外文翻译-将Servlet和JSP组合使用

0外文原文CombiningJSPandServletsThetechnologyofJSPandServletisthemostimportanttechnologywhichuseJavatechnologytoexploitrequestofserver,anditisalsothestandardwhichexploitbusinessapplication.Javadevelopersprefertouseitforavarietyofreasons,oneofwhichisalreadyfamiliarwiththeJavalanguageforthedevelopmentofthistechnologyareeasytolearnJavatotheotherisapreparation,runeverywheretobringtheconceptofWebapplications,Toachieveaone-preparedeverywhererealized.Andmoreimportantly,iffollowedsomeoftheprinciplesofgooddesign,itcanbesaidofseparatingandcontenttocreatehigh-quality,reusable,easytomaintainandmodifytheapplication.Forexample,ifthedocumentinHTMLembeddedJavacodetoomuch(script),willleadthedevelopedapplicationisextremelycomplex,difficulttoread,itisnoteasyreuse,butalsoforfuturemaintenanceandmodificationwillalsocausedifficulties.Infact,CSDNtheJSP/Servletforum,canoftenseesomequestions,thecodeisverylong,canlogicisnotveryclear,alargenumberofHTMLandJavacodemixedtogether.Thisistherandomdevelopmentofthedefects.EarlydynamicpagesmainlyCGI(CommonGatewayInterface,publicGatewayInterface)technology,youcanusedifferentlanguagesoftheCGIprograms,suchasVB,C/C+orDelphi,andsoon.ThoughthetechnologyofCGIisdevelopedandpowerful,becauseofdifficultiesinprogramming,andlowefficiency,modifycomplexshortcomings,itisgraduallybeingreplacedbythetrend.Ofallthenewtechnology,JSP/Servletwithmoreefficientandeasytoprogram,morepowerful,moresecureandhasagoodportability,theyhavebeenmanypeoplebelievethatthefutureisthemostdynamicsiteofthefuturedevelopmentoftechnology.SimilartoCGI,Servletsupportrequest/responsemodel.Whenacustomersubmitarequesttotheserver,theserverpresentedtherequestServlet,Servletresponsibleforhandlingrequestsandgeneratearesponse,andthengavetheserver,andthenfromtheserversenttothecustomer.AndtheCGIisdifferent,Servletnotgenerateanewprocess,butwithHTTPServeratthesameprocess.Itthreadsthroughtheuseoftechnology,reducetheservercosts.Servlethandlingoftherequestprocessisthis:Whenreceivedfromtheclientsrequest,callingservicemethods,themethodofServletarrivalofthefirstjudgementiswhattypeofrequest(GET/POST/HEAD),thencallstheappropriatetreatment(DoGet/doPost/doHead)andgeneratearesponse.1Althoughsuchacomplex,infact,simplysaidtoServletisaJavaclass.AndthegeneralcategoryofthedifferenceisthatthistypeoperatinginaServletcontainer,whichcanprovidesessionmanagementandtargetedlife-cyclemanagement.SothatwhenyouusetheServlet,youcangetallthebenefitsoftheJavaplatform,includingthesafetyofthemanagement,useJDBCaccessthedatabaseandcross-platformcapability.Moreover,Servletusingthread,andcandevelopmoreefficientWebapplications.JSPtechnologyisakeyJ2EEtechnology,itatahigherlevelofabstractionofaServlet.ItallowsconventionalstaticanddynamicHTMLcontentgeneratedbycombininganHTMLpagelookslike,butasaServlettorun.TherearemanycommercialapplicationserversupportJSPtechnology,suchasBEAWebLogic,IBMWebSphere,JRun,andsoon.JSPandServletusemorethansimple.IfyouhaveaJSPsupportforWebservers,andaJSPdocument,youcanputitFangdaoanystaticHTMLfilescanbeplaced,donothavetocompile,donothavetopack,donothavetoClassPathsettings,youcanvisitasordinaryWebItdidvisit,theserverwillautomaticallyhelpyoutodootherwork.JSPdocumentlookslikeanordinarystaticHTMLdocument,butinsidecontainsanumberofJavacode.Ituses.Jspthesuffix,usedtotelltheserverthisdocumentinneedofspecialtreatment.WhenwevisitaJSPpage,thedocumentwillfirstbetranslatedintoaJSPengineJavasourcefiles,isactuallyaServlet,andcompiler,andthen,likeotherServlet,fromServletenginetohandle.Servletengineofthistypeloading,handlingrequestsfromcustomers,andtheresultsreturnedtothecustomer,asshownbelow:Figure1:CallingtheprocessofJSPpagesAfteranothervisitthispagetothecustomer,aslongasthepapertherehavebeennochanges,JSPenginehasbeenloadeddirectlycalltheServlet.Ifyouhavealreadybeenmodified,itwillbeonceagaintheimplementationoftheaboveprocess,translate,compileandload.Infact,thisistheso-calledfirstpersontopunishment.Becausewhenthefirstvisittotheimplementationofaseriesoftheaboveprocess,sowillspendsometimeaftersuchavisitwouldnot.2JavaservletsofferapowerfulAPIthatprovidesaccesstoalltheinformationabouttherequest,thesession,biningJSPwithservletsletsyouclearlyseparatetheapplicationlogicfromthepresentationoftheapplication;inotherwords,itletsyouusethemostappropriatecomponenttypefortherolesofModel,ViewandController.Servlets,Filters,andListenersAservletisaJavaclassthatextendsaserverwithfunctionalityforprocessingarequestandproducingaresponse.ItsimplementedusingtheclassesandinterfacesdefinedbytheServletAPI.TheAPIconsistsoftwopackages:thejavax.servletpackagecontainsclassesandinterfacesthatareprotocol-independent,whilethejavax.servlet.httppackageprovidesHTTP-specificextensionsandutilityclasses.Whatmakesaservletaservletisthattheclassimplementsaninterfacenamedjavax.servlet.Servlet,eitherdirectlyorbyextendingoneofthesupportclasses.Thisinterfacedefinesthemethodsusedbythewebcontainertomanageandinteractwiththeservlet.AservletforprocessingHTTPrequeststypicallyextendsthejavax.servlet.http.HttpServletclass.ThisclassimplementstheServletinterfaceandprovidesadditionalmethodssuitableforHTTPprocessing.ServletLifecycleThewebcontainermanagesallaspectsoftheservletslifecycle.Itcreatesaninstanceoftheservletclasswhenneeded,passesrequeststotheinstanceforprocessing,andeventuallyremovestheinstance.ForanHttpServlet,thecontainercallsthefollowingmethodsattheappropriatetimesintheservletlifecycle.BesidesthedoGet()anddoPost()methods,therearemethodscorrespondingtotheotherHTTPmethods:doDelete(),doHead(),doOptions(),doPut(),anddoTrace().Typicallyyoudontimplementthesemethods;theHttpServletclassalreadytakescareofHEAD,OPTIONS,andTRACErequestsinawaythatssuitableformostservlets,andtheDELETEandPUTHTTPmethodsarerarelyusedinawebapplication.Itsimportanttorealizethatthecontainercreatesonlyoneinstanceofeachservlet.Thismeansthattheservletmustbethreadsafe-abletohandlemultiplerequestsatthesametime,eachexecutingasaseparatethreadthroughtheservletcode.Withoutgettinglostindetails,yousatisfythisrequirementwithregardstoinstancevariablesifyoumodifythereferencedobjectsonlyintheinit()anddestroy()methods,andjustreadthemintherequestprocessingmethods.CompilingandInstallingaServlet3Tocompileaservlet,youmustfirstensurethatyouhavetheJARfilecontainingallServletAPIclassesintheCLASSPATHenvironmentvariable.TheJARfileisdistributedwithallwebcontainers.Tomcatincludesitinafilecalledservlet.jar,locatedinthecommon/libdirectory.OnaWindowsplatform,youincludetheJARfileintheCLASSPATH.ReadingaRequestOneoftheargumentspassedtothedoGet()anddoPost()methodsisanobjectthatimplementstheHttpServletRequestinterface.Thisinterfacedefinesmethodsthatprovideaccesstoawealthofinformationabouttherequest.GeneratingaResponseBesidestherequestobject,thecontainerpassesanobjectthatimplementstheHttpServletResponseinterfaceasanargumenttothedoGet()anddoPost()methods.Thisinterfacedefinesmethodsforgettingawriterorstreamfortheresponsebody.Italsodefinesmethodsforsettingtheresponsestatuscodeandheaders.UsingFiltersandListenersTheservletspecificationdefinestwocomponenttypesbesideservlets:filtersandlisteners.ThesetwotypeswereintroducedintheServlet2.3specification,soifyoureusingacontainerthatdoesntyetsupportthisversionofthespecification,Imafraidyoureoutofluck.FiltersAfilterisacomponentthatcaninterceptarequesttargetedforaservlet,JSPpage,orstaticpage,aswellastheresponsebeforeitssenttotheclient.Thismakesiteasytocentralizetasksthatapplytoallrequests,suchasaccesscontrol,logging,andchargingforthecontentortheservicesofferedbytheapplication.Afilterhasfullaccesstothebodyandheadersoftherequestandresponse,soitcanalsoperformvarioustransformations.OneexampleiscompressingtheresponsebodyiftheAccept-Languagerequestheaderindicatesthattheclientcanhandleacompressedresponse.AfiltercanbeappliedtoeitheraspecificservletortoallrequestsmatchingaURLpattern,suchasURLsstartingwiththesamepathelementsorhavingthesameextension.ListenersListenersallowyourapplicationtoreacttocertainevents.PriortoServlet2.3,youcouldhandleonlysessionattributebindingevents(triggeredwhenanobjectwasaddedorremovedfromasession).Youcoulddothisbylettingtheobjectsavedasasessionattribute(usingthe4