Mapstruct bidirectional mapping

Question

I have the following example in which i have a separate domain layer and a separate persistence layer. I am using Mapstruct for mapping and I get StackOverflow when mapping from domain to entity or from entity to domain because of the bidirectional reference that always gets called on -> infinite loop scenario. How can I use Mapstruct for this scenario?

class User {
  private UserProfile userProfile;
}

class UserProfile {
 private User user;
}

@Entity
class UserEntity {
  @OneToOne
  @PrimaryKeyJoinColumn
  private UserProfileEntity userProfile;
}

@Entity
class UserProfileEntity {
  @OneToOne(mappedBy = "userProfile")
  private UserEntity userEntity;
}

class for mapping is pretty basic

@Mapper
interface UserMapper {

UserEntity mapToEntity(User user);

User mapToDomain(UserEntity userEntity);
}

Answer 1

Check out the Mapstruct mapping with cycles example.

A solution to your problem is also demonstrated in the documentation for Context annotation.

Example

A complete example: https://github.com/jannis-baratheon/stackoverflow--mapstruct-mapping-graph-with-cycles.

Reference

Mapper:

@Mapper
public interface UserMapper {

    @Mapping(target = "userProfileEntity", source = "userProfile")
    UserEntity mapToEntity(User user,
                           @Context CycleAvoidingMappingContext cycleAvoidingMappingContext);

    @InheritInverseConfiguration
    User mapToDomain(UserEntity userEntity,
                     @Context CycleAvoidingMappingContext cycleAvoidingMappingContext);

    @Mapping(target = "userEntity", source = "user")
    UserProfileEntity mapToEntity(UserProfile userProfile,
                                  @Context CycleAvoidingMappingContext cycleAvoidingMappingContext);

    @InheritInverseConfiguration
    UserProfile mapToDomain(UserProfileEntity userProfileEntity,
                            @Context CycleAvoidingMappingContext cycleAvoidingMappingContext);
}

where CycleAvoidingMappingContext keeps track of the already mapped objects and reuses them avoiding the stack overflow:

public class CycleAvoidingMappingContext {
    private final Map<Object, Object> knownInstances = new IdentityHashMap<>();

    @BeforeMapping
    public <T> T getMappedInstance(Object source,
                                   @TargetType Class<T> targetType) {
        return targetType.cast(knownInstances.get(source));
    }

    @BeforeMapping
    public void storeMappedInstance(Object source,
                                    @MappingTarget Object target) {
        knownInstances.put(source, target);
    }
}

Mapper usage (mapping single object):

UserEntity mappedUserEntity = mapper.mapToEntity(user, new CycleAvoidingMappingContext());

You can also add a default method on your mapper:

@Mapper
public interface UserMapper {

    // (...)

    default UserEntity mapToEntity(User user) {
        return mapToEntity(user, new CycleAvoidingMappingContext());
    }

    // (...)
}