package java.util.concurrent;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
import java.util.AbstractQueue;
import java.util.Collection;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
/**
*
* 无界，链式，阻塞，线程安全的队列
*
* @since 1.5
* @author Doug Lea
* @param <E>
* the type of elements held in this collection
*/
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -6903933977591709194L;
/**
* 队列节点数据结构
*/
static class Node<E> {
E item;
/**
* One of: - the real successor Node - this Node, meaning the successor
* is head.next - null, meaning there is no successor (this is the last
* node)
*/
Node<E> next;
Node(E x) { item = x; }
}
/**
* LinkedBlockingQueue构造的时候若没有指定大小，则默认大小为Integer.MAX_VALUE，当然也可以在构造函数的参数中指定大小。
* LinkedBlockingQueue不接受null。
*/
private final int capacity;
/** 当前元素的数量，线程安全的 */
private final AtomicInteger count = new AtomicInteger();
/**
* Head of linked list. Invariant: head.item == null 头不存东西
*/
transient Node<E> head;
/**
* Tail of linked list. Invariant: last.next == null 尾指向空
*/
private transient Node<E> last;
/** take, poll, 操作的锁 */
private final ReentrantLock takeLock = new ReentrantLock();
/** 是一个锁定状态,take,poll 需要判断是否为空， */
private final Condition notEmpty = takeLock.newCondition();
/** put, offer, 操作的锁 */
private final ReentrantLock putLock = new ReentrantLock();
/** 链队一般不会满，这里是指阻塞 */
private final Condition notFull = putLock.newCondition();
/**
* Signals a waiting take. Called only from put/offer (which do not
* otherwise ordinarily lock takeLock.)
*
* 表示唤醒take/poll等待,只能在put/offer中被调用,也就是元素入队后,可以通过这个来通知队列，不再是空了
*/
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
/**
* Signals a waiting put. Called only from take/poll.
*
* 表示唤醒put/offer等待,只能在take/poll中被调用,也就是出队,可以通过这个方法通知队列不再是满状态了
*/
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
// 唤醒
notFull.signal();
} finally {
putLock.unlock();
}
}
/**
* 内部方法，向队列尾加入节点
*
* @param node
* the node
*/
private void enqueue(Node<E> node) {
// assert putLock.isHeldByCurrentThread();
// assert last.next == null;
last = last.next = node;
}
/**
*
* 内部方法，从队列头移除一个元素
*
* @return the node
*/
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
// assert head.item == null;
Node<E> h = head;
Node<E> first = h.next;
h.next = h; // help GC
head = first;
E x = first.item;
first.item = null;
return x;
}
/**
* 锁定，以防止元素出队
*/
void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* 解锁，以允许元素入队
*/
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
// /**
// * Tells whether both locks are held by current thread.
// */
// boolean isFullyLocked() {
// return (putLock.isHeldByCurrentThread() &&
// takeLock.isHeldByCurrentThread());
// }
/**
* 默认构造
*/
public LinkedBlockingQueue() {
// 可见默认队列大写就是MAX_VALUE
this(Integer.MAX_VALUE);
}
/**
* 可以指定大小的
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();// 不可以小于0，直接抛异常
this.capacity = capacity;
last = head = new Node<E>(null);
}
/**
* 通过先有实现Collection接口的集合构造阻塞队列
*/
public LinkedBlockingQueue(Collection<? extends E> c) {
this(Integer.MAX_VALUE);// 调用默认构造
final ReentrantLock putLock = this.putLock;
putLock.lock(); // 从未发生竞争，但是对于可见性来说是有必要的，很明显，此时队列不可能满
try {
int n = 0;
for (E e : c) {// 遍历c集合
if (e == null)
throw new NullPointerException();// 不能存空值
if (n == capacity)
throw new IllegalStateException("Queue full");
enqueue(new Node<E>(e));// 从尾入队
++n;
}
count.set(n);// 修改元素个数
} finally {
putLock.unlock();// 释放入队锁定
}
}
/**
* 返回队列中元素的个数
*/
public int size() {
return count.get();
}
/**
* 返回可用，容量== 初始容量-现有元素个数，不能通过该方法判断是否入队成功，因为可能被其他线程修改，简单来说就是不精确不可靠的方法
*/
public int remainingCapacity() {
return capacity - count.get();
}
/**
* 将指定的元素插入到此队列的尾部，如果阻塞，则等待空间变得可用。
*
* @throws InterruptedException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
*/
public void put(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
// Note: convention in all put/take/etc is to preset local var
// holding count negative to indicate failure unless set.
