Java：如何处理异常

在处理异常时，应该区分checked异常和unchecked异常。对于checked异常，我们应该提供健壮的异常恢复机制，而对于unchecked异常，这就是编程错误即bug，应该在调试阶段很好的发现和处理它们。
1. Java[/b]异常层次结构[/b][/b]



上图（注：该图引自http://dev2dev.bea.com.cn/techdoc/200702364792.html）标出了Java异常层次结构，也指出了哪些异常是unchecked，哪些异常是checked。下面给出几段常见的异常处理代理，试图总结日常开发中应该如何处理异常。
2.[/b]针对[/b]checked[/b]异常的恢复机制[/b][/b]
checked异常并不是编程错误，它的出现是软件运行阶段所不可避免的。最常见的这类异常如socket连接超时。
对于此类异常，我们应该在程序的运行态下试图从异常中恢复过来。下面这段代码(Recover.java)的主要逻辑是，对目标值protected int current进行判断，如果该值大于2则成功，否则抛出NotBigEnoughException异常。
在执行程序的过程中，在每次catch到NotBigEnoughException异常时，我们对current值递增，试图从异常中恢复过来。
NotBigEnoughException.java

[align=left]package com.zj.exception.types;[/align] [align=left] [/align] [align=left]public class NotBigEnoughException extends Exception {[/align] [align=left] public NotBigEnoughException() {[/align] [align=left] super();[/align] [align=left] }[/align] [align=left] [/align] public NotBigEnoughException(String msg) { [align=left] super(msg);[/align] [align=left] }[/align] }

Recover.java

[align=left]package com.zj.exception;[/align] [align=left]import com.zj.exception.types.NotBigEnoughException;[/align] [align=left] [/align] [align=left]public class Recover {[/align] [align=left] protected int current = 0;[/align] [align=left] protected boolean accept = false;[/align] [align=left] [/align] [align=left] public Recover() {}[/align] [align=left] [/align] [align=left] public Recover(int cur) {[/align] [align=left] current = cur;[/align] [align=left] }[/align] [align=left] [/align] [align=left] public void increment() {[/align] [align=left] ++current;[/align] [align=left] }[/align] [align=left] [/align] [align=left] public boolean passed() {[/align] [align=left] return accept;[/align] [align=left] }[/align] [align=left] [/align] public void passing() throws NotBigEnoughException { [align=left] if (current > 2) {[/align] [align=left] accept = true;[/align] [align=left] System.out.println("accept " + current);[/align] [align=left] } else[/align] [align=left] throw new NotBigEnoughException("reject " + current);[/align] [align=left] }[/align] [align=left] [/align] [align=left] public static void main(String[] args) {[/align] [align=left] Recover re = new Recover();[/align] [align=left] while (!re.passed()) {[/align] [align=left] try {[/align] [align=left] re.passing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] System.out.println(e);[/align] [align=left] re.increment();[/align] [align=left] }[/align] [align=left] }[/align] [align=left] }[/align] }

结果：
[align=left]com.zj.exception.types.NotBigEnoughException: reject 0[/align]
[align=left]com.zj.exception.types.NotBigEnoughException: reject 1[/align]
[align=left]com.zj.exception.types.NotBigEnoughException: reject 2[/align]
accept 3
3.[/b]继承异常[/b][/b]
在子类继承父类的情况下，子类override方法的异常声明只能取自（小于等于）父类该方法的异常声明；对于子类构造方法的异常声明必须包含（大于等于）父类构造方法的异常声明。
类Inheritor继承自类Recover，它的方法passing()试图声明一个父类没有的异常UnknowException，这样做是不允许的。
UnknowException.java

