线程安全的无界阻塞队列LinkedBlockingQueue
LinkedBlockingQueue 是一个基于链表的无界队列（理论上有界），队列按照先进先出的顺序进行排序。LinkedBlockingQueue不同于ArrayBlockingQueue，它如果不指定容量，默认为 Integer.MAX_VALUE，也就是无界队列。所以为了避免队列过大造成机器负载或者内存爆满的情况出现，我们在使用的时候建议手动传一个队列的大小。
队列创建
BlockingQueue blockingQueue = new LinkedBlockingQueue<>();上面这段代码中，blockingQueue 的容量将设置为 Integer.MAX_VALUE 。
应用场景
多用于任务队列，单线程发布任务，任务满了就停止等待阻塞，当任务被完成消费少了又开始负责发布任务。
我们来看一个例子：
public class TestLinkedBlockingQueue {
private static LinkedBlockingQueue<String> queue = new LinkedBlockingQueue<String>();
// 线程控制开关
private final CountDownLatch latch = new CountDownLatch(1);
// 线程池
private final ExecutorService pool;
// AtomicLong 计数 生产数量
private final AtomicLong output = new AtomicLong(0);
// AtomicLong 计数 销售数量
private final AtomicLong sales = new AtomicLong(0);
// 是否停止线程
private final boolean clear;
public TestLinkedBlockingQueue(boolean clear) {
this.pool = Executors.newCachedThreadPool();
this.clear = clear;
}
public void service() throws InterruptedException {
Consumer a = new Consumer(queue, sales, latch, clear);
pool.submit(a);
Producer w = new Producer(queue, output, latch);
pool.submit(w);
latch.countDown();
}
public static void main(String[] args) {
TestLinkedBlockingQueue t = new TestLinkedBlockingQueue(false);
try {
t.service();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
/**
* 消费者（销售产品）
*/
class Consumer implements Runnable {
private final LinkedBlockingQueue<String> queue;
private final AtomicLong sales;
private final CountDownLatch latch;
private final boolean clear;
public Consumer(LinkedBlockingQueue<String> queue, AtomicLong sales, CountDownLatch latch, boolean clear) {
this.queue = queue;
this.sales = sales;
this.latch = latch;
this.clear = clear;
}
public void run() {
try {
latch.await(); // 放闸之前老实的等待着
for (; ; ) {
sale();
Thread.sleep(500);
}
} catch (InterruptedException e) {
if (clear) { // 响应中断请求后,如果有要求则销售完队列的产品后再终止线程
cleanWarehouse();
} else {
System.out.println("Seller Thread will be interrupted...");
}
}
}
public void sale() {
System.out.println("==取take=");
try {
String item = queue.poll(50, TimeUnit.MILLISECONDS);
System.out.println(item);
if (item != null) {
sales.incrementAndGet(); // 可以声明long型的参数获得返回值,作为日志的参数
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
/**
* 销售完队列剩余的产品
*/
private void cleanWarehouse() {
try {
while (queue.size() > 0) {
sale();
}
} catch (Exception ex) {
System.out.println("Seller Thread will be interrupted...");
}
}
}
/**
* 生产者（生产产品）
*
*/
class Producer implements Runnable {
private LinkedBlockingQueue<String> queue;
private CountDownLatch latch;
private AtomicLong output;
public Producer() {
}
public Producer(LinkedBlockingQueue<String> queue, AtomicLong output, CountDownLatch latch) {
this.queue = queue;
this.latch = latch;
this.output = output;
}
public void run() {
try {
latch.await(); // 线程等待
for (; ; ) {
work();
Thread.sleep(100);
}
} catch (InterruptedException e) {
System.out.println("Producer thread will be interrupted...");
}
}
/**
* 工作
*/
public void work() {
try {
String product = RandomStringUtils.randomAscii(3);
boolean success = queue.offer(product, 100, TimeUnit.MILLISECONDS);
if (success) {
output.incrementAndGet();// 可以声明long型的参数获得返回值,作为日志的参数
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}工作原理
LinkedBlockingQueue内部由单链表实现，只能从head取元素，从tail添加元素。添加元素和获取元素都有独立的锁，也就是说LinkedBlockingQueue是读写分离的，读写操作可以并行执行。LinkedBlockingQueue采用可重入锁(ReentrantLock)来保证在并发情况下的线程安全。
