Java程序設計JavaProgrammingSpring,20131成都信息工程學院計算機系ContentsWhyExceptions?WhatareExceptions?HandlingExceptionsCreatingExceptionTypes2chapter8ExceptionsWhyExceptions(異常)?Duringexecution(執行),programscanrunintomanykindsoferrors;Whatdowedowhenanerroroccurs?Javausesexceptionstoprovidetheerror-handlingcapabilitiesforitsprograms.3chapter8ExceptionsExceptions(異常)ErrorClassCritical(嚴重的)

errorwhichisnotacceptableinnormalapplicationprogram.

ExceptionClassPossibleexceptioninnormalapplicationprogramexecution;

Possibletohandlebyprogrammer.

4chapter8ExceptionsExceptions(異常)Treatexceptionasanobject.AllexceptionsareinstancesofaclassextendedfromThrowable

classoritssubclass.

Generally,aprogrammermakesnewexceptionclasstoextendtheExceptionclasswhichisasubclassofThrowableclass.6chapter8ExceptionsExceptionClass繼承關系ThrowableErrorExceptionRuntimeExceptionIOExceptionObject7異常類的層次結構：8Classifying(分類)JavaExceptionsUncheckedExceptions(非檢查性異常)Itisnotrequiredthatthesetypesofexceptionsbecaughtordeclaredonamethod.RuntimeexceptionscanbegeneratedbymethodsorbytheJVMitself.Errors

aregeneratedfromdeepwithintheJVM,andoftenindicateatrulyfatalstate.CheckedExceptions(檢查性異常)Musteitherbecaughtbyamethodordeclaredinitssignaturebyplacingexceptionsinthemethodsignature.9Exception的分類非檢查性異常(uncheckedexception)：以RuntimeException為代表的一些類，編譯時發現不了，只在能運行時才能發現。檢查性異常(checkedexception)：一般程序中可預知的問題，其產生的異常可能會帶來意想不到的結果，因此Java編譯器要求Java程序必須捕獲或聲明所有的非運行時異常。以IOException為代表的一些類。如果代碼中存在檢查性異常，必須進行異常處理，否則編譯不能通過。如：用戶連接數據庫SQLException、FileNotFoundException。10chapter8ExceptionsException的分類什么是RuntimeException：Java虛擬機在運行時生成的異常，如：被0除等系統錯誤、數組下標超范圍等，其產生比較頻繁，處理麻煩，對程序可讀性和運行效率影響太大。因此由系統檢測,用戶可不做處理，系統將它們交給默認的異常處理程序(當然，必要時，用戶可對其處理)。Java程序異常處理的原則是：對于Error和RuntimeException，可以在程序中進行捕獲和處理，但不是必須的。對于IOException及其他異常，必須在程序進行捕獲和處理。異常處理機制主要處理檢查性異常。11chapter8ExceptionsJavaExceptionTypeHierarchy(層次)virtualmachineerrors12系統異常類的層次結構：13chapter8ExceptionsException的分類System-DefinedException(系統定義的異常)Programmer-DefinedException(程序員自定義異常)14chapter8ExceptionsSystem-DefinedException

(系統定義的異常)Raisedimplicitlybysystembecauseofillegalexecutionofprogram;CreatedbyJavaSystemautomatically;ExceptionextendedfromErrorclassandRuntimeExceptionclass.

15chapter8ExceptionsSystem-DefinedExceptionIndexOutOfBoundsException:Whenbeyondtheboundofindexintheobjectwhichuseindex,suchasarray,string,andvectorArrayStoreException:WhenassignobjectofincorrecttypetoelementofarrayNegativeArraySizeException:

WhenusinganegativesizeofarrayNullPointerException:WhenrefertoobjectasanullpointerSecurityException:Whenviolatesecurity.CausedbysecuritymanagerIllegalMonitorStateException:Whenthethreadwhichisnotownerofmonitorinvolveswaitornotifymethod16Programmer-DefinedException

(程序員自定義異常)

Exceptionsraisedbyprogrammer

SubclassofException

classCheckbycompilerwhethertheexceptionhandlerforexceptionoccurredexistsornotIfthereisnohandler,itiserror.

