特殊的队列:BlockingQueue 使用

本例介绍一个特殊的队列:BlockingQueue,如果BlockQueue是空的,从BlockingQueue取东西的操作将会被阻断进入等待状态,直到BlockingQueue进了东西才会被唤醒.同样,如果BlockingQueue是满的,任何试图往里存东西的操作也会被阻断进入等待状态,直到BlockingQueue里有空间才会被唤醒继续操作.
    本例再次实现11.4线程----条件Condition中介绍的篮子程序,不过这个篮子中最多能放的苹果数不是1,可以随意指定.当篮子满时,生产者进入等待状态,当篮子空时,消费者等待.



    使用BlockingQueue的关键技术点如下:
    1.BlockingQueue定义的常用方法如下:
        1)add(anObject):把anObject加到BlockingQueue里,即如果BlockingQueue可以容纳,则返回true,否则招聘异常
        2)offer(anObject):表示如果可能的话,将anObject加到BlockingQueue里,即如果BlockingQueue可以容纳,则返回true,否则返回false.
        3)put(anObject):把anObject加到BlockingQueue里,如果BlockQueue没有空间,则调用此方法的线程被阻断直到BlockingQueue里面有空间再继续.
        4)poll(time):取走BlockingQueue里排在首位的对象,若不能立即取出,则可以等time参数规定的时间,取不到时返回null
        5)take():取走BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到Blocking有新的对象被加入为止
    2.BlockingQueue有四个具体的实现类,根据不同需求,选择不同的实现类
        1)ArrayBlockingQueue:规定大小的BlockingQueue,其构造函数必须带一个int参数来指明其大小.其所含的对象是以FIFO(先入先出)顺序排序的.
        2)LinkedBlockingQueue:大小不定的BlockingQueue,若其构造函数带一个规定大小的参数,生成的BlockingQueue有大小限制,若不带大小参数,所生成的BlockingQueue的大小由Integer.MAX_VALUE来决定.其所含的对象是以FIFO(先入先出)顺序排序的
        3)PriorityBlockingQueue:类似于LinkedBlockQueue,但其所含对象的排序不是FIFO,而是依据对象的自然排序顺序或者是构造函数的Comparator决定的顺序.
        4)SynchronousQueue:特殊的BlockingQueue,对其的操作必须是放和取交替完成的.
    3.LinkedBlockingQueue和ArrayBlockingQueue比较起来,它们背后所用的数据结构不一样,导致LinkedBlockingQueue的数据吞吐量要大于ArrayBlockingQueue,但在线程数量很大时其性能的可预见性低于ArrayBlockingQueue.



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    import java.util.concurrent.ArrayBlockingQueue; 
    import java.util.concurrent.BlockingQueue; 
    import java.util.concurrent.ExecutorService; 
    import java.util.concurrent.Executors; 
      
    public class BlockingQueueTest { 
           /**定义装苹果的篮子*/ 
           public static class Basket{ 
                  //篮子,能够容纳3个苹果 
                  BlockingQueue<String> basket = new ArrayBlockingQueue<String>(3); 
                  //生产苹果,放入篮子 
                  public void produce() throws InterruptedException{ 
                         //put方法放入一个苹果,若basket满了,等到basket有位置 
                         basket.put("An apple"); 
                  } 
                  //消费苹果,从篮子中取走 
                  public String consume() throws InterruptedException{ 
                         //take方法取出一个苹果,若basket为空,等到basket有苹果为止 
                         return basket.take(); 
                  } 
           } 
           //测试方法 
           public static void testBasket(){ 
                  final Basket basket = new Basket();//建立一个装苹果的篮子 
                  //定义苹果生产者 
                  class Producer implements Runnable{ 
                         public void run(){ 
                                try{ 
                                       while(true){ 
                                              //生产苹果 
                                              System.out.println("生产者准备生产苹果: " + System.currentTimeMillis()); 
                                              basket.produce(); 
                                              System.out.println("生产者生产苹果完毕: " + System.currentTimeMillis()); 
                                              //休眠300ms 
                                              Thread.sleep(300); 
                                       } 
                                }catch(InterruptedException ex){ 
                                } 
                         } 
                  } 
                  //定义苹果消费者 
                  class Consumer implements Runnable{ 
                         public void run(){ 
                                try{ 
                                       while(true){ 
                                              //消费苹果 
                                              System.out.println("消费者准备消费苹果: " + System.currentTimeMillis()); 
                                              basket.consume(); 
                                              System.out.println("消费者消费苹果完毕: " + System.currentTimeMillis()); 
                                              //休眠1000ms 
                                              Thread.sleep(1000); 
                                       } 
                                }catch(InterruptedException ex){ 
                                } 
                         } 
                  } 
                  ExecutorService service = Executors.newCachedThreadPool(); 
                  Producer producer = new Producer(); 
                  Consumer consumer = new Consumer(); 
                  service.submit(producer); 
                  service.submit(consumer); 
                  //程序运行5s后,所有任务停止 
                  try{ 
                         Thread.sleep(5000); 
                  }catch(InterruptedException ex){ 
                  } 
                  service.shutdownNow(); 
           } 
           public static void main(String[] args){ 
                  BlockingQueueTest.testBasket(); 
           } 
    }