向无限队列添加元素的所有操作都将永远不会阻塞，[注意这里不是说不会加锁保证线程安全]，因此它可以增长到非常大的容量。
使用无限 BlockingQueue 设计生产者 - 消费者模型时最重要的是 消费者应该能够像生产者向队列添加消息一样快地消费消息。否则，内存可能会填满，然后就会得到一个 OutOfMemory 异常。
源码分析
定义
LinkedBlockingQueue的类继承关系如下：
其包含的方法定义如下：
成员属性
/**
* 节点类，用于存储数据
*/
static class Node<E> {
E item;
Node<E> next;
Node(E x) { item = x; }
}
/** 阻塞队列的大小, 默认为Integer.MAX_VALUE */
private final int capacity;
/** 当前阻塞队列中的元素个数 */
private final AtomicInteger count = new AtomicInteger();
/**
* 阻塞队列的头节点
*/
transient Node<E> head;
/**
* 阻塞队列的尾节点
*/
private transient Node<E> last;
/** 获取并移除元素时使用的锁，如take，poll，etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** notEmpty 条件对象，当队列没有数据时用于挂起执行删除的线程 */
private final Condition notEmpty = takeLock.newCondition();
/** 添加元素时使用的锁，如 put，offer，etc */
private final ReentrantLock putLock = new ReentrantLock();
/** notFull 条件对象，每当队列数据已满时用于挂起执行添加的线程 */
private final Condition notFull = putLock.newCondition();从上面的属性我们知道，每个添加到LinkedBlockingQueue队列中的数据都将被封装成Node节点，添加的链表队列中，其中head和last分别指向队列的头结点和尾结点。与ArrayBlockingQueue不同的是，LinkedBlockingQueue内部分别使用了takeLock 和 putLock 对并发进行控制，也就是说，添加和删除操作并不是互斥操作，可以同时进行，这样也就可以大大提高吞吐量。
这里如果不指定队列的容量大小，也就是使用默认的Integer.MAX_VALUE，如果存在添加速度大于删除速度时候，有可能会内存溢出，这点在使用前希望慎重考虑。
另外，LinkedBlockingQueue对每一个lock锁都提供了一个Condition用来挂起和唤醒其他线程。
构造函数
默认的构造函数和最后一个构造函数创建的队列大小都为 Integer.MAX_VALUE，只有第二个构造函数用户可以指定队列的大小。第二个构造函数最后初始化了last和head节点，让它们都指向了一个元素为null的节点。
最后一个构造函数使用了putLock来进行加锁，但是这里并不是为了多线程的竞争而加锁，只是为了放入的元素能立即对其他线程可见。
public LinkedBlockingQueue() {
// 默认大小为Integer.MAX_VALUE
this(Integer.MAX_VALUE);
}
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(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();
}
}入队方法
LinkedBlockingQueue提供了多种入队操作的实现来满足不同情况下的需求，入队操作有如下几种：
- void put(E e)；
- boolean offer(E e)；
- boolean offer(E e, long timeout, TimeUnit unit)。
其中：
- offer方法有两个重载版本，只有一个参数的版本，如果队列满了就返回false，否则加入到队列中，返回true，add方法就是调用此版本的offer方法；另一个带时间参数的版本，如果队列满了则等待，可指定等待的时间，如果这期间中断了则抛出异常，如果等待超时了则返回false，否则加入到队列中返回true；
- put方法跟带时间参数的offer方法逻辑一样，不过没有等待的时间限制，会一直等待直到队列有空余位置了，再插入到队列中，返回true。
1、put(E e)
public void put(E e) throws InterruptedException {
if (e == null) throw new NullPointerException();
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
// 获取锁中断
putLock.lockInterruptibly();
try {
//判断队列是否已满，如果已满阻塞等待
while (count.get() == capacity) {
notFull.await();
}
// 把node放入队列中
enqueue(node);
c = count.getAndIncrement();
// 再次判断队列是否有可用空间，如果有唤醒下一个线程进行添加操作
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
// 如果队列中有一条数据，唤醒消费线程进行消费
if (c == 0)
signalNotEmpty();
}从put方法来看，它总共做了以下情况的考虑：
- 队列已满，阻塞等待。
- 队列未满，创建一个node节点放入队列中，如果放完以后队列还有剩余空间，继续唤醒下一个添加线程进行添加。如果放之前队列中没有元素，放完以后要唤醒消费线程进行消费。
我们再看看put方法中用到的几个其他方法，先来看看 enqueue(Node node) 方法：
private void enqueue(Node<E> node) {
last = last.next = node;
}用一张图来看看往队列里依次放入元素A和元素B，如下：
接下来我们看看signalNotEmpty，顺带着看signalNotFull方法。
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}为什么要这么写？因为signal的时候要获取到该signal对应的Condition对象的锁才行。
2、offer(E e)
public boolean offer(E e) {
if (e == null) throw new NullPointerException();
final AtomicInteger count = this.count;
if (count.get() == capacity)
return false;
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
// 队列有可用空间，放入node节点，判断放入元素后是否还有可用空间，
// 如果有，唤醒下一个添加线程进行添加操作。
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}可以看到offer仅仅对put方法改动了一点点，当队列没有可用元素的时候，不同于put方法的阻塞等待，offer方法直接方法false。
