中北大学毕业设计英文文献及中文翻译.docx
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中北大学毕业设计英文文献及中文翻译.docx
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中北大学毕业设计英文文献及中文翻译
毕业设计说明书
英文文献及中文翻译
学生姓名:
学号南社区0906064109
学院:
电子与计算机科学技术学院
专业:
网络工程
指导教师:
刘爽英
2018年6月
AnOverviewofServletandJSPTechnology
GildasAvoineandPhilippeOechslin
EPFL,Lausanne,Switzerland
1.1AServlet'sJob
ServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowing
tasks,asillustratedinFigure1-1b5E2RGbCAP
WebServer
(ServletsJSP)
Figure1-1
1.Readtheexplicitdatasentbytheclient.
TheendusernormallyentersthisdatainanHTMLformonaWebpage.However,thedatacouldalsocomefromanappletoracustomHTTPclientprogramEanqFDPw
2.ReadtheimplicitHTTPrequestdatasentbythebrowserXDiTa9E3d
Figure1-1showsasinglearrowgoingfromtheclienttotheWebserver(thelayerwhereservletsandJSPexecute〉,buttherearereallytwovarietiesofdata:
theexplicitdatathattheenduserentersinaformandthebehind-the-scenesHTTPinformation.Bothvarietiesarecritical.TheHTTPinformationincludescookies,informationaboutmediatypesandcompressionschemesthebrowserunderstands,andscRTCTpUDGiT
3.Generatetheresults.
Thisprocessmayrequiretalkingtoadatabase,executinganRMIorEJBcall,invokingaWebservice,orcomputingtheresponsedirectly.Yourrealdatamaybeinarelationaldatabase.Fine.Butyourdatabaseprobablydoesn'tspeakHTTPorreturnresultsinHTML,sotheWebbrowsercan'ttalkdirectlytothedatabase.Evenifitcould,forsecurityreasons,youprobablywouldnotwantitto.Thesameargumentappliestomostotherapplications.YouneedtheWebmiddlelayertoextracttheincomingdatafromtheHTTPstream,talktotheapplication,andembedtheresultsinsideadocument5PCzVD7HxA
4.Sendtheexplicitdata(i.e.,thedocument>totheclient-BHrnAiLg
Thisdocumentcanbesentinavarietyofformats,includingtext(HTMLorXML>,binary(GIFimages>,orevenacompressedformatlikegzipthatislayeredontopofsomeotherunderlyingformat.But,HTMLisbyfarthemostcommonformat,soanimportantservlet/JSPtaskistowraptheresultsinsideofHTMLHAQX74J0X
5.SendtheimplicitHTTPresponsedata.
Figure1-1showsasinglearrowgoingfromtheWebmiddlelayer(theservletorJSPpage>totheclient.But,therearereallytwovarietiesofdatasent:
thedocumentitselfandthebehind-the-scenesHTTPinformation.Again,bothvarietiesarecriticaltoeffectivedevelopment.SendingHTTPresponsedatainvoIvestellingthebrowserorotherclientwhattypeofdocumentisbeingreturned(e.g.,HTML>,settingcookiesandcachingparameters,andothersuchtaskLDAYtRyKfE
1.2WhyBuildWebPagesDynamically?
manyclientrequestscanbesatisfiedbyprebuiltdocuments,andtheserverwouldhandletheserequestswithoutinvokingservlets.Inmanycases,however,astaticresultisnotsufficient,andapageneedstobegeneratedforeachrequest.ThereareanumberofreasonswhyWebpagesneedtobebuilton-the-fZyz6ZB2Ltk
1.TheWebpageisbasedondatasentbythecliedvZfvkwMi1
Forinstanee,theresultspagefromsearchenginesandorder-confirmationpagesatonlinestoresarespecifictoparticularuserrequests.Youdon'tknowwhattodisplayuntilyoureadthedatathattheusersubmits.Justrememberthattheusersubmitstwokindsofdata:
explicit(i.e.,HTMLformdata>andimplicit(i.e.,HTTPrequestheaders>.Eitherkindofinputcanbeusedtobuildtheoutputpage.Inparticular,itisquitecommontobuildauser-specificpagebasedonacookievalue.ni4ZNxi
2.TheWebpageisderivedfromdatathatchangesfrequentfy.xvxotoc。
Ifthepagechangesforeveryrequest,thenyoucertainlyneedtobuildtheresponseatrequesttime.Ifitchangesonlyperiodically,however,youcoulddoittwoways:
youcouldperiodicallybuildanewWebpageontheserver(independentlyofclientrequests>,oryoucouldwaitandonlybuildthepagewhentheuserrequestsit.Therightapproachdependsonthesituation,butsometimesitismoreconvenienttodothelatter:
waitfortheuserrequest.Forexample,aweatherreportornewsheadlinessitemightbuildthepagesdynamically,perhapsreturningapreviouslybuiltpageifthatpageisstilluptodatesixE2yxpq5
3.TheWebpageusesinformationfromcorporatedatabasesorotherserver-sideSourceS6ewMyirQFL
Iftheinformationisinadatabase,youneedserver-sideprocessingeveniftheclientisusingdynamicWebcontentsuchasanapplet.Imagineusinganappletbyitselfforasearchenginesite:
avU42VRUs"Downloading50terabyteapplet,pleasewait!
