Hibernate 简化继承映射 外文翻译Word文档格式.docx

Hibernate 简化继承映射 外文翻译Word文档格式.docx

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lldescribethatprocessinthisarticle.）

3.CopytheXMLconfigurationfilestoyourapplication'

sCLASSPATH.

You'

llnoticethatyoudon'

thavetomodifyanyJavaobjectstosupporttheframework.Imagine,forinstance,thatyouneededtosomehowchangeadatabasetablethatyourJavaapplicationused--byrenamingacolumn,forexample.Onceyou'

dchangedthetable,allyou'

dhavetodotoupdateyourJavaapplicationwouldbetoupdatetheappropriateXMLconfigurationfile.Youwouldn'

tneedtorecompileanyJavacode.

HibernateQueryLanguage（HQL）

HibernateprovidesaquerylanguagecalledHibernateQueryLanguage（HQL）,whichisquitesimilarSQL.Forthoseofyouwhoprefergoodold-fashionedSQLqueries,Hibernatestillgivesyoutheopportunitytousethem.ButoursupportingexamplewilluseHQLexclusively.

HQLisquitesimpletouse.YouwillfindallthefamiliarkeywordsyouknowfromSQL,likeSELECT,FROM,andWHERE.HQLdiffersfromSQLinthatyoudon'

twritequeriesdirectlyonyourdatamodel（thatis,onyourtables,columns,etc.）,butratheronyouJavaobjects,usingtheirpropertiesandrelationships.

Listing1illustratesabasicexample.ThisHQLcoderetrievesallIndividualswhosefirstNameis"

John."

Listing1.BasicHQLQuery

SELECT*FROMeg.hibernate.mapping.dataobject.IndividualWHEREfirstName="

John"

YoucanrefertotheHQLreferencematerialontheHibernateWebsiteifyouwanttolearnmoreaboutHQLsyntax.

XMLconfigurationfiles

ThecoreofHibernate'

sfunctionalityresidesinsideXMLconfigurationfiles.Thesefilesmustresideinyourapplication'

sCLASSPATH.Weplacedthemintheconfigdirectoryofoursamplecodepackage

Thefirstfilethatwe'

llexamineishibernate.cfg.xml.Itcontainsinformationregardingyourdatasource（DBURL,schemaname,username,password,etc.）andreferencestootherconfigurationfilesthatwillcontainyourmappinginformation.

TheremainingXMLfilesallowyoutomapJavaclassesagainstdatabasetables.Wewilltakeacloserlookatthosefileslater,butitisimportanttoknowthattheirfilenamesfollowthepatternClassName.hbm.xml.

Oursupportingexample

Inthisarticle,we'

llexamineabasicexamplethatillustrateshowHibernateworksandputstogoodusethreedifferentstrategiesunderwhichyoucanuseHibernatetodoyourobject-relationalmapping.Ourexampleapplicationwillbeusedbyaninsurancecompanythatmustkeeplegalrecordsofallthepropertyrightsthatitscustomersareinsuredfor.We'

veprovidedthefullsourcecodewiththisarticle;

thiscodeprovidesbasicfunctionalityfromwhichyoucouldbuildafull-fledgedapplication,suchasaWeborSwingapplication.

Ourexampleassumesaclassicusecaseforthiskindofapplication.Theuserwouldprovidethesearchcriteriaforanytypeofcustomer（individual,corporation,governmentagency,etc.）,andwouldthenbepresentedwithalistofallcustomersmatchingthespecifiedcriteria--evenifthesearedifferenttypesofcustomers.Theusercouldaccessamoredetailedviewofaspecificcustomerfromthatsamelist.

Inourapplication,apropertyrightisrepresentedbytheRightclass.ARightcaneitherbeaLeaseoraProperty.ARightisownedbyacustomer.Torepresentourcustomer,we'

llusethegenericclassPerson.APersoncaneitherbeanIndividualoraCorporation.Ofcourse,theinsurancecompanymustknowtheEstatestowhichthoseRightsareassigned.AnEstateisaverygenericterm,you'

llagree.So,we'

llusetheLandandBuildingclassestogiveourdevelopersmorecomprehensiveobjectstoworkwith.

Fromthisabstract,wecandeveloptheclassmodelshowninFigure1:

Figure1.CompleteClassModel

Ourdatabasemodelwasdesignedtocoverthethreedifferentstrategieswe'

lldiscussinthisarticle.FortheRighthierarchy,we'

lluseasingletable（TB_RIGHT）andmaptothecorrectclassusingtheDISCRIMINATORcolumn.ForthePersonhierarchy,we'

llusewhatwecallasupertable（TB_PERSON）thatwillsharethesameIDswithtwoothertables（TB_CORPORATIONandTB_INDIVIDUAL）.Thethirdhierarchy（Estate）usestwodifferenttables（TB_BUILDINGandTB_LAND）linkedbyaforeignkeydefinedbythecombinationoftwocolumns（REF_ESTATE_IDandREF_ESTATE_TYPE）.

