阻塞队列详解
本文详细介绍了ArrayBlockingQueue、LinkedBlockingQueue和DelayedQueue三种阻塞队列的数据结构、构造方式及核心方法实现原理，包括入队、出队操作的具体流程，并通过示例代码帮助理解。
文章目录
ArrayBlockingQueue
ArrayBlockingQueue是一个有界阻塞队列，数据结构基于数组、使用ReentrantLock、Condition保证并发同步。
所谓阻塞队列
当队列满了，则会对生产线程产生阻塞直到有空位可插入；
当队列空了，则会对消费队列产生阻塞直到有新的元素被加入队列。
方法中含有字母t的都会产生阻塞waiting；
方法中含有o的都会返回 true/false;
剩下add、remove的会抛出异常;
peek（）会从队列头部观察头结点，但并不会对队列造成影响。
我们通过一个简单的应用，来逐步分析ArrayBlockingQueue队列的代码：
public class ArrayBlockingQueueTest {
public static void main(String[] args) throws InterruptedException {
ExecutorService ex = Executors.newFixedThreadPool(50);
ArrayBlockingQueue<CustomizedTask> tasksQueue = new ArrayBlockingQueue<CustomizedTask>(100);//有界队列 100个元素
// 生产者线程
new Thread(new Runnable() {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
try {
tasksQueue.put(new CustomizedTask());
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
// 消费者线程
new Thread(new Runnable() {
@Override
public void run() {
CustomizedTask task;
try {
while ((task = tasksQueue.take()) != null && !Thread.currentThread().isInterrupted()) {
ex.submit(task);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
System.out.println("Main Thread is terminated");
}
static class CustomizedTask implements Runnable {
@Override
public void run() {
System.out.println(System.currentTimeMillis());
}
}
}
1.构造：
/** The queued items */
final Object[] items;
/** items index for next take, poll, peek or remove */
int takeIndex;
/** items index for next put, offer, or add */
int putIndex;
/** Number of elements in the queue */
int count;
/*
* Concurrency control uses the classic two-condition algorithm
* found in any textbook.
*/
/** Main lock guarding all access */
final ReentrantLock lock;
/** Condition for waiting takes */
private final Condition notEmpty;
/** Condition for waiting puts */
private final Condition notFull;
/**
* Creates an {@code ArrayBlockingQueue} with the given (fixed)
* capacity and default access policy.
*
* @param capacity the capacity of this queue
* @throws IllegalArgumentException if {@code capacity < 1}
*/
public ArrayBlockingQueue(int capacity) {
this(capacity, false);
}
public ArrayBlockingQueue(int capacity, boolean fair) {
if (capacity <= 0)
throw new IllegalArgumentException();
this.items = new Object[capacity];//全局变量，一个Object[]数组用来维护入队元素
lock = new ReentrantLock(fair);//ReentrantLock.Condition实现等待\通知
notEmpty = lock.newCondition();
notFull = lock.newCondition();
}
2.入队列。生产者生产消息并放入队列
public void put(E e) throws InterruptedException {
checkNotNull(e);//入队元素正确性判断
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();//获取锁
try {
while (count == items.length)//如果队列中数据已经达到队列上限
notFull.await();//阻塞并释放锁(此时当前线程进入Condition队列并产生park阻塞)
enqueue(e);//当队列中有空位存在的时，执行入队
} finally {
lock.unlock();
}
}
/**
* Inserts element at current put position, advances, and signals.
* Call only when holding lock.
*/
private void enqueue(E x) {
// assert lock.getHoldCount() == 1;
// assert items[putIndex] == null;
final Object[] items = this.items;
items[putIndex] = x;//putIndex初始化为0,每次插入元素后递增
if (++putIndex == items.length)//达到上限
putIndex = 0;
count++;//Number of elements in the queue
//通知阻塞在队列上的消费者(AQS：在获取到锁的情况下，将阻塞在Condition队列的结点放入sync队列中，等待被唤醒再次尝试锁获取)
notEmpty.signal();
}
3.出队列。消费者如果阻塞会被唤醒，并且进行锁获取和取队列元素
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
while (count == 0)//如果是个空队列
notEmpty.await();//阻塞直到队列进入元素同时释放锁
return dequeue();
} finally {
lock.unlock();
}
}
/**
* Extracts element at current take position, advances, and signals.
