JUC系列学习：阻塞队列BlockingQueue介绍及其相关实现ArrayBlockingQueue、LinkedBlockingQueue等的使用及源码分析

BlockingQueue

public interface Queue extends Collection {
   
    boolean add(E e);

    boolean offer(E e);
    
    E remove();

    E poll();

    E element();

    E peek();
}

public interface BlockingQueue extends Queue {
 
    boolean add(E e);

    boolean offer(E e);

    void put(E e) throws InterruptedException;

    boolean offer(E e, long timeout, TimeUnit unit) throws InterruptedException;

    E take() throws InterruptedException;

    E poll(long timeout, TimeUnit unit) throws InterruptedException;

    int remainingCapacity();

    boolean remove(Object o);

    public boolean contains(Object o);

    int drainTo(Collection c);

    int drainTo(Collection c, int maxElements);
}

BlockingQueue是一个接口，定义了元素的添加和删除等操作，其实现类ArrayBlockingQueue、LinkedBlockingQueue等通常用做阻塞队列，使用场景用在生产者-消费者模式中：

• 生产者往队列中添加元素，通过add/put/offer实现往队列中添加元素，当队列满时，添加元素的线程会阻塞等待队列至可用为止；
• 消费者在队列中取出元素并消费，通过remove/take/poll实现队列中删除元素当队列为空时，消费元素的线程会阻塞等待队列至不为空为止。

添加或删除元素时有四种不同的表现形式：

• 抛异常(Throws Exception)：当队列为空时，调用remove(e)删除元素会抛出异常；当队列满时，调用add(e)添加元素也会抛出异常

• 返回特殊值(false或者null) ：调用offer(e)添加元素或者调用poll()删除元素时，如果不能马上执行，将返回一个特殊的值，一般为false或null。

• 阻塞当前线程直到被唤醒：当队列为空时，消费者线程调用take()方法时会阻塞当前线程，直到队列不为空时重新被唤醒；或者当队列满时，生产者线程调用put(e)方法时会阻塞生产者线程，直到队列不满时会重新被唤醒。

• 在某个时间段内阻塞等待，超时失败：当队列为空时，线程调用poll(timeouot,unit)尝试取元素会直接阻塞；当队列满时，线程调用offer(e,timeout,unit)添加元素时会阻塞。如果poll和offer在timeout时间内没有被唤醒，则直接退出。

总结如下：

添加(Insert)	删除(Remove)	检查(Examine)
抛异常(Throws Exception)	add(e)	remove(o)
阻塞(Blocked)	put(e)	take()
返回特殊值(Special value)	offer(e)	poll()
超时(Times out)	offer( e, timeout, unit)	poll( timeout, unit)

BlockingQueue相关实现类

ArrayBlockingQueue实现原理

构造函数

final Object[] items;//数组队列

int takeIndex;//取元素对应的索引(take/poll/peek/remove)

int putIndex;//添加元素对应的索引(put/offer/add)

int count;//队列中的元素数量

final ReentrantLock lock;//ReentrantLock锁

private final Condition notEmpty;//消费线程对应的condition

private final Condition notFull;//生产线程对应的condition

public ArrayBlockingQueue(int capacity) {
    this(capacity, false);
}

//capacity表示队列的初始化容量，一旦设置后就不能再改变
//fair是可选参数 默认是非公平锁 传入true的话变为公平锁
public ArrayBlockingQueue(int capacity, boolean fair) {
    if (capacity <= 0)
        throw new IllegalArgumentException();
    //初始化一个容量为capacity的数组
    this.items = new Object[capacity];
    lock = new ReentrantLock(fair);
    notEmpty = lock.newCondition();
    notFull =  lock.newCondition();
}

添加元素add/put/offer

//队列满时，阻塞当前线程，当有空余元素时被唤醒；队列不满时，直接添加元素到队列中
public void put(E e) throws InterruptedException {
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        while (count == items.length)
            notFull.await();
        enqueue(e);
    } finally {
        lock.unlock();
    }
}

//队列未满时，添加元素到队列中并返回true；队列满时，抛出异常
public boolean add(E e) {
    return super.add(e);
}

//添加元素到队列中，成功返回true；失败返回false
public boolean offer(E e) {
    checkNotNull(e);
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        if (count == items.length)
            return false;
        else {
            enqueue(e);
            return true;
        }
    } finally {
        lock.unlock();
    }
}

