In a single query, how can Springboot fill in the DTO class?

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Springboot provides various ways to fill DTO classes in a single query. One way is by using Hibernate's Criteria API, which allows you to build complex queries based on your DTO class. Another way is by using Spring Data JPA, which provides query methods that can return DTO objects directly.

For example, let's assume we have a PersonEntity class with the following properties: id, firstName, lastName, and age, and we want to map these properties to a DTO class named PersonDTO. We can use Spring Data JPA to do this as follows:

@Repository
public interface PersonRepository extends JpaRepository<PersonEntity, Long> {
    @Query("SELECT new com.example.dto.PersonDTO(p.id, p.firstName, p.lastName, p.age) FROM PersonEntity p WHERE p.age > 18")
    List<PersonDTO> findAdults();
}

In the query above, we are selecting all adult persons (age > 18) and mapping their properties to the PersonDTO class using the constructor. We can then inject this repository into our service class and use it to retrieve DTO objects in a single query.

Alternatively, we can also use Hibernate's Criteria API to build complex queries that can map properties to DTO classes. For example:

Criteria crit = session.createCriteria(PersonEntity.class)
        .add(Restrictions.gt("age", 18))
        .setProjection(Projections.projectionList()
            .add(Projections.property("id"), "id")
            .add(Projections.property("firstName"), "firstName")
            .add(Projections.property("lastName"), "lastName")
            .add(Projections.property("age"), "age"))
        .setResultTransformer(Transformers.aliasToBean(PersonDTO.class));
List<PersonDTO> adults = crit.list();

In the query above, we are using the Criteria API to select all adult persons (age > 18) and map their properties to the PersonDTO class using the aliasToBean() method. We can then retrieve DTO objects in a single query.