从实体中获取DTO数据，也就是从DDD聚合中获取DDD值对象有多种方式，如果我们不使用DTO方式，直接返回完整实体聚合，容易导致性能损失，使用DTO可以让我们只提取所需的数据。在这个应用程序中，我们展示如何依赖于javax.persistence.Tuple原生SQL生成DTO。点击#DTO标签可获得更多生成DTO的方式。
主要点：
编制自己的Dao调用EntityManager.createNativeQuery()，返回java.persistence.Tuple类型：
假设Car实体：
@Entity
@Table(name = "car")
public class Car implements Serializable {
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private String name;
private String engine;
private String color;
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编制自己的DAO:
@Repository
@Transactional
public class Dao<T, ID extends Serializable> implements GenericDao<T, ID> {
@PersistenceContext
private EntityManager entityManager;
@Override
public <S extends T> S persist(S entity) {
Objects.requireNonNull(entity, "Cannot persist a null entity");
entityManager.persist(entity);
return entity;
}
@Transactional(readOnly = true)
public List<Tuple> fetchCars() {
List<Tuple> result = entityManager
.createNativeQuery(
"select name, color from car", Tuple.class
).getResultList();
return result;
}
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Service调用：
@Service
public class CarService {
private final Dao dao;
public CarService(Dao dao) {
this.dao = dao;
}
public List<Tuple> fetchCars() {
return dao.fetchCars();
}
}
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最终Spring Boot中调用：
@SpringBootApplication
public class TupleAndSqlApplication {
private static final Logger logger
= Logger.getLogger(TupleAndSqlApplication.class.getName());
@Autowired
private CarService carService;
public static void main(String args) {
SpringApplication.run(TupleAndSqlApplication.class, args);
}
@Bean
public ApplicationRunner init() {
return args -> {
carService.populateCars();
List<Tuple> cars = carService.fetchCars();
cars.forEach((e) -> logger.info(() -> "Car: " + e.get("name") + "," + e.get("color")));
};
}
}
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源码下载