mvc 的一种开放源码实现外文翻译本科学位论文.docx
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mvc 的一种开放源码实现外文翻译本科学位论文.docx
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mvc的一种开放源码实现外文翻译本科学位论文
A.1英文原文
anopen-sourceMVCimplementation
ThisarticleintroducesStruts,aModel-View-ControllerimplementationthatusesservletsandJavaServerPages(JSP)technology.StrutscanhelpyoucontrolchangeinyourWebprojectandpromotespecialization.EvenifyouneverimplementasystemwithStruts,youmaygetsomeideasforyourfutureservletsandJSPpageimplementation.
Introduction
KidsingradeschoolputHTMLpagesontheInternet.However,thereisamonumentaldifferencebetweenagradeschoolpageandaprofessionallydevelopedWebsite.Thepagedesigner(orHTMLdeveloper)mustunderstandcolors,thecustomer,productflow,pagelayout,browsercompatibility,imagecreation,JavaScript,andmore.Puttingagreatlookingsitetogethertakesalotofwork,andmostJavadevelopersaremoreinterestedincreatingagreatlookingobjectinterfacethanauserinterface.JavaServerPages(JSP)technologyprovidesthegluebetweenthepagedesignerandtheJavadeveloper.
Ifyouhaveworkedonalarge-scaleWebapplication,youunderstandthetermchange.Model-View-Controller(MVC)isadesignpatternputtogethertohelpcontrolchange.MVCdecouplesinterfacefrombusinesslogicanddata.StrutsisanMVCimplementationthatusesServlets2.2andJSP1.1tags,fromtheJ2EEspecifications,aspartoftheimplementation.YoumayneverimplementasystemwithStruts,butlookingatStrutsmaygiveyousomeideasonyourfutureServletsandJSPimplementations.
Model-View-Controller(MVC)
JSPtagssolvedonlypartofourproblem.Westillhaveissueswithvalidation,flowcontrol,andupdatingthestateoftheapplication.ThisiswhereMVCcomestotherescue.MVChelpsresolvesomeoftheissueswiththesinglemoduleapproachbydividingtheproblemintothreecategories:
Model
Themodelcontainsthecoreoftheapplication'sfunctionality.Themodelencapsulatesthestateoftheapplication.Sometimestheonlyfunctionalityitcontainsisstate.Itknowsnothingaboutthevieworcontroller.
View
Theviewprovidesthepresentationofthemodel.Itisthelookoftheapplication.Theviewcanaccessthemodelgetters,butithasnoknowledgeofthesetters.Inaddition,itknowsnothingaboutthecontroller.Theviewshouldbenotifiedwhenchangestothemodeloccur.
Controller
Thecontrollerreactstotheuserinput.Itcreatesandsetsthemodel.
MVCModel2
TheWebbroughtsomeuniquechallengestosoftwaredevelopers,mostnotablythestatelessconnectionbetweentheclientandtheserver.Thisstatelessbehaviormadeitdifficultforthemodeltonotifytheviewofchanges.OntheWeb,thebrowserhastore-querytheservertodiscovermodificationtothestateoftheapplication.
Anothernoticeablechangeisthattheviewusesdifferenttechnologyforimplementationthanthemodelorcontroller.Ofcourse,wecoulduseJava(orPERL,C/C++orwhatever)codetogenerateHTML.Thereareseveraldisadvantagestothatapproach:
Javaprogrammersshoulddevelopservices,notHTML.
Changestolayoutwouldrequirechangestocode.
Customersoftheserviceshouldbeabletocreatepagestomeettheirspecificneeds.
Thepagedesignerisn'tabletohavedirectinvolvementinpagedevelopment.
HTMLembeddedintocodeisugly.
FortheWeb,theclassicalformofMVCneededtochange.Figure1displaystheWebadaptationofMVC,alsocommonlyknownasMVCModel2orMVC2.
Figure1.MVCModel2
Struts,anMVC2implementation
Strutsisasetofcooperatingclasses,servlets,andJSPtagsthatmakeupareusableMVC2design.ThisdefinitionimpliesthatStrutsisaframework,ratherthanalibrary,butStrutsalsocontainsanextensivetaglibraryandutilityclassesthatworkindependentlyoftheframework.Figure2displaysanoverviewofStruts.
Figure2.Strutsview
Strutsoverview
Clientbrowser
AnHTTPrequestfromtheclientbrowsercreatesanevent.TheWebcontainerwillrespondwithanHTTPresponse.
Controller
TheControllerreceivestherequestfromthebrowser,andmakesthedecisionwheretosendtherequest.WithStruts,theControllerisacommanddesignpatternimplementedasaservlet.Thestruts-config.xmlfileconfigurestheController.
Businesslogic
Thebusinesslogicupdatesthestateofthemodelandhelpscontroltheflowoftheapplication.WithStrutsthisisdonewithanActionclassasathinwrappertotheactualbusinesslogic.
Modelstate
Themodelrepresentsthestateoftheapplication.Thebusinessobjectsupdatetheapplicationstate.ActionFormbeanrepresentstheModelstateatasessionorrequestlevel,andnotatapersistentlevel.TheJSPfilereadsinformationfromtheActionFormbeanusingJSPtags.
View
TheviewissimplyaJSPfile.Thereisnoflowlogic,nobusinesslogic,andnomodelinformation--justtags.TagsareoneofthethingsthatmakeStrutsuniquecomparedtootherframeworkslikeVelocity.
Strutsdetails
DisplayedinFigure3isastripped-downUMLdiagramoftheorg.apache.struts.actionpackage.Figure6showstheminimalrelationshipsamongActionServlet(Controller),ActionForm(FormState),andAction(ModelWrapper).
Figure3:
therelationshipbetweenActionServlet(Controller)、ActionForm(FormState)andAction(ModelWrapper)
TheActionServletclass
Doyourememberthedaysoffunctionmappings?
Youwouldmapsomeinputeventtoapointertoafunction.Ifyouwhereslick,youwouldplacetheconfigurationinformationintoafileandloadthefileatruntime.FunctionpointerarrayswerethegoodolddaysofstructuredprogramminginC.
LifeisbetternowthatwehaveJavatechnology,XML,J2EE,andallthat.TheStrutsControllerisaservletthatmapsevents(aneventgenerallybeinganHTTPpost)toclasses.Andguesswhat--theControllerusesaconfigurationfilesoyoudon_thavetohard-codethevalues.Lifechanges,butstaysthesame.
ActionServletistheCommandpartoftheMVCimplementationandisthecoreoftheFramework.ActionServlet(Command)createsandusesAction,anActionForm,andActionForward.Asmentionedearlier,thestruts-config.xmlfileconfigurestheCommand.DuringthecreationoftheWebproject,ActionandActionFormareextendedtosolvethespecificproblemspace.Thefilestruts-config.xmlinstructsActionServletonhowtousetheextendedclasses.Thereareseveraladvantagestothisapproach:
Theentirelogicalflowoftheapplicationisinahierarchicaltextfile.Thismakesiteasiertoviewandunderstand,especiallywithlargeapplications.
ThepagedesignerdoesnothavetowadethroughJavacodetounderstandtheflowoftheapplication.
TheJavadeveloperdoesnotneedtorecompilecodewhenmakingflowchanges.
CommandfunctionalitycanbeaddedbyextendingActionServlet.
TheActionFormclass
ActionFormmaintainsthesessionstatefortheWebapplication.ActionFormisanabstractclassthatissub-classedforeachinputformmodel.WhenIsayinputformmodel,IamsayingActionFormrepresentsageneralconceptofdatathatissetorupdatedbyaHTMLform.Forinstance,youmayhaveaUserActionFormthatissetbyanHTMLForm.TheStrutsframeworkwill:
ChecktoseeifaUserActionFormexists;ifnot,itwillcreateaninstanceoftheclass.
StrutswillsetthestateoftheUserActionFormusingcorrespondingfieldsfromtheHttpServletRequest.Nomoredreadfulrequest.getParameter()calls.Forinstance,theStrutsframeworkwilltakefnamefromrequeststreamandcallUserActionForm.setFname().
TheStrutsframeworkupdatesthestateoftheUserActionFormbeforepassingittothebusinesswrapperUserAction.
BeforepassingittotheActionclass,Strutswillalsoconductformstatevalidationbycallingthevalidation()methodonUserActionForm.Note:
Thisisnotalwayswisetodo.TheremightbewaysofusingUserActionForminotherpagesorbusinessobjects,wherethevalidationmightbedifferent.ValidationofthestatemightbebetterintheUserActionclass.
TheUserActionFormcanbemaintainedatasessionlevel.
Notes:
Thestruts-config.xmlfilecontrolswhichHTMLformrequestmapstowhichActionForm.
MultiplerequestscanbemappedUserActionForm.
UserActionFormcanbemappedovermultiplepagesforthingssuchaswizards.
TheActionclass
TheActionclassisawrapperaroundthebusinesslogic.ThepurposeofActionclassistotranslatetheHttpServletRequesttothebusinesslogic.TouseAction,subclassandoverwritetheprocess()method.
TheActionServlet(Command)passestheparameterizedclassestoActionFormusingtheperform()method.Again,nomoredreadfulrequest.getParameter()calls.Bythetimetheeventgetshere,theinputformdata(orHTMLformdata)hasalreadybeentranslatedoutoftherequeststreamandintoanActionFormclass.
Note:
"Thinkthin"whenextendingtheActionclass.TheActionclassshouldcontroltheflowandnotthelogicoftheapplication.ByplacingthebusinesslogicinaseparatepackageorEJB,weallowflexibilityandreuse.
AnotherwayofthinkingaboutActionclassisastheAdapterdesignpattern.ThepurposeoftheActionisto"Converttheinterfaceofaclassintoanotherinterfacetheclientsexpect.Adapterletsclassesworktogetherthatcouldn_totherwisebecauseofincompatibilityinterface"(fromDesignPatterns-ElementsofReusableOOSoftwarebyGof).TheclientinthisinstanceistheActionServletthatknowsnothingaboutourspecificbusinessclassinterface.Therefore,Strutsprovidesabusinessinterfaceitdoesunderstand,Action.ByextendingtheAction,wemakeourbusinessinterfacecompatiblewithStrutsbusinessinterface.(AninterestingobservationisthatActionisaclassandnotaninterface.Actionstartedasaninterfaceandchangedintoaclassovertime.Nothin
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