XMLOBjEctsprojectXOBELK02overcomesthedifferencesbetweenXMLstructuresas - PDF document

1 XMLOBjEctsproject(XOBE)[LK02]overcomesth
XMLOBjEctsproject(XOBE)[LK02]overcomesthedifferencesbetweenXMLstructuresasstringsandcorrespondingobjectsbyanextensionoftheobjectorientedpro-gramminglanguageJava.Inotherwords,whenusingXMLsyntaxinXOBE,italwaysdenotesXMLobjects,i.e.generatingandanalyzingXMLisdoneconceptuallyonlyonthebasisofobjects.ThereforeXOBEintroducesaclassforeveryelementtypeofausedschema,butdoesnotgeneratetheseclassesexplicitlyasJavaclasses.Insteadofthattheycanbeusedlikebuilt-indatatypes.Theyarede®nedinsuchawaythatthegenerationofXMLobjectsisdoneinasyntaxorientedmannerallowingtocheckmostportionsofthepropertyvalidity,whichwecallstaticvalidity,ofallgeneratedXMLstructures,i.e.XMLobjects,atcompiletime.ThepresentpaperfocusesontheXOBEtypesystem.ThemainproblemoftheXOBEtypesystemisindecidingifanXMLobjectisallowedatthepositionitappears.Thisdecisionproblemforsubtypingisalgorithmicallydif®cult.WeintroduceanalgorithmwhichchecksthesubtyperelationshipofXMLobjects.ThealgorithmcanbeviewedasanimprovementofAntimirov'salgorithmsforth

2 edecisionproblemofregularexpressions.Eve
edecisionproblemofregularexpressions.EvenonXOBEapplicationsthatinvolvequitelargetypes,suchasthecompleteschemaofXHTML,ouralgorithmcompletesinreasonabletime.OuralgorithmenablesXOBEtoguaranteestaticvalidityatcompiletimewhichimpliesthefollowingadvantages:1.XOBEprogramsaremoreef®cientbecauseweavoidexpensiveruntimecheckstoguaranteevalidity.2.XOBEprogramsaremorereliablebecausewecanomittheprogrammingofrecov-eryprocedureswhichareneededifruntimechecksfail.3.XOBEenablesafasterdevelopmentofimplementations,becausewecanavoidex-tensivetestruns,whicharenecessarytomakethevalidityofthegeneratedXMLdocumentsfeasible.4.XOBEimprovesthemaintenanceofwebservicesandwebapplicationsbecauseitleadstoasimplersourcecodestructure. Thepaperisorganizedasfollows.Inthenextsection,wegiveanintroductiontotheprogrammingwithXMLobjects.InSection3,wedescribetheconnectionbetweenXMLobjectsandregularhedgeexpressionsandintroducesubtyping.InSection4,wepresentoursubtypingalgorithm,themainpartofthepaper.Section5explainsourimplementa

3 tiontechniquesandSection6discussessomepe
tiontechniquesandSection6discussessomeperformancemeasurements.WesurveyrelatedworkinSection7,i.e.wesummarizethestateoftheartofprogrammingwebapplicationsandwebservices.Section8concludesthepaperandgivesanoutlookonfuturework. XMLobjectsinXOBE.ThereforeXMLobjectsare®rst-classdatavaluesthatmaybepassedandstoredlikeanyotherdatavalues.ThegivenschemaisusedtotypedifferentXMLobjects.XOBEprogramsimportschemade®nitions,declaredbyanimportstatement CartdepictedinFigure1.Theclasshasamember®eld intarticleNr) request /account done /shoppingCart /shopResponse ;returnresponse; XOBEincorporatesXPath[W3C99]foraccessingXMLobjectsinXOBEprograms.XPathprovidesamechanismtoexpresspathexpressionsextractingsomenodesinanXMLstructure.Apathexpressioningeneralconsistsofa processRequestprocessinganincom-ingshoprequest.Therequestispassedtotheaddressedcart,whichreturnstheresultingresponse.NotethataglobalvariableallCartsisusedwhichgivesaccesstoallregis-teredCart-objects.1shopResponseprocessRequest(shopRequestrq) //proc

