java并发编程——阻塞队列与非阻塞队列

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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。