java.util.concurrent多线程框架 （转）

JDK5中的一个亮点就是将Doug Lea的并发库引入到Java标准库中。DougLea确实是一个牛人，能教书，能出书，能编码，不过这在国外还是比较普遍的，而国内的教授们就相差太远了。

<wbr><wbr><wbr>一般的服务器都需要线程池，比如Web、FTP等服务器，不过它们一般都自己实现了线程池，比如以前介绍过的Tomcat、Resin和Jetty等，现在有了JDK5，我们就没有必要重复造车轮了，直接使用就可以，何况使用也很方便，性能也非常高。<br><br></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
3. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
4. public<wbr></wbr>class<wbr>TestThreadPool<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
5. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String<wbr>args[])<wbr></wbr></wbr></wbr>throws<wbr>InterruptedException<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
6. //<wbr>only<wbr>two<wbr>threads</wbr></wbr></wbr><wbr><wbr></wbr></wbr>
7. ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr>2);<wbr><wbr></wbr></wbr>
8. for(int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>100;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
9. Runnable<wbr>run<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
10. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
11. long<wbr>time<wbr>=<wbr>(</wbr></wbr></wbr>long)<wbr>(Math.random()<wbr>*<wbr></wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
12. System.out.println(“Sleeping<wbr>”<wbr>+<wbr>time<wbr>+<wbr>“ms”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
13. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
14. Thread.sleep(time);<wbr><wbr></wbr></wbr>
15. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
16. } <wbr><wbr></wbr></wbr>
17. } <wbr><wbr></wbr></wbr>
18. }; <wbr><wbr></wbr></wbr>
19. exec.execute(run);<wbr><wbr></wbr></wbr>
20. } <wbr><wbr></wbr></wbr>
21. //<wbr>must<wbr>shutdown</wbr></wbr><wbr><wbr></wbr></wbr>
22. exec.shutdown();<wbr><wbr></wbr></wbr>
23. } <wbr><wbr></wbr></wbr>
24. }<wbr></wbr>

<wbr><wbr><wbr>上面是一个简单的例子，使用了2个大小的线程池来处理100个线程。但有一个问题：在for循环的过程中，会等待线程池有空闲的线程，所以主线程会阻塞的。为了解决这个问题，一般启动一个线程来做for循环，就是为了避免由于线程池满了造成主线程阻塞。不过在这里我没有这样处理。[重要修正：经过测试，即使线程池大小小于实际线程数大小，线程池也不会阻塞的，这与Tomcat的线程池不同，它将Runnable实例放到一个“无限”的BlockingQueue中，所以就不用一个线程启动for循环，DougLea果然厉害]<br><br><wbr><wbr><wbr>另外它使用了Executors的静态函数生成一个固定的线程池，顾名思义，线程池的线程是不会释放的，即使它是Idle。这就会产生性能问题，比如如果线程池的大小为200，当全部使用完毕后，所有的线程会继续留在池中，相应的内存和线程切换（while(true)+sleep循环）都会增加。如果要避免这个问题，就必须直接使用ThreadPoolExecutor()来构造。可以像Tomcat的线程池一样设置“最大线程数”、“最小线程数”和“空闲线程keepAlive的时间”。通过这些可以基本上替换Tomcat的线程池实现方案。<br><br><wbr><wbr><wbr>需要注意的是线程池必须使用shutdown来显式关闭，否则主线程就无法退出。shutdown也不会阻塞主线程。<br><br><wbr><wbr><wbr>许多长时间运行的应用有时候需要定时运行任务完成一些诸如统计、优化等工作，比如在电信行业中处理用户话单时，需要每隔1分钟处理话单；网站每天凌晨统计用户访问量、用户数；大型超时凌晨3点统计当天销售额、以及最热卖的商品；每周日进行数据库备份；公司每个月的10号计算工资并进行转帐等，这些都是定时任务。通过java的并发库concurrent可以轻松的完成这些任务，而且非常的简单。<br><br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. import<wbr></wbr>static<wbr>java.util.concurrent.TimeUnit.SECONDS;<wbr><wbr></wbr></wbr></wbr>
3. import<wbr>java.util.Date;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.ScheduledExecutorService<wbr>;<wbr><wbr></wbr></wbr></wbr></wbr>
6. import<wbr>java.util.concurrent.ScheduledFuture;<wbr><wbr></wbr></wbr></wbr>
7. public<wbr></wbr>class<wbr>TestScheduledThread<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
8. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
9. final<wbr>ScheduledExecutorService<wbr><wbr>scheduler<wbr>=<wbr>Executors<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
10. .newScheduledThreadPool(2);<wbr><wbr></wbr></wbr>
11. final<wbr>Runnable<wbr>beeper<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
12. int<wbr>count<wbr>=<wbr></wbr></wbr></wbr>0;<wbr><wbr></wbr></wbr>
13. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
14. System.out.println(new<wbr>Date()<wbr>+<wbr>”<wbr>beep<wbr>”<wbr>+<wbr>(++count));<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
15. } <wbr><wbr></wbr></wbr>
16. }; <wbr><wbr></wbr></wbr>
17. //<wbr>1秒钟后运行，并每隔2秒运行一次</wbr><wbr><wbr></wbr></wbr>
18. final<wbr>ScheduledFuture<wbr>beeperHandle<wbr>=<wbr>scheduler.scheduleAtFixedRate(<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>
19. beeper,<wbr></wbr>1,<wbr></wbr>2,<wbr>SECONDS);<wbr><wbr></wbr></wbr></wbr>
20. //<wbr>2秒钟后运行，并每次在上次任务运行完后等待5秒后重新运行</wbr><wbr><wbr></wbr></wbr>
21. final<wbr>ScheduledFuture<wbr>beeperHandle2<wbr>=<wbr>scheduler<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>
22. .scheduleWithFixedDelay(beeper,<wbr></wbr>2,<wbr></wbr>5,<wbr>SECONDS);<wbr><wbr></wbr></wbr></wbr>
23. //<wbr>30秒后结束关闭任务，并且关闭Scheduler</wbr><wbr><wbr></wbr></wbr>
24. scheduler.schedule(new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
25. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
26. beeperHandle.cancel(true);<wbr><wbr></wbr></wbr>
27. beeperHandle2.cancel(true);<wbr><wbr></wbr></wbr>
28. scheduler.shutdown();<wbr><wbr></wbr></wbr>
29. } <wbr><wbr></wbr></wbr>
30. },<wbr></wbr>30,<wbr>SECONDS);<wbr><wbr></wbr></wbr></wbr>
31. } <wbr><wbr></wbr></wbr>
32. }<wbr><wbr></wbr></wbr>

