;
                nanos = notFull.awaitNanos(nanos);
            }
            enqueue(new Node<E>(e));
            c = count.getAndIncrement();
            if (c + 1 < capacity)
                notFull.signal();
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return true;
    }

這個方法帶有超時失敗，當等待時間超過給定限制時，就返回False。

下面這個就沒有時間限制了，當不能放入元素時，直接返回False，不等待。

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<E> node = new Node<E>(e);
        final ReentrantLock putLock = this.putLock;
        putLock.lock();
        try {
            if (count.get() < capacity) {
                enqueue(node);
                c = count.getAndIncrement();
                if (c + 1 < capacity)
                    notFull.signal();
            }
        } finally {
            putLock.unlock();
        }
        if (c == 0)
            signalNotEmpty();
        return c >= 0;
    }

take ，poll，peek方法

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) {
                notEmpty.await();
            }
            x = dequeue();
            c = count.getAndDecrement();
            if (c > 1)
                notEmpty.signal();
        } finally {
            takeLock.unlock();
        }
        if (c == capacity)
            signalNotFull();
        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 <= 0L)
                    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;
    }

    public E peek() {
        if (count.get() == 0)
            return null;
        final ReentrantLock takeLock = this.takeLock;
        takeLock.lock();
        try {
            return (count.get() > 0) ? head.next.item : null;
        } finally {
            takeLock.unlock();
        }
    }

這三個方法均需要獲得takeLock鎖，其他和put方法沒有區別，想法都一樣。

remove是一個需要同時獲得takeLock和putLock的方法

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

contains同樣需要獲得fullyLock

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();
        }
    }

還有toArray方法

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();
        }
    }

clear等，就不全部列舉了。

使用LinkedBlockingQueue最好限制鏈表的長度，否則鏈表過長不利於查找。

LinkedBlockingQueue相比於ArrayBlockingQueue，使用了兩把鎖，將put和take操作分開，效率會有提升。