Validation

Validation is a critical part of building reliable and secure REST APIs. It ensures that the data sent by the client is correct, complete, and safe before the application processes or stores it.

Without proper validation, invalid or malformed input can cause runtime errors, corrupt the database, or expose security vulnerabilities.

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What is Validation?

Validation is the process of verifying that incoming client data meets predefined rules before it is processed by the server.

Validation helps to:

• Ensure data correctness
• Prevent invalid or malicious input
• Avoid runtime errors
• Protect database and business logic
• Improve API reliability and stability

Simple Examples:

• Email must be valid
• Name must not be blank
• Age must be positive

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How Validation Works in Spring Boot

Spring Boot provides built-in validation support using Bean Validation (JSR 380), typically implemented via Hibernate Validator.

Validation Flow

ClientJSON
   ↓
@RequestBody
   ↓
@Valid
   ↓
BeanValidationAnnotations
   ↓
✔ControllerLogic
❌400BadRequest

How it works internally:

• Validation rules are defined using annotations on DTO or model fields
• @Valid triggers validation in the controller
• If validation fails, Spring automatically returns 400 Bad Request
• No manual if-else checks are required

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Common Validation Annotations

Annotation	Description
@NotNull	Field must not be null
@NotBlank	String must not be null, empty, or whitespace
@Size(min, max)	Length constraints
@Email	Valid email format
@Min(value)	Minimum numeric value
@Max(value)	Maximum numeric value
@Positive	Must be a positive number
@Pattern(regex)	Must match a regex
@Past / @Future	Date validation

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Practical Example: User Registration API

We are creating a User Registration API.

The client sends user details, and the server must validate the input before saving.

User DTO with Validation Rules

publicclassUserDTO {

@NotBlank(message = "Name is required")
private String name;

@Email(message = "Invalid email address")
private String email;

@Min(value = 18, message = "Age must be at least 18")
privateint age;

// getters and setters
}

What happens here?

• @NotBlank → Name cannot be empty
• @Email → Email format must be valid
• @Min(18) → Age must be 18 or above

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Controller Using @Valid

@RestController
@RequestMapping("/users")
publicclassUserController {

@PostMapping("/register")
public ResponseEntity<String>registerUser(
@Valid@RequestBody UserDTO userDTO) {

// Executes only if validation passes
return ResponseEntity
                .status(201)
                .body("User registered successfully");
    }
}

What happens here?

• @RequestBody → Converts JSON into Java object
• @Valid → Applies validation rules
• Validation success → Controller executes
• Validation failure → 400 Bad Request

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Valid Request (Success Case)

{
"name":"Alice",
"email":"[email protected]",
"age":25
}

Response

HTTP/1.1201 Created
User registered successfully

The request data satisfies all validation rules, so the controller method executes successfully.

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Invalid Request (Failure Case)

{
"name":"",
"email":"alice.com",
"age":10
}

Automatic Error Response (400 Bad Request)

{
"timestamp":"2026-01-01T12:00:00",
"status":400,
"errors":[
"Name is required",
"Invalid email address",
"Age must be at least 18"
]
}

What Spring Boot Does Automatically

• Detects invalid request data
• Applies validation rules using @Valid
• Stops controller execution
• Returns 400 Bad Request with validation error messages

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Handling Validation Errors (Advanced)

For better control and standardized error responses, you can use:

• @ControllerAdvice
• @ExceptionHandler(MethodArgumentNotValidException.class)
• Custom error response objects

This improves API usability and developer experience, especially in large applications.

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Best Practices

• Always validate client input
• Use DTOs, not entities, for validation
• Combine @Valid with @RequestBody
• Use ResponseEntity for proper HTTP status codes
• Provide clear and meaningful error messages
• Never rely only on client-side validation

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Conclusion

Validation in Spring Boot REST APIs ensures that your application receives clean, safe, and predictable data.

By using Bean Validation annotations with @Valid, Spring Boot automatically handles input validation and error responses, making your APIs robust, secure, and production-ready.