//添加元素到队列中，并唤醒消费者线程
private void enqueue(E x) {
    final Object[] items = this.items;
    items[putIndex] = x;
    //添加元素的索引putIndex在队尾时直接变为队首，即数组可循环使用
    if (++putIndex == items.length)
        putIndex = 0;
    count++;
    notEmpty.signal();
}

父类AbstractQueue中的add方法：

public boolean add(E e) {
    if (offer(e))
        return true;
    else
        throw new IllegalStateException("Queue full");
}

删除元素remove/take/poll

//删除队列中takeIndex位置处的元素并返回该元素，如果该位置没有元素，返回null
public E poll() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return (count == 0) ? null : dequeue();
    } finally {
        lock.unlock();
    }
}

//队列为空时，直接阻塞当前线程并在队列中有元素时被唤醒；队列不为空时直接删除takeIndex位置处元素并返回该元素
public E take() throws InterruptedException {
    final ReentrantLock lock = this.lock;
    lock.lockInterruptibly();
    try {
        while (count == 0)
            notEmpty.await();
        return dequeue();
    } finally {
        lock.unlock();
    }
}

//队列不为空，直接取出takeIndex所在的元素并返回；队列为空时，阻塞等待timeout时间，如果在等待时间内队列中有新添加元素，那么该线程被唤醒并去消费该元素，如果timeout内依然没有新元素进入队列，直接超时返回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);
        }
        return dequeue();
    } finally {
        lock.unlock();
    }
}

//删除队列中的元素o，如果o存在且不为null，删除并返回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 {
         //判断队列不为空，为空直接返回false
        if (count > 0) {
            final int putIndex = this.putIndex;
            int i = takeIndex;
            do {
                //如果在遍历队列时找到目标元素，直接删除并返回true
                if (o.equals(items[i])) {
                    removeAt(i);
                    return true;
                }
                if (++i == items.length)
                    i = 0;
             //遍历队列并且i!=putIndex(相等时表示队列已经遍历完)
            } while (i != putIndex);
        }
        return false;
    } finally {
        lock.unlock();
    }
}

//删除队列中removeIndex位置的元素
void removeAt(final int removeIndex) {
    final Object[] items = this.items;
    //如果删除的位置在队首，直接删除并且索引takeIndex后移
    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) {
                //删除的元素不在队尾，直接把队列后面的元素前移一位，然后继续循环
                items[i] = items[next];
                i = next;
            } else {
                //遍历到队尾的元素，将队尾元素置空，并将该位置赋值给添加索引putIndex并跳出循环
                items[i] = null;
                this.putIndex = i;
                break;
            }
        }
        count--;
        if (itrs != null)
            itrs.removedAt(removeIndex);
    }
    notFull.signal();
}

//取元素索引takeIndex对应的元素置空，并唤醒生产者线程
private E dequeue() {
    final Object[] items = this.items;
    @SuppressWarnings("unchecked")
    E x = (E) items[takeIndex];
    items[takeIndex] = null;
    //循环队列
    if (++takeIndex == items.length)
        takeIndex = 0;
    //队列元素数量减1    
    count--;
    if (itrs != null)
        itrs.elementDequeued();
    notFull.signal();
    return x;
}

其他操作peek/element等

//队列为空时，返回null；队列不为空时返回队首takeIndex位置上的元素
public E peek() {
    final ReentrantLock lock = this.lock;
    lock.lock();
    try {
        return itemAt(takeIndex); 
    } finally {
        lock.unlock();
    }
}

final E itemAt(int i) {
    return (E) items[i];
}

//父类AbstractQueue中 对于ArrayBlockingQueue来说，如果队列不为空，返回队首takeIndex位置上的元素；如果队列为空，直接抛出异常
public E element() {
    E x = peek();
    if (x != null)
        return x;
    else
        throw new NoSuchElementException();
}

LinkedBlockingQueue实现原理

构造函数

private final int capacity;//队列容量，默认是Integer.MAX_VALUE

private final AtomicInteger count = new AtomicInteger();//元素个数

transient Node head;//链表头结点

private transient Node last;//链表尾结点

//取元素的锁 如take/poll中使用
private final ReentrantLock takeLock = new ReentrantLock();