<wbr><wbr><wbr>为了退出进程，上面的代码中加入了关闭Scheduler的操作。而对于24小时运行的应用而言，是没有必要关闭Scheduler的。<br><br><wbr><wbr><wbr>在实际应用中，有时候需要多个线程同时工作以完成同一件事情，而且在完成过程中，往往会等待其他线程都完成某一阶段后再执行，等所有线程都到达某一个阶段后再统一执行。<br><br><wbr><wbr><wbr>比如有几个旅行团需要途经深圳、广州、韶关、长沙最后到达武汉。旅行团中有自驾游的，有徒步的，有乘坐旅游大巴的；这些旅行团同时出发，并且每到一个目的地，都要等待其他旅行团到达此地后再同时出发，直到都到达终点站武汉。<br><br><wbr><wbr><wbr>这时候CyclicBarrier就可以派上用场。CyclicBarrier最重要的属性就是参与者个数，另外最要方法是await()。当所有线程都调用了await()后，就表示这些线程都可以继续执行，否则就会等待。<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. import<wbr>java.text.SimpleDateFormat;<wbr><wbr></wbr></wbr></wbr>
3. import<wbr>java.util.Date;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.BrokenBarrierException;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.CyclicBarrier;<wbr><wbr></wbr></wbr></wbr>
6. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
7. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
8. public<wbr></wbr>class<wbr>TestCyclicBarrier<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
9. //<wbr>徒步需要的时间:<wbr>Shenzhen,<wbr>Guangzhou,<wbr>Shaoguan,<wbr>Changsha,<wbr>Wuhan</wbr></wbr></wbr></wbr></wbr></wbr><wbr><wbr></wbr></wbr>
10. private<wbr></wbr>static<wbr></wbr>int[]<wbr>timeWalk<wbr>=<wbr>{<wbr></wbr></wbr></wbr></wbr>5,<wbr></wbr>8,<wbr></wbr>15,<wbr></wbr>15,<wbr></wbr>10<wbr>};<wbr><wbr></wbr></wbr></wbr>
11. //<wbr>自驾游</wbr><wbr><wbr></wbr></wbr>
12. private<wbr></wbr>static<wbr></wbr>int[]<wbr>timeSelf<wbr>=<wbr>{<wbr></wbr></wbr></wbr></wbr>1,<wbr></wbr>3,<wbr></wbr>4,<wbr></wbr>4,<wbr></wbr>5<wbr>};<wbr><wbr></wbr></wbr></wbr>
13. //<wbr>旅游大巴</wbr><wbr><wbr></wbr></wbr>
14. private<wbr></wbr>static<wbr></wbr>int[]<wbr>timeBus<wbr>=<wbr>{<wbr></wbr></wbr></wbr></wbr>2,<wbr></wbr>4,<wbr></wbr>6,<wbr></wbr>6,<wbr></wbr>7<wbr>};<wbr><wbr></wbr></wbr></wbr>
15. <wbr><wbr></wbr></wbr>
16. static<wbr>String<wbr>now()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
17. SimpleDateFormat<wbr>sdf<wbr>=<wbr></wbr></wbr></wbr>new<wbr>SimpleDateFormat(“HH:mm:ss”);<wbr><wbr></wbr></wbr></wbr>
18. return<wbr>sdf.format(</wbr>new<wbr>Date())<wbr>+<wbr>“:<wbr>“;<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>
19. } <wbr><wbr></wbr></wbr>
20. <wbr><wbr></wbr></wbr>
21. static<wbr></wbr>class<wbr>Tour<wbr></wbr></wbr>implements<wbr>Runnable<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
22. private<wbr></wbr>int[]<wbr>times;<wbr><wbr></wbr></wbr></wbr>
23. private<wbr>CyclicBarrier<wbr>barrier;<wbr><wbr></wbr></wbr></wbr></wbr>
24. private<wbr>String<wbr>tourName;<wbr><wbr></wbr></wbr></wbr></wbr>
25. public<wbr>Tour(CyclicBarrier<wbr>barrier,<wbr>String<wbr>tourName,<wbr></wbr></wbr></wbr></wbr></wbr>int[]<wbr>times)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
