@Override
    public void initialize(final ValidDirectoryPath constraintAnnotation) {
        this.ensureExistence = constraintAnnotation.ensureExistence();
    }
Initializes the validator in preparation for
{@link #isValid(String, ConstraintValidatorContext)} calls.
The constraint annotation for a given constraint declaration
is passed.
<p/>
This method is guaranteed to be called before any use of this instance for
validation.

@param constraintAnnotation annotation instance for a given constraint declaration
stroom-util/src/main/java/stroom/util/validation/ValidDirectoryPathValidatorImpl.java