인증 여부 확인 중복 코드 개선

public final class AuthenticationPrincipalArgumentResolver implements HandlerMethodArgumentResolver {

	private ExpressionParser parser = new SpelExpressionParser();

	private BeanResolver beanResolver;

	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		return findMethodAnnotation(AuthenticationPrincipal.class, parameter) != null;
	}

	@Override
	public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
			NativeWebRequest webRequest, WebDataBinderFactory binderFactory) {
		Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
		if (authentication == null) {
			return null;
		}
		Object principal = authentication.getPrincipal();
		AuthenticationPrincipal annotation = findMethodAnnotation(AuthenticationPrincipal.class, parameter);
		String expressionToParse = annotation.expression();
		if (StringUtils.hasLength(expressionToParse)) {
			StandardEvaluationContext context = new StandardEvaluationContext();
			context.setRootObject(principal);
			context.setVariable("this", principal);
			context.setBeanResolver(this.beanResolver);
			Expression expression = this.parser.parseExpression(expressionToParse);
			principal = expression.getValue(context);
		}
		// -> 여기에서 CastException을 자동
		if (principal != null && !ClassUtils.isAssignable(parameter.getParameterType(), principal.getClass())) {
			if (annotation.errorOnInvalidType()) {
				throw new ClassCastException(principal + " is not assignable to " + parameter.getParameterType());
			}
			return null;
		}
		return principal;
	}

	/**
	 * Sets the {@link BeanResolver} to be used on the expressions
	 * @param beanResolver the {@link BeanResolver} to use
	 */
	public void setBeanResolver(BeanResolver beanResolver) {
		this.beanResolver = beanResolver;
	}

	/**
	 * Obtains the specified {@link Annotation} on the specified {@link MethodParameter}.
	 * @param annotationClass the class of the {@link Annotation} to find on the
	 * {@link MethodParameter}
	 * @param parameter the {@link MethodParameter} to search for an {@link Annotation}
	 * @return the {@link Annotation} that was found or null.
	 */
	private <T extends Annotation> T findMethodAnnotation(Class<T> annotationClass, MethodParameter parameter) {
		T annotation = parameter.getParameterAnnotation(annotationClass);
		if (annotation != null) {
			return annotation;
		}
		Annotation[] annotationsToSearch = parameter.getParameterAnnotations();
		for (Annotation toSearch : annotationsToSearch) {
			annotation = AnnotationUtils.findAnnotation(toSearch.annotationType(), annotationClass);
			if (annotation != null) {
				return annotation;
			}
		}
		return null;
	}

}