读PriorityBlocking源码

//一个基于而为堆的优先级队列,它是无界的。
//先看构造函数：
 public PriorityBlockingQueue() {
        this(DEFAULT_INITIAL_CAPACITY, null);
    }

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(int initialCapacity) {
        this(initialCapacity, null);
    }


 public PriorityBlockingQueue(Collection<? extends E> c) {
        this.lock = new ReentrantLock();
        this.notEmpty = lock.newCondition();
	//是否需要堆化 
        boolean heapify = true; // true if not known to be in heap order
        //是否需要检测null
	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)
	    //生成堆
            heapify();
    }
//添加元素
     public boolean add(E e) {
        return offer(e);
    }
//添加元素不阻塞
    public boolean offer(E e) {
        if (e == null)
            throw new NullPointerException();
        final ReentrantLock lock = this.lock;
        lock.lock();
        int n, cap;
        Object[] array;
	//扩容
        while ((n = size) >= (cap = (array = queue).length))
            tryGrow(array, cap);
        try {
            Comparator<? super E> cmp = comparator;
            if (cmp == null)
                siftUpComparable(n, e, array);
            else
                siftUpUsingComparator(n, e, array, cmp);
	   //长度+1
            size = n + 1;
	    //当前队列已经不为空了唤醒在notEmpty上等待的线程。
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
        return true;
    }

//扩容。这里处理的很精巧利用CAS进行扩容。
     private void tryGrow(Object[] array, int oldCap) {
       //这里释放了锁
        lock.unlock(); // must release and then re-acquire main lock
        Object[] newArray = null;
	/**利用CAS设置allocationSpinLock状态成功则扩容。注意看这段代码虽然采用CAS机制但是还是有并发问题.
          关键在于下面的一句判断&& queue == array关键就在这里,这里判断了相等才会复制 */
        if (allocationSpinLock == 0 &&
            UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
                                     0, 1)) {
            try {
                int newCap = oldCap + ((oldCap < 64) ?
                                       (oldCap + 2) : // grow faster if small
                                       (oldCap >> 1));
                if (newCap - MAX_ARRAY_SIZE > 0) {    // possible overflow
                    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 {
	      //扩容完后置为0
                allocationSpinLock = 0;
            }
        }
	//其他线程扩容了交给其他线程执行
        if (newArray == null) // back off if another thread is allocating
            Thread.yield();
        lock.lock();
	// queue == array这个判断很重要这个判断判断了只有一个线程扩容成功,其他扩容的线程都失败。
        if (newArray != null && queue == array) {
            queue = newArray;
            System.arraycopy(array, 0, newArray, 0, oldCap);
        }
    }
 //添加元素不阻塞
  public boolean offer(E e, long timeout, TimeUnit unit) {
        return offer(e); // never need to block
    }

//只获取头元素不阻塞
    public E peek() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return (size == 0) ? null : (E) queue[0];
        } finally {
            lock.unlock();
        }
    }

//获取头元素并移除
public E poll() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return dequeue();
        } finally {
            lock.unlock();
        }
    }

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;
        }
    }

//在一定时间内获取并移除某个元素
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        E result;
        try {
	   //当队列中有元素并且没有超时的情况下,在notEmpty条件上等待队列变为非空。
            while ( (result = dequeue()) == null && nanos > 0)
                nanos = notEmpty.awaitNanos(nanos);
        } finally {
            lock.unlock();
        }
        return result;
    }

//可以阻塞的获取元素直接队列非空。
 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;
    }

//获取队列大小
public int size() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return size;
        } finally {
            lock.unlock();
        }
    }

//无界队列所以返回最大值。
public int remainingCapacity() {
        return Integer.MAX_VALUE;
    }

//返回元素的比较器
 public Comparator<? super E> comparator() {
        return comparator;
    }

//返回是否包含某元素
 public boolean contains(Object o) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return indexOf(o) != -1;
        } 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;
    }

//删除某个元素
     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();
        }
    }

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;
    }

//转化为数组
   public Object[] toArray() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            return Arrays.copyOf(queue, size);
        } finally {
            lock.unlock();
        }
    }


//转化为带类型的数组
 public <T> T[] toArray(T[] a) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = size;
	    //如果数组长度小于队列长度重新生成一个队列长度的数组
            if (a.length < n)
                // Make a new array of a's runtime type, but my contents:
                return (T[]) Arrays.copyOf(queue, size, a.getClass());
            System.arraycopy(queue, 0, a, 0, n);
            if (a.length > n)
                a[n] = null;
            return a;
        } finally {
            lock.unlock();
        }
    }
//清空队列
public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            Object[] array = queue;
            int n = size;
            size = 0;
            for (int i = 0; i < n; i++)
                array[i] = null;
        } finally {
            lock.unlock();
        }
    }


//将队列中的所有元素移除封装到集合中
public int drainTo(Collection<? super E> c) {
        return drainTo(c, Integer.MAX_VALUE);
    }

public int drainTo(Collection<? super E> c, int maxElements) {
        if (c == null)
            throw new NullPointerException();
        if (c == this)
            throw new IllegalArgumentException();
        if (maxElements <= 0)
            return 0;
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            int n = Math.min(size, maxElements);
            for (int i = 0; i < n; i++) {
                c.add((E) queue[0]); // In this order, in case add() throws.
                dequeue();
            }
            return n;
        } finally {
            lock.unlock();
        }
    }