DTO validation in Spring Boot - Step-by-Step Execution

Concept Flow - DTO validation

Receive HTTP Request

↓

Bind data to DTO object

↓

Validate DTO fields

↓

Process request

↓

Send response

The flow shows how Spring Boot receives data, binds it to a DTO, validates it, and either processes or returns errors.

Execution Sample

Spring Boot

import jakarta.validation.constraints.Email;
import jakarta.validation.constraints.NotBlank;

public record UserDTO(
  @NotBlank String name,
  @Email String email
) {}

@PostMapping("/users")
public ResponseEntity<String> createUser(@Valid @RequestBody UserDTO user) {
  return ResponseEntity.ok("User created");
}

This code defines a UserDTO with validation annotations and a controller method that validates the DTO on request.

Execution Table

Step	Action	DTO Field	Validation Check	Result	Next Step
1	Receive HTTP POST /users	-	-	-	Bind data to UserDTO
2	Bind JSON to UserDTO	name	Check @NotBlank	Pass if not empty	Validate next field
3	Bind JSON to UserDTO	email	Check @Email format	Pass if valid email	Validation complete
4	Validation result	-	-	All fields valid	Call createUser method
5	Process request	-	-	Return 200 OK with message	Send response
6	If validation fails	-	-	Return 400 Bad Request with errors	Send error response

💡 Validation stops request processing if any field fails; otherwise, controller method runs.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
user.name	null	"Alice"	"Alice"	"Alice"
user.email	null	null	"[e-mail]"	"[e-mail]"
validationErrors	empty	empty	empty	empty or error list

Key Moments - 3 Insights

Why does the request fail if the 'name' field is empty?

Because @NotBlank on 'name' requires a non-empty string. Execution table step 2 shows validation fails if empty, stopping processing.

What happens if the 'email' field is not a valid email format?

Step 3 in the execution table shows @Email validation fails, causing the request to return a 400 error before the controller method runs.

How does Spring Boot know to validate the DTO automatically?

The @Valid annotation on the controller method parameter triggers validation after binding, as shown between steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the result of validation at step 3 if the email is 'not-an-email'?

A Pass validation

B Fail validation

C Skip validation

D Partial validation

💡 Hint

Check step 3 in the execution table where @Email validation is performed.

At which step does the controller method 'createUser' get called?

A Step 4

B Step 2

C Step 5

D Step 6

💡 Hint

Look for the step where the request is processed after validation passes.

If the 'name' field is empty, how does the variable 'validationErrors' change?

A Contains error for 'email'

B Remains empty

C Contains error for 'name'

D Clears all errors

💡 Hint

Refer to variable_tracker and key_moments about validation failure on 'name'.

Concept Snapshot

DTO validation in Spring Boot:
- Use @Valid on controller method parameter
- Annotate DTO fields with validation annotations like @NotBlank, @Email
- Spring binds JSON to DTO, then validates automatically
- If validation fails, returns 400 error with messages
- If valid, controller method executes normally

Full Transcript

In Spring Boot, when a request comes in, the JSON data is bound to a DTO object. The DTO fields have validation annotations like @NotBlank and @Email. The @Valid annotation on the controller method parameter triggers automatic validation after binding. Each field is checked in order. If all fields pass, the controller method runs and returns success. If any field fails, Spring Boot stops processing and returns a 400 Bad Request with error details. This flow ensures data is correct before business logic runs.