Java并发编程与技术内幕:ArrayBlockingQueue、LinkedBlockingQueue及SynchronousQueue源码解析

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一、BlockingQueue介绍与常用方法

BlockingQueue是一个阻塞队列。在高并发场景是用得非常多的，在线程池中。如果运行线程数目大于核心线程数目时，也会尝试把新加入的线程放到一个BlockingQueue中去。队列的特性就是先进先出很容易理解，在java里头它的实现类主要有下图的几种，其中最常用到的是ArrayBlockingQueue、LinkedBlockingQueue及SynchronousQueue这三种，这三个也是今天主要讲的类。

它主要的方法有

BlockingQueue的核心方法：
1、放入数据

（1） add(object)

队列没满的话，放入成功。否则抛出异常。

（2）offer(object):

表示如果可能的话,将object加到BlockingQueue里,即如果BlockingQueue可以容纳,则返回true,否则返回false.（本方法不阻塞当前执行方法的线程）
（3）offer(E o, long timeout, TimeUnit unit)

可以设定等待的时间，如果在指定的时间内，还不能往队列中加入BlockingQueue，则返回失败。
（4）put(object)

把object加到BlockingQueue里,如果BlockQueue没有空间,则调用此方法的线程阻塞。直到BlockingQueue里面有空间再继续.
2、获取数据
（1）poll(time)

取走BlockingQueue里排在首位的对象,若不能立即取出,则可以等time参数规定的时间,取不到时返回null;
（2）poll(long timeout, TimeUnit unit)

从BlockingQueue取出一个队首的对象，如果在指定时间内，队列一旦有数据可取，则立即返回队列中的数据。否则知道时间超时还没有数据可取，返回失败。

（3）take()

取走BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到BlockingQueue有新的数据被加入;
（4）drainTo()

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。

二、ArrayBlockingQueue

一个基本数组的阻塞队列。可以设置列队的大小。

ArrayBlockingQueue的源码是比较简单的，下面是笔者抽取了一部分源码并加以注释。它的基本原理实际还是数组，只不过存、取、删时都要做队列是否满或空的判断。然后加锁访问。

