JAVA外文文献+翻译精品管理资料.docx

JAVA外文文献+翻译精品管理资料.docx

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JAVA外文文献+翻译精品管理资料

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，movingtoVBScript