* Call only when holding lock.
*/
private E dequeue() {
// assert lock.getHoldCount() == 1;
// assert items[takeIndex] != null;
final Object[] items = this.items;
@SuppressWarnings("unchecked")
E x = (E) items[takeIndex];//数组中取数
items[takeIndex] = null;//取数后释放占用
if (++takeIndex == items.length)
takeIndex = 0;
count--;//队列中总元素数目减1
if (itrs != null)
itrs.elementDequeued();
notFull.signal();//唤醒阻塞的等待消费的线程
return x;
}
LinkedBlockingQueue
LinkedBlockingQueue是一个有界阻塞队列，基于链表结构实现，默认capacity为Integer.MAX_VALUE。
我们通过一个简单的应用，来逐步分析LinkedBlockingQueue队列的代码：
public class LinkedBlockingQueueTest {
public static void main(String[] args) throws InterruptedException {
ExecutorService ex = Executors.newFixedThreadPool(50);
LinkedBlockingQueue<CustomizedTask> tasksQueue = new LinkedBlockingQueue<CustomizedTask>(100);
// 生产者线程
new Thread(new Runnable() {
@Override
public void run() {
while (!Thread.currentThread().isInterrupted()) {
try {
tasksQueue.put(new CustomizedTask());
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}).start();
// 消费者线程
new Thread(new Runnable() {
@Override
public void run() {
CustomizedTask task;
try {
while ((task = tasksQueue.take()) != null && !Thread.currentThread().isInterrupted()) {
ex.submit(task);
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}).start();
System.out.println("Main Thread is terminated");
}
static class CustomizedTask implements Runnable {
@Override
public void run() {
System.out.println(System.currentTimeMillis());
}
}
}
1.初始化构造：
/** Current number of elements */
private final AtomicInteger count = new AtomicInteger();
/** Lock held by take, poll, etc */
private final ReentrantLock takeLock = new ReentrantLock();
/** Wait queue for waiting takes */
private final Condition notEmpty = takeLock.newCondition();
/** Lock held by put, offer, etc */
private final ReentrantLock putLock = new ReentrantLock();
/** Wait queue for waiting puts */
private final Condition notFull = putLock.newCondition();
/**
* 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);//构造链表的头尾结点，链表的初始化
}
1.1 链表数据结构
/**
* Linked list node class
* 一个简单的单向链表
*/
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;
}
}
2.入队列。生产者生产消息并放入队列
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);//插入的对象包装为一个结点
final ReentrantLock putLock = this.putLock;
final AtomicInteger count = this.count;
putLock.lockInterruptibly();//获取putLcok
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) {//队列内元素达到上限
notFull.await();//condition等待
}
enqueue(node);//在队列不满的情况下 插入元素
c = count.getAndIncrement();//容量计数
if (c + 1 < capacity)//队列是否可以再插入一个元素
notFull.signal();//唤醒在 putLock.condition等待的线程，线程执行插入操作。
} finally {
putLock.unlock();
}
if (c == 0)//如果队列再进入这个操作之前是空的，那么现在不空了(刚插入一个元素)，唤醒因为队列空而阻塞的取数线程
signalNotEmpty();
}
private void enqueue(Node<E> node) {
// assert putLock.isHeldByCurrentThread();
// assert last.next == null;
last = last.next = node;//尾部插入一个元素，并且把last引用指向这个元素
}
private void signalNotEmpty() {