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
/*
*
* 直到队列可用【链队如果不设置默认值，就是MAX，几乎用不完】
*/
while (count.get() == capacity) {
notFull.await();
}
enqueue(node);// 进行尾入队
c = count.getAndIncrement();// 计数=元素的个数，然后再+1
if (c + 1 < capacity)
notFull.signal();// 加1后再判断与初始化的容量大小，如果小，则发出唤醒队列【非满】操作
} finally {
putLock.unlock();
}
if (c == 0) // 前面getandset操作，导致只要前面进行成功，后面c必须等于0，而-1表示失败
signalNotEmpty(); // 通知队列不再是空，可以进行下一步的take,poll操作
}
/**
* 在这个队列的尾部插入指定的元素，如果阻塞，等待指定的时间，直到空间可用
*
* @return {@code true} if successful, or {@code false} if the specified
* waiting time elapses before space is available
* @throws InterruptedException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
*/
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
if (e == null) throw new NullPointerException();
long nanos = unit.toNanos(timeout);// 获得纳秒数
int c = -1;
final ReentrantLock putLock = this.putLock;// 得到写入锁
final AtomicInteger count = this.count;// 得到当前元素个数
putLock.lockInterruptibly();// 优先响应中断，而不是响应锁的重入和获取
try {
while (count.get() == capacity) {
// 满队
if (nanos <= 0)
return false;// 小于0即不允许等待，直接返回false
nanos = notFull.awaitNanos(nanos);// 在满队状态上发起指定时间的等待
}
enqueue(new Node<E>(e));// 进行队尾入队
c = count.getAndIncrement();// 当前元素个数+1
if (c + 1 < capacity)
notFull.signal();// 判断是否在加了这个元素之后，变成满队
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();// 唤醒元素非空【第一次添加元素的时候生效】
return true;
}
/**
* 如果立即可行,插入,失败返回false,而不是抛出异常
*/
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity)// 如果是满，直接返回false
return false;
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;// 得到写入锁
putLock.lock();
try {
if (count.get() < capacity) {// 如果小于容量，进行队尾入队
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();// 唤醒可能存在的满队列上的等待
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();// 唤醒可能存在的空队列上的等待
return c >= 0;
}
/**
* 获取/出队
*
* 可响应中断
*/
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
while (count.get() == 0) {
// 空队，进行等待
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
/**
* 出队，阻塞
*
* 可响应中断
*/
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
E x = null;
int c = -1;
long nanos = unit.toNanos(timeout);
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
while (count.get() == 0) {
if (nanos <= 0)
return null;// 如果为空且不允许等待，则直接返回null
nanos = notEmpty.awaitNanos(nanos);// 否则进行指定时间的等待
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
/**
* 出队，无参数的，同理，不可响应中断
*/
public E poll() {
final AtomicInteger count = this.count;
if (count.get() == 0)
return null;
E x = null;
int c = -1;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
if (count.get() > 0) {
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
}
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
/**
* 获取队首元素，但不删除原元素
*/
public E peek() {
if (count.get() == 0)
return null;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
Node<E> first = head.next;
if (first == null)
return null;// 不存在首，返回null
else
return first.item;// 返回首元素
} finally {
takeLock.unlock();
}
}
/**
* 将P从trail链上断开
*/
void unlink(Node<E> p, Node<E> trail) {
// assert isFullyLocked();
// p.next is not changed, to allow iterators that are
// traversing p to maintain their weak-consistency guarantee.