26. this.times<wbr>=<wbr>times;<wbr><wbr></wbr></wbr></wbr></wbr>
27. this.tourName<wbr>=<wbr>tourName;<wbr><wbr></wbr></wbr></wbr></wbr>
28. this.barrier<wbr>=<wbr>barrier;<wbr><wbr></wbr></wbr></wbr></wbr>
29. } <wbr><wbr></wbr></wbr>
30. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
31. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
32. Thread.sleep(times[0]<wbr>*<wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
33. System.out.println(now()<wbr>+<wbr>tourName<wbr>+<wbr>”<wbr>Reached<wbr>Shenzhen”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
34. barrier.await();<wbr><wbr></wbr></wbr>
35. Thread.sleep(times[1]<wbr>*<wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
36. System.out.println(now()<wbr>+<wbr>tourName<wbr>+<wbr>”<wbr>Reached<wbr>Guangzhou”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
37. barrier.await();<wbr><wbr></wbr></wbr>
38. Thread.sleep(times[2]<wbr>*<wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
39. System.out.println(now()<wbr>+<wbr>tourName<wbr>+<wbr>”<wbr>Reached<wbr>Shaoguan”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
40. barrier.await();<wbr><wbr></wbr></wbr>
41. Thread.sleep(times[3]<wbr>*<wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
42. System.out.println(now()<wbr>+<wbr>tourName<wbr>+<wbr>”<wbr>Reached<wbr>Changsha”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
43. barrier.await();<wbr><wbr></wbr></wbr>
44. Thread.sleep(times[4]<wbr>*<wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
45. System.out.println(now()<wbr>+<wbr>tourName<wbr>+<wbr>”<wbr>Reached<wbr>Wuhan”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
46. barrier.await();<wbr><wbr></wbr></wbr>
47. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
48. }<wbr></wbr>catch<wbr>(BrokenBarrierException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
49. } <wbr><wbr></wbr></wbr>
50. } <wbr><wbr></wbr></wbr>
51. } <wbr><wbr></wbr></wbr>
52. <wbr><wbr></wbr></wbr>
53. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
54. //<wbr>三个旅行团</wbr><wbr><wbr></wbr></wbr>
55. CyclicBarrier<wbr>barrier<wbr>=<wbr></wbr></wbr></wbr>new<wbr>CyclicBarrier(</wbr>3);<wbr><wbr></wbr></wbr>
56. ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr>3);<wbr><wbr></wbr></wbr>
57. exec.submit(new<wbr>Tour(barrier,<wbr>“WalkTour”,<wbr>timeWalk));<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
58. exec.submit(new<wbr>Tour(barrier,<wbr>“SelfTour”,<wbr>timeSelf));<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
59. exec.submit(new<wbr>Tour(barrier,<wbr>“BusTour”,<wbr>timeBus));<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
60. exec.shutdown();<wbr><wbr></wbr></wbr>
61. } <wbr><wbr></wbr></wbr>
62. }<wbr><wbr></wbr></wbr>

