Custom validator annotation in Spring Boot - Mini Project: Build & Apply

Create a Custom Validator Annotation in Spring Boot

📖 Scenario: You are building a Spring Boot application that needs to validate user input. The default validators do not cover your specific rule, so you want to create a custom validator annotation to check if a string contains only uppercase letters.

🎯 Goal: Build a custom validator annotation called @Uppercase that can be applied to string fields. This validator should check if the string contains only uppercase letters.

📋 What You'll Learn

Create a custom annotation called @Uppercase

Create a validator class that implements ConstraintValidator

Use the custom annotation on a field in a model class

Ensure the validator checks if the string is uppercase only

💡 Why This Matters

🌍 Real World

Custom validators are useful when built-in validation rules do not cover your specific business requirements, such as enforcing uppercase-only codes or special formats.

💼 Career

Knowing how to create and use custom validation annotations is important for backend developers working with Spring Boot to ensure data integrity and user input validation.

1

Create the @Uppercase annotation

Create a custom annotation called @Uppercase with these exact elements: annotate it with @Constraint(validatedBy = UppercaseValidator.class), @Target({ElementType.FIELD}), @Retention(RetentionPolicy.RUNTIME), and include a message element with default value "Must be uppercase". Also include Class[] groups() default {}; and Class[] payload() default {};.

Spring Boot

// Your code here

Need a hint?

Remember to import jakarta.validation.Constraint and set the validatedBy to your validator class.

2

Create the UppercaseValidator class

Create a class called UppercaseValidator that implements ConstraintValidator<Uppercase, String>. Override the isValid method to return true if the input string is null or contains only uppercase letters (use input.equals(input.toUpperCase())).

Spring Boot

import jakarta.validation.ConstraintValidator;
import jakarta.validation.ConstraintValidatorContext;

// Your code here

Need a hint?

Check for null first, then compare the string to its uppercase version.

3

Use @Uppercase on a model field

Create a class called User with a private string field called code. Annotate the code field with @Uppercase. Include public getter and setter methods for code.

Spring Boot

public class User {
    // Your code here
}

Need a hint?

Remember to add the @Uppercase annotation exactly above the code field.

4

Complete the Spring Boot validation setup

In the User class, add the @Validated annotation on the class level to enable validation. Also, add the @NotNull annotation on the code field to require a value.

Spring Boot

@Validated
public class User {
    @Uppercase
    // Your code here
}

Need a hint?

Add @Validated on the class and @NotNull on the code field to complete validation setup.