CustomGoogleEmailValidator.java

public class CustomGoogleEmailValidator implements ConstraintValidator<CustomGoogleEmailConstraint, String> {
  @Override
  public void initialize(CustomGoogleEmailConstraint email) { }
  
  @Override
  public boolean isValid(String email, ConstraintValidatorContext constraintValidatorContext) { 
    return email != null && email.matches("^[a-zA-Z0–9.]+@gmail.com"); 
  }
}