博弈Java讲义 - Java线程之uncontrolled exceptions_JAVA_编程开发_程序员俱乐部

前两天翻阅《Effective Java》发现一条提示慎用线程组。ThreadGroup提供的很多功能的实现是有瑕疵的。例如，我们可以调用activeCount获得该组中活动线程的数量，一旦这个数组进行了分配，并用enumerate方法遍历，如果线程数增加了，就有可能忽略掉调用activeCount后新增的线程。关于处理线程组逻辑，可以用线程池的executor代替。class="Apple-converted-space">?
? 也许ThreadGroup提供的有用的功能之一就是uncaughtException方法了。Java提供了强大的异常处理机制，有些异常可以通过try/catch捕获或者re-throw，这些都是checked exception,比如IOException和ClassNotFoundException,还有一些是不必要捕获处理的，如NumberFormatException.?
? 在Java多线程中提供了一个层次化的机制帮助我们有效的处理uncaught exception. 当一个线程抛出异常时，JVM首先会调用thread里面指定的uncaught exception handler处理异常，如果在thread级没有设置handler，JVM会到当前线程所在的组的exception handler处理异常，如果线程组没有定义uncaughtException方法，JVM会继续寻找上一级的exception handler处理。如果一个handler都没有找到，这默认打出错误栈并推出程序。?
? 让们看一个具体的例子：?
??

1. ??import?java.lang.Thread.UncaughtExceptionHandler;??
2. ??
3. public?class?MyUncaughtExceptionHandler?implements?UncaughtExceptionHandler?{??
4. ??
5. ????@Override??
6. ????public?void?uncaughtException(Thread?t,?Throwable?e)?{??
7. ????????System.out.printf("Uncaught?exception?raised?and?captured?in?thread??%s?:?\n",?t.getName());??
8. ????}??
9. ??
10. }??
11. ??
12. import?java.lang.Thread.UncaughtExceptionHandler;??
13. ??
14. public?class?MyUncaughtExceptionHandler?implements?UncaughtExceptionHandler?{??
15. ??
16. ????@Override??
17. ????public?void?uncaughtException(Thread?t,?Throwable?e)?{??
18. ????????System.out.printf("Uncaught?exception?raised?and?captured?in?thread??%s?:?\n",?t.getName());??
19. ????}??
20. ??
21. }??
22. ??
23. public?class?MyThreadGroupWithExceptionHandler?extends?ThreadGroup?{??
24. ??
25. ????public?MyThreadGroupWithExceptionHandler(String?name)?{??
26. ????????super(name);??
27. ????}??
28. ??
29. ????@Override??
30. ????public?void?uncaughtException(Thread?t,?Throwable?e)?{??
31. ????????System.out.printf("Uncaught?exception?caught?in?thread?group's?uncaughtException?%s.\n",this.getName());??
32. ????????interrupt();??
33. ????}??
34. ??????
35. }??
36. public?class?TestUncaughtException?{??
37. ??
38. ????public?static?void?main(String[]?args){??
39. ????????MyTask?task1?=?new?MyTask();??
40. ????????Thread?t1?=?new?Thread(task1,"UncaughtException?Task");??
41. ????????t1.setUncaughtExceptionHandler(new?MyUncaughtExceptionHandler());??
42. ????????t1.start();??
43. ??????
44. ??????????
45. ????????MyThreadGroupWithExceptionHandler?group1?=?new?MyThreadGroupWithExceptionHandler("mythread");??
46. ????????MyTask?task2?=?new?MyTask();??
47. ????????Thread?t2?=?new?Thread(group1,?task2,"Task?with?group?exception?handler");??
48. ????????t2.start();??
49. ??????????
50. ????????ThreadGroup?group2?=?new?ThreadGroup("mythread");??
51. ????????MyTask?task3?=?new?MyTask();??
52. ????????Thread.setDefaultUncaughtExceptionHandler(new?MyDefaultExceptionHandler());??
53. ????????Thread?t3?=?new?Thread(group2,?task3,?"Task?with?exception?caught?by?Thread.default");??
54. ????????t3.start();??
55. ??????????
56. ????}??
57. ??????
58. }??

? 输出可见异常分别在Thread实例,ThreadGroup,Thread级别被捕获。?