private static String[] optionalVar(String[] arr, String[] defaultArr) {
        if (arr == null || arr.length==0) {
            return defaultArr;
        }
        return arr;
    }
List A - List B

@param allowed allowed items
@param disallowed disallowed items
@return allowed items minus disallowed items
gateway/platforms/servlet/src/main/java/io/apiman/gateway/platforms/servlet/connectors/ssl/SSLSessionStrategyFactory.java