英文文件及翻译Servlet及JSP技术简述AnOverviewofServletandJSPTechnologyNagle,WiegleyAbstract:Servletprogramrunningintheserver-side,dynamicallygeneratedWebpagewiththetraditionalCGIandmanyothersimilarcomparedtoCGItechnology,JavaServletwithamoreefficient,easiertouse,morepowerfulandhasbetterportability,moresavingstoinvest.Keywords:JSPTechnology;Servlet;HTTPserver1.AServletsJobServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowingtasks,asillustratedinFigure1-1.1.1Readtheexplicitdatasentbytheclient.TheendusernormallyentersthisdatainanHTMLformonaWebpage.However,thedatacouldalsocomefromanappletoracustomHTTPclientprogram.1.2ReadtheimplicitHTTPrequestdatasentbythebrowser.Figure1-1showsasinglearrowgoingfromtheclienttotheWebserver(thelayerwhereservletsandJSPexecute),buttherearereallytwovarietiesofdata:theexplicitdatathattheenduserentersinaformandthebehind-the-scenesHTTPinformation.Bothvarietiesarecritical.TheHTTPinformationincludescookies,informationaboutmediatypesandcompressionschemesthebrowserunderstands,andsoon.1.3Generatetheresults.Thisprocessmayrequiretalkingtoadatabase,executinganRMIorEJBcall,invokingaWebservice,orcomputingtheresponsedirectly.Yourrealdatamaybeinarelationaldatabase.Fine.ButyourdatabaseprobablydoesntspeakHTTPorreturnresultsinHTML,sotheWebbrowsercanttalkdirectlytothedatabase.Evenifitcould,forsecurityreasons,youprobablywouldnotwantitto.Thesameargumentappliestomostotherapplications.YouneedtheWebmiddlelayertoextracttheresultsinsideadocument.1.4Sendtheexplicitdata(i.e.,thedocument)totheclient.Thisdocumentcanbesentinavarietyofformats,includingtext(HTMLorXML),binary(GIFimages),orevenacompressedformatlikegzipthatislayeredontopofsomeotherunderlyingformat.But,HTMLisbyfarthemostcommonformat,soanimportantservlet/JSPtaskistowraptheresultsinsideofHTML.1.5SendtheimplicitHTTPresponsedata.Figure1-1showsasinglearrowgoingfromtheWebmiddlelayer(theservletorJSPpage)totheclient.But,therearereallytwovarietiesofdatasent:thedocumentitselfandthebehind-the-scenesHTTPinformation.Again,bothvarietiesarecriticaltoeffectivedevelopment.SendingHTTPresponsedatainvolvestellingthebrowserorotherclientwhattypeofdocumentisbeingreturned(e.g.,HTML),settingcookiesandcachingparameters,andothersuchtasks.2WhyBuildWebPagesDynamically?manyclientrequestscanbesatisfiedbyprebuiltdocuments,andtheserverwouldhandletheserequestswithoutinvokingservlets.Inmanycases,however,astaticresultisnotsufficient,andapageneedstobegeneratedforeachrequest.ThereareanumberofreasonswhyWebpagesneedtobebuilton-the-fly:2.1TheWebpageisbasedondatasentbytheclient.Forinstance,theresultspagefromsearchenginesandorder-confirmationpagesatonlinestoresarespecifictoparticularuserrequests.Youdontknowwhattodisplayuntilyoureadthedatathattheusersubmits.Justrememberthattheusersubmitstwokindsofdata:explicit(i.e.,HTMLformdata)andimplicit(i.e.,HTTPrequestheaders).Eitherkindofinputcanbeusedtobuildtheoutputpage.Inparticular,itisquitecommontobuildauser-specificpagebasedonacookievalue.2.2TheWebpageisderivedfromdatathatchangesfrequently.Ifthepagechangesforeveryrequest,thenyoucertainlyneedtobuildtheresponseatrequesttime.Ifitchangesonlyperiodically,however,youcoulddoittwoways:youcouldperiodicallybuildanewWebpageontheserver(independentlyofclientrequests),oryoucouldwaitandonlybuildthepagewhentheuserrequestsit.Therightapproachdependsonthesituation,butsometimesitismoreconvenienttodothelatter:waitfortheuserrequest.Forexample,aweatherreportornewsheadlinessitemightbuildthepagesdynamically,perhapsreturningapreviouslybuiltpageifthatpageisstilluptodate.2.3TheWebpageusesinformationfromcorporatedatabasesorotherserver-sidesources.Iftheinformationisinadatabase,youneedserver-sideprocessingeveniftheclientisusingdynamicWebcontentsuchasanapplet.Imagineusinganappletbyitselfforasearchenginesite:Downloading50terabyteapplet,pleasewait!Obviously,thatissilly;youneedtotalktothedatabase.GoingfromtheclienttotheWebtiertothedatabase(athree-tierapproach)insteadoffromanappletdirectlytoadatabase(atwo-tierapproach)providesincreasedflexibilityandsecuritywithlittleornoperformancepenalty.Afterall,thedatabasecallisusuallytherate-limitingstep,sogoingthroughtheWebserverdoesnotslowthingsdown.Infact,athree-tierapproachisoftenfasterbecausethemiddletiercanperformcachingandconnectionpooling.3TheAdvantagesofServletsOverTraditionalCGIJavaservletsaremoreefficient,safer,andcheaperthantraditionaleasiertouse,morepowerful,moreportable,CGIandmanyalternativeCGI-liketechnologies.3.1EfficientWithtraditionalCGI,anewprocessisstartedforeachHTTPrequest.IftheCGIprogramitselfisrelativelyshort,theoverheadofstartingtheprocesscandominatetheexecutiontime