PriotityBlockingQueue源码解析

PriotityBlockingQueue是待优先级的无界阻塞队列，每次出队都返回优先级最高或者最低的元素。其内部是使用平衡二叉树堆实现的，所以直接遍历队列元素不保证有序。默认使用对象的compareTo方法提供比较规则，如果需要自定义比较规则，可以自定义Comparators。

类图

PriotityBlockingQueue的类图如下所示：

PriotityBlockingQueue实现了BlockingQueue接口和继承了AbstractQueue类。

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

BlockingQueue接口只是定义了一些阻塞队列的公共方法，如下：

public interface BlockingQueue<E> extends Queue<E> {
    // 将给定元素设置到队列中，如果设置成功返回true, 否则抛出异常。如果是往限定了长度的队列中设置值，推荐使用offer()方法。
    boolean add(E e);

    // 将给定的元素设置到队列中，如果设置成功返回true, 否则返回false. e的值不能为空，否则抛出空指针异常。
    boolean offer(E e);

    //  将元素设置到队列中，如果队列中没有多余的空间，该方法会一直阻塞，直到队列中有多余的空间。
    void put(E e) throws InterruptedException;

    // 将给定元素在给定的时间内设置到队列中，如果设置成功返回true, 否则返回false.
    boolean offer(E e, long timeout, TimeUnit unit)
        throws InterruptedException;

    // 从队列中获取值，如果队列中没有值，线程会一直阻塞，直到队列中有值，并且该方法取得了该值。
    E take() throws InterruptedException;

    // 获取并移除此队列的头元素，可以在指定的等待时间前等待可用的元素，timeout表明放弃之前要等待的时间长度，用 unit 的时间单位表示，如果在元素可用前超过了指定的等待时间，则返回null，当等待时可以被中断
    E poll(long timeout, TimeUnit unit)
        throws InterruptedException;

    // 获取队列中剩余的空间。
    int remainingCapacity();

    // 从队列中移除指定的值。
    boolean remove(Object o);

    // 判断队列中是否拥有该值。
    public boolean contains(Object o);

    // 将队列中值，全部移除，并发设置到给定的集合中。
    int drainTo(Collection<? super E> c);

    // 将队列中值，全部移除，并发设置到给定的集合中。最多设置maxElements个元素
    int drainTo(Collection<? super E> c, int maxElements);
}

AbstractQueue也实现了Queue接口，还提供了某些方法的默认实现

// 从这可以发现，Add方法，其实就是调用的offer方法
public boolean add(E e) {
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
    }

    // 调用poll方法,如果为null抛出NoSuchElementException异常
    public E remove() {
        E x = poll();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

    // 调用peek方法，如果为null，抛出NoSuchElementException异常
    public E element() {
        E x = peek();
        if (x != null)
            return x;
        else
            throw new NoSuchElementException();
    }

    // 按个从队列中删除node
    public void clear() {
        while (poll() != null)
            ;
    }

    public boolean addAll(Collection<? extends E> c) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }

主要属性

/**
     * Default array capacity.
     */
    // 默认数组大小
    private static final int DEFAULT_INITIAL_CAPACITY = 11;

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    // 默认最大数组大小
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * Priority queue represented as a balanced binary heap: the two
     * children of queue[n] are queue[2*n+1] and queue[2*(n+1)].  The
     * priority queue is ordered by comparator, or by the elements'
     * natural ordering, if comparator is null: For each node n in the
     * heap and each descendant d of n, n <= d.  The element with the
     * lowest value is in queue[0], assuming the queue is nonempty.
     */
    // 数组实现的二叉堆
    private transient Object[] queue;

    /**
     * The number of elements in the priority queue.
     */
    // 优先级队列的大小
    private transient int size;

    /**
     * The comparator, or null if priority queue uses elements'
     * natural ordering.
     */
    // 比较器
    private transient Comparator<? super E> comparator;

    /**
     * Lock used for all public operations
     */
    private final ReentrantLock lock;

    /**
     * Condition for blocking when empty
     */
    // 取线程等待队列
    private final Condition notEmpty;

    /**
     * Spinlock for allocation, acquired via CAS.
     */
    // 扩容的时候使用的控制变量，CAS更新这个值，谁更新成功了谁扩容，其它线程让出CPU
    private transient volatile int allocationSpinLock;

    /**
     * A plain PriorityQueue used only for serialization,
     * to maintain compatibility with previous versions
     * of this class. Non-null only during serialization/deserialization.
     */
    // 不阻塞的优先级队列，非存储元素的地方，仅用于序列化/反序列化时
    private PriorityQueue<E> q;

