Thinking in Java 3 Edition 此外你还可以用 Throwable的基类,也就是 Object(所有对象的基类) 的方法。对异常来说有一个很有用的方法, getclass()。它会返回一个 “表示这个对象是属于哪种类型的” class对象。接下来,你可以用 getName()查询这个 Class对象的名字。你还可以用 Class对象作 更复杂的事情,不过对异常处理来说,就没这个必要了 下面这段程序演示了这些基本的 Exception方法的用法 //: c09: ExceptionMethods java / Demonstrating the Exception Methods import com. bruceeckel simpletest*i public class ExceptionMethods t private static Test monitor new Test() public static void main(String[] args) i ception("My Exe 1 catch(Exception e)t println (" Caught e getMessage() e get LocalizedMessage()ig System. err println("getLocali mEssage(): stem.err println("print stackTrace():")i e print stackTrace()i String[] t Caught Exception getMessage () My Exception" tostring () java. lang Except My Exception "printstackTrace(): 号号\ tat ExceptionMethods,main\(.*\)” 你能看到这些方法一个比一个提供更多的信息—实际上它们每个都是前 个的超集 重抛异常 有时你需要重新抛出那个刚捕捉到的,用 Exception捕捉到的异常。由 于你已经有了当前异常的 reference,因此你可以直接将那个 reference 重抛出来 System. err println("An exception was thrown") 第11页共11页 www.wgqqh.com/shhgs/tij.html
Thinking in Java 3 rd Edition ✞ 11 ✟ ✠ 11 ✟ www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com ±³S Th rowable #$ Object(,-÷`) @n÷èZ`a-^\ä-@n#getCl ass( )9~op^\ øùR\÷`jkã?ÐCl ass ÷`|`#±S getN am e( )¨1R\ Cl ass ÷`±³S Cl ass ÷`» ói2¢ #=÷èZ78`a# ÀR\; R¨%&3ù R2 Exception @nnÊ //: c09:ExceptionMethods.java // Demonstrating the Exception Methods. import com.bruceeckel.simpletest.*; public class ExceptionMethods { private static Test monitor = new Test(); public static void main(String[] args) { try { throw new Exception("My Exception"); } catch(Exception e) { System.err.println("Caught Exception"); System.err.println("getMessage():" + e.getMessage()); System.err.println("getLocalizedMessage():" + e.getLocalizedMessage()); System.err.println("toString():" + e); System.err.println("printStackTrace():"); e.printStackTrace(); } monitor.expect(new String[] { "Caught Exception", "getMessage():My Exception", "getLocalizedMessage():My Exception", "toString():java.lang.Exception: My Exception", "printStackTrace():", "java.lang.Exception: My Exception", "%% \tat ExceptionMethods.main\\(.*\\)" }); } } ///:~ ±'R2@n^\^\;ó[GHãg9:¸\.* ^\4 -± ;ÔLK\5# Exception èZ k±ËÌ- F*èZ referen ce#ƱSÇ|DK\ referen ce L`Ê catch(Exception e) { System.err.println("An exception was thrown"); throw e; }
hapter 9: Error Handl 重抛会把异常送到更高一层的异常处理程序去。同一个try区块的其它 catch子句都将被忽略。此外,它还会保留异常对象里的所有信息,这 样捕获这个异常的上一层的异常处理程序就能够提取这个对象中的所有信 息了 如果你直接重抛当前的异常,则 printstackTrace()所打印出来的那 些保存在异常对象里的信息,还会指向异常发生的地方,它们不会被指到 你重抛异常的地点。如果你要装载新的栈轨迹信息,你可以调用 fillIn stackTrace(),这个方法会将当前栈的信息塞进旧的异常对象 中,并返回一个 Throwable对象。下面就是它的工作方式 //: c09: Rethrowing. java // Demonstrating fillInStackTrace() import com. bruceeckel. simpletest * i public class Rethrowing i Test oi System. out. println ("originating the exception in f()") throw new Exception("thrown from f()")i public static void g() throws Throwable t f(); h(Exception e)t System. err println ("Inside g(,e. print stackTrace()) e print stackTrace()i / thr 1lInstackT public static void main(String[] args)throws Throwable t g(); 1 catch(Exception e)( System. err. println( Caught in main, e printStackTrace()")i e print stackTrace() monitor. expect (new String[] i originating the exception in f()", () java. lang Exception: thrown from f()", 号号\ tat Rethrowing.f(,*?)”, 号号\ tat Throwing.q(.*?)”, 号号\ tat Rethrowing,main(.*?) "Caught in main, e. print stackTrace()", thrown from f() 号号\ tat Rethrowing.f(.*?)", 号号\ tat Throwing,g(,*?) 号号\ tat Throwing,main(.*?) 第12页共12页 www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com
