JPA - Beyond copy-paste

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

WHOAMI
CO-FOUNDER OF DEVSKILLER / CODEARTE
TRAINER AT BOTTEGA
DEVOXX.PL PROGRAM COMMITTEE
FORMER CONFITURA.PL

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

@Entity
    @Id
    @GeneratedValue
    private Long id;
    @Column // it does exactly nothing
    // ...
}

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

@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
    }
}

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

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

@Entity
public class Order {
}

@Entity(name = "orders")
}

}
insert into orders ...

}
em.createQuery("SELECT o FROM Order o")

}
em.createQuery("SELECT o FROM orders o")

@Entity
@Table(name = "orders")
}

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

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

ONE TO MANY
@Entity
@OneToMany
private Set<Address> addresses;
}

ONE TO MANY
@Entity
    @OneToMany
    @JoinColumn(name = "customer_id")
    private Set<Address> addresses;
}

MANY TO MANY
@Entity
    @ManyToMany
    private Collection<Address> addresses;
}
@Entity
    @ManyToMany
    private Collection<Customer> customers;
}

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

LAZY LOADING
Custom collections
PersistentSet
PersistentBag
PersistentList

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

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

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

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()) {
        ...
    }
}

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

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

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

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

OPTIMISTIC LOCKING IN REST
NEVER HEARD OF IT

IDENTITY
EQUALS() AND HASHCODE()

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);
     }
}

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

FLUSH MODES
MANUAL
COMMIT
AUTO
ALWAYS

LEVEL 1 CACHE PITFALLS
FLUSH() AND CLEAR()

LEVEL 2 CACHE PITFALLS
DISTRIBUTED ENVIRONMENT

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

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

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

JPA - Beyond copy-paste

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