index.md

title	date
Spring Data Substitutes ObjectMapper	2018-04-27 17:00:00

Most times when we need a DTO mapping from our persistent entity, we would think of using an ObjectMapper. However, with the help of the Spring Data, you might not need an ObjectMapper. This lesson presents a solution to use Spring Data to replace simple ObjectMapper use case.

Story

Given a Book class and one of it's DTO class BookMeta as shown below, create a repository method that retrieve all BookMeta by category.

@Entity
public class Book {
	private String title;
	private String author;
	private String category;
	private String intro;

	// getter and setters are ommitted for brevity
}

@Entity
public class BookMeta {
	private String title;
	private String author;

	public BookMeta(String title, String author) {
		this.title = title;
		this.author = author;
	}

	// getter and setters are ommitted for brevity
}

Solution

As part of JPA, JPQL now supports constructor expressions. What we could do is to new a full-classpath-constructor in the named query.

@Repository
public interface BookRepository extends JpaRepository<Book, Long> {
    @Query("SELECT new com.leafyjava.domains.dtos.BookMeta(b.title, b.author) FROM Book b WHERE b.category = :category")
    List<BookMeta> findAllMetaByCategory(@Param("category") String category);
}

Lesson Learnt

• DTO class full classpath must be used in the @Query annotation
• Use @Param to set query parameters.