java面试-阻塞队列

一、阻塞队列

当阻塞队列是空，从队列中获取元素的操作会被阻塞

当阻塞队列是满，往队列中添加元素的操作会被阻塞

二、为什么用，有什么好处？

我们不需要关心什么时候需要阻塞线程，什么时候需要唤醒线程，因为这一切阻塞队列都包办了。

三、常见的阻塞队列

ArrayBlockingQueue由数组构成的有界阻塞队列.

LinkedBlockingQueue由链表构成的有界阻塞队列（默认值为Integer.MAX_VALUE）

public class BlockingQueueDemo {

    public static void main(String[] args) throws InterruptedException {
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<String>(3);
        /**
         * 1、抛出异常 add()/remove()
         */
//        System.out.println(blockingQueue.add("a"));
//        System.out.println(blockingQueue.add("b"));
//        System.out.println(blockingQueue.add("c"));
//        System.out.println(blockingQueue.add("d"));

//        System.out.println(blockingQueue.element()); //检查队首元素

//        System.out.println(blockingQueue.remove());
//        System.out.println(blockingQueue.remove());
//        System.out.println(blockingQueue.remove());
//        System.out.println(blockingQueue.remove());

        /**
         * 2、返回布尔类型 offer()/pull()
         */
//        System.out.println(blockingQueue.offer("a"));
//        System.out.println(blockingQueue.offer("b"));
//        System.out.println(blockingQueue.offer("c"));
//        System.out.println(blockingQueue.offer("d"));
//
//        System.out.println(blockingQueue.peek()); //检查队首元素
//
//        System.out.println(blockingQueue.poll());
//        System.out.println(blockingQueue.poll());
//        System.out.println(blockingQueue.poll());
//        System.out.println(blockingQueue.poll());

        /**
         * 3、阻塞 put()/take()
         */
//        blockingQueue.put("a");
//        blockingQueue.put("b");
//        blockingQueue.put("c");
//        System.out.println("############");
//        blockingQueue.put("d");
//
//        System.out.println(blockingQueue.take());
//        System.out.println(blockingQueue.take());
//        System.out.println(blockingQueue.take());
//        System.out.println(blockingQueue.take());

        /**
         *4、超时
         */

        System.out.println(blockingQueue.offer("a",2L, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("b",2L, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("c",2L, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("d",2L, TimeUnit.SECONDS));
    }
}

SynchronousQueue是一个不存储元素的阻塞队列,也即单个元素的队列

public class SynchronousQueueDemo {
    public static void main(String[] args) {
        BlockingQueue<String> blockingQueue = new SynchronousQueue<>();

        new Thread(() -> {
            try {
                System.out.println(Thread.currentThread().getName() + " put 1");
                blockingQueue.put("1");

                System.out.println(Thread.currentThread().getName() + " put 2");
                blockingQueue.put("2");

                System.out.println(Thread.currentThread().getName() + " put 3");
                blockingQueue.put("3");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "AAA").start();

        new Thread(() -> {
            try {

                TimeUnit.SECONDS.sleep(2);
                System.out.println(Thread.currentThread().getName() + " take " + blockingQueue.take());

                TimeUnit.SECONDS.sleep(2);
                System.out.println(Thread.currentThread().getName() + " take " + blockingQueue.take());

                TimeUnit.SECONDS.sleep(2);
                System.out.println(Thread.currentThread().getName() + " take " + blockingQueue.take());