运行结果：
00:02:25: SelfTour Reached Shenzhen
00:02:25: BusTour Reached Shenzhen
00:02:27: WalkTour Reached Shenzhen
00:02:30: SelfTour Reached Guangzhou
00:02:31: BusTour Reached Guangzhou
00:02:35: WalkTour Reached Guangzhou
00:02:39: SelfTour Reached Shaoguan
00:02:41: BusTour Reached Shaoguan

<wbr><wbr><wbr>并发库中的BlockingQueue是一个比较好玩的类，顾名思义，就是阻塞队列。该类主要提供了两个方法put()和take()，前者将一个对象放到队列中，如果队列已经满了，就等待直到有空闲节点；后者从head取一个对象，如果没有对象，就等待直到有可取的对象。<br><br><wbr><wbr><wbr>下面的例子比较简单，一个读线程，用于将要处理的文件对象添加到阻塞队列中，另外四个写线程用于取出文件对象，为了模拟写操作耗时长的特点，特让线程睡眠一段随机长度的时间。另外，该Demo也使用到了线程池和原子整型（AtomicInteger），AtomicInteger可以在并发情况下达到原子化更新，避免使用了synchronized，而且性能非常高。由于阻塞队列的put和take操作会阻塞，为了使线程退出，特在队列中添加了一个“标识”，算法中也叫“哨兵”，当发现这个哨兵后，写线程就退出。<br><br><wbr><wbr><wbr>当然线程池也要显式退出了。<br><br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. import<wbr>java.io.File;<wbr><wbr></wbr></wbr></wbr>
3. import<wbr>java.io.FileFilter;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.BlockingQueue;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
6. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
7. import<wbr>java.util.concurrent.LinkedBlockingQueue;<wbr><wbr></wbr></wbr></wbr>
8. import<wbr>java.util.concurrent.atomic.AtomicInteger;<wbr><wbr></wbr></wbr></wbr>
9. <wbr><wbr></wbr></wbr>
10. public<wbr></wbr>class<wbr>TestBlockingQueue<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
11. static<wbr></wbr>long<wbr>randomTime()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
12. return<wbr>(</wbr>long)<wbr>(Math.random()<wbr>*<wbr></wbr></wbr></wbr>1000);<wbr><wbr></wbr></wbr>
13. } <wbr><wbr></wbr></wbr>
14. <wbr><wbr></wbr></wbr>
15. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
16. //<wbr>能容纳100个文件</wbr><wbr><wbr></wbr></wbr>
17. final<wbr>BlockingQueue<wbr>queue<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>LinkedBlockingQueue(</wbr>100);<wbr><wbr></wbr></wbr>
18. //<wbr>线程池</wbr><wbr><wbr></wbr></wbr>
19. final<wbr>ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr></wbr>5);<wbr><wbr></wbr></wbr>
20. final<wbr>File<wbr>root<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>File(“F:\\JavaLib”);<wbr><wbr></wbr></wbr></wbr>
21. //<wbr>完成标志</wbr><wbr><wbr></wbr></wbr>
22. final<wbr>File<wbr>exitFile<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>File(“”);<wbr><wbr></wbr></wbr></wbr>
23. //<wbr>读个数</wbr><wbr><wbr></wbr></wbr>
24. final<wbr>AtomicInteger<wbr>rc<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>AtomicInteger();<wbr><wbr></wbr></wbr></wbr>
25. //<wbr>写个数</wbr><wbr><wbr></wbr></wbr>
26. final<wbr>AtomicInteger<wbr>wc<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>AtomicInteger();<wbr><wbr></wbr></wbr></wbr>
27. //<wbr>读线程</wbr><wbr><wbr></wbr></wbr>
28. Runnable<wbr>read<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
29. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
30. scanFile(root);<wbr><wbr></wbr></wbr>
31. scanFile(exitFile);<wbr><wbr></wbr></wbr>
32. } <wbr><wbr></wbr></wbr>
33. <wbr><wbr></wbr></wbr>
34. public<wbr></wbr>void<wbr>scanFile(File<wbr>file)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
