java错误机制、错误栈、错误处理方式、错误链、错误丢失

java异常机制、异常栈、异常处理方式、异常链、异常丢失

1、java标准异常概述

Throwable表示任何可以作为异常被抛出的类，有两个子类Error和Exception。从这两个类的源代码中可以看出，这两个类并没有添加新的方法，Throwable提供了所以方法的实现。Error表示编译时和系统错误。Exception是可以被抛出的异常类。RuntimeException继承自Exception（如NullPointerException），表示运行时异常，JVM会自动抛出.

2、自定义异常类

自定义异常类方法：?通过继承Throwable或Exception。异常类的所有实现都是基类Throwable实现的，所以构造自定义异常类完全可以参考Exception和Error类。我们只要添加上自定义异常类的构造方法就可以了

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. /**?
4. ?*?自定义异常类方法?
5. ?*?1、通过继承Throwable?
6. ?*?2、通过继承Exception?
7. ?*??
8. ?*?@author?Touch?
9. ?*/??
10. public?class?MyExceptionDemo?extends?Exception?{??
11. ??
12. ????private?static?final?long?serialVersionUID?=?1L;??
13. ??
14. ????public?MyExceptionDemo()?{??
15. ????????super();??
16. ????}??
17. ??
18. ????public?MyExceptionDemo(String?message)?{??
19. ????????super(message);??
20. ????}??
21. ??
22. ????public?MyExceptionDemo(String?message,?Throwable?cause)?{??
23. ????????super(message,?cause);??
24. ????}??
25. ??
26. ????public?MyExceptionDemo(Throwable?cause)?{??
27. ????????super(cause);??
28. ????}??
29. }??
30. </span>??

?

3、异常栈及异常处理方式

可以通过try、catch来捕获异常。捕获到的异常。下面的示例演示了几种常用异常处理方式

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. import?mine.util.exception.MyException;??
4. ??
5. public?class?ExceptionDemo1?{??
6. ????public?void?f()?throws?MyException?{??
7. ????????throw?new?MyException("自定义异常");??
8. ????}??
9. ??
10. ????public?void?g()?throws?MyException?{??
11. ????????f();??
12. ????}??
13. ??
14. ????public??void?h()?throws?MyException??{??
15. ????????try?{??
16. ????????????g();??
17. ????????}?catch?(MyException?e)?{??
18. ????????????//1、通过获取栈轨迹中的元素数组来显示异常抛出的轨迹??
19. ????????????for?(StackTraceElement?ste?:?e.getStackTrace())??
20. ????????????????System.out.println(ste.getMethodName());??
21. ????????????//2、直接将异常栈信息输出至标准错误流或标准输出流??
22. ????????????e.printStackTrace();//输出到标准错误流??
23. ????????????e.printStackTrace(System.err);??
24. ????????????e.printStackTrace(System.out);??
25. ????????????//3、将异常信息输出到文件中??
26. ????????????//e.printStackTrace(new?PrintStream("file/exception.txt"));??
27. ????????????//4、重新抛出异常,如果直接抛出那么栈路径是完整的，如果用fillInStackTrace()??
28. ????????????//那么将会从这个方法（当前是h()方法）作为异常发生的原点。??
29. ????????????//throw?e;??
30. ????????????throw?(MyException)e.fillInStackTrace();??
31. ????????}??
32. ????}??
33. ????public?static?void?main(String[]?args)?{??
34. ????????????try?{??
35. ????????????????new?ExceptionDemo1().h();??
36. ????????????}?catch?(MyException?e)?{??
37. ????????????????//?TODO?Auto-generated?catch?block??
38. ????????????????e.printStackTrace();??
39. ????????????}??
40. ????}??
41. }??
42. </span>??

运行结果：

f
g
h
main
mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo1.f(ExceptionDemo1.java:7)
?at demo.others.ExceptionDemo1.g(ExceptionDemo1.java:11)
?at demo.others.ExceptionDemo1.h(ExceptionDemo1.java:16)
?at demo.others.ExceptionDemo1.main(ExceptionDemo1.java:35)
mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo1.f(ExceptionDemo1.java:7)
?at demo.others.ExceptionDemo1.g(ExceptionDemo1.java:11)
?at demo.others.ExceptionDemo1.h(ExceptionDemo1.java:16)
?at demo.others.ExceptionDemo1.main(ExceptionDemo1.java:35)
mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo1.f(ExceptionDemo1.java:7)
?at demo.others.ExceptionDemo1.g(ExceptionDemo1.java:11)
?at demo.others.ExceptionDemo1.h(ExceptionDemo1.java:16)
?at demo.others.ExceptionDemo1.main(ExceptionDemo1.java:35)
mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo1.h(ExceptionDemo1.java:30)
?at demo.others.ExceptionDemo1.main(ExceptionDemo1.java:35)

