Java IO 系统.docx

Java IO 系统.docx

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JavaIO系统

9．4翻译

TheJavaI/OSystem

Creatingagoodinput/output（I/O）systemisoneofthemoredifficulttasksforthelanguagedesigner.

Thisisevidencedbythenumberofdifferentapproaches.Thechallengeseemstobeincoveringalleventualities.NotonlyaretheredifferentsourcesandsinksofI/Othatyouwanttocommunicatewith（files,theconsole,networkconnections,etc.）,butyouneedtotalktotheminawidevarietyofways（sequential,random-SQLServer2000,buffered,binary,character,bylines,bywords,etc.）.

TheJavalibrarydesignersattackedthisproblembycreatinglotsofclasses.Infact,therearesomanyclassesforJava’sI/Osystemthatitcanbeintimidatingatfirst（ironically,theJavaI/Odesignactuallypreventsanexplosionofclasses）.TherewasalsoasignificantchangeintheI/OlibraryafterJava1.0,whentheoriginalbyte-orientedlibrarywassupplementedwithchar-oriented,Unicode-basedI/Oclasses.InJDK1.4,thenioclasses（for“newI/O,”anamewe’llstillbeusingyearsfromnow）wereaddedforimprovedperformanceandfunctionality.Asaresult,thereareafairnumberofclassestolearnbeforeyouunderstandenoughofJava’sI/Opicturethatyoucanuseitproperly.Inaddition,it’sratherimportanttounderstandtheevolutionhistoryoftheI/Olibrary,evenifyourfirstreactionis“don’tbothermewithhistory,justshowmehowtouseit!

”Theproblemisthatwithoutthehistoricalperspective,youwillrapidlybecomeconfusedwithsomeoftheclassesandwhenyoushouldandshouldn’tusethem.

ThischapterwillgiveyouanintroductiontothevarietyofI/OclassesinthestandardJavalibraryandhowtousethem.

TheFileclass

Beforegettingintotheclassesthatactuallyreadandwritedatatostreams,we’lllookatautilityprovidedwiththelibrarytoassistyouinhandlingfiledirectoryissues.

TheFileclasshasadeceivingname;youmightthinkitreferstoafile,butitdoesn’t.Itcanrepresenteitherthenameofaparticularfileorthenamesofasetoffilesinadirectory.Ifit’sasetoffiles,youcanaskforthatsetusingthelist（ ）method,whichreturnsanarrayofString.Itmakessensetoreturnanarrayratherthanoneoftheflexiblecontainerclasses,becausethenumberofelementsisfixed,andifyouwantadifferentdirectorylisting,youjustcreateadifferentFileobject.Infact,“FilePath”wouldhavebeenabetternamefortheclass.Thissectionshowsanexampleoftheuseofthisclass,includingtheassociatedFilenameFilterinterface.

Adirectorylister

Supposeyou’dliketoseeadirectorylisting.TheFileobjectcanbelistedintwoways.Ifyoucalllist（ ）withnoarguments,you’llgetthefulllistthattheFileobjectcontains.However,ifyouwantarestrictedlist—forexample,ifyouwantallofthefileswithanextensionof.java—thenyouusea“directoryfilter,”whichisaclassthattellshowtoselecttheFileobjectsfordisplay.

TheDirFilterclass“implements”theinterfaceFilenameFilter.It’susefultoseehowsimpletheFilenameFilterinterfaceis:

publicinterfaceFilenameFilter{

booleanaccept（Filedir,Stringname）;

}

Itsaysallthatthistypeofobjectdoesisprovideamethodcalledaccept（ ）.Thewholereasonbehindthecreationofthisclassistoprovidetheaccept（ ）methodtothelist（ ）methodsothatlist（ ）can“callback”accept（ ）todeterminewhichfilenamesshouldbeincludedinthelist.Thus,thisstructureisoftenreferredtoasacallback.Morespecifically,thisisanexampleoftheStrategyPattern,becauselist（ ）implementsbasicfunctionality,andyouprovidetheStrategyintheformofaFilenameFilterinordertocompletethealgorithmnecessaryforlist（ ）toprovideitsservice.Becauselist（ ）takesaFilenameFilterobjectasitsargument,itmeansthatyoucanpassanobjectofanyclassthatimplementsFilenameFiltertochoose（evenatruntime）howthelist（ ）methodwillbehave.Thepurposeofacallbackistoprovideflexibilityinthebehaviorofcode.

DirFiltershowsthatjustbecauseaninterfacecontainsonlyasetofmethods,you’renotrestrictedtowritingonlythosemethods.（Youmustatleastprovidedefinitionsforallthemethodsinaninterface,however.）Inthiscase,theDirFilterconstructorisalsocreated.

Theaccept（ ）methodmustacceptaFileobjectrepresentingthedirectorythataparticularfileisfoundin,andaStringcontainingthenameofthatfile.Youmightchoosetouseorignoreeitherofthesearguments,butyouwillprobablyatleastusethefilename.Rememberthatthelist（ ）methodiscallingaccept（ ）foreachofthefilenamesinthedirectoryobjecttoseewhichoneshouldbeincluded;thisisindicatedbythebooleanresultreturnedbyaccept（ ）.