[align=left]package com.zj.exception.types;[/align] [align=left] [/align] [align=left]public class UnknowException extends Exception {[/align] [align=left] public UnknowException() {[/align] [align=left] super();[/align] [align=left] }[/align] [align=left] [/align] public UnknowException(String msg) { [align=left] super(msg);[/align] [align=left] }[/align] }

error in: Inheritor.java

//couldn't throws new exceptions where not found in its base class [align=left]public void passing() throws NotBigEnoughException, UnknowException {[/align] [align=left] if (current > 2) {[/align] [align=left] accept = true;[/align] [align=left] System.out.println("accept " + current);[/align] [align=left] } else if (current >= 0)[/align] [align=left] throw new NotBigEnoughException("reject " + current);[/align] [align=left] else[/align] [align=left] throw new UnknowException("i don't know how to deal with "[/align] [align=left] + current);[/align] }

之所以覆盖这个方法的目的是对父类的passing()方法做进一步扩展，对0<=current<=2的情况抛出NotBigEnoughException，而对current<0的情况则抛出一个新的异常UnknowException。
此时，提供两种解决方法。
方法一，使用恢复异常机制，overrding passing（）方法，这样可以处理掉所有的异常，因此不需要异常声明。
ok in: Inheritor.java

//sure passing(),so not have to throw exceptions [align=left]public void passing(){[/align] [align=left] while (!passed()) {[/align] [align=left] try {[/align] [align=left] super.passing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] increment();[/align] [align=left] }[/align] [align=left] }[/align] }

方法二，写一个加强的passing（）方法，即fortifiedPassing()，对于在父类passing（）中捕获的异常，进行再判断。如果是0<=current<=2的情况则重新抛出NotBigEnoughException，如果是current<0的情况则抛出一个新的异常UnknowException。
ok in: Inheritor.java

public void fortifiedPassing() throws NotBigEnoughException, UnknowException{ [align=left] try {[/align] [align=left] super.passing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] if(current>=0)[/align] [align=left] throw e;[/align] [align=left] else[/align] [align=left] throw new UnknowException("i don't know how to deal with "[/align] [align=left] + current);[/align] [align=left] }[/align] }

Inheritor.java

[align=left]package com.zj.exception;[/align] [align=left]import com.zj.exception.types.NotBigEnoughException;[/align] [align=left]import com.zj.exception.types.UnknowException;[/align] [align=left] [/align] [align=left]public class Inheritor extends Recover {[/align] [align=left] public Inheritor(int cur) {[/align] [align=left] super(cur);[/align] [align=left] }[/align] [align=left] [/align] //couldn't throws new exceptions where not found in its base class [align=left] /**[/align] [align=left] public void passing() throws NotBigEnoughException, UnknowException {[/align] [align=left] if (current > 2) {[/align] [align=left] accept = true;[/align] [align=left] System.out.println("accept " + current);[/align] [align=left] } else if (current >= 0)[/align] [align=left] throw new NotBigEnoughException("reject " + current);[/align] [align=left] else[/align] [align=left] throw new UnknowException("i don't know how to deal with "[/align] [align=left] + current);[/align] [align=left] }*/[/align] [align=left] [/align] //sure passing(),so not have to throw exceptions [align=left] public void passing(){[/align] [align=left] while (!passed()) {[/align] [align=left] try {[/align] [align=left] super.passing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] increment();[/align] [align=left] }[/align] [align=left] }[/align] [align=left] }[/align] [align=left] [/align] public void fortifiedPassing() throws NotBigEnoughException, UnknowException{ [align=left] try {[/align] [align=left] super.passing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] if(current>=0)[/align] [align=left] throw e;[/align] [align=left] else[/align] [align=left] throw new UnknowException("i don't know how to deal with "[/align] [align=left] + current);[/align] [align=left] }[/align] [align=left] }[/align] [align=left] [/align] [align=left] public static void main(String[] args) {[/align] [align=left] // not required try-catch[/align] [align=left] new Inheritor(3).passing();[/align] [align=left] new Inheritor(1).passing();[/align] [align=left] new Inheritor(-1).passing(); [/align] [align=left] //no exceptions[/align] [align=left] try {[/align] [align=left] new Inheritor(3).fortifiedPassing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] e.printStackTrace();[/align] [align=left] } catch (UnknowException e) {[/align] [align=left] System.out.println(e);[/align] [align=left] }[/align] [align=left] //NotBigEnoughException:[/align] [align=left] try {[/align] [align=left] new Inheritor(1).fortifiedPassing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] e.printStackTrace();[/align] [align=left] } catch (UnknowException e) {[/align] [align=left] System.out.println(e);[/align] [align=left] }[/align] [align=left] //UnknownException:[/align] [align=left] try {[/align] [align=left] new Inheritor(-1).fortifiedPassing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] e.printStackTrace();[/align] [align=left] } catch (UnknowException e) {[/align] [align=left] System.out.println(e);[/align] [align=left] }[/align] [align=left] }[/align] }

