【注意】最后更新于 June 4, 2017，文中内容可能已过时，请谨慎使用。
LinkBlockingQueue源码分析
简介
LinkedBlockingQueue是一个单向链表实现的阻塞队列。该队列按FIFO（先进先出）排序元素，新元素插入到队列的尾部，并且队列获取操作会获得位于队列头部的元素。
链接队列的吞吐量通常要高于基于数组的队列，但是在大多数并发应用程序中，其可预知的性能要低。此外，LinkedBlockingQueue还是可选容量的(防止过度膨胀)，即可以
指定队列的容量。如果不指定，默认容量大小等于Integer.MAX_VALUE。
一、LinkedBlockingQueue 数据结构实现
/**
* Linked list node class
*/
static class Node<E> {
E item;
/**
* One of:
* - the real successor
* - this Node, meaning the successor is head.next
* - null, meaning there is no successor (this is the last node) (null，意味着没有后续节点(这是最后一个节点))
*/
Node<E> next;
Node(E x) { item = x; }
}
```
- LinkedBlockingQueue 内部结构实现
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public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -6903933977591709194L;
/** The capacity bound, or Integer.MAX_VALUE if none */
//容量
private final int capacity;
/** Current number of elements */
//当前的元素数量(原子Int)
private final AtomicInteger count = new AtomicInteger(0);
/**
* Head of linked list.
* Invariant: head.item == null
*/
//链表头节点
private transient Node<E> head;
/**
* Tail of linked list.
* Invariant: last.next == null
*/
//链表的尾节点
private transient Node<E> last;
/** Lock held by take, poll, etc */
//重入锁，控制出队列
private final ReentrantLock takeLock = new ReentrantLock();
/** Wait queue for waiting takes */
//不为空等待条件
private final Condition notEmpty = takeLock.newCondition();
/** Lock held by put, offer, etc */
//重入锁，控制添加队列
private final ReentrantLock putLock = new ReentrantLock();
/** Wait queue for waiting puts */
//队列满等待条件
private final Condition notFull = putLock.newCondition();
/**
* Creates a {@code LinkedBlockingQueue} with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
/**
* Creates a {@code LinkedBlockingQueue} with the given (fixed) capacity.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if {@code capacity} is not greater
* than zero
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
/**
* Creates a {@code LinkedBlockingQueue} with a capacity of
* {@link Integer#MAX_VALUE}, initially containing the elements of the
* given collection,
* added in traversal order of the collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public LinkedBlockingQueue(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock putLock = this.putLock;
putLock.lock(); // Never contended, but necessary for visibility
try {
int n = 0;
for (E e : c) {
if (e == null)
throw new NullPointerException();
if (n == capacity)
throw new IllegalStateException("Queue full");
enqueue(new Node<E>(e));
++n;
}
count.set(n);
} finally {
putLock.unlock();
}
}
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}
```
LinkedBlockingQueue结构如图下:
二、入队
1. offer方法
- offer(E e)
- offer(E e, long timeout, TimeUnit unit) 将指定元素插入该队列的尾部，如果队列已满，则等待空间的指定等待时间
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/**
* Inserts the specified element at the tail of this queue if it is
* possible to do so immediately without exceeding the queue's capacity,
* returning {@code true} upon success and {@code false} if this queue
* is full.
* 如果在队列尾部插入指定元素，如果不超过队列的容量，就可以立即执行，返回{@代码true}，如果此队列已满，则返回{@ code false}。
* When using a capacity-restricted queue, this method is generally
* preferable to method {@link BlockingQueue#add add}, which can fail to
* insert an element only by throwing an exception.
*
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity) //判断容量是否满
return false;
int c = -1;
Node<E> node = new Node(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
if (count.get() < capacity) { //双重判断容量是否满
enqueue(node); // 队列尾端的链接节点
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();//等待条件唤醒
}
} finally {
putLock.unlock();
}
if (c == 0) //队列为空情况，不为空条件等待线程唤醒
signalNotEmpty();
return c >= 0;
}
/**
* Links node at end of queue.
*
* @param node the node
*/
private void enqueue(Node<E> node) {
// assert putLock.isHeldByCurrentThread();
// assert last.next == null;
last = last.next = node;
}
/**
* Signals a waiting take. Called only from put/offer (which do not
* otherwise ordinarily lock takeLock.)