35. if<wbr>(file.isDirectory())<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
36. File[]<wbr>files<wbr>=<wbr>file.listFiles(</wbr></wbr></wbr>new<wbr>FileFilter()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
37. public<wbr></wbr>boolean<wbr>accept(File<wbr>pathname)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
38. return<wbr>pathname.isDirectory()<wbr><wbr></wbr></wbr></wbr>
39. ||<wbr>pathname.getPath().endsWith(“.java”);<wbr><wbr></wbr></wbr></wbr>
40. } <wbr><wbr></wbr></wbr>
41. }); <wbr><wbr></wbr></wbr>
42. for<wbr>(File<wbr>one<wbr>:<wbr>files)<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>
43. scanFile(one);<wbr><wbr></wbr></wbr>
44. }<wbr></wbr>else<wbr>{<wbr><wbr></wbr></wbr></wbr>
45. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
46. int<wbr>index<wbr>=<wbr>rc.incrementAndGet();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
47. System.out.println(“Read0:<wbr>”<wbr>+<wbr>index<wbr>+<wbr>”<wbr>“<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
48. +<wbr>file.getPath());<wbr><wbr></wbr></wbr></wbr>
49. queue.put(file);<wbr><wbr></wbr></wbr>
50. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
51. } <wbr><wbr></wbr></wbr>
52. } <wbr><wbr></wbr></wbr>
53. } <wbr><wbr></wbr></wbr>
54. }; <wbr><wbr></wbr></wbr>
55. exec.submit(read);<wbr><wbr></wbr></wbr>
56. //<wbr>四个写线程</wbr><wbr><wbr></wbr></wbr>
57. for<wbr>(</wbr>int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>4;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
58. //<wbr>write<wbr>thread</wbr></wbr><wbr><wbr></wbr></wbr>
59. final<wbr></wbr>int<wbr>NO<wbr>=<wbr>index;<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
60. Runnable<wbr>write<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
61. String<wbr>threadName<wbr>=<wbr>“Write”<wbr>+<wbr>NO;<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
62. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
63. while<wbr>(</wbr>true)<wbr>{<wbr><wbr></wbr></wbr></wbr>
64. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
65. Thread.sleep(randomTime());<wbr><wbr></wbr></wbr>
66. int<wbr>index<wbr>=<wbr>wc.incrementAndGet();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
67. File<wbr>file<wbr>=<wbr>queue.take();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
68. //<wbr>队列已经无对象</wbr><wbr><wbr></wbr></wbr>
69. if<wbr>(file<wbr>==<wbr>exitFile)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr>
70. //<wbr>再次添加”标志”，以让其他线程正常退出</wbr><wbr><wbr></wbr></wbr>
71. queue.put(exitFile);<wbr><wbr></wbr></wbr>
72. break;<wbr><wbr></wbr></wbr>
73. } <wbr><wbr></wbr></wbr>
74. System.out.println(threadName<wbr>+<wbr>“:<wbr>”<wbr>+<wbr>index<wbr>+<wbr>”<wbr>“<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
75. +<wbr>file.getPath());<wbr><wbr></wbr></wbr></wbr>
76. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
77. } <wbr><wbr></wbr></wbr>
78. } <wbr><wbr></wbr></wbr>
79. } <wbr><wbr></wbr></wbr>
80. }; <wbr><wbr></wbr></wbr>
81. exec.submit(write);<wbr><wbr></wbr></wbr>
82. } <wbr><wbr></wbr></wbr>
83. exec.shutdown();<wbr><wbr></wbr></wbr>
84. } <wbr><wbr></wbr></wbr>
85. }<wbr><wbr></wbr></wbr>

