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1、AjaxAjaxisshortfor‘‘AsynchronousJavaScriptandXML.’’Itiscross-platform.Itiseffectivelyopensource.Itissupportedbytheseversionsofthemajorbrowsers:■InternetExplorer5.0andup■Netscape7andup■Firefox,Opera8+■Safari1.2andupAlthoughitwouldbeastretchtocallAjaxanewtechnology,anditi
2、scertainlynotaprogramminglanguage,itisanadaptiveimplementationofpreexistingtoolstoprovideanintegrated,efficientnewtoolthatcanbeusedtomakebetter,faster,moreversatileWebapplications.Asyoumayknow,Ajax,whichisamixorreformulationofexistingtechnologies,facilitatesserverintera
3、ctionwithelementsofaWebpage.This,inturn,resultsinamoreresponsiveapplicationwithlowerbandwidthusage,and,hence,lowercosts,resultinginawin-winsituationforallconcerned.Itisalsothesefeaturesthatmakeitattractivetotheso-calledWeb2.0crowdand,thosewhoareinvolvedinsoftwaredesigna
4、nddevelopmentformobilehandhelddevices.FollowingarethemajorcomponentsofAjax:■JavaScriptasthescriptinglanguagetocallfunctions■XMLHttpRequestisusedforasynchronousdatatransferfromaserver■XHTMLandCascadingStyleSheets(CSS)areusedforuniformencodingandpresentationcross-platform
5、■DocumentObjectModel(DOM)isusedforprecisemappinganddatadisplay■XMLandXSLT(although,ironically,doesnotrequireuseofXMLfordatafetchandtransfer)Ajaxprovidesthatslightmarginofadvantagetogetaheadofthecompetition.Butyoumustknowwhentouseit,andthenuseitjudiciously.Youwouldtendto
6、defeatoneofitsappealingfeatures—fast,efficientdownloadsofsmallsegmentsofdata—ifyourepeatedlycalledforupdatesorrefreshesofinformation.Youwouldconstantlybetyingupsystemresourceswithrepeatedcallsfornewdata,whichmaynotbenewatall(ornecessary).Wouldyou,forexample,finditnecess
7、arytorefreshthetemperaturefieldevery3secondsonaweatherpage?Itcouldbebetterdonewherearefreshofthatfieldisonlydonewhenthenewtemperaturedoesnotequaltheoldtemperature.Async(asynchronous)justmeansthatrequestsanddatamaybeexchangedacrosstheInternetfromtheclientWebpagetotheserv
8、erandbackatanytime.TheJavaScriptobjectusedtodothisiscalledXMLHttpRequest.However,thereisnorequirementthattheas