分析上面的程序，首先main函数被调用，然后是调用h函数，再g函数、f函数，f函数抛出异常，并在h函数捕获，这时将依次从栈顶到栈底输出异常栈路径。

4、异常链

有时候我们会捕获一个异常后在抛出另一个异常，如下代码所示：

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. import?java.io.IOException;??
4. ??
5. import?mine.util.exception.MyException;??
6. ??
7. public?class?ExceptionDemo2?{??
8. ????public?void?f()?throws?MyException?{??
9. ????????throw?new?MyException("自定义异常");??
10. ????}??
11. ??
12. ????public?void?g()?throws?Exception??{??
13. ????????try?{??
14. ????????????f();??
15. ????????}?catch?(MyException?e)?{??
16. ????????????e.printStackTrace();??
17. ????????????throw?new?Exception("重新抛出的异常1");??
18. ????????}??
19. ????}??
20. ??
21. ????public??void?h()?throws?IOException????{??
22. ????????try?{??
23. ????????????g();??
24. ????????}?catch?(Exception?e)?{??
25. ????????????//?TODO?Auto-generated?catch?block??
26. ????????????e.printStackTrace();??
27. ????????????throw?new?IOException("重新抛出异常2");??
28. ????????}??
29. ????}??
30. ????public?static?void?main(String[]?args)?{??
31. ????????????try?{??
32. ????????????????new?ExceptionDemo2().h();??
33. ????????????}?catch?(IOException?e)?{??
34. ????????????????//?TODO?Auto-generated?catch?block??
35. ????????????????e.printStackTrace();??
36. ????????????}??
37. ????}??
38. }??
39. </span>??

运行结果：

mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo2.f(ExceptionDemo2.java:9)
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:14)
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:23)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:32)
java.lang.Exception: 重新抛出的异常1
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:17)
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:23)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:32)
java.io.IOException: 重新抛出异常2
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:27)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:32)

从结果中我们可以看出，异常栈变小了。也就是说丢失了最原始的异常信息。怎样保存最原始的异常信息呢？Throwable类中有个Throwable? cause属性，表示原始异常。通过接收cause参数的构造器可以把原始异常传递给新异常，或者通过initCause()方法。如下示例：

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. import?java.io.IOException;??
4. ??
5. import?mine.util.exception.MyException;??
6. ??
7. public?class?ExceptionDemo2?{??
8. ????public?void?f()?throws?MyException?{??
9. ????????throw?new?MyException("自定义异常");??
10. ????}??
11. ??
12. ????public?void?g()?throws?Exception??{??
13. ????????try?{??
14. ????????????f();??
15. ????????}?catch?(MyException?e)?{??
16. ????????????e.printStackTrace();??
17. ????????????throw?new?Exception("重新抛出的异常1",e);??
18. ????????}??
19. ????}??
20. ??
21. ????public??void?h()?throws?IOException????{??
22. ????????try?{??
23. ????????????g();??
24. ????????}?catch?(Exception?e)?{??
25. ????????????//?TODO?Auto-generated?catch?block??
26. ????????????e.printStackTrace();??
27. ????????????IOException?io=new?IOException("重新抛出异常2");??
28. ????????????io.initCause(e);??
29. ????????????throw?io;??
30. ????????}??
31. ????}??
32. ????public?static?void?main(String[]?args)?{??
33. ????????????try?{??
34. ????????????????new?ExceptionDemo2().h();??
35. ????????????}?catch?(IOException?e)?{??
36. ????????????????//?TODO?Auto-generated?catch?block??
37. ????????????????e.printStackTrace();??
38. ????????????}??
39. ????}??
40. }??
41. </span>??

?

