Difference between Entity and DTO

Question

What is the difference between a DTO and an Entity? In details these are my questions:

1. What fields should the DTOs have? For example my entity classes are:

   @Entity
   public class MyFirstEntity implements Serializable {
   
       @Id @GeneratedValue
       private Long id;
   
       private String stringData;
   
       @OneToOne
       private MySecondEntity mySecondEntity;
   
       @OneToMany
       private List<MySecondEntity> mySecondEntitesList;
   
   }
   
   @Entity
   public class MySecondEntity implements Serializable {
   
       @Id @GeneratedValue
       private Long id;
   
       private Integer integerData;
   
       @ManyToOne
       private MyFirstEntity myFirstEntity;
   
   }
   

There is a one-sided connection (One-to-one) and a two-sided connection (Many-to-one), a simple String and Integer data and of course the ids. What to put from them in the MyFirstDTO and MySecondDTO classes?

2. If there is an inheritance between the entities, then how should I represent it in the DTOs? For example:

   @Entity
   public class MyFirstEntity extends MySecondEntity {
       ....
   }
   
   @Entity
   public class MyFirstDTO extends MySecondDTO {
       ....
   }
   

3. How should I use them? For example, I find out this: I'm working on a web project. The user of the webpage wants to register. He/She fills the forms, and sends it to the server. On the server side I create first a DTO, because its fields have the validations. From the DTO I create an Entity and persist it to the database. When there is a request for an entity, I convert the requested entity to DTO, and give it to the user on the client side. Is it a good imagination, or not?

Answer 1

Short answer:

• Entities may be part of a business domain. Thus, they can implement behavior and be applied to different use cases within the domain.

• DTOs are used only to transfer data from one process or context to another. As such, they are without behavior - except for very basic and usually standardised storage and retrieval functions.

Long answer:

While the term "Data Transfer Object" (DTO) is defined quite unambiguously, the term "Entity" is interpreted differently in various contexts.

The most relevant interpretations of the term "Entity", in my opinion, are the following three:

1. In the context of enterprise java and jpa:
   "An object that represents persistent data maintained in a database."

2. In the context of "domain-driven-design" (by Eric Evans):
   "An object defined primarily by its identity, rather than its attributes."

3. In the context of "clean architecture" (by Robert C. Martin):
   "An object that encapsulates enterprise-wide critical business rules."

The Jee- and Jpa-community sees entities primarily as objects mapped to a database table. This point of view is very close to the definition of a DTO - and that's where much of the confusion probably stems from.

In the context of domain-driven-design, as well as Robert Martins point of view, however, Entities are part of a business domain and thus can and should implement behavior.