JSP应用框架中英文对照外文翻译文献.docx

JSP应用框架中英文对照外文翻译文献.docx

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JSP应用框架中英文对照外文翻译文献

中英文对照外文翻译文

JSPAPPLICATIONFRAMEWORKS

1.1WHATAREAPPLICATIONFRAMEWORKS:

Aframeworkisareusable,semi-completeapplicationthatcanbespecializedtoproducecustomapplications[Johnson].Likepeople,softwareapplicationsaremorealikethantheyaredifferent.Theyrunonthesamecomputers,expectinputfromthesamedevices,outputtothesamedisplays,andsavedatatothesameharddisks.Developersworkingonconventionaldesktopapplicationsareaccustomedtotoolkitsanddevelopmentenvironmentsthatleveragethesamenessbetweenapplications.Applicationframeworksbuildonthiscommongroundtoprovidedeveloperswithareusablestructurethatcanserveasthefoundationfortheirownproducts.

Aframeworkprovidesdeveloperswithasetofbackbonecomponentsthathavethefollowingcharacteristics:

Frameworksaretheclassicbuild-versus-buyproposition.Ifyoubuildit,youwillunderstanditwhenyouaredone—buthowlongwillitbebeforeyoucanrollyourown?

Ifyoubuyit,youwillhavetoclimbthelearningcurve—andhowlongisthatgoingtotake?

Thereisnorightanswerhere,butmostobserverswouldagreethatframeworkssuchasStrutsprovideasignificantreturnoninvestmentcomparedtostartingfromscratch,especiallyforlargerprojects.

1.2OTHERTYPESOFFRAMEWORKS:

Theideaofaframeworkappliesnotonlytoapplicationsbuttoapplicationcomponentsaswell.Throughoutthisarticle,weintroduceothertypesofframeworksthatyoucanusewithStruts.TheseincludetheLucenesearchengine,theScaffoldtoolkit,theStrutsvalidator,andtheTilestaglibrary.Likeapplicationframeworks,thesetoolsprovidesemi-completeversionsofasubsystemthatcanbespecializedtoprovideacustomcomponent.

Someframeworkshavebeenlinkedtoaproprietarydevelopmentenvironment.ThisisnotthecasewithStrutsoranyoftheotherframeworksshowninthisbook.YoucanuseanydevelopmentenvironmentwithStruts:

VisualAgeforJava,JBuilder,Eclipse,Emacs,andTextpadareallpopularchoicesamongStrutsdevelopers.IfyoucanuseitwithJava,youcanuseitwithStruts.

1.3ENABLINGTECHNOLPGIES:

ApplicationsdevelopedwithStrutsarebasedonanumberofenablingtechnologies.ThesecomponentsarenotspecifictoStrutsandunderlieeveryJavawebapplication.AreasonthatdevelopersuseframeworkslikeStrutsistohidethenastydetailsbehindacronymslikeHTTP,CGI,andJSP.AsaStrutsdeveloper,youdon’tneedtobeanalphabetsoupguru,butaworkingknowledgeofthesebasetechnologiescanhelpyoudevisecreativesolutionstotrickyproblems.

1.4HYPERTEXTTRANSFERPROTOCOL（HTTP）:

Whenmediatingtalksbetweennations,diplomatsoftenfollowaformalprotocol.

Diplomaticprotocolsaredesignedtoavoidmisunderstandingsandtokeepnegotiationsfrombreakingdown.Inasimilarvein,whencomputersneedtotalk,theyalsofollowaformalprotocol.Theprotocoldefineshowdataistransmittedandhowtodecodeitonceitarrives.WebapplicationsusetheHypertextTransferProtocol（HTTP）tomovedatabetweenthebrowserrunningonyourcomputerandtheapplicationrunningontheserver.

ManyserverapplicationscommunicateusingprotocolsotherthanHTTP.Someofthesemaintainanongoingconnectionbetweenthecomputers.Theapplicationserverknowsexactlywhoisconnectedatalltimesandcantellwhenaconnectionisdropped.Becausetheyknowthestateofeachconnectionandtheidentityofeachpersonusingit,theseareknownasstatefulprotocols.

Bycontrast,HTTPisknownasastatelessprotocol.AnHTTPserverwillacceptanyrequestfromanyclientandwillalwaysprovidesometypeofresponse,eveniftheresponseisjusttosayno.Withouttheoverheadofnegotiatingandretainingaconnection,statelessprotocolscanhandlealargevolumeofrequests.ThisisonereasonwhytheInternethasbeenabletoscaletomillionsofcomputers.

AnotherreasonHTTPhasbecometheuniversalstandardisitssimplicity.AnHTTPrequestlookslikeanordinarytextdocument.ThishasmadeiteasyforapplicationstomakeHTTPrequests.YoucanevensendanHTTPrequestbyhandusingastandardutilitysuchasTelnet.WhentheHTTPresponsecomesback,itisalsoinplaintextthatdeveloperscanread.

ThefirstlineintheHTTPrequestcontainsthemethod,followedbythelocationoftherequestedresourceandtheversionofHTTP.ZeroormoreHTTPrequestheadersfollowtheinitialline.TheHTTPheadersprovideadditionalinformationtotheserver.Thiscanincludethebrowsertypeandversion,acceptabledocumenttypes,andthebrowser’scookies,justtonameafew.Ofthesevenrequestmethods,GETandPOSTarebyfarthemostpopular.