p.item = null;
trail.next = p.next;
if (last == p)
last = trail;
if (count.getAndDecrement() == capacity)
notFull.signal();
}
/**
* 从队列删除元素
*
* 返回true/false
*/
public boolean remove(Object o) {
if (o == null) return false;// 空返回false
fullyLock();
try {
for (Node<E> trail = head, p = trail.next;
p != null;
trail = p, p = p.next) {
if (o.equals(p.item)) {
unlink(p, trail);
return true;
}
}
return false;
} finally {
fullyUnlock();
}
}
/**
* 判断队列是否含有元素
*
* true/false
*/
public boolean contains(Object o) {
if (o == null) return false;// 空返回false
fullyLock();
try {
for (Node<E> p = head.next; p != null; p = p.next)
if (o.equals(p.item))
return true;
return false;
} finally {
fullyUnlock();
}
}
/**
* 将队列转换为数组
*/
public Object[] toArray() {
fullyLock();
try {
int size = count.get();
Object[] a = new Object[size];// new一个元素大小的对象数组
int k = 0;
for (Node<E> p = head.next; p != null; p = p.next)
a[k++] = p.item;
return a;
} finally {
fullyUnlock();
}
}
/**
* 转换为数组
*/
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
fullyLock();
try {
int size = count.get();
if (a.length < size)
a = (T[])java.lang.reflect.Array.newInstance
(a.getClass().getComponentType(), size);
int k = 0;
for (Node<E> p = head.next; p != null; p = p.next)
a[k++] = (T)p.item;
if (a.length > k)
a[k] = null;
return a;
} finally {
fullyUnlock();
}
}
/**
* 默认实现的toString方法
*/
public String toString() {
fullyLock();
try {
Node<E> p = head.next;
if (p == null)
return "[]";
StringBuilder sb = new StringBuilder();
sb.append('[');
for (;;) {
E e = p.item;
sb.append(e == this ? "(this Collection)" : e);
p = p.next;
if (p == null)
return sb.append(']').toString();
sb.append(',').append(' ');
}
} finally {
fullyUnlock();
}
}
/**
*
* 清空队列
*/
public void clear() {
fullyLock();
try {
for (Node<E> p, h = head; (p = h.next) != null; h = p) {
h.next = h;
p.item = null;
}
head = last;
// assert head.item == null && head.next == null;
if (count.getAndSet(0) == capacity)
notFull.signal();
} finally {
fullyUnlock();
}
}
/**
* @throws UnsupportedOperationException
* {@inheritDoc}
* @throws ClassCastException
* {@inheritDoc}
* @throws NullPointerException
* {@inheritDoc}
* @throws IllegalArgumentException
* {@inheritDoc}
*/
public int drainTo(Collection<? super E> c) {
return drainTo(c, Integer.MAX_VALUE);
}
/**
*
* 一次性从BlockingQueue获取所有可用的数据对象（还可以指定获取数据的个数），
* 通过该方法，可以提升获取数据效率；不需要多次分批加锁或释放锁。
*/
public int drainTo(Collection<? super E> c, int maxElements) {
if (c == null)
throw new NullPointerException();// 最大个数为空，抛出异常
if (c == this)
throw new IllegalArgumentException();// 非本对象，抛出异常
if (maxElements <= 0)// 最大个数小于0 ，返回0
return 0;
boolean signalNotFull = false;
final ReentrantLock takeLock = this.takeLock;// 加上读锁
takeLock.lock();
try {
int n = Math.min(maxElements, count.get()); // 获取最先可用的元素个数
// count.get provides visibility to first n Nodes
Node<E> h = head;
int i = 0;
try {
while (i < n) {
Node<E> p = h.next;
c.add(p.item);
p.item = null;
h.next = h;
h = p;
++i;
}
return n;
} finally {
// 如果add抛出了异常。则进行恢复
if (i > 0) {
// assert h.item == null;
head = h;
signalNotFull = (count.getAndAdd(-i) == capacity);
}
}
} finally {
takeLock.unlock();
if (signalNotFull)
signalNotFull();
}
}
/**
* Returns an iterator over the elements in this queue in proper sequence.
* The elements will be returned in order from first (head) to last (tail).
*
* <p>
* The returned iterator is <a href="package-summary.html#Weakly"><i>weakly
* consistent</i></a>.
*
* @return an iterator over the elements in this queue in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}
private class Itr implements Iterator<E> {
/*
* Basic weakly-consistent iterator. At all times hold the next item to
* hand out so that if hasNext() reports true, we will still have it to
* return even if lost race with a take etc.
*/
private Node<E> current;
private Node<E> lastRet;
private E currentElement;
Itr() {
fullyLock();
try {
current = head.next;
if (current != null)
currentElement = current.item;
} finally {
fullyUnlock();
}
}
public boolean hasNext() {
return current != null;
}
/**
* Returns the next live successor of p, or null if no such.
