How to apply custom validation on a field using javax validation only if it is not null?

I am working on javax validation API in Spring Boot Application. I have a User bean and i have to validate that username given in request is unique and doesn't exist into Database.

I have created custom annotation (UniqueUser.java) and custom Validator(UniqueUserValidator.java) for this requirement.

public class User  {
    @NotNull
    @UniqueUser
    private String username;

    @NotNull
    private String password;

    @NotNull
    @Email
    private String email;

    @NotNull
    private String phone;
}

UniqueUser.java

@Target({TYPE, ANNOTATION_TYPE})
@Retention(RUNTIME)
@Constraint(validatedBy = UniqueUserValidator.class)
@Documented
public @interface NameMatch
{
    String message() default "User id already exists";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};
}

UniqueUserValidator.java

public class UniqueUserValidator implements ConstraintValidator<NameMatch, Object>
{
    @Autowired
    UserRepository userRepository;
   
    @Override
    public boolean isValid(String userName, final ConstraintValidatorContext context)
    {
        boolean isValidUser = false;
        if(userName!=null && !userName.isEmpty()) {
        Optional<User> user= userRepository.findByUserId(userName);
           isValidUser = !user.isPresent(); 
        }
        return isValidUser;
    }
}

In above code, the unique user validation get called for username field even if the field is null and shows Username already exists error message. I want the custom validator to get called only when username has some value. Is it possible to avoid this call.

I can fix the error by modifying the below method and returning true if username is null, but i want't to avoid this unnecessary call.

public class UniqueUserValidator implements ConstraintValidator<NameMatch, Object>
{
    @Autowired
    UserRepository userRepository;
   
    @Override
    public boolean isValid(String userName, final ConstraintValidatorContext context)
    {
        boolean isValidUser = false;
        if(userName!=null && !userName.isEmpty()) {
        Optional<User> user= userRepository.findByUserId(userName);
           isValidUser = !user.isPresent(); 
        } else {
           isValidUser = true; 
        }
        return isValidUser;
    }
} 

I think what you are looking for is Grouping constraints;

> Groups allow you to restrict the set of constraints applied during
> validation. One use case for validation groups are UI wizards where in
> each step only a specified subset of constraints should get validated.
> The groups targeted are passed as var-arg parameters to the
> appropriate validate method.

In your case it should look like this:

@GroupSequence({ FirstConstaint.class, SecondConstaint.class })
public class User  {
    @NotNull(groups = FirstConstaint.class)
    @UniqueUser(groups = SecondConstaint.class)
    private String username;

    // the rest of your fields
}

interface FirstConstaint {
}

interface SecondConstaint {
}

This way, it will check @UniqueUser only if the field is not null, otherwise, it will return the message of @NotNull validation.

Otherwise like @User said you can use this checks in service layer ^^