4 essRequestListing2:MethodprocessRequestF
essRequestListing2:MethodprocessRequestFromtheincomingrequestrqthemethodextractstheshoppingcartandassignsittovariablesc.Afterwardsthetargetedcartischosenfromthesetofavailablecartsall-Carts.Anestedconditionalstatementdifferentiatesthethreepossiblerequestsandcallsthesuitablemethods.Therequiredparametersaregatheredfromtherequest.InthemethodprocessRequestweuseXPathconstructsforaccessingXMLobjectcontent.Usingthepathnotations(4-8)itispossibletonavigatethroughXMLobjects.BecausetheXPathnodetestreturnsalistofresultingXMLobjects,wehavetoaccessthe®rstitemofthesequencewiththeXPathintegerpredicate(4-7).WedeterminethesizeofsuchalistusingmethodgetLength(6-8).TheimpactofourapproachonthehostlanguageJavaisanXMLconsistentextensionofthetypesystem.BecauseoftheoutstandingroleofXMLinthewebapplicationandwebservicesprogrammingworld,andmaybethewholesoftwaredevelopmentworldinthefuture,thisseemstobeaconsequentstep.Webelievethatthetrade-offsbetweentheextensionofanexistingprogramminglanguageononehandandtheapproa

5 chofde®ninganewprogramminglanguagearoun
chofde®ninganewprogramminglanguagearoundXMLontheotherareminimal.Insteadthebene®tsofusingalreadydevelopedcodearesigni®cant.3BasicDe®nitionsAsseeninthelastsectionXOBEallowsXMLsyntaxinexpressions,assignmentsandmethodparameters.DuringcompilationtheXOBEsystemveri®esthecorrectnessofthe assignmentintwosteps.Firstitdeterminesthetypesoftherightandlefthandsidesusingtypeinference.Secondly,thesubtyperelationshipoftheinferredtypesischeckedbyasubtypingalgorithm.InXOBEweformalizeandrepresenttypesasregularhedgeexpressionsrepresentingreg-ularhedgelanguages[BKMW01].Consequentlyaschemaisformalizedandrepresentedinternallybyaregularhedgegrammar. withaset with Anon-nullableexpressionisaregularhedgeexpressionwhichdoesnotcontaintheemptyhedge.Wede®netheregularhedgeexpressions,referredtoinshortasregularexpressions,similartothenotationusedin[W3C01a].De®nition2(regularhedgeexpression)Givenasetofterminalsymbols andaset ofnonterminalsymbols,theset theemptyset, thesimpletypes, thecomplextypes, theelements,

6 theKleenestaroperation.forall , , , . As
theKleenestaroperation.forall , , , . AsanexampleweformalizetheschemaSIF(AppendixA)introducedinthelastsectionasaregularhedgegrammaras with: , shopRequest shopResponse shoppingCart account add 1Thetwoconstraintsensureregularity. get request items article description andthesetofcomplextypes havetobedisjunct.Inthispaperweusedifferentfontstoseparateelementnamesfromcomplextypenames.Mixedcontentisformalizedasasimpletypestring.Wedealwithattributessimilartoelements,butwithaspeciallymarkedname,rewritingthemwithregularhedgeconstructors.Any-typesarerewrittentoaregularunionofalldeclaredelementtypesaswell.AsmentionedaboveXOBEinfersatcompiletimebothtypesoftherightandlefthandsidesofanassignment.Becauseallvariableshavetobedeclared,thetypeinferenceofvariablesissimple.InourexampleofListing1avariable )isusedasconcatenationoperationinregularexpression,weusethesemicolon( exampleXOBEhastochecktheso-called shoppingCart account integer where standsforthesubtyperelationship.Note,thatthename ontherighthandsid

7 estandsforthecomplextype .ThenameshopRes
estandsforthecomplextype .ThenameshopResponseonthelefthandsideisanelementname. leadingNames.Theoperationleadingnamesisde®nedasfollows.De®nition3(leadingnames)Givenaregularexpression theoperation returnsthesetofallleadingterminalsymbols: with , , Itisde®nedrecursivelyandreturnstheleadingsimpletypenamesandelementnames.Ifacomplextypenameoccurstheoperationusestheproductionde®nition.Foraregularconcatenationtheoperationneedsthe predicatetochecktheemptyhedgeinclusion. Foreachdeterminednamethealgorithmtriestoreducebothsidesoftheinequalitybythisname.Theresultingreducedinequalitiesaresimplerthanthestartinginequalityinthemajorityofcasesandcanbecheckedbyarecursiveapplicationofthealgorithm.Thealgorithmtracksalreadytreatedinequalitiesinasetofinequalities,whichisemptyinthebeginning.Thisensuresterminationifweencounterthesameinequalitylateron.Wedonotprovesubtypingdirectly,insteadwecalculatethesetofallpossiblereducedinequalitiesofthegiveninequality.Ifwereceiveatriviallyinconsistentinequalityweconc

