外文翻译3Stateless Session Beans的基础.docx

外文翻译3Stateless Session Beans的基础.docx

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外文翻译3StatelessSessionBeans的基础

TheBasicsofStatefulSessionBeans

Statefulsessionbeansareconversationalbeansbecausetheyholdconversations

withclientsthatspanmultiplemethodinvocations.Statefulsessionbeansstore

conversationalstatewithinthebean.Thatconversationalstateisspecifictoa

particularclient.Thischapterwillteachyouthebasicsofwritingstatefulsession

beans.We’llbeginattheconceptuallevelandquicklymoveintoacodeexample

illustratingsessionbeandevelopment.We’llwrapupwithacomparison

ofthestatefulandstatelessprogrammingmodels.

CharacteristicsofStatefulSessionBeans

Let’sbeginwithalookatthecharacteristicsthatallstatefulsessionbeansshare.

AchievingtheEffectofPoolingwithStatefulBeans

Imagineascenariowherethousandsofclientsarehavingconversationswith

differentstatefulsessionbeansrunningwithinacontainer.Let’sassumethat

theclientsaretypicalclientsandtakealongtimeto“think”betweenmethod

calls.Perhapsaclientisfaraway,connectingfromanothercountry,andthe

networklagishigh.Orperhapstheclientrepresentsahumaninteractingwith

aJavaapplet,andthehumanisdecidingonthenextbuttontoclick.

Thousandsofclientsimplythousandsofstatefulsessionbeans,eachholding

conversationalstateonbehalfofaspecificclient.And,ofcourse,ourEJBcontainer/

serverhasonlyafiniteamountofresourcesavailable,suchasmemory,

databaseconnections,andsocketconnections.Iftheconversationalstatethat

thebeansareholdingislarge,thecontainer/servercouldeasilyrunoutofresources.

Thiswasnotaproblemwithstatelesssessionbeansbecausethecontainer

couldpoolonlyafewbeanstoservicethousandsofclients.

Withstatefulsessionbeans,poolingisnotassimple.Whenaclientinvokesa

methodonabean,heorsheisstartingaconversationwiththebean,andthe

conversationalstatestoredinthebeanmustbeavailableforthatsameclient’s

nextmethodrequest.Therefore,thecontainercannoteasilypoolbeansand

dynamicallyassignthemtohandlearbitraryclientmethodrequests,sinceeach

beanisstoringstateonbehalfofaparticularclient.Butwestillneedtoachieve

theeffectofpoolingforstatefulsessionbeans,sothatwecanconserveresources

andenhancetheoverallscalabilityofthesystem.

Thisproblemshouldsoundquitefamiliartooperatingsystemsgurus.Whenever

yourunanapplicationonacomputer,youhaveonlyafixedamountofphysical

memoryinwhichtorun.Theoperatingsystemstillmustprovideawayfor

manyapplicationstorun,eveniftheapplicationstakeupmoreaggregate

memorythanisavailablephysically.Toprovideforthis,operatingsystemsuse

yourharddiskasanextensionofphysicalmemory.Thiseffectivelyextendsthe

amountofvirtualmemorythatyoursystemhas.Whenanapplicationgoesidle,

itsmemorycanbeswappedoutfromphysicalmemoryandontotheharddisk.

Whentheapplicationbecomesactiveagain,anyneededdataisswappedinfrom

theharddiskandintophysicalmemory.Thistypeofswappinghappensvery

oftenwhenswitchingbetweenapplications（calledcontextswitching）.

EJBcontainersexploitthisveryparadigmtoconservestatefulsessionbean

resources.Tolimitthenumberofstatefulsessionbeaninstancesinmemory,

thecontainercanswapoutastatefulbean,savingitsconversationalstatetoa

harddiskorotherstorage.Thisiscalledpassivation.Afterpassivatingastateful

bean,theconversationalstateissafelystoredaway,allowingresourcessuch

asmemorytobereclaimed.Whentheoriginalclientinvokesamethod,thepassivated

conversationalstateisswappedintoabean.Thisiscalledactivation.

Thisbeannowresumestheconversationwiththeoriginalclient.Notethatthe

beanthatreceivestheactivatedstatemaynotbetheoriginalbeaninstance.But

that’sallrightbecausetheinstanceisresumingitsconversationfromthepoint

wheretheoriginalinstancewaspassivated.

Thus,EJBdoesindeedsupporttheeffectofpoolingstatefulsessionbeans.Only

afewinstancescanbeinmemorywhenthereareactuallymanyclients.Butthis

poolingeffectdoesnotcomeforfree—thepassivation/activationstepscould

entailanI/Obottleneck.Contrastthistostatelesssessionbeans,whichareeasily

pooledbecausethereisnostatetosave.

Sohowdoesthecontainerdecidewhichbeanstoactivateandwhichbeansto

passivate?

Theanswerisspecifictoeachcontainer.Mostcontainerswillemploy

aLeastRecentlyUsed（LRU）passivationstrategy,whichsimplymeansto

passivatethebeanthathasbeencalledtheleastrecently.Thisisagoodalgorithm

becauseremoteclientshavethehabitofdisconnectingfromthenetwork,

leavingbeansstrandedwithoutaclient,readytobepassivated.Ifabeanhasn’t

beeninvokedinawhile,thecontainerwillwriteittodisk.

