Validation error responses in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when a Spring Boot controller receives invalid input?

Consider a Spring Boot REST controller method annotated with @Valid on its input DTO. What will the client receive if the input fails validation?

Spring Boot

public record UserDTO(@NotBlank String name, @Min(18) int age) {}

@PostMapping("/users")
public ResponseEntity<String> createUser(@Valid @RequestBody UserDTO user) {
    return ResponseEntity.ok("User created");
}

A HTTP 500 Internal Server Error due to validation failure

B HTTP 200 OK with an empty response body

C HTTP 400 Bad Request with a JSON body listing validation errors

D HTTP 404 Not Found because the resource is missing

💡 Hint

Think about how Spring Boot handles @Valid and validation exceptions by default.

Spring Boot automatically returns HTTP 400 Bad Request with details about validation errors when @Valid fails.

📝 Syntax

intermediate

Which code snippet correctly customizes validation error responses in Spring Boot?

You want to return a custom JSON structure for validation errors. Which code snippet correctly implements this using @ControllerAdvice?

A

@ControllerAdvice
public class ValidationHandler {
  public ResponseEntity<String> handleValidation(MethodArgumentNotValidException ex) {
    return ResponseEntity.badRequest().body("Error");
  }
}

B

@ControllerAdvice
public class ValidationHandler {
  @ExceptionHandler(MethodArgumentNotValidException.class)
  public ResponseEntity<Map<String, String>> handleValidation(MethodArgumentNotValidException ex) {
    Map<String, String> errors = new HashMap<>();
    ex.getBindingResult().getFieldErrors().forEach(error ->
      errors.put(error.getField(), error.getDefaultMessage()));
    return ResponseEntity.badRequest().body(errors);
  }
}

C

@RestController
public class ValidationHandler {
  @ExceptionHandler(MethodArgumentNotValidException.class)
  public String handleValidation() {
    return "Validation failed";
  }
}

D

@ControllerAdvice
public class ValidationHandler {
  @ExceptionHandler(Exception.class)
  public ResponseEntity<String> handleAll(Exception ex) {
    return ResponseEntity.status(500).body("Server error");
  }
}

💡 Hint

Look for the method signature that catches the right exception and returns a detailed map.

Option B correctly uses @ExceptionHandler for MethodArgumentNotValidException and builds a map of field errors.

🔧 Debug

advanced

Why does this Spring Boot validation error handler not work as expected?

Given this code, why does the custom validation error handler never get called?

Spring Boot

@ControllerAdvice
public class CustomValidationHandler {
  @ExceptionHandler(BindException.class)
  public ResponseEntity<String> handleBind(BindException ex) {
    return ResponseEntity.badRequest().body("Bind error");
  }
}

@PostMapping("/submit")
public ResponseEntity<String> submit(@Valid @RequestBody UserDTO user) {
  return ResponseEntity.ok("Success");
}

A The exception thrown is MethodArgumentNotValidException, not BindException, so the handler is not triggered

B The @ControllerAdvice class is missing @RestController annotation

C The @Valid annotation should be on the controller class, not the method parameter

D The handler method must return void to work correctly

💡 Hint

Check which exception Spring Boot throws for @Valid on @RequestBody parameters.

Spring Boot throws MethodArgumentNotValidException for @Valid on @RequestBody, so the handler for BindException is never called.

❓ state_output

advanced

What is the JSON response body when a validation error occurs with this handler?

Given this handler, what JSON will the client receive if the 'email' field is empty?

Spring Boot

@ControllerAdvice
public class ValidationErrorHandler {
  @ExceptionHandler(MethodArgumentNotValidException.class)
  public ResponseEntity<Map<String, String>> handleValidationErrors(MethodArgumentNotValidException ex) {
    Map<String, String> errors = new HashMap<>();
    ex.getBindingResult().getFieldErrors().forEach(error ->
      errors.put(error.getField(), error.getDefaultMessage()));
    return ResponseEntity.badRequest().body(errors);
  }
}

// UserDTO with @NotBlank on 'email' field

A {"email":null}

B {"error":"Validation failed"}

C {"message":"Invalid input"}

D {"email":"must not be blank"}

💡 Hint

The handler builds a map with field names and their error messages.

The handler returns a JSON object with the field name as key and the validation message as value.

🧠 Conceptual

expert

Which statement about Spring Boot validation error handling is true?

Select the correct statement about how Spring Boot handles validation errors by default and how to customize them.

A Spring Boot automatically returns HTTP 400 with a default error body on validation failure; customizing requires @ControllerAdvice with @ExceptionHandler for MethodArgumentNotValidException

B Spring Boot returns HTTP 500 on validation errors unless you add @ResponseStatus on the DTO class

C Validation errors are ignored by default and must be manually checked in controller methods

D You must implement HandlerInterceptor to catch validation errors globally

💡 Hint

Think about default behavior and how to override it cleanly.

Spring Boot returns HTTP 400 with a default error response for validation failures. To customize, use @ControllerAdvice and handle MethodArgumentNotValidException.