Java EE Lecture

8/17/2019 Java EE Lecture

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Week 2

Object-Relational Mapping

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The slides in this presentation aredeveloped from:

Chapter 5 Object-Relational Mapping (page125 to 159)

Beginning Java EE 7ntonio !oncalve"

pre""#

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A CKNOWLEDGMEN

TS


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OBJECTIVES

Interpret the basics of object-relationalmapping (OR! in terms of ho# attrib$tesand relationships are mapped from a

domain model to a database% &evelop di'erent mappings $sing )*

annotations or +, mapping descriptors%

Investigate di'erent #as for comple.relationship mappings%


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ORM B ASIC R ULES

The entit class m$st be annotated #[email protected] annotation%

Or denoted in an +, descriptor as an entit @javax.persistence.Id annotation m$st

be $sed to denote a simple primar 0e%

The entit class m$st have a no-arg

constr$ctor that has to be p$blic orprotected% The entit class ma have other constr$ctors

as #ell%


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ORM B ASIC R ULES

The entit class m$st be a top-level class% *n en$m or interface cannot be designated as

an entit%

The entit class m$st not be nal% o methods or persistent instance variables of

the entit class ma be nal%

If an entit instance has to be passed bval$e as a detached object (e%g% thro$gha remote interface! the entit class m$stimplement the Serializable interface%


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ORM B ASIC R ULES

The mapping of the Book entit


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ORM B ASIC R ULES

7ince the 8oo0 entit follo#s these simpler$les the persistence provider can 7nchronise the data bet#een the attrib$tes

of the 8oo0 entit and the col$mns of the8OO9 table%

%g% if the attrib$te isbn is modied b theapplication the I78 col$mn #ill be

snchronised provided:o The entit is managedo The transaction conte.t is set


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CONFIGURATION BY

EXCEPTION


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CONFIGURATION BY

EXCEPTION ?itho$t an annotation the 8oo0 entit#o$ld be treated j$st li0e a )OO and notbe persisted%

?ith @Entity the entit is persisted% ?ith @Id the id attrib$te has to be

mapped to a primar 0e%

@or all the other attrib$tes: The entit name is mapped to a relationaltable nameo e%g% the 8oo0 entit ↔ the 8OO9 table

*ttrib$te names are mapped to col$mn


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CONFIGURATION BY

EXCEPTION &8A r$les appl for mapping avaprimitives to relational data tpes% 7tring↔ B*RA=*R(2//!

,ong ↔ 8ICIT 8oolean ↔ 7*,,IT

&efa$lt mapping r$les are di'erentfrom one database to another%

* 7tring is mapped to a B*RA=*R in &erbb$t a B*RA=*R2 in Oracle%

*n Integer is mapped to an ITCR in &erband a


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T ABLE

R$les for cong$ration-b-e.ceptionmapping state that the entit and thetable name are the same%

Do$ ma #ant to map data to a di'erenttable or even map a single entit tom$ltiple tables%

The @javax.persistence.Tableannotation ma0es it possible to changethe defa$lt val$es related to the table% The name of the table

The catalog and the database schema @UniqueConstraint in conjunction it!


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T ABLE

The Book entit being mapped to a T"B##$ table


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SECONDARY T ABLE

*n entit can be mapped to a single table(primar table!%

&ata can be spread across m$ltiple tables

(secondar tables!%o


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SECONDARY T ABLE

*ttrib$tes of the &ddress entit aremapped into three di'erent tables%


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SECONDARY T ABLE

ach table contains di'erent attrib$tesb$t the all have the same primar 0e (to

join the tables together!%


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PRIMARY K EYS

In relational database a primar 0e$niE$el identies each ro# in a table%

It comprises either a single col$mn or set

of col$mns% )* reE$ires entities to have an identier

mapped to a primar 0e%


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PRIMARY K EYS

@javax.persistence.Id annotates anattrib$te as being a $niE$e identier%

The val$e of this identier can be

generated an$all b the application *$tomaticall b the persistence provider

$sing the @'enerated(alue annotation%

If the @'enerated(alue annotation is notdened the application has to create itso#n identier b appling an algorithmthat #ill ret$rn a $niE$e val$e%


