JAVA毕业论文-外文文献+翻译-Java和因特网（精品doc）.docx

JAVA毕业论文-外文文献+翻译-Java和因特网（精品doc）.docx

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JavaandtheInternet

IfJavais,infact,yetanothercomputerprogramminglanguage,youmayquestionwhyitissoimportantandwhyitisbeingpromotedasarevolutionarystepincomputerprogramming.Theanswerisn'timmediatelyobviousifyou'recomingfromatraditionalprogrammingperspective.AlthoughJavaisveryusefulforsolvingtraditionalstand-aloneprogrammingproblems,itisalsoimportantbecauseitwillsolveprogrammingproblemsontheWorldWideWeb.

1. Client-sideprogramming

TheWeb'sinitialserver-browserdesignprovidedforinteractivecontent,buttheinteractivitywascompletelyprovidedbytheserver.Theserverproducedstaticpagesfortheclientbrowser,whichwouldsimplyinterpretanddisplaythem.BasicHTMLcontainssimplemechanismsfordatagathering:

text-entryboxes,checkboxes,radioboxes,listsanddrop-downlists,aswellasabuttonthatcanonlybeprogrammedtoresetthedataontheformor''submit"thedataontheformbacktotheserver.ThissubmissionpassesthroughtheCommonGatewayInterface（CGI）providedonallWebservers.ThetextwithinthesubmissiontellsCGIwhattodowithit.Themostcommonactionistorunaprogramlocatedontheserverinadirectorythat'stypicallycalled"cgi-bin."（IfyouwatchtheaddresswindowatthetopofyourbrowserwhenyoupushabuttononaWebpage,youcansometimessee"cgi-bin"withinallthegobbledygookthere.）Theseprogramscanbewritteninmostlanguages.Perlisacommonchoicebecauseitisdesignedfortextmanipulationandisinterpreted,soitcanbeinstalledonanyserverregardlessofprocessororoperatingsystem.

ManypowerfulWebsitestodayarebuiltstrictlyonCGI,andyoucaninfactdonearlyanythingwithit.However,WebsitesbuiltonCGIprogramscanrapidlybecomeoverlycomplicatedtomaintain,andthereisalsotheproblemofresponsetime.TheresponseofaCGIprogramdependsonhowmuchdatamustbesent,aswellastheloadonboththeserverandtheInternet.（Ontopofthis,startingaCGIprogramtendstobeslow.）TheinitialdesignersoftheWebdidnotforeseehowrapidlythisbandwidthwouldbeexhaustedforthekindsofapplicationspeopledeveloped.Forexample,anysortofdynamicgraphingisnearlyimpossibletoperformwithconsistencybecauseaGIFfilemustbecreatedandmovedfromtheservertotheclientforeachversionofthegraph.Andyou'venodoubthaddirectexperiencewithsomethingassimpleasvalidatingthedataonaninputform.Youpressthesubmitbuttononapage;thedataisshippedbacktotheserver;theserverstartsaCGIprogramthatdiscoversanerror,formatsanHTMLpageinformingyouoftheerror,andthensendsthepagebacktoyou;youmustthenbackupapageandtryagain.Notonlyisthisslow,it'sinelegant.

Thesolutionisclient-sideprogramming.MostmachinesthatrunWebbrowsersarepowerfulenginescapableofdoingvastwork,andwiththeoriginalstaticHTMLapproachtheyaresittingthere,justidlywaitingfortheservertodishupthenextpage.Client-sideprogrammingmeansthattheWebbrowserisharnessedtodowhateverworkitcan,andtheresultfortheuserisamuchspeedierandmoreinteractiveexperienceatyourWebsite.

Theproblemwithdiscussionsofclient-sideprogrammingisthattheyaren'tverydifferentfromdiscussionsofprogrammingingeneral.Theparametersarealmostthesame,buttheplatformisdifferent:

aWebbrowserislikealimitedoperatingsystem.Intheend,youmuststillprogram,andthisaccountsforthedizzyingarrayofproblemsandsolutionsproducedbyclient-sideprogramming.Therestofthissectionprovidesanoverviewoftheissuesandapproachesinclient-sideprogramming.

2.Plug-ins

Oneofthemostsignificantstepsforwardinclient-sideprogrammingisthedevelopmentoftheplug-in.Thisisawayforaprogrammertoaddnewfunctionalitytothebrowserbydownloadingapieceofcodethatplugsitselfintotheappropriatespotinthebrowser.Ittellsthebrowser"fromnowonyoucanperformthisnewactivity.,,（Youneedtodownloadtheplug-inonlyonce.）Somefastandpowerfulbehaviorisaddedtobrowsersviaplug-ins,butwritingaplug-inisnotatrivialtask,andisn'tsomethingyou'dwanttodoaspartoftheprocessofbuildingaparticularsite.Thevalueoftheplug-inforclient-sideprogrammingisthatitallowsanexpertprogrammertodevelopanewlanguageandaddthatlanguagetoabrowserwithoutthepermissionofthebrowsermanufacturer.Thus,plug-insprovidea"backdoor"thatallowsthecreationofnewclient-sideprogramminglanguages（althoughnotalllanguagesareimplementedasplug-ins）.

