The document provides an overview of key concepts in Java Persistence API (JPA) and Hibernate including: - Entity mappings for fields vs getters, naming strategies, access types - Relationship mappings for one-to-one, one-to-many, many-to-many - Lazy loading, collections, N+1 problem - Optimistic locking, identity, caching - Common pitfalls around flushing, queries, and injections

1. @jkubrynski / kubrynski.com
JPA - BEYOND COPY-PASTE
JAKUB KUBRYNSKI
jk@devskiller.com / @jkubrynski / http://kubrynski.com

2. WHOAMI
CO-FOUNDER OF DEVSKILLER
TRAINER AT BOTTEGA
DEVOXX.PL PROGRAM COMMITTEE
CO-AUTHOR OF SPRING CLOUD CONTRACT

4. @Entity
public class Product {
@Id
@GeneratedValue
private Long id;
@Column
private String name;
// ...
}

5. @Entity
public class Product {
@Id
@GeneratedValue
private Long id;
@Column // it does exactly nothing
private String name;
// ...
}

6. @Transactional @Service
public class ProductService {
public void updatePrice(Long productId, Money newPrice) {
Product product = productRepository.find(productId);
product.setPrice(newPrice);
productRepository.save(product);
}
}

7. @Transactional @Service
public class ProductService {
public void updatePrice(Long productId, Money newPrice) {
Product product = productRepository.find(productId);
product.setPrice(newPrice);
productRepository.save(product); // it does exactly nothing
}
}

8. FIELDS VS GETTERS
@Entity
public class Product {
private Long id;
private String name;
@Id
public Long getId() {
return id;
}
}

9. MIXED ACCESS
@Entity
@Access(AccessType.FIELD)
public class Product {
private Long id;
private String name;
@Id
@Access(AccessType.PROPERTY)
public Long getId() {
return id;
}
}

10. @Entity
public class Order {
}

11. @Entity(name = "orders")
public class Order {
}

12. @Entity(name = "orders")
public class Order {
}
insert into orders ...

13. @Entity(name = "orders")
public class Order {
}
em.createQuery("SELECT o FROM Order o")

14. @Entity(name = "orders")
public class Order {
}
em.createQuery("SELECT o FROM orders o")

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

16. MAPPINGS

17. ONE TO ONE
@Entity
public class Customer {
@OneToOne
private Address address;
}
@Entity
public class Address {
@OneToOne
private Customer customer;
}

18. ONE TO ONE
@Entity
public class Customer {
@OneToOne
private Address address;
}
@Entity
public class Address {
@OneToOne(mappedBy = "address")
private Customer customer;
}

19. ONE TO MANY
@Entity
public class Customer {
@OneToMany
private Set<Address> addresses;
}

20. ONE TO MANY
@Entity
public class Customer {
@OneToMany
@JoinColumn(name = "customer_id")
private Set<Address> addresses;
}

21. MANY TO MANY
@Entity
public class Customer {
@ManyToMany
private Collection<Address> addresses;
}
@Entity
public class Address {
@ManyToMany
private Collection<Customer> customers;
}

22. LAZY LOADING
element is loaded only when required
proxy by subclassing, but not always needed

23. LAZY LOADING
Custom collections
PersistentSet
PersistentBag
PersistentList

24. SET, BAG OR LIST?
@OneToMany
Set<Product> products;

25. SET, BAG OR LIST?
@OneToMany
List<Product> products;

26. SET, BAG OR LIST?
@OneToMany
@OrderColumn
List<Product> products;

27. N+1
@Entity
public class User {
@OneToMany
private List<Address> addresses;
}
List<User> users = em.createQuery("SELECT u FROM User u").getResultList();
for (User user : users) {
for (Address address : user.getAddresses()) {
...
}
}

28. N+1
@Entity
public class User {
@OneToMany
@BatchSize(size = 10)
private List<Address> addresses;
}

29. N+1
SELECT DISTINCT u FROM User u
JOIN FETCH u.addresses

30. HOW TO SAVE
EntityManager.persist()
EntityManager.merge()

31. OPTIMISTIC LOCKING
@Entity
public class User {
@Version
private int version;
}

32. OPTIMISTIC LOCKING IN REST
NEVER HEARD OF IT

33. IDENTITY
EQUALS() AND HASHCODE()

34. BASE CLASS
@MappedSuperclass
public abstract class BaseEntity implements Serializable {
@Id @GeneratedValue
private Long id;
private String uuid = UUID.randomUUID().toString();
public int hashCode() {
return Objects.hash(uuid);
}
public boolean equals(Object that) {
return this == that || that instanceof BaseEntity
&& Objects.equals(uuid, ((BaseEntity) that).uuid);
}
}

35. CACHING
L1 - EntityManager cache / Session cache
L2 - EntityManagerFactory cache / SessionFactory cache
QueryCache

36. FLUSH MODES
MANUAL
COMMIT
AUTO
ALWAYS

37. LEVEL 1 CACHE PITFALLS
FLUSH() AND CLEAR()

38. LEVEL 2 CACHE PITFALLS
DISTRIBUTED ENVIRONMENT

39. HQL INJECTION
String hqlQuery = "SELECT p FROM Product p where p.category = '" + cat + "'";
List<Product> products = em.createQuery(hqlQuery, Product.class)
.getResultList();

40. DYNAMIC UPDATES
@Entity
@DynamicUpdate
public class MyEntity {
// ...
}

41. DYNAMIC INSERTS
@Entity
@DynamicInsert
public class MyEntity {
// ...
}

42. MATERIALS
Hibernate logs analysis
Hibernate code debugging :-)
https://leanpub.com/high-performance-java-persistence
https://vladmihalcea.com
http://www.jooq.org

43. QUESTIONS?

44. THANKS