java多线程基本概述（二十一）——BlockingQueue
public interface BlockingQueue<E>extends Queue<E>
支持两个附加操作的 Queue，这两个操作是：获取元素时等待队列变为非空，以及存储元素时等待空间变得可用。
BlockingQueue 方法以四种形式出现，对于不能立即满足但可能在将来某一时刻可以满足的操作，这四种形式的处理方式不同：第一种是抛出一个异常，第二种是返回一个特殊值（null 或 false，具体取决于操作），第三种是在操作可以成功前，无限期地阻塞当前线程，第四种是在放弃前只在给定的最大时间限制内阻塞。下表中总结了这些方法：
抛出异常 特殊值 阻塞 超时
插入 add(e) offer(e) put(e) offer(e, time, unit)
移除 remove() poll() take() poll(time, unit)
检查 element() peek() 不可用 不可用
BlockingQueue 不接受 null 元素。试图 add、put 或 offer 一个 null 元素时，某些实现会抛出 NullPointerException。null 被用作指示 poll 操作失败的警戒值。
BlockingQueue 可以是限定容量的。它在任意给定时间都可以有一个 remainingCapacity，超出此容量，便无法无阻塞地 put 附加元素。没有任何内部容量约束的 BlockingQueue 总是报告 Integer.MAX_VALUE 的剩余容量。
BlockingQueue 实现主要用于生产者-使用者队列，但它另外还支持 Collection 接口。因此，举例来说，使用 remove(x) 从队列中移除任意一个元素是有可能的。然而，这种操作通常不 会有效执行，只能有计划地偶尔使用，比如在取消排队信息时。
BlockingQueue 实现是线程安全的。所有排队方法都可以使用内部锁或其他形式的并发控制来自动达到它们的目的。然而，大量的 Collection 操作（addAll、containsAll、retainAll 和 removeAll）没有 必要自动执行，除非在实现中特别说明。因此，举例来说，在只添加了 c 中的一些元素后，addAll(c) 有可能失败（抛出一个异常）。
BlockingQueue 实质上不 支持使用任何一种“close”或“shutdown”操作来指示不再添加任何项。这种功能的需求和使用有依赖于实现的倾向。例如，一种常用的策略是：对于生产者，插入特殊的 end-of-stream 或 poison 对象，并根据使用者获取这些对象的时间来对它们进行解释。
以下是基于典型的生产者-使用者场景的一个用例。注意，BlockingQueue 可以安全地与多个生产者和多个使用者一起使用。
class Producer implements Runnable { private final BlockingQueue queue; Producer(BlockingQueue q) { queue = q; } public void run() { try { while(true) { queue.put(produce()); } } catch (InterruptedException ex) { ... handle ...} } Object produce() { ... } } class Consumer implements Runnable { private final BlockingQueue queue; Consumer(BlockingQueue q) { queue = q; } public void run() { try { while(true) { consume(queue.take()); } } catch (InterruptedException ex) { ... handle ...} } void consume(Object x) { ... } } class Setup { void main() { BlockingQueue q = new SomeQueueImplementation(); Producer p = new Producer(q); Consumer c1 = new Consumer(q); Consumer c2 = new Consumer(q); new Thread(p).start(); new Thread(c1).start(); new Thread(c2).start(); } }
例子： LiftOff.java
package tij; /** * Created by huaox on 2017/3/31. * */ public class LiftOff implements Runnable { protected int countDown = 10; private static int taskDown = 0; private final int id = taskDown++; public LiftOff() { } public LiftOff(int countDown) { this.countDown = countDown; } public String status(){ return "【#"+id+"("+countDown+")】"; } @Override public void run() { try { while (countDown-->0){ //Thread.sleep(5000); System.out.println("liftoff= "+status()+" was used "+System.currentTimeMillis()); Thread.yield(); } } catch (Exception e) { System.out.println("interrupted !!!!liftOff"); } } }
Test.java
