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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.Figure4displaystheWebadaptationofMVC,alsocommonlyknownasMVCModel2orMVC2.
Strutsdetails
DisplayedinFigure6isastripped-downUMLdiagramoftheorg.apache.struts.actionpackage.Figure6showstheminimalrelationshipsamongActionServlet(Controller),ActionForm(FormState),andAction(ModelWrapper).
Figure6.UMLdiagramoftherelationshipoftheCommand(ActionServlet)totheModel(Action&
ActionForm)
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.
Figure4.MVCModel2
Struts,anMVC2implementation
Strutsisasetofcooperatingclasses,servlets,andJSPtagsthatmakeupareusableMVC2design.ThisdefinitionimpliesthatStrutsisaframework,ratherthanalibrary,butStrutsalsocontainsanextensivetaglibraryandutilityclassesthatworkindependentlyoftheframework.Figure5displaysanoverviewofStruts.
Figure5.Strutsoverview
Strutsoverview
∙Clientbrowser
AnHTTPrequestfromtheclientbrowsercreatesanevent.TheWebcontainerwillrespondwithanHTTPresponse.
TheControllerreceivestherequestfromthebrowser,andmakesthedecisionwheretosendtherequest.WithStruts,theControllerisacommanddesignpatternimplementedasaservlet.Thestruts-config.xmlfileconfigurestheController.
∙Businesslogic
Thebusinesslogicupdatesthestateofthemodelandhelpscontroltheflowoftheapplication.WithStrutsthisisdonewithanActionclassasathinwrappertotheactualbusinesslogic.
∙Modelstate
Themodelrepresentsthestateoftheapplication.Thebusinessobjectsupdatetheapplicationstate.ActionFormbeanrepresentstheModelstateatasessionorrequestlevel,andnotatapersistentlevel.TheJSPfilereadsinformationfromtheActionFormbeanusingJSPtags.
TheviewissimplyaJSPfile.Thereisnoflowlogic,nobusinesslogic,andnomodelinformation--justtags.TagsareoneofthethingsthatmakeStrutsuniquecomparedtootherframeworkslikeVelocity.
Note:
"
Thinkthin"
whenextendingtheActionclass.TheActionclassshouldcontroltheflowandnotthelogicoftheapplication.ByplacingthebusinesslogicinaseparatepackageorEJB,weallowflexibilityandreuse.
AnotherwayofthinkingaboutActionclassisastheAdapterdesignpattern.ThepurposeoftheActionisto"
Converttheinterfaceofaclassintoanotherinterfacetheclientsexpect.Adapterletsclassesworktogetherthatcouldn_totherwisebecauseofincompatibilityinterface"
(fromDesignPatterns-ElementsofReusableOOSoftwarebyGof).TheclientinthisinstanceistheActionServletthatknowsnothingaboutourspecificbusinessclassinterface.Therefore,Strutsprovidesabusinessinterfaceitdoesunderstand,Action.ByextendingtheAction,wemakeourbusinessinterfacecompatiblewithStrutsbusinessinterface.(AninterestingobservationisthatActionisaclassandnotaninterface.