翻译文献-有状态会话bean

外文原文5THEBASICSOFSTATEFULSESSIONBEANSSTATEFULSESSIONBEANSARECONVERSATIONALBEANSBECAUSETHEYHOLDCONVERSATIONSWITHCLIENTSTHATSPANMULTIPLEMETHODINVOCATIONSSTATEFULSESSIONBEANSSTORECONVERSATIONALSTATEWITHINTHEBEANTHATCONVERSATIONALSTATEISSPECIFICTOAPARTICULARCLIENTTHISCHAPTERWILLTEACHYOUTHEBASICSOFWRITINGSTATEFULSESSIONBEANSWELLBEGINATTHECONCEPTUALLEVELANDQUICKLYMOVEINTOACODEEXAMPLEILLUSTRATINGSESSIONBEANDEVELOPMENTWELLWRAPUPWITHACOMPARISONOFTHESTATEFULANDSTATELESSPROGRAMMINGMODELS51CHARACTERISTICSOFSTATEFULSESSIONBEANSLETSBEGINWITHALOOKATTHECHARACTERISTICSTHATALLSTATEFULSESSIONBEANSSHARE511ACHIEVINGTHEEFFECTOFPOOLINGWITHSTATEFULBEANSIMAGINEASCENARIOWHERETHOUSANDSOFCLIENTSAREHAVINGCONVERSATIONSWITHDIFFERENTSTATEFULSESSIONBEANSRUNNINGWITHINACONTAINERLETSASSUMETHATTHECLIENTSARETYPICALCLIENTSANDTAKEALONGTIMETO“THINK”BETWEENMETHODCALLSPERHAPSACLIENTISFARAWAY,CONNECTINGFROMANOTHERCOUNTRY,ANDTHENETWORKLAGISHIGHORPERHAPSTHECLIENTREPRESENTSAHUMANINTERACTINGWITHAJAVAAPPLET,ANDTHEHUMANISDECIDINGONTHENEXTBUTTONTOCLICKTHOUSANDSOFCLIENTSIMPLYTHOUSANDSOFSTATEFULSESSIONBEANS,EACHHOLDINGCONVERSATIONALSTATEONBEHALFOFASPECIFICCLIENTAND,OFCOURSE,OUREJBCONTAINER/SERVERHASONLYAFINITEAMOUNTOFRESOURCESAVAILABLE,SUCHASMEMORY,DATABASECONNECTIONS,ANDSOCKETCONNECTIONSIFTHECONVERSATIONALSTATETHATTHEBEANSAREHOLDINGISLARGE,THECONTAINER/SERVERCOULDEASILYRUNOUTOFRESOURCESTHISWASNOTAPROBLEMWITHSTATELESSSESSIONBEANSBECAUSETHECONTAINERCOULDPOOLONLYAFEWBEANSTOSERVICETHOUSANDSOFCLIENTSWITHSTATEFULSESSIONBEANS,POOLINGISNOTASSIMPLEWHENACLIENTINVOKESAMETHODONABEAN,HEORSHEISSTARTINGACONVERSATIONWITHTHEBEAN,ANDTHECONVERSATIONALSTATESTOREDINTHEBEANMUSTBEAVAILABLEFORTHATSAMECLIENTSNEXTMETHODREQUESTTHEREFORE,THECONTAINERCANNOTEASILYPOOLBEANSANDDYNAMICALLYASSIGNTHEMTOHANDLEARBITRARYCLIENTMETHODREQUESTS,SINCEEACHBEANISSTORINGSTATEONBEHALFOFAPARTICULARCLIENTBUTWESTILLNEEDTOACHIEVETHEEFFECTOFPOOLINGFORSTATEFULSESSIONBEANS,SOTHATWECANCONSERVERESOURCESANDENHANCETHEOVERALLSCALABILITYOFTHESYSTEMTHISPROBLEMSHOULDSOUNDQUITEFAMILIARTOOPERATINGSYSTEMSGURUSWHENEVERYOURUNANAPPLICATIONONACOMPUTER,YOUHAVEONLYAFIXEDAMOUNTOFPHYSICALMEMORYINWHICHTORUNTHEOPERATINGSYSTEMSTILLMUSTPROVIDEAWAYFORMANYAPPLICATIONSTORUN,EVENIFTHEAPPLICATIONSTAKEUPMOREAGGREGATEMEMORYTHANISAVAILABLEPHYSICALLYTOPROVIDEFORTHIS,OPERATINGSYSTEMSUSEYOURHARDDISKASANEXTENSIONOFPHYSICALMEMORYTHISEFFECTIVELYEXTENDSTHEAMOUNTOFVIRTUALMEMORYTHATYOURSYSTEMHASWHENANAPPLICATIONGOESIDLE,ITSMEMORYCANBESWAPPEDOUTFROMPHYSICALMEMORYANDONTOTHEHARDDISKWHENTHEAPPLICATIONBECOMESACTIVEAGAIN,ANYNEEDEDDATAISSWAPPEDINFROMTHEHARDDISKANDINTOPHYSICALMEMORYTHISTYPEOFSWAPPINGHAPPENSVERYOFTENWHENSWITCHINGBETWEENAPPLICATIONSCALLEDCONTEXTSWITCHINGEJBCONTAINERSEXPLOITTHISVERYPARADIGMTOCONSERVESTATEFULSESSIONBEANRESOURCESTOLIMITTHENUMBEROFSTATEFULSESSIONBEANINSTANCESINMEMORY,THECONTAINERCANSWAPOUTASTATEFULBEAN,SAVINGITSCONVERSATIONALSTATETOAHARDDISKOROTHERSTORAGETHISISCALLEDPASSIVATIONAFTERPASSIVATINGASTATEFULBEAN,THECONVERSATIONALSTATEISSAFELYSTOREDAWAY,ALLOWINGRESOURCESSUCHASMEMORYTOBERECLAIMEDWHENTHEORIGINALCLIENTINVOKESAMETHOD,THEPASSIVATEDCONVERSATIONALSTATEISSWAPPEDINTOABEANTHISISCALLEDACTIVATIONTHISBEANNOWRESUMESTHECONVERSATIONWITHTHEORIGINALCLIENTNOTETHATTHEBEANTHATRECEIVESTHEACTIVATEDSTATEMAYNOTBETHEORIGINALBEANINSTANCEBUTTHATSALLRIGHTBECAUSETHEINSTANCEISRESUMINGITSCONVERSATIONFROMTHEPOINTWHERETHEORIGINALINSTANCEWASPASSIVATEDTHUS,EJBDOESINDEEDSUPPORTTHEEFFECTOFPOOLINGSTATEFULSESSIONBEANSONLYAFEWINSTANCESCANBEINMEMORYWHENTHEREAREACTUALLYMANYCLIENTSBUTTHISPOOLINGEFFECTDOESNOTCOMEFORFREETHEPASSIVATION/ACTIVATIONSTEPSCOULDENTAILANI/OBOTTLENECKCONTRASTTHISTOSTATELESSSESSIONBEANS,WHICHAREEASILYPOOLEDBECAUSETHEREISNOSTATETOSAVESOHOWDOESTHECONTAINERDECIDEWHICHBEANSTOACTIVATEANDWHICHBEANSTOPASSIVATETHEANSWERISSPECIFICTOEACHCONTAINERMOSTCONTAINERSWILLEMPLOYALEASTRECENTLYUSEDLRUPASSIVATIONSTRATEGY,WHICHSIMPLYMEANSTOPASSIVATETHEBEANTHATHASBEENCALLEDTHELEASTRECENTLYTHISISAGOODALGORITHMBECAUSEREMOTECLIENTSHAVETHEHABITOFDISCONNECTINGFROMTHENETWORK,LEAVINGBEANSSTRANDEDWITHOUTACLIENT,READYTOBEPASSIVATEDIFABEANHASNTBEENINVOKEDINAWHILE,THECONTAINERWILLWRITEITTODISKPASSIVATIONCANOCCURATANYTIME,SOLONGASABEANISNOTINVOLVEDINAMETHODCALLITSUPTOTHECONTAINERTODECIDEWHENPASSIVATIONMAKESSENSENOTETHATTHEREISONEEXCEPTIONTOTHISRULEANYBEANINVOLVEDINATRANSACTIONSEECHAPTER10