异常
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转自:http://blog.csdn.net/touch_2011/article/details/6860043
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
/** * 自定义异常类方法 * 1、通过继承Throwable * 2、通过继承Exception * * @author Touch */ public class MyExceptionDemo extends Exception { private static final long serialVersionUID = 1L; public MyExceptionDemo() { super(); } public MyExceptionDemo(String message) { super(message); } public MyExceptionDemo(String message, Throwable cause) { super(message, cause); } public MyExceptionDemo(Throwable cause) { super(cause); } } </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
import mine.util.exception.MyException; public class ExceptionDemo1 { public void f() throws MyException { throw new MyException("自定义异常"); } public void g() throws MyException { f(); } public void h() throws MyException { try { g(); } catch (MyException e) { //1、通过获取栈轨迹中的元素数组来显示异常抛出的轨迹 for (StackTraceElement ste : e.getStackTrace()) System.out.println(ste.getMethodName()); //2、直接将异常栈信息输出至标准错误流或标准输出流 e.printStackTrace();//输出到标准错误流 e.printStackTrace(System.err); e.printStackTrace(System.out); //3、将异常信息输出到文件中 //e.printStackTrace(new PrintStream("file/exception.txt")); //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() //那么将会从这个方法(当前是h()方法)作为异常发生的原点。 //throw e; throw (MyException)e.fillInStackTrace(); } } public static void main(String[] args) { try { new ExceptionDemo1().h(); } catch (MyException e) { // TODO Auto-generated catch block e.printStackTrace(); } } } </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:
import java.io.IOException; import mine.util.exception.MyException; public class ExceptionDemo2 { public void f() throws MyException { throw new MyException("自定义异常"); } public void g() throws Exception { try { f(); } catch (MyException e) { e.printStackTrace(); throw new Exception("重新抛出的异常1"); } } public void h() throws IOException { try { g(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); throw new IOException("重新抛出异常2"); } } public static void main(String[] args) { try { new ExceptionDemo2().h(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
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:
import java.io.IOException; import mine.util.exception.MyException; public class ExceptionDemo2 { public void f() throws MyException { throw new MyException("自定义异常"); } public void g() throws Exception { try { f(); } catch (MyException e) { e.printStackTrace(); throw new Exception("重新抛出的异常1",e); } } public void h() throws IOException { try { g(); } catch (Exception e) { // TODO Auto-generated catch block e.printStackTrace(); IOException io=new IOException("重新抛出异常2"); io.initCause(e); throw io; } } public static void main(String[] args) { try { new ExceptionDemo2().h(); } catch (IOException e) { // TODO Auto-generated catch block e.printStackTrace(); } } }
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:
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
package demo.others; import mine.util.exception.MyException; public class ExceptionDemo3 { public void f() throws MyException { throw new MyException("异常1"); } public void g() throws MyException { throw new MyException("异常2"); } public static void main(String[] args) { try { ExceptionDemo3 ex = new ExceptionDemo3(); try { ex.f(); } finally { 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 } } catch (MyException e) { System.out.println(e); } } }
结果:mine.util.exception.MyException: 异常2
此时异常1就丢失了
或者这样写:
package demo.others; import mine.util.exception.MyException; public class ExceptionDemo3 { public void g() throws MyException { throw new MyException("异常2"); } public static void main(String[] args) { ExceptionDemo3 ex = new ExceptionDemo3(); try { ex.g(); } finally { //直接return会丢失所以抛出的异常 return; } } }
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。
7、异常的匹配
异常匹配并不要求与抛出的异常完全匹配,也可以匹配该异常的基类。
如果故意把基类异常放在前面,导致子类异常的catch子句永远得不到执行,编译器会报错。
class Annoyance extends Exception {}class Sneeze extends Annoyance {}public class Human {public static void main(String[] args) {// Catch the exact type:try {throw new Sneeze();} catch(Sneeze s) {System.out.println("Caught Sneeze");} catch(Annoyance a) {System.out.println("Caught Annoyance");}// Catch the base type:try {throw new Sneeze();} catch(Annoyance a) {System.out.println("Caught Annoyance");}}} /* Output:Caught SneezeCaught Annoyance
8、其它可选方式
1、将异常传递给控制台
2、用RuntimeException来包装“被检查的异常”。
class WrapCheckedException { void throwRuntimeException(int type) { try { switch(type) { case 0: throw new FileNotFoundException(); case 1: throw new IOException(); case 2: throw new RuntimeException("Where am I?"); default: return; } } catch(Exception e) { // Adapt to unchecked: throw new RuntimeException(e); } } } class SomeOtherException extends Exception {} public class TurnOffChecking { private static Test monitor = new Test(); public static void main(String[] args) { WrapCheckedException wce = new WrapCheckedException(); // You can call f() without a try block, and let // RuntimeExceptions go out of the method: wce.throwRuntimeException(3); // Or you can choose to catch exceptions: for(int i = 0; i < 4; i++) try { if(i < 3) wce.throwRuntimeException(i); else throw new SomeOtherException(); } catch(SomeOtherException e) { System.out.println("SomeOtherException: " + e); } catch(RuntimeException re) { try { throw re.getCause(); } catch(FileNotFoundException e) { System.out.println( "FileNotFoundException: " + e); } catch(IOException e) { System.out.println("IOException: " + e); } catch(Throwable e) { System.out.println("Throwable: " + e); } } monitor.expect(new String[] { "FileNotFoundException: " + "java.io.FileNotFoundException", "IOException: java.io.IOException", "Throwable: java.lang.RuntimeException: Where am I?", "SomeOtherException: SomeOtherException" }); } } ///:~