用 join 或 CountDownLatch 让主线程等待所有子线程完成

原文出处：http://blog.chenlb.com/2008/11/join-or-countdownlatch-make-main-thread-wait-all-sub-thread.html

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在编写多线程的工作中，有个常见的问题：主线程（main) 启动好几个子线程（task）来完成并发任务，主线程要等待所有的子线程完成之后才继续执行main的其它任务。

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默认主线程退出时其它子线程不会停，如果想让main退出时其它子线程终止，可以用subThread.setDaemon(true) 设置子线程为“守护线程”。但现在要的是主线程等待所有子线程完成后，还要执行其它操作（比如：结果合并）。记得可以用join()方法来等待所有子线程 完成后，才继续执行。如果不用join()，main线程与子线程是并发的，要稍加处理使main线程暂停。简单点用Thread.sleep(long millis) 了，当然用“等待-通知”机制也可以。

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下面是用join的实现main等待所有子线程完成了，示例代码：WaitAllSubThread.java。

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1. package?com.chenlb;????
2. import?java.util.Random;????
3. /**??*?@author?chenlb?2008-11-1?下午11:32:43?
4. ?*/??public?class?WaitAllSubThread?{??
5. ??????/*int?liveThreadNum;//记录运行的子线程数?
6. ????*/??????int?n;??//工作线程数??
7. ??????public?WaitAllSubThread(int?n)?{??
8. ????????this.n?=?n;??????}??
9. ??????class?Worker?implements?Runnable?{??
10. ??????????String?name;??
11. ????????int?sleep;????
12. ????????public?Worker(String?name,?int?sleep)?{??????????????this.name?=?name;??
13. ????????????this.sleep?=?sleep;??????????}??
14. ??????????public?void?run()?{??
15. ????????????/*upLive();?//计算此线程已经工作.?????????????*/??
16. ????????????System.out.println(name+",?start?to?work.");??????????????try?{??
17. ????????????????Thread.sleep(sleep);????//虚拟工作.?10s?随机时间??????????????}?catch?(InterruptedException?e)?{??
18. ????????????????System.out.println(name+"?interrupted.");??????????????}??
19. ????????????System.out.println(name+",?end?to?work?["+sleep+"]?sleep.");??????????????/*downLive();???//此线程工作完成?
20. ????????????*/??????????}??
21. ????}??/*??//记录线程数的同步方法.?
22. ????private?synchronized?void?downLive()?{?????????liveThreadNum--;?
23. ????}??
24. ????private?synchronized?void?upLive()?{?????????liveThreadNum++;?
25. ????}??
26. ????private?synchronized?boolean?isLive()?{?????????return?liveThreadNum?>?0;?
27. ????}*/????
28. ????public?void?run()?{??????????System.out.println("-------------main?run?start-------------");??
29. ????????int?sleepSaid?=?10?*?1000;??//每个工作线程虚拟工作最大时间??????????Random?rm?=?new?Random();??
30. ????????for(int?i=0;?i<ths.length;?i++)?{??????????????ths[i]?=?new?Thread(new?MyTask(rm.nextInt(sleep)+1));??
31. ??????????????ths[i].start();??
32. ????????}????
33. ????????for(Thread?th?:?ths)?{??????????????try?{??
34. ????????????????th.join();//join方式??????????????}?catch?(InterruptedException?e)?{??
35. ????????????????//?TODO?Auto-generated?catch?block??????????????????e.printStackTrace();??
36. ????????????}??????????}??
37. ????????/*//等待所有工作线程完成.?????????while(isLive())?{?
38. ????????????try?{?????????????????Thread.sleep(1000);?//每隔1s查看下是否所有线程完成.?
39. ????????????}?catch?(InterruptedException?e)?{?????????????????System.out.println("main?thread?sleep?interrupted.");?
40. ????????????}?????????}*/??
41. ????????System.out.println("---------------main?run?end--------------");??????}??
42. ??????public?static?void?main(String[]?args)?{??
43. ????????WaitAllSubThread?wast?=?new?WaitAllSubThread(10);??????????wast.run();??
44. ????}??}??

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如果不用join，上面的代码会使先输出“main run end”，原因是：main 与 所有sub thread并发工作，不等待所有子线程继续工作。而所有子线程完成了，main线程才会退出。

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用比较笨的方式：把上面/* */的注释去掉，把 th.join();块注释掉。这样可以使等待所有子线程完成了才去执行其它后续的（比如：这里是输出“main run end”）。分析：程序中加工作的子线程的计数（liveThreadNum）。main不断轮询是否所有子线程完成，所有完成就执行剩下的。

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上面的是昨天写的，今天发现一个更加简洁的方式去处理main线程阻塞（等待所有子线程），那就是java.util.concurrent.CountDownLatch类。现在重新实现上面的功能，CountDownLatchUse.java。

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1. package?com.chenlb;????
2. import?java.util.Random;??import?java.util.concurrent.CountDownLatch;??
3. ??/**?
4. ?*?@author?chenlb?2008-11-1?下午11:43:31??*/??
5. public?class?CountDownLatchUse?{????
6. ????final?CountDownLatch?downLatch;??????int?n;??//工作线程数??
7. ??????public?CountDownLatchUse(int?n)?{??
8. ????????this.downLatch?=?new?CountDownLatch(n);??????????this.n?=?n;??
9. ????}????
10. ????class?Worker?implements?Runnable?{????
11. ????????String?name;??????????int?sleep;??
12. ??????????public?Worker(String?name,?int?sleep)?{??
13. ????????????this.name?=?name;??????????????this.sleep?=?sleep;??
14. ????????}????
15. ????????public?void?run()?{??????????????System.out.println(name+",?start?to?work.");??
16. ????????????try?{??????????????????Thread.sleep(sleep);????//虚拟工作.?10s?随机时间??
17. ????????????}?catch?(InterruptedException?e)?{??????????????????System.out.println(name+"?interrupted.");??
18. ????????????}??????????????System.out.println(name+",?end?to?work?["+sleep+"]?sleep.");??
19. ????????????meDone();???//某个工作线程完成??????????}??
20. ????}????
21. ????private?void?meDone()?{??????????downLatch.countDown();??
22. ????}????
23. ????public?void?run()?{??????????System.out.println("-------------main?run?start-------------");??
24. ????????int?sleepSaid?=?10?*?1000;??//每个工作线程虚拟工作最大时间??????????Random?rm?=?new?Random();??
25. ????????for(int?i=0;?i<n;?i++)?{??????????????new?Thread(new?Worker("worker-"+i,?rm.nextInt(sleepSaid)+1)).start();??
26. ????????}????
27. ????????try?{??????????????downLatch.await();??//等待所有工作线程完成.??
28. ????????}?catch?(InterruptedException?e)?{??????????????System.out.println("main?interrupted.");??
29. ????????}??????????System.out.println("---------------main?run?end--------------");??
30. ????}????
31. ????public?static?void?main(String[]?args)?{??????????CountDownLatchUse?mtu?=?new?CountDownLatchUse(10);??
32. ????????mtu.run();??????}??
33. }??

CountDownLatch.countDown();完成线程的计数。CountDownLatch.await();完成了主线程阻塞。简洁就是好，以后就这种方式了。