外文翻译-Java和因特网

0外文原文JavaandtheInternetIfJavais,infact,yetanothercomputerprogramminglanguage,youmayquestionwhyitissoimportantandwhyitisbeingpromotedasarevolutionarystepincomputerprogramming.Theanswerisntimmediatelyobviousifyourecomingfromatraditionalprogrammingperspective.AlthoughJavaisveryusefulforsolvingtraditionalstandaloneprogrammingproblems,itisalsoimportantbecauseitwillsolveprogrammingproblemsontheWorldWideWeb.WhatistheWeb?TheWebcanseemabitofamysteryatfirst,withallthistalkof“surfing,”“presence,”and“homepages.”Itshelpfultostepbackandseewhatitreallyis,buttodothisyoumustunderstandclient/serversystems,anotheraspectofcomputingthatsfullofconfusingissues.Client/ServercomputingTheprimaryideaofaclient/serversystemisthatyouhaveacentralrepositoryofinformationsomekindofdata,ofteninadatabasethatyouwanttodistributeondemandtosomesetofpeopleormachines.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.Thismeansyoumustensurethatoneclientsnewdatadoesntwalkoveranotherclientsnewdata,orthatdataisntlostintheprocessofaddingittothedatabase(thisis1calledtransactionprocessing).Asclientsoftwarechanges,itmustbebuilt,debugged,andinstalledontheclientmachines,whichturnsouttobemorecomplicatedandexpensivethanyoumightthink.Itsespeciallyproblematictosupportmultipletypesofcomputersandoperatingsystems.Finally,therestheall-importantperformanceissue:Youmighthavehundredsofclientsmakingrequestsofyourserveratanyonetime,soanysmalldelayiscrucial.Tominimizelatency,programmersworkhardtooffloadprocessingtasks,oftentotheclientmachine,butsometimestoothermachinesattheserversite,usingso-calledmiddleware.(Middlewareisalsousedtoimprovemaintainability.)Thesimpleideaofdistributinginformationhassomanylayersofcomplexitythatthewholeproblemcanseemhopelesslyenigmatic.Andyetitscrucial:Client/servercomputingaccountsforroughlyhalfofallprogrammingactivities.Itsresponsibleforeverythingfromtakingordersandcredit-cardtransactionstothedistributionofanykindofdatastockmarket,scientific,government,younameit.Whatwevecomeupwithinthepastisindividualsolutionstoindividualproblems,inventinganewsolutioneachtime.Thesewerehardtocreateandhardtouse,andtheuserhadtolearnanewinterfaceforeachone.Theentireclient/serverproblemneedstobesolvedinabigway.TheWebasagiantserverTheWebisactuallyonegiantclient/serversystem.Itsabitworsethanthat,sinceyouhavealltheserversandclientscoexistingonasinglenetworkatonce.Youdontneedtoknowthat,becauseallyoucareaboutisconnectingtoandinteractingwithoneserveratatime(eventhoughyoumightbehoppingaroundtheworldinyoursearchforthecorrectserver).Initiallyitwasasimpleone-wayprocess.Youmadearequestofaserverandithandedyouafile,whichyourmachinesbrowsersoftware(i.e.,theclient)wouldinterpretbyformattingontoyourlocalmachine.Butinshortorderpeoplebeganwantingtodomorethanjustdeliverpagesfromaserver.Theywantedfullclient/servercapabilitysothattheclientcouldfeedinformationbacktotheserver,forexample,tododatabaselookupsontheserver,toaddnewinformationtotheserver,ortoplaceanorder(whichrequiredmoresecuritythan2theoriginalsystemsoffered).ThesearethechangeswevebeenseeinginthedevelopmentoftheWeb.TheWebbrowserwasabigstepforward:theconceptthatonepieceofinformationcouldbedisplayedonanytypeofcomputerwithoutchange.However,browserswerestillratherprimitiveandrapidlyboggeddownbythedemandsplacedonthem.Theywerentparticularlyinteractive,andtendedtoclogupboththeserverandtheInternetbecauseanytimeyouneededtodosomethingthatrequiredprogrammingyouhadtosendinformationbacktotheservertobeprocessed.Itcouldtakemanysecondsorminutestofindoutyouhadmisspelledsomethinginyourrequest.Sincethebrowserwasjustavieweritcouldntperformeventhesimplestcomputingtasks.