8 ludethatthegiveninequalityisincorrect.In
ludethatthegiveninequalityisincorrect.Inallothercasesweassumethatthegiveninequalityholds.Thisisastandardproceedinginsubtypingalgorithmsofrecursivetypes.Therearetwodifferentresultsofourrecursivealgorithm.Firstthealgorithmrespondsfalseiftheinequalityinquestionistriviallyinconsistentusingthe De®nition4(subtypingalgorithm)Givenaset isde®nedbythefollowingpseudocode:boolprove(r if((isNullable(r)&&ÄisNullable(s)) )returnfalse;elsif((r s) partialDerivatives(n,r s);A:=A r s ) ) pd)ok:=ok&&(prove( ,A) prove( ,A));returnok; Antimirovintroducesso-calledpartialderivativesofregularexpressionstoexpressre-ducedregularexpressions.Apartialderivativereducesaregularexpressionbyagiventypenameorelementname.Inthehedgegrammarsettingwemodifypartialderivativesconcerningtypenamesandelementnames.Forexample,ifwehavetheregularexpres-sion integer request t request integer .Theresultisasetingeneral,becausewecangetmultiplederivativesforagivenregularexpression.Theelementsofthesetarepairscorrespondingtothetwodimens

9 ionsinaregularhedge,theparent-childdimen
ionsinaregularhedge,theparent-childdimensionandthesiblingdimension.Intheexamplethe®rstcomponentofthepairisthetypeofthecontentofelementaccount.Thesecondistheregularexpressionreducedbyelementaccount.AdditionallyAntimirovintroducesso-calledpartialderivativesofregularinequalitiestoexpressreducedinequalities.Inthehedgegrammarcasethesebecomemorecomplicated.Becausethepartialderivativesofregularexpressionsarepairswehavetoperformaset- Theorem5(subsetrelationonCartesianproduct)Givensomesets , ,..., ,..., thefollowingholds: .Aswecansee,thistheoremreducesthesubsetrelationonCartesianproductstoasubsetrelationonsets.Becausetypescanbeinterpretedassetsofvalues,wecanwritethefollowingrelationemergingafterreducingtheleftandrighthandsidesofaninequalitybytheleadingname with .Tothisrelationwecanapplythegiventheoremandreceive and and .Inthede®nitionofthepartialderivativesofregularinequalitieswecollectthedisjunctionsseparatelyinaset.Additionallywerewritetherelationusingtheregularexpressionop-eratorsregular

10 union insteadofsubsetonsets .De®nition6
union insteadofsubsetonsets .De®nition6(partialderivativesofregularinequality)Givenaregularinequality with andaterminalsymbol and proverecursively.Intheremainingsectionweapplyoursubtypingalgorithmtoasmallexample.Con-sidertheexamplewherewehavethetwotypes integer inthefollowing,forwhichwestartthesubtypingalgorithmwithanemptysetofinequalities .Inthe®rstrecursionthealgorithmcalculatesthesetofleadingnames description .Additionallytheset ofalreadyprocessedinequalitiesisenlargedto .Weget string andDe®nition6leadsto string string (1) account integer account integer and account integer forfurtherdescription.Forprovinginequality8thealgorithmcalculatesthesetsofleadingnames account .Againthesetofinequalitiesisenlargedto .Thefollowingpartialderivativesaregeneratedduringthealgorithm: integer integer (9) (10) integer (11) .(12)Itiseasytosee,thatinequality9holdsandinequalities10and11evaluatetofalse.Thelastinequality12istrue,becauseitisalreadyinoursetofalreadyprocessedinequalities 5ImplementationI

