JPA

Java Persistence API

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JPA Specification

• Part of Java EE spec (Enterprise Edition)
• Contained in the javax.persistence package
• Implemented by many other libraries

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Hibernate

• Popular implementation of JPA
• Handles mapping Entity classes to DB tables
• Included in spring-data-jpa dependency by default
• Finds entities with entity scanning

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ORM (definition)

• Object Relational Mapping
• The practice of/tool for relating objects to DB rows

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Spring repositories

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Repositories

• Spring’s DAO objects
• The way to use your data
• Part of Spring framework
• Used to provide JPA implementation
• Automatically implemented by Spring Boot (during startup)
• Extend JPARepository or CRUDRepository

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Existing repository types

• Repository - parent marker interface
• CRUDRepository - repository defines CRUD operations
• JpaRepository - extended from CRUDRepository, defines 18 methods and can contain custom methods

- Method names parsed and automatically implemented

<verb><subject?>By<predicate>()
eg: findCarByYear() or countDistictPersonByLastName()

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Verbs

• get, read, find – queries and returns objects (synonyms)
• count – returns a count of matching objects

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Subject

• Optional
• Produces distinct query if the word distinct is present

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Predicate

• references a property
• may specify comparison operations (equals is implied)
• ignoresCase/ignoringCase usable for case-insensitive comparison
• predicate properties map to arguments

- Repositories provide indirect JPA access – we can still directly access using @persistencecontext/@entitymanager (later)

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JPA Annotations

• @Entity
• @ID
• @PersistenceContext
• @OneToMany, @ManyToOne, @OneToOne, @ManyToMany
• Full list

- @Entity

• Domain objects are “entities”
• Entities are managed by the EntityManager
• Table name defaults to class name
  • use @Table(name = "...") to change this

- @ID

• Sets the field (usually int or long) as the primary key
• Usually paired with @GeneratedValue

- @OneToMany, @ManyToOne, @OneToOne, @ManyToMany

Indicate relationships between entities eg:

@Entity
public class Room{
...
@OneToMany
private List<Window> windows;

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More about Repository methods

- Access Sub-properties in predicates

• findBy<PropertyName><SubpropertyName>
• eg: findByAddressState()
• Optional underscore separator: findByAddress_ZipCode()

- Specify custom queries

• @Query annotation on method signature
• Queries written in JPQL
• Serves as a hint for the Spring-generated implementation

- Custom query example

@Query("select c from Customer c where c.emailAddress = ?1")
Customer findByEmailAddress(EmailAddress email);

- Custom implementations (example to follow)

• Create an interface to define the desired method(s)
• Implement custom JPA method in <RepositoryName>Impl, implementing interface
• Extend the same inteface in <RepositoryName>

- Custom JPA example

interface ClassicCarUpdater {
  int updateClassics();
}

public class CarRepositoryImpl implements ClassicCarUpdater {
  @PersistenceContext
  private EntityManager em;

  public int updateClassics() {
    String update =
      "UPDATE Cars car SET car.isClassic = true " +
      "WHERE car.isClassic = false " +
      "AND car.id IN (" +
      "SELECT c from Cars c WHERE c.year < 1992)";
    return em.createQuery(update).executeUpdate();
  }
}

- Custom JPA example (repository)

public interface CarRepository extends
        JPARepository<Car, Integer>,
        ClassicCarUpdater {
    ...
    }

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Configuration hints:

• Show hibernate generated queries with spring.jpa.show-sql=true in application.properties

- @PersistenceContext and @PersistenceUnit

• @PersistenceUnit injects an EntityManagerFactory
• @PersistenceContext injects an EntityManager