主要方法

无参构造函数

/**
     * Creates a {@code PriorityBlockingQueue} with the default
     * initial capacity (11) that orders its elements according to
     * their {@linkplain Comparable natural ordering}.
     */
    public PriorityBlockingQueue() {
        this(DEFAULT_INITIAL_CAPACITY, null);
    }

创建一个默认大小为11的优先级阻塞队列，使用默认比较器

有参构造函数-指定大小

/**
     * Creates a {@code PriorityBlockingQueue} with the specified
     * initial capacity that orders its elements according to their
     * {@linkplain Comparable natural ordering}.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @throws IllegalArgumentException if {@code initialCapacity} is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity) {
        this(initialCapacity, null);
    }

生成一个指定大小的队列，使用默认比较器

有参构造函数-指定比较器

/**
     * Creates a {@code PriorityBlockingQueue} with the specified initial
     * capacity that orders its elements according to the specified
     * comparator.
     *
     * @param initialCapacity the initial capacity for this priority queue
     * @param  comparator the comparator that will be used to order this
     *         priority queue.  If {@code null}, the {@linkplain Comparable
     *         natural ordering} of the elements will be used.
     * @throws IllegalArgumentException if {@code initialCapacity} is less
     *         than 1
     */
    public PriorityBlockingQueue(int initialCapacity,
                                 Comparator<? super E> comparator) {
        if (initialCapacity < 1)
            throw new IllegalArgumentException();
        this.lock = new ReentrantLock();
        this.notEmpty = lock.newCondition();
        this.comparator = comparator;
        this.queue = new Object[initialCapacity];
    }

指定队列大小，以及比较器

有参构造函数-指定集合

根据指定集合创建一个优先级队列，如果指定的集合是SortedSet或者是另外一个PriorityQueue，那么就会保持原集合的排序顺序。否则按照Comparator指定的排序规则或者自然排序

/**
     * Creates a {@code PriorityBlockingQueue} containing the elements
     * in the specified collection.  If the specified collection is a
     * {@link SortedSet} or a {@link PriorityQueue}, this
     * priority queue will be ordered according to the same ordering.
     * Otherwise, this priority queue will be ordered according to the
     * {@linkplain Comparable natural ordering} of its elements.
     *
     * @param  c the collection whose elements are to be placed
     *         into this priority queue
     * @throws ClassCastException if elements of the specified collection
     *         cannot be compared to one another according to the priority
     *         queue's ordering
     * @throws NullPointerException if the specified collection or any
     *         of its elements are null
     */
    public PriorityBlockingQueue(Collection<? extends E> c) {
        this.lock = new ReentrantLock();
        this.notEmpty = lock.newCondition();
        boolean heapify = true; // true if not known to be in heap order
        boolean screen = true;  // true if must screen for nulls
        if (c instanceof SortedSet<?>) {
            SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
            this.comparator = (Comparator<? super E>) ss.comparator();
            heapify = false;
        }
        else if (c instanceof PriorityBlockingQueue<?>) {
            PriorityBlockingQueue<? extends E> pq =
                (PriorityBlockingQueue<? extends E>) c;
            this.comparator = (Comparator<? super E>) pq.comparator();
            screen = false;
            if (pq.getClass() == PriorityBlockingQueue.class) // exact match
                heapify = false;
        }
        Object[] a = c.toArray();
        int n = a.length;
        // If c.toArray incorrectly doesn't return Object[], copy it.
        if (a.getClass() != Object[].class)
            a = Arrays.copyOf(a, n, Object[].class);
        if (screen && (n == 1 || this.comparator != null)) {
            for (int i = 0; i < n; ++i)
                if (a[i] == null)
                    throw new NullPointerException();
        }
        this.queue = a;
        this.size = n;
        if (heapify)
            heapify();
    }

size方法

public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return size;
        } finally {
            lock.unlock();
        }
    }

锁定，返回大小，解锁

remainingCapacity方法

返回Integer.MAX_VALUE.等同于没有边界

/**
     * Always returns {@code Integer.MAX_VALUE} because
     * a {@code PriorityBlockingQueue} is not capacity constrained.
     * @return {@code Integer.MAX_VALUE} always
     */
    public int remainingCapacity() {
        return Integer.MAX_VALUE;
    }

put方法

内部调用offer方法

/**
     * Inserts the specified element into this priority queue.
     * As the queue is unbounded, this method will never block.
     *
     * @param e the element to add
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public void put(E e) {
        offer(e); // never need to block
    }

offer方法

添加一个元素到队列，因为队列是无界的，所以，该方法永不会返回false.