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PRIMARY K EYS

The Book entit #ith an a$tomaticallgenerated identier


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COMPOSITE PRIMARY

K EYS It is a good practice to designate a singlededicated col$mn as the primar 0e% There are cases #here a composite

primar 0e is reE$ired% apping to a legac database )rimar 0es have to follo# certain b$siness

r$leso * date and a val$e or a co$ntr code and a

time stamp need to be incl$ded% @or e.ample

The A&-8oo07tore application needs to postne#s%

The ne#s has content a title and a lang$agecode%


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COMPOSITE PRIMARY

K EYS The primar 0e class )esId is composedof t#o attrib$tes of tpe 7tring: title andlang$age%

)rimar 0e classes m$st incl$de method

denitions for equals*+ and !as!Code*+% The m$st also be p$blic and have a no-

arg constr$ctor% The m$st implement serialiHable if need

to cross architect$ral laers (e%g%persistence presentation!

The primar 0e class is annotated #ith

Jmbeddable%


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COMPOSITE PRIMARY

K EYS The entit )es embeds the primar 0eclass #ith @E%beddedId annotation%

ver JmbeddedId m$st refer to anembeddable class

7implied code to nd an entit thro$ghits composite primar 0e%


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A TTRIBUTES

* primar 0e (simple or composite! is am$st an entit

*n entit has all sorts of di'erentattrib$tes ma0ing $p its state that haveto be mapped to the table%

This state can incl$de almost ever ava

tpe: ava primitive tpeso int do$ble Koat etc%

The #rapper classes of primitive java tpeso Integer &o$ble @loat etc%

L *rras of btes and characters


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A TTRIBUTES

7tring large n$meric and temporal tpeso java.lan,.Strin,o java.%at!.Bi,Inte,er

java.%at!.Bi,-eci%al java.util.-ate

java.util.Calendar java.sql.-atejava.sql.Ti%e java.sql.Ti%esta%p

n$merated tpes and $ser-dened tpes thatimplement the Serializable interface

Aollections of basic and embeddable tpes *n entit can also have entit attrib$tes collections of entities or embeddable classes%

This reE$ires introd$cing relationships

bet#een entities%


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A TTRIBUTES

The @javax.persistence.Basicannotation has t#o parameters

(optional and fetch! The val$e of the attrib$te ma be n$ll(optional!

The @etch element can ta0e t#o val$es:o ,*D: data sho$ld be fetched laHil

(onl #hen the application as0s forthe propert!

o *CR: data sho$ld be fetched

eagerl (#hen the entit is initiall


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A TTRIBUTES

The Track entit #ith azy loading on the#av attrib$te


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A TTRIBUTES

A$stomising mapping for the Book entit


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A TTRIBUTES

@javax.persistence.Te%poralannotation has three possible val$es:

&*T TI or TI7T*) %ap java.util.-ate and

java.util.Calendar represent date ho$r or second


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A TTRIBUTES

*nnotated #ith @Entity all attrib$tes area$tomaticall mapped to a table%

?hile no need to map an attrib$te $se

the @javax.persistence.Transient*@Transient+ annotation or the javatransient 0e#ord%

@or e.ample in the A$stomer entit *ge can be calc$lated from the date of birth% The age attrib$te does not need to be

mapped% Therefore the age attrib$te can be transient%


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A TTRIBUTES

The Custo%er entit #ith a Transient age


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A TTRIBUTES

The en$meration tpes are no# sofreE$entl $sed

The val$es of an en$m *re constantsF =ave an implicit ordinal assignment that is

determined b the order in #hich the aredeclared%

This ordinal cannot be modied at r$ntime b$tcan be $sed to store the val$e of theen$merated tpe in the database%


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A TTRIBUTES

In the follo#ing AreditAard entit and #iththe defa$lt mapping The ordinals assigned to the val$es of this

en$merated tpe at compile time are > forBI7* 1 for *7TRPA*R& and 2 forRIA*P+)R77%

