Chapitre 4 JPA

Chapitre 5 : JPA and EclipseLink
Note
• CecourscouvrelesbesoinslesplusimportantspourlapersistanceEn
Java(vouspourrezretrouveruncourstrèscompletsurJPA2etsurla
persistanceenjavaengénéralsurinternet).
• UnebonneréférenceenlignesurJPA2
• http://www.objectdb.com/api/java/jpa,enparticulierlasectionJPA2
annotations
Lapersistanceparsérialisation
• Sérialisation=sauvegardedel'étatd'unobjetsousformed'octets.
• Apartird'unétatsérialisé,onpeutreconstruirel'objet
• Enjava,autraversdel'interfacejava.io.Serializable, desméthodesde
java.io.ObjectInputStream etjava.io.ObjectOutputStream
Lapersistanceparsérialisation
• Défautstrèsnombreux…
• Gestiondesversions,maintenance…
• Pasderequêtescomplexes…
• Ex:onsérializemillecomptesbancaires.Commentretrouverceuxquiontun
soldenégatif?
• Solution:stockerlesobjetsdansunebasededonnée!
Ils’agitde:Object-Relational Mapping(ORM)
• Theprocess ofmapping Javaobjects todatabase tablesandviceversa
is called "Object-relational mapping"(ORM).
• TheJavaPersistence API(JPA)is onepossibleapproach toORM.
• JPAis aspecification andseveral implementations areavailable.
• Popular implementations areHibernate,EclipseLink andApache
OpenJPA.
• Thereference implementation ofJPAis EclipseLink !
Principe:Object-Relational Mapping(ORM)
• Onstockel'étatd'unobjetdansunebasededonnée.
• Ex:laclassePersonnepossèdedeuxattributsnom etprenom,onassocie
cetteclasseàunetable quipossèdedeuxcolonnes:nom etprenom.
• Ondécomposechaqueobjetenunesuitedevariablesdonton
stockeralavaleurdansuneouplusieurstables.
• Permetdesrequêtescomplexes.
Exemple(comptebancaire):Object-Relational
Mapping(ORM)
Qu'est-cequ'unEntityBean
• Cesontdesobjetsquisaventsemapper dansunebasededonnée.
• Ilsutilisentunmécanismedepersistance
• Ilsserventàreprésentersousformed'objetsdesdonnéessituées
dansunebasededonnée
• Leplussouventunobjet=uneouplusieursligne(s)dansuneouplusieurs
table(s)
Qu'est-cequ'unEntityBean
• Allentityclassesmustdefineaprimarykey,musthaveanon-arg
constructorandornotallowedtobefinal.Keyscanbeasinglefield
oracombinationoffields.
• JPAallowstoauto-generatetheprimarykeyinthedatabasevia
the @GeneratedValue annotation.
• Bydefault,thetablenamecorrespondstotheclassname.Youcan
changethiswiththeadditiontotheannotation
@Table(name="NEWTABLENAME").
Exempleavecuncomptebancaire
Atraversuneentité« Compte_bancaire »
• Onlitlesinformationsd'uncomptebancaireenmémoire,dansune
instanced'uneentitybean,
• Onmanipulecesdonnées,onlesmodifieenchangeantlesvaleurs
desattributsd'instance,
• Lesdonnéesserontmisesàjourdanslabasededonnées
automatiquement!
Persistancedeschamps
• By default each field is mapped to a column with the name of the field. You can change the default
name via @Column (name="newColumnName").
• The following annotations can be used.
Table 1.Annotationsforfields/getter andsetter
@Id
Identifies theunique IDofthedatabase entry
@GeneratedValue
Together with an ID this annotation defines that this
value is generated automatically.
@Transient
Field will not be saved in database
MappingRelationnel
• JPAallows todefine relationships between classes,e.g.it can be
defined that aclassis partofanother class(containment).
• Classescan haveonetoone,onetomany,many toone,andmany tomany
relationships with other classes.
• Arelationship can be bidirectional orunidirectional,e.g.
• inabidirectional relationship both classesstoreareference toeach other
while inanunidirectional caseonly oneclasshasareference totheother
class.Within abidirectional relationship you need tospecify theowning side
ofthis relationship intheother classwith theattribute "mappedBy",e.g.
@ManyToMany(mappedBy="attributeOfTheOwningClass".
MappingRelationnel
Relationshipannotations:
• @OneToOne
• @OneToMany
• @ManyToOne
• @ManyToMany
Persistence units
• Thepersistence unitis described viathe persistence.xml filein
the META-INF directoryofthesourcefolder.
• Asetofentities which arelogical connected will be grouped viaa
persistence unit.
• The persistence.xml filedefines theconnection datatothedatabase,
e.g.thedriver,theuserandthepassword.
Entity Manager
• The EntityManager iscreatedbythe EntityManagerFactory whichis
configuredbythepersistenceunit.
• Theentity manager javax.persistence.EntityManager provides the
operations from andtothedatabase,e.g.find objects,persists them,
remove objects from thedatabase,etc.
• InaJavaEE applicationtheentity manageris automatically inserted inthe
webapplication.Outside JavaEE you need tomanagetheentity manager
yourself.
• Entities which aremanaged byanEntity Managerwill automatically
propagate these changestothedatabase (ifthis happens within acommit
statement).
• IftheEntity Manageris closed (viaclose())then themanaged entities are
inadetached state.Ifsynchronize them again with thedatabase aEntity
Managerprovides themerge()method.