AbstractQueue简介： http://donald-draper.iteye.com/blog/2363608
ConcurrentLinkedQueue解析： http://donald-draper.iteye.com/blog/2363874
BlockingQueue接口的定义： http://donald-draper.iteye.com/blog/2363942
LinkedBlockingQueue解析： http://donald-draper.iteye.com/blog/2364007
ArrayBlockingQueue解析： http://donald-draper.iteye.com/blog/2364034
package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.*;
/**
* An unbounded {@linkplain BlockingQueue blocking queue} that uses
* the same ordering rules as class {@link PriorityQueue} and supplies
* blocking retrieval operations. While this queue is logically
* unbounded, attempted additions may fail due to resource exhaustion
* (causing {@code OutOfMemoryError}). This class does not permit
* {@code null} elements. A priority queue relying on {@linkplain
* Comparable natural ordering} also does not permit insertion of
* non-comparable objects (doing so results in
* {@code ClassCastException}).
*
PriorityBlockingQueue是一个与PriorityQueue具有相同排序策略的无界阻塞队列，
提供阻塞检索操作。队列虽说是无界的，但当内存资源耗尽时，尝试添加元素，则
将会失败。队列不允许null元素的存在。优先级队列插入的元素依据元素的Comparable，
不允许插入一个不可比较的元素。
* <p>This class and its iterator implement all of the
* [i]optional[/i] methods of the {@link Collection} and {@link
* Iterator} interfaces. The Iterator provided in method {@link
* #iterator()} is [i]not[/i] guaranteed to traverse the elements of
* the PriorityBlockingQueue in any particular order. If you need
* ordered traversal, consider using
* {@code Arrays.sort(pq.toArray())}. Also, method {@code drainTo}
* can be used to [i]remove[/i] some or all elements in priority
* order and place them in another collection.
队列实现了集合接口和迭代器的所有方法。迭代器的#iterator方法不能保证，
不能以特殊的顺序traverse（移动）元素。如果需要以特殊的方式traverse（移动）元素，则
可以使用Arrays.sort(pq.toArray()方法。drainTo用于将一些元素，或所有元素以优先级的
顺序移动到另一个集合中。
*
* <p>Operations on this class make no guarantees about the ordering
* of elements with equal priority. If you need to enforce an
* ordering, you can define custom classes or comparators that use a
* secondary key to break ties in primary priority values. For
* example, here is a class that applies first-in-first-out
* tie-breaking to comparable elements. To use it, you would insert a
* {@code new FIFOEntry(anEntry)} instead of a plain entry object.
*
队列的所有操作不能保证按照元素优先级的顺序。如果需要重新定义一个以原始优先级作为key的
比较器，保证顺序。比如原始为FIFO队列，你可以用 FIFOEntry代替原始的Entry
* <pre> {@code
* class FIFOEntry<E extends Comparable<? super E>>
* implements Comparable<FIFOEntry<E>> {
* static final AtomicLong seq = new AtomicLong(0);
* final long seqNum;
* final E entry;
* public FIFOEntry(E entry) {
* seqNum = seq.getAndIncrement();
* this.entry = entry;
* }
* public E getEntry() { return entry; }
* public int compareTo(FIFOEntry<E> other) {
* int res = entry.compareTo(other.entry);
* if (res == 0 && other.entry != this.entry)
* res = (seqNum < other.seqNum ? -1 : 1);
* return res;
* }
* }}</pre>
*
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea
* @param <E> the type of elements held in this collection
*/
public class PriorityBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = 5595510919245408276L;
/*
* The implementation uses an array-based binary heap, with public
* operations protected with a single lock. However, allocation
* during resizing uses a simple spinlock (used only while not
* holding main lock) in order to allow takes to operate
* concurrently with allocation. This avoids repeated
* postponement of waiting consumers and consequent element
* build-up. The need to back away from lock during allocation
* makes it impossible to simply wrap delegated
* java.util.PriorityQueue operations within a lock, as was done
* in a previous version of this class. To maintain
* interoperability, a plain PriorityQueue is still used during
* serialization, which maintains compatibility at the espense of
* transiently doubling overhead.