package tij; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.util.concurrent.*; /** * Created by huaox on 2017/3/31. * */ class RunnerBlockingQueue implements Runnable { private BlockingQueue<LiftOff> rockets; RunnerBlockingQueue(BlockingQueue<LiftOff> rockets) { this.rockets = rockets; } void add(LiftOff liftOff){ try { System.out.println("liftoff= "+liftOff.status()+/*"queue size="+rockets.size()+*/" put start "+System.currentTimeMillis()); rockets.put(liftOff); //System.out.println("liftoff= "+liftOff.status()+" put end "+System.currentTimeMillis()); }catch (InterruptedException e){ System.out.println("interrupting during put()"); } } @Override public void run() { try { while (!Thread.interrupted()){ // System.out.println("liftoff get "+System.currentTimeMillis()); LiftOff liftOff = rockets.take(); System.out.println("liftoff= "+liftOff.status()+" start run "+System.currentTimeMillis()); //line 1 liftOff.run(); //System.out.println("liftoff= "+liftOff.status()+" run end "+System.currentTimeMillis()); //line 2 } }catch (InterruptedException e){ System.out.println("waking from take()"); } System.out.println("Exiting runner"); } } public class Test{ static void getKey(String msg){ System.out.println(msg); try { String result = new BufferedReader(new InputStreamReader(System.in)).readLine(); System.out.println("finish block io "+result); }catch (IOException e){ throw new RuntimeException(e); } } static void test(String msg,BlockingQueue<LiftOff> queue) throws InterruptedException { System.out.println("using queue test"+msg); RunnerBlockingQueue runner = new RunnerBlockingQueue(queue); Thread thread = new Thread(runner); thread.start();//这时候开始的时候，队列里面并没有任务。那么取队列任务运行的时候会被阻塞 TimeUnit.SECONDS.sleep(1); for (int i = 0; i < 5; i++) { runner.add(new LiftOff(5)); } getKey("Press 'Enter'("+msg+")");//阻塞直到控制台输入后才发送中断消息 thread.interrupt(); System.out.println("Finished "+msg+" test"); } public static void main(String[] args) throws InterruptedException { test("LinkedBlockingDeque",new LinkedBlockingDeque<LiftOff>()); //未限制大小 // test("ArrayBlockingQueue",new ArrayBlockingQueue<LiftOff>(3)); //限制大小为3 // test("SynchronousQueue",new SynchronousQueue<>()); //大小为1 } }
输出结果：
using queue testLinkedBlockingDeque liftoff= 【#0(5)】 put start 1492681266001 put 0 liftoff= 【#1(5)】 put start 1492681266001 put 1 liftoff= 【#0(5)】 start run 1492681266001 start use 0 liftoff= 【#2(5)】 put start 1492681266001 put 2 liftoff= 【#0(4)】 was used 1492681266001 use 0 liftoff= 【#3(5)】 put start 1492681266001 put 3 liftoff= 【#0(3)】 was used 1492681266001 use 0 liftoff= 【#4(5)】 put start 1492681266001 put 4 liftoff= 【#0(2)】 was used 1492681266001 use 0 Press 'Enter'(LinkedBlockingDeque) liftoff= 【#0(1)】 was used 1492681266001 use 0 liftoff= 【#0(0)】 was used 1492681266001 use 0 liftoff= 【#1(5)】 start run 1492681266001 start use 1 liftoff= 【#1(4)】 was used 1492681266001 use 1 liftoff= 【#1(3)】 was used 1492681266001 use 1 liftoff= 【#1(2)】 was used 1492681266001 use 1 liftoff= 【#1(1)】 was used 1492681266001 use 1 liftoff= 【#1(0)】 was used 1492681266001 use 1 liftoff= 【#2(5)】 start run 1492681266001 start use 2 liftoff= 【#2(4)】 was used 1492681266001 use 2 liftoff= 【#2(3)】 was used 1492681266001 use 2 liftoff= 【#2(2)】 was used 1492681266001 use 2 liftoff= 【#2(1)】 was used 1492681266001 use 2 liftoff= 【#2(0)】 was used 1492681266001 use 2 liftoff= 【#3(5)】 start run 1492681266001 start use 3 liftoff= 【#3(4)】 was used 1492681266001 use 3 liftoff= 【#3(3)】 was used 1492681266001 use 3 liftoff= 【#3(2)】 was used 