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1JSPAPPLICATIONFRAMEWORKS1.1WHATAREAPPLICATIONFRAMEWORKSAframeworkisareusable,semicompleteapplicationthatcanbespecializedtoproducecustomapplicationsJohnson.Likepeople,softwareapplicationsaremorealikethantheyaredifferent.Theyrunonthesamecomputers,expectinputfromthesamedevices,outputtothesamedisplays,andsavedatatothesameharddisks.Developersworkingonconventionaldesktopapplicationsareaccustomedtotoolkitsanddevelopmentenvironmentsthatleveragethesamenessbetweenapplications.Applicationframeworksbuildonthiscommongroundtoprovidedeveloperswithareusablestructurethatcanserveasthefoundationfortheirownproducts.AframeworkprovidesdeveloperswithasetofbackbonecomponentsthathavethefollowingcharacteristicsFrameworksaretheclassicbuildversusbuyproposition.Ifyoubuildit,youwillunderstanditwhenyouaredonebuthowlongwillitbebeforeyoucanrollyourownIfyoubuyit,youwillhavetoclimbthelearningcurveandhowlongisthatgoingtotakeThereisnorightanswerhere,butmostobserverswouldagreethatframeworkssuchasStrutsprovideasignificantreturnoninvestmentcomparedtostartingfromscratch,especiallyforlargerprojects.1.2OTHERTYPESOFFRAMEWORKSTheideaofaframeworkappliesnotonlytoapplicationsbuttoapplicationcomponentsaswell.Throughoutthisarticle,weintroduceothertypesofframeworksthatyoucanusewithStruts.TheseincludetheLucenesearchengine,theScaffoldtoolkit,theStrutsvalidator,andtheTilestaglibrary.Likeapplicationframeworks,thesetoolsprovidesemicompleteversionsofasubsystemthatcanbespecializedtoprovideacustomcomponent.Someframeworkshavebeenlinkedtoaproprietarydevelopmentenvironment.ThisisnotthecasewithStrutsoranyoftheotherframeworksshowninthisbook.YoucanuseanydevelopmentenvironmentwithStrutsVisualAgeforJava,JBuilder,Eclipse,Emacs,andTextpadareallpopularchoicesamongStrutsdevelopers.IfyoucanuseitwithJava,youcanuseitwithStruts.1.3ENABLINGTECHNOLPGIESApplicationsdevelopedwithStrutsarebasedonanumberofenablingtechnologies.ThesecomponentsarenotspecifictoStrutsandunderlieeveryJavawebapplication.Areason2thatdevelopersuseframeworkslikeStrutsistohidethenastydetailsbehindacronymslikeHTTP,CGI,andJSP.AsaStrutsdeveloper,youdontneedtobeanalphabetsoupguru,butaworkingknowledgeofthesebasetechnologiescanhelpyoudevisecreativesolutionstotrickyproblems.1.4HYPERTEXTTRANSFERPROTOCOLHTTPWhenmediatingtalksbetweennations,diplomatsoftenfollowaformalprotocol.Diplomaticprotocolsaredesignedtoavoidmisunderstandingsandtokeepnegotiationsfrombreakingdown.Inasimilarvein,whencomputersneedtotalk,theyalsofollowaformalprotocol.Theprotocoldefineshowdataistransmittedandhowtodecodeitonceitarrives.WebapplicationsusetheHypertextTransferProtocolHTTPtomovedatabetweenthebrowserrunningonyourcomputerandtheapplicationrunningontheserver.ManyserverapplicationscommunicateusingprotocolsotherthanHTTP.Someofthesemaintainanongoingconnectionbetweenthecomputers.Theapplicationserverknowsexactlywhoisconnectedatalltimesandcantellwhenaconnectionisdropped.Becausetheyknowthestateofeachconnectionandtheidentityofeachpersonusingit,theseareknownasstatefulprotocols.Bycontrast,HTTPisknownasastatelessprotocol.AnHTTPserverwillacceptanyrequestfromanyclientandwillalwaysprovidesometypeofresponse,eveniftheresponseisjusttosayno.Withouttheoverheadofnegotiatingandretainingaconnection,statelessprotocolscanhandlealargevolumeofrequests.ThisisonereasonwhytheInternethasbeenabletoscaletomillionsofcomputers.AnotherreasonHTTPhasbecometheuniversalstandardisitssimplicity.AnHTTPrequestlookslikeanordinarytextdocument.ThishasmadeiteasyforapplicationstomakeHTTPrequests.YoucanevensendanHTTPrequestbyhandusingastandardutilitysuchasTelnet.WhentheHTTPresponsecomesback,itisalsoinplaintextthatdeveloperscanread.ThefirstlineintheHTTPrequestcontainsthemethod,followedbythelocationoftherequestedresourceandtheversionofHTTP.ZeroormoreHTTPrequestheadersfollowtheinitialline.TheHTTPheadersprovideadditionalinformationtotheserver.Thiscanincludethebrowsertypeandversion,acceptabledocumenttypes,andthebrowserscookies,justtonameafew.Ofthesevenrequestmethods,GETandPOSTarebyfarthemostpopular.Oncetheserverhasreceivedandservicedtherequest,itwillissueanHTTPresponse.ThefirstlineintheresponseiscalledthestatuslineandcarriestheHTTPprotocolversion,anumericstatus,andabriefdescriptionofthestatus.Followingthestatusline,theserverwillreturnasetofHTTPresponseheadersthatworkinawaysimilartotherequestheaders.Aswementioned,HTTPdoesnotpreservestateinformationbetweenrequests.The3serverlogstherequest,sendstheresponse,andgoesblissfullyontothenextrequest.Whilesimpleandefficient,astatelessprotocolisproblematicfordynamicapplicationsthatneedtokeeptrackoftheirusers.CookiesandURLrewritingaretwocommonwaystokeeptrackofusersbetweenrequests.Acookieisaspecialpacketofinformationontheuserscomputer.URLrewritingstoresaspecialreferenceinthepageaddressthataJavaservercanusetotrackusers.Bothapproachesareseamless,andusingeithermeansextraworkwhendevelopingawebapplication.Onitsown,astandardHTTPwebserverdoesnottrafficindynamiccontent.Itmainlyusestherequesttolocateafileandthenreturnsthatfileintheresponse.ThefileistypicallyformattedusingHypertextMarkupLanguageHTMLW3C,HTMLthatthewebbrowsercanformatanddisplay.TheHTMLpageoftenincludeshypertextlinkstootherwebpagesandmaydisplayanynumberofothergoodies,suchasimagesandvideos.Theuserclicksalinktomakeanotherrequest,andtheprocessbeginsanew.Standardwebservershandlestaticcontentandimagesquitewellbutneedahelpinghandtoprovideuserswithacustomized,dynamicresponse.DEFINITIONStaticcontentontheWebcomesdirectlyfromtextordatafiles,likeHTMLorJPEGfiles.Thesefilesmightbechangedfromtimetotime,buttheyarenotalteredautomaticallywhenrequestedbyawebbrowser.Dynamiccontent,ontheotherhand,isgeneratedonthefly,typicallyinresponsetoanindividualizedrequestfromabrowser.1.5COMMONGATEWAYINTERFACECGIThefirstwidelyusedstandardforproducingdynamiccontentwastheCommonGatewayInterfaceCGI.CGIusesstandardoperatingsystemfeatures,suchasenvironmentvariablesandstandardinputandoutput,tocreateabridge,orgateway,betweenthewebserverandotherapplicationsonthehostmachine.Theotherapplicationscanlookattherequestsenttothembythewebserverandcreateacustomizedresponse.WhenawebserverreceivesarequestthatsintendedforaCGIprogram,itrunsthatprogramandprovidestheprogramwithinformationfromtheincomingrequest.TheCGIprogramrunsandsendsitsoutputbacktotheserver.Thewebserverthenrelaystheresponsetothebrowser.CGIdefinesasetofconventionsregardingwhatinformationitwillpassasenvironmentvariablesandhowitexpectsstandardinputandoutputtobeused.LikeHTTP,CGIisflexibleandeasytoimplement,andagreatnumberofCGIawareprogramshavebeenwritten.ThemaindrawbacktoCGIisthatitmustrunanewcopyoftheCGIawareprogramforeachrequest.Thisisarelativelyexpensiveprocessthatcanbogdownhighvolumesiteswherethousandsofrequestsareservicedperminute.AnotherdrawbackisthatCGIprograms4tendtobeplatformdependent.ACGIprogramwrittenforoneoperatingsystemmaynotrunonanother.1.6JAVASERVLETSSunsJavaServletplatformdirectlyaddressesthetwomaindrawbacksofCGIprograms.First,servletsofferbetterperformanceandutilizationofresourcesthanconventionalCGIprograms.Second,thewriteonce