IT Boxing Championship

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ORM with JPA/Hibernate overview
Petar Milev
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Contents

• ORM Concepts
• Java class to database table
• Java object to database tables row
• Classes vs. tables
• (OR) Mapping with Hibernate
• Two ways to go
• Annotations vs. XML
• Create, Retrieve, Update, Delete and JPA QL

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ORM Concepts
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Class To Table

• Associate Java POJO with database table
• Table PERSON (SQL may be generated by Hibernate)
• You can do it without even using SQL

ID NAME DESCRIPTION
1 Pesho Java man
2 Gosho Just man
class Person Long id String name String
description //getter, setters
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Instances Are Rows

• Java instances are represented as rows in the
  table

ID NAME DESCRIPTION
1 Pesho Java man
2 Gosho Just man
void mymethod(Person p) System.out.println(p.g
etId()) //1 System.out.println(p.getName())
//Pesho System.out.println(p.getDescription()
)//Java man
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Classes vs. Tables

• Classes inherit from each other
• Classes have associations between them
• What about tables?
• The differences lead to the need for meta
  information

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(OR) Mappings
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Ways To Go - XML

• XML files for every mapped class

ltclass namewww.com.Person" tablePERSON"gt  
ltid columnID" name"id" type"java.lang.Long/gt
ltproperty columnNAME" namename"
type"java.lang.String"/gt ltproperty
columnDESCRIPTION" namedescription"
type"java.lang.String"/gt ltset
namehairs"gt ltkey columnPERSON_ID"
not-null"true"/gt ltone-to-many
classwww.com.Hair"/gt lt/setgtlt/classgt
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Ways To Go Java Annotations

• Java Annotations used at the mapped classes

_at_Entity(PERSON) public class Person
private Long id private String name
private String description private SetltHairgt
hairs //..code continued
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Java Annotations 2

• Java Annotations used at the mapped classes

//... _at_Id public Long getId()
return id _at_Column(nameNAME) public
String getName() return name
_at_Column(nameDESCRIPTION) public String
getDescription() return description
_at_OneToMany(mappedByperson) public
SetltHairgt getHairs() return hairs
//normal setters
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Annotations vs. XML

• Annotations advantages
• Everything is in one place
• Annotations are refactoring-friendly for renaming
  and moving of classes
• Annotations are controlled by the compiler
• Annotations shortcomings
• Cluttered source code

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XML vs. Annotations

• XML advantages
• Java code is more readable
• XML shortcomings
• More things to write
• Support simultaneously two files .xml and .java

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More about mappings in JPA

• Configuration by exception
• You can tell JPA how to fetch the relations
• Transient properties
• Map relations
• Map maps
• Embedded objects
• Compound primary keys

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Create, Read, Update, Delete
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(CR)UD

• Create
• Read

void create(EntityManager entityManager)
Person person new Person(pesho, java
programmer) entityManager.persist(person)
void read(EntityManager entityManager)
Person person entityManager.find(Person.class,
1)

• Query

void query(EntityManager entityManager)
Query query entityManager.createQuery(from
Person where name like pesho ) List
persons query.getResultList()
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CR(UD)

• Update

void update(EntityManager entityManager)
Person person entityManager.find(Person.class,
1) person.setDescription(new
description) . entityManager.getTransac
tion().commit()

• Delete

void delete(EntityManager entityManager)
Person person entityManager.find(Person.class,
1) entityManager.remove(person)
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Query Language Examples

• Polymorphic queries

from Shape //where Shape is an abstract
class from java.lang.Object //get the whole
database, why not?

• Where, like, in, functions

from Person p where p.name like P and
p.description like J or p.name in (John,
Ben, Petar) or lower(p.name) pesho
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More Query Examples

• Sub selects with any, some, all and in

from Box b where 10 lt any ( select b.amount from
i.bets b ) from Box b where 10 gt all ( select
b.amount from i.bets b ) from Box b where 10
some ( select b.amount from i.bets b ) from Box
b where 10 in ( select b.amount from i.bets b )

• Projection

select a, b from Animal a, Banica b //returns
List of Object, the array is with
2 elements
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More Query Examples

• Implicit joins

from Bet bet where bet.item.category.name like
hundred' and bet.item.firstBet.quantity gt 10

• SQL alternative

select . . . . from BET B inner join ITEM I on
B.ITEM_ID I.ITEM_ID inner join CATEGORY C on
I.CATEGORY_ID C.CATEGORY_ID inner join BET SB
on I.FIRST_BET_ID SB.BET_ID where C.NAME like
hundred' and SB.QUANTITY gt 100
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More Query Examples

• Normal joins

from Person p left join p.hairs h with h.length gt
10 where p.description like 'Foo'

• Dynamic instantiation

select new BetInfo( bet.item.id, count(bet),
avg(bet.amount) ) from Bet bet where
bet.current.ammount is null group by
bet.current.id