Should Validation Logic Reside in Spring MVC Controllers or the Service Layer?

Where should validation logic be implemented in a Spring MVC application: within the controllers or the service layer?

@Valid
public String createUser(@Valid User user, BindingResult result) {
    if (result.hasErrors()) {
        return "form";
    }
    userService.save(user);
    return "redirect:/users";
}

In Spring MVC applications, deciding where to place validation logic is crucial for maintaining clean architecture and separation of concerns. Generally, validation can be conducted in both controllers and the service layer, but each approach has its own implications.

@Service
public class UserService {
    public void save(@Valid User user) {
        //Business logic and validation
        if (user.getAge() < 18) {
            throw new IllegalArgumentException("User must be at least 18 years old.");
        }
        userRepository.save(user);
    }
}

Causes

• Controllers are designed to handle HTTP requests and responses, making it convenient to validate user input immediately before processing it.

Solutions

• Place simple, presentation-specific validations directly in controllers to quickly prevent invalid data from reaching the service layer.
• Use the service layer for complex business logic validation to ensure that business rules are enforced consistently across different entry points.

Mistake: Validating the same data in both the controller and service layer, leading to redundancy.

Solution: Centralize validation in the service layer and keep controller validations minimal.

Mistake: Neglecting to return specific error messages to the user on validation failure.

Solution: Use BindingResult in controllers to display proper validation error messages to users.