Entity to DTO mapping in Spring Boot - Step-by-Step Execution

Concept Flow - Entity to DTO mapping

Start with Entity object

↓

Create DTO instance

↓

Copy data from Entity to DTO

↓

Return DTO for use in API or UI

↓

End

This flow shows how data moves from an Entity object to a DTO by creating a new DTO and copying relevant fields.

Execution Sample

Spring Boot

public class UserEntity {
  private Long id;
  private String name;

  public Long getId() {
    return id;
  }

  public String getName() {
    return name;
  }
}

public class UserDTO {
  private String name;

  public String getName() {
    return name;
  }

  public void setName(String name) {
    this.name = name;
  }
}

public static UserDTO mapToDTO(UserEntity entity) {
  UserDTO dto = new UserDTO();
  dto.setName(entity.getName());
  return dto;
}

This code copies the 'name' field from a UserEntity to a UserDTO, leaving out the 'id' field.

Execution Table

Step	Action	Entity State	DTO State	Result
1	Receive UserEntity with id=1, name='Alice'	id=1, name='Alice'	empty	Ready to map
2	Create new UserDTO instance	id=1, name='Alice'	name=null	DTO created
3	Copy name from Entity to DTO	id=1, name='Alice'	name='Alice'	Name copied
4	Return DTO	id=1, name='Alice'	name='Alice'	DTO ready for use

💡 Mapping complete, DTO returned without id field

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
entity	id=1, name='Alice'	id=1, name='Alice'	id=1, name='Alice'	id=1, name='Alice'
dto	null	name=null	name='Alice'	name='Alice'

Key Moments - 2 Insights

Why doesn't the DTO have the 'id' field from the Entity?

The DTO is designed to expose only needed data, so 'id' is omitted intentionally as shown in step 3 of the execution_table.

What happens if the Entity's 'name' is null?

The DTO's 'name' will also be null after copying, as the mapping copies values directly (see step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what is the DTO's 'name' after step 2?

A empty string

B 'Alice'

C null

D undefined

💡 Hint

Check the 'DTO State' column at step 2 in the execution_table

At which step is the 'name' copied from Entity to DTO?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column describing copying in the execution_table

If the Entity had an extra field 'email', what would happen to the DTO after mapping?

A DTO would have 'email' copied automatically

B DTO would ignore 'email' unless explicitly copied

C Mapping would fail with error

D DTO would have 'email' set to null

💡 Hint

Mapping copies only fields shown in the code example, see execution_sample

Concept Snapshot

Entity to DTO mapping:
- Create a new DTO object
- Copy only needed fields from Entity
- DTO hides sensitive or unnecessary data
- Used to send clean data to UI or API
- Mapping can be manual or with tools like MapStruct

Full Transcript

Entity to DTO mapping means taking data from a database entity object and copying it into a simpler object called a DTO. This DTO only has the fields needed for the user interface or API, hiding others like database IDs. The process starts by receiving an entity, then creating a new DTO instance. Next, the relevant fields like 'name' are copied from the entity to the DTO. Finally, the DTO is returned for use. This keeps data clean and secure. The example showed copying the 'name' field from a UserEntity to a UserDTO. The 'id' field was left out intentionally. If the entity's field is null, the DTO's field will also be null after copying. This mapping can be done manually or with libraries. Understanding this helps keep backend and frontend data separate and safe.