翻译文献-第二章 http 和 servlet 基础知识

外文原文CHAPTER2HTTPANDSERVLETBASICSLETSSTARTOFFCHAPTERBYDEFININGTHETERMWEBAPPLICATIONWEVEALLSEENREGULARCLIENTSIDEAPPLICATIONS,BUTWHATEXACTLYISAWEBAPPLICATIONLOOSELY,ITCANBEDEFINEDASANAPPLICATIONRUNNINGONASERVERAUSERACCESSESTHROUGHATHIN,GENERALPURPOSECLIENTTODAY,THEMOSTCOMMONCLIENTISAWEBBROWSERONAPCORWORKSTATION,BUTOTHERKINDSOFCLIENTSARERAPIDLYJOININGTHEPARTY,SUCHASWIRELESSPDAS,CELLPHONES,ANDOTHERSPECIALIZEDDEVICESTHELOFTYGOALHEREISTOACCESSALLTHEINFORMATIONANDSERVICESYOUNEEDFROMANYTYPEOFDEVICETHATHAPPENSTOBEINFRONTOFYOUTHISMEANSTHATTHESAMESIMPLECLIENTPROGRAMMUSTBEABLETOTALKTOMANYDIFFERENTSERVERAPPLICATIONS,ANDTHEAPPLICATIONMUSTBEABLETOWORKWITHMANYDIFFERENTTYPESOFCLIENTSTOSATISFYTHISNEED,THEPROTOCOLOFHOWACLIENTANDASERVERTALKTOEACHOTHERMUSTBEDEFINEDINDETAILTHATSEXACTLYWHATTHEHYPERTEXTTRANSPORTPROTOCOLHTTPISFORTHECOMMUNICATIONMODELDEFINEDBYHTTPFORMSTHEFOUNDATIONFORALLWEBAPPLICATIONDESIGNABASICUNDERSTANDINGOFHTTPISKEYTODEVELOPINGAPPLICATIONSTHATFITWITHINTHECONSTRAINTSOFTHEPROTOCOL,NOMATTERWHICHSERVERSIDETECHNOLOGYYOUUSEINTHISCHAPTER,WELOOKATTHEMOSTIMPORTANTDETAILSOFHTTPYOUNEEDTOBEAWAREOFASAWEBAPPLICATIONDEVELOPERONEOTHERITEMTHISBOOKISABOUTUSINGJSPASTHESERVERSIDETECHNOLOGY,SOTHATSWHATWELLFOCUSONASYOUSAWINCHAPTER1,JSPISBASEDONTHEJAVASERVLETTECHNOLOGYBOTHTECHNOLOGIESSHAREALOTOFTERMINOLOGYANDCONCEPTS,SOKNOWINGABITABOUTSERVLETSWILLHELPYOUEVENWHENYOUDEVELOPPUREJSPAPPLICATIONSTOREALLYUNDERSTANDANDUSETHEFULLPOWEROFJSP,YOUNEEDTOKNOWAFAIRBITABOUTSERVLETSHENCE,WELOOKATSERVLETFUNDAMENTALSINTHELASTSECTIONOFTHISCHAPTER21THEHTTPREQUEST/RESPONSEMODELHTTPANDALLEXTENDEDPROTOCOLSBASEDONHTTPAREBASEDONAVERYSIMPLECOMMUNICATIONSMODELHERESHOWITWORKSACLIENT,TYPICALLYAWEBBROWSER,SENDSAREQUESTFORARESOURCETOASERVER,ANDTHESERVERSENDSBACKARESPONSECORRESPONDINGTOTHERESOURCEORARESPONSEWITHANERRORMESSAGEIFITCANTPROCESSTHEREQUESTFORSOMEREASON,ARESOURCECANBEANUMBEROFTHINGS,SUCHASASIMPLEHTMLFILERETURNEDVERBATIMTOTHEBROWSERORAPROGRAMTHATGENERATESTHERESPONSEDYNAMICALLYTHISREQUEST/RESPONSEMODELISILLUSTRATEDINFIGURE21THISSIMPLEMODELIMPLIESTHREEIMPORTANTFACTSYOUNEEDTOBEAWAREOFHTTPISASTATELESSPROTOCOLTHISMEANSTHATTHESERVERDOESNTKEEPANYINFORMATIONABOUTTHECLIENTAFTERITSENDSITSRESPONSE,ANDTHEREFOREITCANTRECOGNIZETHATMULTIPLEREQUESTSFROMTHESAMECLIENTMAYBERELATEDWEBAPPLICATIONSCANTEASILYPROVIDETHEKINDOFIMMEDIATEFEEDBACKTYPICALLYFOUNDINSTANDALONEGUIAPPLICATIONSSUCHASWORDPROCESSORSORTRADITIONALCLIENT/SERVERAPPLICATIONSEVERYINTERACTIONBETWEENTHECLIENTANDTHESERVERREQUIRESAREQUEST/RESPONSEEXCHANGEPERFORMINGAREQUEST/RESPONSEEXCHANGEWHENAUSERSELECTSANITEMINALISTBOXORFILLSOUTAFORMELEMENTISUSUALLYTOOTAXINGONTHEBANDWIDTHAVAILABLETOMOSTINTERNETUSERSTHERESNOTHINGINTHEPROTOCOLTHATTELLSTHESERVERHOWAREQUESTISMADECONSEQUENTLY,THESERVERCANTDISTINGUISHBETWEENVARIOUSMETHODSOFTRIGGERINGTHEREQUESTONTHECLIENTFOREXAMPLE,HTTPDOESNTALLOWAWEBSERVERTODIFFERENTIATEBETWEENANEXPLICITREQUESTCAUSEDBYCLICKINGALINKORSUBMITTINGAFORMANDANIMPLICITREQUESTCAUSEDBYRESIZINGTHEBROWSERWINDOWORUSINGTHEBROWSERSBACKBUTTONINADDITION,HTTPDOESNTCONTAINANYMEANSFORTHESERVERTOINVOKECLIENTSPECIFICFUNCTIONS,SUCHASGOINGBACKINTHEBROWSERHISTORYLISTORSENDINGTHERESPONSETOACERTAINFRAMEALSO,THESERVERCANTDETECTWHENTHEUSERCLOSESTHEBROWSEROVERTHEYEARS,PEOPLEHAVEDEVELOPEDVARIOUSTRICKSTOOVERCOMETHEFIRSTPROBLEMHTTPSSTATELESSNATUREWELLLOOKATTHEMINCHAPTER10THEOTHERTWOPROBLEMSNOIMMEDIATEFEEDBACKANDNODETAILSABOUTHOWTHEREQUESTISMADEAREHARDERTODEALWITH,BUTSOMEAMOUNTOFINTERACTIVITYCANBEACHIEVEDBYGENERATINGARESPONSETHATINCLUDESCLIENTSIDECODECODEEXECUTEDBYTHEBROWSER,SUCHASJAVASCRIPTORAJAVAAPPLETTHISAPPROACHISDISCUSSEDBRIEFLYINCHAPTER17211REQUESTINDETAILLETSTAKEACLOSERLOOKATREQUESTSAUSERSENDSAREQUESTTOTHESERVERBYCLICKINGALINKONAWEBPAGE,SUBMITTINGAFORM,ORTYPINGINAWEBPAGEADDRESSINTHEBROWSERSADDRESSFIELDTOSENDAREQUEST,THEBROWSERNEEDSTOKNOWWHICHSERVERTOTALKTOANDWHICHRESOURCETOASKFORTHISINFORMATIONISSPECIFIEDBYANHTTPUNIFORMRESOURCELOCATORURLHTTP/WWWGEFIONSOFTWARECOM/INDEXHTMLTHEFIRSTPARTOFTHEURLSHOWNSPECIFIESTHATTHEREQUESTISMADEUSINGTHEHTTPPROTOCOLTHISISFOLLOWEDBYTHENAMEOFTHESERVER,INTHISCASEWWWGEFIONSOFTWARECOMTHEWEBSERVERWAITSFORREQUESTSTOCOMEINONASPECIFICTCP/IPPORTPORTNUMBER80ISTHESTANDARDPORTFORHTTPREQUESTIFTHEWEBSERVERUSESANOTHERPORT,THEURLMUSTSPECIFYTHEPORTNUMBERINADDITIONTOTHESERVERNAMEFOREXAMPLEHTTP/WWWGEFIONSOFTWARECOM8080/INDEXHTMLTHISREQUESTISSENTTOASERVERTHATUSESPORT8080INSTEADOF80THELASTPARTOFTHEURL,/INDEXHTML,IDENTIFIESTHERESOURCETHATTHECLIENTISREQUESTINGAURLISACTUALLYASPECIALIZATIONOFAUNIFORMRESOURCEIDENTIFIERURI,DEFINEDINTHERFC23961SPECIFICATIONAURLIDENTIFIESARESOURCEPARTLYBYITSLOCATION,FORINSTANCETHESERVERTHATCONTAINSTHERESOURCEANOTHERTYPEOFURIISAUNIFORMRESOURCENAMEURN,WHICHISAGLOBALLYUNIQUEIDENTIFIERTHATISVALIDNOMATTERWHERETHERESOURCEISLOADEDHTTPDEALSONLYWITHTHEURLVARIETYTHETERMSURIANDURLAREOFTENUSEDINTERCHANGEABLE,ANDUNFORTUNATELY,THEYHAVESLIGHTLYDIFFERENTDEFINITIONSINDIFFERENTSPECIFICATIONSIMTRYINGTOUSETHETERMSASDEFINEDBYTHEHTTP/11SPECIFICATIONRFC2616,WHICHISPRETTYCLOSETOHOWTHEYAREALSOUSEDINTHESERVLETANDJSPSPECIFICATIONSH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