用一个二进制的数组堆来实现，用一个lock来保护public方法操作。
然而，为take操作与扩容操作的并发，我们用一个自旋锁来控制空间分配。
这可以避免消费者的重复等待和元素的包装。
The need to back away from lock during allocation
makes it impossible to simply wrap delegated
java.util.PriorityQueue operations within a lock, as was done
in a previous version of this class.
为了保证互操作性，在序列化的时候用了一个空白的PriorityQueue，
which maintains compatibility at the espense of
transiently doubling overhead.
*/
/**
* Default array capacity.
*/
默认容量
private static final int DEFAULT_INITIAL_CAPACITY = 11;
/**
* The maximum size of array to allocate.
* Some VMs reserve some header words in an array.
* Attempts to allocate larger arrays may result in
* OutOfMemoryError: Requested array size exceeds VM limit
*/
//最大队列容量
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
/**
* Priority queue represented as a balanced binary heap: the two
* children of queue[n] are queue[2*n+1] and queue[2*(n+1)]. The
* priority queue is ordered by comparator, or by the elements'
* natural ordering, if comparator is null: For each node n in the
* heap and each descendant d of n, n <= d. The element with the
* lowest value is in queue[0], assuming the queue is nonempty.
优先级队列，表示一个平衡二叉树堆：
queue[n]的两个字节点为queue[2*n+1] and queue[2*(n+1)]。优先级队列以元素
的比较作为排序依据，如果比较器为null，则以元素的自然属性排序。每个堆元素n的
子孙d满足n <= d。如果队列为非空，最低优先级元素放在queue[0]。
*/
private transient Object[] queue;
/**
* The number of elements in the priority queue.队列元素数量
*/
private transient int size;
/**
* The comparator, or null if priority queue uses elements'
* natural ordering.
比较器，为空，则按自然属性
*/
private transient Comparator<? super E> comparator;
/**
* Lock used for all public operations，public操作保护lock
*/
private final ReentrantLock lock;
/**
* Condition for blocking when empty，队列非空条件
*/
private final Condition notEmpty;
/**
* Spinlock for allocation, acquired via CAS,通过CAS获取分配的自旋锁
*/
private transient volatile int allocationSpinLock;
/**
* A plain PriorityQueue used only for serialization,
* to maintain compatibility with previous versions
* of this class. Non-null only during serialization/deserialization.
空有限级队列用于序列化
*/
private PriorityQueue q;
/**
* Creates a {@code PriorityBlockingQueue} with the default
* initial capacity (11) that orders its elements according to
* their {@linkplain Comparable natural ordering}.
*/
public PriorityBlockingQueue() {
this(DEFAULT_INITIAL_CAPACITY, null);
}
/**
* Creates a {@code PriorityBlockingQueue} with the specified
* initial capacity that orders its elements according to their
* {@linkplain Comparable natural ordering}.
*
* @param initialCapacity the initial capacity for this priority queue
* @throws IllegalArgumentException if {@code initialCapacity} is less
* than 1
*/
public PriorityBlockingQueue(int initialCapacity) {
this(initialCapacity, null);
}
/**
* Creates a {@code PriorityBlockingQueue} with the specified initial
* capacity that orders its elements according to the specified
* comparator.
*
待比较器和容量参数的构造
* @param initialCapacity the initial capacity for this priority queue
* @param comparator the comparator that will be used to order this
* priority queue. If {@code null}, the {@linkplain Comparable
* natural ordering} of the elements will be used.
* @throws IllegalArgumentException if {@code initialCapacity} is less
* than 1
*/
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];
}
来看put操作:
/**
* Inserts the specified element into this priority queue.
* As the queue is unbounded, this method will never block.
*
* @param e the element to add
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according to the
* priority queue's ordering
* @throws NullPointerException if the specified element is null
*/
public void put(E e) {
//委托给put操作
offer(e); // never need to block
}
//offer操作
/**
* Inserts the specified element into this priority queue.
