@Nullable
    @Override
    public OAuth2Token apply(RequestHeaders headers) {
        final String authorization = requireNonNull(headers, "headers").get(header);
        if (Strings.isNullOrEmpty(authorization)) {
            return null;
        }

        final Matcher matcher = AUTHORIZATION_HEADER_PATTERN.matcher(authorization);
        if (!matcher.matches()) {
            logger.debug("Invalid authorization header: {}", authorization);
            return null;
        }

        return AuthToken.ofOAuth2(matcher.group("accessToken"));
    }
Extracts {@link OAuth2Token} from {@link RequestHeaders}, in order to be used by
{@link AuthServiceBuilder}.
core/src/main/java/com/linecorp/armeria/server/auth/OAuth2TokenExtractor.java