JPA - Beyond Copy-Paste

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@jkubrynski / kubrynski.com JPA - BEYOND COPY-PASTE JAKUB KUBRYNSKI [email protected] / @jkubrynski / http://kubrynski.com

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

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No content

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ALL NON-TRIVIAL ABSTRACTIONS, TO SOME DEGREE, ARE LEAKY. JOEL SPOLSKY

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

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

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

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

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

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@Entity public class Order { }

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

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

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

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

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

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MAPPINGS

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

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

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ONE TO MANY @Entity public class Customer { @OneToMany private Set

addresses; }

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ONE TO MANY @Entity public class Customer { @OneToMany @JoinColumn(name = "customer_id") private Set

addresses; }

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MANY TO MANY @Entity public class Customer { @ManyToMany private Collection

addresses; } @Entity public class Address { @ManyToMany private Collection customers; }

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

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LAZY LOADING Custom collections PersistentSet PersistentBag PersistentList

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SET, BAG OR LIST? @OneToMany Set products;

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SET, BAG OR LIST? @OneToMany List products;

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SET, BAG OR LIST? @OneToMany @OrderColumn List products;

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N+1 @Entity public class User { @OneToMany private List

addresses; } List users = em.createQuery("SELECT u FROM User u").getResultList(); for (User user : users) { for (Address address : user.getAddresses()) { ... } }

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N+1 @Entity public class User { @OneToMany @BatchSize(size = 10) private List

addresses; }

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

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

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

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OPTIMISTIC LOCKING IN REST NEVER HEARD OF IT

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IDENTITY EQUALS() AND HASHCODE()

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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); }

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

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FLUSH MODES MANUAL COMMIT AUTO ALWAYS

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LEVEL 1 CACHE PITFALLS FLUSH() AND CLEAR()

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LEVEL 2 CACHE PITFALLS DISTRIBUTED ENVIRONMENT

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

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