外文翻译Servlet和JSP技术简述.docx

外文翻译Servlet和JSP技术简述.docx

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外文翻译Servlet和JSP技术简述

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AnOverviewofServletandJSPTechnology

Nagle,Wiegley

Abstract:

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

Keywords:

JSPTechnology;Servlet;HTTPserver

ServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowingtasks,asillustratedinFigure1-1.

Figure1-1　Webmiddlewarerole

AServlet'sJob

Readtheexplicitdatasentbytheclient.

TheendusernormallyentersthisdatainanHTMLformonaWebpage.However,thedatacouldalsocomefromanappletoracustomHTTPclientprogram.

ReadtheimplicitHTTPrequestdatasentbythebrowser.

Figure1-1showsasinglearrowgoingfromtheclienttotheWebserver（thelayerwhereservletsandJSPexecute）,buttherearereallytwovarietiesofdata:

theexplicitdatathattheenduserentersinaformandthebehind-the-scenesHTTPinformation.Bothvarietiesarecritical.TheHTTPinformationincludescookies,informationaboutmediatypesandcompressionschemesthebrowserunderstands,andsoon.

Generatetheresults.

Thisprocessmayrequiretalkingtoadatabase,executinganRMIorEJBcall,invokingaWebservice,orcomputingtheresponsedirectly.Yourrealdatamaybeinarelationaldatabase.Fine.Butyourdatabaseprobablydoesn'tspeakHTTPorreturnresultsinHTML,sotheWebbrowsercan'ttalkdirectlytothedatabase.Evenifitcould,forsecurityreasons,youprobablywouldnotwantitto.Thesameargumentappliestomostotherapplications.YouneedtheWebmiddlelayertoextracttheresultsinsideadocument.

Sendtheexplicitdata（i.e.,thedocument）totheclient.

Thisdocumentcanbesentinavarietyofformats,includingtext（HTMLorXML）,binary（GIFimages）,orevenacompressedformatlikegzipthatislayeredontopofsomeotherunderlyingformat.But,HTMLisbyfarthemostcommonformat,soanimportantservlet/JSPtaskistowraptheresultsinsideofHTML.

SendtheimplicitHTTPresponsedata.

Figure1-1showsasinglearrowgoingfromtheWebmiddlelayer（theservletorJSPpage）totheclient.But,therearereallytwovarietiesofdatasent:

thedocumentitselfandthebehind-the-scenesHTTPinformation.Again,bothvarietiesarecriticaltoeffectivedevelopment.SendingHTTPresponsedatainvolvestellingthebrowserorotherclientwhattypeofdocumentisbeingreturned（e.g.,HTML）,settingcookiesandcachingparameters,andothersuchtasks.

WhyBuildWebPagesDynamically?

manyclientrequestscanbesatisfiedbyprebuiltdocuments,andtheserverwouldhandletheserequestswithoutinvokingservlets.Inmanycases,however,astaticresultisnotsufficient,andapageneedstobegeneratedforeachrequest.ThereareanumberofreasonswhyWebpagesneedtobebuilton-the-fly:

TheWebpageisbasedondatasentbytheclient.

Forinstance,theresultspagefromsearchenginesandorder-confirmationpagesatonlinestoresarespecifictoparticularuserrequests.Youdon'tknowwhattodisplayuntilyoureadthedatathattheusersubmits.Justrememberthattheusersubmitstwokindsofdata:

explicit（i.e.,HTMLformdata）andimplicit（i.e.,HTTPrequestheaders）.Eitherkindofinputcanbeusedtobuildtheoutputpage.Inparticular,itisquitecommontobuildauser-specificpagebasedonacookievalue.

TheWebpageisderivedfromdatathatchangesfrequently.

Ifthepagechangesforeveryrequest,thenyoucertainlyneedtobuildtheresponseatrequesttime.Ifitchangesonlyperiodically,however,youcoulddoittwoways:

youcouldperiodicallybuildanewWebpageontheserver（independentlyofclientrequests）,oryoucouldwaitandonlybuildthepagewhentheuserrequestsit.Therightapproachdependsonthesituation,butsometimesitismoreconvenienttodothelatter:

waitfortheuserrequest.Forexample,aweatherreportornewsheadlinessitemightbuildthepagesdynamically,perhapsreturningapreviouslybuiltpageifthatpageisstilluptodate.

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,servletsarenotrestrictedtoWeborapplicationserversthathandleHTTPrequestsbutcanbeusedforothertypesofserversaswell.Forexample,servletscouldbeembeddedinFTPormailserverstoextendtheirfunctionality.And,aservletAPIforSIP（SessionInitiationProtocol）serverswasrecentlystandardized（seehttp:

//jcp.org/en/jsr/detail?

id=116）.Inpractice,however,thisuseofservletshasnotcaughton,andwe'llonlybediscussingHTTPservlets.

TheAdvantagesofServletsOver"Traditional"CGI

Javaservletsaremoreefficient,easiertouse,morepowerful,moreportable,safer,andcheaperthantraditionalCGIandmanyalternativeCGI-liketechnologies.

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,soitisstraightforwardtostorearbitrarilycomplexdatabetweenclientrequests.

Convenient

ServletshaveanextensiveinfrastructureforautomaticallyparsinganddecodingHTMLformdata,readingandsettingHTTPheaders,handlingcookies,trackingsessions,andmanyothersuchhigh-levelutilities.InCGI,youhavetodomuchofthisyourself.Besides,ifyoualreadyknowtheJavaprogramminglanguage,whylearnPerltoo?

You'realreadyconvincedthatJavatechnologymakesformorereliableandreusablecodethandoesVisualBasic,VBScript,orC++.Whygobacktothoselanguagesforserver-sideprogramming?

Powerful

ServletssupportseveralcapabilitiesthataredifficultorimpossibletoaccomplishwithregularCGI.ServletscantalkdirectlytotheWebserver,whereasregularCGIprogramscannot,atleastnotwithoutusingaserver-specificAPI.CommunicatingwiththeWebservermakesiteasiertotranslaterelativeURLsintoconcretepathnames,forinstance.Multipleservletscanalsosharedata,makingiteasytoimplementdatabaseconnectionpoolingandsimilarresource-sharingoptimizations.Servletscanalsomaintaininformationfromrequesttorequest,simplifyingtechniqueslikesessiontrackingandcachingofpreviouscomputations.

Portable

ServletsarewrittenintheJavaprogramminglanguageandfollowastandardAPI.ServletsaresupporteddirectlyorbyapluginonvirtuallyeverymajorWebserver.Consequently,servletswrittenfor,say,MacromediaJRuncanrunvirtuallyunchangedonApacheTomcat,MicrosoftInternetInformationServer（withaseparateplugin）,IBMWebSphere,iPlanetEnterpriseServer,Oracle9iAS,orStarNineWebStar.TheyarepartoftheJava2Platform,EnterpriseEdition（J2EE;seesoindustrysupportforservletsisbecomingevenmorepervasive.

Inexpensive

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（orh