<wbr><wbr><wbr>从名字可以看出，CountDownLatch是一个倒数计数的锁，当倒数到0时触发事件，也就是开锁，其他人就可以进入了。在一些应用场合中，需要等待某个条件达到要求后才能做后面的事情；同时当线程都完成后也会触发事件，以便进行后面的操作。</wbr></wbr></wbr>

<wbr>CountDownLatch最重要的方法是countDown()和await()，前者主要是倒数一次，后者是等待倒数到0，如果没有到达0，就只有阻塞等待了。<br><br><wbr><wbr><wbr>一个CountDouwnLatch实例是不能重复使用的，也就是说它是一次性的，锁一经被打开就不能再关闭使用了，如果想重复使用，请考虑使用CyclicBarrier。<br><br><wbr><wbr><wbr>下面的例子简单的说明了CountDownLatch的使用方法，模拟了100米赛跑，10名选手已经准备就绪，只等裁判一声令下。当所有人都到达终点时，比赛结束。<br><br><wbr><wbr><wbr>同样，线程池需要显式shutdown。<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. <wbr><wbr></wbr></wbr>
3. import<wbr>java.util.concurrent.CountDownLatch;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
6. <wbr><wbr></wbr></wbr>
7. public<wbr></wbr>class<wbr>TestCountDownLatch<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
8. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr></wbr></wbr></wbr>throws<wbr>InterruptedException<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
9. //<wbr>开始的倒数锁</wbr><wbr><wbr></wbr></wbr>
10. final<wbr>CountDownLatch<wbr>begin<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>CountDownLatch(</wbr>1);<wbr><wbr></wbr></wbr>
11. //<wbr>结束的倒数锁</wbr><wbr><wbr></wbr></wbr>
12. final<wbr>CountDownLatch<wbr>end<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>CountDownLatch(</wbr>10);<wbr><wbr></wbr></wbr>
13. //<wbr>十名选手</wbr><wbr><wbr></wbr></wbr>
14. final<wbr>ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr></wbr>10);<wbr><wbr></wbr></wbr>
15. for(int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>10;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
16. final<wbr></wbr>int<wbr>NO<wbr>=<wbr>index<wbr>+<wbr></wbr></wbr></wbr></wbr></wbr>1;<wbr><wbr></wbr></wbr>
17. Runnable<wbr>run<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Runnable(){<wbr><wbr></wbr></wbr></wbr>
18. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
19. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
20. begin.await();<wbr><wbr></wbr></wbr>
21. Thread.sleep((long)<wbr>(Math.random()<wbr>*<wbr></wbr></wbr></wbr>10000));<wbr><wbr></wbr></wbr>
22. System.out.println(“No.”<wbr>+<wbr>NO<wbr>+<wbr>”<wbr>arrived”);<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
23. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
24. }<wbr></wbr>finally<wbr>{<wbr><wbr></wbr></wbr></wbr>
25. end.countDown();<wbr><wbr></wbr></wbr>
26. } <wbr><wbr></wbr></wbr>
27. } <wbr><wbr></wbr></wbr>
28. }; <wbr><wbr></wbr></wbr>
29. exec.submit(run);<wbr><wbr></wbr></wbr>
30. } <wbr><wbr></wbr></wbr>
31. System.out.println(“Game<wbr>Start”);<wbr><wbr></wbr></wbr></wbr>
32. begin.countDown();<wbr><wbr></wbr></wbr>
33. end.await();<wbr><wbr></wbr></wbr>
34. System.out.println(“Game<wbr>Over”);<wbr><wbr></wbr></wbr></wbr>
35. exec.shutdown();<wbr><wbr></wbr></wbr>
36. } <wbr><wbr></wbr></wbr>
37. }<wbr></wbr>

运行结果:
Game Start
No.4 arrived
No.1 arrived
No.7 arrived
No.9 arrived
No.3 arrived
No.2 arrived
No.8 arrived
No.10 arrived
No.6 arrived
No.5 arrived
Game Over

<wbr><wbr><wbr>有时候在实际应用中，某些操作很耗时，但又不是不可或缺的步骤。比如用网页浏览器浏览新闻时，最重要的是要显示文字内容，至于与新闻相匹配的图片就没有那么重要的，所以此时首先保证文字信息先显示，而图片信息会后显示，但又不能不显示，由于下载图片是一个耗时的操作，所以必须一开始就得下载。<wbr><br><br><wbr><wbr><wbr>Java的并发库的Future类就可以满足这个要求。Future的重要方法包括get()和cancel()，get()获取数据对象，如果数据没有加载，就会阻塞直到取到数据，而cancel()是取消数据加载。另外一个get(timeout)操作，表示如果在timeout时间内没有取到就失败返回，而不再阻塞。<br><br><wbr><wbr><wbr>下面的Demo简单的说明了Future的使用方法：一个非常耗时的操作必须一开始启动，但又不能一直等待；其他重要的事情又必须做，等完成后，就可以做不重要的事情。<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. <wbr><wbr></wbr></wbr>
3. import<wbr>java.util.concurrent.Callable;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.ExecutionException;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
6. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
7. import<wbr>java.util.concurrent.Future;<wbr><wbr></wbr></wbr></wbr>
8. <wbr><wbr></wbr></wbr>
9. public<wbr></wbr>class<wbr>TestFutureTask<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
10. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)</wbr></wbr>throws<wbr>InterruptedException,<wbr><wbr></wbr></wbr></wbr>
11. ExecutionException<wbr>{<wbr><wbr></wbr></wbr></wbr>
12. final<wbr>ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr></wbr>5);<wbr><wbr></wbr></wbr>
13. Callable<wbr>call<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Callable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
14. public<wbr>String<wbr>call()<wbr></wbr></wbr></wbr>throws<wbr>Exception<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
15. Thread.sleep(1000<wbr>*<wbr></wbr></wbr>5);<wbr><wbr></wbr></wbr>
16. return<wbr>“Other<wbr>less<wbr>important<wbr>but<wbr>longtime<wbr>things.”;<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
17. } <wbr><wbr></wbr></wbr>
18. }; <wbr><wbr></wbr></wbr>
19. Future<wbr>task<wbr>=<wbr>exec.submit(call);<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
20. //<wbr>重要的事情</wbr><wbr><wbr></wbr></wbr>
21. Thread.sleep(1000<wbr>*<wbr></wbr></wbr>3);<wbr><wbr></wbr></wbr>
22. System.out.println(“Let’s<wbr></wbr>do<wbr>important<wbr>things.”);<wbr><wbr></wbr></wbr></wbr></wbr>
23. //<wbr>其他不重要的事情</wbr><wbr><wbr></wbr></wbr>
24. String<wbr>obj<wbr>=<wbr>task.get();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
25. System.out.println(obj);<wbr><wbr></wbr></wbr>
26. //<wbr>关闭线程池</wbr><wbr><wbr></wbr></wbr>
27. exec.shutdown();<wbr><wbr></wbr></wbr>
28. } <wbr><wbr></wbr></wbr>
29. }<wbr><wbr></wbr></wbr>

