在Restful Web服务中将列表作为基本名称值对发送为HttpPost

Question

我正在尝试将httpPost写入Web服务。我想将列表作为数组发送。这是我到目前为止所做的。

public Response searchMessages(@QueryParam("tags") List<String> tags, @QueryParam("senders") List<String> senders, @QueryParam("api_keys") List<String> apiKeys) throws Exception

{Iterator<String> tagsIt = tags.iterator();
        Iterator<String> sendersIt = senders.iterator();
        Iterator<String> apiKeysIt = apiKeys.iterator();
CloseableHttpClient httpClient  = null;
        HttpPost httpPost =null;
        List<NameValuePair> nvps=null;
        CloseableHttpResponse response=null;
        InputStream  in=null;
        String myResponse = "";
        try
        {
            httpClient  = HttpClients.createDefault();
            httpPost = new HttpPost(url);

            while(tagsIt.hasNext())
            {
                String keyIt = tagsIt.next();
                nvps.add(new BasicNameValuePair("tags",hashMap.get(keyIt)));
            }
            while(sendersIt.hasNext())
            {
                String keySend = sendersIt.next();
                nvps.add(new BasicNameValuePair("senders",hashMap.get(keySend)));
            }
            while(apiKeysIt.hasNext())
            {
                String keySend = sendersIt.next();
                nvps.add(new BasicNameValuePair("apiKeys",hashMap.get(keySend)));
            }   
                //System.out.println(key+" "+hashMap.get(key));



            httpPost.setEntity(new UrlEncodedFormEntity(nvps,Consts.UTF_8));
            response = httpClient.execute(httpPost);

            BufferedReader buffer=null;
            try{


                //System.out.println(response.toString());
                in= response.getEntity().getContent();
                buffer = new BufferedReader(new InputStreamReader(in));
                  String s = "";
                  while ((s = buffer.readLine()) != null) {
                    myResponse += s+"\n";
                  }
                  status= response.getStatusLine().getStatusCode();
                  resp = Response.status(status).entity(myResponse).build();
            }
            catch(Exception e)
            {
                e.printStackTrace();
            }
        }
        finally
        {
            in.close();
            response.close();
            httpClient.close();

        }

我的问题是我在in.close（）行中得到一个空指针异常。 我不知道我的错误在哪里。请帮助。