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，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-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,movingtoVBScriptwill