Validation error responses in Spring Boot - Mini Project: Build & Apply

Handling Validation Error Responses in Spring Boot

📖 Scenario: You are building a simple Spring Boot REST API for user registration. You want to ensure that when users send invalid data, the API responds with clear validation error messages.

🎯 Goal: Create a Spring Boot controller that validates user input and returns structured validation error responses.

📋 What You'll Learn

Create a User class with validation annotations

Create a REST controller with a POST endpoint to accept User data

Add a configuration to handle validation errors and return a JSON response with error details

Ensure the error response contains field names and corresponding error messages

💡 Why This Matters

🌍 Real World

APIs often need to validate user input and provide clear error messages so clients can fix their requests.

💼 Career

Understanding validation and error handling is essential for backend developers building reliable and user-friendly APIs.

1

Create User class with validation annotations

Create a class called User with two fields: name of type String and email of type String. Add the annotation @NotBlank on name and @Email on email.

Spring Boot

public class User {
    // Your code here
}

Need a hint?

Use @NotBlank on the name field and @Email on the email field.

2

Create REST controller with POST endpoint

Create a class called UserController annotated with @RestController. Add a POST mapping method called registerUser that accepts a @Valid @RequestBody User user parameter and returns ResponseEntity<String> with body "User registered".

Spring Boot

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

@RestController
public class UserController {
    // Your code here
}

Need a hint?

Use @RestController on the class and @PostMapping("/register") on the method.

3

Create exception handler for validation errors

Create a class called ValidationExceptionHandler annotated with @RestControllerAdvice. Add a method called handleMethodArgumentNotValid that takes MethodArgumentNotValidException ex and returns a Map<String, String> of field errors. Use ex.getBindingResult().getFieldErrors() to collect field names and default messages into the map.

Spring Boot

import org.springframework.web.bind.annotation.RestControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.bind.MethodArgumentNotValidException;
import java.util.Map;
import java.util.HashMap;

@RestControllerAdvice
public class ValidationExceptionHandler {
    // Your code here
}

Need a hint?

Use @RestControllerAdvice and @ExceptionHandler(MethodArgumentNotValidException.class) to catch validation errors.

4

Complete the project with proper imports and class structure

Ensure all classes have the necessary imports and are in the same package or properly imported. Confirm that the User, UserController, and ValidationExceptionHandler classes are complete and ready to run in a Spring Boot application.

Spring Boot

/* All classes and imports are present above. 
   Review and add any missing imports or annotations here. */
// Your code here

Need a hint?

Make sure all classes are in the same package and have all required imports.