HttpSession.setAttribute()method)implementtheHttpSessionBindingListenerinterface.Withthenewinterfacesintroducedinthe2.3versionofthespecification,youcancreatelistenersforservletcontextandsessionlifecycleeventsaswellassessionactivationandpassivationevents(usedbyacontainerthattemporarilysavessessionstatetodiskormigratesasessiontoanotherserver).Anewsessionattributeeventlisteneralsomakesitpossibletodealwithattributebindingeventsforallsessionsinoneplace,insteadofplacingindividuallistenerobjectsineachsession.ThenewtypesoflistenersfollowthestandardJavaeventmodel.Inotherwords,alistenerisaclassthatimplementsoneormoreofthelistenerinterfaces.Theinterfacesdefinemethodsthatcorrespondtoevents.Thelistenerclassisregisteredwiththecontainerwhentheapplicationstarts,andthecontainerthencallstheeventmethodsattheappropriatetimes.InitializingSharedResourcesUsingaListenerBeanslikethistypicallyneedtobeinitializedbeforetheycanbeused.Forinstance,theymayneedareferencetoadatabaseorsomeotherexternaldatasourceandmaycreateaninitialinformationcacheinmemorytoprovidefastaccesseventothefirstrequestfordata.YoucanincludecodeforinitializationofthesharedresourcesintheservletandJSPpagesthatneedthem,butamoremodularapproachistoplaceallthiscodeinoneplaceandlettheotherpartsoftheapplicationworkontheassumptionthattheresourcesarealreadyinitializedandavailable.Anapplicationlifecyclelistenerisaperfecttoolforthistypeofresourceinitialization.Thistypeoflistenerimplementsthejavax.servlet.ServletContextListenerinterface,withmethodscalledbythecontainerwhentheapplicationstartsandwhenitshutsdown.PickingtheRightComponentTypeforEachTaskTheProjectBillboardapplicationintroducedisafairlycomplexapplication.Halfthepagesarepurecontrollerandbusinesslogicprocessing,itaccessesadatabasetoauthenticateusers,andmostpagesrequireaccesscontrol.Inreallife,itwouldlikelycontainevenmorepages,forinstance,pagesforaccesstoashareddocumentarchive,timeschedules,andasetofpagesforadministration.Astheapplicationevolves,itmaybecomehardtomaintainasapureJSPapplication.Itseasytoforgettoincludetheaccesscontrolcodeinnewpages.ThisisclearlyanapplicationthatcanbenefitfromusingacombinationofJSPpagesandthecomponenttypesdefinedbytheservletspecificationfortheMVCroles.Letslookatthemainrequirementsandseehowwecanmapthemtoappropriatecomponenttypes:Databaseaccessshouldbeabstracted,toavoidknowledgeofaspecificdataschema5ordatabaseengineinmorethanonepartoftheapplication:beansintheroleofModelcanbeusedtoaccomplishthis.Thedatabaseaccessbeansmustbemadeavailabletoallotherpartsoftheapplicationwhenitstarts:anapplicationlifecycleeventlisteneristheperfectcomponenttypeforthistask.Onlyauthenticatedusersmustbeallowedtousetheapplication:afiltercanperformaccesscontroltosatisfythisrequirement.RequestprocessingisbestdonewithJavacode:aservlet,actingastheController,fitsthebill.Itmustbeeasytochangethepresentation:thisiswhereJSPshines,actingastheView.Addingservlets,listeners,andfilterstothemixminimizestheneedforcomplexlogicintheJSPpages.PlacingallthiscodeinJavaclassesinsteadmakesitpossibletousearegularJavacompileranddebuggertofixpotentialproblems.CentralizedRequestProcessingUsingaServletWithaservletasthecommonentrypointforallapplicationrequests,yougaincontroloverthepageflowoftheapplication.Theservletcandecidewhichtypeofresponsetogeneratedependingontheoutcomeoftherequestedaction,suchasreturningacommonerrorpageforallrequeststhatfail,ordifferentresponsesdependingonthetypeofclientmakingtherequest.Withthehelpfromsomeutilityclasses,itcanalsoprovideservicessuchasinputvalidation,I18Npreparations,andingeneral,encourageamorestreamlinedapproachtorequesthandling.WhenyouuseaservletasaController,youmustdealwiththefollowingbasicrequirements:AllrequestsforprocessingmustbepassedtothesingleControllerservlet.Theservletmustbeabletodistinguishrequestsfordifferenttypesofprocessing.Hereareotherfeaturesyouwillwantsupportfor,eventhoughtheymaynotberequirementsforallapplications:Astrategyforextendingtheapplicationtosupportnewtypesofprocessingrequestsinaflexiblemanner.Amechanismforchangingthepageflowoftheapplicationwithoutmodifyingcode.MappingApplicationRequeststotheServletThefirstrequirementforusingaControllerservletisthatallrequestsmustpassthrough6it.Thiscanbesatisfiedinmanyways.Ifyouhaveplayedaroundabitwithservletspreviously,youreprobablyusedtoinvokingaservletwithaURIthatstartswith/myApp/servlet.ThisisaconventionintroducedbySunsJavaWebServer(JWS),thefirstproducttosupportservletsbeforetheAPIwasstandardized.Mostservletcontainerssupportthisconventiontoday,eventhoughitsnotformallydefinedintheservletspecification.7中文翻译将Servlet和JSP组合使用Servlet和JSP技术是用Java开发服务器端应用的主要技术，是开发商务应用表示端的标准。Java开发者喜欢使用它有多种原因，其一是对于已经熟悉Java语言的开发者来说这个技术容易学习；其二是Java把“一次编写，到处运行”的理念带入到Web应用中，实现了“一次编写，到处实现”。而且更为重要的是，如果遵循一些良好的设计原则的话，就可以把表示和内容相分离，创造出高质量的、可以复用的、易于维护和修改的应用程序。比方说，在HTML文档中如果嵌入过多的Java代码（scriptlet），就会导致开发出来的应用非常复杂、难以阅读、不容易复用，而且对以后的维护和修改也会造成困难。事实上，在CSDN的JSP/Servlet论坛中，经常可以看到一些提问，代码很长，可以逻辑却不是很清晰，大量的HTML和Java代码混杂在一起，让人看得一头雾水。这就是随意开发的弊端。早期的动态网页主要采用CGI（CommonGatewayInterface，公共网关接口）技术，你可以使用不同的语言编写CGI程序，如VB、C/C+或Delphi等。虽然CGI技术发展成熟且功能强大，但由于编程困难、效率低下、修改复杂等缺点，所以有逐渐被取代的趋势。在所有的新技术中，JSP/Servlet具备更高效、更容易编程、功能更强、更安全和具有良好的可移植性，因而被许多人认为是未来最有发展前途的动态网站技术。与CGI相似，Servlet支持请求/响应模型。当一个客户向服务器递交一个请求时，服务器把请求送给Servlet，Servlet负责处理请求并生成响应，然后送给服务器，再由服务器发送给客户。与CGI不同的是，Servlet没有生成新的进程，而是与HTTPServer处于同一进程中。它通过使用线程技术，减小了服务器的开销。Servlet处理请求的过程是这样的：当收到来自客户端的请求后，调用service方法，该方法中Servlet先判断到来的请求是什么类型的（GET/POST/HEAD），然后调用相应的处理方法（doGet/doPost/doHead）并生成响应。