Servlet和JSP技术简述(英文翻译)

AnOverviewofServletandJSPTechnologyGildasAvoineandPhilippeOechslinEPFL,Lausanne,SwitzerlandHISTORYOFJSPTheJavaServerPages(JSP)isakindofaccordingtowebofthescriptplaitdistancetechnique,similarcarriesthescriptlanguageofJavaintheserveroftheNetscapecompanyofserver-sideJavaScript(SSJS)andtheActiveServerPages(ASP)oftheMicrosoft.JSPcomparestheSSJSandASPtohavebettercanexpandsex,anditisnomoreexclusivethananyfactoryorsomeoneparticularserverofWeb.ThoughthenormofJSPistobedrawupbytheSuncompanyof,anyfactorycancarryouttheJSPonownsystem.TheAfterSunreleasetheJSP(theJavaServerPages)formally,thethiskindofnewWebapplicationdevelopmenttechniqueveryquicklycausedthepeople'sconcern.JSPprovidedaspecialdevelopmentenvironmentfortheWebapplicationthatestablishesthehighdynamicstate.AccordingtotheSunparlance,theJSPcanadapttoincludetheApacheWebServer,IIS4.0onthemarketatinsideof85%serverproduct.ThischapterwillintroducetherelatedknowledgeofJSPandDatabases,andJavaBeanrelatedcontents,isallcertainlyrougherintroductionamongthembasiccontents,sayperhapstoisaGuideonly,ifthereaderneedsthemoredetailedinformation,pleasingthebookofconsultthehomologousJSP.1.1.1GENERALIZETheJSP(JavaServerPages)isfromthecompanyofSunMicrosystemsinitiate,themanycompaniestheparticipatetothebuildupthetogetheroftheakindtheofdynamicthestatewebthepagetechniquestandard,theithavetheitintheconstructiontheofthedynamicstatethewebpagethestrongbutthedonottheespeciallyofthefunction.JSPandthetechniqueofASPoftheMicrosoftisveryalike.BothallprovidetheabilitythatmixeswithacertainprocedurecodeandisexplainbythelanguageenginetocarryouttheprocedurecodeinthecodeofHTML.Underneathwearesimpleofcarryontheintroductiontoit.JSPpagesaretranslatedintoservlets.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,youcanenforceaweakerorstrongerseparationbetweenthestaticHTMLandthedynamiccontent.Now,thisdiscussionisnottosaythatyoushouldstopusingservletsanduseonlyJSPinstead.Bynomeans.Almostallprojectswilluseboth.Forsomerequestsinyourproject,youwilluseservlets.Forothers,youwilluseJSP.Forstillothers,youwillcombinethemwiththeMVCarchitecture.Youwanttheappropriatetoolforthejob,andservlets,bythemselves,donotcompleteyourtoolkit.1.1.2SOURCEOFJSPThetechniqueofJSPofthecompanyofSun,makingthepageofWebdevelopthepersonnelcanusetheHTMLperhapsmarkingofXMLtodesigntoturntheendpagewithformat.UsetheperhapssmallscriptfuturelifeofmarkingofJSPbecomesthedynamicstateonthepagecontents.(thecontentschangesaccordingtotheclaimof)TheJavaServletisatechnicalfoundationofJSP,andthelargeWebappliesthedevelopmentoftheproceduretoneedtheJavaServlettomatchwithwiththeJSPandthencancomplete,thisnameofServletcomesfromtheApplet,thelocaltranslationmethodofnowisalotof,thisbookinordernottomisconstruction,decidethedirectadoptionServletbutdon'tdoanytranslation,ifreaderwouldliketo,cancallitas"smallserviceprocedure".TheServletissimilartotraditionalCGI,ISAPI,NSAPIetc.Webproceduredevelopmentthefunctionofthetoolinfact,atusetheJavaServlethereafter,thecustomerneednotuseagainthelowlymethodofCGIofefficiency,alsoneednotuseonlytheabilitycometobornpageofWebofdynamicstateinthemethodofAPIthatacertainfixedWebserverterracecirculate.ManyserversofWeballsupporttheServlet,evennotsupporttheServletserverofWebdirectlyandcanalsopasstheadditionalappliedserverandthemoldpiecestosupporttheServlet.ReceivebenefitinthecharacteristicoftheJavacross-platform,theServletisalsoaterraceirrelevant,actually,aslongasmatchthenormofJavaServlet,theServletiscompletetohavenothingtodowithterraceandistohavenothingtodowithserverofWeb.BecausetheJavaServletisinternaltoprovidetheservicebythelinedistance,neednotstartaprogresstotheeachclaimses,andmakeuseofthemulti-threadingmechanismcanatthesametimeforseveralclaimservice,thereforetheefficiencyofJavaServletisveryhigh.ButtheJavaServletalsoisnottohasnoweakness,similartotraditionalCGI,ISAPI,theNSAPImethod,theJavaServletistomakeuseoftooutputtheHTMLlanguagesentencetocarryoutthedynamicstatewebpageof,ifdevelopthewholewebsitewiththeJavaServlet,theintegrationprocessofthedynamicstatepartandthestaticstatepageisanevil-forebodingdreamsimply.ForsolvingthiskindofweaknessoftheJavaServlet,theSUNreleasedtheJSP.Anumberofyearsago,Martywasinvitedtoattendasmall20-personindustryroundtablediscussiononsoftwaretechnology.SittingintheseatnexttoMartywasJamesGosling,inventoroftheJavaprogramminglanguage.Sittingseveralseatsawaywasahigh-levelmanagerfromaverylargesoftwarecompanyinRedmond,Washington.Duringthediscussion,themoderatorbroughtupthesubjectofJini,whichatthattimewasanewJavatechnology.Themoderatoraskedthemanagerwhathethoughtofit,andthemanagerrespondedthatitwastooearlytotell,butthatitseemedtobeanexcellentidea.Hewentontosaythattheywouldkeepaneyeonit,andifitseemedtobecatchingon,theywouldfollowhiscompany'susual"embraceandextend"strategy.Atthispoint,Goslinglightheartedlyinterjected"Youmeandisgraceanddistend."Now,thegrievancethatGoslingwasairingwasthathefeltthatthiscompanywouldtaketechnologyfromothercompaniesandsubornitfortheirownpurposes.Butguesswhat?Theshoeisontheotherfoothere.TheJavacommunitydidnotinventtheideaofdesigningpagesasamixtureofstaticHTMLanddynamiccodemarkedwithspecialtags.Forexample,ColdFusiondidityearsearlier.EvenASP(aproductfromtheverysoftwarecompanyoftheaforementionedmanager)popularizedthisapproachbeforeJSPcamealonganddecidedtojumponthebandwagon.Infact,JSPnotonlyadoptedthegeneralidea,itevenusedmanyofthesamespecialtagsasASPdid.TheJSPisanestablishmentatthemodelofJavaservletsonoftheexpressionlayertechnique,itmakestheplaitwritetheHTMLtobecomemoresimple.BeliketheSSJS,italsoallowsyoucarrythestaticstateHTMLcontentsandserversthescriptmixtoputtogethertheborndynamicstateexportation.JSPthescriptlanguagethattheJavaisthetacitapproval,however,beliketheASPandcanuseotherlanguages(suchasJavaScriptandVBScript),thenormofJSPalsoallowstouseotherlanguages.1.1.3JSPCHARACTERISTICSIsaserviceaccordingtothescriptlanguageinsomeonelanguageofthestaturessystemthiskindofdiscuss,theJSPshouldbeseemakeisakindofscriptlanguage.However,beakindofscriptlanguage,theJSPseemedtobetoostrongagain,almostcanuseallJavasintheJSP.Beakindofaccordingtotextoriginallyof,takemanifestationasthecentraldevelopmenttechnique,theJSPprovidedalladvantagesoftheJavaServlet,and,whencombinewithaJavaBeanstogether,providingakindofmakecontentsandmanifestationthatsimplewaythatlogicseparate.Separatethecontentsandadvantageoflogicalmanifestationsis,thepersonnelwhorenewsthepageexternalappearanceneednotknowthecodeofJava,andrenewtheJavaBeanspersonnelalsoneednotbedesignthewebpageofexpertinhand,canusetotakethepageofJavaBeansJSPtodefinethetemplateofWeb,tobuildupafromhavethealikeexternalappearanceofthewebsitethatpageconstitute.JavaBeanscompletesthedatatoprovide,havingnocodeofJavainthetemplatethus,thismeansthatthesetemplatescanbewrittenthepersonnelbyaHTMLplaittosupport.Certainly,canalsomakeuseoftheJavaServlettocontrolthelogicofthewebsite,adjustthroughtheJavaServlettousethewayofthedocumentofJSPtoseparatewebsiteoflogicandcontents.Generallyspeaking,