17chapter8ExceptionsUser-definedExceptions

(用戶自定義異常)classUserErrextendsException

{ …}CreatingExceptionTypes:18Example1:publicclassInsufficientFundsException

extendsException

{ privateBankAccountexcepbank;privatedoubleexcepAmount;

InsufficientFundsException(Bankba,doubledAmount) { excepbank=ba;excepAmount=dAmount; }}19chapter8Exceptions程序運行時發生異常，系統會拋出異常，如果程序中未處理和捕獲異常，異常拋出后程序運行中斷。publicclassTest{publicint[]bar(){ inta[]=newint[2]; for(intx=0;x<=2;x++){

a[x]=0;//拋出異常，程序中斷} returna;}publicstaticvoidmain(String[]args){ Testt=newTest(); t.bar(); //程序中斷 System.out.println(“Method:foo”);//不被執行}}系統給出的錯誤信息：Exceptioninthread"main"java.lang.ArrayIndexOutOfBoundsException:2atexception.Test.bar(Test.java:7)atexception.Test.main(Test.java:25)20HandlingExceptions(處理異常)Throwinganexception(拋出異常)Whenanerroroccurswithinamethod.Anexceptionobjectiscreatedandhandedofftotheruntimesystem(運行時系統).Theruntimesystemmustfindthecodetohandletheerror.Catchinganexception(捕獲異常)Theruntimesystem(運行時系統)searchesforcodetohandlethethrownexception.Itcanbeinthesamemethodorinsomemethodinthecall(調用)stack(堆棧).21chapter8ExceptionsKeywordsforJavaExceptionstry

Marksthestartofablockassociatedwithasetofexceptionhandlers.catch

Iftheblockenclosedbythetrygeneratesanexceptionofthistype,controlmoveshere;watchoutforimplicitsubsumption.finally

Alwayscalledwhenthetryblockconcludes,andafteranynecessarycatchhandleriscomplete.throws

Describestheexceptionswhichcanberaisedbyamethod.throw

Raisesanexceptiontothefirstavailablehandlerinthecallstack,unwindingthestackalongtheway.22拋出異常–throw,throws在一個方法的運行過程中，如果一個語句引起了錯誤時，含有這個語句的方法就會創建一個包含有關異常信息的異常對象，并將它傳遞給Java運行時系統。我們把生成異常對象并把它提交給運行時系統的過程稱為拋出(throw)異常。throw

—在方法體中用throw手工拋出異常；throws—在方法頭部間接拋出異常，即：申明方法中可能拋出的異常。231.在方法體中用throw手工拋出異常throw拋出異常,可以是系統定義的異常，也可以是用戶自定義的異常。語法格式：或例如:下面語句就拋出了一個IOException異常：thrownewIOException()；異常類名對象名＝new異常類構造函數；throw對象名；thrownew異常類構造函數；24throw

－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr){ try{ if(ptr==0)

thrownewNullPointerException();

} catch(NullPointerExceptione){}}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}252.throws—間接拋出異常(申明異常)一個方法不處理它產生的異常,而是沿著調用層次向上傳遞,由調用它的方法來處理這些異常,叫聲明異常。在定義方法時用throws關鍵字將方法中可能產生的異常間接拋出。若一個方法可能引發一個異常，但它自己卻沒有處理，則應該聲明異常，并讓其調用者來處理這個異常，這時就需要用throws關鍵字來指明方法中可能引發的所有異常。類型方法名(參數列表)throws

異常列表{ //…代碼}26Example1：publicclassExceptionTest{ voidProc(intsel)throwsArrayIndexOutOfBoundsException{

System.out.println(“InSituation"+sel); if(sel==1){ intiArray[]=newint[4];iArray[10]=3;//拋出異常 }

}

}27異常向上傳遞－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr)throws

NullPointerException{if(ptr==0)

thrownewNullPointerException();}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}運行結果：

java.lang.NullPointerExceptionatThrowStatement.exp(ThrowStatement.java:4)atThrowStatement.main(ThrowStatement.java:8)28Exceptions-throwingmultiple(多個)

exceptionsAMethodcanthrowmultipleexceptions.Multipleexceptionsareseparatedbycommasafterthethrowskeyword:publicclassMyClass{ publicintcomputeFileSize()

throwsIOException,ArithmeticException { ...}}29HandlingExceptions在一個方法中，對于可能拋出的異常，處理方式有兩種：一個方法不處理它產生的異常,只在方法頭部聲明使用throws拋出異常，使異常沿著調用層次向上傳遞,由調用它的方法來處理這些異常。用try-catch-finally語句對異常及時處理；30HandlingExceptionspublicvoidreplaceValue(Stringname,Objectvalue)

throwsNoSuchAttributeException{Attrattr=find(name);if(attr==null)

thrownewNoSuchAttributeException(name);

attr.setValue(value);}try{…

replaceValue(“att1”,“newValue”);…}catch(NoSuchAttributeExceptione){e.printStackTrace();}當replaceValue(…)方法被調用時，調用者需處理異常。31處理異常語句try-catch-finally的基本格式為：try{ //可能產生異常的代碼；}//不能有其它語句分隔catch(異常類名異常對象名){ //異常處理代碼；//要處理的第一種異常}catch(異常類名異常對象名){ //異常處理代碼；//要處理的第二種異常}…finally{ //最終處理（缺省處理）}32Exceptions–Syntax(語法)示例try{