Passivationcanoccuratanytime,solongasabeanisnotinvolvedinamethod

call.It’suptothecontainertodecidewhenpassivationmakessense.Notethat

thereisoneexceptiontothisrule:

Anybeaninvolvedinatransaction（see

Chapter10）cannotbepassivateduntilthetransactioncompletes.

Toactivatebeans,mostcontainerswillusuallyuseajust-in-timealgorithm.Just

intimemeansthatbeansshouldbeactivatedondemand,asclientrequestscome

in.Ifaclientrequestcomesinbutthatclient’sconversationhasbeenpassivated,

thecontainerwillactivatethebeanondemand,readingthepassivatedstateback

intomemory.

Ingeneral,passivationandactivationarenotusefulforstatelesssessionbeans.

Statelessbeansdonothaveanystatetopassivate/activate,andsostatelessbeans

cansimplybedestroyedarbitrarilybythecontainer.Passivation/activationalso

appliestoentitybeans,whichwe’lllearnaboutinChapters7–9.

TheRulesGoverningConversationalState

Aswehavesaid,statefulsessionbeansholdconversationalstateonbehalfof

oneclient.Let’sdefinethatstateabitmorerigorously.It’simportanttoknow

therulesforspecifyingconversationalstate,sothatthecontainerwillpassivate

andactivateproperly.

TheconversationalstateofabeanfollowstheruleslaidoutbyJavaobjectserialization

（seeAppendixA）.Whenacontainerpassivatesabean,itusesobject

serialization（oranequivalentprotocol）toconvertthebean’sconversational

stateintoabit-blob.Itcanthenwritethatblobouttostorage.Oncethebeanis

writtentostorage,thememoryisavailabletobefreedbythegarbagecollector.

Activationreversestheprocess—aserializedblobthathadbeenwrittentostorage

isreadbackintomemoryandconvertedtoin-memorybeandata.What

makesthiswholeprocessworkisthejavax.ejb.EnterpriseBeaninterfaceextends

java.io.Serializable,andeveryenterprisebeanclassindirectlyimplements

thisinterface.

Moreconcretely,everymembervariableinabeanisconsideredtobepartof

thebean’sconversationalstateifthefollowingapply:

__Themembervariableisanontransientprimitivetype,or

__ThemembervariableisanontransientJavaobject（extends

java.lang.Object）.

ForeveryJavaobjectthat’spartofabean’sconversationalstate,theprevious

algorithmisreappliedrecursivelyonthoseobjects.Thus,objectserialization

constructsanentiregraphofdatareferredtobythemainbean.Youshouldnote

thatwhileyourbeansmustfollowtherulesforobjectserialization,theEJB

containeritselfdoesnotnecessarilyneedtousethedefaultserializationprotocol;

itcoulduseacustomprotocolforthis,toallowflexibilityanddifferentiation

betweencontainervendors.

Activation/PassivationCallbacks

Let’snowlookatwhatactuallyhappenstoyourbeanduringpassivationand

activation.WhenanEJBcontainerpassivatesabean,thecontainerwritesthe

bean’sconversationalstatetosecondarystorage,suchasafileordatabase.The

containerinformsthebeanthatit’sabouttoperformpassivationbycalling

thebean’srequiredejbPassivate（）callbackmethod.ejbPassivate（）isawarning

tothebeanthatitsheldconversationalstateisabouttobeswappedout.

It’simportantthatthecontainerinformthebeanusingejbPassivate（）sothat

thebeancanrelinquishheldresources.Theseheldresourcesincludedatabase

connections,opensockets,openfiles,orotherresourcesthatdonotmakesense

tobesavedtodiskorcannotbetransparentlysavedusingobjectserialization.

TheEJBcontainercallstheejbPassivate（）methodtogivethebeanachance

toreleasetheseresourcesordealwiththeresourcesasthebeanseesfit.Once

thecontainer’sejbPassivate（）callbackmethodintoyourbeaniscomplete,your

beanmustbeinastatesuitableforpassivation.ThisisshowninFigure5.1.

Theexactoppositeprocessoccursduringtheactivationprocess.Here,theserialized

conversationalstateisreadbackintomemory,andthecontainerreconstructs

thein-memorystateusingobjectserializationortheequivalent.The

containerthencallsthebean’srequiredejbActivate（）method.ejbActivate（）gives

thebeanachancetorestoretheopenresourcesitreleasedduringejbPassivate（）.

ThisentireprocessisshowninFigure5.2.

DoyouhavetoworryaboutimplementingejbPassivate（）andejbActivate（）?

Probablynot,unlessyouareusingopenresources,suchassocketconnections

ordatabaseconnections,thatmustbereestablishedafteractivation.Inmost

cases,youcansimplyleavethesemethodsempty.

ASimpleStatefulSessionBean

Let’sputourstatefulsessionbeanknowledgetousebyprogrammingasimple

statefulbean.Ourbeanwillbeacounterbean,anditwillsimplyberesponsible

forcountinguponebyone.Thecurrentcountwillbestor