前言
PriorityBlockingQueue是BlockingQueue接口的实现类，它是一种优先级阻塞队列，每次出队都返回优先级最高或最低的元素，其内部是用平衡二叉树堆实现的。这里的优先级指的是元素类必须实现Comparable接口，然后用compareTo()方法比较元素的优先级大小，当然也可指定自定义的比较器comparator。
实现原理
先来看看它的重要属性：
// 队列默认容量为11
private static final int DEFAULT_INITIAL_CAPACITY = 11;
// 队列最大容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
// 存放元素的数组
private transient Object[] queue;
// 队列长度
private transient int size;
// 自定义比较器
private transient Comparator comparator;
// 操作元素数组的互斥锁
private final ReentrantLock lock;
// 数组非空条件
private final Condition notEmpty;
// 数组扩容操作的自璇锁，1表示正在扩容，0表示没有在扩容
private transient volatile int allocationSpinLock;
// 优先级队列
private PriorityQueue q; 再来看它的几个构造函数：
public PriorityBlockingQueue() {
this(DEFAULT_INITIAL_CAPACITY, null);
}public PriorityBlockingQueue(int initialCapacity) {
this(initialCapacity, null);
}public PriorityBlockingQueue(int initialCapacity,
Comparator comparator) {
if (initialCapacity < 1)
throw new IllegalArgumentException();
this.lock = new ReentrantLock();
this.notEmpty = lock.newCondition();
this.comparator = comparator;
this.queue = new Object[initialCapacity];
}再来看重要方法：
put()：
public void put(E e) {
offer(e); // never need to block
}offer()：
public boolean offer(E e) {
if (e == null)
throw new NullPointerException();
final ReentrantLock lock = this.lock;
lock.lock();
int n, cap;
Object[] array;
// 队列当前长度>=队列容量时，进行扩容
while ((n = size) >= (cap = (array = queue).length))
tryGrow(array, cap);
try {
Comparator cmp = comparator;
// 未指定比较器时，则使用默认的compareTo()来计算插入元素的位置
if (cmp == null)
siftUpComparable(n, e, array);
else
// 指定了时，则使用指定的比较器计算位置
siftUpUsingComparator(n, e, array, cmp);
size = n + 1;
// 唤醒某个等待在notEmpty条件的线程
notEmpty.signal();
} finally {
lock.unlock();
}
return true;
}其中tryGrow()方法如下：
private void tryGrow(Object[] array, int oldCap) {
// 先释放操作数组的互斥锁，去尝试获取扩容的自璇锁
lock.unlock(); // must release and then re-acquire main lock
Object[] newArray = null;
// 尝试获取扩容的锁
if (allocationSpinLock == 0 &&
UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
0, 1)) {
try {
// 计算扩容后的新容量
int newCap = oldCap + ((oldCap < 64) ?
(oldCap + 2) : // grow faster if small
(oldCap >> 1));
// 新容量超出最大容量时，则取最大容量
if (newCap - MAX_ARRAY_SIZE > 0) { // possible overflow
// 旧容量加1仍然溢出时，抛内存溢出异常
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;
}
}
// 其他线程抢到了扩容锁并正在扩容时，当前线程则让出CPU调度权
if (newArray == null) // back off if another thread is allocating
Thread.yield();
// 获取操作数组的互斥锁
lock.lock();
// 扩容操作成功时，将旧数组元素拷贝到扩容后的新数组
if (newArray != null && queue == array) {
queue = newArray;
System.arraycopy(array, 0, newArray, 0, oldCap);
}
}siftUpComparable()方法如下：
private static void siftUpComparable(int k, T x, Object[] array) {
Comparable key = (Comparable) 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;
} siftUpUsingComparator()方法如下：
private static void siftUpUsingComparator(int k, T x, Object[] array,
Comparator 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;
} take():
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
E result;
try {
// 队列元素为空时，阻塞等待
while ( (result = dequeue()) == null)
notEmpty.await();
} finally {
lock.unlock();
}
return result;
}poll():
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
// 非阻塞，队列为空时返回null
return dequeue();
} finally {
lock.unlock();
}
}其中dequeue()方法如下：
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 cmp = comparator;
if (cmp == null)
siftDownComparable(0, x, array, n);
else
siftDownUsingComparator(0, x, array, n, cmp);
size = n;
return result;
}
}其中siftDownComparable()方法如下：
private static void siftDownComparable(int k, T x, Object[] array,
int n) {
if (n > 0) {
Comparable key = (Comparable)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) 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;
}
} siftDownUsingComparator()方法如下：
private static void siftDownUsingComparator(int k, T x, Object[] array,
int n,
Comparator 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;
}
} 先睡了，明天再分析优先级的具体代码。晚安全世界！