Snippet

Service-Aware Custom Constraint Validators

By marking a ConstraintValidator as a Spring @Component, the framework automatically handles dependency injection. This allows performing complex business logic validation, such as checking database uniqueness, directly via JSR-303 annotations.

snippet.java

java

@Component
public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, String> {
    private final UserRepository userRepository;
    public UniqueEmailValidator(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
    @Override
    public boolean isValid(String email, ConstraintValidatorContext context) {
        if (email == null) return true;
        return !userRepository.existsByEmail(email);
    }
}

spring

Breakdown

implements ConstraintValidator<UniqueEmail, String>

Binds the validator to a specific custom annotation and the target data type.

private final UserRepository userRepository;

Injected Spring bean used to perform lookups during validation.

return !userRepository.existsByEmail(email);

The actual validation logic checking state against a persistent data store.

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Service-Aware Custom Constraint Validators

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