Article 10: Request Validation in Spring Boot

1. Introduction

APIs should never trust input.

Without validation:

• Invalid data enters the system
• Databases get corrupted
• Bugs appear far away from the source

Spring Boot provides Bean Validation to validate requests cleanly and declaratively.

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2. What Is Bean Validation?

Bean Validation:

• Uses annotations to define constraints
• Automatically validates request bodies
• Returns meaningful error responses

Spring Boot integrates Bean Validation seamlessly.

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3. Adding Validation Dependency

If you are using spring-boot-starter-web, validation is usually included. If not, add:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-validation</artifactId>
</dependency>

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4. Validating Request Body with @Valid

4.1 Update Entity / DTO

package com.example.demo.dto;

import jakarta.validation.constraints.*;

public class UserRequest {

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

    @Email(message = "Email should be valid")
    @NotBlank(message = "Email is mandatory")
    private String email;

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

    // getters and setters
}

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4.2 Use @Valid in Controller

@PostMapping("/users")
public User createUser(@Valid @RequestBody UserRequest request) {
    return userService.createUser(request);
}

Spring automatically:

• Validates request
• Stops execution if validation fails

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5. Common Validation Annotations

Annotation	Purpose
@NotNull	Value must not be null
@NotBlank	Non-empty string
@Size	Length constraint
@Email	Email format
@Min, @Max	Numeric limits

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6. Validation Error Response (Default)

If validation fails, Spring returns:

• HTTP 400 (Bad Request)
• Field-specific error messages

Example:

{
  "errors": [
    "Email should be valid",
    "Name is mandatory"
  ]
}

(Default format may vary)

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7. Validating Path Variables and Request Params

@GetMapping("/users/{id}")
public User getUser(@PathVariable @Min(1) Long id) {
    return userService.getUser(id);
}

Enable method validation:

@Validated
@RestController
public class UserController {
}

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8. Custom Validation Messages

Add to application.properties:

spring.messages.basename=messages

Create:

messages.properties

NotBlank.userRequest.name=Name cannot be empty

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9. Common Beginner Mistakes

❌ Forgetting @Valid
❌ Validating entity instead of DTO
❌ No error handling strategy
❌ Ignoring validation errors

✔ Always validate at API boundary

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10. Best Practices

✔ Use DTOs for validation
✔ Keep entities clean
✔ Fail fast on invalid input
✔ Provide meaningful error messages

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11. What You Should Understand Before Moving On

You should now know:

• Why validation matters
• How @Valid works
• Common validation annotations
• How Spring handles validation errors

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12. What’s Next?

➡ Article 11: Exception Handling the Right Way

• @ExceptionHandler
• @ControllerAdvice
• Standard error responses

Type Next when you’re ready.