Spring Data JPA Entity

Entity?

• JPA Entity is a Java Class that is mapped to a table in the database

 

Example

@Entity
@Table(name = "users")
public class User {
	@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(name = "username")
    private String name;
    
    @Column
    private String phone;
}

SQL query ran

table created in H2 database

Annotations

@Entity

• Specifies that this class is an entity

@Table

• Specifies the primary table for the annotated entity.
• Without this annotation, JPA entity is mapped to a table named entity's name
• We can specify additional tables with @SecondaryTable annotation.

@Column

• Specifies the mapped column for a persistent property or field
• We can specify properties of the column (name, nullable, etc)

@Id

• Specifies the primary key of an entity
• with @GeneratedValue, the database will automatically assign primary key based on the strategy selected

@GeneratedValue

• GenerationType.AUTO
  • Default generation type
  • Let the persistence provider (ex Hibernate) choose the generation strategy
• GenerationType.IDENTITY
  • Relies on an auto-incremented database column and lets the database generate a new value with each INSERT operation
• GenerationType.SEQUENCE
  • Uses a database sequence to generate unique values
  • Requires @SequenceGenerator annotation to define the generator
  • Sequence: database object which allows the automatic generation of the values
  • Can be used for database that supports sequence: Oracle, PostgreSQL, H2, etc
• GenerationType.TABLE
  • Requires @TableGenerator annotation
  • Similar to SEQUENCE, but creates a table that is only used for key generation
  • Rarely used nowadays