,runanywherenatureofJavameansthatservletsareportablebetweenoperatingsystemsthathaveaJavaVirtualMachineJVM.AServletlooksandfeelslikeaminiaturewebserver.Itreceivesarequestandrendersaresponse.But,unlikeconventionalwebservers,theServletapplicationprogramminginterfaceAPIisspecificallydesignedtohelpJavadeveloperscreatedynamicapplications.TheServletitselfissimplyaJavaclassthathasbeencompiledintobytecode,likeanyotherJavaobject.TheServlethasaccesstoarichAPIofHTTPspecificservices,butitisstilljustanotherJavaobjectrunninginanapplicationandcanleverageallyourotherJavaassets.Togiveconventionalwebserversaccesstoservlets,theservletsarepluggedintocontainers.TheServletcontainerisattachedtothewebserver.EachServletcandeclarewhatURLpatternsitwouldliketohandle.Whenarequestmatchingaregisteredpatternarrives,thewebserverpassestherequesttothecontainer,andthecontainerinvokestheServlet.ButunlikeCGIprograms,anewServletisnotcreatedforeachrequest.OncethecontainerinstantiatestheServlet,itwilljustcreateanewthreadforeachrequest.JavathreadsaremuchlessexpensivethantheserverprocessesusedbyCGIprograms.OncetheServlethasbeencreated,usingitforadditionalrequestsincursverylittleoverhead.Servletdeveloperscanusetheinitmethodtoholdreferencestoexpensiveresources,suchasdatabaseconnectionsorEJBHomeInterfaces,sothattheycanbesharedbetweenrequests.Acquiringresourceslikethesecantakeseveralsecondswhichislongerthanmanysurfersarewillingtowait.Theotheredgeoftheswordisthat,sinceservletsaremultithreaded,Servletdevelopersmusttakespecialcaretobesuretheirservletsarethreadsafe.1.7JAVASERVERPAGESWhileJavaservletsareabigstepupfromCGIprograms,theyarenotapanacea.Togeneratetheresponse,developersarestillstuckwithusingprintlnstatementstorendertheHTML.Codethatlookslikeout.printlnOnelineofHTML.out.printlnAnotherlineofHTML.ItisalltoocommoninservletsthatgeneratetheHTTPresponse.TherearelibrariesthatcanhelpyougenerateHTML,butasapplicationsgrowmorecomplex,Javadevelopersend5upbeingcastintotheroleofHTMLpagedesigners.Meanwhile,giventhechoice,mostprojectmanagersprefertodividedevelopmentteamsintospecializedgroups.TheylikeHTMLdesignerstobeworkingonthepresentationwhileJavaengineerssweatthebusinesslogic.Usingservletsaloneencouragesmixingmarkupwithbusinesslogic,makingitdifficultforteammemberstospecialize.Tosolvethisproblem,Sunturnedtotheideaofusingserverpagestocombinescriptingandtemplatingtechnologiesintoasinglecomponent.TobuildJavaServerPages,developersstartbycreatingHTMLpagesinthesameoldway,usingthesameoldHTMLsyntax.Tobringdynamiccontentintothepage,thedevelopercanalsoplaceJSPscriptingelementsonthepage.ScriptingelementsaretagsthatencapsulatelogicthatisrecognizedbytheJSP.YoucaneasilypickoutscriptingelementsonJSPpagesbylookingforcodethatbeginswith.TobeseenasaJSPpage,thefilejustneedstobesavedwithanextensionofjsp.WhenaclientrequeststheJSPpage,thecontainertranslatesthepageintoasourcecodefileforaJavaServletandcompilesthesourceintoaJavaclassfilejustasyouwoulddoifyouwerewritingaServletfromscratch.Atruntime,thecontainercanalsocheckthelastmodifieddateoftheJSPfileagainsttheclassfile.IftheJSPfilehaschangedsinceitwaslastcompiled,thecontainerwillretranslateandrebuildthepagealloveragain.ProjectmanagerscannowassignthepresentationlayertoHTMLdevelopers,whothenpassontheirworktoJavadeveloperstocompletethebusinesslogicportion.TheimportantthingtorememberisthataJSPpageisreallyjustaServlet.AnythingyoucandowithaServlet,youcandowithaJSP.1.8JAVABEANSJavaBeansareJavaclasseswhichconformtoasetofdesignpatternsthatmakethemeasiertousewithdevelopmenttoolsandothercomponents.DEFINITIONAJavaBeanisareusablesoftwarecomponentwritteninJava.ToqualifyasaJavaBean,theclassmustbeconcreteandpublic,andhaveanonargumentconstructor.JavaBeansexposeinternalfieldsaspropertiesbyprovidingpublicmethodsthatfollowaconsistentdesignpattern.Knowingthatthepropertynamesfollowthispattern,otherJavaclassesareabletouseintrospectiontodiscoverandmanipulateJavaBeanproperties.TheJavaBeandesignpatternsprovideaccesstothebeansinternalstatethroughtwoflavorsofmethodsaccessorsareusedtoreadaJavaBeansstatemutatorsareusedtochangeaJavaBeansstate.Mutatorsarealwaysprefixedwithlowercasetokensetfollowedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.Thereturnvalueisalways6voidmutatorsonlychangepropertyvalues,theydonotretrievethem.Themutatorforasimplepropertytakesonlyoneparameterinitssignature,whichcanbeofanytype.Mutatorsareoftennicknamedsettersaftertheirprefix.ThemutatormethodsignatureforaweightpropertyofthetypeDoublewouldbepublicvoidsetWeightDoubleweightAsimilardesignpatternisusedtocreatetheaccessormethodsignature.Accessormethodsarealwaysprefixedwiththelowercasetokenget,followedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.Thereturnvaluewillmatchthemethodparameterinthecorrespondingmutator.Accessorsforsimplepropertiescannotacceptparametersintheirmethodsignature.Notsurprisingly,accessorsareoftencalledgetters.TheaccessormethodsignatureforourweightpropertyispublicDoublegetWeightIftheaccessorreturnsalogicalvalue,thereisavariantpattern.Insteadofusingthelowercasetokenget,alogicalpropertycanusetheprefixis,followedbythepropertyname.Thefirstcharacterinthepropertynamemustbeuppercase.ThereturnvaluewillalwaysbealogicalvalueeitherbooleanorBoolean.Logicalaccessorscannotacceptparametersintheirmethodsignature.ThebooleanaccessormethodsignatureforanonpropertywouldbepublicbooleanisOnThecanonicalmethodsignaturesplayanimportantrolewhenworkingwithJavaBeans.OthercomponentsareabletousetheJavaReflectionAPItodiscoveraJavaBeanspropertiesbylookingformethodsprefixedbyset,is,orget.IfacomponentfindssuchasignatureonaJavaBean,itknowsthatthemethodcanbeusedtoaccessorchangethebeansproperties.SunintroducedJavaBeanstoworkwithGUIcomponents,buttheyarenowusedwitheveryaspectofJavadevelopment,includingwebapplications.WhenSunengineersdevelopedtheJSPtagextensionclasses,theydesignedthemtoworkwithJavaBeans.ThedynamicdataforapagecanbepassedasaJavaBean,andtheJSPtagcanthenusethebeanspropertiestocustomizetheoutput.Resourcehttp//book.huihoo.com/pdf/introductiontomachinelearning/7JSP应用框架1.1什么是应用框架框架(framework)是可以被重用的一种半成品的应用程序,我们可以用它来制作专门的定制程序。这就如同人类一样,软件应用的相同点要远远大于不同点。它们在相同的机器上运行,把信息输入相同的设备,用相同的显示设备显示,并将数据村存储到相同的硬件设备中。传统的桌面开发应用的开发人员更习惯于使用可以涵盖应用开发同一性的工具包和开发环境。框架提供给开发人员这些公共基础上的应用框架从而为他们的产品提供可重用服务的基本架构。框架是经典的构建购买命题。如果你独立构建它那当你可以独立完成的时候你就会理解它,但是你又花费了多长时间来用之自如呢如果你购买了它,你必须得克服学习曲线并且学习它将话费你大约多长时间这里没有明确的答案,因此许多专家会认为,例如Struts这样的框架能提供比全新的开发更显著的投资回报,特别是对大型项目来说尤为显著。1.2其它类型的框架框架的思想不仅仅用于应用程序也可以应用于组件。通过这些,我们以前也提到过一些可以与在Struts一起使用的框架。其中包括Lucene搜索引擎,Scaffold工具包,Struts验证器,以及Tiles标签库等等。就像应用框架一样,这些工具也提供了一些可以用在用户的定制组件之中的半成品。某些框架已经呗限定于专门的开发环境之中。而Struts以及本文中涉及的其他框架却不是这样的。你可以在各种各样的开发环境中来使用Struts例如VisualAgeforJava,JBuilder,Eclipse,Emacs,甚至在Textpad中都可以使用Struts。只要是可以用来开发Java的工具都可以用来开发Struts。1.3使用的技术使用Struts的应用开发基于大量的其它技术。这些技术并不专属于Struts而是针对所有Javaweb应用的。开发人员之所以使用Struts之类的框架的一个原因是为了隐藏在HTTP,CGI,以及JSP之类技术后面的繁琐细节。作为一个Struts开发人员,你并不需要完全了解Struts的所有相关知识,但是它们有助于解决比较麻烦的问题。1.4超文本传输协议HTTP当两个国家质检进行交涉时,外交官们总是遵循一定的正式协议。设计外交协议的主要目的是避免误会从而防止谈判破裂。同样,当两台计算机需要进行会话时,它们也要遵循一个正式协议。这个协议定义数据如何传输以及到达后如何
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