Java - 使用循环依赖关系映射DTO

Question

Currently settings up classic mappers, converting Entities to DTOs. Some of the entities (and thus DTOs) reference each other (after specific JPA entity definition).

Let's say:

public class Person {
  private String name;
  private List<State> states; // All states with this person
  // Getters & Setters
}

public class State {
  private String name;
  private List<Person> persons; // All persons with this state
   // Getters & Setters
}

With such circular dependencies I have to set up mappers like so:

public class PersonMapper {
   public PersonDTO toDTO(Person p) { // not null
     PersonDTO dto = new PersonDTO();
     dto.setName(p.getName());
     dto.setStates(p.States().stream().map(stateMapper::toDTO).collect(Collectors.toList());
     return dto;
   }

public class StateMapper {
   public StateDTO toDTO(State s) { // not null
     StateDTO dto = new StateDTO();
     dto.setName(s.getName()); 
     dto.setPersons(s.getPersons().stream().map(personMapper::toDTO).collect(Collectors.toList());
     return dto;
   }

One way to easily avoid this is to create a new method in PersonMapper or StateMapper and make it NOT map the persons or states. But I was wondering if there was a known design pattern or a more generic way of doing this?

Thanks

Answer 1

The only way is to construct one side first:

public class PersonMapper {
   public PersonDTO toDTO(Person p) { // not null
     PersonDTO dto = new PersonDTO();
     dto.setName(p.getName());
     dto.setStates(new ArrayList<>());
     states.forEach(state -> state.getPersons().add(dto)) //todo: uniqueness check
     return dto;
   }
}

public class StateMapper {
   //todo: maybe Person.name is better than the whole Person object in the map
   public StateDTO toDTO(State s, Map<Person, PersonDTO> personMapping) { 
     StateDTO dto = new StateDTO();
     dto.setName(s.getName()); 

     List<PersonDTO> persons = s.getPersons().stream().map(personMapping::get)
                               .collect(Collectors.toList();
     persons.forEach(p -> p.getStates().add(dto)) //todo: uniqueness check for states or use a set
     dto.setPersons(persons);

     return dto;
  }
}

Everything else would use some Reference<State> like data structure but I don't think thats worth it. You could also break the circular dependency by removing one side and correlate a Person to its States when needed or store a Map<State, List<Person>> on another level. But I still prefer the approach outlined at the start of my post.

On a side-note: Maybe try datus (disclaimer: I am the author) and see if it fits your other conversion tasks and ease your work :)