Return Entity object or POJO to UI?

Question

I have JPA Employee persistent entity wjich has 50 fields. Sometimes I need to display all 50 fields to UI or sometime much less than that.

There are two approaches I can think of

Approach 1:-

I will continue return the Employee entity to UI. There are ways(with annotation) where I can mention which specific fields under Employee needs to be returned

Example :-

If I want to ignore MiddleName from Employee class before returning json to browser, I will create EmployeeMixin with required @JsonIgnoreProperties like below

@JsonIgnoreProperties({"MiddleName"})
public abstract class EmployeeMixin extends Employee{

}




Employee employee = new Employee ();
Map<Class<?>, Class<?>> mixinMap = new HashMap<Class<?>, Class<?>>();
mixinMap.put(Employee.class, EmployeeMixin.class);
ObjectMapper mapper = new ObjectMapper(mixinMap);
return mapper.writeValueAsString(employee);

Approach 2:-

I will make new POJO containing again all 5o fields and retuen to UI.

My question is which is better design approach ? Should I return Entity object or new Pojo object(constructed from Entity) to UI ?

Answer 1

I would prefer Approach 3, projection

• Create a DTO object for each case with a variable set of fields you need.
• Create a corresponding JPQL or CriteriaQueryto populate appropriate DTO.

Having an entity like

@Entity
@Getter
public class Pojo3 {
   @Id
   @GeneratedValue
   private Long id;
   @Setter
   private String f1, f2,f3;
}

you can make a DTO class with subset of fields like

@Getter @Setter
@AllArgsConstructor // Pojo2DTO(id,f1,f2)
public class Pojo2DTO {
   private Long id;
   private String f1,f2;
}

that can be populated in JPQL like this test shows

@Test
@Transactional
public void test() {
   Pojo3 pojo3 = new Pojo3();
   pojo3.setF1("f1");
   pojo3.setF2("f2");
   pojo3.setF3("f3");      
   em.persist(pojo3);

   // **NOTE** the full package path to DTO object
   TypedQuery<Pojo2DTO> tq = em.createQuery(
         "SELECT NEW org.example.jpa.manyfields.Pojo2DTO(p3.id, p3.f1, p3.f2) "
      + " FROM Pojo3 p3 WHERE p3.id=:id", Pojo2DTO.class);
   tq.setParameter("id", pojo3.getId());

   Pojo2DTO pojo2dto = tq.getSingleResult(); 
   log.info("{}, {}, {}", pojo2dto.getId(), pojo2dto.getF1(), pojo2dto.getF2());

}