docs: public static Pool retrievePool(UrlParser urlParser) {
      if (!poolMap.containsKey(urlParser)) {
        synchronized (poolMap) {
          if (!poolMap.containsKey(urlParser)) {
            if (poolExecutor == null) {
              poolExecutor = new ScheduledThreadPoolExecutor(1,
                  new MariaDbThreadFactory("MariaDbPool-maxTimeoutIdle-checker"));
            }
            Pool pool = new Pool(urlParser, poolIndex.incrementAndGet(), poolExecutor);
            poolMap.put(urlParser, pool);
            return pool;
          }
        }
      }
      return poolMap.get(urlParser);
    }