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

Concept Flow - Request validation preview

Client sends HTTP request

↓

Spring Boot Controller receives request

↓

Request body mapped to DTO object

↓

Validation annotations checked

↓

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.

Practice

1. What is the main purpose of using @Valid annotation in Spring Boot request handling?

easy

A. To trigger validation of the incoming request data based on annotations in the DTO

B. To automatically log the request data for debugging

C. To convert the request body into a JSON string

D. To disable validation for the request parameters

Solution

1. Step 1: Understand the role of @Valid

   The @Valid annotation tells Spring Boot to check the incoming data against validation rules defined in the DTO class.

2. Step 2: Identify what @Valid does not do

   It does not log data, convert to JSON, or disable validation. Its sole purpose is to trigger validation.

3. Final Answer:

   To trigger validation of the incoming request data based on annotations in the DTO -> Option A

4. Quick Check:

   @Valid triggers validation [OK]

Hint: Remember: @Valid means 'check this data' [OK]

Common Mistakes:

• Thinking @Valid logs or converts data
• Confusing @Valid with JSON serialization
• Assuming @Valid disables validation

2. Which of the following is the correct way to apply validation annotations in a Spring Boot DTO class?

easy

A. public class UserDTO {
@Valid
private String name;
}

B. public class UserDTO {
@RequestBody
private String name;
}

C. public class UserDTO {
@NotNull
private String name;
}

D. public class UserDTO {
@Autowired
private String name;
}

Solution

1. Step 1: Recognize validation annotations

   Annotations like @NotNull are used on fields to define validation rules in DTO classes.

2. Step 2: Identify incorrect annotations

   @Valid is used on method parameters, not fields; @RequestBody and @Autowired are unrelated to field validation.

3. Final Answer:

   public class UserDTO { @NotNull private String name; } -> Option C

4. Quick Check:

   Use validation annotations like @NotNull on fields [OK]

Hint: Validation annotations go on fields, not on parameters [OK]

Common Mistakes:

• Using @Valid on fields instead of method parameters
• Confusing @RequestBody with validation
• Applying @Autowired to DTO fields

3. Given the following DTO and controller method, what will happen if the client sends a request with an empty email field? public class UserDTO { @NotBlank private String email; // getters and setters } @PostMapping("/users") public ResponseEntity<String> createUser(@Valid @RequestBody UserDTO user) { return ResponseEntity.ok("User created"); }

medium

A. The request will be ignored silently

B. The request will succeed and return "User created"

C. The server will throw a NullPointerException

D. The request will fail validation and return a 400 Bad Request with an error message

Solution

1. Step 1: Understand @NotBlank validation

   The @NotBlank annotation requires the field to be non-null and contain at least one non-whitespace character.

2. Step 2: Analyze the empty email input

   An empty string fails @NotBlank validation, so Spring Boot will reject the request and return a 400 error with validation details.

3. Final Answer:

   The request will fail validation and return a 400 Bad Request with an error message -> Option D

4. Quick Check:

   @NotBlank empty input causes 400 error [OK]

Hint: Empty strings fail @NotBlank, causing 400 errors [OK]

Common Mistakes:

• Assuming empty strings pass validation
• Expecting NullPointerException instead of validation error
• Thinking server ignores invalid input

4. Identify the error in this Spring Boot controller method that uses request validation: @PostMapping("/register") public ResponseEntity<String> registerUser(@Valid UserDTO user) { // method body } Assuming UserDTO has validation annotations.

medium

A. Missing @Valid annotation on the method parameter

B. Missing @RequestBody annotation on the method parameter

C. Method should return void instead of ResponseEntity

D. UserDTO should not have validation annotations

Solution

1. Step 1: Check parameter annotations for validation

   To validate request body data, the parameter must be annotated with @RequestBody so Spring can deserialize JSON into the DTO.

2. Step 2: Confirm @Valid usage

   While @Valid triggers validation, without @RequestBody the data won't be bound from the request body, so validation won't run.

3. Final Answer:

   Missing @RequestBody annotation on the method parameter -> Option B

4. Quick Check:

   Use @RequestBody to bind JSON, then @Valid to validate [OK]

Hint: Always use @RequestBody with @Valid for JSON validation [OK]

Common Mistakes:

• Forgetting @RequestBody causes no data binding
• Assuming @Valid alone binds data
• Thinking return type affects validation

5. You want to validate a user registration request with fields username (not blank), age (minimum 18), and email (valid email format). Which combination of annotations in the DTO class is correct?

hard

A. public class UserDTO {
@NotBlank
private String username;
@Min(18)
private int age;
@Email
private String email;
}

B. public class UserDTO {
@NotNull
private String username;
@Max(18)
private int age;
@Pattern(regexp = ".*@.*")
private String email;
}

C. public class UserDTO {
@NotBlank
private String username;
@Min(18)
private String age;
@Email
private String email;
}

D. public class UserDTO {
@NotEmpty
private String username;
@Min(18)
private Integer age;
@Email
private String email;
}

Solution

1. Step 1: Validate username with @NotBlank

   Username must not be null or empty, so @NotBlank is correct.

2. Step 2: Validate age with @Min(18) and correct type

   Age must be at least 18 and an integer type; int with @Min(18) fits.

3. Step 3: Validate email format with @Email

   @Email annotation checks for valid email format.

4. Final Answer:

   public class UserDTO { @NotBlank private String username; @Min(18) private int age; @Email private String email; } -> Option A

5. Quick Check:

   Use @NotBlank, @Min(18), and @Email for these fields [OK]

Hint: Match annotation to field type and validation need [OK]

Common Mistakes:

• Using @Max instead of @Min for age
• Using String type for age with @Min
• Using @NotNull instead of @NotBlank for username