DTO vs Entity: Why You Should Separate Concerns

• The Data Transfer Object is a design pattern used to transfer data across different layers or components of an application. It is a simple, flat, serializable object that contains only fields and accessors-appropriately getters and setters-without business logic. The main role of DTOs is to enable the effective encapsulation and transfer of data.

• Clean and maintainable code is the key to modern software development for scalable and efficient applications. In any ordinary application, two very commonly used concepts, Data Transfer Objects and Entities, play an important role in segregating the concerns within an application. This article will explain the differences between Data Transfer Objects and Entities, why separating their responsibilities is important, and how to implement them.

What is an Entity?

An Entity represents a real-world object or concept in the domain of your application. It directly maps onto the database table and contains the business logic associated with that object. These are typically part of a persistence layer and are managed by an ORM framework like Hibernate or JPA in Java.

@Entity
@Data
@Builder
@AllArgsConstructor
@NoArgsConstructor
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;
    private String email;

}

Key Characteristics of Entities:

• Mapped directly to a database table.
• Contains both data and business logic.
• Changes in the database structure can directly impact entities.

What is a DTO?

A Data Transfer Object, or DTO, is just a simple, flat object used to transfer data between different layers of an application-from the backend to the frontend, for example. It is designed to carry data but not business logic. They are used to shape the data according to the specific requirements of a certain use case or API endpoint.

@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class UserDTO {

    @NotNull(message = "Name is required")
    private String name;
    @NotNull(message = "Email is required")
    private String email;

}

Key Characteristics of DTOs:

• Do not map directly to database tables.
• Used for transferring data, especially between layers or across networks (e.g., APIs).
• Contains only fields, getters, and setters (no business logic).

Why Separate DTOs and Entities?

1. Encapsulation and Security
Entities often contain sensitive information-for example, password, internal IDs-that you do not want to expose to external clients or other layers of the application.
Using DTOs enables you to control what data gets exposed and the structure of the data.
2. Decoupling Layers
DTO is a bridge between layers like service layer and API layer, that decouples each of them; in case some layer faces changes, for example, database schema changes, the other layer wouldn't be affected.
Assume there's some kind of change in your database structure, your DTO might remain unchanged because of that it saves your API from breaking changes.
3. Use Case Customization
DTOs allow the developer to adapt the structure of the data to his needs. For instance, you might want to return only the name and email fields for a user, while you would exclude fields like id and password from the return.
4. Performance Optimization
By including only the necessary fields, DTOs reduce the size of the data transferred over the network, improving performance in distributed systems, such as REST APIs.
5. Testability and Maintainability
DTOs make unit testing more straightforward and easy since they are simple and independent of any persistence layer dependencies.
DTOs also make the code base more maintainable due to a separation of concerns.
6. Avoids Tight Coupling
The approach of using entities directly inside APIs tightly couples your persistence model with your API contract. Changes in the entity structure may inadvertently break API consumers.

Service Layer with Mapping

public class UserMapper {
    public static UserDTO toDTO(User user) {
        UserDTO dto = new UserDTO();
        dto.setName(user.getName());
        dto.setEmail(user.getEmail());
        return dto;
    }

    public static User toEntity(UserDTO dto) {
        User user = new User();
        user.setName(dto.getName());
        user.setEmail(dto.getEmail());
        return user;
    }
}

Service Example:

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public UserDTO getUserById(Long id) {
        User user = userRepository.findById(id).orElseThrow(() -> new RuntimeException("User not found"));
        return UserMapper.toDTO(user);
    }
}

Best Practices of Using DTOs and Entities

Never Expose the Entities in APIs:
Always map the entities to DTOs before returning data to the external client.

Using Libraries for Mapping:
These tools, like MapStruct or ModelMapper, enable ease of mapping between DTOs and Entities and reduce boilerplate code.

Keep DTOs Simple:
Don't add business logic or complicated behavior to your DTOs.

Use Validation in DTOs:
Let validation of the incoming data at DTO level be done with the use of annotations such as @Size and @notblank beforehand and map it to an entity.

Create Different DTOs for Different Use Cases:
You could have a UserResponseDTO for API responses, a UserCreateRequestDTO in cases of handling user creation requests.

Summary of DTO

The separation of DTOs from entities keeps the code base clean and maintainable, decouples the concerns, enhances security, and ensures performance. While entities will care about the business logic and database interaction logic, DTOs know only about the data transfer between layers or systems. The separation of these elements follows best practices in software design and protects your application from unexpected side effects of tightly coupled components.