英文文献及翻译Servlet和JSP技术简述.docx
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英文文献及翻译Servlet和JSP技术简述
AnOverviewofServletandJSPTechnology
Nagle,Wiegley
Abstract:
Servletprogramrunningintheserver-side,dynamicallygeneratedWebpagewiththetraditionalCGIandmanyothersimilarcomparedtoCGItechnology,JavaServletwithamoreefficient,easiertouse,morepowerfulandhasbetterportability,moresavingstoinvest..
Keywords:
JSPTechnology;Servlet;HTTPserver
1.AServlet'sJob
ServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowingtasks,asillustratedinFigure1-1.
Figure1-1 Webmiddlewarerole
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.Butyourdatabaseprobablydoesn'tspeakHTTPorreturnresultsinHTML,sotheWebbrowsercan'ttalkdirectlytothedatabase.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.Youdon'tknowwhattodisplayuntilyoureadthedatathattheusersubmits.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.
Inprinciple,servletsarenotrestrictedtoWeborapplicationserversthathandleHTTPrequestsbutcanbeusedforothertypesofserversaswell.Forexample,servletscouldbeembeddedinFTPormailserverstoextendtheirfunctionality.And,aservletAPIforSIP(SessionInitiationProtocol)serverswasrecentlystandardized(seehttp:
//jcp.org/en/jsr/detail?
id=116).Inpractice,however,thisuseofservletshasnotcaughton,andwe'llonlybediscussingHTTPservlets.
3TheAdvantagesofServletsOver"Traditional"CGI
Javaservletsaremoreefficient,easiertouse,morepowerful,moreportable,safer,andcheaperthantraditionalCGIandmanyalternativeCGI-liketechnologies.
3.1Efficient
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,soitisstraightforwardtostorearbitrarilycomplexdatabetweenclientrequests.
3.2Convenient
ServletshaveanextensiveinfrastructureforautomaticallyparsinganddecodingHTMLformdata,readingandsettingHTTPheaders,handlingcookies,trackingsessions,andmanyothersuchhigh-levelutilities.InCGI,youhavetodomuchofthisyourself.Besides,ifyoualreadyknowtheJavaprogramminglanguage,whylearnPerltoo?
You'realreadyconvincedthatJavatechnologymakesformorereliableandreusablecodethandoesVisualBasic,VBScript,orC++.Whygobacktothoselanguagesforserver-sideprogramming?
3.3Powerful
ServletssupportseveralcapabilitiesthataredifficultorimpossibletoaccomplishwithregularCGI.ServletscantalkdirectlytotheWebserver,whereasregularCGIprogramscannot,atleastnotwithoutusingaserver-specificAPI.CommunicatingwiththeWebservermakesiteasiertotranslaterelativeURLsintoconcretepathnames,forinstance.Multipleservletscanalsosharedata,makingiteasytoimplementdatabaseconnectionpoolingandsimilarresource-sharingoptimizations.Servletscanalsomaintaininformationfromrequesttorequest,simplifyingtechniqueslikesessiontrackingandcachingofpreviouscomputations.
3.4Portable
ServletsarewrittenintheJavaprogramminglanguageandfollowastandardAPI.ServletsaresupporteddirectlyorbyapluginonvirtuallyeverymajorWebserver.Consequently,servletswrittenfor,say,MacromediaJRuncanrunvirtuallyunchangedonApacheTomcat,MicrosoftInternetInformationServer(withaseparateplugin),IBMWebSphere,iPlanetEnterpriseServer,Oracle9iAS,orStarNineWebStar.TheyarepartoftheJava2Platform,EnterpriseEdition(J2EE;seesoindustrysupportforservletsisbecomingevenmorepervasive.
3.5Inexpensive
AnumberoffreeorveryinexpensiveWebserversaregoodfordevelopmentuseordeploymentoflow-ormedium-volumeWebsites.Thus,withservletsandJSPyoucanstartwithafreeorinexpensiveserverandmigratetomoreexpensiveserverswithhigh-performancecapabilitiesoradvancedadministrationutilitiesonlyafteryourprojectmeetsinitialsuccess.ThisisincontrasttomanyoftheotherCGIalternatives,whichrequireasignificantinitialinvestmentforthepurchaseofaproprietarypackage.
Priceandportabilityaresomewhatconnected.Forexample,Martytriestokeeptrackofthecountriesofreadersthatsendhimquestionsbyemail.Indiawasnearthetopofthelist,probablybehindtheU.S.MartyalsotaughtoneofhisJSPandservlettrainingcourses(seeinManila,andtherewasgreatinterestinservletandJSPtechnologythere.
Now,whyareIndiaandthePhilippinesbothsointerested?
Wesurmisethattheansweristwofold.First,bothcountrieshavelargepoolsofwell-educatedsoftwaredevelopers.Second,both