添加成功之后，唤醒等待的获取线程

/**
     * Inserts the specified element into this priority queue.
     * As the queue is unbounded, this method will never return {@code false}.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Queue#offer})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean offer(E e) {
        if (e == null)
            // 不能添加null元素
            throw new NullPointerException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        int n, cap;
        Object[] array;
        // 如果当前元素大于等于数组大小，扩容
        while ((n = size) >= (cap = (array = queue).length))
            tryGrow(array, cap);
        try {
            Comparator<? super E> cmp = comparator;
            if (cmp == null)
                // 如果没有比较器，使用默认比较规则
                siftUpComparable(n, e, array);
            else
                // 否则使用指定的比较规则
                siftUpUsingComparator(n, e, array, cmp);
            // size 加1
            size = n + 1;
            // 唤醒取线程
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
        return true;
    }

tryGrow

/**
     * Tries to grow array to accommodate at least one more element
     * (but normally expand by about 50%), giving up (allowing retry)
     * on contention (which we expect to be rare). Call only while
     * holding lock.
     *
     * @param array the heap array
     * @param oldCap the length of the array
     */
    private void tryGrow(Object[] array, int oldCap) {
        // 首先释放锁，便于在扩容期间其它线程使用所
        lock.unlock(); // must release and then re-acquire main lock
        Object[] newArray = null;
        // CAS成功则开启扩容，否则调用yield方法，让出cpu
        if (allocationSpinLock == 0 &&
            UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
                                     0, 1)) {
            try {
                // 如果原大小小于64，每次自增2，否则，每次扩容两倍
                int newCap = oldCap + ((oldCap < 64) ?
                                       (oldCap + 2) : // grow faster if small
                                       (oldCap >> 1));
                if (newCap - MAX_ARRAY_SIZE > 0) {    // possible overflow
                    // 如果新大小，超过最大大小
                    int minCap = oldCap + 1;
                    // 溢出了，抛出OOM异常
                    if (minCap < 0 || minCap > MAX_ARRAY_SIZE)
                        throw new OutOfMemoryError();
                    // 大小设置为最大的大小
                    newCap = MAX_ARRAY_SIZE;
                }
                if (newCap > oldCap && queue == array)
                    newArray = new Object[newCap];
            } finally {
                allocationSpinLock = 0;
            }
        }
        if (newArray == null) // back off if another thread is allocating
            Thread.yield();
        // 加锁
        lock.lock();
        if (newArray != null && queue == array) {
            queue = newArray;
            // 复制原来的元素至新数组
            System.arraycopy(array, 0, newArray, 0, oldCap);
        }
    }

siftUpComparable

父节点的索引是叶子节点索引的两倍，比较父节点是否比子节点大，否则交换

/**
     * Inserts item x at position k, maintaining heap invariant by
     * promoting x up the tree until it is greater than or equal to
     * its parent, or is the root.
     *
     * To simplify and speed up coercions and comparisons. the
     * Comparable and Comparator versions are separated into different
     * methods that are otherwise identical. (Similarly for siftDown.)
     * These methods are static, with heap state as arguments, to
     * simplify use in light of possible comparator exceptions.
     *
     * @param k the position to fill
     * @param x the item to insert
     * @param array the heap array
     */
    private static <T> void siftUpComparable(int k, T x, Object[] array) {
        Comparable<? super T> key = (Comparable<? super T>) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = array[parent];
            if (key.compareTo((T) e) >= 0)
                break;
            array[k] = e;
            k = parent;
        }
        array[k] = key;
    }

siftUpUsingComparator

使用自定义的比较器，其他同上

private static <T> void siftUpUsingComparator(int k, T x, Object[] array,
                                       Comparator<? super T> cmp) {
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = array[parent];
            if (cmp.compare(x, (T) e) >= 0)
                break;
            array[k] = e;
            k = parent;
        }
        array[k] = x;
    }

add方法

内部调用offer方法

/**
     * Inserts the specified element into this priority queue.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Collection#add})
     * @throws ClassCastException if the specified element cannot be compared
     *         with elements currently in the priority queue according to the
     *         priority queue's ordering
     * @throws NullPointerException if the specified element is null
     */
    public boolean add(E e) {
        return offer(e);
    }

take方法

从队列中获取一个元素，并从队列中删除该元素，如果队列为空，阻塞当前线程

public E take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        E result;
        try {
            while ( (result = dequeue()) == null)
                notEmpty.await();
        } finally {
            lock.unlock();
        }
        return result;
    }

