【文章內(nèi)容簡介】 theapplicationlogicfromthepresentationoftheapplication。inotherwords,itletsyouusethemostappropriateponenttypefortherolesofModel,ViewandController. Servlets,Filters,andListeners AservletisaJavaclassthatextendsaserverwithfunctionalityforprocessingarequestandproducingaresponse.It＇simplementedusingtheclassesandinterfacesdefinedbytheServletAPI.TheAPIconsistsoftwopackages:thepackagecontainsclassesandinterfacesthatareprotocolindependent,whilethepackageprovidesHTTPspecificextensionsandutilityclasses. Whatmakesaservletaservletisthattheclassimplementsaninterfacenamed,eitherdirectlyorbyextendingoneofthesupportclasses.Thisinterfacedefinesthemethodsusedbythewebcontainertomanageandinteractwiththe servlet.AservletforprocessingHTTPrequeststypicallyextendsthe class.ThisclassimplementstheServletinterfaceandprovidesadditionalmethodssuitableforHTTPprocessing. ServletLifecycle Thewebcontainermanagesallaspectsoftheservlet＇slifecycle.Itcreatesaninstanceoftheservletclasswhenneeded,passesrequeststotheinstanceforprocessing,andeventuallyremovestheinstance.ForanHttpServlet,thecontainercallsthefollowingmethodsattheappropriatetimesintheservletlifecycle. BesidesthedoGet()anddoPost()methods,therearemethodscorrespondingtotheotherHTTPmethods:doDelete(),doHead(),doOptions(),doPut(),anddoTrace().Typicallyyoudon＇timplementthesemethods。theHttpServletclassalreadytakescareofHEAD,OPTIONS,andTRACErequestsinawaythat＇ssuitableformostservlets,andtheDELETEandPUTHTTPmethodsarerarelyusedinawebapplication. It＇simportanttorealizethatthecontainercreatesonlyoneinstanceofeachservlet.Thismeansthattheservletmustbethreadsafeabletohandlemultiplerequestsatthesametime,eachexecutingasaseparatethreadthroughtheservletcode.Withoutgettinglostindetails,yousatisfythisrequirementwithregardstoinstancevariablesifyoumodifythereferencedobjectsonlyintheinit()anddestroy()methods,andjustreadthemintherequestprocessingmethods. CompilingandInstallingaServlet Topileaservlet,youmustfirstensurethatyouhavetheJARfilecontainingallServletAPIclassesintheCLASSPATHenvironmentvariable.TheJARfileisdistributedwithallwebcontainers.Tomcatincludesitinafilecalled,locatedinthemon/libdirectory.OnaWindowsplatform,youincludetheJARfileintheCLASSPATH. ReadingaRequest OneoftheargumentspassedtothedoGet()anddoPost()methodsisanobjectthatimplementstheHttpServletRequestinterface.Thisinterfacedefinesmethodsthatprovideaccesstoawealthofinformationabouttherequest. GeneratingaResponse Besidestherequestobject,thecontainerpassesanobjectthatimplementstheHttpServletResponseinterfaceasanargumenttothedoGet()anddoPost()methods.Thisinterfacedefinesmethodsforgettingawriterorstreamfortheresponsebody.Italsodefinesmethodsforsettingtheresponsestatuscodeandheaders. UsingFiltersandListeners Theservletspecificationdefinestwoponenttypesbesideservlets:filtersandlisteners.ThesetwotypeswereintroducedintheServletspecification,soifyou＇reusingacontainerthatdoesn＇tyetsupportthisversionofthespecification,I＇mafraidyou＇re outofluck. Filters Afilterisaponentthatcaninterceptarequesttargetedforaservlet,JSPpage,orstaticpage,aswellastheresponsebeforeit＇ssenttotheclient.Thismakesiteasytocentralizetasksthatapplytoallrequests,suchasaccesscontrol,logging,andchargingforthecontentortheservicesofferedbytheapplication.Afilterhasfullaccesstothebodyandheadersoftherequestandresponse,soitcanalsoperformvarioustransformations.Oneexampleispressingtheresponsebodyifthe Acceptlanguagerequestheaderindicatesthattheclientcanhandleapressedresponse. AfiltercanbeappliedtoeitheraspecificservletortoallrequestsmatchingaURLpattern,suchasURLsstartingwiththesamepathelementsorhavingthesameextension. Listeners Listenersallowyourapplicationtoreacttocertainevents.PriortoServlet,youcouldhandleonlysessionattributebindingevents(triggeredwhenanobjectwasaddedorremovedfromasession).Youcoulddothisbylettingtheobjectsavedasasessionattributimplementthe1