JPQL

Methods to interact with Database

flowchart LR
SDJPA["Spring Data JPA"]

Methods --> JPA
Methods --> Hibernate
Methods --> SDJPA

JPA --> JPQL
JPA --> EntityManager
JPA --> Criteria

Hibernate --> HQL
Hibernate --> Session

SDJPA --> Repository
SDJPA --> Specification
SDJPA --> QueryDsl

JPA

• Jakarta Persistence API (formerly Java Persistence API)
  • package names: javax.*
• In 2019 renamed to Jakarta Persistence
  • package names: jakarta.*
• Refer Java History

EntityManager

• jakarta.persistence.EntityManager

public interface EntityManager extends AutoCloseable {}

• JPA interface used to interact with the persistence context
• Low-level, versatile, but verbose and less type-safe
• provides methods to create and execute queries using”
  • JPQL
  • native SQL
  • Criteria

List<User> users = entityManager.createNativeQuery("SELECT * FROM User", User.class).getResultList();

JPQL

• org.springframework.data.jpa.repository.Query
• Java Persistence Query Language
• We use @Query
• For native queries we use @Query("...", native=true)
• Can be executed using:
  • EntityManager
  • @NamedQuery
  • Spring Data JPA Repository @Query

Criteria

• jakarta.persistence.criteria.CriteriaQuery
• Part of JPA
• nice clean object oriented API
• Can detect errors in compile time
• JPQL and Criteria have same efficiency and performance
• Criteria is more flexible and provide better support for writing dynamic queries as compared to HQL and JPQL
• Disadvantage: Criteria does not support native queries
• CriteriaBuilder is used to build CriteriaQuery objects
• Predicate is used to construct clauses which can be used in CriteriaQuery#where

Hibernate

Session

• org.hibernate.Session

public interface Session extends SharedSessionContract, EntityManager {}

• It is specific to Hibernate
• It extends EntityManager
• EntityManager will invoke Session behind the hood if implementation is hibernate

HQL

• org.hibernate.annotations.NamedQuery
• Hibernate Query Language
• We use @NamedQuery
• For native queries we use @NamedNativeQuery
• Disadvantage:
  • Hibernate specific
  • code’s unreadability
• Can be executed using:
  • Session.createQuery or EntityManager
  • @NamedQuery

Spring Data JPA

Repository

• https://docs.spring.io/spring-data/jpa/reference/repositories/definition.html
• Interfaces:
  • JpaRepositry — CRUD + pagination + sorting + flush persistence/delete record in batch etc., returns List<>
  • PagingAndSortingRepository — CRUD + pagination + sorting
  • CrudRepository — Supports only CRUD, returns Iterable<>
  • ListCrudRepository — returns List<>
  • ReactiveCrudRepository — Flux Mono
  • RxJava3CrudRepository — RxJava

Specification

• org.springframework.data.jpa.domain.Specification
• https://stackoverflow.com/questions/47469861/what-is-the-difference-between-a-criteria-a-predicate-and-a-specification

public interface Specification<T> extends Serializable {}

• It is an Interface which is part of Spring Data JPA
• while Criteria is part of JPA
• It is based on Domain Driven Design book by Eric Evans
• It is developed to define reusable Predicate and use Criteria API in type-safe way
• Disadvantage: Specification does not support native queries
• You need to have JpaSpecificationExecutor on your repository

interface CustomerRepository extends JpaRepository<Customer>, JpaSpecificationExecutor {
  // Your query methods here
}

• Use

public CustomerSpecifications {
 
  public static Specification<Customer> customerHasBirthday() {
    return new Specification<Customer> {
      public Predicate toPredicate(Root<T> root, CriteriaQuery query, CriteriaBuilder cb) {
        return cb.equal(root.get(Customer_.birthday), today);
      }
    };
  }
}
 
customerRepository.findAll(hasBirthday());

Querydsl

• org.springframework.data.jpa.repository.support.Querydsl
• https://spring.io/blog/2011/04/26/advanced-spring-data-jpa-specifications-and-querydsl
• It is third party library and also part of Spring Data JPA
• It is useful for more complex queries
• Provides fluent API for constructing type-safe queries
• Disadvantage: Querydsl does not support native queries
• You need to have QueryDslPredicateExecutor on your repository

interface CustomerRepository extends JpaRepository<Customer>, QueryDslPredicateExecutor {
  // Your query methods here
}

• Use

BooleanExpression customerHasBirthday = customer.birthday.eq(today);
BooleanExpression isLongTermCustomer = customer.createdAt.lt(today.minusYears(2));
customerRepository.findAll(customerHasBirthday.and(isLongTermCustomer));