*/
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
/**
* Inserts the specified element at the tail of this queue, waiting if
* necessary up to the specified wait time for space to become available.
*
* @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);
int c = -1;
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
while (count.get() == capacity) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos); //按指定时间等待，等队列未满情况。
}
enqueue(new Node<E>(e));
c = count.getAndIncrement(); ////count.getAndIncrement()得到为修改值
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return true;
}
```
### 2. add方法
add(E e) 使用AbstractQueue抽象类中add方法,具体调用offer方法做判断，队列满了，throw IllegalStateException异常
```java
// AbstractQueue.add
public boolean add(E e) {
if (offer(e))
return true;
else
throw new IllegalStateException("Queue full");
}
|
3. put方法
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/**
* Inserts the specified element at the tail of this queue, waiting if
* necessary for space to become available.
* 将指定元素插入该队列的尾部，等待空间可用的必要条件。
* @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);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();
try {
/*
* Note that count is used in wait guard even though it is
* not protected by lock. This works because count can
* only decrease at this point (all other puts are shut
* out by lock), and we (or some other waiting put) are
* signalled if it ever changes from capacity. Similarly
* for all other uses of count in other wait guards.
*/
while (count.get() == capacity) { //判断队列满，线程等待！
notFull.await();
}
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
}
|
二、出队
1. poll方法
- poll() 获取并移除此队列的头，如果此队列为空，则返回 null
- E poll(long timeout, TimeUnit unit) 获取并移除此队列的头部，队列为空时，在指定的等待时间前等待可用的元素。
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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;
nanos = notEmpty.awaitNanos(nanos); //指定等待时间，
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity) //
signalNotFull();
return x;
}
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;
}
/**
* Removes a node from head of queue.
* 移除头节点
* @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;
}
/**
* Signals a waiting put. Called only from take/poll.
*/
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
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2. take方法
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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) { //队列为空时，线程等待
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();//count.getAndDecrement()得到为修改值
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
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3. peek方法
peek方法是java.util.Queue接口方法。获取队列的头部，不删除队列头部元素
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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;
else
return first.item;
} finally {
takeLock.unlock();
}
}
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三、移除队列
1. remove方法
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/**
* 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 this queue contained the specified element
* (or equivalently, if this queue changed as a result of the call).
* 删除队列中指定元素
* @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) {
if (o == null) return 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); //删除一个节点
return true;
}
}
return false;
} finally {
fullyUnlock();
}
}
/**
* Lock to prevent both puts and takes.
*
*/
void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* Unlinks interior Node p with predecessor trail.
*/
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) //count.getAndDecrement()得到以前为修改值，判断未修改值等于队列容量，唤醒等到队列满线程
notFull.signal();
}
/**
* Unlock to allow both puts and takes.
*/
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
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2. clear方法
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/**
* Atomically removes all of the elements from this queue.
* The queue will be empty after this call returns.
* 从原子上删除该队列中的所有元素。
*/
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();
}
}
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3. drainTo方法
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/**
* @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);
}
/**
* @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();
boolean signalNotFull = false;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
int n = Math.min(maxElements, count.get());//获取最小值
// count.get provides visibility to first n Nodes
Node<E> h = head;
int i = 0;
try {
//获取区间值，更新链表值
while (i < n) {
Node<E> p = h.next;
c.add(p.item);
p.item = null;
h.next = h;
h = p;
++i;
}
return n;
} finally {
// Restore invariants even if c.add() threw
if (i > 0) {
// assert h.item == null;
head = h;
signalNotFull = (count.getAndAdd(-i) == capacity);
}
}
} finally {
takeLock.unlock();
if (signalNotFull)
signalNotFull();
}
}
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五、Iterator 迭代器实现
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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; //当前节点
private Node<E> lastRet; //最后节点
private E currentElement; //当前值
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.
* 返回p的next节点，如果没有，则返回null。
* Unlike other traversal methods, iterators need to handle both:
* - dequeued nodes (p.next == p)
* - (possibly multiple) interior removed nodes (p.item == null)
* 与其他遍历方法不同，迭代器需要同时处理两个问题:
* 删除节点时 (p.next == p)
* (可能是多个)interior removed 节点 (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);//删除节点
break;
}
}
} finally {
fullyUnlock();
}
}
}
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