Request validation preview in Spring Boot - Step-by-Step Execution

Concept Flow - Request validation preview

Client sends HTTP request

↓

Spring Boot Controller receives request

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Request body mapped to DTO object

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Validation annotations checked

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Response sent

This flow shows how Spring Boot receives a request, maps it to an object, validates it, and either processes it or returns errors.

Execution Sample

Spring Boot

import jakarta.validation.constraints.Min;
import jakarta.validation.constraints.NotBlank;
import org.springframework.http.ResponseEntity;
import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RestController;
import jakarta.validation.Valid;

@RestController
@Validated
public class UserController {
    public record UserRequest(@NotBlank String name, @Min(18) int age) {}

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

This code defines a request object with validation and a controller method that validates the request before processing.

Execution Table

Step	Action	Input Data	Validation Check	Result	Response
1	Receive HTTP POST /users	{"name":"Alice", "age":20}	Check @NotBlank name	Valid	Proceed
2	Validate age	20	Check @Min(18)	Valid	Proceed
3	All validations passed	-	-	Valid	Return 200 OK with 'User created'
4	Receive HTTP POST /users	{"name":"", "age":17}	Check @NotBlank name	Invalid (blank)	Error
5	Validate age	17	Check @Min(18)	Invalid (less than 18)	Error
6	Validation failed	-	-	Invalid	Return 400 Bad Request with error details

💡 Execution stops when validation fails or all validations pass and controller returns response.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3/6
name	null	"Alice" or ""	"Alice" or ""	"Alice" or ""
age	0	20 or 17	20 or 17	20 or 17
validationResult	null	Valid or Invalid	Valid or Invalid	Valid or Invalid

Key Moments - 2 Insights

Why does the controller method not run when validation fails?

Because Spring Boot intercepts the request after mapping and before the method runs, checking validation annotations. If invalid, it returns errors immediately as shown in steps 4-6.

What happens if the request body is missing a required field?

Validation fails on that field's annotation (like @NotBlank), so the request is rejected before the controller method executes, as seen in the validation checks in the execution table.

Visual Quiz - 3 Questions

Look at the execution table, what is the validation result at step 2 for age=20?

A Invalid (less than 18)

B Valid

C Not checked

D Error

💡 Hint

Check the 'Validation Check' and 'Result' columns at step 2 in the execution table.

At which step does the controller return a 400 Bad Request?

A Step 3

B Step 1

C Step 6

D Step 2

💡 Hint

Look at the 'Response' column for error responses in the execution table.

If the name is blank but age is 25, what happens according to the execution flow?

A Validation fails at name check and returns error

B Validation passes and user is created

C Validation fails at age check

D Controller ignores validation

💡 Hint

Refer to the validation checks for name and age in the execution table and concept flow.

Concept Snapshot

Spring Boot request validation:
- Use @Valid on @RequestBody parameter
- Define validation annotations on DTO fields (e.g., @NotBlank, @Min)
- Spring validates before controller method runs
- If invalid, returns 400 with errors
- If valid, controller executes normally

Full Transcript

In Spring Boot, when a client sends a request, the controller receives it and maps the request body to a data object. Spring then checks validation annotations on that object. If all validations pass, the controller method runs and returns a success response. If any validation fails, Spring stops execution and returns an error response immediately. This process ensures only valid data reaches your business logic.