JDK源碼-PriorityBlockingQueue（優先阻塞隊列）

數據結構

//二叉堆
private transient Object[] queue;
//外部操作加鎖
private final ReentrantLock lock;
//優先隊列的元素數目
private transient int size;

//阻塞消費者線程
private final Condition notEmpty;
//通過CAS獲取的自旋鎖
private transient volatile int allocationSpinLock;

初始化

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];
}
    
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;
        //比較器的賦值，二叉堆插入操作時會使用
        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();
}

插入數據

public boolean add(E e) {
    return offer(e);
}
public void put(E e) {
    offer(e);
}

public boolean offer(E e) {
    if (e == null)
        throw new NullPointerException();
    final ReentrantLock lock = this.lock;
    lock.lock();
    int n, cap;
    Object[] array;
    //如果隊列元素不小於二叉堆數組的元素數，就進行二叉堆擴容操作
    while ((n = size) >= (cap = (array = queue).length))
        tryGrow(array, cap);  // 擴容操作
    try {
        Comparator<? super E> cmp = comparator;
        if (cmp == null) //新節點插入，二叉堆的節點向上比較
            siftUpComparable(n, e, array);
        else
            siftUpUsingComparator(n, e, array, cmp);
        size = n + 1;
        notEmpty.signal();
    } finally {
        lock.unlock();
    }
    return true;
}

//二叉堆插入新節點時的向上比較操作（上濾） 新節點最初在堆末尾
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];
        //比較新節點與其父節點，只要比起父節點小，就與之交換
        if (key.compareTo((T) e) >= 0)
            break;
        array[k] = e;
        k = parent;
    }
    array[k] = key;
}

private void tryGrow(Object[] array, int oldCap) {
	//需要先釋放鎖然後，上游已經加鎖
    lock.unlock(); // must release and then re-acquire main lock
    Object[] newArray = null;
    //CAS競爭allocationSpinLock
    if (allocationSpinLock == 0 &&
        UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
                                 0, 1)) {
        try {
        	//新尺寸，大容量時增50%
            int newCap = oldCap + ((oldCap < 64) ?
                                   (oldCap + 2) : // grow faster if small
                                   (oldCap >> 1));
            if (newCap - MAX_ARRAY_SIZE > 0) {    // 內存可能溢出時的保底策略
                int minCap = oldCap + 1;
                if (minCap < 0 || minCap > MAX_ARRAY_SIZE)
                    throw new OutOfMemoryError();
                newCap = MAX_ARRAY_SIZE;
            }
            if (newCap > oldCap && queue == array)
                newArray = new Object[newCap];
        } finally {
            allocationSpinLock = 0;
        }
    }
    if (newArray == null) // 以上if爲false，說明沒有成功競爭獲取擴容權限，即有其他線程正在擴容
        Thread.yield(); //讓出調度權
    lock.lock();
    if (newArray != null && queue == array) {
        queue = newArray;
        System.arraycopy(array, 0, newArray, 0, oldCap);
    }
}

獲取數據

//加鎖+出隊操作
public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return dequeue();
    } finally {
        lock.unlock();
    }
}

//如果隊列爲空就一直阻塞等待
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;
}

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    E result;
    try {
        while ( (result = dequeue()) == null && nanos > 0)
            nanos = notEmpty.awaitNanos(nanos);
    } finally {
        lock.unlock();
    }
    return result;
}

//出隊操作
private E dequeue() {
    int n = size - 1; //隊列元素樹扣減
    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) //堆的根節點刪除後，執行下濾操作，填補根節點
            siftDownComparable(0, x, array, n);
        else
            siftDownUsingComparator(0, x, array, n, cmp);
        size = n;
        return result;
    }
}

//二叉堆移除根節點後的填補操作（填補根節點）
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
        //從原來根節點的兒子開始，逐層處理，先找到每一層的較小節點，將其上移到父節點，直到堆末尾
        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]; //確保child始終爲較小節點
            if (key.compareTo((T) c) <= 0) //直到堆末尾位置
                break;
            array[k] = c; //將當前節點上移到父節點
            k = child;
        }
        array[k] = key;
    }
}

獲取最高優先級元素

獲取二叉堆的根節點，O(1)時間複雜度

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

移除特定數據

public boolean remove(Object o) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        int i = indexOf(o); //遍歷獲取數據對應的元素下標
        if (i == -1)
            return false;
        removeAt(i);
        return true;
    } finally {
        lock.unlock();
    }
}
    
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;
}

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;
        //從i開始執行節點下濾，填補i位置
        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;
}

阻塞隊列使用場景

• 生產者-消費者模式使用阻塞隊列實現任務池
• JDK線程池中使用阻塞隊列實現線任務隊列（java.util.concurrent.ThreadPoolExecutor#workQueue）
• 弱引用