*
* Unlike other traversal methods, iterators need to handle both: -
* dequeued nodes (p.next == p) - (possibly multiple) interior removed
* nodes (p.item == null)
*/
private Node<E> nextNode(Node<E> p) {
for (;;) {
Node<E> s = p.next;
if (s == p)
return head.next;
if (s == null || s.item != null)
return s;
p = s;
}
}
public E next() {
fullyLock();
try {
if (current == null)
throw new NoSuchElementException();
E x = currentElement;
lastRet = current;
current = nextNode(current);
currentElement = (current == null) ? null : current.item;
return x;
} finally {
fullyUnlock();
}
}
public void remove() {
if (lastRet == null)
throw new IllegalStateException();
fullyLock();
try {
Node<E> node = lastRet;
lastRet = null;
for (Node<E> trail = head, p = trail.next;
p != null;
trail = p, p = p.next) {
if (p == node) {
unlink(p, trail);
break;
}
}
} finally {
fullyUnlock();
}
}
}
/** A customized variant of Spliterators.IteratorSpliterator */
static final class LBQSpliterator<E> implements Spliterator<E> {
static final int MAX_BATCH = 1 << 25; // max batch array size;
final LinkedBlockingQueue<E> queue;
Node<E> current; // current node; null until initialized
int batch; // batch size for splits
boolean exhausted; // true when no more nodes
long est; // size estimate
LBQSpliterator(LinkedBlockingQueue<E> queue) {
this.queue = queue;
this.est = queue.size();
}
public long estimateSize() { return est; }
public Spliterator<E> trySplit() {
Node<E> h;
final LinkedBlockingQueue<E> q = this.queue;
int b = batch;
int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
if (!exhausted &&
((h = current) != null || (h = q.head.next) != null) &&
h.next != null) {
Object[] a = new Object[n];
int i = 0;
Node<E> p = current;
q.fullyLock();
try {
if (p != null || (p = q.head.next) != null) {
do {
if ((a[i] = p.item) != null)
++i;
} while ((p = p.next) != null && i < n);
}
} finally {
q.fullyUnlock();
}
if ((current = p) == null) {
est = 0L;
exhausted = true;
}
else if ((est -= i) < 0L)
est = 0L;
if (i > 0) {
batch = i;
return Spliterators.spliterator
(a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL |
Spliterator.CONCURRENT);
}
}
return null;
}
public void forEachRemaining(Consumer<? super E> action) {
if (action == null) throw new NullPointerException();
final LinkedBlockingQueue<E> q = this.queue;
if (!exhausted) {
exhausted = true;
Node<E> p = current;
do {
E e = null;
q.fullyLock();
try {
if (p == null)
p = q.head.next;
while (p != null) {
e = p.item;
p = p.next;
if (e != null)
break;
}
} finally {
q.fullyUnlock();
}
if (e != null)
action.accept(e);
} while (p != null);
}
}
public boolean tryAdvance(Consumer<? super E> action) {
if (action == null) throw new NullPointerException();
final LinkedBlockingQueue<E> q = this.queue;
if (!exhausted) {
E e = null;
q.fullyLock();
try {
if (current == null)
current = q.head.next;
while (current != null) {
e = current.item;
current = current.next;
if (e != null)
break;
}
} finally {
q.fullyUnlock();
}
if (current == null)
exhausted = true;
if (e != null) {
action.accept(e);
return true;
}
}
return false;
}
public int characteristics() {
return Spliterator.ORDERED | Spliterator.NONNULL |
Spliterator.CONCURRENT;
}
}
/**
* Returns a {@link Spliterator} over the elements in this queue.
*
* <p>
* The returned spliterator is <a href="package-summary.html#Weakly"><i>weakly
* consistent</i></a>.
*
* <p>
* The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
*
* @implNote The {@code Spliterator} implements {@code trySplit} to permit
* limited parallelism.
*
* @return a {@code Spliterator} over the elements in this queue
* @since 1.8
*/
public Spliterator<E> spliterator() {
return new LBQSpliterator<E>(this);
}
/**
* Saves this queue to a stream (that is, serializes it).
*
* @param s
* the stream
* @throws java.io.IOException
* if an I/O error occurs
* @serialData The capacity is emitted (int), followed by all of its
* elements (each an {@code Object}) in the proper order,
* followed by a null
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
fullyLock();
try {
// Write out any hidden stuff, plus capacity
s.defaultWriteObject();
// Write out all elements in the proper order.
for (Node<E> p = head.next; p != null; p = p.next)
s.writeObject(p.item);
// Use trailing null as sentinel
s.writeObject(null);
} finally {
fullyUnlock();
}
}
/**
* Reconstitutes this queue from a stream (that is, deserializes it).
*
* @param s
* the stream
* @throws ClassNotFoundException
* if the class of a serialized object could not be found
* @throws java.io.IOException
* if an I/O error occurs
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
// Read in capacity, and any hidden stuff
s.defaultReadObject();
count.set(0);
last = head = new Node<E>(null);
// Read in all elements and place in queue
for (;;) {
@SuppressWarnings("unchecked")
E item = (E)s.readObject();
if (item == null)
break;
add(item);
}
}
}
