DTOs, or data-transfer objects, are commonly used. What is not со commonly-known is that they originate from DDD (Domain-driven design). There it makes a lot of sense – domain objects have state, identity and business logic while DTOs have only state.
But many projects today are using the anemic data model approach (my opinion) and still use DTOs. They are used whenever an object “leaves” the service layer or “leaves” the system (through web services, rmi, etc.). There are three approaches:
- every entity has at least one corresponding DTO. Usually more than one, for different scenarios in the view layer. When you display a user in a list you have one DTO, when you display it in a “user details” window you need a more extended DTO. I am not in favour of this approach because in too many cases the DTO and the domain structure have exactly the same structure and as a result there’s a lot of duplicated code + redundant mapping. Another thing is the variability of multiple DTOs. Even if they differ from the entity, they differ from one another with one or two fields. Why duplication is a bad thing? Because changes are to be made in two places, issues are traced harder when data passes through multiple objects, and because..it is duplication. Copy & paste within the same project is a sin.
- DTOs are only created when their structure significantly differs
from the that of the entity. In all other cases the entity itself is
used. The cases when you don’t want to show some fields (especially when
exposing via web services to 3rd parties) exist, but are not that
common. This can sometimes be handled via the serialization mechanism –
mark them as @JsonIgnore or @XmlTransient for example – but in other
cases the structures are just different. In these cases a DTO is due.
For example you have a User and UserDetails, where UserDetails holds the
details + the relations of the currently logged user to the given user.
The latter has nothing to do with the entity, so you create a DTO.
However in the case of a DirectMessage you have sender, recipient, text
and datetime both in the DB and in the UI. No need to have a DTO.
One caveat with this approach (as well as with the next one). Anemic entities usually come with an ORM (JPA in the case of Java). Whenever they exit the service layer they may be invalid, because of lazy collections that require an open session. You have two options here:
- use OpenSessionInView / OpenEntityManagerInView
- Don’t use lazy collections. Lazy collections are unneeded. Either make them eager, if they are supposed to hold a small list of items (for example – the list of roles for a user), or if the data is likely to grow use queries. Yes, you are not going to show 1000 records at on go anyway, you will have to page it. Without lazy associations (@*ToOne are eager by default) you won’t have invalid objects when the session is closed
– thus your session stays open until you are finished preparing the response. This is easy to configure but is not my preferred option – it violates layer boundaries in a subtle way, and this sometimes leads to problems especially for novice developers
- Don’t use DTOs at all. Applicable a soon as there aren’t significantly varying structures. For smaller projects this is usually a good way to go. Everything mentioned in the above point applies here.
So my preferred approach is the “middle way”. But it requires a lot of consideration in each case, which may not be applicable for bigger and/or less experienced teams. So one of the two “extremes” should be picked. Since the “no DTOs” approach also requires consideration – what to make @Transient, how does lazy collections affect the flow, etc, the “All DTOs” is usually chosen. But even though it is seemingly the safest approach, it has many pitfalls.
First, how do you map from DTOs to entities and vice-versa? Three options:
- dedicated mapper classes
- constructors – the DTO constructor takes the entity and fills itself, and vice-versa (remember to also provide a default constructor)
- declarative mapping (e.g. Dozer). This is practically the same as the first option – it externalizes the mapping. It can even be used together with a dedicated mapper class
- map them in-line (whenever needed). This can generate unmaintainable code and is not preferred
I prefer the constructor approach, at least because fewer classes are created. But they are essentially the same (DTOs are not famous for encapsulation, so all of your properties are exposed anyway). Here is a list of guidelines when using DTOs and either of the “mapping” approaches:
- Don’t generate too much redundant code. If two scenarios require slightly different DTOs, reuse. No need to create a new DTO for a difference of one or two fields
- Don’t put presentation logic in mappers/constructors. For example if (entity.isActive()) dto.setStatus("Active"); This should happen in the view layer
- Don’t sneak entities together with DTOs. DTOs should not have members which are entities. Generally, entities should not be used outside the service layer (this is a bit extreme, but if we use DTOs everywhere we should be consistent and stick to that practice)
- Don’t use the mappers/entity-to-dto constructors in controllers, use them in the service layer. The reason DTOs are used in the first place is that entities may be ORM-bound, and they may not valid outside a session (i.e. outside the service layer).
- If using mappers, prefer static mapper methods. Mappers don’t have state, so no need for them to be instantiated. (And they don’t have to be mocked, wrapped, etc).
- If using mappers, there’s no need for a separate mapper for each entity(+its multiple DTOs). Related entities can be grouped in one mapper. For example Company, CompanyProfile, CompanySubsidiary can use the same mapper class
Just make sure you make all these decisions at the beginning of a project and figure out which is applicable in your scenario (team size and experience, project size, domain complexity).