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Introduction

Validation is one of the most important aspects of enterprise applications.

It ensures that only valid and consistent data enters the system before any business logic is executed.

Without validation, applications may suffer from:

• Invalid customer data
• Database inconsistencies
• Security vulnerabilities
• Business rule violations
• Runtime exceptions

Spring Boot integrates with Jakarta Bean Validation (Jakarta Validation API) and Hibernate Validator to provide declarative validation.

Validation is commonly applied to:

• REST APIs
• DTOs
• Request Parameters
• Path Variables
• Service Methods
• Configuration Properties

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Validation Architecture

flowchart LR

Client --> Controller

Controller --> Validation

Validation --> Success

Validation --> ValidationException

Success --> Service

ValidationException --> GlobalExceptionHandler

GlobalExceptionHandler --> ErrorResponse

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Q1. What is Bean Validation?

Answer

Bean Validation is the standard Java validation specification (Jakarta Validation).

Spring Boot uses Hibernate Validator as its default implementation.

Dependency

<dependency>

<groupId>
org.springframework.boot
</groupId>

<artifactId>
spring-boot-starter-validation
</artifactId>

</dependency>

Benefits

• Declarative validation
• Less boilerplate
• Standard annotations
• Automatic request validation

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Q2. What is @Valid?

@Valid triggers validation for request objects.

Example

@PostMapping

public Customer save(

@Valid

@RequestBody

CustomerRequest request){

}

Flow

flowchart TD

Request --> Validation

Validation --> Success

Validation --> MethodArgumentNotValidException

Success --> Controller

If validation fails,

Spring throws MethodArgumentNotValidException.

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Q3. What is @Validated?

@Validated is a Spring annotation that supports:

• Validation Groups
• Method Validation
• Class-level validation

Example

@Service

@Validated

public class CustomerService {

}

Difference

@Valid	@Validated
Jakarta Validation	Spring Validation
No groups	Supports validation groups
Common for DTOs	Common for services

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Q4. What are commonly used validation annotations?

Annotation	Purpose
@NotNull	Cannot be null
@NotBlank	Non-empty string
@NotEmpty	Collection/String not empty
@Size	Length validation
@Min	Minimum value
@Max	Maximum value
@Positive	Positive number
@Email	Email validation
@Pattern	Regex validation
@Past	Date must be in past
@Future	Date must be in future

Example

@NotBlank

private String name;

@Email

private String email;

@Min(18)

private int age;

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Q5. How do you validate nested objects?

Use @Valid on nested fields.

Example

public class Customer{

    @Valid

    private Address address;

}

Nested Validation

flowchart LR

Customer --> Address

Address --> Validation

Validation --> Success

Validation cascades to nested objects automatically.

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Q6. What are Validation Groups?

Validation Groups apply different rules in different scenarios.

Example

public interface Create{

}

public interface Update{

}

Usage

@NotNull(

groups = Update.class)

Typical scenarios

• Create
• Update
• Admin Operations

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Q7. How do you create a Custom Validator?

Step 1

Create annotation

@Target(

FIELD)

@Retention(

RUNTIME)

public @interface

ValidAccount{

}

Step 2

Implement validator

public class

AccountValidator

implements

ConstraintValidator

Typical use cases

• Account Number
• PAN Number
• IFSC Code
• Aadhaar
• Business Rules

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Q8. How are Validation Errors handled?

Validation failures throw

MethodArgumentNotValidException

Global Handler

@ExceptionHandler(

MethodArgumentNotValidException.class)

Flow

flowchart TD

Request --> Validation

Validation --> Exception

Exception --> GlobalExceptionHandler

GlobalExceptionHandler --> JSONError

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Q9. How do you validate Path Variables and Request Parameters?

Example

@GetMapping(

"/customers/{id}")

public Customer find(

@PathVariable

@Positive

Long id){

}

Example

@RequestParam

@NotBlank

String name

Enable method validation

@Validated

@RestController

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Q10. Validation Best Practices

Validate DTOs

Avoid validating entity classes directly.

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Use Bean Validation

Avoid manual if validations.

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Create Custom Validators

For business-specific rules.

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Return Standard Error Responses

Use ProblemDetail or a consistent API error model.

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Keep Validation Close to Input

Validate at API boundaries before invoking business logic.

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Banking Example

flowchart TD

TransferRequest --> Validation

Validation --> Success

Validation --> ErrorResponse

Success --> TransferService

TransferService --> Database

Only validated requests reach the service layer.

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Common Interview Questions

• What is Bean Validation?
• What is @Valid?
• What is @Validated?
• @Valid vs @Validated?
• What are Validation Groups?
• How do you validate nested objects?
• How do you create custom validators?
• How are validation exceptions handled?
• How do you validate path variables?
• Validation best practices?

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Quick Revision

Topic	Summary
Bean Validation	Jakarta Validation API
Hibernate Validator	Default implementation
@Valid	Validate request object
@Validated	Spring validation + groups
Validation Groups	Different rules per scenario
Custom Validator	Business validation
Nested Validation	Cascade using @Valid
MethodArgumentNotValidException	Validation failure
ProblemDetail	Standard validation error response
DTO Validation	Recommended practice

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Validation Lifecycle

sequenceDiagram
Client->>Controller: HTTP Request
Controller->>Validation Engine: Validate DTO
Validation Engine-->>Controller: Success
Controller->>Service: Business Logic
Service-->>Controller: Response
Controller-->>Client: Success Response

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Production Example – Banking Account Registration

A customer submits a new account registration request.

Validation Rules

• Name must not be blank.
• Email must be valid.
• Age must be at least 18.
• Mobile number must match the organization's format.
• Address object is validated recursively.
• PAN number is validated using a custom validator.

If validation fails

• Spring throws MethodArgumentNotValidException.
• @RestControllerAdvice catches the exception.
• A ProblemDetail response is returned.
• The service layer is never invoked.

flowchart LR

MobileApp --> RegistrationAPI

RegistrationAPI --> BeanValidation

BeanValidation --> Success

BeanValidation --> ValidationError

Success --> RegistrationService

ValidationError --> GlobalExceptionHandler

GlobalExceptionHandler --> ProblemDetail

This approach prevents invalid data from entering the business layer and ensures consistent error handling.

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Key Takeaways

• Spring Boot uses Jakarta Bean Validation with Hibernate Validator as the default implementation.
• @Valid validates request objects, while @Validated adds support for validation groups and method-level validation.
• Standard validation annotations such as @NotBlank, @Email, @Size, and @Positive simplify input validation.
• Nested objects can be validated recursively using @Valid.
• Validation Groups allow different rules for operations such as create, update, and administrative workflows.
• Custom validators support organization-specific business rules such as account numbers or regulatory identifiers.
• Validation failures should be handled centrally using @RestControllerAdvice and ProblemDetail.
• Validate DTOs at API boundaries to improve application security, maintainability, and data integrity.