(Ontheotherhand,itwassafe,becauseitcouldntexecuteanyprogramsonyourlocalmachinethatmightcontainbugsorviruses.)Tosolvethisproblem,differentapproacheshavebeentaken.Tobeginwith,graphicsstandardshavebeenenhancedtoallowbetteranimationandvideowithinbrowsers.Theremainderoftheproblemcanbesolvedonlybyincorporatingtheabilitytorunprogramsontheclientend,underthebrowser.Thisiscalledclient-sideprogramming.Client-sideprogrammingTheWebsinitialserver-browserdesignprovidedforinteractivecontent,buttheinteractivitywascompletelyprovidedbytheserver.Theserverproducedstaticpagesfortheclientbrowser,whichwouldsimplyinterpretanddisplaythem.BasicHyperTextMarkupLanguage(HTML)containssimplemechanismsfordatagathering:text-entryboxes,checkboxes,radioboxes,listsanddrop-downlists,aswellasabuttonthatcanonlybeprogrammedtoresetthedataontheformor“submit”thedataontheformbacktotheserver.ThissubmissionpassesthroughtheCommonGatewayInterface(CGI)providedonallWebservers.ThetextwithinthesubmissiontellsCGIwhattodowithit.Themostcommonactionistorunaprogramlocatedontheserverinadirectorythatstypicallycalled“cgi-bin.”(IfyouwatchtheaddresswindowatthetopofyourbrowserwhenyoupushabuttononaWebpage,youcansometimessee“cgi-bin”withinallthegobbledygookthere.)Theseprogramscanbewritteninmostlanguages.Perlhasbeenacommonchoicebecauseitisdesignedfortextmanipulationandisinterpreted,soitcanbeinstalledonanyserverregardlessof3processororoperatingsystem.However,Python(myfavoriteseewww.P)hasbeenmakinginroadsbecauseofitsgreaterpowerandsimplicity.ManypowerfulWebsitestodayarebuiltstrictlyonCGI,andyoucaninfactdonearlyanythingwithCGI.However,WebsitesbuiltonCGIprogramscanrapidlybecomeoverlycomplicatedtomaintain,andthereisalsotheproblemofresponsetime.TheresponseofaCGIprogramdependsonhowmuchdatamustbesent,aswellastheloadonboththeserverandtheInternet.(Ontopofthis,startingaCGIprogramtendstobeslow.)TheinitialdesignersoftheWebdidnotforeseehowrapidlythisbandwidthwouldbeexhaustedforthekindsofapplicationspeopledeveloped.Forexample,anysortofdynamicgraphingisnearlyimpossibletoperformwithconsistencybecauseaGraphicsInterchangeFormat(GIF)filemustbecreatedandmovedfromtheservertotheclientforeachversionofthegraph.Andyouvenodoubthaddirectexperiencewithsomethingassimpleasvalidatingthedataonaninputform.Youpressthesubmitbuttononapage;thedataisshippedbacktotheserver;theserverstartsaCGIprogramthatdiscoversanerror,formatsanHTMLpageinformingyouoftheerror,andthensendsthepagebacktoyou;youmustthenbackupapageandtryagain.Notonlyisthisslow,itsinelegant.Thesolutionisclient-sideprogramming.MostmachinesthatrunWebbrowsersarepowerfulenginescapableofdoingvastwork,andwiththeoriginalstaticHTMLapproachtheyaresittingthere,justidlywaitingfortheservertodishupthenextpage.Client-sideprogrammingmeansthattheWebbrowserisharnessedtodowhateverworkitcan,andtheresultfortheuserisamuchspeedierandmoreinteractiveexperienceatyourWebsite.Theproblemwithdiscussionsofclient-sideprogrammingisthattheyarentverydifferentfromdiscussionsofprogrammingingeneral.Theparametersarealmostthesame,buttheplatformisdifferent;aWebbrowserislikealimitedoperatingsystem.Intheend,youmuststillprogram,andthisaccountsforthedizzyingarrayofproblemsandsolutionsproducedbyclient-sideprogramming.Therestofthissectionprovidesanoverviewoftheissuesandapproachesinclient-sideprogramming.4Plug-insOneofthemostsignificantstepsforwardinclient-sideprogrammingisthedevelopmentoftheplug-in.Thisisawayforaprogrammertoaddnewfunctionalitytothebrowserbydownloadingapieceofcodethatplugsitselfintotheappropriatespotinthebrowser.Ittellsthebrowser“fromnowonyoucanperformthisnewactivity.”