inactualengineofJSP,thepageofJSPistheeditandtranslatetypewhilecarryout,notexplainthetypeof.Explainthedynamicstatewebpagedevelopmenttoolofthetype,suchasASP,PHP3etc.,becausespeedetc.reason,havealreadycan'tsatisfycurrentthelargeelectroniccommerceneedsappliedly,traditionaldevelopmenttechniquesareallattoeditandtranslatetheexecutivewaychange,suchastheASP→ASP+;PHP3→PHP4.IntheJSPnormbook,didnotrequesttheprocedureintheJSPcodepart(becalledtheScriptlet)andmustwritewiththeJavadefinitely.Actually,havesomeenginesofJSPareadoptiveotherscriptlanguagessuchastheEMAC-Script,etc.,butactuallythisafewscriptlanguagesalsoaretosetupontheJava,editandtranslatefortheServlettocarryoutof.WriteaccordingtothenormofJSP,havenoScriptletofrelationwithJavaalsoiscanof,however,mainlylieintheabilityandJavaBeans,theEnterpriseJavaBeansesbecauseoftheJSPstrongfunctiontoworktogether,soevenistheScriptletpartnottousetheJava,editandtranslateofperformancecodealsoshouldisrelatedwithJava.1.1.4JSPMECHANISMTocomprehendtheJSPhowunitethetechnicaladvantagethatabovevariousspeakof,cometocarryoutvariousresulteasily,thecustomermustunderstandthedifferentiationof"themoduledevelopsforthewebpageofthecenter"and"thepagedevelopsforthewebpageofthecenter"first.TheSSJSandASPareallinseveralyearagotorelease,thenetworkofthattimeisstillveryyoung,nooneknowstostillhaveinadditiontomakingallbusiness,datasandtheexpressionlogicentertheoriginalwebpageentirelyheapwhatbettersolvethemethod.Thiskindofmodelthattakepageasthecenterstudiesandgetstheveryfastdevelopmenteasily.However,alongwithchangeoftime,thepeopleknowthatthiskindofmethodisunwellinsetuplarge,theWebthatcanupgradeappliestheprocedure.Theexpressionlogicwriteinthescriptenvironmentwaslockinthepage,onlypassingtosheartosliceandgluetostickthencandriveheavyuse.Expressthelogictousuallymixtogetherwithbusinessandthedatalogics,whenthismakesbetheproceduremembertotrytochangeanexternalappearancethatappliestheprocedurebutdonotwanttobreakwithitslliedbusinesslogic,applytheprocedureofmaintenancebeliketowalkthesimilardifficultyontheeggshell.Infactinthebusinessenterprise,heavyusetheapplicationofthemodulealreadythroughverymature,noonewouldliketorewritethoselogicsfortheirappliedprocedure.HTMLandsketchthedesignerhandedovertotheimplementworkoftheirdesigntheWebplaittheonewhowrite,maketheyhavetodoublework-Usuallyisthehandicraftplaittowrite,becausehavenofittoolandcancarrythescriptandtheHTMLcontentsknottotheservertoputtogether.Chienbutspeech,applythecomplexityoftheprocedurealongwiththeWebtopromotecontinuously,thedevelopmentmethodthattakepageasthecenterlimitssextobecometogetupobviously.Atthesametime,thepeoplealwaysatlookforthebettermethodofbuilduptheWebapplicationprocedure,themodulespreadsincustomer'smachine/servertherealm.JavaBeansandActiveXwerepublishedthecompanytoexpandtoapplytheproceduredeveloperforJavaandWindowstousetocometodevelopthecomplicatedprocedurequicklyby"thefastapplicationproceduredevelopment"(RAD)tool.Thesetechniquesmaketheexpertinthesomerealmbeabletowritethemodulefortheperpendicularapplicationplaitintheskillarea,butthedevelopercangofetchtheusagedirectlybutneednotcontroltheexpertiseofthisrealm.Beakindoftakemoduleasthecentraldevelopmentterrace,theJSPappeared.ItwiththeJavaBeansandEnterpriseJavaBeans(EJB)moduleincludesthemodelofthebusinessandthedatalogicforfoundation,provideagreatdealoflabelandascriptterracestousetocometoshowintheHTMLpagefromthecontentsofJavaBeanscreationorsendapresentinreturn.BecauseofthepropertythatregardsthemoduleasthecenteroftheJSP,itcandriveJavaandnotthedeveloperofJavausesequally.NotthedeveloperofJavacanpasstheJSPlabel(Tags)tousetheJavaBeansthatthedeluxedeveloperofJavaestablish.ThedeveloperofJavanotonlycanestablishandusetheJavaBeans,butalsocanusethelanguageofJavatocometocontrolmoreaccuratelyintheJSPpageaccordingtotheexpressionlogicofthefirstfloorJavaBeans.SeenowhowJSPishandleclaimof.Inbasicclaimmodel,aclaimdirectlywassendtoJSPpagein.ThecodeofJSPcontrolstocarryonhourofthelogicprocessingandmoduleofJavaBeanses'handoverwitheachother,andthemanifestationresultindynamicstatebornly,mixingwiththeHTMLpageofthestaticstateHTMLcode.TheBeanscanbeJavaBeansormoduleofEJBs.Moreover,themorecomplicatedclaimmodelcanseemakefromisrequestotherJSPpagesofthepagecallsignorJavaServlets.TheengineofJSPwantstochasethecodeofJavathatthelabelofJSP,codeofJavaintheJSPpageevenallconvertsintothebigpiecetogetherwiththestaticstateHTMLcontentsactually.ThesecodespiecewasorganizedtheJavaServletthatcustomercannotseetogotobytheengineofJSP,thentheServleteditsandtranslatethemautomaticallybytecodeofJava.Thus,thevisitantthatisthewebsiterequestsaJSPpage,undertheconditionofitisnotknowing,analreadyborn,theServletactualfullgeneralthatpreparedtoeditandtranslatecompletesallworks,veryconcealmentbutagainandefficiently.TheServletistoeditandtranslateof,sothecodeofJSPinthewebpagedoesnotneedwhentheeverytimerequeststhatpageisexplain.TheengineofJSPneedtobeeditandtranslateafterServletthecodeendismodifyonlyonce,thenthisServletthatedittedandtranslatecanbecarryout.TheinviewofthefactJSPengineautoisborntoeditandtranslatetheServletalso,neednotprocedurememberbeginstoeditandtranslatethecode,sotheJSPcanbringvividsexthatfunctionandfastdevelopmentsneedthatyouareefficiently.ComparedwiththetraditionalCGI,theJSPhastheequaladvantage.First,onthespeed,thetraditionalprocedureofCGIneedstousethestandardimportationofthesystemtooutputtheequipmentstocarryoutthedynamicstatewebpageborn,buttheJSPisdirectismutuallytheconnectionwithserver.AndsayfortheCGI,eachinterviewneedstoaddtoaddaprogresstohandle,theprogressbuildupanddestroybyburningconstantlyandwillbeanotsmallburdenforcalculatorofbetheserverofWeb.Thenextinorder,theJSPisspecializedtodevelopbutdesignfortheWebof,itspurposeisforbuildingupaccordingtotheWebappliedprocedure,includedthenormandthetoolofathewholeset.UsethetechniqueofJSPcancombinealotofJSPpagestobecomeaWebapplicationprocedureveryexpediently.1.2AServlet'sJobServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherclientsanddatabasesorapplicationsontheserver.Theirjobistoperformthefollowingtasks,asillustratedinFigure1-1.Figure1-11．Readtheexplicitdatasentbytheclient.TheendusernormallyentersthisdatainanHTMLformonaWebpage.However,thedatacouldalsocomefromanappletoracustomclientprogram.2．Readtheimplicitrequestdatasentbythebrowser.Figure1-1showsasinglearrowgoingfromtheclienttotheWebserver(thelayerwhereservletsandJSPexecute),buttherearereallytwovarietiesofdata:theexplicitdatathattheenduserentersinaformandthebehind-the-scenesinformation.Bothvarietiesarecritical.Theinformationincludescookies,informationaboutmediatypesandcompressionschemesthebrowserunderstands,andsoon.3．Generatetheresults.Thisprocessmayrequiretalkingtoadatabase,executinganRMIorEJBcall,invokingaWebservice,orcomputingtheresponsedirectly.Yourrealdatamaybeinarelationaldatabase.Fine.Butyourdatabaseprobablydoesn'tspeakorreturnresultsinHTML,sotheWebbrowsercan'ttalkdirectlytothedatabase.Evenifitcould,forsecurityreasons,youprobablywouldnotwantitto.Thesameargumentappliestomostotherapplications.YouneedtheWebmiddlelayertoextracttheincomingdatafromthestream,talktotheapplication,andembedtheresultsinsideadocument.4．