Custom validator annotation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when validating a field with a custom annotation?

Consider a Spring Boot application with a custom annotation @StartsWithA that validates if a string starts with the letter 'A'. What happens when a field annotated with @StartsWithA contains the value "Apple"?

Spring Boot

public class Fruit {
    @StartsWithA
    private String name;

    // constructor, getters, setters
}

// Assume @StartsWithA is correctly implemented to check if the string starts with 'A'.

A Validation passes because "Apple" starts with 'A'.

B Validation fails because custom annotations require manual validation calls.

C Validation fails because "Apple" is not exactly 'A'.

D Validation passes only if the field is public.

💡 Hint

Think about what the custom validator checks and the value given.

The custom validator checks if the string starts with 'A'. Since "Apple" starts with 'A', validation passes.

📝 Syntax

intermediate

Which option correctly defines a custom validator annotation in Spring Boot?

You want to create a custom annotation @StartsWithA to validate strings starting with 'A'. Which option shows the correct annotation definition?

A

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = StartsWithAValidator.class)
public class StartsWithA {
    String message() default "Must start with A";
}

B

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.CLASS)
@Constraint(validatedBy = StartsWithAValidator.class)
public @interface StartsWithA {
    String message() default "Must start with A";
}

C

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface StartsWithA {
    String message() default "Must start with A";
}

D

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = StartsWithAValidator.class)
public @interface StartsWithA {
    String message() default "Must start with A";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

💡 Hint

Remember that custom annotations for validation must specify target, retention, and the validator class.

Option D correctly uses @Target, @Retention, and @Constraint with validatedBy, and defines required elements for a validation annotation.

🔧 Debug

advanced

Why does the custom validator not trigger during validation?

Given a custom annotation and validator, the validation does not trigger when saving an entity. What is the most likely cause?

Spring Boot

public class User {
    @StartsWithA
    private String username;

    // getters and setters
}

// Validator class and annotation are implemented correctly.

A The annotation is missing the @Documented meta-annotation.

B The field username is private and needs to be public for validation.

C The validator class is not registered as a Spring bean or not referenced correctly in the annotation.

D The validation only works on controller methods, not on entity fields.

💡 Hint

Check how Spring Boot finds and uses validator classes.

If the validator class is not properly linked or registered, Spring Boot won't run it during validation.

❓ state_output

advanced

What is the validation error message when the input fails the custom validator?

If a field annotated with @StartsWithA(message = "Must start with A letter") contains "Banana", what error message will be shown?

A Must start with A

B Must start with A letter

C Validation failed for field username

D Banana is invalid

💡 Hint

Look at the message attribute in the annotation.

The message attribute in the annotation overrides the default message and is shown on validation failure.

🧠 Conceptual

expert

Why use custom validator annotations instead of manual validation in Spring Boot?

Which is the best reason to create a custom validator annotation rather than manually checking values in service methods?

A Custom annotations integrate with Spring's validation framework, enabling automatic checks and error reporting.

B Manual validation is faster and more reliable than annotations.

C Custom annotations require less code but cannot be reused.

D Manual validation is the only way to validate complex objects.

💡 Hint

Think about integration and reusability benefits.

Custom validator annotations work with Spring's validation system, allowing automatic validation and consistent error handling across the app.