1492681266001 use 3 liftoff= 【#3(1)】 was used 1492681266001 use 3 liftoff= 【#3(0)】 was used 1492681266001 use 3 liftoff= 【#4(5)】 start run 1492681266001 liftoff= 【#4(4)】 was used 1492681266001 liftoff= 【#4(3)】 was used 1492681266001 liftoff= 【#4(2)】 was used 1492681266001 liftoff= 【#4(1)】 was used 1492681266001 liftoff= 【#4(0)】 was used 1492681266001 finish Task finish block io finish Task Finished LinkedBlockingDeque test waking from take() Exiting runner Process finished with exit code 0
图中结果状态描绘的很清除了，相信一看就能明白吧！
如果把Task也就是任务具体的运行者，LiftOff的run方法里面的加个休眠。
@Override public void run() { try { while (countDown-->0){ Thread.sleep(500); System.out.println("liftoff= "+status()+" was used "+System.currentTimeMillis()); Thread.yield(); } } catch (Exception e) { System.out.println("interrupted !!!!liftOff"); } }
那么运行结果：
using queue testLinkedBlockingDeque liftoff= 【#0(5)】 put start 1492682392952 put 0 liftoff= 【#0(5)】 start run 1492682392952 start 0 liftoff= 【#1(5)】 put start 1492682392952 put 1 liftoff= 【#2(5)】 put start 1492682392952 put 2 liftoff= 【#3(5)】 put start 1492682392952 put 3 liftoff= 【#4(5)】 put start 1492682392952 put 4 Press 'Enter'(LinkedBlockingDeque) liftoff= 【#0(4)】 was used 1492682393452 use 0 liftoff= 【#0(3)】 was used 1492682393962 use 0 liftoff= 【#0(2)】 was used 1492682394462 use 0 liftoff= 【#0(1)】 was used 1492682394962 use 0 liftoff= 【#0(0)】 was used 1492682395478 use 0 liftoff= 【#1(5)】 start run 1492682395478 start use 1 liftoff= 【#1(4)】 was used 1492682395978 use 1 liftoff= 【#1(3)】 was used 1492682396485 use 1 liftoff= 【#1(2)】 was used 1492682396995 use 1 liftoff= 【#1(1)】 was used 1492682397496 use 1 liftoff= 【#1(0)】 was used 1492682397996 use 1 liftoff= 【#2(5)】 start run 1492682397996 start use 2 liftoff= 【#2(4)】 was used 1492682398496 use 2 liftoff= 【#2(3)】 was used 1492682399012 use 2 liftoff= 【#2(2)】 was used 1492682399512 use 2 liftoff= 【#2(1)】 was used 1492682400027 use 2 liftoff= 【#2(0)】 was used 1492682400528 use 2 liftoff= 【#3(5)】 start run 1492682400528 liftoff= 【#3(4)】 was used 1492682401043 liftoff= 【#3(3)】 was used 1492682401543 liftoff= 【#3(2)】 was used 1492682402043 liftoff= 【#3(1)】 was used 1492682402543 liftoff= 【#3(0)】 was used 1492682403057 liftoff= 【#4(5)】 start run 1492682403057 liftoff= 【#4(4)】 was used 1492682403557 liftoff= 【#4(3)】 was used 1492682404057 liftoff= 【#4(2)】 was used 1492682404558 liftoff= 【#4(1)】 was used 1492682405058 liftoff= 【#4(0)】 was used 1492682405558 exit finish block io exit Finished LinkedBlockingDeque test waking from take() Exiting runner Process finished with exit code 0
LinkedBlockingDeque 可以允许同时多个任务添加进来，但是取的时候会一个接着一个来。
下面测试：把line 1注释掉，line 2取消注释。并且打开下面代码的注释
test("ArrayBlockingQueue",new ArrayBlockingQueue<LiftOff>(3)); //限制大小为3
输出结果：