11 ssuesTheprevioussectionspresentedtherepr
ssuesTheprevioussectionspresentedtherepresentationoftypesinXOBE.WenowdescribeourXOBEimplementationarchitecture[Kra02],whichisshowninFigure2.Althoughit Java compiler Java transformation Type checking XOBE Java parser Java with DOMXOBE preprocessorXOBE programXML schemaFigure2:XOBEJavaArchitectureispossibletointegratethefunctionalityofXOBEintotheJavacompiler,wehavechosentoimplementXOBEasaJavapreprocessor.TheXOBEpreprocessorconsistsofthefollowingthreecomponents:1.TheXOBEparser,2.thetypecheckinganalysisand3.thetransformationtostandardJavacode.TheXOBEparserreadstheXOBEprogramandconvertstheXMLportionsofthepro-gramtoaninternalrepresentation.TheparserincludesinadditiontoastandardJavaparser aschemaparserandaslightlymodi®edXMLparser.TheschemaparserisnecessarytoscantheschemasimportedbytheXOBEprogram.TheXMLparserisneededtorecog-nizetheXMLconstructorsdistributedovertheprogramsourcecode.BecausetheXMLconstructorscanincludeXMLvariableswehavetomodifythestandardXMLparser.Inourimplementationweutilizet

12 heJavacompilercompilerJavaCC[Web02]togen
heJavacompilercompilerJavaCC[Web02]togeneratetheXOBEparser.AdditionallyweusetheXMLparserXerces[The01]torecognizetheusedschemas.TheinternalrepresentationoftheprocessedXOBEprogramisdonewiththeJavatreebuilderJTB[TWP00].Inthetypeanalysisphasethepreprocessordetermineswhethertheparsedprogramiswell-typedornot.Well-typedinXOBEmeansthattheprocessedXMLobjectsarevalidaccordingtothedeclaredschemas.At®rstthetypeanalysisphasevalidatestheimportedschemas.Afterwards,thetypecheckofJavaexpressionsusingXMLobjects,likeassign-mentsormethodcalls,isperformedaccordingtothedescriptionoftheprevioussection.ThetypeinferenceofXMLconstructorsandXMLvariablesisfollowedbysubtypingproofstoverifytheexpressions.BecausethetypesystemofstandardXMLisstrictandcanbeformalizedbyrestrictedregularhedgeexpressions,wecanusetheregularhedgegrammarbasedalgorithmtodecidetheequivalenceofregularexpressionsforthatpurpose.XMLSchemaweakensthestrictnessbyintroducingtypeextensionandtyperestriction,whichrequiresamoresophisticatedtypeinferenc

13 estrategy.Thedetaileddescriptionofthisex
estrategy.Thedetaileddescriptionofthisextendedalgorithmwillbeintroducedinthesis[Kem03].ThelasttaskthepreprocessorperformsisthetransformationoftheXOBEprogramintoJavasourcecode,whichisacceptedbythestandardJavacompiler.ForthisresultingJavacodeseveralimplementationalternativesexist,dependingontheXMLrepresentation.WechosethestandardrepresentationoftheDocumentObjectModel,orDOM[W3C98a],recommendedbytheW3Consortium.ThetransformationreplacestheXMLconstructorsandXPathexpressionsoftheXOBEprogramwithsuitableDOMcode.Theexacttrans-formationruleswillbepresentedin[Kem03].PleasenotethateventhoughDOMisanuntypedXMLimplementationnotguaranteeingstaticvalidity,thetransformedXMLob-jectsintheXOBEprogramarevalid.Thisholdsbecauseourtypecheckingalgorithmguaranteesthisproperty.InourimplementationthetransformationisperformedontheinternalJTBrepresentation,wherewereplacethesubtreesrepresentingXOBEconstructsbynewlycreatedsubtrees,whichrepresentthesuitableDOMcode. isasmallprogramwhichgeneratesXHTMLwebpagesforanestat

14 ebroker.RealestatesarestoredinanXML®lef
ebroker.RealestatesarestoredinanXML®lefollowingasmallnon-standardschema.These®lesaretakenasinputandconvertedintofree-standingXHTMLdocuments.Loginisaservletwhichrealizestheloginforanacademicexerciseadministrationsys-tem.Itrequestsloginandpasswordandpassestheinputtothesystem.MobileArchiveisaservlet-basedwebapplicationrealizingaWML-connectiontoamed-icalmediaarchive.Anavigationthroughthearchivestructuresimilartoa®lesys-temtreeispossible,inadditiontoamediasearch.Somemediaobjectsofspeci®cformatscanbeviewedaswell.The®rstapplicationiswrittenasseveralsimpleiterativemethods,performingastraight-forwardtraversaloftheinputtree.ThesecondapplicationcommunicatesoverJDBCwithanInformixdatabasemanagementsystem.TheclientinputispassedtothedatabasetherespondofwhichiswrappedintoXHTMLandsenttotheclient.ThelastapplicationisaWMLconnectiontoamediaarchive.ThemediaarchiveisaccessedthroughagivenAPI.Theapplicationmemorizesthepositionoftheactualclientinthestructureofthearchive.WeuseXHTML(morepreciseXHTML-trans

