Why use request and response DTOs in a Spring API?

Keeps JSON binding, validation, writable fields, and response evolution separate from domain and persistence models.

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A request DTO defines exactly which fields a client may send and where transport validation applies. A response DTO defines the representation the API promises without exposing internal entity shape, lazy relationships, or implementation-only fields. Separate create and update DTOs prevent writable fields from being inferred from a persistence model. The controller maps transport data to an application operation and maps the result back to the response contract.

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Overposting prevention, serialization stability, validation ownership, mapping cost, and contract evolution.

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A DTO is the API's explicit contract, not a duplicate class created by habit.
A request DTO defines exactly which fields a client may send and where transport validation applies. A response DTO defines the representation the API promises without exposing internal entity shape, lazy relationships, or implementation-only fields. Separate create and update DTOs prevent writable fields from being inferred from a persistence model. The controller maps transport data to an application operation and maps the result back to the response contract.
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Binding a JPA entity directly can enable overposting, expose bidirectional graphs, trigger lazy-loading surprises, and couple schema refactors to clients. DTOs also let create, patch, list, and detail operations have different shapes. I avoid one universal DTO that accumulates nullable fields for every use case. Mapping can be manual or generated, but important transformations and defaults remain explicit and tested.

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Returning persistence entities directly.
Using one nullable DTO for every operation.
Letting an automatic mapper hide business defaults.

Why use request and response DTOs in a Spring API?

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