3、offer(E e, long timeout, TimeUnit unit)
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 {
// 等待超时时间nanos，超时时间到了返回false
while (count.get() == capacity) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
enqueue(new Node<E>(e));
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return true;
}该方法只是对offer方法进行了阻塞超时处理，使用了Condition的awaitNanos来进行超时等待，这里为什么要用while循环？因为awaitNanos方法是可中断的，为了防止在等待过程中线程被中断，这里使用while循环进行等待过程中中断的处理，继续等待剩下需等待的时间。
出队方法
入队列的方法说完后，我们来说说出队列的方法。LinkedBlockingQueue提供了多种出队操作的实现来满足不同情况下的需求，如下：
- E take();
- E poll();
- E poll(long timeout, TimeUnit unit);
1、take()
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
// 队列为空，阻塞等待
while (count.get() == 0) {
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
// 队列中还有元素，唤醒下一个消费线程进行消费
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
// 移除元素之前队列是满的，唤醒生产线程进行添加元素
if (c == capacity)
signalNotFull();
return x;
}take方法看起来就是put方法的逆向操作，它总共做了以下情况的考虑：
- 队列为空，阻塞等待
- 队列不为空，从对首获取并移除一个元素，如果消费后还有元素在队列中，继续唤醒下一个消费线程进行元素移除。如果放之前队列是满元素的情况，移除完后需要唤醒生产线程进行添加元素。
我们来看看dequeue方法：
private E dequeue() {
// 获取到head节点
Node<E> h = head;
// 获取到head节点指向的下一个节点
Node<E> first = h.next;
// head节点原来指向的节点的next指向自己，等待下次gc回收
h.next = h; // help GC
// head节点指向新的节点
head = first;
// 获取到新的head节点的item值
E x = first.item;
// 新head节点的item值设置为null
first.item = null;
return x;
}我们结合注释和图来看一下链表算法：
其实这个写法看起来很绕，我们其实也可以这么写：
private E dequeue() {
// 获取到head节点
Node<E> h = head;
// 获取到head节点指向的下一个节点，也就是节点A
Node<E> first = h.next;
// 获取到下下个节点，也就是节点B
Node<E> next = first.next;
// head的next指向下下个节点，也就是图中的B节点
h.next = next;
// 得到节点A的值
E x = first.item;
first.item = null; // help GC
first.next = first; // help GC
return x;
}2、poll()
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;
}poll方法去除了take方法中元素为空后阻塞等待这一步骤，这里也就不详细说了。同理，poll(long timeout, TimeUnit unit)也和offer(E e, long timeout, TimeUnit unit)一样，利用了Condition的awaitNanos方法来进行阻塞等待直至超时。这里就不列出来说了。
获取元素方法
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();
}
}加锁后，获取到head节点的next节点，如果为空返回null，如果不为空，返回next节点的item值。
删除元素方法
public boolean remove(Object o) {
if (o == null) return false;
// 两个lock全部上锁
fullyLock();
try {
// 从head开始遍历元素，直到最后一个元素
for (Node<E> trail = head, p = trail.next;
p != null;
trail = p, p = p.next) {
// 如果找到相等的元素，调用unlink方法删除元素
if (o.equals(p.item)) {
unlink(p, trail);
return true;
}
}
return false;
} finally {
// 两个lock全部解锁
fullyUnlock();
}
}
void fullyLock() {
putLock.lock();
takeLock.lock();
}
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}因为remove方法使用两个锁全部上锁，所以其他操作都需要等待它完成，而该方法需要从head节点遍历到尾节点，所以时间复杂度为O(n)。我们来看看unlink方法。
void unlink(Node<E> p, Node<E> trail) {
// p的元素置为null
p.item = null;
// p的前一个节点的next指向p的next，也就是把p从链表中去除了
trail.next = p.next;
// 如果last指向p，删除p后让last指向trail
if (last == p)
last = trail;
// 如果删除之前元素是满的，删除之后就有空间了，唤醒生产线程放入元素
if (count.getAndDecrement() == capacity)
notFull.signal();
}完整源码
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;
public class LinkedBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -6903933977591709194L;
/**
* 节点类，用于存储数据
*/
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; }
}
/** 阻塞队列的大小, 默认为Integer.MAX_VALUE */
private final int capacity;
/** 当前阻塞队列中的元素个数 */
private final AtomicInteger count = new AtomicInteger();
/**
* 阻塞队列的头节点
*/
transient Node<E> head;
/**
* 阻塞队列的尾节点
*/
private transient Node<E> last;