(Youneedtodownloadtheplug-inonlyonce.)Somefastandpowerfulbehaviorisaddedtobrowsersviaplug-ins,butwritingaplug-inisnotatrivialtask,andisntsomethingyoudwanttodoaspartoftheprocessofbuildingaparticularsite.Thevalueoftheplug-inforclient-sideprogrammingisthatitallowsanexpertprogrammertodevelopanewlanguageandaddthatlanguagetoabrowserwithoutthepermissionofthebrowsermanufacturer.Thus,plug-insprovidea“backdoor”thatallowsthecreationofnewclient-sideprogramminglanguages(althoughnotalllanguagesareimplementedasplug-ins).ScriptinglanguagesPlug-insresultedinanexplosionofscriptinglanguages.Withascriptinglanguage,youembedthesourcecodeforyourclient-sideprogramdirectlyintotheHTMLpage,andtheplug-inthatinterpretsthatlanguageisautomaticallyactivatedwhiletheHTMLpageisbeingdisplayed.Scriptinglanguagestendtobereasonablyeasytounderstandand,becausetheyaresimplytextthatispartofanHTMLpage,theyloadveryquicklyaspartofthesingleserverhitrequiredtoprocurethatpage.Thetrade-offisthatyourcodeisexposedforeveryonetosee(andsteal).Generally,however,youarentdoingamazinglysophisticatedthingswithscriptinglanguages,sothisisnottoomuchofahardship.ThispointsoutthatthescriptinglanguagesusedinsideWebbrowsersarereallyintendedtosolvespecifictypesofproblems,primarilythecreationofricherandmoreinteractivegraphicaluserinterfaces(GUIs).However,ascriptinglanguagemightsolve80percentoftheproblemsencounteredinclient-sideprogramming.Yourproblemsmightverywellfitcompletelywithinthat80percent,andsincescriptinglanguagescanalloweasierandfasterdevelopment,youshouldprobablyconsiderascriptinglanguagebeforelookingatamoreinvolvedsolutionsuchasJavaorActiveXprogramming.5ThemostcommonlydiscussedbrowserscriptinglanguagesareJavaScript(whichhasnothingtodowithJava;itsnamedthatwayjusttograbsomeofJavasmarketingmomentum),VBScript(whichlookslikeVisualBASIC),andTcl/Tk,whichcomesfromthepopularcross-platformGUI-buildinglanguage.Thereareothersoutthere,andnodoubtmoreindevelopment.JavaScriptisprobablythemostcommonlysupported.ItcomesbuiltintobothNetscapeNavigatorandtheMicrosoftInternetExplorer(IE).Unfortunately,theflavorofJavaScriptonthetwobrowserscanvarywidely(theMozillabrowser,freelydownloadablefromwww.M,supportstheECMAScriptstandard,whichmayonedaybecomeuniversallysupported).Inaddition,thereareprobablymoreJavaScriptbooksavailablethantherearefortheotherbrowserlanguages,andsometoolsautomaticallycreatepagesusingJavaScript.However,ifyourealreadyfluentinVisualBASICorTcl/Tk,youllbemoreproductiveusingthosescriptinglanguagesratherthanlearninganewone.(YoullhaveyourhandsfulldealingwiththeWebissuesalready.)JavaIfascriptinglanguagecansolve80percentoftheclient-sideprogrammingproblems,whatabouttheother20percentthe“reallyhardstuff?”Javaisapopularsolutionforthis.Notonlyisitapowerfulprogramminglanguagebuilttobesecure,cross-platform,andinternational,butJavaisbeingcontinuallyextendedtoprovidelanguagefeaturesandlibrariesthatelegantlyhandleproblemsthataredifficultintraditionalprogramminglanguages,suchasmultithreading,databaseaccess,networkprogramming,anddistributedcomputing.Javaallowsclient-sideprogrammingviatheappletandwithJavaWebStart.Anappletisamini-programthatwillrunonlyunderaWebbrowser.TheappletisdownloadedautomaticallyaspartofaWebpage(justas,forexample,agraphicisautomaticallydownloaded).Whentheappletisactivated,itexecutesaprogram.Thisispartofitsbeautyitprovidesyouwithawaytoautomaticallydistributetheclientsoftwarefromtheserveratthetimetheuserneedstheclientsoftware,andnosooner.Theusergetsthelatestversionoftheclientsoftwarewithoutfailandwithoutdifficultreinstallation.BecauseofthewayJavaisdesigned,theprogrammerneedstocreateonlyasingleprogram,andthat6programautomaticallyworkswithallcomputersthathavebrowserswithbuilt-inJavainterpreters.(Thissafelyincludesthevastmajorityofmachines.)SinceJavaisafull-fledgedprogramminglanguage,youcandoasmuchworkaspossibleontheclientbeforeandaftermakingrequestsoftheserver.Forexample,youwontneedtosendarequestformacrosstheInternettodiscoverthatyouvegottenadateorsomeotherparameterwrong,andyourclientcomputercanquicklydotheworkofplottingdatainsteadofwaitingfortheservertomakeaplotandshipagraphicimagebacktoyou.Notonlydoyougettheimmediatewinofspeedandresponsiveness,butthegeneralnetworktrafficandloadonserverscanbereduced,preventingtheentireInternetfromslowingdown.OneadvantageaJavaapplethasoverascriptedprogramisthatitsincompiledform,sothesourcecodeisntavailabletotheclient.Ontheotherhand,aJavaappletcanbedecompiledwithouttoomuchtrouble,buthidingyourcodeisoftennotanimportantissue.Twootherfactorscanbeimportant.Asyouwillseelaterinthisbook,acompiledJavaappletcanrequireextratimetodownload,ifitislarge.AscriptedprogramwilljustbeintegratedintotheWebpageaspartofitstext(andwillgenerallybesmallerandreduceserverhits).ThiscouldbeimportanttotheresponsivenessofyourWebsite.Anotherfactoristheall-importantlearningcurve.Regardlessofwhatyouveheard,Javaisnotatriviallanguagetolearn.IfyoureaVISUALBASICprogrammer,movingtoVBScriptwillbeyourfastestsolution(assumingyoucanconstrainyourcustomerstoWindowsplatforms),andsinceitwillprobablysolvemosttypicalclient/serverproblems,youmightbehardpressedtojustifylearningJava.IfyoureexperiencedwithascriptinglanguageyouwillcertainlybenefitfromlookingatJavaScriptorVBScriptbeforecommittingtoJava,becausetheymightfityourneedshandilyandyoullbemoreproductivesooner.SecurityAutomaticallydownloadingandrunningprogramsacrosstheInternetcansoundlikeavirus-buildersdream.IfyouclickonaWebsite,youmightautomaticallydownloadanynumberofthingsalongwiththeHTMLpage:GIFfiles,scriptcode,compiledJavacode,andActiveXcomponents.Someofthesearebenign;GIFfilescantdoanyharm,andscriptinglanguagesaregenerallylimitedinwhattheycando.Javawasalsodesignedtorunitsapplets7withina“sandbox”ofsafety,whichpreventsitfromwritingtodiskoraccessingmemoryoutsidethesandbox.MicrosoftsActiveXisattheoppositeendofthespectrum.ProgrammingwithActiveXislikeprogrammingWindowsyoucandoanythingyouwant.SoifyouclickonapagethatdownloadsanActiveXcomponent,thatcomponentmightcausedamagetothefilesonyourdisk.Ofcourse,programsthatyouloadontoyourcomputerthatarenotrestrictedtorunninginsideaWebbrowsercandothesamething.VirusesdownloadedfromBulletin-BoardSystems(BBSs)havelongbeen.Thesolutionseemstobe“digitalsignatures,”wherebycodeisverifiedtoshowwhotheauthoris.Thisisbasedontheideathatavirusworksbecauseitscreatorcanbeanonymous,soifyouremovetheanonymity,individualswillbeforcedtoberesponsiblefortheiractions.Thisseemslikeagoodplanbecauseitallowsprogramstobemuchmorefunctional,andIsuspectitwilleliminatemaliciousmischief.If,however,aprogramhasanunintentionaldestructivebug,itwillstillcauseproblems.TheJavaapproachistopreventtheseproblemsfromoccurring,viathesandbox.TheJavainterpreterthatlivesonyourlocalWebbrowserexaminestheappletforanyuntowardinstructionsastheappletisbeingloaded.Inparticular,theappletcannotwritefilestodiskorerasefiles(oneofthemainstaysofviruses).Appletsaregenerallyconsideredtobesafe,andsincethisisessentialforreliableclient/serversystems,anybugsintheJavalanguagethatallowvirusesarerapidlyrepaired.