Figure2.Completedatamodel

Settingupthedatabase

HibernatesupportsawidevarietyofRDBMSs,andanyofthemshouldworkwithoursample.However,thesamplecodeandthetextofthisarticlehavebeentailoredforHSQLDB,afullyfunctionalrelationaldatabasewrittenentirelyintheJavalanguage.Inthesqldirectoryofthesamplecodepackage,youwillfindafilenameddatamodel.sql.ThisSQLscriptwillcreatethedatamodelusedinourexample.

SettinguptheJavaproject

Althoughyoucanalwaysbuildandexecutethesamplecodeusingthecommandline,youmaywanttoconsidersettinguptheprojectinanIDEforbetterintegration.Withinthesamplecodepackage,you'

llfindthefollowingdirectories:

∙config,whichcontainsallthesample'

sXMLconfigurationfiles（mapping,Log4J,etc.）

∙data,whichcontainsconfigurationfilesusedbyHSQLDB.YouwillalsofindabatchfilenamedstartHSQLDB.batthatyoumayusetolaunchthedatabase.

∙src,containsallthesample'

ssourcecode.

BesuretocopytherequiredJavalibrariesandXMLconfigurationfilestoyouapplication'

sCLASSPATH.ThecodeneedsonlytheHibernateandHSQLDBlibrariesinordertocompileandrunproperly.

Strategy1:

Onetablepersubclass（Persons）

Inourfirststrategy,we'

lllookathowtomapourPersonhierarchy.You'

llnoticethatthedatamodelisveryclosetoourclassmodel.Asaresult,we'

lluseadifferenttableforeachclassinthehierarchy,butallthesetablesmustsharethesameprimarykey（we'

llexplainthatinmoredetailmomentarily）.Hibernatewillthenusethisprimarykeywhenitinsertsnewrecordsintothedatabase.ItwillalsomakeuseofthissameprimarykeytoperformJOINoperationswhenaccessingthedatabase.

Nowweneedtomapourobjecthierarchytoourtablemodel.Wehavethreetables（TB_PERSON,TB_INDIVIDUAL,andTB_CORPORATION）.Aswementionedabove,theyallhaveacolumnnamedIDasaprimarykey.It'

snotmandatorytohaveasharedcolumnnamelikethis,butitisconsideredgoodpractice--anditmakesitaloteasiertoreadthegeneratedSQLqueries.

IntheXMLmappingfile,showninListing2,you'

llnoticethatthetwoconcreteclassesaredeclaredas<

joined-subclass>

insidethePersonmappingdefinition.TheXMLelement<

id>

ismappedtotheprimarykeyforthetop-leveltableTB_PERSON,whilethe<

key>

elements（fromeachsubclass）aremappedtothematchingprimarykeysoftheTB_INDIVIDUALandTB_CORPORATIONtables.

Listing2.Person.hbm.xml

<

?

xmlversion="

1.0"

encoding="

UTF-8"

>

!

DOCTYPEhibernate-mappingPUBLIC"

-//Hibernate/HibernateMappingDTD2.0//EN"

"

hibernate-mapping>

<

classname="

eg.hibernate.mapping.dataobject.Person"

table="

TB_PERSON"

polymorphism="

implicit"

idname="

id"

column="

ID"

generatorclass="

assigned"

/>

/id>

setname="

rights"

lazy="

false"

keycolumn="

REF_PERSON_ID"

one-to-manyclass="

eg.hibernate.mapping.dataobject.Right"

/>

/set>

joined-subclassname="

eg.hibernate.mapping.dataobject.Individual"

TB_INDIVIDUAL"

propertyname="

firstName"

FIRST_NAME"

type="

java.lang.String"

lastName"

LAST_NAME"

/joined-subclass>

eg.hibernate.mapping.dataobject.Corporation"

TB_CORPORATION"

name"

NAME"

string"

registrationNumber"

REGISTRATION_NUMBER"

/class>

/hibernate-mapping>

SavinganewinstanceofIndividualformourJavacodewithHibernateisprettystraightforward,asshowninListing3:

Listing3.SavinganewinstanceofIndividual

publicObjectcreate（Objectobject）{

Sessionsession=null;

try{

session=sessionFactory.openSession（）;

Transactiontx=session.beginTransaction（）;

session.save（object）;

session.fl