Tomakesuretheelementyou’reworkingwithisonlythefilenameandcontainsnopathinformation,allyouhavetodoistaketheStringobjectandcreateaFileobjectoutofit,thencallgetName（ ）,whichstripsawayallthepathinformation（inaplatform-independentway）.Thenaccept（ ）usesaregularexpressionmatcherobjecttoseeiftheregularexpressionregexmatchesthenameofthefile.Usingaccept（ ）,thelist（ ）methodreturnsanarray.

Inputandoutput

I/Olibrariesoftenusetheabstractionofastream,whichrepresentsanydatasourceorsinkasanobjectcapableofproducingorreceivingpiecesofdata.ThestreamhidesthedetailsofwhathappenstothedatainsidetheactualI/Odevice.

TheJavalibraryclassesforI/Oaredividedbyinputandoutput,asyoucanseebylookingattheclasshierarchyintheJDKdocumentation.Byinheritance,everythingderivedfromtheInputStreamorReaderclasseshavebasicmethodscalledread（ ）forreadingasinglebyteorarrayofbytes.Likewise,everythingderivedfromOutputStreamorWriterclasseshavebasicmethodscalledwrite（ ）forwritingasinglebyteorarrayofbytes.However,youwon’tgenerallyusethesemethods;theyexistsothatotherclassescanusethem—theseotherclassesprovideamoreusefulinterface.Thus,you’llrarelycreateyourstreamobjectbyusingasingleclass,butinsteadwilllayermultipleobjectstogethertoprovideyourdesiredfunctionality.ThefactthatyoucreatemorethanoneobjecttocreateasingleresultingstreamistheprimaryreasonthatJava’sstreamlibraryisconfusing.

It’shelpfultocategorizetheclassesbytheirfunctionality.InJava1.0,thelibrarydesignersstartedbydecidingthatallclassesthathadanythingtodowithinputwouldbeinheritedfromInputStream,andallclassesthatwereassociatedwithoutputwouldbeinheritedfromOutputStream.

TypesofInputStream

InputStream’sjobistorepresentclassesthatproduceinputfromdifferentsources.Thesesourcescanbe:

1.Anarrayofbytes.

2.AStringobject.

3.Afile.

4.A“pipe,”whichworkslikeaphysicalpipe:

Youputthingsinatoneendandtheycomeouttheother.

5.Asequenceofotherstreams,soyoucancollectthemtogetherintoasinglestream.

6.Othersources,suchasanInternetconnection.（ThisiscoveredinThinkinginEnterpriseJava.）

EachofthesehasanassociatedsubclassofInputStream.Inaddition,theFilterInputStreamisalsoatypeofInputStream,toprovideabaseclassfor"decorator"classesthatattachattributesorusefulinterfacestoinputstreams.Thisisdiscussedlater.

TypesofOutputStream

Thiscategoryincludestheclassesthatdecidewhereyouroutputwillgo:

anarrayofbytes（noString,however;presumably,youcancreateoneusingthearrayofbytes）,afile,ora“pipe.”

Inaddition,theFilterOutputStreamprovidesabaseclassfor"decorator"classesthatattachattributesorusefulinterfacestooutputstreams.

Addingattributesandusefulinterfaces

TheuseoflayeredobjectstodynamicallyandtransparentlyaddresponsibilitiestoindividualobjectsisreferredtoastheDecoratorpattern.（PatternsarethesubjectofThinkinginPatterns（withJava）atwww.BruceE.）Thedecoratorpatternspecifiesthatallobjectsthatwraparoundyourinitialobjecthavethesameinterface.Thismakesthebasicuseofthedecoratorstransparent—yousendthesamemessagetoanobjectwhetherithasbeendecoratedornot.Thisisthereasonfortheexistenceofthe“filter”classesintheJavaI/Olibrary:

Theabstract“filter”classisthebaseclassforallthedecorators.（Adecoratormusthavethesameinterfaceastheobjectitdecorates,butthedecoratorcanalsoextendtheinterface,whichoccursinseveralofthe“filter”classes）.

Decoratorsareoftenusedwhensimplesubclassingresultsinalargenumberofclassesinordertosatisfyeverypossiblecombinationthatisneeded—somanyclassesthatitbecomesimpractical.TheJavaI/Olibraryrequiresmanydifferentcombinationsoffeatures,andthisisthejustificationforusingthedecoratorpattern.Thereisadrawbacktothedecoratorpattern,however.Decoratorsgiveyoumuchmoreflexibilitywhileyou’rewritingaprogram（sinceyoucaneasilymixandmatchattributes）,buttheyaddcomplexitytoyourcode.ThereasonthattheJavaI/Olibraryisawkwardtouseisthatyoumustcreatemanyclasses—the“core”I/Otypeplusallthedecorators—inordertogetthesingleI/Oobjectthatyouwant.

TheclassesthatprovidethedecoratorinterfacetocontrolaparticularInputStreamorOutputStreamaretheFilterInputStreamandFilterOutputStream,whichdon’thaveveryintuitivenames.FilterInputStreamandFilterOutputStreamarederivedfromthebaseclassesoftheI/Olibrary,InputStreamandOutputStream,whichisthekeyrequirementofthedecorator（sothatitprovidesthecommoninterfacetoalltheobjectsthatarebeingdecorated）.

ReadingfromanInputStreamwithFilterInputStream

TheFilterInputStreamclassesaccomplisht