* As the queue is unbounded, this method will never return {@code false}.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Queue#offer})
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according to the
* priority queue's ordering
* @throws NullPointerException if the specified element is null
*/
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))
//如果队列已满，释放锁，获取扩容锁，成功则扩容，扩容后则重新获取锁，将原始队列拷贝的新的队列中。
tryGrow(array, cap);
try {
Comparator<? super E> cmp = comparator;
//比较，确定元素存储的位置
if (cmp == null)
siftUpComparable(n, e, array);
else
siftUpUsingComparator(n, e, array, cmp);
//容量自增
size = n + 1;
//唤醒等待take的线程
notEmpty.signal();
} finally {
lock.unlock();
}
return true;
}
这里有几点要关注：
1.
while ((n = size) >= (cap = (array = queue).length))
//如果队列已满
tryGrow(array, cap);
2.
Comparator<? super E> cmp = comparator;
if (cmp == null)
siftUpComparable(n, e, array);
3.
else
siftUpUsingComparator(n, e, array, cmp);
先看第一点
1.
while ((n = size) >= (cap = (array = queue).length))
//如果队列已满
tryGrow(array, cap);
/**
* Tries to grow array to accommodate at least one more element
* (but normally expand by about 50%), giving up (allowing retry)
* on contention (which we expect to be rare). Call only while
* holding lock.
尝试调整至少一个元素，释放锁。在需要的时候再持有锁
* @param array the heap array
* @param oldCap the length of the array
*/
private void tryGrow(Object[] array, int oldCap) {
lock.unlock(); // must release and then re-acquire main lock，先释放锁，需要时，在重新获取
Object[] newArray = null;
//获取扩容锁，成功则，重新扩展队列容量，扩容后则重新获取锁，将原始队列拷贝的新的队列中。
if (allocationSpinLock == 0 &&
UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
0, 1)) {
try {
//当容量小于64时，则增长容量为原来的2倍，大于64则每次增长两个。
int newCap = oldCap + ((oldCap < 64) ?
(oldCap + 2) : // grow faster if small
(oldCap >> 1));
if (newCap - MAX_ARRAY_SIZE > 0) { // possible overflow
//如果添加元素后，容量移除，或大于MAX_ARRAY_SIZE，则抛出OutOfMemoryError
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
//如果分配失败，则暂定当前线程
Thread.yield();
//自旋后，重新获取锁
lock.lock();
if (newArray != null && queue == array) {
queue = newArray;
//将原始队列拷贝的新的队列中
System.arraycopy(array, 0, newArray, 0, oldCap);
}
}
这一步，首先释放锁，获取扩容锁，成功则，重新扩展队列容量，扩容后则重新获取锁，将原始队列拷贝的新的队列中。
2.
Comparator<? super E> cmp = comparator;
if (cmp == null)
siftUpComparable(n, e, array);
/**
* Inserts item x at position k, maintaining heap invariant by
* promoting x up the tree until it is greater than or equal to
* its parent, or is the root.
*
* To simplify and speed up coercions and comparisons. the
* Comparable and Comparator versions are separated into different
* methods that are otherwise identical. (Similarly for siftDown.)
* These methods are static, with heap state as arguments, to
* simplify use in light of possible comparator exceptions.
*
* @param k the position to fill
* @param x the item to insert
* @param array the heap array
* @param n heap size
*/
private static <T> void siftUpComparable(int k, T x, Object[] array) {
//获取元素的比较器
Comparable<? super T> key = (Comparable<? super T>) x;
while (k > 0) {
//数组k叶节点的父节点为(k - 1) >>> 1，无符号右移，左补0
int parent = (k - 1) >>> 1;
Object e = array[parent];
//比较，确定元素存储的位置
if (key.compareTo((T) e) >= 0)
break;
array[k] = e;
k = parent;
}
array[k] = key;
}
3.
else
siftUpUsingComparator(n, e, array, cmp);
这个以上一步没有什么太大的区别，唯一区别是，使用的自定义的比较器
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;
}
小节：
offer操作，获取锁，如果队列已满，释放锁，获取扩容锁，成功则扩容，扩容后则重新获取锁，将原始队列拷贝的新的队列中。获取当前队列中的尾元素的父节点，将当前要添加的元素与父节点比较，确定存储的位置。比较，确定元素存储的位置。最后容量自增，唤醒等待take的线程。从offer操作来看，数组队列存放的逻辑结构实际上是一个平衡二叉树（堆排序）。
add操作：
/**
* Inserts the specified element into this priority queue.
