IS-907 Java EEJPA: Simple Object-Relational Mapping

Object-Relational-Mapping

• Description of how an object model “maps” to a relational data model

• Class to table(s)

• Object properties (attributes) to columns or relations

• The mapping can be described as

• java source code annotations, or

• in xml mapping files

• xml mapping files override the source code annotations

IS-202 - Course Info 2

JPA Entity annotations

• To mark a class as a persistent entity (i.e. that objects of the class can be stored in a database), use the @Entity annotation:

import javax.persistence.Entity;

@Entity

public class Employee {

...

}

• JPA will attempt to use the classname as a table name, and the attribute names as column names

IS-202 - Course Info 3

Persistence Unit and entities

• All classes annotated with @Entity are included in the PU

• Additional classes may be included:

• Classes with entries in xml mapping files

• Classes listed in <class> elements in persistence.xml

• You can use persistence.xml to exclude classes:

• use an <exclude-unlisted-classes> element

• use <class> elements to include the entities you want

• No other classes will be included!

Even Åby Larsen (even.larsen@uia.no) IS-102 Introduksjon 4

JPA Id annotations

• The @Id annotations is used to specify the primary key.

• The @GeneratedValue annotation can be used to specify automatic generation of the primary key values

import javax.persistence.*;

@Entity

public class Employee {

@Id @GeneratedValue

private long empNo;

...

}

IS-202 - Course Info 5

@Id, primary key, and object identity

• The primary key defines the identity of an entity

• The equals method defines the identity of an object

• They should yield the same result!

@Entity public class Employee {

@Id @GeneratedValue private long empNo;

public boolean equals(Object o) {

...

return this.empNo == that.empNo;

}

}

Even Åby Larsen (even.larsen@uia.no) IS-102 Introduksjon 6

Mapping simple types

• Fields of the following types are mapped automatically to a column with the same name as the field:

• byte, int, short, long, boolean, char, float, double

• Byte, Integer, Short, Long, Boolean, Character, Float, Double

• byte[], Byte[], char[], Character[]

• java.math.BigInteger, java.math.BigDecimal

• String

• java.util.Date, java.util.Calendar

• java.sql.Date, java.sql.Time. java.sql.Timestamp

• enum types

• Serializable classes

Even Åby Larsen (even.larsen@uia.no) IS-102 Introduksjon 7

Field access mode

• Mapping annotations can be put on the field itself, or on the get methods

• If the fields are annotated, JPA will access the fields directly (bypassing the private restriction) to get or set their value

• If the get methods are annotated, JPA will use the get and set methods

• all persistent fields must have getters and setters

Even Åby Larsen (even.larsen@uia.no) IS-102 Introduksjon 8