翻译文献-stateless session beans的基础

外文原文THEBASICSOFSTATEFULSESSIONBEANSSTATEFULSESSIONBEANSARECONVERSATIONALBEANSBECAUSETHEYHOLDCONVERSATIONSWITHCLIENTSTHATSPANMULTIPLEMETHODINVOCATIONSSTATEFULSESSIONBEANSSTORECONVERSATIONALSTATEWITHINTHEBEANTHATCONVERSATIONALSTATEISSPECIFICTOAPARTICULARCLIENTTHISCHAPTERWILLTEACHYOUTHEBASICSOFWRITINGSTATEFULSESSIONBEANSWELLBEGINATTHECONCEPTUALLEVELANDQUICKLYMOVEINTOACODEEXAMPLEILLUSTRATINGSESSIONBEANDEVELOPMENTWELLWRAPUPWITHACOMPARISONOFTHESTATEFULANDSTATELESSPROGRAMMINGMODELSCHARACTERISTICSOFSTATEFULSESSIONBEANSLETSBEGINWITHALOOKATTHECHARACTERISTICSTHATALLSTATEFULSESSIONBEANSSHAREACHIEVINGTHEEFFECTOFPOOLINGWITHSTATEFULBEANSIMAGINEASCENARIOWHERETHOUSANDSOFCLIENTSAREHAVINGCONVERSATIONSWITHDIFFERENTSTATEFULSESSIONBEANSRUNNINGWITHINACONTAINERLETSASSUMETHATTHECLIENTSARETYPICALCLIENTSANDTAKEALONGTIMETO“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,WHICHWELLLEARNABOUTINCHAPTERS79THERULESGOVERNINGCONVERSATIONALSTATEASWEHAVESAID,STATEFULSESSIONBEANSHOLDCONVERSATIONALSTATEONBEHALFOFONECLIENTLETSDEFINETHATSTATEABITMORERIGOROUSLYITSIMPORTANTTOKNOWTHERULESFORSPECIFYINGCONVERSATIONALSTATE,SOTHATTHECONTAINERWILLPASSIVATEANDACTIVATEPROPERLYTHECONVERSATIONALSTATEOFABEANFOLLOWSTHERULESLAIDOUTBYJAVAOBJECTSERIALIZATIONSEEAPPENDIXAWHENACONTAINERPASSIVATESABEAN,ITUSESOBJECTSERIALIZATIONORANEQUIVALENTPROTOCOLTOCONVERTTHEBEANSCONVERSATIONALSTATEINTOABITBLOBITCANTHENWRITETHATBLOBOUTTOSTORAGEONCETHEBEANISWRITTENTOSTORAGE,THEMEMORYISAVAILABLETOBEFREEDBYTHEGARBAGECOLLECTORACTIVATIONREVERSESTHEPROCESSASERIALIZEDBLOBTHATHADBEENWRITTENTOSTORAGEISREADBACKINTOMEMORYANDCONVERTEDTOINMEMORYBEANDATAWHATMAKESTHISWHOLEPROCESSWORKISTHEJAVAXEJBENTERPRISEBEANINTERFACEEXTENDSJAVAIOSERIALIZABLE,ANDEVERYENTERPRISEBEANCLASSINDIRECTLYIMPLEMENTSTHISINTERFACEMORECONCRETELY,EVERYMEMBERVARIABLEINABEANISCONSIDEREDTOBEPARTOFTHEBEANSCONVERSATIONALSTATEIFTHEFOLLOWINGAPPLYTHEMEMBERVARIABLEISANONTRANSIENTPRIMITIVETYPE,ORTHEMEMBERVARIABLEISANONTRANSIENTJAVAOBJECTEXTENDSJAVALANGOBJECTFOREVERYJAVAOBJECTTHATSPARTOFABEANSCONVERSATIONALSTATE,THEPREVIOUSALGORITHMISREAPPLIEDRECURSIVELYONTHOSEOBJECTSTHUS,OBJECTSERIALIZATIONCONSTRUCTSANENTIREGRAPHOFDATAREFERREDTOBYTHEMAINBEANYOUSHOULDNOTETHATWHILEYOURBEANSMUSTFOLLOWTHERULESFOROBJECTSERIALIZATION,THEEJBCONTAINERITSELFDOESNOTNECESSARILYNEEDTOUSETHEDEFAULTSERIALIZATIONPROTOCOLITCOULDUSEACUSTOMPROTOCOLFORTHIS,TOALLOWFLEXIBILITYANDDIFFERENTIATIONBETWEENCONTAINERVENDORSACTIVATION/PASSIVATIONCALLBACKSLETSNOWLOOKATWHATACTUALLYHAPPENSTOYOURBEANDURINGPASSIVATIONANDACTIVATIONWHENANEJBCONTAINERPASSIVATESABEAN,THECONTAINERWRITESTHEBEANSCONVERSATIONALSTATETOSECONDARYSTORAGE,SUCHASAFILEORDATABASETHECONTAINERINFORMSTHEBEANTHATITSABOUTTOPERFORMPASSIVATIONBYCALLINGTHEBEANSREQUIREDEJBPASSIVATECALLBACKMETHODEJBPASSIVATEISAWARNINGTOTHEBEANTHATITSHELDCONVERSATIONALSTATEISABOUTTOBESWAPPEDOUTITSIMPORTANTTHATTHECONTAINERINFORMTHEBEANUSINGEJBPASSIVATESOTHATTHEBEANCANRELINQUISHHELDRESOURCESTHESEHELDRESOURCESINCLUDEDATABASECONNECTIONS,OPENSOCKETS,OPENFILES,OROTHERRESOURCESTHATDONOTMAKESENSETOBESAVEDTODISKORCANNOTBETRANSPARENTLYSAVEDUSINGOBJECTSERIALIZATIONTHEEJBCONTAINERCALLSTHEEJBPASSIVATEMETHODTOGIVETHEBEANACHANCETORELEASETHESERESOURCESORDEALWITHTHERESOURCESASTHEBEANSEESFITONCETHECONTAINERSEJBPASSIVATECALLBACKMETHODINTOYOURBEANISCOMPLETE,YOURBEANMUSTBEINASTATESUITABLEFORPASSIVATIONTHISISSHOWNINFIGURE51THEEXACTOPPOSITEPROCESSOCCURSDURINGTHEACTIVATIONPROCESSHERE,THESERIALIZEDCONVERSATIONALSTATEISREADBACKINTOMEMORY,ANDTHECONTAINERRECONSTRUCTSTHEINMEMORYSTATEUSINGOBJECTSERIALIZATIONORTHEEQUIVALENTTHECONTAINERTHENCALLSTHEBEANSREQUIREDEJBACTIVATEMETHODEJBACTIVATEGIVESTHEBEANACHANCETORESTORETHEOPENRESOURCESITRELEASEDDURINGEJBPASSIVATETHISENTIREPROCESSISSHOWNINFIGURE52DOYOUHAVETOWORRYABOUTIMPLEMENTINGEJBPASSIVATEANDEJBACTIVATEPROBABLYNOT,UNLESSYOUAREUSINGOPENRESOURCES,SUCHASSOCKETCONNECTIONSORDATABASECONNECTIONS,THATMUSTBEREESTABLISHEDAFTERACTIVATIONINMOSTCASES,YOUCANSIMPLYLEAVETHESEMETHODSEMPTYASIMPLESTATEFULSESSIONBEANLETSPUTOURSTATEFULSESSIONBEANKNOWLEDGETOUSEBYPROGRAMMINGASIMPLESTATEFULBEANOURBEANWILLBEACOUNTERBEAN,ANDITWILLSIMPLYBERESPONSIBLEFORCOUNTINGUPONEBYONETHECURRENTCOUNTWILLBESTOREDWITHINTHEBEAN,ANDITWILLINCREMENTASCLIENTREQUESTSARRIVETHUS,OURBEANWILLBESTATEFUL,ANDITWILLHOLDAMULTIMETHODCONVERSATIONWITHAPARTICULARCLIENTTHECOUNTBEANSREMOTEINTERFACEFIRST,LETSDEFINEOURBEANSREMOTEINTERFACETHECODEISSHOWNINSOURCE51OURREMOTEINTERFACEDEFINESASINGLEBUSINESSMETHODCOUNTWHICHWEWILLIMPLEMENTINTHEENTERPRISEBEANCLASSTHECOUNTBEANOURBEANIMPLEMENTATIONHASONEBUSINESSMETHOD,COUNT,WHICHISRESPONSIBLEFORINCREMENTINGANINTEGERMEMBERVARIABLE,CALLEDVALTHECONVERSATIONALSTATEISTHEVALMEMBERVARIABLEWESHOWTHECODEFOROURCOUNTERBEANINSOURCE52HEREARESOMETHINGSTONOTICEABOUTOURBEANTHEBEANIMPLEMENTSJAVAXEJBSESSIONBEANDEFINEDINCHAPTER3THISMEANSTHEBEANMUSTDEFINEALLMETHODSINTHESESSIONBEANINTERFACEINDEED,BYLOOKINGATTHEBEAN,YOUCANSEEWEVEDEFINEDTHEMBUTKEPTTHEMFAIRLYTRIVIALOUREJBCREATEINITIALIZATIONMETHODTAKESAPARAMETER,VALTHISISINSTARKCONTRASTTOSTATELESSSESSIONBEANS,WHICHNEVERTAKEPARAMETERSINEJBCREATEOURINITIALIZATIONMETHODISRESPONSIBLEFORBEGINNINGACONVERSATIONWITHTHECLIENTITUSESVALASTHESTARTINGSTATEOFTHECOUNTERTHEVALMEMBERVARIABLEOBEYSTHERULESFORCONVERSATIONALSTATEBECAUSEITISSERIALIZABLETHUS,ITWILLLASTACROSSMETHODCALLS,ANDITWILLAUTOMATICALLYBEPRESERVEDDURINGPASSIVATION/ACTIVATIONTHECOUNTBEANSHOMEINTERFACETOCOMPLETEOURSTATEFULBEANCODE,WEMUSTDEFINEAHOMEINTERFACETHEHOMEINTERFACEWILLDETAILHOWTOCREATEANDDESTROYOURCOUNTEJBOBJECTTHECODEFOROURHOMEINTERFACEISINSOURCE53BECAUSEWEIMPLEMENTJAVAXEJBEJBHOME,OURHOMEINTERFACEGETSTHEREMOVEDESTROYMETHODFORFREETHECOUNTBEANSDEPLOYMENTDESCRIPTORNOWTHATWEVEGOTALLOURJAVAFILESFOROURBEAN,WENEEDTODEFINETHEDEPLOYMENTDESCRIPTORTOIDENTIFYTHEBEANSSETTINGSTOTHECONTAINERTHEDEPLOYMENTDESCRIPTORSETTINGSWEUSEARELISTEDINTABLE51NOTICETHATWEVEADDEDACONTAINERSPECIFICDEPLOYMENTDESCRIPTORSETTING,“MAXBEANSLOADEDATANYTIME”NOTALLCONTAINERSWILLSUPPORTTHISTHEBEAWEBLOGICSERVERWETESTEDAGAINSTDOESWEARERESTRICTINGTHEMAXIMUMNUMBEROFBEANSLOADEDSOTHATWECANFORCETHEEJBCONTAINERTOPASSIVATEANDACTIVATEOURBEANSOURSTRATEGYWILLBETOLOADTHREEBEANSATONCE,THUSFORCINGTHECONTAINERTOPASSIVATEINORDERTOREDUCETHENUMBEROFBEANSINMEMORYALSONOTICETHATOURBEANSSTATEFULNATUREISDEFINEDDECLARATIVELYINTHEDEPLOYMENTDESCRIPTORWENEVERINTRODUCETHENOTIONOFABEANBEINGSTATEFULINTHEBEANCODEITSELFTHISALLOWSUSTOEASILYSWITCHFROMTHESTATEFULTOSTATELESSPARADIGMANDBACKTHECOUNTBEANSENVIRONMENTPROPERTIESTHISDEMONSTRATIONBEANISQUITESIMPLE,SOWEDONOTUSEENVIRONMENTPROPERTIESTOCUSTOMIZETHEBEANONDEPLOYMENTCHAPTER6DEMONSTRATESBEANSWITHPROPERTIESFILESTHECOUNTBEANSMANIFESTFILE,EJBJARFILE,ANDDEPLOYMENTTOCOMPLETEOURCOMPONENT,WENEEDTOWRITEAMANIFESTFILEFORTHEEJBJARFILETHEJARSMANIFESTISSTRAIGHTFORWARDANDNODIFFERENTREALLYFROMTHEONEWEUSEDFOROUR“HELLO,WORLD”BEANINCHAPTER4THEGENERATIONOFANEJBJARFILEISSIMILARTOOUR“HELLO,WORLD”EXAMPLEONCEWEGENERATETHEEJBJARFILE,WEMUSTDEPLOYOURBEANINANEJBCONTAINERTOTESTITOUTTHISSTEPVARIESWIDELYFROMCONTAINERTOCONTAINERIFYOUREUSINGTHEBEASWEBLOGICSERVER,TAKEALOOKATTHEMAKEFILEONTHEINCLUDEDCDROMFORTHISDEPLOYMENTSTEPIFYOUHAVEADIFFERENTPREFERREDCONTAINERVENDOR,CONSULTYOURVENDORSPRODUCTDOCUMENTATIONTOSAVESPACE,INFUTUREEXAMPLESWELLCONSIDERTHATMANIFESTGENERATION,EJBJARFILEGENERATION,ANDBEANDEPLOYMENTAREIMPLIEDSTEPSTHECOUNTBEANSCLIENTCODENOWTHATOURBEANISDEPLOYED,WECANWRITESOMEJAVACODETOTESTOURBEANSOURCLIENTCODEPERFORMSTHEFOLLOWINGSTEPS1WEACQUIREAJNDIINITIALCONTEXT2WELOCATETHEHOMEOBJECTUSINGJNDI3WEUSETHEHOMEOBJECTTOCREATETHREEDIFFERENTCOUNTEJBOBJECTSTHUS,WEARECREATINGTHREEDIFFERENTCONVERSATIONSANDARESIMULATINGTHREEDIFFERENTCLIENTS4OURDEPLOYMENTDESCRIPTORLIMITSTHEBEANPOOLSIZETOTWOBEANS,SODURINGTHEPREVIOUSSTEPSOMEOFTHETHREEBEANSMUSTHAVEBEENPASSIVATEDWEPRINTOUTAMESSAGEDURINGTHEEJBPASSIVATECALLBACKTOILLUSTRATETHIS5NEXT,WECALLCOUNTONEACHEJBOBJECTTHISFORCESTHECONTAINERTOACTIVATETHEINSTANCES,RESTORINGTHECONVERSATIONSTOMEMORYONCEAGAINWEPRINTOUTAMESASGEDURINGTHEEJBACTIVATECALLBACKTOILLUSTRATETHIS6FINALLY,ALLTHEEJBOBJECTSAREREMOVEDTHECODEAPPEARSINSOURCE54RUNNINGTHECLIENTTORUNTHECLIENT,YOUNEEDTOKNOWTHEPARAMETERSYOURJNDISERVICEPROVIDERUSESTHISSHOULDALSOBEPARTOFYOURCONTAINERSDOCUMENTATIONWITHTHEBEAWEBLOGICSERVER,WETYPEDTHEFOLLOWINGJAVADJAVANAMINGFACTORYINITIALWEBLOGICJNDITENGAHINITIALCONTEXTFACTORYDJAVANAMINGPROVIDERURLT3/LOCALHOST7001COMWILEYCOMPBOOKSROMANSESSIONCOUNTCOUNTCLIENTTHECLIENTSIDEOUTPUTAFTERRUNNINGTHECLIENT,WESEETHEFOLLOWINGOUTPUTINSTANTIATINGBEANS123CALLINGCOUNTONBEANS234WEFIRSTCREATEDTHREEBEANSANDTHENCALLEDCOUNTONEACHASEXPECTED,THEBEANSINCREMENTEDTHEIRVALUESBYONEEACHDURINGTHESECONDPASS,SOOUTPUTISASEXPECTEDBUTWEREOURBEANSREALLYPASSIVATEDANDACTIVATEDLETSCHECKTHESERVERLOGTHESERVERSIDEOUTPUTTHECONTAINERLOGYIELDSTHEFOLLOWINGRESULTSEJBCREATECOUNTEJBCREATECOUNTEJBCREATEEJBPASSIVATECOUNTEJBPASSIVATEEJBACTIVATECOUNTEJBPASSIVATEEJBACTIVATECOUNTEJBPASSIVATEEJBACTIVATECOUNTEJBPASSIVATEEJBACTIVATEEJBREMOVEEJBACTIVATEEJBREMOVEEJBREMOVEASYOUCANSEEFROMTHEPASSIVATION/ACTIVATIONMESSAGESINTHELOG,THECONTAINERISINDEEDPASSIVATINGANDACTIVATINGBEANSTOCONSERVESYSTEMRESOURCESBECAUSETHECLIENTSIDEOUTPUTISCORRECT,EACHOFOURBEANSCONVERSATIONALSTATEWASRETAINEDPROPERLYSTATEFULORSTATELESSNOWTHATWEVEGONETHROUGHAFEWEXAMPLES,YOUMAYBEWONDERINGWHENSTATEFULBEANSSHOULDBEUSEDANDWHENSTATELESSBEANSCANGETTHEJOBDONETHEREAREADVANTAGESANDDRAWBACKSTOBOTHSTATELESSANDSTATEFULDESIGNMYTHSANDFACTSABOUTSTATELESSNESSLATELY,THERESBEENALOTOFFUSSOVERSTATELESSNESSTHELIMITATIONSOFSTATELESSNESSAREOFTENEXAGGERATED,ASWELLASITSBENEFITSMANYSTATELESSNESSPROPONENTSBLINDLYDECLARETHATSTATELESSNESSLEADSTOINCREASEDSCALABILITY,WHILESTATEFULBACKERSARGUEABOUTHAVINGTOREARCHITECTENTIRESYSTEMSTOACCOMMODATESTATELESSNESSWHATSTHEREALSTORYDESIGNEDRIGHT,STATELESSNESSHASTWOVIRTUESWITHSTATELESSBEANS,THEEJBCONTAINERISABLETOEASILYPOOLANDREUSEBEANS,ALLOWINGAFEWBEANSTOSERVICEMANYCLIENTSWHILETHESAMEPARADIGMAPPLIESTOSTATEFULBEANS,THEBEANSTATEMUSTBEPASSIVATEDANDACTIVATEDBETWEENMETHODCALLS,POSSIBLYRESULTINGINI/OBOTTLENECKSSOONEPRACTICALVIRTUEOFSTATELESSNESSISTHEABILITYTOEASILYPOOLANDREUSECOMPONENTSATLITTLEORNOOVERHEADBECAUSEASTATEFULSESSIONBEANCACHESACLIENTCONVERSATIONINMEMORY,ABEANFAILUREMAYENTAILLOSINGYOURCONVERSATIONTHISCANHAVESEVEREREPRECUSSIONSIFYOUDONTWRITEYOURBEANSWITHTHISINMINDORIFYOUDONTUSEANEJBPRODUCTTHATPROVIDESSTATEFULRECOVERYTHELARGESTDRAWBACKTOSTATELESSNESSISTHATYOUNEEDTOPUSHCLIENTSPECIFICDATAINTOTHESTATELESSBEANFOREACHMETHODINVOCATIONMOSTSTATELESSSESSIONBEANSWILLNEEDTORECEIVESOMEINFORMATIONTHATISSPECIFICTOACERTAINCLIENT,SUCHASABANKACCOUNTNUMBERFORABANKINGBEANTHISINFORMATIONMUSTBERESUPPLIEDTOSTATELESSBEANSEACHTIMEACLIENTREQUESTARRIVESBECAUSETHEBEANCANNOTHOLDANYSTATEONBEHALFOFAPARTICULARCLIENTONEWAYTOSUPPLYTHEBEANWITHCLIENTSPECIFICDATAISTOPASSTHEDATAASPARAMETERSINTOTHEBEANSMETHODSTHISCANLEADTOPERFORMANCEDEGRADATION,HOWEVER,ESPECIALLYIFTHEDATABEINGPASSEDISLARGETHISALSOCLOGSTHENETWORK,REDUCINGAVAILABLEBANDWIDTHFOROTHERPROCESSESANOTHERWAYTOGETCLIENTSPECIFICDATATOASTATELESSBEANISFORTHEBEANTOSTOREDATAPERSISTENTLYONBEHALFOFACLIENTTHECLIENTTHENDOESNOTNEEDTOPASSTHEENTIRESTATEINAMETHODINVOCATION,BUTSIMPLYNEEDSTOSUPPLYANIDENTIFIERTORETRIEVETHEDATAFROMPERSISTENTSTORAGETHETRADEOFFHEREIS,AGAIN,PERFORMANCESTORINGCONVERSATIONSPERSISTENTLYCOULDLEADTOSTORAGEI/OBOTTLENECKS,RATHERTHANNETWORKI/OBOTTLENECKSYETANOTHERWAYTOWORKAROUNDTHELIMITATIONSOFSTATELESSNESSISFORABEANTOSTORECLIENTSPECIFICDATAINADIRECTORYSTRUCTUREUSINGJNDITHECLIENTCOULDLATERPASSTHEBEANANIDENTIFIERFORLOCATINGTHEDATAINTHEDIRECTORYSTRUCTURETHISISQUITESIMILARTOSTORINGDATAINADATABASETHEBIGDIFFERENCEISTHATAJNDIIMPLEMENTATIONCOULDBEANINMEMORYIMPLEMENTATIONTHISWOULDGIVEASIMILAREFFECTTOASHAREDPROPERTYMANAGER,FAMILIARTOMTS/COMREADERSIFCLIENTDATAISSTOREDINMEMORY,THEREISNODATABASEHITWHENCHOOSINGBETWEENSTATEFULANDSTATELESS,YOUSHOULDASKYOURSELFWHATTYPEOFBUSINESSPROCESSYOURSESSIONBEANSAREATTEMPTINGTOEMULATEDOESTHEBUSINESSPROCESSSPANMULTIPLEINVOCATIONS,REQUIRINGACONVERSATIONIFSO,THESTATEFULMODELFITSVERYNICELYBECAUSECLIENTSPECIFICCONVERSATIONSCANBEPARTOFTHEBEANSTATEONTHEOTHERHAND,IFYOURBUSINESSPROCESSLASTSFORASINGLEMETHODCALL,THESTATELESSPARADIGMWILLBETTERSUITYOURNEEDSINREALITY,MOSTSOPHISTICATEDDEPLOYMENTSARELIKELYTOHAVEACOMPLEXANDINTERESTINGCOMBINATIONOFSTATELESSANDSTATEFULBEANSTHECHOICEBETWEENSTATEFULORSTATELESSMAYALSOPALEINCOMPARISONTOOTHERFACTORSINYOUREJBDEPLOYMENT,SUCHASPROPERUSEOFTRANSACTIONSWELLFINDOUTHOWTOAPPROPRIATELYUSETRANSACTIONSINCHAPTER10SUMMARYINTHISCHAPTER,WELEARNEDHOWTOPROGRAMWITHSTATEFULSESSIONBEANSWEBEGANWITHALOOKATTHECONCEPTSBEHINDSTATEFULSESSIONBEANSANDHOWTHEYAREDIFFERENTFROMSTATELESSSESSIONBEANSWETHENCODEDUPASIMPLECOUNTINGPROGRAM中文翻译STATELESSSESSIONBEANS的基础在这一节,我们将学会如和编写“STATELESSSESSIONBEANS“。“STATELESSSESSIONBEANS“是由一个单独的方法命令执行典型商业的模式执行。本节从着眼于STATELESSSESSIONBEANS共享特性开始。我们将在后文用一个STATELESSBEAN的例子和简单的服务器代码来实现它。STATELESSSESSIONBEANS的特性在我进入编写的细节之前，我从编写STATELESSSESSIONBEANS不同于其他BEANS的细节的总观开始。无会话状态STATELESSSESSION停止了会话状态服务器的利益，尽管他们能包括互联网状态，他们的状态不是为一个详细的服务器而定制化。这说明所有STATELESSBEANS出现在同一的服务器，服务器不能分辨它们。要想使一个STATELESSSESSIONBEAN能对服务器有用，服务器必须通过BEAN所需关于商业逻辑方法的参数所有服务器数据。有选择性的，BEAN能从外部来源重新找回它所需的数据，与数据库相似。唯一初始化STATELESSSESSION的方法我们已经学会了用EJBCREATE方法初始化SESSIONBEANSSTATELESSSESSIONBEANS不能保持状态连接方法命令，他们也不能保持在服务器通过了EJBCREATE命令数据的状态。它因此不能使支持奇异的EJBCREATE方法有意义由于并发命令BEAN的实例，这个实例会对之前的EJBCREATE方法有纪录。从而所有的STATELESSSESSIONBEANS受到唯一一个EJBCREATE方法，不带参数。主对象也会受到与其匹配的不带参数的CREATE方法。容器能共享个再利用STATELESSSESSION由于STATELESSSESSIONBEANS的EJBCREATE方法不带有参数，服务期从不提供任何需要开动的“BEAN“实例的鉴定信息。因此EJB容器能在服务期连接前共享STATELESSSESSIONBEANS实例。当服务期命令一个方法时，容器能从共享中找回实例，让其为方法服务，然后把它返回到共享中。这个使容器能够有力的指派你的BEAN的实例到不同的服务期。另一个作用是来自服务期的每次请求，一个不同的STATELESSSESSIONBEANS实例能服务这个请求。这是因为STATELESSSESSION保持会话在单一方法请求的范围内并且在每个方法命令后摆脱了服务器特有状态。所有STATELESSSESSIONBEANS都认为在一个方法命令之后处于同一状态，他们不会有效的执行就算先前的方法命令发生。因此，容器能有力地再指定BEANS到服务器请求在每个方法级别。不同的STATELESSSESSIONBEANS能服务来自服务器的每个方法命令。当然，重新指定BEANS到服务器的实际执行是容器特有的。BEAN实例共享的好处是使BEANS的共享比服务器连接的实际数量少很多。这取决于服务器的“思考时间“，就像网络延迟或在服务器另一边人类的决定时间。在服务器运行的同时，容器能利用BEANS实例服务器他的服务器，储存先前系统资源。记录BEAN共享不需要有静止的大小。高级容器提供有力的重定大小的共享随时间，允许共享大小随着读取要求的波动而变化。举个例子，如果比晚上有更多的服务器在白天连接到你的设备。这些免费的系统资源不休止的被其他进