15 itional)asschemainprogramsEstateandLogin
itional)asschemainprogramsEstateandLoginandWMLinMobileArchive.ThelasttwoapplicationshavebeenmigratedfromastandardJavaservletimplementation.Manysillymistakesinthenon-XOBEimplementa-tionhavebeenfounddoingthistask.Despitecarefultestrunsofthepreviousimplemen-tation,manymistakeshavebeenoverlooked. linesofcode compiletime(s) executiontime(s)Application XOBE schema total subtyping standard XOBE Estate 158 1231 2.16 0.05 - 0.9 Login 195 1196 4.61 0.07 0.01 0.01 MobileArchive 1045 355 4.49 0.16 0.03 0.04InthetablethenumberoflinesofthewholeXOBEprogramsandthenumberoflinesoftheimportedschemasispresentedinthe®rsttwocolumns.InthesecondgroupofcolumnsthetableshowsthetimespentforprecompilingtheXOBEprogram.Itincludestheparsing,thetypeinference,thesubtypingalgorithmandthecodetransformationintostandardJava,asdescribedinSection5.Thetimespentduringthesubtypingalgorithmisshownincolumnªsubtypingº.ThethirdcolumngroupofthetablegivesanimpressionofhowtheperformanceoftheservletsisaffectedbytheDOM-basedimpleme

16 ntation.Thecolumnªstandardºshowsthetim
ntation.Thecolumnªstandardºshowsthetimewhichhaselapsedevaluatingthestandardnon-XOBEservletimplementation.ThelastcolumngivestherunningtimeoftheXOBEprogram.Asindicatedbythetableourtypecheckingalgorithmrunsatacceptablespeedfortheseapplications.EventheapplicationswhichusequitelargetypesfromtheXHTMLorWMLschemaarecompiledinencouragingtime.TheexecutionspeedofourDOM-basedservletimplementationsisslowerthanthestandardservletsasexpectedbutstillinaconvenientrange. 7RelatedWork CompiletimevalidationWeareawareofonlythreeapproacheswhicharereallycomparabletoXOBE.TheXDucelanguage[HVP00]isafunctionallanguagedevelopedasanXMLprocessinglanguage.Itintroducessocalledregularexpressiontypesthevaluesofwhicharecompa-rabletoourXMLobjects.Elementsarecreatedbyspeci®cconstructorsandthecontentcanbeaccessedthroughpatternmatching.XDucesupportstypeinferenceforpatternsandvariablesandperformsasubtypinganalysistoensurevalidityofregularexpressiontypeinstancesatcompiletime.BigWig[BMS01]isaprogramminglanguagefordevelopin

17 ginteractivewebservices.ItcompilesBigWig
ginteractivewebservices.ItcompilesBigWigsourcecodeintoacombinationofstandardwebtechnologies,likeHTML,CGI,applets,andJavaScript.TypedXMLdocumenttemplateswithgapsareintroduced.InordertogenerateXMLdocumentsdynamically,gapscanbesubstitutedatruntimebyothertemplatesorstrings.ForalltemplatesBigWigvalidatesalldynamicallycomputeddocumentsaccordingtoagivenDTD.Thisisdonebytwodata¯owanalysesconstructingagraphwhichsummarizesallpossibledocuments.Thisgraphisanalyzedtodeterminevalidityofthosedocuments.IncomparisontoourapproachtemplatescanbeseenasmethodsreturningXMLobjects.Theargumentsofthemethodscorrespondtothegapsofthetemplates.Anotherchallengingapproachispresentedbythelanguagespeci®cationofXL[FGK02].XLisanXMLprogramminglanguagefortheimplementationofwebservices.Incontrasttoourapproach,itisastand-aloneprogramminglanguage,i.e.itisnotanextensionofanexistinglanguagelikeJava.Itprovideshigh-levelanddeclarativeconstructsadopt-ingXQuery.AdditionallyimperativelanguagestatementsareintroducedmakingXLacombin