别看这么复杂，其实简单说来Servlet就是一个Java类。与一般类的不同之处是，这个类运行在一个Servlet容器内，可以提供session管理和对象生命周期管理。因而当你使用Servlet的时候，你可以得到Java平台的所有好处，包括安全性管理、使用JDBC访问数据库以及跨平台的能力。而且，Servlet使用线程，因而可以开发出效率更高的Web应用。JSP技术是J2EE的一个关键技术，它在更高一级的层次上抽象Servlet。它可以让常规静态HTML与动态产生的内容相结合，看起来像一个HTML网页，却作为Servlet来运行。现在有许多商业应用服务器支持JSP技术，比如BEAWebLogic、IBMWebSphere、JRun等等。使用JSP比用Servlet更简单。如果你有一个支持JSP的Web服务器，并且有一个JSP文件，你可以把它放倒任何静态HTML文件可以放置的位置，不用编译，不用打包，也不用进行ClassPath的设置，就可以像访问普通网页那样访问它，服务器会自动帮你做好其他的工作。JSP文件看起来就像一个普通静态HTML文件，只不过里面包含了一些Java代码。它使用.jsp的后缀，用来告诉服务器这个文件需要特殊的处理。当我们访问一个JSP页面的时候，这个文件首先会被JSP引擎翻译为一个Java源文件，8其实就是一个Servlet，并进行编译，然后像其他Servlet一样，由Servlet引擎来处理。Servlet引擎装载这个类，处理来自客户的请求，并把结果返回给客户，如下图所示：图1：调用JSP页面的流程以后再有客户访问这个页面的时候，只要该文件没有发生过更改，JSP引擎就直接调用已经装载的Servlet。如果已经做过修改的话，那就会再次执行以上过程，翻译、编译并装载。其实这就是所谓的“第一人惩罚”。因为首次访问的时候要执行一系列以上的过程，所以会耗费一些时间；以后的访问就不会这样了。Javaservlet提供了一种强有力的API，用这个API可以访问关于请求、会话和应用程序的所有信息。将servlet和JSP页面组合起来使用，可以把应用程序的逻辑部分和外观呈现部分清楚地分开；换句话，利用这个方式可以对模型、视图和控制器这三种角色分别使用最合适的组件类型。Servlet、过滤器和监听器Servlet是一种Java类，它使得服务器的功能可扩展至处理请求和生成应答。它是用ServletAPI定义的类和接口实现的。API由两个程序包组成：jvavax.servlet程序包包含独立于协议的类和接口，而javax.servlet.http程序包则提供HTTP特定的扩展的实用程序类。Servlet的实质是实现了接口javax.servlet.Servlet的类，实现是直接完成或通过扩展某个支持类来完成的。该接口定义了Web容器用来管理servlet和与之交互的方法。用于处理HTTP请求的servlet一般情况下都会扩展javax.servlet.http.HttpServlet类。该类实现了Servlet接口，并提供了使用HTTP处理的附加方法。Servlet的生命周期Web容器管理servlet生命周期的所有方面。它根据需要创建servlet类的实例、将请求传递给实例进行处理，最终删除实例。对于HttpServlet来说，容器会在servlet生命周期的适当时间调用方法。除了doGet（）和doPost（）方法之外，还有一些对应于其他HTTP方法的9方法：doDelete（）、doHead（）、doOptiongs（）、doPut（）和doTrace（）。一般情况下不用实现这些方法，因为HttpServlet类已经用适用于大多数servlet的方法考虑到了HEAD、OPTIONS和TRACE请求，而且DELETE和PUT这两种HTTP方法很少用在Web应用程序中。容器只为每个Servlet创建一个实例非常重要。这意味着servlet必须是线程安全的即，能够同时处理多个请求，每个处理都通过servlet代码作为单独的线程来执行。如果只在init（）和destroy（）方法中修改参考的对象，而且只在请求处理方法中读取他们，那么不用丧失任何细节就可以满足关于实例变量的这个要求。编译和安装servlet要编译servlet，必须首先确保JAR文件包含着CLASSPATH环境变量中所有ServletAPI类。该JAR文件将随所有的Web容器一起发布。Tomcat中包含了一个名为servlet.jar的JAR文件，位于common/lib目录中。在Windows平台中，应在CLASSPATH中包含JAR文件。读取请求传递到doGet（）和doPost（）方法的参数之一是实现了HttpServletRequest接口的对象。该接口定义的方法可提供对关于请求的许多信息的访问。生成应答除应答对象之外，容器还将实现HttpServletRequest接口的对象作为icanshu传递给doGet（）和doPost（）方法。该接口定义了为应答行为体获取数序程序或流的方法。它还定义了设置应答状态代码和首部的方法。使用过滤器和监听器Servlet规范servlet内定义了两种组件类型：过滤器和监听器。这两种类型是在Servlet2.3规范中引入的，因此，如果你使用的是不支持该版本规范的容器，恐怕就不能继续学习了。过滤器过滤器是一种组件，可以解释对servlet、JSP页面或静态页面的请求以及发送给客户端之前的应答。这样可以很容易地将应用于所有请求的任务集中在一起，例如访问控制、登录和内容的开销或应用提供的服务等。过滤器对请求与应答的行为体和首部具有完全访问权限，因此还可以执行各种转换。例如，如果Accept-Language请求首部指出客户端可以处理压缩的应答，那么过滤器就可以压缩应答的行为体。过滤器可以应用在特定servlet上，或匹配某种URL模式的所有请求上，例如以相同的路径元素开头或具有相同扩展名的URL。监听器监听器允许应用程序对特定事件做出回应。Servlet2.3之前，只能处理会话属性绑定事件（在添加对象或从会话中删除对象时）。实现监听器的方式是用保存为会话属性（使用HttpSession.setAttribute()方法）的对象实现HttpSessionBinding-Listener接口。随着Servlet规范的2.3版本中新接