*
* @param e the element to add
* @return {@code true} (as specified by {@link Collection#add})
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according to the
* priority queue's ordering
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {
return offer(e);
}
超时offer操作：
**
* Inserts the specified element into this priority queue.
* As the queue is unbounded, this method will never block or
* return {@code false}.
*
* @param e the element to add
* @param timeout This parameter is ignored as the method never blocks
* @param unit This parameter is ignored as the method never blocks
* @return {@code true} (as specified by
* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
* @throws ClassCastException if the specified element cannot be compared
* with elements currently in the priority queue according to the
* priority queue's ordering
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e, long timeout, TimeUnit unit) {
return offer(e); // never need to block
}
从上面来看无论是add，put，还是超时offer操作，都是委托给offer操作。
再来看take操作：
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
//以可中断方式获取锁
lock.lockInterruptibly();
E result;
try {
while ( (result = extract()) == null)
//如果队列为空，则等待非空条件notEmpty
notEmpty.await();
} finally {
lock.unlock();
}
return result;
}
这里我们有一点要关注的是
while ( (result = extract()) == null)
//如果队列为空，则等待非空条件notEmpty
notEmpty.await();
/**
* Mechanics for poll(). Call only while holding lock.
*/
private E extract() {
E result;
int n = size - 1;
if (n < 0)
//队列为空
result = null;
else {
Object[] array = queue;
从队列头获取元素
result = (E) array[0];
E x = (E) array[n];
array[n] = null;
Comparator<? super E> cmp = comparator;
//从队列头去除元素，则重新调整平衡二叉树
if (cmp == null)
siftDownComparable(0, x, array, n);
else
siftDownUsingComparator(0, x, array, n, cmp);
size = n;
}
return result;
}
//调整平衡二叉树
/**
* Inserts item x at position k, maintaining heap invariant by
* demoting x down the tree repeatedly until it is less than or
* equal to its children or is a leaf.
*
* @param k the position to fill
* @param x the item to insert
* @param array the heap array
* @param n heap size
*/
private static <T> void siftDownComparable(int k, T x, Object[] array,
int n) {
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;
}
//使用比较器调整平衡二叉树
private static <T> void siftDownUsingComparator(int k, T x, Object[] array,
int n,
Comparator<? super T> cmp) {
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;
}
从上面可以看出，take操首先以可中断方式获取锁，如果获取成功，则从队列头部获取元素，
并重新调整平衡二叉树，如果从队列头取的元素为null，则等待非空条件notEmpty。
poll操作
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
E result;
try {
result = extract();
} finally {
lock.unlock();
}
return result;
}
poll操作与take的区别时，直接从队列头部获取元素为null，直接返回，而不是等待非空条件notEmpty。
再看超时poll，
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 = extract()) == null && nanos > 0)
nanos = notEmpty.awaitNanos(nanos);
} finally {
lock.unlock();
}
return result;
}
超时poll与take的区别为当队列为空时，超时等待非空条件notEmpty。
再看peek操作：
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
E result;
try {
result = size > 0 ? (E) queue[0] : null;
} finally {
lock.unlock();
}
return result;
}
peek操作获取锁，返回队头元素。
再来看remove操作:
/**
* Removes a single instance of the specified element from this queue,
* if it is present. More formally, removes an element {@code e} such
* that {@code o.equals(e)}, if this queue contains one or more such
* elements. Returns {@code true} if and only if this queue contained
* the specified element (or equivalently, if this queue changed as a
* result of the call).
*
* @param o element to be removed from this queue, if present
* @return {@code true} if this queue changed as a result of the call
*/
public boolean remove(Object o) {
boolean removed = false;
final ReentrantLock lock = this.lock;
lock.lock();
try {
//定位数据在队列中的索引
int i = indexOf(o);
if (i != -1) {
//如果存在对应的元素，则移除
removeAt(i);
removed = true;
}
} finally {
lock.unlock();
}
return removed;
}
有两点要看：
1.