Sendtheexplicitdata(i.e.,thedocument)totheclient.Thisdocumentcanbesentinavarietyofformats,includingtext(HTMLorXML),binary(GIFimages),orevenacompressedformatlikegzipthatislayeredontopofsomeotherunderlyingformat.But,HTMLisbyfarthemostcommonformat,soanimportantservlet/JSPtaskistowraptheresultsinsideofHTML.5．Sendtheimplicitresponsedata.Figure1-1showsasinglearrowgoingfromtheWebmiddlelayer(theservletorJSPpage)totheclient.But,therearereallytwovarietiesofdatasent:thedocumentitselfandthebehind-the-scenesinformation.Again,bothvarietiesarecriticaltoeffectivedevelopment.Sendingresponsedatainvolvestellingthebrowserorotherclientwhattypeofdocumentisbeingreturned(e.g.,HTML),settingcookiesandcachingparameters,andothersuchtasks.1.3WhyBuildWebPagesDynamically?manyclientrequestscanbesatisfiedbyprebuiltdocuments,andtheserverwouldhandletheserequestswithoutinvokingservlets.Inmanycases,however,astaticresultisnotsufficient,andapageneedstobegeneratedforeachrequest.ThereareanumberofreasonswhyWebpagesneedtobebuilton-the-fly:1．TheWebpageisbasedondatasentbytheclient.Forinstance,theresultspagefromsearchenginesandorder-confirmationpagesatonlinestoresarespecifictoparticularuserrequests.Youdon'tknowwhattodisplayuntilyoureadthedatathattheusersubmits.Justrememberthattheusersubmitstwokindsofdata:explicit(i.e.,HTMLformdata)andimplicit(i.e.,requestheaders).Eitherkindofinputcanbeusedtobuildtheoutputpage.Inparticular,itisquitecommontobuildauser-specificpagebasedonacookievalue.2．TheWebpageisderivedfromdatathatchangesfrequently.Ifthepagechangesforeveryrequest,thenyoucertainlyneedtobuildtheresponseatrequesttime.Ifitchangesonlyperiodically,however,youcoulddoittwoways:youcouldperiodicallybuildanewWebpageontheserver(independentlyofclientrequests),oryoucouldwaitandonlybuildthepagewhentheuserrequestsit.Therightapproachdependsonthesituation,butsometimesitismoreconvenienttodothelatter:waitfortheuserrequest.Forexample,aweatherreportornewsheadlinessitemightbuildthepagesdynamically,perhapsreturningapreviouslybuiltpageifthatpageisstilluptodate.3．TheWebpageusesinformationfromcorporatedatabasesorotherserver-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,servletsarenotrestrictedtoWeborapplicationserversthathandlerequestsbutcanbeusedforothertypesofserversaswell.Forexample,servletscouldbeembeddedinFTPormailserverstoextendtheirfunctionality.And,aservletAPIforSIP(SessionInitiationProtocol)serverswasrecentlystandardized(see:///en/jsr/detail?id=116).Inpractice,however,thisuseofservletshasnotcaughton,andwe'llonlybediscussingservlets.1.4TheAdvantagesofServletsOver"Traditional"CGIJavaservletsaremoreefficient,easiertouse,morepowerful,moreportable,safer,andcheaperthantraditionalCGIandmanyalternativeCGI-liketechnologies.1．EfficientWithtraditionalCGI,anewprocessisstartedforeachrequest.IftheCGIprogramitselfisrelativelyshort,theoverheadofstartingtheprocesscandominatetheexecutiontime.Withservlets,theJavavirtualmachinestaysrunningandhandleseachrequestwithalightweightJavathread,notaheavyweightoperatingsystemprocess.Similarly,intraditionalCGI,ifthereareNrequeststothesameCGIprogram,thecodefortheCGIprogramisloadedintomemoryNtimes.Withservlets,however,therewouldbeNthreads,butonlyasinglecopyoftheservletclasswouldbeloaded.Thisapproachreducesservermemoryrequirementsandsavestimebyinstantiatingfewerobjects.Finally,whenaCGIprogramfinisheshandlingarequest,theprogramterminates.Thisapproachmakesitdifficulttocachecomputations,keepdatabaseconnectionsopen,andperformotheroptimizationsthatrelyonpersistentdata.Servlets,however,remaininmemoryevenaftertheycompletearesponse,soitisstraightforwardtostorearbitrarilycomplexdatabetweenclientrequests.2．