The persistence providers #ill map thisen$merated tpe to the database ass$mingthat the col$mn is of tpe Integer%


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A TTRIBUTES

A


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A TTRIBUTES

In the follo#ing AreditAard entit and #iththe defa$lt mapping The ordinal assignment is determined b the

order in #hich val$es are declared% If introd$cing a ne# constant to the top of the

en$meration the val$es alread stored in thedatabase #ill no longer match theen$meration%

* better sol$tion #o$ld be to store thename of the val$e as a string #ith@Enu%erated annotation instead ofstoring the ordinal%

A


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A TTRIBUTES

apping an en$merated tpe #ithStrin,

A


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A TTRIBUTES

@Ele%entCollection annotation is $sedto indicate that an attrib$te contains acollection of instances of basic tpes or

embeddables java.util.Collection java.util.Set java.util.ist

@CollectionTable annotation allo#s toc$stomise details of the collection tables$ch as its name%


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A TTRIBUTES

Relationship bet#een the 8OO9 and the T*C tables

M XML


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M APPING WITH XML

)* also o'ers an +, snta. to mapentities%

ver single annotation has an +,

eE$ivalent% If both is $sed +, overrides annotations%

(the length of the QdescriptionG col$mn(slide 42! in +, mapping #ill override

that declared in Jntit (slide 41! ItGs a matter of taste as the behavio$r of

both is e.actl the same% etadata (e%g% col$mn length! can be

chan ed de endin on

M XML


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M APPING WITH XML

This ma be better e.pressed in e.ternal+, deploment descriptors

7o the code doesnGt have to be modied%

M XML


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M APPING WITH XML

The mapping le /ET&0I)12book"%appin,.x%l

M XML


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M APPING WITH XML

B##$"3/"/&44I)' table str$ct$re

In persistence%.ml le o$ need to

reference the book"%appin,.x%l le% The persistence.x%l denes

The entit persistence conte.t The database it sho$ld be mapped to


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E S


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EMBEDDABLES

mbeddables are objects that donGt have apersistent identit on their o#n%

The can be embedded onl #ithin o#ningentities%

The o#ning entit can have collections ofembeddables or a single embeddable

attrib$te% The are stored as an intrinsic part of ano#ning entit and share the identit of thisentit%

ach attrib$te of the embedded object is

EMBEDDABLES


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EMBEDDABLES

The @E%beddable annotation species that&ddress can be embedded in anotherentit class (or another embeddable!%

mbeddables donGt have a persistent

identit on their o#n

EMBEDDABLES


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EMBEDDABLES

The Custo%er entit $ses the @E%bedded annotation to specif &ddress is a persistentattrib$te that #ill be stored as an intrinsicpart and share its identit%

Structure of the CUSTOMER table

with all ADDRESS attributes

RELATIONSHIP MAPPING


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R ELATIONSHIP M APPING

Object-oriented programming abo$nds#ith classes and associations bet#eenclasses%

These associations lin0 objects and allo#

one object to ca$se another to perform anaction on its behalf%

*n association has a direction


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In ava o$ $se the dot (%! snta. tonavigate thro$gh objects% c$stomer%get*ddress(!%getAo$ntr(!

$nidirectional

bidirectional

8idirectionalassociationrepresented

#ith t#oarro#s



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/>


*n association also has a m$ltiplicit (orcardinalit!%

ach end of an association can specifho# man referring objects are involved in

the association%

In ava an association that representsmore than one object $ses collections oftpe java.util.Collection5 java.util.Set5

java.util.ist5 or even java.util./ap



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* relationship has an o#nership (i%e% theo#ner of the relationship!


