外文翻译 - JSP技术概述与应用框架

0英文文献OverviewofJSPTechnologyandJSPapplicationframeworks1.BenefitsofJSPJSPpagesaretranslatedintoservlets.So,fundamentally,anytaskJSPpagescanperformcouldalsobeaccomplishedbyservlets.However,thisunderlyingequivalencedoesnotmeanthatservletsandJSPpagesareequallyappropriateinallscenarios.Theissueisnotthepowerofthetechnology,itistheconvenience,productivity,andmaintainabilityofoneortheother.Afterall,anythingyoucandoonaparticularcomputerplatformintheJavaprogramminglanguageyoucouldalsodoinassemblylanguage.Butitstillmatterswhichyouchoose.JSPprovidesthefollowingbenefitsoverservletsalone:ItiseasiertowriteandmaintaintheHTML.YourstaticcodeisordinaryHTML:noextrabackslashes,nodoublequotes,andnolurkingJavasyntax.YoucanusestandardWeb-sitedevelopmenttools.EvenHTMLtoolsthatknownothingaboutJSPcanbeusedbecausetheysimplyignoretheJSPtags.Youcandivideupyourdevelopmentteam.TheJavaprogrammerscanworkonthedynamiccode.TheWebdeveloperscanconcentrateonthepresentationlayer.Onlargeprojects,thisdivisionisveryimportant.Dependingonthesizeofyourteamandthecomplexityofyourproject,youcanenforceaweakerorstrongerseparationbetweenthestaticHTMLanddynamiccontentonaweakseparation(weakerseparation)andstrongseparation(strongerseparation).Inthiscase,thisdiscussiondoesnotallowyoutostopusingservlets,usingonlyJSP.Almostalloftheprojectswillbeusedbothtechnologiessimultaneously.Somerequestsfortheproject,youmaybeusedincombinationintheMVCframeworkofthesetwotechnologies.Wealwayswanttousethepropertoolstocompletethejobthatcorrespondsmerelyservletdoesnotfillyourtoolbox.2.JSPadvantagerelativetocompetingtechnologies1Manyyearsago,Martywasinvitedtoparticipateinasoftwaretechnologyrelatedtosmall(20people)seminartodonextinMartyJamesGosling-whoistheinventoroftheJavaprogramminglanguage.Everyfewpositions,aseniormanagerfromWashington,alargesoftwarecompany.Duringthediscussion,theChairmanoftheseminartopicspresentedJini,whichatthetimewasanewJavatechnology.Chairmanaskedhisthoughtstothemanagerandhewentontosaythattheywillcontinuetofocusthistechnology,Ifthistechnologybecomespopular,theywillfollowtheacceptandexpansion(embraceandextend)strategyofthecompanyatthistime,Goslingcasuallyinterjected,Youmeaninfactthatisnotacceptableanddoesnotexpand(disgraceanddistend).Here,Goslingcomplaintshowsthathefeltthatthecompanywilltakethetechnologyfromwhereothercompaniesfortheirownpurposes,buthowdoyouthinkthiskind?Theshoesontheotherfoot.Javacommunitydidnotinventthisidea-thepagedesignedbythestaticanddynamicHTMLcodelabelmarkedwithaspecialblendcomposition.ColdFusionyearsago,hasalreadydoneso.(Aproductfromthepreviousmanagerscompany)evenbeforetheJSPASPhaveappearedtopromotethisapproach.Infact,JSPnotonlyadoptedthisgeneralconcept,itevenusesthesamelotandASPspeciallabel.Sothequestionbecomes:WhyuseJSP,withouttheuseofothertechnologiesitourfirstreactionisthatwearenotarguingthatallpeopleshoulddo?.Theseothertechnologies,therearesomeverygood,indeedinsomecasesitisreasonablechoice,however,inothercases,JSPsignificantlybetter.Hereareafewreasons.2.1comparedwith.NETandActiveServerPages(ASP).NETisatechnologythatMicrosoftdesigned.ASP.NETisindirectcompetitionwithservletsandJSPtechnologies.TheadvantagesofJSPreflectedintwoaspects.First,JSPcanbeportedtomultipleoperatingsystemsandWebservers,youdonotjustlimitedtothedeploymenteventhoughthecore.NETplatformcanrunonseveralnon-WindowsplatformsonWindowsandIIS,butnotthispartofASP.YoucannotexpecttobeabletodeployanASP.NETapplicationtoavarietyofimportantserversandoperatingsystems.Forsomeapplications,thisdifferencehasnoeffect.However,someapplications,thisdifferenceisveryimportant.Secondly,