Layered Architecture

• Domain Model
• Repository
• Service
• Controller

Domain Model

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;
    private String email;
}

Repository

The repository is an abstraction of the database. It provides CRUD operations.

public interface StudentRepository extends JpaRepository<Student, Integer> {
    List<Student> findByName(String name);
}

Service

The service is the business logic. It can use multiple repositories. It can also use other services.

@Service
public class StudentService {
    @Autowired
    private StudentRepository studentRepository;
    
    public List<Student> findByName(String name) {
        return studentRepository.findByName(name);
    }
}

Controller

The controller is the entry point of the application. It handles HTTP requests.

@RestController
public class StudentController {
    @Autowired
    private StudentService studentService;
    
    @GetMapping("/students")
    public List<Student> findByName(@RequestParam String name) {
        return studentService.findByName(name);
    }
}

JPQL Synthax

Selecting objects

public OptionalDouble getAvgSpeed(Runner runner,EntityManager em) {
    var q = em.createQuery("""
        SELECT sum(km)*60/sum(r.minutes) as min FROM Run r WHERE true""", runner)
    ...
}

Pre-Remove

@PreRemove
@PreUpdate
@PrePersist
public void beforeAnyChange() {
    System.out.println("Before any change");
}

@PostRemove
@PostUpdate
@PostPersist
public void afterAnyChange() {
    System.out.println("After any change");
}

Selects

new Objects

public List<Runner> getRunners(EntityManager em) {
    var q = em.createQuery("""
        SELECT new Runner(r.name, sum(km)*60/sum(r.minutes)) FROM Run r WHERE true GROUP BY r.name""", Runner.class)
    ...
}

Embeddable

Embeddable classes are used to group fields together.

@Embeddable
public class Address {
    private String street;
    private String city;
    private String zip;
}

@EmbeddaedId

@Embeddable
public class Address {
    private String street;
    private String city;
    private String zip;
}

@Entity
public class Person {
    @EmbeddedId
    private Address address;
    private String name;
}

Inheritance

There are three types of inheritance in JPA:

• Single Table
• Joined
• Table Per Class

Single Table

In single table inheritance, all the classes are stored in the same table.

Joined

In joined inheritance, each class is stored in its own table.

Table Per Class

In table per class inheritance, each class is stored in its own table.

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;
}