Spring Validation – Validator

In Spring, the Validator interface is used for custom validation logic on objects, typically before saving them to a database or processing user input.

🧩 Purpose

• Validate fields of a Java object (e.g., form input).
• Can implement complex rules not possible with simple annotations (@NotNull, @Size).
• Works with Spring MVC’s @Valid or programmatically.

Key Interface

public interface Validator {
    boolean supports(Class<?> clazz);       // Check if the validator can validate this class
    void validate(Object target, Errors errors);  // Perform validation
}

Example

public class UserValidator implements Validator {

    @Override
    public boolean supports(Class<?> clazz) {
        return User.class.equals(clazz);
    }

    @Override
    public void validate(Object target, Errors errors) {
        User user = (User) target;

        if (user.getUsername().isEmpty()) {
            errors.rejectValue("username", "username.empty", "Username is required");
        }

        if (user.getPassword().length() < 6) {
            errors.rejectValue("password", "password.short", "Password must be at least 6 characters");
        }
    }
}

Controller Usage:

@PostMapping("/register")
public String register(@ModelAttribute User user, BindingResult result) {
    new UserValidator().validate(user, result);
    if (result.hasErrors()) {
        return "registerForm";
    }
    userService.save(user);
    return "success";
}

🧠 Key Points

• supports() ensures the validator only validates compatible classes.
• validate() adds errors to Errors object, which Spring MVC binds to the view.
• Can be combined with annotation-based validation for flexibility.