如果要共享连接，请不要在请求后关闭客户端。
也就是说，你可能在做这样的事情：

PoolingHttpClientConnectionManager pool = new PoolingHttpClientConnectionManager();
...
CloseableHttpClient httpclient = HttpClients.custom()
     .setConnectionManager(pool)
     .build();

try { // try-with-resources
    HttpGet httpget = new HttpGet(url.toURI());
    try (CloseableHttpResponse response = httpclient.execute(httpget);
             InputStream fis = response.getEntity().getContent();
            ReadableByteChannel channel = Channels.newChannel(fis)) {
             // ... get data ...
     } finally {
         httpclient.close(); <====== !!
     }
} catch (IOException | URISyntaxException e) {
    // exception handling ...
}

那个 httpclient.close() 正在导致下一个池连接失败。