JPA Basics — Practical Demo

Hands-on examples for JPA Basics. We use a small Product/Order/Customer domain throughout to keep context consistent.

Prerequisites

You should be comfortable with basic Spring Boot setup and understand what @Entity and @Id mean before running these examples. See JPA Basics for the theory.

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Example 1: Define a Basic Entity

The simplest entity — a Product with an auto-generated ID and a few columns.

Product.java

@Entity
@Table(name = "products")
public class Product {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)  // ← DB auto-increment
    private Long id;

    @Column(nullable = false, length = 200)
    private String name;

    @Column(nullable = false, precision = 10, scale = 2)
    private BigDecimal price;

    @Column(columnDefinition = "TEXT")  // ← maps to TEXT column type
    private String description;

    // Getters and setters (or use Lombok @Getter @Setter)
}

application.yml

spring:
  jpa:
    hibernate:
      ddl-auto: create-drop   # ← auto-creates schema on startup, drops on stop (dev only)
    show-sql: true
    properties:
      hibernate:
        format_sql: true

Expected SQL logged on startup:

create table products (
    id bigint generated by default as identity,
    name varchar(200) not null,
    price numeric(10,2) not null,
    description text,
    primary key (id)
)

Key takeaway

@Column(nullable = false) maps to NOT NULL in DDL. The @GeneratedValue(strategy = IDENTITY) delegates ID generation to the database — the most common choice for modern databases.

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Example 2: Add a ManyToOne Relationship

An Order belongs to one Customer. Keep LAZY loading on the @ManyToOne (it's EAGER by default — override it).

Customer.java

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

    @Column(nullable = false)
    private String name;

    private String email;
}

Order.java

@Entity
@Table(name = "orders")
public class Order {

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

    @ManyToOne(fetch = FetchType.LAZY)          // ← override EAGER default; load on demand
    @JoinColumn(name = "customer_id")           // ← foreign key column name in orders table
    private Customer customer;

    @Enumerated(EnumType.STRING)
    private OrderStatus status;

    @OneToMany(mappedBy = "order",              // ← "mappedBy" = inverse side (no FK here)
               cascade = CascadeType.ALL,
               fetch = FetchType.LAZY)
    private List<OrderItem> items = new ArrayList<>();
}

OrderRepository.java

public interface OrderRepository extends JpaRepository<Order, Long> {}

Demo

// Saving an order
Customer customer = customerRepo.save(new Customer("Alice", "alice@example.com"));
Order order = new Order();
order.setCustomer(customer);
order.setStatus(OrderStatus.PENDING);
orderRepo.save(order);
// ↑ two INSERTs: one for customer (if not saved), one for order

Key takeaway

cascade = CascadeType.ALL on @OneToMany means saving or deleting an Order will cascade to its OrderItem children automatically. Without this, you'd need to save items separately.

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Example 3: Lifecycle Callbacks and Auditing

Track when entities are created and updated automatically using Spring Data auditing.

AuditableEntity.java

@MappedSuperclass          // ← not an entity itself; fields inherited by subclasses
@EntityListeners(AuditingEntityListener.class)  // ← Spring Data auditing listener
public abstract class AuditableEntity {

    @CreatedDate
    @Column(updatable = false)
    private LocalDateTime createdAt;

    @LastModifiedDate
    private LocalDateTime updatedAt;

    @CreatedBy
    @Column(updatable = false)
    private String createdBy;
}

Product.java (updated)

@Entity
@Table(name = "products")
public class Product extends AuditableEntity {  // ← inherits audit fields
    // ... id, name, price, description as before
}

Application.java

@SpringBootApplication
@EnableJpaAuditing(auditorAwareRef = "auditorProvider")  // ← activates audit infrastructure
public class Application { ... }

@Bean
public AuditorAware<String> auditorProvider() {
    // In a real app, return the current user from SecurityContextHolder
    return () -> Optional.of("system");
}

Expected behaviour: After productRepo.save(product):

product.getCreatedAt() → 2026-03-08T10:00:00
product.getCreatedBy() → "system"

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Exercises

1. Easy: Add a @Column(unique = true) constraint to the Product name field and verify the generated DDL.
2. Medium: Add a @PreUpdate lifecycle callback to Product that logs the entity ID and the timestamp when it is about to be updated.
3. Hard: Implement a @ManyToMany between Product and Tag (using a join table product_tags), save a product with two tags, and verify that deleting the product does not delete the tags (configure cascade appropriately).

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Back to Topic

Return to JPA Basics for theory, trade-offs, interview questions, and further reading.