CANNOTBEPASSIVATEDUNTILTHETRANSACTIONCOMPLETESTOACTIVATEBEANS,MOSTCONTAINERSWILLUSUALLYUSEAJUSTINTIMEALGORITHMJUSTINTIMEMEANSTHATBEANSSHOULDBEACTIVATEDONDEMAND,ASCLIENTREQUESTSCOMEINIFACLIENTREQUESTCOMESINBUTTHATCLIENTSCONVERSATIONHASBEENPASSIVATED,THECONTAINERWILLACTIVATETHEBEANONDEMAND,READINGTHEPASSIVATEDSTATEBACKINTOMEMORYINGENERAL,PASSIVATIONANDACTIVATIONARENOTUSEFULFORSTATELESSSESSIONBEANSSTATELESSBEANSDONOTHAVEANYSTATETOPASSIVATE/ACTIVATE,ANDSOSTATELESSBEANSCANSIMPLYBEDESTROYEDARBITRARILYBYTHECONTAINERPASSIVATION/ACTIVATIONALSOAPPLIESTOENTITYBEANS,WHICHWELLLEARNABOUTINCHAPTERS79PASSIVATETHEBEANTHATHASBEENCALLEDTHELEASTRECENTLYTHISISAGOODALGORITHMBECAUSEREMOTECLIENTSHAVETHEHABITOFDISCONNECTINGFROMTHENETWORK,LEAVINGBEANSSTRANDEDWITHOUTACLIENT,READYTOBEPASSIVATEDIFABEANHASNTBEENINVOKEDINAWHILE,THECONTAINERWILLWRITEITTODISKPASSIVATIONCANOCCURATANYTIME,SOLONGASABEANISNOTINVOLVEDINAMETHODCALLITSUPTOTHECONTAINERTODECIDEWHENPASSIVATIONMAKESSENSENOTETHATTHEREISONEEXCEPTIONTOTHISRULEANYBEANINVOLVEDINATRANSACTIONSEECHAPTER10CANNOTBEPASSIVATEDUNTILTHETRANSACTIONCOMPLETESTOACTIVATEBEANS,MOSTCONTAINERSWILLUSUALLYUSEAJUSTINTIMEALGORITHMJUSTINTIMEMEANSTHATBEANSSHOULDBEACTIVATEDONDEMAND,ASCLIENTREQUESTSCOMEINIFACLIENTREQUESTCOMESINBUTTHATCLIENTSCONVERSATIONHASBEENPASSIVATED,THECONTAINERWILLACTIVATETHEBEANONDEMAND,READINGTHEPASSIVATEDSTATEBACKINTOMEMORYINGENERAL,PASSIVATIONANDACTIVATIONARENOTUSEFULFORSTATELESSSESSIONBEANSSTATELESSBEANSDONOTHAVEANYSTATETOPASSIVATE/ACTIVATE,ANDSOSTATELESSBEANSCANSIMPLYBEDESTROYEDARBITRARILYBYTHECONTAINERPASSIVATION/ACTIVATIONALSOAPPLIESTOENTITYBEANS,WHICHWELLLEARNABOUTINCHAPTERS79512THERULESGOVERNINGCONVERSATIONALSTATEASWEHAVESAID,STATEFULSESSIONBEANSHOLDCONVERSATIONALSTATEONBEHALFOFONECLIENTLETSDEFINETHATSTATEABITMORERIGOROUSLYITSIMPORTANTTOKNOWTHERULESFORSPECIFYINGCONVERSATIONALSTATE,SOTHATTHECONTAINERWILLPASSIVATEANDACTIVATEPROPERLYTHECONVERSATIONALSTATEOFABEANFOLLOWSTHERULESLAIDOUTBYJAVAOBJECTSERIALIZATIONSEEAPPENDIXAWHENACONTAINERPASSIVATESABEAN,ITUSESOBJECTSERIALIZATIONORANEQUIVALENTPROTOCOLTOCONVERTTHEBEANSCONVERSATIONALSTATEINTOABITBLOBITCANTHENWRITETHATBLOBOUTTOSTORAGEONCETHEBEANISWRITTENTOSTORAGE,THEMEMORYISAVAILABLETOBEFREEDBYTHEGARBAGECOLLECTORACTIVATIONREVERSESTHEPROCESSASERIALIZEDBLOBTHATHADBEENWRITTENTOSTORAGEISREADBACKINTOMEMORYANDCONVERTEDTOINMEMORYBEANDATAWHATMAKESTHISWHOLEPROCESSWORKISTHEJAVAXEJBENTERPRISEBEANINTERFACEEXTENDSJAVAIOSERIALIZABLE,ANDEVERYENTERPRISEBEANCLASSINDIRECTLYIMPLEMENTSTHISINTERFACEMORECONCRETELY,EVERYMEMBERVARIABLEINABEANISCONSIDEREDTOBEPARTOFTHEBEANSCONVERSATIONALSTATEIFTHEFOLLOWINGAPPLY_THEMEMBERVARIABLEISANONTRANSIENTPRIMITIVETYPE,OR_THEMEMBERVARIABLEISANONTRANSIENTJAVAOBJECTEXTENDSJAVALANGOBJECTFOREVERYJAVAOBJECTTHATSPARTOFABEANSCONVERSATIONALSTATE,THEPREVIOUSALGORITHMISREAPPLIEDRECURSIVELYONTHOSEOBJECTSTHUS,OBJECTSERIALIZATIONCONSTRUCTSANENTIREGRAPHOFDATAREFERREDTOBYTHEMAINBEANYOUSHOULDNOTETHATWHILEYOURBEANSMUSTFOLLOWTHERULESFOROBJECTSERIALIZATION,THEEJBCONTAINERITSELFDOESNOTNECESSARILYNEEDTOUSETHEDEFAULTSERIALIZATIONPROTOCOLITCOULDUSEACUSTOMPROTOCOLFORTHIS,TOALLOWFLEXIBILITYANDDIFFERENTIATIONBETWEENCONTAINERVENDORS513ACTIVATION/PASSIVATIONCALLBACKSLETSNOWLOOKATWHATACTUALLYHAPPENSTOYOURBEANDURINGPASSIVATIONANDACTIVATIONWHENANEJBCONTAINERPASSIVATESABEAN,THECONTAINERWRITESTHEBEANSCONVERSATIONALSTATETOSECONDARYSTORAGE,SUCHASAFILEORDATABASETHECONTAINERINFORMSTHEBEANTHATITSABOUTTOPERFORMPASSIVATIONBYCALLINGTHEBEANSREQUIREDEJBPASSIVATECALLBACKMETHODEJBPASSIVATEISAWARNINGTOTHEBEANTHATITSHELDCONVERSATIONALSTATEISABOUTTOBESWAPPEDOUTITSIMPORTANTTHATTHECONTAINERINFORMTHEBEANUSINGEJBPASSIVATESOTHATTHEBEANCANRELINQUISHHELDRESOURCESTHESEHELDRESOURCESINCLUDEDATABASECONNECTIONS,OPENSOCKETS,OPENFILES,OROTHERRESOURCESTHATDONOTMAKESENSETOBESAVEDTODISKORCANNOTBETRANSPARENTLYSAVEDUSINGOBJECTSERIALIZATIONTHEEJBCONTAINERCALLSTHEEJBPASSIVATEMETHODTOGIVETHEBEANACHANCETORELEASETHESERESOURCESORDEALWITHTHERESOURCESASTHEBEANSEESFITONCETHECONTAINERSEJBPASSIVATECALLBACKMETHODINTOYOURBEANISCOMPLETE,YOURBEANMUSTBEINASTATESUITABLEFORPASSIVATIONTHISISSHOWNINFIGURE51THEEXACTOPPOSITEPROCESSOCCURSDURINGTHEACTIVATIONPROCESSHERE,THESERIALIZEDCONVERSATIONALSTATEISREADBACKINTOMEMORY,ANDTHECONTAINERRECONSTRUCTSTHEINMEMORYSTATEUSINGOBJECTSERIALIZATIONORTHEEQUIVALENTTHECONTAINERTHENCALLSTHEBEANSREQUIREDEJBACTIVATEMETHODEJBACTIVATEGIVESTHEBEANACHANCETORESTORETHEOPENRESOURCESITRELEASEDDURINGEJBPASSIVATETHISENTIREPROCESSISSHOWNINFIGURE52DOYOUHAVETOWORRYABOUTIMPLEMENTINGEJBPASSIVATEANDEJBACTIVATEPROBABLYNOT,UNLESSYOUAREUSINGOPENRESOURCES,SUCHASSOCKETCONNECTIONSORDATABASECONNECTIONS,THATMUSTBEREESTABLISHEDAFTERACTIVATIONINMOSTCASES,YOUCANSIMPLYLEAVETHESEMETHODSEMPTY514ASIMPLESTATEFULSESSIONBEANLETSPUTOURSTATEFULSESSIONBEANKNOWLEDGETOUSEBYPROGRAMMINGASIMPLESTATEFULBEANOURBEANWILLBEACOUNTERBEAN,ANDITWILLSIMPLYBERESPONSIBLEFORCOUNTINGUPONEBYONETHECURRENTCOUNTWILLBESTOREDWITHINTHEBEAN,ANDITWILLINCREMENTASCLIENTREQUESTSARRIVETHUS,OURBEANWILLBESTATEFUL,ANDITWILLHOLDAMULTIMETHODCONVERSATIONWITHAPARTICULARCLIENTTHECOUNTBEANSREMOTEINTERFACEFIRST,LETSDEFINEOURBEANSREMOTEINTERFACETHECODEISSHOWNINSOURCE51OURREMOTEINTERFACEDEFINESASINGLEBUSINESSMETHODCOUNTWHICHWEWILLIMPLEMENTINTHEENTERPRISEBEANCLASSTHECOUNTBEANOURBEANIMPLEMENTATIONHASONEBUSINESSMETHOD,COUNT,WHICHISRESPONSIBLEFORINCREMENTINGANINTEGERMEMBERVARIABLE,CALLEDVALTHECONVERSATIONALSTATEISTHEVALMEMBERVARIABLEWESHOWTHECODEFOROURCOUNTERBEANINSOURCE52HEREARESOMETHINGSTONOTICEABOUTOURBEAN_THEBEANIMPLEMENTSJAVAXEJBSESSIONBEANDEFINEDINCHAPTER3THISMEANSTHEBEANMUSTDEFINEALLMETHODSINTHESESSIONBEANINTERFACEINDEED,BYLOOKINGATTHEBEAN,YOUCANSEEWEVEDEFINEDTHEMBUTKEPTTHEMFAIRLYTRIVIAL_OUREJBCREATEINITIALIZATIONMETHODTAKESAPARAMETER,VALTHISISINSTARKCON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是JAVAXEJBENTERPRISEBEAN接口，并且每一个企业级的BEAN类都间接的继承这个接口。更加具体的是，在以下条件成立的情况下，每一个在BEAN中的成员变量都被认为是BEAN的会话状态的一个组成部分。每一个成员变量的属性都是私有的。每一个成员变量都是一个JAVA对象（继承于JAVALANGOBJECT。对于每一个隶属于BEAN的会话状态的JAVA对象来说，之前的那些算法是在这些对象上进行了再一次的递归，如此说来，集合对象构造了一个完整的数据提交到主干BEAN的图表。我们应该注意到在我们的BEAN必须遵循集合对象的这些规则的时候，EJB的容器本身没有必要使用这个默认的集合协议；它可以为此使用一个自定义的协议，用来承受不同容器使用者之间的反复和不同。513钝化和激活的调用现在让我们来观察一下在你的BEAN进行钝化和激活的时候到底发生了什么。容器将BEAN的会话状态写到二级存储中，例如一个文件或者数据库中。容器通告这个BEAN，这个BEAN准备用BEAN的EJBPASSIVATE方法进行钝化。EJBPASSIVATE对那些保持会话状态并将会被交换出的BEAN来说是一个警告。对于BEAN来说，容器利用EJBPASSIVATE方法对其进行警告是非常重要的，这样这些BEAN就能够选择放弃持有这些资源。这些资源包括数据库的连接，开源的文件，或者一些其它的对节约硬盘资源或者不能节约对象集合毫无意义的资源。EJB的容器调用这个EJBPASSIVATE方法以保证给这些BEAN一个释放这些资源或者对他们进行处理的机会。一旦在你的BEAN中容器的这些EJBPASSIVATE方法调用完成了，那么你的BEAN就必须