HttpURLConnection returning 400 status on post call without payload

Question

Actually there are two POST URLs. First POST url is login as follows and even it contains body as json as follows.

String loginUrl = "http://00.00.00.00:0000/url/vs1/login";
json = `{"email": "qqqq@mail.com", "password": "/JGgdwd6vhsvJJFGDF7ttd="}`

Second POST url without payload as follows.

String SiteLoginUrl = "http://00.00.00.00:0000/url/vs1/site/1/login";

So first, above first url need to login then only i can login to second url. Hence first URL is logging in successfully but after getting response i need to login for second URL which doesn't consists of payload or json. So after calling second URL with POST is returning 400 error. For second URL i have written code below.

// java HttpURLConnection for POST without json or payload data     
        String SiteLoginUrl = "http://00.00.00.00:0000/url/vs1/site/1/login";
        HttpURLConnection conn = (HttpURLConnection) new URL(SiteLoginUrl).openConnection();
        conn.setDoOutput(true); 
        conn.setUseCaches(true);
        conn.setRequestMethod("POST");
        conn.setRequestProperty("ContentType","application/json;charset=UTF-8");
        conn.setDoOutput(true);
        DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
        JSONObject obj = new JSONObject();
        wr.writeBytes(obj.toString()); //String Empty Object
        wr.flush();
        wr.close();

        if(conn.getResponseCode() == HttpURLConnection.HTTP_OK){
            BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String inputLine;
            StringBuffer response = new StringBuffer();
            while ((inputLine = in.readLine()) != null) {
                response.append(inputLine);
            }
            in.close();         
            return 1;
        } else {
            return 0;
        }

In POSTMAN i checked it is working fine (returning successful login response) with text {}(String Empty Object) payload.