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

Validation error response formatting

📖 Scenario: You are building a Spring Boot REST API for user registration. You want to ensure that when users send invalid data, the API returns a clear, structured error response showing which fields failed validation and why.

🎯 Goal: Create a Spring Boot controller that validates a user registration request and returns a formatted JSON error response listing all validation errors with field names and messages.

📋 What You'll Learn

Create a User DTO with validation annotations

Add a controller method to accept User data

Configure a global exception handler to catch validation errors

Format the validation error response as JSON with field names and error messages

💡 Why This Matters

🌍 Real World

APIs often need to validate user input and return clear error messages so clients can fix mistakes easily.

💼 Career

Backend developers must handle validation and error formatting to build robust, user-friendly APIs.

1

Create User DTO with validation annotations

Create a class called User with two fields: String username and String email. Add validation annotations @NotBlank on username and @Email on email.

Spring Boot

// Create User class with fields and validation annotations
// Your code here

Need a hint?

Use @NotBlank on username and @Email on email fields.

2

Add controller method to accept User data

Create a Spring REST controller class called UserController. Add a method registerUser annotated with @PostMapping("/register") that accepts a @Valid @RequestBody User user parameter and returns ResponseEntity<String> with body "User registered".

Spring Boot

import org.springframework.http.ResponseEntity;
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;

// Create UserController class with registerUser method
// Your code here

Need a hint?

Use @RestController and @PostMapping("/register"). Add @Valid and @RequestBody to the User parameter.

3

Create global exception handler for validation errors

Create a class called GlobalExceptionHandler annotated with @RestControllerAdvice. Add a method handleValidationExceptions annotated with @ExceptionHandler(MethodArgumentNotValidException.class) that takes MethodArgumentNotValidException ex and returns a Map<String, String> mapping field names to error messages.

Spring Boot

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

// Create GlobalExceptionHandler class with handleValidationExceptions method
// Your code here

Need a hint?

Use @RestControllerAdvice and @ExceptionHandler to catch validation exceptions. Extract field errors and map field names to messages.

4

Return formatted JSON error response

Ensure the handleValidationExceptions method returns a Map<String, String> where keys are field names and values are validation error messages. This map will be serialized as JSON in the response.

Spring Boot

// Confirm the error map is returned for JSON serialization
// Your code here

Need a hint?

Return the map of errors directly so Spring Boot serializes it as JSON in the response.