Spring Data JPA repositories

Key points

• Extend JpaRepository<Entity, Id> and Spring generates the implementation at startup. No code to write.
• Method names become queries: findByStatusAndTotalGreaterThan is parsed into JPQL.
• For anything complex, write it yourself with @Query. A ten-word method name is a smell.
• Put @Transactional on the service method so several repository calls commit as one unit.
• Use ddl-auto: validate and a real migration tool (Flyway or Liquibase) in production — never update.
• Paginate list endpoints with Pageable. Unbounded findAll() is how services fall over.

Example

import jakarta.persistence.*;
import org.springframework.data.jpa.repository.*;
import org.springframework.data.domain.*;

@Entity
@Table(name = "orders")
public class Order {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String customerEmail;

    @Enumerated(EnumType.STRING)     // store the name, never the ordinal
    private Status status;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<LineItem> items = new ArrayList<>();
    // getters / setters omitted
}

public interface OrderRepository extends JpaRepository<Order, Long> {

    // Derived query - name is the specification
    List<Order> findByStatusOrderByCreatedAtDesc(Status status);

    Page<Order> findByCustomerEmail(String email, Pageable pageable);

    @Query("select o from Order o join fetch o.items where o.id = :id")
    Optional<Order> findWithItems(@Param("id") Long id);   // avoids N+1
}

Declare the interface, get the queries — but own the schema and the fetch strategy yourself.