513计算机专业App应用HTML设计的毕业设计论文英文英语外文文献翻译成品面向对象Web应用程序开发中英文双语.docx

513计算机专业App应用HTML设计的毕业设计论文英文英语外文文献翻译成品面向对象Web应用程序开发中英文双语.docx

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513计算机专业App应用HTML设计的毕业设计论文英文英语外文文献翻译成品面向对象Web应用程序开发中英文双语

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OBJECT-ORIENTEDWEBAPPLICATIONDEVELOPMENT

TheWebhasevolvedintoaglobalenvironmentfordeliveringallkindsofapplications,rangingfromsmall-scaleandshort-livedservicestolarge-scale,enterpriseworkflowsystemsdistributedovermanyservers.ApplicationsthatuseHTML-basedfrontendsbenefitfromthepervasivedistributionofWebbrowsersforuniversal,cross-platformaccess.AnotherstrikingadvantageofWebdeliveryliesintheconceptofthinclientsandcentralizedmaintenance,facilitatinginstantaneousdeploymentofsoftwareupdatesatminimalcost.WhilethepopularityoftheWebanditsadvantagesasaclient-serverplatformhaveledtocountlessHTML-basedapplications,thedevelopmentofWebapplicationsisstillmostlyadhoc.Thereisnorigorous,systematicapproach,andmostcurrentWebapplicationdevelopmentandmanagementpracticesrelyontheknowledgeandexperienceofindividualdevelopers.

OnereasonforthelackofastructuredapproachmaybeintheWeb’slegacyasaninformationmediumratherthananapplicationplatform.Webdevelopmentisseenprimarilyasanauthoringproblemratherthanasoftwaredevelopmentproblemtowhichwell-establishedsoftwareengineeringprinciplesshouldapply.

WebelieveanotherreasonisthattheWebimplementationmodeldoesnotrelatewelltostate-of-the-artsoftwaredevelopmentmodels.Webimplementationisbasedonlow-leveltechnologiesthatdonotprovidehigh-levelabstractionsforsharingandreuse.Thelackofsuitableabstractionsmakesitdifficulttoconstructframeworksthatcapturearchitecturaldesigndecisionsforreuseindifferentpartsofanapplicationorindifferentapplicationprojects.ThelackofabstractionalsomakesitdifficulttomaintainandevolveWeb-basedapplications.Designdecisionsareveryhardtotrackinalow-levelimplementation;astheapplicationevolves,changescaneasilyleadtoinconsistencies.BecauseWeb-basedapplicationstendtoevolvequickly,withfrequentupdatesandredesigns,poormaintainabilityisacriticalproblem.

InthisarticlewesummarizeworkreportedearlieronWebComposition,1amodelforWebapplicationdevelopment,thenintroducetheWeb-CompositionMarkupLanguage—anXML-basedlanguagethatimplementsthemodel.WCMLembodiesobject-orientedprinciplessuchasmodularity,abstraction,andencapsulation.ItfacilitatesthedescriptionofhigherlevelconceptsandframeworksforreusethatcanbridgethegapwecurrentlyperceiveinWebapplicationdevelopmentbetweenhigh-leveldesignandlow-levelimplementation.WebeginwithadiscussionofwhythebasicWebimplementationmodeldoesnotworkasadevelopmentmodelforWebapplications.Relatedworkispresentedinthesidebar,“StructuredWebApplicationDevelopment,”onpage62.

WEBAPPLICATIONDEVELOPMENTPROCESS

WedefineaWebapplicationasanysoftwareapplicationthatdependsontheWebforitscorrectexecution.Obviously,softwareexplicitlydesignedfordeliveryovertheWebfallsunderthisdefinition—forexample,WebsitesorWeb-basedjournals.Thesekindsofsoftwarearecharacterizedbyastrongnotionofcontent.

WealsoincludesoftwarethatusestheWebinfrastructureforitsexecution.Forexample,manyinformationsystemsthatweredesignedandbuiltpriortotheWebarenowmadeavailableasWebapplicationsthroughtheuseofbrowsers.Beyondlegacyinformationsystems,thereisalsosoftwarethatislesstypicalfordeliveryovertheWebbutstilldependentonitastheclient-serverplatformforitsexecution.Tworecentlyreportedexamplesaddressedthecoordinationofdistributedapplica-tions2andremotemonitoring.

GeneralSoftwareDevelopmentConventionalsoftwareprocessesusuallyaddressfourdevelopmentphases:

analysis,design,implementation,andmaintenance/evolution.Duringanalysis,developersbuildamodelofanapplicationintermsofthedomain.Ideally,thisanalysisconcentratesontheapplication’sproblemspace—separatefromsoftwareconsiderations,whicharepartofthesolutionspace.Accordingly,thisphaseshouldnotbeaffectedbywhetherornottheapplicationistobedeliveredovertheWeb.Basedontheanalysis,amodelofthesoftwaresolutionisdefinedduringthedesignphase.Obviously,theWebapplicationdevelopmentprocess—incontrasttoageneralsoftwareprocess—assumesaWeb-basedsoftwaresolutionandmustaccommodatethis.

