Best Practices - Spring Framework - Should Validation Be implemented in Business Service Or Be In Separated?

Recently I've run into a debate with one of my colleagues about where we should keep business validations. I have searched on stackoverflow for similar questions but none of them gives me a comparison between the 2 approaches which I'm gonna explain.

Approach 1: Validation should be separated from the business service (my approach)

We have a Spring Rest application. I use the interface org.springframework.validation.Validator. Let's call the validator as CreateUserValidator. And the business rule is email address has to be unique.

I then hooked up the validator to the controller by InitBinder as follows:

@Autowired
private CreateUserValidatorvalidator;

@InitBinder
private void initBinder(WebDataBinder binder) {
    binder.addValidators(validator);
}

@RequestMapping(method = RequestMethod.POST)
public ResponseEntity<User> createUser(
        @Valid @RequestBody User user)
        throws MethodArgumentNotValidException {
    logger.debug("Creating User");

    user = userService.createUser(user);

    return new ResponseEntity<User>(user, HttpStatus.OK);

}

Pay attention to @Valid in the method createUser(). It means that the user object will be validated through a series of validators (Hibernates, Spring,...) and my validator which is CreateUserValidator.

Hence, the userService doesn't have to do the validation jobs.

Approach 2: Validation should be in the Service Layer.

For this, please look at the code below:

@Override
public User createUser(final User user) {
    if(this.findByName(user.getName())!=null){
        throw new DuplicateKeyException("user.name.duplicate",new Object[]{user.getName()});
    }
    return userRepository.save(user);
}

The whole idea is validation should be in the service.

Arguments

Approach 1:

1. Pros
   
   
   
   • Clearer
   
   
   • Validators can be reused
   
   
   
   


2. Cons
   
   
   
   • If we expose the service as an API, we are missing the validations
   
   
   
   

Approach 2:

1. 
   

   Pros

   
   
   
   
   • Validations are bound to Service
   
   
   
   


2. 
   

   Cons

   
   
   
   
   • Validators cant be reused
   
   
   • Mix of validations and business code in the service
   
   
   
   

Can you give your opinions on this?

Thanks,