Repository layer with Spring Data JPA

Introduction

Spring Data JPA can implement an interface that extends JpaRepository at runtime (JDK proxy or subclass). You get save, findById, findAll, delete without writing implementation classes. This is the repository layer: persistence only, no HTTP.

Real-world explanation

• JpaRepository<T, ID> — T is the entity, ID is the id type
• Derived query methods — findByName(String name) is parsed to a WHERE clause
• Optional as return for one result—matches 0 or 1 without null checks
• Heavier queries use @Query (JPQL or native) or EntityManager in a custom impl

Step-by-step: repository for Item

com.example.demoservice.repository.ItemRepository

package com.example.demoservice.repository;
 
import com.example.demoservice.model.Item;
import org.springframework.data.jpa.repository.JpaRepository;
 
import java.util.Optional;
 
public interface ItemRepository extends JpaRepository<Item, Long> {
 
  Optional<Item> findByName(String name);
 
  boolean existsByName(String name);
}

Service will call

// inside a @Service, injected via constructor
private final ItemRepository itemRepository;
 
public Item createIfAbsent(String name) {
  if (itemRepository.existsByName(name)) {
    throw new IllegalStateException("already exists");
  }
  return itemRepository.save(new Item(name));
}

(We will move exceptions to global handler in a later post.)

Common mistakes

• Putting @Service on the repository interface—only extend the Spring Data marker; Spring Data creates the bean itself
• Wrong ID type (int vs Integer vs Long); JPA GenerationType.IDENTITY usually Long
• Transactions: read methods in simple paths work without readOnly but for larger reads, @Transactional(readOnly = true) on the service is better

Best practices

• Query in service-appropriate terms; keep the repository as dumb data access; or a thin query layer if the team allows complex @Query here
• Indexes: if you findByName a lot, add a DB index on the column; JPA is not a substitute for index design
• Pagination: use Pageable in repository methods (article 18) instead of findAll() on large tables

Final summary

Repositories extend JpaRepository and expose persistence operations. Services call them and orchestrate work. The next article is the service layer: where transactions and use cases live.