18 ationofanimperativeanddeclarativeprogram
ationofanimperativeanddeclarativeprogramminglanguage.AdditionallyitisworthnoticingthattheupcomingstandardofanXMLquerylanguageXQuery[W3C02]hastosupportstaticvalidityaswell. Evaluation Literaturverzeichnis[ABS00]SergeAbiteboul,PeterBuneman,andDanSuciu. http://www.rpbourret.com/xml/XMLDataBinding.htm,28.July2002.[Exo01]ExoLabGroup.Castor.ExoLabGroup,http://castor.exolab.org/,11De-cember2001.[FGK02]DanielaFlorescu,AndreasGrÈunhagen,andDonaldKossmann.XL:AnXMLProgram-mingLanguageforWebServiceSpeci®cationandComposition.InProceedingsof InternationalWorldWideWebConference(WWW2002),May7-11,Honolulu,Hawaii,USA http://www.jdom.org/docs/faq.html.[Kem03]MartinKempa.Schema-abhÈangigeProgrammierungvonXML-basiertenWeb-Anwendungen.PhDthesis,InstitutfÈurInformationssysteme,UniversitÈatzuLÈubeck,2003.toappear,ingerman.[KL01]MartinKempaandVolkerLinnemann.V-DOMandP-XML±TowardsAValidPro-grammingOfXML-basedApplications.InAkmalB.ChaudhriandAwaisRashid,ed-itors,OOPSLA'01WorkshoponObjects,XMLandDatabase

19 s,TambaBay,Florida,USA,October2001.[KL02
s,TambaBay,Florida,USA,October2001.[KL02]MartinKempaandVolkerLinnemann.OnXMLObjects.InInformalProceedingsoftheWorkshoponProgrammingLanguageTechnologiesforXML(PLAN-X2002),PLI2002,Pittsburgh,USA,pages44±54,3.-8.October2002.[Kra02]JensKramer.ErzeugunggarantiertgÈultigerServer-SeitenfÈurDokumentederExtensibleMarkupLanguageXML.Master'sthesis,InstitutfÈurInformationssysteme,UniversitÈatzuLÈubeck,2002.ingerman.[Lin81]VolkerLinnemann.Context-freeGrammarsandDerivationTreesinAlgol68.InPro-ceedingsInternationalConferenceonALGOL68,Amsterdam,pages167±182,1981.[LK02]VolkerLinnemannandMartinKempa.SprachenundWerkzeugezurGenerierungvonHTML-undXML-Dokumenten.InformatikSpektrum,25(5):349±358,2002.ingerman.[Mic01]MicrosoftCorporation..NETFrameworkDeveloper'sGuide.webdocument,http://msdn.microsoft.com/library/default.asp,2001.[Net97]NetscapeCommunicationsCorporation.JavaScript1.1LanguageSpeci®cation.http://www.netscape.com/eng/javascript/index.html,1997.[Ora01]OracleCorporation.Oracle9i,Applicat

20 ionDeveloper'sGuide-XML,Release1(9.0.1).
ionDeveloper'sGuide-XML,Release1(9.0.1).RedwoodCity,CA94065,USA,June2001.ShelleyHiggins,PartNumberA88894-01.[PLC99]EduardoPelegrÂi-LlopartandLarryCable.JavaServerPagesSpeci®cation,Version1.1.JavaSoftware,SunMicrosystems,http://java.sun.com/products/jsp/download.html,30.November1999.[Sun01]SunMicrosystems,Inc.TheJavaArchitectureforXMLBinding,UserGuide.http://www.sun.com,May2001.[The01]TheApacheXMLProject.XercesJavaParser.http://xml.apache.org/xerces-j/index.html,15.November2001.Version1.4.4.[TWP00]KevinTao,WanjunWang,andDr.JensPalsberg.JavaTreeBuilderJTB.http://www.cs.purdue.edu/jtb/,15.May2000.Version1.2.2.[W3C98a]W3Consortium.DocumentObjectModel(DOM)Level1Speci®cation,Version1.0.Recommendation,http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/,1.October1998. [W3C98b]W3Consortium.ExtensibleMarkupLanguage(XML)1.0.Recommendation, JavaServletsbyExample.ManningPublicationsCo.,Greenwich,1999.[WS92]L.WallandR.L.Schwartz.ProgrammingPerl.O'Reilly&Associates,Inc.,Sebastipol,California,199