Chapter 124: DTO in Java

DTO (Data Transfer Object) is a purpose-built type for crossing a boundary—HTTP JSON, message queue payloads, or batch export rows. It should not carry Hibernate proxies or lazy-loading baggage your clients never asked for.

Java record types are a natural fit for immutable read models and small request bodies.

1. Topic title

DTO: contract-first payload; map early, serialize predictably, version consciously

2. What it means

Request DTO — what @RequestBody deserializes into—validation annotations land here (Chapter 126).

Response DTO — what @RestController returns—hide internal ids, embed computed fields (displayName), omit secrets.

Mapping — ItemEntity → ItemResponse inside ItemService (or a dedicated mapper class) keeps controllers dumb.

3. Why it is used

Decouple database refactors from mobile apps still on old JSON.

Security — no accidental serialization of password hashes or internal flags.

4. Mental sketch

Entity is the kitchen prep table—messy, hot, staff-only. DTO is the plate that leaves the swing door—only what the guest should see.

5. Java code example

public record CreateItemRequest(String name, String note) {
}
 
public record ItemResponse(long id, String name) {
}
 
public final class ItemMapper {
    private ItemMapper() {
    }
 
    public static ItemResponse toResponse(ItemEntity entity) {
        return new ItemResponse(entity.getId(), entity.getName());
    }
}

note might exist on entity but stay absent from ItemResponse if clients do not need it yet—explicit omission is a feature.

6. Explanation of code

Records give equals/hashCode/toString for free—great for DTOs; entities often stay classes for mutability and JPA callbacks.

7. Common mistakes

One mega-DTO reused for create, update, admin, and public views—split types when fields diverge.

Entity graphs as JSON—serialization annotations on entities become coupling tar.

8. Best practices

Name types after role (PublicItemResponse, AdminItemDetail), not only ItemDto.

When API v2 ships, new package or suffix types rather than overloading ItemResponse semantics.

9. Small practice task

Add ItemAdminResponse including note and map it only in an admin controller—keep ItemResponse lean for public GET.

Beginner tip

MapStruct and manual mappers both work—start manual until boilerplate hurts, then automate with compile-time codegen.