英文技术资料翻译模版文档格式.docx

英文技术资料翻译模版文档格式.docx

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AServlet'

sJob

ServletsareJavaprogramsthatrunonWeborapplicationservers,actingasamiddlelayerbetweenrequestscomingfromWebbrowsersorotherHTTPclientsanddatabasesorapplicationsontheHTTPserver.Theirjobistoperformthefollowingtasks,asillustratedinFigure1-1.

Figure1-1

1．Readtheexplicitdatasentbytheclient.

TheendusernormallyentersthisdatainanHTMLformonaWebpage.However,thedatacouldalsocomefromanappletoracustomHTTPclientprogram.

2．ReadtheimplicitHTTPrequestdatasentbythebrowser.

Figure1-1showsasinglearrowgoingfromtheclienttotheWebserver（thelayerwhereservletsandJSPexecute）,buttherearereallytwovarietiesofdata:

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

3．Generatetheresults.

Thisprocessmayrequiretalkingtoadatabase,executinganRMIorEJBcall,invokingaWebservice,orcomputingtheresponsedirectly.Yourrealdatamaybeinarelationaldatabase.Fine.Butyourdatabaseprobablydoesn'

tspeakHTTPorreturnresultsinHTML,sotheWebbrowsercan'

ttalkdirectlytothedatabase.Evenifitcould,forsecurityreasons,youprobablywouldnotwantitto.Thesameargumentappliestomostotherapplications.YouneedtheWebmiddlelayertoextracttheincomingdatafromtheHTTPstream,talktotheapplication,andembedtheresultsinsideadocument.

4．Sendtheexplicitdata.,thedocument）totheclient.

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

5．SendtheimplicitHTTPresponsedata.

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

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

WhyBuildWebPagesDynamically?

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

1．TheWebpageisbasedondatasentbytheclient.

Forinstance,theresultspagefromsearchenginesandorder-confirmationpagesatonlinestoresarespecifictoparticularuserrequests.Youdon'

tknowwhattodisplayuntilyoureadthedatathattheusersubmits.Justrememberthattheusersubmitstwokindsofdata:

explicit.,HTMLformdata）andimplicit.,HTTPrequestheaders）.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,servletsarenotrestrictedtoWeborapplicationserversthathandleHTTPrequestsbutcanbeusedforothertypesofserversaswell.Forexample,servletscouldbeembeddedinFTPormailserverstoextendtheirfunctionality.And,aservletAPIforSIP（SessionInitiationProtocol）serverswasrecentlystandardized（seeen/jsr/detail?

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

llonlybediscussingHTTPservlets.

TheAdvantagesofServletsOver"

Traditional"

CGI

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

1．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.

2．Convenient

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

You'

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

3．Powerful

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

4．Portable

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

5．Inexpensive

AnumberoffreeorveryinexpensiveWebserversaregoodfordevelopmentuseordeploymentoflow-ormedium-volumeWebsites.Thus,withservletsandJSPyoucanstartwithafreeorinexpensiveserverandmigratetomoreexpensiveserverswithhigh-performancecapabilitiesoradvancedadministrationutilitiesonlyafteryourprojectmeetsinitialsuccess.ThisisincontrasttomanyoftheotherCGIalternatives,whichrequireasignificantinitialinvestmentforthepurchaseofaproprietarypackage.

Priceandportabilityaresomewhatconnected.Forexample,Martytriestokeeptrackofthecountriesofreadersthatsendhimquestionsbyemail.Indiawasnearthetopofthelist,probably#2behindthe.MartyalsotaughtoneofhisJSPandservlettrainingcourses（seeinManila,andtherewasgreatinterestinservletandJSPtechnologythere.

Now,whyareIndiaandthePhilippinesbothsointerested?

Wesurmisethattheansweristwofold.First,bothcountrieshavelargepoolsofwell-educatedsoftwaredevelopers.Second,bothcountrieshave（orhad,atthattime）highlyunfavorablecurrencyexchangeratesagainstthe.dollar.So,buyingaspecial-purposeWebserverfroma.companyconsumedalargepartofearlyprojectfunds.

But,withservletsandJSP,theycouldstartwithafreeserver:

ApacheTomcat（eitherstandalone,embeddedintheregularApacheWebserver,orembeddedinMicrosoftIIS）.Oncetheprojectstartstobeco