翻譯文獻-第9章 編寫容器：管理持久性實體bean

外文原文CHAPTER9WRITINGCONTAINERMANAGEDPERSISTENTENTITYBEANSINCHAPTER8,WEWROTEABEANMANAGEDPERSISTENTENTITYBEANREPRESENTINGABANKACCOUNTINTHISCHAPTER,WELLSEEHOWTHINGSCHANGEWHENWEMOVETOACONTAINERMANAGEDMODELWITHCONTAINERMANAGEDPERSISTENCE,YOUDONTIMPLEMENTANYPERSISTENCELOGICINTHEENTITYBEANITSELFRATHER,THEEJBCONTAINERPERFORMSSTORAGEOPERATIONSFORYOUASYOUWILLSEE,THISGREATLYSIMPLIFIESBEANDEVELOPMENTBEFOREREADINGTHISCHAPTER,YOUSHOULDBEFAMILIARWITHTHEENTITYBEANCONCEPTSWECOVEREDINCHAPTER7CONTAINERMANAGEDFIELDSACONTAINERMANAGEDPERSISTENTENTITYBEANALLOWSTHECONTAINERTOHANDLESOMEORALLOFITSDATAACCESSLOGICRATHERTHANCODINGJDBCORSQL/JOPERATIONSINYOURBEANCLASS,YOURCONTAINERIMPLICITLYPERFORMSALLDATABASEOPERATIONSBEHINDTHESCENESWITHCONTAINERMANAGEDPERSISTENCE,YOUMUSTMAKESOMEOFYOURENTITYBEANCLASSSFIELDSPUBLICSOTHATTHECONTAINERCANSETTHEFIELDSWHENITPERFORMSDATABASEOPERATIONSONBEHALFOFYOURBEANTHEFIELDSTHATYOUWANTTOBEPERSISTENTARECALLEDCONTAINERMANAGEDFIELDSYOUDONTHAVETOWORRYABOUTSETTINGTHESEFIELDSTHEEJBCONTAINERWILLAUTOMATICALLYMANIPULATETHEMFORYOUBEHINDTHESCENESWHENITPERFORMSSTORAGEOPERATIONSONERESTRICTIONOFCONTAINERMANAGEDFIELDSISTHATEVERYFIELDYOUWANTTOBEMANAGEDBYTHECONTAINERMUSTFOLLOWTHERULESFORJAVAOBJECTSERIALIZATIONWEDESCRIBETHESERULESINFULLINAPPENDIXATHISMEANSTHATPRIMITIVETYPESSUCHASDOUBLESANDBOOLEANS,ASWELLASSERIALIZABLECLASSESSUCHASPRIMARYKEYCLASSESOREJBHANDLESTOOTHERENTITYBEANS,CANBECONTAINERMANAGEDFIELDSFOREXAMPLE,THEFOLLOWINGISASNIPPETOFCODEFROMOURBANKACCOUNTENTITYBEANCLASSTHATWEWROTEINCHAPTER8WITHCONTAINERMANAGEDPERSISTENCE,THECONTAINERCANPERSISTEACHOFTHESEFIELDSFORYOUBEHINDTHESCENESWHENSAVINGYOURBEANINSTANCESFIELDS,THECONTAINERISRESPONSIBLEFORQUERYINGYOURBEANINSTANCEFORTHESEFIELDVALUESWHENLOADINGDATAINTOYOURBEANINSTANCE,THECONTAINERSETSTHESEFIELDSTHISISPOSSIBLEBECAUSEEACHOFTHEFIELDSISDECLAREDASPUBLICOFCOURSE,YOUSTILLMUSTINFORMTHECONTAINERABOUTWHICHFIELDSITSHOULDMANIPULATEYOUSPECIFYTHISINYOURBEANSDEPLOYMENTDESCRIPTORTHEEJBCONTAINERWILLINSPECTTHEDEPLOYMENTDESCRIPTORTOFIGUREOUTWHICHOFYOURENTITYBEANSFIELDSTOMANIPULATENOTETHATNOTALLFIELDSWITHINTHEBEANHAVETOBEMANAGEDBYTHECONTAINERYOUMIGHTBEPULLINGDATAMANUALLYFROMASECONDARYSOURCE,ORYOUMIGHTHAVECALCULATEDFIELDSTHEEJBCONTAINERWILLAUTOMATICALLYNOTIFYYOURBEANCLASSDURINGPERSISTENTOPERATIONS,ALLOWINGYOUTOMANAGETHESEFIELDSPRIMARYKEYCLASSASWITHBEANMANAGEDPERSISTENCE,CONTAINERMANAGEDPERSISTENCEDICTATESTHATYOURPRIMARYKEYCLASSMUSTBESERIALIZABLEBECAUSETHEEJBCONTAINERWILLWORKWITHYOURPRIMARYKEY,THEREARENEWRESTRICTIONSFORHOWYOUWRITEYOURPRIMARYKEYCLASSTHEMOSTIMPORTANTRESTRICTIONISTHATTHEFIELDSYOUHAVEINYOURPRIMARYKEYMUSTCOMEFROMTHECONTAINERMANAGEDFIELDSOFYOURENTITYBEAN,WHICHWEDESCRIBEDPREVIOUSLYTHISRESTRICTIONALLOWSTHEEJBCONTAINERTOSET,ASWELLASEXTRACT,YOURENTITYBEANSPRIMARYKEYFIELDSFOREXAMPLE,TAKEOURPRIMARYKEYCLASSFROMOURCHAPTER8SBANKACCOUNTTHISISAVALIDPRIMARYKEYCLASSFORCONTAINERMANAGEDPERSISTENCEBECAUSEITSSERIALIZABLEANDBECAUSEITSPUBLICFIELDSCOMEFROMOURBEANCLASSSCONTAINERMANAGEDFIELDSIMPLEMENTATIONGUIDELINESFORCONTAINERMANAGEDPERSISTENCETHEMETHODIMPLEMENTATIONSOFYOURENTITYBEANSSHOULDBEDIFFERENTFORCONTAINERMANAGEDPERSISTENTENTITIESNOLONGERAREYOUCONTROLLINGTHEROUTINEPERSISTENTOPERATIONSOFYOURBEANS,ANDSOMANYOFTHEMETHODSCANBELEFTEMPTYTHECONTAINERWILLDOITFORYOUTABLE91ISASUMMARYOFWHATYOUSHOULDIMPLEMENTINEACHMETHOD,ASSUMINGYOURENTITYBEANSPERSISTENCEISCONTAINERMANAGEDTAKEAQUICKGLANCEATTHECHARTFORNOWASYOUCANSEEFROMTHETABLE,MANYOFTHEDATABASEINTENSIVEOPERATIONSHAVEBEENREDUCEDINSCOPESIGNIFICANTLYYOUSHOULDREFERBACKTOTHECHARTWHENREADINGTHROUGHTHECODEINTHISCHAPTERORWHENPROGRAMMINGYOUROWNENTITYBEANCLASSESTHEORDEROFMETHODSLISTEDVERYROUGHLYMODELSTHEFLOWOFCONTROLOFANENTITYBEANINSTANCESLIFECYCLETHATWESAWATTHEENDOFCHAPTER7CONTAINERMANAGEDPERSISTENCEEXAMPLEAPRODUCTLINELETSSEEAQUICKDEMONSTRATIONOFCONTAINERMANAGEDPERSISTENCEINACTION,APPLIEDTOTHECONCEPTOFAPRODUCTLINEIFYOUREWORKINGFORAPRODUCTBASEDCOMPANY,YOURCOMPANYSPRODUCTLINEISTHESUITEOFPRODUCTSYOURCOMPANYOFFERSFOREXAMPLE,IFYOUREANAPPLIANCECOMPANY,YOUMIGHTOFFERADISHWASHER,ASTOVE,ANDADRYERIFYOUREACOMPUTERHARDWARECOMPANY,YOUMIGHTOFFERMEMORY,HARDDISKS,ANDPROCESSORSWEREGOINGTOMODELAGENERICPRODUCTASANENTITYBEANTHATUSESCONTAINERMANAGEDPERSISTENCETHEOBJECTMODELFOROURPRODUCTLINEISDETAILEDINFIGURE91LETSTAKEALOOKATEACHOFTHEFILESTHATWEMUSTCREATEFOROURENTITYBEANCOMPONENTPRODUCTJAVAOURREMOTEINTERFACEISSPECIFIEDBYPRODUCTJAVA,SHOWNINSOURCE91OURREMOTEINTERFACEISVERYSIMILARTOCHAPTER8SBANKACCOUNTREMOTEINTERFACEITHASMETHODSTOMODIFYTHEENTITYBEANINSTANCESFIELDSANDTHROWSREMOTEEXCEPTIONSTOINDICATESYSTEMLEVELERRORSPRODUCTHOMEJAVANEXT,WEHAVETHEPRODUCTSHOMEINTERFACE,PRODUCTHOMEJAVA,PRESENTEDINSOURCE92OURHOMEINTERFACEDEFINESASINGLECREATEMETHODTOCREATEANEWPRODUCTINTHEDATABASEITRETURNSAPRODUCTEJBOBJECTSOTHECLIENTCANMANIPULATETHEENTITYBEANDATAANDTHROWSAJAVAXEJBCREATEEXCEPTIONTOINDICATEANAPPLICATIONLEVELPROBLEMWEALSOEXPOSEALLSORTSOFFINDERMETHODSTOFINDEXISTINGPRODUCTSSOMEOFTHEFINDERSRETURNASINGLEEJBOBJECT,WHILEOTHERSRETURNAJAVAUTILENUMERATIONOFMULTIPLEEJBOBJECTSTHISISNEEDEDIFTHEFINDERMETHODSFINDMORETHANONEMATCHINGOBJECTNOTETHATFINDBYPRIMARYKEYSHOULDNEVERRETURNANENUMERATIONBECAUSEPRIMARYKEYSMUSTBEUNIQUEPRODUCTPKJAVAOURPRIMARYKEYCLASSISDEFINEDBYPRODUCTPKJAVA,SHOWNINSOURCE93ASWITHOURBANKACCOUNT,OURPRIMARYKEYISASIMPLESTRINGANDASWEVEFOUNDOUT,THEREARERESTRICTIONSFORWHATOURPRIMARYKEYCANBEOURPRIMARYKEYFIELDSARECOMINGFROMTHECONTAINERMANAGEDFIELDSOFTHEENTITYBEANCLASS,ASISREQUIREDWITHCONTAINERMANAGEDPERSISTENCEINPARTICULAR,OURPRIMARYKEYREPRESENTSTHEIDSTRINGOFAPRODUCTSUCHASAPRODUCTSKUNUMBERPRODUCTBEANJAVANEXT,WEHAVEOURCONTAINERMANAGEDENTITYBEANIMPLEMENTATION,PRODUCTBEANJAVA,SHOWNINSOURCE94THISBEANISMORECOMPLEXTHANOURBANKACCOUNTEXAMPLEWEVEDEFINEDMANYFINDERMETHODS,ANDWEHAVEFOURPERSISTENTFIELDSYETEVENTHOUGHWEVEADDEDALLTHISCOMPLEXITY,OURBEANISLESSTHAN40PERCENTOFTHESIZEOFOURBANKACCOUNTBEANTHISISANAMAZINGREDUCTIONINCODECOMPLEXITYANDBECAUSEOURBEANHASNODATABASECODEINIT,WEHAVEREDUCEDTHECHANCEFORBUGSINOURBEANTHATWOULDBEDUETOUSERERRORWORKINGWITHJDBCCODETHISISAHUGESAVINGSINDEVELOPMENTANDTESTINGTIMEWEHAVEFOURCONTAINERMANAGEDFIELDS,ALLWITHPUBLICSCOPETHEYREPUBLICSOTHATTHECONTAINERCANMANIPULATETHEMOUREJBCREATEMETHODSIMPLYSETSOURCONTAINERMANAGEDFIELDSTOTHEPASSEDINCLIENTPARAMETERSTHEEJBCONTAINERWILLEXTRACTTHOSEFIELDSANDSETUPTHEDATABASEDATAFORUSNOTICETHATOUREJBCREATEMETHODDOESNOTRETURNAPRIMARYKEYBECAUSETHEEJBCONTAINERDOESTHATFORUSTHERESTOFOURBEANISJUSTEMPTYMETHODSANDCOMMENTSTHERESALMOSTNOLOGICATALLOURBEANCLASSISJUSTDATAWITHSOMEACCESSORMETHODSCLIENTJAVAOURCLIENTCODEISASIMPLESUITEOFTESTCASESTOTRYOUTOURBEAN,ASSHOWNINSOURCE95WEPERFORMAJNDILOOKUPTOACQUIRETHEHOMEOBJECTANDCREATESOMEENTITYBEANDATAWETHENTRYOUTACOUPLEOFFINDERMETHODSWECANLOOPTHROUGHTHEFINDERSRETURNEDNUMERATIONSANDCALLBUSINESSMETHODSONEACHEJBOBJECTWETHENDESTROYALLTHEEJBOBJECTSWECREATEDINAFINALLYCLAUSETHEDEPLOYMENTDESCRIPTORWENOWNEEDTOWRITEOURDEPLOYMENTDESCRIPTORINADDITIONTODEFININGTHESTANDARDENTITYBEANFIELDS,WENOWNEEDTOINFORMTHECONTAINERABOUTOURPUBLICCONTAINERMANAGEDFIELDSTHEDEPLOYMENTDESCRIPTORISSHOWNINTABLE92NOTICETHATWENOLONGERHAVEANYJDBCAPPLICATIONSPECIFICPROPERTIESBECAUSEWEVEEXTERNALIZEDALLDATABASEACTIVITYTOTHECONTAINERINADDITIONTOTHEDEPLOYMENTDESCRIPTOR,WENEEDTOTELLTHECONTAINEREXACTLYHOWTOPERFORMPERSISTENTOPERATIONSTHISISONETRADEOFFOFCONTAINERMANAGEDPERSISTENCEYOUSTILLNEEDTODECLAREPERSISTENTRULES,RATHERTHANCODETHEMINTOYOURBEANUSINGJDBCORSQL/JIFYOUREUSINGARELATIONALDATASTORE,YOULLNEEDTODEFINEEXACTLYHOWYOURENTITYBEANSPUBLICFIELDSMAPTOTHATDATABASETHUS,WEMUSTDEFINEASERIESOFOBJECTRELATIONALMAPPINGENTRIESTHESEENTRIESMAPENTITYBEANFIELDSTORELATIONALDATABASECOLUMNNAMESTHEEJBCONTAINERINTHISCASE,BEAWEBLOGICWILLUSETHISMAPPINGWHENSTORINGORRETRIEVINGOURCONTAINERMANAGEDFIELDSFROMTHEDATABASENOTETHATTHISISVERYEJBCONTAINERSPECIFICSOMEEJBCONTAINERSWILLSUPPORTOBJECTDATABASESANDTHUSWILLNOTHAVEAMAPPINGINTOATWODIMENSIONALRELATIONALDATABASECONSULTYOUREJBCONTAINERSDOCUMENTATIONFORMOREINFORMATIONOURPRODUCTLINESPERSISTENTENTRIESFORBEASWEBLOGICSERVERARESHOWNINTABLE93WEALSONEEDTOSPECIFYTHEIMPLEMENTATIONOFOURHOMEOBJECTSFINDERMETHODSTHISISALSO,UNFORTUNATELY,PROPRIETARYFOREACHEJBCONTAINERBEAWEBLOGICHASASIMPLESCRIPTINGLANGUAGEFORTHISPURPOSEFOREXAMPLETHECOMPLETESCRIPTISSHOWNINTABLE94THECONTAINERWILLIMPLEMENTTHISLOGIC,PERHAPSUSINGJDBCORSQL/JWHENEVERACLIENTWANTSTOEXECUTEAFINDERMETHODONTHEHOMEOBJECT,THECONTAINERWILLAUTOMATICALLYRUNTHEIMPLEMENTEDJDBCORSQL/JCODERUNNINGTHECLIENTPROGRAMTORUNTHECLIENTPROGRAM,TYPEACOMMANDSIMILARTOTHEFOLLOWINGDEPENDINGONWHATYOUREJBCONTAINERJAVANAMINGANDDIRECTORYINTERFACE,ORJNDI,INITIALIZATIONPARAMETERSARETHEINITIALIZATIONPARAMETERSAREREQUIREDBYJNDITOFINDTHEHOMEOBJECT,ASWELEARNEDINCHAPTER4SERVERSIDEOUTPUTWHENYOURUNTHECLIENT,YOUSHOULDSEESOMETHINGSIMILARTOTHEFOLLOWINGONTHESERVERSIDENOTETHATYOURPARTICULAROUTPUTMAYVARY,DUETOVARIANCESINEJBCONTAINERBEHAVIORWECREATEDANUMBEROFNEWPRODUCTSINOURCLIENTCODEFOREACHNEWPRODUCT,OUREJBCONTAINERCREATEDADEDICATEDBEANINSTANCEITDIDNTHAVETODOTHISITCOULDHAVEPASSIVATED/ACTIVATEDTHESAMEBEANANDSWITCHEDCONTEXTBETWEENCLIENTSWHENCREATINGABEAN,OURCONTAINERFIRSTCALLEDNEWINSTANCE,FOLLOWEDBYSETENTITYCONTEXT,WHICHGOTTHEBEANINTOTHEPOOLITTHENCALLEDEJBCREATE,SETUPTHEDATABASEDATA,BOUNDTHEBEANTOANEJBOBJECT,ANDFINALLYCALLEDEJBPOSTCREATEALLASEXPECTEDITTHENSERVICEDAFEWBUSINESSCALLS,INSTANTIATEDAFEWNEWBEANS,ANDOCCASIONALLYSYNCHRONIZEDTHEBEANSWITHTHEUNDERLYINGDATABASECLIENTSIDEOUTPUTFORTHECLIENTSIDE,AFTERCREATINGSOMEPRODUCTS,WEPERFORMEDAFINDFORALLPRODUCTSTHATCOST200INDEED,MULTIPLEENTITYBEANSWERERETURNEDINOURENUMERATION,ASISSHOWNBELOWPROMISESANDREALITIESBEANMANAGEDPERSISTENCEVERSUSCONTAINERMANAGEDPERSISTENCENOWTHATYOUVESEENBOTHBEANMANAGEDANDCONTAINERMANAGEDPERSISTENTENTITYBEANS,YOUMUSTBECONVINCEDTHATCONTAINERMANAGEDBEANSARETHEWAYTOGOALLTHATJDBCCODEWASELIMINATEDFROMOURBEANCLASS,SAVINGUSSIGNIFICANTDEVELOPMENTTIMEHOWEVER,THECHOICEBETWEENCONTAINERANDBEANMANAGEDPERSISTENCEISNOTNECESSARILYCLEARCUTBOTHBEANMANAGEDANDCONTAINERMANAGEDBEANSHAVEVIRTUESCONTAINERMANAGEDPERSISTENCEMAYPROMISEALOT,BUTITSCURRENTMANIFESTATIONFAILSTODELIVERONNUMEROUSCOUNTS,ABOUTWHICHYOUMUSTBEINFORMEDLETUSLOOKATTHRE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的客戶端參數(shù)，簡單地設定我們的容器管理作用域。EJB容器將提取這些作用域，并為我們從數(shù)據(jù)庫中提取數(shù)據(jù)。注意，我們EJBCREATE方法并不返回一個主鍵，因為EJB容器已經(jīng)為我們執(zhí)行了這樣的操作。我們BEAN中其余的只是空的方法和注釋。這些幾乎沒有邏輯可言。我們的BEAN類只是數(shù)據(jù)與一些存取器方法。CLIENTJAVA我們的客戶端代碼是一套簡單測試案例，用來測試我們的BEAN，見源代碼95。我們執(zhí)行一個JNDI查找獲得有效的對象并創(chuàng)造一些實體BEAN數(shù)據(jù)。然后，我們嘗試一系列的搜索方法。我們可以通過循環(huán)調(diào)用搜索方法返回編號并可以對每個EJB的對象調(diào)用業(yè)務方法。最后，我們在我們創(chuàng)造的FINALLY子句中銷毀所有EJB對象。部署描述符我們現(xiàn)在需要編寫我們的部署描述符。除了界定標準實體BEAN作用域，我們現(xiàn)在還需要通知容器我們公共的容器管理作用域。部署描述符列在表92中。注意，我們不再有任何JDBC的明確應用性能，因為我們已經(jīng)向容器具體化數(shù)據(jù)庫的所有行為。除上述部署描述符，我們需要告訴容器到底履行持久化操作。這是一個容器管理持久化的交替使用你依然需要定義持久化規(guī)則，而不只是把他們添加到你使用JDBC或SQL/J編寫的BEAN中。如果你使用的是關系數(shù)據(jù)存儲，你需要嚴格定義如何把你的實體BEAN的公共作用域映射到數(shù)據(jù)庫。因此，我們必須定義一系列的關系對象映射到實體。這些實體把實體BEAN作用域映射到關系型數(shù)據(jù)庫的字段。該EJB容器（在此我們使用的是BEA公司的WEBLOGIC）在向數(shù)據(jù)庫存儲或從數(shù)據(jù)庫中讀取我們的容器管理作用域時將利用這個映射。注意，這是這個特殊EJB容器的特性一些EJB的容器支持對象數(shù)據(jù)庫，并因此將不會有一個二維關系型數(shù)據(jù)庫的映射。查詢你的EJB容器文檔可以獲取更多信息。我們使用BEA公司的WEBLOGICSERVER的生產(chǎn)線持久化的實體，列在表93中。我們還需要指定實現(xiàn)的是我