Chapter 9: Error Handling with Exceptions www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com ✞ 12 ✟ ✠ 12 ✟ L~ èZöór^KèZ78%&´^\ try B«U9 catch N¹.D/67#9³~8èZ÷`,-GH#R O R\èZg^KèZ78%& 'GR\÷`!,-G H ¶·±Ç|LF*èZ#ï printStackTrace( ),ñò`K 2x èZ÷`GH#³~Å÷èZ0ü@#9:~/Å ±LèZ¯¶·±;9Ô()GH#±Sº fil l InStackTrace( )#R\@n~DF*GH:±;èZ÷` !#iop^\ Th rowable ÷` 9<»@AÊ //: c09:Rethrowing.java // Demonstrating fillInStackTrace() import com.bruceeckel.simpletest.*; public class Rethrowing { private static Test monitor = new Test(); public static void f() throws Exception { System.out.println("originating the exception in f()"); throw new Exception("thrown from f()"); } public static void g() throws Throwable { try { f(); } catch(Exception e) { System.err.println("Inside g(),e.printStackTrace()"); e.printStackTrace(); throw e; // 17 // throw e.fillInStackTrace(); // 18 } } public static void main(String[] args) throws Throwable { try { g(); } catch(Exception e) { System.err.println( "Caught in main, e.printStackTrace()"); e.printStackTrace(); } monitor.expect(new String[] { "originating the exception in f()", "Inside g(),e.printStackTrace()", "java.lang.Exception: thrown from f()", "%% \tat Rethrowing.f(.*?)", "%% \tat Rethrowing.g(.*?)", "%% \tat Rethrowing.main(.*?)", "Caught in main, e.printStackTrace()", "java.lang.Exception: thrown from f()", "%% \tat Rethrowing.f(.*?)", "%% \tat Rethrowing.g(.*?)", "%% \tat Rethrowing.main(.*?)"
Thinking in Java 3 Edition }///: 最重要的几行都已经标出来了。如果第17行没有注释掉(也就是现在的情 况),那么无论这个异常被重抛了多少次,异常的栈轨迹都会一直记得它 是从最初那个抛出的 如果把第17行注释掉,并且把第18行注释回来,那么就会用到 fillIn StackTrace(),结果是: oriaNa n f( Inside g(,e. print StackTrace( java. lang. Exception: thrown from f() at Rethrowing. f(Rethrowing. java: 9) at Rethrowing g(Rethrowing. java: 12) at Rethrowing. main(Rethrowing. java: 23) Caught in main, e. print stackTrace() java. lang. Exception: thrown from f() at Rethrowing g(Rethrowing java: 18) at Rethrowing. main(Rethrowing. java: 23) (此外还有一些τ ext expect()输出的出错信息。)由于 fillIn StackTrace(),第18行成了异常的新的发生地了。 方法g()和main()的异常说明中必须包括 Throwable,这是因为 fillInstacktrace()方法返回的是 Throwable对象的 reference 而由于 Throwable是 Exception的基类,所以有可能会得到一个 Throwable而不是 Exception,因此man()里面的 Exception 的处理程序就有可能会捕捉不到这个重抛出来的对象。为了确保一切都能 正常运行,编译器会强制要求在异常说明中使用 Throwable。例如,在 下面这段程序中的main()就没能捉到抛出来的异常 / C09: Throwout. java //(ThrowsException public class ThrowOut t main(String[] args) throws Throwable try i throw new Throwable ( t catch(Exception e)i System. err println("Caught in main()") }///: 第13页共13页 www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com
