JUC之BlockingQueue（阻塞队列）

一、BlockingQueue概述

两种情况：

• 当队列是空的，从队列中获取元素的操作将会被阻塞
• 当队列是满的，从队列中获取元素的操作将会被阻塞

阻塞队列的好处：

• 阻塞也叫挂起，之前的wait就是挂起线程，等条件满足，被挂起的线程又会被唤醒。
• 而阻塞队列可以帮我们把何时阻塞何时需要唤醒的事情都给做了，不用我们自己关心阻塞和唤醒。

二、BlockingQueue常用方法

方法类型	抛出异常	特殊值	阻塞	超时
插入	add(e)	offer(e)	put(e)	offer(e,timeunit)
移除	remove(e)	poll()	take()	poll(time,unit)
检查	element()	peek()	不可用	不可用

• 抛出异常：

  • 当队列满时，再往队列里add插入元素会抛IllegalStateException: Queue full异常
  • 当队列为空，再从队列里remove移除元素会抛NoSuchElementException异常

• 特殊值：

  • 插入方法，成功返回true，失败false
  • 移除方法，成功返回元素，没有元素就返回null

• 阻塞：

  • 当阻塞队列满时，生产者线程继续往队列put元素，队列会一直阻塞生产者线程直到pull数据或响应中断退出。
  • 当阻塞队列空时，消费者线程试图从队列里take元素，队列会一直阻塞消费者线程直到队列可用

• 超时退出：当阻塞队列满时，队列会阻塞生产者线程一段时间，超过时间后生产者线程会自动退出

抛出异常代码演示：

add

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        //返回类型是boolean
        System.out.println(blockingQueue.add("a"));
        System.out.println(blockingQueue.add("c"));
        System.out.println(blockingQueue.add("d"));
        System.out.println(blockingQueue.add("d"));
    }
}

结果：

true
true
true
Exception in thread "main" java.lang.IllegalStateException: Queue full
	at java.util.AbstractQueue.add(AbstractQueue.java:98)
	at java.util.concurrent.ArrayBlockingQueue.add(ArrayBlockingQueue.java:312)
	at juc.BlockingQueueDemo.main(BlockingQueueDemo.java:16)

remove

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.add("a"));
        System.out.println(blockingQueue.add("c"));
        System.out.println(blockingQueue.add("d"));

        //返回类型是E
        System.out.println(blockingQueue.remove());
        System.out.println(blockingQueue.remove());
        System.out.println(blockingQueue.remove());
        System.out.println(blockingQueue.remove());
    }
}

结果：

true
true
true
a
c
d
Exception in thread "main" java.util.NoSuchElementException
	at java.util.AbstractQueue.remove(AbstractQueue.java:117)
	at cn.yylm._04blockingQueue.BlockingQueueDemo.main(BlockingQueueDemo.java:18)

element 检查队首元素是什么

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.add("a"));
        System.out.println(blockingQueue.add("c"));
        System.out.println(blockingQueue.add("d"));

        System.out.println(blockingQueue.element());
    }
}

结果

true
true
true
a

特殊值代码演示

offer 插入，此时插入失败不会报错，而是返回false

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.offer("a"));
        System.out.println(blockingQueue.offer("b"));
        System.out.println(blockingQueue.offer("c"));
        System.out.println(blockingQueue.offer("d"));
    }
}

结果

true
true
true
false

poll 移除 同样 移除的元素不存在也不会报错，而是返回null

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.offer("a"));
        System.out.println(blockingQueue.offer("b"));
        System.out.println(blockingQueue.offer("c"));
        System.out.println(blockingQueue.poll());
        System.out.println(blockingQueue.poll());
        System.out.println(blockingQueue.poll());
        System.out.println(blockingQueue.poll());
    }
}

结果

true
true
true
a
b
c
null

peek() 和element()一样，不再演示

阻塞代码演示

put 插入，没有返回值，当发生阻塞无法插入时会一直等待pull或线程退出

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        blockingQueue.put("a");
        blockingQueue.put("b");
        blockingQueue.put("c");
        blockingQueue.put("d");
    }
}

结果

take 移除元素，当阻塞队列没有元素时会一直等待

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        blockingQueue.put("a");
        blockingQueue.put("b");
        blockingQueue.put("c");

        System.out.println(blockingQueue.take());
        System.out.println(blockingQueue.take());
        System.out.println(blockingQueue.take());
        System.out.println(blockingQueue.take());
    }
}

结果

超时代码演示

offer 插入，指定超时时间，到期后自动终止

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.offer("a", 3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("b", 3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("c", 3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("d", 3, TimeUnit.SECONDS));
    }
}

poll 移除 指定超时时间，过期自动终止

public class BlockingQueueDemo {
    public static void main(String[] args) throws InterruptedException {
        //创建一个容量为3的阻塞队列
        BlockingQueue<String> blockingQueue = new ArrayBlockingQueue<>(3);
        System.out.println(blockingQueue.offer("a", 3000, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("b", 3000, TimeUnit.SECONDS));
        System.out.println(blockingQueue.offer("c", 3000, TimeUnit.SECONDS));
        System.out.println(blockingQueue.poll(3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.poll(3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.poll(3, TimeUnit.SECONDS));
        System.out.println(blockingQueue.poll(3, TimeUnit.SECONDS));
    }
}

结果