//取元素锁takeLock对应的条件队列(condition queue),链表为空时阻塞，不为空时被唤醒消费
private final Condition notEmpty = takeLock.newCondition();

//添加元素的锁 在put/offer中使用
private final ReentrantLock putLock = new ReentrantLock();

//添加元素锁putLock对应的条件队列(condition queue),链表满时阻塞，不满时被唤醒执行添加操作
private final Condition notFull = putLock.newCondition();

//初始化队列 默认容量是
public LinkedBlockingQueue() {
    this(Integer.MAX_VALUE);
}

//初始化队列容量及头结点 尾结点
public LinkedBlockingQueue(int capacity) {
    if (capacity <= 0) throw new IllegalArgumentException();
    this.capacity = capacity;
    last = head = new Node(null);
}

静态内部类Node

static class Node {
    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 next;

    Node(E x) { item = x; }
}

添加元素add/put/offer

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 node = new Node(e);
    final ReentrantLock putLock = this.putLock;
    final AtomicInteger count = this.count;
    putLock.lockInterruptibly();
    try {
        while (count.get() == capacity) {
            notFull.await();
        }
        enqueue(node);
        c = count.getAndIncrement();
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        putLock.unlock();
    }
    if (c == 0)
        signalNotEmpty();
}

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 node = new Node(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;
}

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));
        c = count.getAndIncrement();
        if (c + 1 < capacity)
            notFull.signal();
    } finally {
        putLock.unlock();
    }
    if (c == 0)
        signalNotEmpty();
    return true;
}

//尾节点指向新入节点 尾指针指向新入节点
private void enqueue(Node node) {
    last = last.next = node;
}

删除元素remove/take/poll

public boolean remove(Object o) {
    if (o == null) return false;
    fullyLock();
    try {
        for (Node 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();
    }
}

void fullyLock() {
    putLock.lock();
    takeLock.lock();
}

void fullyUnlock() {
    takeLock.unlock();
    putLock.unlock();
}

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;
}

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;
}

void unlink(Node p, Node 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();
}

//删除链表队首元素
private E dequeue() {
    Node h = head;
    Node first = h.next;
    h.next = h; // help GC
    head = first;
    E x = first.item;
    first.item = null;
    return x;
}

其他操作peek/element等

//返回链表队首元素
public E peek() {
    if (count.get() == 0)
        return null;
    final ReentrantLock takeLock = this.takeLock;
    takeLock.lock();
    try {
        Node first = head.next;
        if (first == null)
            return null;
        else
            return first.item;
    } finally {
        takeLock.unlock();
    }
}

ArrayBlockingQueue、LinkedBlockingQueue的异同

相同点：

• 都是先进先出队列(FIFO)，任务的执行顺序与他们到达队列的顺序相同。

区别：

• ArrayBlockingQueue底层实现是数组，LinkedBlockingQueue底层实现是链表
• ArrayBlockingQueue是有界队列，LinkedBlockingQueue默认是无界队列(Integer.MAX_VALUE),当然也可以传入count数量变成有界队列。

SynchronousQueue实现原理

SynchronousQueue并不是一个真正的队列，而是一种在线程间进行移交的机制。SynchronousQueue可以避免任务排队，可以直接将任务从生产者移交给消费者。一个线程(生产者线程)要将一个元素放入SynchronousQueue中，必须有另一个线程(消费者线程)等待接收这个元素。

SynchronousQueue的使用

public static void main(String[] args) throws InterruptedException {

    //初始化SynchronousQueue 默认是非公平队列
    SynchronousQueue queue = new SynchronousQueue<>();
    //添加操作
    PutRunnable putRunnable = new PutRunnable(queue);
    //删除操作
    TakeRunnable takeRunnable = new TakeRunnable(queue);

    new Thread(putRunnable).start();
    Thread.sleep(1500);
    new Thread(takeRunnable).start();
}

static class PutRunnable implements Runnable {
    private SynchronousQueue queue;

    PutRunnable(SynchronousQueue queue) {
        this.queue = queue;
    }

    @Override
    public void run() {
        System.out.println("++生产者线程开始执行");
        try {
            System.out.println("++生产者线程添加元素：10");
            queue.put(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("++生产者线程结束");
        }
    }
}
static class TakeRunnable implements Runnable {

    private SynchronousQueue queue;

