PriorityBlockingQueue 是一个支持优先级的无界阻塞队列。在Thread中，我们可以通过 setPriority(int newPriority) 来设置优先级，线程优先级高的线程先执行，优先级低的后执行。 PriorityBlockingQueue 默认情况下元素采用自然顺序升序排序，当然我们也可以通过构造函数来指定Comparator来对元素进行排序。需要注意的是PriorityBlockingQueue不能保证同优先级元素的顺序。
PriorityBlockingQueue底层是二叉堆构成实现的，下面先介绍一些二叉堆知识点。
二叉堆
二叉堆是一种特殊的堆，就结构性而言就是完全二叉树或者是近似完全二叉树，满足树结构性和堆序性。树机构特性就是完全二叉树应该有的结构，堆序性则是：父节点的键值总是保持固定的序关系于任何一个子节点的键值，且每个节点的左子树和右子树都是一个二叉堆。
队列结构定义
public class PriorityBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = 5595510919245408276L;
//默认初始化大小11
private static final int DEFAULT_INITIAL_CAPACITY = 11;
//定义列表最大容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//二叉堆数组
private transient Object[] queue;
private transient int size;//队列个数
// 比较器，如果为空，则为自然顺序
private transient Comparator<? super E> comparator;
private final ReentrantLock lock;
private final Condition notEmpty;
}
入队
PriorityBlockingQueue是无界的，所以不可能会阻塞。内部调用offer(E e)。
public void put(E e) {
offer(e); // never need to block
}
public boolean add(E e) {
return 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;
while ((n = size) >= (cap = (array = queue).length))
tryGrow(array, cap);
try {
Comparator<? super E> cmp = comparator;
// 根据比较器是否为null，做不同的处理
if (cmp == null)
siftUpComparable(n, e, array);
else
siftUpUsingComparator(n, e, array, cmp);
size = n + 1;
notEmpty.signal();
} finally {
lock.unlock();
}
return true;
}
//采用自然排序，调用siftUpComparable方法
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;
}
//当比较器不为null时，采用所指定的比较器，调用siftUpUsingComparator方法
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;
}
扩容：tryGrow
private void tryGrow(Object[] array, int oldCap) {
lock.unlock(); // must release and then re-acquire main lock
Object[] newArray = null;
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 {
allocationSpinLock = 0;
}
}
if (newArray == null) // back off if another thread is allocating
Thread.yield();
lock.lock();
if (newArray != null && queue == array) {
queue = newArray;
System.arraycopy(array, 0, newArray, 0, oldCap);
}
}
出队
PriorityBlockingQueue提供put()、add()、offer()方法向队列中加入元素。我们这里从put()入手：put(E e) ：将指定元素插入此优先级队列。
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;
}
}
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;
}
}
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;
}
}