Spring Data Substitutes ObjectMapper


Zhenyang Hua

Updated: Apr 27, 2018

Most of the 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. ```java @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. ```java @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.