package java.util.concurrent;
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.lang.ref.WeakReference;
import java.util.Spliterators;
import java.util.Spliterator;
public class ArrayBlockingQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -817911632652898426L;
/** 真正存入数据的数组*/
final Object[] items;
/** take, poll, peek or remove的下一个索引 */
int takeIndex;
/** put, offer, or add的下一个索引 */
int putIndex;
/**队列中元素个数*/
int count;
/**可重入锁 */
final ReentrantLock lock;
/** 队列不为空的条件 */
private final Condition notEmpty;
/** 队列未满的条件 */
private final Condition notFull;
transient Itrs itrs = null;
/**
*当前元素个数-1
*/
final int dec(int i) {
return ((i == 0) ? items.length : i) - 1;
}
/**
* 返回对应索引上的元素
*/
@SuppressWarnings("unchecked")
final E itemAt(int i) {
return (E) items[i];
}
/**
* 非空检查
*
* @param v the element
*/
private static void checkNotNull(Object v) {
if (v == null)
throw new NullPointerException();
}
/**
* 元素放入队列，注意调用这个方法时都要先加锁
*
*/
private void enqueue(E x) {
final Object[] items = this.items;
items[putIndex] = x;
if (++putIndex == items.length)
putIndex = 0;
count++;//当前拥有元素个数加1
notEmpty.signal();//有一个元素加入成功，那肯定队列不为空
}
/**
* 元素出队，注意调用这个方法时都要先加锁
*
*/
private E dequeue() {
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E x = (E) items[takeIndex];
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;/当前拥有元素个数减1
if (itrs != null)
itrs.elementDequeued();
notFull.signal();//有一个元素取出成功，那肯定队列不满
return x;
}
/**
* 指定删除索引上的元素
*
*/
void removeAt(final int removeIndex) {
final Object[] items = this.items;
if (removeIndex == takeIndex) {
items[takeIndex] = null;
if (++takeIndex == items.length)
takeIndex = 0;
count--;
if (itrs != null)
itrs.elementDequeued();
} else {
final int putIndex = this.putIndex;
for (int i = removeIndex;;) {
int next = i + 1;
if (next == items.length)
next = 0;
if (next != putIndex) {
items[i] = items[next];
i = next;
} else {
items[i] = null;
this.putIndex = i;
break;
}
}
count--;
if (itrs != null)
itrs.removedAt(removeIndex);
}
notFull.signal();//有一个元素删除成功，那肯定队列不满
}
/**
*
* 构造函数，设置队列的初始容量
*/
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
}
/**
* 构造函数。capacity设置数组大小，fair设置是否为公平锁
* capacity and the specified access policy.
*/
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
this.items = new Object[capacity];
lock = new ReentrantLock(fair);//是否为公平锁，如果是的话，那么先到的线程先获得锁对象。
//否则，由操作系统调度由哪个线程获得锁，一般为false，性能会比较高
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
/**
*构造函数，带有初始内容的队列
*/
public ArrayBlockingQueue(int capacity, boolean fair,
Collection<? extends E> c) {
this(capacity, fair);
final ReentrantLock lock = this.lock;
lock.lock(); //要给数组设置内容，先上锁
try {
int i = 0;
try {
for (E e : c) {
checkNotNull(e);
items[i++] = e;//依次拷贝内容
}
} catch (ArrayIndexOutOfBoundsException ex) {
throw new IllegalArgumentException();
}
count = i;
putIndex = (i == capacity) ? 0 : i;//如果putIndex大于数组大小，那么从0重新开始
} finally {
lock.unlock();//最后一定要释放锁
}
}
/**
* 添加一个元素，其实super.add里面调用了offer方法
*/
public boolean add(E e) {
return super.add(e);
}
/**
*加入成功返回true,否则返回false
*
*/
public boolean offer(E e) {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lock();//上锁
try {
if (count == items.length) //超过数组的容量
return false;
else {
enqueue(e); //放入元素
return true;
}
} finally {
lock.unlock();
}
}
/**
* 如果队列已满的话，就会等待
*/
public void put(E e) throws InterruptedException {
checkNotNull(e);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();//和lock()方法的区别是让它在阻塞时也可抛出异常跳出
try {
