信息科学与工程学院毕业设计科技文献翻译Word下载.docx

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专业计算机科学与技术　

指导教师李双双　　　

2018年3月

Struts——AnOpen-sourceMVCImplementation

ThisarticleintroducesStruts,aModel-View-ControllerimplementationthatusesservletsandJavaServerPages（JSP）technology.StrutscanhelpyoucontrolchangeinyourWebprojectandpromotespecialization.EvenifyouneverimplementasystemwithStruts,youmaygetsomeideasforyourfutureservletsandJSPpageimplementation

1.Introduction

Ifyouhaveworkedonalarge-scaleWebapplication,youunderstandthetermchange.Model-View-Controller（MVC）isadesignpatternputtogethertohelpcontrolchange.MVCdecouplesinterfacefrombusinesslogicanddata.StrutsisanMVCimplementationthatusesServlets2.2andJSP1.1tags,fromtheJ2EEspecifications,aspartoftheimplementation.YoumayneverimplementasystemwithStruts,butlookingatStrutsmaygiveyousomeideasonyourfutureServletsandJSPimplementations.

2.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.

3.StrutsSummary

3.1Strutsdetails

DisplayedinFigure1isastripped-downUMLdiagramoftheorg.apache.Struts.actionpackage.Figure6showstheminimalrelationshipsamongActionServlet（Controller）,ActionForm（FormState）,andAction（ModelWrapper）.

Figure1UMLdiagramoftherelationshipoftheCommand（ActionServlet）totheModel（Action&

ActionForm）

3.1.1TheActionServletclass

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:

1）ThepagedesignerdoesnothavetowadethroughJavacodetounderstandtheflowoftheapplication.

2）TheJavadeveloperdoesnotneedtorecompilecodewhenmakingflowchanges.

3）CommandfunctionalitycanbeaddedbyextendingActionServlet.

3.1.2TheActionFormclass

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.

3.1.3TheActionclass

TheActionclassisawrapperaroundthebusinesslogic.ThepurposeofActionclassistotranslatetheHttpServletRequesttothebusinesslogic.TouseAction,subclassandoverwritetheprocess（）method.

TheActionServlet（Command）passestheparameterizedclassestoActionFormusingtheperform（）method.Again,nomoredreadfulrequest.getParameter（）calls.Bythetimetheeventgetshere,theinputformdata（orHTMLformdata）hasalreadybeentranslatedoutoftherequeststreamandintoanActionFormclass.

3.1.4TheErrorclasses

ActionErrorencapsulatesanindividualerrormessage.ActionErrorsisacontainerofActionErrorclassesthattheViewcanaccessusingtags.ActionErrorsisStrutswayofkeepingupwithalistoferrors.

Figure2UMLdiagramoftherelationshipoftheCommand（ActionServlet）totheModel（Action）

3.1.5TheActionMappingclass

AnincomingeventisnormallyintheformofanHTTPrequest,whichtheservletContainerturnsintoanHttpServletRequest.TheControllerlooksattheincomingeventanddispatchestherequesttoanActionclass.TheStruts-config.xmldetermineswhatActionclasstheControllercalls.TheStruts-config.xmlconfigurationinformationistranslatedintoasetofActionMapping,whichareputintocontainerofActionMappings.（Ifyouhavenotnoticedit,classesthatendwithsarecontainers）

TheActionMappingcontainstheknowledgeofhowaspecificeventmapstospecificActions.TheActionServlet（Command）passestheActionMappingtotheActionclassviatheperform（）method.ThisallowsActiontoaccesstheinformationtocontrolflow.

3.2Strutspros

3.2.1UseofJSPtagmechanism

ThetagfeaturepromotesreusablecodeandabstractsJavacodefromtheJSPfile.ThisfeatureallowsniceintegrationintoJSP-baseddevelopmenttoolsthatallowauthoringwithtags.

3.2.2Taglibrary

Whyre-inventthewheel,orataglibrary?

Ifyoucannotfindsomethingyouneedinthelibrary,contribute.Inaddition,StrutsprovidesastartingpointifyouarelearningJSPtagtechnology.

3.2.3Opensource

Youhavealltheadvantagesofopensource,suchasbeingabletoseethecodeandhavingeveryoneelseusingthelibraryreviewingthecode.Manyeyesmakeforgreatcodereview.

3.2.4SampleMVCimplementation

StrutsofferssomeinsightifyouwanttocreateyourownMVCimplementation.

3.2.5Managetheproblemspace

Divideandconquerisanicewayofsolvingtheproblemandmakingtheproblemmanageable.Ofcourse,theswordcutsbothways.Theproblemismorecomplexandneedsmoremanagement.

3.3Strutscons

3.3.1Youth

Strutsdevelopmentisstillinpreliminaryform.Theyareworkingtowardreleasingaversion1.0,butaswithany1.0version,itdoesnotprovideallthebellsandwhistles.

3.3.2Change

Theframeworkisundergoingarapidamountofchange.AgreatdealofchangehasoccurredbetweenStruts0.5and1.0.YoumaywanttodownloadthemostcurrentStrutsnightlydistributions,toavoiddeprecatedmethods.Inthelast6months,IhaveseentheStrutslibrarygrowfrom90Ktoover270K.IhadtomodifymyexamplesseveraltimesbecauseofchangesinStruts,andIamnotgoingtoguaranteemyexampleswillworkwiththeversionofStrutsyoudownload.

3.3.3Correctlevelofabstraction

DoesStrutsprovidethecorrectlevelofabstraction?

Whatistheproperlevelofabstractionforthepagedesigner?

Thatisthe$64Kquestion.ShouldweallowapagedesigneraccesstoJavacodeinpagedevelopment?

SomeframeworkslikeVelocitysayno,andprovideyetanotherlanguagetolearnforWebdevelopment.ThereissomevaliditytolimitingJavacodeaccessinUIdevelopment.Mostimportantly,giveapagedesigneralittlebitofJava,andhewillusealotofJava.IsawthishappenallthetimeinMicrosoftASPdevelopment.InASPdevelopment,youweresupposedtocreateCOMobjectsandthenwritealittleASPscripttoglueitalltogether.Instead,theASPdeveloperswouldgocrazywithASPscript.

3.3.4Limitedscope

StrutsisaWeb-basedMVCsolutionthatismeantbeimplementedwithHTML,JSPfiles,andservlets.

3.3.5J2EEapplicationsupport

StrutsrequiresaservletcontainerthatsupportsJSP1.1andServlet2.2specifications.Thisalonewillnotsolveallyourinstallissues,unlessyouareusingTomcat3.2.IhavehadagreatdealofproblemsinstallingthelibrarywithNetscapeiPlanet6.0,whichissupposedlythefirstJ2EE-compliantapplicationserver.IrecommendvisitingtheStrutsUserMailingListarchive（seeResources）whenyourunintoproblems.

3.3.6Complexity

Separatingtheproblemintopartsintroducescomplexity.ThereisnoquestionthatsomeeducationwillhavetogoontounderstandStruts.Withtheconstantchangesoccurring,thiscanbefrustratingattimes.WelcometotheWeb.

4.FutureofStruts

Thingschangerapidlyinthisnewageofsoftwaredevelopment.Inlessthan5years,Ihaveseenthingsgofromcgi/perl,toISAPI/NSAPI,toASPwithVB,andnowJavaandJ2EE.SunisworkinghardtoadaptchangestotheJSP/servletarchitecture,justastheyhaveinthepastwiththeJavalanguageandAPI.