Custom Validators in Java

neat

So, you’ve got a signup form, and someone tries to register as kingkong123 for the 14th time. What do you do? Throw the whole server away? Nope. You create a custom validator to ensure usernames are unique.

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Let’s go step by step — short, sweet, and practical.

1. Entity Setup

@Entity
public class User {
@Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @UniqueUsername // <- Our custom annotation
    private String username;
    private String email;
    private String password;
    // Getters and setters
}

2. Custom Annotation

@Constraint(validatedBy = UniqueUsernameValidator.class)
@Target({ ElementType.FIELD })
@Retention(RetentionPolicy.RUNTIME)
public @interface UniqueUsername {
    String message() default "Username already exists";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

3. Validator Class

@Component
@RequiredArgsConstructor
public class UniqueUsernameValidator implements ConstraintValidator<UniqueUsername, String> {

    private final UserRepository userRepository;

    @Override
    public boolean isValid(String username, ConstraintValidatorContext context) {
        return username != null && !userRepository.existsByUsername(username);
    }
}

4. Repository Layer

public interface UserRepository extends JpaRepository<User, Long> {
    boolean existsByUsername(String username);
}

5. Controller Layer

@RestController
@RequestMapping("/api/users")
@RequiredArgsConstructor
public class UserController {

private final UserService userService;

    @PostMapping("/register")
    public ResponseEntity<String> registerUser(@Valid @RequestBody User user) {
        userService.saveUser(user);
        return ResponseEntity.ok("User registered successfully!");
    }
}

6. Service Layer

@Service
@RequiredArgsConstructor
public class UserService {

    private final UserRepository userRepository;
    public void saveUser(User user) {
        userRepository.save(user);
    }
}

7. Global Exception Handling (Make it Reusable)

@RestControllerAdvice
public class GlobalExceptionHandler {
@ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Object> handleValidationException(MethodArgumentNotValidException ex) {
        Map<String, Object> errors = new HashMap<>();
        ex.getBindingResult().getFieldErrors().forEach(error ->
                errors.put(error.getField(), error.getDefaultMessage()));
        return buildErrorResponse(HttpStatus.BAD_REQUEST, "Validation failed", errors);
    }
    private ResponseEntity<Object> buildErrorResponse(HttpStatus status, String message, Map<String, Object> errors) {
        Map<String, Object> response = new HashMap<>();
        response.put("status", status.value());
        response.put("message", message);
        response.put("errors", errors);
        return new ResponseEntity<>(response, status);
    }
}

Final Words

There you go. You now have a working custom validator in Java. Say goodbye to duplicate usernames and hello to clean data.