JAVA外文文献+翻译.docx

JAVA外文文献+翻译.docx

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JAVA外文文献+翻译

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,anysortofdynamicgraphingisnearlyimpossibletoperformwithconsistencybecauseaGIFbecreatedandmovedfromtheservertotheclientforeachversionofthegraph、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—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,movingtoVBScriptwillbeyourfa