JUC：堵塞队列BlockingQueue和同步SynchronousQueue

BlockingQueue

堵塞队列的特点：

• 写入：如果队列满了，就必须堵塞等待
• 取出：如果队列是空的，必须堵塞等待生产

堵塞队列的应用场景：线程池

继承关系图：

四组API：

方式	抛出异常	有返回值，不抛出异常	堵塞等待	超时等待
添加	add	offer	put	offer(obj,int,TimeUnit)
移除	remove	poll	take	poll(int,TimeUnit)
首个元素	element	peek

1. 抛出异常

ArrayBlockingQueue<Object> objectArrayBlockingQueue = new ArrayBlockingQueue<>(3);
System.out.println(objectArrayBlockingQueue.add("a"));
System.out.println(objectArrayBlockingQueue.add("b"));
System.out.println(objectArrayBlockingQueue.add("c"));
System.out.println(objectArrayBlockingQueue.add("d"));
System.out.println(objectArrayBlockingQueue.element());
System.out.println("==================================");
System.out.println(objectArrayBlockingQueue.remove());
System.out.println(objectArrayBlockingQueue.remove());
System.out.println(objectArrayBlockingQueue.remove());

true
true
true
a
==================================
a
b
c

如果超过capacity或者取元素时队列为空，抛出异常：

Exception in thread "main" java.lang.IllegalStateException: Queue full

2. 有返回值，不抛出异常

ArrayBlockingQueue<Object> objectArrayBlockingQueue = new ArrayBlockingQueue<>(3);
System.out.println(objectArrayBlockingQueue.offer("a"));
System.out.println(objectArrayBlockingQueue.offer("b"));
System.out.println(objectArrayBlockingQueue.offer("c"));

System.out.println(objectArrayBlockingQueue.peek());
System.out.println("==================================");
System.out.println(objectArrayBlockingQueue.poll());
System.out.println(objectArrayBlockingQueue.poll());
System.out.println(objectArrayBlockingQueue.poll());

true
true
true
a
==================================
a
b
c

当从空队列取值/超过容量：

3. 堵塞等待

ArrayBlockingQueue<Object> objectArrayBlockingQueue = new ArrayBlockingQueue<>(3);
objectArrayBlockingQueue.put("a");
objectArrayBlockingQueue.put("b");
objectArrayBlockingQueue.put("c");
System.out.println("放入三个元素");
System.out.println("================");
System.out.println(objectArrayBlockingQueue.take());
System.out.println(objectArrayBlockingQueue.take());
System.out.println(objectArrayBlockingQueue.take());
System.out.println(objectArrayBlockingQueue.take());
System.out.println("结束");

如果取出元素时队列为空/超过队列容量，程序等待：

4. 超时等待

 ArrayBlockingQueue blockingQueue = new ArrayBlockingQueue<>(3);
 blockingQueue.offer("a");
 blockingQueue.offer("b");
 blockingQueue.offer("c");
  blockingQueue.offer("d",2,TimeUnit.SECONDS); // 等待超过2秒就退出
 System.out.println("===============");
 System.out.println(blockingQueue.poll());
 System.out.println(blockingQueue.poll());
 System.out.println(blockingQueue.poll());
blockingQueue.poll(2,TimeUnit.SECONDS);

SynchronousQueue

SynchronousQueue类实现了BlockingQueue接口。SynchronousQueue是一个内部只能包含一个元素的队列。插入元素到队列的线程被阻塞，直到另一个线程从队列中获取了队列中存储的元素。同样，如果线程尝试获取元素并且当前不存在任何元素，则该线程将被阻塞，直到线程将元素插入队列。

/**
 * 同步队列
 * 和其他的BlockingQueue 不一样， SynchronousQueue 不存储元素
 * put了一个元素，必须从里面先take取出来，否则不能在put进去值！
 */
public class SynchronousQueueDemo {
    public static void main(String[] args) {
        BlockingQueue<String> blockingQueue = new SynchronousQueue<>(); // 同步队
        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();
            }
        },"T1").start();
        new Thread(()->{
            try {
              TimeUnit.SECONDS.sleep(3);
                System.out.println(Thread.currentThread().getName()+"=>"+blockingQueue.take());
               TimeUnit.SECONDS.sleep(3);
                System.out.println(Thread.currentThread().getName()+"=>"+blockingQueue.take());
              TimeUnit.SECONDS.sleep(3);
                System.out.println(Thread.currentThread().getName()+"=>"+blockingQueue.take());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        },"T2").start();
    }
}

T1 put 1
T2=>1
T1 put 2
T2=>2
T1 put 3
T2=>3