ConvenientServletshaveanextensiveinfrastructureforautomaticallyparsinganddecodingHTMLformdata,readingandsettingheaders,handlingcookies,trackingsessions,andmanyothersuchhigh-levelutilities.InCGI,youhavetodomuchofthisyourself.Besides,ifyoualreadyknowtheJavaprogramminglanguage,whylearnPerltoo?You'realreadyconvincedthatJavatechnologymakesformorereliableandreusablecodethandoesVisualBasic,VBScript,orC++.Whygobacktothoselanguagesforserver-sideprogramming?3．PowerfulServletssupportseveralcapabilitiesthataredifficultorimpossibletoaccomplishwithregularCGI.ServletscantalkdirectlytotheWebserver,whereasregularCGIprogramscannot,atleastnotwithoutusingaserver-specificAPI.CommunicatingwiththeWebservermakesiteasiertotranslaterelativeURLsintoconcretepathnames,forinstance.Multipleservletscanalsosharedata,makingiteasytoimplementdatabaseconnectionpoolingandsimilarresource-sharingoptimizations.Servletscanalsomaintaininformationfromrequesttorequest,simplifyingtechniqueslikesessiontrackingandcachingofpreviouscomputations.4．PortableServletsarewrittenintheJavaprogramminglanguageandfollowastandardAPI.ServletsaresupporteddirectlyorbyapluginonvirtuallyeverymajorWebserver.Consequently,servletswrittenfor,say,MacromediaJRuncanrunvirtuallyunchangedonApacheTomcat,MicrosoftInternetInformationServer(withaseparateplugin),IBMWebSphere,iPlanetEnterpriseServer,Oracle9iAS,orStarNineWebStar.TheyarepartoftheJava2Platform,EnterpriseEdition(J2EE;see://java.sun/j2ee/),soindustrysupportforservletsisbecomingevenmorepervasive.5．InexpensiveAnumberoffreeorveryinexpensiveWebserversaregoodfordevelopmentuseordeploymentoflow-ormedium-volumeWebsites.Thus,withservletsandJSPyoucanstartwithafreeorinexpensiveserverandmigratetomoreexpensiveserverswithhigh-performancecapabilitiesoradvancedadministrationutilitiesonlyafteryourprojectmeetsinitialsuccess.ThisisincontrasttomanyoftheotherCGIalternatives,whichrequireasignificantinitialinvestmentforthepurchaseofaproprietarypackage.Priceandportabilityaresomewhatconnected.Forexample,Martytriestokeeptrackofthecountriesofreadersthatsendhimquestionsbyemail.Indiawasnearthetopofthelist,probably#2behindtheU.S.MartyalsotaughtoneofhisJSPandservlettrainingcourses(see://courses.coreservlets/)inManila,andtherewasgreatinterestinservletandJSPtechnologythere.Now,whyareIndiaandthePhilippinesbothsointerested?Wesurmisethattheansweristwofold.First,bothcountrieshavelargepoolsofwell-educatedsoftwaredevelopers.Second,bothcountrieshave(orhad,atthattime)highlyunfavorablecurrencyexchangeratesagainsttheU.S.dollar.So,buyingaspecial-purposeWebserverfromaU.S.companyconsumedalargepartofearlyprojectfunds.But,withservletsandJSP,theycouldstartwithafreeserver:ApacheTomcat(eitherstandalone,embeddedintheregularApacheWebserver,orembeddedinMicrosoftIIS).Oncetheprojectstartstobecomesuccessful,theycouldmovetoaserverlikeCauchoResinthathadhigherperformanceandeasieradministrationbutthatisnotfree.ButnoneoftheirservletsorJSPpageshavetoberewritten.Iftheirprojectbecomesevenlarger,theymightwanttomovetoadistributed(clustered)environment.Noproblem:theycouldmovetoMacromediaJRunProfessional,whichsupportsdistributedapplications(Webfarms).Again,noneoftheirservletsorJSPpageshavetoberewritten.Ifthe