PriorityBlockingQueue

PriorityBlockingQueue简介

PriorityBlockingQueue是一种优先级，无界堵塞，线程安全的队列，每次出队返回优先级高或低的元素，内部通过堆实现。

源码详解

属性分析


//队列默认大小
   private static final int DEFAULT_INITIAL_CAPACITY = 11;

	//数组最大容量
   private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

//存放队列元素
   private transient Object[] queue;

//队列元素个数
   private transient int size;

//比较器
   private transient Comparator<? super E> comparator;

//独占锁
   private final ReentrantLock lock;

//当队列为空时，进行出队操作的线程会被放进这个条件队列等待
   private final Condition notEmpty;

构造方法分析

//无参构造方法，设置队列大小为默认11，比较器为null
   public PriorityBlockingQueue() {
       this(DEFAULT_INITIAL_CAPACITY, null);
   }
//比较器为null
   public PriorityBlockingQueue(int initialCapacity) {
       this(initialCapacity, null);
   }

//主要构造方法，初始化各组件
   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];
   }

入队操作

offer()

public boolean offer(E e) {
    if (e == null)
        throw new NullPointerException();//抛出NPE异常
    final ReentrantLock lock = this.lock;
    lock.lock();//上锁
    int n, cap;
    Object[] array;
    //如果当前元素个数大于等于队列大小则扩展
    while ((n = size) >= (cap = (array = queue).length))
        tryGrow(array, cap);
    try {
        Comparator<? super E> cmp = comparator;
        if (cmp == null)
            siftUpComparable(n, e, array);//调用对象的compareTo方法
        else
            siftUpUsingComparator(n, e, array, cmp);//使用比较器
        size = n + 1;
        notEmpty.signal();//唤醒因出队操作而休眠的线程
    } finally {
        lock.unlock();
    }
    return true;
}

tryGrow()

private void tryGrow(Object[] array, int oldCap) {
    lock.unlock(); //释放获得的锁
    Object[] newArray = null;
    
    //尝试获得扩容的锁把
    if (allocationSpinLock == 0 &&
        UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
                                 0, 1)) {
        //如果队列大小小于64则扩大oldCap+2，否则扩大成原来的50%，最大值为MAX_ARRAY_SIZE
        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 {
            allocationSpinLock = 0;
        }
    }
    if (newArray == null) //如果newArray为空则代表尝试获得扩容的锁失败了，需要把自己尝试挂起，让给扩容线程工作。
        Thread.yield();
    lock.lock();//获得锁
    if (newArray != null && queue == array) {
        queue = newArray;
        System.arraycopy(array, 0, newArray, 0, oldCap);//真正扩容
    }
}

这里比较难理解的地方主要是为什么要释放锁。。。这里主要是因为扩容队列需要时间，比较耗时，因为锁的原因，扩容时无法进行入队操作和出队操作。所以为了提高并发性能就要释放锁拉,小编我也感到很惊讶。

put()

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public void put(E e) {
    offer(e); // 直接调用offer()。。两个入队方法一样的
}

siftUpComparable()

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;//其实就是(k-1)/2,获得父类节点
        Object e = array[parent];
        if (key.compareTo((T) e) >= 0)//如果入队元素大于等于父类节点则直接跳出循环
            break;
        array[k] = e;
        k = parent;
    }
    array[k] = key;
}

出队操作

poll()

//这方法没啥就是个壳，作用就上锁。。
  public E poll() {
      final ReentrantLock lock = this.lock;
      lock.lock();
      try {
          return dequeue();
      } finally {
          lock.unlock();
      }
  }

dequeue()

private E dequeue() {
    int n = size - 1;
    if (n < 0)//队列为空直接返回null
        return null;
    else {
        Object[] array = queue;
        E result = (E) array[0];//获得首部元素（出队元素）
        E x = (E) array[n];
        array[n] = null;
        Comparator<? super E> cmp = comparator;
        //开始向下调整
        if (cmp == null)
            siftDownComparable(0, x, array, n);
        else
            siftDownUsingComparator(0, x, array, n, cmp);
        size = n;
        return result;
    }
}

siftDownComparable()

private static <T> void siftDownComparable(int k, T x, Object[] array,                                       int n) {
    //判断队列长度是否大于0
    if (n > 0) {
        Comparable<? super T> key = (Comparable<? super T>)x;
        int half = n >>> 1//获得队列长度的一半;          
        while (k < half) {
            int child = (k << 1) + 1; //获得k*2+1（也就是获得左子节点）
            Object c = array[child];//获得左子节点的值
            int right = child + 1;//获得右子节点
            if (right < n &&
                ((Comparable<? super T>) c).compareTo((T) array[right]) > 0)
                c = array[child = right];//右子节点比较小
            if (key.compareTo((T) c) <= 0)//如果key值比选出来的节点还小就直接跳出循环
                break;
            array[k] = c;//赋值
            k = child;//获得当前最小节点
        }
        array[k] = key;
    }
}

take()

public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    E result;
    try {
        while ( (result = dequeue()) == null)//如果当前队列为空则堵塞
            notEmpty.await();
    } finally {
        lock.unlock();
    }
    return result;
}

其他操作

size()

public int size() {
    final ReentrantLock lock = this.lock;
    lock.lock();//上锁，保证获取队列长度的时候没有入队出队操作。
    try {
        return size;
    } finally {
        lock.unlock();
    }
}

remove()

public boolean remove(Object o) {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        int i = indexOf(o);//获得移除元素的下标
        if (i == -1)
            return false;//为空直接返回
        removeAt(i);
        return true;
    } finally {
        lock.unlock();
    }
}

removeat()

private void removeAt(int i) {
    Object[] array = queue;
    int n = size - 1;
    if (n == i) // 如果移除的是最后的元素则直接设为null
        array[i] = null;
    else {
        E moved = (E) array[n];//队列最后一个元素
        array[n] = null;
        Comparator<? super E> cmp = comparator;
       	 //向下调整
        if (cmp == null)
            siftDownComparable(i, moved, array, n);
        else
            siftDownUsingComparator(i, moved, array, n, cmp);
        //如果向下调整不了则向上调整
        if (array[i] == moved) {
            if (cmp == null)
                siftUpComparable(i, moved, array);
            else
                siftUpUsingComparator(i, moved, array, cmp);
        }
    }
    size = n;
}