Spring Data JPA

The Repository pattern is commonly used in Java Spring applications. It abstracts the story and retrieval of data from a data store. The repository is typically called from a Service class. For a web application, the Service class is called from the Controller class.

First, we define the entity class with the fields that we have to store:

    package com.pockettheories;

    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;

    @Entity
    public class CarEntity {
        @Id
        @GeneratedValue(strategy = GenerationType.AUTO)
        public int ID;
        public String makeName;
        public Integer modelYear;
        public String modelName;
    }

When building a console application, the Main class can be used to call the Service class, and to inject a Service object instance, we @Autowired it.

    @Autowired
    private CarService carService;

The Service class contains methods that in turn call the Repository class:

    package com.pockettheories;

    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Component;
    import org.springframework.stereotype.Service;

    import java.util.List;

    @Service
    public class CarService {
        @Autowired
        private CarRepository carRepository;

        public void addCar(CarEntity car) {
            carRepository.addCar(car);
        }
        public List getAllCars() {
            return carRepository.getAllCars();
        }
    }

The Repository class interacts with the EntityManager, from JPA. The EntityManager can be used to execute JPQL queries.

    package com.pockettheories;

    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Repository;
    import org.springframework.transaction.annotation.Transactional;

    import javax.persistence.EntityManager;
    import javax.persistence.PersistenceContext;
    import java.util.List;

    @Transactional
    @Repository
    public class CarRepository implements ICarRepository{
        @PersistenceContext
        private EntityManager entityManager;

        @Override
        public void addCar(CarEntity car) {
            entityManager.persist(car);
        }

        @Override
        public CarEntity getCarById(int carId) {
            return entityManager.find(CarEntity.class, carId);
        }

        @Override
        public List getAllCars() {
            return entityManager.createQuery("select c from CarEntity c").getResultList();
        }
    }