结果：

mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo2.f(ExceptionDemo2.java:9)
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:14)
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:23)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:34)
java.lang.Exception: 重新抛出的异常1
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:17)
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:23)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:34)
Caused by: mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo2.f(ExceptionDemo2.java:9)
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:14)
?... 2 more
java.io.IOException: 重新抛出异常2
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:27)
?at demo.others.ExceptionDemo2.main(ExceptionDemo2.java:34)
Caused by: java.lang.Exception: 重新抛出的异常1
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:17)
?at demo.others.ExceptionDemo2.h(ExceptionDemo2.java:23)
?... 1 more
Caused by: mine.util.exception.MyException: 自定义异常
?at demo.others.ExceptionDemo2.f(ExceptionDemo2.java:9)
?at demo.others.ExceptionDemo2.g(ExceptionDemo2.java:14)
?... 2 more

从结果中看出当获取到“重新抛出异常2的时候，同时可以输出原始异常“重新抛出的异常1“和原始异常”自定义异常，这就是异常链。

5、finally的使用

finally子句总是执行的，通常用来做一些清理工作，如关闭文件，关闭连接等

下面举几个finally的例子：

[java]?view plaincopy?

1. <span?style="font-size:16px;">//?读取指定路径文本文件??
2. ????public?static?String?read(String?filePath)?{??
3. ????????StringBuilder?str?=?new?StringBuilder();??
4. ????????BufferedReader?in?=?null;??
5. ????????try?{??
6. ????????????in?=?new?BufferedReader(new?FileReader(filePath));??
7. ????????????String?s;??
8. ????????????try?{??
9. ????????????????while?((s?=?in.readLine())?!=?null)??
10. ????????????????????str.append(s?+?'\n');??
11. ????????????}?finally?{??
12. ????????????????in.close();??
13. ????????????}??
14. ????????}?catch?(IOException?e)?{??
15. ????????????//?TODO?Auto-generated?catch?block??
16. ????????????e.printStackTrace();??
17. ????????}??
18. ????????return?str.toString();??
19. ????}</span>??

分析：如果调用in = new BufferedReader(new FileReader(filePath));时发生异常，这时是一个文件路径不存在的异常，也就是说并没有打开文件，这时将会直接跳到catch块，而不会执行try...finally块(并不是finally子句)里面的语句in.close();此时不需要关闭文件。

再看一个例子，会导致异常的丢失

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. import?mine.util.exception.MyException;??
4. ??
5. public?class?ExceptionDemo3?{??
6. ????public?void?f()?throws?MyException?{??
7. ????????throw?new?MyException("异常1");??
8. ????}??
9. ??
10. ????public?void?g()?throws?MyException?{??
11. ????????throw?new?MyException("异常2");??
12. ????}??
13. ??
14. ????public?static?void?main(String[]?args)?{??
15. ??
16. ????????try?{??
17. ????????????ExceptionDemo3?ex?=?new?ExceptionDemo3();??
18. ????????????try?{??
19. ????????????????ex.f();??
20. ????????????}?finally?{??
21. ????????????????ex.g();//此时捕获g方法抛出的异常，f方法抛出的异常丢失了??
22. ????????????}??
23. ????????}?catch?(MyException?e)?{??
24. ????????????System.out.println(e);??
25. ????????}??
26. ??
27. ????}??
28. }??
29. </span>??

结果：mine.util.exception.MyException: 异常2

此时异常1就丢失了
或者这样写：

[java]?view plaincopy?

1. <span?style="font-size:16px;">package?demo.others;??
2. ??
3. import?mine.util.exception.MyException;??
4. ??
5. public?class?ExceptionDemo3?{??
6. ??
7. ????public?void?g()?throws?MyException?{??
8. ????????throw?new?MyException("异常2");??
9. ????}??
10. ??
11. ????public?static?void?main(String[]?args)?{??
12. ????????ExceptionDemo3?ex?=?new?ExceptionDemo3();??
13. ????????try?{??
14. ????????????ex.g();??
15. ????????}?finally?{??
16. ????????????//直接return会丢失所以抛出的异常??
17. ????????????return;??
18. ????????}??
19. ??
20. ????}??
21. }??
22. </span>??

6、异常的限制

（1）当覆盖方法时，只能抛出在基类方法的异常说明里列出的那些异常，有些基类的方法声明抛出异常其实并没有抛出异常，这是因为可能在其子类的覆盖方法中会抛出异常

（2）构造器可以抛出任何异常而不必理会基类构造器所抛出的异常，派生类构造器异常说明必须包含基类构造器异常说明，因为构造派生类对象时会调用基类构造器。此外，派生类构造器不能捕获基类构造器抛出的异常。