package java.util.concurrent;
//无界阻塞优先级队列，每次出队都返回优先级最高的元素
//是二叉树最小堆的实现
public class PriorityBlockingQueue<E> extends AbstractQueue<E>
    implements BlockingQueue<E>, java.io.Serializable

1、常用方法

构造方法

//默认队列容量
private static final int DEFAULT_INITIAL_CAPACITY = 11;

public PriorityBlockingQueue() {
    this(DEFAULT_INITIAL_CAPACITY, null);
}

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

public PriorityBlockingQueue(int initialCapacity,
                             Comparator<? super E> comparator) {
    if (initialCapacity < 1)
        throw new IllegalArgumentException();
    //lock独占锁对象用来控制同时只能有一个线程可以进行入队出队操作
    this.lock = new ReentrantLock();
    //notEmpty条件变量用来实现take方法阻塞模式
    //put操作是非阻塞的（无界队 ）
    this.notEmpty = lock.newCondition();
    //比较器comparator用来比较元素大小
    this.comparator = comparator;
    //数组queue用来存放队列元素
    this.queue = new Object[initialCapacity];
}

入队方法

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

public void put(E e) {
    offer(e); // never need to block
}

/**
 * 增加一个元索
 */
public boolean offer(E e) {
    if (e == null)
        throw new NullPointerException();
    final ReentrantLock lock = this.lock;
    lock.lock();
    int n, cap;
    Object[] array;
    while ((n = size) >= (cap = (array = queue).length))
        //如果size大小已经达到队列的大小 则 扩容
        tryGrow(array, cap);
    try {
        Comparator<? super E> cmp = comparator;
        if (cmp == null)
            siftUpComparable(n, e, array);
        else
            siftUpUsingComparator(n, e, array, cmp);
        size = n + 1;
        notEmpty.signal();
    } finally {
        lock.unlock();
    }
    return true;
}

 /**
  * 如果size大小已经达到队列的大小 则 扩容
  */
 private void tryGrow(Object[] array, int oldCap) {
     //出于性能考虑，队列此时的出队操作还可以获取锁
     lock.unlock(); // must release and then re-acquire main lock
     Object[] newArray = null;
     //allocationSpinLock用cas控制只有一个线程可以进行扩容
     if (allocationSpinLock == 0 &&
         UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
                                  0, 1)) {
         try {
             //oldGap<64则扩容新增oldcap+2,否者扩容50%
             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 {
             allocationSpinLock = 0;
         }
     }
     if (newArray == null) // back off if another thread is allocating
         //其它扩容线程会从执行状态转为就绪状态，让出cpu
         Thread.yield();
     lock.lock();
     if (newArray != null && queue == array) {
         //这里是在获取锁后操作的目的是 确保获取的数组则是最新的（可能有元素出队）
         queue = newArray;
         System.arraycopy(array, 0, newArray, 0, oldCap);
     }
 }

出队方法

/**
 * 移除并返问队列头部的元素，如果队列为空，则返回null
 */
public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return dequeue();
    } finally {
        lock.unlock();
    }
}

/**
 * 移除并返回队列头部的元素，如果队列为空，则阻塞
 */
public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    E result;
    try {
        //如果队列为空，则阻塞
        while ( (result = dequeue()) == null)
            notEmpty.await();
    } finally {
        lock.unlock();
    }
    return result;
}    

/**
 * 出队
 */
private E dequeue() {
    int n = size - 1;
    if (n < 0)
        return null;
    else {
        Object[] array = queue;
        //队头元素
        E result = (E) array[0];
        //对尾元素
        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;
    }
}
   

其它

/**
 * 返回队列头部的元素，如果队列为空，则返回null
 */
public E peek() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return (size == 0) ? null : (E) queue[0];
    } finally {
        lock.unlock();
    }
} 

/**
 * 获取队列元素个数
 */
public int size() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return size;
    } finally {
        lock.unlock();
    }
}