Object-Relational Mapping (ORM)

Object-Relational Mapping (ORM)

• ORM provides the foundational concept for mapping objects to relational databases
• Java Persistence API (JPA) standardizes ORM in Java, defining a set of rules and interfaces
• Hibernate implements JPA, offering the actual tools and functionalities for ORM
• Spring Data JPA builds on JPA and Hibernate, making it easier to implement data access layers with minimal configuration and code

  • @Entity on POJO

    @Data
    @NoArgsConstructor
    @Entity
    @Table(name = "authors")
    public class Author {
    
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;
    
        @Column(nullable = false)
        private String name;
    
        @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
        @JsonManagedReference
        private List<Book> books;
    }
    
    @Data
    @NoArgsConstructor
    @Entity
    @Table(name = "books")
    public class Book {
    
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long id;
    
        @Column(nullable = false)
        private String title;
    
        @ManyToOne
        @JoinColumn(name = "author_id", nullable = false)
        @JsonBackReference
        private Author author;
    }
    

  • @Repository

    • With Spring Data JPA, marks a interface as a data access layer component

    @Repository
    public interface AuthorRepository extends JpaRepository<Author, Long> {
    }
    

  • CRUD

    • eg. findAll(), findById(id), save(author), deleteById(id)

  • @Query on method can run native SQL query in Spring Data JPA

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