Java PriorityBlockingQueue 原理分析
PriorityBlockingQueue是一个支持优先级的无界阻塞队列，直到系统资源耗尽。默认情况下元素采用自然顺序升序排列。也可以自定义类实现compareTo()方法来指定元素排序规则，或者初始化PriorityBlockingQueue时，指定构造参数Comparator来对元素进行排序。但需要注意的是不能保证同优先级元素的顺序。PriorityBlockingQueue也是基于最小二叉堆实现，使用基于CAS实现的自旋锁来控制队列的动态扩容，保证了扩容操作不会阻塞take操作的执行。
PriorityBlockingQueue有四个构造方法：
// 默认的构造方法，该方法会调用this(DEFAULT_INITIAL_CAPACITY, null)，即默认的容量是11
public PriorityBlockingQueue()
// 根据initialCapacity来设置队列的初始容量
public PriorityBlockingQueue(int initialCapacity)
// 根据initialCapacity来设置队列的初始容量，并根据comparator对象来对数据进行排序
public PriorityBlockingQueue(int initialCapacity, Comparator comparator)
// 根据集合来创建队列
public PriorityBlockingQueue(Collection c)
public class PriorityBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = 5595510919245408276L;
private static final int DEFAULT_INITIAL_CAPACITY = 11;
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
private transient Object[] queue;
private transient int size;
private transient Comparator<? super E> comparator;
private final ReentrantLock lock;
private final Condition notEmpty;
private transient volatile int allocationSpinLock;//扩容时候用到，自旋锁
private PriorityQueue<E> q;//数组实现的最小堆，writeObject和readObject用到。 为了兼容之前的版本，只有在序列化和反序列化才非空
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]; //构造函数没有初始化allocationSpinLock，q
}
public PriorityBlockingQueue(Collection<? extends E> c) {
this.lock = new ReentrantLock();
this.notEmpty = lock.newCondition();
boolean heapify = true; // true if not known to be in heap order
boolean screen = true; // true if must screen for nulls
if (c instanceof SortedSet<?>) {// 如果传入集合是有序集，则无须进行堆有序化
SortedSet<? extends E> ss = (SortedSet<? extends E>) c;
this.comparator = (Comparator<? super E>) ss.comparator();
heapify = false;//不需要重建堆
}// 如果传入集合是PriorityBlockingQueue类型，则不进行堆有序化
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;//不需要重建堆
}
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)
heapify();//重建堆
}
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;
}
private static <T> void siftDownComparable(int k, T x, Object[] array,
int n) {//元素x放到k的位置
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)//比子节点小就不动，小堆
break;
array[k] = c;
k = child;
}
array[k] = key;
}
}
private static <T> void siftUpComparable(int k, T x, Object[] array) {//元素x放到k的位置
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)//比父亲大就不动，小堆
break;
array[k] = e;
k = parent;
}
array[k] = key;
}
public boolean offer(E e) {
if (e == null)// 若插入的元素为null，则直接抛出NullPointerException异常
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;
if (cmp == null)
siftUpComparable(n, e, array);//准备放在最后size位置处
else
siftUpUsingComparator(n, e, array, cmp);
size = n + 1;
notEmpty.signal();// 唤醒等待在空上的线程
} finally {
lock.unlock();
}
return true;
}
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;
}
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
E result;
try {
while ( (result = dequeue()) == null && nanos > 0)
nanos = notEmpty.awaitNanos(nanos);
} finally {
lock.unlock();
}
return result;
}
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (size == 0) ? null : (E) queue[0];
} finally {
lock.unlock();
}
}
public int size() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return size;
} 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;
}
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();
}
}
public boolean contains(Object o) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return indexOf(o) != -1;
} 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 void heapify() {
Object[] array = queue;
int n = size;
int half = (n >>> 1) - 1;
Comparator<? super E> cmp = comparator;
if (cmp == null) {
for (int i = half; i >= 0; i--)
siftDownComparable(i, (E) array[i], array, n);//数组重建为堆
}
else {
for (int i = half; i >= 0; i--)
siftDownUsingComparator(i, (E) array[i], array, n, cmp);
}
}
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();
}
public int drainTo(Collection<? super E> c, int maxElements) {//批量获取元素
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++) {// 循环遍历，不断弹出队首元素；
c.add((E) queue[0]); // In this order, in case add() throws.
dequeue();
}
return n;
} finally {
lock.unlock();
}
}
}
放，取，移除 的时候都加锁，同时只能一个线程操作。
private PriorityQueue q;//数组实现的最小堆，writeObject和readObject用到。
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
lock.lock();
try {
// avoid zero capacity argument
q = new PriorityQueue<E>(Math.max(size, 1), comparator);
q.addAll(this);
s.defaultWriteObject();
} finally {
q = null;
lock.unlock();
}
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
try {
s.defaultReadObject();
int sz = q.size();
SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, sz);
this.queue = new Object[sz];
comparator = q.comparator();
addAll(q);
} finally {
q = null;
}
}
private transient volatile int allocationSpinLock;//扩容时候用到
不扩容就是正常的获取锁之后加入元素。
扩容时候释放了锁，如果取的线程获取了锁可以取，如果offer的线程获取了锁可以放（方法中释放了锁，别的线程就可以进去这个方法，也可以进去其他需要锁的方法）
释放了lock锁加了一把allocationSpinLock 锁（这个锁：获取到的走进去，没有获取到的跳过。）