网络工程专业毕业外文翻译.docx

网络工程专业毕业外文翻译.docx

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网络工程专业毕业外文翻译

XX大学

毕业设计（论文）说明书

英文翻译

专业

网络工程

班级

网X

姓名

X

学号

X

指导教师

X

二O一五年六月十七日

外文原文

出自《IBMSystemJournal.2014》作者：

MalcolmDavis

Struts——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.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.

Controller

TheControllerreceivestherequestfromthebrowser,andmakesthedecisionwheretosendtherequest.WithStruts,theControllerisacommanddesignpatternimplementedasaservlet.Thestruts-config.xmlfileconfigurestheController.

Businesslogic

Thebusinesslogicupdatesthestateofthemodelandhelpscontroltheflowoftheapplication.WithStrutsthisisdonewithanActionclassasathinwrappertotheactualbusinesslogic.

Modelstate

Themodelrepresentsthestateoftheapplication.Thebusinessobjectsupdatetheapplicationstate.ActionFormbeanrepresentstheModelstateatasessionorrequestlevel,andnotatapersistentlevel.TheJSPfilereadsinformationfromtheActionFormbeanusingJSPtags.

View

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.（