JPA Notes

JPA

The Java Persistence API (JPA) is the Java standard for mapping Java objects to a relational database. 

• The basic unit of persistence in JPA is the entity
•  Every entity class should have an @Entity marker and primary key column identifier field, indicated by @Id.
• you are not required to specify table names and column names that an entity is mapped. JPA will take default values as same as entitiy class having.
• To override default values, you can use mapping matadata.

@Table on the entity class -> the table name explicit

@Column on the field->maps particular column.

@JoinColumn on the field ->override the name of the foreign key column for relationship references.

Ex:

An example of two mapped entities are the Pet and Owner classes shown in Listings 1 and 2.

Listing 1 - Pet entity class

@Entity

@Table(name="PET_INFO")

public class Pet {

@Id

@Column(name="ID")

int licenseNumber;

String name;

PetType type;

@ManyToOne

@JoinColumn(name="OWNER_ID")

Owner owner;

...

}

Listing 2 - Owner entity class

@Entity

public class Owner {

@Id

int id;

String name;

@Column(name="PHONE_NUM")

String phoneNumber;

@OneToOne

Address address;

@OneToMany(mappedBy="owner")

List<Pet> pets;

...

}

In a bidirectional relationship pair,

@OneToMany relationship in Owner to Pet and

@ManyToOne relationship back from Pet to Owner,

only one foreign key is required in one of the tables to manage both sides of the relationship. As a general rule, the side that does not have the foreign key in it specifies a mappedBy attribute in the relationship annotation and specifies the field in the related entity that maps the foreign key.

The possible mapping annotations that can be used are:

@Basic @Enumerated @ManyToOne @Temporal @Embedded @Lob @OneToMany @Transient @EmbeddedId @ManyToMany @OneToOne

@AttributeOverride(s) @PrimaryKeyJoinColumn(s) @AssociationOverride(s) @SecondaryTable(s) @Column @SequenceGenerator @DiscriminatorColumn @Table @JoinColumn(s) @TableGenerator @JoinTable

@Entity @IdClass @MappedSuperclass @Embeddable @Inheritance @OrderBy @GeneratedValue @DiscriminatorValue @Version @Id @MapKey