//Codewhichmightthrowanexception //...}catch(FileNotFoundExceptionx){

//codetohandleaFileNotFoundexception}catch(IOExceptionx){

//codetohandleanyotherI/Oexceptions}catch(Exceptionx){

//Codetocatchanyothertypeofexception}finally{

//ThiscodeisALWAYSexecutedwhetheranexceptionwasthrown //ornot.Agoodplacetoputclean-upcode.ie.close //anyopenfiles,etc...}33HandlingExceptions(處理異常)try-catch或try-catch-finally.Threestatementshelpdefinehowexceptionsarehandled:tryidentifiesablockofstatementswithinwhichanexceptionmightbethrown;Atrystatementcanhavemultiplecatchstatementsassociatedwithit.catchmustbeassociatedwithatrystatementandidentifiesablockofstatementsthatcanhandleaparticulartypeofexception.Thestatementsareexecutedifanexceptionofaparticulartypeoccurswithinthetryblock.34HandlingExceptionsfinally(可選項)mustbeassociatedwithatrystatementandidentifiesablockofstatementsthatareexecutedregardlessofwhetherornotanerroroccurswithinthetryblock.Evenifthetryandcatchblockhaveareturnstatementinthem,finallywillstillrun.35用try-catch-finally語句對異常及時處理－ExampleclassThrowStatementextendsException{publicstaticvoidexp(intptr){ try{ if(ptr==0)

thrownewNullPointerException(); } catch(NullPointerExceptione){}}

publicstaticvoidmain(String[]args){inti=0;

ThrowStatement.exp(i);}}36系統拋出異常后，捕獲異常，運行finally塊，程序運行繼續。publicclassTest{ publicvoidfoo(){

try{ inta[]=newint[2];

a[4]=1;/*causesaruntimeexceptionduetotheindex*/System.out.println(“Method:foo”);//若異常發生，不執行}catch(ArrayIndexOutOfBoundsExceptione){System.out.println("exception:"+e.getMessage()); e.printStackTrace();}finally{System.out.println("Finallyblockalwaysexecute!!!");}}publicstaticvoidmain(String[]args){Testt=newTest();t.foo();

System.out.println(“ExcecutionafterException!!!”);

//繼續執行}}37運行結果：exception:4java.lang.ArrayIndexOutOfBoundsException:4 atexception.Test.foo(Test.java:8) atexception.Test.main(Test.java:20)Finallyblockalwaysexecute!!!ExcecutionafterException!!!38Exceptions-throwingmultipleexceptionspublicvoidmethod1(){ MyClassanObject=newMyClass(); try{ inttheSize=anOputeFileSize(); }catch(ArithmeticExceptionx){ //... }catch(IOExceptionx){ //...}}39Exceptions-catchingmultipleexceptionsEachtryblockcancatchmultipleexceptions.StartwiththemostspecificexceptionsFileNotFoundExceptionisasubclassofIOExceptionItMUSTappearbeforeIOException

inthecatchlistpublicvoidmethod1(){ FileInputStreamaFile; try{ aFile=newFileInputStream(...); intaChar=aFile.read(); //... }catch(FileNotFoundExceptionx){ //... }catch(IOExceptionx){ //...}}40Exception-Thecatch-allHandlerSinceallExceptionclassesareasubclassoftheExceptionclass,acatchhandlerwhichcatches"Exception"willcatchallexceptions.ItmustbethelastinthecatchList.publicvoidmethod1(){ FileInputStreamaFile; try{ aFile=newFileInputStream(...); intaChar=aFile.read(); //... } catch(IOExceptionx){ //... } catch(Exceptionx){ //CatchAllExceptions}}41User-definedExceptions(用戶自定義異常)的處理classUserErrextendsException{ …}用戶自定義異常是檢查性異常(checkedexception)，必須用throw手工拋出并處理。classUserClass{ …

UserErr

x=newUserErr();...if(val<1)

throw

x;}42//ThrowExample.javaclassIllegalValueException extendsException{}

classUserTrial{intval1,val2;publicUserTrial(inta,intb){val1=a;val2=b;}voidshow()throwsIllegalValueException{

if((val1<0)||(val2>0)) thrownewIllegalValueException();System.out.println(“Value1=”+val1);//不運行System.out.println("Value2="+val2);}}classThrowExample{publicstaticvoidmain(Stringargs[]){

UserTrialvalues=newUserTrial(-1,1);try{ values.show();}catch(IllegalValueExceptione){Syst