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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,WHICHISPRETTYCLOSETOHOWTHEYAREALSOUSEDINTHESERVLETANDJSPSPECIFICATIONSHENCE,IUSETHETERMURLONLYWHENTHEURIMUSTSTARTWITHHTTPORHTTPS,FORHTTPOVERANENCRYPTEDCONNECTIONFOLLOWEDBYASERVERNAMEANDPOSSIBLYAPORTNUMBER,ASINTHEPREVIOUSEXAMPLESIUSEURIASAGENERICTERMFORANYSTRINGTHATIDENTIFIESARESOURCE,WHERETHELOCATIONCANBEDEDUCEDFROMTHECONTEXTANDISNTNECESSARILYPARTOFTHEURIFOREXAMPLE,WHENTHEREQUESTHASBEENDELIVEREDTOTHESERVER,THELOCATIONISAGIVEN,ANDONLYTHERESOURCEIDENTIFIERISIMPORTANT1AVAILABLEATHTTP/WWWIETFORG/RFC/RFC2396TXTTHEBROWSERUSESTHEURLINFORMATIONTOCREATETHEREQUESTMESSAGEITSENDSTOTHESPECIFIEDSERVERUSINGTHESPECIFIEDPROTOCOLANHTTPREQUESTMESSAGECONSISTSOFTHREETHINGSAREQUESTLINE,REQUESTHEADERS,ANDPOSSIBLYAREQUESTBODYTHEREQUESTLINESTARTSWITHTHEREQUESTMETHODNAME,FOLLOWEDBYARESOURCEIDENTIFIERANDTHEPROTOCOLVERSIONUSEDBYTHEBROWSERGET/INDEXHTMLHTTP/11THEMOSTCOMMONLYUSEDREQUESTMETHODISNAMEDGETASTHENAMEIMPLIES,AGETREQUESTISUSEDTORETRIEVEARESOURCEFROMTHESERVERITSTHEDEFAULTREQUESTMETHOD,SOIFYOUTYPEAURLINTHEBROWSERSADDRESSFIELD,ORCLICKONALINK,THEREQUESTISSENTASAGETREQUESTTOTHESERVERTHEREQUESTHEADERSPROVIDEADDITIONALINFORMATIONTHESERVERMAYUSETOPROCESSTHEREQUESTTHEMESSAGEBODYISINCLUDEDONLYINSOMETYPESOFREQUESTS,LIKETHEPOSTREQUESTDISCUSSEDLATERHERESANEXAMPLEOFAVALIDHTTPREQUESTMESSAGEGET/INDEXHTMLHTTP/11HOSTWWWGEFIONSOFTWARECOMUSERGENTMOZILLA/50WINDOWSUWIN9X490ENUSRV102ACCEPTIMAGE/GF,IMAGE/JPEG,IMAGE/PJPEG,IMAGE/PNG,/ACCEPTLANGUAGEENACCEPTCHARSETISO88591,UTF8THEREQUESTLINESPECIFIESTHEGETMETHODANDASKSFORTHERESOURCENAMED/INDEXHTMLTOBERETURNEDUSINGTHEHTTP/11PROTOCOLVERSIONTHEVARIOUSHEADERSPROVIDEADDITIONALINFORMATIONTHEHOSTHEADERTELLSTHESERVERTHEHOSTNAMEUSEDINTHEURLASERVERMAYHAVEMULTIPLENAMES,SOTHISINFORMATIONISUSEDTODISTINGUISHBETWEENMULTIPLEVIRTUALWEBSERVERSSHARINGTHESAMEWEBSERVERPROCESSTHEUSERAGENTHEADERCONTAINSINFORMATIONABOUTTHETYPEOFBROWSERMAKINGTHEREQUESTTHESERVERCANUSETHISTOSENDDIFFERENTTYPESOFRESPONSESTODIFFERENTTYPESOFBROWSERFORINSTANCE,IFTHESERVERKNOWSWHETHERINTERNETEXPLORERORNETSCAPENAVIGATORISUSED,ITCANSENDARESPONSETHATTAKESADVANTAGEOFEACHBROWSERSUNIQUEFEATURESITCANALSOTELLIFACLIENTOTHERTHANANHTMLBROWSERISUSED,SUCHASAWIRELESSMARKUPLANGUAGEWMLBROWSERONACELLPHONEORAPDADEVICE,ANDGENERATEANAPPROPRIATERESPONSETHEACCEPTHEADERSPROVIDEINFORMATIONABOUTTHELANGUAGEANDFILEFORMATSTHEBROWSERACCEPTSTHESEHEADERSCANBEUSEDTOADJUSTTHERESPONSETOTHECAPABILITIESOFTHEBROWSERANDTHEUSERSPREFERENCES,SUCHASUSEASUPPORTEDIMAGEFORMATANDTHEPREFERREDLANGUAGETHESEAREJUSTAFEWOFTHEHEADERSTHATCANBEINCLUDEDINAREQUESTMESSAGETHEHTTPSPECIFICATION,AVAILABLEATHTTP/WWWW3CORG/,DESCRIBESALLOFTHEMTHERESOURCEIDENTIFIERURIDOESNTNECESSARILYCORRESPONDTOASTATICFILEONTHESERVERITCANIDENTIFYANEXECUTABLEPROGRAM,ARECORDINADATABASE,ORPRETTYMUCHANYTHINGTHEWEBSERVERKNOWSABOUTTHATSWHYTHEGENERICTERMRESOURCEISUSEDINFACT,THERESNOWAYTOTELLIFTHE/INDEXHTMLURICORRESPONDSTOAFILEORSOMETHINGELSEITSJUSTANAMETHATMEANSSOMETHINGTOTHESERVERTHEWEBSERVERISCONFIGUREDTOMAPTHESEUNIQUENAMESTOTHEREALRESOURCE212RESPONSESINDETAILWHENTHEWEBSERVERRECEIVESTHEREQUEST,ITLOOKSATTHEURIANDDECIDES,BASEDINCONFIGURATIONINFORMATION,HOWTOHANDLEITITMAYHANDLEITINTERNALLYBYSIMPLYREADINGANHTMLFILEFROMTHEFILESYSTEM,ORITCANFORWARDTHEREQUESTTOSOMECOMPONENTTHATISRESPONSIBLEFORTHERESOURCECORRESPONDINGTOTHEURITHISCANBEAPROGRAMTHATUSESDATABASEINFORMATION,FORINSTANCE,TODYNAMICALLYGENERATEANAPPROPRIATERESPONSETOTHEBROWSERITMAKESNODIFFERENCEHOWTHEREQUESTISHANDLEDALLITCARESABOUTISGETTINGARESPONSETHERESPONSEMESSAGELOOKSSIMILARTOTHEREQUESTMESSAGEITCONSISTSOFTHREETHINGSASTATUSLINE,RESPONSEHEADERS,ANDANOPTIONALRESPONSEBODYHERESANEXAMPLEHTTP/11200OKLASTMODIFIEDMON,20DEC200222642GMTDATETUE,11JAN2003205240GMTSTATUS200CONTENTTYPETEXT/HTMLSERVLETENGLISHTOMCATWEBSERVER/50CONTENTLENGTH59HELLOWORLDTHESTATUSLINESTARTSWITHTHENAMEOFTHEPROTOCOL,FOLLOWEDBYASTATUSCODEANDASHORTDESCRIPTIONOFTHESTATUSCODEHERETHESTATUSCODEIS200,MEANINGTHEREQUESTWASEXECUTEDSUCCESSFULLYTHERESPONSEMESSAGEHASHEADERSJUSTLIKETHEREQUESTMESSAGEINTHISEXAMPLE,THELASTMODIFIEDHEADERGIVESTHEDATAANDTIMEFORWHENTHERESOURCEWASLASTMODIFIEDTHEBROWSERCANUSETHISINFORMATIONASATIMESTAMPINALOCALCACHETHENEXTTIMETHEUSERASKSFORTHISRESOURCE,HECANASKTHESERVERTSENDITONLYIFITSBEENUPDATEDSINCETHELASTTIMEITWASREQUESTEDTHECONTENTTYPEHEADERTELLSTHEBROWSERWHATTYPEOFRESPONSEDATATHEBODYCONTAINSANDTHECONTENTLENGTHHEADERHOWLARGEITISTHEOTHERHEADERSARESELFEXPLANATORYABLANKLINESEPARATESTHEHEADERSFROMTHEMESSAGEBODYHERETHEBODYISASIMPLEHTMLPAGEHELLOWORLDOFCOURSE,THEBODYCANCONTAINAMORECOMPLEXHTMLPAGEORANYOTHERTYPEOFCONTENTFOREXAMPLE,THEREQUESTMAYRETURNANHTMLPAGEWITHELEMENTSWHENTHEBROWSERREADSTHEFIRSTRESPONSEANDFINDTHEELEMENT,OFTENINPARALLELTHESERVERRETURNONERESPONSEFORTHEEACHIMAGEREQUEST,WITHACONTENTTYPEHEADERTELLINGWHATTYPEOFIMAGEITISFORINSTANCEIMAGE/GIFANDTHEBODYCONTAININGTHEBYTESTHATMAKEUPTHEIMAGETHEBROWSERTHENCOMBINESALLRESPONSETORENDERTHECOMPLETEPAGETHISINTERACTIONISILLUSTRATEDINFIGURE22213REQUESTPARAMETERSBESIDESTHEURIANDHEADERS,AREQUESTMESSAGECANCONTAINADDITIONALINFORMATIONINTHEFORMOFPARAMETERSIFTHEURIIDENTIFIESASERVERSIDEPROGRAMFORDISPLAYINGWEATHERINFORMATION,FOREXAMPLE,REQUESTPARAME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