Theimplementationtransformsthedesignintoactualsoftware.ForWebapplications,thisphasehastorelyonavailableWebimplementationtechnology.Finally,maintenanceisconcernedwithmodificationstothesoftware,whichmayoccurattheimplementation,design,orevenanalysislevels.GeneralsoftwareprocessesarehardtorelatetotheWebimplementationmodel,aswillbediscussedbelow.TheWebimplementationmodelisbasedonflatdecompositionofapplicationsintoresources.Resourceshaveauniqueaddress,andtheyaredeliveredonrequestfromWebservertoWebclient.Theycanbestaticordynamicallygeneratedfromascript,buttheyareinherentlyspecific,whichmeansthattheycannotcaptureabstractions.

DocumentDeliveryApplications

TheWorldWideWebwasoriginallydesignedasaninformationmediumfordistributedresearchteams.4Akeyobjectivewastomakeitaseasyaspossibleforauthorstodeliverdocuments,andthenotionofWebapplicationdevelopmentessentiallyboileddowntodocumentdevelopmentbyanauthororsmallgroupofauthors.Forthiskindofdevelopment,thelifecyclecomprisesinformalanalysisofwhatistobepresented,informaldesignofhowtostructureitintohyper-linkedchunksofinformation,andimplementationthroughmarkup.Followingimplementation,thedocumentsaremaintainedbytheauthorsthemselves.

TheWebimplementationmodelwasdesignedtomeettheselife-cyclerequirements.Itisdeliberatelysimple,basedonthenotionofresourcesthatmodelmostlyself-containedchunksofinformation.Resourcesareauthoredandmaintainedratherindependentlyofotherresources,andlinksarethemeansbywhichresourcescanbecombinedintocoherentsetsofdocuments—forexample,aWebsite.

Theresourceconceptfitsthedocument-developmentlifecycleverywell,andresourcessupporttheprinciplesofmodularityinthiscontext.How-ever,theuseoftheWebhasmovedfarbeyonditsoriginalscope,evenindocumentdelivery.Ithasbecomeatoolforhigh-endpublishingwithcomplexrequirementsrelatedtolayout,corporateidentity,andtheintegrityoflargewebsofinformation.ThelifecycleofacompanyWebsiteisnowtypicallybasedonintensiverequirementsanalysisintermsofcontent,structure,access,andcorporateidentity.Thedesignmustdecomposetheserequirementsintoresourcesandhyperlinks,andalsoaddresslayout.Maintenancemustfocusontheevlutionofcontent,butalsoonsiteintegrity.

Thelifecycleisnolongerauthor-centric.Requirementsanalysisinvolves,forexample,informationanalystsaswellasmarketingpeopleandotherstakeholders.Designaddressesbothdatabasedevelopmentandgraphicpresentation.ImplementationrequiresWebprogrammerstousefeaturesbeyondsimplemarkup,andmaintenanceinvolvessitemanagersandWebmasters.

TheresourcenotionunderlyingtheWebimplementationmodeldoesnotmeettherequirementsofthiskindoflifecycle.Mostnotably,thereisnoseparationofconcernsregardingcontentandlay-out.Further,theWebimplementationmodeldoesnotprovideabstractionstocapturestructuraldesignforreuse,eventhoughlayoutandnavigationstructuresarecommonlyreusedindifferentpartsofasite.

LifeCycleofGeneralApplicationsDeliveredovertheWeb

NordoestheresourcemodeladdressthelifecycleofgeneralapplicationsdeliveredovertheWeb.Decomposingapplicationsintoresourcesisnotlogical.Theresourcemodelrequiresseparateconcernssuchasuserinterfaces,applicationlogic,and—forexample—databaseback-endstobeembeddedinanintermingledway.

Becauseoftherequest-responsestyleprotocolbetweenclientandserver,Webapplicationsarestructured,ineffect,asfinitestatemachines（FSM）:

Thenodescorrespondtoresourcesandthetransitionsleadingawayfromanodecorrespondtohyperlinksorformelementswithinthenode.Thisdistributestheapplicationlogicoveranumberofresourcesinchunksofscriptcode,indocument-embeddedlinks,andinformelements.Contentismixedwithapplicationlogicandtypicallyembeddedinscriptcodethatimplementstheapplicationlogic.Further,theuserinterfaceisdeclarativeandspecifiedasdocumentstoberenderedinstandardbrowsers.AWebapplicationtypicallygeneratestheuserinterfacedeclarationdynamicallytoreflectapplicationandinteractionstate.

Insummary,thedeliveryofapplicationsintheWebenvironmentisradicallydifferentfromtheusualwaysofdeliveringsoftware,andimposesacompletelydifferentstructureandapproachonapplicationdevelopment.Obviously,designandmaintenanceofsuchapplicationsshouldbeintermsofuserinterface,applicationlogic,anddata-basebackendratherthanresources.CurrentWebimplementationtechnologyistoolowleveltosup-portaproperdevelopmentprocess.

CaseStudyinWebApplicationDevelopment

Toillustratesomeproblemswe’vefoundtobetypicalofWebapplicationdevelopment,wewilldescribeasmallpartofatravelassistantsystemthatwedevelopedoverthepastyear.Theapplicationintegratestravelbookingandroutingsystemsforintermodaltravelplanning.TheWebwaschosenastheintegrationplatformtocapitalizeonalreadyexistingtravelinformationsystemsandonHTMLbrowsersasameansofglobalsystemaccess.

Onerequirementwa