Request validation preview in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a Spring Boot controller receives invalid input with @Valid?

Consider a Spring Boot REST controller method annotated with @Valid on a request body. What is the default behavior when the input fails validation?

Spring Boot

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

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

A Spring Boot automatically returns a 400 Bad Request with a validation error message without entering the controller method.

B The controller method runs and returns a 200 OK with "User created" regardless of validation errors.

C The controller method throws a NullPointerException if validation fails.

D The controller method returns a 500 Internal Server Error when validation fails.

💡 Hint

Think about how Spring Boot handles validation exceptions by default.

When @Valid detects invalid input, Spring Boot triggers a validation exception before the controller method executes. This results in a 400 Bad Request response with details about the validation errors.

📝 Syntax

intermediate

Which code snippet correctly applies validation annotations to a Spring Boot request DTO?

Choose the code snippet that correctly uses validation annotations on a Spring Boot record used as a request DTO.

A public record ProductRequest(@NotNull String name, @Size(min=1) int quantity) {}

B public record ProductRequest(@NotBlank int name, @Min(1) String quantity) {}

C public record ProductRequest(@Min(1) String name, @NotEmpty int quantity) {}

D public record ProductRequest(@NotEmpty String name, @Positive int quantity) {}

💡 Hint

Remember which annotations apply to which data types.

@NotEmpty and @NotBlank apply to Strings or collections, while @Positive and @Min apply to numbers. Option D correctly applies @NotEmpty to a String and @Positive to an int.

🔧 Debug

advanced

Why does this Spring Boot controller not return validation errors as expected?

Given the controller method below, why might validation errors not trigger a 400 response?

Spring Boot

public record OrderRequest(@NotNull String productId, @Min(1) int quantity) {}

@PostMapping("/orders")
public ResponseEntity<String> placeOrder(OrderRequest order) {
    return ResponseEntity.ok("Order placed");
}

A The <code>@Min</code> annotation cannot be used on primitive int types.

B The <code>OrderRequest</code> record must be a class, not a record, for validation to work.

C The <code>@RequestBody</code> annotation is missing, so Spring does not bind the JSON to the object and skip validation.

D The <code>@Valid</code> annotation is missing on the <code>OrderRequest</code> parameter, so validation is not triggered.

💡 Hint

Think about how Spring binds HTTP request data to method parameters.

Without @RequestBody, Spring does not deserialize the JSON request body into the OrderRequest object, so validation annotations are not processed. Adding @RequestBody and @Valid triggers validation.

❓ state_output

advanced

What is the HTTP response status and body when validation fails in this Spring Boot controller?

Given the controller method below, what is the HTTP status code and response body when the client sends {"name":"", "age":15}?

Spring Boot

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

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

A Status 400 Bad Request with a JSON body listing 'name' must not be blank and 'age' must be at least 18.

B Status 200 OK with body 'User created'.

C Status 500 Internal Server Error with empty body.

D Status 422 Unprocessable Entity with plain text error message.

💡 Hint

Recall the default error response Spring Boot sends on validation failure.

Spring Boot returns a 400 Bad Request with a JSON body describing each validation error, including that 'name' must not be blank and 'age' must be at least 18.

🧠 Conceptual

expert

How can you customize the validation error response in Spring Boot globally?

Which approach allows you to globally customize the JSON structure returned when validation errors occur in a Spring Boot REST API?

A Add a custom message attribute to each validation annotation on the DTO fields.

B Implement a @ControllerAdvice class with an @ExceptionHandler for MethodArgumentNotValidException to build a custom response body.

C Override the toString() method of the request DTO to return custom error messages.

D Use @ResponseStatus on the controller method to change the HTTP status code.

💡 Hint

Think about how Spring Boot handles exceptions globally.

Using a @ControllerAdvice with an @ExceptionHandler for MethodArgumentNotValidException lets you intercept validation errors globally and customize the JSON response structure.