How to use JPA API Specification?

#java #spring #jpa #api

Hey, this is my first article here. I hope that I'll be able to post content about technology more consistently from now on.

I've chosen to bring you an advanced JPA 2 topic: the Specification API.

Specification was a concept that Eric Evans brought up in his book Domain Driven Design(DDD). Based on this, JPA brings this concept as a set of interfaces that facilitate the reuse of predicates, using the Criteria API. That's it, simples as that.

Let's look an example:

We have a book model with id, author, title, isbn and publication year.

	@Entity
	@Data
	public class Book {
	@Id
	private Integer id;
	private String author;
	private String title;
	private String isbn;
	private Integer publicationYear;
	}

view raw Book.java hosted with ❤ by GitHub

and We have a book repository interface that extends JpaSpecificationExecutor

	public interface BookRepository extends JpaRepository<Book, Integer>, JpaSpecificationExecutor<Book> {
	}

view raw BookRepository.java hosted with ❤ by GitHub

The JpaSpecificationExecutor provides two methods:

	List<T> findAll(Specification<T> specification);
	Page<T> findAll(Specification<T> specification, Pageable pageable);

view raw JpaSpecificationExecutorMethods.java hosted with ❤ by GitHub

Now to build our query, We can define the class that implements the Specification interface.

	public class BookSpecifications {
	public static Specification<Book> booksByAuthor(String authorName) {
	return (root, query, cb) -> cb.like(cb.lower(root.get(Book_.author)), "%" + authorName.toLowerCase() + "%");
	}
	}

view raw BookSpecifications.java hosted with ❤ by GitHub

With this static method booksByAuthor, we can use this specification to filter for all books that have an author in common. Let's see how simple it is below:

bookRepository.findAll(booksByAuthor("George R. R. Martin"));

view raw UseCaseBookSpecifications.java hosted with ❤ by GitHub

One aspect of the specification that makes it more interesting is the fact that we can combine the queries. Let's look an example:

bookRepository.findAll(where(booksByAuthor("George R. R. Martin")).and(booksByPublicationYear(1996)));

view raw CombineQueriesUseCase.java hosted with ❤ by GitHub

And the specification class would look like this:

	public class BookSpecifications {
	public static Specification<Book> booksByAuthor(String authorName) {
	return (root, query, cb) -> cb.like(cb.lower(root.get(Book_.author)), "%" + authorName.toLowerCase() + "%");
	}

	public static Specification<Book> booksByPublicationYear(int publicationYear) {
	return (root, query, cb) -> cb.equal(root.get(Book_.publicationYear), publicationYear);
	}
	}

view raw BookSpecifications2.java hosted with ❤ by GitHub

That's it. Thank you very much for reading this content. I usually bring content with a more hands-on approach and if something wasn't so clear, feel free to comment and suggest improvements.

If you want to know more about this content, I recommend the official documentation:
JPA Specifications