运行结果：
Let’s do important things.
Other less important but longtime things.

<wbr><wbr><wbr>考虑以下场景：浏览网页时，浏览器了5个线程下载网页中的图片文件，由于图片大小、网站访问速度等诸多因素的影响，完成图片下载的时间就会有很大的不同。如果先下载完成的图片就会被先显示到界面上，反之，后下载的图片就后显示。<wbr><br><br><wbr><wbr><wbr>Java的并发库的CompletionService可以满足这种场景要求。该接口有两个重要方法：submit()和take()。submit用于提交一个runnable或者callable，一般会提交给一个线程池处理；而take就是取出已经执行完毕runnable或者callable实例的Future对象，如果没有满足要求的，就等待了。CompletionService还有一个对应的方法poll，该方法与take类似，只是不会等待，如果没有满足要求，就返回null对象。<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. <wbr><wbr></wbr></wbr>
3. import<wbr>java.util.concurrent.Callable;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.CompletionService;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.ExecutionException;<wbr><wbr></wbr></wbr></wbr>
6. import<wbr>java.util.concurrent.ExecutorCompletionServic<wbr>e;<wbr><wbr></wbr></wbr></wbr></wbr>
7. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
8. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
9. import<wbr>java.util.concurrent.Future;<wbr><wbr></wbr></wbr></wbr>
10. <wbr><wbr></wbr></wbr>
11. public<wbr></wbr>class<wbr>TestCompletionService<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
12. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr></wbr></wbr></wbr>throws<wbr>InterruptedException,<wbr><wbr></wbr></wbr></wbr>
13. ExecutionException<wbr>{<wbr><wbr></wbr></wbr></wbr>
14. ExecutorService<wbr>exec<wbr>=<wbr>Executors.newFixedThreadPool(</wbr></wbr></wbr>10);<wbr><wbr></wbr></wbr>
15. CompletionService<wbr>serv<wbr>=<wbr><wbr></wbr></wbr></wbr></wbr>
16. new<wbr>ExecutorCompletionServic<wbr>e(exec);<wbr><wbr></wbr></wbr></wbr></wbr>
17. <wbr><wbr></wbr></wbr>
18. for<wbr>(</wbr>int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>5;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
19. final<wbr></wbr>int<wbr>NO<wbr>=<wbr>index;<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
20. Callable<wbr>downImg<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Callable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
21. public<wbr>String<wbr>call()<wbr></wbr></wbr></wbr>throws<wbr>Exception<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
22. Thread.sleep((long)<wbr>(Math.random()<wbr>*<wbr></wbr></wbr></wbr>10000));<wbr><wbr></wbr></wbr>
23. return<wbr>“Downloaded<wbr>Image<wbr>”<wbr>+<wbr>NO;<wbr><wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>
24. } <wbr><wbr></wbr></wbr>
25. }; <wbr><wbr></wbr></wbr>
26. serv.submit(downImg);<wbr><wbr></wbr></wbr>
27. } <wbr><wbr></wbr></wbr>
28. <wbr><wbr></wbr></wbr>
29. Thread.sleep(1000<wbr>*<wbr></wbr></wbr>2);<wbr><wbr></wbr></wbr>
30. System.out.println(“Show<wbr>web<wbr>content”);<wbr><wbr></wbr></wbr></wbr></wbr>
31. for<wbr>(</wbr>int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>5;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
32. Future<wbr>task<wbr>=<wbr>serv.take();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
33. String<wbr>img<wbr>=<wbr>task.get();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
34. System.out.println(img);<wbr><wbr></wbr></wbr>
35. } <wbr><wbr></wbr></wbr>
36. System.out.println(“End”);<wbr><wbr></wbr></wbr>
37. //<wbr>关闭线程池</wbr><wbr><wbr></wbr></wbr>
38. exec.shutdown();<wbr><wbr></wbr></wbr>
39. } <wbr><wbr></wbr></wbr>
40. }<wbr><wbr></wbr></wbr>

