向指定URL发送POST请求的方法

此方法只支持HTTP请求，HTTPS请求会报错！
示例如下：

    /**
     * 向指定URL发送POST请求，格式为JSON
     * @param url
     * @param jsonStr
     * @return
     * @throws Exception
     */
public static String sendHttpPost(String url, String jsonStr) throws Exception {
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpPost httpPost = new HttpPost(url);
        httpPost.addHeader("Content-Type", "application/json");
        httpPost.setEntity(new StringEntity(jsonStr));

        CloseableHttpResponse response = httpClient.execute(httpPost);
        System.out.println(response.getStatusLine().getStatusCode() + "\n");
        HttpEntity entity = response.getEntity();
        String responseContent = EntityUtils.toString(entity, "UTF-8");
        System.out.println("loginResultJson:" + responseContent);

        response.close();
        httpClient.close();
        return responseContent;
    }