(Itsworthnotingthatthebrowsersoftwareactuallyenforcesthesesecurityrestrictions,andsomebrowsersallowyoutoselectdifferentsecuritylevelstoprovidevaryingdegreesofaccesstoyoursystem.)Youmightbeskepticalofthisratherdraconianrestrictionagainstwritingfilestoyourlocaldisk.Forexample,youmaywanttobuildalocaldatabaseorsavedataforlateruseoffline.Theinitialvisionseemedtobethateventuallyeveryonewouldgetonlinetodoanythingimportant,butthatwassoonseentobeimpractical(althoughlow-cost“Internetappliances”mightsomedaysatisfytheneedsofasignificantsegmentofusers).Thesolutionisthe“signedapplet”thatusespublic-keyencryptiontoverifythatanappletdoesindeedcomefromwhereitclaimsitdoes.Asignedappletcanstilltrashyourdisk,butthetheoryis8thatsinceyoucannowholdtheappletcreatorsaccountable,theywontdoviciousthings.Javaprovidesaframeworkfordigitalsignaturessothatyouwilleventuallybeabletoallowanapplettostepoutsidethesandboxifnecessary.Chapter14containsanexampleofhowtosignanapplet.Inaddition,JavaWebStartisarelativelynewwaytoeasilydistributestandaloneprogramsthatdontneedawebbrowserinwhichtorun.Thistechnologyhasthepotentialofsolvingmanyclientsideproblemsassociatedwithrunningprogramsinsideabrowser.WebStartprogramscaneitherbesigned,ortheycanasktheclientforpermissioneverytimetheyaredoingsomethingpotentiallydangerousonthelocalsystem.Chapter14hasasimpleexampleandexplanationofJavaWebStart.Digitalsignatureshavemissedanimportantissue,whichisthespeedthatpeoplemovearoundontheInternet.Ifyoudownloadabuggyprogramanditdoessomethinguntoward,howlongwillitbebeforeyoudiscoverthedamage?Itcouldbedaysorevenweeks.Bythen,howwillyoutrackdowntheprogramthatsdoneit?Andwhatgoodwillitdoyouatthatpoint?IranetTheWebisthemostgeneralsolutiontotheclient/serverproblem,soitmakessensetousethesametechnologytosolveasubsetoftheproblem,inparticulartheclassicclient/serverproblemwithinacompany.Withtraditionalclient/serverapproachesyouhavetheproblemofmultipletypesofclientcomputers,aswellasthedifficultyofinstallingnewclientsoftware,bothofwhicharehandilysolvedwithWebbrowsersandclient-sideprogramming.WhenWebtechnologyisusedforaninformationnetworkthatisrestrictedtoaparticularcompany,itisreferredtoasanintranet.IntranetsprovidemuchgreatersecuritythantheInternet,sinceyoucanphysicallycontrolaccesstotheserverswithinyourcompany.Intermsoftraining,itseemsthatoncepeopleunderstandthegeneralconceptofabrowseritsmucheasierforthemtodealwithdifferencesinthewaypagesandappletslook,sothelearningcurvefornewkindsofsystemsseemstobereduced.Thesecurityproblembringsustooneofthedivisionsthatseemstobeautomaticallyformingintheworldofclient-sideprogramming.IfyourprogramisrunningontheInternet,9youdontknowwhatplatformitwillbeworkingunder,andyouwanttobeextracarefulthatyoudontdisseminatebuggycode.Youneedsomethingcross-platformandsecure,likeascriptinglanguageorJava.Ifyourerunningonanintranet,youmighthaveadifferentsetofconstraints.ItsnotuncommonthatyourmachinescouldallbeIntel/Windowsplatforms.Onanintranet,youreresponsibleforthequalityofyourowncodeandcanrepairbugswhentheyredis