MVC外文翻译(C4-1)Word格式.doc
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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
lClientbrowser
AnHTTPrequestfromtheclientbrowsercreatesanevent.TheWebcontainerwillrespondwithanHTTPresponse.
lController
TheControllerreceivestherequestfromthebrowser,andmakesthedecisionwheretosendtherequest.WithStruts,theControllerisacommanddesignpatternimplementedasaservlet.Thestruts-config.xmlfileconfigurestheController.
lBusinesslogic
Thebusinesslogicupdatesthestateofthemodelandhelpscontroltheflowoftheapplication.WithStrutsthisisdonewithanActionclassasathinwrappertotheactualbusinesslogic.
lModelstate
Themodelrepresentsthestateoftheapplication.Thebusinessobjectsupdatetheapplicationstate.ActionFormbeanrepresentstheModelstateatasessionorrequestlevel,andnotatapersistentlevel.TheJSPfilereadsinformationfromtheActionFormbeanusingJSPtags.
lView
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.Actionstartedasaninterfaceandchan