while (count == items.length)
notFull.await(); //这里就是阻塞了，要注意。如果运行到这里，那么它会释放上面的锁，一直等到notify
enqueue(e);
} finally {
lock.unlock();
}
}
/**
* 带有超时时间的插入方法，unit表示是按秒、分、时哪一种
*/
public boolean offer(E e, long timeout, TimeUnit unit)
throws InterruptedException {
checkNotNull(e);
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == items.length) {
if (nanos <= 0)
return false;
nanos = notFull.awaitNanos(nanos);//带有超时等待的阻塞方法
}
enqueue(e);//入队
return true;
} finally {
lock.unlock();
}
}
//实现的方法，如果当前队列为空，返回null
public E poll() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return (count == 0) ? null : dequeue();
} finally {
lock.unlock();
}
}
//实现的方法，如果当前队列为空，一直阻塞
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)
notEmpty.await();//队列为空，阻塞方法
return dequeue();
} finally {
lock.unlock();
}
}
//带有超时时间的取元素方法，否则返回Null
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);//超时等待
}
return dequeue();//取得元素
} finally {
lock.unlock();
}
}
//只是看一个队列最前面的元素，取出是不删除队列中的原来元素。队列为空时返回null
public E peek() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return itemAt(takeIndex); // 队列为空时返回null
} finally {
lock.unlock();
}
}
/**
* 返回队列当前元素个数
*
*/
public int size() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return count;
} finally {
lock.unlock();
}
}
/**
* 返回当前队列再放入多少个元素就满队
*/
public int remainingCapacity() {
final ReentrantLock lock = this.lock;
lock.lock();
try {
return items.length - count;
} finally {
lock.unlock();
}
}
/**
* 从队列中删除一个元素的方法。删除成功返回true,否则返回false
*/
public boolean remove(Object o) {
if (o == null) return false;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i])) {
removeAt(i); //真正删除的方法
return true;
}
if (++i == items.length)
i = 0;
} while (i != putIndex);//一直不断的循环取出来做判断
}
return false;
} finally {
lock.unlock();
}
}
/**
* 是否包含一个元素
*/
public boolean contains(Object o) {
if (o == null) return false;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
if (count > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
if (o.equals(items[i]))
return true;
if (++i == items.length)
i = 0;
} while (i != putIndex);
}
return false;
} finally {
lock.unlock();
}
}
/**
* 清空队列
*
*/
public void clear() {
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int k = count;
if (k > 0) {
final int putIndex = this.putIndex;
int i = takeIndex;
do {
items[i] = null;
if (++i == items.length)
i = 0;
} while (i != putIndex);
takeIndex = putIndex;
count = 0;
if (itrs != null)
itrs.queueIsEmpty();
for (; k > 0 && lock.hasWaiters(notFull); k--)
notFull.signal();
}
} finally {
lock.unlock();
}
}
/**
* 取出所有元素到集合
*/
public int drainTo(Collection<? super E> c) {
return drainTo(c, Integer.MAX_VALUE);
}
/**
* 取出所有元素到集合
*/
public int drainTo(Collection<? super E> c, int maxElements) {
checkNotNull(c);
if (c == this)
throw new IllegalArgumentException();
if (maxElements <= 0)
return 0;
final Object[] items = this.items;
final ReentrantLock lock = this.lock;
lock.lock();
try {
int n = Math.min(maxElements, count);
int take = takeIndex;
int i = 0;
try {
while (i < n) {
@SuppressWarnings("unchecked")
E x = (E) items[take];
c.add(x);
items[take] = null;
if (++take == items.length)
take = 0;
i++;
}
return n;
} finally {
// Restore invariants even if c.add() threw
if (i > 0) {
count -= i;
takeIndex = take;
if (itrs != null) {
if (count == 0)
itrs.queueIsEmpty();
else if (i > take)
itrs.takeIndexWrapped();
}
for (; i > 0 && lock.hasWaiters(notFull); i--)
notFull.signal();
}
}
} finally {
lock.unlock();
}
}
}

