String key = "non_existent";
        crawlerStatsHelper.discard(key);

        assertNull(localLogMsg.get());
    }

    static class TestUrlQueue implements UrlQueue<String> {
        private String id;
        private String url;

        public TestUrlQueue(String id, String url) {
            this.id = id;
            this.url = url;
        }

        @Override
        public String getId() {