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毕业设计(论文)外文翻译题目:基于Java的网上商城系统的设计与实现院(系):计算机科学与工程专业:网络工程班级:070607学生:冯宁学号:070607105指导教师:王辉2011年05月09日EnglishOnlyJavaandtheInternetIfJavais,infact,yetanothercomputerprogramminglanguage,youmayquestionwhyitissoimportantandwhyitisbeingpromotedasarevolutionarystepincomputerprogramming.Theanswerisn’timmediatelyobviousifyou’recomingfromatraditionalprogrammingperspective.AlthoughJavaisveryusefulforsolvingtraditionalstandaloneprogrammingproblems,itisalsoimportantbecauseitwillsolveprogrammingproblemsontheWorldWideWeb.WhatistheWeb?TheWebcanseemabitofamysteryatfirst,withallthistalkof“surfing,”“presence,”and“homepages.”It’shelpfultostepbackandseewhatitreallyis,buttodothisyoumustunderstandclient/serversystems,anotheraspectofcomputingthat’sfullofconfusingissues.1.Client/ServercomputingTheprimaryideaofaclient/serversystemisthatyouhaveacentralrepositoryofinformation—somekindofdata,ofteninadatabase—thatyouwanttodistributeondemandtosomesetofpeopleormachines.Akeytotheclient/serverconceptisthattherepositoryofinformationiscentrallylocatedsothatitcanbechangedandsothatthosechangeswillpropagateouttotheinformationconsumers.Takentogether,theinformationrepository,thesoftwarethatdistributestheinformation,andthemachine(s)wheretheinformationandsoftwareresideiscalledtheserver.Thesoftwarethatresidesontheremotemachine,communicateswiththeserver,fetchestheinformation,processesit,andthendisplaysitontheremotemachineiscalledtheclient.Thebasicconceptofclient/servercomputing,then,isnotsocomplicated.Theproblemsarisebecauseyouhaveasingleservertryingtoservemanyclientsatonce.Generally,adatabasemanagementsystemisinvolved,sothedesigner“balances”thelayoutofdataintotablesforoptimaluse.Inaddition,systemsoftenallowaclienttoinsertnewinformationintoaserver.Thismeansyoumustensurethatoneclient’snewdatadoesn’twalkoveranotherclient’snewdata,orthatdataisn’tlostintheprocessofaddingittothedatabase(thisiscalledtransactionprocessing).Asclientsoftwarechanges,itmustbebuilt,debugged,andinstalledontheclientmachines,whichturnsouttobemorecomplicatedandexpensivethanyoumightthink.It’sespeciallyproblematictosupportmultipletypesofcomputersandoperatingsystems.Finally,there’stheall-importantperformanceissue:Youmighthavehundredsofclientsmakingrequestsofyourserveratanyonetime,soanysmalldelayiscrucial.Tominimizelatency,programmersworkhardtooffloadprocessingtasks,oftentotheclientmachine,butsometimestoothermachinesattheserversite,usingso-calledmiddleware.(Middlewareisalsousedtoimprovemaintainability.)Thesimpleideaofdistributinginformationhassomanylayersofcomplexitythatthewholeproblemcanseemhopelesslyenigmatic.Andyetit’scrucial:Client/servercomputingaccountsforroughlyhalfofallprogrammingactivities.It’sresponsibleforeverythingfromtakingordersandcredit-cardtransactionstothedistributionofanykindofdata—stockmarket,scientific,government,younameit.Whatwe’vecomeupwithinthepastisindividualsolutionstoindividualproblems,inventinganewsolutioneachtime.Thesewerehardtocreateandhardtouse,andtheuserhadtolearnanewinterfaceforeachone.Theentireclient/serverproblemneedstobesolvedinabigway.2.TheWebasagiantserverTheWebisactuallyonegiantclient/serversystem.It’sabitworsethanthat,sinceyouhavealltheserversandclientscoexistingonasinglenetworkatonce.Youdon’tneedtoknowthat,becauseallyoucareaboutisconnectingtoandinteractingwithoneserveratatime(eventhoughyoumightbehoppingaroundtheworldinyoursearchforthecorrectserver).Initiallyitwasasimpleone-wayprocess.Youmadearequestofaserverandithandedyouafile,whichyourmachine’sbrowsersoftware(i.e.,theclient)wouldinterpretbyformattingontoyourlocalmachine.Butinshortorderpeoplebeganwantingtodomorethanjustdeliverpagesfromaserver.Theywantedfullclient/servercapabilitysothattheclientcouldfeedinformationbacktotheserver,forexample,tododatabaselookupsontheserver,toaddnewinformationtotheserver,ortoplaceanorder(whichrequiredmoresecuritythantheoriginalsystemsoffered).Thesearethechangeswe’vebeenseeinginthedevelopmentoftheWeb.TheWebbrowserwasabigstepforward:theconceptthatonepieceofinformationcouldbedisplayedonanytypeofcomputerwithoutchange.However,browserswerestillratherprimitiveandrapidlyboggeddownbythedemandsplacedonthem.Theyweren’tparticularlyinteractive,andtendedtoclogupboththeserverandtheInternetbecauseanytimeyouneededtodosomethingthatrequiredprogrammingyouhadtosendinformationbacktotheservertobeprocessed.Itcouldtakemanysecondsorminutestofindoutyouhadmisspelledsomethinginyourrequest.Sincethebrowserwasjustavieweritcouldn’tperformeventhesimplestcomputingtasks.(Ontheotherhand,itwassafe,becauseitcouldn’texecuteanyprogramsonyourlocalmachinethatmightcontainbugsorviruses.)Tosolvethisproblem,differentapproacheshavebeentaken.Tobeginwith,graphicsstandardshavebeenenhancedtoallowbetteranimationandvideowithinbrowsers.Theremainderoftheproblemcanbesolvedonlybyincorporatingtheabilitytorunprogramsontheclientend,underthebrowser.Thisiscalledclient-sideprogramming.Client-sideprogrammingTheWeb’sinitialserver-browserdesignprovidedforinteractivecontent,buttheinteractivitywascompletelyprovidedbytheserver.Theserverpro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