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* relational database is a collection ofrelations (also called tables S anthing o$model is a table!%

In )* #hen o$ have an associationbet#een one class and another in thedatabase o$ #ill get a table reference%

This reference can be modelled in t#o

di'erent #as:


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Do$ #o$ldnGt $se a join table torepresent a one-to-one relationship

as this co$ld have performance iss$es @or e.ample o$ al#as need to access

a third table to get the address of ac$stomer%

oin tables are generall $sed #heno$ have one-to-man or man-to-man cardinalities%


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ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL * one-to-one $nidirectional relationship

bet#een entities =as a reference of cardinalit 1 Aan be reached in onl one direction

*ss$me the c$stomer has onl oneaddress%

It is important to navigate from the

c$stomer to#ard the address% Do$ donGt need to be able to navigate inthe opposite direction ?hich c$stomer lives at a given address

ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL * Custo%er #ith one &ddress

ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL *s o$ see in A


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ONE-TO-ONE

UNIDIRECTIONAL The CUST#/E6 Table #ith a foreign 0e

pointing to the &--6ESS table

ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL

To c$stomise the mapping o$ can $set#o annotations%

@#neTo#ne It can modif some attrib$tes of theassociation itself s$ch as the #a it has to befetched%

@7oinColu%n It is $sed to c$stomise the join col$mn -the

foreign 0e of the o#ning side%

ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL The Custo%er entit #ith c$stomised

relationship mapping

ONE TO ONE


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ONE-TO-ONE

UNIDIRECTIONAL

In )* a foreign 0e col$mn is called a joincol$mn%

The @7oinColu%n annotation allo#s toc$stomise the mapping of a foreign 0e% The previo$s e.ample:

Renames the foreign 0e col$mn to &--"1$

a0es the relationship obligator b ref$singthe n$ll val$e (nullable89alse!% Cives the persistence provider a hint to fetch

the relationship laHil%

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL ?ith a one-to-man relationship one

so$rce object refers to an ensemble oftarget objects%

@or e.ample a p$rchase order iscomposed of several order lines%

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL

The cardinalit is m$ltiple and thenavigation is done onl from Order to#ardOrder,ine%

In ava this m$ltiplicit is described bthe Collection5 ist and Set interfacesof the java.util packa,e%


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ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL In the previo$s e.ample a collection of an

entit tpe is being $sed as an attrib$teon this entit triggers:

* OneToan relationship mapping b defa$ltF One-to-man $nidirectional relationshipsF * join table to 0eep the relationship

information #ith t#o foreign 0e col$mns%o One foreign 0e col$mn refers to the table

OR&RFo The other refers to OR&RP,I%

The join table is named OR&RPOR&RP,I(name of both tables separated b the

$nderscore P smbol!%

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL oin table bet#een #6-E6 and #6-E6"I)E

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL If o$ li0e to rename the defa$lt join table

and foreign 0e names or to c$stomisemapping o$ can $se )* annotations to

redene these defa$lt val$es% The @7oinColu%n annotation can be $sed to

change the foreign 0e col$mns% The @7oinTable annotation can do the same

for the join table mapping% The @#neTo/any annotation c$stomises therelationship itself%

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL The #rder entit #ith annotated One-to-

an relationship

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL 7tr$ct$re of the join table

The defa$lt r$le for a one-to-man$nidirectional relationship is to $se a join

table% To change to $sing foreign 0es the #rder entit has to provide a @7oinColu%n annotation instead of a @7oinTable.

ONE TO MANY


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ONE-TO-M ANY

UNIDIRECTIONAL The #rder entit #ith a join col$mn

ONE-TO-MANY


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ONE-TO-M ANY

UNIDIRECTIONAL In the previo$s e.ample: The @#neTo/any annotation is overriding the

defa$lt fetch mode (t$rning it to E&'E6 insteadof &:;!%

8 $sing @7oinColu%n the $nidirectional

association is then mapped $sing the foreign0e strateg% The foreign 0e is renamed to #6-E6"1$ b the

annotation and e.ists in the target table(#6-E6"I)E!%

SUMMARY


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SUMMARY

The foc$s of this #ee0 is OR% ?ith cong$ration b e.ception not m$ch

is reE$ired to map entities to tables% )* $ses @Entity informs the persistence

provider that a class is an entit% )* $ses @Id informs that an attrib$te is

its identier% )* has a ver rich set of annotations to

c$stomise ever little detail of OR% lementar annotations can be $sed on

attrib$tes or classes to c$stomise the

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Java EE Lecture