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外文文献原文

AnOverviewofServletandJSPTechnology

Nagle,Wiegley

Abstract:

Servletprogramrunningintheserver-side,dynamicallygeneratedWebpagewiththetraditionalCGIandmanyothersimilarcomparedtoCGItechnology,JavaServletwithamoreefficient,easiertouse,morepowerfulandhasbetterportability,moresavingstoinvest.

Keywords:

JSPTechnology;Servlet;HTTPserver1.

1．AServlet'sJob

ServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowingtasks：

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.

2．WhyBuildWebPagesDynamically?

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.

3．TheAdvantagesofServletsOver"Traditional"CGIJavaservletsaremoreefficient,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,bothcountrieshave（orhad,atthattime）highlyunfavorablecurrencyexchangera