final ReentrantLock takeLock = this.takeLock;
takeLock.lock();
try {
notEmpty.signal();
} finally {
takeLock.unlock();
}
}
3.出队列。消费者如果阻塞会被唤醒，并且进行锁获取和取队列元素
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) {//队列为空，则阻塞取操作直到队列不空
notEmpty.await();
}
x = dequeue();
c = count.getAndDecrement();
if (c > 1)//如果进入这个操作之前队列中元素超过1个(比如2个),则表示这个操作取数后依旧不为空(起码还有1个)，那么可以唤醒其他因为队列为空而阻塞的线程
notEmpty.signal();
} finally {
takeLock.unlock();
}
//唤醒这个操作执行之前因为队列慢而产生的阻塞，起码这个操作之后会有一个空位
if (c == capacity)
signalNotFull();
return x;
}
private E dequeue() {
// assert takeLock.isHeldByCurrentThread();
// assert head.item == null;
Node<E> h = head;
Node<E> first = h.next;//head的下个元素。可以看到是按照 FIFO队列排序获取的
//将这个元素从队列中清除(出队)
h.next = h; // help GC
head = first;
E x = first.item;
first.item = null;
return x;
}
private void signalNotFull() {
final ReentrantLock putLock = this.putLock;
putLock.lock();
try {
notFull.signal();
} finally {
putLock.unlock();
}
}
DelayedQueue
一个无界的阻塞队列，其中的元素需要是先Delayed接口，对元素的提取加入了延期限制
当元素的过期时间到了才允许从队列中取出。队列头部的元素是等待时间最久的元素。
如果插入数据增加会自动扩容，创建新的更大的数组并将原数组数据放入(PriorityQueue)。
如果没有元素到了过期时间，那么队列头head不存在,并且poll操作返回null。
当一个元素到了过期时间，那么它的getDelay(TimeUnit.NANOSECONDS)方法将会返回一个小于0的数字。队列中不允许放入null元素。
还是用一个Demo来入手源码的分析:
public class DelayQueueTest {
public static void main(String[] args) {
DelayQueue<DelayedElement> delayQueue = new DelayQueue<DelayedElement>();
producer(delayQueue);
consumer(delayQueue);// Consumer 1
consumer(delayQueue);// Consumer 2
}
/**
* 每100毫秒创建一个对象，放入延迟队列，延迟时间1毫秒
* @param delayQueue
*/
private static void producer(final DelayQueue<DelayedElement> delayQueue) {
// offer
new Thread(new Runnable() {
@Override
public void run() {
int i = 0;
while (true) {
i++;
try {
TimeUnit.MILLISECONDS.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
DelayedElement element = new DelayedElement(1000 * 60 * 2, "test" + i);// 2min
System.out.println("offer success " + delayQueue.offer(element));
}
}
},"Producer").start();
/**
* 每秒打印延迟队列中的对象个数
*/
new Thread(new Runnable() {
@Override
public void run() {
while (true) {
try {
TimeUnit.MILLISECONDS.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("delayQueue size:" + delayQueue.size());
}
}
},"Watcher").start();
}
/**
* take
*
* 消费者，从延迟队列中获得数据,进行处理
* @param delayQueue
*/
private static void consumer(final DelayQueue<DelayedElement> delayQueue) {
new Thread(new Runnable() {
@Override
public void run() {
while (true) {
DelayedElement element = null;
try {
element = delayQueue.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(System.currentTimeMillis() + "---" + element);
}
}
},"Consumer").start();
}
}
class DelayedElement implements Delayed {
private final long delay; // 延迟时间
private final long expire; // 到期时间
private final String msg; // 数据
private final long now; // 创建时间
public DelayedElement(long delay, String msg) {
this.delay = delay;
this.msg = msg;
expire = System.currentTimeMillis() + delay; // 到期时间 = 当前时间+延迟时间
now = System.currentTimeMillis();
}
/**
* 需要实现的接口，获得延迟时间 用过期时间-当前时间