theselectionofcertainapplications,theunderlyinglanguageis2essential.Forexample,although.NETC#languagewelldesigned,andthelike,andJava,butfewarefamiliarwiththecoreC#syntaxandnumeroustoolslibraryprogrammers.Inaddition,manydevelopersstillusetheoriginalversionofASP.Relativetothisversion,JSPhasaclearadvantageintermsofdynamiccode.UseJSP,dynamicpartiswritteninJava,ratherthanaproprietaryASPVBScriptlanguageovertheother,soJSPstronger,moresuitableforcomplexapplicationsrequiredcomponentreuse.WhentheJSPcomparisonwithpreviousversionsofColdFusion,youmaygetthesameconclusion.ApplicationJSP,youcanuseJavatowriterealcode,donotdependonaspecificserverproducts.However,thecurrentversionofColdFusionmeettheJ2EEserverenvironmentthatallowsdeveloperstoeasilymixColdFusionandServlet/JSPcode.ComparedwithPHP2.2PHP(PHP:HypertextPreprocessorisarecursiveacronym)isafree,opensource,HTMLembeddedscriptinglanguage,similartoASPandJSPhassomedegree.OneadvantageofthedynamicpartoftheJSPiswritteninJava,andJavaalreadyinnetworking,databaseaccessareas,suchasdistributedobjecthasanextensiveAPI,andPHPneedtolearnanewapplicationofarelativelywiderangeoflanguages.ThesecondadvantageisthatJSPandPHPcompared,JSPhasaverywiderangeofsupporttoolsandserverproviders.ComparedwithpureServlet2.3Inprinciple,JSPServletcannotprovidecompletefunctionality.Infact,JSPdocumentsareautomaticallyinthebackgroundConvertedintoaServlet.Butthewriting(andmodify)theconventionalHTML,HTMLgenerationthanmanyprintlnstatementismuchmoreconvenient.Inaddition,byseparatingthesaidcontent,youcanassigndifferentpeoplefordifferenttasks:WebdesignerstousefamiliartoolstobuildHTML,orleavespacefortheServletprogrammerstoinsertdynamiccontent,dynamiccontentorindirectlybycallingtheXMLtags.DoesthismeanthatyoucanjustlearnJSP,Servletwillbethrownintothesideofit?Ofcoursenot!Asthefollowingfourreasons,JSPdevelopersneedtounderstandServlet:(1)JSPpagesareconvertedintoServlet.UnderstandJSPServletcannotknowhowtowork.3(2)JSPfromstaticHTML,JSPtagsandJava-specificcode.WhattypeofJavacode?OfcourseServletcode!IfyoudonotunderstandtheServletprogramming,thenyoucannotwritethiscode.(3)tocompletesometasksbetterbyusingJSPServletcompletedratio.JSPpagesgeneratedbyalargenumberofgoodorganizedHTMLorotherstructureddataconsistingofcharacter.Servletspecializesgeneratebinarydata,buildstructuresanddiversepages,andtaskexecutionoutputwithlittleornooutput(suchasredirection).(4)SometasksaremoresuitableforcombinationwithServletandJSPtocomplete,ratherthanaloneServletorJSP.ComparedwithJavaScript2.4JavaScriptandJavaprogramminglanguageistotallydifferent,theformerisgenerallyusedtodynamicallygenerateHTMLontheclient,tobuildpartofthecontentofthepagewhenthebrowserloadsthedocument.Thisisausefulfeature,generallyoverlapwithJSPfunction(onlyrunsontheserverside)doesnotoccur.LikeconventionalHTMLpages,JSPpagescanstillincludeaSCRIPTtagforJavaScript.Infact,JSPcanbeusedtodynamicallygenerateevensenttotheclientJavaScript.Thus,JavaScriptisnotacompetingtechnology,whichisacomplementarytechnology.JavaScriptcanalsobeusedontheserverside,themostattentionbecausepeopleareSUNONE(formerlytheiPlanet),IIS,andBroadVisionserver.However,Javaismorepowerful,flexible,reliableandportable.ComparedwithWebMacroandVelocity2.5JSPbynomeansperfect.ManypeoplehavepointedouttheJSPcanbeimprovedfeatures.Thisisagoodthing,oneoftheadvantagesisthattheJSPspecificationbymanydifferentcompaniescontrolledsocieties.Therefore,insubsequentversions,thistechnologycanbecoordinatedimprovements.However,someorganizationshavedevelopedaJava-basedalternatives,tryingtomakeupforthesedeficiencies.Accordingtoourjudgment,thisiswrong.JSPandServlettechnologiestoexpandtheuseofthird-partytoolssuchasApacheStructs,isagoodidea,aslongasthetooltobringthebenefitsofcompensationtoolscanbringadditionalcomplexity.4However,insteadofattemptingtouseanon-standardtoolforJSPisnotideal.Whenselectingatechnology,manyaspectsneedtoweighfactors:standardization,portability,integration,industrysupportandtechnicalcharacteristics.JSPalternativetechnologiesfordebatealmostexclusivelyfocusedonthetechnicalcharacteristics,andportability,standardizationandintegrationisalsoveryimportant.Forexample,ServletandJSPspecificationdefinesastandardWebapplicationdirectorystructure,andprovideastandardfile(.Warfile)fordeployingWebapplications.AllJSP-compliantservermustsupportthesestandards.WecancreateafiltereffecttoanytreesServletandJSPpages,butnotfornon-standardresources.Webapplicationsecuritysettingsarethesame.Inaddition,theindustrytremendoussupportJSPandServlettechnologiestomakethesetwotechnologieshavemadetremendousprogress,thusreducingmanyofthecriticismsoftheJSP.Forexample,JSPStandardTagLibraryandJSP2.0expressionlanguagetosolve.Two,themostwidespreadcriticism:thelackofgooditerativestructure;withoutusingexplicitJavacodeorlengthyjsp:useBeanelementisdifficulttoaccessdynamicresults.Third,themisunderstandingoftheJSPForgetJSPtechnologyistheserver-sidetechnologyHerearesometypicalquestionsMartyreceived(mostoftheproblemsoccurmorethanonce).OurserverisrunningJDK1.4.HowdoIuseSwingcomponentsJSPpages?HowdoIputanimageJSPpage?IdonotknowwhattoreadimagefilesshouldJavaI/Ocommands.TomcatdoesnotsupportJavaScript,whentheusermovesthemouseovertheimage,howdoImaketheimageshighlightit?OurcustomersdonotunderstandtheJSPusingolderbrowsers.HowshouldIdo?SourceWhen(viewsourcecode),andhowtostopthemseeJSPtags?WhenourcustomersusethebrowserViewAllofthesequestionsarebasedonthebrowsertotheserverintheprocessofunderstandingtheassumptions.Butinfactbrowserdoesnotunderstandtheprocessontheserverside.Therefore:5IfyouwanttouseSwingcomponentsappletonaWebpage,itisimportantthattheJavaversionofthebrowser,andJavaversionsoftheserver-independent.IfyourbrowsersupportsJava2platform,youcanusethenormalAPPLET(orJavaplug-in)label,evenwithnon-Javatechnologyontheservershouldalsobetrue.YoudonotneedJavaI/Otoreadimagefiles,youcansimplystoretheimageonthecatalogWebresources(ieWEB-INF/classesdirectorylevelsup),andoutputsanormalIMGtag.YoushouldusetheSCRIPTtag,useclient-sideJavaScripttocreatetheimageunderthemousewillchange,andthiswillnotchangebecausetheserverusingJSP.BrowserdoesnotsupportJSP-theyseeonlytheoutputoftheJSPpage.Thus,astreatedasstaticHTMLpages,justmakesuretheoutputJSPandHTMLcompatiblebrowser.Ofcourse,youdonotneedtodoanythingtopreventcustomersseeJSPtagsthatareprocessedontheserverandsenttothecustomersoutputdoesnotappear.ConfusionduringthetransitionperiodandrequestJSPpageneedstobeconvertedintoaservlet.ServletAftercompiling,loadingthecontentserver,initializeandexecute.Butwhathappensineachstepwhen?Toanswerthisquestion,wemustrememberthefollowingpoints:JSPpagesonlywhenfirstaccessedaftermodification,willbeconvertedintoaservletandcompiled;loadedintomemory,initializationandexecutionfollowthegeneralrulesoftheservlet.Themostcommonlymisunderstoodtermhashighlightedmarked.Whenreferringtothetable,weshouldnotethattheservletgeneratedbytheJSPpagesusing_jspServicemethods(GETandPOSTrequestsarecallingthefunction),notthedoGetordoPostmethods.Similarly,forinitialization,jspInitmethodstheyuse,insteadoftheinitmethod.TheJSPpageistranslatedintoaservletcompiledServletServletloadedintoservermemorywillcalljspInitcall_jspService.Whatistheapplicationframework:Framework(framework)isareusable,semi-finishedapplicationthatcanbeusedto6producespecializedcustomprograms.Likepeople,softwareapplicationstobemoresimilaritiesthandifferences.Theyrunonsimilarmachines,thedesiredinputinformationfromthesamedevice,adisplayoutputtothesamedevice,anddatastoredintheharddiskdevicetothesame.Developmentoftraditionaldesktopapplicationdevelopersmoreaccustomedtotheidentityofthosewhocancoverapplicationdevelopmenttoolkitanddevelopmentenvironment.Frameworkapplicationframeworkbasedonthesepubliccanprovidedeveloperscanprovidereusableinfrastructureservicesfortheirproducts.Skeletonframeworkprovidesarangeofcomponentswiththefollowingcharacteristicstodevelopers:Theyhavebeenknowntoworkwellonotherprograms;Theycanalwaysusethenextproject;Theycanbeusedbyotherteamorganization;Theframeisatypicalbuildorbuyproposition.Ifyoubuildit,whenyouredoneyoullunderstandit,butbeforeyouspendmuchtimeinturnbeintegratedintoit?Ifyouwanttobuy,youhavetoovercomethelearningcurve,too,beforeyoucanuseitforworkandhavetospendmuchtime?Thereisnorightanswerhere,butmanyobserverswouldagreethatsuchaframeworklikeStrutsthandevelopedfromscratchtoprovideamoresignificantreturnoninvestment,especiallyforlargeprojects.Othertypesofframes:Notonlytheconceptualframeworkcanalsobeusedforapplicationcomponents.InotherdatawhichwehaveintroducedanumberofStrutsframeworkcanbeusedtogether.TheseincludetheLucenesearchengine,Scaffoldtoolkit,StrutsValidatorandTilestaglibrary.Andapplicationframework,thesetoolsalsoprovidesomesemi-finishedversion,canbeusedincustomcomponentsamongusers.Someframeworksarelimitedtospecificdevelopmentenvironment.Strutsandcomponentsinvolvedinthisarticleisnotso.YoucaninmanyenvironmentstodevelopStruts:VisualAgeforJava,JBuilder,Eclipse,Emacs,andevenuseTextpad.Foryourtools,ifyoucanusetodevelopJava,youcanuseittodevelopStruts.Techniquesused:7UseStrutsapplicationdevelopmentusingalotofotherbasictechniques.ThesetechniquesarenotspecifictoStruts,butforallJavawebapplicationcanuse.DevelopersusingStrutsframeworkliketohideinareassuchasHTTP,CGI,JSPandtediousdetailslikethetechnologybehind.AsaStrutsdeveloper,youdonotneedtoknowalltherelevantknowledge,butthesebasictechnologyworksmayhelpyoudesignathornyissueforcreativesolutions.HypertextTransferProtocol(HTTP):Whenmediationbetweenthetwocountries,diplomatsalwaysfollowaformalagreement.Diplomaticprotocolismainlydesignedtoavoidmisunderstandingsandpreventthebreakdownofnegotiations.Similarly,whentheneedfordialoguebetweencomputers,theyalsofollowaformalAgreement.Thisprotocoldefineshowdataistransmitted,andhowtodecodeaftertheyarrive.WebapplicationsthatusetheHTTPprotocoltotransferdatabetweenthecomputerandruntheserverprogramrunningthebrowser.ManyserverapplicationsusingotherprotocolsotherthanHTTP.Theymaintainapersistentconnectionbetweencomputers.Applicationservercanknowwhoisconnectedup,andwhentodisconnect.Becausetheyknowthateverystateoftheconnection,aswellaseverypersonusingit.Thisiscalledstatusagreement.Instead,HTTPisastatelessprotocol.HTTPServercanacceptavarietyofrequestsfromavarietyofcustomers,andprovideavarietyofresponse,eveniftheresponseisjusttosayNo.Notalotofnegotiationandconnectionpersistence,statelessprotocolcanhandlealargenumberofrequests.ThisistheInternetcanbeextendedtomanyreasonsacomputer.HTTPbecometheuniversalstandardbecauseofitssimplicity.HTTPrequestlookslikeanormaltextdocument.ThismakestheapplicationveryeasytocreateHTTPrequests.YoucaneventhroughthestandardproceduressuchasTelnettomanuallypassaHTTPrequest.WhentheHTTPresponseisreturned,itisalsoadevelopercandirectlyreadtheflattext.ThefirstlinecontainstheHTTPrequestmethod,followedbythesourceaddressandtheHTTPversionoftherequest.HTTPrequestheadersinthefirstrowbehindnow,cannotandtherecanbemultiple.HTTPheadersprovideadditionalinformationtotheserver.May8includethetypeandversionofthebrowser,acceptabledocumenttype,browsercookies,andsoon.Sevenkindsofrequestmethods,GETandPOSTarethemostused.Oncetheserverreceivestherequest,itisnecessarytogenerateaHTTPresponse.Thefirstlineofresponseiscalledthestatusline,includingtheHTTPprotocolversion,numericstate,andabriefdescriptionofthestate.Afterthestatusline,theserverwillreturnanHTTPresponseheader,similartotheHTTPrequestheader.Asmentionedabove,HTTPdoesnotmaintainstateinformationbetweenrequests.Theserveracceptstherequest,aresponse,andcontinuehappilyhandletextrequests.Becauseitissimpleandefficient,statelessprotocolisnotsuitablefortheneedtotracktheusersstateofdynamicapplications.CookiesandURLrewritingistrackinguserrequestsbetweenthetwostatesintheway.Acookieisaspecialkindofinformationpacketsstoredintheuserscomputer.URLrewritingisstoredinthepageaddressaspecialmark,Javaservercanuseittotrackusers.Thesetwomethodsarenotseamless,whichisusedinthedevelopmentofameanextraworkshouldbecarriedout.Onitsown,thestandardHTTPwebserverdoesnottransmitdynamiccontent.Itismostlyusedtolocatedocumentsrequestedresourceandreturnsthisresourceintheresponse.HeresthefilenormallyuseHypertextMarkupLanguage(HTML)W3C,HTMLformat,sothatthebrowsercandisplaythem.HTMLpagestypicallycontainhypertextconnectionstootherpages,youcanalsodisplaysomeothercontentsuchasimagesandvideo.Usersclickingonthelinkwillhaveanotherrequest,theybeginanewprocess.Standardwebserverhandlesstaticcontenthandlingitwell,butwhendealingwithdynamiccontentyouneedextrahelpmeansit.Definition:Staticcontentdirectlyfromthetextordatafiles,suchasHTMLorJPEGfiles.Thesefilescanbechangedatanytime,butbythetimethebrowserrequests,butcannotchangeautomatically.Instead,thedynamiccontentistemporarilygenerated,typically,itisaresponsetoindividualrequestsofthebrowser.CommonGatewayInterface(CGI).:ThefirstwidelyusedtogeneratedynamiccontentstandardCommonGatewayInterface(CommonGatewayInterface(CGI).CGIusingstandardoperatingsystemfeatures,suchas9environmentvariablesandstandardinputandoutput,andcreateabridgebetweenthegatewaybetweentheWebserverandthehostsystem.Otherprogramscanbeseenpassingoverthewebserverrequests,andcreateacustomizedresponse.WhenawebserverreceivesarequestforaCGIprogram,itwillruntheprogramandprovideinformationwhichitrequestsincluded.CGIprogramrunsandreturnstheoutputtotheWebserver,webserverresponseisoutputtothebrowser.CGIdefinesasetofinformationaboutwhatispassedasenvironmentvariables,andhowitwantstousethestandardinputandoutputpractice.LikewithHTTP,CGIisflexibleandeasytoimplement,andhasalargenumberofexistingCGIprograms.ThemaindrawbackisthatitmustCGIrequesttorunaprogramforeach.Thisisarelativelyexpensiveprocessingmethodforhigh-capacitysite,therearethousandsofrequestsperminute,itispossibletomakethecrashsite.AnotherdrawbackisplatformdependentCGIprograms,programsdevelopedononeplatformmaynotbeabletorunonanotherplatform.Javaservlet:SunsJavaServletplatformdirectlyaddressesthetwomaindisadvantagesofCGIprograms:First,servletprovidesbetterperformanceandresourceutilizationthanconventionalCGIprograms.Secondly,oncewritten,JAVAservletfeaturesmeanttorunanywhereintheroomwithJVMoperatingsystemislightweightmovable.Servletlookslikeatinywebserver.Itacceptstherequestandgeneratearesponse.However,differentfromconventionalwebserver,servletAPIisspecificallydesignedtohelpJavadeveloperstocreatedynamicapplications.ServletitselfistobecompiledintoJavabytecodeclass,justlikeanyotherJavaobjects.HTTPServletaccessspecificservicesAPI,butit