PriorityBlockingQueue
最近一直在忙公司新业务流程引擎，耽误了最后一个阻塞队列，之后把流程引擎结合springboot总结一下
PriorityBlockingQueue
描述：
带排序的BlockingQueue实现，采用ReentrantLock控制并发，队列为无界队列，与其他阻塞队列不同的是PriorityBlockingQueue只能指定初始队列大小，后面插入元素的时候，如果空间不够的话会自动扩容。ArrayBlockingQueue是有界队列，LinkedBlockingQueue可以是无界队列（就是容量为Integer.MAX_VALUE），其实就是容量非常大的有界队列。是PriorityBlockingQueue的贤臣安全版本。不可以插入null值(阻塞队列应该都不允许插入null值，印象中，还有去校验)。插入队列的对象必须是可以比较大小的(comparable)，否则报ClassCastException异常。
存储结构介绍
PriorityBlockingQueue是基于数组实现的二叉堆，对于元素a[i]，其左子节点a[2*i+1],其右子节点a[2*i+2],其父节点a[(i-1)/2],而且每个节点的值都小于左右节点,也就是说根节点是二叉堆中最小的值。
示意图
属性介绍
//默认的容量
private static final int DEFAULT_INITIAL_CAPACITY = 11;
//最大容量 是不是 也被上面的无界队列欺骗了
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//数组
private transient Object[] queue;
//当前队列大小
private transient int size;
//大小比较器
private transient Comparatorsuper E> comparator;
//用于控制并发的锁
private final ReentrantLock lock;
//
private final Condition notEmpty;
//再进行扩容时 需要获取这个锁
private transient volatile int allocationSpinLock;
//用于序列化和反序列化时使用
private PriorityQueue<E> q;
构造器
//默认容量 没有指定比较器
public PriorityBlockingQueue() {
this(DEFAULT_INITIAL_CAPACITY, null);
}
//指定容量 没有指定比较器
public PriorityBlockingQueue(int initialCapacity) {
this(initialCapacity, null);
}
//指定容量 比较器
public PriorityBlockingQueue(int initialCapacity,
Comparatorsuper E> comparator) {
if (initialCapacity < 1)
throw new IllegalArgumentException();
this.lock = new ReentrantLock();
this.notEmpty = lock.newCondition();
this.comparator = comparator;
this.queue = new Object[initialCapacity];
}
//可以传入一个集合
public PriorityBlockingQueue(Collectionextends 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) {
SortedSetextends E> ss = (SortedSetextends E>) c;
this.comparator = (Comparatorsuper E>) ss.comparator();
heapify = false;
}
else if (c instanceof PriorityBlockingQueue) {
PriorityBlockingQueueextends E> pq =
(PriorityBlockingQueueextends E>) c;
this.comparator = (Comparatorsuper E>) pq.comparator();
screen = false;
if (pq.getClass() == PriorityBlockingQueue.class) // exact match
heapify = false;
}
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();
}
扩容方法 tryGrow
位置：PriorityBlockingQueue 289
方法目的：扩容
方法流程：
private void tryGrow(Object[] array, int oldCap) {
//先释放锁
lock.unlock(); // must release and then re-acquire main lock
Object[] newArray = null;
//cas 将 allocationSpinLock设置为1 成功代表获取锁
if (allocationSpinLock == 0 &&
UNSAFE.compareAndSwapInt(this, allocationSpinLockOffset,
0, 1)) {
try {
//如果 现在的节点数 小于 64增加 oldCap + 2 如果大于等于增加一半
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 {
//释放锁
allocationSpinLock = 0;
}
}
//如果其他线程有扩容操作
if (newArray == null) // back off if another thread is allocating
Thread.yield();
lock.lock();
//将原来数组中的元素复制到新的数组中
if (newArray != null && queue == array) {
queue = newArray;
System.arraycopy(array, 0, newArray, 0, oldCap);
}
}
Put方法
位置：PriorityBlockingQueue 511
public void put(E e) {
offer(e); // never need to block
}
位置：PriorityBlockingQueue 511
方法目的：元素进队
方法流程：
public boolean offer(E e) {
if (e == null)
throw new NullPointerException();
final ReentrantLock lock = this.lock;
lock.lock();
int n, cap;
Object[] array;
//判断是否需要扩容 数组中的元素个数 >= 当前容量
while ((n = size) >= (cap = (array = queue).length))
tryGrow(array, cap);
try {
Comparatorsuper E> cmp = comparator;
//节点进入二叉堆
if (cmp == null)
siftUpComparable(n, e, array);
else
siftUpUsingComparator(n, e, array, cmp);
//更新当前队列元素数
size = n + 1;
//唤醒读线程
notEmpty.signal();
} finally {
lock.unlock();
}
return true;
}
进入二叉堆方法
//插入的节点比父节点小 那么交换 然后重新比较父节点
//那么怎么选择第一个父节点呢 二叉堆 除了最下面一层 其他层都是满二叉树 比如上面的二//叉堆 如果要进入新元素那么13是第一个父节点
private static <T> void siftUpComparable(int k, T x, Object[] array) {
Comparablesuper T> key = (Comparablesuper T>) x;
while (k > 0) {
//父节点
int parent = (k - 1) >>> 1;
Object e = array[parent];
if (key.compareTo((T) e) >= 0)
break;
array[k] = e;
k = parent;
}
array[k] = key;
}
private static <T> void siftUpUsingComparator(int k, T x, Object[] array,
Comparatorsuper 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;
}
Take方法
位置：PriorityBlockingQueue 544
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
E result;
try {
//如果队列为空 等待
while ( (result = dequeue()) == null)
notEmpty.await();
} finally {
lock.unlock();
}
return result;
}
位置：PriorityBlockingQueue 323
方法目的：元素出队
方法流程：
private E dequeue() {
//队列元素大小
int n = size - 1;
if (n < 0)
return null;
else {
Object[] array = queue;
//对头 返回结果 也就是说先出对头 毕竟是 FIFO队列嘛
E result = (E) array[0];
//队尾出队
E x = (E) array[n];
array[n] = null;
Comparatorsuper E> cmp = comparator;
if (cmp == null)
siftDownComparable(0, x, array, n);
else
siftDownUsingComparator(0, x, array, n, cmp);
size = n;
return result;
}
}
出二叉堆方法
怎么出二叉堆呢，移除一个节点之后，将节点的左右子节点中较小的位置，”上滑”替换原来父节点的位置，然后依次执行.又由于结构除了最后一层满足满二叉树结构，所以最下面一层的最右边节点会补充位置
private static <T> void siftDownComparable(int k, T x, Object[] array,
int n) {
if (n > 0) {
Comparablesuper T> key = (Comparablesuper T>)x;
//a[n]是最后一个元素，父节点是a[(n-1)/2] 我们结合上面的图分析 17出队
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 &&
((Comparablesuper 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,
Comparatorsuper 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;
}
}