Thinking in Java 3 rd Edition ✞ 13 ✟ ✠ 13 ✟ www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com }); } } ///:~ ;^.ËÌv` ¶·{ 1 7 À-Æ= ($ 1 «)#K *R\èZ/L [$¹#èZ().~^Ç9 bpK\L ¶· { 1 7 Æ= #i¬ { 1 8 Æ=p`#K ~ fil l InStackTrace( )#Â·Ê originating the exception in f() Inside g(),e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.f(Rethrowing.java:9) at Rethrowing.g(Rethrowing.java:12) at Rethrowing.main(Rethrowing.java:23) Caught in main, e.printStackTrace() java.lang.Exception: thrown from f() at Rethrowing.g(Rethrowing.java:18) at Rethrowing.main(Rethrowing.java:23) (³-^2 Text. expect( )øGH)Âk fil l InStackTrace( )#{ 1 8 Í èZÔ0ü @n g( )³ m ain( )èZaü!äå Th rowable#RÆ fil l InStackTrace( )@nop Th rowable ÷` referen ce µÂk Th rowable Exception #,S-'~^\ Th rowable µ Exception#Æ m ain( ) Exception 78%& -'~R\L`÷` Q^>.' PZ(# !1~Ò2;3 èZaü! Th rowable6¶# R¨%&! m ain( ) À'L`èZÊ //: c09:ThrowOut.java // {ThrowsException} public class ThrowOut { public static void main(String[] args) throws Throwable { try { throw new Throwable(); } catch(Exception e) { System.err.println("Caught in main()"); } } } ///:~
hapter 9: Error Handl 你也可以抛出一个与你捕捉到的异常不同的异常。这么做的效果同使用 fillIn StackTrace()的差不多—异常最初在哪里发生的信息被仍 了,现在里面保存的是抛出新异常的地点。 //: c09: RethrowNew. java // Rethrow a different object from the one that was /IThrowsException) import com. bruceeckel. simpletest. class OneException extends Exception t public OneException(String s) super(s) class TwOException extends Exception t public TwoException(string s) super(s)i public class RethrowNew i rivate static test monitor =n public static void f() throws OneException t Syste itIn("originating the exceptio throw new OneException("thrown from f())i main(String[ args) throws TwoException i i catch(one ) Caught in main, e. printstackTrace()")i e print stackTrace() throw new TwoException ("from main()") monitor. expect(new String[] i originating the exception in f() Caught in main, e.print stackTrace()", m\tat RethrowNew. f(RethrowNew java: 18) w\tat RethrowNew main(RethrowNew. java: 22) "Exception in thread \"main\""+ TwOException: from main()", "\tat RethrowNew. main(RethrowNew java: 28 最后那个异常只知道它是从main()而不是f()来的 你永远也不需要为清理前一个异常或是其它异常而担心。它们都是用 new在堆里创建的,因此垃圾收集器会把它们自动清理干净 异常链 第14页共14页 www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com
Chapter 9: Error Handling with Exceptions www.wgqqh.com/shhgs/tij.html email:shhgs@sohu.com ✞ 14 ✟ ✠ 14 ✟ ±$SL^\±èZèZRm×· fil l InStackTrace( )?[èZp ã0üGH/Õ #1 xLÔèZ¯ //: c09:RethrowNew.java // Rethrow a different object from the one that was caught. // {ThrowsException} import com.bruceeckel.simpletest.*; class OneException extends Exception { public OneException(String s) { super(s); } } class TwoException extends Exception { public TwoException(String s) { super(s); } } public class RethrowNew { private static Test monitor = new Test(); public static void f() throws OneException { System.out.println("originating the exception in f()"); throw new OneException("thrown from f()"); } public static void main(String[] args) throws TwoException { try { f(); } catch(OneException e) { System.err.println( "Caught in main, e.printStackTrace()"); e.printStackTrace(); throw new TwoException("from main()"); } monitor.expect(new String[] { "originating the exception in f()", "Caught in main, e.printStackTrace()", "OneException: thrown from f()", "\tat RethrowNew.f(RethrowNew.java:18)", "\tat RethrowNew.main(RethrowNew.java:22)", "Exception in thread \"main\" " + "TwoException: from main()", "\tat RethrowNew.main(RethrowNew.java:28)" }); } } ///:~ yK\èZ5LM9b m ain( )µ f( )` ±@$ ;8*^\èZtU9èZµA9:. new aÎb#ÆBC} 1~ 9:ð8D