"Obviously,thatissilly。
youneedtotalktothedatabase.GoingfromtheclienttotheWebtiertothedatabase(athree-tierapproach>insteadoffromanappletdirectlytoadatabase(atwo-tierapproach>providesincreasedflexibilityandsecuritywithlittleornoperformaneepenalty.Afterall,thedatabasecallisusuallytherate-limitingstep,sogoingthroughtheWebserverdoesnotslowthingsdown.Infact,athree-tierapproachisoftenfasterbecausethemiddletiercanperformcachingandconnectionpooling6v3ALoS89
Inprinciple,servletsarenotrestrictedtoWeborapplicationserversthathandleHTTPrequestsbutcanbeusedforothertypesofserversaswell.Forexample,servletscouldbeembeddedinFTPormailserverstoextendtheirfunctionality.And,aservletAPIforSIP(SessionInitiationProtocol>serverswasrecentlystandardized(seehttp:
//jcp.org/en/jsr/detail?
id=116>.Inpractice,however,thisuseofservletshasnotcaughton,andwe'llonlybediscussingHTTPservlet^^2ub6vSTnP
1.3TheAdvantagesofServletsOver"Traditional"CGIoYujefmuew
Javaservletsaremoreefficient,easiertouse,morepowerful,moreportable,safer,andcheaperthantraditionalCGIandmanyalternativeCGI-liketechnologieseuts8ZQVRd
1.Efficient
WithtraditionalCGI,anewprocessisstartedforeachHTTPrequest.IftheCGIprogramitselfisrelativelyshort,theoverheadofstartingtheprocesscandominatetheexecutiontime.Withservlets,theJavavirtualmachinestaysrunningandhandleseachrequestwithalightweightJavathread,notaheavyweightoperatingsystemprocess.Similarly,intraditionalCGI,ifthereareNrequeststothesameCGIprogram,thecodefortheCGIprogramisloadedintomemoryNtimes.Withservlets,however,therewouldbeNthreads,butonlyasinglecopyoftheservletclasswouldbeloaded.Thisapproachreducesservermemoryrequirementsandsavestimebyinstantiatingfewerobjects.Finally,whenaCGIprogramfinisheshandlingarequest,theprogramterminates.Thisapproachmakesitdifficulttocachecomputations,keepdatabaseconnectionsopen,andperformotheroptimizationsthatrelyonpersistentdata.Servlets,however,remaininmemoryevenaftertheycompletearesponse,soitisstraightforwardtostorearbitrarilycomplexdatabetweenclientrequestsQsAEjkW5T
2.Convenient
ServletshaveanextensiveinfrastructureforautomaticallyparsinganddecodingHTMLformdata,readingandsettingHTTPheaders,handlingcookies,trackingsessions,andmanyothersuchhigh-levelutilities.InCGI,youhavetodomuchofthisyourself.Besides,ifyoualreadyknowtheJavaprogrammingIanguage,whylearnPerltoo?
You'realreadyconvincedthatJavatechnologymakesformorereliableandreusablecodethandoesVisualBasic,VBScript,orC++.WhygobacktothoseIanguagesforserver-sideprogrammingGMsIasNXkA
3.Powerful
ServletssupportseveralcapabilitiesthataredifficultorimpossibletoaccomplishwithregularCGI.ServletscantalkdirectlytotheWebserver,whereasregularCGIprogramscannot,atleastnotwithoutusingaserver-specificAPI.CommunicatingwiththeWebservermakesiteasiertotranslaterelativeURLsintoconcretepathnames,forinstanee.Multipleservletscanalsosharedata,makingiteasytoimplementdatabaseconnectionpoolingandsimilarresource-sharingoptimizations.Servletscanalsomaintaininformationfromrequesttorequest,simplifyingtechniqueslikesessiontrackingandcachingofpreviouscomputationsirRGchYzg
4.Portable
ServletsarewrittenintheJavaprogrammingIanguageandfollowastandardAPI.ServletsaresupporteddirectlyorbyapluginonvirtuallyeverymajorWebserver.Consequently,servletswrittenfor,say,MacromediaJRuncanrunvirtuallyunchangedonApacheTomcat,MicrosoftInternetInformationServer(withaseparateplugin>,IBMWebSphere,iPlanetEnterpriseServer,Oracle9iAS,orStarNineWebStar.TheyarepartoftheJava2Platform,EnterpriseEdition,soindustrysupportforservletsisbecomingevenmorepervasiv&.EqzcWLZNx
5.Inexpensive
AnumberoffreeorveryinexpensiveWebserversaregoodfordevelopmentuseordeploymentoflow-ormedium-volumeWebsites.Thus,withservletsandJSPyoucanstartwithafreeorinexpensiveserverandmigratetomoreexpensiveserverswithhigh-performancecapabilitiesoradvaneedadministrationutilitiesonlyafteryourprojectmeetsinitialsuccess.ThisisincontrasttomanyoftheotherCGIalternatives,whichrequireasignificantinitialinvestmentforthepurchaseofaproprietarypackage!
zq7IGf02E
Priceandportabilityaresomewhatconnected.Forexample,Martytriestokeeptrackofthecountriesofreadersthatsendhimquestionsbyemail.Indiawasnearthetopofthelist,probably#2behindtheU.S.MartyalsotaughtoneofhisJSPandservlettrainingcourses(seeManila,andtherewasgreatinterestinservletandJSPtechnolo
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