3.Scriptinglanguages

Plug-insresultedinanexplosionofscriptinglanguages.Withascriptinglanguageyouembedthesourcecodeforyourclient-sideprogramdirectlyintotheHTMLpage,andtheplug-inthatinterpretsthatlanguageisautomaticallyactivatedwhiletheHTMLpageisbeingdisplayed.Scriptinglanguagestendtobereasonablyeasytounderstandand,becausetheyaresimplytextthatispartofanHTMLpage,theyloadveryquicklyaspartofthesingleserverhitrequiredtoprocurethatpage.Thetrade-offisthatyourcodeisexposedforeveryonetosee（andsteal）.Generally,however,youaren'tdoingamazinglysophisticatedthingswithscriptinglanguagessothisisnottoomuchofahardship.

ThispointsoutthatthescriptinglanguagesusedinsideWebbrowsersarereallyintendedtosolvespecifictypesofproblems,primarilythecreationofricherandmoreinteractivegraphicaluserinterfaces（GUIs）.However,ascriptinglanguagemightsolve80percentoftheproblemsencounteredinclient-sideprogramming.Yourproblemsmightverywellfitcompletelywithinthat80percent,andsincescriptinglanguagescanalloweasierandfasterdevelopment,youshouldprobablyconsiderascriptinglanguagebeforelookingatamoreinvolvedsolutionsuchasJavaorActiveXprogramming.

ThemostcommonlydiscussedbrowserscriptinglanguagesareJavaScript（whichhasnothingtodowithJava;it'snamedthatwayjusttograbsomeofJava'smarketingmomentum）,VBScript（whichlookslikeVisualBasic）,andTcl/Tk,whichcomesfromthepopularcross-platformGUI-buildinglanguage.Thereareothersoutthere,andnodoubtmoreindevelopment.

JavaScriptisprobablythemostcommonlysupported.ItcomesbuiltintobothNetscapeNavigatorandtheMicrosoftInternetExplorer（IE）.Inaddition,thereareprobablymoreJavaScriptbooksavailablethantherearefortheotherbrowserlanguages,andsometoolsautomaticallycreatepagesusingJavaScript.However,ifyou'realreadyfluentinVisualBasicorTcl/Tk,you'llbemoreproductiveusingthosescriptinglanguagesratherthanlearninganewone.（You'llhaveyourhandsfulldealingwiththeWebissuesalready.）

4.Java

Ifascriptinglanguagecansolve80percentoftheclient-sideprogrammingproblems,whatabouttheother20percent—the"reallyhardstuff?

"ThemostpopularsolutiontodayisJava.Notonlyisitapowerfulprogramminglanguagebuilttobesecure,cross-platform,andinternational,butJavaisbeingcontinuallyextendedtoprovidelanguagefeaturesandlibrariesthatelegantlyhandleproblemsthataredifficultintraditionalprogramminglanguages,suchasmultithreading,databaseaccess,networkprogramming,anddistributedcomputing.Javaallowsclient-sideprogrammingviatheapplet.

Anappletisamini-programthatwillrunonlyunderaWebbrowser.TheappletisdownloadedautomaticallyaspartofaWebpage（justas,forexample,agraphicisautomaticallydownloaded）.Whentheappletisactivateditexecutesaprogram.Thisispartofitsbeauty一itprovidesyouwithawaytoautomaticallydistributetheclientsoftwarefromtheserveratthetimetheuserneedstheclientsoftware,andnosooner.Theusergetsthelatestversionoftheclientsoftwarewithoutfailandwithoutdifficultreinstallation.BecauseofthewayJavaisdesigned,theprogrammerneedstocreateonlyasingleprogram,andthatprogramautomaticallyworkswithallcomputersthathavebrowserswithbuilt-inJavainterpreters.（Thissafelyincludesthevastmajorityofmachines.）SinceJavaisafull-fledgedprogramminglanguage,youcandoasmuchworkaspossibleontheclientbeforeandaftermakingrequestsoftheserver.Forexample,youwon'tneedtosendarequestformacrosstheInternettodiscoverthatyou'vegottenadateorsomeotherparameterwrong,andyourclientcomputercanquicklydotheworkofplottingdatainsteadofwaitingfortheservertomakeaplotandshipagraphicimagebacktoyou.Notonlydoyougettheimmediatewinofspeedandresponsiveness,butthegeneralnetworktrafficandloadonserverscanbereduced,preventingtheentireInternetfromslowingdown.

OneadvantageaJavaapplethasoverascriptedprogramisthatit'sincompiledform,sothesourcecodeisn'tavailabletotheclient.Ontheotherhand,aJavaappletcanbedecompiledwithouttoomuchtrouble,buthidingyourcodeisoftennotanimportantissue.Twootherfactorscanbeimportant.Asyouwillseelaterinthisbook,acompiledJavaappletcancomprisemanymodulesandtakemultipleserver''hits"（accesses）todownload.（InJava1.1andhigherthisisminimizedbyJavaarchives,calledJARfiles,thatallowalltherequiredmodulestobepackagedtogetherandcompressedforasingledownload.）AscriptedprogramwilljustbeintegratedintotheWebpageaspartofitstext（andwillgenerallybesmallerandreduceserverhits）.ThiscouldbeimportanttotheresponsivenessofyourWebsite.Anotherfactoristheall-importantlearningcurve.Regardlessofwhatyou'veheard,Javaisnotatriviallanguagetolearn.Ifyou'reaVisualBasicprogrammer,movingtoVBScriptwillbeyourf