//定位数据在队列中的索引
int i = indexOf(o);
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;
}
2.
if (i != -1) {
//如果存在对应的元素，则移除
removeAt(i);
removed = true;
}
/**
* Removes the ith element from queue.
*/
private void removeAt(int i) {
Object[] array = queue;
int n = size - 1;
//移除元素
if (n == i) // removed last element
array[i] = null;
else {
E moved = (E) array[n];
array[n] = null;
Comparator<? super E> cmp = comparator;
//左旋
if (cmp == null)
siftDownComparable(i, moved, array, n);
else
siftDownUsingComparator(i, moved, array, n, cmp);
//右旋
if (array[i] == moved) {
if (cmp == null)
siftUpComparable(i, moved, array);
else
siftUpUsingComparator(i, moved, array, cmp);
}
}
size = n;
}
从上可以看出，remove操作，首先获取锁，再次定位元素的位置，移除元素，调整平衡二叉树。
再看contains
public boolean contains(Object o) {
int index;
final ReentrantLock lock = this.lock;
lock.lock();
try {
index = indexOf(o);
} finally {
lock.unlock();
}
return index != -1;
}
这个不需要多讲了吧。
有了上面的分析，下面的drainTo和clear应该很容易理解了吧。
drainTo操作：
/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
if (c == null)
throw new NullPointerException();
if (c == this)
throw new IllegalArgumentException();
final ReentrantLock lock = this.lock;
lock.lock();
try {
int n = 0;
E e;
while ( (e = extract()) != null) {
c.add(e);
++n;
}
return n;
} finally {
lock.unlock();
}
}
/**
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
*/
public int drainTo(Collection<? super E> c, int maxElements) {
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 = 0;
E e;
while (n < maxElements && (e = extract()) != null) {
c.add(e);
++n;
}
return n;
} finally {
lock.unlock();
}
}
clear操作：
/**
* Atomically removes all of the elements from this queue.
* The queue will be empty after this call returns.
*/
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();
}
}
序列化：
/**
* Saves the state to a stream (that is, serializes it). For
* compatibility with previous version of this class,
* elements are first copied to a java.util.PriorityQueue,
* which is then serialized.
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
lock.lock();
try {
int n = size; // avoid zero capacity argument
q = new PriorityQueue<E>(n == 0 ? 1 : n, comparator);
q.addAll(this);
s.defaultWriteObject();
} finally {
q = null;
lock.unlock();
}
}
反序列：
/**
* Reconstitutes the {@code PriorityBlockingQueue} instance from a stream
* (that is, deserializes it).
*
* @param s the stream
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
try {
s.defaultReadObject();
this.queue = new Object[q.size()];
comparator = q.comparator();
addAll(q);
} finally {
q = null;
}
}
序列化与反序列主要是将元素放在一个PriorityQueue中，进行序列化与反序列操作。
总结：
offer操作，获取锁，如果队列已满，释放锁，获取扩容锁，成功则扩容，扩容后则重新获取锁，将原始队列拷贝的新的队列中。获取当前队列中的尾元素的父节点，将当前要添加的元素与父节点比较，确定存储的位置。比较，确定元素存储的位置。最后容量自增，唤醒等待take的线程。从offer操作来看，数组队列存放的逻辑结构实际上是一个平衡二叉树（堆排序）。无论是add，put，还是超时offer操作，都是委托给offer操作。
take操首先以可中断方式获取锁，如果获取成功，则从队列头部获取元素，
并重新调整平衡二叉树，如果从队列头取的元素为null，则等待非空条件notEmpty。
poll操作与take的区别时，直接从队列头部获取元素为null，直接返回，而不是等待非空条件notEmpty。超时poll与take的区别为当队列为空时，超时等待非空条件notEmpty。
peek操作获取锁，返回队头元素。
remove操作，首先获取锁，再次定位元素的位置，移除元素，调整平衡二叉树。
序列化与反序列主要是将元素放在一个PriorityQueue中，进行序列化与反序列操作。