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Zhenyang Hua

This is a space that holds a collection of my personal work and ideas

Spring Data Substitutes ObjectMapper

Posted on 04/27/2018

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.

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