dequeue

取数组的第一个出队列，然后根据比较器是否为null，将首位置下沉。

/**
     * Mechanics for poll().  Call only while holding lock.
     */
    private E dequeue() {
        int n = size - 1;
        if (n < 0)
            return null;
        else {
            Object[] array = queue;
            E result = (E) array[0];
            E x = (E) array[n];
            array[n] = null;
            Comparator<? super E> cmp = comparator;
            if (cmp == null)
                siftDownComparable(0, x, array, n);
            else
                siftDownUsingComparator(0, x, array, n, cmp);
            size = n;
            return result;
        }
    }

siftDownComparable

计算出子节点的位置，循环比较，如果子节点比父节点大，就跳出

/**
     * Inserts item x at position k, maintaining heap invariant by
     * demoting x down the tree repeatedly until it is less than or
     * equal to its children or is a leaf.
     *
     * @param k the position to fill
     * @param x the item to insert
     * @param array the heap array
     * @param n heap size
     */
    private static <T> void siftDownComparable(int k, T x, Object[] array,
                                               int n) {
        if (n > 0) {
            Comparable<? super T> key = (Comparable<? super T>)x;
            int half = n >>> 1;           // loop while a non-leaf
            while (k < half) {
                int child = (k << 1) + 1; // assume left child is least
                Object c = array[child];
                int right = child + 1;
                if (right < n &&
                    ((Comparable<? super T>) c).compareTo((T) array[right]) > 0)
                    c = array[child = right];
                if (key.compareTo((T) c) <= 0)
                    break;
                array[k] = c;
                k = child;
            }
            array[k] = key;
        }
    }

siftDownUsingComparator

private static <T> void siftDownUsingComparator(int k, T x, Object[] array,
                                                    int n,
                                                    Comparator<? super T> cmp) {
        if (n > 0) {
            int half = n >>> 1;
            while (k < half) {
                int child = (k << 1) + 1;
                Object c = array[child];
                int right = child + 1;
                if (right < n && cmp.compare((T) c, (T) array[right]) > 0)
                    c = array[child = right];
                if (cmp.compare(x, (T) c) <= 0)
                    break;
                array[k] = c;
                k = child;
            }
            array[k] = x;
        }
    }

poll方法

从队列中获取一个元素，并从队列中删除该元素，如果队列为空，直接返回null

public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

peek方法

从队列中获取一个元素，如果队列为空，直接返回null

public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (size == 0) ? null : (E) queue[0];
        } finally {
            lock.unlock();
        }
    }

remove方法

从队列中删除指定的元素，有返回true，否则返回false

/**
     * Removes a single instance of the specified element from this queue,
     * if it is present.  More formally, removes an element {@code e} such
     * that {@code o.equals(e)}, if this queue contains one or more such
     * elements.  Returns {@code true} if and only if this queue contained
     * the specified element (or equivalently, if this queue changed as a
     * result of the call).
     *
     * @param o element to be removed from this queue, if present
     * @return {@code true} if this queue changed as a result of the call
     */
    public boolean remove(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int i = indexOf(o);
            if (i == -1)
                return false;
            removeAt(i);
            return true;
        } finally {
            lock.unlock();
        }
    }

removeAt

/**
     * Removes the ith element from queue.
     */
    private void removeAt(int i) {
        Object[] array = queue;
        int n = size - 1;
        if (n == i) // removed last element
            array[i] = null;
        else {
            E moved = (E) array[n];
            array[n] = null;
            Comparator<? super E> cmp = comparator;
            if (cmp == null)
                siftDownComparable(i, moved, array, n);
            else
                siftDownUsingComparator(i, moved, array, n, cmp);
            if (array[i] == moved) {
                if (cmp == null)
                    siftUpComparable(i, moved, array);
                else
                    siftUpUsingComparator(i, moved, array, cmp);
            }
        }
        size = n;
    }

contains方法

/**
     * Returns {@code true} if this queue contains the specified element.
     * More formally, returns {@code true} if and only if this queue contains
     * at least one element {@code e} such that {@code o.equals(e)}.
     *
     * @param o object to be checked for containment in this queue
     * @return {@code true} if this queue contains the specified element
     */
    public boolean contains(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return indexOf(o) != -1;
        } finally {
            lock.unlock();
        }
    }

indexOf

顺序遍历

private int indexOf(Object o) {
        if (o != null) {
            Object[] array = queue;
            int n = size;
            for (int i = 0; i < n; i++)
                if (o.equals(array[i]))
                    return i;
        }
        return -1;
    }