Java Persistence API(JPA)

JPA (Java Persistence API) is a specification for managing relational data in Java applications. It provides an abstraction over SQL-based databases, making it easier to work with data using object-oriented concepts. Here’s a quick breakdown of key JPA concepts:

Key Components of JPA

1. Entity – A Java class mapped to a database table.
2. Persistence Context – The cache where entity instances are managed.
3. Entity Manager – The core interface for interacting with the database.
4. JPQL (Java Persistence Query Language) – A query language similar to SQL but works with entity objects.
5. Annotations – Used for mapping Java classes to database tables.

Example of JPA in Action

1. Define an Entity

import jakarta.persistence.*;

@Entity
@Table(name = "cars")
public class Car {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false)
    private String brand;
    
    @Column(nullable = false)
    private String model;
    
    // Getters and Setters
    public Long getId() { return id; }
    public void setId(Long id) { this.id = id; }

    public String getBrand() { return brand; }
    public void setBrand(String brand) { this.brand = brand; }

    public String getModel() { return model; }
    public void setModel(String model) { this.model = model; }
}

2. Use an EntityManager to Persist Data

import jakarta.persistence.*;

public class CarRepository {
    private static final EntityManagerFactory emf = Persistence.createEntityManagerFactory("my-persistence-unit");

    public void saveCar(Car car) {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        em.persist(car);
        em.getTransaction().commit();
        em.close();
    }
}

3. Query Data Using JPQL

public List<Car> getCarsByBrand(String brand) {
    EntityManager em = emf.createEntityManager();
    List<Car> cars = em.createQuery("SELECT c FROM Car c WHERE c.brand = :brand", Car.class)
                        .setParameter("brand", brand)
                        .getResultList();
    em.close();
    return cars;
}

Additional Features

• Relationships: JPA supports @OneToOne, @OneToMany, @ManyToOne, and @ManyToMany relationships.
• Cascade Operations: Control how changes in one entity affect related entities.
• Criteria API: Type-safe queries instead of JPQL.