运行结果:
Show web content
Downloaded Image 1
Downloaded Image 2
Downloaded Image 4
Downloaded Image 0
Downloaded Image 3
End

<wbr><wbr><wbr>操作系统的信号量是个很重要的概念，在进程控制方面都有应用。Java并发库的Semaphore可以很轻松完成信号量控制，Semaphore可以控制某个资源可被同时访问的个数，acquire()获取一个许可，如果没有就等待，而release()释放一个许可。比如在Windows下可以设置共享文件的最大客户端访问个数。<br><br><wbr><wbr><wbr>Semaphore维护了当前访问的个数，提供同步机制，控制同时访问的个数。在数据结构中链表可以保存“无限”的节点，用Semaphore可以实现有限大小的链表。另外重入锁ReentrantLock也可以实现该功能，但实现上要负责些，代码也要复杂些。<br><br><wbr><wbr><wbr>下面的Demo中申明了一个只有5个许可的Semaphore，而有20个线程要访问这个资源，通过acquire()和release()获取和释放访问许可。<br></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr></wbr>

Java代码

1. package<wbr>concurrent;<wbr><wbr></wbr></wbr></wbr>
2. <wbr><wbr></wbr></wbr>
3. import<wbr>java.util.concurrent.ExecutorService;<wbr><wbr></wbr></wbr></wbr>
4. import<wbr>java.util.concurrent.Executors;<wbr><wbr></wbr></wbr></wbr>
5. import<wbr>java.util.concurrent.Semaphore;<wbr><wbr></wbr></wbr></wbr>
6. <wbr><wbr></wbr></wbr>
7. public<wbr></wbr>class<wbr>TestSemaphore<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
8. public<wbr></wbr>static<wbr></wbr>void<wbr>main(String[]<wbr>args)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
9. //<wbr>线程池</wbr><wbr><wbr></wbr></wbr>
10. ExecutorService<wbr>exec<wbr>=<wbr>Executors.newCachedThreadPool();<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
11. //<wbr>只能5个线程同时访问</wbr><wbr><wbr></wbr></wbr>
12. final<wbr>Semaphore<wbr>semp<wbr>=<wbr></wbr></wbr></wbr></wbr>new<wbr>Semaphore(</wbr>5);<wbr><wbr></wbr></wbr>
13. //<wbr>模拟20个客户端访问</wbr><wbr><wbr></wbr></wbr>
14. for<wbr>(</wbr>int<wbr>index<wbr>=<wbr></wbr></wbr></wbr>0;<wbr>index<wbr>&lt;<wbr></wbr></wbr></wbr>20;<wbr>index++)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
15. final<wbr></wbr>int<wbr>NO<wbr>=<wbr>index;<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
16. Runnable<wbr>run<wbr>=<wbr></wbr></wbr></wbr>new<wbr>Runnable()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
17. public<wbr></wbr>void<wbr>run()<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr>
18. try<wbr>{<wbr><wbr></wbr></wbr></wbr>
19. //<wbr>获取许可</wbr><wbr><wbr></wbr></wbr>
20. semp.acquire();<wbr><wbr></wbr></wbr>
21. System.out.println(“Accessing:<wbr>”<wbr>+<wbr>NO);<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
22. Thread.sleep((long)<wbr>(Math.random()<wbr>*<wbr></wbr></wbr></wbr>10000));<wbr><wbr></wbr></wbr>
23. //<wbr>访问完后，释放</wbr><wbr><wbr></wbr></wbr>
24. semp.release();<wbr><wbr></wbr></wbr>
25. }<wbr></wbr>catch<wbr>(InterruptedException<wbr>e)<wbr>{<wbr><wbr></wbr></wbr></wbr></wbr></wbr>
26. } <wbr><wbr></wbr></wbr>
27. } <wbr><wbr></wbr></wbr>
28. }; <wbr><wbr></wbr></wbr>
29. exec.execute(run);<wbr><wbr></wbr></wbr>
30. } <wbr><wbr></wbr></wbr>
31. //<wbr>退出线程池</wbr><wbr><wbr></wbr></wbr>
32. exec.shutdown();<wbr><wbr></wbr></wbr>
33. } <wbr><wbr></wbr></wbr>
34. }<wbr><wbr></wbr></wbr>

运行结果：
Accessing: 0
Accessing: 1
Accessing: 2
Accessing: 3
Accessing: 4
Accessing: 5
Accessing: 6
Accessing: 7
Accessing: 8
Accessing: 9
Accessing: 10
Accessing: 11
Accessing: 12
Accessing: 13
Accessing: 14
Accessing: 15
Accessing: 16
Accessing: 17
Accessing: 18
Accessing: 19