Cassandra Asyn连接限制

Question

我从一个表中获取数据然后迭代该ResultSet以从另一个表中获取数据（executeAsync（））。我想控制异步任务的数量。

for(Row row : resultSites) {
        String siteName = row.getString("site_name");
        System.out.println(siteName);
        ResultSetFuture future = casCon.session.executeAsync(casCon.boundStatement.bind(siteName));
        Futures.addCallback(future,fCR,MoreExecutors.listeningDecorator(executorService));
    }

我的执行者和FutureCallback是

    ExecutorService executorService = Executors.newFixedThreadPool(2);
    FutureCallback<ResultSet> fCR = new FutureCallback<ResultSet>() {
                @Override public void onSuccess(ResultSet result) {
                    System.out.println(result.one());
                }
                @Override public void onFailure(Throwable t) {
                System.out.println("Error while reading Cassandra version: " + t.getMessage());
            }
        };

现在通过搜索我发现了这个并在我的代码中实现了它

PoolingOptions poolingOpts = new PoolingOptions();
            poolingOpts.setCoreConnectionsPerHost(HostDistance.REMOTE, 2);
               poolingOpts.setMaxConnectionsPerHost(HostDistance.REMOTE, 4);
               poolingOpts.setMaxSimultaneousRequestsPerConnectionThreshold(HostDistance.REMOTE, 10);
               poolingOpts.setMinSimultaneousRequestsPerConnectionThreshold(HostDistance.REMOTE, 2);
               poolingOpts.setMinSimultaneousRequestsPerConnectionThreshold(HostDistance.LOCAL, 2);
               poolingOpts.setMaxSimultaneousRequestsPerConnectionThreshold(HostDistance.LOCAL, 10);


            cluster = Cluster.builder().addContactPoint(ip)
                    .withLoadBalancingPolicy(new DCAwareRoundRobinPolicy())
                    .withPoolingOptions(poolingOpts)
                    .withCredentials(user, pass)
                    .withRetryPolicy(DefaultRetryPolicy.INSTANCE)
                    .build();

但是当我打电话时

System.out.println(session.getCluster().getConfiguration().getPoolingOptions().getMaxConnectionsPerHost(HostDistance.REMOTE));

我得到空指针异常所以我不确定我是否已正确实现它？有任何帮助或建议吗？