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Article:

In Spring Boot applications, separating your data access and business logic layers is a best practice that leads to clean, maintainable code. In this article, we will create a JPA repository and service layer for an entity class CollegeCourse that manages the many-to-many relationship between College and Course.

🧱 Step 1: Define the CollegeCourse Entity

java

import javax.persistence.*;
import org.hibernate.annotations.OnDelete;
import org.hibernate.annotations.OnDeleteAction;

@Entity
public class CollegeCourse {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne(fetch = FetchType.EAGER, optional = true)
    @JoinColumn(name = "collegeId", referencedColumnName = "id", nullable = true)
    @OnDelete(action = OnDeleteAction.CASCADE)
    private College college;

    @ManyToOne(fetch = FetchType.EAGER, optional = true)
    @JoinColumn(name = "courseId", referencedColumnName = "id", nullable = true)
    @OnDelete(action = OnDeleteAction.CASCADE)
    private Course course;

    // Getters and Setters
}

This entity links a College and a Course, allowing you to model a many-to-many relationship explicitly using a join table (CollegeCourse).

📁 Step 2: Create the CollegeCourseRepository

java

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface CollegeCourseRepository extends JpaRepository<CollegeCourse, Long> {
    // Add custom query methods if needed
}

The repository layer provides built-in methods for CRUD operations. It extends JpaRepository, which saves you from writing boilerplate DAO logic.

🔧 Step 3: Define the CollegeCourseService Interface

java

import java.util.List;

public interface CollegeCourseService {
    CollegeCourse saveCollegeCourse(CollegeCourse collegeCourse);
    CollegeCourse getCollegeCourseById(Long id);
    List<CollegeCourse> getAllCollegeCourses();
    void deleteCollegeCourse(Long id);
}

This interface abstracts your business logic operations and makes the service layer testable and easier to mock.

🚀 Step 4: Implement the CollegeCourseServiceImpl

java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;
import java.util.Optional;

@Service
public class CollegeCourseServiceImpl implements CollegeCourseService {

    private final CollegeCourseRepository collegeCourseRepository;

    @Autowired
    public CollegeCourseServiceImpl(CollegeCourseRepository collegeCourseRepository) {
        this.collegeCourseRepository = collegeCourseRepository;
    }

    @Override
    public CollegeCourse saveCollegeCourse(CollegeCourse collegeCourse) {
        return collegeCourseRepository.save(collegeCourse);
    }

    @Override
    public CollegeCourse getCollegeCourseById(Long id) {
        Optional<CollegeCourse> optional = collegeCourseRepository.findById(id);
        return optional.orElseThrow(() ->
            new RuntimeException("CollegeCourse not found with id: " + id));
    }

    @Override
    public List<CollegeCourse> getAllCollegeCourses() {
        return collegeCourseRepository.findAll();
    }

    @Override
    public void deleteCollegeCourse(Long id) {
        collegeCourseRepository.deleteById(id);
    }
}

This class provides actual implementations for all methods defined in the interface, delegating the persistence logic to CollegeCourseRepository.

✅ Summary

With this structure, you achieve a clean separation between the entity, repository, and service layers. This approach:

• Keeps your code modular
• Promotes good testing practices
• Makes your logic reusable and maintainable

This pattern is widely used in enterprise-level Spring Boot applications.