            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "BBB").start();
    }
}　　

PriorityBlockingQueue：支持优先级排序的无界阻塞队列

DelayQueue：使用优先级队列实现的延迟无界阻塞队列

LinkedTransferQueue：由链表构成的无界阻塞队列

LinkedBlockingDeque：由链表构成的双向阻塞队列

四、BlockQueue的核心方法

add()/remove()/element()：抛出异常

offer()/pull()：返回布尔类型/支持超时

put()/take()：阻塞

peek() 检查队列首元素

五、使用场景

1、生产者-消费者模式

问题： 一个初始值为零的变量，两个线程对其交替操作，一个加1一个减1，来5轮

1）传统版的生产者-消费者模式

/**
 * Created by wujuhong on 2019/7/3.
 * 传统版的生产者消费者模式
 */
public class ProductConsumer_TraditionDemo {

    public static void main(String[] args) {
        ShareData shareData = new ShareData();
        new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                try {
                    shareData.increment();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "AA").start();

        new Thread(() -> {
            for (int i = 0; i < 5; i++) {
                try {
                    shareData.decrement();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "BB").start();
    }
}

class ShareData {
    private int number = 0;
    private Lock lock = new ReentrantLock();
    private Condition condition = lock.newCondition();

    public void increment() throws InterruptedException {
        lock.lock();
        try {
            //1、判断
            while (number != 0) {
                //等待，不能生产
                condition.await();
            }
            //2、干活
            number++;
            System.out.println(Thread.currentThread().getName() + " " + number);
            //3、通知唤醒
            condition.signalAll();

        } finally {
            lock.unlock();
        }
    }

    public void decrement() throws InterruptedException {
        lock.lock();
        try {
            //1、判断
            while (number == 0) {
                //等待，不能生产
                condition.await();
            }
            //2、干活
            number--;
            System.out.println(Thread.currentThread().getName() + " " + number);
            //3、通知唤醒
            condition.signalAll();

        } finally {
            lock.unlock();
        }
    }
}

　　2）阻塞队列的生产者-消费者模式

class MyResource {
    private volatile boolean FLAG = true; //默认生产，进行生产+消费
    private AtomicInteger atomicInteger = new AtomicInteger();
    BlockingQueue<String> blockingQueue = null;

    public MyResource(BlockingQueue<String> blockingQueue) {
        this.blockingQueue = blockingQueue;
        System.out.println(blockingQueue.getClass().getName());
    }

    public void myProduct() throws InterruptedException {
        String data = "";
        boolean returnValue;
        while (FLAG) {
            data = atomicInteger.incrementAndGet() + "";
            returnValue = blockingQueue.offer(data, 2, TimeUnit.SECONDS);
            if (returnValue) {
                System.out.println(Thread.currentThread().getName() + " 插入队列" + data + "成功");
            } else {
                System.out.println(Thread.currentThread().getName() + " 插入队列" + data + "失败");

            }
            TimeUnit.SECONDS.sleep(1);
        }
        System.out.println(Thread.currentThread().getName() + " 生产动作结束，FLAG = false");
    }

    public void myConsume() throws InterruptedException {
        String result = "";
        while (FLAG) {
            result = blockingQueue.poll(2, TimeUnit.SECONDS);
            if (null == result || result.equalsIgnoreCase("")) {
                FLAG = false;
                System.out.println(Thread.currentThread().getName() + " 超过2s钟没有取到蛋糕，消费队列退出");
                return;

            }
            System.out.println(Thread.currentThread().getName() + " 消费队列" + result + "成功");

            TimeUnit.SECONDS.sleep(1);
        }
        System.out.println(Thread.currentThread().getName() + " 消费动作结束，FLAG = false");
    }

    public void stop() {
        this.FLAG = false;
    }
}

public class ProductConsumer_BlockQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        MyResource myResource = new MyResource(new ArrayBlockingQueue<String>(10));
        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + " 生产线程启动");
            try {
                myResource.myProduct();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "product").start();

        new Thread(() -> {
            System.out.println(Thread.currentThread().getName() + " 消费线程启动");
            try {
                myResource.myConsume();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, "consume").start();

        TimeUnit.SECONDS.sleep(5);

        System.out.println("5秒钟时间到，main线程叫停，活动结束");

        myResource.stop();
    }
}　　

2、线程池

3、消息中间件

阻塞队列有没有好的一面？

不得不阻塞，你如何管理？