public static Pool getDefaultPool() {
        Pool defaultPool = poolMap.get(DEFAULT);
        if (defaultPool == null) {
            synchronized (HttpClientPoolUtils.class) {
                defaultPool = poolMap.get(DEFAULT);
                if (defaultPool == null) {
                    defaultPool = new Pool(null);
                    poolMap.put(DEFAULT, defaultPool);
                }
            }
        }
        return defaultPool;
    }