三、LinkedBlockingQueue

接下来看看LinkedBlockingQueue的部分源码。

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;
/**
* 链表节点类
*/
static class Node<E> {
E item;
Node<E> next;//下一节点
Node(E x) { item = x; }
}
/** 链表大小，默认大小是Integer.MAX_VALUE */
private final int capacity;
/**当前队列中存放的元素个数，注意是原子类*/
private final AtomicInteger count = new AtomicInteger();
/**
* 链表队列头节点
*/
transient Node<E> head;
/**
* 链表队列尾节点
*/
private transient Node<E> last;
/** 取元素时的可重入锁 */
private final ReentrantLock takeLock = new ReentrantLock();
/**不为空条件*/
private final Condition notEmpty = takeLock.newCondition();
/**放元素是时的重入锁 */
private final ReentrantLock putLock = new ReentrantLock();
/** 不为满的条件 */
private final Condition notFull = putLock.newCondition();
/**
* 不为空通知方法
*/
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
/**
* 不为满通知方法
*/
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
/**
* 进队
*
* @param node the node
*/
private void enqueue(Node<E> node) {
last = last.next = node;
}
/**
* 出队
*/
private E dequeue() {
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;
}
/**
* 取和入都上锁，此时无法取和放
*/
void fullyLock() {
putLock.lock();
takeLock.lock();
}
/**
* 释放锁
*/
void fullyUnlock() {
takeLock.unlock();
putLock.unlock();
}
/**
* 构造函数
*/
public LinkedBlockingQueue() {
this(Integer.MAX_VALUE);
}
/**
* 构造函数
*
*/
public LinkedBlockingQueue(int capacity) {
if (capacity <= 0) throw new IllegalArgumentException();
this.capacity = capacity;
last = head = new Node<E>(null);
}
/**
* 构造函数
*/
public LinkedBlockingQueue(Collection<? extends E> c) {
this(Integer.MAX_VALUE);
final ReentrantLock putLock = this.putLock;
putLock.lock(); //取得放入锁
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();
}
}
//阻塞等待放入
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;
putLock.lockInterruptibly(); //取得放入锁
try {
while (count.get() == capacity) {//队列已满
notFull.await();
}
enqueue(node);//入队
c = count.getAndIncrement();//当前队列中元素个数加1
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
}
/**
*带超时时间的阻塞等待放入，队列不满。放入成功返回true，否则返回fasle
*/
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);
}
enqueue(new Node<E>(e));
c = count.getAndIncrement();
if (c + 1 < capacity)
notFull.signal();
} finally {
putLock.unlock();
}
if (c == 0)
signalNotEmpty();
return true;
}
/**
* 非阻塞放入。队列不满放入成功返回true，否则返回fasle
*/
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 {
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;
}
//阻塞等待取出元素
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) {
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
//带有超时时间等待的取出元素
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();//等待时可抛出异常跳出
try {
while (count.get() == 0) {
if (nanos <= 0)
return null;
nanos = notEmpty.awaitNanos(nanos);//超时等待
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)
notEmpty.signal();//不这空条件成立
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
//取队头元素。没有的话返回null，有的话返回元素，并将队列中删除此元素
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();//获得取得锁
try {
if (count.get() > 0) {
x = dequeue();//出队
c = count.getAndDecrement();//当前队列中元素个数减去1
if (c > 1)
notEmpty.signal();//不为空条件成功
}
} finally {
takeLock.unlock();
}
if (c == capacity)
signalNotFull();
return x;
}
//取队头元素，但不从队列中删除，没有的话返回null,不阻塞
public E peek() {
if (count.get() == 0)
return null;
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();//获得取得锁
try {
Node<E> first = head.next;
if (first == null)
return null;
else
return first.item;
} finally {
takeLock.unlock();
}
}
/**
* 删除时要同时取得放入锁和取得锁
*/
public boolean remove(Object o) {
if (o == null) return false;
fullyLock();//同时取得放入锁和取得锁
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();
}
}
/**
* 是否包含
*/
public boolean contains(Object o) {
if (o == null) return false;
fullyLock();//同时取得放入锁和取得锁
try {
for (Node<E> p = head.next; p != null; p = p.next)
if (o.equals(p.item))
return true;
return false;
} finally {
fullyUnlock();
}
}
}

从LinkedBlockingQueue的源码中，我们可以看出他和ArrayBlockingQueue主要有以下两点区别：

1、ArrayBlockingQueue数据是放在一个数组中。LinkedBlockingQueue是放在一个Node节点中，构成一个链接。

2、ArrayBlockingQueue取元素和放元素都是同一个锁，而LinkedBlockingQueue有两个锁，一个放入锁，一个取得锁。分别对应放入元素和取得元素时的操作。这是由链表的结构所确定的。但是删除一个元素时，要同时获得放入锁和取得锁。

四、SynchronousQueue

SynchronousQueue 这个队列实现了 BlockingQueue接口。该队列的特点
1.容量为0，无论何时 size方法总是返回0
2. put操作阻塞，直到另外一个线程取走队列的元素。
3.take操作阻塞，直到另外的线程put某个元素到队列中。
4. 任何线程只能取得其他线程put进去的元素，而不会取到自己put进去的元素