How do you usually map objects from entity to DTO and back?

The following approaches are commonly used for mapping between entity and DTO in Java:

1. Manual mapping — writing methods that explicitly copy the necessary fields from entity to DTO and vice versa. It's simple, but becomes laborious with a large number of fields and classes.

2. Using mapping libraries — for example, MapStruct or ModelMapper. MapStruct generates code at compile time, providing good performance and type safety.

Example with MapStruct:

@Mapper
public interface UserMapper {
    UserDTO toDto(UserEntity entity);
    UserEntity toEntity(UserDTO dto);
}

MapStruct will generate an implementation that copies fields with the same names.

3. Using constructors or static factory methods in DTO — sometimes DTOs have constructors that accept an entity, and vice versa.

The choice of approach depends on the project size and performance requirements. In large projects, using MapStruct is preferable for convenience and support.