/** 获取并移除元素时使用的锁，如take，poll，etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** notEmpty 条件对象，当队列没有数据时用于挂起执行删除的线程 */
private final Condition notEmpty = takeLock.newCondition();
/** 添加元素时使用的锁，如 put，offer，etc */
private final ReentrantLock putLock = new ReentrantLock();
/** notFull 条件对象，每当队列数据已满时用于挂起执行添加的线程 */
private final Condition notFull = putLock.newCondition();
/**
* 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();
}
}
/**
* 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();
}
}
/**
* 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;
}
/**
* Removes a node from head of queue.
*
* @return the node
*/
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
// assert head.item == null;
// 获取到head节点
Node<E> h = head;
// 获取到head节点指向的下一个节点
Node<E> first = h.next;
// head节点原来指向的节点的next指向自己，等待下次gc回收
h.next = h; // help GC
// head节点指向新的节点
head = first;
// 获取到新的head节点的item值
E x = first.item;
// 新head节点的item值设置为null
first.item = null;
return x;
}
/**
* Locks to prevent both puts and takes.
*/
void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* Unlocks to allow both puts and takes.
*/
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
// /**
// * Tells whether both locks are held by current thread.
// */
// boolean isFullyLocked() {
// return (putLock.isHeldByCurrentThread() &&
// takeLock.isHeldByCurrentThread());
// }
/**
* Creates a {@code LinkedBlockingQueue} with a capacity of
* {@link Integer#MAX_VALUE}.
*/
public LinkedBlockingQueue() {
// 默认大小为Integer.MAX_VALUE
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();
}
}
// this doc comment is overridden to remove the reference to collections
// greater in size than Integer.MAX_VALUE
/**
* Returns the number of elements in this queue.
*
* @return the number of elements in this queue
*/
public int size() {
return count.get();
}
// this doc comment is a modified copy of the inherited doc comment,
// without the reference to unlimited queues.
/**
* Returns the number of additional elements that this queue can ideally
* (in the absence of memory or resource constraints) accept without
* blocking. This is always equal to the initial capacity of this queue
* less the current {@code size} of this queue.
*
* <p>Note that you <em>cannot</em> always tell if an attempt to insert
* an element will succeed by inspecting {@code remainingCapacity}
* because it may be the case that another thread is about to
* insert or remove an element.
*/
public int remainingCapacity() {
return capacity - count.get();
}
/**
* Inserts the specified element at the tail of this queue, waiting if
* necessary for space to become available.
*
* @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;
// 获取锁中断
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.
*/
//判断队列是否已满，如果已满阻塞等待
while (count.get() == capacity) {
notFull.await();
}
// 把node放入队列中
enqueue(node);
c = count.getAndIncrement();
// 再次判断队列是否有可用空间，如果有唤醒下一个线程进行添加操作
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
// 如果队列中有一条数据，唤醒消费线程进行消费
if (c == 0)
signalNotEmpty();
}
/**
* 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 {
// 等待超时时间nanos，超时时间到了返回false
while (count.get() == capacity) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);
}
enqueue(new Node<E>(e));
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return true;
}
/**
* 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.