using queue testArrayBlockingQueue liftoff= 【#0(5)】 put end 1492683884689 put queue size =1 liftoff= 【#0(5)】 start run 1492683884689 take queue size =0 liftoff= 【#1(5)】 put end 1492683884689 put queue size =1 liftoff= 【#2(5)】 put end 1492683884689 put queue size =2 liftoff= 【#3(5)】 put end 1492683884689 put queue size =3 liftoff= 【#0(4)】 was used 1492683885190 liftoff= 【#0(3)】 was used 1492683885690 liftoff= 【#0(2)】 was used 1492683886205 liftoff= 【#0(1)】 was used 1492683886705 liftoff= 【#0(0)】 was used 1492683887206 此刻。0将运行完成，将运行下一个任务，队列长度变为2 liftoff= 【#1(5)】 start run 1492683887206 任务1开始运行 liftoff= 【#4(5)】 put end 1492683887206 put 于此同时，添加新的任务到队列中来 Press 'Enter'(ArrayBlockingQueue) liftoff= 【#1(4)】 was used 1492683887721 liftoff= 【#1(3)】 was used 1492683888221 liftoff= 【#1(2)】 was used 1492683888721 liftoff= 【#1(1)】 was used 1492683889227 liftoff= 【#1(0)】 was used 1492683889727 liftoff= 【#2(5)】 start run 1492683889727 liftoff= 【#2(4)】 was used 1492683890227 liftoff= 【#2(3)】 was used 1492683890743 liftoff= 【#2(2)】 was used 1492683891243 liftoff= 【#2(1)】 was used 1492683891743 liftoff= 【#2(0)】 was used 1492683892243 liftoff= 【#3(5)】 start run 1492683892243 liftoff= 【#3(4)】 was used 1492683892743 liftoff= 【#3(3)】 was used 1492683893243 liftoff= 【#3(2)】 was used 1492683893759 liftoff= 【#3(1)】 was used 1492683894259 liftoff= 【#3(0)】 was used 1492683894759 liftoff= 【#4(5)】 start run 1492683894759 liftoff= 【#4(4)】 was used 1492683895259 liftoff= 【#4(3)】 was used 1492683895759 liftoff= 【#4(2)】 was used 1492683896271 liftoff= 【#4(1)】 was used 1492683896787 liftoff= 【#4(0)】 was used 1492683897287 Process finished with exit code 1
ArrayBlockingQueue 一个有界队列，大小固定，那么队列中同时允许只有大小的任务，只有任务出队后才能添加新任务
下面测试：
test("SynchronousQueue",new SynchronousQueue<>())
输出结果：
using queue testSynchronousQueue liftoff= 【#0(5)】 start run 1492685180178 liftoff= 【#0(4)】 put end 1492685180178 put 0 liftoff= 【#0(4)】 was used 1492685180679 liftoff= 【#0(3)】 was used 1492685181179 liftoff= 【#0(2)】 was used 1492685181690 虽然此过程中取出了，任务队列中数量为0.但也要最新出队列的任务完成后，才能接收下个任务。这是一个保证，否则也就没意思了 liftoff= 【#0(1)】 was used 1492685182205 liftoff= 【#0(0)】 was used 1492685182705 liftoff= 【#1(5)】 start run 1492685182705 liftoff= 【#1(5)】 put end 1492685182705 put 1 liftoff= 【#1(4)】 was used 1492685183206 liftoff= 【#1(3)】 was used 1492685183706 liftoff= 【#1(2)】 was used 1492685184206 liftoff= 【#1(1)】 was used 1492685184706 liftoff= 【#1(0)】 was used 1492685185206 liftoff= 【#2(5)】 start run 1492685185206 liftoff= 【#2(5)】 put end 1492685185206 put 2 liftoff= 【#2(4)】 was used 1492685185706 liftoff= 【#2(3)】 was used 1492685186221 liftoff= 【#2(2)】 was used 1492685186721 liftoff= 【#2(1)】 was used 1492685187222 liftoff= 【#2(0)】 was used 1492685187724 liftoff= 【#3(5)】 put end 1492685187724 put 3 liftoff= 【#3(5)】 start run 1492685187724 liftoff= 【#3(4)】 was used 1492685188239 liftoff= 【#3(3)】 was used 1492685188740 liftoff= 【#3(2)】 was used 1492685189240 liftoff= 【#3(1)】 was used 1492685189741 liftoff= 【#3(0)】 was used 1492685190241 liftoff= 【#4(5)】 start run 1492685190241 liftoff= 【#4(5)】 put end 1492685190241 put 4 Press 'Enter'(SynchronousQueue) liftoff= 【#4(4)】 was used 1492685190742 liftoff= 【#4(3)】 was used 1492685191256 liftoff= 【#4(2)】 was used 1492685191762 liftoff= 【#4(1)】 was used 1492685192274 liftoff= 【#4(0)】 was used 1492685192774 Process finished with exit code 1
可见 SynchronousQueue 一次只允许一个任务进行。