结果：
[align=left]accept 3[/align]
[align=left]accept 3[/align]
[align=left]accept 3[/align]
[align=left]accept 3[/align]
[align=left]com.zj.exception.types.UnknowException: i don't know how to deal with -1[/align]
[align=left]com.zj.exception.types.NotBigEnoughException: reject 1[/align]
at
com.zj.exception.Recover.passing(Recover.java:28)
at
com.zj.exception.Inheritor.fortifiedPassing(Inheritor.java:38)
at
com.zj.exception.Inheritor.main(Inheritor.java:63)
4.RuntimeException[/b]与包装异常[/b][/b]
RuntimeException是unhecked异常，它们由JVM抛出（你也可以抛出它），并且不必在异常声明（throws）中列出。
如果RuntimeException没有被catch而到达mian（）方法时，那么在程序退出前会自动调用该异常的printStackTrace()方法，打印该异常。
RuntimeException代表的是编程错误（如0除数，数组越界），是应该在调试阶段解决的。
当你在捕获某些异常，而不知道该如果处理时，你可以将它包装为RuntimeException，这样在后续的方法调用过程中就不用声明(throws)该方法了。
在类Wrapper中，我们override fortifiedPassing()方法，并将它可能抛出的异常包装为RuntimeException。
Wrapper.java

[align=left]package com.zj.exception;[/align] [align=left]import com.zj.exception.types.NotBigEnoughException;[/align] [align=left]import com.zj.exception.types.UnknowException;[/align] [align=left] [/align] [align=left]public class Wrapper extends Inheritor {[/align] [align=left] public Wrapper(int cur) {[/align] [align=left] super(cur);[/align] [align=left] }[/align] [align=left] [/align] [align=left] public void fortifiedPassing() {[/align] [align=left] try {[/align] [align=left] super.fortifiedPassing();[/align] [align=left] } catch (NotBigEnoughException e) {[/align] [align=left] throw new RuntimeException(e);[/align] [align=left] } catch (UnknowException e) {[/align] [align=left] throw new RuntimeException(e);[/align] [align=left] }[/align] [align=left] }[/align] [align=left] [/align] [align=left] public static void main(String[] args) {[/align] [align=left] // not required try-catch[/align] [align=left] new Wrapper(3).fortifiedPassing();[/align] [align=left] new Wrapper(1).fortifiedPassing();[/align] [align=left] new Wrapper(-1).fortifiedPassing();[/align] [align=left] }[/align] }

结果：
[align=left]accept 3[/align]
Exception in thread
"main" java.lang.RuntimeException: com.zj.exception.types.NotBigEnoughException: reject 1
at
com.zj.exception.Wrapper.fortifiedPassing(Wrapper.java:14)
at
com.zj.exception.Wrapper.main(Wrapper.java:23)
[align=left]Caused by: com.zj.exception.types.NotBigEnoughException: reject 1[/align]
at
com.zj.exception.Recover.passing(Recover.java:28)
at
com.zj.exception.Inheritor.fortifiedPassing(Inheritor.java:38)
at
com.zj.exception.Wrapper.fortifiedPassing(Wrapper.java:12)
... 1
more

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