* 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)
return false;
int c = -1;
Node<E> node = new Node<E>(e);
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
// 队列有可用空间，放入node节点，判断放入元素后是否还有可用空间，
// 如果有，唤醒下一个添加线程进行添加操作。
if (count.get() < capacity) {
enqueue(node);
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
}
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return c >= 0;
}
public E take() throws InterruptedException {
E x;
int c = -1;
final AtomicInteger count = this.count;
final ReentrantLock takeLock = this.takeLock;
takeLock.lockInterruptibly();
try {
// 队列为空，阻塞等待
while (count.get() == 0) {
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
// 队列中还有元素，唤醒下一个消费线程进行消费
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
// 移除元素之前队列是满的，唤醒生产线程进行添加元素
if (c == capacity)
signalNotFull();
return x;
}
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);
}
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;
}
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();
}
}
/**
* 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的元素置为null
p.item = null;
// p的前一个节点的next指向p的next，也就是把p从链表中去除了
trail.next = p.next;
// 如果last指向p，删除p后让last指向trail
if (last == p)
last = trail;
// 如果删除之前元素是满的，删除之后就有空间了，唤醒生产线程放入元素
if (count.getAndDecrement() == capacity)
notFull.signal();
}
/**
* 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).
*
* @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;
// 两个lock全部上锁
fullyLock();
try {
// 从head开始遍历元素，直到最后一个元素
for (Node<E> trail = head, p = trail.next;
p != null;
trail = p, p = p.next) {
// 如果找到相等的元素，调用unlink方法删除元素
if (o.equals(p.item)) {
unlink(p, trail);
return true;
}
}
return false;
} finally {
// 两个lock全部解锁
fullyUnlock();
}
}
/**
* Returns {@code true} if this queue contains the specified element.
* More formally, returns {@code true} if and only if this queue contains
* at least one element {@code e} such that {@code o.equals(e)}.
*
* @param o object to be checked for containment in this queue
* @return {@code true} if this queue contains the specified element
*/
public boolean contains(Object o) {
if (o == null) return 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();
}
}
/**
* Returns an array containing all of the elements in this queue, in
* proper sequence.
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this queue. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this queue
*/
public Object[] toArray() {
fullyLock();
try {
int size = count.get();
Object[] a = new Object[size];
int k = 0;
for (Node<E> p = head.next; p != null; p = p.next)
a[k++] = p.item;
return a;
} finally {
fullyUnlock();
}
}
/**
* Returns an array containing all of the elements in this queue, in
* proper sequence; the runtime type of the returned array is that of
* the specified array. If the queue fits in the specified array, it
* is returned therein. Otherwise, a new array is allocated with the
* runtime type of the specified array and the size of this queue.
*
* <p>If this queue fits in the specified array with room to spare
* (i.e., the array has more elements than this queue), the element in
* the array immediately following the end of the queue is set to
* {@code null}.
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose {@code x} is a queue known to contain only strings.
* The following code can be used to dump the queue into a newly
* allocated array of {@code String}:
*
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the queue are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose
* @return an array containing all of the elements in this queue
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this queue
* @throws NullPointerException if the specified array is null
*/
@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();
}
}
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();
}
}
/**
* Atomically removes all of the elements from this queue.
* The queue will be empty after this call returns.
*/
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);
}
/**
* @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;
boolean signalNotFull = false;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
int n = Math.min(maxElements, count.get());
// 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 {
// 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();
}
}
/**
* 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);
}
}
}总结
LinkedBlockingQueue是一个阻塞队列，内部由两个ReentrantLock来实现出入队列的线程安全，由各自的Condition对象的await和signal来实现等待和唤醒功能。它和ArrayBlockingQueue的不同点在于：
- 队列大小有所不同，ArrayBlockingQueue是有界的初始化必须指定大小，而LinkedBlockingQueue可以是有界的也可以是无界的(Integer.MAX_VALUE)，对于后者而言，当添加速度大于移除速度时，在无界的情况下，可能会造成内存溢出等问题。
- 数据存储容器不同，ArrayBlockingQueue采用的是数组作为数据存储容器，而LinkedBlockingQueue采用的则是以Node节点作为连接对象的链表。
- 由于ArrayBlockingQueue采用的是数组的存储容器，因此在插入或删除元素时不会产生或销毁任何额外的对象实例，而LinkedBlockingQueue则会生成一个额外的Node对象。这可能在长时间内需要高效并发地处理大批量数据的时，对于GC可能存在较大影响。
- 两者的实现队列添加或移除的锁不一样，ArrayBlockingQueue实现的队列中的锁是没有分离的，即添加操作和移除操作采用的同一个ReenterLock锁，而LinkedBlockingQueue实现的队列中的锁是分离的，其添加采用的是putLock，移除采用的则是takeLock，这样能大大提高队列的吞吐量，也意味着在高并发的情况下生产者和消费者可以并行地操作队列中的数据，以此来提高整个队列的并发性能。
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