DTO conveter pattern in Spring Boot

Question

The main question is how to convert DTOs to entities and entities to Dtos without breaking SOLID principles.
For example we have such json:

{ id: 1,
  name: "user", 
  role: "manager" 
} 

DTO is:

public class UserDto {
 private Long id;
 private String name;
 private String roleName;
}

And entities are:

public class UserEntity {
  private Long id;
  private String name;
  private Role role
} 
public class RoleEntity {
  private Long id;
  private String roleName;
}

And there is usefull Java 8 DTO conveter pattern.

But in their example there is no OneToMany relations. In order to create UserEntity I need get Role by roleName using dao layer (service layer). Can I inject UserRepository (or UserService) into conveter. Because it seems that converter component will break SRP, it must convert only, must not know about services or repositories.

Converter example:

@Component
public class UserConverter implements Converter<UserEntity, UserDto> {
   @Autowired
   private RoleRepository roleRepository;    

   @Override
   public UserEntity createFrom(final UserDto dto) {
       UserEntity userEntity = new UserEntity();
       Role role = roleRepository.findByRoleName(dto.getRoleName());
       userEntity.setName(dto.getName());
       userEntity.setRole(role);
       return userEntity;
   }

   ....

Is it good to use repository in the conveter class? Or should I create another service/component that will be responsible for creating entities from DTOs (like UserFactory)?

Answer 1

I think the way to do it cleanly is to include a Role DTO that you convert to the RoleEntity. I might use a simplified User DTO in case that it is read only. For example, in case of unprivileged access.

To expand your example

public class UserDto {
 private Long id;
 private String name;
 private RoleDto role;
}

with the Role DTO as

public class RoleDto {
  private Long id;
  private String roleName;
}

And the JSON

{ 
  id: 1,
  name: "user", 
  role: {
   id: 123,
   roleName: "manager"
}

Then you can convert the RoleDto to RoleEntity while converting the User in your UserConverter and remove the repository access.