    TakeRunnable(SynchronousQueue queue) {
        this.queue = queue;
    }

    @Override
    public void run() {
        System.out.println("--消费者线程开始执行");
        try {
            System.out.println("--消费者线程取出元素：" + queue.take());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            System.out.println("--消费者线程结束");
        }
    }
}

执行结果：

++生产者线程开始执行
++生产者线程添加元素：10
--消费者线程开始执行
--消费者线程取出元素：10
--消费者线程结束
++生产者线程结束

从结果上可以看到：当生产者开始执行并调用put方法后，发现没有线程来消费(take)，此时生产者线程没有继续执行，而是等待消费者线程来获取此元素并唤醒自己，最后生产者线程和消费者线程双双执行完毕并退出。

浅析SynchronousQueue

SynchronousQueue的构造参数：

public SynchronousQueue() {
    this(false);
}

//如果传入的是true,所有的生产者和消费者是按顺序一一对应的，即先到的生产者会先被消费；反之如果是false，就生产者和消费者的对应没有了顺序。
public SynchronousQueue(boolean fair) {
    transferer = fair ? new TransferQueue() : new TransferStack();
}

不同于ArrayBlockingQueue、LinkedBlockingQueue的内部缓存队列，SynchronousQueue内部并没有缓存数据，生产者线程进行添加操作(put)必须等待消费者线程的移除操作(take)，反之一样。

总结一下：与SynchronousQueue关联的添加(put)操作线程和消费(take)操作线程必须成对出现，并双双继续执行，如果只有一个操作(put或take)，那么此线程会阻塞等待，直到另一个线程执行对应的操作时才会唤醒自己。SynchronousQueue适合在两个线程之间做数据交换工作。

PriorityBlockingQueue实现原理

PriorityBlockingQueue是一个无界的、基于堆的并发安全优先级队列。PriorityBlockingQueue中传入的元素不允许是null，并且必须要实现Comparable接口。

public static void main(String[] args) throws InterruptedException {

    //Example2
    ArrayList list = new ArrayList<>();
    list.add(new User("张三", 20));
    list.add(new User("李四", 40));
    list.add(new User("王五", 30));
    list.add(new User("赵六", 10));

    //初始化队列
    PriorityBlockingQueue priorityBlockingQueue = new PriorityBlockingQueue<>();
    //添加元素 添加时是没有顺序的
    priorityBlockingQueue.addAll(list);

    while (priorityBlockingQueue.size() > 0) {
        User user = priorityBlockingQueue.take();
        System.out.println("name:" + user.getName() + ",age:" + user.getAge() + ",队列元素个数：" + priorityBlockingQueue.size());
        }

    }

static class User implements Comparable {

    User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    String name;
    int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    @Override
    public int compareTo(Object o) {
        if (o instanceof User) {
            User user = (User) o;
            return age > user.getAge() ? -1 : 1;
        }
        return 0;
    }
}

打印结果：

name:李四,age:40,队列元素个数：3
name:王五,age:30,队列元素个数：2
name:张三,age:20,队列元素个数：1
name:赵六,age:10,队列元素个数：0

传入的元素时没有顺序的，但是通过compareTo排了优先级，age越大的优先级越高，所有最后的输出结果是按age的大小进行排序的。

总结：

• 传入PriorityBlockingQueue中的元素必须实现Comparable接口，通过此接口的compareTo方法来确定优先级，如果当前元素优先级高于比较的元素，返回一个负数(如-1)，反之返回一个正数(如1)。
• PriorityBlockingQueue中只有take的时候会加锁，put的时候并不会加锁，因为PriorityBlockingQueue是无界队列，支持在并发情况下去执行put操作。队列为空时，take方法会阻塞当前线程。

参考

【1】https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/BlockingQueue.html
【2】SynchronousQueue实现原理：https://zhuanlan.zhihu.com/p/29227508
【3】SynchronousQueue使用实例：https://segmentfault.com/a/1190000011207824
【4】J.U.C之阻塞队列：PriorityBlockingQueue：http://cmsblogs.com/?p=2407
【5】无界阻塞优先级队列PriorityBlockingQueue原理探究：http://ifeve.com/java-priorityblockingqueu/