使用springboot的RestTemplate发送post请求，为啥报如下错误，代码如下？

Question

public class HttpClientTemplate {
    public static String sendPOSTRequest(String url, Map<String, String> params) {
        RestTemplate client = new RestTemplate();
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_JSON);
        HttpEntity<Map<String, String>> r = new HttpEntity<>(params, headers);
        String data= client.postForObject(url, r, String.class);
        return data;
    }

}