Making JPA great again

Réda Housni Alaoui

• Coding Architect @Cosium
• OSS Contributor
• Github: reda-alaoui

Cosium

• SaaS editor, LIR, Health sector, International
• Engineers company, strong technical skills
• Java, Web, TypeScript, PostgreSQL, Linux, 3D, hardware,...
• Software factory: IntelliJ, Maven, Git, Jenkins, Jira, SonaQube, Archiva... Gerrit and Vet!

We're hiring

How to

• Tackle JPA verbosity
• Eliminate useless joins
• Avoid N+1 query issue

JPA

• Java Persistence API
• Version 1.0 released in 2006
• Implemented by Object-relational mapping libs:
  • Hibernate
  • EclipseLink
  • OpenJPA
  • ...

Our JPA playground

@Entity
public class Author {
  @Id
  private long id;
  private String name;
}

create table Author (
  id bigserial primary key, 
  name varchar
);

@Entity
public class Book {
  @Id
  private long id;
  private String title;
  // Association
  @ManyToOne
  private Author author;
}

create table Book (
  id bigserial primary key, 
  title varchar, 
  author_id bigint references author(id) not null
);
					

JPA semantic is cumbersome

// Let's print all books titles
    
// Begin JPA plumbing    
CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery cq = cb.createQuery(Book.class);
entityManager
    .createQuery(cq)
// End JPA plumbing    
    .getResultList()
    .forEach(Book::printTitle);
    
// And we did not yet introduce pagination    

Spring Data to the rescue !

[Spring Data] makes it easy to use data access technologies, relational and non-relational databases, map-reduce frameworks, and cloud-based data services

Spring Data

• Supports JPA via Spring Data JPA project
• Centered around the concept of repository
• A repository is a facade over a data source

Introducing the BookRepository

public interface BookRepository extends CrudRepository<Book, Long> {}

bookRepository
  .findAll()
  .forEach(Book::printTitle);
// Pretty neat :)  

select 
    book.id, 
    book.title, 
    author.name 
from book 
join author 
    on author.id = book.author_id; 

select 
    book.id, 
    book.title
from book;

JPA associations fetch type

• Possible values:
  • EAGER
  • LAZY
• @ManyToOne defaults to EAGER fetching

@Entity
public class Book {
  // ...
  @ManyToOne(/* default to fetch = EAGER */)
  private Author author;
  // ...  
}

Lazy loading Book's Author

@Entity
public class Book {
  // id + title omitted
  @ManyToOne(fetch = LAZY)
  private Author author;
}

bookRepository
  .findAll()
  .forEach(Book::printTitle);

select 
    book.id, 
    book.title
from book;

select 
    book.id, 
    book.title
from book;

N+1 query issue

bookRepository
  .findAll()
  .forEach(book -> {
    book.printTitle();
    book.printAuthorName();
  });

select book.id, 
       book.title, 
       book.author_id
from book;
select author.name 
    from author 
    where author.id = 1;    
select author.name 
    from author 
    where author.id = 2;

select 
    book.id, 
    book.title
    author.name
from book
join author
    on book.author_id = author.id;

We need to select fetching behaviour at runtime

JPA EntityGraph to the rescue !

• Introduced in JPA 2.1
• Runtime fetch type
• Describes a graph of the entities that will be eagerly fetched
• Can be defined programmatically or by annotation

book-with-author entity graph

@Entity
@NamedEntityGraphs({
  @NamedEntityGraph(
    name = "book-with-author",
    attributeNodes = {
      @NamedAttributeNode("author")
    }
  )
})    
public class Book {
  // id + title omitted
  @ManyToOne(fetch = LAZY)
  private Author author;
}

Applying book-with-author to the query

public interface BookRepository extends CrudRepository<Book, Long> {
    @EntityGraph("book-with-author") // Can't be passed dynamically
    List<Book> findAll();    
}

bookRepository.findAll().forEach(book -> {
    book.printTitle();
    book.printAuthorName();
  });

select 
    book.id, 
    book.title
    author.name
from book
join author --...;

select 
    book.id, 
    book.title
    author.name
from book
join author --...;

Spring Data neutralizes EntityGraph

• Forces us to choose the EntityGraph before compilation
• Makes a repository method forever bound to a unique EntityGraph
• Removes the EntityGraph dynamic aspect
• DATAJPA-749 Context enabled JPA 2.1 EntityGraph issue closed as won't fix

spring-data-jpa-entity-graph to the rescue !

• github.com/Cosium/spring-data-jpa-entity-graph
• Spring Data JPA extension
• Pass EntityGraph as argument to ANY repository method

// The new BookRepository
public interface BookRepository 
    extends EntityGraphCrudRepository<Book, Long> {}

Printing all books titles and authors names one last time

bookRepository
  .findAll(EntityGraphs.named("book-with-author"))
  .forEach(book -> {
    book.printTitle();
    book.printAuthorName();
  });

select 
    book.id, 
    book.title
    author.name
from book
join author --...;

select 
    book.id, 
    book.title
    author.name
from book
join author --...;

Printing all books titles one last time

bookRepository
  .findAll()
  .forEach(Book::printTitle);

select 
    book.id, 
    book.title
from book;

select 
    book.id, 
    book.title
from book;