Oncetheserverhasreceivedandservicedtherequest,itwillissueanHTTPresponse.ThefirstlineintheresponseiscalledthestatuslineandcarriestheHTTPprotocolversion,anumericstatus,andabriefdescriptionofthestatus.Followingthestatusline,theserverwillreturnasetofHTTPresponseheadersthatworkinawaysimilartotherequestheaders.

Aswementioned,HTTPdoesnotpreservestateinformationbetweenrequests.Theserverlogstherequest,sendstheresponse,andgoesblissfullyontothenextrequest.Whilesimpleandefficient,astatelessprotocolisproblematicfordynamicapplicationsthatneedtokeeptrackoftheirusers.

CookiesandURLrewritingaretwocommonwaystokeeptrackofusersbetweenrequests.Acookieisaspecialpacketofinformationontheuser’scomputer.URLrewritingstoresaspecialreferenceinthepageaddressthataJavaservercanusetotrackusers.Bothapproachesareseamless,andusingeithermeansextraworkwhendevelopingawebapplication.Onitsown,astandardHTTPwebserverdoesnottrafficindynamiccontent.Itmainlyusestherequesttolocateafileandthenreturnsthatfileintheresponse.ThefileistypicallyformattedusingHypertextMarkupLanguage（HTML）[W3C,HTML]thatthewebbrowsercanformatanddisplay.TheHTMLpageoftenincludeshypertextlinkstootherwebpagesandmaydisplayanynumberofothergoodies,suchasimagesandvideos.Theuserclicksalinktomakeanotherrequest,andtheprocessbeginsanew.

Standardwebservershandlestaticcontentandimagesquitewellbutneedahelpinghandtoprovideuserswithacustomized,dynamicresponse.

DEFINITION:

StaticcontentontheWebcomesdirectlyfromtextordatafiles,likeHTMLorJPEGfiles.Thesefilesmightbechangedfromtimetotime,buttheyarenotalteredautomaticallywhenrequestedbyawebbrowser.Dynamiccontent,ontheotherhand,isgeneratedonthefly,typicallyinresponsetoanindividualizedrequestfromabrowser.

1.5COMMONGATEWAYINTERFACE（CGI）:

ThefirstwidelyusedstandardforproducingdynamiccontentwastheCommonGatewayInterface（CGI）.CGIusesstandardoperatingsystemfeatures,suchasenvironmentvariablesandstandardinputandoutput,tocreateabridge,orgateway,betweenthewebserverandotherapplicationsonthehostmachine.Theotherapplicationscanlookattherequestsenttothembythewebserverandcreateacustomizedresponse.

Whenawebserverreceivesarequestthat’sintendedforaCGIprogram,itrunsthatprogramandprovidestheprogramwithinformationfromtheincomingrequest.TheCGIprogramrunsandsendsitsoutputbacktotheserver.Thewebserverthenrelaystheresponsetothebrowser.

CGIdefinesasetofconventionsregardingwhatinformationitwillpassasenvironmentvariablesandhowitexpectsstandardinputandoutputtobeused.LikeHTTP,CGIisflexibleandeasytoimplement,andagreatnumberofCGI-awareprogramshavebeenwritten.

ThemaindrawbacktoCGIisthatitmustrunanewcopyoftheCGI-awareprogramforeachrequest.Thisisarelativelyexpensiveprocessthatcanbogdownhigh-volumesiteswherethousandsofrequestsareservicedperminute.AnotherdrawbackisthatCGIprogramstendtobeplatformdependent.ACGIprogramwrittenforoneoperatingsystemmaynotrunonanother.

1.6JAVASERVLETS:

Sun’sJavaServletplatformdirectlyaddressesthetwomaindrawbacksofCGIprograms.First,servletsofferbetterperformanceandutilizationofresourcesthanconventionalCGIprograms.Second,thewrite-once,run-anywherenatureofJavameansthatservletsareportablebetweenoperatingsystemsthathaveaJavaVirtualMachine（JVM）.

AServletlooksandfeelslikeaminiaturewebserver.Itreceivesarequestandrendersaresponse.But,unlikeconventionalwebservers,theServletapplicationprogramminginterface（API）isspecificallydesignedtohelpJavadeveloperscreatedynamicapplications.

TheServletitselfissimplyaJavaclassthathasbeencompiledintobytecode,likeanyotherJavaobject.TheServlethasaccesstoarichAPIofHTTP-specificservices,butitisstilljustanotherJavaobjectrunninginanapplicationandcanleverageallyourotherJavaassets.

Togiveconventionalwebserversaccesstoservlets,theservletsarepluggedintocontainers.TheServletcontainerisattachedtothewebserver.EachServletcandeclarewhatURLpatternsitwouldliketohandle.Whenarequestmatchingaregisteredpatternarrives,thewebserverpassestherequesttothecontainer,andthecontainerinvokestheServlet.

ButunlikeCGIprograms,anewServletisnotcreatedforeachrequest.OncethecontainerinstantiatestheServlet,itwilljustcreateanewthreadforeachrequest.JavathreadsaremuchlessexpensivethantheserverprocessesusedbyCGIprograms.OncetheServlethasbeencreated,usingitforadditionalrequestsincursverylittleoverhead.Servletdeveloperscanusetheinit（）methodtoholdreferencestoexpensiveresources,suchasdatabaseconnectionsorEJBHomeInterfaces,sothatthe