Why you should use DTOs in your REST API

DTO stands for Data Transfer Object.

This pattern was created with a very well defined purpose: transfer data to remote interfaces, just like web services. This pattern fits very well in a REST API and DTOs will give you more flexibility in the long run.

The models that represent the domain of your application and the models that represent the data handled by your API are (or at least should be) different concerns and should be decoupled from each other. You don’t want to break your API clients when you add, remove or rename a field from the application domain model.

While your service layer operates over the domain/persistence models, your API controllers should operate over a different set of models. As your domain/persistence models evolve to support new business requirements, for example, you may want to create new versions of the API models to support these changes. You also may want to deprecate the old versions of your API as new versions are released. And it’s perfectly possible to achieve when the things are decoupled.

Just to mention a few benefits of exposing DTOs instead of persistence models:

  • Decouple persistence models from API models.

  • DTOs can be tailored to your needs and they are great when exposing only a set of attributes of your persistence entities. You won’t need annotations such as @XmlTransient and @JsonIgnore to avoid the serialization of some attributes.

  • By using DTOs, you will avoid a hell of annotations in your persistence entities, that is, your persistence entities won’t be bloated with non persistence related annotations.

  • You will have full control over the attributes you are receiving when creating or updating a resource.

  • If you are using Swagger, you can use @ApiModel and @ApiModelProperty annotations to document your API models without messing your persistence entities.

  • You can have different DTOs for each version of your API.

  • You’ll have more flexibility when mapping relationships.

  • You can have different DTOs for different media types.

  • Your DTOs can have a list of links for HATEOAS. That’s the kind of thing that shouldn’t be added to persistence objects. When using Spring HATEOAS, you can make your DTO classes extend ResourceSupport or wrap them with Resource<T>.

Dealing with the boilerplate code #

You won’t need to map your persistence entities to DTOs and vice versa mannually. There are many mapping frameworks you can use to do it. For instance, have a look at MapStruct, which is annotation based and works as a Maven Annotation Processor. It works well in both CDI and Spring-based applications.

You also may want to consider Lombok to generate getters, setters, equals(), hashcode() and toString() methods for you.

Related: To give better names to your DTO classes, refer to this post or to this answer on Stack Overflow.