protected URL getResourceForOptionalProperty(final IClientConfigKey<String> configKey) {
        final String propValue = clientConfig.get(configKey);
        URL result = null;

        if (propValue != null) {
            result = Resources.getResource(propValue);
            if (result == null) {
                throw new IllegalArgumentException("No resource found for " + configKey + ": "
                        + propValue);
            }
        }
        return result;
    }
Resolve the final property value from,
1. Request specific configuration
2. Default configuration
3. Default value

@param key
@param requestConfig
@param defaultValue
@return
ribbon-transport/src/main/java/com/netflix/ribbon/transport/netty/LoadBalancingRxClient.java