* @param unit
* @return
*/
@Override
public long getDelay(TimeUnit unit) {
return unit.convert(this.expire - System.currentTimeMillis(), TimeUnit.MILLISECONDS);
}
/**
* 用于延迟队列内部比较排序 当前时间的延迟时间 - 比较对象的延迟时间
* @param o
* @return
*/
@Override
public int compareTo(Delayed o) {
return (int) (this.getDelay(TimeUnit.MILLISECONDS) - o.getDelay(TimeUnit.MILLISECONDS));
}
@Override
public String toString() {
final StringBuilder sb = new StringBuilder("DelayedElement{");
sb.append("delay=").append(delay);
sb.append(", expire=").append(expire);
sb.append(", msg='").append(msg).append('\'');
sb.append(", now=").append(now);
sb.append('}');
return sb.toString();
}
}
1.构造初始化DelayedQ
private final transient ReentrantLock lock = new ReentrantLock();
private final PriorityQueue<E> q = new PriorityQueue<E>()；//内部通过一个PriorityQueue存储元素，而PriorityQueue内部通过数组实现。这个priority会自动通过移动数组元素进行扩容，类似ArrayList
private final Condition available = lock.newCondition();//同样是通过condition实现
public DelayQueue() {
}
/**
* 线程被设计来用来等待队列头部的元素
*
* 这是 leader-follower模式的变体，为了最大限度减小不必要的时间等待
* 当一个线程成为 leader，它会等待直到头结点过期，而其他线程会无限期的等待下去，直到这个leader被释放并唤醒其他线程。
* leader 线程必须在从take()或者poll()等其他方法中返回前，通知激活其他线程，并释放leader引用
*
* 无论什么时候头结点被替换了一个更早过期的时间。
* 这个leader field 通过设置为null，被置为无效。
* 其他线程被唤醒然后准备获取到接着释放leadship。
*
*/
private Thread leader = null;
2.offer插入元素
public boolean offer(E e) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
q.offer(e);//队尾插入
if (q.peek() == e) {//队列中仅有一个元素
leader = null;
available.signal();//可能存在其他线程因为队列控而阻塞
}
return true;
} finally {
lock.unlock();
}
}
3.take提取数组元素
/**
* Retrieves and removes the head of this queue, waiting if necessary until
* an element with an expired delay is available on this queue.
*
* @return the head of this queue
* @throws InterruptedException {@inheritDoc}
*/
public E take() throws InterruptedException {
final ReentrantLock lock = this.lock;
lock.lockInterruptibly();
try {
for (;;) {
E first = q.peek();//查看队列中的头元素
if (first == null)//为null表示没有可获取的元素
available.await();//condition await
else {
long delay = first.getDelay(NANOSECONDS);//查看这个元数据的过期时间
if (delay <= 0)//已过期 可获取
return q.poll();
first = null; // don't retain ref while waiting
if (leader != null)
available.await();//如果不是leader则进入等待状态，直到之前的leader被释放后被唤醒
else {
Thread thisThread = Thread.currentThread();
leader = thisThread;//当前获取队列元素的线程
try {
available.awaitNanos(delay);
} finally {
if (leader == thisThread)
leader = null;//线程获取到元素后释放leader引用
}
}
}
}
} finally {
if (leader == null && q.peek() != null)//leader已被释放 && 下个结点存在
available.signal();//leader线程获取了元素 并且释放了leader引用，退出方法前唤醒其他线程。
lock.unlock();
}
}
小结
加上之前对ArrayBlockingQueue、LinkedBlockingQueue的介绍，阻塞队列常用类型基本介绍完了,下边对其他阻塞队列做个简介。
SynchronousQueue:
这个队列不存储元素，当一个线程向这个队列插入一个元素，另一个队列需要立刻从这个队列里取出，否则无法继续插入元素。适合传递型场景。
LinkedTransferQueue:
一个由链表构成的无界阻塞队列
LinkedBlockingDeque：
一个链表结构的 双向阻塞队列。可以满足两个线程分别从头尾进行插入或移除操作，应用于“工作窃取”算法：允许一个线程从头部插入\移除元素，另一个窃取线程从尾部窃取元素。
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