2外文翻译编写容器管理持久性实体Bean.docx

2外文翻译编写容器管理持久性实体Bean.docx

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2外文翻译编写容器管理持久性实体Bean

WritingContainer-ManagedPersistentEntityBeans

Container-ManagedFields

Acontainer-managedpersistententitybeanallowsthecontainertohandlesomeorallofitsdataaccesslogic.RatherthancodingJDBCorSQL/Joperationsinyourbeanclass,yourcontainerimplicitlyperformsalldatabaseoperationsbehindthescenes.

Withcontainer-managedpersistence,youmustmakesomeofyourentitybeanclass’sfieldspublicsothatthecontainercansetthefieldswhenitperformsdatabaseoperationsonbehalfofyourbean.Thefieldsthatyouwanttobepersistentarecalledcontainer-managedfields.Youdon’thavetoworryaboutsettingthesefields—theEJBcontainerwillautomaticallymanipulatethemforyou

behindthesceneswhenitperformsstorageoperations.

Onerestrictionofcontainer-managedfieldsisthateveryfieldyouwanttobemanagedbythecontainermustfollowtherulesforJavaobjectserialization（wedescribetheserulesinfullinAppendixA）.ThismeansthatprimitivetypessuchasdoublesandBooleans,aswellasserializableclassessuchasprimarykeyclassesorEJBhandlestootherentitybeans,canbecontainer-managedfields.

Forexample,thefollowingisasnippetofcodefromourbankaccountentitybeanclassthatwewroteinChapter8:

Withcontainer-managedpersistence,thecontainercanpersisteachofthesefieldsforyoubehindthescenes.Whensavingyourbeaninstance’sfields,thecontainerisresponsibleforqueryingyourbeaninstanceforthesefieldvalues.Whenloadingdataintoyourbeaninstance,thecontainersetsthesefields.Thisispossiblebecauseeachofthefieldsisdeclaredaspublic.

Ofcourse,youstillmustinformthecontaineraboutwhichfieldsitshouldmanipulate.Youspecifythisinyourbean’sdeploymentdescriptor.TheEJBcontainerwillinspectthedeploymentdescriptortofigureoutwhichofyourentitybean’sfieldstomanipulate.

Notethatnotallfieldswithinthebeanhavetobemanagedbythecontainer.Youmightbepullingdatamanuallyfromasecondarysource,oryoumighthavecalculatedfields.TheEJBcontainerwillautomaticallynotifyyourbeanclassduringpersistentoperations,allowingyoutomanagethesefields.

PrimaryKeyClass

Aswithbean-managedpersistence,container-managedpersistencedictatesthatyourprimarykeyclassmustbeserializable.BecausetheEJBcontainerwillworkwithyourprimarykey,therearenewrestrictionsforhowyouwriteyourprimarykeyclass.Themostimportantrestrictionisthatthefieldsyouhaveinyourprimarykeymustcomefromthecontainer-managedfieldsofyourentitybean,whichwedescribedpreviously.ThisrestrictionallowstheEJBcontainertoset,aswellasextract,yourentitybean’sprimarykeyfields.

Forexample,takeourprimarykeyclassfromourChapter8’sbankaccount:

Thisisavalidprimarykeyclassforcontainer-managedpersistencebecauseit’sserializableandbecauseitspublicfieldscomefromourbeanclass’scontainer-managedfields.

ImplementationGuidelinesforContainer-ManagedPersistence

Themethodimplementationsofyourentitybeansshouldbedifferentforcontainer-managedpersistententities.Nolongerareyoucontrollingtheroutinepersistentoperationsofyourbeans,andsomanyofthemethodscanbeleftempty—thecontainerwilldoitforyou.Table9.1isasummaryofwhatyoushouldimplementineachmethod,assumingyourentitybean’spersistenceiscontainermanaged.Takeaquickglanceatthechartfornow.Asyoucanseefromthetable,manyofthedatabase-intensiveoperationshavebeenreducedinscopesignificantly.Youshouldreferbacktothechartwhenreadingthroughthecodeinthischapterorwhenprogrammingyourownentitybeanclasses.Theorderofmethodslistedveryroughlymodelstheflowofcontrolofanentitybeaninstance’slifecyclethatwesawattheendofChapter7.

Container-ManagedPersistenceExample:

AProductLine

Let’sseeaquickdemonstrationofcontainer-managedpersistenceinaction,appliedtotheconceptofaproductline.

Ifyou’reworkingforaproduct-basedcompany,yourcompany’sproductlineisthesuiteofproductsyourcompanyoffers.Forexample,ifyou’reanappliancecompany,youmightofferadishwasher,astove,andadryer.Ifyou’reacomputerhardwarecompany,youmightoffermemory,harddisks,andprocessors.We’regoingtomodelagenericproductasanentitybeanthatusescontainer-managedpersistence.

TheobjectmodelforourproductlineisdetailedinFigure9.1.

Let’stakealookateachofthefilesthatwemustcreateforourentitybeancomponent.

Product.java

OurremoteinterfaceisspecifiedbyProduct.java,showninSource9.1.

OurremoteinterfaceisverysimilartoChapter8’sbankaccountremoteinterface.Ithasmethodstomodifytheentitybeaninstance’sfieldsandthrowsremoteexceptionstoindicatesystem-levelerrors.

