Why am I getting HTTP 400 bad request

Question

I am using an HTTP client (code copied from http://www.mkyong.com/java/apache-httpclient-examples/ ) to send post requests. I have been trying to use it with http://postcodes.io to look up a bulk of postcodes but failed. According to http://postcodes.io I should send a post request to http://api.postcodes.io/postcodes in the following JSON form: {"postcodes" : ["OX49 5NU", "M32 0JG", "NE30 1DP"]} but I am always getting HTTP Response Code 400. I have included my code below. Please tell me what am I doing wrong? Thanks

private void sendPost() throws Exception {

    String url = "http://api.postcodes.io/postcodes";
    HttpClient client = HttpClientBuilder.create().build();
    HttpPost post = new HttpPost(url);

    List<NameValuePair> urlParameters = new ArrayList<NameValuePair>();
    urlParameters.add(new BasicNameValuePair("postcodes", "[\"OX49 5NU\", \"M32 0JG\", \"NE30 1DP\"]"));
    post.setEntity(new UrlEncodedFormEntity(urlParameters));
    HttpResponse response = client.execute(post);

    System.out.println("Response Code : " 
                + response.getStatusLine().getStatusCode());
    System.out.println("Reason : " 
            + response.getStatusLine().getReasonPhrase());
    BufferedReader br = new BufferedReader(
            new InputStreamReader(response.getEntity().getContent()));
    StringBuffer result = new StringBuffer();
    String line = "";
    while ((line = br.readLine()) != null) {
        result.append(line);
    }
    br.close();
    System.out.println(result.toString());
}

Answer 1

This works, HTTP.UTF_8 is deprecated:

String url = "http://api.postcodes.io/postcodes";
HttpClient client = HttpClientBuilder.create().build();
HttpPost post = new HttpPost(url);

StringEntity params =new StringEntity("{\"postcodes\" : [\"OX49 5NU\", \"M32 0JG\", \"NE30 1DP\"]}");
post.addHeader("Content-Type", "application/json");
post.setEntity(params);

Answer 2

Jon Skeet is right (as usual, I might add), you are basically sending a form and it defaults to form-url-encoding. You could try something like this instead:

String jsonString = "{\"postcodes\" : [\"OX49 5NU\", \"M32 0JG\", \"NE30 1DP\"]}";
StringEntity entity = new StringEntity(jsonObj.toString(), HTTP.UTF_8);
entity.setContentType("application/json");
post.setEntity(entity);