ProductHome.java

Next,wehavetheproduct’shomeinterface,ProductHome.java,presentedinSource9.2

Ourhomeinterfacedefinesasinglecreate（）methodtocreateanewproductinthedatabase.ItreturnsaProductEJBobjectsotheclientcanmanipulatetheentitybeandataandthrowsajavax.ejb.CreateExceptiontoindicateanapplication-levelproblem.

Wealsoexposeallsortsoffindermethodstofindexistingproducts.SomeofthefindersreturnasingleEJBobject,whileothersreturnajava.util.EnumerationofmultipleEJBobjects.Thisisneededifthefindermethodsfindmorethanonematchingobject.NotethatfindByPrimaryKey（）shouldneverreturnanenumerationbecauseprimarykeysmustbeunique.

ProductPK.java

OurprimarykeyclassisdefinedbyProductPK.java,showninSource9.3.

AswithourBankAccount,ourprimarykeyisasimplestring.Andaswe’vefoundout,therearerestrictionsforwhatourprimarykeycanbe.Ourprimarykeyfieldsarecomingfromthecontainer-managedfieldsoftheentitybeanclass,asisrequiredwithcontainer-managedpersistence.Inparticular,ourprimarykeyrepresentstheIDstringofaproduct（suchasaproductSKUnumber）.

ProductBean.java

Next,wehaveourcontainer-managedentitybeanimplementation,ProductBean.java,showninSource9.4.

Thisbeanismorecomplexthanourbankaccountexample.We’vedefinedmanyfindermethods,andwehavefourpersistentfields.Yeteventhoughwe’veaddedallthiscomplexity,ourbeanislessthan40percentofthesizeofourBankAccountbean.Thisisanamazingreductionincodecomplexity.Andbecauseourbeanhasnodatabasecodeinit,wehavereducedthechanceforbugsinourbeanthatwouldbeduetousererrorworkingwithJDBCcode.Thisisahugesavingsindevelopmentandtestingtime.

Wehavefourcontainer-managedfields,allwithpublicscope.They’republicsothatthecontainercanmanipulatethem.OurejbCreate（）methodsimplysetsourcontainer-managedfieldstothepassed-inclientparameters.TheEJBcontainerwillextractthosefieldsandsetupthedatabasedataforus.NoticethatourejbCreate（）methoddoesnotreturnaprimarykeybecausetheEJBcontainerdoesthatforus.

Therestofourbeanisjustemptymethodsandcomments.There’salmostnologicatall.Ourbeanclassisjustdatawithsomeaccessormethods.

Client.java

Ourclientcodeisasimplesuiteoftestcasestotryoutourbean,asshowninSource9.5.

WeperformaJNDIlookuptoacquirethehomeobjectandcreatesomeentitybeandata.Wethentryoutacoupleoffindermethods.Wecanloopthroughthefinders’returnednumerationsandcallbusinessmethodsoneachEJBobject.WethendestroyalltheEJBobjectswecreatedinafinally{}clause.

TheDeploymentDescriptor

Wenowneedtowriteourdeploymentdescriptor.Inadditiontodefiningthestandardentitybeanfields,wenowneedtoinformthecontaineraboutourpubliccontainer-managedfields.ThedeploymentdescriptorisshowninTable9.2.NoticethatwenolongerhaveanyJDBCapplication-specificpropertiesbecausewe’veexternalizedalldatabaseactivitytothecontainer.

Inadditiontothedeploymentdescriptor,weneedtotellthecontainerexactlyhowtoperformpersistentoperations.Thisisonetrade-offofcontainer-managedpersistence—youstillneedtodeclarepersistentrules,ratherthancodethemintoyourbeanusingJDBCorSQL/J.

Ifyou’reusingarelationaldatastore,you’llneedtodefineexactlyhowyourentitybean’spublicfieldsmaptothatdatabase.Thus,wemustdefineaseriesofobject-relationalmappingentries.Theseentriesmapentitybeanfieldstorelationaldatabasecolumnnames.TheEJBcontainer（inthiscase,BEAWebLogic）willusethismappingwhenstoringorretrievingourcontainer-managedfieldsfromthedatabase.NotethatthisisveryEJBcontainer-specific!

SomeEJBcontainerswillsupportobjectdatabasesandthuswillnothaveamappingintoatwo-dimensionalrelationaldatabase.ConsultyourEJBcontainer’sdocumentationformoreinformation.Ourproductline’spersistententriesforBEA’sWebLogicserverareshowninTable9.3.

Wealsoneedtospecifytheimplementationofourhomeobject’sfindermethods.Thisisalso,unfortunately,proprietaryforeachEJBcontainer.BEAWebLogichasasimplescriptinglanguageforthispurpose.Forexample:

ThecompletescriptisshowninTable9.4.Thecontainerwillimplementthislogic,perhapsusingJDBCorSQL/J.Wheneveraclientwantstoexecuteafindermethodonthehomeobject,thecontainerwillautomaticallyruntheimplementedJDBCorSQL/Jcode.

RunningtheClientProgram

Toruntheclientprogram,typeacommandsimilartothefollowing（dependingonwhatyourEJBcontainerJavaNamingandDirectoryInterface,orJNDI,initializationparametersare）:

TheinitializationparametersarerequiredbyJNDItofindthehomeobject,aswelearnedin